Why is Emission Monitoring
necessary?
The need to reduce emissions is now of paramount importance in
protecting health and the environment. The last decade has seen a rapid
change in regulation for controlling gas turbine emissions. The
monitoring of emissions has therefore become an issue that can no
longer be ignored. Measuring gas turbine emissions from the exhaust can
be costly. We need a cost effective method of determining gas turbine
emissions.
Parametric Models
Many parametric models have been proposed and validated in predicting
emissions. However, these models often need engine measurements which
are difficult or almost impossible to measure on an engine operating on
a site. Such measurements often refer to combustion air flow and
combustion exit temperature (turbine entry temperature). If we can
derive these measurements then parametric models are a cost effective
solution for determining gas turbine emissions.
Gas Path Analysis
Techniques
The use of gas path analysis techniques enable us to determine these
parameters using the measurements that are readily obtained from the
engine. The technique solves the necessary equations using these
measurements to derive other parameters such as airflow and turbine
entry temperature. Close agreement has been achieved between measured
and derived measurements for airflow and turbine entry temperature for
various gas turbines. These techniques are extensively used and have
been validated using GPA’s gas turbine performance monitoring system'
The figure above shows the variation of oxides of nitrogen or NOx with
engine power output for a LM2500 PE gas turbine using gas path analysis
and parametric models. The parametric model used here is that proposed
by L E Bakken and L Skogly. There is close agreement between their data
and that generated by GPA. Production of NOx is dependant on turbine
entry temperature, fuel-air ratio and combustion pressure. Higher the
values of these parameters greater the NOx produced in the combustion
process. These parameters increase with engine power output, hence an
increase in NOx with engine power output as shown in the figure above.
Another pollutant emitted by gas turbines combustors is Carbon Monoxide
or CO. Again parametric models together with gas path analysis
techniques can be used to predict CO. The figure above predicts the
variation of CO with engine power output for LM2500 PE. Also shown is
some OEM values for comparative purpose. The parametric model used to
predict CO is that given by N K Rizk and H C Mongia. CO emissions
decreases with increase in primary zone temperature, combustion
pressure and combustion pressure loss. These parameters increase with
engine power output, which results in a reduction in CO with increase
in power output as shown above.
Gas Turbine Performance Deterioration
Gas turbine performance deterioration has a big impact
on NOx
and a small effect on CO. This is primarily due to the increase in
turbine entry temperature resulting from performance deterioration. For
example a moderately fouled compressor can increase the NOx emissions
by about 5%.
Since gas path analysis techniques are used to determine
performance deterioration (Fault Indices), the prediction of emissions
when performance deterioration is present can be accounted for.
XEMn
Gas path analysis and parametric modelling techniques
have now
been combined to produce GPA’s emissions monitoring software product
XEMn. It also predicts the emissions when gas turbine performance
deterioration is present. The XEMn will also predict the NOx emissions
when water or steam injection is use for NOx suppression. When used in
conjunction with XPGTn instrument fault can be highlighted.
Required Measurements
Measurements required for the calculation of emissions
for a typical two shaft gas turbine are as follows
- Compressor inlet temperature
- Compressor inlet pressure
- Compressor exit temperature
- Compressor exit pressure
- Gas Generator Exit Temperature EGT/TGT
- GG turbine exhaust pressure
- Gas generator speed
- Fuel flow Rate
- Lower Calorific Value of the Fuel (LCV)
Note that the system does not require the power or
compressor air inlet flow rate measurements.
