Kinetics of reactions involved in pvrolvsis of cellulose II. The modified kilzer-bioid model

At temperatures below 300°C, it has been shown that the pyrolysis of cellulose can be modeled in terms of a modified Kilzer-Broido ((K-B) Model. According to this model, cellulose decomposes via two competitive reactions, a dehydration reaction to form anhydrocellulose and a depolymerization reaction to form levoglucosan. Anhydrocellulose later decomposes to chars and gaseous products via two competitive reactions. Arrhenius parameters and a technique of estimating empirical constants from experimental data for the K-B type reactions are presented. Comparison of the ability of the modified K-B model and the three reaction model to predict weight loss, product yields and heat pretreated results are also made.     

Structural relationships between kerogens and their pyrolysis products determined with a wire-mesh reactor

Two samples, of different geological age, of each of the three major types of kerogen were pyrolysed in a wire-mesh reactor, allowing tars to be recovered quantitatively under conditions minimizing extraparticle secondary reactions. To gain improved understanding of the reactions leading from kerogens to oil, the structures of the kerogens and their pyrolysis tars were compared by FT-i.r. spectroscopy. Tars were also characterized by size exclusion and planar chromatography. For each type of kerogen, the geologically younger sample gave the greater oil and total volatile yields. The aliphatic hydrogen contents of the kerogen samples were correlated with the oil yields from the wire-mesh reactor. The infrared spectra showed that the tars had substantially higher aliphatic hydrogen and slightly higher aromatic hydrogen contents than those of the original kerogens. Concentrations of phenolic-OH and carbonyl groups were also somewhat higher than in the original kerogen, at the expense of etheric structures, which tend to be present in very low concentrations in kerogen tars. Initial results indicate that a more comprehensive treatment needs to be formulated than that provided by conventional elemental analysis in predicting hydrocarbon production from kerogens.     

Structural evolution of chars from biomass components pyrolysis in a xenon lamp radiation reactor

The structural evolution of the chars from pyrolysis of biomass components(cellulose, hemicellulose and lignin)in a xenon lamp radiation reactor was investigated. The elemental composition analysis showed that the C content increased at the expense of H and O contents during the chars formation. The values of ΔH/C/ΔO/Cfor the formation of cellulose and hemicellulose chars were close to 2, indicating that dehydration was the dominant reaction. Meanwhile, the value was more than 3 for lignin char formation, suggesting that the occurrence of demethoxylation was prevalent. FTIR and XRD analyses further disclosed that the cellulose pyrolysis needed to break down the stable crystal structure prior to the severe depolymerization. As for hemicellulose and lignin pyrolysis, the weak branches and linkages decomposed firstly, followed by the major decomposition. After the devolatilization at the main pyrolysis stage, the three components encountered a slow carbonization process to form condensed aromatic chars. The SEM results showed that the three components underwent different devolatilization behaviors, which induced various surface morphologies of the chars.     

纤维素热解机理研究进展：以中间态纤维素为核心的纤维素演变

纤维素热解的机理研究对于生物质能的热利用至关重要,能够有效指导工业实际应用。基于著名的Broido-Shafizadeh模型,纤维素热解被分为两步,首先转变为活性的熔融态中间体（中间态纤维素）,然后通过解聚和开环生成左旋葡聚糖、5-羟甲基糠醛、羟基乙醛等重要的化工原料。在这两步转变中,主要涉及低温段氢键网络的断裂、中间态纤维素的生成,以及高温段的解聚和吡喃环开环反应。本文从这3个部分对前人的研究进行了综述,着重介绍了中间态纤维素的生成和表征,综述了纤维素热解几个研究方向：结晶度和结晶形态对热解的影响、纤维素解聚反应方式、吡喃环开环方式等,详细阐述了二次反应对纤维素热解的影响,并提出了部分解决方案。关于纤维素热解依然存在诸多未知和争论,需要进一步的实验研究和理论计算对其进行揭示。     

A comparative reactivity and kinetic study on the combustion of coal-biomass char blends

The combustion behavior and kinetics of various biomass chars, a lignite and a hard coal char and their blends were investigated. Pure fuel chars were compared to blended chars with respect to their performance during combustion. Non-isothermal thermogravimetry experiments were performed in air atmosphere, over a temperature range of 25-850 °C and at a heating rate of 10 °C/min. Kinetic evaluation was performed using a power law model. Reaction kinetic parameters were obtained by modeling the combustion of biomass and coal chars as a single reaction, with the exception of lignite and olive kernel chars, the combustion of which was modeled by two partial reactions. A single reaction model was used in the case of coal-wood char blends, while for the lignite-biomass char blends two partial reactions were used. Reactivity was assessed using the specific reaction rate, as a function of conversion. Biomass chars were generally more reactive than those of hard coal and lignite. The combustion behavior of the blends was greatly influenced by the rank of each coal (hard coal or lignite) and the proportion of each component in the blend. Combustion performance of the blends showed some deviation from the expected weighted average of the constituent chars. An attempt was made to estimate the kinetics of the blends using, as a basis, the parameters estimated for the individual components. In this case, because of the interactions between the components of the blends, the kinetic parameters needed to be slightly modified. Alteration in reactivity was more pronounced in the case of lignite-biomass chars than coal-wood chars.     

Evolution prediction of coal-nitrogen in high pressure pyrolysis processes

A pyrolysis model which can describe the effects of pressure on the evolution of coal-nitrogen has been constructed based on the FLASHCHAIN^® model in order to relate the gas release mechanism under high pressure conditions to the polymer reactions in coal. Various kinds of nitrogen-containing gaseous species in the evolved volatiles and their secondary decomposition and coupling processes have been also clarified by considering the elementary reactions of pyrrole-type nitrogen as the primary type of bound nitrogen in the first evoluted heavy species (tar vapor). The results show that the recombination reactions of metaplast are activated in a coal by the increase in pressure, resulting in a lesser amount of tar vapor and more intermediate chars. Thus, the conversion ratio of coal-nitrogen to gaseous volatile-nitrogen increases with the increase of pressure and N-gas converted from the fuel-N is much larger than the tar-N, and becomes more significant in high pressure conditions. Due to the chemical kinetics of the gas phase reactions, a shift in the distributions of tar-N vapor and gas-N with the increase of pressure can be predicted, and larger amounts of H₂CCHCN and bipyrrole gases are rapidly formed through three-body reactions, while HCN gas decreases greatly compared with the reaction at normal pressures. The changes of gas composition are in close agreement with the experimental results.     

Influence of inorganic additives on oxygen chemisorption on cellulosic chars

Chemisorption of oxygen on cellulosic chars is the initial step leading to gasification and is a significant factor in controlling chemical reactivity and heat release in smoldering and glowing combustion of cellulose. Oxygen chemisorption kinetics have been determined for chars (HTT 550°C) prepared from cellulose and cellulose treated with inorganic additives. Elovich kinetic analysis indicates that combustion behavior can be correlated with chemisorption kinetics. Addition of the same inorganic additives by grinding with pure cellulose chars had little or no effect on chemisorption kinetics. These data indicate that the mode of action on inorganic additives in enhancing or inhibiting the solid phase combustion of cellulose chars involves their influence on char functionality developed during pyrolysis. Chemisorption of oxygen on chars results in a decrease in free radical concentration, and heat treatment at 400°C in flowing nitrogen restores the original concentration. However, free radical concentrations do not differ significantly between additive treatments over most of the temperature range studied. Therefore, combustion behavior cannot be explained strictly in terms of changes in free radical concentration and other functional groups must also play a significant role.     

Kinetic analysis of NO-Char reaction

Two Chinese coals were used to prepare chars in a flat flame flow reactor which can simulate the temperature and gas composition of a real pulverized coal combustion environment. Acid treatment on the YB and SH chars was applied to obtain demineralized chars. Kinetic characterization of NO-char reaction was performed by isothermal thermogravimetry in the temperature range of 973–1,573 K. Presence of catalytic metal matter can increase the reactivity of chars with NO, which indicates that the catalytic effects of inherent mineral matter play a significant role in the NO-char reaction. The discrete random pore model was applied to describe the NO-char reactions and obtain the intrinsic kinetics. The model can predict the data for all the chars at various temperatures well, but underestimate the reaction rates at high carbon conversions for the raw YB and SH chars, which can be attributed to the accumulation of metal catalyst on char surface. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

基于不同三组分模型解析生物质热解过程

三组分模型（three pseudocomponent model）通常被用来表征生物质热解过程。传统三组分模型中单个模型的反应级数被限定为1或3。在本研究过程中，利用非线性最小二乘法，在不限定反应级数的前提下回归三组分模型动力学参数（活化能、指前因子、反应级数）。通过研究发现，纤维素（cellulose）分解反应级数接近1，与前人结果相一致。木质素（lignin）分解级数与生物质种类有关，接近于1或3。半纤维素（hemicellulose）的分解过程最复杂，其反应级数在1.5～4之间变化。以Ozawa方法计算得到的活化能作为相对标准，对3种三组分模型进行比较，发现反应级数未确定时的模型比其他两种模型更精确地表征生物质热解过程。     

Calcium catalysis in air gasification of cellulosic chars

Cellulosic chars containing calcium have been prepared from carboxymethyl cellulose in the calcium form and from pure cellulose containing sorbed calcium acetate, at several heat treatment temperatures (HTTs) in the range 400–900°C. The chars have been gasified in air at 300°C. The results indicated a general decrease in reactivity with increasing HTT. However, instead of a monotonic decrease in reactivity reported previously for coal chars containing indigenous or added calcium, the reactivity versus HTT curve consisted of three distinct regions which probably reflect the transformations being undergone by the catalyst species on increasing the HTT. Crystalline CaO was detected by XRD only in chars heated to 1000°C, at which temperature the catalyst was no longer effective. The relationships between gasification rate and catalyst concentration and also mode of addition have been determined using chars of HTT 600°C. As found by earlier workers with some coal chars, the rate reached a maximum with increasing calcium content and then declined. Chars containing sorbed calcium showed a relatively early decline of rate. They also showed less reactivity than the chars containing ion-exchanged calcium at all calcium concentrations.     

Modelling of the pyrolysis of biomass particles. Studies on kinetics,thermal and heat transfer effects

The present work provides a rationally-based model to describe the pyrolysis of a single solid particle of biomass. As the phenomena governing the pyrolysis of a biomass particle are both chemical (primary and secondary reactions) and physical (mainly heat transfer phenomena), the presented model couples heat transport with chemical kinetics. The thermal properties included in the model are considered to be linear functions of temperature and conversion, and have been estimated from literature data or by fitting the model with experimental data. The heat of reaction has been found to be represented by two values: one endothermic, which prevails at low conversions and the other exothermic, which prevails at high conversions. Pyrolysis phenomena have been simulated by a scheme consisting of two parallel reactions and a third reaction for the secondary interactions between charcoal and volatiles. The model predictions are in agreement with experimental data regarding temperature and mass-loss histories of biomass particles over a wide range of pyrolysis conditions.     

Application of CFD to model fast pyrolysis of biomass

The article deals with the CFD modelling of fast pyrolysis of biomass in an Entrained Flow Reactor (EFR). The Lagrangian approach is adopted for the particle tracking, while the flow of the inert gas is treated with the standard Eulerian method for gases. The model includes the thermal degradation of biomass to char with simultaneous evolution of gases and tars from a discrete biomass particle. The chemical reactions are represented using a two-stage, semi-global model. The radial distribution of the pyrolysis products is predicted as well as their effect on the particle properties. The convective heat transfer to the surface of the particle is computed using the Ranz-Marshall correlation.     

纤维素热裂解反应机理数值模拟

A detailed mechanism analysis of cellulose pyrolysis was carried out according to the previous experimental results. On the basis of the Brodio-Shafizadeh model, a modified two-stage model was proposed to simulate the formation and decomposition of active cellulose (AC) and several main organic compounds, such as levoglucosan (LG), hydroxyl-acetaldehyde (HAA), acetol and furfural etc. During pryolysis, the temperature rise of cellulose can be divided into three stages. In the second stage, cellulose undergoes a main decomposition process in which the reaction temperature remains rather low because of the endothermic cracking of glucosidic bond of AC during the formation of LG. The components density of bio-oil, including LG and other competitive compounds, increased rapidly with the increase of temperature during the first stage. However, in the main decomposition process, LG density in bio-oil had an obvious decrease, while the competitive products appeared to increase gradually, which means the ring-opening and reforming reaction of pyranoid ring are superior to LG formation in high temperature.The secondary reaction of volatile components occurs largely in gaseous phase rather than in the solid phase. Short residence time of volatile materials in high temperature region will be advantageous to a high production of LG,which may otherwise decompose quickly under high temperature. An optimum yield of LG could be obtained when radiant source temperature is in the range of 730---920K and gas residence time is less than 1 s. In addition, the reaction temperature has a stronger effect than gas residence time on the formation of HAA, acetol, formaldehyde and furfural etc.     

The significance of heating rate on char yield and char properties in the pyrolysis of cellulose

The carbonization of powdered cellulose was investigated in the temperature range 200–950°C by measuring weight loss, carbon and hydrogen content, BET-adsorption of nitrogen and carbon dioxide, mercury penetration and particle-size distribution. Evidence is presented in support of a kinetic model according to which cellulose decomposition is controlled by dehydration at low temperature and by cleavage/scission at high temperature. Increased char yield and lower $\text{O}\text{C}$ ratio at low heating rate, as well as kinetic investigations into the effect of potential catalysts, support this model. The difference in reaction mechanism according to the heating rate appeared to influence the char properties considerably. Yield in micropore volume and surface area of slowly carbonized cellulose is up to four times larger than that of rapidly heated cellulose. Mercury pore volume, density and particle diameter depend on the heating rate, also. By adsorption of various gases, differences in relative size of the pore openings of different chars can be discerned. Micropore volumes measured with carbon dioxide were as much as seventy times larger than the corresponding volume measured with nitrogen. Thus, it is possible to obtain chars with molecular sieve properties by simple pyrolysis heating schemes.     

Effect of oxidation on the chemical nature and distribution of low-temperature pyrolysis products from bituminous coal

Two bituminous coals, a high volatile Eastern Canadian and a medium volatile Western Canadian, were used to investigate the effect of oxidation on yields and chemical composition of gases, liquids and chars produced during coal pyrolysis. Pyrolysis experiments were performed at 500 °C using the Fischer assay method. Mild oxidation of coals resulted in a decrease of liquid hydrocarbon yields. Further coal oxidation increased the proportion of aromatic carbon in liquid products as determined by n.m.r., and also increased the content of oxygen in liquid products. The content of oxygen in chars was markedly lower than in corresponding coals. An attempt is made to explain reactions occurring during oxidation and subsequent pyrolysis of coal on the basis of differences in chemical composition of gases, liquids and chars.     

包含反应阶数变化的分形煤焦颗粒燃烧模型的建立与实验验证

为了深入研究煤焦燃烧的机理并提高对煤焦燃烧过程的预测精度,建立了一个综合的煤焦燃烧模型。该模型考虑了煤焦颗粒孔隙内二次反应与扩散的耦合作用、煤焦燃烧反应阶数的变化和反应过程中CO/CO₂比例等问题。使用热天平(TGA)对11种煤焦的燃烧特性进行分析,测得各种煤焦的表观活化能与指前因子,以确定模型中的待定参数。在沉降炉(DTF)中对这11种煤焦做燃烧实验,用TGA基于灰分守恒测得DTF炉管出口处的煤焦样品的转化率。运用建立的模型模拟这些煤焦的燃烧过程,预测的转化率与实验结果有较好的吻合度,相比传统的本征动力学模型,该模型预测的精度有了较大提高,证明了该模型能适用于从褐煤到无烟煤的较广煤焦范围。研究还发现,煤焦燃烧的表观反应阶数在燃烧过程中不断减小并最终趋于稳定。     

Microwave effect on kinetics of paper cups pyrolysis

Not only are microwaves notorious in food heating, but they exhibit interesting properties in different domains including chemical engineering. Their ability to concentrate heat transfer inside dielectric materials enhances process efficiency and permits high heating rates. Nonetheless, their effect on reactions is still controversial. While some researchers believe in non-thermal effects due to the efficient conversion of microwave energy to enhance reactions without heat dissipation, others assert that microwave frequencies are not high enough to excite molecular bonds. In this study, paper cups pyrolysis is achieved in electrical and microwave TGA using two heating modes. The effect of microwaves on the kinetics of paper cups pyrolysis is shown to depend on the heating regime: at a moderate heating rate, microwave pyrolysis started at a lower temperature, while the pyrolysis in the electrical and microwave TGA have similar kinetic parameters at high heating conditions. This difference is linked with reaction mechanisms. At moderate heating conditions, cellulose decomposes first to an intermediate compound then to final products. The intermediate has a short reaction time and interacts with the microwave. Hence, hot spots at the molecular scale are generated in a short amount of time below the detectable limits of existing temperature measurements media. At a high heating rate, the decomposition of cellulose is direct and no effect is observed.     

Chemisorption of oxygen on cellulose char

Investigation of a series of chars prepared by rapid pyrolysis of cellulose in the temperature range of 400–800°C has shown that they have a high chemisorptive affinity for oxygen. Maximum chemisorption occured on chars prepared at a HTT of 550°C. The Elovich equation was used to describe the kinetics of the process. The extent of chemisorption decreased with increasing HTT of the chars, although the surface area of the chars stayed approximately constant; indicating the presence of less reactive areas on the surface of chars formed at higher temperatures. As chemisorption progressed there was a corresponding increase in the intensity of several IR absorption bands, which were attributed to the formation of stable oxygen-containing functional groups. The chemisorption process, preceded by physical adsorption, does not influence the gasification reaction. The presence of impurities from pre-pyrolysis doping of cellulose could promote or inhibit the rate of gasification but had negligible effect on the initial rate of chemisorption. The role of these two processes in char combustion was discussed in the light of known concepts for the carbon-oxygen reaction.     

Kinetics of CO2/Char gasification at elevated temperatures: Part I: Experimental results

The gasification kinetics of char has so far been mainly studied based on data measured at low temperatures and low heating rates with a thermo-gravimetric analyzer. The results cannot be directly applied to high temperature gasifiers such as entrained flow gasifiers. In this work, gasification of seven types of chars in CO₂ at elevated temperatures and high heating rates was investigated with a uniquely made fluidized bed. It was found that the reaction rates for various chars were very different in low and high temperature ranges, and two orders of magnitude more pronounced in the lower temperature range. From 1273 K to 1673 K, all chars demonstrated a strong tendency to increase reaction rate with temperature. However, at a high temperature range (1773 to 1873 K), different chars demonstrated different temperature dependences. The seven types of chars studied can be roughly separated into three groups based on ash fusion temperature. Each group demonstrated a different temperature dependence at a high temperature range. For chars with low ash fusion temperatures, the reaction rate leveled off, or even decreased a little as temperature increased, which was presumed to be because of the ash fusion at elevated temperatures. These results suggest that a high temperature does not necessarily raise the gasification rate.     

Chemical nature of pre-asphaltenes from flash pyrolysis tars and supercritical gas extracts

Pre-asphaltenes from flash pyrolysis tars of three Australian coals and a supercritical gas extract of one of these coals were studied by ¹³C and ¹H n.m.r. of the silylated pre-asphaltenes. Further information was obtained by hydrogenolysis of one of the pre-asphaltenes followed by g.c. analysis. ¹³C n.m.r. studies of the silylated derivatives and g.c. analysis of the hydrogenated pre-asphaltene showed the presence of long alkyl chains. The aromaticity of the pre-asphaltenes from the flash pyrolysis tars increased as the rank of the coal increased. The pre-asphaltene from the supercritical gas extract was less aromatic than that from the flash pyrolysis tar of the same coal. Average structural data for each of the pre-asphaltenes are reported.