SYSTEM DESIGN AND COST ESTIMATIONS FOR NO_xSELECTIVE CATALYTIC REDUCTION (SCR) ON COAL-FIRED BOILERS

摘要

Nitrogen oxides (NO_x) are gaseous pollutants that are primarily formed through combustion process. While flue gas is within the combustion unit, about 95% of the NO_x exists in the form of nitric oxide (NO). The balance is nitrogen dioxide (NO_2), which is unstable at high temperatures. Once the flue gas is emitted into the atmosphere, most of the NO is ultimately converted to NO_2. NO_x in the atmosphere reacts in the presence of sunlight to form ozone (O_3), one of the criteria pollutants for which health-based National Ambient Air Quality Standards have been established. NO_x is generated in one of three forms; fuel NO_x, thermal, NO_x and prompt NO_x. Fuel NO_x is produced by oxidation of nitrogen in the fuel source. Combustion of fuels with high nitrogen content such as coal and residual oils produces greater amounts of NO_x than those with low nitrogen content such as distillate oil and natural gas. Thermal NO_x is formed by the fixation of molecular nitrogen and oxygen at temperatures greater than 3600°F (2000°C). Prompt NO_x forms from the oxidation of hydrocarbon radicals near the combustion flame and produces an insignificant amount of NO_x. Selective Noncatalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) are post-combustion control technologies based on the chemical reduction of nitrogen oxides (NO_x) into molecular nitrogen (N_2) and water vapour (H_2O). The primary difference between the two technologies is that SCR utilizes a catalyst to increase the NO_x removal efficiency, which allows the process to occur at lower temperatures. The technologies can be used separately or in combination with other NO_x combustion control technologies such as low NO_x burners (LNB) and natural gas reburn (NGR). SNCR and SCR can be designed to provide NO_x reductions year-round or only during summer months, when ozone concerns are greatest. The paper presents design specifications and a costing methodology for SCR applications for large industrial boilers. The work describes the process chemistry, performance parameters, and system components of SCR. In addition, impacts to the boiler performance and facility operations resulting from the installation of SCR are presented. The paper also estimates important underlying design parameters including the normalized stoichiometric ratio, catalyst volume and reagent consumption. Lastly, it presents assumptions and equations for estimating capital costs, annual operation and maintenance costs, and annualized costs.