The measurement of pollutant emissions species from industrial plants, and demonstration of compliance to various standards, is an important consideration for most plant operators. However, measurement of these emissions is a complex and expensive process, especially if continuous monitoring is required.
Therefore if emission levels can be predicted using selected (pre-installed) plant operating parameters, then a significant cost saving can be made compared to a dedicated measurement system. Systems which predict emissions in this way are generally known as Predictive Emissions Monitoring Systems (PEMS)
Siemens Industrial Turbomachinery have developed a prototype PEMS which is a parametric model derived from an underlying chemical kinetics model to predict emission levels using engine control system parameters. Such a system has potential to represent the ‘next generation’ of PEMS, and may result in significant increases in accuracy over current PEMS systems. The introduction of more intelligent engine control systems also means that PEMS models which are more integrated with the engine controls system are potentially more capable of accurately predicting emissions compared to PEMS models which use parameters significantly ‘upstream’ or ‘downstream’ of the engine
National Grid are keen to develop PEMS capabilities as it potentially can yield the following benefits:
- PEMS represent a significant cost saving over continuous actual measurement
- The Siemens prototype may provide significantly more accurate results than current systems
- A ‘next generation’ PEMS system may help lead to acceptance of PEMS systems in legislation where it is not currently included
Siemens in-house results of the prototype model are encouraging, but a field application test is required to assess the viability of the model over a range of operating conditions.
The scope of this project is therefore to install and run the prototype PEMS system alongside an actual measurement system on an SGT-400 running at one of National Grid’s sites. The results will be periodically analysed and the suitability of the PEMS model assessed. Improvements to the model may result from this testing period.
Benefits
An assessment of the PEMS model based on a comparison to measured values taken over the same period, which should include a range of ambient and operating conditions. Modification to the PEMS model periodically is expected as more data becomes available.
In-house trials suggest that an accuracy of +/-5ppm relative to measured values can be achieved on NOx and CO levels down to 50% loads. It is expected that a similar accuracy can be obtained from the field trial. The stability of the PEMS software should also be assessed, although it is not possible to assign a value to this.
The PEMS should deliver at the emission limit value 95% confidence intervals of a single result that shall not exceed 10% for carbon monoxide and 20% for nitrogen oxides to be in line with the uncertainty requirements of continuous emission monitoring systems in Annex 5 Part III of the Industrial Emissions Directive.
