Experimental validation of in situ CO2 capture with CaO during the low temperature combustion of biomass in a fluidized bed reactor

A novel concept for capturing CO2 from biomass combustion using CaO as an active solid sorbent of CO2 is discussed and experimentally tested. According to the CaO/CaCO3 equilibrium, if a fuel could be burned at a sufficiently low temperature (below 700 °C) it would be possible to capture CO2 “in situ” with the CaO particles at atmospheric pressure. A subsequent step involving the regeneration of CaCO3 in a calciner operating at typical conditions of oxyfired-circulating fluidized combustion would deliver the CO2 ready for purification, compression and permanent geological storage. Several series of experiments to prove this concept have been conducted in a 30 kW interconnected fluidized bed test facility at INCAR-CSIC, made up of two interconnected circulating fluidized bed reactors, one acting as biomass combustor-carbonator and the other as air-fired calciner (which is considered to yield similar sorbent properties than those of an oxyfired calciner). CO2 capture efficiencies in dynamic tests in the combustor-carbonator reactor were measured over a wide range of operating conditions, including different superficial gas velocities, solids circulation rates, excess air above stoichiometric, and biomass type (olive pits, saw dust and pellets). Biomass combustion in air is effective at temperatures even below the 700 °C, necessary for the effective capture of CO2 by carbonation of CaO. Overall CO2 capture efficiencies in the combustor-carbonator higher than 70% can be achieved with sufficiently high solids circulation rates of CaO and solids inventories. The application of a simple reactor model for the combined combustion and CO2 capture reactions allows an efficiency factor to be obtained from the dynamic experimental test that could be valuable for scaling up purposes.