Pressurized In Situ CO2 Capture from Biomass Combustion via the Calcium Looping Process in a Spout-Fluidized-Bed Reactor

Biomass combustion with in situ CO2 capture via the calcium looping cycle is a novel process for the production of low- (or even negative-) carbon heat and power. Both processes can take place in the same unit vessel because of their compatible operating temperatures (600-700 degrees C). Combining the two process steps is beneficial in terms of reduced number of unit operations and process complexity. However, biomass combustion at this lower temperature range can result in the production of tar. In addition to reduced combustion efficiency, the presence of tar can lead to the blockages and fouling of downstream process equipment and loss of sorbent reactivity because of coking. Higher temperatures are known to increase tar conversion; however, this is detrimental to the rate of carbonation. Pressurizing the process allows higher CO2 partial pressures to be achieved and therefore alleviates the thermodynamic limitations on the process. Our work employed a novel high-temperature, pressurized fluidized-bed reactor to investigate the influence of temperature, pressure, and the presence of CaO on biomass combustion and tar yield. Higher operating pressures and temperatures or the addition of CaO were found to significantly reduce the gravimetric tar yield. However, the extent of CO2 capture appeared to have been limited by rapid combustion kinetics at the higher end of the investigated temperature and O-2 partial pressure ranges. Size exclusion chromatography and ultraviolet fluorescence analysis of the product tars helped to provide insights into tar production and destruction pathways under different conditions.