Primary Fragmentation Behavior of Indian Coals and Biomass during Chemical Looping Combustion

Abstract

Devolatilization and fragmentation are important physical phenomena occurring during solid fuel chemical looping combustion (CLC). Primary fragmentation during devolatilization strongly affects the rate of fuel conversion, emissions, and fine particulates generation in a fuel reactor of a fluidized bed CLC unit, thus forming a critical design input. The present study focuses on investigating the primary fragmentation behavior of large coal and biomass (wood) particles during the devolatilization phase of CLC. Three types of coals (two Indian coals, one Indonesian coal) and one type of Casuarina wood of three sizes in the range of 8–25 mm, at different fuel reactor bed temperatures (800, 875, and 950 °C) are studied for primary fragmentation. Iron ore with 64% Fe is used as the oxygen carrier bed material, with steam as the fluidizing medium in the fuel reactor. The fragmentation behavior is expressed in terms of the number of fragments, fragmentation index, frequency of fragmentation, and particle size distribution of fragments at different residence times of coal during devolatilization in the fuel reactor. Under the conditions of study, the number of fragments increases with an increase in particle size and temperature, for all fuels studied. Also, it is found that the number of fragments increases with the decrease in compressive strength of both coal and biomass particles. The Indian coals are found to fragment in the earlier stages of devolatilization, while the Indonesian coal and the biomass particles begin to fragment in the later stages of devolatilization. The maximum fragmentation index is found with Indian coal - IC1, which has the highest fixed carbon content among the fuels studied, and the least value is observed in biomass. Different modes of fragmentation exhibited by each fuel type is discussed. Indian coals do not show any volumetric changes as such, whereas Indonesian coal indicates some degree of volumetric expansion.

Chemical Looping for Combustion of Solid Biomass: A Review

“…Primary fragmentation affects the rate of fuel conversion, emissions, and fine particulates generation in an FR, thus constituting a critical design input. Two specific works were focused on the investigation of the char conversion rate and the attrition of various fuel particles under fluidized bed CLC conditions. , Fragmentation of fuel particles immediately after the introduction into a combustion environment was attributed to the thermal shock created by the temperature gradient, which often results in a large number of fragments. During the later stages of devolatilization, fragmentation occurred due to the mechanical stresses created by the evolution of volatile matter at high temperatures.…”

“…Fragmentation pathway of biomass particles [Reproduced from ref . Copyright 2018 American Chemical Society].…”