16 April 1999

Making methane pays – but

Muck – a product to be

valued, or an irritating

substance which is little

more than an unfortunate

but inevitable by-product of

stock production? Our Muck

Special takes a practical

look at the situation and

starts with the prospects for

methane production – does

it have a future?

TECHNOLOGY for powering an engine with farm produced methane is available off the shelf, but research in Lincolnshire suggests the production process needs plenty of care and attention. There are also questions to be asked whether such systems can be financially viable.

Methane production at Caythorpe, Grantham, has been working reliably so far this year, but teething problems in the early stages had already caused delays and increased start-up costs.

Production from pig slurry and other farm wastes is part of a research project to show how farms and small villages can generate their own electricity by using renewable energy resources. It is a project, which also includes a wind powered generator and a gasifier fuelled by wood chips, organised by the De Montfort Universitys Innovative Technology Centre based at the university farm at Caythorpe.

Renewable resources will play an increasingly important role in the production of energy during the next century, the research team believes, and the Caythorpe project was awarded more than £500,000 from EC funds to investigate ways of integrating production from different resources.

Wind power is the main energy source for the project, with the methane process and wood chip gasifier providing essential back-up when there is not enough wind to power the generator.

Energy production started last autumn and Andrew Chick, the projects research technician, says the wind generator was much easier to manage than either of the gas production processes.

"Equipment used to generate electricity from wind power is basically simple and reliable, and it needs very little maintenance," he points out. "The problem is that wind is not a reliable energy source, so you need an alternative in order to maintain a supply of electricity.

"There is nothing new about the process for producing methane. Every sewage works has an anaerobic digester producing methane – and methane production on a very small scale is used in some countries to produce energy for cooking. Some farmers have already installed equipment for producing methane, but our experience suggests it needs a lot of management input and it probably needs to be an enthusiast to get the best out of it."

Methane production needs an optimum temperature of 35C, and some of the energy available in the gas is used to maintain the required temperature in the digester. Methane produced at Caythorpe powers one of the two six-cylinder Waukesha spark ignition engines which were originally designed for natural gas but have been modified to run on biogas.

The engines are used to drive generators producing electricity, but waste heat in the water from the cooling system is circulated to two heat exchangers in the digester. Capacity of the Caythorpe digester is 120cu m which allows a 1.5 days average retention time. It is fuelled with slurry from part of the farms 1000-head pig fattening unit, with added waste from a vegetable processing plant.

Peak output from the methane powered engine is 40kW of electricity from 20cu m of methane per hour. Calorific value of the fuel is 27 megajoules per cu m, equivalent to about 150kW of energy. Of this about 40kW is electrical energy from the generator, 100kW is thermal energy including heat for the digester, with the rest waste energy.

"This project is attracting considerable interest, and I think there are probably a lot of farmers who would like to have a go at methane production," says Mr Chick, "but there are important factors to consider before investing in a digester."

One of the priorities is making sure there is an adequate supply of a suitable waste material, he warns. On a dairy farm, for example, there is usually plenty of slurry available through the winter, but very little during the rest of the year.

It is also easy to underestimate the amount of care and attention needed to keep the equipment working efficiently, he insists.

"In theory the equipment can run on its own, but in practice it is quite demanding. Its no use treating it as a sideline and checking it over when youve a few minutes to spare – it needs regular attention to get it working properly again," says Mr Chick.

The economics can also look unattractive, he warns. Anyone who is thinking of producing methane simply to generate electricity to sell to the Grid should think again – the economics simply dont add up.

"You need to think of methane production as part of waste management. The digestion process has virtually no effect on the fertiliser replacement value of the manure. I think the environmental aspects of manure disposal will be increasingly important in the future."