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Mechanism of Layer Formation on Olivine Bed Particles in Industrial-Scale Dual Fluid Bed Gasification of Wood
Abstract
Utilization of biomass as feedstock in dual fluidized bed steam gasification is a promising technology for the substitution of fossil energy carriers. Experience from industrial scale power plants showed an alteration of the olivine bed material due to interaction with biomass ash components. This change results mainly in the formation of Ca-rich layers on the bed particles. In this paper, a mechanism for layer formation is proposed and compared to the better understood mechanism for layer formation on quartz bed particles. Olivine bed material was sampled at an industrial scale power plant before the start of operation and at pre-defined times after the operation had commenced. Therefore, time dependent layer formation in industrial-scale conditions could be investigated. The proposed mechanism suggests that the interaction between wood biomass ash and olivine bed particles is based on a solid-solid substitution reaction, where Ca2+ is incorporated into the crystal structure. As a consequence Fe2+/3+ and Mg2+ ions are expelled as oxides. This substitution results in the formation of cracks in the particle layer due to a volume expansion in the crystal structure once Ca2+ is incorporated. The results of this work are compared to relevant published results including those related to quartz bed particles.
... Within the usual composition of woody biomass ash, the most reactive elements providing catalytic properties are calcium and potassium [8]. These ash elements are proven to interact with the bed material changing its physicochemical properties with time of exposure [10] and different layers with predominant species are usually identified over the bed material particles [11,12]. The catalytic activity of the bed material will thus be different with time as the ash layer develops, and simultaneously the bed material will be able to transport inorganic species between the reactors, e.g., sulphur [8], or potassium [13]. ...
... In this regard, oxides of transition metals act as oxygen carriers which oxidize with the air in the combustion side and get reduced by reacting with the feedstock in the gasifier [14]. Olivine is to date the bed material of choice in most largescale DFB gasifiers due to its high acquired catalytic activity by interaction with biomass ash and its mechanical strength [11,15,16]. ...
... The study of the ash layer formation and the distribution of the different ash elements over the ash layer is beyond the scope of this work. The reader is referred to the works of Faust et al. and Kuba et al. for a comprehensive characterization of the ash layer over olivine particles exposed to biomass ash in large DFB units [11,32,33]. ...
In this work, the effect of ash activated olivine on olefinic products cracking and aromatization was assessed. The experiments were carried out in the Chalmers 2–4 MWth dual fluidized bed gasifier, where the feedstock was cracked using steam as fluidization agent, at a reaction temperature of ca. 780–790 °C. Three activation states of the olivine, representing three consecutive days of the campaign, were evaluated. The changes of the permanent gas composition along with the reduction in aromatic species with the time of exposure to biomass ash demonstrate a clear effect of the ash activated olivine on the conversion of olefinic cracking products. The ash activation of the olivine clearly promoted the reactions involving steam. As a consequence, higher yields of permanent gases, mainly H2, CO and CO2, were produced at expenses of the yields of the total aromatic compounds and C4 hydrocarbons and larger. It is concluded that the biomass ash activated olivine promotes the steam reforming path of the C4 and larger hydrocarbon fragments, while avoiding the alternative aromatization route. The results presented here provide useful insights on the opportunities and limitations of ash activated materials in DFB systems when steam cracking linear hydrocarbon feedstocks, e.g., polyolefin-based materials.
... On the surface of the particles, the outer layer is heterogeneous consisting mainly of elements found in the fuel ash. However, ash layer characteristics are dependent on the bed material, fuel ash and the selected conversion process [17][18]. For feldspar, the developed inner layer comprises both Ca-Al-silicates and Ca-silicate while in the outer layer Ca 3 Mg(SiO 4 ) 2 and MgSiO 3 could be detected [19][20]. ...
... For feldspar, the developed inner layer comprises both Ca-Al-silicates and Ca-silicate while in the outer layer Ca 3 Mg(SiO 4 ) 2 and MgSiO 3 could be detected [19][20]. In the inner layer formed around olivine bed particles, the formation of CaMgSiO 4 was found [17,21]. In both cases, incorporation of higher amounts of Ca into the inner layer structure was associated with cracks in the inner layer perpendicular to the bed particle surface [17,19]. ...
... In the inner layer formed around olivine bed particles, the formation of CaMgSiO 4 was found [17,21]. In both cases, incorporation of higher amounts of Ca into the inner layer structure was associated with cracks in the inner layer perpendicular to the bed particle surface [17,19]. ...
Olivine and alkali-feldspar were utilized in separate campaigns in an indirect dual fluidized bed gasification campaign with woody biomass as fuel. After three days, both bed materials were reported to be active towards tar removal and exhibited oxygen-carrying abilities and had formed an ash layer consisting of an outer ash deposition layer and an inner interaction layer.
X-ray microtomography analysis concluded that a preferred deposition of ash happens onto convex regions of the bed particles, which results in an increase in thickness of the ash layer over convex regions. This effect is most pronounced for the outer layer which is a product of ash deposition. The inner layer exhibits a homogeneous thickness and is probably formed by interaction of Ca from the outer layer with the particles. Transmission electron microscopy revealed the presence of Fe and Mn on the surface of the particles in a solid solution with Mg. The oxygen-carrying effect which is found for aged particles is therefore attributed to the presence of Fe and Mn on the surface of aged particles. Alkali were found on the surface of both particles which are likely contributing to the catalytic activity of the material towards tar removal.
... On the surface of the particles, the outer layer is heterogeneous consisting mainly of elements found in the fuel ash. However, ash layer characteristics are dependent on the bed material, fuel ash and the selected conversion process [17][18]. For feldspar, the developed inner layer comprises both Ca-Al-silicates and Ca-silicate while in the outer layer Ca 3 Mg(SiO 4 ) 2 and MgSiO 3 could be detected [19][20]. ...
... For feldspar, the developed inner layer comprises both Ca-Al-silicates and Ca-silicate while in the outer layer Ca 3 Mg(SiO 4 ) 2 and MgSiO 3 could be detected [19][20]. In the inner layer formed around olivine bed particles, the formation of CaMgSiO 4 was found [17,21]. In both cases, incorporation of higher amounts of Ca into the inner layer structure was associated with cracks in the inner layer perpendicular to the bed particle surface [17,19]. ...
... In the inner layer formed around olivine bed particles, the formation of CaMgSiO 4 was found [17,21]. In both cases, incorporation of higher amounts of Ca into the inner layer structure was associated with cracks in the inner layer perpendicular to the bed particle surface [17,19]. ...
... Öhman, et al. [112] examined coating distribution across the particles examined with SEM/EDX, as summarised in More recently, several groups have observed calcium layering when using olivine in fluidized bed gasifiers, mostly when using woody fuels [115,116,117,118]. For example, Kirnbauer & Hofbauer [117] investigated use of olivine with a wood fuel in an industrial-scale dual fluidized bed gasification. ...
... Olivine has been the focus of some studies of agglomeration phenomena with biomass fuels, but these are overwhelmingly focused on its use with woody fuels, and in gasifiers where olivine also offers catalytic benefits [114,116,117,118,233,234]. ...
... Regardless, calcium is clearly driven to the olivine particle surface to form this calcium-rich layer. Others have observed the formation of a calcium-rich layer when using olivine in different contexts, such as fluidized bed gasification of wood [117,118], miscanthus [116], or fluidized bed combustion of wood [114]. These other groups generally observed calcium layers to arise as a standalone feature on olivine particles, whereas here they were only observed on bed particles embedded in a large ash melt phase and was observed uniquely with the combustion of wheat straw. ...
Fluidized bed combustion technology is increasingly used for biomass fuels, due to the high variability of their energy density and composition. However, this technology is still susceptible to ash-related issues. Agglomeration is caused by ash melting onto or reacting with bed material to form alkali silicate melts, allowing bed particles to adhere together. Accumulation of agglomerates causes bed defluidization, and consequently unscheduled downtime. This thesis investigates agglomeration mechanisms and mitigation measures at the pilot-scale, focusing on agricultural fuels that have received less attention in literature and may be of interest for boiler operators. When burning wheat straw, the magnesium-iron silicate bed material olivine lengthened defluidization times versus silica sand, though this was not sufficient to make the fuel viable. The additives kaolin and dolomite prevented bed defluidization entirely when burning miscanthus, but had no effect with wheat straw, despite chemically reacting with both fuel ashes. In combination with thermochemical modelling, it was proposed that the poor breakdown of wheat straw pellet sand release of ash to their surface allows the pellet to act as a seed for agglomerate formation, hence additives proving ineffective. Agglomeration mechanisms were studied with different fuels, bed materials and additives. This included a novel analysis of agglomerates from different bed locations, and a spatially defined study of agglomerates from tests with additives, both of which revealed mechanisms in greater detail than previously reported. A novel thermochemical modelling approach using FactSage was applied to agglomerate compositional data, together with an appraisal of the software for agglomeration studies. Through collaboration with project sponsor Sembcorp Energy UK on their “Wilton 10” bubbling fluidized bed boiler, a 5-year fuel data set was studied to determine fuel quality improvement potential. Several analytical methods were applied, including a machine learning algorithm. Recommendations were made regarding fuel quality and sampling.
... Due to the high number of reaction pathways of the inorganic components, expected for different residual fuels, the knowledge needed for further developing step-by-step reaction models describing ash transformation is still gathered. The development of Boström's original work to consider tertiary reactions using Lewis acid/base classification of reactants, has proven generally applicable in the discussion of driving forces for cationic substitution in silicate-based ashes and bed materials [45][46][47][48]. In particular, it has aided the identification of chemical driving forces responsible for compositional changes in layer formation where the mobility of Ca 2+ in silicate matrices has proven important for layer growth, composition, as well as process conditions. ...
... The exchangeabe sites M1 or M2 are occupied by either Fe 2+ or Mg 2+ ions. Fe 2+ is statistically more likely to be replaced by Ca 2+ than Mg 2+ [48]. ...
... Thus, Fig. 10. Olivine bed material particle displaying a Ca/Mg-rich inner layer and a Ca-rich outer layer from industrial-scale DFB combustion of woody-type biomass, adapted from Kuba et al. [48]. ...
Bed material particle layer formation plays a significant role in thermo-chemical conversion of biomass. The interaction between biomass ash and bed material in fluidized bed conversion processes has been described for a variety of different applications and spans from fundamental research of formation mechanisms to effects of this layer formation on long-term operation in industrial-scale. This review describes the current state of the research regarding the mechanisms underlying layer formation and the positive influence of bed material particle layer formation on the operation of thermo-chemical conversion processes. Thus, the main focus lies on its effect on the catalytic activity towards gasification reactions and the impact on oxygen transport in chemical looping combustion. The review focuses on the most commonly investigated bed materials, such as quartz, feldspar or olivine. While the most relevant results for both the underlying mechanisms and the subsequently observed effects on the operation are presented and discussed, knowledge gaps where further research is necessary are identified and described.
... Biomass ash plays a major role in fluidized bed gasification, since it interacts with the bed material resulting in ash layer formation on the bed particles' surface [11][12][13]. This ash layer formation has been studied for different types of bed materials [14], such as olivine [15], quartz [16], or Kfeldspar [14,17], describing the mechanisms underlying the formation of those layers. While ash layer formation is often associated with negative consequences, such as increased agglomeration tendency [11,18,19] or deposit formation [20,21], in steam gasification an increase in its catalytic activity leading to an improvement of the product gas quality could be observed [22,23]. ...
... Ash-layered olivine is showing brighter color, which means an increase in local particle density. This can be caused by structural changes in the layer originating from the layer formation mechanism, which is based on a substitution reaction, where Ca 2+ ions are incorporated into the crystal structure of olivine, while Fe 2+ and Mg 2+ ions are excluded from the crystal structure [15]. Furthermore, other ash components, present to a minor extent in comparison to CaO, such as K or Si, can lead to sintering on the surface further increasing the density of the layer. ...
... The more clearly defined, three-dimensional, landscape on the particle surface shows the change of morphology through ash layer formation. Deposited ash particles forming the outer particle layer-compared to the inner reaction layer described in Kuba et al. [15]-redefines the newly developed surface. Table 2 shows measurements from the used layered olivine sample which is investigated in this study. ...
The present study aims to present a comprehensive characterization of the surface of ash-layered olivine bed particles from dual fluidized bed gasification. It is well known from operation experience at industrial gasification plants that the bed material is activated during operation concerning its positive influence on gasification reactions. This is due to the built up of ash layers on the bed material particles; however, the chemical mechanisms are not well understood yet. Olivine samples from long-term operation in an industrial-scale gasification plant were investigated in comparison to fresh unused olivine. Changes of the surface morphology due to Ca-enrichment showed a significant increase of their surface area. Furthermore, the Ca-enrichment on the ash layer surface was distinctively associated to CaO being present. The presence of CaO on the surface was proven by adsorption tests of carbon monoxide as model compound. The detailed characterization contributes to a deeper understanding of the surface properties of ash layers and forms the basis for further investigations into their influence on gasification reactions.
... The aging of olivine, a bed material that is used in several industrial-scale DFBG units, 27,28 has been studied by several authors. [29][30][31] A proposed mechanism for the layer formation can be found elsewhere. 30 Olivine that has been used in the gasification of woody biomass is rich in Ca and K, whereas fresh olivine does not contain measurable amounts of these elements. ...
... [29][30][31] A proposed mechanism for the layer formation can be found elsewhere. 30 Olivine that has been used in the gasification of woody biomass is rich in Ca and K, whereas fresh olivine does not contain measurable amounts of these elements. 31,32 Ca is mainly present in the ash layer coating the activated olivine particle, whereas K exists primarily inside the particle. ...
... The build-up of a Ca-rich ash layer around the activated olivine particles is clearly visible, whereas K is mainly present in the particle core together with Si, in agreement with previous findings. 30 A line-scan of an activated olivine particle ( Figure 8) shows that the Ca concentration is high in the ash layer and decreases towards the particle core. The high concentration of Ca in the ash layer suggests that Ca can potentially catalyse char gasification through a solid-solid contact between the external surface of the char particle and either the ash layer around the olivine particle or fly ash from the ash layer (see Figure 1). ...
The use of certain bed materials has been found to increase the steam gasification rate of biomass char. The present work investigates how this phenomenon is influenced by different parameters (e.g., temperature, fuel type, and fuel moisture content), using a laboratory-scale bubbling fluidized bed gasifier. Silica sand, fresh olivine, and activated olivine were employed as bed materials, and three biomass fuels (wood chips, wood pellets, and forest residue pellets) were considered. Switching the bed material from silica sand to activated olivine resulted in a significant increase in the char gasification rate for all three fuels, with further increases noted as the fuel particle size was decreased. The observed effect was strongest (up to 4-fold) during the initial conversion phase (char gasification degrees < 20%) when the temperature was relatively low (≤ 800 °C). The moisture content of the wood chips (0%–40%) had no significant effect on the char gasification rate.
... In order for olivine to operate optimally it must undergo an "activation" process, which includes an ash layer formation [19]. This involves formation of a calcium rich layer, migration of iron to the surface of the particle and interaction with ash components [20][21][22]. The formation of an ash layer has been investigated by several researchers [15,21,23,24]. ...
... This involves formation of a calcium rich layer, migration of iron to the surface of the particle and interaction with ash components [20][21][22]. The formation of an ash layer has been investigated by several researchers [15,21,23,24]. During biomass gasification potassium enrichments have been observed in the formed cracks of olivine particles [24] along with calcium rich outer layer [15]. ...
Gasification experiments in this study were performed in a 2–4 MW indirect gasifier coupled to a semi-commercial CFB combustor at Chalmers University of Technology. Experiments were carried out during 13 days with automotive shredder residue (ASR), giving a unique opportunity to investigate the bed material under realistic conditions and with long residence times. The metal rich ash was accumulated in the bed, gaining some oxygen carrying capabilities, creating a chemical looping gasification (CLG) process. This study aims to expand the knowledge about the chemistry of zinc, copper, lead and antimony during CLG of ASR. Several experimental methods have been utilized, such as XRD, SEM-EDX and XPS along with detailed thermodynamic calculations to study chemical transformations that can occur in the system. Thermodynamic calculations showed that the reduction potential affect the phase distribution of these elements, where highly reduction conditions result in heavy metals dissolving in the slag phase. Copper and zinc ferrites, lead silicates and antimony oxides were identified at the particle surfaces in the bottom ash. The formation of an iron rich ash layer plays an important role, especially for copper and zinc speciation. The main pathways in the complex CLG system have been discussed in detail.
... The agglomeration behaviour of macroalgae can be different from that of terrestrial biomass due to the difference in ashforming elemental composition. The agglomeration behaviour of terrestrial biomass has been investigated extensively [22][23][24][25][26][27][28][29][30][31][32][33][34][35]. The agglomeration behaviour for terrestrial biomass with a high K and a low Si content, tends to be coating-induced and to result from the reaction of K and Si from the bed material [32]. ...
... The investigation of the time-dependent biomass ash-bed material interaction behaviours or layer formation contributes to a better understanding of agglomeration. The layer formation process for different bed materials including quartz sand, olivine, ilmenite, manganese ore, Na-Feldspar and K-Feldspar has been assessed with terrestrial biomass as the feedstock [30,31,34,[43][44][45][46][47][48]. The agglomeration tendency of quartz sand is significantly higher than that of olivine and K-feldspar with woody biomass as the feedstock in a dual fluidized bed gasifier [49]. ...
... Furthermore, inner layer formation can be based on a substitution reaction, as observed for olivine, where Ca 2+ ions substitute for Fe 2+ and Mg 2+ ions in the crystal structure of olivine. Therefore, inner layers typically grow inwards into the bed particles [35]. ...
... Based on the results presented here, woody fuels typically shows both inner and outer layer formation (Fig. 7 a)), whereas P-rich fuels show a stronger tendency towards the formation of an outer layer only (Fig. 7 b)). Mechanisms for layer formation frequently proposed in the literature [34,35] typically refer to inner layer formation, since the outer layers are merely the accumulation of ash components and are therefore highly dependent on the fuel ash content itself. ...
Understanding layer formation on bed materials used in fluidized beds is a key step for advances in the application of alternative fuels. Layers can be responsible for agglomeration-caused shut-downs but they can also improve the gas composition in fluidized bed gasification. Layers were observed on K-feldspar (KAlSi3O8) impurities originating from the combined heat and power plant Senden which applies the dual fluidized bed (DFB) steam gasification technology. Pure K-feldspar was therefore considered as alternative bed material in DFB steam gasification. Focusing on the interactions between fuel ash and bed material, K-feldspar was tested in combustion and DFB steam gasification atmospheres using different fuels, namely Ca-rich bark, Ca- and P-rich chicken manure, and an admixture of chicken manure to bark. The bed particle layers formed on the bed material surface were characterized using combined scanning electron microscopy and energy-dispersive X-ray spectroscopy; area mappings and line scans were carried out for all samples. The obtained data show no essential influence of operational mode on the layer-formation process. During the combustion and DFB steam gasification of Ca-rich bark, a layer rich in Ca formed while K was diffusing out of the layer. The use of Ca- and P-rich chicken manure inhibited the diffusion of K, and a layer rich in Ca and P formed. The addition of P to bark via chicken manure also changed the underlying layer-formation processes to reflect the same processes as observed for pure chicken manure.
... Furthermore, inner layer formation can be based on a substitution reaction, as observed for olivine, where Ca 2+ ions substitute for Fe 2+ and Mg 2+ ions in the crystal structure of olivine. Therefore, inner layers typically grow inwards into the bed particles [35]. ...
... Based on the results presented here, woody fuels typically shows both inner and outer layer formation (Fig. 7 a)), whereas P-rich fuels show a stronger tendency towards the formation of an outer layer only (Fig. 7 b)). Mechanisms for layer formation frequently proposed in the literature [34,35] typically refer to inner layer formation, since the outer layers are merely the accumulation of ash components and are therefore highly dependent on the fuel ash content itself. ...
Today, mainly wood-based feedstocks are used in thermo-chemical biomass conversion since they have a comparably high heating value and contain a small amount of ash. Fluidized beds allow a greater variety of fuels to be used, since they are rather flexible regarding their fuel input. The use of biogenic waste streams (chicken manure, horse manure, etc.) and sewage sludge would not only increase the fuel diversity in fluidized beds but might also enhance the usability of side products. The contained essential nutrients like phosphorus, potassium, calcium, etc. in these fuels are enriched in the ash after thermochemical conversion. Thus, in the near future it may be possible to apply this ash as secondary resource for fertilizer. Especially the recovery of phosphorus is of importance due to the imminent phosphorus scarcity.
Due to its tendency to react with ash forming elements in fuels, phosphorus influences the ash chemistry severely. Especially the agglomeration and layer formation on bed materials during biomass combustion and gasification is highly dependent on the predominant ash forming elements. Phosphorus therefore has a significant impact on those mechanisms. Until now, the behavior of phosphorus-rich fuels in fluidized beds has not been studied in much detail.
To develop a basic understanding of the behavior, phosphorus-rich feedstock was combusted in a bench-scale fluidized bed reactor. Ash layers on bed particles, which were formed during these experiments, were studied and compared to results with phosphorus-lean fuels. Furthermore, layer formation of phosphorus-rich and phosphorus-lean fuels from dual fluid bed gasification were compared to those from fluidized bed combustion.
The studied layers on bed materials showed significant amounts of phosphorus. The data also indicates a change in layer formation as soon as phosphorus is present. An increased catalytic activity due ash-layer formation was observed for both phosphorus-rich and phosphorus-lean feedstock, independent from the presence of phosphorus in the ash layer.
... The heterogeneous outer layer grows outward on the surface of bed particles with a similar composition to fly ash. The less heterogeneous inner layer is often dominated by Ca and/or K, which can diffuse into the bed material and reciprocally react together [48,49,51,53]. Most research on ash-bed material interaction focused on Si-based bed particles, while few studies have been centered on the interaction between OC and coal ash [54][55][56]. ...
As a new concept, oxygen carrier aided combustion (OCAC) technology proposed in 2013 by Chalmers University of Technology’s group, can alleviate the problem of uneven distribution of oxygen in the reactors. In the past 10 years,various research institutions, including Chalmers University of Technology, University of Cambridge, Tsinghua University, Friedrich-Alexander University and University of Nottingham, have conducted a series of studies on OCAC technology. It is worth mentioning that Chalmers University of Technology has complied with most of these studies from laboratory to industry scales. In particular, they carried out a serious of semi-industrial scale experiments in the 12 MW th CFB boiler, which is well-known research boiler. OCAC technology is comprehensively introduced from six aspects: combustion characteristics, NO x /SO x emission, ash-related issues, aging of oxygen carrier, oxygen carrier recovery and physicochemical characteristics of oxygen carrier. In this chapter, allsummarized studies were performed under traditional air-combustion conditions without much consideration of CO 2 capture.
... Thereafter, the heat is transported to the gasifier by means of a sand-like material that circulates continuously between the two reactors. The concept of the DFB gasifier has been successfully demonstrated in several projects [1][2][3]. ...
Potassium (K) is a notorious villain among the ash components found in the biomass, being the cause of bed agglomeration and contributing to fouling and corrosion. At the same time, K is known to have catalytic properties towards fuel conversion in combustion and gasification environments. Olivine (MgFe silicate) used as gasifier bed material has a higher propensity to form catalytically active K species than traditional silica sand beds, which tend to react with K to form stable and inactive silicates. In a dual fluidized bed (DFB) gasifier, many of those catalytic effects are expected to be relevant, given that the bed material becomes naturally enriched with ash elements from the fuel. However, a comprehensive overview of how enrichment of the bed with alkali affects fuel conversion in both parts of the DFB system is lacking. In this work, the effects of ash-enriched olivine on fuel conversion in the gasification and combustion parts of the process are mapped. The work is based on a dedicated experimental campaign in a Chalmers DFB gasifier, wherein enrichment of the bed material with K is promoted by the addition of a reaction partner, i.e., sulfur, which ensures K retention in the bed in forms other than inactive silicates. The choice of sulfur is based on its affinity for K under combustion conditions. The addition of sulfur proved to be an efficient strategy for capturing catalytic K in olivine particles. In the gasification part, K-loaded olivine enhanced the char gasification rate, decreased the tar concentration, and promoted the WGS equilibrium. In the combustion part, K prevented full oxidation of CO, which could be mitigated by the addition of sulfur to the cyclone outlet.
... Meanwhile, the AAEMs and biomass ash can also reduce the tar formation [100]. It was reported that the catalytic activity of ash-coated olivine (bed material in fluidized gasifiers) even progressively increases with exposure to biomass ash [230], and a mechanism based on a solid-solid substitution reaction was proposed [231]. The AAEMs can also enhance the rate of soot gasification. ...
Biomass gasification is a promising technology in current and future low carbon energy systems. Soot formation is a great technical challenge for the industrialization of biomass gasification that is inevitable at high temperature and fuel rich conditions. In this review, a comprehensive summary of soot formation in biomass gasification is provided with special focus on entrained flow technologies. The topics covered the state of the art knowledge of soot formation in different gasifiers, the fundamental knowledge, experimental methods and recent control strategies. Soot generation and oxidation mechanism are discussed while the relationship between soot, tar and char in biomass gasification are analyzed in detail. Reaction models for soot formation coupled to the gasification process are introduced, including (semi-)empirical and detailed models. Effect of biomass components and ash forming elements on soot formation are highlighted. This is followed by a detailed description of in-situ and ex-situ experimental measurements, such as the optical diagnostics, aerosol particle mass analyzer and mass spectrometer. Soot formation characteristics and properties in different types of gasifiers are then addressed in detail with an emphasis of entrained flow gasifiers. Finally, the soot control strategies in biomass gasification are reviewed and evaluated. This review concludes by summarizing the available knowledge and challenges in soot formation during biomass gasification.
... The research gasifier at Chalmers with 2-4 MW th input has been included, as it has enabled investigations over a wider range of operation than commercial plants with complete downstream equipment and it also provided data points with unique operation features, e.g., silica sand as bed material. The plants in Güssing [29], Oberwart [30] and Villach [36] have been described in previous publications, and only single points are included in the following diagrams. ...
Steam gasification enables the thermochemical conversion of solid fuels into a medium calorific gas that can be utilized for the synthesis of advanced biofuels, chemicals or for heat and power production. Dual fluidized bed (DFB) gasification is at present the technology applied to realize gasification of biomass in steam environment at large scale. Few large-scale DFB gasifiers exist, and this work presents a compilation and analysis of the data and operational strategies from the six DFB gasifiers in Europe. It is shown that the technology is robust, as similar gas quality can be achieved despite the differences in reactor design and operation strategies. Reference concentrations of both gas components and tar components are provided, and correlations in the data are investigated. In all plants, adjusting the availability and accessibility to the active ash components (K and Ca) was the key to control the gas quality. The gas quality, and in particular the tar content of the gas, can conveniently be assessed by monitored the concentration of CH 4 in the produced gas. The data and experience acquired from these plants provide important knowledge for the future development of the steam gasification of biomass.
... The quantity of magnesium, silicon and iron in the layered Chinese olivine is lower than in the used Austrian olivine. The lower quantity of iron and magnesium observed came from the substitution of calcium for the iron and magnesium [20,46,47]. For the used Austrian olivine, the active surface, including the calcium-rich layer of the used Austrian olivine, was removed due to attrition because it was used many times during the Nong Bua Plant commissioning. ...
Bed materials and their catalytic activity are two main parameters that affect the performance of the dual fluidized bed (DFB) gasification system in terms of product gas composition and tar levels. Two sources of bed materials were used for the operation of a commercial DFB gasification system in Thailand, using woodchips as a biomass feedstock. One source of the bed materials was the calcined olivine which had been used in the Gussing Plant, Austria, and the other activated bed material was a mixture of fresh Chinese olivine and used Austrian olivine with additives of biomass ash, calcium hydroxide and dolomite. These bed materials were collected and analysed for morphological and chemical composition using a scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray fluorescence spectroscopy (XRF). The product gas was cleaned in a scrubber to remove tars, from which the samples were collected for gravimetric tar analysis. Its composition data was automatically recorded at the operation site before it entered the gas engine. From the SEM, EDS and XRF analyses, calcium-rich layers around the bed materials were observed on the activated bed material. The inner layers of bed materials collected were homogeneous. Biomass ash, which was generally added to the bed materials, had significant calcium and potassium content. These calcium-rich layers of the bed materials, from the calcium hydroxide, biomass ash and dolomite, influenced system performance, which was determined by observing lower tar concentration and higher hydrogen concentration in the product gas.
... In the case of melt-induced agglomeration, the bed particles are glued together with molten ash, whereas the coating-induced agglomeration is preceded by formation of an alkali-silicate-dominated ash layer on the particle surface. Upon temperature increase, the ash layer melts, increasing its stickiness for other particles, and forms necks between bed material particles [11][12][13][14]. ...
Thermal conversion of ash-rich fuels in fluidized bed systems is often associated with extensive operation problems caused by the high amount of reactive inorganics. This paper investigates the behavior of wheat straw lignin—a potential renewable fuel for dual fluidized bed gasification. The formation of coherent ash residues and its impact on the operation performance has been investigated and was supported by thermochemical equilibrium calculations in FactSage 7.3. The formation of those ash residues, and their subsequent accumulation on the surface of the fluidized bed, causes temperature and pressure fluctuations, which negatively influence the steady-state operation of the fluidized bed process. This paper presents a detailed characterization of the coherent ash residues, which consists mostly of silica and partially molten alkali silicates. Furthermore, the paper gives insights into the formation of these ash residues, dependent on the fuel pretreatment (pelletizing) of the wheat straw lignin, which increases their stability compared to the utilization of non-pelletized fuel.
... A detailed assessment of the ash transformation reactions in thermochemical conversion of biomass was published by Boström et al. [13]. This ash layer formation has been studied for different types of bed materials, such as olivine [14], quartz [15], and K-feldspar [16,17], describing the mechanisms underlying the formation of those layers. The mechanisms differentiate significantly from another in the initiating steps of the layer formation. ...
Interaction of biomass ash and bed materials in thermochemical conversion in fluidized beds leads to changes of the bed particle surface due to ash layer formation. Ash components present on the bed particle surface strongly depend on the ash composition of the fuel. Thus, the residual biomass used has a strong influence on the surface changes on bed particles in fluidized bed conversion processes and, therefore, on the catalytic performance of the bed material layers. Ash layer formation is associated with an increase in the catalytic activity of the bed particles in gasification and plays a key role in the operability of different biomass fuels. The catalytic activation over time was observed for K-feldspar used as the bed material with bark, chicken manure, and a mixture of bark and chicken manure as fuels. The changes on the bed material surfaces were further characterized by SEM/EDS and BET analyses. Raman, XPS, and XRD analyses were used to characterize the crystal phases on the bed material surface. An increase in surface area over time was observed for K-feldspar during the interaction with biomass ash. Additionally, a more inhomogeneous surface composition for fuels containing chicken manure in comparison to pure bark was observed. This was due to the active participation of phosphorus from the fuel ash in the ash transformation reactions leading to their presence on the particle surface. A decreased catalytic activity was observed for the same BET surface area compared to bark combustion, caused by the different fuel ash composition of chicken manure.
Interactions between fuel ash and K-feldspar increase the particle surface area and the catalytic activity regarding the water-gas-shift reaction
... The layers themselves are composed of several sublayers, formed as a result of the differences in diffusion rates of species within the particle crystalline structure and the replacement of atoms by those migrating ash species. Such mechanisms have been investigated for silica sand [37,38], olivine [39][40][41], feldspar [42][43][44][45][46], and bauxite [47], with respect to DFB gasification. In all these bed materials, the formation of the layers is characterized by the inner diffusion of Ca into the particles and its reaction with the crystalline structure, leading to the differentiation of the inner layer into two sublayers. ...
... To convert biomass into gaseous fuels there are essentially three thermochemical methods: gasification, pyrolysis and combustion. Many authors [8][9][10][11][12][13][14][15][16] are considering and applying the gasification that presents many advantages especially if the power plant is integrated with a solar energy system [17]. A state of the art of the fluidization technology for the biomass gasification is reviewed in [18], where different gasifier types, technologies and influence of process parameters on the syngas are discussed. ...
Sustainable biomass exploitation for combined heat and power (CHP) generation is fundamental to address concerns about climate changes related to energy conversion systems. The aim of the work is to design a power plant able to satisfy energy demands exclusively through renewable resources. A layout of a hybrid micro-gas turbine–solarplant fed by biomass from agriculture products (olive pits) is modelled. It provides the coupling between a gasifier, dedicated to the biomass conversion and a micro gas turbine, optimized for the operation with syngas. In particular, the MGT plant is integrated with a solar tower field, able to provide a partial or complete fuel heating replacement depending on the amount of solar heat provided to the working fluid reaching the turbine inlet temperature. After the solar field design able to reduce fuel consumption during diurnal hours, a gasifier was integrated to obtain a syngas from biomass in place of the traditional natural gas. A parametric analysis aimed to define the gasifier parameters is reported and allowed to obtain a syngas able to satisfy the power demands of the MGT with good performances. The Thermoflex® commercial software is used to model the full plant. The results in terms of efficiency and environmental impact are reported and compared to those obtained with the traditional natural gas-fuelled MGT. Keywords: Biomass, Gasification, Micro gas turbine, Solar energy, Syngas
... This crucial topic has never yet been explored for dolomite whereas it has been extensively studied for olivine. Indeed, several studies showed that olivine is coated by ash materials during biomass gasification and this ash layer considerably increases the reactivity of olivine for tar cracking and WGS reactions [82][83][84][85][86][87][88]. This present work reveals that the impact of ashes may be very different for dolomite than for olivine. ...
Dolomite is a cheap and robust catalyst used for biomass gasification, but its deactivation under relevant conditions of pilot-scale gasifiers has still been poorly understood. For this reason, the catalytic activity of fresh and used dolomites produced from an industrial air-blow fluidized bed was investigated. Fresh and used dolomites were characterized by BET, SEM-EDX, XPS, ICP-MS, XRD, TPD and TPO. Benzene steam reforming was selected as a surrogate reaction of tar conversion in order to probe the reactivity of the two dolomites. The activity of used dolomite was 25% lower than that of fresh dolomite. This difference could be explained by: (1) the deposition of a Si-based layer from biomass ashes at the surface of used dolomite, and (2) the production of coke during gasification. The reaction mechanism of benzene steam reforming over fresh and used dolomites was discussed. For used dolomite, the Si and coke depositions reduced the availability of the active sites (CaO, MgO) thus lowering the conversion of benzene. These deposits could also inhibit the interactions between CaO and MgO and enhance the formation of a stable coke.
... Olivine bed particle displaying a Ca/Mg-rich inner layer and a Ca-rich outer layer from industrial-scale DFB combustion of woody-type biomass, adapted with permission from Kuba et al[12]. ...
The role of fluidized beds is increasingly important for challenging and ash-rich fuels, such as fast-growing biomass and waste streams. From a biomass perspective, the relatively homogeneous woody-type fuels are most commonly used in fluidized beds today whereas the fuel feedstock for waste streams is more heterogeneous. A key issue in enabling a broader fuel feedstock for existing and planned fluidized beds is how the fuel ash interacts with bed materials of different types during combustion or gasification. The resulting bed particle coating, layers, and cracks formed in bed grains are responsible for bed agglomeration and bed material deposition mechanisms, but studies have suggested that there is a possibility to affect melting temperatures of bed ash and reduce interaction between fuel ash and bed material through additives or by fuel blend design. Furthermore, it is of interest to extend the lifetime of bed materials in the reactor to reduce the amount of material that is generated as waste streams, as well as increase the timespan between bed replacements. The aim of this review is therefore to summarize some of our previous research in this topic, to discuss current knowledge concerning layer formation and bed agglomeration mechanisms, address the benefit for different bed materials, and discuss how fuel ash composition can be used to reduce bed agglomeration issues. This is achieved by comparing studies from different combustion and gasification facilities using different biomasses as well as agricultural residues and waste streams. In particular, the possibility of using fuel blend design to reduce interaction of fuel ash with bed material will be highlighted. Using such approaches, coupled with a fundamental understanding of how differences between bed materials affect layer formation mechanisms, has the potential to reduce operational issues caused by interactions between fuel ash and bed materials as well as increase the potential fuel feedstock.
... The dilution of the carbonate material in the FB can be advantageous for (i) reducing the amount of fines produced by attrition and carried over from the reactor and (ii) taking advantage of the better mechanical properties of silica sand in the FB [40]. It is also possible that the two materials synergistically combine during long-term operation, to form mechanically robust particles with a Ca-rich catalytic coating layer, as reported by Kuba et al. [41][42][43][44], for ash enhancement of quartz or olivine during longterm operation of the dual FB gasifier. ...
Fluidised bed (FB) technology can be advantageously used for the gasification of solid fuels. Calcined carbonate materials, such as limestone or dolomite, can be used directly in the fluidised bed of the gasification reactor to reform tars in situ and to enhance carbon conversion and cold gas efficiency of the gasification process. However, they exhibit poorer mechanical stability, they tend to be attrited and carried over from the reactor, and their catalytic activity decreases over time. Therefore, a portion of the material has to be replenished continuously or periodically. To decrease the amount of carbonate material that has to be replenished, a lower amount of lime (calcined limestone or dolomite) can be used in the FB, diluted by a mechanically robust material, such as silica sand or olivine. According to the literature, even concentrations in the order of 10–35% wt. of carbonate material in the FB of silica sand have a substantial effect on the decomposition of tars during steam or air gasification. However, the effect of the concentration of lime in the FB has not yet been described for CO2 gasification. In this paper, we focus on the effect of the ratio of calcined dolomitic limestone and silica sand in the FB (0%, 25%, 50% and 100% vol. of dolomitic limestone) for CO2+O2 gasification of biomass and compare it with H2O+O2 gasification at the temperature of 850 °C. The experiments were performed in a semi-autothermal spouting FB reactor, gasifying 1.4 kg h−1 of woody biomass. The effects of the concentration of dolomitic lime in the fluidised bed differed for H2O+O2 and CO2+O2 gasification. When gasifying with H2O+O2, optimal results were found with 50% vol. (35% wt.) lime in the FB, when the yield of tar was similar to the use of pure lime in the FB. When gasifying with CO2+O2, a substantial decrease in tar yield was observed when using 50% vol. (35% wt.) lime in the FB (compared to the use of pure silica sand); nevertheless, the use of undiluted lime in the FB remains the best option to attain minimal tar yield when gasifying with a CO2+O2 gasifying agent. In this case, the tar yield was decreased 8.7-fold, and the tar dew point was decreased by 124 °C (to 71 °C) compared to the non-catalysed case with silica sand being the fluidised bed of the gasifier.
Gasification of lignocellulosic biomass in a fluidized bed gasifier is a preferred option for gaseous fuel generation and solid waste management. Erosion, slagging and agglomeration limit the wide adoption of this process. Agglomeration needs serious attention as it leads to defluidization and plant shut down. The present study considers the various aspects of agglomerate formation by the interaction of the inorganic elements (K, Na, Ca) and bed materials. Inexpensive and widely used bed material like silica sand is prone to agglomeration due to the formation of lower-melting alkali silicates. The mechanism involving the formation of agglomerates and ash behavior has been reviewed. K2SiO3 is the commonly formed lower-melting eutectic by the reaction of bed materials and compounds like KCl, K2SO4 and K2CO3 present in the bed. Investigations on various bed materials and their combined effects on bed agglomeration are summarized in this paper. Among the preventive measures, the use of different types of bed materials plays a vital role in the reduction of agglomerate formation, thereby increasing fluidization time. Olivine, dolomite, alumina sand, mullite, etc. are identified as some of the alternatives to the conventional bed materials. The detection of agglomeration using various experimental and online methods is also reviewed. This could further help in the abatement of agglomeration in the reactor without affecting process parameters.
Renewable biomass fuels are frequently used for power generation. Biomass ash causes bed agglomeration in fluidized bed boilers due to the formation of alkali silicate melts. Very few prior studies have tested dolomite and kaolin bed additives for agglomeration mitigation with agricultural biomasses. In this work, pelletized miscanthus and wheat straw were tested in a pilot-scale 65kWth fluidized bed combustor with varying dosages of dolomite and kaolin on a silica sand bed. Neither additive improved defluidization time with wheat straw, whereas additive use at all dosages prevented bed defluidization with miscanthus. Agglomerates were studied through a novel, detailed SEM/EDX analysis across structural features. SEM/EDX analysis presented evidence of chemical reaction between both additives and fuels. Potassium in ash migrated into kaolin particle at depths of up to 60μm. With dolomite, calcium and magnesium raised melt temperatures. Thermochemical modelling of the ash and additive combinations predicted that additive use would substantially reduce ash melt formation. Wheat straw pellet acts as a “ready-made” agglomerate structure due to release of molten ash to the pellet surface which bed material then sticks to, hence the lack of change to defluidization time regardless of additive use. Future studies into this behaviour would improve additive use.
Fluidized beds have been used as equipment for promoting agglomeration in industrial processes. In general, particle enlargement is desirable because it improves the properties of powders. In some cases, agglomeration can be inconvenient it because can reduce the reaction rate in processes such as combustion. Bibliometric analysis was conducted to examine the global panorama of publications and identify trends in agglomeration in a fluidized bed in the last four decades. China, Sweden, Germany, Canada, and the United States raised their status as the most productive countries in the total number of publications. Keyword analysis showed that fluidization, biomass, granulation, defluidization, and coating were important to agglomeration in fluidized beds. Two main focuses were also identified in publications: granulation and combustion. Research emphasis on agglomeration in fluidized beds is progressing to evaluate the operating parameters of processing new materials and modern measurement techniques to enhance theoretical approaches.
Thermal conversion of automotive shredder residue (ASR) using indirect fluidized bed gasification was conducted in the Chalmers semi-industrial 2–4-MWth gasifier. The bed material consisted of olivine that was activated through the deposition of biomass ash prior to a 13-day exposure to ASR. The interactions between the bed material and the ASR ash were investigated using XRD, SEM-EDS, and thermodynamic modeling. The deposition of iron (Fe) onto the olivine particles was noted, and this is likely to increase the oxygen-carrying ability of the particles. Furthermore, at the end of the campaign, about one-third of the particles in the bed were found to originate from the ASR ash. These particles were rich in Fe and Si, as well as elements found exclusively in the ASR ash, such as Zn, Ti, and Cu. Some of these particles exhibited a hollow morphology, suggesting a melt state during their formation in the gasifier. In addition, a low level of agglomeration of the ash and olivine particles was detected. Thermodynamic modeling with the FactSage software indicated the formation of slag. This study presents a detailed investigation of the interactions that occur between the bed material and an ash-rich fuel such as ASR. The findings may have applications in demonstrating the induction of oxygen-carrying ability in bed materials or for metal recycling through the separation of ash particles from the bed material.
This paper presents an experimental study of various bed materials for oxy-fuel combustion of biomass in a bubbling fluidized bed (BFB). A silica sand with three particle size distributions (PSDs) and a lightweight ceramic aggregate (LWA) made of thermally expanded clay with two particle size distributions were chosen as potentially suitable materials. The experiments of biomass combustion in oxy-fuel mode with these materials were carried out to study their suitability as fluidized bed materials. The experiments were performed using a 30 kWth lab-scale BFB facility with A1 wood pellets as a fuel.
The fluidized bed temperature was controlled at approx. 800 °C and the volumetric fraction of O2 in dry off-gas at 10%. The resulting volumetric fraction of CO2 in dry off-gas was approximately 85%. At these conditions, there were no problems with sintering and forming of agglomerates. In the case of ceramic materials, it was possible to control the combustion process in a significantly wider range of operating parameters. Compared to silica sand, the experimental facility could be operated at a lower excess of oxygen (below 6% in dry off-gas) without significant influence on the temperatures in the facility. In terms of the consumption of the combustion facility itself, ceramic materials seem to be more suitable, as they are lighter. The pressure drop of the fluidized bed made of LWA is lower than that of the bed of the same volume made of silica sand, which means that less power is required to drive the fluidization fan.
In dual fluidized bed (DFB) gasification, the interaction of the bed material with the fuel ash leads to the development of a bed catalytic activity toward tar-abating reactions. However, the formation of ash layers may also be detrimental to the process, especially in terms of the uncontrolled transport of oxygen from the combustor to the gasifier. A few previous studies investigating the development of catalytic activity in bed materials have also reported the development of oxygen transport, although the latter was not the focus of these studies. This work verifies that olivine and feldspar, which are bed materials with limited and no intrinsic oxygen transport capacities, respectively, develop the capacity to transport oxygen by interacting with the fuel ash. We correlate this development in oxygen transport to the development of bed catalytic activity. Our results imply that the volatile species that are released by the bed material to the gas phase in the gasifier contribute to the developed oxygen transport. Sulfur is proposed as one of the components of these volatile species, and its potential contribution is investigated. For feldspar, the results support the notion that sulfur is involved in the transport of oxygen, both as a volatile species and as a species remaining within the ash layer. The results also suggest that other species, including volatile ones, are involved. These aspects are investigated based on experimental results obtained from the Chalmers gasifier—a semi-industrial-scale DFB gasifier—and are isolated in laboratory-scale experiments.
Gasification is a thermochemical process that transforms carbonaceous matter into a gaseous secondary energy carrier, referred to as product gas. This product gas can be used for heat and power generation but also for syntheses. One possible gasification technology suitable for further synthesis is dual fluidised bed (DFB) steam gasification. The H2:CO ratio, which determines the suitability of the product gas for further synthesis, is influenced by the catalytic activity inside the gasification reactor. Eleven DFB steam gasification experiments were performed comparing the catalytic activity for various bed material and fuel combinations. The bed materials used were K-feldspar, fresh and layered olivine, and limestone, and the fuels gasified were softwood, chicken manure, a bark–chicken manure mixture and a bark-straw-chicken manure mixture. The water-gas-shift (WGS) equilibrium deviation was used to evaluate the catalytic activity inside the gasification reactor. It was shown that both the fuel ash and bed material have an effect on the catalytic activity during gasification. Scanning electron microscopy and energy dispersive X-ray spectrometry showed the initial layer formation for experiments with ash-rich fuels. Isolated WGS experiments were performed to further highlight the influence of bed material, fuel ash and fuel ash layers on the WGS equilibrium.
Techniques that produce chemicals and fuels from sustainable carbon sources will have to maximize the carbon recovery to support circularity. In dual fluidized bed (DFB) gasification, to facilitate carbon recovery, the CO2 from the flue gas can be concentrated using pure oxygen as an oxidant. The heat required by the process can also be provided electrically or by oxidizing an oxygen-carrying bed material, rather than combusting part of the char, thereby concentrating all of the carbon in the syngas. In this work, the three configurations of oxyfuel, electrical, and chemical-looping gasification (CLG) are compared to each other, as well as to the standard or “air” configuration, which corresponds to the combustion of char with air and the separation of CO2 from both the flue gas and syngas. The configurations are compared based on their carbon distributions and energy demands for CO2 separation. We show that the air and oxyfuel configurations lead to similar carbon distributions, whereas the CLG configuration gives the lowest carbon recovery in the form of an end product. The oxyfuel and CLG configurations show the lowest energy demands for CO2 separation, while the air configuration exhibits the highest. The electrical configuration has the lowest potential to benefit from heat integration to cover this energy demand. An investigation into the optimal gasification temperature for the air and oxyfuel configurations shows that there is no driver for operation at high temperatures.
Oxygen carrier aided combustion (OCAC) is a combustion concept which utilises oxygen carriers as bed material in existing fluidised bed boilers. In this study, a manganese ore was used in a 12 MWth CFB boiler. During the experimental session with the manganese ore, the boiler was operated with wood chips as fuel for more than a week without replacement of the bed material. Bed samples were extracted each day in order to investigate interactions between the manganese ore and the wood ash components. The samples were examined with SEM/EDX to follow the chemical distribution of ash elements in the bed particles. Physical properties such as density, size distribution and attrition resistance were followed as well. The impact on the reactivity of the oxygen-carrier bed particles was examined in a batch fluidised bed reactor at laboratory scale with gaseous fuels.
Elemental composition analysis of the samples showed that common ash elements such as silicon, calcium, potassium, magnesium and sulphur had been accumulated in the manganese ore. Silicon, calcium and potassium were found throughout the particles as well as in formed surface layers. Sulphur was only found at the surface of the particles. The reactivity of the oxygen-carrying particles was affected during operation and showed a continuous decrease with increasing residence time in the boiler. The decrease in reactivity could be coupled to the layers of ash formed. Thus, this is an important issue when developing novel combustion concepts, such as OCAC and chemical-looping combustion (CLC), for biomass fuels.
Bubbling fluidized bed combustion of biomass has benefits of fuel flexibility, high combustion efficiency, and lower emissions. Bed agglomeration is where bed particles adhere together with alkali silicate melts and can lead to unscheduled plant shutdown. This pilot-scale study investigates performance and agglomeration when varying fuel (white wood, oat hull waste, miscanthus, wheat straw), bed height, bed material, and includes a novel spatial analysis of agglomerates from different bed locations. White wood was the best performing fuel and did not undergo bed defluidization due to its low ash content (0.5% mass), whereas wheat straw (6.67% mass ash) performed worst (defluidization times <25 min). Olivine was a superior bed material to silica sand, with 25%+ longer defluidization times with the worst performing fuel (wheat straw). Calcium-rich layers formed at olivine particle surfaces within wheat straw ash melts, and capillary action drew potassium silicate melt fractions into olivine particle fractures. An analysis of agglomerate samples from different bed spatial locations following tests with oat hull waste showed that ash layers on agglomerates retrieved further from the landing point of fuel onto the bed had reduced potassium and elevated calcium, likely due to reduced availability of fresh fuel ash for reaction with bed material.
Quartz, feldspar and olivine are minerals commonly used as bed material for dual fluidized bed gasification of biomass. During their interaction with biomass ash, the materials develop surface layers rich in ash-derived elements. These layers decrease the concentration of tar which is an unwanted side product of gasification. The interaction of quartz, feldspar and olivine with woody biomass ash leading to the formation of active layers was studied with XRD, SEM-EDS and ToF-SIMS and the results compared to calculations done with FactSage. It was found that the interaction causes the formation of three-layered structures for all materials: A Mg-rich surface layer, a Ca-rich intermediate layer and an inner layer which varies between the three materials. For quartz and feldspar, the integration of Ca and Mg into the structure causes a transition by depolymerizing the tectosilicate structure via an inosilicate intermediate to finally a nesosilicate. As the olivine structure is a nesosilicate from the beginning, no further depolymerization of the silicate structure can occur and a substitution of Mg by Ca occurs, leading to an accumulation of expelled MgO on the surface. The interaction of the materials with K was found to differ, causing melt formation for quartz, a substitution of Na-rich feldspar by K-rich feldspar and the formation of feldspathoids for alkali-feldspar, or retention as a separate phase for olivine.
Olivine bed material used in an aging experiment for indirect gasification in the Chalmers 2–4-MWth DFB gasifier was investigated with the aim to determine the mechanism of layer formation around the particles upon exposure to gasification conditions. The collected samples were exposed for 1, 2 and 4 days. The development of ash layer around the bed material particles was studied with different analysis methods. Formation of Ca3Mg(SiO4)2 and MgO was confirmed by X-ray diffraction (XRD). Cross-sections of the bed material samples were prepared using Broad Ion Beam (BIB) milling and were further analyzed with Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM-EDS). The analysis of the produced cross-sections revealed the formation of a Mg-rich surface layer on top of the Ca-rich ash layer. Minor amounts of K were also found near the sample surface. Based on the results from the characterization techniques, a reaction mechanism involving the transition of Mg2SiO4 with CaO to MgO and Ca3Mg(SiO4)2 was suggested which was confirmed by equilibrium calculations. This mechanism was supported by Transmission Electron Microscopy (TEM) analysis where diffraction patterns corresponding to MgO were found. TEM-EDS line-scan revealed the presence of ash components in the ash layer such as P and Ti at locations coinciding with high levels of Ca which indicates the formation of Ca3(PO4)2 and CaTiO3. The results presented provide detailed information on the composition of the ash layer which can be used to fully understand the mechanism responsible for the formation of catalytically active ash layers.
The choice of bed material for biomass gasification plays a crucial role for the overall efficiency of the process. Olivine is the material conventionally used for biomass gasification due to the observed activity of olivine towards cracking of unwanted tars. Despite its catalytic activity, olivine contains high levels of chromium which complicates the deposition of used bed material. Feldspar has shown the same activity as olivine when used as bed material in biomass gasification. As opposed to olivine, feldspar does not contain environmentally hazardous compounds which makes it a preferred alternative for further applications. The interaction of bed material and ash heavily influences the properties of the bed material. In the present study interactions between feldspar and main ash compounds of woody biomass in an indirect gasification system were investigated. Bed materials samples were collected at different time intervals and analyzed with SEM-EDS and XRD. The obtained analysis results were then compared to thermodynamic models. The performed study was divided in two parts: in part 1 (the present paper), K-rich feldspar was investigated, whereas Na-rich feldspar is presented in part 2 of the study. From the performed material analysis, it can be seen that, as a result of the bed material interactions with the formed ash compounds, the latter were first deposited on the surface of the K-feldspar particles and on a later stage resulted in the formation of Ca- and Mg-rich layers. The Ca enriched in the layers further reacted with the feldspar, which led to its diffusion into the particles and the formation of CaSiO3 and KAlSiO4. Contrary to the Ca, Mg did not react with the feldspar and remained on the surface of the particles, where it was found as Mg- or Ca-Mg-silicates. As a result of the described interactions layer separation was noted after 51 hours with an outer Mg-rich layer and an inner Ca-rich layer. Due to the development of the Ca and Mg-rich layers and the bed material-ash interactions crack formation was observed on the particles’ surface.
Selecting a suitable bed material for the thermochemical conversion of a specific feedstock in a fluidized bed system requires identification of the characteristics of potential bed materials. An essential part of these characteristics is the interaction of the bed material with feedstock ash in a fluidized bed, which leads to layer formation and morphology changes. For this purpose, the interaction of feldspar bed material with the main ash-forming elements in wood ash (Ca, K, Mg, Si) in an indirect gasification system was analyzed using SEM/EDS, XRD, and thermodynamic modeling. In the first part of this work, the layer formation on K-feldspar dominated by Ca-reaction and ash deposition was investigated. The aim of this second part of the work was the determination of time-dependent layer formation on Na-feldspar and comparison with the findings for K-feldspar. Interaction of Na-feldspar with ash-derived elements resulted in different layers on Na-feldspar: K-reaction layers, where K replaced Na and Si-shares decreased; Ca-reaction layers, where Ca enriched and react-ed with the Na-feldspar; and ash deposition layers, where wood ash elements accumulated on the surface. Ca-reaction layers were formed first and became continuous on the surface before K-reaction layers and ash deposition layers were detected. Cracks and crack layer formation in the Na-feldspar particles were found after several days of operation. The layer compositions and growth rates indicate that the diffusion of Ca and K plays an essential role in the formation of Ca-reaction and K-reaction layers. The reaction with Ca and the crack formation coincides with the interaction previously found for quartz and K-feldspar. In contrary to K-feldspar, Na-feldspar showed high reaction potential with K. The findings indicate that the reaction of Na-feldspar with ash-derived K makes Na-feldspar a less stable bed material than K-feldspar during the thermochemical conversion of K-rich feedstocks in a fluidized bed system.
Catalytic gasification is a method for converting carbon feeds to usable products such as hydrogen, carbon monoxide, and methane. This review summarizes the research on potassium, sodium, calcium, magnesium, and heavy metal catalysts, the interactions between these catalysts and other species in typical feeds, and gasifier design considerations. Phase behaviour is a critical element in the performance of a gasification catalyst - the active phase of the catalyst must be thermodynamically favoured. Formation of stable but inactive forms and/or highly volatile forms of the catalyst are detrimental. Given that real feeds are complex and the conditions in a gasifier not uniform, laboratory results must be carefully translated to the larger scale.
Biomass is recognized as a CO2 neutral energy resource. However, biomass is a challenging fuel to combust due to its heterogeneity, with regard to the content of inorganic constituents, volatiles and moisture. Oxygen carrier aided combustion (OCAC) is a process advancement that provides enhanced combustion in existing circulating fluidized bed (CFB) units. The oxygen carrier has a central role in the OCAC concept through the oxygen transport it provides. The natural mineral ilmenite (FeTiO3) has been identified as a promising potential oxygen carrier. In order to assure the feasibility even for long-term operation in industrial scale processes, it is imperative to understand the evolution of the material during an OCAC process. In the present study ilmenite was used as bed material in Chalmers 12 MWth CFB-boiler during OCAC with woody biomass as fuel. Bed material samples were extracted from the bed inventory at different time intervals ranging from 5 to over 300 hours. This paper proposes a mechanism for migration and layer growth of biomass ash on the ilmenite used as oxygen carrier in a CFB combustor. It was found that, with increased time of exposure, potassium migrated into the particle core. Longer process time led to the formation of a calcium layer around the particle, simultaneously as migration of calcium inward on the particle was observed. Thermodynamic calculation was used along with analysis techniques in order to build a hypothesis around possible mechanism of ash-bed material interaction.
Relevant waste flows like sewage sludge, farmyard manure, digestion residues, and humid residues from food and feed production are known for food safety issues and for environmental and waste management problems. If used as a resource for crop nutrients and soil fertility, distribution is the main issue: urbanization and intensive livestock farming produce mass flows requiring extended cropland typically not available in the densely populated regions of our planet. Thermal conversion is an acknowledged option for concentration and recycling of mineral residues including phosphates, but the typical moisture content of >70 wt% makes it difficult to yield relevant surplus energy flows. This challenge is approached by increasing the efficiency of drying and replacing combustion by gasification, in essence by making effective use of the hydrogen (H2) molecules of water in the process chain. Outotec’s technology approach aims at keeping H2 molecules in the loop and eventually using them in the form of a hydrogen-rich gas in a variety of energy and biochemical applications. The approach is intrinsically circular, and the related processes – closed-loop steam drying and steam gasification – are well known but have not been applied to the waste flows and in the configuration as outlined in this book. If successfully implemented, waste flows in the order of 1–1.5 billion cubic meters in the EU28 may be recycled to a relevant building block of a future hydrogen economy with a vast array of applications in the energy and biochemistry sector.
Tar measurements at two industrial-scale DFB gasification plants showed clear trends regarding the influence of the above mentioned parameters on the product gas and tar composition. Since data was gathered during tar measurement campaigns over the course of four years the density of information in industrial-scale was increased significantly. As different operation points, e.g. different capacities of the power plant, are included in the consideration, the verisimilitude is comparably high.
It was shown, that reducing the operation temperature leads to an increase of the total tar amounts. However, while the concentration of the tar compounds benzofuran, styrene, and 1H-indene was increased when lowering the temperature, the concentration of naphthalene was decreased. These results were in good correlation with previous work from lab-scale investigations. The temperature did not have a measureable influence on the concentration of the tar compounds anthracene and ace-naphthalene, which was against former experience from lab-scale. The concentration of those larger PAHs anthracene and ace-naphthalene was more dominantly influenced by the bed height in the gasification reactor. Increasing the bed height led to a decrease of the concentration of larger PAHs while it did not have a distinctive influence on benzofuran, styrene, and 1H-indene.
The reactor design was identified as an influencing effect, due to the presence of a moving bed section above the inclined wall, where no fluidization is ensured. Thus, additional fluidization nozzles were installed to reduce the effect of the inclined wall. Finally, two operation points for optimized long-term operation were derived from the results.
Substitution of fossil fuels for production of electricity, heat, fuels for transportation and chemicals can be realized using biomass steam gasification in a dual fluidized bed (DFB).
Interaction between biomass ash and bed material in a fluidized bed leads to transformation of the bed particle due to enrichment of components from the biomass ash resulting in the development of ash layers on the bed particle surface. These ash-rich particle layers enhance the catalytic activity of the bed material regarding the water-gas-shift reaction and the reduction of tars.
The water-gas-shift reaction at conditions typical for dual fluidized bed biomass gasification at a temperature of 870 °C was investigated. Diffusion and heat transfer limitations were minimized using a lab-scale experimental set-up consisting of a gas mixing section and a quartz glass reactor in which the catalyst is investigated.
The following new rate expression for the water-gas-shift reaction in dual fluidized bed gasification of biomass was empirically developed, which takes ash layer formation into account: rOlivine_ash-layer=8,9·10-6exp-95000RTpCO1,96pH2O1,81pCO2-0,75pH2-1,69
In this paper, the ability of an ash-coated olivine to catalyze the steam gasification of biomass-derived char is investigated in a laboratory reactor. The olivine investigated is a sample from the Chalmers dual fluidized bed gasifier and it has been activated by the in-bed addition of S and K2CO3. The char and bed material samples were analyzed by Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS). It is shown that the ash layer coating of the olivine can catalyze the steam gasification of char by transferring catalytic potassium (K) to the char particles. The mobilities of the catalytic species from the olivine ash-layer are discussed. This work furthers the current understanding of the catalytic activities of ash-coated bed material particles during the thermochemical conversion of carbonaceous feedstocks in fluidized beds. In addition, it complements the existing literature on catalytic bed materials, which to date have focused on tar removal and improving gas quality.
The time-dependent layer-formation process of the agglomerates for three common agricultural residues in Australia with different ash-forming elements, together with quartz sand as the bed material, were investigated in a lab-scale, fixed-bed reactor under combustion (5% v/v O2) and steam-gasification (50% v/v steam) atmospheres at 900 °C. The impact of the atmosphere on the ash–bed material interaction was studied from the elemental composition and the morphology of the agglomerates, which were characterized with scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy. The ash–bed material interaction mechanisms for the three feedstock were identified as part of the alkali metals react to form ash particles, which, for wheat straw and cotton stalks, consist of Na, Mg, Si, P, K, and Ca and, for grape marc, is composed mostly of KCaPO4; the remaining alkali metals react with either Si from the quartz sand (for grape marc and cotton stalk) or reactive Si from the fuel (for wheat straw) to form a low-melting-point alkali silicate coating layer; Ca dissolves or diffuses into the coating layer (for wheat straw and cotton stalk); and the ash particles formed in the first step then deposit on, and progressively embed in, the coating layer. The elemental composition of the coating layer is relatively independent of both the reaction time and the gas atmosphere. The coating layer increases in thickness with an increase in the reaction time. The addition of steam results in the production of more liquid alkali silicates, which augment the agglomeration. Any residual S may form sulfate particles with K, Ca, or Na in a combustion atmosphere, while in a steam-gasification atmosphere, the S is released to the gas phase so that more alkali metal may remain to form the low-melting-point alkali silicate.
Gasification in fluidized bed systems is considered to be a highly promising alternative for the thermal conversion of biomass. A major challenge for this process is the formed tars, which represent a loss of energy from the product gas and entail additional costs for their removal. Olivine is considered to be the most effective catalytic bed material in terms of its impact on tar levels in the product gas. Additions and modifications to olivine have revealed the potential to enhance its catalytic activity. In the present study, the effect of the addition of K2CO3 to the gasification process on the tar decomposition capability of olivine were evaluated. The effect of the added K2CO3 on the product gas was assessed in the 30-MWth pilot gasification plant GoBiGas. Once decreases in the tar level were detected, samples of the bed material were extracted from the system and evaluated for morphological and chemical changes related to the observed catalytic effect. SEM-EDX and XPS analyses of the surfaces of the olivine particles indicate that the additive is involved in the formation of mixed oxides of Ca and K within the outermost layer of the olivine particles. DFT modeling showed that the formation of mixed Ca and K oxides changes the oxidation potential of the surface, which may explain the increased activity of ash-coated olivine towards tar reduction.
Steam gasification of solid biomass in dual fluidized bed systems is a suitable technology for the production of chemicals, fuels for transportation, electricity, and district heating. Interaction between biomass ash and bed material leads to the development of Ca-rich bed particle layers. Furthermore, incomplete decomposition of biomass leads to the formation of tar components; among these are stable intermediate products such as 1H-indene and stable gaseous hydrocarbons such as methane. In this work, the influence of bed particle layers on the conversion of intermediate products such as 1H-indene and methane via steam reforming was investigated by conducting experiments in a lab-scale test rig. Satisfying conversion of 1H-indene into gaseous molecules (e.g., CO, CO2, H2) was achieved with used, layered olivine, whereas fresh olivine showed significantly poorer performance. Since steam reforming was connected to the water-gas-shift reaction for the tested hydrocarbons, investigations regarding carbon monoxide conversion in the presence of steam were conducted as well. Furthermore, a comparison of the influence of fresh and used bed material concerning the conversion of methane is presented, showing that methane is not affected by the bed material, independent of the presence of particle layers.
Biomass is available from many sources or can be mass-produced. Moreover, biomass has a high energy-generation potential, produces less toxic emissions than some other fuels, is mostly carbon neutrality, and burns easily. Biomass has been widely utilized as a raw material in thermal chemical conversion, replacing coal and oil, including power generation. Biomass firing and co-firing in pulverized coal boilers, fluidized bed boilers, and grate furnaces or stokerfed boilers have been developed around the world because of the worsening environmental problems and developing energy crisis. However, many issues hinder the efficient and clean utilization of biomass in energy applications. They include preparation, firing and co-firing, and ash-related issues during and after combustion. In particular, ash-related issues, including alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, and ash utilization, are among the most challenging problems. The current review provides a summary of knowledge and research developments concerning these ash-related issues. It also gives an in-depth analysis and discussion on the formation mechanisms, urgent requirements, and potential countermeasures including the use of additives, co-firing, leaching, and alloying.
Since 2011, a pilot plant for the production of mixed alcohols (MAs) from biomass-derived synthesis gas has been in operation. The pilot plant uses synthesis gas provided by a biomass-based combined heat and power plant (CHP) in Güssing, in which the fixed bed reactor is filled with a sulfidized molybdenum catalyst (MoS2). The advantage of a sulfidized catalyst is resistance to sulfur components. Sulfur resistance significantly lowers the operating costs of such a plant. The main parts of the mixed alcohol synthesis plant in Güssing are a steam reforming unit, a glycol scrubber, a compression step, a fixed bed reactor for the synthesis itself, a condensation vessel for the separation of alcohols from the gas stream, and an expansion valve. The main aim of this project in the last year was to perform parameter variation to investigate the optimal operation point of the plant and to use these parameters to conduct long-time trials over several days. Parameter variation showed the impact of reaction temperature, pressure, and space velocity on product yield and distribution. The influence of the gas composition on the product distribution and side products was also investigated. This study was dedicated to investigating the influence of the ethylene content in the synthesis gas on the formation of propanol and ethyl mercaptan (C2H6S). During the experiments, several liters of MAs were produced. The alcohols consisted mainly of alcohols from carbon numbers between one and three. One of the main results of the first experiments was that the ethylene content in the synthesis gas has a high impact on the product distribution and is also responsible for side reactions to sulfur components.
The bed agglomeration characteristics during combustion of typical biomass fuels were determined in a bench-scale bubbling fluidized-bed reactor (5 kW) using olivine and quartz sand as bed material. The fuels studied include willow, logging residues, wheat straw, and wheat distiller’s dried grain with solubles (wheat DDGS). Bed material samples and agglomerates were analyzed by means of scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), for morphology and elemental composition. Furthermore, bed ash particles were separated by sieving from the bed material samples and analyzed for elemental composition by SEM–EDS and for determination of crystalline phases by powder X-ray diffraction (XRD). Chemical equilibrium calculations were performed to interpret the experimental findings of layer formation and reaction tendencies in both bed materials. Significant difference in the agglomeration tendency between olivine and quartz was found during combustion of willow and logging residues. These fuels resulted in inner layers that were more dependent on the bed material composition, and outer layers that have a composition similar to the fuel ash characteristics. The elemental composition of the inner layer formed on the quartz bed particles was dominated by Si, K, and Ca. In the olivine bed, the inner layer consisted mainly of Mg, Si, and Ca. Chemical equilibrium calculations made for both bed materials showed a low chemical driving force for K to react and be retained by the olivine bed particles, which is in accordance to the experimental findings. For the quartz case, the inner layer was found responsible for the initiation of the agglomeration process. The composition of the fewer and more porous agglomerates found after the experiments in the olivine bed showed neck composition and characteristics similar to the individual bed ash particles found in the bed or outer bed particle coating composition. For DDGS (rich in S, P, K, and Mg) and wheat straw (rich in Si and K), no significant differences in the bed agglomeration tendency between olivine and quartz bed materials were found. The results show that the bed particle layer formation and bed agglomeration process were associated to direct adhesion of bed particles by partly molten fuel ash derived K–Mg phosphates for DDGS and K-silicates for wheat straw.
The bed agglomeration characteristics during combustion of phosphorus-rich biomass fuels and fuel mixtures were determined in a fluidized (quartz) bed reactor (5 kW). The fuels studied (separately and in mixtures) included logging residues, bark, willow, wheat straw, and phosphorus-rich fuels, like rapeseed meal (RM) and wheat distillers dried grain with solubles (DDGS). Phosphoric acid was used as a fuel additive. Bed material samples and agglomerates were studied by means of scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX), in order to analyze the morphological and compositional changes of coating/reaction layers and necks between agglomerated bed particles. Furthermore, bed ash particles were separated by sieving from the bed material samples and analyzed with SEM/EDS and powder X-ray diffraction (XRD). For logging residues, bark, and willow, with fuel ash rich in Ca and K but with low contents of P and organically bound Si, the bed layer formation is initiated by reactions of gaseous or liquid K compounds with the surface of the bed material grains, resulting in the formation of a potassium silicate melt. The last process is accompanied by the diffusion/dissolving of Ca into the melt and consequent viscous flow sintering and agglomeration. The addition of high enough phosphorus content to convert the available fuel ash basic oxides into phosphates reduced the amount of K available for the reaction with the quartz bed material grains, thus preventing the formation of an inner bed particle layer in the combustion of logging residues, bark, and willow. Some of the phosphate-rich ash particles, formed during the fuel conversion, adhered and reacted with the bed material grains to form noncontinuous phosphate−silicate coating layers, which were found responsible for the agglomeration process. Adding phosphorus-rich fuels/additives to fuels rich in K and Si (e.g., wheat straw) leads to the formation of alkali-rich phosphate−silicate ash particles that also adhered to the bed particles and caused agglomeration. The melting behavior of the bed particle layers/coatings formed during combustion of phosphorus-rich fuels and fuel mixtures is an important controlling factor behind the agglomeration tendency of the fuel and is heavily dependent on the content of alkaline earth metals in the fuel. A general observation is that phosphorus is the controlling element in ash transformation reactions during biomass combustion in fluidized quartz beds because of the high stability of phosphate compounds.
Naturally occurring catalytic substances are employed in biomass steam-gasification processes to enhance the yield of fuel gas and reduce its tar content by cracking and reforming the high molecular weight organic components. Calcined dolomite is widely used for this purpose; it exhibits good catalytic activity under the operating conditions of the gasifier. However, due to its poor mechanical strength, it gives rise to a large production of fines in a fluidised-bed environment. This work reports an investigation into the catalytic behaviour of olivine, a common, naturally occurring mineral containing magnesium, iron oxides and silica: iron is known to play a positive role in tar decomposition reactions. The gasification runs, performed with a laboratory scale, biomass gasification unit, show that the olivine activity is close to that exhibited by dolomite under comparable operating conditions. Olivine has the additional advantage, however, that its resistance to attrition in the fluidised bed is much greater, similar to that of sand. Parametric sensitivity studies of a gasification process, utilising olivine as the fluidised-bed inventory, indicate an optimum gasification temperature of just above 800°C, and little influence of the steam/biomass ratio in the range 0.5–1.
Major- and minor-element site occupancies have been determined for Ni- and Mn-rich synthetic forsteritic olivines that were equilibrated to 300, 600, 900, or 1,050 C, by analyzing electron-channeling-enhanced x-ray emissions. Site preferences for minor elements (Ni>Cr for M1; Ca>Mn for M2) are consistent with those reported in previous studies. Degrees of ordering at different elevated temperatures differ less than analytical errors, although there may be significant differences in ordering between olivines that have been rapidly quenched from 300 C or higher and those that have undergone normal lava-flow cooling histories. An x-ray structure refinement of San Carlos olivine (R=0.031) gives major-element occupancies similar to those determined by electron-channeling on the same material. Thus, the channeling technique gives results that are consistent with other methods, and it may be possible to use the technique to identify olivines that have undergone very rapid quenching.
High oil prices and peak oil, next to ecological aspects, increase the necessity of governmental support regarding the use
of renewable energy resources. Biomass is a renewable energy source, which allows a sustainable utilization for several reasons.
Its carbon dioxide neutrality and high availability in countries across Europe make economic usage of this source possible.
Nowadays, biomass is used in rather conservative ways to produce heat and/or electric power. A more sophisticated way of using
wood is transforming it into a secondary energy source by liquefaction and gasification. The product of the gasification process—considered
in this paper—is a medium calorific product gas, which is nearly free of nitrogen and has a H2/CO ratio favourable for synthesis processes. Therefore, the product gas can be converted into a synthetic natural gas (BioSNG).
In Güssing (Austria), the concept of a steam blown dual fluidized bed gasifier coupled to a catalytic conversion of the product
gas to BioSNG could be proven successfully. A slipstream was used to run a demonstration unit with a capacity of 1MW BioSNG.
The resulting BioSNG exceeded the regulations for injection into the natural gas grid. The compressed BioSNG was stored in
a fuelling station to supply CNG cars with energy. Thus, the applicability of using BioSNG in CNG cars was proven as well.
The simulation software IPSEpro was used to model the overall system of gasification, gas cleaning, methanation and upgrading
to BioSNG. The aim of this modelling work was to evaluate the optimization potential within the system and improve the economic
and ecologic situation. Moreover, this tool will also be used to scale-up the process hereafter.
KeywordsBioSNG–Güssing gasifier–Methanation
Dieses Standardwerk beschreibt umfassend und detailliert die biologischen, physikalischen, chemischen und technischen Grundlagen einer Energiegewinnung aus Biomasse. Es werden die Möglichkeiten der Bereitstellung von Nutz- bzw. Endenergie aus organischen Stoffen sachlich und mit Hilfe aussagekräftiger Abbildungen dargestellt. Die Autoren gehen konkret ein auf die unterschiedlichen Biomasseressourcen und ihre Verfügbarmachung sowie auf deren thermo-chemische, physikalisch-chemische sowie bio-chemische Umwandlung in Sekundärenergieträger bzw. in End- oder Nutzenergie.
> Die 2. Auflage wurde vollständig überarbeitet und teilweise neu strukturiert. Hinzu gekommen sind u.a. folgende Themen: die Bereitstellung flüssiger und gasförmiger Biokraftstoffe über die thermo-chemische Biomasseumwandlung, die Torrefizierung fester Biomassen, die Optionen zur Hydrierung von Pflanzenölen und die Technik der Einspeisung von Biogas in Erdgasnetze. Das Buch bietet einen soliden und umfassenden Überblick nach dem Stand der Technik und informiert über Trends und neuere Entwicklungen. Es ist den Herausgebern gelungen, unter Mitarbeit einer Vielzahl kompetenter Fachleute ein solides Werk "aus einem Guss" zu erarbeiten.
> Es ist geeignet für Studierende, Anlagenbetreiber, Berater, Wissenschaftler und interessierte Laien.
Agglomeration is among one of the major problems in the operation of fluidized bed boilers. The formation of bed particle layers is thought to play an important role on the occurrence of agglomeration in wood-fired fluidized (quartz) beds. In spite of frequent experimental reports on the quartz bed particle layer characteristics, the underlying bed layer formation process has not yet been presented. By combining our previously experimental results on layer characteristics for samples with durations from 4 h to 23 days, with phase diagrams, thermochemical equilibrium calculations, and a diffusion model, a mechanism of quartz bed particle layer formation was proposed. For younger bed particles (<around 1 day), the layer growth process is accelerated due to a high diffusion of calcium in a K-rich silicate melt. However, with continuous addition of calcium into the layer, the amount of melt decreases and crystalline Ca-silicates starts to form. Ca2SiO4 is the dominating crystalline phase in the inner layer, while the formation of CaSiO3 and possibly Ca3SiO5 are favored for younger and older bed particles, respectively. The decreasing amount of melt and formation of crystalline phases result in low diffusion rates of calcium in the inner layer and the layer growth process becomes diffusion controlled after around 1 day.
In this paper, the performance of a water gas shift (WGS) pilot plant which processed tar-rich product gas for about 2250 h is investigated. The WGS pilot plant employed a commercial Fe/Cr based catalyst (ShiftMax® 120). The product gas was generated by the industrial scale and commercial dual fluidized bed (DFB) biomass steam gasification plant in Oberwart, Austria. A partial flow of tar-rich product gas was extracted for the WGS pilot plant before the tar scrubber of the gasification plant. The extracted product gas had a temperature of about 150 °C and a GCMS tar content between 2.7 and 8.2 g m−3 (d.b.). In order to investigate the stability of the catalyst and to observe the performance of the WGS pilot plant, extensive chemical analyses were carried out: CO, CO2, CH4, N2, O2, C2H6, C2H4, C2H2, H2S, COS, and C4H4S were measured. In addition, GCMS tar and NH3 analyses were performed. Furthermore, the catalyst's activity was observed by measuring the temperature profiles along the reactors of the three stage WGS pilot plant. During the about 2250 h of operation, no significant catalyst deactivation could be observed. A CO conversion of up to 92% as well as a GCMS tar reduction along the WGS pilot plant was obtained. The results showed that the application of a commercial Fe/Cr based catalyst in a WGS unit seems to be a suitable way for increasing the hydrogen content in a product gas generated by dual fluidized bed biomass steam gasification. Furthermore, with such a technique, it is possible to optimally adjust the required CO/H2 ratio for several synthesis reactions, for example, methanation and Fischer-Tropsch synthesis.
A promising technology replacing fossil energy carriers for the production of electricity, heat, fuels for transportation and synthetic chemicals is steam gasification of biomass in a dual fluid bed (DFB). The principle of this technology is the separation into a gasification and a combustion reactor. Bed material, nowadays olivine, circulates between them, and has two functions. It acts as a heat carrier from the combustion to the gasification zone and as catalyst regarding gasification reactions. Today, an alternative to olivine does yet not exist.
In this work, experiments in a lab-scale test rig were performed investigating the catalytic activity of different fresh and used bed materials, such as olivine and quartz. The enhancement of the catalytic activity due to particle coatings was tested regarding the water-gas-shift reaction and steam reforming of toluene. Calcite is known as an active material in this respect and is therefore used as a benchmark substance.
Experiments revealed a correlation between the catalytic activity of bed particle coatings towards both the water-gas-shift reaction and the reduction of toluene by steam reforming. Results showed bed material particles with a calcium (Ca)-rich layer achieve satisfactory conversion of carbon monoxide and reduction of toluene. Furthermore, a qualitative comparison regarding hydrogen production relative to the benchmark material CaO is given for the water-gas-shift and steam reforming of toluene and ethene - used as model substance for lighter hydrocarbons. These results are the basis for further research on the catalytic properties of potential bed materials for DFB gasification of biomass.
In this paper, the performance of a commercial Fe/Cr based catalyst for the water gas shift reaction was investigated. The catalyst was used in a water gas shift pilot plant which processed real product gas from a commercial biomass steam gasification plant with two different qualities: extracted before and extracted after scrubbing with a rapeseed methyl ester gas scrubber. The performance of the WGS pilot plant regarding these two different gas qualities was investigated. For this reason, extensive chemical analyses were carried out. CO, CO2, CH4, N2, O2, C2H6, C2H4, and C2H2 and H2S, COS, and C4H4 S were measured. In addition, GCMS tar and NH3 analyses were performed. Furthermore, the catalyst's activity was observed by measuring the temperature profiles along the reactors of the water gas shift pilot plant. During the 200 h of operation with both product gas qualities, no catalyst deactivation could be observed. A CO conversion up to 93% as well as a GCMS tar reduction (about 28%) along the water gas shift pilot plant was obtained. Furthermore, a specific H2 production of 63 g H2 per kg biomass (dry and ash free) was reached with both product gas qualities. No significant performance difference could be observed.
Agglomeration of bed materials at high temperature is one of the most important and challenging problems for fluidized-bed biomass boilers for thermal/power generation. Inorganic alkali components from the fuel can be problematic as they form low-melting alkali compounds. In the present study, the critical amount of liquid (molten ash in real biomass boiler operations) that would result in severe bed agglomeration and defluidization was studied in a small pilot-scale bubbling fluidized bed (BFB) rig, operated at elevated temperatures (415–420 °C) using low melting-point salt (KOH) to simulate molten ash. In the BFB fluidization system tested, the critical liquid amount of simulate molten ash that could cause the bed materials start to form agglomeration/channeling, and form severe agglomeration/channeling (poor fluidization) is approx. 0.5 wt.% and 0.8 wt.% at a lower fluidizing gas velocity (3.9 Umf) and a higher fluidizing gas velocity (5.9 Umf), respectively. With more low-melting-point compound (KOH) present in the BFB, more agglomerates of bed material are formed. Increasing the amount of liquid could change the fluidization behavior of group B (bubbling) particles towards group A (Aerated) and even C (Cohesive) depending on the amount of the low-melting-point compound in the bed material.
The central theme of this book is the development of the largest renewable energy source at present to efficient applications in modern and developing society: biomass. Energy from solar radiation, fixated by self–assembling plant structures, creating biomass which is converted to energy carriers fit for application in today’s and tomorrow’s energy generating equipment. The book is understandable for the non–expert but is nevertheless revealing the true challenges in this extremely broad area. You will find in it the interesting history of the use biomass by mankind in general and how the future of its modern use was shaped by active support of the European Union told by passionate pioneers and (ex) EU officials. The main emphasis is on specific technologies both biological and thermo–chemical, from simple to extremely complex. These are explained in a clear way, including future prospects, by recognized experts. Climb on the shoulders of all these authors (>25) and look into the close and distant future where interaction with other renewable sources will occur and discover: no renewable energy future without an important role of the oldest one: bio-energy. © 2015 by Pan Stanford Publishing Pte. Ltd. All rights reserved.
Olivine is one of the refractory materials well-suited for fluidized-bed reactor technology. However, this material agglomerates at high temperatures due to the presence of sticky molten ash. The aim of this work is to investigate the mechanism of olivine agglomeration in fluidized-bed reactors and to determine the risk factors for agglomeration. A laboratory fluidized-bed reactor was designed to study the agglomeration effect between the ash and the refractory bed material. A systematic experiment was performed to determine the agglomeration ratio as a function of different parameters (operating time, bed materials, ash content, temperature, gas flow and additives). The mechanism of adhesion between the molten ash and the bed material is described, and the optimization of parameters to prevent this agglomeration is determined. © 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
A promising way to substitute fossil fuels for production of electricity, heat, fuels for transportation and synthetic chemicals is biomass steam gasification in a dual fluidized bed (DFB). Using lower-cost feedstock, such as logging residues, instead of stemwood, improves the economic operation. In Senden, near Ulm in Germany, the first plant using logging residues is successfully operated by Stadtwerke Ulm. The major difficulties are slagging and deposit build-up. This paper characterizes inorganic components of ash forming matter and draws conclusions regarding mechanisms of deposit build-up. Olivine is used as bed material. Impurities, e.g., quartz, brought into the fluidized bed with the feedstock play a critical role. Interaction with biomass ash leads to formation of potassium silicates, decreasing the melting temperature. Recirculation of coarse ash back into combustion leads to enrichment of critical fragments. Improving the management of inorganic streams and controlling temperature levels is essential for operation with logging residues.
A test campaign was carried out to generate renewable hydrogen based on wood gas derived from the commercial biomass steam gasification plant in Oberwart, Austria. The implemented process consisted of four operation units: (I) catalyzed water–gas shift (WGS) reaction, (II) gas drying and cleaning in a wet scrubber, (III) hydrogen purification by pressure swing adsorption, and (IV) use of the generated biohydrogen (BioH2) in a proton exchange membrane (PEM) fuel cell. For almost 250 h, a reliable and continuous operation was achieved. A total of 560 (Ln dry basis (db))/h of wood gas were extracted to produce 280 (Ln db)/h of BioH2 with a purity of 99.97 vol %db. The catalyzed WGS reaction enabled a hydrogen recovery of 128% (ṅBioH2)/(ṅH2,wood gas) over the whole process chain. An extensive chemical analysis of the main gas components and trace components (sulfur, CxHy, and ammonia) was carried out. No PEM fuel cell poisons were measured in the generated BioH2. The only detectable impurities in the product were 0.02 vol %db of O2 and 0.01 vol %db of N2.Keywords: Biohydrogen; Biomass; Gasification; Product gas; Water−gas shift; Gas scrubbing; Pressure swing adsorption; Latex
The use of untreated olivine as the bed material in a biomass gasifier is investigated in this work, in which activation of the material is the main focus. The experiments were carried out in the Chalmers 2–4-MWth indirect biomass gasification unit and comprised analyses of the gas composition and bed material, as well as changes in tar yield. Starting from the raw material, the first signs of activation, in the form of a reduction in the tar yield, were observed already during the second day of the operation. The tar yield continued to decrease with time, and by the fourth day it was reduced by 30%, as compared to the yield on the first day of the operation. Analysis of the bed samples showed accumulation of inorganics within the bed material, with a share of potassium being present in leachable form. Thermodynamic calculations support the indications from the experiment that potassium can be released under gasification conditions and may play an important role in the activation of olivine. To examine the impacts of S and silica on the activity of olivine, two experiments were conducted. The addition of S to the combustion side gave a positive effect in terms of the tar levels in the raw gasification gas. The addition of silica sand revealed, as expected given the affinity of potassium for silicone, negative influences on the tar yield and gas composition that could not be attributed to mere dilution, as compared with the gas produced during operation with pure olivine.
Interactions between fuel ash and bed material, which lead to agglomeration, are impeding a widespread use of alternative biomass fuels for energy conversion in fluidized bed boilers. Reactions of certain ash components (alkaline metals in the case of biomass) with silicon of the quartz sand, commonly used as bed material, have been identified as main drivers for this phenomenon. Still, there is no consensus on the detailed mechanisms causing the attachment of ash on bed particles and the consequent growth of agglomerates. Only through a better understanding of these processes will it be possible to cast precise predictions and implement effective countermeasures for agglomeration issues. This paper reviews the current state of knowledge and delivers new insights into the mecha-nisms of coating induced agglomeration. In particular, the influence of heterogeneous reactions on coating formation and the metamorphism of the coatings during agglomeration are investigated. The results are derived from experiments performed on lab-scale reactors described in our earlier publications. In addition, coated particles and agglomerates from large scale plants, or such extracted during the lab scale experiments, were evaluated using SEM/EDX analyses. The findings are summarized in an updated description of the agglomeration process, eliminating heterogeneous chemical reactions from the major influencing factors and proposing a hypothesis for the behavior of the typical two-layer coatings during agglomeration.
One of the main concerns about biomass fluidized bed gasification and combustion is the risk of bed particle agglomeration due to ash melting. Although many studies have been conducted about the agglomeration mechanism using silica sand, olivine is mostly mentioned as an alternative bed material for tar decomposition and its interaction with biomass ash has not been yet fully understood. The aim of this work is to investigate the agglomeration of miscanthus ashes focusing on thermophysical and thermochemical aspects. Three different bed materials (silica sand (SiO2), raw and calcined olivine ((Mg,Fe)2SiO4)) and an additive to prevent agglomeration (dolomite (CaMg(CO3)2) were tested. The effects of atmosphere and miscanthus harvest time were also investigated. It was found that the key parameter of agglomeration is the wettability of bed particles by molten ashes. In contact with ashes all three bed material showed good wetting tendencies, while dolomite had non-wetting properties. The adhesion between bed materials and molten ashes increases in the order of silica, olivine and calcined olivine. While in the case of silica sand only physical adhesion occurred, the diffusion of iron oxide into the molten ash was observed using olivine. Calcined olivine has a roughened surface which further increased the adhesion. The atmosphere did not influence the mechanism of ash/bed material interaction. On the other hand, miscanthus harvest time had significant effect on ash reactivity and interaction with raw and calcined olivine.
Formation of sticky layers on bed particles has been considered as a prerequisite for bed agglomeration in fluidized bed combustion of wood-derived fuels. The present investigation was undertaken to determine the quartz bed particle layer formation process in fluidized bed combustion of wood-derived fuels. Bed material samples from three different appliances, bench-scale bubbling fluidized bed, full-scale bubbling fluidized bed, and full-scale circulating fluidized bed, at different sampling times from startup with a fresh bed were collected. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) were used to explore layer morphology and chemical composition and to gain information on crystalline phases of the layers and coatings. Significant differences in layer morphology and composition were found for quartz bed particles with different ages. For bed samples with operational duration of less than 1 day, only one thin Ca-, Si-, O-, and K-rich homogeneous quartz bed particle layer that has a relatively high K/Ca molar ratio was found. For quartz bed particles with an age from around 1 day to 2 weeks, an outer more particle-rich coating layer was also found. During the initial days of this period, the layer growth rate was high but decreased over time, and decreasing K/Ca and increasing Ca/Si molar ratios in the inner bed particle layer were observed. For bed particles with age between 2 and 3 weeks, a much lower layer growth rate was observed. At the same time, the Ca/Si molar ratio reached high values and the K concentration remained on a very low level. In addition to these layer formation processes mentioned, also an inner–inner/crack layer was also formed in the circulating fluidized bed quartz bed particles simultaneously with the inner bed particle layer.
Ash components of biomass fuels can cause fouling, slagging, and bed material agglomeration during thermal utilization in fluidized bed combustion and gasification plants. The influence of ash components on these problems in dual fluidized bed biomass gasification plants is investigated in an industrial scale plant in Güssing, Austria. Samples of fouling are analyzed, and the results are evaluated. The samples were analyzed by X-ray fluorescence analysis and thermal analyses such as thermogravimetric analysis, differential thermal analysis, and differential scanning calorimetry. Mass balances of inorganic matter are presented, evaluating different loop configurations. The analyses showed high potassium contents compared to the fuel ash composition in fouling of up to 23% by weight. The potassium content of fly ash with a particle size smaller than 200 μm is half that of coarse fly ash with a particle size larger than 200 μm. The thermal analyses showed a large difference between samples of inorganic streams such as fly ash or fly char and fouling. Different fractions of fly ash samples (particle fraction smaller than 200 μm and particle fraction larger than 200 μm) showed similar thermal behavior: endothermic weight losses at around 400 °C and around 720–820 °C caused by decomposition of carbonates. The composition of inorganic matters of fly ash and fly char is similar. The elemental composition of deposits at the cyclone wall and the first heat exchanger in the flue gas path showed high potassium contents up to 23.6%. While samples of fly ash and fly char did not show significant melting in their thermal behavior, melting could be detected with fouling at temperatures higher than 1000 °C. Mass balances of inorganic matter showed a flow of potassium oxide from the combustion reactor to the gasification reactor, which leads to unexpected high potassium concentrations in the fly char. A reduction of ash loops reduces the amount of potassium that is transferred from the combustion reactor to the gasification reactor. Recommendations are made for the operation of dual fluidized bed gasification plants in terms of ash handling to reduce tendencies for fouling, slagging, and bed material agglomeration.
This is the first in a series of three papers describing combustion of biosolids in a 5-kW bubbling fluidized bed, the ash chemistry, and possible application of the ash produced as a fertilizing agent. This part of the study aims to clarify whether the distribution of main ash forming elements from biosolids can be changed by modifying the fuel matrix, the crystalline compounds of which can be identified in the raw materials and what role the total composition may play for which compounds are formed during combustion. The biosolids were subjected to low-temperature ashing to investigate which crystalline compounds that were present in the r