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Effect of freeboard residence time on the molecular mass distributions of fluidized bed pyrolysis tars
Abstract
Yields and molecular mass distributions are reported for tars obtained in an atmospheric pressure fluidized bed pyrolysis reactor operated at two different freeboard residence times (0.8 s and 4.5 s). Experimentally, significant tar loss through covalent bond scission for the longer freeboard residence time (4.5 s) was observed to occur at temperatures as low as 500 °C. At higher temperatures (580–750 °C) the molecular mass distributions of the tars appear to shift to lower values. The larger molecular mass fractions of the tars (typically the > 2500 Da fraction) are found in greater abundance in tars thought to have undergone the least extent of secondary reactions. Taken together, the data appear consistent with pyrolysis models describing tar evolution as being primarily limited by bond scission and evaporation of tar precursors.
... Doolan et al. [7], Katheklakis et al. [8], Hayashi et al. [9,10] and Zieliński et al. [11] studied the volatiles' reaction in fluidized-bed reactors. They reported that an increase in temperature or in reaction time decreased the tar's yield and H/C ratio, and increased the tar's heavy component content. ...
... They reported that an increase in temperature or in reaction time decreased the tar's yield and H/C ratio, and increased the tar's heavy component content. Katheklakis et al. [8] noted that the loss of tar via volatiles' reaction was significant at temperatures as low as 500°C. In a range of 580-750°C, the amount of tar loss due to extended reaction time, approximately 9.35-26.02% ...
... Temperature effect similar to those discussed above can be found in the literature. For example, Katheklakis et al. [8] studied the reaction of volatiles produced from pyrolysis of Linby coal (81.5 wt.% carbon), and observed the maximum tar yield at 580°C in a temperature range of 400-750°C. Xu and Tomita [4] found that in pyrolysis of Liddell coal (83.5 wt.% carbon) the tar yield began to decrease and the gas yield began to increase when the volatiles temperature was higher than 600°C. ...
The reaction of volatiles generated from coal during pyrolysis determines the yield and quality of products. To understand this reaction in detail particularly on the condensation reaction that forms coke, this paper studies the reaction of volatiles generated from a lignite and a subbituminous coal using a two-stage fix-bed reactor, with the first stage for coal pyrolysis and the second stage for reaction of volatiles. The yields of tar, gas and coke formed on the wall and presented in tar at a second-stage temperature range of 440-700. °C and a residence time range of 1.5-6.9. s are quantified. The concentration, g value and linewidth of radicals in tars, mainly in coke presented in tars, are measured by electron spin resonance (ESR) and discussed to explore the transformation of coke in structure. It is found that the reactions of volatiles generated from the lignite and the subbituminous coal are different in the temperature range studied but the structure of coke formed in these two volatiles differs mainly at temperatures lower than 600. °C. The proportion of coke formed on the reactor wall is small at 440. °C, but large at 650 and 700. °C, close to 50% in 4.2. s. The increase in concentration and decreases in g value and linewidth of radicals with increasing temperature over time suggest that the coke undergoes condensation, leading to loss in oxygen and increase in aromaticity.
... Despite extensive characterization of tar decomposition at moderate temperatures in laboratory studies [1][2][3][4][5][6][7][8][9], no kinetics for tar decomposition have yet been reported, so this chemistry has not yet been incorporated into design simulations for coal utilization technologies. The FG-DVC pyrolysis mechanism contains an option that allows primary tars to continue to lose their functional groups in the vapor phase surrounding the fuel at the same rates applied in the condensed coal phase [10], which is fundamentally incorrect. ...
... No parameters were adjusted to improve the agreement with any of the data in this section. The first validation study monitored secondary pyrolysis along the freeboard of a fluidized bed operated at atmospheric pressure with a hv bituminous coal [4]. Both tar yields and M n -values were reported. ...
... Validation of predicted tar yields for secondary pyrolysis in a freeboard after 0.8 s (d) and 4.5 s (s) from a hv bituminous coal[4]. ...
... A well known example is the Char Oil Energy Development (COED) process developed in 1970s which employs 4 fluidized-bed reactors in series with one of the reactors consisting of two stages [16]. The reactor analyzed here, however, is a single stage fluidized-bed reactor not only for simplicity but also for its frequent use in coal pyrolysis studies [4,5,23]. Fig. 4(a) is a schematic diagram of a fluidized-bed reactor which consists of two phases, the dense phase and the dilute phase, which are very close in temperature. ...
... For a reactor operated at 750°C the temperature increase in volatiles is about 450°C for the volatiles generated at a coal temperature of 300°C while 150°C for the volatiles generated at a coal temperature of 600°C. The residence time of volatiles, regardless of their generation temperature, is very close and usually in a range of a few seconds, such as 0.8-4.5 s reported by Katheklakis et al. [23]. These data suggest that cracking of volatiles in a fluidized-bed reactor may be similar to that in L-R reactor, depending upon the operating temperature of course, but the particulate content of tars from a fluidized-bed reactor is higher due to strong solid agitation and gas entrainment [24]. ...
... Since it is not possible for us to carry out such experiments or to find such data in the literature, literature data on coals of similar carbon content and similar volatile matter content are compared. Fig. 9 shows tar yields of coals with carbon contents Tar yield (g g of 79-83 wt% (dry-ash-free (daf) base) and volatiles matter contents of 39-41 wt% (daf base) in all the reactors discussed above [4,5,12,15,23,[27][28][29]31,[36][37][38]. The tar yields are restricted to pyrolysis with the highest coal temperature of 600°C or higher to ensure that most of volatiles are released from the coals. ...
Coal pyrolysis is regarded by many as a simple method to produce liquid fuels and chemicals and has been studied extensively in the past. However, fast coal pyrolysis technologies intended for higher tar yields and higher productivities, in comparison to the successful slow pyrolysis technologies, had common problems such as poor tar quality and plugging of the volatile products lines. The failing of many technical efforts in solving these problems calls for more fundamental studies, especially on the reaction of volatiles in major pyrolysis reactors. This is because that thermal cleavage of covalent bonds in coal that generates volatiles is a single step and depends mainly on the temperature of the coal, while the reactions of the volatiles involve multiple steps and depend on many factors especially the gas phase temperature that is generally higher than that of the coal due to the large temperature gradient in fast pyrolysis reactors. This article analyzes temperature increases in volatiles’ upon their generation from coal in various pyrolysis reactors and correlates the temperature increases with tar yield and composition. Experimental results on reaction of volatiles are also presented to enlighten the importance of the volatiles’ reactions.
... The conversion degrees and physicochemical transformations of tars, which occur during a gasification process, have been investigated by many researchers [8][9][10][11][12] and are summarized in Fig. 1. ...
... The mechanism by which a carbonaceous material is degraded and evolves in the gas phase in a fluidized bed reactor is very similar to that proposed by Katheklakis et al. [8] except that the initial material here is a refuse derived fuel. According to this mechanism, the degradation process starts at 300-400°C with the desorption of moisture, some light gases (e.g. ...
... Tar yields in a generic gasification process as a function of temperature. Results are qualitative and given on the basis of wt.% dry ash-free (daf) lignin-plastic mixture, as representative for waste material [8][9][10][11][12]. compact and relatively stable molecules able to evaporate completely at temperatures around 500°C [9]. ...
This work focuses on systematic studies of the plasma reforming of newly evolved vapors from a fluid bed gasifier, and on the resulting evolution of individual gaseous cracking products to hydrogen-rich syngas. The aim of this study is to compare some previously developed mechanisms of thermal cracking and to identify the main elementary reactions and the most sensible ones for tar decomposition in a two-stage process. Evaluation of plasma chemistry is performed by a comparison between experimental data and thermal kinetic predicted results. Distribution analysis of condensable organics shows that for all the representative species, the levels of tars are distinct in the first stage and almost negligible after the plasma treatment. Under the given reaction conditions, the organic cracking products such as methane and C2-species are completely converted to carbon monoxide and hydrogen, and no soot significantly formed. Oxygen atoms initially formed from CO2 were identified as the major active species involved in the oxidative decomposition of hydrocarbon intermediates and soot precursors. As a result, a two-stage system shows better reforming results, large treatment capacity and almost complete carbon conversion.
... Doolan et al. [22] reported a conversion of C 2 H 4 to C 2 H 2 at temperatures higher than 1000°C. Katheklakis et al. [23] showed increases in the formation of CH 2 Cl 2 and CH 3 OH insoluble matters, coke and gas due to the volatiles' reaction at high temperatures. ...
... It should be noted that in Fig. 7 the largest change in tar fractions occurred in t 2nd < 4.2 s at both temperatures for HLBE tar (Fig. 7(b) and 7(d)) and t 2nd < 2.2 s at 440°C for SM tar (Fig. 7(c)), indicating that the tars were reactive even at such a low T 2nd and short t 2nd . These conditions are much milder than that used in most pyrolysis studies in laboratory and in pilot scales reported in the literatures [2,23,[39][40][41]. The slight changes in R IC and in the fraction of b.p. of 200-538°C for SM tar obtained at T 2nd of 650°C (shown in Fig. 7(a)) may suggest that the major volatiles' reaction for SM tar at this temperature occurred in < 1.5 s because the reaction rate at 650°C should be much faster than that at 440°C. ...
In coal pyrolysis, the reaction of volatiles plays an important role in determination of tar yield and composition but has not been studied in detail in the literature. This work studies the volatiles’ reaction in pyrolysis of a bituminous coal (SM) and a lignite (HLBE) using a two-stage fixed-bed laboratory reactor with the second-stage temperature of 440–700 °C and the residence time of 1.5–6.9 s. The tars were characterized by solvent extraction with tetrahydrofuran (THF) and hexane, and by simulated distillation (SIMDIST) of the THF soluble matter and high performance liquid chromatography (HPLC) analysis of the hexane soluble matter. It was found that the main component of the tars was oil, which was more in the HLBE tar than in the SM tar. As much as >50 wt% oil and tar retained in the chromatograph columns. The volatiles reacted at temperatures as low as 440 °C and the rate was more pronounced at 600 °C in 4.2 s. The volatiles’ reaction reduced the oil yield while increased the pitch yield. The aromatics in the hexane soluble matter were mainly 2-ring compounds and their content increased by the volatiles’ reaction. The major tar fractions have boiling points of 200–538 °C and their content varied systematically in the volatiles reaction. The findings are important for design of efficient coal pyrolysis reactors.
... The volatiles' reaction in the space beyond the solid bed has also been studied in the literature. It was reported that the volatiles' reaction in coal pyrolysis in the dilute phase of fluidized-bed reactors decreased the yield and H/C ratio and increased the heavy fraction of tars [5][6][7][8]; the volatiles' reaction downstream of a fixed-bed coal pyrolysis reactor decreased the tar yield and increased the gas and coke yields [9,10]. The volatiles' reactions in coal pyrolysis are significant to alter the products yield at temperatures as low as 500 °C [6,10,11], but are significant to alter the tars' coke and radical concentrations at lower temperatures [12][13][14], as low as 350 °C. ...
... It was reported that the volatiles' reaction in coal pyrolysis in the dilute phase of fluidized-bed reactors decreased the yield and H/C ratio and increased the heavy fraction of tars [5][6][7][8]; the volatiles' reaction downstream of a fixed-bed coal pyrolysis reactor decreased the tar yield and increased the gas and coke yields [9,10]. The volatiles' reactions in coal pyrolysis are significant to alter the products yield at temperatures as low as 500 °C [6,10,11], but are significant to alter the tars' coke and radical concentrations at lower temperatures [12][13][14], as low as 350 °C. However, little study can be found on reaction of volatiles generated in co-pyrolysis of different organic matter. ...
... Higher temperature and pressure and longer residence time could intensify the secondary reaction, thus affecting the tar obtained from pyrolysis. 24 Katheklakis et al. 25 studied the effect of the residence time of volatiles on the molecular weight of tar in a fluidized bed reactor. The results showed that a higher pyrolysis temperature and longer residence time would significantly reduce the molecular weight of tar. ...
To develop the in situ underground pyrolysis process of tar-rich coal more scientifically, the effect of temperature and pressure on the distribution of pyrolysis products should be clarified. This paper selected the typical components in five distillates of light tar, phenol tar, naphthalene tar, washing tar, and anthracene tar as the main reaction products. 32 typical secondary reactions were constructed. Based on the thermodynamic analysis strategy, the variation of the Gibbs free energy and equilibrium constant of secondary reactions was investigated. The results showed that pressure mainly affected the reaction characteristics of molecule-increasing reactions. The Gibbs free energy value of the molecule-increasing reactions increased with increasing pressure. The trend that the reaction could proceed spontaneously gradually weakened. The initial temperature of some reactions that could proceed spontaneously would need to increase by dozens or even hundreds of degrees. Due to the influence of formation pressure, the generation of related components of light tar, naphthalene tar, washing tar, and anthracene tar would be inhibited to varying degrees in the in situ underground pyrolysis process. The secondary reactions related to phenol tar were equimolecular reactions, which were almost unaffected by stratal pressure. Axial pressure and confining pressure of different coal seam depths should be considered in the process of in situ underground pyrolysis.
... Thermal cleavage of covalent bonds in ADR produced volatiles in a single step that primarily depends on the temperature, while the volatile reactions involve multiple steps and depend on many factors especially the gas phase temperature. Different heating techniques, such as EH and IH, had substantial effects on primary pyrolysis products and volatile repolymerization, resulting in a variation in products distribution Katheklakis et al. 1990). Figure 4 shows the temperature profiles in reactors. ...
Fast pyrolysis via rapid infrared heating may significantly enhance the heat transfer and suppress the secondary reaction of the volatiles. The effects of various pyrolysis temperatures on pyrolysis behaviors of anaerobic digestion residues (ADR) were studied in this research utilizing a fixed-bed reactor equipped with rapid infrared heating (IH), as well as to compare the pyrolysis products produced by rapid infrared heating (IH) to those produced by conventional electric heating (EH). Thermogravimetric (TG) analysis revealed that pyrolysis of ADR occurred in three decomposition stages. The results of pyrolysis experiments showed that increasing temperature first raised the bio-oil yield for IH and EH, peaking at 500–600 °C, but thereafter decreased the yield. In contrast to the findings achieved with EH, infrared heating (IH) presented a greater overall bio-oil yield but a lower gas yield. The bio-oil produced by IH increased from 8.35 wt.% at 400 °C to 12.56 wt.% at 500 °C before dropping to 11.22 wt.% at 700 °C. Gaseous products produced by IH have a higher heating value than those generated by EH. Nitrogenous compounds, ketones, and phenols make up the majority of the bio-oil. In the IH bio-oil, nitrogen compounds rose with increasing temperature, while those varied slightly in the EH bio-oil. The phenols content in IH bio-oil was much more than that of EH, exhibiting values of 8.63% and 2.95%, respectively. The findings of the FTIR spectra of biochar indicated that as the temperature increased, the chains of aliphatic side professedly reduced and the structure of biochar became considerably ordered for both heating techniques. The Raman spectra of IH biochar showed that the ratio of AG/AD rose progressively from 0.17 to 0.20 as pyrolysis temperature rose from 500 to 700 °C.
... And at a given temperature, an increase in residence time results in a decrease in tar yield and an increase in gas and coke production. Katheklakis et al. [21] studied the effect of temperature and residence time on the molecular weight distribution of tar from pyrolysis of Linby coal. It is found that the tar yield dropped significantly at 500°C and residence time of 4.5 s. ...
... While for the high heating rate, the trend to generate new and stronger bonds decreased, leading to the release of more volatiles in a very short period. It means that the higher heating rate can lower the secondary reaction of volatiles to generate some small-molecule gas components and char [24][25][26]. Similar results can be also seen in other literature [17,20,27]. ...
... 。 图 1 慢速热解机理 [20] Fig. 1 Mechanism of slow pyrolysis [20] 煤首先在 Fig. 3 Mechanism of caking coal pyrolysis [23] 面的分子内部输送，对于硬煤来说主要是通过孔道 的扩散作用，而软煤主要通过液相扩散或气泡运动 来实现的。最终不能释放出去的挥发分通过缩聚反 应形成半焦。 上述各种煤热解机理实际是自由基反应机理， 通过弱键的断裂产生自由基碎片，然后通过自由基 碎片之间的反应最终产生稳定的产品。图 4 描述了 煤热解过程中自由基浓度的变化过程 [25] [25] Fig. 4 Change of free radicals in coal pyrolysis [25] Fig. 6 Network of volatiles secondary reactions [27] 解产物在颗粒内部的传质过程中会发生二次反应， 尤其是大分子自由基碎片通过重聚反应形成更多的 半焦产品，使得焦油在颗粒内有一个提质过程 [28] ， 这个过程主要受颗粒粒径和内外压差的影响。 在快速热解过程中初次热解产物瞬间释放，颗 粒内的二次反应可以忽略不计，气相中的二次反应 占主导地位。随着气相二次反应温度和停留时间的 增加，焦油产率减少，气体产率和焦炭增加 [29][30][31][32] 。 这个过程使得焦油组成发生明显变化，不同的反应 条件得出的结果不尽相同。有文献 [33] 报道随热解温 度的升高，焦油的 H/C 比例下降，稠环芳烃含量提 高。也有文献 [34] 报道延长停留时间和适当提高温度 可以提高焦油中的轻质组分。 Serio 等 [35] 认为焦油的 二次裂解是分段进行的，焦油根据自身的活性可相 互无影响地进行裂解，活性最高的在温和条件下即 可反应，活性最差的即使在很苛刻的条件下也不会 发生反应。可见，通过适当调控二次反应发生的程 度来实现煤的定向热解是可行的。已有文献 [36] 报道 可以通过调节二次反应发生的程度来制备高附加值 的 BTX 和乙烯。 [37] Fig. 7 Relative yields of pyrolysis product in inert atmosphere as functions of temperature, time and heating rate [37] 图 8 煤颗粒在反应器中的传热方向和挥发物传质方向 [39] Fig. 8 Transport behaviors of heat and coal volatiles during heating of coal particle [39] 了煤颗粒进入热解反应器中被加热的情况 [39] Fig. 10 Different types of particle size changes during high temperature process [79] 图 9 加氢热解机理和芳环化合物加氢和加氢裂解实例 [47] Fig. 9 Mechanisms of coal hydropyrolysis and examples for hydrogenation and hydrocracking of aromatics [47] 化 工 学 报 Fig. 11 Possible fragmentation modes [83] (a) exfoliation; (b) fragmentation at particle center; (c) fragmentation at an internal radial position; (d) exfoliation and fragmentation at center; ...
Coal pyrolysis is an efficient route for the clean utilization of low-rank coals due to its producing scarce
gas and oil resources. This paper focuses on the key technical problems of the unmanageable yield and quality of
tar and the high dust content in tar. From coal pyrolysis mechanism, the types and occurrence conditions of
volatile secondary reactions and the major influence factors on pyrolysis are discussed in detail. Combining the
application of coal pyrolysis technology, it is summarized that gas phase secondary reactions of volatiles are
responsible for the decrease of tar yield. Meanwhile, the coal particle fragmentation mechanism and the source of
dust during pyrolysis are analyzed. Based on previous research results, we propose that the control methods for
coal pyrolysis process, such as changing volatile flow direction from high temperature zone to low temperature
zone, coupling pyrolysis with gasification and in situ tar upgrading and dust removal, which can inhibit heavy tar
formation, improve light tar content and reduce dust content in tar, and thus realize oriented coal pyrolysis.
... For systems with fluidized beds, the entire ultimate volatile yield was subjected to more severe conditions in a freeboard or TFR. Most fluidized tests had a low-temperature bed coupled to a TFR 2,9 or freeboard whose temperature covered a broad range, 6 although the temperatures of both the bed and freeboard were the same and changed together in one test series. 8 Wherever possible, thermal histories estimated by the testing teams were implemented in the simulations. ...
This paper extends FLASHCHAIN theory with a mechanism for tar decomposition with any coal at any temperature under inert gases at atmospheric pressure, including (i) continuous elimination of heteroatoms as noncondensables, which transforms primary tar into polynuclear aromatic hydrocarbons at moderate temperatures; (ii) disintegration of tar monomers with attached hydrogen sources into oils and additional noncondensables; and (iii) nucleation and addition of oils and tars to a nascent soot phase at elevated temperatures. The aromatic nuclei, labile bridges, char links, and peripheral groups in FLASHCHAIN describe tar decomposition without modification, and seven of eleven proposed reactions were also transferred from FLASHCHAIN, albeit with markedly different kinetic parameters. Only the channel for oil production and the three for soot production are new and distinctive. All stoichiometric coefficients can be evaluated from a primary tar composition from FLASHCHAIN, and the associated variations in these coefficients are the distinguishing factors in the distinctive behavior of individual primary tar samples. Conversely, kinetics for tar decomposition are far less sensitive to coal quality than primary devolatilization kinetics, and the thermal response is much narrower. The mechanism accurately interprets the dynamics and product distributions throughout a validation database representing heating rates from 60 to roughly 50,000C/s; temperatures from 500 to 1300C; tar contact times from 40 ms through 14 s; and coal ranks from brown coal through anthracite (without subbituminous samples). The validation work also demonstrates that the kinetics for two reaction processes, bimolecular recombination of tar molecules and addition of oils and tars to soot, must explicitly depend on coal loading and, by inference, pressure in the subject tests or applications under consideration.
... Based on extensive reviews of experimental results both Kim (2015) and Brown (2015) note that vapor phase tar reactions can significantly affect overall yields of bio-oil, although the details of these reactions are still poorly understood. The effect of these reactions appears to be similar to reactions noted in earlier studies with coal pyrolysis, which also led to losses in liquid yield [Kathelakis et al (1990)]. ...
... It is commonly accepted that these problems are resulted mainly from the reaction of volatiles as soon as they are formed from cleavage of weak covalent bonds in coals. The reaction of volatiles has been studied in various reactors and found responsible for low tar yield and high coke formation [1,[4][5][6][7]. It was recently shown that the volatiles' reactions are mainly promoted by the high temperature environment surrounding the coal particles, which occurs in all types of pyrolysis reactors. ...
The reaction of volatiles in a coal pyrolysis reactor determines the yield and quality of tar. This paper studies the yield, group composition and radical concentration of tars obtained from pyrolysis of a subbituminous coal, mainly on the effect of the tars' residence time in the pyrolysis reactor and in post-pyrolysis heating. The latter operation simulates the environment of tars, if they were in a large fast pyrolysis reactor. It is found that an increase in the volatiles' residence time in the pyrolysis reactor reduces the yield and increases the radical concentration of tars. The pitch fraction of the tars is mainly responsible for coke formation which occurs significantly at temperatures higher than 420 °C. The tars contain radicals, mainly in their coke and pitch fractions. The coke concentration in the tars increases with increasing heating time, linearly at 420 and 450 °C but in a sigmoidal pattern at higher temperatures. The coking behavior within 10 min can be expressed by the zero-order or zero-order + autocatalytic kinetics with activation energies of 128–138 kJ/mol.
... However, in the pyrolysis of waste tyres, the formation of aromatic hydrocarbons can also be attributed to double bonds produced during the scission of polymer chain. Conjugated double bonds tend to form chromatics after cyclization and non-conjugated ones tend to form olefinic compounds [26,27]. With the effect of zeolite catalyst, USY catalyst or ZSM-5 catalyst, degradation tends to form many double bonds, including conjugated bonds, so a high concentration of aromatics is produced in this paper. ...
Pyrolysis of waste tyres was carried out with a two-stage bed reactor. The waste tyres were first pyrolysed in a fixed bed reactor, and then the evolved pyrolysis gases were passed through a secondary catalytic reactor filled with two types of zeolite catalysts, which were ultrastable Y-type (USY) and ZSM-5 catalyst. The catalytic temperatures were examined to determine their influence on the yield of product from the pyrolysis–catalyst of waste tyres. The results indicated that with the increase of catalytic temperature, the gas yield increased at the expense of oil yield. The light fractions (<220 °C) distilled from the derived oils with the two catalyst were analyzed by gas chromatography/mass spectrometry (GC/MS). It showed that there was a dramatic increase in the concentration of single ring aromatic compounds in the light fractions of the derived oils after catalysis. For example, toluene reached a value in the light fraction of 11.62 wt%, benzene 1.6 wt%, m/p-xylene 12.27 wt% and o-xylene 4.42 wt%, with the catalysis of USY catalyst, at the pyrolysis temperature, catalysis temperature and catalyst/tyre ratio of 500, 400 °C and 0.5, where the light fraction was about 71 wt% in the derived oil. The yield of aromatic hydrocarbons in the derived oils were related to the different properties of the two catalysts such as the pore size which influenced the selectivity, and the silica/alumina (Si/Al) ratio which influenced the number of active sites on the catalyst surface.
Fast pyrolysis technology can reduce the secondary reactions, improve the volatile product yield, and reduce the semicoke yield. Still, the high proportion of heavy tar components affects the development of fast pyrolysis industrialization. Therefore, this paper put forward a catalytic upgrading method of coal based on the solid waste bauxite residue (BR) as a catalyst. This study investigated the impact of varying particle sizes of pulverized coal and the addition of the BR catalyst on the product distribution and kinetics of coal fast pyrolysis. The results found that the tar yield was the highest at 600 °C when the particle size of pulverized coal was 75–150 μm, which was 19.44%. In the range of 550–650 °C, the relative content of benzene and toluene xylene (BTX) in liquid products increased with the temperature. With the increase of the proportion of the BR catalyst, the yield of semicoke in coal pyrolysis products increased, the yield of the gas phase also increased, and the yield of the liquid phase decreased.
The presence of waste tires on open fields or households creates an ideal breeding ground for disease-carrying vermin, threatening human well-being. There are various technologies studied for efficient use of waste tires, such as pyrolysis, which results in char, oil, and non-condensable gases. Tire pyrolytic oil (TPO) has been reported to be similar to commercial diesel fuel. The current hurdle for using TPO in commercial diesel engines is the available sulfur content (>1.0 wt%). The disadvantages of sulfur in liquid fuels are its ability to reduce the engine's life due to corrosion and the undesirable emission of SOx that subsequently damages public health and property. There is a rising need to develop efficient technologies for the desulfurization of such liquid fuels. Besides conventional hydrodesulfurization, other emerging technologies include adsorption, oxidation, photocatalytic degradation, and biological desulfurization. This paper reviews the status of pyrolysis of waste tires and desulfurization technologies for TPO.
The resource characteristics of coal-rich, oil-deficient, and low-gas have determined the need to fully exploit the advantages of coal resources in China. China holds a large amount of low-rank coal with high volatile content and high tar yield. Based on the abundant oil-rich and low-rank coal resources, resource evaluation and research on its development and utilization are of great significance to the coal geology. According to the estimated reserves, the low-rank coal reserves are about 63.86 billion tons. Among the low-rank coals, the tar yield is greater than 7%, which is called oil-rich coal. Gas and semi-coke resources, which can greatly increase the application value of this type of coal resources. The oil-rich coal resources are widely distributed in the Carboniferous-Permian, Triassic, Jurassic, Cretaceous, and Neogene in China. They are mainly distributed in the three provinces of Inner Mongolia, Shaanxi, Xinjiang, and also Qinghai and Gansu in space. The Carboniferous-Permian oil-rich coal is mainly distributed in Shaanxi Fugu mining area, and the Triassic oil-rich coal is mainly distributed in the Zichang mining area of Shaanxi Province. It shows that the oil-rich coals are mainly lignite and long-flame coal with low metamorphism but also contains a small amount of gas coal and gas-fat coal. The tar yield and volatile content generally have a positive correlation. It has great significance to further study on the oil-rich coal resources.
A novel vacuumed hermetic (VH) reactor is designed and compared with the common Gray-King (GK) and flow-through (FT) reactors for lignite pyrolysis under different operation modes. The quantity and quality analyses of pyrolysis products, such as char, tar and its fractions, water and gas show that the VH reactor with pre-evacuation (PE) by a pump and cooling of volatiles by liquid nitrogen (VH-PE-77 K) greatly reduces the volatiles reaction time and collects all of products, and the pyrolysis results are closer to the coal-to-volatiles reaction (or the so called primary pyrolysis) than other reactors commonly used in the literature. Compared with the yields of VH-PE-77 K in the temperature ranges from 50 °C to 500, 550 and 600 °C, the GK reactor allows more volatiles reaction (or the so called secondary reaction) and generally yields about 10% less tar, 7% less water, 3% more char and 10% more gas. It is found that the volatiles reaction contributes little to CO2 yield but significantly to H2, CH4 and CO yields. The water generated from coal-to-volatiles reaction participated in the volatiles reaction. The volatiles reaction reduces the chars’ surface area and the higher heating value (HHV) of gas (on volume basis), but increases the chars’ HHV (on mass basis).
Electron spin resonance (ESR) is a spectroscopic method to detect paramagnetic species, which has been applied in coal and coal conversion research over 60 years. The major advancements made before 1990s were reviewed but the studies and findings reported since then have not been reviewed. The recent studies not only showed new phenomena on ESR radical behaviors associated with coal conversion processes, but also revealed some misunderstanding of earlier studies on the nature of ESR radicals and its relationship with the coal conversion processes, especially with the total radicals generated in coal conversion processes. This article provides a dedicated review of ESR studies in coal research from 1954 to now, with emphases on research advancements in this century. It also includes ESR studies on other heavy organic matters, such as biomass and oil shale, because their reactions are similar in principle with coal reactions. It starts with the basic knowledge on ESR analysis, including signal assignment, spectra analysis, and cautions needed to ensure the reliability of spectra, and then moves to the significance of ESR spectra for coals and coal conversions especially new phenomena and understandings reported in recent decade, and finally ends with the challenging researches needed in future studies.
High ash-containing paper sludge ash (PSA) has been investigated in terms of its use as a low-cost catalyst for primary pyrolysis tar cracking in the two-stage pyrolysis of biomass. Experimental results showed that tar yields decreased significantly using PSA as catalyst in comparison with the thermal cracking experiments. A consequent increase in total gas yield was obtained. The yields of H2, CH4, and CO2 increased significantly under the catalytic effect of PSA, particularly the H2 yield almost doubled, while CO yields showed a slight decrease due to the consumption reactions. Thermogravimetric analysis of the fresh and spent PSA showed that carbon particles formed along with the tar decomposition, while the catalytic reactivity of the PSA was not reduced because of the participation of carbon particles in tar reforming. XRD spectrograms revealed that calcium-ferrite oxide (Ca2Fe2O5) was formed in PSA, which has excellent stability and catalytic effect on tar cracking and H2 production. The PSA still showed high tar catalytic conversion efficiency and stable yields of the gas components after use for three cycles. GC-MS results of the tar samples showed that using PSA as catalyst has less effect on the composition of the tar. The tar conversion using PSA, CaO, Fe2O3, and CaO/Fe2O3 reached 62.8%, 73.8%, 63.7%, and 68.9% at 700 °C, implying that the performance of PSA as a catalyst for biomass tar cracking could be comparable to that of the pure metal oxides.
Fluidised beds are the most popular technologies for gasification as they are considered to be more robust and versatile than other more conventional reactors. Most of the commercial fluid bed gasifiers (FBG) were originally developed for operation coal and/or pure biomass. The translation of the systems to operation on waste is the issue which represents the most significant technical difficulty, and from this cascade particular concerns regarding the ability to achieve long term stable operation, as well as wider confidence in commercial viability. The purpose of this Chapter is to appraise the fluidized bed gasification systems in light of the exotic characteristics of alternative fuels derived from waste materials. Given the technology’s heritage there is extensive technical information available regarding the modus operandi of fluidized bed reactors. However, given the relatively limited track record of operation on waste, this depth of information is helpful in appraising the expected performance of a waste-fed fluid bed system and the technical issues associated therein.
Coal partial gasification of CFBB gas and steam cogeneration is proposaled, which coal is partially gasificated, heat is carried by circulating ash, char is the fuel of boiler. One of the key technology of coal gasification is to reduce the yield of tar and to increase the yield of gas. The cracking reactions of the two main components of tar: benzene and toluene over circulating ash of CFBB in fixed bed reactor are experimental investigated to understand catalytic effect of ash particles. Cracking reactions kinetic parameters are measured. And the catalytic mechanic is also discussed. The experimental results shows that circulating ash promotes the cracking reactions of tar and increases gas products.
Shenmu bituminous coal (sized 45-80 mesh) was pyrolyzed in a fluidized bed reactor. Temperatures of dense phase and freeboard of the fluidized bed can be controlled independently. In this study, the dense phase temperature was maintained at 600°C and the freeboard temperature was varied in the range of 580-1000°C. The gas residence time in the freeboard was changed from 0.07 to 12s. The effects of temperature and gas residence time in the freeboard on gaseous product composition were investigated. A simple kinetic model for gaseous product formation in the freeboard was given.
Shenmu coal (-45 + 80 mesh) was pyrolyzed in a specially designed fluidized bed reactor with an I.D. of 48mm and a cone-shaped distributor. The temperature of the dense phase and the freeboard can be controlled individually. The relationship between the gas phase product yield and the dense phase temperature was obtained. The results show that the yields of H2, CO, CH4 and C2H4 increase dramatically with increasing dense phase temperature, however the liquid yield reaches the maximum at the dense phase temperature of 600∼650°C. By comparing with the results of former researchers, it can be seen that the volatile can undergo significant secondary reactions when the freeboard temperature is high, even at a very short residence time. The results of this paper are believed to represent the more accurate trend of the effect of bed temperature on gas phase product yield because the secondary reactions in the vapour phase were minimized in this experiment.
Secondary pyrolysis of coal tar produced from commercial coke oven was performed with a flow reactor at 700-900°C for 5-22 s and the effects of reaction conditions on the products yields and the composition of the light fraction has been investigated. The products were separated by solvent extractions into HS (hexane soluble). HI-CFS (hexane insoluble-chloroform soluble), CFI-THFS (chloroform insoluble-tetrahydrofuran soluble) and THFI (tetrahydrofuran insoluble= coke). The yield of HS decreased and that of THFI increased remarkably at 800 and 900°C. The HS fractions were analyzed by glass-capillary column gas chromatography equipped with a flame ionization detector. The HS fraction was made up of unsubstituted, alkyl-substituted and hydrogenated derivatives of polycyclic aromatic hydrocarbons (PAHs), heterocyclic and aliphatic compounds. It was found that the conversions of substituted PAHs and heterocyclic compounds containing oxygen and nitrogen, such as dibenzofuran and quinoline, increased remarkably at 900°C, and most part of these compounds disappeared in 13 s. Further, naphthalene which was considered as a most stable compound in unsubstituted PAHs in HS fraction also converted remarkably into the condensed compound at 900 deg;C.
To improve the knowledge about pyrolysis behavior of reed black liquor (RBL), reed black liquor particles (RBLP) pyrolysis experiments were performed in nitrogen atmosphere at 530, 580, 630, 680, 730 and 780 °C in fluidized bed at atmospheric pressure. The effects of bed temperature on yields of pyrolysis product, gaseous product characteristics, carbon and hydrogen conversions, NOx and SO2 emissions, and the micro-morphology of RBLP char were investigated. The results showed that the main components of the gaseous products were CO, CH4, CO2 and H2. The contents (% by volume) of H2 and CO were increased with the increase of bed temperature, but CH4 and CO2 had an opposite trend. In addition, the combustible gases concentration increased with temperature increasing, which was from an initial 64.03% (at 530 °C) increase to 77.36% (at 780 °C). At last, the great impact of pyrolysis temperatures on micro-morphology of RBLP char had been found.
This paper aims to investigate the rapid pyrolysis characteristics of Huainan coal using a tube furnace. Influence of temperature on yields and compounds of tar and char are tested. The result shows that aliphatic chains break gradually with increasing pyrolysis temperature. This leads an increase in aromaticity. Maximum tar yields were obtained at about 550 degrees C, the char yields decreased and the gas yields increased with the pyrolysis temperature. The tar was isolated to aliphatic hydrocarbon aromatic hydrocarbon, non hydrocarbon and asphaltene, and characterized by chromatography-mass spectrometry (GC/MS). The main content of tar include normal alkanes from C16-C30, two-,three- and four-ring aromatic hydrocarbons and alkyl-substituted hydroaromatic derivatives of polycyclic, phenols, indoles, quinines esters and others compounds.
The growing amount of rubber waste, such as that from tires and cables, has resulted in serious environmental problems. Since rubber waste is not easily biodegradable even after a long period of landfill treatment, material and energy recovery is the preferable alternative to disposal. The potential offered by waste tire pyrolysis for solving both energy and waste treatment problems is widely recognized. Pyrolysis is one method of inducing thermal decomposition without using any oxidizing agent, or using such a limited supply of the agent that oxidization does not proceed to an appreciable extent. The latter may be described as autothermal pyrolysis and will be studied in the present work.The main objective of this research was to study the operating parameters of autothermal pyrolysis of scrap tires in a laboratory-scale fluidized bed reactor with a 100-cm bed height (10 cm I.D.) and a 100-cm freeboard (25 cm I.D.). Scrap tires were pyrolyzed in a limited oxygen supply, so that the heat for pyrolysis of the scrap tires was provided by combustion of some portion of the scrap tires. The operating parameters evaluated included the effect on the pyrolysis oil products and their relative proportions of (1) the air factor (O.O7–O35); (2) the pyrolysis temperature (370–570 °C); and (3) the catalyst added (zeolite and calcium carbonate). The results show that: (1) the composition of the liquid hydrocarbon obtained is affected significantly by the air factor; (2) the higher operating temperature caused a higher yield of gasoline and diesel; (3) the yield of gasoline increased due to the catalyst zeolite added, and the yield of diesel increased due to the addition of the catalyst calcium carbonate; (4) the principal constituents of gasoline included dipentene and diprene.
Experiments conducted at bench scale can throw new light on the operation of pilot and plant scale processes. In the coal gasification/combustion area, such experiments have ranged from determinations of coal reactivities for novel entrained-flow gasifiers, to investigating the fate of injectant coal during blast furnace operation. Meanwhile, the accurate mimicking of conditions within pilot and plant scale equipment requires careful design. This paper attempts an overview of challenges faced in designing experiments where coal thermal breakdown plays an important role. Particular emphasis is placed on characterising the underlying behaviour of the pyrolysing material itself, with as little reference to sample or reactor configuration as possible. [Received: September 11, 2007; Accepted: January 8, 2008]
In this study, polyethylene was pyrolysed in the fluidized-bed pyrolysis system in a limited oxygen supply. Thus, the heat to pyrolyse the polyethylene was made available by partial incineration of the polyethylene, which can be referred to as autothermal pyrolysis. The fluidized bed was selected as the reactor owing to its high capacity, homogeneity and low tar content. Experimental parameters evaluated included the effect of: (1) the pyrolysis temperatures; (2) the air factors; and (3) the catalyst on the liquid hydrocarbon (gasoline, die sel, fuel oil, residual) formation, and B. T. X. (benzene, tolu ene, xylene) concentration in gasoline. Moreover, the pri mary composition in gasoline was analyzed. The results indi cated that it is practical to recover oil from plastic waste using the autothermal pyrolysis system.
Coal resource is abundant in China, while the reserves of natural gas and petroleum are limited. Due to the rapid increase in the number of automobiles, a competitive way to produce liquid fuels from coal is urgently needed in China. A so-called 'coal topping process' is under development at the Institute of Process Engineering, Chinese Academy of Sciences, from which liquid products can be obtained by flash pyrolysis in an integrated circulating fluidized bed system. In order to achieve a high yield of liquid products from high volatile coal, controlling the residence time of coal particles and produced gas may be of importance for minimizing the degree of the secondary reactions; i.e., polymerization and cracking of the liquid products. Experiments of the flash pyrolysis of coal have been conducted in an entrained bed reactor which is especially designed to study the influence of the coal particle residence time on the product distribution. The results show that the gaseous, liquid, and solid product distribution, the gas compositions as well as the liquid compositions depend strongly on the gas and particle residence time.
To trace the behavior of naphthalene in the pyrolysis of coal tar, the pyrolysis of coal tar containing C-14-labeled naphthalene was carried out at 800-950 degrees C for 40-50 s. It was found that naphthalene has significant pyrolysis reactivity. The distributions of the radioactivities in compounds with 3-5 rings were 21.6, 30.3, and 18.6% at 800, 900, and 950 degrees C for 50 s, respectively. Furthermore, the distributions of the recovered radioactivity in THFI at 800, 900, and 950 degrees C for 50 s were 1.6, 5.3, and 47%, respectively, indicating that about half of the C-14-naphthalene can be converted into tetrahydrofuran insoluble (THFI) in 50 s at 950 degrees C. This C-14 tracer method using C-14-labeled naphthalene is convenient for tracing the reactivity of coal tar and for elucidating the reaction mechanism of coal tar pyrolysis.
Substances such as CuO, Fe3O4 and even metallurgical coke (termed ‘receptors’) heat rapidly in a microwave oven at 2.45 GHz. The receptor, when mixed with Creswell coal and subjected to microwave radiation, induces rapid pyrolysis of the coal. Condensable tar yields of 20 wt% are obtained with coke, 27 wt% with Fe3O4 and as high as 49 wt% in some experiments with CuO. Despite the high final temperature (1200–1300°C after 3 min), analyses suggest that the volatiles are released in the lower part of the temperature regime but that some secondary cracking does occur. The tars are similar in composition, although with coke the proportion of aromatic hydrogen is greater than with CuO and Fe3O4. X-ray photoelectron spectroscopy shows that both pyridinic and pyrrolic nitrogen are present in the tars and chars, and that the dominant form of tar sulfur is thiophenic. There is evidence that mineral sulfur is immobilized when CuO in particular is the receptor. The chars formed show a degree of graphitization and are themselves excellent microwave receptors. In the presence of oxide receptors, char-oxide redox reactions occur, with loss of char, reduction of oxide and enhanced yields of CO and CO2. Of the lighter hydrocarbons identified in the gas phase, methane predominates. The data obtained are compared with those for other pyrolysis methods.
Product distributions from the atmospheric pressure pyrolysis of sugar-cane bagasse and silver birch were determined in a wire-mesh reactor as a function of temperature (300–900 °C) and heating rate (1–1000 K s−1), over a range of holding times between 0 and 100 s. Major groups of components present in the tars were identified by g.c.-m.s.. Above 500 °C when increasing the heating rate from 1 to 1000 K s−1 in experiments using holding times longer than several seconds, tar yields were observed to increase by about 10 wt% to nearly 54% w/w daf bagasse; total volatile yields increased by similar amounts. Number average molecular masses (MMn) of tars determined by vapour pressure osmometry (v.p.o.) were in the range 150–250 Da, those of silver birch tars being systematically larger than those of sugar-cane bagasse by 15–30 Da. For both sets of tars, MMn were observed to decrease with increasing temperature between 400 °C and 600 °C, but remained unchanged between 600 °C and 900 °C. The molecular masses were also observed to decline with increasing heating rate. Lignocellulosic substrates appear to cleave readily to give relatively small, compact and relatively stable molecules able to evaporate completely at temperatures around 500 °C. Extensive chemical modification of primary products appears to result from intraparticle secondary reactions, these reactions being intensified during pyrolysis at higher heating rates. Whilst the wire-mesh reactor configuration appears not to alter tar yields or structures subsequent to release from parent substrate particles, comparison with tars obtained at 600 °C in a ‘hot-rod’ (fixed-bed) reactor suggests that whilst overall yields may be adversely affected and the relative abundance of some major components altered, extraparticle secondary reactions do not substantially change the character of the tar at this temperature.
Yallourn brown coal particles were heated at rates in the order of 103Ks−1 and pyrolyzed in two different reactors, a drop-tube reactor (DTR) and a Curie-point reactor (CPR). In DTR the vapor-phase secondary (extra-particle) reactions of volatiles occurred concurrently with the primary reactions within the particle, while in CPR the volatiles were swept out of the heating zone immediately after formed so that their secondary reactions were efficiently suppressed. The char yields for the pyrolyses in these two reactors were described by the same functions of temperature. This enabled to evaluate the decrease or increase in the yield of volatile products due to the secondary reactions that proceeded within the volatiles' residence for time shorter than 2s. The evaluation was done by a parameter, ΔY(i), defined as ΔY(i)=Y(i)DTY(i)CP where Y(i)DT and Y(i)CP are the yields of volatile product i for pyrolyses in DTR and CPR, respectively. ΔY(tar) was found to decrease with increasing pyrolysis temperature and reached −19mol-C per 100mol-C in the coal, while it was negligible at temperatures lower than 873K. ΔY(tar) at 1173K was much lower than that expected when Y(tar)DT was assumed to decrease only due to dealkylation to form gaseous hydrocarbons and deoxygenation to form carbon monoxide, −10mol-C. The difference between the above two ΔY(tar)s was explained well by the reaction of tar with steam (water formed by the primary pyrolysis) occurring above 1073K and resulting in a considerable decrease in ΔY(H2O) and the corresponding increase in ΔY(CO) and ΔY(H2). The addition of steam to the carrier nitrogen gas further promoted the reduction of Y(tar)DT and ΔY(tar) to 1 and −23mol, respectively. The reduction of ΔY(tar) for the pyrolysis of acid-washed coal was much less significant than that for the original coal. Thus the observed steam reforming of tar was found to be catalyzed by Fe, Ca and/or Mg species that were initially dispersed in the coal matrix as ion-exchanged cations. The addition of steam also increased the total conversion of carbon into volatiles at 1173K from 44 to 56mol, indicating the gasification of char by steam within an estimated residence time of coal/char particles.
The behaviour of hydrogen in naphthalene in the pyrolysis of coal tar was investigated using a tritium tracer technique to elucidate the pyrolysis mechanism of coal tar. The pyrolysis of coal tar containing tritiated naphthalene was carried out using a batch reactor at 800–950°C for 50 sec. The ratios of amounts of naphthalene in the tars pyrolysed at 800, 900, and 950°C to that in the feed tar were 80.1, 73.4, and 42.5 wt%, respectively. However, the ratios of the radioactivity of naphthalene in tars after pyrolysis to that in the feed tar also decreased significantly with increasing temperature and it was only 12.9% for the reaction at 950°C. The ratio of the recovered radioactivity of naphthalene to the initial radioactivity of naphthalene in the feed tar was about one-third of that of the recovered amount of naphthalene to the initial amount of naphthalene in the feed tar at 950°C. The results indicated that the hydrogen in naphthalene transferred to other compounds in the pyrolysis of coal tar. It is suggested that naphthalene could play an important part in the radical reaction mechanism in the pyrolysis of coal tar.
Pyrolysis tar and char yields were measured at atmospheric pressure between 400–800 °C in three reactors with significantly different design characteristics: a fluidized bed; a wire-mesh reactor; and a ‘hot-rod’ fixed bed reactor. Ranges of conditions under which results were generally in good agreement were identified using a common sample of Linby (low rank British bituminous) coal. Differences highlighted important non-ideal characteristics in the behaviour of all three systems. The causes of these non-idealities and their implications for the interpretation of pyrolysis data from the three types of reactors are discussed in detail.
A bench-scale high-pressure fluidized-bed reactor designed for pyrolysis, gasification, and combustion experiments (up to 1000 °C, 40 bar) has been constructed and commissioned. The system is intended to generate fuel reactivity data rapidly and cheaply, under realistic experimental conditions; it is relatively simple to construct and capable of operation by a single researcher. The reactor body (Incoloy Alloy 800HT, 34 mm i.d., 504 mm long) serves as the resistance heater and is designed to withstand the reaction pressure, obviating use of a “cold” pressure casing. The reactor is lined with a loosely fitting quartz tube to limit corrosion and catalytic effects. Sample (up to 2000 mg) is held between two air actuated valves and injected batchwise (“single-slug”) through a water-cooled probe. The design of the reactor allows determination of tar/oil and char yields. Exhaust gases are passed through a dryer before entering the analytical stage. The present report focuses on equipment design and preliminary data from pyrolysis and gasification experiments with Daw Mill coal (UK) at up to 1000 °C and 30 bar. Extents of gasification were calculated by subtraction of total volatile yields in CO2 from mass loss during pyrolysis in helium, performed under otherwise identical conditions. Combustion reactivities of chars were determined in an atmospheric pressure TGA instrument. A number of design adjustments have been described, enabling total volatile yields from the FB reactor to match those from a high-pressure wire-mesh reactor. As expected, tar yields from the fluidized-bed reactor were significantly lower than those from the wire-mesh instrument, due to longer residence times of volatiles in the bed of solids and the reactor freeboard.
Changes in the molecular structure of tar are investigated in a fluidized-bed reactor divided into two regions: a dense bed for the primary reaction and a freeboard for the secondary reaction in the gas phase. The temperature in the dense bed is kept at 600-degrees-C and that in the freeboard is varied in the range of 600-900-degrees-C. A pulverized subbituminous coal (C = 76 wt %) is continuously fed into the dense bed, and tar is recovered at the reactor outlet. The tar is separated by solvent extraction into preasphaltene and asphaltene. The asphaltene is further classified by column chromatography into saturates, aromatics, phenolic ethers, nitrogen-containing compounds, and hydroxylic compounds. The aromatics are fractionated by HPLC on the basis of the number of double bonds per molecule (db) into dicyclic (5 db), dicyclic (6 db), tricyclic (7 db), tetracyclic (8 db), tetracyclic (9 db), pentacyclic (10 db), and pentacyclic and greater (+11 db) PAHs (polycyclic aromatic hydrocarbons). Each homologue is characterized by FIMS and H-1 NMR to determine molar yield, number-based distribution of aliphatic carbons per molecule, and number-based distribution of alpha-, beta-, and gamma-carbons per molecule. At freeboard temperatures of 600-700-degrees-C, detachment of aliphatic substituents proceeds with bond cleavage at aryl-alpha positions rather than alpha-beta and remote positions. Decomposition of alpha-methyl groups is not observed below 700-degrees-C. At a freeboard temperature of 600-degrees-C, PAHs with three to four aliphatic carbons per molecule are most abundant, and the mole fraction of unsubstituted PAHs is only ca. 5% for each homologue. At 900-degrees-C, however, the fraction of unsubstituted PAHs is more than 50%. The structural distribution in pyrolysis products is thus controlled by changing the freeboard temperature.
Two pairs of raw and acid-washed coal samples were prepared from Yallourn and Loy Yang brown coals, and subjected to rapid pyrolysis in a drop-tube reactor at 1073–1173 K in a stream of N2 or H2O/N2 mixture. Examinations were made on the roles of the inherent metallic species in the secondary reactions of nascent tar and char that were formed by the intraparticle primary reactions. The experimental results revealed that the inherent metallic species were essential for vary rapid steam reforming/gasification of the nascent tar/char and simultaneous suppression of soot formation. In the absence of the metallic species, the soot formation from the tar accounted as much as 15–19 and 6–13% of the carbon in coal in N2 and H2O/N2, respectively. The metallic species reduced the yield of soot to 6–8% in N2 by enhancing the reforming of tar by H2O generated from the pyrolysis of coal. In the H2O/N2 stream, instead of soot formation, a net gasification conversion up to 17% within 4.3 s was observed in the presence of the metallic species as a result of catalytic gasification of the nascent char. Moreover, the metallic species catalyzed the steam reforming of the nascent tar, giving its conversion up to 99%. Over the range of the conditions employed, a one-to-one stoichiometry was established between the steam consumption and the yield of carbon oxides formed by the steam reforming/gasification and water-gas-shift reaction.
The courses of the thermal breakdown reactions in coal pyrolysis and liquefaction were compared using similar coal and maceral concentrate samples in parallel experiments. A wire-mesh pyrolysis and a flowing-solvent liquefaction reactor were used, both reactors providing for rapid removal of products from the reaction zone in order to suppress secondary reactions; product structures were therefore expected to be relatively free of reactor-related effects. An attempt was made to distinguish between the effects of density, solvent power and donor ability of media surrounding the particles during thermal breakdown. Products were characterized by size exclusion chromatography, u.v.-fluorescence and FT-i.r. spectroscopy. The results suggest that the sensitivity of pyrolysis yields to changes in heating rate is related to competing rates of volatile release and resolidification within the pyrolysing mass: the presence of locally available hydrogen — e.g. from hydrogen donor species — appears to shift this competition in favour of greater product release and slower resolidification. This explains the absence of a direct effect of heating rate on liquefaction yields at high tetralin:coal ratios. Broad similarity was found between liquefaction yields in hexadecane and yields from pyrolysis experiments, where effects of sample resolidification similar to those in ‘dry’ pyrolysis were observed. Molecular mass distributions extending to higher levels were observed in liquefaction extracts than in pyrolysis tars; however, it remains possible that aromatic nuclei with ranges of cluster sizes similar to those available in the original coal may be found in both tar and extract molecules.
Shredded automotive tyre waste was pyrolysed in a 200 cm3 static batch reactor in a N2 atmosphere. The compositions and properties of the derived gases, pyrolytic oils and solid char were determined in relation to pyrolysis temperatures up to 720 °C and at heating rates between 5 and 80 °C min−1. As the pyrolysis temperature was increased the percentage mass of solid char decreased, while gas and oil products increased until 600 °C after which there was a minimal change to product yield, the scrap tyres producing approximately 55% oil, 10% gas and 35% char. There was a small effect of heating rate on the product yield. The gases were identified as H2, CO, CO2, C4H6, CH4 and C2H6, with lower concentrations of other hydrocarbon gases. Chemical class composition analysis by liquid chromatography showed that an increase in temperature produced a decrease in the proportion of aliphatic fractions and an increase in aromatic fractions for each heating rate. The molecular mass range of the oils, as determined by size exclusion chromatography, was up to 1600 mass units with a peak in the 300–400 range. There was an increase in molecular mass range as the pyrolysis temperature was increased. FT-i.r. analysis of the oils indicated the presence of alkanes, alkenes, ketones or aldehydes, aromatic, polyaromatic and substituted aromatic groups. Surface area determination of the solid chars showed a significant increase with increasing pyrolysis temperature and heating rate.
Used tyres were thermally decomposed under vacuum in a process development unit. At 510°C and total pressure 2–20 kPa, the process yielded 50 wt% oil, 25 wt% carbon black, 9 wt% steel, 5 wt% fibres and 11 wt% gas. Distillation of the pyrolytic oil yielded ∼20 wt% light naphtha (i.b.p. 160°C), 6.8 wt% heavy naphtha (160–204°C), 30.7 wt% middle distillate (204–350°C) and 42.5 wt% of bottom residue (>350°C). d,l-Limonene was one of the major chemicals in the naphtha fraction, with a concentration of ∼7 wt%. The naphtha also had high contents of aromatics, olefins and iso-alkanes (45, 22 and 15 vol. % respectively). Its relatively high levels of sulfur, nitrogen, olefinic and diolefinic compounds would make it unsuitable as a blending component for gasoline without hydrofining and reforming. However, ∼2 vol. % of the naphtha could be blended with hydrofiner feedstock without significantly affecting the process requirements. Approximately 71.1 and 68 wt% of the pyrolytic and petroleum light naphthas respectively were quantified.
A wire-mesh reactor, with the capability of virtually eliminating secondary reactions, has been used as base-case in the study of product yields and structures from the pyrolysis and hydropyrolysis of a sample of sugar cane bagasse in a fixed-bed `hot-rod' reactor. Results from the two reactors have been compared to determine how best to assess bench-scale data which might be used for eventual process development. Experiments have been carried out at 600°C at pressures up to 70 bar. Structural features of the bio-oils have been examined by size exclusion chromatography and FT-infrared spectroscopy. In both reactors the effect of increasing pressure was to reduce the bio-oil and total volatile yields; hydropyrolysis bio-oil yields were marginally higher than pyrolysis yields under equivalent operating conditions. About 5 to 6% bio-oil product is lost in the fixed-bed reactor, compared with the wire-mesh reactor, with consequent increase in recovered chars. Pressure and reactor bed depth appear to affect only the thermally more sensitive components of the bio-oils, and increasing pressure beyond 40 bar, or bed depth beyond the level reported, was found not to affect yields to an appreciable extent. Taken together, these data indicate that about one-third of the original biomass may be converted to oil by direct pyrolysis. Size exclusion chromatograms and FT-infrared spectra of bio-oils from the hydropyrolysis experiments conducted at 1 and 70 bar suggest that oils from the hot-rod reactor experience a greater degree of secondary reactions than those from the wire-mesh reactor. Using tetrahydrofuran as eluent, the highest molecular masses were found to be around 1000 u in terms of polystyrene standards. However, comparison with size exclusion chromatograms using 1-methyl-2-pyrrolidinone indicated partial loss of sample when operating in THF. Our findings strongly suggest that THF is not a suitable eluent for the characterisation of biomass-derived pyrolysis oils by size exclusion chromatography.
The yield of tar from a given coal is the same for a laboratory-scale rig and three pilot-scale pyrolysers. For bituminous and sub-bituminous coals the tar yield can be related to the H/C ratio of the coal; for brown coals the influence of inorganic constituents must also be considered. Tars from bituminous and sub-bituminous coals have been successfully converted to refinery-feedstock grade oil in two different types of hydrogenator; brown coal tars are difficult to hydrogenate.
The molecular weight distributions of coal tars and coal char extracts were examined in an effort to learn more about the process of mass transfer during coal pyrolysis. Evidence was obtained which suggests that the majority of the tar evolved during rapid pyrolysis of pulverized coal escapes by a process limited by gas film diffusion. However, there is also some evidence that the tar includes a small amount of heavy material which could have been ejected from the particle in a condensed phase. Data were also obtained which suggest that the tar precursors (within the parent coal) are formed over a wide range of temperature and do not seem to be present as such in the raw coal. The rather large effect of pressure on yields of tar from bituminous coal pyrolysis has previously been attributed to the effect of pressure on evaporation rates of tar precursors from the particle surface. This study shows that the molecular weight distributions of both the tar and extractable tar precursors within the particle are consistent with such a mechanism.
The sulfur-containing components in coals of various ranks were studied by using Pyroprobe GC/MS and continuous isothermal flash pyrolysis conditions. Volatile sulfur-containing products are H/sub 2/S, CS/sub 2/, COS, CH/sub 3/SH (in a few cases SO/sub 2/), and many heterocyclic sulfur compounds including thiophene, benzothiophene, and dibenzothiophene and most of the possible methyl derivatives of these compounds. Comparison of pyrolysis products of coals as a function of pyrolysis temperature with those of low molecular weight and a polymeric model sulfur compounds suggests the organic sulfur-containing species in coal to be present as both aliphatic sulfur-containing side chains (thioethers) and heteroatoms in single-ring or multiring aromatic clusters. Little aromatic nonheterocyclic structures were detected. The relative proportion of aliphatic sulfur structures to total organic sulfur appears to be high (60-80%) in low-rank coals (lignite), in the range of 30-50% in high-sulfur bituminous coals, and almost zero in high-rank (anthracite) coals. The reaction of elemental sulfur, H/sub 2/S, or pyrite with coal, coke, or char to form thermally stable nonvolatile structures is observed. 26 references, 6 figures, 6 tables.
The origin and magnitude of errors in molecular mass (MM) distributions of coal-derived materials (tars, asphaltenes, and pre-asphaltenes) determined by size exclusion chromatography (SEC) are described. The largest errors in M vector/sub w/ and M vector/sub n/ arise from the use of solvents that allow self-association of solute or adsorption on the gel; while tetrahydrofuran is preferred, association with low MM polar constituents was observed. Other large errors originate from inappropriate calibration standards: narrow preparative SEC fractions of materials similar to those under study are recommended. Among a number of detectors investigated, UV spectrophotometry is most appropriate, but corrections for variation of response with MM are necessary. A totally excluded internal standard restricts errors from flow rate and column-temperature deviation. Errors arising from mass of coal derivative injected, band spreading, and base line uncertainty are also reported. Errors in values of M vector/sub n/ for coal derivatives determined by SEC with appropriate precautions are similar to those in M vector/sub n/ determined by vapor pressure osmometry.
The molecular weight distributions of tars produced during flash pyrolysis of coal have been determined by a combination of gel permeation chromatography and vapour phase osmometry. Small particles (62–88 μm) of two high-volatile and one low-volatile bituminous coal, and a lignite have been pyrolysed at heating rates of ≈1000 K s−1 at temperatures from 600 to 1300 K in a heated wire mesh apparatus. The molecular weight distributions range from ≈100 to 4000 and peak in the range from 250 to 750 in all cases. The evidence gathered on a softening bituminous coal clearly indicates a selective evaporation of light fractions of the metaplast.
The devolatilization behaviour of finely-ground (< 0.2 mm) Loy Yang brown coal was investigated under rapid heating conditions using a small-scale fluidized-bed pyrolyser. The pyrolyser operated continuously, coal being fed at rates of 1–3 g/h directly into a bed of sand fluidized by nitrogen. Particle heating rates probably exceeded 104 °C/s. The yields of tar, C1-C3 hydrocarbons and total volatile matter are reported for a pyrolyser-temperature range of 435 to 900 °C. A maximum tar yield of 23% w/w (dry ash-free coal), 60% more than the Fischer assay, was obtained at 580 °C. Yields of C1-C3 hydrocarbons increased with increasing temperature, reaching 8% at 900 °C. Elemental analyses showed that the composition of the tar and char products was strongly dependent on pyrolysis temperature. The effects on the devolatilization behaviour of the coal produced by the moisture associated with the coal, by hydrogen, and by the replacement of the sand by a fluidized bed of petroleum coke were investigated.
The devolatilization behaviour of ten bituminous coals was investigated under rapid heating conditions using a small-scale fluidized-bed pyrolyser. The pyrolyser operated continuously, coal particles being injected at a rate of 1–3 g h−1 directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C1–C3 hydrocarbon gases, and total volatile-matter and an agglomeration index are reported for all coals. Maximum tar yields were obtained at about 600 °C and were always substantially higher than those from the Gray-King assay. Total volatile-matter yields were also substantially higher than the proximate analysis values. The maximum tar yields appear to be directly proportional to the coal atomic ratio. The elemental analysis of the tar is strongly dependent on pyrolysis temperature. The tar atomic ratio is proportional to that of the parent coal. The effect on the devolatilization behaviour of two coals produced by changes in the pyrolyser atmosphere and the nature of the fluidized-bed material were also investigated. Substituting an atmosphere of hydrogen, helium, carbon dioxide or steam for nitrogen, has no effect on tar yield and, with one exception, little effect on the hydrocarbon gas yields. In the presence of hydrogen the yield of methane was increased at temperatures above 600 °C. Tar yields were significantly reduced on substituting petroleum coke for sand as the fluid-bed material. A fluidized bed of active char virtually eliminated the tar yield.
The effect of heating rates between 1 and 1000 K/s on coal pyrolysis in helium at 1.2 bar has been examined in a wire-mesh pyrolysis apparatus. The apparatus used a microcomputer for feedback temperature control and had a sweep gas flow through the sample holder to give rapid product removal. A significant increase in total volatile yields between 1 and 1000 K/s was found for three of the four coals tested, provided that sufficient time was allowed at the peak temperature for reactions to run to completion. For Pittsburgh No. 8, the only coal for which tar measurements were made, the increase in total volatiles could mostly be attributed to an increase in tar yield. Experiments under vacuum confirmed the effect of heating rate observed in helium, suggesting that the lower volatile yield observed at 1 K/s is not an artifact of external mass-transfer resistance.
A continuous fluidized bed bench scale flash pyrolysis unit operating at atmospheric pressure and feed rates of about 15 g/h has been successfully designed and operated. A unique solids feeder capable of delivering constant low rates of biomass has also been developed. Extensive pyrolysis tests with hybrid aspen-popular sawdust (105–250 μm) have been carried out to investigate the effects of temperature, particle size, pyrolysis atmosphere and wood pretreatment on yields of tar, organic liquids, gases and char. At optimum pyrolysis conditions high tar yields of up to 65% of the dry wood weight fed are possible at residence times of less than one second.
On a conçu et employé avec succès, à l'échelle du laboratoire, une unité de pyrolyse-éclair à lit fluidisé continu; le dispositif fonctionnait à la pression atmosphérique et à des débits d'alimentation d'environ 15 g/h. On a aussi mis au point un dispositif unique d'alimentation en matières solides, capable d'assurer de faibles débits constants de biomasse. On a fait des expériences poussées de pyrolyse sur des sciures d'hybrides de peuplier-faux tremble (105—250 μm), dans le but d'étudier les effets de la température, de la granulométrie des particules, de l'atmosphère de la pyrolyse et d'un traitement préalable du bois sur les rendements en goudron, liquides organiques, gaz et matières carbonisées. Il est possible, dans les conditions optimales de pyrolyse, d'obtenir des rendements élevés en goudron, qui peuvent atteindre 65% du bois sec d'alimentation en poids pour des temps de séjour de moins d'une seconde.
A continuous atmospheric pressure flash pyrolysis process for the production of organic liquids from cellulosic biomass has been demonstrated at a scale of 1–3 kg/hr of dry feed. Organic liquid yields as high as 65–70% of the dry feed can be obtained from hardwood waste material, and 45–50% from wheat straw. The fluidized sand bed pyrolysis reactor operates on a unique principle so that char does not accumulate in the bed and treatment of the sand is not necessary. The product gas, about 15% of the yield, has a medium heating value. The liquid product is an acidic fluid, which pours easily and appears to be stable. A preliminary economic analysis suggests that if the pyrolysis oil can be used directly as a fuel, its production cost from wood waste is probably competitive with conventional fuel oil at the present time.
On a fait la démonstration d'un procédé continu de pyrolyse éclair à l'échelle de l à 3 kg/h d'alimentation sèche, à la pression atmosphérique, pour la production de liquides organiques à partir d'une biomasse cellulosique. On peut obtenir des rendements en liquide atteignant 65 à 70% de l'alimentation sèche à partir de déchets de bois dur et de 45 à 50% à partir de paille de blé. Le réacteur de pyrolyse, à lit de sable fluidisé, fonctionne sur un principe unique, de sorte que le charbon ne s'accumule pas dans le lit et qu'il n'est pas nécessaire de traiter le sable. Le produit gazeux, qui correspond à environ 15% de rendement, a une valeur calorifique moyenne. Le produit liquide est un fluide acide qui se déverse facilement et semble assez stable. Une analyse économique préliminaire indique que, si l'on peut employer directement l'huile de pyrolyse comme combustible, son coǔt de production à partir de déchets de bois est probablement compétitif actuellement avec celui de l'huile combustible classique.
A fluidized bed pyrolysis reactor with variable freeboard height was constructed to investigate the effect of time-temperature history on freshly formed pyrolysis tars. The apparatus allowed direct measurement of tar loss through gas phase reactions in the freeboard, and calculation of the quantities of tar cracked or coked within the fluidized bed itself. At 600 °C about 30 wt% more tar (daf cellulose) appears to crack in the freeboard for 3.5 s residence time compared with 0.25 s. At this temperature, 19 wt% (daf cellulose) tar is destroyed within the fluidized bed before reaching the freeboard. The amount of tar destroyed within the bed was found to increase sharply with temperature, reaching nearly 50 wt% (daf cellulose) at 700 °C. Four other substrates were studied: silver birch, pulverized municipal solid waste, a Turkish lignite (Can), and a low rank British bituminous coal (Linby). Results indicate that the temperatures corresponding to maximum tar yields and thermal stabilities of the tars broadly correlate with substrate rank. At higher reactor temperatures, biomass tars lose their predominantly carbohydrate character and heteroatom content, and become more aromatic in character, with increasing contents of condensed aromatic ring systems.
An electrically-heated wire-mesh pyrolysis reactor is described. The apparatus is capable of heating rates ranging from virtually zero to 5000 K s−1 as well as multi-step heating with variable heating rates and holding times at intermediate temperatures and at the peak temperature. An alternating current power supply is used, allowing thermocouples to be attached directly to the wire mesh to give close tracking of temperature changes. The heating circuit is driven by a micro-computer for real-time logging and control. A continuous stream of carrier gas is forced through the sample holder, giving mean volatiles residence times of around 2ms in the heated zone. The gas sweep gives sample cooling rates of 500–1000 K s−1; slower cooling rates may be selected by pre-programming the heating circuit. Pyrolysis yields of tar and total volatiles from a low rank British bituminous coal (Linby) obtained with single-step heating, show clear changes in the product distribution with increasing heating rate. The effect of holding times, relatively unimportant during slow heating (1 K s−1) runs, is quite pronounced for fast heating rates (l000 K s−1) at peak temperatures below 650–700 °C. Because of this, unless pyrolysis reactions are allowed to run to completion, the effect of the heating rate on product distributions may be masked.
The applicability of the evaporative analyser in the size-exclusion chromatography (SEC) of coal derivatives has been explored. The detector, in which the intensity is measured for the light scattered from the finely divided solute particles resulting from evaporation of droplets of chromatographic eluate, is mass responsive, sensitive, and linear for both narrow and gross coal-extract fractions. For molecular masses above 300 the mass response is almost uniform, and there are considerable advantages over the detectors commonly used in SEC. Application to lower molecular mass coal materials is limited by the evaporation of solute along with solvent in the detector.
Time resolved kinetics of the generation and destruction of pyridine extractables within a Pittsburgh No. 8 seam bituminous coal was measured as a function of pyrolysis-time history. Tentative activation energies for the generation and destruction processes are 28.8 kcal mol−1 and 42.2 kcal mol−1, respectively. Extractables are formed by pyrolytic bond breaking and depleted from the coal through tar evaporation or destroyed by coke formation. The development and disappearance of coal plasticity, as measured with a fast response plastometer, correlates well with the generation and removal of the pyridine extractables. Extractables molecular weight increased with heating time, implying that repolymerization reactions contribute to the coke formation.
A number of procedures have been investigated for the calibration of a size-exclusion chromatography (s.e.c.) column in the determination of molecular mass (MM) distributions of coal derivatives. The behaviour of narrow fractions of coal extracts in the MM range 200–3000 was compared with a variety of the more generally available calibration standards. Calibration with preparative s.e.c. subfractions of materials similar to those under study has been recommended. Polystyrene standards are satisfactory, however, for MM < 1000, but above this range other polymer standards should be sought. Universal and molar volume calibration do not apply to coal-derived materials.
Fuel submitted for publication
- Z S Gonenc
- J R Gibbins
- I E Katheklakis
- R Kandiyoti