By Dr. J. Andrew Maxson, GE Energy Services,
and Dr. Herek Clack, Ill. Institute of Technology
In three years federal regulations will likely require reductions in mercury emissions of up to 90 percent from current levels. For coal-fired plants, these new standards pose one of the most vexing challenges in recent memory. Currently proposed solutions are expensive and may be insufficient to meet the new emission limits by themselves.
However, achieving the required mercury limits is critical for coal-fired plants. Plants that fail to meet the new standards may suffer from lower production, or worse, a prolonged shutdown. In today’s competitive environment, no power generator can afford that costly mistake.
In order to comply with these regulations, coal-fired plants should consider leveraging developing mercury control technologies by combining them with advanced coal blending, which can be used to tailor coal quality to operating conditions to make mercury removal more efficient and cost-effective. Together, both solutions represent the most promising way to operate successfully in this new reality.
Reducing mercury output presents special challenges because emission rates vary greatly by coal type and plant. Research shows that for a hypothetical limit of 0.03 tons of mercury emitted per million tons of coal, some plants wouldn’t need additional controls while others would have to reduce mercury levels by more than 80 percent. While existing controls for sulfur and NOx are relatively effective in removing oxidized forms of mercury, they are largely ineffective in capturing elemental mercury. Because the ratio of elemental mercury to oxidized forms of mercury is unique to each plant, comprehensive mercury control cannot solely rely on using existing emission control devices. As a result, considerable work has been done to develop technologies to improve the remediation of mercury emissions for existing boilers.
Leading developing mercury control technologies include:
- Injection of powdered activated carbon (PAC). PAC is the leading technology for mercury control. Retrofitting existing boilers would require, at a minimum, installation of sorbent injection ports upstream of the particulate control device. Although PAC is available commercially, cheaper waste-derived activated carbons have also been produced. Additional considerations are based on the particulate control device used.
- Electrostatic precipitator (ESP). Mercury capture occurs within the mixture of flue gas and powdered sorbent, making this approach sensitive to operating conditions. In particular, higher temperatures are detrimental to sorbent effectiveness, making spray cooling mandatory for hot-side ESPs and a possibility even for cold-side ESPs.
- ESP with wet flue gas desulfurization (FGD). Issues are similar to ESP-only installations. Wet FGD is only effective in capturing oxidized forms of mercury, which may or may not constitute a significant fraction of the total mercury.
- Fabric filter. Without lengthening the exhaust train, mercury adsorption occurs primarily across the sorbent bed on the fabric filter. Retrofit may require increased filter rapping frequency and increased draft capacity to compensate for the increased pressure drop across the fabric filter.
- Fly ash from coal combustion. Under certain conditions, coal combustion produces fly ash that has the high internal surface area and pore structure characteristic of activated carbon. Studies are ongoing to determine the combination of coal type and combustion environment needed to produce sorbent-quality ash.
- Impregnated activated carbons. Ordinary activated carbons are doped with elements (e.g., sulfur) that may increase sorbent reactivity, capacity, or high-temperature resiliency.
- Wet FGD catalysts. Additives promote oxidation reactions, converting elemental mercury to an oxidized form that is removed as part of the wet FGD process.
While these technologies have the potential to reduce mercury emissions, they are expensive and have variable removal efficiencies depending on the site. For example, PAC has reduced mercury emissions at some sites by up to 90 percent, while at others the removal efficiency drops below 50 percent. The operating costs of using PAC have been estimated at $1 million per kWh. These two factors will make it difficult for plants to be profitable without carefully considering all options for mercury control. As a result, compliance will likely require customized solutions for each plant.
Add the right coal
Effective coal blending can also be used to help reduce mercury emissions. In particular, dynamic coal blending, whereby coal quality is changed with time to match operating conditions, can dramatically reduce costs and improve efficiency. Using this methodology, for example, a plant can use a lower-quality coal-with higher mercury and poor ash quality-at part load, where the removal efficiency of retrofit devices can be lower because the mercury load on the unit is less. At full load, dynamic blending would provide a higher-quality fuel-with lower mercury content and more favorable ash quality-to help improve removal efficiency. Operating costs would be reduced because of the increase in efficiency, and fuel costs would drop because the plant would be capitalizing on using lower-cost fuels without derating. Dynamic coal blending is an excellent complement to the long-term solution for mercury control.
Dr. Maxson, applications manager for GE Energy Services, can be reached at firstname.lastname@example.org. Dr. Clack, a professor at the Illinois Institute of Technology, can be reached at email@example.com.