Innovation: Pumping iron to clean air

A BREAKTHROUGH in the removal of nitrogen oxides from flue gases now promises cleaner skies for a fraction of the current price.

Every year, power stations worldwide belch out millions of tons of nitrogen oxides, and efforts to reduce their emissions are costing the industry dearly.

Nitrogen oxides, collectively denoted NOx, are the gases that put the 'nox' into noxious. Along with sulphur dioxide, another substantial by-product of coal combustion, they contribute to some of the most serious current environmental problems. As well as being produced by coal-fired power plants, NOx is pumped into the atmosphere from vehicle exhausts.

In the lower atmosphere, nitrogen and sulphur oxides undergo chemical reactions which convert them to nitric and sulphuric acids. These are then washed out of the sky by rainwater, falling back on the planet's surface as acid rain, with drops as acidic as vinegar reported in the US and Europe.

But the inimical effects of NOx do not stop here. It is one of the prime causes of smog; sunlight induces it to react with other atmospheric gases, generating toxic ozone and an eye irritant called peroxyacetyl nitrate. Although the famous Los Angeles smogs are caused mainly by vehicle fumes, the areas hardest hit by NOx are those downwind of power plants.

Scandinavia is in a particularly bad position, receiving the combined airborne pollution of Northern Europe and the former Eastern bloc. Building higher smokestacks to reduce local pollution has simply meant that the outflow is dispersed further afield.

There are reasonably cheap and effective ways of removing sulphur dioxide from flue gases - involving the gas reacting with lime - but NOx is a more difficult customer. The most effective methods at present cost between dollars 2,000 ( pounds 1,300) and dollars 4,000 per ton of NOx removed. As the emissions run into millions of tons, the cost is potentially astronomical. The US Clean Air Act of 1990 stipulates NOx emissions in the US must be reduced by 30-40 per cent or 2 million tons by the year 2000, and the EU's Large Combustion Plant directive sets much the same limits for 1998.

Now two researchers at the Energy and Environment Division of the Lawrence Berkeley Laboratory in California have found a simple way to extract one of the principal nitrogen oxides, NO, which they say will cost less than dollars 400 per ton. After combustion, the gases pass through a solution containing a water-soluble iron compound that seizes and binds NO tightly; the bound NO is then converted by electrochemical means to relatively innocuous ammonia, which is easily disposed of.

The idea is not new; the breakthrough is in finding a compound that binds NO sufficiently well. Writing in Nature earlier this month, Eric Pham and Shih-Ger Chang estimate their compound has the potential to remove 80 per cent of the NO that passes through the solution. Because it operates in water, the process can be incorporated with existing methods for removing sulphur dioxide in 'wet scrubbers'.

Whether or not this really is the panacea the power industry has been awaiting will become apparent only when the scheme is taken out of the laboratory and used in a power station. Dr Chang says a pilot test in a small plant will soon be underway.