UV treatment promises mercury removal by Catherine Early
A promising technology to remove mercury from coal-fired power plant emissions has been developed and is currently being tested at the US Department of Energy's National Energy Technology Laboratory (NETL).
Known as the GP-254 process, the mercury is removed using ultraviolet light to induce various components of power plant stack gas to react with the metal.
This changes the composition of the mercury into a form that can be removed easily and economically.
The team hope that the process, which uses equipment similar to that used in water treatment plants to kill microbes, may become a cheap and effective way of reducing mercury emissions.
When power plant flue gases are exposed to the ultraviolet light in the GP-254 process, the elemental mercury becomes excited, making it more likely to react with other compounds in the gas.
In laboratory experiments with simulated flue gas, the excited mercury reacted with oxygen and sulphur dioxide to form mercurous sulphate and mercuric oxide, both of which were easily removed.
The process is designed to work with existing pollution-control devices. Depending on where the ultraviolet light is applied, it could enhance mercury removal in the particulate collector or wet scrubber of a coal-fired plant.
The technology could be especially attractive to power plants that burn low rank coals – softer coals with lower energy content such as sub-bituminous coals and lignite.
Not only do low rank coals release more mercury per unit of energy than higher rank coals, but more of the mercury in low rank coals is harder to remove.
A preliminary cost analysis, based on small-scale laboratory tests, indicates that operating costs for the process will be lower than those for other methods.
The next step will be to test and optimise the process at pilot scale using emissions from NETL’s onsite combustor.
NETL’s Mercury Control Technology Research and Development Programme is conducting full-scale field testing of mercury-control technologies, and continuing bench and pilot-scale development of a number of novel control concepts, including the GP-254 process.
Other plans include developing technologies that reduce emissions by 50-70% by 2005 and 90% by 2010 – all at costs of 25-50% less than current estimates.