CaO-based chemical looping gasification of biomass for hydrogen-enriched gas production with in situ CO2 capture and tar reduction

Abstract

Steam gasification of biomass undergoes the problem of undesirable CO2 and tar formation. Calcium oxide (CaO), when added to the gasification, could play the dual role of tar reforming catalyst and CO2 sorbent, and thereby produce more hydrogen. However, the deactivation of CaO after carbonation reaction is challenging for continuous hydrogen production and economical perspective. The concept of CaO-based chemical looping gasification (CaO-CLG) plays a key role in overcoming such a challenge. This work primarily aims at studying steam gasification of biomass with the presence of CaO in a uniquely designed chemical looping gasification (CLG) system for hydrogen production with in situ CO2 capture and tar reduction. The effect of solid circulation rates on gas and tar production is studied. A comparison of CaO-CLG, sand-based chemical looping gasification (Sand-CLG) and CaO-based bubbling fluidized bed gasification (CaO-BFBG) is presented mainly focusing on gas and tar production. The maximum H2 and minimum CO2 concentrations as well as maximum H2 yields of 78%, 4.98% and 451.11ml (STP)/g of biomass, respectively, are obtained at the solid circulation rate of 1.04kg/m2s. At the same point, the maximum total gas yield was 578.38ml (STP)/g of biomass and the tar content of 2.48g/Nm3 was the lowest. 30% higher concentration of H2 and triple yield of H2 were found in CaO-CLG compared to Sand-CLG. Compared to CaO-BFBG, CaO-CLG resulted in 15% higher concentration of H2 and almost double yield of H2. Moreover, the lowest tar content of 2.48g/Nm3 was obtained for CaO-CLG while the tar content was 68.5g/Nm3 for Sand-CLG and 26.71g/Nm3 for CaO-BFBG. CO2 concentration obtained for CaO-CLG also significantly reduced by 13–17% as compared to both Sand-CLG and CaO-BFBG.