Carbon Capture by Sorption-Enhanced Water−Gas Shift Reaction Process using Hydrotalcite-Based Material

A novel route for precombustion decarbonization is the sorption-enhanced water−gas shift (SEWGS) process. In this process carbon dioxide is removed from a synthesis gas at elevated temperature by adsorption. Simultaneously, carbon monoxide is converted to carbon dioxide by the water−gas shift reaction. The periodic adsorption and desorption of carbon dioxide is induced by a pressure swing cycle, and the cyclic capacity can be amplified by purging with steam. From previous studies is it known that for SEWGS applications, hydrotalcite-based materials are particularly attractive as sorbent, and commercial high-temperature shift catalysts can be used for the conversion of carbon monoxide. Tablets of a potassium promoted hydrotalcite-based material are characterized in both breakthrough and cyclic experiments in a 2 m tall fixed-bed reactor. When exposed to a mixture of carbon dioxide, steam, and nitrogen at 400 °C, the material shows a breakthrough capacity of 1.4 mmol/g. The sharp adsorption front is accompanied by an exotherm that travels along the bed. Even after breakthrough carbon dioxide continues to be taken up by the bed albeit at a much lower rate. It is shown that the total capacity of this material can exceed 10 mmol/g, which has not been reported before. Desorption curves indicate efficiencies of removing additional carbon dioxide by purging with low-pressure, superheated steam at various flow rates. During cyclic operation for more than 1400 adsorption and desorption cycles, the carbon dioxide slip is very low and remains stable which indicates that carbon recoveries well above 90% can be obtained. The sorbent shows a stable cyclic capacity of 0.66 mmol/g. In subsequent experiments the material was mixed with tablets of promoted iron−chromium shift catalyst and exposed to a mixture of carbon dioxide, carbon monoxide, steam, hydrogen, and nitrogen. It is demonstrated that carbon monoxide conversion can be enhanced to 100% in the presence of a carbon dioxide sorbent. At breakthrough, carbon monoxide and carbon dioxide simultaneously appear at the end of the bed. During more than 300 cycles of adsorption/reaction and desorption, the capture rate, and carbon monoxide conversion are confirmed to be stable. Two different cycle types are investigated: one cycle with a CO 2 rinse step and one cycle with a steam rinse step. The performance of both SEWGS cycles are discussed. These experimental results will allow optimization of process conditions and cycle parameters, and especially the reduction of steam consumption needed for sorbent regeneration. The results will provide the basis for scale-up to a pilot unit, which will demonstrate precombustion decarbonization in fossil-fuels-based power generation or hydrogen production.