Mechanisms governing alkali metal capture by kaolinite in a downflow combustor.

Abstract

Experimental work was carried on a 17 kW, 600 cm long, gas fired 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 SO₂ and Cl₂ 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 μm 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°e. Larger reductions at lower temperatures are attributed to the elevated dewpoint of sodium sulfate (∼850°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. An optimum temperature window between 900°C-1100°C, in which the formation of water insoluble alkali alumino silicates is favored, has been reported. Above 1200°C the sorbent used here, melted. Sorbent melt at such low temperatures is probably due to the impurities (Ti, Fe, K) in kaolinite. A simple first order kinetic model is proposed for sodium capture by metakaolinite and is best fitted by volume rate k = 4.5±0.5x10⁷ exp (-5300±150 kcal/RT) cm³ gas/ cm³ porous solid.sec. Activation energies in the range of 6-10 kCal/mol gave model prediction results that compared very well with experimental data. Kinetics is the rate controlling mechanism for small particles under 3.00μ with intraphase pore diffusion shown to be the controlling mechanism for particles larger than 3 μm. A maximum sorbent utilization of 50% was realized.