生物质热解机理研究进展

总结了生物质热解动力学模型的研究进展,指出了全局反应动力学模型的不足之处;由此讨论了包含具体热解产物的热解机理反应模型,介绍了近年来热解机理研究的新方法,同时列举了包含新方法的实验例子;最后指出了热解机理研究中存在的关键问题。     

Numerical simulation on biomass-pyrolysis and thermal cracking of condensable volatile component

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.     

The kinetics of vapour-phase cellulose fast pyrolysis reactions

Biomass fast pyrolysis reactions consist of primary activation and fragmentation reactions, followed by secondary vapour-phase cracking reactions. Kinetic data derived from in-house experiments and published literature have clearly indicated that under true fast pyrolysis conditions, the primary reaction rates exceed those of the secondary reactions by several orders of magnitude. Therefore, since the cracking reactions are rate-limiting, an estimation of the rate of conversion of biomass to secondary products is in fact an estimation of the secondary reaction rate. This paper focuses on the determination of the key kinetic parameters (rate constants, pre-exponential constant and activation energy) for the vapour-phase cracking reactions which occur during cellulose pyrolysis. The parameters were determined using a first-order kinetic model and a non-linear regression routine. The experimental work was conducted in the Ultrapyrolysis equipment at the University of Western Ontario in London, Canada.     

Kinetic modeling and simulation: Pyrolysis of Jatropha residue de-oiled cake

An improved kinetic model based on thermal decomposition of biomass constituents, i.e. cellulose, hemicellulose and lignin, is developed in the present study. The model considers the independent parallel reactions of order n producing volatiles and charcoal from each biomass constituent. While estimating the kinetic parameters, the order of degradation of biomass constituents is also checked and found to be matching with the order of degradation reported in the literature. The results of thermo-gravimetric analysis of Jatropha de-oiled cakes are used to find the kinetic parameters. The experimental runs are carried out using a thermo-gravimetric analyzer (TGA 4000, Perkin Elmer). TGA study is performed in a nitrogen atmosphere under non-isothermal conditions at different heating rates and the thermal decomposition profiles are used. The model is simulated using finite difference method to predict the pyrolysis rate. The corresponding parameters of the model are estimated by minimizing the square of the error between the model predicted values of residual weight fraction and the experimental data of thermogravimetry. The minimization of square of the error is performed using non-traditional optimization technique logarithmic differential evolution (LDE).     

Detailed kinetic modeling of the thermal degradation of lignins

The aim of this kinetic work is to provide a better understanding of the pyrolysis of lignin and biomasses not only in terms of devolatilazation rate but also of the volatile species released. The complexity of both lignin structure and its degradation mechanism meant that a lumping approach suitable for handling the huge amount of initial, intermediate and final products had to be used. Despite these simplifications, the proposed semi-detailed kinetic scheme involves about 100 molecular and radical species in 500 elementary and lumped reactions. It has already been proved that this lignin devolatilization model correctly predicts the degradation rates and the detail of the released products. This work constitutes an initial yet significant step towards deriving a complete kinetic scheme of biomass devolatilization.     

热解条件对生物质焦气化活性的影响及等温气化动力学参数求解方法

生物质气化是生物质利用研究的一个重点。生物质气化包含生物质的热解和热解所得焦炭的气化两个过程。不同的热解条件将得到具有不同气化活性的生物质焦炭,不同热解条件制取的焦炭的动力学参数也不相同。本文主要概述了热解条件对生物质焦气化活性的影响。同时基于阿伦尼乌斯公式介绍了生物质焦等温气化动力学参数的两种获取方法,非等转化率法是通过选择动力学模型中的结构因子f(x) 来获取动力学参数,而等转化率法是通过避开选择动力学模型中的结构因子f(x) 来获取动力学参数。基于简单碰撞理论提出了获取等温气化动力学参数的新方法,对阿伦尼乌斯公式中的指数项、指前因子A提出了明确的物理意义。基于简单碰撞理论的等温求解气化动力学参数方法类似于基于阿伦尼乌斯公式的等温求解气化动力学参数方法。     

农林生物质废弃物的热重实验研究

利用TGA对甘蔗渣、花生壳、谷壳、松木屑等4种农林废弃物进行热重实验研究,并建立了生物质热解反应的表观动力学模型.实验研究发现:甘蔗渣的DTG曲线不同于其他3种生物质的DTG曲线,它呈现出两个明显的双峰形状.对于甘蔗渣,由于其DTG曲线的双失重峰,其动力学模型在低温段的反应级数为2.5,高温段则为1.5;其他3种生物质的热解动力学模型的反应级数为2.5.     

Kinetic study of the catalytic reforming of biomass pyrolysis volatiles over a commercial Ni/Al2O3 catalyst

An original kinetic model has been proposed for the reforming of the volatiles derived from biomass fast pyrolysis over a commercial Ni/Al₂O₃ catalyst. The pyrolysis-reforming strategy consists of two in-line steps. The pyrolysis step is performed in a conical spouted bed reactor (CSBR) at 500 °C, and the catalytic steam reforming of the volatiles has been carried out in-line in a fluidized bed reactor. The reforming conditions are as follows: 600, 650 and 700 °C; catalyst mass, 0, 1.6, 3.1, 6.3, 9.4 and 12.5 g; steam/biomass ratio, 4, and; time on stream, up to 120 min. The integration of the kinetic equations has been carried out using a code developed in Matlab. The reaction scheme takes into account the individual steps of steam reforming of bio-oil oxygenated compounds, CH₄ and C₂-C₄ hydrocarbons, and the WGS reaction. Moreover, a kinetic equation for deactivation has been derived, in which the bio-oil oxygenated compounds have been considered as the main coke precursors. The kinetic model allows quantifying the effect reforming conditions (temperature, catalyst mass and time on stream) have on product distribution.     

Co-pyrolysis of textile dyeing sludge and four typical lignocellulosic biomasses: Thermal conversion characteristics,synergetic effects and reaction kinetics

In this work, the thermal conversion characteristics, interactive effects and reaction kinetics during co-pyrolysis of textile dyeing sludge (TDS) and four typical lignocellulosic biomasses (peanut vine – PV, wheat straw – WS, cotton stalks – CS and sawdust – SD) were comparatively investigated based on thermogravimetric analysis. The results indicated that the more contents of cellulose and hemicelluloses in the raw materials, the larger pyrolysis characteristic index D was. Meanwhile, the type of biomass played an important role on the interactive effect during co-pyrolysis process, which could be inhibitive and accelerative. Moreover, CS pyrolysis with the simulation ash showed that the metallic oxide in TDS ash would react with the residue carbon to increase the mass loss at the final stage. According to kinetic analysis result, the reaction mechanism of TDS, biomasses and their blends can be well predicted by the reaction order model and the diffusion models, i.e. 0, 3rd and 1-D model. The kinetic analysis also suggests TDS co-pyrolysis with biomass could reduce the theoretical activation energy for thermochemical conversion. About 50% CS content was turned out to be the optimal additive for the co-pyrolysis.     

The chemistry of chemical recycling of solid plastic waste via pyrolysis and gasification: State-of-the-art,challenges, and future directions

Chemical recycling of solid plastic waste (SPW) is a paramount opportunity to reduce marine and land pollution and to enable the incorporation of the circular economy principle in today's society. In addition to more conscious behaviors and wiser product design (“design for recycling”), a key challenge is the identification of the leading recycling technologies, minimizing the global warming potential in an industrially relevant context. Chemical recycling technologies based on pyrolysis and gasification are leading the way because of their robustness and good economics, but an improved understanding of the chemistry and more innovative reactor designs are required to realize a potential reduction of greenhouse gas emissions of more than 100 million tonnes of CO₂-eq., primarily by more efficient use of valuable natural resources. The feed flexibility of thermal processes supports the potential of pyrolysis and gasification, however, the strong variability in time and space of blending partners such as multiple and co-polymers, additives, and contaminants (such as inorganic materials) calls for accurate assessment through fundamental experiments and models. Such complex and variable mixtures are strongly sensitive to the process design and conditions: temperature, residence time, heating rates – severity, mixing level, heat and mass transfer strongly affect the thermal degradation of SPW and its selectivity to valuable products. A prerequisite in improving design and performance is the ability to model conversion profiles and product distributions based on accurate rate coefficients for the dominating reaction families established using first-principle derived transport and thermodynamic properties. These models should also help with the “design for recycling” strategy to increase recyclability, for example by identifying additives that make chemical recycling difficult. Fundamental experiments of increased quality (accuracy, integrity, validity, replicability, completeness) together with improved deterministic kinetic models, systematically developed according to the reaction classes and rate rules approach, provide insights to identify optimal process conditions. This will allow shedding some light upon the important pathways involved in the thermal degradation of the feedstock and the formation/disappearance of desired or unwanted products. In parallel, the intrinsic kinetics of the dominating elementary reaction steps should be determined with higher accuracy, moving beyond single step kinetics retrieved from thermogravimetric analysis experiments. Together with more accurate kinetic parameters, better models to account for heat and mass transfer limitations also need to be further developed, since plastic degradation involves at least three phases (solid, liquid, gas), whose interactions should be accounted for in a more rigorous way. Novel experimental approaches (e.g. detailed feedstock and product characterization using comprehensive chromatographic techniques and photoionization mass spectrometry) and available computational tools (e.g. kinetic Monte Carlo, liquid phase, and heterogeneous theoretical kinetics) are needed to tackle these problems and improve our fundamental understanding of chemical recycling of SPW.     

Kinetic study on biomass gasification (A 1991 ISES Solar World Congress honors paper)

An experimental apparatus, with the features of fast heating rate and continuous record of reaction parameters, was developed to study kinetics of fast pyrolysis. The temperature effects, at a range of 400°C to 900°C, on pyrolysis rate, products profile, gas quality and quantity, and so on, were studied and the results are listed and analyzed. The effect of secondary reaction of gas phase at 700°C was tested and the regression result is expressed in an experimental formula. Based on the experimental results, the three-stage-reaction mechanism module is suggested. The kinetic expression to calculate gas formation rate is concluded as: .The kinetic parameters of A, E, and n at different temperatures are given in the paper.     

流化床中单颗粒纤维素热解模型研究

为了研究生物质热解过程，该文对纤维素这种生物质中主要组份的流化床热解过程进行了数值模拟。模型在合理选取动力学模型的基础上考虑了单颗粒纤维素在流化床热解过程中由扩散和对流所引起的热量传递，包括了各种重要的气、液相热解产物的质量传递以及颗粒内部压力对过程的影响。计算结果显示，即使是对非常小的颗粒，热解反应热对热解过程的影响也至关重要；而无论是在大颗粒还是小颗粒中，热解液相中间产物流动对能量、质量传递的影响以及挥发份参加颗粒内二次反应的份额则可以忽略。计算还得到不同粒径颗粒热解的产物分布。总体来说，该模型为我们提供了一个探究纤维素热解细节的机会。计算结果可以为实际热解反应器的设计和运行提供依据。     

Hydrogen-rich syngas production via integrated configuration of pyrolysis and air gasification processes of various algal biomass: Process simulation and evaluation using Aspen Plus software

In the present study, hydrogen-rich syngas production via integrated configuration of pyrolysis and air gasification processes of different algal biomass is investigated at relevant industrial condition. A comprehensive steady state equilibrium simulation model is developed using Aspen Plus software, to investigate and evaluate the performance of pyrolysis and air gasification processes of different algal biomass (Algal waste, Chlorella vulgaris, Rhizoclonium sp and Spirogyra). The model can be used as a predictive tool for optimization of the gasifier performance. The developed process consists of three general stages including biomass drying, pyrolysis and gasification. The model validation using reported experimental results for pyrolysis of algal biomass indicated that the predicted results are in good agreement with experimental data. The effect of various operational parameters, such as gasifier temperature, gasifier pressure and air flow rate on the gas product composition and H₂/CO was investigated by sensitivity analysis of parameters. The achieved optimal operating condition to maximize the hydrogen and carbon monoxide production as the desirable products were as follows: gasifier temperature of 600 °C, gasifier pressure of 1 atm and air flow rate of 0.01 m³/h.     

常压及加压条件下生物质热解特性的热重研究

在常压热重分析仪和自行研制的加压热重分析仪上进行了生物质热解特性的系统研究 ,得到了升温速率、压力等因素对生物质热解过程的影响规律。对不同试验条件下的反应动力学参数进行了求解和比较 ,并作了机理分析。试验表明 :与常压热解相比 ,在加压条件下 ,生物质的反应速率有明显提高 ;随着升温速率的增加 ,热解反应趋于更加激烈。上述研究结果为生物质的合理利用提供了一定的理论基础。     

A comprehensive mathematical model of a serial composite process for biomass and coal Co-gasification

A comprehensive mathematical model to simulate a serial composite process for biomass and coal co-gasification has been built. The process is divided into combustion stage and gasification stage in the same gasifier, it is a new process for the co-gasification of biomass and coal. The model is based on reaction kinetic, hydrodynamics, mass and energy balances, it is a one-dimensional, K-L three-phase, unsteady state model. The model is divided into two sub-models, one is the combustion sub-model, the other is the coal-biomass serial gasification sub-model. Combustion sub-model includes coal pyrolysis, dense phase combustion, and dilute phase combustion model. Gasification sub-model includes biomass pyrolysis, dense phase coal gasification, dense phase biomass gasification, and dilute phase gasification model. The model studies the effects of key parameters on gasification properties, including gasification temperature, S/B, B/C, and predicts the composition of product gas and gas calorific value along the reactor's axis at different time. The model predictions agree well with experimental results and can be used to study and optimize the operation of the process.     

Current Technologies for Biomass Conversion into Chemicals and Fuels

Thermal depolymerization and decomposition of woody biomass structural components, such as cellulose, hemicelluloses and lignin, form liquids and gas products as well as a solid residue of charcoal. The direct thermo-chemical conversion technologies that have been developed are classified into two groups: high pressure liquefaction and fast pyrolysis. Bio-oils derived from biomass have been increasingly attracting attention as alternative sources of fuels and chemicals. The bio-oils were composed of a range of cyclopentanone, methoxyphenol, acetic acid, methanol, acetone, furfural, phenol, formic acid, levoglucosan, guaiocol and their alkylated phenol derivatives. The structural components of the biomass samples mainly affect pyrolytic degradation products.     

Manipulation of chemical species in bio-oil using in situ catalytic fast pyrolysis in both a bench-scale fluidized bed pyrolyzer and micropyrolyzer

In situ catalytic fast pyrolysis (CFP) of biomass was conducted with base or acid catalysts in a bench-scale fluidized bed pyrolyzer. Complete mass balances were performed, allowing for quantitatively investigating the catalytic impacts on the final bio-oil composition. Acidic catalysts exhibited relatively higher activities for decomposition of sugar and pyrolytic lignin, dehydration, decarbonylation, and coke formation, as relative to base catalysts. Carbon balances revealed that a significant amount of carbon in bio-oil was transformed to coke during CFP. Due to the decrease in the bio-oil yield during CFP, significantly less energy was recovered in CFP products than in control fast pyrolysis products. CFP was also performed in micropyrolyzer and the results were compared with those in the bench-scale reactor to determine the consistency across the experimental systems. Different from the bench-scale pyrolyzer, the basic catalyst more strongly influenced the micropyrolyzer products and the discrepancies suggest a more rapid deactivation of the basic catalyst.     

A model of wood flash pyrolysis in fluidized bed reactor

With a view of exploiting renewable biomass energy as a highly efficient and clean energy, liquid fuel from biomass pyrolysis, called bio-oil, is expected to play a major role in future energy supply. At present, fluidized bed technology appears to have maximum potential in producing high-quality bio-oil. A model of wood pyrolysis in a fluidized bed reactor has been developed. The effect of main operation parameters on wood pyrolysis product distribution was well simulated. The model shows that reaction temperature plays a major important role in wood pyrolysis. And a good agreement between experimental and theoretical results was obtained. It was shown that particles less than 500 μm could achieve a high heating-up rate to meet flash pyrolysis demand.     

Chemical kinetic modeling of hydrocarbon combustion

Chemical kinetic modeling of high temperature hydrocarbon oxidation in combustion is reviewed. First, reaction mechanisms for specific fuels are discussed, with emphasis on the hierarchical structure of reaction mechanisms for complex fuels. The concept of a comprehensive mechanism is developed, requiring model validation by comparison with data from a wide range of experimental regimes. Fuels of increasing complexity from hydrogen to n-butane are described in detail, and further extensions of the general approach to other fuels are discussed.Kinetic modification to fuel oxidation kinetics is considered, including both inhibition and promotion of combustion. Simplified kinetic models are then described by comparing their features with those of detailed kinetic models. Finally, application of kinetic models to study real combustions systems are presented, beginning with purely kinetic-thermodynamic applications, in which transport effects such as diffusion of heat and mass can be neglected, such as shock tubes, detonations, plug flow reactors, and stirred reactors. Laminar flames and the coupling between diffusive transport and chemical kinetics are then described, together with applications of laminar flame models to practical combustion problems.