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# . 215 South Cascade Street PO Box 496 Fergus Falls, Minnesota 56538-0496 218739-8200 www.otpco.com (web site)

May 21,1998 Mr. Brian Gustafson Natural Resources Engineer Air Quality Program South Dakota Department ofEnvironment andNatural Resources

_**_.~** - Joe Foss Building 523 East Capitol Pierre, SD 57501-3Un Dear Mr. Gustafson: PowerCompany

SUBJECT: BIG STONE PLANT

- PERMIT NUMBER 28.0801-29 This letter is in response to yourMay 9, 1997 letter ofapproval for installation ofan over-fire air system at Big Stone Plant. The following is a list ofDENR requirements for the project and the corresponding Otter Tail Power Company responses: '. 1.
. OtterTail Power Company shall notify.the department, in writing, within 5 days of the initial start-up date ofthe over-fire air system; OTP Response: Notification ofsystem startup was provided via letter ofOctober 9, 1997. 2. OtterTail Power Company shall evaluate the affects ofthe over-fire air system on the air emission from the cyclone-fired boiler; OTP Response: Baseline emission tests were conducted priorto installation ofthe over-fire air system. Emissiontests were also conducted while using the over-fire system. Those test results are documented in the enclosed data sheet. Please note that during the tests, NOx emissions were reduced from approximately 1.28 Ibs/mmBtu to approximately 0.73 Ib/mmBtu, which is a reduction ofapproximately 43%. Also note thatthe emissions ofthe otherregulated air pollutants are not materially affected by operation ofthe over-fire air system. 3. OtterTail Power Company shall submit the proper documentation for revising the Title V air quality operating and acid rain permit within one year ofthe initial start-up ofthe over-fire air system. The documentation shall identify ifthere were any increases or decreases i.n emissions, quantify the increase or decrease for each pollutant, ifapplicable, and identify the appropriate permit revisioIl process; and
. ," " " . , , .... , . OTP Response: Itis OtterTail's opinion that installation ofthe over-fire air system is consistent with the Phase II NOx Compliance Planthat was filed with the Department of

An Equal OpportunityEmployer

### Mr. Brian Gustafson May 21, 1998 Page 2 Environment andNatural Resources on October 29, 1997. Based onthat plan, Big Stone Plant must meet an annual average NOx emission rate ofO.86lbs/mmBtu beginning with calendar year 2000. It is ourunderstanding that the DENR will have to reopen the Title V permitby January 1, 1999, to incorporatethe Phase IINOx Compliance Plan. No additional permit revisions are necessary for the project. 4. Otter Tail Power Company shall at all times during the use ofthe over-fire air system, comply with its existing air quality operating permit and acid rain permit. OTP Response: The only purpose for installation ofthe over-fire air system is for compliance with regulatory requirements. In absence ofthe Phase II NOx reduction requirements, the project would not likely have been completed. Should you have any questions on ourresponses oron the project, please contact me at 218- 739-8407.

# ~~ Terry Graumann Manager, Environmental Engineering Enclosure**--------------------~----** ---_._- ---