OCR Text |
Show structures are assumed to be unimportant. The char NOx path is much less efficient at converting N2 to NOx. However, there are less possibilities to reduce the efficiency than for the volatile-N2 route. The NOx formed by the char combustion can be reduced by the mechanism presented in Figure 1, but the char combustion takes place in an oxygen rich environment. Air-staged burner designs do not alter this. In summary, the fuel N2 can be converted to NOx via two paths, volatile matter and char, each having a distinct set of mechanisms. Up to 80% of the fuel NOx comes from the volatile path. This arises from a higher percentage of fuel N2 in the volatile matter and a high efficiency by this path. However, NOx emissions from volatile N2 species can be reduced by controlling the oxygen concentration to which the species are exposed. The char NOx can not be significantly altered by combustion staging. Both char and volatile NOx mechanisms assume that the form of the N2 is not important. For the volatile path, the starting point is HeN; for the char path, the starting point is nitrogen bound in heterocyclic compounds. Thus, the main difference between coals is the distribution of N2 between the volatile and char fractions. The amount of N2 contained in the volatile and char fractions of any coal is a function of total N2 and volatile content. Total N2 in coal ranges from about 0.5 to 2% with no correlation with coal rank. The volatile matter content of the coal is a function of coal rank and of the conditions to which the coal is subjected. In conditions similar to pulverized coal flames, 40 to 80% of the coal was found to be volatile material from coals with ranks of Low-Volatile Bituminous through Lignite (13]. For the same coals, the fraction of N2 reporting to the volatile phase was between 50 and 95%. In general the amount of volatile nitrogen is usually found to be proportional to the amount of volatile matter (11-13]. In addition to affecting the relative quantities of reactants, the coal quality can also affect the environment to which the reactants are exposed. The relative proportion of volatile material can affect the oxygen concentration and gas temperature to which the volatile-N2 species are exposed. Thus, the NOx yield can be affected. Although the effect of coal characteristics on gas composition and temperature appears to be a secondary effect, some researchers have postulated that these "secondary" effects are the main way that coal quality affects NOx [19]. 4 |