LIMESTONE AND DOLOMITE AS SULFUR ABSORBENTS UNDER PRESSURIZED GASIFICATION CONDITIONS

The capture of H2S by both limestone and dolomite was studied with a p ressurized thermogravimetric apparatus. The experimental conditions we re chosen to correspond to the conditions typical in a pressurized flu idized bed gasifier. Depending on these conditions the limestone react s either uncalcined (CaCO3) or calcined (CaO), and the dolomite either half-calcined (CaCO3 + MgO) or fully calcined (CaO + MgO), with H2S. The reaction product is calcium sulfide (CaS). MgO does not react with H2S under these circumstances, due to thermodynamic limitations. The H2S absorption capacities of three dolomites and two limestones of par ticle size 125-180 mu m were compared at 950 degrees C and 2 MPa. The capture of H2S by calcined limestone and fully calcined dolomite was f ast and 80-90 wt% of the Ca was converted to CaS. H2S capture by uncal cined limestone was slow and after 2 h no more than 20 wt% of the Ca h ad reacted to CaS. Half-calcined dolomite was significantly more effic ient than uncalcined limestone and 60-80 wt% of the Ca was converted t o CaS within 60 min. Experiments were also carried out at lower temper atures. For calcined limestone and fully calcined dolomite the reactio n rate was still too fast for kinetic studies at 750 degrees C. Howeve r, for uncalcined limestone and half-calcined dolomite the reaction ra te clearly decreased at lower temperatures. The observed activation en ergy for uncalcined limestone was similar to 100 kJ mol(-1) and for ha lf-calcined dolomite 300-400 kJ mol(-1).