COMBUSTION AND SO2-NOX EMISSIONS OF BITUMINOUS COAL PARTICLES TREATED WITH CALCIUM-MAGNESIUM ACETATE

Experiments were conducted in an externally heated drop-tube furnace to assess the effectiveness of the chemical calcium magnesium acetate [CMA, CaMg2(CH3COO)6] as a combustion catalyst and a coal pretreatment agent for reducing SO2 emissions. Bituminous coal particles of two distinct sizes, pulverized (75-90 mum) or micronized and beneficiated (mean diameter approximately 3.5 mum) were burned. To measure the coal particle temperatures and burn times, combustion traces were recorded for single pulverized coal particles and clusters of micronized coal particles using a three-colour near-infrared optical pyrometer. The volatile and the char phase combustion temperatures of untreated pulverized-grind particles, in air at a gas temperature of 1450 K, were determined to be 2200 and 1800 K, respectively. Particles treated with CMA under the same conditions burned hotter, with the temperature of volatile and char phase being 2400 and 2000 K, respectively. SO2 and NO(x) concentrations were measured at the exit of the furnace for both the pulverized and the micronized coals. For furnace gas temperatures between 1250 and 1450 K, in a background gas containing 10-50 ppm SO2 and equivalence ratios, phi, between 0.4 to 0.7, untreated micronized and pulverized bituminous coal particles produced SO2 emissions in the range 100-200 ppm and NO(x) emissions in the range 200-450 ppm. In contrast, combustion of pulverized particles treated with CMA, burning under the same conditions, not only did not produce any SO2 but also eliminated the background SO2 concentration. The combustion of micronized coal treated with CMA produced less SO2 than untreated micronized coal, but complete reduction of SO2 was not achieved. Experiments with CMA-treated micronized coal in atmospheres containing 40% oxygen suggested that the primary mechanism for sulfur removal in this virtually ash-free coal was the sulfation of resulting CaO/MgO fly ash/aerosols. Experiments in which the effluent of the combustion of pulverized coal was quenched immediately after the heated furnace zone suggest that a fraction of the fuel sulfur (up to 50% of the released SO2) may have been encapsulated by ash during the combustion of treated pulverized coals, with the remaining released SO2 being removed by sulfation of Ca/Mg-containing ash or submicron CaO/MgO aerosols in the cool-down region of the furnace.