An innovative, large-scale waste to energy project in Germany is successfully employing fuel cell technology to efficiently produce electricity and heat from biogas.

In the German city of Leonberg, biological waste is now being efficiently converted into energy through the use of high-temperature fuel cell technology, in the first major example of a carbonate fuel cell being used to convert biogas from a digester plant into electricity and heat.

Since early 2005, biological waste generated in the district of Böblingen, Baden-Wurttemberg, which amounts to nearly 30,000 tonnes per year, has been prepared and processed at the digestion plant in Leonberg. The biomass is digested in a fermentation unit to produce a biogas, which is then utilized by two cogeneration units to produce electricity and heat.

In early 2006, the Waste Management Corporation (WMC), the operator of the digestion plant, commissioned the installation of a molten carbonate fuel cell (MCFC) to support the two cogeneration units, with the key objective to increase electricity yield by more than 25 per cent through an increase in efficiency.

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The MCFC installed is the Hot Module, which was developed by CFC Solutions GmbH (previously MTU CFC Solutions GmbH). It operates in the 250 kW class and generates in the region of 170 kW of heat from the methane gas produced by the digestion of compostable waste.

Fuel cell design

The Hot Module primarily consists of three separate components – a central cylindrical steel container, a gas cleaner and an electrical equipment enclosure.

The central steel container houses the horizontally configured stack of fuel cells, along with starting equipment, catalytic burner and mixing chamber for the fresh air. The gas cleaner is situated upstream of the system and conditions the fuel gas for the fuel cell process, and the electrical equipment enclosure contains the system controls and an inverter to convert the direct current from the fuel cell into an alternating current output ready to be fed into the AC grid.

The fuel cells use molten carbonate as an electrolyte. Each cell is constructed as a flat sandwich with two electrodes – an anode and a cathode – that enclose a foil filled with the lithium/potassium carbonate electrolyte. Tie rods hold together the 350 individual cells that make up the complete stack.

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A major emphasis of CFC Solutions’ fuel cell programme has been to reduce manufacturing costs, so that the cost/kW can be kept as low as possible. The design of the HotModule has made it possible to integrate many components into the central steel container that had previously been located outside of it. Thus, the HotModule has a much simpler design, and operates with fewer of these peripheral items.

Operation mode

As in all fuel cells, the electrochemical process is based on a reaction between hydrogen and oxygen, which releases electric power and heat. On the cathode side of the fuel cell, ambient oxygen is reduced and the oxygen ions combine with carbon dioxide to form carbonate ions, which then migrate via the electrolyte to the anode.

Here, they react with the hydrogen to form water and carbon dioxide. Electron surplus forms at the anode, a voltage is created between the anode and the cathode and a current flows. The oxygen is present in the air, but the hydrogen must first be released from the methane-containing biogas. This involves a reforming process that takes place in the fuel cell stack.

In the process the biogas and water are brought together and hydrogen is released. The reformation and the electrochemical process both occur at 650 °C, making the HotModule a high-temperature fuel cell.

Furthermore, the HotModule operates directly with either biogas or natural gas reaching a net electrical efficiency of 47 per cent, and depending on the heat usage an overall efficiency of 70 per cent can be achieved.

The central steel container houses the hoirzontally configured stack of fuel cells
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The emissions from the fuel cell system are classified as exhaust air in accordance with the Federal Emission Control Act (Bundes-Immisionsschutzgesetz, BImSchG).

This is because the waste gas primarily contains air, water vapour and a low level of carbon dioxide. The hot exhaust air is above 400 °C, and is utilized via a heat exchanger system.

Plant performance

The HotModule has been in operation in the Leonberg digestion plant since September 2006, operating with more than a 200 kW output. By the beginning of this year, it has accumulated 2500 operating hours, and a total of 320 MW of electricity has been fed into the electrical grid.

Project partners

CFC Solutions GmbH had a number of partners in the project, which RWE Fuel Cells GmbH, which acted as prime contractor for the overall fuel cell system; WMC and EnBW Energie Baden-Württemberg. WMC established a subsidiary named Biogas-Brennstoffzellen GmbH for the construction and operation of the fuel cell.

The project was regarded as a demonstration project, so more than 80 per cent of the total cost was financed by the Federal Republic of Germany, the State of Baden-Württemberg, EnBW Energie Baden-Württemberg, RWE Fuel Cells GmbH and DaimlerChrysler AG.

In addition, consultation support was provided by the Wirtschaftsförderung (Economic Development Corporation) of the Stuttgart Region, the Kompetenz- und Innovationszentrum Brennstoffzelle (Fuel Cell Competence and Innovation Centre), and the German Aerospace Centre. The remainder of the investment cost of the project was borne by the Waste Management Corporation, which includes a share of the fuel cell costs and expenditures relating to the installation of the fuel cell in the digestion plant and operational infrastructure.

Technological award

With the objective of giving public recognition to outstanding developments in fuel cell technology and stimulating further development, the Environment Ministry of Baden-Württemberg and Wirtschaftsförderung Region Stuttgart GmbH joined forces with EnBW Energie Baden-Württemberg to issue the ‘f-cell’ award, which was founded by the State of Baden-Württemberg. The award is given in recognition of practically-oriented developments in the innovative sector of fuel cells.

The commitment of the Waste Management Corp-oration to the Leonberg biogas project was recognized when it was awarded the third-place f-cell bronze award.