Mechanical strength evolution of biomass pellet during chemical looping gasification in fluidized bed

Abstract

Due to a large particle size and a small specific surface, biomass pellet fragmentation during Chemical looping gasification (CLG) process plays a critical role in the internal devolatilization rate and its conversion. To investigate the mechanical strength evolution of biomass pellet during CLG process, a gasification reactor of bubbling fluidized bed capable of controlling the gasification time arbitrarily is established. Sawdust and rice-husk pellets with different ash content are selected as fuels. More than 720 biomass samples undergoing different gasification time ranging from 15 s to 180 s are collected at different conditions. A porous and brittle morphology of char samples is revealed to be a gradual evolutionary process from the surface to the internal structure during CLG process. Uni-axial compression test shows that the reduction of the peak compressive force for crushing the samples mainly depends on the consumption and destruction of the overall carbon structure including internal skeleton and external epidermis. The penetration of oxygen carrier through pores and/or cracks and the internal overpressure because of rapid volatiles release are the remarkable boosts to the breakage and attrition of the internal carbon skeleton. A corresponding thermal-damage model is developed to predict the mechanical strength of pellet during CLG process.