TGA and macro-TGA characterisation of biomass fuels and fuel mixtures

The thermal behaviour of selected biomass fuels and mixtures as wood, demolition wood, coffee waste and glossy paper was investigated using a thermogravimetric analyzer (TGA) and a macro-thermobalance (macro-TGA). A kinetic model, involving first-order independent parallel reactions, was applied to results obtained from pyrolysis TGA experiments. The pyrolysis rate was considered as the sum of the main biomass pseudo-components, namely cellulose, hemicellulose and lignin. Additionally, the thermal behaviour of the same fuels was investigated at combustion conditions in the TGA, including ignition behaviour. The thermogravimetric analysis showed that each single fuel had pyrolysis and combustion characteristics based on its own main pseudo-components (hemicellulose, cellulose and lignin). The pyrolysis and combustion characteristics of selected fuel mixtures and the gas composition analysis from macro-TGA experiments showed respectively quantitative and qualitative summative behaviour based on the single fuels.     

Evaluating effects of applying different kinetic models to pyrolysis modeling of fiberglass‐reinforced polymer composites

This research evaluates the effects of applying different kinetic models (KMs), developed based on thermal analysis using thermogravimetric analysis data, when used in typical 1D pyrolysis models of fiberglass‐reinforced polymer (FRP) composites. The effect of different KMs is isolated from the FRP heating by conducting pyrolysis modeling based on measured temperature gradients. Mass loss rate simulations from this pyrolysis modeling with various KMs show changes in the simulations due to applying different KM approaches are minimal in general. Pyrolysis simulations with the most complex KM are conducted at several heat flux levels. Mass loss rate comparison shows there is good overlap between simulations and the experimental data at low incident heat fluxes. Comparison shows there is poor overlap at high incident heat fluxes. These results indicate that increasing complexity of KMs to be used in pyrolysis modeling is unnecessary for these FRP samples and that the basic assumption of considering thermal decomposition of each computational cell in comprehensive pyrolysis modeling as equivalent to that in a thermogravimetric analysis experiment becomes inapplicable at depth and higher heating rates. Copyright © 2014 John Wiley & Sons, Ltd.     

Modelling decomposition and fire behaviour of small samples of a glass‐fibre‐reinforced polyester/balsa‐cored sandwich material

This publication presents the experimental and numerical methods to model the devolatilization process of a glass‐fibre‐reinforced polyester/balsa‐cored sandwich material on small scale. The fundamental modelling of the source term in pyrolysis‐based fire simulations requires as input data the thermochemical properties of solid fuel and the kinetic parameters of the devolatilization process. First, the thermal decomposition of both elements composing the sandwich structure was studied by thermogravimetry coupled with gas analysis, in air and pure nitrogen atmospheres at several heating rates, in order to define a comprehensive multi‐step reaction pathway. A differential equation system is defined to model these decomposition processes. The kinetic parameters were then estimated by solving the system of equations by an inverse problem. Second, the fire behaviour of each element was studied separately and then combined in the sandwich structure on the cone calorimeter. In addition, numerical simulations with Fire Dynamics Simulator were performed to gradually assess the ability of the model(s) to reproduce each element composing the sandwich structure. Numerical and experimental results are compared and then discussed. Overall, the model provides a good agreement with the experimental data and encourages to model higher scales. Copyright © 2012 John Wiley & Sons, Ltd.     

Modelling the combustion of solid‐phase fuels in cone calorimeter experiments

The process of ablation often forms a key part in many mathematical models describing the combustion of a solid‐phase fuel. The phrase was first used to describe the thermal erosion of glaciers over 140 years ago. In recent times it has been applied to the thermal degradation of a solid when exposed to a large flux of heat. Two fundamental assumptions in the treatment are that mass is lost only from surface regions and that the temperature of the surface remains constant throughout the period of mass loss. The two critical parameters in this model are the critical temperature T_p at which mass loss occurs and the heat required to convert matter from solid to gas at temperature T_p. In this paper we report mathematical models designed to simulate the loss of mass of a solid fuel (such as polyethylene) in cone calorimeter experiments. The models do not compromise on the thermal properties of the solid fuel or the heat loss mechanisms and consequently they require a numerical method of solution. Initially we explore a relatively simple ablation‐based model. Although conceptually simple in approach, the model reproduces many qualitative features observed in experiments. We go on to consider a different approach, where the thermal degradation of the solid is governed by a set of kinetic rate laws. We show that this approach removes many of the unrealistic features and assumptions of ablation‐based models. Furthermore, we demonstrate that the agreement between theory and experiment is improved by this approach. We also show how the mathematical models may be used as aids in the interpretation of cone calorimeter data. For example, we show that a steady state measurement of mass loss rate is a more reliable indicator of a material response than a peak measurement. In fact we show how peak measurements are strongly affected by the thermal properties of the sample holder, but steady state measurements are insensitive to the particular choice of sample holder. Copyright © 1999 John Wiley and Sons, Ltd.     

Catalytic upgrading of bio‐oils by esterification

Biomass is the term given to naturally‐produced organic matter resulting from photosynthesis, and represents the most abundant organic polymers on Earth. Consequently, there has been great interest in the potential exploitation of lignocellulosic biomass as a renewable feedstock for energy, materials and chemicals production. The energy sector has largely focused on the direct thermochemical processing of lignocellulose via pyrolysis/gasification for heat generation, and the co‐production of bio‐oils and bio‐gas which may be upgraded to produce drop‐in transportation fuels. This mini‐review describes recent advances in the design and application of solid acid catalysts for the energy efficient upgrading of pyrolysis biofuels. © 2015 Society of Chemical Industry     

Modelling wood combustion under fixed bed conditions

A computer model describing the conversion of wood under packed-bed conditions is presented. The packed bed is considered to be an arrangement of a finite number of particles, typically sized between 5 and 25 mm, with a void space left between them. Each particle is undergoing a thermal conversion process, which is described by a one-dimensional and transient model.Within the single-particle model, heating, drying, pyrolysis, gasification and combustion are considered, whereby each particle exchanges energy due to conduction and radiation with its neighbours. Because of the one-dimensional discretization of the particles, heat transfer and mass transfer is taken into account explicitly. Therefore, no macrokinetic data are needed within the model. For ease of implementation and access, kinetic data and property data are stored in a database. The global conversion of the packed bed is represented by the contributions of single particles, where each particle is coupled to the surrounding gas phase by heat and mass transfer. For gas phase flow through the porous bed, the conservation equations for mass, momentum and energy are solved on a Cartesian mesh by a Finite Volume method.Experiments have been performed to validate the single particle model for the conversion of beech wood during pyrolysis and char combustion. Agreement between experimental and predictions obtained by the model is very satisfactory. However, for wet wood, changes in structure seem to enhance the heat transfer to the solid which is not yet covered in the model.     

固体热载体法褐煤热解过程中的传质传热特性

为揭示在固定床反应器中固体热载体法快速热解褐煤工艺过程中的热、质传递机理,建立了固体热载体法褐煤热解过程中的传质传热模型。模型包括球型颗粒的一维非稳态导热方程和基于分布活化能模型的动力学模块,分别采用有限容积法与Matlab软件中遗传算法工具箱对二者进行数值计算。通过呼伦贝尔褐煤热重实验数据与温度测定实验数据分别验证了预测的动力学参数及颗粒传热模型结果。研究发现,热、质变化在固体热载体法褐煤热解工艺中呈现复杂的耦合特性。此外,考察了在不同初始温度、热载体进料比与煤颗粒半径条件下,褐煤在热解过程中颗粒内部温度场在径向上随时间的变化规律,并分析了产物释放速率与温度场的关联性,结果表明热历程改变是工艺条件对热解产物分布造成影响的根本原因。     

Evaluation of pyrolysis parameters for fiberglass reinforced polymer composites based on multi‐objective optimization

This study was conducted to investigate the ability of global, multi‐objective/variable optimization methods to estimate material parameters for comprehensive pyrolysis models—thermo‐physical and optical properties of two fiberglass reinforced polymer (FRP) composites that share the same fiberglass. With these optimization methods used in pair with a comprehensive pyrolysis model, parameter estimation was carefully conducted with considerations given to applying appropriate thermal decomposition kinetic models (three different models from simple to complex) and optimization targets (cone calorimeter data irradiated at 50 kW/m²). Estimation results are compared with independently measured effective properties—thermal conductivity, specific heat capacity, and emissivity of polymer resins and FRPs. Additionally, fiberglass properties estimated from the two FRPs are compared to analyze for consistency in optimized values. The results show that for a well‐configured parameter estimation exercise using the optimization method described earlier, (1) estimated results are within ±100% of the measurements in general and sometimes comparable to effective property values, (2) increasing complexity of the kinetic modeling for a single component system has insignificant effect on estimated values, and (3) increasing complexity of the kinetic modeling for a multiple component system with each element having different thermal characteristics has positive effect on estimated values. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical study of polyethylene burning in counterflow: Effect of pyrolysis kinetics and composition of pyrolysis products

The burning behavior of polyethylene in the counterflow of oxidizing air has been studied numerically with a coupled model describing feedback heat and mass transfer between gas‐phase flame and polymeric solid fuel. A 2‐dimensional elliptic equation in axisymmetric formulation (revealing the cylindrical shape of the polymer sample used in the experiment) has been employed to simulate heat transfer in solid fuel, and a set of 1‐dimensional hyperbolic equations has been used to determine the solid‐to‐gas conversion degree of the pyrolysis reaction. Four sets of products compositions and two modifications for the kinetic parameters of solid fuel pyrolysis reaction have been taken into account. Gas‐phase formulation is presented by set of 1‐dimensional conservation equations for multi‐component flow with detailed kinetic mechanism of combustion. The profiles of temperature and species concentrations in the flame zone have been calculated and compared with the results of experimental study of combustion of ultrahigh molecular weight polyethylene. Higher hydrocarbon composition (dodecane) has been found to show the best agreement between the temperature and species concentration profiles with the measurements, especially for the low‐level mass fractions of the by‐product components—propylene, butadiene, and benzene.     

Thermal stability and pyrolysis kinetics of lignin‐phenol‐formaldehyde resins

A thermal stability and kinetic study from non‐isothermal experiments of a commercial and a lignin‐novolac resin mixed with two amounts of curing agent has been done employing thermogravimetric analysis technique. Three kinetic models have been tested: a single heating rate method, such as Coats‐Redfern, employing several mechanistic functions and contrasted with Van Krevelen—it is the first time that this method has been employed in polymer degradation. Finally, the Ozawa method allowed the obtaining of the activation energy by the multiple‐heating‐rate without knowing the mechanism. Results show that commercial mixtures of resins lose less weight than lignin‐novolac resins. The calculated kinetic parameters showed that Coats‐Redfern gives similar results to Van Krevelen, which means that these methods are adequate for novolac pyrolysis, and Ozawa shows activation energies in accordance with the last mentioned models. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Evaluation of global biomass devolatilization kinetics in a drop tube reactor with CFD aided experiments

A procedure coupling experimental characterization and computational fluid dynamics (CFD) is developed for providing valuable global kinetic parameters to large applications of biomass fuels (fast pyrolysis, co-combustion and gasification). This is based on an advanced lab-scale apparatus (drop tube reactor), reproducing high heating rates and low residence times at different nominal temperatures (400-800 °C) for particle size of practical interest. Although the relative simplicity of the operation, a detailed and accurate evaluation of the particle residence time and effective thermal history is needed to elaborate suitable global devolatilization kinetics, which differ significantly from low heating rate kinetics (for instance in thermogravimetric balance) and also from those obtained assuming strong hypotheses (e.g. constant particle temperature in the reactor). The developed procedure gives kinetic parameters which are not the intrinsic devolatilization kinetics but global kinetics at high heating rates. These global kinetic parameters are useful to simulate practical systems (characterised by high heating rate) with comprehensive codes (CFD), since detailed particle kinetics require additional sub-models (e.g. of external and internal heat transfer) which may be time consuming and need many data, often known only with uncertainty. In this work CFD is used as both diagnostic and predictive tool; its potentials and drawbacks in aiding advanced experimentation on biomass/coal pyrolysis are discussed.     

The Use of Methane‐Containing Syngas in a Solid Oxide Fuel Cell: A Comparison of Kinetic Models and a Performance Evaluation

The nickel‐based anodes of solid oxide fuel cells (SOFCs) can catalytically reform hydrocarbons, which make natural gas, gasification syngas, etc., become potential fuels in addition to hydrogen. SR and water–gas shift (WGS) often occur inside SOFCs when operated on these fuels. Their reaction rates affect the partial pressures of hydrogen and carbon monoxide, the local temperatures and the related Nernst voltages. Consequently, the reaction rates affect the electrochemical reactions in the fuel cell. Three different kinetic models were used to characterize methane SR in a tubular SOFC; the results of each model were evaluated and compared. The polarizations of the fuel cell results of these models were validated against experimental data. The performance of a fuel cell operated with different fuels and based on a selected kinetic model was further studied in terms of the anode oxygen partial pressure, the thermo‐electrochemical distribution, and the system level performance.     

Safety evaluation of sewage‐sludge‐derived fuels by comparison with other fuels

The utility of sewage sludge as a biomass fuel is taken as a new approach to recycle unwanted wastes as renewable energy and deal with global warming. However, safety caring of this new type of fuel is a premise before it is practically used in boilers. Thermal behaviors of four sludge‐derived fuels which are under development were examined by several calorimeters (such as thermogravimetry/differential thermal analysis, C80 and thermal activity monitor) at temperature ramp and isothermal conditions. Heat generation at relatively low temperatures was observed. The corresponding spontaneous ignition was detected in an adiabatic spontaneous ignition tester at 80^°C in some sludge species. Moreover, a certain amount of gaseous evolution was accompanied when the sludge fuels were stored at room temperature and at 60^°C. Oxidation is mainly responsible for the heat and gas release from the sludge fuels. The hazards of the sewage sludge fuels were also compared with a bituminous coal and a refuse‐derived fuel, which have the main feature of spontaneous ignition. Copyright © 2009 John Wiley & Sons, Ltd.     

Modified carbon‐dioxide measurement to predict the heat release rate of fire burning in a compartment based on the three‐zone model

The heat release rate (HRR) of fuels has been described as the single important variable of fuels in fire hazard, and the HRR experimental measurement remains a key issue in fire science. A modified carbon‐dioxide generation (CDG) method, applying a three‐zone smoke model, is developed to predict the HRR of gas, liquid, and solid fuel fires. The three‐zone smoke model with three layers is determined by the vertical thermal stratification, and their physical thermal properties are computed. The application of modified method on typical gas fuel, liquid fuel, and simple solid‐fuel fires is verified. The prediction accuracy is examined quantitatively by the cosine similarity comparison of predicted results with the experimental data. In addition, the ventilation effects on the predicted results are also explored. Results show that the application of three‐zone model improves the HRR prediction accuracy, because it can accurately capture the mixing behavior from the upper layer to the lower layer. The effect of ventilation on modified CDG method is positive as the ventilation enhances the smoke mixing and the smoke distribution in each layer is relatively uniform.     

Kinetics of isothermal and non‐isothermal degradation of cellulose: model‐based and model‐free methods

BACKGROUND: The kinetics of the thermal decomposition of cellulosic materials is of interest from the viewpoint of flame retardancy for safety, optimization of incineration processes and reducing energy production from fossil sources and associated pollution. One essential step in these processes is the thermal degradation through mass and energy transport, which determines the rate of evolution of various types of products from cellulosic materials. RESULTS: Kinetic parameters have been determined using various model‐based and model‐free methods in the thermal degradation of cellulose up to 700 °C in helium atmosphere. The values of the activation energy obtained in isothermal processes and non‐isothermal processes have been found to be not far from each other. From the integral method, the random nucleation (F₁)‐type mechanism has been found most probable for cellulose degradation having an activation energy, E_a, in the range 156.5–166.5 kJ mol^?1, lnA = 20–23 min^?1, for first‐order reaction during its decomposition process at heating rates of 2, 5 and 10 °C min^?1. Based on the high correlation coefficient, many types of mechanisms seem equally good for non‐isothermal degradation of cellulose. CONCLUSION: The linear correlation coefficient has a limitation for verifying the correctness of a reaction mechanism in the study of degradation kinetics. Therefore, the correctness of a mechanism should be considered on the basis of comparing the kinetic parameters obtained from isothermal as well as non‐isothermal methods. Copyright © 2008 Society of Chemical Industry     

Analysis and modelling of wood pyrolysis

In many industrial processes wood is treated as big fragments or pellets. In such conditions kinetics and yields are different with respect to the case of particles with few mg weight. However most of published kinetic models were based on experimental data obtained with very small samples. In this work pyrolysis of wood pellets was investigated by using a special experimental device which allowed to determine kinetics of total weight loss, gas and tar production. Two different heating rates, 0.05 and 1 K/s, were employed to determine kinetic parameters. Dynamic and isothermal pyrolysis tests were carried out on beech and pine wood previously dried in an oven. A simple but realistic kinetic scheme was proposed able to take into account the phenomena that happen in big wood particles. The numerical parameters were determined from the results of experimentation on beech wood. The proposed kinetic model takes into account the presence of two different stages during pyrolysis: a first one involving only unreacted wood and a second one where the products not yet escaped from the solid matrix react further. This kinetic model allowed to fit the experimental data quite well. The model was successfully validated with tests performed at an elevated heating rate (approximately 60–100 K/s) of the external surface of the pellets. In these conditions, the pellets showed a marked gradient of temperature inside, which was suitably considered.