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 H-2. The H-2 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 degrees 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.