OCR Text |
Show Introduction The goal when burning coal for electric power generation is to maximize conversion of fuel to marketable energy while minimizing the environmental impact of the emissions. Gas-phase pollutants such as nitrogen oxides (NOx), coarse and fine particulate emissions, and the land disposal of gas cleaning residue are all areas of concern. Much research has gone into techniques for minimizing formation of N O x in coal combustors [1] by controlling the mixing of fuel and air. N O x emissions can be reduced from the 1000 ppm concentration that was typical of boilers before the 1970s to below 100 ppm through techniques such as low N Ox burners, low excess air operation, staged combustion, and reburning. All these low-NOx processes involve creating oxygen-deficient regions where conversion of atmospheric and fuel nitrogen to N O is minimized, and where the conversion of N O and intermediate nitrogen species such as H C N to N 2 is favored. Low N O x pulverized coal combustion is associated with high levels of unburned carbon in the fly ash. [2, 3] Increased unburned carbon in the coal fly ash is undesirable because the carbon represents wasted fuel, the mutagenicity of coal fly ash has been associated with the organic content, [4-6] and high carbon ash cannot be recycled for cement production. Char refers to porous, carbon-rich particles which remain in a solid or liquid phase throughout combustion while soot refers to carbon-rich ultrafine primary particles produced from gas-phase precursors. Pulverized coal combustion begins with rapid heatup of the fuel particle followed by decomposition into char and volatiles. The volatiles further decompose to produce soot which creates a luminous zone surrounding the source particles. [7, 8] Char continues to burn after volatile combustion is complete and most studies of residual carbon in coal fly ash have focused on the heterogeneous reaction rates for char oxidation. [9-11] Char particle sizes found in the fly ash range from several microns up to the diameter of the pulverized coal particles depending on the swelling and fragmentation behavior of the coal. Soot consists of aggregates formed from spherical primary particles that are typically 20-100 n m diameter. Char and Soot 2 September 4, 1998 |