Article

A Kinetic Model for the Production of Liquids from the Flash Pyrolysis of Biomass

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Abstract

A kinetically based prediction model for the production of organic liquids from the flash pyrolysis of biomass is proposed. Wood or other biomass is assumed to be decomposed according to two parallel reactions yielding liquid tar and ( gas + char) The tar is then assumed to further react by secondary homogeneous reactions to form mainly gas as a productThe model provides a very good agreement with the experimental results obtained using a pilot plant fluidized bed pyrolysis reactorThe proposed model is shown to be able to predict the organic liquid yield as a function of the operating parameters of the process, within the optimal conditions for maximizing the tar yields, and the reaction rate constants compare reasonably well with those reported in the literature

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... In our work, the pre-exponential factor and the activation temperature for both pyrolysis and cracking are taken from Glaister [51], as cited in Grønli [36] (see Table 4.5 in Grønli [36]). Di Blasi [17] derived the pyrolysis rate from Roberts and Clough [50] and the cracking rate from Liden et al. [52]. Mandl et al. [24] derived them from Grønli [36] and Liden et al. [52], respectively (see the footnote to Table 4). ...
... Di Blasi [17] derived the pyrolysis rate from Roberts and Clough [50] and the cracking rate from Liden et al. [52]. Mandl et al. [24] derived them from Grønli [36] and Liden et al. [52], respectively (see the footnote to Table 4). Figure 3 shows that the rates are similar. ...
... The correction factor ξ, appearing in Equation (52), was already introduced into the modelling in the 1980s and 1990s [18], so it is around forty years old. As shown in Equation (52), the factor governs the energy transfer between the gas-and solid-phases; for ξ = 1, the rate is maximum, while for ξ = 0, there is no energy transfer between the phases. Thus, with ξ approaching 1, the differences between gas-phase and solid-phase temperatures are minimized. ...
Article
The subject of this work is the mathematical modelling of a counter-current moving-bed gasifier fuelled by wood-pellets. Two versions of the model have been developed: the one-dimensional (1D) version-solving a set of Ordinary Differential Equations along the gasifier height-and the three-dimensional (3D) version where the balanced equations are solved using Computational Fluid Dynamics. Unique procedures have been developed to provide unconditionally stable solutions and remove difficulties occurring by using conventional numerical methods for modelling counter-current reactors.The procedures reduce the uncertainties introduced by other mathematical approaches, and they open up the possibility of straightforward application to more complex software, including commercial CFD packages. Previous models of Hobbs et al., Di Blasi and Mandl et al. used a correction factor to tune calculated temperatures to measured values. In this work, the factor is not required. Using the 1D model, the Mandl et al. 16.6 kW gasifier was scaled to 9.5 MW input; the 89% cold-gas efficiency, observed at 16.6 kW input, decreases only slightly to 84% at the 9.5 MW scale.
... Therefore, the use of kinetic data determined in fast pyrolysis reactors under realistic operating conditions is necessary in order to progress in the proposal of reliable models applicable to the design and optimization of industrial scale processes. However, these types of kinetic modeling studies are scarce in the literature, except a few ones involving fluidized bed [17][18][19][20][21][22][23], entrained flow [24][25][26] and conical spouted bed reactors [16]. Most of these models use a single-component single-step reaction mechanism and describe the process either in terms of global devolatilization rates [22][23][24][25][26], which involve the production of gas, liquid and char products [20], or take into account just the formation of certain gaseous products [17,18]. ...
... Most of these models use a single-component single-step reaction mechanism and describe the process either in terms of global devolatilization rates [22][23][24][25][26], which involve the production of gas, liquid and char products [20], or take into account just the formation of certain gaseous products [17,18]. Only the studies carried out by Liden et al. [19] and Font et al. [21] included a secondary cracking reaction of the condensable products (liquid) to yield gases. The kinetic modeling in a conical spouted bed reactor by Olazar et al. [16] was also based on a reaction scheme considering three parallel and independent reactions to yield gas, liquid and char products, but without considering secondary cracking reactions. ...
... Overall and individual mass balances (Eqs. (5)- (19)) have been solved by means of a Matlab code based on a fourth-fifth order Runge-Kutta-Felberg method in order to obtain the evolution of the concentrations of the pyrolysis products with reaction time, although other analytical methods may also have been used to solve these systems [55,56]. The kinetic constants have been expressed by Arrhenius equation, which have been reparameterized to a reference temperature (T * ) of 550°C: ...
Article
Biomass fast pyrolysis in a conical spouted bed reactor has been modeled by coupling a kinetic model with reactor hydrodynamics. The kinetic model proposed considers the influence of the two main parameters affecting the pyrolysis process, which are reaction temperature and residence time. Thus, the reaction scheme contemplates three parallel reactions for the formation of the pyrolysis products (gas, bio-oil and char)and a homogeneous secondary reaction in the gas phase, whose extent depends on the residence time of the pyrolysis volatiles. The experimental results used for calculating the kinetic parameters of best fit were obtained in a bench scale conical spouted bed reactor equipped with a draft tube and operated in the 450–550 °C temperature range and with a gas residence time between 0.56 and 1.69 s. The model suitably predicts product formation rates and pyrolysis product yields in a wide range of reaction conditions. Moreover, a sensitivity analysis was carried out to evaluate the influence of the hydrodynamic parameters, as well as the model robustness.
... (15) balances all existing fragments independent of their physical state divided in different size classes indicated by the number of clusters. The source term is given by percolation theory via equation (14), while for cross-linking a first order Arrhenius approach dy frag,N Cl ,cross dt = r cross · y frag,N Cl ,Fl with r cross = A cross · e − Ea,cross R·T (16) is implemented. Instead of removing the entire evaporated gas phase in each time step like in the original CPD model [3], a gas release model is implemented taking effects of intra-particle gas transport into account. ...
... Parameters A tar,GPR and E a,tar,GPR for the tar cracking mechanism are used for a moderate mechanism from Serio et al. [15] and an intense mechanism from Liden et al. [16]. ...
... In the experiments, all species with a dew point higher than 293 K are accumulated to tar as all species are measured in the gaseous phase at a gas cell temperature of 553 K. Figure 5 reveals two main findings. First, there is a significant difference between the simulated and experimental tar to light gas ratio and second, only the stronger tar cracking mechanism from Liden et al. [16] has a significant impact on the tar to light gas ratio in the investigated temperature range. The discrepancy between simulation and experiments may be caused by several unknown or yet indefinite model parameters. ...
Conference Paper
Full-text available
In the present study, the CPD model, originally developed based on predictions from drop tube (DTR) experiments, has been adapted to analyze pyrolysis kinetics in a small-scale fluidized bed reactor. Impacts of particle feed, particle heat up as well as tar cracking reactions in the gas phase are considered. Furthermore, an optimized solver structure allows a time step independent solution and enables the use of implicit methods. A comparison with experimental results is undertaken for pulverized Rhenish lignite fuel particles in the temperature range from 673 to 973 K in N2 atmosphere. The comparison between experimentally derived volatile release rates and simulated ones reveals a good agreement, indicating that the high temperature derived kinetic parameters from DTR experiments can be extrapolated to lower temperatures. Nevertheless, discrepancies in the tar to light gas ratio occur with the proposed model implementation. Introduction Predicting devolatilization behavior of solid fuels is important for design and optimization of pulverized fuel combustion systems. The release of volatiles strongly impacts ignition times as well as the formation of pollutant precursors. The published mathematical descriptions of the pyrolysis process range in complexity from simple first order Arrhenius approaches up to sophisticated pyrolysis network models. The most commonly used network model is the Chemical Percolation Devolatilization (CPD) model, developed in the early nineties for the prediction of coal flash pyrolysis. Its concept is solely based on the fuel molecular structure and thus, the model is designed to be predictive for any fuel whose chemical structure is known. For the initial calibration of the CPD model, experiments of various fuels have been used. Thereby the main apparatus used for the determination of devolatilization kinetics is an entrained flow reactor (EFR). This reactor type operates in the temperature range from approximately 1300 to 1900 K with particle residence times up to around 500 ms. Grant et al. identified that this database is not sufficient to determine all model parameters as some of them might only be accessible with experiments at lower temperatures. In this study, the Chemical Percolation Devolatilization (CPD) model is chosen as a sophisticated network model for analyzing the performance in predicting volatile release rates at low pyrolysis temperatures. Therefore all relevant model equations for particle movement, particle heat up and secondary gas phase equations have been adapted to the experimental procedure of kinetic investigations undertaken in a small-scale fluidized bed reactor (FBR).
... In our work, the pre-exponential factor and the activation temperature for both pyrolysis and cracking are taken from Glaister [51], as cited in Grønli [36] (see Table 4.5 in Grønli [36]). Di Blasi [17] derived the pyrolysis rate from Roberts and Clough [50] and the cracking rate from Liden et al. [52]. Mandl et al. [24] derived them from Grønli [36] and Liden et al. [52], respectively (see the footnote to Table 4). ...
... Di Blasi [17] derived the pyrolysis rate from Roberts and Clough [50] and the cracking rate from Liden et al. [52]. Mandl et al. [24] derived them from Grønli [36] and Liden et al. [52], respectively (see the footnote to Table 4). Figure 3 shows that the rates are similar. ...
... The correction factor ξ, appearing in Equation (52), was already introduced into the modelling in the 1980s and 1990s [18], so it is around forty years old. As shown in Equation (52), the factor governs the energy transfer between the gas-and solid-phases; for ξ = 1, the rate is maximum, while for ξ = 0, there is no energy transfer between the phases. Thus, with ξ approaching 1, the differences between gas-phase and solid-phase temperatures are minimized. ...
Chapter
Vorwort Der Flammentag ist das etablierte Forum für den Austausch von Wissenschaft und Praxis über die neuesten Betriebs- und Forschungsergebnisse auf dem Gebiet der Verbrennung und der Feuerungen. Teilnehmer aus Industrie und Hochschule treffen sich im zweijährigen Turnus auf dieser Veranstaltung, um bei Fachvorträgen, Posterpräsentationen und Fachgesprächen den Wissenstransfer voranzutreiben. Der 28. Deutsche Flammentag ist zu Gast an der Technischen Universität Darmstadt und lädt Sie ein, aktuelle Ergebnisse der Industrie- und Hochschulforschung mit Praktikern und Forscher aus den Gebieten der Flammenforschung, des Feuerungsbaus, aus Energieversorgungs-, Anlagenbau- und Zulieferunternehmen sowie aus Hochschulen und Forschungseinrichtungen zu diskutieren. Mit insgesamt 60 Vorträgen wird das gesamte Spektrum der Verbrennung und Vergasung von der Großfeuerung im zentralen Bereich bis zu den Fragestellungen der Prozessfeuerungen, thermische Behandlung und Biomassenutzung abged...
... An increase in the length can lead to a longer residence time of pyrolysis volatiles within the structure. It is suspected that above 500 • C such longer residence times can lead to enhanced deposition of secondary char due to cracking of evolved volatiles [47], and in consequence causes a distortion in the PSD of the pyrochar. The mineral matter and its composition is also an adjustable feedstock property, and it affects the development of the pore structures of the pyrochars as well. ...
... The pyrolysis temperatures were selected to contain three scenarios without secondary cracking of volatiles and three scenarios where secondary cracking will most likely occur. A temperature of 500 • C was selected as a threshold above which the secondary cracking of volatiles becomes significant [47]. Additionally, the lowest three temperatures were selected to indirectly investigate the influence of selective degradation of biomass constituents (hemicellulose, cellulose and lignin) [58]. ...
... When prepared at temperatures < 300 • C, the increase in SSA was negligible, and above that temperature, significant microporosity was noticed for the pyrochars. The difference in the microporous SSA between pyrochars prepared from wood cylinders of different length became noticeable for pyrolysis temperatures of 500-900 • C. The differences were consistent with that secondary char formation (char from thermal cracking of high molecular compounds), relevant at temperatures above 500 • C [47] was enhanced by an increased length of the wood cylinder. Vapours that evolved during the wood pyrolysis, due to a pressure difference move through the outer, highly-heated char layer. ...
Article
Char obtained from biomass pyrolysis is an eco-friendly porous carbon, which has potential use as a material for electrodes in supercapacitors. For that application, a high microporous specific surface area (SSA) is desired, as it relates to the accessible surface for an applied electrolyte. Currently, the incomplete understanding of the relation between porosity development and production parameters hinders the production of tailor-made, bio-based pyrochars for use as electrodes. Additionally, there is a problem with the low reliability in assessing textual properties for bio-based pyrochars by gas adsorption. To address the aforementioned problems, beech wood cylinders of two different lengths, with and without pre-treatment with citric acid were pyrolysed at temperatures of 300–900 °C and analysed by gas adsorption. The pyrolyzed chars were characterised with adsorption with N2 and CO2 to assess the influence of production parameters on the textual properties. The new approach in processing the gas adsorption data used in this study demonstrated the required consistency in assessing the micro- and mesoporosity. The SSA of the chars rose monotonically in the investigated range of pyrolysis temperatures. The pre-treatment with citric acid led to an enhanced SSA, and the length of the cylinders correlated with a reduced SSA. With pyrolysis at 900 °C, the micro-SSAs of samples with 10 mm increased by on average 717 ± 32 m²/g. The trends among the investigated parameters and the textual properties were rationalized and provide a sound basis for further studies of tailor-made bio-based pyrochars as electrode materials in supercapacitors.
... The mechanism to predict the decomposition of biomass closely follows the development in references [25,[29][30][31][32][33][34][35][36][37][38][39][40]. In particular, the works by Xue et al. [25,40] have resulted in a model which will be used for this work. ...
... The first-order kinetic rates of Arrhenius form, k i ¼ A i expðE i =ðR u T b ÞÞ, are tabulated for each component in Table 3, and R u is the universal gas constant. The char formations ratios are Y C ¼ 0:35; Y H ¼ 0:60, and Y L ¼ 0:75, for cellulose, hemicellulose, and lignin, respectively [25,29,30,34]. ...
... We conservatively estimate e comminute % 1 MJ/kg by extrapolating from the values given in Mani et al. [53]. 4.25e6 108.0 [25,34] Experimental results from a lab-scale fluidized bed reactor (Lab FBR) are taken from the literature [21] for the purposes of comparison because the RBCR and FBR have nearly the same size/footprint and, thus, by crude assumption, similar capital costs. For context, the reactor volume in the RBCR consists of 8 cylinders each with a bore of 104 mm and a stroke length of 106 mm. ...
Article
In this work, the capabilities of a pilot-scale reciprocating biomass conversion reactor (RBCR) are assessed by modeling. The RBCR is a novel means to produce bio-oil by fast-pyrolysis whose core is a re-purposed 4-cycle internal combustion engine. Inert gas and a small volume-fraction of pulverized biomass are input into the RBCR intake, and the crankshaft is cycled by an external energy source to supply the process heat. The biomass is converted during the compression stroke, and then the bio-products are exhausted. The cycle is: intake, compression/heating, expansion, and exhaust. A control-volume energy balance to assess the steady-state flow of mass and energy through the RBCR is presented. The RBCR calculations are quantitatively compared to experimental results from the state of the art considered to be a lab-scale fluidized-bed reactor (FBR) found in the literature. This FBR is chosen for comparison because the RBCR and FBR have nearly the same size/footprint and, thus, by crude assumption, similar capital costs. Relative to the state-of-the-art (SOA), calculations predict that the RBCR will increase the biomass throughput, decrease the mass-specific energy requirement to thermochemically convert biomass to bio-oil, bio-char, and bio-gas by fast-pyrolysis. Moreover, calculations predict that at bench scale, the RBCR process results in a bio-oil product with a heating value approximately 6.8 times higher than the energy required to drive the cycle (there is a 6.8 times “return on energy investment”).
... The emerging nature of the biomass pyrolysis modeling field (reviews in [18,[26][27][28]) presents a number of options to model the production rates of bio-products. We choose a model which "super poses" cellulose, hemicellulose, and lignin as The mechanism to predict the decomposition of biomass closely follows the development in references [25,[29][30][31][32][33][34][35][36][37][38][39][40]. In particular, the works by Xue et al. [25,40] have resulted in a ...
... , are tabulated for each component in Table 3, and R u is the universal gas constant. The char formations ratios are Y C = 0.35, Y H = 0.60, and Y L = 0.75, for cellulose, hemicellulose, and lignin, respectively [25,29,30,34]. ...
... The fractions of the biomass are defined as α = m c /m b , β = m h /m b , and γ = m l /m b . The evolution of these fractions are modeled simultaneously and independently.Examples of different compositions are given inTable 1. Mechanism for pyrolysis adapted from[25,[29][30][31][32][33][34][35][36][37][38][39][40]. j may be cellulose C, hemicellulose H, or lignin, L. ...
Conference Paper
Full-text available
The capabilities of a pilot-scale reciprocating biomass conversion reactor (RBCR) and its sensitivity to changes in various input parameters are examined. The RBCR is a 4-stroke diesel engine repurposed as a novel reactor to produce bio-oil by fast-pyrolysis of biomass. An external source powers the RBCR through the intake, compression/heating, expansion and exhaust strokes. Biomass is carried to the RBCR by an inert gas and is converted during compression as part of the compression work provides the heat of pyrolysis. A control-volume energy balance is coupled with a biomass decomposition mechanism from literature to predict the evolution of bio-products during compression and expansion strokes. The RBCR calculations are compared to experimental results from the state of the art considered to be a lab-scale fluidized-bed reactor (FBR). Calculations predict that the RBCR will increase the biomass throughput, and decrease the energy requirement for biomass conversion with a 6.8 times return on energy investment. The sensitivity analysis indicates the need for finely pulverized biomass for significant conversion, and highlights the versatility of the RBCR since operation parameters can be adjusted to achieve near complete conversion to bio-gas or to yield bio-oil up to 70% of biomass weight.
... The mechanism to predict the decomposition of biomass closely follows the development in references [25,[29][30][31][32][33][34][35][36][37][38][39][40]. In particular, the works by Xue et al. [25,40] have resulted in a model which will be used for this work. ...
... The first-order kinetic rates of Arrhenius form, k i ¼ A i expðE i =ðR u T b ÞÞ, are tabulated for each component in Table 3, and R u is the universal gas constant. The char formations ratios are Y C ¼ 0:35; Y H ¼ 0:60, and Y L ¼ 0:75, for cellulose, hemicellulose, and lignin, respectively [25,29,30,34]. ...
... We conservatively estimate e comminute % 1 MJ/kg by extrapolating from the values given in Mani et al. [53]. 4.25e6 108.0 [25,34] Experimental results from a lab-scale fluidized bed reactor (Lab FBR) are taken from the literature [21] for the purposes of comparison because the RBCR and FBR have nearly the same size/footprint and, thus, by crude assumption, similar capital costs. For context, the reactor volume in the RBCR consists of 8 cylinders each with a bore of 104 mm and a stroke length of 106 mm. ...
... These mismatches or overprediction at low conversion caused by too high coefficients and especially constants in Eqs. (22) and (25). Thus, using developed regression models with order only 2 (quadratic regression models) could really get better agreement in wider range of conversion than using constants. ...
... The kinetics of pyrolysis analyzed with these moderate heating rates had been widely used in simulations of slow pyrolysis (or conventional pyrolysis) and gasification involving pyrolysis reaction [17,[25][26][27]. From Section 4.1, applying the set of constants (E a = 248.05 ...
Article
Full-text available
This study used thermogravimetric analysis to investigate activation energy and pre-exponential factor of corn cob pyrolysis via various model-free methods. For applying kinetics of pyrolysis reaction in commercial simulation tools, a representative single reaction of overall pyrolysis reaction would be preferable in provided toolboxes. However, the kinetic parameters for this single reaction should be suitable to cover all the conversion range. Thus, the kinetic parameters were developed as polynomial regression models. The results showed that verification of using the quadratic regression models derived via Friedman method (Ea=−488.68X2+587.78X+64.104 and ln(A)=−106.58X2+116.71X+17.232) had very close agreement with the experiments in entire range of conversion. These regression models could be widely applied in slow pyrolysis and gasification. However, the constants derived via Ozawa–Flynn–Wall method (Ea=248.05 kJ mol−1 and A=3.05x10²² min−1) were simpler for use in some applications which had no effect of pyrolysis at temperature below 300 °C or conversion below 50%, like fluidized bed pyrolysis/gasification.
... Modeling studies were also conducted to study the homogeneous decomposition of tar from biomass pyrolysis. 95,96 According to the kinetic model of Linden et al., 95 the wood undergoes flash pyrolysis and decomposes according to two parallel reaction mechanisms to give primary pyrolysis products (gas, char, and tar). These primary products undergo further secondary homogeneous decomposition reactions as shown in Figure 6. ...
... Modeling studies were also conducted to study the homogeneous decomposition of tar from biomass pyrolysis. 95,96 According to the kinetic model of Linden et al., 95 the wood undergoes flash pyrolysis and decomposes according to two parallel reaction mechanisms to give primary pyrolysis products (gas, char, and tar). These primary products undergo further secondary homogeneous decomposition reactions as shown in Figure 6. ...
Article
Full-text available
Biomass pyrolysis is a thermochemical conversion process that undergoes a complex set of concurrent and competitive reactions in oxygen-depleted conditions. A considerable amount of the literature uses lumped kinetic approaches to predict pyrolysis products. Despite the prolonged studies, the science of pyrolysis chemistry and models' capability to simulate the exact conversion phenomenon has unraveled yet. In this review, an initiative was made by compiling existing mathematical models for biomass pyrolysis viz., lumped and distributed kinetic models, particle, and reactor models. An absolute analysis of computational fluid dynamics (CFD), artificial neural network (ANN), and ASPEN Plus models was also conducted. It was observed that the coupling of distributed kinetic models with CFD provides a better understanding of the hydrodynamic reaction of particles under reactive flow with the influence on reactor performance and predicts exact product yield. Furthermore, the pros and cons of each modeling technique are also highlighted individually. Finally, considering the future perspective of biomass pyrolysis with respect to the modeling approach, suggestions have been incorporated.
... Liden et al. (1988) estudaram o modelo cinético para produção de líquidos da pirólise e consideraram que a biomassa se decompõe de acordo com duas reações, produzindo alcatrão líquido (óleo) e gás + carvão. O óleo é então, através de reações homogêneas, convertido em gás como principal produto. ...
... O óleo é então, através de reações homogêneas, convertido em gás como principal produto. O modelo proposto se baseia emBradbury et al. (1979) e outros autores, e mostrou-se capaz de prever o rendimento do líquido orgânico produzido dentro das condições ideais para maximizar os rendimentos de alcatrão.Di Blasi (1994) estudou um modelo matemático para a pirólise da celulose, analisando o efeito do tamanho de partícula sobre a taxa de reação, adotando o modelo desenvolvido porBradbury et al. (1979) eLiden et al. (1988). O modelo proposto Brazilian Journal of Development, Curitiba, v.7, n.8, p. 78706-78719 aug. ...
... Finally, pyrolytic water production was not measured for the PP experiments, in contrast with the HF tests. Pyrolytic water can account for 10-12 wt% of the dry feed [60][61][62] and part of it condenses in the impinger trap for the PP experiments. ...
Article
Full-text available
The present work focuses on the sampling procedure and quantification of the PAH yield from the fast pyrolysis of waste softwood. In particular, fast pyrolysis experiments were conducted using a CDS Pyroprobe 5200 at temperatures between 500 °C and 1000 °C, at a heating rate of 600 °C/s for a sample size of 30 mg. High performance liquid chromatography (HPLC) was used for the determination of the PAH compounds present in the liquid sample fraction, while a micro – GC was employed for the analysis of the main gaseous products (CO, CO2, CH4 and H2). An alternative tar sampling protocol was proposed, which employed the use of a cold trap (50 °C) and an isopropanol filled impinger bottle for the collection of the condensable products. The experiments were compared to heated foil reactor based pyrolysis tests within the same temperature range and heating rate, except for a slightly lower sample size (10 mg). The Pyroprobe and adapted sampling system proved to be more efficient regarding PAH capture and quantification compared to the heated foil reactor. Naphthalene, acenaphthylene and phenanthrene were the main PAH compounds detected. The PAH yields increased with pyrolysis temperature, up to values corresponding to roughly 0.2 wt% of the overall yield at 1000 °C. From the results it was derived that PAH evolution is mainly a product of secondary decomposition of primary tar, since the char yield stabilized for higher temperatures and the yields of CO, H2 and CH4 increased. Overall mass balance closure values were around 80 wt% on average. Char and gas yields were determined with high reproducibility, however gravimetric liquid analysis lacked due to the inability to gravimetrically measure the yield condensing in the impinger bottle. Future work is aimed on improving on this particular aspect. Overall, the alternative tar sampling system proposed was successful in the quantification of PAH from biomass fast pyrolysis experiments offering increased flexibility, accuracy and practicality of use.
... The competing parallel reactions are mainly applicable at low heating rate and for specific fuels, which may create additional challenges for the modeling of fast pyrolysis at high temperatures [74,88,89]. More complex modeling ways were also implemented, which involve additional steps for the tar decomposition in the gas phase [90] or an intermediate product deriving from primary decomposition of biomass [50,52,91]. The proposed models mostly can be applied only for a particular type of biomass. ...
Thesis
This Ph.D. thesis describes experimental and modeling investigations of fast high temperature pyrolysis of biomass. Suspension firing of biomass is widely used for power generation and has been considered as an important step in reduction of greenhouse gas emissions by using less fossil fuels. Fast pyrolysis at high temperatures plays a significant role in the overall combustion process since the biomass type, the reaction kinetics and heat transfer rates during pyrolysis influence the volatile gas release. The solid residue yield and its properties in suspension firing, including particle size and shape, composition, reactivity and burnout depend significantly on the operating conditions of the fast pyrolysis. Biomass fast pyrolysis experiments were performed in a laboratory-scale wire mesh reactor and bench scale atmospheric pressure drop tube / entrained flow reactors with the aim to investigate the effects of operating parameters and biomass types on yields of char and soot, their chemistry and morphology as well as their reactivity using thermogravimetric analysis. The experimental study was focused on the influence of a wide range of operating parameters including heat treatment temperature, heating rate, particle size, residence time, inorganic matter and major organic biomass compounds. Woody and herbaceous biomass were used as fuels. Char yields from the drop tube and entrained flow reactors were lower than those obtained in the wire mesh reactor, emphasizing the importance of heating rate on the product yields. The char yield decreased significantly between 10 and 600 K s^-1, but continued to decrease with increasing heating rate, and was lowest for the drop tube / entrained flow reactors with estimated heating rate of > 10^4 K s^-1. The heat treatment temperature and potassium content affected the char yield stronger than the heating rates and differences in the plant cell wall compounds between 600 and 3000 K s^-1. The heat treatment temperature affected more the herbaceous biomass char yield compared with wood. The differences in the char yield for particle size fractions in the range of 0.05 mm < dp < 0.425 mm were negligible, leading to the conclusion that the biomass particle can be assumed isothermal, when its size did not exceed 0.425 mm. Compared to smaller particles, the larger pinewood particles (dp > 0.85 mm) required more than 1 s holding time for the complete conversion at intermediate and fast heating rates. The influence of heating rate on the char yields was less pronounced for larger particles (from 0.85 to 4 mm) obtained at temperatures > 1250 C in the wire mesh reactor, single particle burner and drop tube reactor, due to the predominance of internal heat transfer control within the large particles. Potassium compared to all other ash elements in the fuels had the highest influence on the char yield. The effect of potassium on the char yield was stronger at low and intermediate heating rates where potassium catalyzed the repolymerization and cross-linking reactions, leading to higher char yields. Silicon compounds abundant in herbaceous biomass had a negligible influence on the char yield and reactivity. However, a very high content of silicon oxides in biomass (> 50 % of the overall biomass inorganic matter) significantly affected the char morphology, as observed for rice husk. For this fuel, the high content of low-temperature melting amorphous silicon oxides led to the formation of a glassy shell on rice husk chars at 1000-1500C. The ability of char to melt in fast pyrolysis followed the order pinewood > beechwood, straw > rice husk, and was related to the formation of metaplast. Different particle shapes of beechwood and leached wheat straw chars produced in the drop tube reactor which have similar potassium content suggested a stronger influence of the major biomass cell wall compounds (cellulose, hemicellulose, lignin and extractives) and silicates on the char morphology than alkali metals. In this study, potassium lean pinewood (0.06 wt. %) produced the highest soot yield (9 and 7 wt.%) at 1250 and 1400C, whereas leached wheat straw with the higher potassium content (0.3 wt. %) generated the lowest soot yield (2 and 1 wt.%). Soot yields of wheat and alfalfa straw at both temperatures were 5 % points lower than wood soot yields and 3 % points higher than leached wheat straw soot yield, indicating that potassium plays a minor role on the soot formation. The leaching of alkali from wheat straw additionally resulted in a removal of lignin, leading to the decreased formation of polycyclic aromatic hydrocarbon precursors, and thereby to lower soot yields. Pinewood soot particles generated at 1250C were significantly larger (77.7 nm) than soot particles produced in pinewood (47.8 nm) pyrolysis at 1400C, beechwood (43 nm) and wheat straw (30.8 nm) devolatilization at both temperatures. The larger pinewood soot particles were related to the formation of tar balls known from smoldering combustion. The major difference in nanostructure of pinewood, beechwood and wheat straw soot was in the formation of multi and single core particles. Pinewood soot particles generated at 1250C were mainly multi core structures compared to pinewood soot generated at 1400C, combining both single and multi core particles. Beechwood and wheat straw soot samples had multi and single core particles at both temperatures. In thermogravimetric analysis, the maximal reaction rate of pinewood soot was shifted to temperatures about 100C higher than for the other samples in both oxidation and CO2 gasification, indicating a significantly lower reactivity. Soot samples produced at 1400C were more reactive than soot generated at 1250C. The beechwood and wheat straw soot samples were more graphitic than pinewood soot based on the electron energy loss spectroscopy (EELS) analysis. In contrast to expectations of graphitic structures to react slower than amorphous samples, beechwood and wheat straw soot were 35 and 571 times more reactive than pinewood soot prepared at 1400C. The presence of potassium in wheat straw soot mainly as water-soluble KCl, KOH, KHCO3 and K2CO3 and to a minor extent bonded to the soot matrix in oxygen-containing surface groups (e.g. carboxyl, phenolate) or intercalated in soot graphene layers led to a higher reactivity in CO2 gasification compared to low-alkali containing pinewood soot. The results showed that potassium has a dominating effect on the soot reactivity compared to nanostructure and particle size. A mathematical model of biomass fast pyrolysis was developed to predict the gas and char yields of wood and herbaceous biomass at heating rates > 600 K s^-1. The model includes both kinetics and external and internal heat transfer assuming that mass transfer is fast. The model relies on the concept applied in fast pyrolysis of cellulose through the formation of an intermediate liquid (so called metaplast) which reacts further to form char and gas. The kinetics of the fast pyrolysis was described through the Broido-Shafizadeh scheme for biomass. The catalytic effect of potassium which is a major ash element influencing the char yield was included in the model.
... These results show that it is not needed to described tar cracking in bed for this technology. Furthermore, for the case M8, with the highest temperatures at the pyrolysis front, a simulation was conducted including the tar cracking kinetics from Liden et al. [38] (A = 4.26 × 10 6 s − 1 , E = 108 kJ⋅mol − 1 ), which are the fastest among the most commonly employed tar cracking kinetics in literature [31]. The results showed that less than 0.2% of the tar was cracked for this case, confirming that tar cracking in the bed is minimal at these conditions. ...
Article
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Fixed-bed biomass conversion with a low primary air ratio and a counter-current configuration has a high feedstock flexibility, as it resembles updraft gasification, and the potential to reduce emissions when integrated in biomass combustion systems. A 1D bed model was validated with experimental results from a biomass combustion boiler with such a bed conversion system, predicting with a good accuracy the temperatures in the reactor and producer gas composition. The model was applied for different cases to investigate the fuel flexibility of this combustion system, including the influence of moisture content and the maximum temperatures achieved in the bed. It was shown that with variations in fuel moisture content from 8 to 30% mass w.b. the producer gas composition, char reduction to CO or maximum temperatures at the grate were not affected due to the separation of the char conversion and pyrolysis/drying zones. Flue gas recirculation was the only possible measure with the tested configuration to reduce the maximum temperatures close to the grate, which is beneficial e.g. to avoid slagging with complicated fuels. A higher tar content was obtained than in conventional updraft gasifiers, which is attributed to the absence of tar condensation in the bed due to the limited height of the reactor and the integration in the combustion chamber. The presented model can support the development of such combustion technologies and is a relevant basis for detailed CFD simulations of the bed or gas phase conversion.
... D'autres mécanismes font intervenir plusieurs étapes comme par exemple le modèle développé par Shafizadeh et Chin [64] (Figure 1.5). Ces réactions primaires qui conduisent à la formation des gaz, des goudrons ou du résidu charbonneux ont été étudiées à de multiples reprises dans la littérature pour différents types de bois [65][66][67][68][69]. En général, des réactions secondaires, c'est-à-dire des réactions qui se font à partir des goudrons ou du résidu charbonneux sont ensuite introduites pour décrire le craquage des goudrons en gaz et leur polymérisation en résidu charbonneux [70][71][72][73][74]. Les réactions secondaires se produisant à partir du résidu charbonneux correspondent à leur oxydation en cendres. ...
Thesis
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Ces travaux de thèse s’inscrivent dans l’amélioration de la compréhension des mécanismes de dégradation des combustibles pour l’incendie. Ils ont pour objectif d’étudier à travers une approche multi-échelle la dégradation thermique de plaques de bois.La dégradation thermique de deux types de bois : le chêne blanc (Quercus alba) et l’eucalyptus commun (Eucalyptus globulus), a tout d’abord été étudiée à l’échelle matière, où des échantillons de faibles masses ont été chauffés dans un analyseur thermogravimétrique. Les résultats ont montré que la dégradation thermique de ces deux bois pouvait se représenter en quatre étapes. A partir de ces résultats expérimentaux, quatre mécanismes réactionnels ont été développés : le mécanisme par constituants, le mécanisme global, le mécanisme actif et le mécanisme simplifié avec seulement deux étapes. Les paramètres cinétiques associés ont été déterminés par optimisation avec un algorithme du gradient descendant. La simulation a révélé que l’ensemble des mécanismes représente de manière efficace la perte de masse des deux bois aux différentes vitesses de chauffe étudiées. La meilleure performance est obtenue par le mécanisme global et la moins bonne par le mécanisme simplifié.La dégradation thermique des deux bois a également été étudiée à l’échelle matériau à l’aide d’un cône calorimètre. Différentes densités de flux variant entre 18 et 28,5 kW/m² ont été appliquées afin d’éviter l’auto-inflammation du bois. Deux conditions limites ont été imposées à la face inférieure des plaques de bois. La température des plaques de bois a été mesurée par thermocouples et par caméra infra-rouge. Les résultats expérimentaux ont révélé que plus la densité de flux augmente, plus la perte de masse est rapide et la température augmente rapidement. L’oxydation du résidu charbonneux prend dans ce cas l’allure d’un front bidimensionnel qui se propage au cours du temps.L’étude numérique menée sur les expériences réalisées à l’échelle matériau a permis de valider les mécanismes réactionnels développés à l’échelle matière, en utilisant le champ de température expérimental des plaques de bois fines. A cette échelle, la performance des mécanismes réactionnels est très proche. Une étude unidimensionnelle a été réalisée avec le code GPYRO afin de prédire la température et la perte de masse des plaques thermiquement fines. Les résultats obtenus sont très satisfaisants, grâce à une optimisation des propriétés thermiques du bois et une convolution pour représenter les phénomènes bidimensionnels. Pour les plaques thermiquement épaisses, les mécanismes à quatre étapes permettent de représenter la perte de masse durant la phase de gazéification mais ne permettent pas cependant de prédire la totalité de la phase d’oxydation du résidu charbonneux.
... In their subsequent work, they derived the yield of gas, tar and solid residue [4]. A similar contribution can be recognized in the study of Liden et al. [5] focusing on the kinetic prediction model for the liquid fraction from fast pyrolysis. On the same issue, but focussing on slow pyrolysis, Hu et al. [6] compared different kinetic models on the ground of data derived from thermogravimetric analyses (TGA). ...
Article
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In this study, the biomass degradation and the evolution of chemical species during pyrolysis are analysed with the main aim of evaluating the energy performance of a micro-cogeneration unit fed by biogas. The decomposition of the feedstock material is modelled as a two-stage process: Firstly, in the reactor, the biomass is decomposed in a residual solid fraction (char) and a gaseous mixture; then, the condensable gases are divided from permanent gases generating the pyro-oil. The mathematical model proposed in this work has been developed considering the dependence of the pyrolysis process from the temperature and within the interval 500-900°C. The kinetic of the reactions involved during the pyrolysis was also taken into account. Simulations run in AspenPlus exploiting the R-yield reactor supported by a calculator block. Afterwards, the energy recovery line for the valorisation of the pyroproducts has been analysed. The gas fraction obtained at the end of the cycle was firstly characterized and then used to feed a micro-CHP system. Results are very promising, with great potential in terms of thermal recovery; more than 60% of the initially fed biogas and about 30% power output can be derived.
... e Di Blasi (1993). f Liden et al. (1988). ...
Chapter
Biomass pyrolysis is a technology that uses high temperatures to break down biomaterials like wood without completely burning them. Products include char, combustible gas, and vapors; the vapors can be condensed into a liquid with myriad uses. This chapter serves as a primer for the fundamental physics and chemistry involved in biomass pyrolysis, common reactor systems, and some reported uses for the products.
... (2) cellulose depolymerization to levoglucosan that could be additionally pyrolyzed to several small-molecular materials and secondary char (Dufour et al. 2011;Liden et al. 1988;Scott et al. 1988;Lin et al. 2019). In comparison with the untreated fabrics, the samples coated by the product of HMTS hydrolysis, NH 4 -HMP and LAP exhibited lower T 5% . ...
Article
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Fabrics with high flame-retardancy have been extensively applied for numerous applications including textile, garments, automobile industries, pants, shirts, suits bed sheets, and indoor decorations. Coatings consisting of ammonium hexametaphosphate (NH4-HMP), laponite (LAP), and hexadecyltrimethoxysilane were synthesized through sol–gel method; they were then employed on the cotton fabrics to enhance their hydrophobicity and flame retardancy. The influences of LAP concentration on fire-retardancy of the samples were evaluated. The combustion behavior, morphological structures, thermal stability, and hydrophobic properties of the cotton fabrics were studied. Results indicated the excellent flame retardant property of the treated cotton fabrics as they immediately extinguished upon removal of the flame source. The limiting oxygen index of the treated cotton was enhanced to 29% in comparison to that of the pure one (19.5%). The findings also indicated that a higher concentration of LAP is useful for improving flame retardancy of the coated substrate. In addition, the hydrophobicity of the fabric surface was measured by a water contact angle of 138°, while its superoleophilicity was assessed by an oil contact angle of 0°. To separate water–oil mixtures, the as-prepared cotton sample was utilized as operative substances. Overall, in this study a facile technique is provided for preparing cotton fabrics with considerably enhanced flame retardancy and superior self-cleaning features toward different fluids making them suitable as a promising candidate for water–oil separation.
... Single-component competitive models are covering only primary biomass degradation reactions, which have an influence on the prediction accuracy of product's yields [24]. Further development of the single-component competitive models was made by the introduction of cracking reactions of high molecular mass vapours (tars) at temperatures higher than 500 C [25]. The most often used kinetic scheme is the one proposed by Shafizadeh and Chin [26]. ...
Chapter
Modelling is a complex task combining elements of knowledge in the field of computer science, mathematics and natural sciences (fluid dynamics, mass and heat transfer, chemistry). In order to correctly model the process of biomass thermal degradation, in-depth knowledge of multi-scale unit processes is necessary. A biomass conversion model can be divided into three main submodels depending on the scale of the unit processes: the molecular model, single particle model and reactor model. Molecular models describe the chemical changes in the biomass constituents. Single-particle models correspond to the description of the biomass structure and its influence on the thermo-physical behaviour and the subsequent reactions of the compounds released during decomposition of a single biomass particle. The largest scale submodel and at the same time, the most difficult to describe is the reactor model, which describes the behaviour of a vast number of particles, the flow of the reactor gases as well as the interaction between them and the reactor. This chapter contains a basic explanation about which models are currently available and how they work from a practical point of view.
... Parameters A tar;GPR and E a;tar;GPR for the tar cracking mechanism are used for a moderate mechanism from Serio, Peters, and Howard (1987b) and an intense mechanism from Liden, Berruti, and Scott (1988). ...
Article
In the present study, the CPD model originally developed based on predictions from heated grid (HGR) and entrained flow (EFR) experiments, has been adapted to analyze pyrolysis kinetics in a small-scale fluidized bed reactor. Impacts of particle feed, particle heat up as well as tar cracking reactions in the gas phase are considered. Furthermore, an optimized solver structure allows a time step independent solution and enables the use of implicit methods. A comparison with experimental results is undertaken for pulverized Rhenish lignite fuel particles in the temperature range from 673 to 973 K in N2 atmosphere. The comparison between simulated and experimentally derived volatile release rates reveals a good agreement, indicating that the high temperature derived kinetic parameters from HRG and EFR experiments can be extrapolated to lower temperatures. Nevertheless, discrepancies in the tar to light gas ratio occur with the proposed model implementation.
... The chemical equilibrium equation can be refereed in Refs. [30,31]. ...
Article
In order to mitigate inherent intermittency of solar energy and satisfy the energy demands, a novel steam/air biomass gasification combined cooling, heating and power system with solar energy is proposed in this work. In the system concentrated solar energy collected with dish optical configuration is used to generate high temperature steam, which acts as a gasification agent to drive the biomass gasification. When solar radiation intensity is lower than the design point (the maximum in whole year), the hybrid steam/air biomass gasification model is conducted for continuous syngas production. Syngas based chemical energy is released for trigeneration in a distributed system. The reaction kinetics models of steam-biomass gasification using solar energy are developed and the thermodynamics performances of the system are numerical investigated. Under the designate conditions, the primary energy efficiency reaches 51.34%. Based on the local weather data, the off-design performances of the system and the annual energy-saving potential are evaluated. Compared with the reference system that combines a conventional air biomass gasification trigeneration system and a solar dish/Stirling engine system, the new system has remarkable advantage in thermal performances. An economic analysis is conducted to evaluate the technical feasibility of the proposed system. This research provides a promising method for the efficient utilization of biomass and solar energy.
... Thurner and Mann [10] assumed that the activation energy for the char formation reaction is similar to the activation energy for mass loss reactions to gas and tar, and therefore, that the final residual weight (e.g. the char yield) is independent of the heating rate and heat treatment temperature. More complex models involve additional steps for tar decomposition in the gas phase [22] or an intermediate product derived from primary decomposition of biomass [15,23,24]. These models can be generally applied only for a specific type of biomass. ...
Article
h i g h l i g h t s A single set of kinetic parameters was used for woody and herbaceous biomass. Heating rate and potassium content influence char yield the most. Particle shrinkage has a negligible effect on char yields. Catalytic effects of potassium on char yields most apparent at high heating rates. At high heating rates, lower mass fractions of metaplast were formed in the core. a b s t r a c t This study presents a combined kinetic and particle model that describes the effect of potassium and heating rate during the fast pyrolysis of woody and herbaceous biomass. The model calculates the mass loss rate, over a wide range of operating conditions relevant to suspension firing. The shrinking particle model considers internal and external heat transfer limitations and incorporates catalytic effects of potassium on the product yields. Modeling parameters were tuned with experimentally determined char yields at high heating rates (>200 K s À1) using a wire mesh reactor, a single particle burner, and a drop tube reactor. The experimental data demonstrated that heating rate and potassium content have significant effects on the char yield. The importance of shrinkage on the devolatilization time becomes greater with increasing particle size, but showed little influence on the char yields.
... However, in those experiments the reactor temperatures 274 above the single particle may have been lower (as already shown for combustion in 275 [16]). The kinetics from Liden et al. [25] are faster and better suited for the application 276 in the stove. 277 ...
Article
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Wood log stoves are a common residential heating technology that produce comparably high pollutant emissions. Within this work, a detailed CFD model for transient wood log combustion in stoves was developed, as a basis for its optimization. A single particle conversion model previously developed by the authors for the combustion of thermally thick biomass particles, i.e. wood logs, was linked with CFD models for flow and turbulence, heat transfer and gas combustion. The sub-models were selected based on a sensitivity analysis and combined into an overall stove model, which was then validated by simulations of experiments with a typical wood log stove, including emission measurements. The comparison with experimental results shows a good accuracy regarding flue gas temperature as well as CO2 and O2 flue gas concentrations. Moreover, the characteristic behavior of CO emissions could be described, with higher emissions during the ignition and burnout phases. A reasonable accuracy is obtained for CO emissions except for the ignition phase, which can be attributed to model simplifications and the stochastic nature of stove operation. Concluding, the CFD model allows a transient simulation of a stove batch for the first time and hence, is a valuable tool for process optimization.
Article
Primary organic aerosols (POA) are abundant in the atmosphere. POA are mainly emitted during the pre-ignition phase of wood combustion. In this work we demonstrate that the thermal degradation of wood that occurs during pre-ignition can be understood and predicted using pyrolysis modeling. We model the pyrolysis of 14 × 3.8 × 2.9 cm maple wood samples using Gpyro software to predict the emission of classes of gaseous products throughout the process. We define two classes of gases emitted that can be predicted by the model: light and heavy gases, where heavy gases include OA precursors. The validation experiments of wood pyrolysis were performed in a cylindrical reactor of the wood samples at three temperatures 400, 500, and 600 °C. Temperature and mass change were predicted by the model. The release rate of gaseous products showed two peaks, that were due to heat transfer at the surface and pressure evolution at the center. The validated model enables the comparison of atmospherically relevant quantities predicted in the model with emission data from atmospheric studies. The predicted emission factor of heavy gases occurring before the first peak of release rate can account for the measured POA emission factor from wood combustion during the pre-ignition phase.
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In this work, a two-dimensional (2D) model for wood particle drying, devolatilization, and char conversion is presented and validated against experimental studies of devolatilization of a large, dry, cylindrical birch wood log. The wood log’s internal distributions of temperature, solid species densities, gas density, and gas species mass fractions, together with the internal pressure and the gas flow velocities in radial and longitudinal directions, were studied. It was found that the internal pressure peak significantly depended on the permeability of char. The velocity profiles changed as the pressure wave traveled radially inward in the same manner as the devolatilization zone. The wood log internal pressure was not only dependent on wood and char permeabilities but also depended on the devolatilization rate, which itself is a heat-controlled conversion stage. In addition to the study on wood anisotropy, which can be accounted for in detail by the presented 2D model, the authors compared the 2D results to results obtained from a corresponding 1D version of the model, where only radial heat and mass transfer properties were used. It was found that even though the predictions of the 2D model were in better agreement with experimental observations than the 1D modeling results, for these parameters, 1D models can still be used without too much loss in accuracy. Finally, the paper concludes with a clear recommendation of when higher dimensional models should be used and when they should not.
Article
The need to achieve renewable alternatives for energy production is pressing and new technologies have been developed. Biomass is a feasible feedstock for thermal conversion techniques like gasification. This technique converts carbonaceous fuels into energy, producing a synthetic gas (syngas) with further commercial uses such as electricity generation, fuels or the chemical industry among others. In this work, a numerical model was developed in order to optimize the experimental parameters in the gasification of agricultural residues. Hence, oxygen content (OC) in the gasifying agent, equivalence ratio (ER) and fluidization velocity were varied so as to assess the effect of each parameter in syngas quality. Lower ER favored higher CO and H2 yields, enhancing also the lower heating value (LHV). Higher fluidization velocity also promoted these features, as well as gasification conversion efficiency (GCE) and carbon conversion efficiency (CCE). Higher OC in the gasifying agent improve syngas quality. The optimal gasification performance was achieved for OC = 40%. The results obtained in this work are essential to describe scenarios relating to the potential use of agricultural residues as a source of energy via gasification.
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Food waste (FW) is a severe environmental problem all over the world, and the recalcitrant organic residues (ROR) from FW treatment plant operations are also a critical environmental issue due to unsustainable treatment and disposal techniques. Requirements for FW and ROR complete exploitation with the establishment of recycling‐renewable technologies are very crucial. This paper review AD and pyrolysis as two promising technologies to degrade FW and its residues, creating numerous renewable bioenergy yields with value‐added. Existing oil/tar application methods in the AD suffered from various problems such as microorganism toxicity and limited productivity. Future upgrading techniques considering the second‐stage pyrolysis process to decompose oil/tars for syngas with high hydrogen content and enhanced bio‐methanation in the AD were addressed. Simultaneous pyrolysis by‐product recycle in the AD during the valorization of FW are aimed to have the features of sustainability toward increased bioenergy production, reactor efficiency, and agricultural application. Routes for utilization of ROR and FW via pyrolysis and AD.
Article
We studied the physical and chemical properties of the condensable volatiles of biomass pyrolysis products. We redefine the liquid product and divide the condensable volatiles into two categories, biomass oil and tar, the latter of which comes from the secondary pyrolysis or cracking reaction of the former. We further establish a kinetic model of biomass pyrolysis and secondary cracking. The chemical reaction kinetics equation and heat transfer equation are coupled to simulate the biomass pyrolysis process. For biomass solid particles, the model not only considers the initial reaction of biomass and secondary cleavage reaction of condensable gas, but also introduces a reaction mode in which biomass oil is converted into tar. When the pyrolysis temperature is below 500 °C, the pyrolysis products are essentially biomass oil. However, when the pyrolysis temperature exceeds 500 °C, the biomass oil gradually converts into tar. The model also considers characteristics of the reaction medium (porosity, intrinsic permeability, thermal conductivity) and the unsteady gas phase process based on Darcy's law of velocity and pressure, heat convection, diffusion, and radiation transfer. We analyze the relationships among the internal temperature of the particles, particle size and position, mass fraction of the reactants and products, the gas mixture, the production share of tar and biomass oil, and the relationship between gas pressure and time. The results show that the effects of the secondary cracking reaction and internal convective flow in the biomass pyrolysis process are coupled because the flow field in the porous medium determines the volatile residence time and thus species that affect the secondary cracking reaction. The rate of volatile formation in the initial and secondary cracking reactions affects the pressure gradient and gas diffusion. Additionally, the endothermic effect influences the temperature field of the pyrolysis reaction but has no apparent effect on small particles whose chemical reaction is the control mechanism. For large particles, heat transfer inside the particles is the diffusion control mechanism and the chemical reaction on the particle surface is the speed control mechanism. Two peaks are observed in the pyrolysis gas mass proportion curve, which result from the consumption of biomass oil and tar as they flow toward hot surfaces. The first peak is the decomposition of biomass oil into non-condensable volatile matter and tar, and the second peak is the further cracking of tar into gas and coke at high temperature.
Article
A compartmental one‐dimensional model of a fluidized bed pyrolytic converter of biomass is presented. Reference conditions are those of non‐catalytic fast pyrolysis of biomass in a shallow fluidized bed with external regeneration of the bed material. The fate of biomass and of the resulting char has been modelled by considering elutriation of biomass and char particles, char attrition as well as bed drain/regeneration. The course of primary and secondary pyrolitic reactions is modelled according to a semi‐lumped reaction network using well established kinetic parameters taken from the literature. A specific focus of the present study is the role of the heterogeneous volatile‐char secondary reactions, whose rate has been modelled borrowing a kinetic expression from the neighbouring area of tar adsorption/decomposition over char. Results of computations highlight the relevance of heterogeneous volatile‐char secondary reactions and of the closely associated control of char loading in the bed. The sensitivity of the reactor performance on char elutriation and attrition, on proper management of bed drain/regeneration, on control of gas phase backmixing is demonstrated. Model results provide useful guidelines for optimal design and control of fluidized bed pyrolyzers and pinpoint future research priorities.
Article
In this study, a micro fluidized bed reactor analyzer (MFBRA) was adopted to test the isothermal reaction characteristics of tar catalytic reforming by char and thermal cracking by quartz sand from 1023 to 1223 K. The behaviors of tar conversion, the evolution of gaseous products generation, and the corresponding reaction kinetics were comprehensively analyzed. Compared to thermal cracking, the yield of catalytic cracking is higher. It can not only promote the tar conversion but also upgrade the fuel gas quality even at a low temperature of 1023 K, especially for CO, CH4, CO2, and C3H6. The tested activation energies (Ea) of tar conversion and gas generation (CH4, C2H6, C3H6, H2, CO2, and CO) in catalytic reforming were about 60.27, 45.48, 56.22, 71.34, 62.35, 51.51, and 60.75 kJ/mol, respectively, corresponding that of 78.16, 63.19, 71.16, 83.19, 84.33, 90.81 and 130.72 kJ/mol in thermal cracking. The lower Ea indicated the good catalytic activity of char on tar conversion and the change of gas generation paths by choosing char as a catalyst. Finally, the kinetics was compared with the results in the literature to verify the feasibility of MFBRA and the accuracy of testing results. This research deepened the understanding of tar catalytic reforming by char and was beneficial for developing a biomass gasification process with low tar generation.
Article
This work presents a numerical study of biomass pyrolysis in turbulent riser flow. Eulerian–Lagrangian simulations of unbounded sedimenting gas-solid flows are performed to isolate the effects of particle clustering on the production of syngas and tar. This configuration provides a framework to resolve the relevant length- and time-scales associated with thermal, chemical and multiphase processes taking place in the fully-developed region of a circulating fluidized bed riser. A four-step kinetic scheme is employed to model the devolatilization of biomass particles and secondary cracking of tar. Two-way coupling between the phases leads to clusters of sand particles that generate and sustain gas-phase turbulence and transport biomass particles. Neglecting the heterogeneity caused by clusters was found to lead to a maximum over-prediction of syngas yield of 33%. Further, it was found that two-dimensional simulations over-predict the level of clustering, resulting in an under-prediction of syngas and tar yields.
Article
Devolatilization kinetics were determined using a modified micropyrolyzer reactor for several biomass feedstocks: switchgrass, corn stover, red oak, and pine. The micropyrolyzer was directly coupled to a flame ionization detector (FID) to track the release of volatiles from the biomass. Time series data from these experiments was analyzed to determine apparent devolatilization rates. Care was taken to assure the experiments were isothermal and kinetically limited calculating a Biot number less than 0.1 and pyrolysis numbers greater than 10, which simplifies the derivation of devolatilization rates. A single, first order reaction was able to model devolatilization rates at temperatures up to 500 °C. No correlation was found between the inorganic content of the biomass and its rate of devolatilization. Apparent activation energies were in the range of 54.9–88.4 kJ mol⁻¹. The rate coefficient at 500 °C was calculated as 1.90–5.14 s⁻¹ for the four feedstocks.
Thesis
As part of a European project, a new assembly using densified dowels to hold wooden slats is currently being validated at the structural level. This type of assembly has the advantage of not using glues and of making it possible to manufacture large-sized structures consisting only of wood. The principle consists of positioning the wooden planks as desired, then drilling and inserting densified dowels: under the effect of moisture absorption, the densified dowels swell and block the assembly, making the structure rigid.The use of this type of assembly requires a multitude of sizing and behavior checks under various stresses, including thermomechanical variations. Thus, within the framework of this thesis work, the objective will be to characterize the behavior of wood lamellar assemblies by densified dowels subjected to significant thermo-hydric stresses, in particular during the fire. To do this, we propose an approach coupled with experiments and numerical modeling. The experiments will first allow the acquisition of the basic data to develop the model. Numerical modeling will then make it possible to better understand the mechanisms involved in the fire of these types of structures in order to improve their performance. This will also reduce the number of expensive trials. The model will be validated by temperature measurements at different depths in the section of the lamellae, but also within the densified dowels. These results will then be compared to experimental tests for validation on a few fire tests under mechanical stress.This model can then be used to estimate the behavior of more complex structures subjected to fire and to provide basic data for the sizing of complete buildings. The results can also serve as a basis for amending regulatory texts such as Eurocode 5.
Article
Under unprecedented environmental crisis associated with greenhouse gas emission, biomass has attracted a great deal of attention due to renewable and carbon neutral nature. In this study, the premixed combustion of various types of wood and its derived syngases are examined for steady and oscillating sates. For this purpose, the poplar, birch, beech and pin sawdust woods and syngases composing of H2, CH4 and CO are considered. To model dust cloud combustion, a novel and comprehensive flame structure consisting of drying, two-step pyrolysis and homogeneous and heterogeneous reactions is proposed. Afterward, the governing equations and their appropriate boundary conditions are derived and solved analytically-numerically. The oscillating combustion is also modeled by exerting an external perturbation on the velocity field. The results indicate that due to the occurrence of heterogeneous reactions in wood combustion, the flame propagation velocity of wood is higher than that of syngases which contributes to high oscillations amplitude of syngases. When the mixture initial temperature changes between 300 and 550 K, the flame velocities of woods and syngases vary in the ranges of 0.4–0.7 m/s and 0.1–0.27 m/s, respectively. The maximum amplitude of temperature oscillation of syngases is approximately 8 times more than that of woods.
Article
The past decades have witnessed increasing interest in developing pyrolysis pathways to produce high-quality chemicals from biomass. However, the numerous defects of bio-oil produced by biomass pyrolysis severely restrict its further application. A promising upgrading technology is considered to be the application of catalytic methods in biomass pyrolysis. Among various catalysts, metal catalysts have been well developed due to their flexible tailorability and unique characteristics. This review is dedicated to revealing the reasons for the biomass conversion pathways catalyzed by different metal catalysts (single and multi-metal catalysts, including Na, Mg, Al, K, Ca, Ti, Fe, Co, Ni, Cu and their combinations) and the conversion mechanisms of metal catalysts in the catalytic process. The interaction between different metal catalysts and the reasons why the combination improves catalytic activity were also introduced. At last, the research trends and perspectives on future development of metal catalysis in the field of biomass pyrolysis were proposed.
Article
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The study presents a parametric study of pyrolysis of poultry litters under isothermal heating conditions using a two-stage pyrolysis scheme and a downer separation technique. The study aims to increase the yield of biochar through pyrolysis using a two-stage kinetic process. The thermo- physical properties of mild steel and poultry litter were used to develop a simulation with a finite element method and COMSOL multi-physics to predict biochar yield at different temperatures and residence time. The numerical model was validated by comparing the predicted and the experimental results from the literature with a percentage difference of 1.3 to 11 % and an R2 value of 0.9124. Further parametric studies showed that for the pyrolysis of 0.5 kg of poultry litters, a maximum yield of biochar of 42.9 % was obtained at a lower temperature of 573.15 oK with a residence time of 9000s. Higher temperature favoured gas yield while biochar yield declined.
Article
Fast pyrolysis is an intricate process due to the variability and anisotropy of lignocellulosic biomass and the complicated chemistry and physics during conversion in a bubbling fluidized bed reactor (BFBR). The complexity of biomass fast pyrolysis lends itself well to computational fluid dynamics (CFD) and discrete element (DEM) analysis, which promises to reduce experimental time and its associated cost. This study investigated switchgrass fast pyrolysis simulated by computational fluid dynamics coupled with a discrete element method to track individual reacting biomass particles throughout a bench-scale BFBR reactor. We accounted for the fast pyrolysis chemistry through a comprehensive reaction scheme with secondary cracking reactions. We performed a three-step reduction for secondary cracking reactions to convert the full cracking scheme into a reduced scheme easily incorporated into our model. We assessed the impact of operational conditions on the steady-state yields of liquid bio-oil, non-condensable gases (NCG), at 550 °C over a range of fluidization numbers (2 – 6 Umf), reported as a ratio to the minimum fluidization velocity (Umf). At steady-state, the volatile bio-oil yield had a range of 49.3–50.4 wt. %. Levoglucosan was the primary volatile component present with 21 wt. % of the bio-oil while water was the second largest with 20 wt. %. The reduction of the secondary reaction schemes did not appreciably affect the overall yields of switchgrass pyrolysis compared to the full secondary scheme.
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This investigation provides a comprehensive experimental dataset and kinetic model for biomass gasification, over a wide temperature range (1150-1350 °Ϲ) in CO2, H2O and the combination of these two reactant gases over the mole fraction ranges of 0 to 0.5 for H2O and 0 to 0.9 for CO2. The data come from a unique experimental facility that tracks continuous mass loss rates for poplar wood, corn stover and switchgrass over the size range of 6-12.5 mm. In addition, the data include char size, shape, surface and internal temperature and discrete measurements of porosity, total surface area, pore size distribution and composition. This investigation also includes several first-ever observations regarding char gasification that probably extend to char reactivity of all types and that are quantified in the model. These include: the effect of ash accumulation on the char surface slowing the apparent reaction rate, changes in particle size, porosity and density as functions of burnout, and reaction kinetics that account for all of these changes. Nonlinear least-squares regression produces optimized power-law model parameters that describe gasification with respect to both CO2 and H2O separately and in combination. A single set of parameters reasonably describes rates for all three chars. Model simulations agree with measured data at all stages of char conversion. This investigation details how ash affects biomass char reactivity, specifically the late-stage burnout. The ash contents ratios in the raw fuels in these experiments are as high as 40:1, providing a clear indication of the ash effect on the char reactivity. The experimental results definitively indicate a decrease in char reaction rate with increasing initial fuel ash content and with increasing char burnout – most pronounced at high burnout. This investigation postulates that an increase in the fraction of the surface covered by refractory material associated with either higher initial ash contents or increased burnout decreases the surface area available for reaction and thus the observed reaction rate. A quantitative model that includes this effect predicts the observed data at any one condition within the data uncertainty and over a broad range of fuel types, particle sizes, temperatures, and reactant concentrations slightly less accurately than the experimental uncertainty. Surface area, porosity, diameter, and density predictions from standard models do not adequately describe the experimental trends. Total surface area increases slightly with conversion, with most of the increase in the largest pores or channels/vascules not measurable by standard surface area techniques but most of the surface area is in the small pores. Porosity also increases with char conversion except for abrupt changes associated with char and ash collapse at the end of char conversion. Char particle diameters decrease during these kinetically controlled reactions, in part because the reaction is endothermic and therefore proceeds more rapidly at the comparatively warmer char surface. SEM images qualitatively confirm the quantitative measurements and imply that the biomass microstructure does not appreciably change during conversion except for the large pore diameters. Extant char porosity, diameter, surface area, and related models do not predict these trends. This investigation suggests alternative models based on these measurements.
Article
This study presents a three-dimensional finite element model to describe the thermomechanical behaviour of flax chipboards under fire conditions. The model is based on kinetic models considering the thermal degradation during the pyrolysis phase and the evolution of the physico-mechanical properties as functions of temperature. The numerical model is integrated into Abaqus via user subroutines (Umat and Umatht) and applied to the analysis of the fire behaviour of panels made of flax chipboards. Thermogravimetric tests are performed on flax particles to serve for the identification of the kinetic parameters of the pyrolysis models. Once these kinetic parameters are determined, they are integrated into a complete numerical model to simulate the behaviour under fire of flax chipboards on small and large scales. The obtained trends in the predicted values indicate good agreements when compared to the measured values. The simulations show that the numerical model is capable of accurately modelling the thermomechanical transfers taking place within the material during exposure to fire.
Article
In the future, renewable energy technologies will have a significant role in catering to energy security concerns and a safe environment. Among the various renewable energy sources available, biomass has high accessibility and is considered a carbon-neutral source. Pyrolysis technology is a thermo-chemical route for converting biomass to many useful products (biochar, bio-oil, and combustible pyrolysis gases). The composition and relative product yield depend on the pyrolysis technology adopted. The present review paper evaluates various types of biomass pyrolysis. Fast pyrolysis, slow pyrolysis, and advanced pyrolysis techniques concerning different pyrolyzer reactors have been reviewed from the literature and are presented to broaden the scope of its selection and application for future studies and research. Slow pyrolysis can deliver superior ecological welfare because it provides additional bio-char yield using auger and rotary kiln reactors. Fast pyrolysis can produce bio-oil, primarily via bubbling and circulating fluidized bed reactors. Advanced pyrolysis processes have good potential to provide high prosperity for specific applications. The success of pyrolysis depends strongly on the selection of a specific reactor as a pyrolyzer based on the desired product and feedstock specifications.
Article
Reactions between char and gasifying agents are usually the controlling step and of a core role for the overall biomass gasification process due to the relatively low reaction rate. Char reactivity will be greatly affected via interacting with volatiles, such as steam, hydrocarbons, tarry compounds, and other light gas species. By taking the updraft/downdraft moving bed and fluidized bed gasifier as examples, this review discussed the effects of char generation and evolution in various reactor environments on its following reaction behaviors. The characteristics of biomass and subsequent char gasification are further examined in terms of feedstock types and their inherent inorganics. Then, the effects of operation conditions and gasifying reagents on biomass gasification are outlined mainly from the point of char production and the subsequent volatiles-char interaction. Finally, some directions and suggestions considering char conversion and utilization are addressed for the design and betterment of the biomass gasification process.
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Around 2.7 billion people worldwide have no access to clean cooking equipment, which leads to major health problems due to high emissions of unburned products (VOC, CO and soot). A top-lit updraft gasifier cookstove with forced draft was identified as the technology with the highest potential for reducing harmful emissions from incomplete combustion in simple cookstoves. The basic variant of the stove was equipped with a fan for efficient mixing of product gas with air and fired with pellets to increase the energy density of low-grade residues. The development was conducted based on water boiling test experiments for wood and rice hull pellets and targeted CFD simulations of flow, heat transfer and gas phase combustion with a comprehensive description of the reaction kinetics, which were validated by the experiments. Emphasis was put on the reduction of CO emissions as an indicator for the burnout quality of the flue gas. The optimisation was carried out in several steps, the main improvements being the design of a sufficiently large post-combustion chamber and a supply of an appropriate amount of primary air for a more stable fuel gasification. The experiments showed CO emissions <0.2 g/MJdel for wood and rice hull pellets, which corresponds to a reduction by a factor of about 15 to 20 compared to the basic forced draft stove concept. Furthermore, these values are between 5 and 10 times lower than published water boiling test results of the best available cookstove technologies and are already close to the range of automatic pellet furnaces for domestic heating, which are considered to be the benchmark for the best possible reduction of CO emissions.
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A novel integrated biorefinery system consists of (1) pyrolysis of biomass into gas, bio-oil and char; (2) bio-oil hydrodeoxygenation and hydrocracking (hydroprocessing) producing renewable jet fuel and small chain alkanes; (3) alkane steam reforming and pressure swing adsorption (PSA) producing green hydrogen and carbon monoxide; (4) mixed ionic electronic conducting membrane (MIEC) splitting high pressure superheated steam (HPSS) into green hydrogen and oxygen; and (5) combined heat and power generation (CHP) using pyrolysis gas and carbon monoxide from PSA as fuel with oxygen from MIEC, to fulfil the demand for HPSS and electricity. Comprehensive mathematical models are shown for the design simulation of the integrated system: (1) kinetic model of biomass pyrolysis at temperature 300−500 °C, (2) stoichiometric chemical reaction model of hydroprocessing, (3) renewable aviation fuel property correlations from its chemical compositions for the ASTM D7566 standard, (4) mass and energy balance analyses of the integrated biorefinery system. Economic value and overall avoided environmental and social impacts have been analysed for sustainability. The ratios of mass and energy flows between biomass, bio-oil, renewable jet fuel, CHP-fuel, char and hydrogen are 1.33:1:0.45:0.3:0.16:0.05 and 1:0.82:0.7:0.41:0.14:0.22, respectively. For 10tph bio-oil processing, the capital cost of the plant is $13.7 million, the return on investment is 19% and the cost of production of renewable jet fuel is $0.07/kg, which is lower than its market price, $0.27/kg. This production can curb 108 kt CO2 equivalent and 1.44 PJ fossil energy per annum. To enable the biorefinery simulation, user-friendly open-source TESARREC™ https://tesarrec.web.app/sustainability/bio-jet-fuel has been developed.
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Fast pyrolysis of biomass converts it mainly into bio-oil, which is incapable of being utilized directly as drop-in fuel because of high oxygen content, unstable nature, and lower heating value. The composition of bio-oil de-cides its quality, fitness for upgrading, and environmental influence. However, it is controlled by numerous essential pyrolysis reactions, which are difficult to characterize because of the multiphase thermal degradation of biomass happening in short time scales with inter-related reaction chemistry and transport effects. This review paper critically analyses the current progress on essential pyrolysis reactions, from reaction-controlled pyrolysis experiments and molecular simulations. In experiments, recently employed Frontier Micropyrolyzer, PHASR reactor, Wire mesh reactor, and Pyroprobe with the allied analytical system revealed essential pyrolysis reactions (i.e., glycosidic bond cleavage, dehydration, and successive fragmentation of C6 or C5 compounds, etc.). The effect of transport on individual pyrolysis products, especially forming bio-oil, is described using transport- controlled experiments. Besides, the role of catalysts in altering biomass pyrolysis reactions, and hence bio-oil composition, is highlighted through experimental and theoretical findings. The mechanistic insight of biomass compounds breaking (validated with experiments), with and without catalysts, is presented. Eventually, the particle level reaction-transport models capturing the inter-related effects of pyrolysis reactions (as reaction kinetics) and transport processes, under different pyrolysis conditions, are discussed. The collective information provided in this review would be beneficial for biomass pyrolysis investigators in designing operating conditions for the conversion of several biomass feedstocks into bio-oil, similar to drop-in fuel
Article
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Numerical modeling of biomass pyrolysis is becoming a cost and timesaving alternative for experimental investigations, also to predict the yield of the by-products of the entire process. . In the present study, a two-step parallel kinetic model was used to predict char yield under isothermal condition. MATLAB ODE45 function codes were employed to solve a set of differential equations that predicts the %char at varying residence times and temperatures. The code shows how the various kinetic parameters and mass of pyrolysis products were determined. Nevertheless, the algorithm used for the prediction was validated with experimental data and results from past works. At 673.15K, the numerical simulation using ODE45 function gives a char yield of 27.84%. From 573.15K to 673.15K, char yield ranges from 31.7 – 33.72% to 27.84% while experimental yield decreases from 44% to 22%. Hence, the error between algorithm prediction and experimental data from literature is -0.26 and 0.22. Again, comparing the result of the present work with the analytical method from the literature showed a good agreement.
Article
This paper reports the results of a complete kinetic study, based on thermogravimetric characterisation, to compare the performance of Nannochloropsis sp. and Tetraselmis sp. microalgae during pyrolysis and combustion with air, enriched air and oxygen. The analysis has been carried out including both the single- and multi-step approach studying the effect of different model-free methods and heating rates. In addition, the study brings together the pseudo-components (obtained by peaks deconvolution) model and master plot methodology to discriminate the kinetic model followed by the different processes with the aim to determine the kinetic triplet (activation energy, reaction order and pre-exponential factor). It results that the thermal decomposition of the microalgae cannot be represented by a single reaction mechanism for the whole conversion range, but several parallel decomposition reactions have been taken into account, and the kinetics have been assessed from each decomposition kinetic of the pseudo-components. The kinetic profiles can be interpreted as the combined effects of reaction-order (F), nucleation (A), exponential nucleation (P) and geometrical contraction (R) mechanisms.
Article
The pyrolysis of woody biomass has received extensive attention due to its importance in the thermal chemical conversion and energy utilization. Due to the complexity of actual reaction scheme, the pyrolysis of wood is generally modeled on the basis of apparent kinetic models. In this paper, six classic lumped kinetic models (Models I-VI) were applied and compared with each other to analyze their sensitivity coupled with Shuffled Complex Evolution method based on the measured thermogravimetric data. The optimized kinetic parameters were used to predict the pyrolysis process, and then verify the validity of different reaction kinetic models. The results show that the existence of second tar reaction for one-component mechanism and the refinement of reactant components for multi-component mechanism can be helpful to improve the reasonability of reaction models. Eventually, the simplest global reaction Model I and second tar reaction involved one-component Models III, IV, as well as three-component parallel reaction Model V can achieve the better compatibility of reaction kinetic models and Shuffled Complex Evolution optimization, with the acceptable efficiency to describe the pyrolysis of woody biomass.
Article
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The kinetics of wood pyrolysis into gas, tar, and char was investigated in the range of 300 to 400/degree/C at atmospheric pressure. An experimental system which facilitates the monitoring of the actual sample temperature, collection of gas and tar, and measurement of the sample weight loss as a function of time was developed. It has been found that, in the range investigated, wood decomposition into gas, tar, and char can be described by three parallel first-order reactions as suggested by F. Shafizadeh and P.P.S. Chin. The activation energies for these reactions are 88.6, 112.7 and 106.5 kJ/mol, respectively and their frequency factors defined on a mass basis are 8.61*10/sup 5/, 2.47*10/sup 8/, and 4.43*10/sup 7/ min/sup -1/. 12 refs.
Article
The pyrolysis of cellulose was studied by electrically heating in helium single strips (0.75 cm×2.5 cm) of low ash ( 9 s −1 . The correlation is slightly improved by use of a multiple-reaction model based on a set of independent parallel first-order reactions represented by a Gaussian distribution of activation energies with a mean of 37.0 kcal/mole and a standard deviation of 1.1 kcal/mole. Selected experiments at reduced pressures (0.0005 atm), elevated heating rates (10,000°C/s), or both resulted in rate constants smaller than those obtained at 400°C/s and 1 atm. The results indicate that the residence time of volatile products within the pyrolyzing cellulose matrix is extremely important in determining conversion. Suggested pathways for cellulose pyrolysis that are consistent with these findings and with much of the pertinent literature involve primary decomposition to an oxygen-rich intermediate (probably levoglucosan) which then participates in three processes to extents depending on experimental conditions: (a) direct escape from the decomposing material into the ambient gas; (b) polymerization, cross-linking and cracking to form char and (c) pyrolysis to smaller volatiles some of which inhibit the char formation in (b) or autocatalyze (c). Accordingly, char is not a primary product, and the yield of char is zero for extremely short residence times of the primary products within the matrix of the decomposing material or for conditions that permit complete inhibition of char formation by secondary pyrolysis products.
Article
The pyrolysis of cellulosic materials is studied by means of the thermal effect of the volatile products on a catalytic sensor which consists of a heated platinum wire. Tests with filter paper samples at different heating rates yield a single set of Arrhenius parameters. Tests with wood show contribution of its individual components. The technique is also used to estimate the effect of migration, condensation and regasification of the volatiles in a pyrolizing material on the apparent Arrhenius kinetics of pyrolysis. Migration of the volatiles into the cooler interior of a test specimen results in reduced gas sensor response but little change in the apparent kinetics. Deposition of the condensed volatiles on a pyrolyzing material tends to slightly reduce the rate of generation of the pyrolysis volatiles and to slightly shift the Arrhenius curve towards lower temperature. The finding does not lend positive support to the proposed explanation for the large variation in local kinetics in a thick pyrolyzing sample (observed with radiographic technique and reported in the literature) in terms of the migration and condensation of the volatiles.
Article
The temperature and surface-density histories of a radiantly heated thermally thin filter-paper sheet held freely in air were measured in order to study the dynamics of the ignition of paper. Analyses of these histories indicate that the chemically complex degradation reactions can be approximately represented for fire dynamics purposes by two competitive first-order reactions with Arrhenius kinetics as observed by Tang [3]. One of these reactions with a preexponential factor 5.9 × 106 sec−1 and an activation energy 26 kcal/gm-mole is dominant at less than about 655°K. At higher temperatures, the other reaction with a preexponential factor 1.9 × 1016 sec−1 and an activation cnergy 54 kcal/gm-mole is dominant. The heat-transfer rates to and from the test sheet were measured in order to estimate the energetics of the reactions. The data were insensitive to the small heat of the low-temperature reaction. Assuming this heat to be −88 cal/g (endothermic), based on DTA measurements of Tang and Neill [8], the heat of the high-temperature reaction is estimated to be about 444 cal/g (exothermic). An approximate formula is developed to predict the spontaneous ignition of a thermally thin sheet under known heating and cooling conditions, provided the Arrhenius kinetics and the heat of a first-order reaction in the sheet are known. Using the measured kinetics and heat of the high-temperature reaction in this formula, the results are compared with the measured data as well as with Martin's [9] ignition data.
Article
Beech sawdust, granular cellulose and sucrose-impregnated pumice have been separately decomposed in a fluidized bed of sand in an atmosphere of nitrogen at temperatures up to 400 °C. Reaction was followed by rapidly withdrawing samples of the bed and analysing for carbon content and ignition loss. The results were converted to weight loss by a correlation developed from subsidiary fixed-bed experiments and mass balances on the fluidized bed. In all cases, decomposition occurred in two stages, the primary stage giving about 85% of the total change at any temperature level in less than 10 min. The kinetics of weight loss in the first stage were approximately first-order with respect to residual weight of organic matter, while the second stage approximated to a second-order process with respect to the weight loss to be completed in reaching equilibrium. In both stages there was a marked change in the temperature dependence of the rate of decomposition of wood in the region 300–350 °C. Below this transition region the response is broadly similar to that found for sucrose in pumice and above it is similar to the behaviour of cellulose. The maximum weight loss at any temperature was a function of temperature, showing that reaction path was temperature-dependent.
Article
Kinetics of cellulose pyrolysis in nitrogen and steam at five different heating rates are presented. A single rate equation for each pyrolysis medium is discussed which provides a good engineering fit to the weight loss curves. The presence of steam in the pyrolysis medium was found to have no measurable affect on cellulose pyrolysis kinetics. The activation energy, pre-exponential factor and reaction order for nitrogen and 1 steam pyrolysis of cellulose are: 36.6 kcal/mol, 6.06 × 10sec, 0.46 and 34.2 kcal/mol, 1.67 / 109 sec, 0.51 respectively. Apparent differences in the data derived using steam rather than nitrogen as a pyrolysis medium are shown to be artifacts of heat transfer phenomenon within the TGA instrumentation used to measure rate of weight loss. Heat transfer effects observed here may explain the large discrepancies in previously reported studies of cellulose pyrolysis kinetics. Kinetic data given for steam pyrolysis are believed to be more accurate due to the more accurate measurement of sample temperature in the reactor system used for the steam experiments.
Article
A bench-scale continuous flash pyrolysis unit using a fluidized bed at atmospheric pressure has been employed to investigate conditions for maximum organic liquid yields from various biomass materials. Liquid yields for poplar-aspen were reported previously, and this work describes results for the flash pyrolysis of maple, poplar bark, bagasse, peat, wheat straw, corn stover, and a crude commercial cellulose. Organic liquid yields of 60-70% mf can be obtained from hardwoods and bagasse, and 40-50% from agricultural residues. Peat and bark with lower cellulose content give lower yields. The effects of the addition of lime and of a nickel catalyst to the fluid bed are reported also. A rough correlation exists between has content and maximum organic liquid yield, but the liquid yield correlates better with the alpha-cellulose content of the biomass. General relationships valid over all reaction conditions appear to exist among the ratios of final decomposition products also, and this correlation is demonstrated for the yields of methane and carbon monoxide.
Article
Systematic studies of the independent effects of temperature (300-1100°C), solids residence time (0-30 s), and heating rate (less than equivalent to 100-15000°C/s) on the yields, compositions, and rates of formation of products from the rapid pyrolysis of 0.0101 cm thick sheets of cellulose under 5 psig pressure of helium have been performed. The experiments mainly probe the primary decomposition of the cellulose, with contributions from post-pyrolysis reactions being confined to those occuring within and closely proximate to the sample. Temperature and sample residence time are the most important reaction conditions in determining the pyrolysis behavior, while heating rate effects are explicable in terms of their influence on these two parameters. A heavy liquid product of complex molecular composition accounted for 40 to 83 wt % of the volatiles above 400°C. Secondary cracking of this material increased with increasing residence time or temperature and was a significant pathway for producing several light gases. 9 refs.
Article
The thermal decomposition of fibrous cellulose powder from 275° to 340°C has been studied by thermogravimetry, scanning electron microscopy, krypton adsorption, and gas-chromatographic analysis of the gaseous products arising from pyrolysis in various oxidizing and inert atmospheres. The reaction kinetics fit a phase boundary model where the rate is controlled by the movement of an interface through a cylindrical particle and the principal kinetic parameters fit a compensation curve described previously for the decomposition of wood products. An explanation of the physical mechanism of pyrolysis is proposed which is consistent with the observed rate data and the structural changes observed by scanning electron microscopy.
Article
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.
Article
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.
Article
Thermal analysis and kinetic studies have shown that oxidative reactions are responsible for acceleration in the rates of weight loss and depolymerization of cellulose on pyrolysis in air at temperatures below 300°C. The oxidative reactions include production of hydroperoxide, carbonyl, and carboxyl groups, which have been investigated at lower temperatures along with the rates of depolymerization and production of carbon monoxide and carbon dioxide. The experimental results are consistent with an autoxidation mechanism involving initiation, propagation, and decomposition reactions. At temperatures above 300°, the rate of pyrolysis is essentially the same in both air and nitrogen, indicating that thermal degradation is independent of the oxidative reactions.
Article
It has been shown that the pyrolysis of cellulose at low pressure (1.5 Torr) can be described by a three reaction model. In this model, it is assumed that an “initiation reaction” leads to formation of an “active cellulose” which subsequently decomposes by two competitive first-order reactions, one yielding volatiles and the other char and a gaseous fraction. Over the temperature range of 259–341°C, the rate constants of these reactions, ki (for cellulose → “active cellulose”), kv (for “active cellulose” → “volatiles”), and kc (for “active cellulose” → char + the gaseous fraction) are given by ki = 1.7 × 1021e− (58,000/RT) min −1, kv = 1.9 × 1016e− (47,300/RT) min−1, and kc = 7.9 × 1011e− (36,600/RT) min−1, respectively.