Coal Ash to Capture Landfill Gas for Power Generation

(The Engineer 4-20-2011) An energy generation scheme that makes use of domestic, plant and industrial waste could help to give new life to former landfill sites. The plan involves covering sites with fly ash from coal power plants to stop landfill gas leaking into the atmosphere and instead using it to turn garden waste into a nutrient-rich soil component called biochar.
The company behind the idea — Cheltenham-based Lichen Renewal — is in advanced talks to regenerate two sites in south-east England, turning them into land for agriculture and leisure while generating heat and electricity for local housing and retail developments.
‘The idea is to integrate waste strategy, methane emissions and renewable energy all into one,’ Ranbir Gill, founder and director of Lichen Renewal, told The Engineer. He initially just wanted to use pulverised fuel ash (PFA), a component of concrete produced by coal power stations, as a cap to stop landfill sites from emitting the greenhouse gas methane. ‘We found that you need energy to produce quite a large percentage of the artificial soil you put on top, so if you’re going to be doing that you might as well integrate it all together,’ said Gill.
The gas collected from the capped landfills will generate electricity for the grid and heat that will dry out the plant waste, break it down into biochar via a process called pyrolysis and create a mixture of hydrogen and carbon monoxide called syngas. This gas can, in turn, be used to produce more electricity and heat for houses and businesses. The biochar will be combined with fly ash, sewage sludge and other material to produce a fertile soil while effectively sequestering carbon for centuries. ‘If we could cap 500 sites, that would stop the equivalent emissions of around 11–15 million tonnes of carbon per annum,’ said Gill.
There is also an economic incentive to the plan, as the government currently pays eight times the subsidy for syngas-sourced electricity as it does for energy generated from landfill gas. Gill is talking to funds and private investors to raise the approximately £35m needed to develop the first site of around 10 hectares, which he estimates could produce up to 10MW of electricity and a similar amount of heat energy.
The company is in discussions with the local authority and the developer of a planned nearby housing site, which would use the energy produced. If planning permission is granted quickly, the facility could be operating by the end of next year following field trials to determine the correct mixture of soil components. All of the technologies to create the system already exist, but no one has yet integrated them in this way, said Cordner Peacocke, director of biomass consultancy CARE, which is working with Lichen Renewal. ‘The biggest issue with these technologies is feedstock preparation and handling. It’s crucial that you match the feedstock with the right technology for what you want to do. It can cost you a lot of money if you get it wrong.’
The gas will be sucked out of the ground using vertical extraction wells, typically 40m apart. Capping the landfill means less air will be drawn into the system from the atmosphere. ‘You’re minimising methane emissions getting into the atmosphere but having better control so the extraction system can suck harder,’ said Ged Duckworth, director of waste management company GD Environmental.
The cap will also prevent water flowing through the landfill, allowing waste to biodegrade faster and stabilising the site while stopping it from becoming saturated, which can reduce gas production. Many landfills were designed to allow water to flow through them, picking up soluble waste components to produce a liquid called leachate that would then be dispersed as the water drains out of the site.
However, leachate is often not dispersed as well as originally intended, so preventing water from entering the landfill in the first place should limit groundwater contamination. The challenge for Gill’s team will be designing a landscape that diverts most of the run-off water without causing flooding, while allowing enough moisture to reach the fly ash cap in order to keep it moist and to prevent it from cracking.
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