Sulfur Removal During Combustion of Solid Fuels In A Fluidized Bed of Dolomite

Abstract

Sulfur removal during combustion of coal or low-temperature coal char using excess air in a fluidized bed of dolomite has been demonstrated in a continuous bench-scale unit. The SO2 produced during the combustion is fixed as CaSO4 by reaction with CaO in the dolomite as follows: CaO + SO2 + ½ O2 = CaSO4. The ranges of conditions explored were calcium oxide-to-feed sulfur mol ratios in the range of 1 to 8, fluidizing velocities of 1.5 and 3.0 ft/sec, superficial gas residence times of 1 and 2 sec, and temperatures from 1700° to 1900°F, with 120% stoichiometric air at the operating pressure of 8 psig. The process was operable at all conditions studied using a highly caking Pittsburgh seam coal as well as two low-temperature chars. Desulfurization efficiency with a Ca-to-S mol ratio of 1 was 78%; at Ca-to-S ratios of 2 or higher desulfurization efficiency exceeded 90%. Continuous regeneration of the sulfated dolomite using partial combustion of carbon monoxide gas at 1950°F was also demonstrated. As high as 97% of the sulfur in the dolomite was liberated as SO2 by this treatment giving a gas that contained SO2 concentrations as high as 23 times that in the combustion gases. Dolomite life was investigated, with the dolomite subjected alternately to SO2-absorption during combustion and subsequent regeneration. It was demonstrated that by regeneration and re-use of the dolomite, the ultimate SO2 capacity can be extended in excess of four times that for a once-through process. Regeneration also permits recovery of elemental sulfur by processing the SO2-rich regenerator off-gas.