Combustion processes produce a variety of undesirable products depending on the type of fuel used. Because combustion is an oxidation process, flue gases contain carbon monoxide, carbon dioxide, sulfur oxides, nitrogen oxides, acids and aerosolized metal oxides. They can also contain organics such as polynuclear aromatic hydrocarbons (PAHs) and even polychlorinated dibenzodioxins such as TCDD, as well as inorganic and carbon and other particulates. Fossil fuels such as coal and oil contain contaminants that produce those products, and municipal solid and hazardous wastes contain plastics such as polyvinyl chloride (PVC) that can produce the TCDDs and acids such as hydrochloric acid, in addition to many other products. Because most of those combustion products are harmful to people and the environment, especially in nearby areas where they may be more concentrated in the air and settle onto land and waterways, it is not surprising that numerous regulatory controls and technologies have been developed and are capable of removing many of the combustion products. In the past, tall stacks were expected to achieve sufficient dilution and distribution to minimize risks. Forest fires are also massive producers of most of these products, but they are not controllable sources.
Electric power plants and other high-volume heat-generating processing plants such as steel mills, cement kilns, other industrial facilities as well as municipal incinerators generate much of the combustion gases subject to regulation. U.S. electricity generation that once was heavily coal-based has been shifting to lesser contaminating sources of energy. The U.S. Energy Administration estimated that the fuel mix in 2010 was 45 percent coal, 24 percent natural gas, 20 percent nuclear, 10 percent renewable and 1 percent petroleum, and the shift has increased since then because of the greater availability of lower-cost natural gas. Any carbon combustion produces carbon monoxide and carbon dioxide, but natural gas usually has smaller amounts of precursors to sulfur, nitrogen and metals releases, and it can be burned more efficiently.
The Clean Air Act (CAA) 42 U.S.C. §7401 (1970), amended in 1977 and 1990, regulates air emissions from stationary and mobile sources, including National Ambient Air Quality Standards (NAAQS) to protect public health and public welfare and to regulate emissions of hazardous air pollutants. Standards have reduced particles, ozone, lead, carbon monoxide, nitrogen dioxide, sulfur dioxide and numerous toxic pollutants. Section 112 deals with emissions of hazardous air pollutants. The 1990 amendments required issuance of technology-based standards for major sources and certain area sources. “Major sources” are stationary sources that emit or have the potential to emit 10 tons per year or more of a hazardous air pollutant or 25 tons per year or more of a combination of hazardous air pollutants. The EPA must establish Maximum Achievable Control Technology emission standards that require the maximum degree of reduction in emissions of hazardous air pollutants. Air toxic standards were issued in 2011 for mercury, acid gases and other toxics from power plants. Several amendments and revisions have been issued since then.
Flue gas treatment technologies
Scrubbers is the generic term applied to flue gas treatment processes. There are both liquid- and solid-type gas treatment processes that are a function of the physical and chemical properties of the contaminant being removed. Among others, they include: wet scrubbers, dry scrubbers, adsorbents and mercury removal processes that chemically convert volatile elemental mercury in the hot flue gas into solid water soluble salts that can be collected. There are also variants such as electrostatic precipitators and desulfurizing processes.
“Scrubbers is the generic term applied to flue gas treatment processes. There are both liquid- and solid-type gas treatment processes that are a function of the physical and chemical properties of the contaminant being removed.”
Flue gas temperatures can range from 590°C to 650°C (1100°F to 1200°F) for annealing furnaces, 870°C to 980°C (1600°F to 1800°F) for fluidized bed combustion, 760°C (1400°F) for conventional incinerators, 982°C to 1538°C (1800°F to 2800°F) for pathological incinerators, 650°C to 870°C (1200°F to 1600°F) for glass-melting furnace and 982°C to 1316°C (1800°F to 2400°F) for ceramic kilns. These high temperatures accelerate sulfur, nitrogen oxide and metal oxidations and particle formation, as well as the alkali/acid neutralization reactions and the gas dehydration processes. Those high-fuel-consuming and high-temperature processes are also obvious opportunities for waste heat recovery to reduce overall process costs. The white vapors seen from scrubbers’ emissions are mostly water vapor generated by the dehydration processes and not particulates or carbon byproducts.
Adsorbers are used for removing toxic chemicals or odorous substances such as hydrogen sulfide and sulfur mercaptans (organic sulfur-containing compounds), as well as carbon dioxide. Adsorbers accumulate chemicals on their surfaces by physical or chemical processes. Granular activated carbon (GAC) is commonly used to remove organic chemicals and other byproducts by surface accumulation from a liquid or gas stream by electrostatic and physical/chemical entrainment mechanisms. GAC can be reactivated and reused by nondestructive heating in an oxygen-free atmosphere. Sometimes, those regenerating thermal processes also require adsorbers to concentrate chemicals and byproducts in the gas stream that are not destroyed by the regeneration heating process.
Carbon dioxide scrubbers that convert CO2 to solid carbonates are even used in space stations and submarines to remove CO2 from exhaled gas and prevent excess accumulation in the air. Some of the carbon dioxide removal processes use basic metal oxides, which are readily converted to stable solid metal carbonates. Larger-scale carbon dioxide sequestration processes for flue gas recovery are also in use and being explored.
Sulfur and its compound forms are ubiquitous in nature, including in proteins. Under reducing conditions such as underground in coal, oil and natural gas formation, they can be present as elemental sulfur or sulfides and mercaptans. Automobile catalytic converters require low sulfur gasoline. Oxidizing conditions, as in combustion, convert them to sulfur oxides (e.g., SO2 and SO3) and sulfate particulates. Sulfur oxides hydrolyze to sulfurous acid and sulfuric acid, respectively, which are corrosive to systems and are irritants; breathable particulates are harmful. So, one of the earliest scrubber applications driven by Clean Air Act legislation was in removing sulfur from flue gases.
Wet scrubbing uses an alkaline medium such as a solution or slurry of limestone (calcium carbonate), lime (calcium oxide/calcium hydroxide) or sea water, which is slightly basic at pH 8.1 and contains sodium, potassium, calcium and magnesium salts that can form carbonates. Sulfur oxides and nitrogen oxides are removed by the wet processes because they are acid anhydrides; other acids such as hydrochloric acid and particulates are also removed. The sulfur compounds are recovered as gypsum (calcium sulfate), which has commercial applications, or even as sulfuric acid in some processes.
Dry scrubbers use injection of alkaline media such as hydrated lime or soda ash (sodium carbonate) for the same purpose. The dry scrubber processes are generally not as efficient or as effective as wet processes, probably because of less favorable contact conditions in the hot, rapidly moving flue gas. They have some applications in typically smaller-scale hospital and municipal incinerators, possibly because of convenience.
Mercury removal from flue gases
Standards were issued in 2011 for hazardous air pollutants emitted by coal- and oil-fired power plants with more than 25 megawatts capacity. Mercury and other metals are commonly present in small quantities in fossil fuels, but many of them are toxic and the volume of fuel used in high-consuming applications such as power plants can result in potentially significant cumulative releases into the air and to the surface environment by deposition and long-range transport. Acid rain is an outcome.
Wet scrubbers are effective for removal of water-soluble mercury species; the elemental mercury is a gas under those conditions and it must be oxidized to mercuric (Hg+2) salts to be sequestered. This is usually accomplished by adding chlorine or bromine or aqueous sodium bromide; the bromine species are more effective than chlorine, partly because chlorine chemistry is more adversely affected by the sulfur oxides that are usually present. Bromides are readily oxidized under those conditions, and bromine forms are added to the coal or to the flue gas where the high temperatures and bromine oxidize elemental mercury, which is volatile, to soluble mercuric bromide that is captured in water that is then reprocessed to remove the mercury compounds. More than 90 percent mercury removal can be achieved, and mercury recovery from the waste stream is also possible.
The Clean Air Act requirements have driven development and installation of many technologies to reduce hazardous air pollutants in many industries. Several common approaches for flue gas cleanup applications have been generally described here. Scrubbers of various types are widely used commercially and have been found to be feasible technologies for numerous combustion applications, particularly for coal and oil electric power generation and other heat-dependent applications. Emissions of sulfur and nitrogen oxides, toxic stable organic chemicals, mercury and particulates can be managed to high degrees of removal.
- EPA Clean Air Act Overview. https://www.epa.gov/clean-air-act-overview/progress-cleaning-air-and-improving-peoples-health.
- EPA Laws and Regulations. https://www.epa.gov/laws-regulations/policy-guidance.
- Flue gas desulphurization detailed process. SlideShare. Feb 16, 2015. https://www.slideshare.net/may021994/flue-gas-desulphurization-detailed-process.
- Fuels Exhaust Temperatures – Engineering ToolBox. https://www.engineeringtoolbox.com/fuels-exhaust-temperatures-d_168.html
- Griggs, M.B (2015). Mercury Scrubbers on Power Plants Clean Up Other Pollutants, too. Popular Science. https://www.popsci.com/mercury-scrubbers-power-plants-clean-additional-pollutants.
- Vosteen B.W., et al. (2005). Bromine-enhanced mercury abatement from Combustion Flue Gases. https://www.yumpu.com/en/document/view/5668779/bromine-enhanced-mercury-abatement-from-combustion-flue-gases.
Joseph Cotruvo, Ph.D., BCES, is president of Joseph Cotruvo and Associates LLC — water, environment and public health consultants — and he is a technical editor of Water Technology. He is a former director of both the EPA Drinking Water Standards and the Risk Assessment Divisions.