Meet gas engine emissions targets

Lower gas prices and increasing demand from the balancing services market, has meant that the market share for gas gensets is rapidly increasing. A by-product of this is that environmental agencies are giving more focus to gas engine emissions, including unburnt hydrocarbons (UHC) such as methane (CH4). This is not surprising, as the greenhouse gas effect of CH4 is 25-100 times that of CO2.

Typical UHC emissions from a natural gas engine can vary between 1-4% of the fuel supplied. UHC emissions from alternate fuel engines such as biogas or landfill gas can be even higher, due to the effects of both CO2 and power cylinder deposits on combustion.

Through applying ETC technology, we can significantly reduce these emissions as well as fuel consumption.

Causes of UHC emissions

  • Fuel short circuiting – This occurs when premixed air and/or fuel flows directly from the intake manifold to the exhaust manifold, due to the simultaneous opening of both intake valve and exhaust valve
  • Crevice volume – As the quenching distance on lean burn natural gas engines is quite large, the flame cannot always propagate into the tight gaps around the combustion chamber. Therefore, combustion does not occur, leaving UHCs behind after combustion which are then expelled directly to the exhaust

How ETC reduces UHC emissions

The ETC system dramatically mitigates these effects by reducing the normally positive scavenging pressure (intake manifold pressure minus exhaust manifold pressure) (P2’-P3) by in the region of 0.8 to 1.2 bar and pushing it negative. This almost entirely eliminates fuel short circuiting due to scavenging and reduces the flow of UHC rich residual gases from the piston and liner/gasket crevices during valve overlap to the exhaust.

Effect of applying the ETC

Additional benefits:

  • In-cylinder and exhaust temperatures are increased, which results in a greater level of post oxidisation of UHCs
  • Reducing UHC emissions has a direct and significant effect on the greenhouse gas emissions of the engine

With relation to meeting hydrocarbon emissions limits:

  • The costly and timely need for OEMs to re-develop power cylinder components to reduce crevice volumes can be avoided
  • The need for combustion chamber cleaning/decoking is decreased as the engine ages
  • Valve timing events can be re-optimised to further increase efficiency

Typical reductions in UHC emissions

Initial results on gas engines indicate that a 25-50% reduction in UHC is achieved by the ETC system. Results are dependent on base engine valve lift profile, piston or combustion chamber design, NOx emissions setting, fuel composition and how aggressively the ETC can be matched to the engine.

We recently achieved a 32% reduction in UHC for a waste to energy company in the UK. UHCs were reduced from 1100 to 750ppm following application of the ETC. This directly equates to approx. 0.5% fuel save or 0.2% electrical efficiency. Further engine output improvements are achieved by the ETC system itself. More information on which can be found here.

Overall reduction in greenhouse emissions

UHC improvementsOur ETC systems have saved over 300,000 tonnes of CO2 emissions as a direct result of fuel consumption improvements.

When considering gas engines it is important to also include CH4 emissions to understand the greenhouse impact. It is estimated that when these emissions are included the ETC system achieves an 8 – 23% reduction* in greenhouse emissions over a 20-year time horizon.

For the specific application mentioned above reducing the UHC from 1100 – 750ppm equates to a reduction in greenhouse gas emissions of 12% when taking both the direct reduction in CO2 emissions from the total fuel savings (~5%) and reduction in CH4 emissions into account.

Through this combined approach to reducing emissions, our customers are able to better meet current and future emissions requirements without the need for costly engine development. For further information on our systems please contact us.

* Based on data from the Intergovernmental Panel on Climate Change, IPCC 2007 – AR4.