Environmental Engineering Reference
In-Depth Information
Reducing emissions from stationary sources
The main emissions from stationary sources are carbon dioxide, nitrogen oxides, sulfur
dioxide, and particle matter. As seen in the previous section, these three offenders can be
reduced in part by selecting the right fuel and the remaining by applying reduction technologies,
which have associated high capital and operational costs.
Carbon dioxide
Net-zero carbon dioxide emission is possible when producing steam or on-site electricity
generation by using renewable resources such as biomass (i.e., wood, wood wastes, saw-
dust, and agricultural wastes), other waste, landfill gas, and methane from anaerobic fer-
mentation of wastewater (biogas). These sources of energy are considered to have zero
emissions because they contain “biogenic” carbon, which is part of the natural carbon bal-
ance that does not add additional carbon dioxide to the atmosphere (IPCC, 1997). The
downside of these sources of energy is that, due to transportation costs, the source has to be
close to the point of consumption. Also, net-zero carbon dioxide emission does not mean no
emissions of other pollutants. When burned, biomass produces significant amounts of par-
ticle matter, and landfill gas and biogas produce almost no particles but usually contain
sulfur that is transformed into sulfur dioxide during combustion. This is covered more in
Chapters 9, 10, and 11.
Another approach to reduce carbon dioxide emissions from stationary sources is cogenera-
tion (also known as heat and power) or a more recent concept called
trigeneration
. These
approaches may use either fossil fuels or renewable sources. These topics are covered in
Chapter 11; but briefly, they consist of producing mechanical energy and heat simultaneously
in a cogeneration system, and energy, heat, and cold in a trigeneration setup using a single fuel
source.
In a cogeneration system, fuel runs an engine or a turbine coupled to a generator that pro-
duces electricity; and the waste heat coming out of the exhaust is used to generate hot water
or low pressure steam that then is used for process applications. In a trigeneration system,
besides heat and electricity, “cold” is produced by running part of the heat through an absorp-
tion chiller.
The advantage of using on-site cogeneration or trigeneration is a highest of fuel utilization
efficiency. In a typical engine, the efficiency is around 30 percent, which means that 30 per-
cent of the fuel burned is converted into mechanical energy and the remaining 70 percent is
emitted into the atmosphere as wasted heat. When energy production is coupled with heat
recovery the efficiencies can be boosted to 75 percent or more (see Chapter 11).
Nitrogen oxides
There are four different methods to abate nitrogen oxides: precombustion, combustion con-
trol, combustion modification, and postcombustion:
Precombustion is about selecting the right fuel with low bound nitrogen content.
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Combustion control can follow several strategies such as reducing the temperature in the
combustion zone, reducing gas residence time in a high-temperature zone, and reducing
oxygen concentration in a combustion zone. These changes in combustion patterns are
achieved either by physical modification of the burners or by changing the operating
conditions (World Bank, 1998).
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