Enhanced hydrogen-rich gas production from steam gasification of coal in a pressurized fluidized bed with CaO as a CO2 sorbent

Steam gasification of a typical Chinese bituminous coal for hydrogen production in a lab-scale pressurized bubbling fluidized bed with CaO as CO2 sorbent was performed over a pressure range of ambient pressure to 4 bar. The compositions of the product gases were analyzed and correlated to the gasification operating variables that affecting H2 production, such as pressure (P), mole ratio of steam to carbon ([H2O]/[C]), mole ratio of CaO to carbon ([CaO]/[C]) and temperature (T). The experimental results indicated that the H2 concentration was enhanced by raising the temperature, pressure and [H2O]/[C] under the circumstances we observed. With the presence of CaO sorbent, CO2 in the production gas was absorbed and converted to solid CaCO3, thus shifting the steam reforming of hydrocarbons and water gas shift reaction beyond the equilibrium restrictions and enhancing the H2 concentration. H2 concentration was up to 78 vol% (dry basis) under a condition of 750 °C, 4 bar, [Ca]/[C] = 1 and [H2O]/[C] = 2, while CO2 (2.7 vol%) was almost in-situ captured by the CaO sorbent. This study demonstrated that CaO could be used as a substantially excellent CO2 sorbent for the pressurized steam gasification of bituminous coal. For the gasification process with the presence of CaO, H2-rich syngas was yielded at far lower temperatures and pressures in comparison to the commercialized coal gasification technologies. SEM/EDX and gas sorption analyses of solid residues sampled after the gasification showed that the pore structure of the sorbent was recovered after the steam gasification process, which was attributed to the formation of Ca(OH)2. Additionally, a coal-CaO-H2O system was simulated with using Aspen Plus software. Calculation results showed that higher temperatures and pressures favor the H2 production within a certain range.