Simultaneous Water-Gas Shift Reaction and Carbon Dioxide Separation for Direct Hydrogen Production From Synthesis Gas.

Abstract

The simultaneous water-gas shift and CaO-carbonation reactions for the direct hydrogen production from synthesis gas have been studied at high temperature and high pressure using a laboratory-scale fixed-bed reactor. Experiments were conducted to evaluate the effect of process parameters--temperature, pressure, gas composition, and space velocity--on system performance during both calcination and shift/carbonation reaction stages. Commercial dolomite was chosen as the primary sorbent while two limestones were tested for comparison. Multicycle tests were conducted to evaluate sorbent durability. Sorbent structure and structural changes associated with reaction were measured using mercury porosimeter. The experiments proved the technical feasibility of combining the shift and carbonation reactions for hydrogen production. Calcination can be conducted over a temperature range of 750$\sp\circ$C to 900$\sp\circ$C under either N$\sb2$ or a mixture of N$\sb2$ and steam. Equilibrium for the combined shift and carbonation reactions can be closely approached at 15 atm, in the temperature range of 500$\sp\circ$C to 650$\sp\circ$C, and at space velocity as high as 3400 hr$\sp{-1}$ (STP) using simulated coal gas feed having a H$\sb2$O to CO ratio as low as 2:1. Fractional carbon oxide removal at these conditions exceeded 0.99, with a maximum of 0.999 at 500$\sp\circ$C and 15 atm. The corresponding total carbon oxide concentration during the prebreakthrough period was below 300 ppm. The CO and CO$\sb2$ concentrations during the prebreakthrough period did not increase in multicycle tests. However, sorbent reactivity loss was evident by a decrease in the duration of the prebreakthrough period. The deterioration rate for dolomite was about 3% to 5% per cycle at favorable reaction conditions. When shift/carbonation pressure decreased from 15 to 1 atm, the fractional carbon oxide removal during the early stage of the reaction decreased to about 0.95, but the sorbent durability did not suffer.