Mercury in the environment has been associated with negative environmental impacts. Elevated concentrations of mercury in fish in the diet can lead to neurological problems, especially in young children.

Coal combustion is a significant, if not the major, source of mercury emissions in many countries. Some areas in the developed world, such as the EU, have managed to reduce mercury emissions significantly (around 70% in the last three decades) due to measures taken to reduce emissions of other pollutants. These reductions are expected to continue for the next decade or so, even without mercury-specific controls, due to further co-benefit effects from existing and impending legislation. Despite an overall reduction in mercury emissions from human activities in the developed world, emissions from developing nations and emerging economies are increasing at such a rate that the overall global mercury emission trend is still upwards.

Mercury-specific legislation has been set in Canada and certain states in the USA. The US EPA is currently working to promulgate a new mercury-specific regulation to replace the recently rejected Clean Air Mercury Rule (CAMR). Despite the rejection of CAMR in the USA, which would have specifically targeted mercury, there is still an impetus for mercury control at many plants in the USA.

Control systems for particulates, SO2 and NOx can result in concomitant mercury reduction.

Since these flue gas treatment systems are not primarily designed to reduce mercury emissions, the amount of mercury captured is variable. These systems can be adjusted to enhance mercury capture, for example by lowering temperatures in flue gas and ESP systems.

However, many of these adjustments can cause detrimental effects to the operation of the control system or to areas of the plant downstream. Whole balance-of-plant effects must be taken into account and this must be done on a unit-by-unit, coal-by-coal basis. By enhancing co-benefit mercury capture in a plant, the requirement for additional, more costly, methods of mercury control can be reduced.

For many plants the amount of mercury reduction achievable through co-benefit effects may not be enough to meet current or future legislative demands. In these cases, more costly mercury-specific control techniques and technologies are required. These include techniques such as activated carbons or sorbent injection, oxidation methods and electrochemical methods. For the most part, these technologies have an equal chance to find a niche in the market, although early demonstrations may prove more effective. At the moment, it is commonly assumed that there is no single BAT/MACT (best available technology/maximum achievable control technology) for mercury on coal-fired plants. Should the new US EPA CAMR-replacement result in a requirement for MACT then the US EPA will face a challenge to select the most appropriate technologies and the market could change dramatically.

Estimating the cost of control technologies for coal-fired plant is problematic since much of the current data are based on either pilot-scale studies or short-term full-scale studies. Further, the variation in mercury behaviour at different plants makes it difficult to make generalised assumptions for cost analyses. As the market for mercury control technologies develops further, prices are likely to drop quite dramatically. Market forces will determine which control technologies will become the most popular, with early successful demonstrations having the most opportunity to take a larger portion of the marketplace.

Economics of mercury control
Lesley Sloss
CCC/134, ISBN 978-92-9029-453-5, June 2008
£255 non-member countries
£85 member countries
£42.50 educational price