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Characteristics of steam gasification of biomass for hydrogen production at ambient pressure in a laboratory-scale external circulating concurrent moving bed (ECCMB) system with CaO as a CO2 absorbent were investigated. In this ECCMB system, steam gasification of biomass, in situ CO2 capture, combustion of the produced char, and calcination of CaCO3 can occur simultaneously. The experimental results verified that the in situ CO2 capture was achieved with calcined limestone as a CO2 absorbent, which increased the extent of the water−gas shift reaction and enhanced the yield of H2. The H2 content of 60−70 mol % in dry gas can be obtained at a steam/biomass weight ratio (S/B ratio) of 0.38–0.59 and a CO2 absorbent/biomass weight ratio (A/B ratio) of 20 at the reactor temperature of 700–800 °C. The results showed that the addition of a CaO-based CO2 absorbent is a good solution to increase hydrogen content in dry gas from the ECCMB process. An irreversible deactivation of the CO2 absorbent occurred in the cyclic carbonation/calcination because of the sinter of CO2 absorbent particles and the formation of inorganic adhesions on the surface of the particles from the solid−solid reactions between biomass ash and CO2 absorbent.

A new process of biomass gasification with a catalytic solid heat carrier is proposed that is called the ECCMB (external circulating concurrent moving bed) gasification system. The system is composed of a moving-bed gasification zone and a combustion zone. A circulation loop of bed material, which acts as catalyst and heat carrier, is achieved between these two zones. The char circulated with the bed material and the cokes deposited on the catalyst are burned off in the combustion zone to regenerate the catalyst and provide the energy for the endothermic gasification reactions with steam in the gasification zone. A lab-scale facility was established to demonstrate this process concept, where steam gasification of biomass and combustion of the produced char can occur simultaneously. The H2 content of 53.3 mol % in dry gas and the tar yield of 0.7 g/Nm3 dry gas can be obtained at a bed height of 400 mm and an S/B ratio (steam/biomass mass ratio) of 0.4 g/g at 800 °C with calcined olivine as the catalyst. The results verify that this new concept of biomass gasification for hydrogen-rich gas is feasible. Concurrent moving of biomass and catalyst in the gasification zone is a good solution that enhances the efficiency of the process and decreases the tar yield.

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