Evaluation of the Technical and Economic Performance of the Calcium Looping Process for CO2 and H2s Capture from Coal-Derived Synthesis Gas

The Calcium Looping Process (CLP) is being developed to reduce the cost and increase the efficiency of producing hydrogen and electricity from coal-derived synthesis gas. It integrates the water-gas shift (WGS) reaction with in-situ carbon dioxide, sulfur, and halide removal at high temperatures in a single reactor, while eliminating the need for a WGS catalyst. In the CLP, a regenerable limestone-derived sorbent (i.e., CaO or Ca(OH)2) is used to chemically absorb CO2, sulfur compounds, and halides from the synthesis gas. The removal of CO2 drives the water-gas shift reaction forward via Le Chatelier's principle, obviating the need for WGS catalyst and enabling the production of high-purity hydrogen. This hydrogen can be sold commercially or combusted in a hydrogen turbine to produce electricity (i.e., in an integrated gasification combined cycle plant). The spent sorbent is heated in a calciner to regenerate CaO for reuse in the process and to release a concentrated CO2 stream that can be dried and compressed for sequestration. The regenerated CaO is hydrated before being reintroduced into the absorber to prevent deterioration of its performance. Laboratory experiments have shown that the CLP can produce >99% purity hydrogen with less than 1 ppm sulfur impurity. The effectiveness of the hydration step in maintaining the activity of the calcium sorbent also has been experimentally demonstrated.

This paper presents a preliminary techno-economic evaluation of the Calcium Looping Process as applied to a commercial-scale coal-to-hydrogen plant with co-production of electricity. The CLP-based plant is compared with a conventional coal-to-hydrogen plant that includes a water-gas shift reactor, a two-stage Selexol unit for H2S and CO2 removal, and a pressure swing adsorber for hydrogen purification. One significant advantage of the CLP over the conventional process is that CO2 absorption occurs at high temperature (e.g., 600-700 °C), allowing the exothermic heat of reaction to be recovered for use in generating steam. Excess heat from the calciner also contributes to steam production in the CLP, as does the exothermic heat of the lime hydration reaction. As a result, the CLP plant produces significantly more net electricity than the conventional plant for a given hydrogen production rate. CO2 emissions from the CLP plant are essentially zero, because the tail gas from hydrogen production is combusted in oxygen in the calciner to produce water and sequestrable CO2. Challenges associated with the development and scale-up of the CLP will be discussed, and the effects of sulfur chemistry on the performance of the process will be explored. Estimated capital costs and fixed and variable operating and maintenance costs for the CLP will also be presented. This preliminary analysis shows that the CLP has the potential to improve the performance and cost of producing hydrogen from coal.