This plant in Stuttgart, Germany, makes biogas out of waste from wholesale markets. Image: Fraunhofer IGB |
Mushy
tomatoes, brown bananas and overripe cherries—to date, waste from
wholesale markets has ended up on the compost heap at best. In future it
will be put to better use: Researchers have developed a new facility
that ferments this waste to make methane, which can be used to power
vehicles.
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. As a result, more and more
motorists are converting their gasoline engines to run on natural gas.
But just like oil, 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 now developed an alternative: They
have found a way to obtain this fuel not from the Earth’s precious
reserves of raw materials, but from fruit and vegetable waste generated
by wholesale markets, university cafeterias and canteens. Fermenting
this food waste produces methane, also known as biogas, which can be
compressed into high-pressure cylinders and used as fuel.
In
early 2012, the researchers will begin operating a pilot plant adjacent
to Stuttgart’s wholesale market. The facility uses various
microorganisms to generate sought-after methane from the food waste in a
two-stage digestion process that lasts just a few days.
“The
waste contains a lot of water and has a very low lignocellulose
content, so it’s highly suitable for rapid fermentation,” says 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,” explains 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 new plant lies in the fact that absolutely everything
it generates can be utilized; the biogas, the liquid filtrate, and even
the sludgy residue that cannot be broken down any further. A second
sub-project in Reutlingen comes into its own here, involving the
cultivation of algae. When the algae in question are provided with an
adequate culture medium, as well as carbon dioxide 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 carbon
dioxide that the algae need in order to grow; while the desired methane
makes up around two thirds of the biogas produced there, some 30% of it
is carbon dioxide. With these products put to good use, all that is left
of the original market waste is the sludgy fermentation residue, which
is itself 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.
The former uses membranes to process the biogas generated in the
market-place plant, while the latter supplies a number of experimental
vehicles designed to run on natural gas. The five-year project is funded
to the tune of six million euros by the German Federal Ministry of
Education and Research (BMBF). If all the different components mesh
together as intended, it is possible that similar plants could in future
spring up wherever large quantities of organic waste are to be found.
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
und the town Stuttgart.
