Public Service Company of Colorado THE SOURCE TO WHICH THIS PERMIT APPLIES IS DESCRIBED AND LOCATED AS FOLLOWS:

COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT AIR POLLUTION CONTROL DIVISION TELEPHONE: (303) 692-3150 CONSTRUCTION PERMIT PERMIT NO: 11PB859 DATE ISSUED: JULY 5, 2005 INITIAL APPROVAL Modification No. 1 ISSUED TO: Public Service Company of Colorado THE SOURCE TO WHICH THIS PERMIT APPLIES IS DESCRIBED AND LOCATED AS FOLLOWS: Comanche Generating Station, 2005 Lime Road, Pueblo, Pueblo County, Colorado THE SPECIFIC EQUIPMENT OR ACTIVITY SUBJECT TO THIS PERMIT INCLUDES THE FOLLOWING: Unit 2: Babcock and Wilcox, Model and Serial No. NB23761, Cross-Fired (Wall-Fired) Dry Bottom Boiler, Nominally Rated at 3,482 mmbtu/hr. Sub-bituminous coal (pulverized) fired, with natural gas used for startup, shutdown and flame stabilization. This permit is issued as part of the Unit 3 Project THIS PERMIT IS GRANTED SUBJECT TO ALL RULES AND REGULATIONS OF THE COLORADO AIR QUALITY CONTROL COMMISSION AND THE COLORADO AIR POLLUTION PREVENTION AND CONTROL ACT C.R.S. (25-7-101 et seq), TO THOSE GENERAL TERMS AND CONDITIONS INCLUDED IN THIS DOCUMENT AND THE FOLLOWING SPECIFIC TERMS AND CONDITIONS: 1. A lime spray dryer and low NO X burners and over-fire air shall be installed on Unit 2 to control SO 2 and NO X emission respectively. The annual emission and fuel consumption limits identified in Conditions 11 and 12 of this permit shall take effect 180 days after initial startup of the last control device for the last unit or upon startup of Unit 3, whichever is earlier. 2. This permit shall expire if the owner or operator does not commence construction of the Unit 3 Project within 18 months after either the date of issuance of the permit or the date on which such construction or activity was scheduled to commence as set forth in the permit, whichever is later; discontinues construction for a period of eighteen months or more; or does not complete construction within a reasonable time of the estimated completion date. (Reference: Colorado Regulation No. 3,Part B, III.F.4.a) Upon showing of good cause by the permittee, the Division may grant extensions of the permit not to exceed eighteen months per extension. (Reference: Colorado Regulation No. 3, Part B, III.F.4.b) 3. The permittee shall notify the Division 30 days prior to startup of each of the control devices. (Reference: Colorado Regulation No. 3, Part B, III.G.1).

page 2 4. Except as provided for in Condition 5, below, no owner or operators of a source shall allow or cause the emission into the atmosphere of any air pollutant which is in excess of 20% opacity (Reference: Colorado Regulation No. 1, Section II.A.1). 5. No owner or operator of a source shall allow or cause to be emitted into the atmosphere any air pollutant resulting from the building of a new fire, cleaning of fire boxes, soot blowing, start-up, process modifications or adjustment or occasional cleaning of control equipment which is in excess of 30% opacity for a period or periods aggregating more than six (6) minutes in any sixty (60) consecutive minutes (Reference: Colorado Regulation No. 1, Section II.A.4). 6. Particulate matter emissions from this unit shall not exceed 0.1 lb/mmbtu (Reference: Colorado Regulation No. 1, Section III. A.1.c). 7. Sulfur dioxide emissions from this unit shall not exceed 1.2 lb/mmbtu on a 3 hour rolling average (Reference: Colorado Regulation No. 1, Section VI.A.3.a.(ii) and VI.A.1). 8. Emissions of Lead (Pb) shall not be such that emissions, from the facility, result in an ambient lead concentration exceeding 1.5 micrograms per standard cubic meter averaged over a onemonth period (Reference: Colorado Regulation No. 8, Part C, Section I.B State-only enforceable). 9. This unit is subject to the continuous emission monitoring requirements as set forth in Colorado Regulation No. 1, Section IV, 40 CFR Part 60 Subpart D (as adopted by reference in Colorado Regulation No. 6, Part A) and 40 CFR Part 75. Continuous emission monitoring systems for opacity, sulfur dioxides and nitrogen oxides shall meet the requirements specified in the Title V Operating Permit. 10. This unit is subject to Colorado Regulation No. 6 - Standards of Performance for New Stationary Sources (NSPS), Part A - Federal Register Regulations Adopted By Reference, Subpart D - Standards of Performance for Fossil-Fuel-fired Steam Generators for Which Construction is Commenced After August 17, 1971, including, but not limited to, the following: a. Particulate Matter: Concentration in the gases discharged into the atmosphere shall not be in excess of 0.1 lbs/mmbtu (3-hour average) The gases discharged into the atmosphere shall not have opacity in excess of 20 percent (6-minute average), except for one 6-minute period per hour of not more than 27 percent opacity. Note that this opacity standard shall apply at all times except during periods of startup, shutdown and malfunction ( 60.11(c)), however, those instances during startup, shutdown and malfunction when the opacity standard is exceeded shall be identified in the Excess Emission Report required by Condition 8.6 of the Title V operating permit. b. Oxides of Nitrogen: Concentration in the gases discharged into the atmosphere shall not be in excess of 0.7 lbs/mmbtu, based on a 3-hr rolling average.

page 3 c. Sulfur Dioxide: Concentration in the gases discharged into the atmosphere shall not be in excess of 1.2 lbs/mmbtu, based on a 3-hr rolling average. In addition, the following requirements of Colorado Regulation No. 6, Part A, Subpart A, General Provisions, apply. d. At all times, including periods of start-up, shutdown, and malfunction, the facility and control equipment shall, to the extent practicable, be maintained and operated in a manner consistent with good air pollution control practices for minimizing emissions. Determination of whether or not acceptable operating and maintenance procedures are being used will be based on information available to the Division, which may include, but is not limited to, monitoring results, opacity observations, review of operating and maintenance procedures, and inspection of the source ( 60.11(d)). e. No article, machine, equipment or process shall be used to conceal an emission which would otherwise constitute a violation of an applicable standard. Such concealment includes, but is not limited to, the use of gaseous diluents to achieve compliance with an opacity standard or with a standard which is based on the concentration of a pollutant in the gases discharged to the atmosphere. ( 60.12). 11. This unit shall be limited to the fuel use rates as listed below and all other activities, operational rates and numbers of equipment as stated in the application. Monthly records of the actual consumption rate shall be maintained by the applicant and made available to the Division for inspection upon request. (Reference: Colorado Regulation No. 3, Part B, II.A.4) Natural Gas 386.5 mmscf/quarter and 1,546 mmscf/yr Coal 473,184 tons/quarter and 1,857,120 tons/yr During the first twelve (12) months of operation, compliance with both the quarterly and annual consumption limitations shall be required. After the first twelve (12) months of operation, compliance with only the annual limitation shall be required. Compliance with the annual consumption limits shall be determined on a rolling twelve (12) month total. 12. Emissions of air pollutants from this unit shall not exceed the following limitations (as calculated in the Division's preliminary analysis). Compliance with the annual limits shall be determined on a rolling twelve (12) month total. By the end of each month a new twelve month total is calculated based on the previous twelve months data. The permit holder shall calculate monthly emissions and keep a compliance record on site for Division review. (Reference: Colorado Regulation No. 3, Part B, II.A.4) Unit 2 Only Particulate Matter (filterable):...390 tons/quarter and 1,525 tons/yr Particulate Matter less than 10 (filterable):...357 tons/quarter and 1,403 tons/yr Carbon Monoxide:...134.1 tons/quarter and 528.3 tons/yr Volatile Organic Compounds...15.3 tons/quarter and 59.9 tons/yr Units 1 and 2, Together Sulfur Dioxide:...939.3 tons/quarter and 3,686 tons/yr Nitrogen Oxides:...1,564.4 tons/quarter and 6,143 tons/yr

page 4 NO X and SO 2 emissions shall be determined using the continuous emission monitoring systems (CEMS) required by Condition 9. PM, PM 10, CO and VOC shall be calculated using the emission factors in the permit notes. During the first twelve (12) months of operation, compliance with both the quarterly and annual emission limitations shall be required. After the first twelve (12) months of operation, compliance with only the annual limitation shall be required. 13. This unit is subject to the following emission limitations as set forth in the Settlement Agreement, signed on December 3, 2004 between Public Service Company of Colorado (PSCo) and Concerned Environmental Community Parties (CECP), as follows (Reference: Colorado Regulation No. 3, Part B, II.A.4) a. Sulfur Dioxide Emissions (i) SO 2 emissions from this unit shall not exceed 0.12 lb/mmbtu, on a 30-day rolling average basis, including emissions from shutdown and malfunction events. The first two hours after coal is first fed to the boiler during a cold startup shall be excluded from the calculation of that day s SO 2 emissions (Paragraph 3.C). (ii) SO 2 emissions from Units 1 and 2 together shall not exceed 0.10 lb/mmbtu, on an annual rolling average basis (rolling on a daily basis) including emissions during startup, shutdown and malfunction events (Paragraph 3.C). b. Nitrogen Oxide Emissions (i) (ii) (iii) NO X emissions from this unit shall not exceed 0.20 lb/mmbtu, on a 30-day rolling average basis, including shutdown and malfunction events (Paragraph 4.C). NO X emissions from Units 1 and 2 together, shall not exceed 0.15 lb/mmbtu on an annual rolling average basis (rolling on a daily basis), including shutdown and malfunction events (paragraph 4.C). During cold startups, the following shall be excluded from the calculation of that day s NO X emissions: A. The first two hours when natural gas-fired igniters are in use, and B. The first four hours after coal is first fed to the boiler (paragraph 4.C). c. Installation and Compliance Schedule (i) The SO 2 and NO X control equipment for Units 1 and 2 shall be operational by December 31, 2008 (paragraph 6). (ii) PSCO shall meet the unit-specific emission limits no later than 180 days after initial startup of the SO 2 and NO X control equipment for each unit or by July 1, 2009, whichever is earlier (Paragraph 6). (iii) PSCO shall begin calculating compliance with the SO 2 and NO X combined annual rolling average emission limits (rolling on a daily basis) for Comanche 1 and 2 no later than 180 days after initial startup of the SO 2 and NO X control equipment for the last unit (paragraph 6). (iv) Compliance with the SO 2 and NO X emission limitations in Conditions 13.a and 13.b above shall be determined by the continuous emission monitoring systems required by Condition 9 (Paragraph 6). d. Monitoring, testing and emission limits for Mercury (i) Within one year after the date that the Unit 3 construction permit (04PB1015) is issued, PSCO shall install, properly maintain and operate a continuous mercury

page 5 emissions monitoring system on Units 1 and 2 using Q-SEMS technology as described at 69 Federal Register at 4694 (January 30, 2004), or such other technology as the Parties may agree (paragraph 7.B). (ii) PSCO shall monitor mercury emissions from Units 1 and 2 beginning 18 months after the issuance of the Unit 3 permit and shall report the quality assured and quality controlled data to CECP and the Department on a calendar quarterly basis thereafter (paragraph 7.B). (iii) PSCo shall operate and maintain the mercury monitoring technology in accordance with EPA requirements and the manufacturing specifications. In the event of any mercury monitoring technology malfunction, PSCo shall either repair or replace such monitoring technology. If the mercury monitoring technology identified in the above paragraph is unable to meet applicable performance requirements despite PSCo s efforts to repair and replace such technology, PSCo agrees to install alternate mercury monitoring technology unless technologically or economically infeasible or to conduct annual stack testing if monitoring technology is technologically or economically infeasible (paragraph 7.C). e. Other Air Permit Issues (i) At all times, including periods of startup, shutdown and malfunction, PSCo shall, to the extent practicable, maintain and operate any emission control equipment required under Condition 1 in a manner consistent with good air pollution control practice for minimizing emissions. Determination of whether acceptable operating and maintenance procedures are being used will be based on information available to the Department which may include, but is not limited to, monitoring results, observations, review of operating and maintenance procedures, and inspection of the source (paragraph 8.G) 14. This source is subject to the provisions of Regulation No. 3. Part C, Operating Permits (Title V of the 1990 Federal Clean Air Act Amendments). The provisions of this construction permit must be incorporated into the operating permit. The application for the modification to the Operating Permit is due within one year of commencing operation of Unit 3. (Reference: Colorado Regulation No. 3, Part C, III.B.2). 15. Prior to issuance of final approval, the applicant shall submit to the Division for approval an operating and maintenance plan for all control equipment and control practices and a proposed recordkeeping format for demonstrating compliance on an ongoing basis. The applicant shall then follow the current Division approved Operating and Maintenance Plan. (Reference: Colorado Regulation No. 3, Part B, III.G.7) 16. Within one hundred and eighty days (180) after commencement of operation, compliance with the conditions contained on this permit shall be demonstrated to the Division. It is the permittee's responsibility to self certify compliance with the conditions. Failure to demonstrate compliance within 180 days may result in revocation of the permit (Reference Colorado Regulation No. 3, Part B, III.G.2). Information on how to certify compliance was mailed with the permit or can be obtained from the Division at 303-692-3209. 17. A revised Air Pollutant Emission Notice (APEN) shall be filed: (Reference: Colorado Regulation No. 3, Part A, II.C) a. Annually whenever a significant increase in emissions occurs as follows: For any criteria pollutant: For sources emitting less than 100 tons per year, a change in actual emissions of five

page 6 tons per year or more, above the level reported on the last APEN; or For sources emitting 100 tons per year or more, a change in actual emissions of five percent or 50 tons per year or more, whichever is less, above the level reported on the last APEN submitted; or For any non-criteria reportable pollutant: If the emissions increase by 50% or five (5) tons per year, whichever is less, above the level reported on the last APEN submitted to the Division. b. Whenever there is a change in the owner or operator of any facility, process, or activity; or c. Whenever new control equipment is installed, or whenever a different type of control equipment replaces an existing type of control equipment; or d. Whenever a permit limitation must be modified; or e. No later than 30 days before the existing APEN expires. Jacqueline Joyce Permit Reviewer Roland C. Hea, P.E. Construction Permits Unit Stationary Sources Program Initial Approval (C-10,140): Issued January 11, 1973 Initial Approval (C-10,140): Issued April 4, 1973 to extend the expiration date Final Approval (C-11,859): Issued April 17, 1978 This Modification: Permit was revised to make SO 2 and NO X reductions federally enforceable. In addition, the permit was revised to be consistent with currently issued construction permits, which entails including all applicable requirements and including annual limits on fuel consumption and emissions.

page 7 Notes to Permit Holder: 1) The production or raw material processing limits and emission limits contained in this permit are based on the production/processing rates requested in the permit application. These limits may be revised upon request of the permittee providing there is no exceedance of any specific emission control regulation or any ambient air quality standard. A revised air pollution emission notice (APEN) and application form must be submitted with a request for a permit revision. 2) This source is subject to the Common Provisions Regulation Part II, Subpart E, Upset Conditions and Breakdowns. The permittee shall notify the Division of any upset condition which causes a violation of any emission limit or limits stated in this permit as soon as possible, but no later than two (2) hours after the start of the next working day, followed by written notice to the Division explaining the cause of the occurrence and that proper action has been or is being taken to correct the conditions causing said violation and to prevent such excess emission in the future. 3) The emission levels contained in this permit are based on the following emission rates, design rate (mmbtu/hr) and 8,760 hrs/yr of operation or permitted fuel consumption limits. SO 2 and NO X emissions for Units 1 and 2 together are based on the following emission rates, the combined design rate of both units and 8,760 hrs/yr of operation. Pollutant Coal Emission Factor - Natural Gas (lb/mmscf) PM 0.1 lb/mmbtu N/A PM 10 0.92(PM) N/A SO 2 0.12 lb/mmbtu N/A NO X 0.20 lb/mmbtu N/A CO 0.50 lb/ton 84 VOC 0.06 lb/ton 5.5 Note that compliance with the annual emission limitations shall be monitored as follows: for CO and VOC the above emission factors and fuel consumption, for PM and PM 10, the most recent Title V stack test emission factors (PM = 0.00514 lb/mmbtu, PM 10 = 0.92 x PM), fuel consumption and the average heat content of the coal and for NO X and SO 2 the CEMS. 4) The source is classified as a: Major Modification At a: Major Stationary Source. 5) For use in the modeling analysis conducted, condensable PM 10 emissions were estimated at 22.9 tons/yr. The source considered that condensable PM 10 emissions are composed of the following compounds. Pollutant Emission Factor (lb/mmbtu) Sulfuric Acid (H 2 SO 4 ) 3 x 10-5 Hydrogen Chloride (HCl) 7 x 10-4 Hydrogen Fluoride (HF) 2 x 10-4 Organic compounds (compounds 1.05 x 10-4 with boiling temperatures of 300 º F or less) Elemental Carbon (presumed to be 4.6 x 10-4 0.5% of filterable PM 10 )

page 8 Total Condensable PM 10 1.5 x 10-3 The emission factors for H 2 SO 4, HCl and HF are based on performance tests. Note that these emission factors do not take credit for any emission reductions that will occur with the addition of the lime spray dryers. The emission factor for organic compounds is the same as the emission factor determined for Unit 3. The emission factor for elemental carbon was determined in the same manner as Unit 3, however, the Reg 1 PM limit (adjusted by 92% to get PM 10 ) was used to determine the emission factor.

page 9 GENERAL TERMS AND CONDITIONS: (IMPORTANT! READ ITEMS 5,6,7 AND 8) 1. This permit is issued in reliance upon the accuracy and completeness of information supplied by the applicant and is conditioned upon conduct of the activity, or construction, installation and operation of the source, in accordance with this information and with representations made by the applicant or applicant's agents. It is valid only for the equipment and operations or activity specifically identified on the permit. 2. Unless specifically stated otherwise, the general and specific conditions contained in this permit have been determined by the APCD to be necessary to assure compliance with the provisions of Section 25-7-114.5(7)(a), C.R.S. 3. Each and every condition of this permit is a material part hereof and is not severable. Any challenge to or appeal of, a condition hereof shall constitute a rejection of the entire permit and upon such occurrence, this permit shall be deemed denied ab initio. This permit may be revoked at any time prior to final approval by the Air Pollution Control Division (APCD) on grounds set forth in the Colorado Air Quality Control Act and regulations of the Air Quality Control Commission (AQCC), including failure to meet any express term or condition of the permit. If the Division denies a permit, conditions imposed upon a permit are contested by the applicant, or the Division revokes a permit, the applicant or owner or operator of a source may request a hearing before the AQCC for review of the Division s action. 4. This permit and any required attachments must be retained and made available for inspection upon request at the location set forth herein. With respect to a portable source which is moved to a new location, a copy of the Relocation Notice (required by law to be submitted to the APCD whenever a portable source is relocated) should be attached to this permit. The permit may be reissued to a new owner by the APCD as provided in AQCC Regulation No. 3, Part B, Section II.B. upon a request for transfer of ownership and the submittal of a revised APEN and the required fee. 5. Issuance (initial approval) of an emission permit does not provide "final" authority for this activity or operation of this source. Final approval of the permit must be secured from the APCD in writing in accordance with the provisions of 25-7-114.5(12)(a) C.R.S. and AQCC Regulation No. 3, Part B, Section III.G. Final approval cannot be granted until the operation or activity commences and has been verified by the APCD as conforming in all respects with the conditions of the permit. If the APCD so determines, it will provide written documentation of such final approval, which does constitute "final" authority to operate. Compliance with the permit conditions must be demonstrated within 180 days after commencement of operation. 6. THIS PERMIT AUTOMATICALLY EXPIRES IF you (1) do not commence construction or operation within 18 months after either the date of issuance of this permit or the date on which such construction or activity was scheduled to commence as set forth in the permit, whichever is later; (2) discontinue construction for a period of 18 months or more; or (3) do not complete construction within a reasonable time of the estimated completion date. Extensions of the expiration date may be granted by the APCD upon a showing of good cause by the permittee prior to the expiration date. 7. YOU MUST notify the APCD at least thirty days (fifteen days for portable sources) prior to commencement of the permitted operation or activity. Failure to do so is a violation of Section 25-7-114.5(12)(a), C.R.S. and AQCC Regulation No. 3, Part B, Section III.G.1., and can result in the revocation of the permit. You must demonstrate compliance with the permit conditions within 180 days after commencement of operation as stated in condition 5. 8. Section 25-7-114.7(2)(a), C.R.S. requires that all sources required to file an Air Pollution Emission Notice (APEN) must pay an annual fee to cover the costs of inspections and administration. If a source or activity is to be discontinued, the owner must notify the Division in writing requesting a cancellation of the permit. Upon notification, annual fee billing will terminate. 9. Violation of the terms of a permit or of the provisions of the Colorado Air Pollution Prevention and control Act or the regulations of the AQCC may result in administrative, civil or criminal enforcement actions under Sections 25-7-115 (enforcement), -121 (injunctions), -122 (civil penalties), -122.1 (criminal penalties), C.R.S.