Carbonation of CaO-Based Sorbents Enhanced by Steam Addition

Abstract

The carbonation reaction has recently been intensively investigated as a means of CO 2 capture from gas mixtures such as flue gas produced during fossil fuel combustion. Unfortunately, this gas−solid reaction is limited due to formation of the solid product (CaCO 3 ) at the reacting surface and sintering, all of which reduce the carrying capacity of the sorbent. In this work the enhancement of carbonation conversion by means of steam addition to the carbonating gas was studied. Seven limestones of different origin and composition as well as one synthetic sorbent (calcium aluminate pellets) were tested. A thermogravimetric analyzer (TGA) was employed for the carbonation tests at different temperatures (350−800 °C) in a gas mixture containing typically 20% CO 2 and 10 or 20% H 2 O (g) . The samples tested were calcined under an N 2 (800 °C) or CO 2 (950 °C) atmosphere to explore the influence of different levels of sample sintering, and the results obtained were compared with those seen for carbonation in dry (no steam) gas mixtures. The morphology of samples after carbonation under different conditions was examined by a scanning electron microscope (SEM). It was found that carbonation is enhanced by steam, but this is more pronounced at lower temperatures and for more sintered samples. With increasing temperature and carbonation time, the enhancement of carbonation becomes negligible because the conversion reaches a “maximum” value (∼75−80% for samples calcined in N 2 ) even without steam. Carbonation of samples calcined in CO 2 is enhanced at different levels depending on the sorbent tested. The shape of carbonation profiles and morphology of carbonated samples show that steam enhances solid state diffusion and, consequently, conversion during carbonation.