Nitrogen Oxides NOX

Seven oxides of nitrogen are present in ambient air [28]. These include nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O), NO3, N2O3, N2O4, and N2O5. Nitric oxide and nitrogen dioxide are collectively referred to as NOx due to their interconvertibility in photochemical smog reactions. The term NOy is often used to represent the sum of the reactive oxides of nitrogen and all other compounds that are atmospheric products of NOx. NOy includes compounds such as nitric acid (HNO3), nitrous acid (HNO2), nitrate radical (NO3), dinitrogen pentoxide (N2O5), and peroxyacetyl nitrate (PAN). It excludes N2O and ammonia (NH3) because they are not normally the products of NOx reactions [28].

Nitrogen oxide emissions from coal combustion are produced from three sources: thermal NOx, fuel NOx, and prompt NOx. Nitrogen oxides are primarily produced as a result of the fixation of atmospheric nitrogen at high temperatures (thermal NOx) and the oxidation of coal nitrogen compounds (fuel NO x). Prompt NOZ is formed when hydrocarbon radical fragments in the flame zone react with nitrogen to form nitrogen atoms, which then form NO. The majority of the oxide species produced is NO, with NO2 accounting for less than 5% of the total [31].

The production of thermal NO is a function of the combustion temperature and fuel-to-air ratio and increases exponentially at temperatures above 2650°F. Thermal NO can be predicted by the following equation [32]:

where T is temperature, t is time, K1 and K2 are constants, and [N2] and [O2] are concentrations in moles. Accordingly, thermal NO can be decreased by reducing the time, temperature, and concentration of N2 and O2. The principal reactions in the formation of thermal NO, which are referred to as the extended Zeldovich mechanism, are:

Reaction (3-14) is assumed to be the rate-determining step due to the high activation energy required to break the triple bond in the nitrogen molecule. Reaction (3-16) has been found to contribute under fuel-rich conditions. The general conclusion is that very little thermal NO is formed in the combustion zone and that the majority is formed in the post-flame region, where the residence time is longer.

Prompt NO is produced by the reaction of hydrocarbon fragments and molecular nitrogen in the flame front. Prompt NO is most significant in fuel-rich flames, where the concentration of radicals such as O and OH can exceed equilibrium values, thereby enhancing the rate of NO formation. Prompt NO occurs due to the collision of hydrocarbons with molecular nitrogen in the fuel-rich flames to form HCN (hydrogen cyanide) and N. The HCN is then converted to NO by a series of reactions among NCO, H, O, OH, NH, and N. The amount of prompt NO generated is proportional to the concentration of N2 and the number of carbon atoms present in the gas phase, but the total amount produced is low in comparison to the total thermal and fuel NO in coal combustion. The two reactions believed to be the most significant with regard to the mechanism for formation of prompt NO are [33]:

Reaction (3-17) was originally proposed, and Reaction (3-18) was added as a minor, but non-negligible, contributor to prompt NO; its importance grows with increasing temperature.

Fuel NO is the primary source of NOZ in flue gas from coal combustion and is formed from the gas-phase oxidation of devolatilized nitrogen-containing species and the heterogeneous combustion of nitrogen-containing char in the tail of the flame [31]. At temperatures below 2650°F, fuel NO can account for more than 75% of the measured NO in coal flames and can be as high as 95%. The reason for the dominance of fuel NO in coal systems is because of the moderate temperatures (2240-3140°F) and the locally fuel-rich nature of most coal flames. Fuel NO is produced more readily than thermal NO because the N-H and N-C bonds common in fuel-bound nitrogen are weaker than the triple bond in molecular nitrogen in the air, which must be dissociated to produce thermal NO. Combustion conditions and the nitrogen content of a coal affect the quantity of NO emissions. During devolatiliza-tion, a portion of the coal nitrogen is released as HCN and to a lesser extent as NH3. HCN readily reacts with oxygen to form NO, but some of this NO can be converted to N2 by reaction with hydrocarbon radicals in fuel-rich zones:

Nitrogen retained in the char is also oxidized to NO, which may react with the char surface or hydrocarbon radicals to form N2. Much of the coal nitrogen is initially converted to NO. The final NO emissions, however, are determined largely by the extent of conversion of NO to N2 in the various regions of the combustion unit. Unlike thermal NO, the production of fuel NO is relatively insensitive to temperature over the range found in pulverized coal flames and more sensitive to the air-to-fuel ratio [32,34].

Environmental Effects

Both NOx and NOy (i.e., HNO3) have been shown to accelerate damage to materials in the ambient air. NOZ affects dyes and fabrics, resulting in fading, discoloration of archival and artistic materials and textile fibers, and loss of textile fabric strength [28]. NO2 absorbs visible light and at a concentration of 0.25 ppmv will cause appreciable reduction in visibility. NO2 affects vegetation, as studies have shown suppressed growth of pinto beans and tomatoes and reduced yields of oranges. In the presence of sunlight, nitrogen oxides react with unburned hydrocarbons—volatile organic compounds (VOCs) that are emitted primarily from motor vehicles but also from chemical plants, refineries, factories, consumer and commercial products, and other industrial sources—to form photochemical smog.

Nitrogen oxides also contribute to the formation of acid rain. NO and NO2 in the ambient air can react with moisture to form NO- and H+ in the aqueous phase (i.e., nitric acid), which can cause considerable corrosion of metal surfaces. The kinetics of nitric acid formation are not as well understood as those for the formation of sulfuric acid discussed earlier. Nitrogen oxides contribute to changes in the composition and competition of some species of vegetation in wetland and terrestrial systems, acidification of freshwater bodies, eutrophication (i.e., explosive algae growth leading to depletion of oxygen in the water) of estuarine and coastal waters, and increases in the levels of toxins harmful to fish and other aquatic life [29].

Health Effects

Nitrogen dioxide acts as an acute irritant and in equal concentrations is more injurious than NO; however, at concentrations found in the atmosphere, NO2 is only potentially irritating and related to chronic obstructive pulmonary disease [28]. EPRI has shown from their early results from the ARIES study that the nitrate components of coal combustion do not have adverse health effects [30]. The EPA reports that short-term exposures (e.g., less than three hours) to current NO2 concentrations may lead to changes in airway responsiveness and lung function in individuals with preexisting respiratory illnesses and increases in respiratory illnesses in children from 5 to 12 years in age [29]. The EPA also reports that long-term exposures to NO2 may lead to increased susceptibility to respiratory infection and may cause alterations in the lung. Atmospheric transformation of NOx can lead to the formation of ozone and nitrogen-bearing particles (most notably in some western United States urban areas), which are associated with adverse health effects [29].

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