Agriculture Reference
In-Depth Information
HCN + 2H
NH
3
+ C
(3)
Formation of fuel rich zone adjacent to the fuel particle effectively reduces NO. Higher
de-volatilisation rate leads to fuel rich zones close to fuel particles and results in NO
reduction [Winter et al. 1999]. HCN is the main precursor to N
2
O. It oxidizes to NCO by O
and OH radicals which is converted to N
2
O and CO by reacting with NO by the following
reaction (4) [Winter et al. 1999]. In the presence of moisture rate of production of NCO can
increase due to the production of more OH radicals and thus conversion of NO to N
2
O by
reaction (4) can increase which can result in lower NO
x
emissions.
NCO + NO
N
2
O + CO
(4)
Volatile nitrogen is predominantly converted to NO but conversion levels are different
for different fuels. Fuels, having higher nitrogen content, show lower nitrogen conversion as
compared to those with lower nitrogen content. NH
3
can convert to NO and also can act as a
reducing agent. That is why it is used in thermal DeNO
x
and Selective Non-Catalytic
Reduction processes to reduce NO
x
emissions by the following reaction [Winter et al. 1999].
2/3NH
3
+ NO
5/6N
2
+ H
2
O
(5)
High nitrogen containing fuels give relatively higher concentrations of HCN and NH
3
during combustion which can reduce already formed NO by reactions (3) to (5).
Hydrocarbons form H-radicals which can also convert HCN to NH
3
. Hydrocarbon radicals
reduce NO to N
2
in the same way as during reburning, so called “intrinsic reburning effect”
[Niksa and Cho, 1996]. Below temperature of 900 °C, interaction of hydrocarbons with NO is
very small [Alzueta et al. 1997]. As all the results presented in this chapter are at 800 - 900
°C, it is possible that hydrocarbon influence on the NO reduction may not be considerable.
Conversion of nitrogen to NH
3
decreases with increase in partial pressure of oxygen as
NH
3
oxidizes to NO and N
2
if enough oxygen is present. With increase in oxygen partial
pressure HCN increases and reaches a maximum then decreases by possible conversion of
HCN to N
2
O [Winter et al. 1999] by reaction (4).Conversion of HCN to NH
3
by reaction (3)
takes place at low oxygen concentrations only [Miller and Brown, 1989]. It is well known
that NH
3
and HCN can act as NO reductants or oxidants depending upon temperature “so
called temperature window” and controlled by radical pools [Kristensen et al. 1996].
Depending upon oxygen partial pressure and concentration of radicals, temperature for
maximum reduction in NO is about 900 °C [Kasuya, et al. 1995].
In the case of coal more HCN is produced at higher temperatures mainly by tar cracking
reactions and NH
3
formation decreases [Nelson et al, 1992]. As NH
3
is mainly oxidized to
NO, the production of more HCN than NH
3
can lead to lower NO emissions.
NO
x
formation during biomass combustion at temperatures between 800 and 1100 °C is
mainly due to fuel bound nitrogen [Sjaak and Jaap, 2008]. Due to the lower nitrogen content
of biomass fuels, combustion of most of the biomass fuels is expected to reduce NO
x
emissions. However, co-firing particularly at low co-firing ratios has resulted in, both higher
and lower NO
x
emissions, from biomass coal blends than those from coal alone.
Biomass fuels have a different chemical structure to coals. In biomass, nitrogen is
predominantly present as aliphatic nitrogen in amino and ammonium structures while, in coal,
