More gas than wind

Landfill contributes the largest proportion of renewable energy in the UK. So why don't we know more about it?

In the shadow of Haytor on Dartmoor, a stream of lorries queues to disgorge its innards on to a 35m high pile of old mattresses, yoghurt pots, garden clippings and assorted junk; the collective rubbish of south Devon and beyond.

Heathfield landfill site opened for business in 1998 and will close in 2016 after burying nine million tonnes of refuse beneath a layer of clay and topsoil.

Yet, if you listen carefully, hidden behind the thumping rhythm of the waiting dumper trucks hangs a different note. Softened into melody by metal containers and thick sound insulation, six massive piston engines pound away, fuelled by nothing more than the sodden methane emitted from the mounds of decaying rubbish. For Heathfield generates 6Mw of electricity, enough for 6,000 simultaneous kettles, 24 hours a day, seven days a week.

It is little known that energy from landfill sites accounts for the biggest proportion of the UK's renewable energy total. According to the department for Business, enterprise and Regulatory Reform, in 2006 landfill production made up 33 per cent of all renewable energy compared to only 8.2 per cent for wind. Landfill gas gives us more than four times the energy than we get from fancy wind turbines, and it's all due to bacterial decay.

When organic matter breaks down in the absence of oxygen - a process known as anaerobic digestion - methane and other gasses making up landfill gas, or LFG, are produced as a by-product. LFG has often been left to disperse in the atmosphere, but with increasing climate change concerns, heightened by the fact that molecule for molecule, methane is 21 times more potent than carbon dioxide as a greenhouse gas, it is becoming unacceptable to allow the methane to escape.

Green campaigning group Friends of the earth would prefer waste to be recycled or reused, but nick Rau on the Foe energy team points out that while government figures show that 62 per cent of municipal solid waste is still directed to landfill, capturing and using the methane has to be a priority. Many older landfill sites produce LFG for decades after the last dumper truck departs and DEFRA estimates that methane from landfill accounted for 3 per cent of total greenhouse gases in 2005.

Explosive consequences

Yet capturing LFG and burning it to produce electricity is not a new idea. Before landfill gas was identified as a greenhouse criminal it was acknowledged as an explosive menace.

In 1965 a young boy playing in a dugout in his backyard in California was burned in a flash fire when he attempted to light a candle. Investigations by the Los Angeles County engineer attributed the presence of gas from the landfill in Monterey Park. In March 1986, an explosion ripped apart a bungalow adjacent to the landfill site at Loscoe, Cumbria. LFG had escaped and collected in the kitchen.

Strategies for dealing with this unwelcome side effect to municipal waste handling include venting, by intercepting the gasses migration routes with permeable 'chimneys', flaring and electricity generation. But it wasn't until the advent of legislation, first the non-fossil fuel obligation (NFFO), then later the renewable obligations (in force April 2002), whereby electricity generators were obligated to use renewable sources, that the commercial possibilities emerged.

Awareness of the explosive tendency of methane combined with the political swing away from landfill, and the climatechange imperative, have resulted in ever tighter legislation. The challenge is on waste management companies to control and profit from LFG.

The first known commercial operator was Thomas Graveson in 1981, a concern now run by waste management company, Infinis. Today, there are hundreds of landfill generation sites dotted across the UK and many large companies investing in this mature technology. Infinis is the market leader with over 80 sites countrywide and an installed capacity of 250Mw, sufficient to power approximately 400,000 homes.

But under the watchful eye of environmentalists and government, operating a landfill site is a far distant experience to that of yesteryear's bury it and forget it mentality. Prospective sites are carefully surveyed and equipped years before the first waste arrives.

Heathfield is operated by waste management company, Viridor. Barry Shaw, the area manager, explains how the site is prepared with a triple composite liner of 300mm of clay, a layer of geosynthetic clay, then 2mm of welded plastic lining destruction tested for air-tightness. On top of this is a drainage layer comprising of gravel and pipework, to remove Leachate - the toxic fluids and water released during decay and settlement. Finally, a 2m layer of protective bagged or shredded waste is spread across the top in readiness for the first trucks.

Once waste has accumulated, methane is emitted almost immediately and can be collected. Like drilling for oil, wells are sunk into the landfill and perforated pipes inserted to within 5m of the bottom. Wells are centred at 40m connected by a plastic ring main. A large vacuum pump sucks out the methane at a rate determined by the slow decay and prevailing conditions; not too quick that reserves are depleted, but not so slowly that gas can collect in pockets. It's a fine balancing act, controlled by well-head valves and operator's experience.

Previously, landfills were capped with an impermeable layer of clay and topsoil, but Shaw is now required to collect the methane as early as possible. Unfortunately, the early gas is most damaging for the engines and equipment. "It's more corrosive, it's quite a brutal environment," he says.

As a result, engine maintenance is his biggest cost, at approximately 0.7p per kW. On top of a five-week service, every six months each of the massive generators is stripped and re-built. The 20 spark plugs are replaced at a price of £8,000 and 700 litres of expensive low-ash oil is changed - sometimes this happens more often.

LFG is not conditioned at Heathfield. Once the moisture has been removed in condensing traps, the generators run directly on the LFG, containing roughly 50 per cent methane. Some sites attempt to clean the gas of its residual corrosive gases and run the generators off purer methane, but Shaw believes it is more economical to suffer engine corrosion and allow for heavy maintenance.

High availability

And yet for all the maintenance pressure, the generators must turn for 92.5 per cent of the time according to contracts with their customer, and each engine can rest for no more than 13 hours a week. It's a relentless target, but the pumps suck out about 600m3 of methane per MWhour and the rate cannot be turned down. It is set by whatever has been chucked away in the landfill and the prevailing weather conditions. Warm, low-pressure weather with rain encourages the most gas, and if generators are not turning the gas is flared off and its energy wasted.

Jonathan Davies, commercial director, sustainable resource management at environmental consultancy enviros, says variability and corrosion are just two of many engineering challenges the industry has learnt to minimise, or perhaps live with. "there has been about 30 years of evolution, which has solved issues including the impact of settlement of waste and stress on wells and pipework, the accumulation and removal of condensate and the management of poor quality gas and emissions from older, low temperature flares."

Davies believes the generation of electricity represents an excellent use of the resource. While schemes exist to use thermal energy from combustion or condition the gas for motor transport, there are few situations where a consumer is located close enough to the source of LFG to justify cost effective supply, "much greater certainty can be provided by conversion to electricity," he says.

A slow decay

Yet it could be a dying industry. Slowly the organic waste required by the generators will be diverted away from landfill. The EU Landfill directive published in 1999 requires biodegradable municipal waste in England be reduced to 11.2 million tonnes in 2010, 7.5 million tonnes in 2013 and 5.2 million tonnes in 2020. The less organic waste is buried, the less LFG is produced.

But given that across the UK some 370kg per person per year is landfilled, according to Eurostat, there will still be a lot of waste going in the ground for decades to come. Shaw says there is potential for electricity generation at Heathfield for 25 years, even after the site is finally closed to tipping.

That energy from landfill gas will be important for years and is by far the biggest contributor to the UK's renewable power sector is probably a consequence of our past behaviour, says Davies. "It is a large source of energy, because of our historic, predominant use of landfill."

But Davies thinks the media is partly to blame for landfill's lowly standing. "Journalists like to stress the negative aspects of waste management believing this to play to readers' fears," he says.

There is no doubt that the darling of the UK's renewable energy sector is wind, but it's status is overblown, certainly at present and until more of its potential is realised. Meanwhile, it is churlish to ignore the valuable contribution from landfill gas; a little-recognised success of British renewable energy.

Recent articles

Info Message

Our sites use cookies to support some functionality, and to collect anonymous user data.

Learn more about IET cookies and how to control them

Close