24 May 2002

Get ahead of the rest, turn slurry into sparks

The Pig & Poultry Fair

2002, held at the NAC,

Stoneleigh last week

produced a number of new


Andy Collings reports

THE Pig & Poultry Fair is not perhaps the venue machinery buffs would put at the top of their list – assuming that pig or poultry production is not on their agenda.

But among the many exhibits aimed specifically at this sector of the industry was a smattering of introductions which perhaps have a more global appeal.

Not least was the launch of a new biogas production system which, supplied as a complete package, provides an opportunity to convert slurry and other farm waste into electricity – which can be sold.

Not a new idea by any means but, says its German manufacturer Weda, one of the first to make economic sense in terms of a realistic pay back period.

The main components of the system comprise an insulated stainless steel fermenting tank and a modified diesel engine/generator coupled to a control unit.

According to Weda, a 200-sow farrow-to-finish operation would produce about 1m gal of slurry/year which could be converted into 366,500kW of electricity worth 7-8p/kWh to total between £25,000 and £29,000. Such a plant would cost about £200,000 which, with maintenance costs would allow the investment to be recouped in less than eight years.

Weda also emphasises that the system results in slurry having less smell, and a product which retains its nitrogen content. The process also produces a significant amount of heat – some of which is used to maintain the slurry at its optimum 45C temperature, and the excess available for other heating tasks such as greenhouses.

So what is the actual process? Slurry is collected from cows or pigs – pig slurry is said to be better – and stored in a holding tank. This is pumped into the fermenter, a tank-like unit which is supplied in a variety of sizes to accommodate different herd sizes – a 150 sow unit would require one with a capacity of about 770cu m.

Gas created by the process is collected in the roof of the fermenter using a diaphragm with air pressure one side and methane the other to create a gasometer effect.

Within the fermenter, run heating pipes to create the optimum temperature for the methane bacteria to work efficiently. With out going into the actual chemistry involved, suffice to say that the methane is produced in the top layers of the slurry following a series of chemical changes.

Frequent top-ups of slurry from the holding tank – as many as six times a day allows the used slurry to overflow into a second holding tank to be spread later.

Meanwhile, the methane is piped to the engine – a modified Deutz 4-cylinder diesel – where it is used with a small amount of diesel fuel to power a generator and produce three-phase electricity. A heat exchange from the exhaust manifold provides the required heat.

A sophisticated computer control system maintains temperatures, slurry levels, electricity output and the timing of engine start up – electricity is worth more at certain times of the day than others.

Payment for electricity is not a simple affair. Unlike Germany where suppliers are guaranteed a fixed rate, the UK supplier is faced with a complicated system with payments fragmented into elements having different criteria. UK suppliers might also like to know that German biogas producers are able to obtain grants for their systems.

Even so, with UK now implementing the EUs Renewable Energy Obligation – a commitment for electricity companies to source 10.4% of their supplies from renewable resources by 2011 – there is an incentive for the government to make biogas production a viable and workable operation for UK farmers. &#42

A new fermenter being erected on a German farm. The country now has over 1500 biogas units in operation with many more planned. Will the same happen in the UK?

The powerhouse. A modified Deutz engine is used to power the generator and create a three-phase supply.