CALCIUM LOOPING PROCESSES FOR CARBON CAPTURE

A growing need for the reduction in anthropogenic CO₂ emission has led to a global push toward the development of efficient, economical, and reliable carbon capture and sequestration technologies (CCS) for application to fossil fuel based power plants. Several options are being investigated for the implementation of CCS on pre-combustion and post-combustion systems including using solvents, sorbents, membranes and chemical looping processes. The calcium looping process (CLP) which is a calcium sorbent based chemical looping process, has the potential to reduce the cost and increase the efficiency of CCS implementation.

In this study, the CLP has been investigated for pre-combustion CO₂ capture through thermodynamic analyses, experimental studies in the lab, bench and sub-pilot scales and system and economic analysis. In the CLP, a regenerable calcium-based sorbent is used to chemically absorb CO₂, sulfur, and halide impurities from syngas or hydrocarbon feedstock during the production of electricity or H₂. The removal of CO₂ drives the conversion of syngas by the water gas shift reaction and the reforming of hydrocarbons in the forward direction enabling the production of high-purity H₂. The process operates at high temperature, eliminating the need for a water gas shift catalyst and allowing the exothermic heat of the CO₂ absorption reaction to be recovered for use in generating electricity. This significantly reduces the energy penalty associated with CO₂ capture. The CLP combines H₂ production, purification and multipollutant capture in a single stage reactor reducing the overall footprint of the process. The spent sorbent consisting mostly of calcium carbonate is heated in a calciner to regenerate calcium oxide for reuse in the process and to release a concentrated CO₂ stream that can be sequestered.

Bench scale studies with a syngas feed show that high purity H₂ is produced with less than 1ppm of sulfur at high temperatures and pressures without the addition of excess steam. For a hydrocarbon feed, the steam reforming of the hydrocarbon is integrated with CO₂ capture in a single reactor. In addition to improving the conversion of hydrocarbons to H₂, the CLP also provides an efficient mode of internal heat integration where the endothermic energy for the reforming reaction is obtained from the exothermic energy released by CO₂ removal.

System analyses using ASPEN Plus have shown that the CLP has a high efficiency for conversion of both coal as well as natural gas to H₂ and electricity. Economic analysis reveals that the CLP has the potential to reduce the cost of H₂ and electricity production from coal when compared to the conventional process. A 25 KW sub-pilot scale unit is being constructed at OSU to conduct continuous testing for the production of H₂ with CO₂ capture.