(358f) Calcium Looping Process (CLP) for High Purity Hydrogen Production From Coal: Results From Sub-Pilot Scale Carbonator Testing

CO₂ capture using calcium sorbents has wide ranging applications in post-combustion and pre-combustion systems, and has the potential of developing into a viable technology. In a typical calcium-based process, the carbonation of calcium oxide (CaO) by reaction with CO₂ from flue or fuel gas results in the formation of calcium carbonate (CaCO₃) and the reverse reaction (calcination) regenerates the CaO releasing a sequestration-ready pure stream of CO₂. Low cost of the naturally occurring CaO precursors and high temperature CO₂ capture capability make this process economically attractive for application in coal-based energy systems.

The Calcium Looping Process (CLP) is one of the clean coal technologies being developed for the production of hydrogen (H₂) and electricity from coal-derived syngas. It integrates the water-gas shift (WGS) reaction with in-situ carbon dioxide (CO₂), sulfur, and halide removal in a single-stage reactor. In the CLP, a regenerable calcium-based sorbent is used to chemically absorb CO₂, sulfur, and halide impurities from the synthesis gas during the production of hydrogen. 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. 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 sequestered.  Overall CO₂ emissions from the process are essentially zero. The regenerated sorbent is reactivated in a hydrator, to reverse the effect of sintering and improve the recyclability of the sorbent, before being reintroduced into the hydrogen production reactor – carbonator. The high temperature operation of the three CLP reactors allows the utilization of high quality heat for electricity production. This significantly reduces the energy penalty associated with CO₂ capture.

Experiments conducted in a laboratory-scale fixed bed reactor have shown that production of high purity H₂ with in-situ CO₂ and contaminant removal is possible at low steam to carbon ratios (~1:1) and moderate pressures (3-5 atm). Based on these results, a sub-pilot scale fluidized bed carbonator has been designed and constructed at The Ohio State University (OSU). Enhanced hydrogen production has been investigated using synthetic syngas mixtures in this reactor and these results will be presented. A simplified model has also been developed to predict the performance of the combined reaction in the fluidized bed carbonator. The predictions of this model are compared to the experimental results.