Your options for mercury emissions control
Based on the negative impacts of mercury on human health, the U.S. Environmental Protection Agency (EPA) began regulating mercury emissions to the environment from a variety of sources. For some sources, including municipal waste-to-energy plants and hazardous wastes incinerators, these emission limits already exist. As more industries throughout the world are faced with the possibility that they may have to reduce their mercury emissions, some understanding of what options exist is necessary.
The Mercury Study Report
Mercury moves through the environment as a result of both natural and human activities. The human activities that are chiefly responsible for causing mercury to enter the environment are burning mercury-containing fuels and wastes and industrial manufacturing processes. Mercury emissions are transported through the air and deposited to water and land where humans and wildlife are exposed. Based on the emissions inventory in EPA's Mercury Study Report
to Congress (Dec. '97), the highest emitting source category is coal burning electric utilities. This group of sources accounts for one-third of the anthropogenic emissions in the United States.
Concentrations of mercury in the air are usually low and of little direct concern. Once mercury enters waters, either directly or through air deposition, it can bioaccumulate in fish and animal tissue in its most toxic form, methylmercury. Bioaccumulation means that the concentration of mercury in predators at the top of the food web can be thousands or even millions of times greater than the concentrations of mercury found in water.
Human exposure to mercury occurs primarily through eating contaminated fish. Exposure to high levels of mercury has been associated with serious neurological and developmental effects in humans. Depending on the dose, the effects can include subtle losses of sensory or cognitive ability, tremors, inability to walk, convulsions and death.
Where do mercury emissions come from?
Mercury emissions from combustion systems are due to their presence in the fuel. In coal, for example, the median mercury concentration in U.S. coals ranges from about 0.03 parts-per-million by weight (ppmw) to 0.24 ppmw. With more than 800 million tons of coal being burned by utilities in the United States this source has potential emissions of more than 100 tons of mercury each year, which pegs utilities as the single largest source of mercury emissions. This is not to say that other combustion emission sources of mercury do not exist, or are not significant. Hazardous and municipal waste incinerators destroying mercury-containing compounds and materials and medical waste incinerators treating wastes from hospitals, clinics and research laboratories are also significant sources of mercury.
Forms of mercury emissions
One of the main problems in determining the best emissions reduction scenario for mercury is that it is emitted as both a vapor and as a particulate. When mercury containing materials are burned, depending upon combustion conditions and other materials in the fuel, a series of reactions take place. Some of these reactions reduce the mercury back to its elemental state (Hg) generally in the form of a vapor or gas. Other reactions, particularly if chlorides or sulfides are present, will produce a particulate mercury that may combine or agglomerate with other ash particles in the gas stream.
This option is one that, for the most part, is only feasible for fossil fuel-fired steam generation and power generation systems. Fuel switching means switching from coal to oil or natural gas, that can virtually eliminate mercury emissions. In order for fuel switching to work, however, there are certain technical and economic factors that must be taken into account. On the technical side, the boiler must either be already capable of multiple fuel combustion or be converted to a multi-fuel boiler. This conversion entails a certain capital expense and will also require re-permitting of the combustion system. A further expense is the difference, $1 million British thermal units (BTU), between the cost of coal and the cost of oil or natural gas. Depending upon the differences in fuel costs and the costs of boiler conversion, if any, this may be an option worth considering.
Coal cleaning. This process has been used for decades, especially by the utility industry, as an approach to improve the quality of boiler/combustor fuels while at the same time reducing emissions. Coal cleaning uses a combination of crushing and media flotation/separation to principally remove ash from coal. Originally looked at as a way to reduce sulfur oxides emissions by eliminating much of the pyrites in the coal, this technique has been reviewed as a way of possibly reducing mercury emissions. EPA tests have indicated that coal cleaning has the potential for reducing mercury emissions by 20 to 40 percent, depending upon coal type.
Coals that have been cleaned will not only be reduced in sulfur, ash and (potentially) mercury, but will also be changed in other ways. Some of these changed characteristics include the potential for slagging and fouling in boilers. Consequently, if someone is operating a coal-fired boiler and is considering coal cleaning as a mercury reduction strategy, then all of the characteristics of the cleaned coal need to be thoroughly studied.
Flue Gas Cleaning. Depending upon the form that mercury takes (particles or vapors) conventional flue gas cleaning equipment can be somewhat effective in reducing its emissions. Obviously, where mercury exists in a gas stream as a particle, then that fraction of the mercury is going to be susceptible to collection with conventional particulate control equipment. In general, the overall collection is dependent upon the fraction of mercury that exists in particulate form and the fraction in the vapor state. Based on EPA, U.S. Department of Energy (DOE) and Electric Power Research Institute tests, it is estimated that the percentage of mercury removal is as follows:
- Particulate matter scrubber - four percent
- Cold-side ESP - 32 percent
- Fabric filter - 44 percent
Flue gas treatment systems, primarily designed to remove sulfur dioxide (SO2
), are also somewhat effective in removing mercury (and other heavy metals) through a combination of adsorption into droplets, agglomeration and separation. Both conventional wet flue gas desulphurization scrubbers and the newer spray dryer-fabric filter systems have been shown to remove mercury at about 30 percent.
Enhanced mercury removal
EPA has indicated it is are considering a 90 percent removal of mercury requirement for regulated combustion sources. Consequently, with conventional gas cleaning equipment unable to achieve this removal level, there has been considerable effort over the last decade to perfect methods of enhancing mercury removal. These techniques have centered around additives for flue gas treatment systems and include sodium sulfide, activated carbon (AC) and proprietary additives.
Activated carbon has been used for about 10 years to enhance mercury removal in the municipal waste and hazardous waste combustion systems. In application, activated carbon is best used in a dry system in which the particulate collection device is a fabric filter. The activated carbon is injected ahead of the fabric filter. If necessary, a water mist is used to cool the flue gas. In cases where a spray dryer-fabric filter system is used the gas cooling occurs in the spray dryer and no cooling mist is necessary.
The effectiveness of activated carbon to remove mercury is dependent upon the effective surface area of the carbon and the mass carbon-to-mercury ratio for the system. Use of activated carbon has been shown to remove more than 90 percent of the mercury in a flue gas stream when used in conjunction with a fabric filter. Additionally, it has been found that by impregnating the activated carbon with sulfur, mercury removal capability can be increased from 93 to 96 percent.
While the use of activated carbon is not suitable for wet scrubbers, the addition of sulfur compounds have been found to enhance mercury removal. In EPA tests performed with wet scrubbing systems, the addition of sodium sulfide (Na2S) has been shown to remove about 80 percent of the mercury in a gas stream. The Na2S reacts with various forms of the mercury to form mercuric sulfide, an extremely insoluble compound. The mercuric sulfide is separated from the scrubber liquor and disposed. As with the use of activated carbon, the effectiveness of sodium sulfide is proportional to the mass ratio of the sodium sulfide to the mercury in the gas stream.
Work with proprietary adsorbents, sponsored by DOE, has given similar results to those achieved through the use of activated carbon with or without sulfur enhancement.
With EPA already having implemented mercury removal and capture criteria for municipal and hazardous waste incinerators and strongly considering similar standards for other combustion sources, an understanding of the techniques available for the capture of mercury from gas streams is important. Coal cleaning, for this specific combustion source, has limited capabilities for mercury removal and may present certain operating problems depending upon the specific coal-boiler combination. Conventional gas cleaning systems have limited capabilities for mercury capture, but can be enhanced through the use of such additives as activated carbon, sodium sulfide or proprietary additives to produce greater than 90 percent removal of mercury. The costs of applying some of these techniques will be dependent upon specific system layouts and characteristics.
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This article appeared in Environmental Protection magazine, August 2000, Vol. 11, No. 8, p. 26.
This article originally appeared in the 08/01/2000 issue of Environmental Protection.