Steam–Coal Gasification Using CaO and KOH for in Situ Carbon and Sulfur Capture

We present experimental results of coal gasification with and without the addition of calcium oxide and potassium hydroxide as dual-functioning catalyst–capture agents. Using two different coal types and temperatures between 700 and 900 °C, we studied the effect of these catalyst–capture agents on (1) the syngas composition, (2) CO₂ and H₂S capture, and (3) the steam–coal gasification kinetic rate. The syngas composition from the gasifier was roughly 20% methane, 70% hydrogen, and 10% other species when a CaO/C molar ratio of 0.5 was added. We demonstrated significantly enhanced steam–coal gasification kinetic rates when adding small amounts of potassium hydroxide to coal when operating a CaO–CaCO₃ chemical looping gasification reactor. For example, the steam–coal gasification kinetic rate increased 250% when dry mixing calcium oxide at a Ca/C molar ratio of 0.5 with a sub-bituminous coal, and the kinetic rate increased 1000% when aqueously mixing calcium oxide at a Ca/C molar ratio of 0.5 along with potassium hydroxide at a K/C molar ratio of 0.06. In addition, we conducted multi-cycle studies in which CaCO₃ was calcined by heating to 900 °C to regenerate the CaO, which was then reused in repeated CaO–CaCO₃ cycles. The increased steam–coal gasification kinetics rates for both CaO and CaO + KOH persisted even when the material was reused in six cycles of gasification and calcination. The ability of CaO to capture carbon dioxide decreased roughly 2–4% per CaO–CaCO₃ cycle. We also discuss an important application of this combined gasifier–calciner to electricity generation and selling the purge stream as a precalcined feedstock to a cement kiln. In this scenario, the amount of purge stream required is fixed not by the degradation in the capture ability but rather by the requirements at the cement kiln on the amount of CaSO₄ and ash in the precalcined feedstock.