Synergistic capture mechanisms for alkali and sulfur species from combustion. Final report

Abstract

Experimental work was carried out on a 17 kW, 600 cm long, gas laboratory combustor, to investigate the post flame reactive capture of alkali species by kaolinite. Emphasis was on alkali/sorbent interactions occurring in flue gas at temperatures above the alkali dewpoint and on the formation of water insoluble reaction products. Time-temperature studies were carried out by injecting kaolinite at different axial points along the combustor. The effect of chlorine and sulfur on alkali capture was investigated by doping the flame with SO{sub 2} and Cl{sub 2} gases to simulate coal flame environments. Particle time and temperature history was kept as close as possible to that which would ordinarily be found in a practical boiler. Experiments designed to extract apparent initial reaction rates were carried using a narrow range, 1-2 {mu}m modal size sorbent, while, a coarse, multi size sorbent was used to investigate the governing transport mechanisms. The capture reaction has been proposed to be between alkali hydroxide and activated kaolinite, and remains so in the presence of sulfur and chlorine. The presence of sulfur reduces sodium capture by under 10% at 1300{degree}C. Larger reductions at lower temperatures are attributed to the elevated dewpoint of sodium ({approximately}850{degree}C) with subsequent reduction in sorbent residence time in the alkali gas phase domain. Chlorine reduces sodium capture by 30% across the temperature range covered by the present experiments. This result has been linked to thermodynamic equilibria between sodium hydroxide, sodium chloride and water.