Laboratory study on the high-temperature capture of HCl gas by dry-injection of calcium-based sorbents

Abstract

This is a laboratory study on the reduction of combustion-generated hydrochloric acid (HCl) emissions by in-furnace dry-injection of calcium-based sorbents. HCl is a hazardous gaseous pollutant emitted in significant quantities by municipal and hazardous waste incinerators, coal-fired power plants, and other industrial furnaces. Experiments were conducted in a laboratory furnace at gas temperatures of 600–1000°C. HCl gas diluted with N 2, and sorbent powders fluidized in a stream of air were introduced into the furnace concurrently. Chlorination of the sorbents occurred in the hot zone of the furnace at gas residence times ≈1 s. The sorbents chosen for these experiments were calcium formate (CF), calcium magnesium acetate (CMA), calcium propionate (CP), calcium oxide (CX), and calcium carbonate (CC). Upon release of organic volatiles, sorbents calcine to CaO at ≈700°C, and react with the HCl according to the reaction CaO+2HCl⇌CaCl 2+H 2O. At the lowest temperature case examined herein, 600°C, direct reaction of HCl with CaCO 3 may also be expected. The effectiveness of the sorbents to capture HCl was interpreted using the “pore tree” mathematical model for heterogeneous diffusion reactions. Results show that the thin-walled, highly porous cenospheres formed from the pyrolysis and calcination of CF, CMA, and CP exhibited high relative calcium utilization at the upper temperatures of this study. Relative utilizations under these conditions reached 80%. The less costly low-porosity sorbents, calcium carbonate and calcium oxide also performed well. Calcium carbonate reached a relative utilization of 54% in the mid-temperature range, while the calcium oxide reached an 80% relative utilization at the lowest temperature examined. The data matched theoretical predictions of sorbent utilization using the mathematical model, with activation energy and pre-exponential factors for the calcination reaction of 17,000 K and 300,000 ( g gas / cm 2/ s/ atm gas ), respectively. Thus, the kinetics of the calcination reaction were found to be much faster (≈500 times) than those of the sulfation reaction examined previously in this laboratory.