Enlarge image

Innocent-looking high efficiency ... from straw!

(© picture-alliance/dpa)

German researchers have developed the first-ever biogas plant to run purely on waste like straw instead of edible raw materials, thereby transforming waste into valuable material.

The plant generates 30 percent more biogas than its predecessors. A fuel cell efficiently converts the gas into electricity.

"Corn belongs in the kitchen, not in biogas facilities" – objections like this can be heard more and more frequently.

They argue against the fermentation of foodstuffs in biogas plants that generate electricity and heat amid fears that generating electricity in this manner will cause food prices to escalate.

So scientists at the Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) in Dresden - along with several small and medium-sized enterprises - have developed the first biogas plant that works entirely without edible raw materials.

"In our pilot plant, we exclusively use agricultural waste such as corn stalks – that is, the corn plants without the cobs. This allows us to generate 30 percent more biogas than in conventional facilities," said IKTS department head Michael Stelter.

Until now, biogas plants have only been able to process a certain proportion of waste material, as this tends to be more difficult to convert into biogas than pure cereal crops or corn, for instance.

This is not the only advantage: The time for which the decomposing waste material, or silage, is stored in the plant can be reduced by 50 to 70 percent. Biomass is usually kept in the fermenter, building up biogas, for 80 days. Thanks to the right kind of pre-treatment, this only takes about 30 days in the new plant.

"Corn stalks contain cellulose which cannot be directly fermented. But in our plant, the cellulose is broken down by enzymes before the silage ferments," Stelter explains.

The researchers have also optimized the conversion of biogas into electricity. They divert the gas into a high-temperature fuel cell with an electrical efficiency of 40 to 55 percent. By comparison, the gas engine normally used for this purpose only achieves an average efficiency of 38 percent.

The fuel cell moreover operates at 850 degrees Celsius. The heat can be used directly for heating or fed into the district heating network.

If the electrical and thermal efficiency are added up, the fuel cell has an overall efficiency of up to 85 percent. The overall efficiency of the combustion engine is usually around 38 percent because its heat is very difficult to harness.

The researchers have already built a pilot plant with an electricity output of 1.5 kilowatts, enough to cover the needs of a family home.

They will present the concept of the new biogas plant at the Hannover Messe, an international series of 10 separate trade fairs in northern Germany that draw participants from over 60 countries, from April 20 to 24 (Hall 13, Stand E20).

In the next phases of the project, the scientists and their industrial partners plan to gradually scale up the biogas plant to two megawatts.

Source: Fraunhofer

Related Link:

Fraunhofer Institute for Ceramic Technologies and Systems (IKTS)

Hannover Messe