Farm Diversification

John Rennie and Sons are agricultural contractors based near Turriff in Aberdeenshire. They have their own land for cereal production and they keep pigs. The company provides a range of services to local farms including spreading fertiliser.

Why Install Renewable Energy?

In the past they used to spread waste from the local slaughterhouses directly onto their own land but a tightening of legislation for disposal of this material in 2001 led to a search for a suitable treatment option.

Researching the Idea

It was at this time that an advertisement from Weltec BioPower, promoting anaerobic digestion (AD), was noticed in the farming press. Andrew Rennie compared this to other options and in 2002 he and his father, John Rennie, decided that AD would be their preferred option. Following this they researched other suppliers of suitable equipment but concluded that the Weltec system, imported from Germany, best suited their farm.

Establishing the Project

It is important to note that, at that time there were no other farm scale AD plants working commercially in the UK. The regulators including the planning authorities, Environmental Health Officers, Scottish Environmental Protection Agency (SEPA) and the State Veterinary Service (SVS) had to develop their knowledge of the process and define their requirements. This took time and effort but, to their credit the Rennies completed almost all the necessary documentation and permitting themselves. This took around 1,000 hrs per year for 4 years.

The plant that was planned was a 450 kW system, costing around £2 million. It comprised a single digestion tank, 6 m high, containing 2400 m3, plus a digestate storage tank, CHP system and necessary mechanisms for feeding and controlling the digester. The digester is fed with pig slurry (about 3000 tonnes per annum) as a base load, with other materials added when available to give a total throughput of around 15,000 tonnes per annum. The material remains in the digester for 50 days before being transferred to the digestate store. The system is set up and authorised to digest Category 3 Food waste which includes fish wastes and some (not infectious) types of waste from slaughterhouses and other sources. For this a system was needed to pasteurise the digestate to kill any pathogens. The extra installation cost of this facility is offset by increased gas yields and gate fees received for disposing of the wastes. The plant was commissioned in 2006 and, with some setbacks, has been working successfully since.

The methane produced is fed into a number of engines that drive the generators allowing the power to be exported to the grid. The grid connection was relatively easy and cost around £80 000. The electricity is sold for an agreed price and, in addition, Renewable Obligation Certificates (ROCs) are claimed for all power generated. The value of these fluctuates with the market: they started at around £36/MWh, peaked at £53/MWh and are now around £48/MWh. The heat produced by the engines is used to keep the digester up to a temperature of around 40 degrees C and to pasteurise the digestate.

The digestate is stored until it can be applied to farmland in place of artificial fertiliser. There are limits on how much can be applied to each hectare so it is important to ensure that adequate area will be available to take all the output of the digester.
With increasing numbers of digesters in the UK authorisation should become easier however for systems treating waste for application to farm land strict controls are still needed and these will add to the cost of the development.

Key considerations for potential developers

• Obtain reliable sources of feedstocks
• Carefully assess the economics of the project from the outset
• Speak to people who have been through the installation process in the UK
• Installations that have a UK agent, who is experience of local requirements, will have fewer problems
• Ideally opt for a turnkey facility, reducing time and input during installation
• Feed the digester a consistent diet and monitor conditions with regular sampling
• Set up a cost effective engine maintenance programme that can be provided by mechanics located nearby.

The Rennies can be contacted via their website: http://www.gaskfarm.co.uk/ and to date have shown the plant to a vast number of interested farmers, potential developers, regulators and legislators.