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Memorandum on Acid Gases 2011-03-02 Page 1 of 10

TO: Janice E. Nolen

Assistant Vice President, National Policy and Advocacy

American Lung Association

FROM: David L. MacIntosh, Sc.D., C.I.H.

DATE: March 2, 2011

RE: Acid Gas Emissions and Coal-Fired Power Plants 

This memorandum addresses acid gas hazardous air pollutants (HAPs) emitted by coal-fired 

electric utility boilers in relation to the Maximum Available Control Technology (Utility MACT)

rules scheduled to be proposed by the U.S. Environmental Protection Agency (USEPA) by

March 16, 2011. The acid gases hydrogen chloride (HCl) and hydrogen fluoride (HF) are 

considered; including their intrinsic hazards, emissions, impacts on the environment, control of 

emissions, and collateral benefits of their control. The Clean Air Act Amendments of 1990 

identified more than 180 HAPs, including HCl and HF, which are known or suspected to cause 

adverse consequences to human health and the environment, making them subject to 

regulation under Section 112 of the Clean Air Act.

Acid Gases

Hydrogen chloride and hydrogen fluoride are strongly corrosive gases produced by coal-burning 

power plants. When combined with water, hydrogen chloride gas forms hydrochloric acid. The 

Agency for Toxic Substances and Disease Registry (ATSDR) characterizes hydrochloric acid as 

―corrosive and can cause irritation and burns‖ at high concentrations (ATSDR, 2010a). Similarly, 

for high exposures to HF, the Agency states that ―hydrogen fluoride is irritating to the skin, eyes, 

and mucous membranes, and inhalation may cause respiratory irritation or hemorrhage‖ 

(ATSDR, 2010b). These effects occur at levels of exposure likely above what are generally 

encountered in outdoor air.

According to the USEPA National Emissions Inventory, 335,000 tons of hydrogen chloride and 

45,000 tons of hydrogen fluoride are likely released each year by coal-burning power plants. 
Memorandum on Acid Gases 2011-03-02 Page 2 of 10

The amounts of hydrogen chloride and hydrogen fluoride produced by a particular power plant 

depend in large part on the concentrations of chloride and fluoride in the coal that is burned and 

whether any emission control systems are in use. Overall however, approximately 60% of the 

chloride and fluoride in coal burned in power plants is estimated to be released to the 

atmosphere as hydrogen chloride and hydrogen fluoride (USEPA, 1998a).

Coal-burning power plants are reported to be the largest anthropogenic source of hydrogen 

chloride and hydrogen fluoride emissions to air (USEPA, 2009a). Due to its high solubility in 

water, the majority of HCl is believed to be rapidly removed from the atmosphere by wet and dry 

deposition as well as by reactions with particles in the atmosphere (Sanhueza, 2001). Hydrogen 

fluoride is emitted as a gas or particle and can be adsorbed onto other particles (USEPA, 

1998b). Hydrogen fluoride containing particles tend to remain suspended in the atmosphere 

longer than hydrogen chloride gas and can travel up to 500 kilometers or more as fine particles 

(USEPA, 1998b).

Because of their high solubility in water, acid gas vapors can readily react with the 

moisture/tissues in the upper airways. Likewise, water bound to microscopic particles can act as 

a ―delivery system‖ for acids to the alveolar regions of the lung (USEPA, 1998b). Controlled 

exposures of people with asthma have shown irritation and restriction of the airways from 

exposure to hydrogen chloride (Fine, 1987). Other studies have shown both acids to irritate 

eyes, nasal passages and the lungs (USEPA, 2000a; USEPA, 2000b).

While the USEPA estimated that hydrogen chloride levels resulting from coal-fired power plant 

emissions are below published health-based guidance levels (USEPA, 1998a), the Agency 

states that it has ―low confidence‖ in the guidance level (called a ―reference concentration‖) due 

to limited study of this compound in human populations (USEPA 2000a). When combined with 

water, 100% of the hydrogen chloride dissociates and forms hydrochloric acid. For our 

purposes think of this complete dissociation as a characteristic of a ―strong acid‖. Other ‖strong 

acids‖ in the atmosphere can result from emissions of nitrogen-based and sulfur-based gases 

released from coal-fired power plants (producing nitric acid and sulfuric acid, respectively).

Strong acids or their precursors that are present in inhaled particles and gases have been linked 

with respiratory effects in large-scale epidemiological studies. A study of 13,000 children in 24 

U.S. and Canadian cities found that strong acidity in particles was associated with increased 

episodes of bronchitis and reduced lung function and acid gases were associated with asthma 
Memorandum on Acid Gases 2011-03-02 Page 3 of 10

and related symptoms in children (Raizenne, 1996; Dockery, 1996). A more recent major 

children’s study found also acid gases and particle pollution were associated with reduced lung 

function (Gauderman et al., 2004). The focus of these landmark studies on children is 

significant; as children are likely more vulnerable than a healthy adult to air pollution, including 

acidic gases and particles. Children have narrower airways, a faster breathing rate and tend to 

spend more time outdoors than adults, resulting in greater overall exposures (ATSDR, 2011; 

AAP, 2004). Results from these studies and other research (USEPA 1998a) show the potential 

harm from HCl vapor, particularly in the vicinity of coal burning power plants, where its 

contribution to acid aerosols and acidic precipitation has been not well-characterized. 

Chloride released from hydrogen chloride can also significantly increase cloud acidity (USEPA, 

1998b) and contribute to acid deposition over a regional scale. While much of the strong acidity 

has generally been thought to be related to sulfur dioxide and nitrogen oxide emissions, 

hydrogen chloride in particular likely plays a significant role in acid deposition in the vicinity of 

coal-burning power plants. Particularly within 15 kilometers of a power plant, USEPA has 

suggested that "...acidity in rainwater near a coal-fired power plant may be predominately the 

result of hydrogen chloride rather than sulfur dioxide or nitrogen oxides."

Acid gas emissions from power plants can also adversely impact ecosystems. A recent study in 

the United Kingdom attributed almost a third of the improvements from acid rain contamination 

since 1986 to reductions in hydrogen chloride from coal burning power plants (Evans, 2011). 

This is a much larger fraction than previously estimated. This study also showed that hydrogen 

chloride likely travels much further in the atmosphere than previously thought. Finally, this paper 

concludes: ―Remarkably, [hydrogen chloride] has largely been overlooked in scientific 

assessments of ecosystem air pollution impacts.‖

Acid Gas Controls

Based on data gathered by the USEPA, control technologies for acid gases currently exist, are 

already in use by some power plants, and are effective at controlling acid gas emissions.

(USEPA, 2009b; USEPA, 2009c). As shown in the chart below, hydrogen chloride and hydrogen 

fluoride emissions for a sample of better controlled coal-fired power plants were five times lower 

than emissions from a randomly selected set of coal-fired power plants.
Memorandum on Acid Gases 2011-03-02 Page 4 of 10

Coal, especially some bituminous coals can have significant chlorine content, which contributes 

to hydrogen chloride emissions when this type of coal is burned (Staudt 2010). Electric utility 

boilers that fire bituminous coal comprise roughly half of the coal-fired electric generating 

capacity of the U.S. Although many of these facilities are equipped with wet scrubbers that are 

highly efficient at capturing hydrogen chloride and other acid gases (e.g., hydrofluoric acid), a 

large number of bituminous fueled units are not equipped with scrubbers—having only 

particulate controls, and could require acid gas controls to meet emission limits set under the 

Utility MACT rules.

In order to meet the MACT standards for hydrogen chloride and hydrogen fluoride, some of the 

uncontrolled facilities may choose to install wet or dry scrubbers, also known as flue gas 

desulfurization. Wet scrubbers are more efficient at removing acid gases, but they are more 

costly than dry scrubbers. Modern wet scrubbers typically reduce sulfur dioxide emissions by 

about 98%

1

; have higher capture rates for hydrogen chloride; and reduce emissions of sulfur 

 

1

2009 EIA Form 860
Memorandum on Acid Gases 2011-03-02 Page 5 of 10

trioxide and sulfate as well by about 50% or more. Reduction of sulfur trioxide and sulfate by 

wet scrubbers is supported by data in USEPA’s Information Collection Request (USEPA 2009a; 

USEPA 2009b) on bituminous coal units equipped with an electrostatic precipitator (ESP). 

A review of these data showed significant reductions in condensable particulate emissions when 

comparing the average emissions of units with wet flue gas desulfurization versus those without 

it, as shown in Table 1. Condensable particulate matter consists of substances, such as many 

metals, that are a vapor in the hottest portions of an exhaust stack, but rapidly condense to form 

primary particulate matter (PM). These values are the results of relatively small data sets and 

may not be representative for all facilities, but they show a large reduction in condensable PM 

from the use of this technology.

Table 1 Comparison of Average Emission Rate of Condensable PM from ICR Data for 

Bituminous Coal Facilities With and Without Wet Flue Gas Desulfurization or 

―Scrubbers‖

Emission Rate, pounds per million British Thermal Units

PM Control Device

Without

Scrubbers

With 

Scrubbers

Percent 

Reduction

Electrostatic Precipitators 0.041* 0.009 78%

*This data included units with selective catalytic reduction, which would increase 

condensable PM somewhat, but typically not more than doubling it.

Because of the cost of wet scrubbers (and to a lesser extent, dry scrubbers), other technologies 

are likely to be deployed for hydrogen chloride capture from unscrubbed bituminous coal fired 

boilers. Many unscrubbed facilities may install dry sorbent injection, which is much less 

expensive to install than a wet scrubber. Dry sorbent injection offers the ability to reduce both 

hydrogen chloride and sulfur oxides, but is generally less effective at removal of sulfur dioxide

than the more costly wet or dry scrubbers. 

At one facility hydrogen chloride and sulfur dioxide were removed using dry sorbent injection 

with both a baghouse (or fabric filter) and an ESP for particle collection (Davidson, 2010). This 

study showed that the unit with an ESP, the most commonly used PM control device on power

plants, removed 95% of the hydrogen chloride and 50% of the sulfur dioxide. At the Mirant 

Potomac River Power Plant, equipped with dry sorbent injection and an ESP, roughly 98% of 
Memorandum on Acid Gases 2011-03-02 Page 6 of 10

hydrogen chloride and greater than 70% of sulfur dioxide were captured (Kong, 2008). A review 

of power plant data showed significant reductions in condensable PM emissions when 

comparing the average emissions of units with dry sorbent injection versus those without, as 

shown in Table 2 (USEPA 2009b; USEPA, 2009c). These values are the results of relatively 

small data sets and may not be representative for all facilities, but they show large reductions in

condensable PM from the use of this technology.

Table 2 Comparison of Average Emission Rate of Condensable PM from ICR Data for 

Facilities With and Without Dry Sorbent Injection (DSI)

Emission Rate, pounds per million British Thermal Units

PM Control Device Without DSI With DSI

Percent 

Reduction

Electrostatic Precipitators** 0.041* 0.007 83%

Fabric Filters*** 0.028* 0.003 91%

* This data included units with SCR, that would increase condensable PM somewhat, but 

typically not more than doubling it.

** Bituminous coal

*** Powder River Basin coal

Regardless of which control technology (scrubbers or dry sorbent injection) is selected, the 

anticipated MACT emission standards are expected to reduce aggregate emissions of HCl and 

HF from coal-fired power plants. Because of the physical and chemical properties of the 

available control technologies the measures taken to reduce HAP acid gases are also 

anticipated to lower emissions of condensable PM and sulfur dioxide. These collateral benefits 

are important because condensable PM and secondary PM formed from sulfur dioxide comprise 

the majority of fine particulate matter in most areas of the United States.

As described in the most recent comprehensive syntheses of the science, both short-term and 

long-term exposure to fine particulate matter has been found to cause cardiovascular effects, 

including mortality (USEPA 2009d; CASAC 2010). In addition, fine particle exposure has been 

found to be a likely cause of adverse respiratory effects, including increased visits to emergency 

departments and hospital admissions; worsening of respiratory symptoms; and reduced lung 

function (USEPA 2009d, CASAC 2010). Long-term exposure to fine particles has also been 

linked to reproductive and developmental effects, cancer, mutagenicity and genotoxicity 
Memorandum on Acid Gases 2011-03-02 Page 7 of 10

(USEPA 2009d). Fine particulate matter has negative impacts on the environment as well. For 

instance, sulfate-laden fine particulate matter is the key contributor to impairment of visibility in 

pristine areas such as National Parks (USEPA 2009d). For these reasons, public health and 

environmental benefits accrued from acid gas controls are expected to be substantial. 

The potential value of collateral benefits from the Utility MACT Rule is indicated EPA’s recent 

Regulatory Impact Analysis for MACT on industrial boilers (USEPA 2011a). In that analysis, 

EPA estimated that public health benefits of at least $22 billion to $54 billion would be achieved 

by MACT controls on industrial boilers. In comparison, the costs of controls were estimated to 

be $1.4 billion. EPA attributed over 90% of the public health benefit to reductions in sulfur 

dioxide emissions, presumably achieved as a by-product of acid gas controls on those boilers. 

With a benefit-cost ratio of at least 16 to 1, the public health and economic value of controlling 

acid gas emissions from those boilers is clear. Although not quantified directly, technologies that 

control acid gas emissions may also reduce emissions of hazardous metals2like mercury and 

selenium, which are not as effectively controlled by conventional particle technologies. (USEPA, 

2010).

While there has been some debate over whether collateral benefits can be considered in 

rulemaking for HAPs, EPA is expressly authorized by Congress to consider the collateral 

benefits of controlling sulfur dioxide and other criteria pollutants when establishing National 

Emission Standards for Hazardous Air Pollutants (Federal Register, 2010). This interpretation of 

the law was recently affirmed. After consideration of extensive public comments on this subject 

EPA concluded it knows of ―no principle in law or common sense‖ that precludes the Agency 

from considering collateral environmental benefits when acting to regulate HAP emissions under 

the Clean Air Act (USEPA 2011b).

 

2

As in some EPA materials, the class of pollutants referred to for simplicity here as ‘metals’ includes some 

elements (e.g. arsenic and selenium) that are not, strictly speaking, fully metallic.
Memorandum on Acid Gases 2011-03-02 Page 8 of 10

References

Agency for Toxic Substances and Disease Registry (ATSDR). 2010a. Medical Management 

Guidelines for Hydrogen Chloride (HCl). Updated September 1, 2010. Accessed February 27, 

2011. http://www.atsdr.cdc.gov/MHMI/mmg173.pdf.

Agency for Toxic Substances and Disease Registry (ATSDR). 2010b. Medical Management 

Guidelines for Hydrogen Fluoride (HF). Updated September 1, 2010. Accessed February 27, 

2011. http://www.atsdr.cdc.gov/mhmi/mmg11.pdf.

Agency for Toxic Substances and Disease Registry (ATSDR). 2011. Toxic Substances Portal: 

Toxicological Profiles. Washington, DC, USA: ATSDR. Web Link: 

http://www.atsdr.cdc.gov/toxprofiles/index.asp [Accessed: 18 February 2011].

American Academy of Pediatrics (AAP), Committee on Environmental Health. 2004. Ambient Air 

Pollution: Health Hazards to Children. Pediatrics 114(6):1699-1707.

CASAC (2010). CASAC Review of Policy Assessment for the Review of the PM NAAQS –

Second External Review Draft (June 2010), Clean Air Scientific Advisory Committee (CASAC) 

Particulate Matter (PM) Review Panel.

Davidson, H. 2010. Dry Sorbent Injection for Multi-pollutant Control Case Study presented at

CIBO Industrial Emissions Control Technology VIII Conference. Portland, ME, USA. August 2-5, 

2010.

Dockery DW, Cunningham J, Damokosh AI, Neas LM, Spengler JD, Koutrakis P, Ware JH, 

Raizenne M, Speizer FE. 1996. Health Effects of Acid Aerosols on North American Children: 

Respiratory Symptoms. Environmental Health Perspectives 104(5):500-504.

Evans CD, Monteith DT, Fowler D, Cape JN, Brayshaw S. 2011. Hydrochloric Acid: An 

Overlooked Driver of Environmental Change. Environmental Science & Technology (In Press,

DOI: dx.doi.org/10.1021/es103574u).

Federal Register. 2010. National Emission Standards for Hazardous Air Pollutants for Major

Sources: Industrial, Commercial, and Institutional Boilers and Process Heaters, Proposed Rule.

75 (4 June 2010):32032.

Fine JM, Gordon T, Thompson JE, Sheppard D. 1987. The role of titratable acidity in acid

aerosol-induced bronchoconstriction. American Review of Respiratory Disease 135(4):826-830.

Gauderman WJ, Avol E, Gilliland F, Vora H, Thomas D, Berhane K, McConnell R, Kuenzli N, 

Lurmann F, Rappaport E, Margolis H, Bates D. 2004. The effect of air pollution on lung function 

development in children aged 10 to 18 years. New England Journal of Medicine.351:1057-1067.
Memorandum on Acid Gases 2011-03-02 Page 9 of 10

Kong Y, de La Hoz JM, Wood M, Atwell M, Lindsay T. 2008. Dry Sorbent Injection of Sodium 

Bicarbonate for SO2 Mitigation presented at International POWER-GEN 2008. Orlando, FL, 

USA. December 2-4, 2008.

Raizenne M, Neas LM, Damokosh AI, Dockery DW, Spengler JD, Koutrakis P, Ware JH, 

Speizer FE. 1996. Health Effects of Acid Aerosols on North American Children: Pulmonary 

Function. Environmental Health Perspectives 104(5):506-514.

Sanhueza E. 2001. Hydrochloric acid from chlorocarbons: A significant global source of 

background rain acidity. Tellus Series B Chemical and Physical Meteorology 53(2):122-132. 

Staudt, J.E., ―Surviving the Power Sector Environmental Regulations‖, Bipartisan Policy Center 

Workshop on Environmental Regulation and Electric System Reliability, October 22, 2010, 

available at www.andovertechnology.com

U.S. Environmental Protection Agency (USEPA). 2011a. Regulatory Impact Analysis: National 

Emissions Standards for Hazardous Air Pollutants: Industrial, Commercial, and Institutional 

Boilers and Process Heaters, Office of Air Quality Planning and Standards, Research Triangle 

Park, NC.

U.S. Environmental Protection Agency (USEPA). 2011b. National Emissions Standards for 

Hazardous Air Pollutants: Industrial, Commercial, and Institutional Boilers and Process Heaters, 

EPA-HQ-OAR-2002-0058. 

U.S. Environmental Protection Agency (USEPA). 2010. Air Toxics Standards for Utilities: Utility 

MACT ICR Data. Part I & II: Final draft (version 2) of selected EU MACT ICR response data 

(excludes facility contact information), including; All Part I (General Facility Information); and All 

Part II (Fuel Analysis and Emission Data); including all Hg CEMs data. Web Link: 

http://www.epa.gov/ttn/atw/utility/utilitypg.html [Accessed 11 January 2011].

U.S. Environmental Protection Agency (USEPA). 2009a. 2002 National-Scale Air Toxics 

Assessment. Web Link: http://www.epa.gov/ttn/atw/nata2002/ [Accessed February 18, 2011].

U.S. Environmental Protection Agency (USEPA). 2009b. Supporting Statement for OMB Review 

of EPA ICR No. 2362.01 (OMB Control Number 2060-0631): Information Collection Effort for 

New and Existing Coal- and Oil-Fired Electric Utility Steam Generating Units, Part A, Final. 

Research Triangle Park, NC, USA: USEPA.

U.S. Environmental Protection Agency (USEPA). 2009c. Supporting Statement for OMB Review 

of EPA ICR No. 2362.01 (OMB Control Number 2060-0631): Information Collection Effort for 

New and Existing Coal- and Oil-Fired Electric Utility Steam Generating Units, Part B, Final. 

Research Triangle Park, NC, USA: USEPA.
Memorandum on Acid Gases 2011-03-02 Page 10 of 10

U.S. Environmental Protection Agency (USEPA). 2009d. Integrated Science Assessment for 

Particulate Matter, Includes Errata Sheet created on 2/10/2010 (EPA/600/R-08/139F). Research 

Triangle Park, NC, USA: USEPA, Office of Air Quality Planning and Standards.

U.S. Environmental Protection Agency (USEPA). 2000a. Air Toxics Web Site: Hydrochloric Acid

(Hydrogen Chloride) Hazards Summary. Web Link:

http://www.epa.gov/ttn/atw/hlthef/hydrochl.html [Accessed February 18, 2011].

U.S. Environmental Protection Agency (USEPA). 2000b. Air Toxics Web Site: Hydrogen 

Fluoride Hazards Summary. Web Link: http://www.epa.gov/ttn/atw/hlthef/hydrogen.html

[Accessed February 18, 2011].

U.S. Environmental Protection Agency (USEPA). 1998a. Study of Hazardous Air Pollutant 

Emissions from Electric Utility Steam Generating Units – Final Report to Congress, Volume 1 

(EPA-453/R-98-004a). Research Triangle Park, NC, USA: USEPA, Office of Air Quality 

Planning and Standards.

U.S. Environmental Protection Agency (USEPA). 1998b. Study of Hazardous Air Pollutant 

Emissions from Electric Utility Steam Generating Units – Final Report to Congress, Volume 2. 

Appendices (EPA-453/R-98-004b). Research Triangle Park, NC, USA: USEPA, Office of Air 

Quality Planning and Standards.