Evolution Of Particle Size And Morphology During Pulverized Biomass Combustion

Burning pulverized biomass in pulverized coal-fired power plants is an important technology to utilize biomass for electricity generation.However,unlike pulverized coal with the particles being approximately spherical,the morphology of pulverized biomass particles is extremely irregular,which has significant impact on particle drying,heating and reaction conversion processes during pulverized biomass combustion.Investigating the evolution of particle size and morphology during pulverized biomass combustion contributes to providing a theoretical basis for the further understanding of combustion mechanisms and ash formation processes,the guidance of the retrofit design and operation of combustion equipments,and the description of combustion processes with modeling and simulation.For this purpose,the size and morphology parameters during devolatilization and combustion of pulverized rice husk(RH),wheat straw(WS)and corn stalk(CS)with a narrowed size were investigated through devolatilization and combustion experiments on a high-temperature drop-tube furnace combined with scanning electron microscopy(SEM)and image analysis on the resulting char and ash particles.The obtained parameters from the characterization and quantification of image analysis were statistically analyzed and systematically compared to explore the particle size and morphology evolution during combustion process and the influence of biomass species,devolatilization and combustion conditions.Observations from SEM images show that the morphology of pulverized biomass particles is significantly various even for the same kind of biomass.There are more squared or rectangular flake particles in pulverized rice husk,while there are more thin strip or flake particles in pulverized wheat straw and corn stalk due to the structure characteristics of the material itself.The morphology of the three kinds of pulverized biomass particles greatly change after devolatilization due to the softening of organic matter and particle rupture.Rice husk char particles turn to curl and appear in small fragments,while wheat straw and corn straw char particles form more nearly spherical particles with the number of the nearly spherical particles increasing with the increase of devolatilization temperature.Comparing to that of char particles,the morphology of rice husk and wheat straw ash particles does not change much on the whole,while that of corn stalk ash particles changes significantly.The effects of combustion temperature and atmosphere on the morphology evolation vary with biomass species.The study and comparision of particle size parameters indicate that the particle sizes of the three kinds of pulverized biomass change in different degrees during the combustion,which is manifested in the fact that the particle size changes of pulverized wheat straw and corn stalk are larger than that of pulverized rice husk.The average width,length and equivalent diameter of the three kinds of pulverized biomass char particles all decrease obviously after devolatilization.The sizes of rice husk char particles decrease mainly in the width direction with a relative change ratio of 11.52%-22.34%,but the influence of devolatilization temperature is little,while the sizes of wheat straw and corn stalk decrease in the length direction with respective relative change ratios of 50.78-67.65% and37.59%-65.38% and the influence of devolatilization temperature is obvious.The size parameters of rice husk ash particles further decrease obviously after char combustion,the size parameters of wheat straw ash particles only decrease slightly,while the size parameters of corn stalk ash particles vary with the combustion atmosphere.The study and comparision of particle shape parameters show that the aspect ratio,circularity,and roundness of the three kinds of pulverized biomass particles change to different degrees due to particle shrinkage,fragmentation or melting during combustion,and the ratios of the changes are generally smaller than those of the size parameters.All the shape factors of pulverized rice husk decrease after devolatilzation and combustion with a relative change ratio of 0.70%-32.92%.In contrast,all the shape parameters of pulverized wheat straw and corn stalk increase,with the relative change ratio of 5.01%-41.54% and 1.07%-25.44%,respectively.The changes of devolatilization temperature,combustion temperature and atmosphere have little influence on the shape factors with no obvious trends.