Pre-treatment potential

What is claimed to be the first biogas plant to run purely on waste instead of edible raw materials has been developed by German researchers.

A team from the Fraunhofer Institute for Ceramic Technologies and Systems in Dresden recently unveiled a new pre-treatment process for agricultural waste, such as corn stalks. The material can then be processed into biogas through a conventional fermentation process.

Michael Stelter, head of the Fraunhofer research team, claims the new pre-treatment process speeds up fermentation from the usual 80 days to only a month and generates 30 per cent more biogas than preceding technologies.

The Fraunhofer team demonstrated a new pilot plant that can convert waste-based biogas into electricity with a fuel cell and achieve an output of 1.5kW, or enough to power a family home.

Agricultural waste cannot be directly fermented because it contains cellulose. The Fraunhofer technology adds extra capabilities to biogas plants so such waste material can be processed.

Stelter explained that the process begins by shredding the material and washing it with water that contains salts and acids that specially selected microorganisms can eat. The slurry-like biomaterial is then put into a reactor to undergo a hydrolysis process. In that reactor, the cellulose is cut into smaller pieces so the microorganisms can eat and digest it better.

Then, high-powered ultrasonic waves are applied to the mix to break up the cells that form the biomass. The material is then ready to put into a fermenter.

Stelter compared the pre-treatment process to chewing food before swallowing it. He said: ‘You chew on a vegetable so it runs through your stomach better. The chewing shortens the time the biomass is in the main fermenter by more than 50 per cent and the biomass you get out is increased.’

For their pilot plant, the researchers diverted the gas into a high-temperature fuel cell with an electrical efficiency of 40 to 55 per cent. Stelter said this was much better than a gas engine, which can only achieve an average efficiency of 38 per cent.

The new biogas plant could operate on not just agricultural waste but also sludge from waste-water treatment facilities. However, Stelter said in order to do that industrially, the 1.5kW pilot plant will need to be scaled up. He said: ‘A reasonable size for a waste-water treatment plant is about 500kW.’

Stelter believes such a scaled-up version could be achieved quickly. ‘If somebody is now in the planning phase for a bigger biogas plant, producing about 200-500kW, we are now ready for discussions,’ he said.

Siobhan Wagner