Food waste makes fuel for cars

February 10th, 2012 by Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB

Researchers have developed a facility that ferments food waste to make methane, which can be used to power vehicles.

This plant in Stuttgart makes biogas from food waste generated by wholesale markets (click image to expand - ©Fraunhofer IGB)

Drivers who fill up with natural gas instead of gasoline or diesel spend less on fuel and are more environmentally friendly.

Natural gas is kinder on the wallet, and the exhaust emissions it produces contain less carbon dioxide and almost no soot particles.

Many German motorists are converting their petrol engines to run on natural gas. But natural gas is also a fossil fuel, and reserves are limited.

Researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart have developed a facility to produce natural gas from fruit and vegetable waste generated by wholesale markets, university cafeterias and canteens.

Fermenting the food waste produces methane, CH₄, also known as biogas, which can be compressed into high-pressure cylinders and used as fuel.

In early 2012, researchers will begin operating a pilot plant adjacent to Stuttgart’s wholesale market.

The facility uses various microorganisms to generate methane from the food waste in a two-stage digestion process that lasts just a few days.

“The food waste contains a lot of water and has a very low lignocellulose content, so it’s highly suitable for rapid fermentation,” said Dr.-Ing. Ursula Schließmann, head of department at the IGB.

“But it still presents a challenge, because its precise composition varies every day.”

“Sometimes it has a high proportion of citrus fruits, while other times there are more cherries, plums and lettuce. On days with a higher citrus fruit content, the researchers have to adjust the pH value through substrate management, because these fruits are very acidic.”

“We hold the waste in several storage tanks, where a number of parameters are automatically calculated – including the pH value. The specially designed management system determines exactly how many liters of waste from which containers should be mixed together and fed to the microorganisms,” said Dr. Schließmann.

It is vital that a correct balance be maintained in the plant at all times, because the various microorganisms require constant environmental conditions to do their job.

Another advantage of the plant is that absolutely everything it generates can be used: the biogas, the liquid filtrate, and the sludgy residue that cannot be broken down any further.

A second sub-project in Reutlingen involves the cultivation of algae. When the selected algae are provided with an adequate culture medium, as well as C02 and sunlight, they produce oil in their cells that can be used to power diesel engines.

The filtrate water from the biogas plant in Stuttgart contains sufficient nitrogen and phosphorus to be used as a culture medium for these algae, and the reactor facility also provides the researchers with the C02 that the algae need to grow.

While the desired methane makes up around two-thirds of the biogas produced there, some 30 percent of it is c02. With these products put to good use, all that is left of the original market waste is the sludgy fermentation residue, which is converted into methane by colleagues at the Paul Scherrer Institute in Switzerland and at the Karlsruhe Institute of Technology.

Others involved in this network project, which goes by the name of ETAMAX, include energy company EnBW Energie Baden-Württemberg and Daimler AG.

EnBW uses membranes to process the biogas generated in the marketplace plant, while Daimler AG supplies a number of experimental vehicles designed to run on natural gas.

The five-year project is funded with €6 million from the German Federal Ministry of Education and Research (BMBF).

If all the different components of this research project come together as intended, it is hoped that similar plants will spring up wherever large quantities of organic waste occur.

Other project partners are the Fraunhofer Institute for Process Engineering and Packaging IVV in Freising, FairEnergie GmbH, Netzsch Mohnopumpen GmbH, Stulz Wasser- und Prozesstechnik GmbH, Subitec GmbH and Stuttgart.


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