(137g) Enhancing the Water Gas Shift Reaction Using Synthetic Sorbents for the Capture of CO2

In order to capture the carbon content of coal, one option would be to
gasify it with mixtures of, e.g. steam, CO₂ and O₂,
to produce synthesis gas: the water-gas shift (WGS) reaction, CO + H₂O
<--> CO₂ + H₂, could then be used to increase the yield of hydrogen.
In principle, if the WGS reaction is operating close to thermodynamic
equilibrium, the yield of H₂ could be enhanced by the simultaneous
removal of the CO₂ from the gas-phase using a suitable sorbent based
on CaO, which is carbonated in: CaO_(s) + CO_2(g) --> CaCO_3(s). This would
move the WGS equilibrium to the right, thereby producing H₂ with only a small contamination by CO₂.
The captured CO₂ would subsequently be recovered by removing the
carbonate and calcining it to generate a pure stream of CO₂ which would be ready for sequestration, e.g.
in deep saline aquifers. The regenerated calcareous sorbent would then be
recycled to the water-gas shift reactor.

This paper describes the results of experiments in which the WGS reaction
was conducted in the presence of a CaO-containing sorbent. A major limitation
of such a scheme is the degradation of the conversion of the CaO over progressive
cycles of carbonation and calcination, particularly with natural sorbents, such
as limestone. In our experiments, the sorbent employed was a synthetic one,
namely calcium magnesium acetate (CMA, Ca : Mg = 7 : 3, molar basis), the
performance of which did not degrade substantially with cycling. The enhanced
WGS reaction was conducted at temperatures of 500, 550, 600 and 650°C, with calcination at 800°C, in a packed bed. The gases fed to the reactor were obtained from
cylinders (in the cases of CO and inert N₂) and steam was generated
from deionised water.

From the experiments performed, it was shown that CaO-containing sorbents
can affect the WGS in two ways: (i) by catalysis of the reaction itself and
(ii) by altering the equilibrium position through abstraction of CO₂
from the gas phase. It was found that a trade-off has to be made between the
production of H₂ and the contamination of the product gas with CO₂
(the CO₂ slip). For the WGS performed at 600°C, equilibrium conditions were reached while at the lowest
experimental temperature of 500°C,
there was very little CO₂ slip.