99/02029 Trace metal partitioning when firing pulverized coal and coal-water slurry fuel: Nodelman, I. G. et al. Prepr. Extract Abstr. ACS Natl. Meet., Am. Chem. Soc., Div. Environ. Chem., 1998, 38, (2), 169–172

Abstract

Major environmental issues are now coming to the forefront in all parts of the globe with increased public awareness of the human health effects and the possible effects on our global environment. Modern control technologies, when implemented, have significantly reduced air pollution emissions that are a result of coal combustion during this century. However, the emissions have not been completely eliminated. On this basis, a study was conducted to determine the efficacy of carboxylic calcium and magnesium salts (e.g., calcium magnesium acetate or CMA, CaMg2 (CH3CO2)6) for the simultaneous removal of SO2 and NOx in oxygen-lean atmospheres. Experiments were performed in a high-temperature furnace that stimulated the post-flame environment of a coal-fired boiler by providing similar temperatures and partial pressures of SO2, NOx, CO2, and O2. When injected into a hot environment, the salts calcined and formed highly porous ‘popcorn’-like cenospheres. Residual MgO and/or CaCO3 and CaO reacted heterogeneously with SO2, and oxygen to form MgSO4 and/or CaSO4. The organic components — which can be manufactured from wastes such as sewage sludge — reduced NOx to N2 efficiently in a moderately fuel rich atmosphere. Dry-injected CMA particles at a CaS ratio of 2, residence time of 1 s and bulk equivalence ratio of 1.3 removed over 90% of SO2 and NOx at gas temperatures ≥950°C. When the furnace isothermal zone was ≤950°C, CaO was essentially inert in the furnace quenching zone, while MgO continued to sorb SO2 as the gas temperature cooled at a rate of — 130°C/s. Hence, the removal of SO2 by CMA could continue for nearly the entire residence time of emissions in the exhaust stream of a power plant. The composition of the calcined salts was used to interpret the results of a cenosphere sulfation model. The sulfation kinetics of Ca-containing calcined residues were found to be bounded by those of pure CaO and pure CaCO3. The high solubility of the carboxylic acid salts makes them excellent candidates for wet injection. Fine mists of CMA sprayed in the furnace at temperatures between 850 and 1050°C, removed 90% of SO2 at a CaS molar ratio of 1, about half of the amount used in the dry injection experiments to achieve a similar SO2 reduction. The NOx reduction chemistry was not affected by water when CMA was sprayed at a CaS ratio of 1, i.e., the same reduction efficiency was achieved as with dry injection (25–30%). Thus, while a substantial degree of success has been achieved in controlling SO2 and NOx emissions, unfortunately, the emissions that are currently unable to be controlled include vapor and particulate emissions within the respirable range. The respirable particulates encompass heavy metals, polyaromatic hydrocarbons, and volatile organics and have the ability to penetrate into the lungs. The vapor phase emissions comprise mercury, arsenic, and selenium. Most of the mercury vapor emissions are unable to be controlled by the current technologies of the electrostatic precipitator or the baghouse. Some of the potential emissions are known to be carcinogens. Therefore, new or improved current technologies for emission control are being developed in hopes of reducing the potential effects. Because previous interest for control technology was directed toward the control of acid rain precursors (sulfur dioxide and nitrogen oxides emissions), and laboratory studies revealed that the injection of calcium magnesium acetate (CMA) into the combustion chamber significantly reduces both levels of emissions, the effect of this sorbent on mercury vapor capture was investigated suing CMA ash after combustion with the gases (SO2 and NOx). Also, the effect of calcium magnesium carbonate (dolomite, CMC), currently being injected into industrial furnaces, was investigated utilizing the same combustion conditions as CMA. In addition, CMA when calcined forms cenospheres which possess thin porous walls with blowholes that enable mercury vapor and respirable particulates to penetrate into the interior of the particle. Thus, these CMA-ash cenospheres have higher porosity and surface area that provides more area for capture and reactions to occur than CMC ash. Experiments for mercury capture have demonstrated that CMA ash combusted with the gases SO2NOx provided the greatest removal (40%); CMC at the same conditions exhibited only 4% removal. Furthermore, in separate experiments using a model particulate (FeSO4), CMA ash was able to capture more of these model air particulates than the CMC ash after combustion suggesting an even further role of CMA in particulate toxin capture. Analysis of ‘spent’ CMA or CMA ‘ash’ from the laboratory furnace, which was combusted with SO2NOx revealed substantial amounts of elemental sulfur, while no elemental sulfur was present on CMA ash which was combusted in the furnace without SO2NOx. These results suggest that CMA pyrolysis serves to reduce SO2 to elemental sulfur and indicating that HgS(s) formation is feasible, thus suggesting the scavenger value of CMA in capturing mercury vapor.