STOPPING toxic mercury vapour from belching into the environment from coal-fired power stations has proved virtually impossible till now. But a chance discovery could provide a solution, although sceptics caution that the remedy has only been observed in the lab.
Mercury pollution is a major threat to health and the environment. The biggest source is the 4 billion tonnes of coal burnt each year for energy. Mercury accumulates in living things and passes up the food chain, sometimes reaching dangerous concentrations in seafood. Mercury poisoning can cause fetal deformities, as well as brain damage and learning difficulties in children and adults.
Removing mercury vapour from exhaust gases is difficult because it is so inert. When released at a power station, it defies chemical attempts to trap it and just sails out of the stack. 鈥淚t really is a phenomenal problem,鈥� says Keith Schofield, a materials chemist at the University of California, Santa Barbara. 鈥淏ut by chance, I think I鈥檝e managed to crack it.鈥�
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Previous attempts to filter mercury have focused on making the mercury react by adding catalyst materials to exhaust gases. But this approach has not been very effective. Now Schofield thinks that mercury might react and condense out if provided with surfaces where it can form solid compounds with other products of burning coal.
Schofield was actually studying how sulphur compounds in flue gases corrode metal components of furnaces. He tried adding various metal vapours to the flue gases and running the mixture over a metal plate. When he added mercury, he was surprised to find that it formed mercuric sulphate on the surface of the plate. He tried the experiment again but without sulphur in the gases, and found that the mercury reacted with oxygen instead to form mercuric oxide (Chemical Physics Letters, vol 386, p 65).
Crucially, both these compounds react with hydrogen chloride gas to form mercuric dichloride 鈥� and hydrogen chloride is abundant in coal exhaust. The dichloride is water-soluble, so it can easily be removed by water filters, or 鈥渟crubbers鈥�, which most power stations already use to remove other pollutants.
Schofield found that all the ingredients for removing mercury vapour are already there in flue gases. All that is missing is a suitable condensation surface. Schofield envisions placing a honeycomb structure in the stack at a height where the temperature has fallen to about 150 to 250 掳C, the ideal temperature for the reactions. He says the technology is ready for testing in a power station.
George Offen, head of mercury abatement programmes at the US Electric Power Research Institute in Palo Alto, California, thinks the stacks are too hot for this to work, and the condensation surface would have to be cooled. 鈥淭he cost and practicality of adding a cooled surface at the appropriate temperature would need to be compared with the costs of other approaches that promote mercury oxidation,鈥� he says.
But Maciek Lewandowski of the UK鈥檚 Environment Agency is more upbeat, and calls the idea 鈥渜uite promising鈥�. 鈥淚t seems that mercuric dichloride could be effectively scrubbed, providing the technology is affordable,鈥� he says.