Page 1 of 10 Comments on

40 CFR Part 49

[EPA–R09–OAR–2009–0598; FRL–8950–6]

Assessment of Anticipated Visi bility Improvements at Surr ounding Class I Areas and
Cost Effectiveness of Best Ava ilable Retrofit Technology for

Four Corners Power Plant and Navajo Generating Station: Advanced Notice of Proposed Rulemaking

AGENCY: Environmental Protection Agency (EPA).

ACTION: Advanced Notice of Proposed Rulemaking.

Federal Register /Vol. 74, No. 166 / Friday, August 28, 2009 / Proposed Rules; Pages 44313-44334

Submitted by

Eladio M. Knipping, Ph.D. Electric Power Research Institute (EPRI) 2000 L St NW, Suite 805 Washington, DC 20036

Naresh Kumar, Ph.D. and Stephanie L. Shaw, Ph.D. Electric Power Research Institute (EPRI)
3420 Hillview Ave. Palo Alto, CA 94304

October 26, 2009Page 2 of 10 The following discussion is presented in response to the following sections in EPA’s Request for Public Comment

II. Request for Public Comment B. Factor 5: Degree of Visibility Improvement
1. FCPP b. EPA Modifications to Emissions Rate Inputs

In addition to the estimates for PM fine described above, EPA additionally revised the mode ling inputs for PM fine to include emissions of hydrogen chloride (HCl) and hydrogen fluoride (HF). AP– 42 (1.1 Bituminous and Subbituminous Coal Combustion) provides a single emission factor each for HCl and HF from all coal and boiler types. APS assumed H2SO4 to be the only contributor to condensable inorganic PM, and the NPS raised concerns about the exclusion of HCl and HF and recommended these two compounds be factored into the CPM–IOR (SO4) modeling input. Method 202 for measuring condensable PM does not capture HCl and HF, theref ore, EPA added these emission s to PM fine rather than SO4. [Page 44323]

and

II. Request for Public Comment B. Factor 5: Degree of Visibility Improvement
2. NGS b. EPA Modifications to Emissions Rate Inputs

For all modeling scenarios, EPA included HCl and HF emissions as PM fine modeling inputs and scaled them in a similar manner described for FCPP. [Page 44329]

Our comment is the following: It is inappropriate to treat HCl and HF as PM Fine for BART Modeling

HCl and HF are emitted as gases and will not form particles that impact visibility on a regional level. Treating HCl and
HF as non-volatile PM Fi ne mass is incorrect.

In this document, we review the physical properties of HCl and HF in detail, an d also use theoretical calculations and
measurement data to demonstr ate that treating these components as PM Fine or H 2 SO 4 is inappropriate for regional
haze calculations.Page 3 of 10 Do HCl and HF emissions from Power Plants contribute to visibility degradation?

It is instructive to first understand how emissions from power plants may contribute to visibility degradation in the atmosphere. Visibility degradation can occur due to primary particles emitted directly or secondary particles formed due to the oxidation of emitted gases such as sulfur dioxide (SO 2 ). Primary particles from power plants are divided into two categories: filterable particulate matter (FPM ) and condensable particulat e matter (CPM). Condensable particulate matter forms when emissions condense up on cooling of stack gases, such as sulfuric acid (H 2 SO 4 ), or when emissions condense upon cooling of stack gases and reacti on with ambient humidity, such as sulfur trioxide (SO 3 ). Secondary particles are formed by reactions in the atmosp here that lead to the oxidation of gases to condensable products. Those condensable prod ucts with very low vapor pressures, such as sulfuric acid, condense fairly readily. However, other condensable products c ontribute to particulate matter only wh en conditions thermodynamically favor the condensed phase. An example is th e formation of ammonium nitrate (NH 4 NO 3 ) which requires

 a molar equivalent excess of ammonium relative to sulfuric acid, and

 cool temperatures that favor the molecular stability of the salt form over its gaseous precursors, nitric acid (HNO 3 ) and ammonia (NH 3 ).

Hydrogen halides, such as hydr ogen chloride (hydrochloric acid, HCl) and hydrogen fluoride (hydrofluoric acid, HF), may also be emitted from power plants. There are three sepa rate, but closely related, issues that arise regarding the ability of these compounds to affect visibility degradation:

1. Can HCl and HF emissions be considered as a comp onent of primary PM fine—either “condensable particulate matter” (CPM) or “filterable partic ulate matter” (FPM)—from industrial stacks?

2. Are the properties of HCl and HF similar to that of H 2 SO 4 such that the hydrogen halide emissions can be
modeled as sulfuric acid in models such as CALPUFF?

3. Does the presence of visible plum e imply a regional haze concern?

These questions are addressed in the following pages by inspecting the thermodynamics properties of HCl and HF and exploring their reactions and tr ansformations in the atmosphere.Page 4 of 10 1. Can HCl and HF emissions be considered as a component of primary PM fine—either “condensable particulate matter” (CPM) or “filterable partic ulate matter” (FPM)—from industrial stacks?

HCl and HF cannot be considered components of primary PM fine, and to understand why we need only look at their thermodynamic properties: th ese compounds are gases under atmospheric conditions.

HCl and HF have fundamentally different thermodyna mic properties compared to the properties of H 2 SO 4 , the main component of CPM. Regarding their potential contributi on to CPM, the following differences should be noted:

As shown in Table 1, at ambient temperatures, sulfuric acid exists either as a solid (at temperatures below 10 °C) or a liquid (at temperatures above 10 °C). Due to its extremely lo w vapor pressure (see Table 2), even when exposed to the open atmosphere at 25 °C, sulfuric acid will favor its condensed form over volatilization to a gas.

Table 1. Freezing (Melting ) and Boiling Points [K]

Compound State at 298K T f T b HF Gas 190 292.7
HCl Gas 161.5 191.1

H 2 SO 4 Liquid 283.5 611.2
HNO 3 Liquid 231.5 356.1

NH 3 Gas 195.4 238.8

Table 2. Pure Compound Va por Pressures [mbar] @ 298K

Compound Pure Compound Vapor Pressure

H 2 SO 4 <0.0014
HNO 3 62

H 2 O 31.7
HF 1213

NH 3 9,986 HCl 47,530
CO 2 57,226

On the other hand, HCl exists solely in the gas form at all ambient temperatures . (The vapor pressure of HCl is also the highest of all compounds listed in Table 2 except for CO 2 , which has a vapor pressure of similar order of magnitude.) HF can exist as a liquid at ambient temperatures below 20 °C. (In fact, it is stable in polymer forms of (HF) n , where n is an integer higher than or equal to 6.) However, due to its high vapor pressure it can only exist in the liquid form in an enclosed space where it can remain at high enough concentrations (mass per volume); upon exposure to the atmosphere, it will ra pidly volatilize to the gas phase.

As a reference, the pure compound (sat uration) vapor pressure of water at 298 K (25 °C) is 31.7 mbar; at that temperature, a pool of water exposed to the open atmosphe re will evaporate (volatilize) to the gas phase—with the rate depending on ambient RH, in an atte mpt to reach an ambient RH of 100%.Page 5 of 10 Due to these fundamental thermodyna mic properties, it is impossible to consider ambient HCl and HF as compounds that fall into the “condensable particulat e matter” (CPM) category—let alone the “filterable particulate matter” FPM category which represents en tirely non-volatile mate rial—of primary PM fine
emissions.

2. Are the properties and reactions of HCl and HF similar to that of H 2 SO 4 such that the hydrogen halide
emissions can be modeled as sulfuric acid in models such as CALPUFF?

We now look at the question of whet her the properties of HCl and HF ar e sufficiently similar to that of H 2 SO 4 such
that the HCl and HF emissions can be modeled as H 2 SO 4 in models such as CALPUFF..

Tables 1 through 7 provide the values for several key ph ysical and thermodynamic properties of HCl, HF and H 2 SO 4 , including the properties for several ammonium-based and so dium-based salts of bisulfate, sulfate and chloride. These properties are necessary to determine the reactions and transformations that occur in the atmosphere. Inspection of these tables show that the properties of the chlorine and fluorine compounds —hydrochloric acid, hydrogen fluoride, and corresponding salts—are quite dissimilar to that of sulfuric acid and its corresponding salts.

In contrast to H 2 SO 4 , HCl and HF are not hygroscopic at ambient conditions due to their gaseous state. However, they are highly soluble in water where they can dissociate into chloride and fluoride anions; thus, at least in theory, HCl and HF emissions have the potential to contribute to both the mass and increase in the hygroscopicity of ambient particles. However, in the real atmosphere, this potential is never realized in fully de liquesced, meta-stable, mixedphase, or crystallized ambient aerosol particles due to competing processes by other less-volatile acids.

We first address the case of fully deliquesced particles. One of the fundamental criteria for equilibrium in aerosol particles is that of charge equilibrium, i.e. cations must ba lance anions. In fully deliques ced, aqueous aerosol particles

, charge equilibrium must be maintained at the same time as acid-base dissociation equilibrium is maintained for all dissolved electrolytic species (including any dissolved CO 2 ), which must in turn maintain equilibrium with their gasphase concentrations as determined by their effective Henry’s Law constants (a measure of solubility that takes into
account the dissociation of electrol ytic species in water). Finally, water equilibrium:, H 2 O ⇋ H + + OH – , must be
maintained.

Table 4 shows that effective solubility of H 2 SO 4 is orders of magnitude higher than that of the other inorganic acids listed, i.e. HNO 3 , HCl and HF. This is in agreement with the vapor pressure property data . The solubility of acids must balance with the other equilibrium considerations, including ch arge balance. Thus, when a less-volatile (more-soluble) acid is present, the solubility of more-vol atile (less-soluble) acids is inhibited. Thus, in industrial plumes, the presence
of H 2 SO 4 (emitted directly or formed from the reaction of SO 3 with H 2 O or the oxidation of SO 2 ) will inhibit the dissolution (in order of increasing streng th of inhibition) the dissolution of HNO 3 , HCl and HF. The dissolution of ammonia—which can form ammonium hydroxide, a base that readily dissociates to yield ammonium (NH 4 + ) cations—
allows for an increase in the amount of acid that can dissolve into the condensed medium, but not to the extent
where HCl and HF dissolution becomes favorable in the at mosphere due to the common presence of sulfuric and
nitric acids.

A similar case occurs with meta-stabl e (i.e. particles that are deliquesced but are between their deliquescence and crystallization points), mixed-phase (l iquid and solid) particles and solid particles. In essence, a well-known phenomenon known as halogen displacement occurs in the atmosphere. When solid or dissolved halogen-containingPage 6 of 10 salts in the atmosphere are exposed to less-volatile acid s, the less-volatile acids displace the halogens and new preferred salts are formed. In fact, this is a very common process in coastal regions, wh ere sea-salt aerosol (which contains pre-existing sulfate from seawat er) experiences chloride depletion. Chlo ride is depleted first (due to masstransfer considerations) from the smallest particles and the displacement continues in order of increasing particle size
until all available sulfuric acid is in the particle phase. When there is no more su lfuric acid left to displace chloride, any available nitric acid will displace chloride from the remaining particles (again, in order of increasing particle size) until
all available nitric acid is consumed. This leads to th e commonly observed formation of coarse sodium nitrate (NaNO 3 ) particles near coastal regions, such as those observ ed in the Tampa Bay Regional Aerosol Characterization Experiment (BRACE). The displaced chloride is released to the gas phase as HCl.

The most common fate of the resulting volatile species HCl (and HF) is dry deposition and washout/rainout during precipitation events. Only under unusual ci rcumstances, (e.g., dust storms) is it likely that conditions could become favorable for the condensation of these species into coarse mode particles (e.g., alkaline dust particles). In contrast,
once H 2 SO 4 condenses onto particles, it does not partition back to the gas phase.

As noted in conversation with Dr. William Malm of th e National Park Service (September 10, 2009), chloride
measurements at non-coastal IMPROVE site s show negligible, virtually zero chlori de in fine particle measurements. This is seen throughout the network, thereby providing suppo rt to the theoretical basis for not expecting chloride aerosols in continental air masses except as in the form of sea-salt chloride. (Fluoride is even more negligible than chloride.)

Due to differences in properties and reactions, it is in appropriate to consider ambient HCl and HF as behaving
similar to H 2 SO 4 and to simulate by using H 2 SO 4 as a surrogate in models such as CALPUFF.

In an industrial plume, th e preference for sulfate-based and (if cond itions are right) nitrate-based salts and
solutions make for conditions that do not favor the existence of chloride/fluoride salts or solutions in aerosol particles reaching Class I areas. Even with reductions in SO 2 and NOx, these salts chloride/fluoride salts or
solutions would still be unfavorable due to other natura l and anthropogenic sources leading to sulfuric and nitric acid and more-favorable organic acids in the pa rticle phase. Observations support this theoretically based conceptual model.

A note on conservatism:

The potential for any compound to form a solid does not suggest that it should be treated as either primary PM or a surrogate compound in the search for a conservative estimate. For example, CO 2 is known to form carbonate salts; a future version of the AIM thermodyna mic model (which can be used for a variety of applications) will include simulation of carbonate salts. However, modeling CO 2 emissions as primary PM or H 2 SO 4 is clearly inappropriate. It is just as inappropriate to model HCl and HF as primary PM or H 2 SO 4 . With enabling science models and data available, the only proper route is to model the comp ounds with their appropriate properties.Page 7 of 10 Table 3. Acid-Base Dissociation Constants [M] @ 298 K (HA ⇋ H + A – )

HA A –Ka pKa
HCl Cl –1.7 × 10 6 -6.23

H 2 SO 4 HSO 4 – 1.0 × 10 3 -3
HNO 3 NO 3 – 1.5 × 10 1 -1.17
HSO 4 – SO 4 2– 1.2 × 10 -2 1.92

HF F –3.5 × 10 -4 3.45

NH 4 + NH 3 5.6 × 10 -10 9.25

Table 4. Solubility in H 2 O @ 298K [M/atm] (C aq,A = p A × H*)

Compound Physical Henry’s Law
H Effective Henry’s Law

H* at pH = 5.6 Effective Henry’s Law

H* at pH = 3.0

H 2 SO 4 2.50 × 10 14 4.77 × 10 25 5.50 × 10 20

HNO 3 2.10 × 10 5 1.25 × 10 12 3.15 × 10 9

HCl 1.10 × 10 0 7.44 × 10 11 1.87 × 10 9

HF 2.74 × 10 4 3.85 × 10 6 3.70 × 10 4

NH 3 6.10 × 10 1 6.96 × 10 1 3.48 × 10 3

Table 5a. Equilibrium Constants (Solid to Gas )

Salt K† a b

NH 4 NO 3 (s) ⇋ NH 3 (g) + HNO 3 (g) 5.47 × 10 -17 -74.38 6.12

NH 4 Cl (s) ⇋ NH 3 (g) + HCl (g) 1.09 × 10 -16 -71.00 2.40

† K(T) = K exp { a (298/T – 1) + b [ 1 + ln (298/T) – 298/T ] }; units of atm 2 for these equilibrium constants

Table 5b. Displacement Reactions

Displacement reactions occur by first allowing for displacemen t of chloride/nitrate by sulfuric acid to form the (more hygroscopic) bisulfate salt and any rema ining sulfuric acid will lead to the form ation of the (less hygroscopic) sulfate salt. Chloride displacement occurs before nitrate displac ement and displacement to form the bisulfate salt occurs before formation of the sulfate salt, with intermediate salts such as letovicite [(NH 4 ) 3 H(SO4) 2 ] forming during the process, in the case of ammonium salts.

K‡ a b
NaCl (s) + HNO 3 (g) ⇋ NaNO 3 (s) + HCl (g) 4.01 5.63 -2.19

‡ K(T) = K exp { a (298/T – 1) + b [ 1 + ln (298/T) – 298/T ] }; unitless for th is equilibrium constant

The rate of following displacement reactions by sulfuric acid are assumed by atmospheric models to be constrained only by mass transfer limitations.Page 8 of 10 NaCl (s) + H 2 SO 4 (g) ⇋ NaHSO 4 (p) + HCl (g) NH 4 Cl (s) + H2SO 4 (g) ⇋ NH 4 HSO 4 (p) + HCl (g) NaNO 3 (s) + H2SO 4 (g) ⇋ NaHSO 4 (p) + HNO 3 (g) NH 4 NO 3(s) + H2SO 4 (g) ⇋ NH 4 HSO 4 (p) + HNO 3 (g)

Table 6. Dissolution Equilibrium Co nstants/Solubility Product at 298 K

Salt K sp (NH 4 ) 2 SO 4 1.82

NH 4 NO 3 1.49 × 10 1

NH 4 Cl 2.21 × 10 1

NH 4 HSO 4 2.55 × 10 2

Na 2 SO 4 4.80 × 10 1

NaNO 3 1.20 × 10 1

NaCl 3.77 × 10 1

NaHSO 4 2.41 × 10 4

Units: M 2 except for (NH 4 ) 2 SO 4 which has units of M 3

Table 7. Deliquescence Points [% RH] at 298 K

Salt DRH

Na 2 SO 4 84.2

NaCl 75.3
NaNO 3 74.3
NaHSO 4 52.0 (NH 4 ) 2 SO 4 79.9

NH 4 Cl 80.0

NH 4 NO 3 61.8

NH 4 HSO 4 40.0

3. Does the presence of visible NH 4 Cl plume imply a regional haze concern?

HCl may react with NH 3 to form NH 4 Cl particles under some conditions (Table 5a, 5b). These particles can form from stack gases under some operating circumstances and lead to visible (opaque) plumes. Does this imply a regional haze
concern?

Visible plumes and regional haze are two distinct issues; the formation of a visible plume does not imply a regional haze concern as these particles are short-lived under atmosp heric conditions. As the plume dilutes, the conditions for the formation of the condensed phase of NH 4 Cl become increasingly unfavo rable. The ambient vapor phase
concentrations of NH 3 and HCl in equilibrium with the particle phase NH 4 Cl decrease due to mixing with ambient air of lower concentrations. As a result, th e thermodynamic equilibrium of the chem ical reaction shifts such that the
condensed NH 4 Cl converts back to the gas phase precursors (NH 3 and HCl).Page 9 of 10 Once the NH 3 and HCl are in the gas phase again, reformation of NH 4 Cl requires a low temperature to overcome the
much diluted concentrations downwind of the em issions source. Thus the fate of gas phase NH 3 and HCl changes as compared to near the stack. As noted earlier, NH 3 preferentially reacts with su lfate-containing, then nitratecontaining, and only subsequently chloride -containing chemicals. Sulfate is usua lly sufficiently elevated at distances far downwind in power plant plumes that virtually all the NH 3 in the stack gases as well as that present from other sources will react with it. Similarly, the fate of the reformed HCl will likely be wet or dry deposition due to its affinity for both deposition processes. This processing me ans that even under favorable conditions for NH 4 Cl formation, the resulting plumes will be quite transient.

The following simple Gaussian plume calculation will demonstrate the short-lived nature of NH 4 Cl particles. Several worst-case assumptions are made to over estimate the likelihood and stability of NH 4 Cl formation.

 No competitive chemistry for reaction with NH 3 is included. This is acco mplished by not including H 2 SO 4 or HNO 3 so that all NH 3 is available for reaction with HCl, thereby favoring formation of NH 4 Cl.

 Concentrations are calculated for the plume center- line in a neutrally stable atmosphere for the least possible amount of diffusion.

 Nucleation is assumed to occur instantaneously, as long as equilibrium conditions are met.

 The atmosphere is fairly stable, thereby reducing turbulent dispersion effects.

Table 8 below shows the distances to which the particulate form of NH 4 Cl would persist. For example, given stack concentrations of 1 ppm NH 3 and 75 ppm HCl, the NH 4 Cl formed would be completely removed within 7 km (or 40 minutes) downwind.

Table 8: Demonstration of short-lived nature of NH 4 Cl particles

NH 3 in
stack (ppm) HCl in
stack (ppm) Distance by Which NH 4 Cl Removed (km) NH 3 in
stack (ppm) HCl in
stack (ppm) Distance by Which NH 4 Cl Removed (km)

1 10 3.4 5 10 5.9

1 75 7 5 75 11.6

1 150 8.5 5 150 14.7
2 10 4.4 10 10 7.4
2 75 8.5 10 75 14.7
2 150 10.8 10 150 18.7 Distances by which NH 4 Cl would decompose and volatilize to NH 3 and HCl for a “worst-case” estimation. Wind speed is assumed to be 3 m s -1 (10.8 km h -1 or 6.7 mi h -1 ).

If the presence of H 2 SO 4 had been considered then the resulting NH 4 Cl particles would begin forming further downwind from the stack and persist for a shor ter period of time, if they form at all.

The second attached document explains in more detail the inadequacy of considering NH 4 Cl from power plants as condensable particulate matter.Page 10 of 10 Summary

We have shown that due to fundamenta l thermodynamic properties, it is impo ssible to consider ambient HCl and HF as compounds that fall into the “condensa ble particulate matter” (CPM ) and due to differences in their properties and reactions, it is inappropriate to consider am bient HCl and HF as behaving similar to H 2 SO 4 and to simulate by using

H 2 SO 4 as a surrogate in models such as CALPUFF. Thus, cons idering HCl and HF as filterable particulate matter (FPM, PM Fine) or condensable particulate matter (CPM, CPM- IOR, SO4) will only produce errors in regional haze calculations and cannot be supported by scientific theory or observations.

References

Finlayson-Pitts, B.J. and Pitts, Jr., J.N. Chemistry of the Upper and Lower Atmo sphere: Theory, Experiments, and Applications, Academic Press, 1999.

Lide, D.R., ed., CRC Handbook of Chemistry and Physics, 89 th Edition (Internet Version 2009), CRC Press, Taylor and Francis, 2009. http://www.hbcpnetbase.com

Sander, R. Compilation of Henry's Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry (Version 3), 1999. http://www.henrys-law.org

Sander, S.P. et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies , NASA/JPL, 2006. http://jpldataeval.jpl.nasa.gov

Seinfeld, J.H. and Pandis, S.N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2 nd Edition, Wiley-Interscience, 2006.

Snoeyink, V.L. and Jenkins, D. Water Chemistry, Wiley, 1980.