Lean Gas CHP

Synergy of climate protection and energy generation

The “Global Methane Pledge” is one of the most important results of the UN Climate Change Conference 2021 in Glasgow. 100 participating countries signed an agreement to strive for a reduction in global methane emissions of at least 30% by the year 2030.

The greenhouse gas methane is considered a climate killer: According to the 6th assessment report of the Intergovernmental Panel on Climate Change (IPCC), the global warming potential (GWP) of methane is 81.2 times higher than that of CO2 over a period of 20 years. The IPCC already published the first quantitative evaluation on the GWP of methane in its status report AR5 in 2014.

The reduction of methane emissions is currently considered to be the fastest and most effective lever to slow down the rate of global warming. According to the United Nations Environment Program (UNEP), substantial measures against methane emissions could reduce the expected mean global temperature increase by almost 0.3°C until 2045.

The combustion of methane is an effective way of its harmless disposal. Ideally, this takes place in internal combustion engines along with the generation of electrical and thermal energy. However, many methane sources contain accompanying substances such as sulfur, chlorine or siloxanes, which can damage internal combustion engines. If the methane content is well below 50%, it can’t be used in gasoline engines any longer. Often, there is only the option of flaring, with the disadvantage of wasting valuable energy.

Lean gas CHP with 7,5 kWel (Picture: Erwin Berghammer)

Stirling engines: suitable for problematic fuels

These relationships were decisive for the development of a Stirling lean gas CHP, which we started in 2016 as part of a research project funded by the FFG. As part of this project, we achieved two significant innovations that quickly led us to our goals:

1. The invention of the alphagamma® technology

The simple and inexpensive technology formed the basis for a highly efficient, out of competition drive unit. The characteristics of the engines with the internal designation “G600” convinced us straight away, but still offered sufficient scope for numerous improvements in detail.

2. The developement of an efficient lean gas burner

Preheating the combustion air by using the waste heat of the exhaust gases is essential to achieve a high overall efficiency of an Stirling engine CHP. Because of the high temperatures of the pre-heated combustion air, premixing burners can not be used. The fuel-air mixture would ignite before it enters the combustion chamber. In the absence of compact devices on the market which include a suitable heat exchanger for air preheating, we decided to develop our own.

The achieved properties of the burner unit confirm that this was the right decision:

• Methane content from 14% to 100% (measurement report)
• Exhaust gas temperature 100 °C
• No formaldehyde or methane in the exhaust gas
• NOx and CO far under the limits for lean gas
• Resistant to siloxanes
• Gas pressure between 576 to 3613 Pa, depending on the methane content

Synergy of climate protection and energy generation

The results of the research project open up a broad market area with a promising economic and environmentally relevant approach. The oxidation of methane in lean and problematic gases can be seen as an effective lever in the future treatment of climate-wrecking gases. This results in a number of applications that allow a CHP unit to be operated with negative fuel prices.

In the course of the research project, the Stirling CHP was operated with gases of different compositions. Take a look at the measurement reports from our research partner BEST Bioenergy:

• Measurement report landfill gas
• Measurement report agricultural biogas
• Measurement report sewage gas