## 113 

# Sulfuric Acid Mist: Regulating Uncertainties

By Matthew Thurlow *

Sulfuric acid mist, also known as H 2 

## SO 4 

## or SO 3 

## , 1 

## is one of the least publ i cized air pollutants associated with emissions from coal

- fired power plants. Long overshadowed by nitrogen oxides, sulfur dioxide, and carbon dioxide, sulfuric acid mist is typically not emitted in the boundary

- crossing and globe

- altering quantities of the more frequently discussed air pollutants. I n the whir l wind of the United States Environmental Protection Agency ’ s (EPA) recent air regulations of coal

- fired power plants including the Mercury and Air Toxic Standards for power pla nts (MATS)
, the New Source Performance Standards and the Tailoring Rule for greenhouse gases, and the recently vacated Cross

- State Air Pollution Rule, sulfuric acid mist has remained relatively untouched. 2 

## But EPA ’ s regulations, which have imposed dramatic

Copyright © 2012 Matthew Thurlow

* Former trial attorney, United States Department of Justice, Env i ronmental Enforcement Section. Special thanks to Meghan Keck, Tracy Perkins, and Alexandra Pressman and the staff of Ecology Law Currents

. The opinions expressed below are my own and do not re p resent the views of my current o r fo rmer employers or clients. 1

. Sulfuric acid mist is often referred to either as H 2 SO 4 (sulfuric acid) or SO 3 (sulfur trioxide). Sulfur trioxide forms in power plants as the result of oxidation of sulfur dioxide; as it cools, sulfur trioxide rapi d ly binds with water vapor in the stack to form H 2 SO 4 , a liquid aer o sol. See U.S. E NVTL

. P ROT

. A GENCY , O FFICE OF P OLLUTION P REVENTION & T OXICS , Emergency Planning & Community Right

- to

- Know Act Section 313, Guidance for Reporting Sulfuric Acid
, E PA .G OV ., http://www.epa.gov/tri/guide_docs/pdf/1998/sulfuric.pdf (last visited Aug. 17, 2012). Sulfuric acid mist is an air pollutant re g ulated pursuant to the Clean Air Act (CAA) as well as the Emergency Planning and Community Right

- to

- Know Act (EPCRA). 42 U.S.C. § 11001

– 50 (1986) . Although industry contends there are no doc u mented health impacts from dilute sulfuric acid mist, sulfuric acid is known to irritate and damage the eyes, skin, nose, and lungs

. EPRI, Chemical Profile: Sulfuric Acid
, AEPS USTAINABILITY

. COM , http://www.aepsustainability.com/ourissues/envperformance/wastemgmt/tri/ chemicalprofiles/SulfuricAcid.pdf (last visited Aug. 13, 2012). 2

. The Cross

- State Air Pollution Rule, which was recently struck down by the D.C. Circuit,

w ould have require d dramatic reductions in sulfur dioxide and nitrogen oxides. See EME Homer City Gener a tion, L.P. v. EPA , No. 11

- 1302 (D.C. Cir. Aug. 21, 2012); U.S. E NVTL

. P ROT

. A GENCY , Cross

State Air Pollution Rule
, E PA .G OV , http://www.epa.gov/airtransport (last updated Aug. 21 , 2012); Fede r al Impleme ntation Plans: Interstate Transport of Fine Particulate Matter and Ozone and Correction of SIP Approvals , 76 Fed. Reg. 48208, 48296 (Aug. 8, 2011); Federal Implementation Plans for I o wa, Michigan, Missouri, Oklahoma, and Wisconsin and Determin a tion for Kan sas Regarding Interstate Transport of Ozone , 76 Fed. Reg. 80760 (Dec. 27, 2011); Revisions to Federal Implementation Plans To Reduce Interstate Transport of Fine Particulate Matter and Ozone , 77 Fed Reg. 10324 (Feb. 21, 2012); Revisions to Federal I m

plemen tation Plans to Reduce Interstate Transport

## 114 ECOLOGY LAW C URRENTS [Vol. 3 9

new emission limits on sulfur dioxide, nitrogen oxides, greenhouse gases, mercury, and hydrochloric acid, are likely to have a significant impact on sulfuric acid mist emission co n trol strategies at coal

- fired power plants. 3 

## Although sulfuric acid mist h as been recognized as an air pollutant for decades, it only emerged as a significant problem for the utility industry in the early 2000s. 4 

## In 2001, after the General James M. Gavin Power Plant installed a type of nitrogen oxide controls called selective ca talytic reduction devices (SCRs)
, sulfuric acid mist emissions unexpectedly spiked from 9 000 to 11,000 pounds per day to allegedly more than 64,000 pounds per day. 5 

## In Cheshire, Ohio, a small village of 200 people in the shadow of the Gavin plant, resident s reported asthma

- like symptoms and noted corrosion and discoloration of paint on cars and houses, as blue plumes of sulfuric acid periodically drifted through the village. 6 

## The owner of the plant, American Electric Power (AEP),

of Fine Partic u late Matter and Ozone , 77 Fed. Reg. 103242 (Feb. 21, 2012). Sulfur dioxide (SO 2 ) reductions, which can be achieved through installation of scrubbers or use of lower sulfur coals, are likely to also reduce su l furic acid mist. Nitrogen oxide reductions, including installation of selective catalytic reduction devices (SCRs), are likely to exacerbate sulfuric acid mist emissions. The new utility hazardous air pollutant rule, the Mercury and Air Toxics S tandards (MATS), may also result in reductions of sulfuric acid mist because it imposes lower emissions limits on mercury and acid gases, which may be controlled using the same controls currently used to reduce sulfuric acid mist emissions. See National Em ission Standards for Hazardous Air Pollutants from Coal and Oil

- Fired Electric Utility Steam Generating Units and Standards of Performance for Fossil

- Fuel

- Fired Elec tric Utility, I n dustrial

- Commer c i al

- Institutional, and Small Industrial

- Commercial

- Institut ional Steam Generating Units
, Final Rule , 77 Fed. Reg. 9304 (Feb. 16, 2012)
, available at https://www.federalregister.gov/ articles/2012/02/16/2012

- 806/national

- emission

- standards

- for

- hazardous

- air

- pollutants

- from

- coal

— and

oil

- fired

- electric

- utility . Power plant operators must also consider the impacts of new and proposed regulations relating to greenhouse gas emissions. See Standards of Performance for Gree n house Gas Emissions for New Stationary Sources: Electric Utility Generating Unit s, P roposed R ule , 77 Fed. Reg. 22392 (April 13, 2012); Prevention of Significant Deterioration and Title V Gree n house Gas Tailoring Rule , 75 Fed. Reg. 31514 (June 3, 2010)

. 3

. See Mark Pastore, Continuous SO 3 Monitoring Can Reduce Sorbent Consumption
, C OAL P OWER M AGAZINE (Feb. 1, 2011), http://www.coalpowermag.com/ops_and_maintenance/308.html ( “ SCRs serv ing another 150 GW are expected to be installed on U.S. coal

- fired power plants by 2020

. . . “ ). 4

. See U.S. E NVTL

. P ROT

. A GENCY , O FFICE OF A IR & W ASTE M ANAGEMENT , Final Guideline Document: Control of Sulfuric Acid Mist Emissions from Existing Sulfuric Aci d Production Units
, E PA

. GOV , available at http://www.epa.gov/ttn/naaqs/ozone/ctg_act/197709_sox_epa450_2

- 77

- 019_sulfuric_acid_mist.pdf

. In June 1995, EPA modified the r e porting requirements for sulfuric acid, but continued to require r e por t ing of sulfuric acid aerosols including sulfuric acid mist. S ee EPCRA Guidance, supra note 1

. 5

. See John Amos Power Plant emitted more sulphuric acid than r e ported, H ERALD

- D ISPAT CH , (Feb. 1 7 , 20 08), http://www. herald

- dispatch.com/x1657941966. 6

. See Health Consultation, Gavin Power Plant, Agency for Toxic Substances and Disease Registry 3 (Aug. 9, 2007), http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd= 3&ved=0CDIQFjAC& url=http%3A%2F%2Fwebapp.psc.state.md.us%2FIntranet%2FCasenum%2FNe wIndex3_VOpenFile.cfm%3Ffilepath%3DC%3A%255CCasenum%255C9000

- 9099%255C9085% 255CItem_56%255C%255CSWA

- ExhibitC

- 1.pdf&ei=CqVCULWbAcL7iwLoooDQDg&usg= AFQjCNFmFSWvTB9JqEUhVy_pLu8NlVzANg

. Sulfuric a cid mist can also cause s e vere corrosion to ductwork and stack liners, or anywhere else the su l furic acid mist vapor condenses. See EPRI, SO 3 Mitigation Guide Update 1

- 9 ( Mar. 2004); Citizens Against Pollution v. Ohio Power Co

. , 484 F. Supp. 2d 800 (S.D. O hio 2007); Walter G. Wright, Does Flue Gas Constitute a RCRA Imminent and Substantial Endange r ment? E NVIRONMENTAL , E NERGY , AND W ATER B LOG (Mar. 7, 2012)
, available at http://www.mitchellwilliamslaw.com/does

- flue

- gas

- constitute

- a

- rcra

- imminent

- and

- substantial

-

## 201 2 ] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 115

eventua l ly paid $20 million to buy out most of Cheshire. A decade later, the village r e mains mostly empty. 7 

## The utility industry responded to the Gavin incident by investing signif i cant time and money to study the sulfuric acid mist problem. 8 

## EPA has al so r e sponded by paying closer attention to sulfuric acid mist from power plants and bringing a handful of sulfuric acid mist enforcement actions

. 9 

## Current and f u ture enforcement cases involving sulfuric acid mist pose a number of challen g es. In cases broug ht under the Clean Air Act ’ s (CAA) Prevention of Significant Deterioration of Air Quality (PSD) Program, utility companies, regulators, and courts may struggle to determine what emissions limits and controls are r e quired for sulfuric acid mist . This strugg le is based on uncertainties about the precise conditions under which sulfuric acid mist forms, how it can be co n trolled, how emissions can be monitored , and most importantly, at what emi s sions levels it poses a threat to human health and the environment. The Gavin incident and subsequent studies have dramatically improved the utility industry ’ s understanding of sulfuric acid mist. Sulfuric acid mist emissions strongly correlate with the sulfur content of coal : the higher the

su l fur content, the higher the sulfuric acid mist emissions. 10 

## But the precise

c i r cumstances that result in the formation of sulfuric acid mist have been much more difficult to unravel

. E xperts believe that vanadium and other constituents in coal may increase sulfuric acid mist formatio n. 11 

## In addition, b oiler design and oxygen levels in the flue gas appear to influence sulfuric acid mist fo r mation. H igh temperatures in boilers increase the formation of sulfuric acid

endangerment ( “ CAP ’ s members experienced watery eyes, burning throats, headaches, and breathing problems during plume touchdowns. ” ); EPRI, Chemical Profile: Sulfuric Acid
, supra note 1 ( “ Sulfuric acid fumes can irritate eyes, skin, and breathing passages, and concentrated fumes can pe r manently damage the nose and lungs. ” ). 7

. See Nicole Cohen, A Disappearing Town in the Shadow of Big Coal
, N ATIONAL P UBLIC R ADIO , (Apr. 4, 2012)
, http://www.npr.org/blogs/pictureshow/2012/04/04/149874232/a

- disappearing

town

- in

- the

- shadow

- of

- big

- coal

. 8

. American Electric Power pioneered the use of dry sorbent controls for sulfuric acid mist. See Douglas P. Ritzenthaler, SO 3 Control: AEP Pioneers and Refines Trona Injection Process for SO 3 Mitigation , C OAL P OWER M AGAZINE (Mar. 1, 2007), http://www.coalpowe rmag.com/ plant_design/29.html

. 9

. See U.S. v. Hoosier Energy Rural Elec. Coop., Inc. , 2010 WL 4250427 (S.D. Ind. July 23, 2010); In re E.ON U.S. & Ky. Util. Co

. , Notice of Violation, Mar. 19, 2009, available at http://psc.ky.gov/pscscf/2011%20cases/2011

- 001 61/20110616_Application%20Exhibits%20and%20 Appendices/GHR%20Exhibits%201%20

- %204/Exhibit%20GHR

- 2%20

- %20Ghent%202009%20NSR

PSD%20NOV.pdf; In re E.ON U.S., Notice of Violation, Sept. 26, 2007, available at http://psc.ky.gov/pscscf/2011%20cases/2011

- 00161/201 10616 ; In re Duke Energy Corporation , Notice and Finding of Violation, Mar. 10, 2008, available at http://yosemite.epa.gov/r5/r5ard.nsf/ 6a2817f3f71298e28625759a0045ba96/e47a671416849529862574c8006fd2ac/$FILE/a rd

- 013713.pdf. 10

. See EPRI, SO 3 Mitigation Guide Update
, supra note 6 ( “ The rea c tion mechanism discussed above for SO 3 formation implies that hig h er sulfur fuels will always produce higher flue gas SO 2 and SO 3 co n centrations than lower sulfur fuels. ” ). 11

. See i d. at 1

- 3, 1

- 4; see also James T. Murphy, SO 3 Control: How Many Coal Plants Might Have Opacity Issues Due to SO 3 Emi s sions? , C OAL P OWER M AGAZINE (Mar. 1, 2007)
, http://www.coalpowermag.com/ plant_design/31.html (discussing va r ious factors that influence sulfuric acid mist formation).

## 116 ECOLOGY LAW C URRENTS [Vol. 3 9

mist with the mist forming at the highest levels in a temperature band above approximately 800 degrees

. 12 

## Finally, a mbient conditions, including wind and water content in the air, also influence sulfuric acid mist formation and its i m pacts

. 13 

## This means that even if all other factors remain constant, weather

co n ditions may resu lt in higher or lower ambient concentrations and can increase the risk of human exposure to sulfuric acid mist. The uncertainties and complexities associated with sulfuric acid mist are further compounded by its relationship to other pollutants. Most trou bling, as discovered at the Gavin plant, there i s a clear relationship between the use of SCRs to reduce nitrogen oxide emissions and increases of sulfuric acid mist. 14 

## A study of power plants equipped with SCRs found that 98 percent of the plants were expe cted to emit sulfuric acid mist at levels above 5 ppm, a level that might result in environmental impacts. 15 

## But enforcement actions brought against plants that have installed SCRs raise the troubling specter of potentially penalizing utilities for the ir ef forts to reduce their environmental impact. In

e x ercising enforcement discretion, regulators may be forced to balance the need to reduce nitrogen oxides and their regional impacts with the need to protect communities from the more localized impacts of sulf uric acid mist. 16 

## And as with any pollutant from power plants, industry, government, and the public must weigh the environmental and health benefits of sulfuric acid mist control on one side, versus energy supply and demand and the potential increased cost of electricity on the other

. This Article briefly outlines the scientific and legal complexities facing the utility industry and environmental regulators in deve l oping sulfuric acid mist control strategies . Next, it compares the economic and environmental tradeoffs of different control strategies . Finally, it recommends control strategies that provide the utility industry operational flexibility while ensuring that human health and the environment are protected from sulfuric a c id mist and recognizes that t here may not be a one

- size

- fits

- all solution to r e duce sulfuric acid mist emissions at power plants.

12

. See EPRI, SO 3 Mitigation Guide Update, supra note 6 , at 1

- 4, 1

- 5. 13

. See Nenad Sarunac, Power 101: Improving the Performance of Boiler Auxiliaries, Part II
, C OAL P OWER M AGAZINE (Feb. 1, 2011) , http://www.coalpowermag.com/ops_and_maintenance/ 300.html (providing that “ atmospheric conditions (the direction of sunlight, temperature, humidity, and wind speed) ” can influence opacity from sulfuric acid mist). 14

. See EPRI, SO 3 Mitigation Guide Update, supra note 6 , at 1

- 5, 1

- 6; EPRI, E stimating Total Sulfuric Acid Emissions from Stationary Power Plants 3

- 4

– 3

- 5 (2010), available at http://m y.epri.com/ portal/server.pt?space=CommunityPage&cached=true&parentname=ObjMgr&parentid=2&control=Set Community&CommunityID=405 (last visited Aug. 13, 2012) (discussing SCR ’ s i n troduction of ammonia into flue gas, which can result in formation of sulfuric ac id mist). 15

. See Murphy, SO 3 Control
, supra note 11

. 16

. Facilities that installed SCRs prior to 2005 and otherwise co m plied with the applicable provisions can argue that the pollution co n trol project exemption to N ew S ource R eview applies. See 40 C.F.R. § 52.21. Although the pollution control project exemption has been vacated by the D.C. Circuit, the decision does not apply retroactively. See New York v. EPA , 413 F.3d 3 ( D.C. Cir. 2005).

## 201 2 ] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 117

I

. R EGULATING 

## S ULFURIC 

## A CID 

## M IST 

## U NDER THE 

## C LEAN 

## A IR 

## A CT


## A. Prevention of Significant Deterioration Provisions One of the most important avenues for regulating sulfuric acid mist under the CAA is through the New Source Review (NSR) program. 17 

## Under NSR
, m ajor new and modified stationary sources in areas that are unclassifiable or that meet the National Ambient Air Quality Standards (NAAQS) are subject t o P SD permitting

. 18 

## Notwithstanding certain exceptions, NSR and PSD are tri g gered either by new construction, or a physical change or change in the method of operation of an existing facility that results in a significant net increase of emissions of a pollutant. The threshold for “ significant increase ” varies by po l lutant. F or sulfuric acid mist , the threshold is an increase of 7 tons per year. 19 

## F acilities subject to the PSD program must submit a PSD permit to the permi t ting authority and i mplement B est A vailable C ontrol T echnology (BACT) for each regulated pollutant . Thus, unless an exception applies, 20 

## any power plant that makes a physical or operational change to its pla nt resulting in

17

. Unlike the criteria air pollutants, there are no NAAQS for sulfuric acid mist. Because sulfuric acid mist always falls within an unclass i fiable area, the prevention of significant deterioration (PSD) permi t ting program applies to new and modified major sources that emit sulfuric acid mist. See Coalition for Responsible Regulation, Inc. v. EPA , 684 F.3d 102; 2012 U.S. App. LEXIS 12980 (D.C. Cir. 2012). In Coalition
, the D.C. Circuit rejected Petitioners ’ arguments that greenhouse gases were not subject to PSD because they are not air pollutants emitted from major emitting facilities. Id. at *64, 72

– 85 ( “ EPA ’ s interpretation of the CAA requires PSD and Title V permits for stationary sources whose potential emissions exceed statutory thresholds for any regulated pollutant

— including greenhouse gases

. . . . [G]iven both the statute ’ s plain language and the Supreme Court ’ s decision in Massachusetts v. EPA , we have little trouble concluding that the phrase “ any air pollutant ” includes all regulated air poll u tants, including greenhouse gases. ” ). 18

. In the Gavin matter , the non

- profit Citizens Against Pollution (CAP) brought a case against the Ohio Power Company (a subsidiary of AEP) under the Resource Conservation and Recovery Act (RCRA), Co mprehensive Environmental Response, Compensation, and Liability Act (CERCLA), and Emergency Planning and Comm u nity Right to Know Act (EPCRA). It appears that the non

- profit brought these claims because U.S. EPA and the Ohio EPA entered into a memorandum of agreement with the power plant to reduce su l furic acid mist emissions and CAA claims were barred. See Opinion and Order, Citizens Against Pollution v. Ohio Power Co. , No. C2

- 04

- CV

- 371 (S.D. Ohio 2007) ( “ OPC took measures to correct the situ a tion, includin g, inter alia , implementing air testing and entering into an Memorandum of Agreement with the United States Environmental Protection Agency and the Ohio Environmental Protection Age n cy. ” ). 19

. 40 C.F.R. § 51:166(23)(i). 20

. While there is little doubt that a s ignificant project like the insta l lation of nitrogen oxide pollution controls can be construed as a phy s ical modification of a power plant, it is less clear if minor tweaks to operations trigger PSD. Power plant owners would likely contend that these tweak s were routine maintenance and that the statute ’ s e x ception for “ routine maintenance, repair, and replacement ” applies. 40 C.F.R. § 51:166 (b)(2)(iii)(a). A more interesting scenario is a sit u ation in which the power plant has merely switched fuel sources from a low sulfur coal source to a high sulfur coal source. Making this type of switch, for instance, from 1

- 2 lb/MMBtu coal source to a coal source three to four times higher in sulfur content, would likely dr a matically increase emissions of sulfuric acid mist (as well as sulfur dioxide). This scenario might seem an obvious trigger of EPA ’ s PSD program because it would appear to be a change in the method o f the plant ’ s operation, but there is a specific carve

- out in the PSD program for fuel switches. A fuel switch exception applies to the use of an a l ternative fuel that a source

“ was capable of accommodating before January 6, 1975. ” 40 C.F.R. § 51.166(b)(2) (iii)(e)(1); see also Hawa i ian Elec. Co., Inc. v. U.S. EPA. , 723 F.2d 1440, 1448 (9th Cir. 1984) (citing 1979 EPA determination that “ an increase in sulfur content does not constitute use of an ‘ alternative ’ fuel ” ).

## 11 8 ECOLOGY LAW C URRENTS [Vol. 3 9

an increase of sulfuric acid mist emissions of more than 7 tons a year must obtain a PSD permit and apply BACT to limit sulfuric acid mist emissions

.
B . Best Available Control Technology

BACT is an emission s limit “ based on the maximum de gree of reduction of each pollutant subject to regulation under ” the CAA “ on a case

- by

- case b a sis, taking into account energy, environmental, and economic impacts and other costs
, ” that the permitting authority “ determines is achievable. ” 21 

## EPA

re c ommends, and most permitting agencies apply, a top

- down BACT analysis that ranks all available control technologies for a regulated pollutant in descending order of effectiveness. 22 

## Following this approach, the most stringent control alternative is selected unless t echnical considerations, energy, environmental, or economic impacts lead the permitting authority to conclude that it is not “ achievable. ” 23 

## Because BACT is assessed on a case

- by

- case basis, it may di f fer significantly from one power plant to another power plant. Geography, fuel sources and types, and plant configuration can impact the technical feasibility of controls and the cost effectiveness of controls

. 24 

## Not surprisingly
, permi t ting authorities, the utility industry , and the public, which can comment on PSD permit s , 25 

## may disagree over how technical, environmental, economic, and site

- specific factors should i nfluence BACT determinations

.
C. Control Technology Available for Sulfuric Acid Mist

One of the most effective control options for sulfuric acid mi st is a wet electrostatic precipitator (WESP)
, a particulate control device t h at removes pa r ticles, including sulfuric acid mist, from flue gas by using an electrostatic charge

. 26 

## WESPs are extremely efficient at removing sulfuric acid mist, but

21

. 42 U.S.C. § 7479(3). 22

. See Alaska Dep ’ t of Envtl. Conservation v. EPA , 540 U.S. 461, 475

– 76 (2004). As the Supreme Court held in Alaska
, “ [n]othing in the Act or its implementing regulations mandates top

- down [BACT] analysis. ” ). 23

. Id. ; see also U.S. E NVTL

. P ROT

. A GENCY , New Source Review Works hop Manual B2 (Oct. 1990), available at http://www.epa.gov/ttn/nsr/gen/wkshpman.pdf . EPA maintains a clearinghouse of best available technologies for power plants and other stationary sources. See RACT/BACT/LAER Clearinghouse, U.S. E NVTL

. P ROT

. A GENCY , http://cfpub.epa.gov/rblc/index.cfm?action=Home.Home

. 24

. Determining the cost

- effectiveness of BACT controls includes an assessment of the capital costs and annual operation and maintenance costs of controls and the difference between baseline emissions and controlled emissions. This allows for an approximat ion of the cost per ton of emissions reductions. Regulated entities also review the i n cremental cost of compliance, or a comparison of the costs o f co m pliance between the best available control method and the next best method. U.S. E NVTL

. P ROT

. A GENCY , PSD and Title V Permitting Guidance for Greenhouse Gases K

- 1 (Mar. 2011), available at http://www.epa.gov/nsr/ghgdocs/ghgpermittingguidance.pdf

. 25

. See U.S. E NVTL

. P ROT

. A GENCY , EPA Region 9 ’ s PSD Permitting Process
, E PA .G OV , http://www.epa.gov/region9/air/permit/psd

- issuing.html (last updated Apr. 3, 2012). 26

. See U.S. E NVTL

. P ROT

. A GENCY , O FFICE OF A IR & R ADIATION , Technical Support Document: Impact on CAIR Analyses of D.C. Ci r cuit Decision in New York v. EPA 5, (Dec. 2005), a vailable at http://www.ade q.state.ar.us/ftproot/Pub/commission/p/Closed%20Permit%20Dockets%202006

- 2012/08

- 006

- P%20AEP%20Service%20Corp.%20%26%20SWEPCO

-

## 201 2 ] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 119
they can cost $50 million to $ 2 0 0 million and require a significant amount of energy , up to 0 .5 percent of the plant ’ s gross output , to operate. 27 

## Because BACT analyses require e conomic asse ssment of control options
, including a calculation of removal costs on a per ton basis, utilities may effectively argue that a WESP is not required under BACT if the amount of sulfuric acid mist removed is less than several thousand tons per year. There is no bright

- line rule for per ton removal costs under BACT, but costs above five or six thousand dollars per ton for controls have been referenced as approaching the upper limit of the threshold of economically feasible technology required under BACT. 28 

## The current preference of the utility industry for sulfuric acid mist control appears to be sorbent injection
, an option that is much more economical than WESPs in the short term. Dry sorbent injection uses nozzles to spray a dry powder, typically magnesium, lime, or trona (a sodium

- based mixture), into the flue gas. 29 

## The sorbent binds with the sulfuric acid mist and removes it from the flue gas stream. But t here are limits on the u se of sorbent control

. E xcessive use of sorbents can clog equipment , so power plant engineers may need to

e x periment with different levels of sorbent injection . They may also need to ba l ance sorbent injection with other control methods , including configura tion changes that increase the amount of time sulfuric acid mist remains in the

Sierra%20Club%20%26%20Audubon(Consolidated)/2009

- 04

- 24_SWEPCO

- BH

- EX.22.pdf (noting that WESP can remove appro x imately 95 percent of sulfuric acid mist from flue gas). 27

. The costs of a WESP are highly plant

- specific and depend on plant size and configuration. The cost of capital projects for power plants is typically expressed as a function of cost per kilowatt of

e n ergy. The estimated capital costs for WESP ’ s range from $20 to $45 per kilowatt, which, for a 2500 megawatt power plant, would tran s late to a cost of $50 million to $112 million. See John Caine & Ha r dik Shah, Membrane WESP

– A Lower Cost Technology to Reduce PM 2.5, SO 3 & HG +2 Emissions (2006), http://www.netl.doe.gov/technologies/coalpower/ewr/mercury/control

- tech/pubs/AQIV

Caine.pdf (last visited Aug. 17, 2012); see also Gary M. Blythe, et al., Economic Comparison of SO 3 Control Options for Coal

- Fired Power Plants
, N ETL .D OE .G OV . (Nov. 25, 2003), http://www.netl.doe.gov/technologies/coalpower/ewr/pm_emissions_control/pubs/SO3%20Control%20 Options%20for%20AQ%20IV.pdf (last visited Sept. 1, 2012)

. Blythe estimates that the capital costs to retrofit a plant and install a WESP would be $40 to $90, for a total cost of $100 to over $200 million for a 2500 megawatt power plant. 28

. Because BACT determinations are made on a case

- by

- case basis, there is no bright line rule regarding the economic feasibility of per ton pollutant removal costs. S ee Brandon A. Mogon, The BACT Analysis Guide: Cost Analysis Considerations
, T HE BACT A NALYSIS G UIDE , ( Oct. 23, 2009), http://bactanalysis.com/bact

- an alysis/the

- bact

- analysis

- guide

- cost

- analysis

- considerations ( “ Each regulatory agency has a different opinion about the maximum economically fe a sible cost effectiveness value, and many (e.g., CTDEP) will not tell you what that value is

. ” ). But some states h ave provided guidance that the general rule of thumb for the upper bound of economic feas i bility for per ton reduction of pollutants approaches $4,000 to $6,000. See, e.g.
, M ASS

. D EP ’ T OF E NVTL

. P ROT

. , Best Available Control Technology Guidance 6 (June 2011), available at http://www.mass.gov/dep/air/approvals/ bactguid.pdf ; N EB

. D EP ’ T OF E NVTL

. Q UALITY , Best Available Control Technology
, http://www.deq.state.ne.us/Publica.nsf/c4afc76e4e077e11862568770059b73f/0949822f884b8ce186257 3bd007da0e9?OpenDocument ; U TAH D EP ’ T OF E NVTL

. Q UALITY , Best Available C ontrol Technol o gy , http://www.airquality.utah.gov/Permits/FORMS/Form01b.pdf . Maximum removal costs should, in theory, relate to pollutants ’ pr o portional threat to the environment and human health with more harmful pollutants having a higher cost per ton th reshold of feasibi l ity under BACT than more innocuous air pollutants. 29

. See EPRI, Estimating Total Sulfuric Acid Emissions from Statio n ary Power Plants
, supra note 14 , at 3

- 6.

## 120 ECOLOGY LAW C URRENTS [Vol. 3 9

stack

. 30 

## The longer the residence time for sulfuric acid mist in the stack , the more opportunity the sulfuric acid mist has to bind with sorbent. 31 

## Facilities

can also maximize s ulfuric acid mist control if they mill sor bent into smaller part i cles that increase the surface area o f sorbent and improve its potential to

ca p ture sulfuric acid mist

. However, even with these measures, there remains a saturation point beyond which increa sing the amount of sorbent injected will not further reduce the amount of sulfuric acid emissions. Other plant improv e ments, including installation of low catalyst SCRs, 32 

## which reduce, but do not eliminate the impacts of SCRs on sulfuric acid mist formation , or switching or blending fuel with low or medium sulfur content coal, 33 

## may be used to su p plement sorbent injection

. Baghouses
, which are large filters designed to capture soot and other pa r ticulates, 34 

## can also reduce sulfuric acid mist emission s. One study indicates that baghouses can remove up to 90 percent of sulfuric acid mist. 35 

## As with WESPs, however, installing baghouses can be a significant capital expend i ture

. 36 

## Utilities may balk at the expense and argue that the technology is not economi cally feasible under a BACT analysis. I m provements in sorbent control may be increasing regulators ’ and indu s try ’ s confidence that sorbent injection, while not achieving the same reductions in sulfuric acid mist as WESPs , can reduce sulfuric mist emissions to levels that are sufficient to protect human health and the environment at a f raction of the cost. 37 

## The effectiveness of sorbent injection, however, may

30

. See EPRI, SO 3 Mitigation Guide Update, supra note 6 , at 1

- 7, 2

- 13, 4

- 2; see also EPRI, SO 3 M ITIGATION : C URRENT U TILITY O PERATING E XPERIENCE A19 (2006). 31

. Residence time is the amount of time that sorbent is present in the gas stream and has an opportunity to bind with sulfuric acid mist and form a precipitate. Injecting the sorbent before the electrostatic pr e cipitator also increases residence time. See Douglas Ritzenthaler, SO 3 Control: AEP Pioneers and Refines Trona Injection for SO 3 Mitig a ti on , C OAL P OWER M AGAZINE (Mar. 1, 2007), http://www.coalpowermag.com/plant_design/SO3

- Control

- AEP

- Pioneers

- and

- Refines

- Trona

- Injection

Process

- for

- SO3

- Mitigation_29.html ; EPRI, Estimating Total Sulfuric Acid Emissions from Stationary Power Plants
, supra not e 14 , at 3

- 6, 4

- 19

. 32

. See EPRI, SO 3 Mitigation: Current Utility Operating Experience
, supra note 30 , at 3

- 19 ( “ Performance testing completed by the man u facturer of the catalyst actually showed less than 0.1 percent SO 2 conversion to SO 3 from three different reactor tests. ” ). 33

. See EPRI, SO 3 Mitigation Guide Update
, supra note 6 , at 2

- 19 (noting that the effectiveness of fuel blending can be difficult to pr e dict, but one facility reduced its sulfuric acid mist emissions to zero by switching to low sulfur, Powder River Basin coal). The economics and cost effectiveness of switching coal can be very site

- specific b e cause switching or blending fuels depends on long

- term coal

co n tracts, coal availability, and other plant

- specific factors. Id. ; see also Gary M. Blythe et al., SO 3 Control Options for Coal

- Fired Power Plants
, supra note 27 , at 2. 34

. Baghouses are air pollution control devices that remove partic u lates from flue gas streams. Baghouses typically use fabric filters to remove and collect dust from the flue gas stream

. See John H. Turner, et al., EPA Air Pollution Control Cost Manual , U.S. E NVTL

. P ROT

. A GENCY (Jan. 2002)
, available at http://www.epa.gov/ttncatc1/dir1/c_allchs.pdf

. 35

. See EPRI, Estimating Total Sulfuric Acid Emissions from Statio n ary Power Plants
, supra note 14 , at 4

- 19. 36

. See EPA Air Pollution Control Cost Manual supra note 34 , at 1

- 1. 37

. Despite their high operation and maintenance costs, WESPs may compare more favorably with sorbent injection over the long

- term, especially if used year

- round. See EPRI, SO 3 Mitigation Guide U p date, supra note 6 , at 3

- 37.

## 201 2 ] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 121

depend on proper calibration and maintenance of so rbent injection rates over time and a co n sistent fuel source

. If these inputs are not constant , sulfuric acid mist emissions could spike. To mitigate the possibility of fluctuations in sulfuric acid mist emissions , operators need to build in some compliance headroom by ensuring that day

- to

- day em issions of sulfuric acid mist are marginally lower than levels that could result in opacity problems or violate permit l imits. Once a control strategy has been adopted, power plants and enforcement authorities need to monitor the effectiveness of the contr ols over time and i n different operating scenarios

. Some p ower plants may need to continue to experiment with a vari e ty of controls to find a solution that provides the best balance of s ulfuric acid mist reduction, control of other pollutant s, and power pl ant performance.
D. Opacity Violations The appearance of the tell

- tale blue plume of sulfuric acid mist from the stack of a power plant often indicates a different violation of the CAA

. In add i tion to, or in lieu of, claims brought under the CAA ’ s PSD pr ovisions, EPA may bring claims against power plant owners and operators for opacity viol a tions under the Act ’ s New Source Performance Standards (NSPS) (Section 111 of the Act) or the applicable State Implementati on Plan

. 38 

## “ Opacity means the degree to which emissions reduce the light and obscure the view of an object in the background. ” 40 CFR § 63.2. At 100 percent opacity, no light is visible through a plume. At zero percent opacity, a plume is completely transparent. While opacity is not itself a polluta nt, it serves as a surrogate for particulate matter pollution , including sulfuric acid mist pollution, from power plants. 39 

## The NSPS for fossil

- fuel

- fired steam generators provide that
, for power plants constructed after August 17, 1971, gases emitted from the facility cannot

“ exhibit greater than 20 percent opacity except for one six

- minute period per hour of not more than 27 percent opacity. ” 40 

## For facilities not subject to the NSPS (built prior to August 1971), opacity limits can vary depending on the app lica ble State Implementation Plan. In Kentucky, for example, facilities are not to exceed 40 percent opacity except for one six

- minute period per hour of not more than 60 percent opacity

. 41 

## In Texas, older facilities cannot exceed 30 percent opacity average d over a six

- minute period. 42 

## Opacity problems associated with sulfuric acid mist can occur with emi s sions as low as 3

- 4 ppm. 43 

## The utility industry reports that sulfuric acid

38

. 40 C.F.R. § 60.42 ; 40 C.F.R. § 52.01 et seq

. 39

. See Sierra Club v. Georgia Power Co. , 443 F.3d 1346, 1350 n.4 (11th Cir. 2006). 40

. 40 C.F.R. § 60.42(a)(2). 41

. 401 KAR 61:015(4); 40 C.F.R. § 52.920

. 42

. 30 T EX

. A DMIN

. C ODE § 111.111(a)(1)(A) (2011); 40 C.F.R. § 52.2270(c)

. 43

. See Pastore, Continuous SO 3 Monitoring Can Reduce Sorbent Consumption, supra note 3 , at 1 ( “ SO 3 also creates a visible blue

- white plume at concentrations as low as 3

- 4 ppm and is often detec t able on an opacity monitor. ” ); but see Sarunac, Power 101
, supra note 13 ( “ Flue gas SO 3

## 122 ECOLOGY LAW C URRENTS [Vol. 3 9

mist concentrations of only 10 ppm can result in opacities greater than 40 perce nt , the upper opacity limit for m any older power plant units

. 44 

## Other industry guidance indicates that opacity problems can occur at sulfuric acid mist co n ce n trations above 5

- 15 ppm. 45 

## The impacts of sulfuric acid mist emissions on stack opacity can fluctuat e depending on operating conditions

. Opacity mon i tors can provide accurate opacity data and send an immediate warning to plant operators if limits are exceeded . B ut many power plants have had to install wet scrubbers to address sulfur dioxide emissions
, which make s it difficult or i m possible to monitor opacity within their stacks

. As a result, m any plants have either removed their opacity monitors or replaced them with particulate mon i tors. Continuous sulfuric acid mist monitors, which would best address the cu r rent monitoring problem, are still under development. 46 

## As a result of the unavailability of effective monitoring devices for sulf u ric acid mist , in many instances the only method to determine opacity at a po w er plant is through visual observation o f the plume. Method 9 reading s , which rel y on the judgment of a trained inspector , are the most common method of visual opacity assessments

. 47 

## Although inspectors are typically experienced and well trained, Method 9 is subject to judgment, memory
, and the h uman eye. 48 

## Inspectors record 24 consecutive observations (typically in six

- minute increments ) and average the results. An accurate opacity test requires ideal weather conditions because the plume cannot be clearly observed on cloudy days. The difficulty an d uncertainty associated with visual opacity readings are a cause for concern for both regulators and the industry. 49 

## On the one hand, regulators have to undertake opacity readings onsite and in clear

concentrations of about 10 ppm v can result in plume opacities greater than 50 percent in some cases; at 5 ppm v , the opacity is about 20 percen t. The specific SO 3 concentration at which a blue plume can be seen is a function of atmospheric conditions and stack characteristics. However, it is generally accepted that if the SO 3 concentration is less than 5 ppm v , there are no visible discoloration effects. ” ). 44

. See EPRI, SO 3 Mitigation Guide Update
, supra note 6 , at 1

- 3. 45

. See EPRI, SO 3 Mitigation: Current Utility Operating Experience
, supra note 30 ; EPRI, Estimating Total Sulfuric Acid Emissions
, supra note 14 , at 3

- 7 ( “ The a lkali injection system usually is operated to r e duce SO 3 emissions to between 5 and 15 ppm, an optimal range to prevent formation of a visible plume ” ) ; SO 3 Mitigation Guide U p date, supra note 6 , at 1

- 8 (providing that flue gas concentrations of sulfuric acid mist of 10 ppm can result in plume opacities above 40 percent). 46

. See Pastore, Continuous SO 3 Monitoring Can Reduce Sorbent Co nsumption , supra note 3

. 47

. U. S. Envtl. Prot. Agency, Stationary Source Compliance Division
, Visible Emissions Field Manual EPA Methods 9 and 22
, EPA.Gov
, available at http://www.epa.gov/ttn/emc/methods/ VEFieldManual.pdf

. 48

. Method 9 inspectors are trained and recertified every six months at “ smoke school, ” in which they observe white and black smoke plumes from a stack with opacity monitoring equipment. As part of th eir training, inspectors are tested on their ability to recognize di f ferent opacity levels of smoke plumes. See, e.g
., S MOKE S CHOOL , I NC

. , http://smokeschoolinc.com (last visited Aug. 13, 2012); E ASTERN T ECHNICAL A SSOCIATES , V ISIBLE E MISSIONS O BSERVER T RAINING M ANUAL (A ug . 200 4 )
, http://www.eta

- is

- opacity.com/ETA_VE_manual.pdf

. 49

. See National Parks Conservation Assoc., Inc. v. TVA , 175 F. Supp. 2d 1071, 1079 (E.D. Tenn. 2001) ( “ Obviously, monitori ng the smokestack emissions continuously with equipment capable of reli a bly measuring the opacity will identify many more exceedances than will be identified by an operator ‘ eyeballing ’ the smokestack emi s sions once a day, or less. ” ); Sierra Club v. Public Service Co of Col o rado , 894 F. Supp. 1455, 1459

- 60 (D. Colo. 1995) (citing the “ rel a tive reliability of CEM data over Method 9 data ” ).

## 201 2 ] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 123

weather to document opacity violations. On the other hand, a n opacity reading made by a regulator visually assessing real time emissions from memory is difficult for a defendant to refute

. If a pattern of opacity violations at a power plant can be established, EPA or state environmental authorities may argu e that significant controls are

r e quired to eliminate the visible sulfuric acid mist plume. Enforcement author i ties may obtain civil penalties for each day a plant exceeds opacity limits
, or obtain significant injunctive relief that , in some cases, approac hes a BACT

- like remedy

.
II. H UMAN 

## H EALTH AND THE 

## E NVIRONMENT AND 

## E MISSIONS 

## L IMITS 

## Perhaps the thorniest issue with regard to regulating sulfuric acid mist is determining at what levels emissions of sulfuric acid mist threaten human health and the environment. Studies indicate that sulfuric acid mist can impact the health of children with asthma at 70 micrograms per cubic meter
, and can impact normal adult lung function at 100 micrograms per cubic meter. 50 

## The impact on health, however, is a function of exposure duration, individual

sens i tivity, and exposure to other air contaminants. In the early 2000s, toxicologists from the Agency for Toxic Substances and Disease Registry evaluated the impacts of sulfur dioxide and sulfuric acid mist on the villag e of Cheshire, Ohio, near the Gavin power plant, and conclu d ed that emissions posed a public health hazard to some residents. 51 

## The highest officially recorded levels of sulfuric acid mist in Cheshire were approximately 120 micrograms per cubic meter of air , but there were unofficial reports of le v els as high as 200 micrograms per cubic meter of air. 52 

## It was difficult for i n vestigators to determine the duration of individual exposure to sulfuric acid mist, but exposures ranged from several minutes to several hours. 53 

## Residents in Cheshire were also exposed to high levels of sulfur dioxide and metal oxide particulates and investigators indicated that the presence of these and other co

- contaminants might also have had health impacts. 54 

## Following the lawsuit bro ught by Citizens Against Pollution, AEP agreed to emissions limits of 14 ppm of sulfuric acid mist at the Gavin plant. By

co m parison, in the only settled case to date in which EPA has directly addressed sulfuric acid mist, involving the Hoosier Energy Comp any, the parties agreed to limits of approximately 2. 5 ppm (.007 lb/mmBTU). 55 

## But this

50

. See A GENCY FOR T OXIC S UBSTANCES AND D ISEASE R EGISTRY , H EALTH C ONSULTATION , G AVIN P OWER P LANT 12 (Apr. 2007). 51

. Id . at 16. 52

. Id. at 5

– 6. 53

. Id. at 5. 54

. Id . at 13. 55

. The Hoosier settlement included a limit of .007 lb/mmBTU, but provided Hoosier with the option to petition for a lower limit. Ho w ever, the limit could not be lower than .009 lb/mmBTU. See U.S. E NVTL

. P ROT

. A GENCY , Consent Decree, U.S. v. Hoosier Energy R u ral Elec. Coop., Inc

. (S.D. Ind.

## 124 ECOLOGY LAW C URRENTS [Vol. 3 9

limit was o n ly one part of a larger settlement that required a number of significant i m provements to the facility at a total cost of $250 million to $300 million. 56 

## A d diti onally, Hoosier had a lengthy compliance period, almost two years, to meet the sulfuric acid mist emissions limit, and an additional year before it was su b ject to stipulated penalties to meet these limits. Although there appears to be some consensus in th e utility industry that emissions of sulfuric acid mist above 5 ppm may result in opacity problems, many power plants may be disinclined to agree to similarly low limits. This hesitation is in part a function of the difficulty of measuring sulfuric acid mi st. 57 

## Although there is an accepted EPA methodology for stack testing for sulfuric acid mist, the industry has expressed concern that this method does not provide accurate results. 58 

## S ulfuric acid mist emissions also may fluctuate b e tween stack tests. While sorbent controls may ensure compliance with a 5 ppm limit most of the time, ambient conditions or variations in fuel could result in higher emissions. 59 

## B oth EPA and the industry are concerned that no contin u ous emissions monitoring device is commercially a vailable for sulfuric acid that has the requisite sensitivity to detect changes of less than 1 ppm in the stack. In the absence of recurrent, visible opacity problems, power plants may remain unaware of the potential significance of sulfuric acid mist emissions

u n til stack testing can be performed, at a relatively high cost, on a quarterly or b i annual basis. Infrequent stack tests threaten both utility operators and regul a tors. Utility operators may worry that one high stack test could be used as eviden ce of continuous non

- compliance with emissions limits, while regulators may be concerned that stack tests do not provide adequate monitoring of sulf u ric acid mist emissions and could fail to identify non

- compliant facilities.

III

. U NCERTAINTY AND 

## B ALANCING


## A s EPA seeks more stringent regulation of other air pollutants , the utility industry and regulators will need to keep close tabs on sulfuric acid mist emi s sions

. Perhaps the most vexing problem for regulators and industry alike is the uneasy relationship bet ween sulfuric acid mist control and control of nitrogen oxides. A significant sulfuric acid mist problem first emerged, at Gavin and elsewhere, with the adoption of SCRs used to reduce nitrogen oxide

July 26, 2010), E PA.G OV , available at http://www.epa.gov/compliance/resources/decrees/civil/ caa/hoosier

- cd.pdf. 56

. See U.S. E NVTL

. P ROT

. A GENCY Hoosier Energy Agreement Marks 20 th Settlement Under EPA ’ s Power Plant Enforcement Initi a tive , EPA. G OV (July 23, 2010); available at http://yosemite.epa.gov/opa/admpress.nsf/f0d7b5b28db5b04985257359003f533b/9dee83d4b674b70385 257769005d7b1e!OpenDocument. 57

. See EPRI, Estimating Total Sulf uric Acid Emissions from Statio n ary Power Plants
, supra note 14 , at 2

- 1

– 2

- 2 (discussing sulfuric acid mist measurement uncertainties). 58

. Id . EPA uses EPA Method 8 to test for sulfuric acid mist. The uti l ity industry prefers the controlled condensate system method because it claims EPA Method 8 results in a positive bias for detection of su l furic acid mist

. See id. at 2

- 1; 3

- 7. 59

. See Murphy, SO 3 Control
, supra note 11

.

## 201 2 ] SULFURIC ACID MIST: REGULATING UNCERTAINTIES 125

emissions. When SCRs are combined with high

- sulfur co al, as they were at the Gavin plant, sulfuric acid mist emissions can increase dramatically. In response to the now decade

- old problem w ith SCRs, the utility industry developed low

- acid conversion catalysts that reduce the sulfuric acid

conve r sion rate. 60 

## But SCRs still involve a trade off with generally higher emissions of sulfuric acid mist. 61 

## Power plants have sought to compensate for the higher sulfuric acid mist emissions with many of the control technologies described above, but several of these strate gies, including WESPs and baghouses, may not be economical if used solely to control sulfuric acid mist. Sorbent injection, while typically the most economically feasible control method for sulfuric acid mist in the short term, may be insufficient to contr ol very high levels of sulfuric acid mist emissions

. Following the recent regulations for hazardous air pollutants, including mercury and acid gases, power plants may increasingly turn to sorbent inje c tion. 62 

## The need to control mercury and acid gas may ha ve a positive impact on sulfuric acid mist control because power plants may not only invest in sup e rior sorbent injection systems, but may also install baghouses t hat increase the effectiveness of sorbent controls. 63 

## Additional controls that target sulfur d io x ide, including scrubbers, should also serve to reduc e sulfuric acid mist emi s sions. In the current regulatory landscape, optimizing the performance of coal

fired power plants and ensuring that they run efficiently within permit limits is increasingly d ifficult. Finding the operational “ sweet spot ” at power plants may require a significant investment of time and money. In order to reduce sulfuric acid mist emissions and balance other regulatory requirements, enforcement authorities and the utility indust ry must develop control strategies tailored to individual power plants. Regulators must allow time for calibration of sulfuric acid mist and other air pollutant control strategies and stack testing. For their part, plant operators must think ahead and cons ider the impacts of implemen t ing control schemes for multiple pollutants. In an environment of regulatory uncertainty, it may be worthwhile for the utility industry to consider adopting conservative control strategie s with multi

- pollutant control benefi ts that i n crease operational flexibility, reduce the risk of future non

- compliance
, and a n ticipate the possibility of strict er, long

- term emissions limits. T he high costs of control equipment, complex maintenance and operation schedules of

60

. See Sarunac, Power 101
, supra note 13 (indicating that high ox i dation catalysts in an SCR can double the concentration of sulfuric acid mist in flue gas; low conversion catalyst significantly reduce su l furic acid mist conversion). 61

. See EPRI, SO 3 Mitigation Guide Update
, supra note 6 , at 1

- 6. 62

. The MATS for power plants sets numerical limits for mercury emissions, other hazardous metal emissions, a nd hydrochloric acid emissions. 63

. 77 Fed. Reg. at 9411 ( “ [T]he EPA agrees that DS I [dry sorbent i n jection] technology is proven and ready for commercial uses in co n trolling acid gases from coal combustion. ” ). As described above, baghouses permit the injection of additional sorbent into the flue gas stream and increase the amount of res idence time in which sorbent can bind with air pollutants. WESPs also can be used to control me r cury. See John Caine & Hardik Shah
, Membrane WESP
, supra note 27
, at 9.

## 126 ECOLOGY LAW C URRENTS [Vol. 3 9

power plants , and t hreat of future enforcement actions l eave little room for trial and error.