Environmental Engineering Reference
In-Depth Information
3.
Fuel NO mechanism
: For nitrogen-bearing fuels such as coal and biomass,
conversion of fuel-bound nitrogen to NO is often the main contribution to
NO formation. The nitrogen is released from the fuel during pyrolysis, mainly
as NH
3
and HCN. These components can subsequently be converted to NO or
N
2
following the same pathways as in the prompt NOmechanism. Additionally,
fuel nitrogen is retained in the char and is largely oxidized to NO in the char
combustion phase but may subsequently be reduced to N
2
by a fast heteroge-
neous reaction with the char. The amount of fuel nitrogen retained in the char
relative to the amount of fuel nitrogen released in the devolatilization phase is
partially determined by the thermal exposure of the fuel.
Nitrous oxide (N
2
O)
emissions are also a result of the oxidation of fuel nitrogen.
Although the N
2
O emission levels measured in various biomass combustion applica-
tions are very low (below ppm level), N
2
O emissions are worth mentioning because
N
2
O has a 310 times higher global warming potential than CO
2
.
Sulfur oxides (SO
x
)
are a result of complete oxidation of sulfur originating from the
fuel. Unlike coal, most biomass types hardly contain any sulfur. Mainly SO
2
(>95%) is
formed. Not all fuel sulfur will be converted to SO
x
; a significant fraction will remain in
the ashes, while a minor fraction is emitted as salt (K
2
SO
4
)orH
2
S at lower tempera-
tures. Sulfur dioxide considerably contributes to air pollution, with an array of adverse
respiratory effects including bronchoconstriction and increased asthma symptoms. In
addition, SO
x
is a precursor to acid rain and atmospheric particulates.
Part of the chlorine content in biomass can be released as
hydrogen chloride
(HCl)
. On contact with water, this forms corrosive hydrochloric acid. Inhalation of
the fumes causes severe respiratory problems. Not all chlorine is converted to HCl;
the main fraction is retained in salts (KCl, NaCl) by reaction with K and Na. Although
the chlorine content of wood is usually very low, significant amounts of HCl
may be formed from biomass fuels containing higher amounts of chlorine, such as
miscanthus, grass, and straw.
Particle
emissions during complete combustion originate from fly ash, which is a
result of entrainment of ash particles in the flue gas, and salts (KCl, NaCl, K
2
SO
4
),
which are a result of reactions between K or Na and Cl or S. Other types of particle
emissions are mentioned in the next section on emissions from incomplete combustion.
Particle emissions from combustion applications, in general, have a negative effect on
the human respiratory system and are carcinogenic. Furthermore, ash particles may have
adverse effects on boiler operation due to agglomeration, slagging, and fouling.
Heavy metals
(most importantly Cu, Pb, Cd, Hg) are present in all biomass fuels to
some degree. In combustion applications, these elements remain in the ash or evap-
orate. They may also attach to emitted particles or be contained inside fly ash particles.
Some heavy metals are toxic, and some are carcinogenic.
9.5.2 Emissions from Incomplete Combustion
When the biomass is not completely oxidized into its final products, intermediate reac-
tion products leave the reactor as part of the flue gas.
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