ATTACHMENT G

Comparison of EPA Mercury MACT Proposal and Latham and Watkins Papers
and March 2003 West Associates Report.

+-----------------------------------+-----------------------------------+
| **Topic: Mercury trading program  |                                   |
| under section 112(n).**           |                                   |
+-----------------------------------+-----------------------------------+
|                                   |                                   |
+-----------------------------------+-----------------------------------+
| **EPA Proposal Language**         | **Latham and Watkins Memo         |
|                                   | Language**                        |
+-----------------------------------+-----------------------------------+
| Page 4661: "While section 112(d)  | Page 3, 9/4/2003: "While section  |
| mandates regulation of all HAP    | 112(d) calls for regulation of    |
| emissions based on the emissions  | all major sources of HAPs based   |
| limitations achieved by similar   | on the emissions limitation       |
| sources, section 112(n) calls for | achieved by similar sources,      |
| regulation of Utility Unit HAP    | section 112(n) calls for          |
| emissions as EPA determines is    | regulation of power plant HAP     |
| "appropriate and necessary after  | emissions only insofar as it is   |
| considering the results of the    | "appropriate and necessary after  |
| study" of public health hazards   | considering the results of the    |
| reasonably anticipated to occur   | study \[of health risk\] required |
| from those Utility Unit HAP       | by this subparagraph, even though |
| emissions."                       | virtually all power plants are    |
|                                   | major sources."                   |
+-----------------------------------+-----------------------------------+
| Page 4661: "Congress provided EPA | Page 3, 9/4/2003: "Congress       |
| with distinct regulatory          | provided EPA with distinct        |
| authority to address HAP          | regulatory mandate for power      |
| emissions from Utility Units      | plant HAPS "because of the logic  |
| "because of the logic of basing   | of basing any decision to         |
| any decision to regulate on the   | regulate on the results of        |
| results of scientific study and   | scientific study and because of   |
| because of the emission           | the emission reductions that will |
| reductions that will be achieved  | be achieved and the extremely     |
| and the extremely high costs that | high costs that electric          |
| electric generators will face     | generators will face under other  |
| under other provisions of the new | provisions of the new Clean Air   |
| Clean Air Act Amendments." 136    | Act amendments." A&P Cong. Record |
| Cong. Rec. E3670,E3671 (November  | E3670,E3671.                      |
| 2, 1990) Statement of Cong.       |                                   |
| Oxley.                            |                                   |
+-----------------------------------+-----------------------------------+
| Page 4661: "Congress' intent to   | Page 3, 9/4/2003: "That Congress  |
| authorize EPA to regulate Utility | intended EPA to regulate HAP      |
| Unit HAP emissions in ways other  | emissions under section 112(n)    |
| than with the prescriptive        | independently of section 112(d)   |
| requirements of section 112(d) is | is further evidenced by section   |
| indicated by the section 112(n)   | 112(n)'s provision for EPA to     |
| requirement that EPA develop      | develop alternative control       |
| alternative control strategies    | strategies."                      |
| for HAP emissions from these      |                                   |
| units."                           |                                   |
+-----------------------------------+-----------------------------------+
| Page 4661: "These alternative     | Page 3, 9/4/2003: "Under the      |
| control strategies must address   | framework of section 112(n), EPA  |
| the hazards to public health that | is to do so by developing and     |
| EPA reasonably anticipates will   | implementing alternative control  |
| occur as a result of Utility Unit | strategies that address           |
| HAP emissions."                   | reasonably anticipated hazards    |
|                                   | posed to public health."          |
+-----------------------------------+-----------------------------------+
| Page 4661: "Congress authorized   | Page 4, 9/4/2003: "(Rather,)      |
| EPA to consider a wider range of  | section 112(n) confers discretion |
| control alternatives for the      | on EPA by permitting it to        |
| utility sector than the           | develop alternative control       |
| source-by-source approach EPA has | strategies for emissions from     |
| prescribed in standards for other | electric utility steam generating |
| source categories under the       | units rather than forcing power   |
| traditional section 112(d) MACT   | plant HAP regulation into the     |
| approach."                        | rigid, technology-based framework |
|                                   | of section 112(d)."               |
+-----------------------------------+-----------------------------------+
| Page 4661: "Because Congress      | Page 3, 9/4/2003: "Congress       |
| directed EPA to develop control   | imposed the requirement that EPA  |
| strategies that would be          | develop and report alternative    |
| alternatives to the usual section | control strategies because it     |
| 112(d) MACT standard, it is       | intended that EPA implement them, |
| reasonable to conclude that       | not that it regulate them under   |
| Congress authorized EPA to        | the framework of section 112(d)." |
| implement such alternatives."     |                                   |
+-----------------------------------+-----------------------------------+
| Page 4662; "As a result, EPA      | Page 4, 9/4/2003: "Section 112(n) |
| believes that section 112(n)      | does not prohibit EPA from        |
| confers on the Agency the         | implementing a system-wide or     |
| authority to develop a            | pooled performance standard with  |
| system-wide or poled performance  | regard to mercury emissions from  |
| standard for HAP emissions from   | power plants."                    |
| Utility Units."                   |                                   |
+-----------------------------------+-----------------------------------+
| # Topic: Subcategorization        |                                   |
+-----------------------------------+-----------------------------------+
| Page 4665: "The American Society  | Page 18, 3/8/02: "The American    |
| for Testing and Materials (ASTM)  | Society for Testing and Materials |
| classifies coals by rank, a term  | (ASTM) classifies coals by rank,  |
| which relates to the carbon       | a term which relates to the       |
| content of the coal and other     | carbon content of the coal and    |
| related parameters such as        | other related parameters such as  |
| volatile-matter content, heating  | volatile-matter content, heating  |
| value, and agglomerating          | value, and agglomerating          |
| properties."                      | properties."                      |
+-----------------------------------+-----------------------------------+
| Page 4665: "The youngest, or      | Page 18, 3/8/02: "The youngest,   |
| lowest rank, coals are termed     | or lowest rank, coals are termed  |
| lignite. Lignites have the lowest | lignite. Lignites have the lowest |
| heating value of the coals        | heating value of the coals        |
| typically used in power plants.   | typically used in power plants.   |
| Their moisture content can be as  | Their moisture content can be as  |
| high as 30 percent, but their     | high as 30 percent, but their     |
| volatile content is also high;    | volatile content is also high;    |
| consequently, they ignite easily. | consequently, they ignite easily. |
| Next in rank are subbituminous    | Next in rank are subbituminous    |
| coals, which also have a          | coals, which also have a          |
| relatively high moisture content, | relatively high moisture content, |
| typically ranging from 15 to 30   | typically ranging from 15 to 30   |
| percent. Subbituminous coals also | percent. Subbituminous coals also |
| are high in volatile matter       | are high in volatile matter       |
| content and ignite easily. Their  | content and ignite easily. Their  |
| heating value is generally in     | heating value is generally in     |
| between that of the lignites and  | between that of the lignites and  |
| the bituminous coals. Bituminous  | the bituminous coals. Bituminous  |
| coals are next in rank, with      | coals are next in rank, with      |
| higher heating values and lower   | higher heating values and lower   |
| moisture and volatile content     | moisture and volatile content     |
| than the subbituminous and        | than the subbituminous and        |
| lignite coals. Anthracites are    | lignite coals. Anthracites are    |
| the highest rank coals. Because   | the highest rank coals. Because   |
| of the difficulty in obtaining    | of the difficulty in obtaining    |
| and igniting anthracite and the   | and igniting anthracite and the   |
| difficulties in maintaining       | difficulties in maintaining       |
| anthracite-fired boilers, only a  | anthracite-fired boilers, only a  |
| single electric utility boiler in | single electric utility boiler in |
| the U.S. burned anthracite as its | the U.S. burned anthracite as its |
| only fuel in 1999.                | only fuel in 1999.                |
+-----------------------------------+-----------------------------------+
| Page 4665: "Although there is     | Page 23, 3/8/02: "Nonetheless,    |
| overlap in some of the ASTM       | the ASTM method of classifying    |
| classification properties, the    | coals by "rank" generally is      |
| ASTM method of classifying coal   | successful in identifying some    |
| by rank has been in use for       | core common characteristics that  |
| decades and generally is          | have implications for power plant |
| successful in identifying some    | design and operation."            |
| common core characteristics that  |                                   |
| have implications for power plant |                                   |
| design and operation."            |                                   |
+-----------------------------------+-----------------------------------+
| Page 4665: "The rank of coal to   | Page 24, 3/8/02: "The type of     |
| be burned has an significant      | coal to be burned has an enormous |
| impact on overall plant design.   | impact on overall plant design.   |
| The goal of the plant designer is | The goal of the plant designer is |
| to arrange boiler components      | to arrange boiler components      |
| (furnace, superheater, reheater,  | (furnace, superheater, reheater,  |
| boiler bank, economizer, and air  | boiler bank, economizer, and air  |
| heater) to provide the rated      | heater) to provide the rated      |
| steam flow, maximize thermal      | steam flow, maximize thermal      |
| efficiency, and minimize cost.    | efficiency and minimize cost.     |
| Engineering calculations are used | Engineering calculations are used |
| to determine the optimum          | to determine the optimum          |
| positioning and sizing of these   | positioning and sizing of these   |
| components, which cool the flue   | components, which cool the flue   |
| gas and generate the superheated  | gas and generate the superheated  |
| steam. The accuracy of the        | steam. The accuracy of the        |
| parameters specified by the       | parameters specified by the       |
| owner/operators is critical to    | owner/operators is critical to    |
| designing and building an         | designing and building an optimal |
| optimally efficient plant."       | plant."                           |
+-----------------------------------+-----------------------------------+
| Page 4665: "For the above         | Page 24, 3/8/02: "Perhaps the     |
| reasons, one of the most          | most significant variation is     |
| important factors in modern       | differences in the types and      |
| electric utility boiler design    | range of fuels to be fired, which |
| involves the differences in the   | requires changes in the details   |
| ranks and range of coals to be    | and overall arrangement of boiler |
| fired and their impact on the     | components. As will be described  |
| details and overall arrangement   | further below, the type of coal   |
| of boiler components. Coal rank   | to be fired has a significant     |
| is so important that plant        | impact on several areas of plant  |
| designers and manufacturers       | design. Fuel type is so important |
| expect to be provided with a      | that plant designers and          |
| complete list of all coal ranks   | manufacturers expect to be        |
| presently available or planned    | provided with a complete list of  |
| for future use, along with their  | all coal types presently          |
| complete chemical and ash         | available or planned for future   |
| analyses, so that the engineers   | use, along with their complete    |
| can properly design and specify   | chemical and ash analyses so that |
| plant equipment."                 | the engineers can properly design |
|                                   | and specify plant equipment."     |
+-----------------------------------+-----------------------------------+
| Page 4665: "For a boiler to       | Page 24, 3/8/02: "For a boiler to |
| operate efficiently, it is        | operate efficiently, it is        |
| critical to recognize the         | critical to recognize the         |
| differences in coals and make the | differences in coals and make the |
| necessary modifications to        | necessary modifications to        |
| provide optimum conditions for    | provide optimum conditions for    |
| efficient combustion."            | efficient combustion."            |
+-----------------------------------+-----------------------------------+
| Page 4666: "The EPA found that    | Page 32, 3/8/02: "In summary, the |
| the characteristics of the coal   | type of coal to be burned in the  |
| rank to be burned was the driving | boiler has a major impact on      |
| factor in how a coal-fired unit   | steam generating equipment and    |
| was designed."                    | plant design."                    |
+-----------------------------------+-----------------------------------+
| Page 4665: "Coal- fired units are | Page 45, 3/8/02: "Coal- fired     |
| designed and constructed with     | units are designed and            |
| different process configurations  | constructed with different        |
| partially because of the          | process configurations because of |
| constraints, including the        | the site-specific requirements or |
| properties of the fuel to be      | constraints placed on the initial |
| used, placed on the initial       | design of the unit. ...           |
| design of the unit. Accordingly,  | Accordingly, these site-specific  |
| these site-specific constraints   | constraints dictate the process   |
| dictate the process equipment     | equipment selected, the component |
| selected, the component order,    | order, the materials of           |
| the materials of construction and | construction and the operating    |
| the operating conditions."        | conditions."                      |
+-----------------------------------+-----------------------------------+
| **Topic: Rationale for Not        |                                   |
| regulating Non-Mercury HAPs.**    |                                   |
+-----------------------------------+-----------------------------------+
| Page 4660: "As explained above,   | Page 5, 8/5/02: Interpreting the  |
| EPA believes interpreting section | CAA as requiring control of *all* |
| 112(n)(1)(A) in this manner would | HAPs from power plants regardless |
| ignore much of the language set   | of the health hazard they pose    |
| forth in that section, and would  | would simply read these phrases   |
| render superfluous the section's  | -- and the limitations on EPA's   |
| processes and requirements. By    | regulatory mandate -- out of the  |
| contrast, EPA's interpretation    | statute. Such an interpretation   |
| gives meaning to all of the words | of the CAA is patently            |
| of section 112(n)(1)(A) and is    | unreasonable under established    |
| consistent with requiring         | rules of statutory construction.  |
| regulation under section 112 of   | Courts "are obliged to give       |
| only those HAP emissions from     | effect, if possible, to every     |
| utility units that the regulatory | word Congress used."              |
| finding identified as appropriate |                                   |
| and necessary to regulate under   |                                   |
| section 112 because they are      |                                   |
| reasonably anticipated to result  |                                   |
| in a hazard to public health      |                                   |
| after imposition of the other     |                                   |
| requirements of the CAA."         |                                   |
+-----------------------------------+-----------------------------------+
| **WEST ASSOCIATES REPORT          |                                   |
| LANGUAGE**                        |                                   |
+-----------------------------------+-----------------------------------+
| **Topic: How did EPA Account for  |                                   |
| Emissions Variability?**          |                                   |
+-----------------------------------+-----------------------------------+
| **EPA Proposal Language**         | **West Report Language**          |
+-----------------------------------+-----------------------------------+
| Page 4672: " In summary, the coal | Section 2, page 2: "In sum, coal  |
| CL content is one of the primary  | chlorine content is one of the    |
| determinants of which             | primary determinants of which     |
| Hg-containing compounds will be   | mercury-containing compounds will |
| present, and in what amounts, in  | be present - and in what          |
| the flue gas of an individual     | amounts - in the flue gas of an   |
| utility unit. The differing       | individual utility unit. The      |
| physical and chemical properties  | differing physical and chemical   |
| of Hg-containing compounds in the | properties of mercury-containing  |
| flue gas result in significant    | compounds in the flue gas result  |
| differences in the feasibility    | in significant differences in the |
| and effectiveness of controls for | feasibility and effectiveness of  |
| removing the compounds from flue  | controls for removing the         |
| gas."                             | compounds from flue gas."         |
+-----------------------------------+-----------------------------------+
| Page 4672: " The EPA determined   | Section 3.3, page 6: "The limited |
| that the stack tests in the ICR   | number of stack tests in the ICR  |
| database alone are insufficient   | III database are insufficient to  |
| to estimate the effect of fuel    | estimate the effect of fuel       |
| variability over time on the      | variability over time on the      |
| emissions of the best-performing  | emissions of the best performing  |
| facilities."                      | facilities."                      |
+-----------------------------------+-----------------------------------+
| Page 4672: "In selecting the      | Section 3.3, page 7: "In the      |
| format of the                     | selection of the format of this   |
|                                   | correlation equation, care was    |
| correlation equation, care was    | taken that the mathematical       |
| taken that                        | expression accurately reflected   |
|                                   | the physical and chemical process |
| the mathematical expression       | by which chlorine contributes to  |
| accurately                        | the controllability of stack      |
|                                   | mercury emissions. Equation (1)   |
| reflected the physical and        | is based on the assumption that   |
| chemical                          | the rate of conversion of mercury |
|                                   | to mercury chloride is            |
| process by which Cl contributes   | proportional to the chlorine      |
| to the                            | concentration in the coal. With   |
|                                   | this expression, the maximum      |
| controllability of stack Hg       | removal fraction is limited to 1, |
| emissions.                        | because the exponent term is      |
|                                   | always nonnegative, regardless of |
| The correlation equation is based | the chlorine concentration. This  |
| on the                            | corresponds to the real-world     |
|                                   | limitation that no more than 100% |
| assumption that the rate of       | of the mercury in flue gas can be |
| conversion                        | removed (*i.e.* there cannot be   |
|                                   | negative mercury emissions). And, |
| of Hg to mercuric chloride (an    | as the coal chlorine              |
| oxidized                          | concentration drops               |
|                                   |                                   |
| form) is proportional to the Cl   | to zero, the mercury removal      |
|                                   | fraction approaches 1-â (this     |
| concentration in the coal,        | value does not of necessity       |
| irrespective of                   | approach zero because some        |
|                                   | mercury removal may be achieved   |
| coal rank. With this expression,  | without reaction with chlorine)." |
| the                               |                                   |
|                                   |                                   |
| maximum removal fraction is       |                                   |
| limited to                        |                                   |
|                                   |                                   |
| 1, because the exponent term is   |                                   |
| always                            |                                   |
|                                   |                                   |
| nonnegative, regardless of the Cl |                                   |
|                                   |                                   |
| concentration. This corresponds   |                                   |
| to the                            |                                   |
|                                   |                                   |
| real-world limitation that no     |                                   |
| more than                         |                                   |
|                                   |                                   |
| 100 percent of the Hg in flue gas |                                   |
| can be                            |                                   |
|                                   |                                   |
| removed (*i.e.*, there cannot be  |                                   |
| negative                          |                                   |
|                                   |                                   |
| Hg emissions). As the coal Cl     |                                   |
|                                   |                                   |
| concentration drops to zero, the  |                                   |
| Hg                                |                                   |
|                                   |                                   |
| removal fraction does not         |                                   |
| approach zero                     |                                   |
|                                   |                                   |
| because some Hg removal is        |                                   |
| achieved                          |                                   |
|                                   |                                   |
| even without reaction with Cl."   |                                   |
+-----------------------------------+-----------------------------------+
| Page 4672: "The purpose of        | Section 3.3, page 8: "The purpose |
| deriving a correlation equation   | of deriving a correlation         |
| for each control configuration    | equation for each control         |
| used by the top performing units  | configuration used by the top     |
| was to provide a numerical means  | performing units was to provide a |
| of predicting the fraction of Hg  | numerical means of predicting the |
| removed for the best performing   | fraction of mercury removed for   |
| sources over the entire range of  | the best performing sources over  |
| fuel variability experienced by   | the entire range of fuel          |
| each of those sources over the    | variability experienced over the  |
| course of a                       | course of a year. Correlation     |
|                                   | equations were derived for each   |
| year. Correlation equations were  | control configuration, but were   |
| derived                           | only used to predict mercury      |
|                                   | removal if they were found to     |
| for each control configuration,   | have acceptable explanatory       |
| but were only used to predict Hg  | power."                           |
| removal if they were found to     |                                   |
| have acceptable explanatory       |                                   |
| power."                           |                                   |
+-----------------------------------+-----------------------------------+
| Page 4673: "To determine whether  | Section 3.3, page 8: "To          |
| the explanatory power of each     | determine whether the explanatory |
| correlation equation warranted    | power of each correlation         |
| its use on a larger range of ICR  | equation warranted its use on a   |
| coal composition data, each       | larger range of ICR II coal       |
|                                   | composition data, ENSR validated  |
| correlation equation was          | each correlation equation against |
| validated                         | ICR III stack test data. For each |
|                                   | of the test chlorine              |
| against the ICR stack test data.  | concentrations in the ICR III     |
| For each                          | stack test database, the mercury  |
|                                   | removal fraction was calculated   |
| of the Cl concentrations in the   | by use of Equation (1) with       |
| ICR stack                         | parameters selected to give the   |
|                                   | best fit to the data. A           |
| test database for 1999, the Hg    | correlation coefficient was then  |
| removal                           | calculated to evaluate the        |
|                                   | accuracy of the fit."             |
| fraction was calculated by using  |                                   |
| the                               |                                   |
|                                   |                                   |
| correlation equation with         |                                   |
| parameters                        |                                   |
|                                   |                                   |
| selected to give the best fit to  |                                   |
| the data.                         |                                   |
|                                   |                                   |
| A correlation coefficient was     |                                   |
| then                              |                                   |
|                                   |                                   |
| calculated to evaluate the        |                                   |
| accuracy of                       |                                   |
|                                   |                                   |
| the fit."                         |                                   |
+-----------------------------------+-----------------------------------+
| Page 4673: "For each of the       | Section 3.4, pages 8-9: "For each |
| best-performing units,            | of the best performing units,     |
| unit-specific coal composition    | unit-specific coal composition    |
| data for a one-year period were   | data for a one-year period were   |
| extracted from the ICR database   | extracted from the ICR II         |
| to find the coal heat content, Hg | database to find the coal heat    |
| content and Cl content. For each  | content, mercury content and      |
| set of coal composition data from | chlorine content. For each set of |
| the ICR database, the controlled  | coal composition data from the    |
| Hg emissions were calculated by   | ICR II database, the controlled   |
| multiplying uncontrolled Hg       | mercury emissions were calculated |
| emissions by (1-Hg removal        | by multiplying uncontrolled       |
| fraction). For each of the        | mercury emissions by (1 --        |
| best-performing sources, this     | mercury removal fraction), as set |
|                                   | forth below:... For each of the   |
| process was repeated for each set | best-performing sources, this     |
| of measured coal composition      | process was repeated for each set |
| values, yielding a range of       | of measured coal composition      |
| controlled Hg emission levels for | values, yielding a range of       |
| that unit over time.              | mercury emission levels for that  |
|                                   | unit over time."                  |
+-----------------------------------+-----------------------------------+
| Page 4673: "The test coal         | Section 3.4, Page 9: "In the      |
| composition data from the ICR     | above formula, the test coal      |
| database (heat and Hg content)    | composition data from the ICR II  |
| was used to calculate the         | database (heat and mercury        |
| uncontrolled Hg emission level.   | content) was used to calculate    |
| The Hg removal fraction was       | the uncontrolled mercury emission |
| calculated in one of the          | level. The mercury removal        |
| following two ways: (1) Where the | fraction was calculated in one of |
| correlation equation was found to | the following two ways: Where the |
| have sufficient explanatory       | correlation equation was found to |
| power, it was used to             | have sufficient explanatory       |
|                                   | power, it was used to estimate    |
| estimate the Hg removal fraction  | the mercury removal fraction      |
| based                             | based on coal chlorine            |
|                                   | composition data from the ICR II  |
| on coal Cl composition data from  | data base. This approach          |
| the                               | accounted for variations in the   |
|                                   | mercury, chlorine and             |
| ICR data base. This approach      |                                   |
| accounted                         | heat content of fuel. Where the   |
|                                   | correlation equation was a poor   |
| for variations in the Hg, Cl, and | fit, the mercury removal fraction |
| heat                              | was based on the average mercury  |
|                                   | removal fraction observed in the  |
| content of fuel. (2) Where the    | ICR III stack tests of that unit. |
| correlation equation was a poor   | This latter approach yielded a    |
| fit, the Hg removal fraction was  | constant removal fraction based   |
| based on the average Hg removal   | upon the source test, and had the |
| fraction observed in the ICR      | effect of reducing the            |
| stack tests of that unit. This    | variability of predicted mercury  |
| latter approach yielded a         | emissions. Under this approach,   |
| constant removal fraction based   | the measured impact of fuel       |
| upon the source test, and had the | variability was limited to the    |
| effect of                         | effect of variations in mercury   |
|                                   | and heat content, while           |
| reducing the variability of       | variations in chlorine            |
| predicted Hg                      | concentration were not explicitly |
|                                   | considered."                      |
| emissions. Under this approach,   |                                   |
| the measured impact of fuel       |                                   |
| variability was                   |                                   |
|                                   |                                   |
| limited to the effect of          |                                   |
| variations in Hg                  |                                   |
|                                   |                                   |
| and heat content, while           |                                   |
| variations in Cl                  |                                   |
|                                   |                                   |
| concentration were not explicitly |                                   |
| considered."                      |                                   |
+-----------------------------------+-----------------------------------+
| Page 47673: "For each of the      | Section 3.5, Pages 14-15: "For    |
| best-performing units, the        | each of the best performing       |
| calculated controlled Hg          | units, the calculated mercury     |
| emissions, calculated in          | emissions calculated in           |
| accordance with the procedures    | accordance with Section 3.5       |
| outlined above, were then sorted  | above, were then sorted from      |
| from smallest to largest to       | smallest to largest to obtain a   |
| obtain a cumulative frequency     | cumulative frequency distribution |
| distribution                      | ("CDF"). The CDF for each unit is |
|                                   | provided in Appendices 1-3. The   |
| (CFD). The 97.5th percentile      | 95th percentile value of this     |
| value of this distribution        | distribution (*i.e.,* an emission |
| (*i.e.*, an emission rate that is | rate that                         |
| expected to be exceeded only 2.5  |                                   |
| percent of the time) was          | is expected to be exceeded only   |
| determined to represent the       | 5% of the time) was determined to |
| operation of the unit under       | represent the                     |
| conditions reasonably expected to |                                   |
| occur at the unit.                | operation of the unit under       |
|                                   | "worst conditions." Because the   |
| It is necessary also to account   | ICR III stack test facilities     |
| for inter-unit variability among  | represent only a small portion of |
| the top performers. The analysis  | the true population of coal-fired |
| of within-unit variability        | utility units, it is necessary    |
| considered only the top units in  | also to account for inter-unit    |
| each subcategory. A focus on      | variability between the top       |
| within unit variability alone is  | performers. The ICR II database   |
| not expected to capture the full  | indicates that the population of  |
| range of emissions variability    | coal-fired units exceeds 1000.    |
| among the best-performing         | Yet, due to the limited size of   |
| sources. The EPA accounted for    | the ICR III database, the         |
| this variability by calculating a | analysis of within-unit           |
| 97.5 percent upper confidence     | variability considered only the   |
| level for the mean by use of the  | top 5 units in each subcategory.  |
| student t-statistic.              | Therefore, the actual number of   |
|                                   | the top 12% of coal-fired units   |
|                                   | in each subcategory is            |
|                                   | significantly larger than the     |
|                                   | number of units used in this      |
|                                   | analysis, particularly with       |
|                                   | respect to units burning          |
|                                   | bituminous and subbituminous      |
|                                   | coal. Under these circumstances,  |
|                                   | a focus on within-unit            |
|                                   | variability alone is not expected |
|                                   | to capture                        |
|                                   |                                   |
|                                   | the full range of emissions       |
|                                   | variability among the best        |
|                                   | performing sources. ENSR          |
|                                   |                                   |
|                                   | accounted for this variability by |
|                                   | calculating a 95% upper           |
|                                   | confidence level for the mean     |
|                                   |                                   |
|                                   | by use of the t-statistic."       |
+-----------------------------------+-----------------------------------+