Burning desire

UK-developed clean waste incineration process combining gasification and plasma arc melting could solve landfill site problem and provide electricity. Stuart Nathan reports.


We have a problem with rubbish. Constant exhortations to recycle more, compost food waste and generally be more careful about what goes into our dustbins are meant to make us more environmentally conscious, but they mask a deeper problem. We are running out of landfill sites — fast.

The UK produces some 30 million tonnes of municipal solid waste (MSW) a year, some 70 per cent of which is landfilled. But according to the Environment Agency, there is only six years-worth of landfill space left in the UK. Meanwhile, the EU’s landfill directive says we must reduce the amount of biodegradable MSW sent to landfill by a quarter by 2010, or pay a fine of £150 a tonne over the limit.

As if to underline this point, Gordon Brown’s latest Budget increased the landfill tax by £8 a tonne a year, rising to £48 a tonne by 2010.

Waste-to-energy technologies offer an enticing way around this problem. Much of the MSW produced by households and collected by local authorities has organic content, so is a potential fuel. Also, it is biodegradable, making it a carbon-neutral fuel, thus counting towards the UK’s goal of generating 10 per cent of its electricity from renewable resources by 2010.

But burning MSW is problematic. It might be a fuel, but it is not a clean fuel; it tends to contain heavy metals and chlorine-containing plastics that create fine particulates and toxic compounds when burned, which then have to be filtered out. The solid residue is a slag, itself a hazardous waste that has to be sent to specialist landfills. It is hardly surprising that incineration is widely seen as a polluting process and concerns over health make it unpopular with affected residents.

Because of this, a UK company, Advanced Plasma Power, has devised a process called Gasplasma. It claims this process can use MSW as an efficient fuel for gas-engines to generate electricity and produces no harmful emissions, while turning the non-flammable components of the waste into an inert substance that can be sold to the building and civil engineering sectors. The process, combining the processes of gasification and plasma arc melting, can be used at a scale that would make it attractive to local authorities, said APP chief executive Andrew Hamilton.

The process starts with the raw waste from bin lorries, still in plastic bags. It is then sorted and shredded outside the plant, and metal and glass are separated out. These materials are easily recyclable and command a fee, explained Hamilton; moreover, if allowed to remain they would absorb energy from the plasma and reduce the process efficiency.

After a drying stage, the waste — a mixture of organic wastes, paper and packaging — is fed into a fluidised bed gasifier, where steam and oxygen are pumped through it at 800-900°C. This converts much of the carbon content of the waste into syngas, a flammable mixture of carbon monoxide and hydrogen.

‘Waste gasification has been tried before,’ said Hamilton, ‘but the problem is that the syngas you produce isn’t clean. It’s full of tarry substances and soot, and it’s very dirty to burn. Also, it still leaves a solid residue of char and ash.’

To solve this problem, the products of gasification are fed into a plasma converter, where a high-voltage arc creates a hot plasma, at 5,000-10,000°C, from a carbon electrode in the converter chamber’s roof. This converts all the residual tarry and sooty carbon products into syngas, while simultaneously melting the solid, non-carbon ash and char into a dense, molten glassy substance.

When this solidifies, it locks any toxic heavy metals into a stable vitrified matrix, said Hamilton. ‘It can’t leach out, and the substance is an ideal material for aggregates for the construction industry, pipe-bedding and road-building,’ he said. The glassy material polishes like granite, he added; it is so stable it could even be used for flooring.

The plasma part of the process was developed by APP’s sister company, Tetronics, based near Swindon. Widely used in the steel industry, the company’s plasma arc system is also used to treat hazardous wastes. in Japan, Hamilton said, it is used to vitrify residues from large waste incinerators and the glassy product is used in roadbuilding. ‘We know there’s a market for this material,’ he added.

The syngas passes through several cooling and cleaning stages, to remove any remaining solids, sulphur compounds and acids, and is then used to power a series of gas engines, equipped with heat recovery and directly coupled to rotary generators. The solids from this cleaning process are the only residues sent to landfill, and comprise about one per cent of the original feed.

A typical plant, handling 50,000tpa of waste, would generate 8MW of electricity, said Hamilton. of this, 3MW would be used to run the plant, with the rest being exported to the National Grid. With heat recovery, the process is about 60 per cent efficient in terms of the energy content of the waste — roughly twice the efficiency of a combined-cycle gas turbine.

‘A Gasplasma plant would be the size of a normal industrial unit, and would produce hardly any emissions,’ he added.

‘A single plant would be about the right size to handle waste from a London borough, whereas a county might need three. ‘But unlike incinerators that have to be built to a large scale these represent a local solution, which wouldn’t require waste to be transported over long distances and should not raise the planning problems associated with large incinerators.’

APP is now operating a pilot-scale plant and is seeking interest from local authorities for its first full-scale plant. This could be operating by 2009, said Hamilton.