Biomass pellet fueled chemical looping gasification: Reaction kinetics study with a fluidized bed - thermogravimetric analysis

Abstract

Biomass pellets show the advantages of higher energy density, lower transportation and storage costs compared to powder biomass, making them attractive for future industrial applications. In this work, a fluidized bed-thermogravimetric analysis (FB-TGA) is developed. The gas-solid process for gram-scale biomass pellets under fluidization state at high temperature is characterized. The effects of gasification agents, bed material components and operating conditions on devolatilization and char gasification are investigated. The devolatilization is revealed to be a rapid phase transition process of volatiles, of which the rate is mainly affected by the intense heat and mass transfer with increasing temperature. While the char gasification is significantly slower, taking more than 600 s for all conditions, therefore be considered as the rate-control step. The hematite oxygen carrier can effectively promote the char gasification by breaking the concentration balance of gas components near the char surface. The char reaction kinetics is fitted based on the isothermal differential method. Results show that the cylindrical geometrical contraction model exhibits an excellent linear correlation. With a mass ratio of oxygen carrier/quartz sand of 1:9, the average activation energies of 130.58 kJ/mol and 104.17 kJ/mol are obtained when CO2 and steam as gasifying agents.