Nitrogen Oxides NOX

Approximately 10 million short tons of NOZ were emitted from fuel combustion in 2002, with power plants contributing less than 4.5 million short tons [19,20]. All sources affected by the EPA's Acid Rain Program NOZ requirements reduced their combined NOZ emissions by 27% over the period 1990 to 2002 [20]. These reductions have been achieved while the amount of fuel burned to produce electricity, as measured by heat input, increased 28% since 1990. In 2002, more than 1000 units complied with emission rate limits. NOZ formation mechanisms are reviewed in this section, followed by technologies used to control NOZ emissions. Similar to the discussion on SO2 control technologies, NOZ control technologies will focus on commercially available, commercially used systems, with the focus on pulverized coal-fired boiler systems.

NOx Formation Mechanisms

NOZ formation from coal combustion was introduced in Chapter 3 (The Effect of Coal Use on Human Health and the Environment), and the discussion is expanded in this section. NOx formation during pulverized coal combustion is also discussed, as this is the most widely used process for power generation. The majority of nitrogen oxides emitted from power plants are in the form of nitric oxide (NO), with only a small fraction as nitrogen dioxide (NO2) and nitrous oxide (N2O). Collectively, these oxides are referred to as NOx. NO originates from the coal-bound nitrogen and nitrogen in the air used in the combustion process and is produced through three mechanisms: thermal NO, prompt NO, and fuel NO. Fuel-bound nitrogen accounts for 75 to 95% of the total NO generated, while thermal and prompt NO account for the balance, with prompt NO being no more than 5% of the total NO [21].

The factors that influence NOx emissions in pulverized coal-fired boilers can be generally categorized as boiler design, boiler operation, and coal properties [21]; however, NOZ formation is complex, and many parameters influence its production [21,22]. Boiler design factors include boiler type, capacity, burner type, number and capacity of the burners, burner zone heat release, residence times, and presence of overfire air ports. Similarly, boiler operation factors include load, mills in operation, excess air level, burner tilt, and burner operation. Coal properties that influence NOz production include the release of volatiles and nitrogen partitioning, ratio of combustibles-to-volatile matter, heating value, rank, and nitrogen content.

Thermal NO Thermal NO formation involves the high-temperature (> 2370°F) reaction of oxygen and nitrogen from the combustion air [21]. The principal reaction governing the formation of NO is the reaction of oxygen atoms formed from the dissociation of O2 with nitrogen. These reactions, referred to as the Zeldovich mechanism, are:

These reactions are sensitive to temperature, local stoichiometry, and residence time. High temperature is required for the dissociation of oxygen and to overcome the high activation energy for breaking the triple bond of the nitrogen molecule. These reactions dominate in fuel-lean, high-temperature conditions. Under fuel-rich conditions, hydroxyl and hydrogen radical concentrations are increased which initiates oxidation of the nitrogen radicals, so at least one additional step should be included in this mechanism:

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