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NOx Control in Pulverized Coal Combustion

Technologies for control of NOZ emissions from pulverized coal-fired power plants can be divided into two groups: (1) combustion modifications where the NOx production is reduced during the combustion process, and (2) flue gas treatment, which removes the NOx from flue gas following its formation. Sometimes the practice of injecting reducing agents to reduce NOx to molecular nitrogen (N2) is classified separately; however, in this section it is included as a flue gas treatment. Table 6-8 lists various NOx control technologies with a summary of their attributes [23]. The abatement or emission control principles for these various control methods include reducing peak flame temperatures, reducing the residence time at peak flame temperatures, chemically reducing NOZ, oxidizing NOZ with subsequent absorption, removing nitrogen, using a sorbent, or a combination of these methods.

Reducing combustion temperature is accomplished by operating at non-stoichiometric conditions to dilute the available heat with an excess of fuel, air, flue gas, or steam [23]. The combustion temperature is reduced by using fuel-rich mixtures to limit the availability of oxygen, using fuel-lean mixtures to dilute energy input, injecting cooled oxygen-depleted flue gas into the combustion air to dilute energy, injecting cooled flue gas with the fuel, or injecting water or steam.

Reducing residence times at high combustion temperatures is accomplished by restricting the flame to a short region to prevent the nitrogen from becoming ionized. Fuel, steam, more combustion air, or recirculating flue gas is then injected immediately after this region. Chemically reducing NOx removes oxygen from the nitrogen oxides. This is accomplished by reducing the valence level of nitrogen to zero after the valence has become higher. Oxidizing NOZ intentionally raises the valence of the nitrogen ion to allow water to absorb to it. This is accomplished by using a catalyst, injecting hydrogen peroxide, creating ozone within the air flow, or injecting ozone into the air flow. Removing nitrogen from combustion is accomplished by removing nitrogen as a reactant either by using low nitrogen content fuels or using oxygen instead of air. The ability to vary coal nitrogen contents, however, is limited. Treatment of flue gas by injection sorbents such as ammonia, limestone, aluminum oxide, or carbon can remove NOx and other pollutants. This type of treatment has been applied in the combustion chamber,

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