Limestone and dolomite as sulfur absorbents under pressurized gasification conditions

The capture of H₂S by both limestone and dolomite was studied with a pressurized thermogravimetric apparatus. The experimental conditions were chosen to correspond to the conditions typical in a pressurized fluidized bed gasifier. Depending on these conditions the limestone reacts 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 particle size 125-180 μm were compared at 950°C and 2 MPa. The capture of H₂S by calcined limestone and fully calcined dolomite was fast and 80-90 wt% of the Ca was converted to CaS. H₂S capture by uncalcined limestone was slow and after 2 h no more than 20 wt% of the Ca had reacted to CaS. Half-calcined dolomite was significantly more efficient than uncalcined limestone and 60-80 wt% of the Ca was converted to CaS within 60 min. Experiments were also carried out at lower temperatures. For calcined limestone and fully calcined dolomite the reaction rate was still too fast for kinetic studies at 750°C. However, for uncalcined limestone and half-calcined dolomite the reaction rate clearly decreased at lower temperatures. The observed activation energy for uncalcined limestone was ∼ 100 kJ mol^-1 and for half-calcined dolomite 300-400 kJ mol^-1.