Cooking on biogas

The prospects for injecting biomethane into the gas grid are really hotting up, according to Megan Darby

“The networks are very much hoping that 2013 is going to be a breakthrough year for biomethane,” says Tim Field, head of public affairs at the Energy Networks Association (ENA). “We would like green gas to be given the same sort of attention as shale, because it has got an equal, if not more important, role.” Indeed, when the government announced in December the creation of an Office for Unconventional Gas to support shale gas development, the ENA called for biomethane to get a share of the perks.

While the scale of recoverable UK shale reserves is hugely uncertain, there is a wealth of untapped energy in organic waste that is just there for the taking. Sewage sludge, agricultural, industrial and household waste – all can be digested or gasified to produce biogas. “There really is no limit in terms of the amount there,” says Field.

While the anaerobic digestion industry has been gathering pace – quadrupling in the past three years, according to the Anaerobic Digestion & Biogas Association – the resulting gas has almost universally been burned in gas engines at less than 50 per cent efficiency. The hoped-for great leap forward will come when the gas is instead cleaned up and turned into biomethane for injection into the grid.

In 2009, a National Grid report suggested that renewable gas could meet up to 48 per cent of UK residential gas demand by 2020, with the right policy support. Some of its suggestions have come to pass. Previously, there was an incentive under the Renewables Obligation to generate power from biogas but none for grid injection. Now, biomethane is covered by the Renewable Heat Incentive (RHI). That is a welcome move, although Field notes “the market is probably cautious about RHI” after last year’s debacle over cuts to solar incentives.

Ofgem’s latest price determination also provided a modicum of encouragement to gas network operators. It offered a discretionary reward of up to £12 million over the next price control period to companies that had gone the extra mile on environmental objectives. There will also be a report on the percentage of biomethane connected by each gas distribution network, to give them a chance to show off.

There is a lot of enthusiasm for producing biomethane and several projects are in the development or planning stage (see boxes). “We are seeing a vast number of commercial and industrial partners wanting to make use of their waste,” says Field.

However, there are a number of potential regulatory and technical stumbling blocks. The route from silage pit, dustbin or sewage works to gas hob cuts across a lot of government departments and regulators’ territories. And different feedstocks produce different types of gas that need treating in different ways to meet network quality standards.

The water company

Northumbrian Water is in talks with Northern Gas Networks about the infrastructure requirements for grid injection at its Howdon sewage works on Tyneside. It expects to go out to tender later this year.

“It would be an industry first on this scale,” says spokesman Alistair Baker, and the company will be able to draw on higher subsidies under the RHI than they could through the Renewables Obligation.

In September, Northumbrian floated plans to introduce food waste for co-digestion with sewage sludge. However, there are regulatory obstacles and the company is now considering a separate food waste plant, says Baker.

The UK water industry treats 66 per cent of sewage sludge with anaerobic digestion, which generated around 1TWh of electricity in 2010, according to government figures.

The waste processor

Advanced Plasma Power is a company whose business is turning solid waste into bio-substitute natural gas (Bio-SNG) . “We are anaerobic digestion on speed,” says chief executive Rolf Stein. Working with National Grid and Progressive Energy, the company starts construction this year on building a plant to inject Bio-SNG into the grid. A similar process using woodchip as a feedstock has been demonstrated in Austria, but this would be a world first for general waste, says Stein.

There is no shortage of solid waste that could be treated in this way. Exactly half the waste collected by local authorities in the UK was sent to landfill in 2010/11, according to Defra. Energy-from-waste companies are paid to take it. “It’s amazing how much people still throw into the black bag,” Stein says, and around 65 per cent is biomass: textiles, paper, food scraps, cotton and even garden waste. The cost of production is “coming down significantly”, he claims, with a levelised cost of £60-£80/MWh.

The network operator

Scotia Gas Networks (SGN) launched the UK’s first commercial-scale biogas-to-grid scheme in November at Poundbury, Dorset, in partnership with JV Energen and the Duchy of Cornwall. The plant takes waste from nearby chocolate, cereal and potato factories, as well as agricultural by-products, and expects to produce enough gas to meet the winter-time demand of 4,000 households.

SGN also worked with Thames Water on a pilot project at the latter’s Didcot sewage works. In terms of getting gas into the grid, the Didcot project has not been a great success, working only fitfully. But SGN commercial project manager Alan Midwinter is at pains to emphasise how much they learned from the experience.

The network company needs to be involved at the start, he says, to make sure it is prepared for the impurities the anaerobic digestion process might throw at it. Some feedstocks produce unwanted chemicals, such as ammonia, which can’t be allowed into the grid.

By working closely with its partners at Poundbury, SGN was able to keep costs down, Midwinter says. Emboldened by that success, it is considering around 20 similar projects.

It is also looked at transporting compressed biogas. A lot of anaerobic digestion plants are in the wrong place for the purpose of putting gas into the network and it is cheaper and less carbon-intensive to transport the compressed gas than the feedstock.

This article first appeared in Utility Week’s print edition of 18th January 2013.

Letter to the editor: Dear Editor: the feature “Cooking on Biogas” (Utility Week 18 January) states that biogas “has almost universally been burned in gas engines at less than 50 per cent efficiency”.

This statement is not correct.

While it is true the electrical efficiency of a modern biogas engine is typically 40-43 per cent, the actual efficiency must also take into consideration the thermal efficiency. Almost all digesters at least have a requirement for process heating and often for pasteurisation. If only the cooling water from the engine is used this will bring the total efficiency of a gas engine to around 60 per cent.

In a large majority of digesters in the UK, the engine’s exhaust heat is also converted into usable thermal energy in order to offset fossil fuels (including distilleries, sewage works and food waste digesters). These CHP plants often have a total efficiency between 80-95 per cent.

Gas engine CHP and biogas upgrading are different technologies which perform different functions. Projects should be evaluated on a case-by-case basis considering the local heat requirements along with the value of the renewable electricity produced by the digester.

Alex Marshall, group marketing manager, Clarke Energy