Ottobrunn--The HotModule carbonate fuel cells made by CFC Solutions GmbH, a Tognum Group company, are a natural choice for operation with biogenic gases. Due to their operating principle, these fuel cells (unlike many other types of fuel cells) are not only insensitive to CO2 in the fuel gas, but in fact have higher electrical efficiency when running on gases rich in CO2, such as biogas or sewage gas. Diverse applications in biowaste utilisation systems and sewage treatment plants have demonstrated the suitability of this electrochemical energy converter for biogenic fuels. The most recent installation of a HotModule in this application area was in late 2007 at the Moosburg sewage treatment plant near Munich (Germany). The CFC Solutions GmbH, Christa-McAuliffe-Straße, 85521 Ottobrunn plant feeds the generated electricity into the grid. The useful heat of the hot exhaust air, which has a temperature of around 400 °C, is used for efficient drying of the sewage sludge.

Using biogenic gases with the HotModule fuel cells instead of natural gas requires only two significant changes from a system engineering perspective. As biogas and sewage gas have a higher proportion of hydrogen sulphide, a purification stage with active carbon filters is fitted ahead of the fuel cell in the gas supply system. This protects the cell stack against damage from sulphur compounds. The second significant change relative to natural gas operation results from the lower caloric value of the biogas or sewage gas. In order to make up for the lower energy content, the gas flow is increased depending on the methane concentration. The engineers of CFC Solutions drew on many years of experience in biogas and sewage gas applications to develop these filtering and fuel gas control solutions.

New HotModule generation provides more power

The HotModule has a high electrical cell efficiency of 55%, which is also one of the positive features of the latest model, the HM320. The HM320 is the first model of a new series first presented publicly in April this year. With this model, CFC boosts the maximum electrical power to 345 kW, and the thermal power increases to 250 kW. ‘Beside the HM 320, we also want to offer systems up to two megawatts in the coming years’, said Michael Bode, Managing Director for engineering at CFC Solutions.

Unlike other types of cogeneration systems, the HotModule generates electricity efficiently under partial load as well as under full load. This makes the fuel cell an ideal ‘base load generator’ in biogas and sewage gas systems. In addition, the HotModule is environmentally friendly in operation, since it CFC Solutions GmbH, Christa-McAuliffe-Straße, 85521 Ottobrunn produces CO2-neutral electricity and heat when running on biogas or sewage gas. The HotModule is practically free of pollutant emissions. With figures of less than 0.01 ppm sulphur dioxide, 2 ppm nitrogen and 9 ppm carbon monoxide, the exhaust can even be classified as ‘exhaust air’ instead of ‘exhaust gas’ according to the German TA Luft regulations.

Hybrid power plant adapts power generation to gas volume

For sewage treatment plant operators and biogas utilisation systems, which have fluctuating gas production volumes, CFC Solutions in cooperation with its affiliated company MDE Dezentrale Energiesysteme GmbH in Augsburg offers the HotModule Hybrid in addition to the HotModule. ‘This is a small power plant that combines our HM320 with a gas-engine cogeneration unit and supplies a total of up to 700 kW of electrical power’, according to Bode. In a hybrid system of this sort, the fuel cell supplies the basic load and runs continuously, preferably at a constant load level. An overall control system governs the use of the engine-driven cogeneration unit according to the fill level of the gas storage tank. There are also good economic reasons for continuous operation of the fuel cell. Due to its high electrical efficiency, it yields higher electrical income* per cubic metre than the gas-engine system.

* In Germany, the fee paid for electricity fed into the grid is set by the Renewable Energy Act.