in excess of 13008C, whereas the fuel-NO is formed from
the nitrogen in the fuel [54]. The prompt- or so-called Fenimore-
NO results from the radical CH formed as an intermediate
at the flame front which reacts with the nitrogen of
the air to form HCN which, at high temperatures, reacts
further to NO [55]. At higher combustion temperatures, as
they are encountered in pulverized coal combustors and
smelting furnaces, the role of the thermal- and prompt-NO
is significant. However, at temperatures below 10008C,
which are recommended for agricultural residues and
especially for those with high contents of alkali oxides
and salts, NO would be expected to originate mainly from
the fuel nitrogen. The nitrogen contents of the agricultural
residues range from 0.6 to 3.5 wt% and are comparable to or
higher than those of coals (cf. Table 2).
The formation of NOx from fuel nitrogen takes place
through the combustion of the nitrogenous species released
with the volatiles and the oxidation of the nitrogen retained
in the char [56]. The dominant nitrogenous volatile species
are NH3 and HCN. Generally, the NH3 may decompose to
NH2 and NH radicals which can either be oxidized by O2 to
form NO or react with available NO and OH radicals to form
N2 [56]. On the contrary, the HCN, may decompose to NCO
with the help of an oxygen radical and thereafter NCO can
react with NO to form N2O [57]. The char-bound nitrogen
burns to form NO, N2O and N2 [58]. N2O is highly temperature
sensitive and at temperatures higher than 9008C it
decomposes to N2.