A challenge for a fuel

Energy is a major part of the cost of cement production - approximately a third of the cost. Cutting the fuel costs offer the largest prospect for savings in the industry. For the past three decades, the focus has been on making the cement-making process more efficient. Using dry proces kilns, cement producers have cut their coal consumption by 40 per cent. However, the process still requires a lot of fuel: Castle Cement alone uses around 400 000tonne of coal per year.

So the cement companies have now switched their focus. If you can't reduce the amount of coal, replace it with something cheaper.

The biggest advantages come from using fuel which would otherwise go to waste — because that's what it is. The industry's preferred alternative fuels would normally all go to a landfill site or an incinerator. If the energy can be used productively, the reasoning goes, so much the better.

The first of the major alternative fuels to gain prominence was Cemfuel, a liquid fuel formulated from residues from the solvent recycling industry. Designed by Castle Cement in conjunction with Her Majesty's Inspectorate of Pollution (now part of the Environment Agency), Cemfuel, and other recycled liquid fuels, are composed mostly of methanol, ethanol and acetone, recovered from materials such as windscreen wash, cosmetics and brush cleansers.

Liquid fuels like Cemfuel are injected into the kiln at 2000°C. The kiln is still fired mainly by coal, natural gas and fuel oil, with the liquid fuel as an adjunct.Castle began using Cemfuel in 1992 at its Ribblesdale works, initially supplying 20 per cent of the energy to the kiln. Emissions monitoring showed reduced production of nitrogen oxides, with no change in emissions of dust, metals, dioxins or furans.

After the success of liquid fuels, cement companies began to turn to other 'waste' fuels. Castle is now proposing to use 'Profuel', a blend of shredded paper, nappy and carpet offcuts and low chlorine plastics.

This approach has its critics. The National Association for Clean Kilns (NACK), a pressure group which represents residents local to cement works who object to the use of waste products in kilns, claims that the cement companies are not monitoring the hazardous content of the flue gases from recycled fuel-burning kilns enough to be sure they are safe. 'There's all sorts of nasty stuff in there,' comments NACK secretary Brian Sullivan, 'like ground-up carbon filters which have been clogged with the residues from other processes, pesticide containers, even ash, which has no thermal advantage whatsoever.'The problem, Sullivan says, is that conditions in kilns are not suitable for combustion of hazardous waste. 'For complete combustion, you've got to have oxygen,' he says. 'Cement companies say that high-quality cement can only be produced in an atmosphere containing less than 2 per cent oxygen — preferably less than 1 per cent. But the European Union's directive on incineration of hazardous waste says that this must be done in an atmosphere of at least 6 per cent oxygen.'

Moreover, he claims, the cement firms do not check that the components of recycled fuels have been combusted completely. 'They ought to check everything that goes in, then analyse the residues and check for the levels of everything that was there before. In fact, they monitor for the same substances that would be present if they were burning coal alone.'

Sullivan's arguments have held sway before — his campaigning led to the first refusal in the UK for a cement kiln to burn recycled fuel, at Blue Circle's Weardale plant. However, cement manufacturers strongly deny his claims. It's true that oxygen levels in cement kilns are below those stipulated for hazardous waste disposal, says Castle Cement's Richard Boarder, but the higher temperatures and longer residence times ensure that combustion is complete. 'We have to have an oxidising environment inside the kiln, and we do get complete combustion,' he says. 'The last thing we want is incomplete combustion — that would mean that we are not making the most efficient use of our fuel,' he adds.

The industry's next target is scrap tyres, whose disposal was becoming a major problem in the UK and Europe. In the UK, 38 million tyres which can no longer be retreaded are scrapped every year, and more than a quarter of them are landfilled, fly-tipped or dumped illegally. Landfills — the traditional and legal disposal route — are doing their job too well: they are filling up. And from 2003, it will be illegal to dump whole tyres in landfill sites. Another route is needed urgently, and the cement companies are keen to provide the opportunity.

Comprising rubber, carbon black and filler materials such as cotton and metals, tyres have a very similar energy value as coal. Burning tyres under atmospheric emissions produces vast clouds of noxious black smoke, but once again the extreme conditions inside a cement kiln break the material down completely, the manufacturers claim, producing no smoke and leaving no ash residue. The metal content is incorporated into the cement clinker. Again, the tyres — whole or shredded — are intended to work with the conventional fuels rather than replacing them completely. Blue Circle Cement's initial application for trial burning of tyres proposed to use them to replacing 24 per cent of the fuel.

The results of the trial were encouraging. Using traditional fuels alone, the Westbury cement works produces 1862mg of nitrogen oxides per cubic metre of flue gas; using a tyre/fuel mixture, this is reduced to 1100mg/m3. Volatile organic compound emissions drop from 16.5mgC/m3 to 10.5mgC/m3; dioxins and furans fall from 0.0313ng/m3 to 0.0177mg/m3. The news isn't all good — sulphur dioxide emissions increase almost tenfold, to 514mg/m3, although this is still well below the proposed Environment Agency emission limit. Particulate levels also rise slightly.

Blue Circle is convinced that partial switching to tyres as fuel is a good thing both for itself and for the local community, and is lobbying the Environment Agency to authorise a permanent switch. 'It's a win, win, win situation,' enthuses technical director Rob Davies. 'The works' impact on the local environment will be permanently reduced. Our high energy bills will be cut, ensuring the factory's competitiveness. And we'll be able to tackle the problem of what to do with all those scrap tyres.'