Effect of Sorbent Type on the Sorption Enhanced Water Gas Shift Process in a Fluidized Bed Reactor

Abstract

The sorption enhanced water gas shift (SEWGS) reaction was proposed to improve CO2 capture from blast furnace (BF) gas. The CO in the BF gas is first catalyzed to CO2 on the sorbent surface, with the formed CO2 then captured by the CaO. This process involves two reactions: the WGS reaction and the CaO carbonation reaction. The WGS reaction is a surface catalytic reaction, while the CaO carbonation is a bulk reaction. This study of the SEWGS reaction in a fluidized bed reactor showed that the sorbent type has an important effect on the SEWGS reaction, with both the CaO and MgO in the sorbent catalyzing the reaction. For calcined limestone, the WGS reaction occurs on the CaO surface with CO2 sorption in situ, but the reaction rate gradually decreases with the CaO conversion due to coverage by the CaCO3 product layer on the CaO surface. Thus, the shift reaction becomes the rate limiting step. For calcined dolomite, MgO in the sorbent does not react with CO2 and can then be used as a catalyst. The shift reaction is not a limiting step, with most of the CO2 for the CaO carbonation coming from the WGS reaction on the MgO surface. Even when the CaO surface is completely covered by the CaCO3 product, the MgO in the calcined dolomite is not completely covered by the CaCO3 product and can still catalyze the WGS reaction. An important phenomenon observed was that not only CaO but also MgO experienced the decay in the catalytic reactivity after multiple cycles. The decay of catalytic reactivity of MgO in dolomite is mainly the gradual covering of the MgO surface by CaO/CaCO3 grains, so most of the MgO grains will lose contact with the CO and the steam in the gas phase. The observation of encapsulation of MgO particles by CaO or CaCO3 is also important for understanding the stabilization of synthetic calcium based sorbent with support addition.