Environmental Engineering Reference
In-Depth Information
Combustion modification has also several alternatives, which include retrofitting existing
equipment with low nitrogen oxide burners; injecting diluents, such as water, steam, or
flue gas; reburning; and pulse firing. Reburning consist of dividing the combustion cham-
ber into two zones, the primary zone and the reburn zone. In the reburn zone, additional
fuel is added and because there is a natural deficiency of oxygen, nitrogen oxides produced
in the primary zone are used as a source of oxygen for combustion, thus converting nitro-
gen oxide into N 2 and water (Maly et al., 1999).
Postcombustion approaches focus on the treatment of the flue gases to strip them from
nitrogen oxides. These are effective techniques but normally more expensive than other
approaches. Selective catalytic reduction is performed by injecting anhydrous ammonia,
or urea, in the presence of a catalytic bed made of metal oxides including vanadium,
platinum, and tungsten. The same technology is also effective to reduce sulfur dioxides
from the flue gases. Selective catalytic reduction is effective at reducing nitrogen oxides
and sulfur dioxide; however, it has several drawbacks such as the formation of secondary
pollutants (ammonia and ammonium sulfate), cost, anhydrous ammonia handling, and
corrosion problems. Another option is carbon adsorption, which also captures VOCs and
sulfur dioxide, in a fluidized bed adsorption (Latta and Weston, 1998; World Bank, 1998).
Sulfur dioxide
The best approach to reduce emissions of sulfur dioxide is avoiding fuels with high content of
sulfur. However, when this is not possible, there are ways to capture sulfur dioxide before fuel
gases are discharged into the atmosphere. Several technologies to capture sulfur dioxide exist,
but all of them work by making this compound react with an alkali (the sorbent) inside a scrubber
or the combustion chamber. Typical sorbents are calcium carbonate (CaCO 3 ) and calcium hydrox-
ide Ca(OH) 2 where sulfur dioxide reacts with the bases according to the following reactions:
SO
+
CaCO
→+
CaSO
CO
[8.18]
2
3
3
2
SO
+
Ca(OH)
+
½ O
CaSO
+
H O
[8.19]
2
2
2
4
2
The reaction with CaCO 3 takes place normally in a wet scrubber with the associated
production of a liquid waste stream that needs further treatment. The reaction with calcium
hydroxide takes place generally in a “dry system” that produces solid particles of calcium
sulfate (CaSO 4 ), which are separated from the gas stream in a bag system (DOE, 2001;
Cheremisinoff, 1993).
Particle matter
One of the by-products of solid fuel burning is the production of particle matter. Several
technologies are available to capture particles before discharging the waste gases to the
atmosphere, including electrostatic precipitators (ESPs), fabric filters (baghouses), wet
particulate scrubbers, mechanical collectors (cyclones), and hot-gas particulate filters (Miller
and Tillman, 2008). However, it is always less expensive to avoid the formation of particles, if
possible, during combustion than cleaning the gases once they leave the combustion chamber.
Particles are not just a problem derived from combustion. In food processing, there are other
sources of particles that result from handling, size reduction, cleaning, and drying (e.g., flour,
sugar, starch, and spices). Some of the technologies mentioned previously can be used to
capture particles from processes. However, containing particles at the source is always a better
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