Gas Turbine Handbook : Principles and Practices

Gas Turbine Exhaust Treatment 133
Carbon Monoxide (CO)
Carbon monoxide emissions are a function of the combustor
design, specifically the combustor’s primary reaction zone. New combustor
designs are being evaluated to reduce these emissions. In the
interim, CO can be effectively treated with a catalytic converter.
Oxides of Nitrogen (NO x
)
Oxides of nitrogen are produced primarily as nitric oxide (NO)
in the hotter regions of the combustion reaction zone of the combustor.
Nitrogen oxides (NO x
) found in the exhaust are the products of
the combustion of hydrocarbon fuels in air. In this process nitrogen
oxides are formed by two mechanisms: Thermal NO and Organic NO.
The predominant mechanism in the formation of NO x
in gas turbine
combustors depends on such conditions as the reaction temperature,
the resident time at high temperature, the fuel/air ratio in and after
the combustion reaction zone, the fuel composition [the fuel bound
nitrogen (FBN) content], the combustor geometry, and the mixing
pattern inside the combustor.
• Thermal NO is extremely temperature dependent, and therefore,
is produced in the hottest regions of the combustor. Water
and steam injection contribute to reducing the combustion temperature
at a given load.
• Organic NO is formed during combustion by chemical combination
of the nitrogen atoms, which are part of the fuel molecule,
and the oxygen in the air. The amount of Organic NO produced
is affected by the nitrogen content of the fuel, a yield factor (a
measure of FBN), the fuel/air ratio, and abatement techniques
such as water and steam injection.
NO x
emissions from burning various fuels, compared to burning
methane (CH 4
) gas, can be estimated as shown in Figure 9-1
for gaseous fuels and Figure 9-2 for liquid fuels. 7 As shown in these
figures, NO x
emissions for most liquid fuels (except liquid methanol)
are higher than methane gas. While the NO x
emissions from most
gaseous fuel corresponds to that of methane. This is directly related
to the amount of fuel bound nitrogen in the fuel. Fuel gas treatment,
such as water scrubbing, can remove some of the nitrogen-bearing