Finding the silver lining in reducing quicksilver
By: Rebecca Saari
As a PhD Candidate researching air pollution, I have enjoyed following the treaty discussions, particularly those focusing on emissions and releases. At MIT, I study the many social and environmental gains from reducing air pollution. Often, targeting reductions of a single pollutant – like mercury – can simultaneously serve to reduce other pollutants as a side-benefit. Finding and quantifying such “co-benefits” is my passion. (My other passions include skiing and chocolate, so it does not hurt that the negotiations are in Switzerland.)
Reducing mercury emissions
If the treaty creates new action to reduce mercury emissions, it can realize gains that go far beyond the direct impacts of mercury alone. Controlling mercury from coal-fired combustion, the second-largest air emissions source, can be achieved with measures that also control other pollutants. In particular, reducing mercury emissions to air can also reduce emissions of particulate matter, sulfur dioxide and nitrogen oxides.
All of these pollutants have significant human health impacts. Estimates of global worldwide deaths due to fine particulate matter exceed 1 million per year. Beijing is currently experiencing extreme levels of fine particulate matter. Countries can use the opportunity presented by this treaty to make progress towards multiple goals in protecting human health and the environment.
Reducing mercury emissions from coal would go a long way towards diminishing the global transport of mercury pollution. Nearly one quarter of all mercury emissions to air arise from the combustion of coal in utility, industrial, and residential boilers.
Many ways to reduce mercury and other pollutants
There are many ways to reduce mercury emissions from coal across the entire combustion process, from start to finish, including pre-treating coal, improving process efficiency, and using post-combustion technologies.
Before coal is burned, several actions can reduce mercury, sulfur compounds, and particulate emissions. There are several different types of coal, and they vary in the amount of pollutants they contain. Coal switching and coal blending can allow mercury emissions to be captured more easily. This is a low-tech, potentially low-cost form of mercury reduction. Coal can also be pre-treated through a variety of processes, including washing, beneficiation, and the application of additives. Depending on the type of cleaning and variety of coal, washing alone can remove about 10-80% of the mercury content in coal before combustion takes place.
We can also improve the efficiency of coal plants through operations and maintenance (O&M) measures that lower the emissions intensity of coal-related pollutants including mercury and greenhouse gases, and potentially lead to more sustainable and cost-effective use of fossil fuels. Various O&M measures are effective options. Typically, these approaches target improved combustion efficiency, improved flue-gas ventilation, and reduced leakage and fouling.
Once coal combustion is complete, mercury can be captured using conventional methods designed for other pollutants. Specifically, wet sulfur scrubbers (a.k.a. wet flue gas desulfurization), particulate capture (including fabric filters, electrostatic precipitators), and NOx controls (i.e. selective catalytic reduction) can aid in mercury removal. Depending on the type of coal and configuration of equipment, more than 90% reduction of mercury can be achieved. For additional mercury removal, mercury-specific sorbent injection can be added to the process.
Looking to the future, multi-pollutant control technologies, which aim to reduce key pollutants simultaneously, may gain in popularity. Several systems already exist, at various stages of development, demonstration and commercialization. The mercury treaty has the potential to sow the seeds for broad protection of human health and the environment, beyond the gains due to mercury alone.
Interested in learning more? Three great resources are the UNEP’s “Process Optimization Guidance”, the International Energy Agency Clean Coal Centre and Pacyna et al. There is also an interactive companion to UNEP POG called iPOG, a tool you can use to learn about options, and estimate your facility’s mercury reduction potential.