Mercurial powers

Simple, inexpensive modifications could have important implications for the environment by drastically reducing mercury emissions from coal-fired boilers, according to recent research.

The findings, from the Energy Research Centre (ERC) of Lehigh University in Pennsylvania, could cut flue-gas treatment costs at coal-fired power stations. This is good news for countries such as China which, with the fastest growing electricity generating market in the world, is almost totally dependent on coal.

Coal-fired power stations are one of the largest single sources of mercury emissions. It is generally emitted as a vapour, and if it enters the atmosphere, can fall to Earth as rain. Mercury is a neurotoxin, and once it enters the body of a living organism, it accumulates. If it enters the food chain, for example if it is ingested by fish, it can be dangerous to humans.

To remove gaseous mercury, power stations use systems such as filters made of activated carbon, which absorb the vapour. However, this is not a cheap process, as a large amount of carbon is needed — some 120kg/hour for a 250MW boiler. It is also expensive to verify levels of mercury emissions, because the toxic levels are very low, in the parts per billion range.

The ERC’s work stems from several years of research into optimising boiler operations to cut emissions of nitrogen and carbon oxides, and particulate materials. These methods, the project’s lead researcher Carlos Romero has found, can also change the properties of the mercury emitted from the burning coal. If the conditions in the furnace are tuned correctly, the mercury is oxidised in the flue gas rather than escaping as a vapour.

Mercury oxide is readily absorbed by the fly ash remaining after the coal is burned, and any oxide remaining in the flue gas stream can be captured easily and efficiently by filters and flue gas scrubbers, said Romero. This reduces the levels of mercury emissions by 70 per cent or more, he claimed, with only a ‘modest impact’ on plant performance and fuel costs.

The ERC team used computer modelling techniques to predict how changes in the furnace conditions could alter the properties of the mercury emitted. The variables included the sizes of the coal particles burned inside the furnace; the flue gas temperature; and the residence time of the fly ash.

In tests using three bituminous coal power plants in Virginia and Massachusetts, these alterations produced 50–75 per cent reductions of total mercury in the flue gases. Further tests are to be carried out next year using sub-bituminous coals.

The system could be used in conjunction with activated carbon injection, a technique where powdered carbon is blown into the slipstream of the flue gases, ahead of a particulate control system such as an electrostatic precipitator or a fabric filter. But this is a rather expensive technology, costing between £570,000 and £1.7m a year for a 250MW unit to reduce its mercury emissions by 50 per cent.

‘We believe that this cost would be offset by adopting the modifications to boiler operations indicated by the ERC technology,’ said Romero.