(621f) High Purity Hydrogen Production with in-Situ Carbon Dioxide and Sulfur Capture

The water gas shift reaction (WGSR) plays a major role in increasing the hydrogen production from fossil fuels. However, the enhanced hydrogen production is limited by thermodynamic constraint posed by the equilibrium limitation of the WGSR. However, this constraint can be overcome by concurrent water-gas shift (WGS) and carbonation reactions to enhance H2 production by incessantly driving the equilibrium-limited WGSR forward and by in-situ CO2 removal from the product gas mixture. This process can effectively and economically produce a pure H2 stream by coal gasification with integrated capture of CO2 emissions, for its subsequent sequestration. From detailed thermodynamic analyses performed for fuel gas streams from typical gasifiers the optimal operating temperature range to prevent CaO hydration and to effect its carbonation is between 575 - 830 oC. While various calcium oxide precursors were tested for CO2 capture, naturally occurring limestones were unable to react completely due to pore pluggage and pore-mouth closure. However, the highly reactive mesoporous precipitated calcium carbonate (PCC) particles, synthesized by a novel wet precipitation technique using surface modifiers, can achieve up to 70 wt% capture during carbonation. Life cycle testing of the sorbent over multiple cycles of carbonation-calcination reactions showed that PCC sorbent attains a capture capacity of 40-36 wt% over 50-100 cycles, which is significantly higher than most of the other high temperature sorbents reported in literature. In contrast, naturally occurring limestone (LC) shows poor performance with a capture capacity of 20% after 50 cycles. The enhanced water gas shift reaction for H2 production with in-situ carbonation was studied using High Temperature Iron Oxide Shift (HTS) catalyst and calcium sorbents. The reactions were investigated over reaction temperatures ranging from 500-750 oC and total pressures varying from 1-20 atm. Experimental evidence clearly shows that the PCC sorbent demonstrates superior performance over that of naturally occurring limestone sorbents. Gas composition analyses show the formation of pure hydrogen stream during the initial part of the breakthrough curve, thus demonstrating the synthesis of pure hydrogen. In addition the effects of varying the steam to carbon monoxide ratios were also investigated. Finally the effects of sulfur (H2S) in the feed stream on the CO2 removal and subsequent hydrogen production were explored. The incessant removal of CO2 from the water gas shift reaction not only enhances the hydrogen production process but it also reduces the requirement for excess steam to drive the WGSR forward. Thus, operating at conditions involving near-stoichiometric steam requirements, augments the H2S removal by CaO.