Clean coal technologies that include CO2 capture during the production of electric power, liquid fuels and hydrogen is one major thrust area. Coal gasification provides options for the conversion of syngas to liquid fuels, hydrogen or efficient power via the IGCC routes. Enhancement in the production of high purity hydrogen is limited by the thermodynamics of the water-gas shift reaction (WGSR). However, this constraint can be overcome by concurrent WGSR and carbonation (of calcium oxide) reaction to enhance H2 production. The carbonation of calcium oxide forming calcium carbonate incessantly drives the equilibrium-limited WGSR forward by removing the carbon dioxide (CO2) product from the reaction mixture. Calcium carbonate can be separately calcined to yield a pure CO2 stream for its subsequent sequestration and the calcium oxide recycled back. This calcium looping scheme not only improves the hydrogen yield and purity but also integrates a gas cleanup and CO2 management scheme in the hydrogen production process.
The proposed calcium looping scheme simplifies high purity H2 production by integrating the water gas shift reaction (WGSR) with in-situ CO2 and sulfur removal from the synthesis gas at high temperatures in a single stage reactor while eliminating the need for WGS catalyst. We have identified a high reactivity OSU patented, mesoporous calcium oxide sorbent for the in-situ capture of CO2 and H2S. The morphological properties of our high reactivity calcium sorbent (PCC) can be tailored using surface modifiers to demonstrate a high CO2 capture capacity of about 70% by weight (~700g of CO2/kg-sorbent) and a high reactivity towards H2S. Life cycle testing of the sorbent over multiple cycles of carbonation-calcination reactions shows that PCC sorbent attains a capture capacity of 40-36 wt% over 50-100 cycles, which is significantly higher than most of the high temperature sorbents reported in literature. Experiments reveal the superior performance of the PCC sorbent over naturally occurring limestone for H2 production. The integrated reaction schemes were investigated over varying pressures (1-20 bar), temperatures (550-700C), steam/carbon ratios and sulfur compositions in the feed stream for catalytic and non-catalytic operations.
The calcium looping process highlights include (1) production of a 99+% high purity H2 stream at high temperatures and pressures (2) simultaneous removal of CO2 as well as sulfur impurities (3) production of a sequestrable CO2 stream by sorbent regeneration (4) reduction in excess steam requirement (5) flexibility in carbon monoxide conversion to produce H2:CO ratios of varying compositions while reducing sulfur to minimal levels in the product gas stream (ppm to ppb levels), suitable for fuels/chemical synthesis through the Fischer Tropsch reaction.