Mechanisms governing trace sodium capture by kaolinite in a downflow combustor

Abstract

A 17-kW, 6-m-long gas-fired laboratory combustor was used to investigate the postflame reactive capture of trace sodium 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. Reaction time-temperature profiles replicated those in typical boilers and were varied by injecting kaolinite at different axial points along the combustor. The effects of chlorine and sulfur on alkali capture were investigated by doping the flame with Cl2 and SO2. Variations in sorbent particle sizes allowed the separate effects of reaction kinetics and transport within the particles to be isolated.

The actual capture reaction is proposed to be between alkali hydroxide and activated kaolinite and remains so in the presence of chlorine and sulfur. Hence, chloride reduces sodium capture significantly because it lowers the gas-phase concentration of the sodium hydroxide. The primary effect of sulfur, on the other hand, is to elevate the sodium species dewpoint and, by thus lessening the time for reaction between gaseous sodium and substrate, to diminish the overall capture. A simple first-order kinetic model is proposed for sodium (hydroxide) capture by metakaolinite and is best fitted by the (global) volumetric rate constant: k0=4.5±0.5×107 exp(−2.657±75/T) cm3 gas/(cm3 porous solid s) Chemical kinetics control for small particles under 2 μm, whereas intraphase pore diffusion controls for larger sorbent particles. A maximum sorbent utilization of 50% was realized, and an optimum sorbent injection window of 1200–1400 K was identified.