Hydrogen and Methane Rich Syngas From a Catalytic Coal Gasifier with in Situ Capture of H2S/CO2 and Ex Situ Regeneration of Catalysts

We present experimental results from a lab-scale, fixed-bed, catalytic, steam-coal gasifier with in situ capture of H₂S/CO₂. While the low-carbon and low-sulfur syngas leaving the gasifier could be sent directly to a gas turbine to generate electricity, the driving force for this research is generating a high hydrogen and high methane content syngas for solid oxide fuel cells (SOFCs). High methane content in the syngas significantly reduces cooling requirements for a SOFC due to the endothermic reforming of methane in the anode of the fuel cell. The catalysts used were calcium oxide and molten alkali hydroxides. These catalysts were chosen for their capability for in situ capture of acid gases. The syngas composition using either calcium oxide and molten alkali hydroxides at a CaO:C ratio of 0.5:1 or a NaOH:C ratio of 1:1, i.e. equal capture capability, was roughly 80% hydrogen and 20% methane at a gasifier temperature of 700ºC and a pressure of 2 MPa. These experiments were performed using both bituminous (medium-sulfur) and sub-bituminous (low-sulfur) coal samples. While both catalysts were equally capable of capturing acid gases, the experimental results found that molten alkali hydroxides were significantly better catalysts for coal conversion into syngas. To measure the amount and the rate of coal conversion to syngas in this lab-scale batch reactor, we measured the chemical oxygen demand of the species exiting the reactor, which was equal to the change in the chemical oxygen demand of the coal inside of the reactor. This method of measuring coal conversion is crucial for gasifiers with in situ capture of carbon species because it is not possible to do a real time carbon balance. In addition, we present experimental results on regenerating the catalysts. For the case of CaO/CaCO₃, the calcium oxide was regenerated using the energy released from the oxy-combustion of the unconverted coal in the gasifier. For the case of alkali carbonate, the alkali hydroxide can be regenerated using cation-selective polymer membranes outside of the gasifier. In both cases, the process of regenerating the catalyst produces a near-pure stream of CO₂ and H₂S/SO₃ for sequestration or conversion.