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Gas turbine emissions using parametric emissions models in conjunction with gas Path Analysis Techniques
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'

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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.
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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.


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