Numerical Simulation Of Multi-components Pyrolysis And Combustion Of Lignocellulosic Biomass Based On OpenFOAM

Lignocellulosic biomass is a typical combustible solid,represented by wood.The pyrolysis and combustion research of lignocellulosic biomass have aroused wide attention due to the high forest coverage rate and wide usage of wood in the contemporary architectural structure in China.Solid combustion,as a complicated process,includes:pyrolysis under external heat source at first,followed by combustible gases,and then the produced gases catch fire under the suitable ignition condition.This process can be divided into chemical reaction kinetics and physical transport(such as heat and mass transfer),which intercoupled with each other.Actually,the lignocellulosic biomass includes four main components:hemicellulose,cellulose and lignin,as well as water.Furthermore,the pyrolysis produced gases are still various,which will take part in the following burning process.The purpose of this paper is to study the influence of multiple components on the pyrolysis and combustion of lignocellulosic biomass,closer the real situation,build up the pyrolysis and combustion models for multiple components coupled with the experiments,and then conduct the numerical simulation study based on an open-source CFD software OpenFOAM.Build up the pyrolysis and combustion models of multiple components.The pyrolysis model of multiple components is based on the Gypro pyrolysis model,and considers the four components(hemicellulose,cellulose,lignin and water)in the meantime by modeling them based on the nth parallel Arrhenius' reaction equations,the influence of char oxidation,and the produced multiple pyrolysis gases,as well as the updated gas-solid boundary conditions.The combustion model of multiple components is based on updated eddy dissipation concept in order to couple the produced multiple pyrolysis gases and consider the influence of flame radiation on pyrolysis.Eventually,a systematic multiple components solver called multiFireFOAM is completed after combining the turbulence model,radiation model and soot model.It should be noted that the chemical reaction kinetic parameters of each component have a vital influence on the simulation,so that this paper also focuses on the estimation of these parameters.It is the first time of SCE optimization algorithm to be applied to the parameter estimation based on the thermogravimetric curves,optimizing 14 parameters in the meantime,and obtaining a set of parameters adapted to various heating rates.Conduct pyrolysis and combustion experiment.The experimental results are used to verify the above-mentioned model and provide input parameters.Beech wood is applied to pyrolysis experiment based on TG-FTIR technique.The pyrolysis properties of lignocellulosic biomass are studied under TG curves of various heating rates.Furthermore,the multiple pyrolysis gases are established by the FTIR experiment,including five main components:formaldehyde,acids,methanol,phenols and CO2,which will be applied to the following combustion simulation.The FPA experimental data in nitrogen atmosphere from the references will be used to verify the pyrolysis model's effectiveness and reliability.Combustion experiment(pool fire)is based on the azeotropic phenomenon,with n-heptane and ethanol blended fuel,including four burning stages:initial development,azeotropic burning,single-component burning and decay stage.The most important stage is the azeotropic buring,in which the fuel temperature is azeotropic point and the vapor composition follows the azeotropic proportion of the blended fuel,extremely suited to verify the combustion model of multiple components.The changes of flame height and fuel temperature also prove the existence of azeotropic phenomenon.Furthermore,the temperature changes on the pool flame axis are also recorded to verify the combustion model.To consider the pyrolysis and combustion process at the same time,cone calorimeter experiment is conducted to obtain the mass loss rate and heat release rate under various radiation heat sources.Eventually,the measurement of density,moisture content,heat conductivity coefficient and combustion heat is supplemented to provide the input parameters in the simulation.Verify the pyrolysis and combustion model of multiple components and conduct the numerical simulation.Based on the optimized kinetic parameters by SCE and the FPA experimental data in nitrogen atmosphere,pyrolysis model is verified without the combustion influence.The predicted results fit well with experimental data on the solid surface temperature and mass loss rate.Based on azeotropic experimental results,combustion model is verified by the flame height and flame temperature along the pool axis.Furthermore,the pyrolysis and combustion model are coupled,and the predicted results are compared with the cone calorimeter experimental data.The predicted results fit well with experimental data of mass loss rate and heat release rate curves,especially capture the two typical experiment peaks.Eventually,the above good predicted results verify the validity of multiFireFOAM.In addition,to establish the influence of five factors(multiple reaction components,optimized reaction kinetic parameters by SCE,moisture,multiple produced pyrolysis gases and EDC combustion model)on the predicted results,further analysis is conducted.The influence of every factor on mass loss rate and heat release rate is tested separately.The results show that no factor can be ignored,so that the five factors have important effects on the combustion behavior of lignocellulosic biomass.