Excess gas pressure becomes electricity

A process that is flexible enough to adapt optimally to the fluctuating gas purchases of municipal utilities, that saves costs, reduces CO₂ emissions and can also contribute to improving the company's image. Thanks to new materials for seals, state-of-the-art control technology and digitalization, the Spilling gas expansion engine still provides municipal utilities today with a cutting-edge tool for highly efficient power generation.
And this is how gas expansion works: The natural gas that the municipal utilities distribute in their networks is compressed to high pressure at the upstream supplier. This is the only way the gas can be transported over long distances. For distribution in the networks of the municipal utilities, however, a much lower pressure is required, so that the gas pressure in gas transfer stations is regulated to the desired pressure level. In many cases, the available excess pressure energy is simply dissipated. This is a pity, because when a gas expansion motor is used, the pressure gradient between natural gas transport networks and distribution systems (or consumers) is used to extract mechanical or electrical power. This type of power plant offers maximum efficiency.

A company in Northern Germany has been using this method successfully since 1988. The plant produces approximately 4800 Megawatt hours of electricity annually. In terms of figures, this amount is sufficient to supply 1200 four-person households with electricity each year. With this process alone, the municipal utilities contribute to an annual reduction of around 2500 tons of CO₂.

The machine mentioned above has been running for over 30 years without any major disruptions and has now been modernised once again. In the past, a little oil had to be added to the gas to lubricate the cylinders, but now it works oil-free thanks to robust new polymer seals. In addition, the control technology has been modernized and is now also state of the art again.

However, the process does not work entirely without additional energy input. Gas cools down considerably during the expansion in a power engine. To prevent icing, the gas must therefore be preheated before the pressure is reduced. Even if the heat for this came from a conventional gas boiler, over 90 percent of the additional thermal energy required for the gas expander would be converted into electricity. An extremely high overall efficiency. The municipal utilities of the North German city have solved the gas preheating problem even more elegantly: there, waste heat from the combined heat and power unit (CHP) on the company´s premises is used to preheat the gas. Overall, the combination of gas expansion engine and CHP unit achieves highly efficient power generation with significantly lower CO₂ emissions than in the conventional German power mix. If waste heat is available for gas preheating that is otherwise not usable anyway, as is often the case in industrial applications, this even results in completely CO₂-free power generation.