Study On The Characteristics Of Biomass Entrained Flow Gasification Based On Char Intrinsic Kinetics

Biomass is a promissing alternative energy source for fossil fuel with the advantages of abundance,renewability,environmental friendliness,etc.and the development of biomass technology is of great potential.Biomass gasification is a thermochemical method for converting low-value biomass mateials into high-value gases and chemicals and it is a clean technology for biomass utilization suitable for a wide variety of applications.Therefore,the present study focusing on basic characteristics of biomass chars and the performances of biomass entrained flow gasification have both theoretical and practical significances on the acquisition of biomass principles and promotion in gasification performances.The experiments of biomass fast pyrolysis were carried out in a micro quartz reactor and the collected chars were analyzed by scanning electron microscope(SEM),energy dispersive spectrometer(EDS),and Raman microscope.The char yields of rice husk(RH),corn stalk(CS)and birche wood(BW)pyrolyzed at 800 oC and the physicochemical properties of chars were investigated,meanwhile the effects of pyrolysis temperature(700,800,900 oC)on char yield and char characteristics of rice husk were studied as well.The results showed that the char yields of three biomass materials were in the order of RH > CS > BW;the silicon concentrations in RH char and CS char were much higher than BW char;CS char had lowest uniformity in carbon structure.With increasing pyrolysis temperature,the char yield from flash pyrolysis of rice husk decreased,furthermore both the O/C ratio and graphitization degree of rice husk char decreased.The intrinsic reaction rates of biomass char-CO2 and char-O2 were investigated through conducting thermogravimetric analysis(TGA)measurements.The effects of reaction temperature and reactant partial pressure on the reaction rates were studied in order to determine the intrinsic kinetic parameters.The results obtained from this study showed that the intrinsic activation energy and intrinsic reaction order for biomass chars gasification with CO2 varied in the ranges of 160.01-238.81 k J/mol and 0.309-0.589,respectively.Pyrolysis temperature for char preparation had slight effect on the intrinsic gasification rate.For biomass char-O2 reaction,the intrinsic activation energy and intrinsic reaction order obtained in this study were in the ranges of 114.59-134.15 k J/mol and 1.431-1.981,respectively.In addition,polynomial regression model was applied to predict the intrinsic reaction rate for three biomass chars to verify the reliability of intrinsic kinetic parameters and chemical reaction model.The biomass gasification model for an entrained flow gasifier was developed with the Euler-Lagrange method including drying,pyrolysis,homogeneous reactions and heterougeneous reactions inside the gasifier.Intrinsic reaction rate submodel was established in this study to consider the effects of diffusion rate and kinetic rate for simulating the char reactions to promote the gasification model.The entrained flow gasification of rice husk and sawdust under various conditions were simulated,and the simulated results were then compared with the experimental values to testify the gasification model developed in this study.The gasification performances among rice husk,sawdust and corn stalk in an entrained flow gasifier were studied by numerical simulation,and the effects of gasification temperature,particle size,equivalence ratio and CO2/biomass mass ratio on gasification process were investigated.The results showed that under the same gasification condition,the concentration of combustible gas in produced gas from corn stalk gasification was least,correspondingly the heating value of produced gas,carbon conversion efficiency and cold gas efficiency of corn stalk gasification were least compared with rice husk and sawdust.With increasing gasification temperature,the heating values of produced gas,gas productions and cold gas efficiencies of all three materials increased.While increasing particle size,the heating values of produced gas,gas productions,carbon conversion efficiencies and cold gas efficiencies of all three materials decreased.As the equivalence ratio increased,the heating values of produced gas and cold gas efficiecies decreased.With increasing mass ratio of CO2/biomass,the CO concentrations in produced gas,heating values and carbon conversion efficiencies increased first and then decreased.Combining gasification model with response surface method(RSM),the gasification temperature,particle size,equivalence ratio and CO2/biomass mass ratio were selected as input factors,and the CO concentration,gas production,cold gas efficiency were selected as responses,then the design of experiments were applied to gasification processes of three biomass materials.The influences of input factors on the responses were analyzed according to the numerical simulations.Finally,multi-objective genetic algorithm was applied to obtain the Pareto frontiers for biomass entrained flow gasifications.