Study On The Kinetic Characteristics And Ignition Model Of Multicomponent Pyrolysis Of Charring Combustible Materials

As one of the most destructive disasters,fire has been threatening the safety of human life.Charring solid combustibles are considered to be the most common carrier of fires,providing a continuous source of fuel for the fires,leading to increasingly intense combustion and ultimately difficult to control.Pyrolysis,as the leading process before ignition,plays a crucial role.Combustible materials generate combustible gases during the pyrolysis stage,and as the temperature increases,the gas concentration also accumulates.When a certain critical point is reached,the combustible materials will generate flames and undergo gas-phase ignition.The flames spread around and inside,exacerbating the fire.Therefore,this article conducts an in-depth study on the leading process before ignition,taking typical charring solid combustible wood as the research object,analyzing this complex process from a theoretical perspective,studying in depth the physical and chemical changes involved,as well as a series of cross coupling reactions such as radiation,convection,heat and mass transfer,revealing the pyrolysis behavior process and deducing its development laws.This paper started from the study of wood pyrolysis kinetics.Based on the multicomponent parallel reaction,the generalized kinetic parameters were solved by optimization for various heating rates to deduce the pyrolysis behavior.A numerical differential model of ignition was developed to analyze the intrinsic laws of such process in comparison with experiments.The ignition behavior of charring combustible materials was investigated from both experimental and theoretical aspects.The specific research contents are as follows:(1)The thermogravimetric experiments of sassafras wood under inert atmosphere were carried out,and the experimental data were first used to obtain the mass loss and mass loss rate at different heating rates.Based on the non-isothermal kinetic theory,the overall global reaction kinetic analysis was performed.The activation energy of the pyrolysis reaction was calculated using the model-free method,while the matching reaction function model was identified applying the model-fitting method,and then the pre-exponential factor was deduced according to the compensation effect.On this basis,a multi-component parallel reactions scheme was put forward to establish a pyrolysis reaction model for sassafras wood degradation,considering that wood is composed of three components:hemicellulose,cellulose and lignin.After that,the approximate range of each reaction kinetic parameter was calculated using the K-K method.A hybrid algorithm combining the Grey Wolf Optimization and Least Squares Fitting method was proposed to optimize the reaction kinetic parameters for multiple heating rates,and were cross-validated with experimental results.(2)Conduction of wood ignition experiments under external radiant heat flux.The surface temperature variations under external radiation and the ignition time were obtained by the temperature collection system,and the mass loss rate of wood was acquired by collecting the data of mass variations from the mass collection system.The effects of radiation heat flux and sample thickness on the ignition characteristic parameters including surface temperature,mass loss rate and ignition time were revealed.Results showed that with the increase of radiation intensity,the surface temperature of wood increases faster,the mass loss rate increases and the ignition time decreases.Meanwhile,with the increase of sample thickness,the surface temperature of wood increases slower and the ignition time increases.While the mass loss rate did not change significantly,and the relationship between mass loss rate and sample thickness was small.Such derived results could also provide as the experimental validation for the establishment of the model.(3)On the basis of thermogravimetric and radiation ignition experiments,the previous model research results were summarized,and an improved one-dimensional pyrolysis numerical partial differential equation model(PDE model)was proposed according to the Fourier heat transfer equation.Additionally,the optimized parameters from the GWO based hybrid optimization were substituted.The equations were discretized according to the energy conservation equation,initial and boundary conditions,the code program was written in MATLAB and the results were output in a final loop iteration.The ignition behavior characteristic parameters:surface temperature,ignition time,and mass loss rate,output from the model were compared with experimental data for validation.Results showed that the predicted and experimental values are in good agreement with high accuracy and consistency,which proves that the proposed improved model can be used to predict the pyrolysis and ignition process of charring combustible materials.