The effect of halides on emissions from circulating fluidized bed combustion of fossil fuels

Abstract

Combustion tests involved addition of chlorides and bromides while burning Highvale coal were carried out in a pilot-scale (152 mm square × 7.3 m tall) circulating fluidized bed combustor (CFBC). The halogens were added in the form of hydrochloric acid (HCl) and calcium bromide ( CaBr 2. 1 2 H 2O ) solutions (10 wt%). The thermodynamic equilibrium composition of CFBC products was calculated using the ASPEN database package. The calculations predict 12% conversion of HCl to CaCl 2 for an HCl feed rate of 9.78 kg h −1 and complete conversion of CaBr 2 to HBr in the vapour phase throughout the range of CaBr 2 solution feed rate investigated. The experimental results indicate that chloride and bromide additions increase the CO and SO 2 concentrations in the flue gases, with corresponding decreases in the NO x level. The halides have no significant effect on N 2O emission. The CO level increased from 27 to 230 ppmv when the chloride concentration in the reactor was ∼4200 ppmv (Cl/fuel = 4.58 wt%). The effect of bromide on CO emission was more dramatic, the CO emission jumping from 56 to 480 ppmv for lower concentrations of bromide (20–400 ppmv). The experiments confirm previous work showing that halide-containing species inhibit CO oxidation through interaction with the hydrogen-oxygen radical pool. The decrease in NO concentration with chloride addition can be explained by surface modification of CaO particles due to formation of a liquid calcium chloride phase favoured by high HCl concentrations near the feed point. The formation of the liquid calcium chloride phase has the potential to make the CaO surface unavailable, thereby reducing catalytic oxidation of volatile nitrogen to NO.