Severn Trent Plc started generating electricity from the U.K.’s first commercial-scale crop-fed power plant as the utility seeks to lower its carbon emissions by using an emerging form of renewable power.
The company’s Severn Trent Water unit began supplying the national power grid from a gas-fueled turbine at a site near Nottingham, Gill Dickinson, a company spokeswoman, said in a telephone interview. The gas comes from anaerobic digestion towers, which use micro-organisms to break down corn and wheat.
Rising oil prices since the 1990s spurred the technology’s growth in Europe though it still needs subsidies in most countries. The water utility, which plans to use government incentives to make money from the 15 million-pound ($23 million) plant, raises its own crops for feedstock after having fully exploited sewage waste as a fuel.
“We’ve maxed out and have sewage-fueled combined heat and power plants deployed at all our major sites,” Renewable Energy Development Manager Martin Dent said in an interview at the Stoke Bardolph site near Nottingham. “This means we need to start going into new areas we haven’t gone to in the past.”
In Britain there are no crop-fed power plants of the scale of Severn Trent’s project, which will be the first to sell electricity to other consumers through the transmission grid.
Anaerobic digestion is also used to make gas from food waste, animal slurry and sewage. Severn Trent has power plants at 34 sites fed by sewage instead of crops. The new plant is fitted with two engines from General Electric Co.’s Jenbacher unit and will help the utility come close to a company target of getting 30 percent of its power from renewable power by 2015.
One of the two turbines began to power the sewage works and feed some energy to the grid on Aug. 16, Dickinson said. The generator is producing about 880 kilowatts of electricity, she said, adding that the quantity exported to the grid varies daily depending on the site’s energy needs. When both engines are generating at capacity, the plant will produce 2 megawatts of power, enough power for more than 4,000 homes.
“We don’t expect to be at maximum capacity with both engines until the end of the year,” Dickinson said.
In the digestion towers, micro-organisms that occur naturally in the crops break them down into gas aided by a temperature of about 40 degrees Celsius (104 degrees Fahrenheit) and air that’s low in oxygen.
“What we’re actually doing is building a concrete cow,” Dent said. “We need to keep it fed, we need to keep the temperature inside constant, and we want gas out of the end.”
There are about 25 anaerobic digestion plants on U.K. farms, running mainly on animal slurry, and 220 anaerobic digesters run by water companies using sewage, according to the government’s Department for Environment, Food and Rural Affairs.
The plants typically serve the farms, breweries and treatment works where they’re located, and produce less than 1 percent of U.K. electricity. Last year, renewable power accounted for about 6.7 percent of U.K. electricity generation.
At present, it wouldn’t be viable to build such a plant without government incentives, Dent said. Severn Trent hasn’t yet decided which incentive plan to use -- the government’s feed-in tariffs, guaranteeing a set rate per unit of electricity generated, or the renewables obligation that awards sellable certificates for energy from renewable sources.
“The asset life of the mechanical plant and equipment is 20 years, and that’s 50 percent of the build cost,” Dent said. “Anything beyond a 20-year payback, you wouldn’t do it.”
Because plants absorb carbon dioxide as they grow, their use as a fuel releases less of the greenhouse gas than do fossil fuels such as coal and natural gas.
German Biogas Boom
In Germany, government incentives and increasing fossil fuel prices spurred growth of the technology to about 4,500 biogas plants in 2010 compared with 100 in 1990, according to IEA Bioenergy, a group set up by the International Energy Agency that says most operate on a mix of manure and crops. France, Switzerland and Austria also subsidize anaerobic digestion.
With incentives in place, Severn Trent expects the plant to pay back the investment in a decade. In the future, the utility may also try to sell the leftover waste, or digestate, as a fertilizer, Dent said.
Severn Trent is already growing corn and some wheat on its own land surrounding the plant. Because the land has been used for more than a century as a repository for sewage sludge, it’s deemed unsuitable for crops for human consumption, so the utility isn’t diverting farmland away from food production.
The utility is cultivating on the site the 2,500 tons of wheat and 34,500 tons of corn needed to run the plant for a year, said John Jackson, Severn Trent’s farms manager. The wheat is being grown on a part of land not suitable for maize, and the corn was chosen after testing 51 varieties, he said.
“To get the most kilowatts per hectare, you work to your strengths,” Jackson said. “Because of its fertility and positioning, the best silage we can use here is maize.”