Nh O No H 654

The main reaction product of hydrocarbon radicals with N2 is HCN, and the amount of NO formed is governed by the reactions of the nitrogen atoms with available radical species. In fuel-rich environments, therefore, the formation of N2 is favored due to the reduced concentrations of hydroxyl and oxygen radical concentrations [21].

Fuel NO Nitrogen in the coal, which typically ranges from 0.5 to 2.0 wt.%, occurs mainly as organically-bound heteroatoms in aromatic rings or clusters [21]. Pyrrolic (5-membered ring) nitrogen is the most abundant form and contributes 50 to 60% of the total nitrogen. Pyridinic (6-membered ring) nitrogen comprises about 20 to 40% of the total nitrogen. The remaining 0 to 20% nitrogen is thought to be in amine or quaternary nitrogen form. Coal nitrogen is first released during volatilization in the coal flame as an element in aromatic compounds referred to as tar. The tar undergoes pyrolysis to convert most of the nitrogen to HCN as well as some NH3 and NH. Some nitrogen is expelled from the char as HCN and occasionally NH3; however, this occurs at a much slower rate than evolution from the volatiles. The partitioning of nitrogen between volatiles and char is important in NOz formation.

Nitric oxide formation proceeds along two paths [21]. The nitrogen from the char reacts with oxygen to form NO. The NH3 and NH released from the volatile matter and, to a lesser extent, the coal reacts with oxygen atoms to form NO. HCN is converted to NO via a pathway of hydrogen abstraction to form ammonia species and subsequently NO. Volatile nitrogen species can also be converted to nitrogen atoms through a series of fuel-rich pyrolysis reactions. Also, reactions between NO and volatile nitrogen species and carbon particles can result in the formation of nitrogen molecules:

In a fuel-rich environment, the main product of the reaction of NO with hydrocarbon radicals is HCN, which is then converted to N2 in an oxygen-deficient environment. This is the basis for reburning, discussed later in this chapter, where a secondary hydrocarbon fuel is injected into combustion products containing NO.

Approximately 15 to 40% of the fuel nitrogen is converted to NO, and the formation of NO is influenced by stoichiometry, flame temperature, coal nitrogen content, and coal volatile matter content [21]. Approximately 25% of the char nitrogen is converted to NO. The reason for the fuel NO dominance (i.e., 75-95% of total NOZ production) is because the N-H and N-C bonds, common in fuel-bound nitrogen, are weaker than the triple bond in molecular nitrogen, which must be dissociated to produce thermal NO.

Nitrogen Dioxide and Nitrous Oxide Small amounts of nitrogen dioxide (NO2) and nitrous oxide (N2O) are formed during coal combustion, but they

comprise less than 5% of the total NOZ production. The oxygen levels are too low and the residence times are too short in high-temperature coal flames for much of the NO to be oxidized to NO2. Nitrous oxide, however, can be formed in the early part of fuel-lean flames by gas-phase reactions [21]:

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