Integrated biomass torrefaction – Chemical looping combustion as a method to recover torrefaction volatiles energy

Hyphenation of biomass torrefaction to chemical looping combustion (CLC) was studied experimentally. The concept consists of burning the torrefaction volatiles using iron oxide as solid oxygen carrier whereby the CLC flue gas, mostly non-diluted CO2 and steam, is recirculated to the torrefaction reactor for heat integration. Also, being deprived of nitrogen and oxygen, the CLC flue gas allows pure CO2 to be recovered and further captured or sequestered thus setting a frame for a negative net CO2 torrefaction in co-firing plants to reduce greenhouse gas emissions. Birch wood was torrefied between 260 and 300 °C and the resulting volatiles were burned over iron oxide solid oxygen carriers between 400 and 600 °C converting up to 99% of volatile carbon into CO2. Oxygen carrier stability for burning torrefaction volatiles and oxygen depletion during CLC were assessed vis-à-vis deactivation, and CO and hydrogen production. Also, the buildup of CO2 and water vapor in the recirculating torrefaction gas and their incidence on the properties of torrefied solid product were quantified in terms of mass and energy yields, equilibrium moisture content and grindability of the torrefied birch wood.