Published by Farmers Guide, December 2007.

Biogas feedstock is constantly pumped in at the bottom of each
of these two digesters. It takes almost six weeks to reach the
top, where it is removed and used as fertiliser.

Subsidies promote maize-derived electricity

THERE may still be only a handful of biogas production plants in the UK, but in Germany there are thousands, many using maize as feedstock, producing electricity for their local area. Farmers Guide recently got the chance to take a close look at one on-farm set-up in the north of the country.

In the drive to produce 'green' electricity, Germany's farmers have been encouraged to invest in biogas plants by generous government subsidies. Consumers pay about 10 cents/kW hour for their power, but if you construct a 500kW biogas power plant, you will get paid 18 cents for every kW hour of electricity you put into the national grid.

The plants are not cheap, costing about €1.50m (£1.03m), but there are now more than 3,600 of them in Germany. Cumulatively, they consume the harvest from about 300,000ha of maize, one-fifth of the country's total area of the crop.

With each plant capable of producing 12,000kW hours of electricity each day (with a value of £1,500), annual revenues of £0.5m are possible with a minimal labour requirement.

The farm Farmers Guide visited ran three biogas plants and dedicated 90% of its 1,000ha of land to growing maize. As well as chopped maize, slurry from a pig unit on the farm also went into the biogas feedstock, as did a small quantity of wheat or barley.

Biogas production in Germany uses continuous anaerobic digestion to turn the feedstock into gas and an inert digestate (anything the mesophilic bacteria used in the process can't break down). The gas is then used to power a generator, while the digestate is spread on the land.

There are three main components to a typical biogas production plant. The first is a mixing unit where the ingredients are combined to produce the feedstock. The plant pictured on these pages mixed 25% chopped maize and 25% slurry with 50% digestate, before adding a little wheat or barley to fine tune the mixture's pH level.

This mixture was then pumped into the second part of the process, the digester. Looking like a covered slurry ring, this is where the bacteria works on the feedstock to release the desired end products. As the process is a continuous one, the feedstock is pumped into the bottom of the digester and digestate is removed from the top. In all it takes between 40-50 days for the feedstock to pass through the process.

The digester is constantly pressurised so that the gas produced (a mixture of methane, CO2 and Hydrogen Sulphide) is sent to the final element in the process, where it is dried and then used to power a biogas engine that drives a 500kW generator. The heat produced by the engine is then recycled back to the digester to keep the bacteria at their ideal working temperature of about 37 degrees Celsius, while the electricity produced is fed into the local supply.

The plants are computer controlled and little human intervention is needed, apart from loading chopped maize into the collection hopper for mixing. The engines and generators require little in the way of maintenance and are designed for a five-year life at 98.5% availability (just five-and-a-half days downtime annually).

With the price paid for the electricity fixed by the German government, the country's biogas-producing farmers are extremely focussed on their costs to maximise profitability. The need for higher outputs from harvesting equipment - and the high demand for harvesters from contractors wanting to serve the sector - has already prompted the introduction of Claas' 830hp flagship Jaguar forager.

Like the latest big square balers from Massey Ferguson, that are increasing efficiency at Denmark's straw-burning power stations, industrial uses of crops and the quest for greener energy sources are helping steer agricultural innovation.

The methane gas powers an engine and
generator set to produce electricity.