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

Experiments were conducted in an externally heated drop-tube furnace t o 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 dis tinct sizes, pulverized (75-90 mum) or micronized and beneficiated (me an diameter approximately 3.5 mum) were burned. To measure the coal pa rticle temperatures and burn times, combustion traces were recorded fo r single pulverized coal particles and clusters of micronized coal par ticles using a three-colour near-infrared optical pyrometer. The volat ile and the char phase combustion temperatures of untreated pulverized -grind particles, in air at a gas temperature of 1450 K, were determin ed to be 2200 and 1800 K, respectively. Particles treated with CMA und er the same conditions burned hotter, with the temperature of volatile and char phase being 2400 and 2000 K, respectively. SO2 and NO(x) con centrations were measured at the exit of the furnace for both the pulv erized and the micronized coals. For furnace gas temperatures between 1250 and 1450 K, in a background gas containing 10-50 ppm SO2 and equi valence ratios, phi, between 0.4 to 0.7, untreated micronized and pulv erized bituminous coal particles produced SO2 emissions in the range 1 00-200 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 t reated with CMA produced less SO2 than untreated micronized coal, but complete reduction of SO2 was not achieved. Experiments with CMA-treat ed micronized coal in atmospheres containing 40% oxygen suggested that the primary mechanism for sulfur removal in this virtually ash-free c oal was the sulfation of resulting CaO/MgO fly ash/aerosols. Experimen ts in which the effluent of the combustion of pulverized coal was quen ched immediately after the heated furnace zone suggest that a fraction of the fuel sulfur (up to 50% of the released SO2) may have been enca psulated by ash during the combustion of treated pulverized coals, wit h the remaining released SO2 being removed by sulfation of Ca/Mg-conta ining ash or submicron CaO/MgO aerosols in the cool-down region of the furnace.