The Wisconsin Department of Natural Resources is taking comment until April 23 on a draft permit that covers higher generating capacity at a peaking power plant built a few years ago by Marshfield Utilities.
This is a modification of a 2006 permit to increase the identified design capacities of the two combustion turbines to authorize the full ‘as-built’ capacities, and to authorize the much higher capacity of the diesel engine emergency generator. This construction permit is proposed to also approve (after the fact) construction of a second diesel fuel oil tank as opposed to the single tank identified in the original application. Due to the prior change to authorize any ‘distillate’ fuel oil (including a number 1 fuel oil), there are additional applicable requirements for each fuel oil tank which require the tanks to be included within the permit.
Marshfield Utilities applied for a construction permit in 2006 to construct a 55 MWe (27.5 MWe each) system with two simple-cycle combustion turbines with maximum heat input capacities of 274 MMBTU per hour (million BTU per hour) each firing natural gas and 259 MMBTU per hour firing fuel oil, a diesel generator with a 350 kWe peak output and one fuel oil storage tank with a 150,000 gallon capacity. The permit was issued in 2008.
The turbine units were restricted to 383.33 hours per month of operation (based on a 12 consecutive month average, equivalent to 4,600 hours per every 12 months) to limit the potential to emit to 245 TPY of carbon monoxide (CO). Had the facility installed catalytic oxidizers, it would be authorized to operate up to 683.33 hours per month (based on a 12 consecutive month average, equivalent to 8,200 hours per year) and could have emitted up to 246 TPY of NOx. Without a construction permit to establish specific limitations on emissions of CO and other pollutants and restrictions on the hours of operation, the facility would have potential emissions sufficient to trigger prevention of significant deterioration (PSD) requirements for CO, NOx, SO2, VOC, PM10, PM2.5, and PM.
The units constructed by Marshfield Utilities had heat input capacities and maximum electrical outputs greater than what was identified in the original air permit application. This constitutes a modification, since these larger emissions units have emissions that are greater than those identified in the original application. In this case, the turbines are now identified as having a peak output of up to 60.5 MWe combined (30.25 MWe each) and an as-built heat input capacity of 325 MMBTU per hour of natural gas, and up to 290 MMBTU per hour of distillate fuel oil. The two storage tanks that were constructed have capacities of 72,000 gallons each.
Permit changes needed to cover this higher capacity
The agency noted: “Although the applicant contends that the source was ‘constructed in accordance with the original application,’ the 06-JAJ-371 application listed significantly lower heat input capacities / power outputs of the combustion turbines, that of the emergency generator, and a different configuration of the tank(s) than were actually installed and operated. Some of the [New Source Performance Standards] and state emission limits are based upon the heat input and/or peak output of the turbine / generator system and these emissions potentials are higher based on the higher capacities now being identified. Constructing and operating these turbines with higher heat inputs / electrical output capacities, and an emergency generator with a much higher heat input / electrical output is both a physical change and a change in method of operation that increases emissions as compared to that identified in the original application upon which permit 06-JAJ-371 was based.”
- The 55 MWe turbine output capacity and 274 MMBTU/hr peak heat input rate identified in the original permit application reflects the peak output during hotter day time temperatures (~90° F) whereas peak electrical output and peak heat input from these and other turbines occurs at cooler temperatures.
- The 60.5 MWe output identified in the current application is more representative of winter, spring, fall or cooler night / early morning temperatures when a turbine should be able to produce at or close to its maximum output as opposed to the significant de-rating that occurs at higher (mid-day / summer) temperatures.
The facility conducted its performance testing in late February and May of 2011, such that this showed that the electrical outputs and heat inputs were higher than those noted in the original application. The Pratt & Whitney Swiftpac 60 FT8-3 detailed tables provided with this new application (for 100% load on natural gas with water injection) noted an anticipated maximum HHV input of 307 MMBTU/hr for a 60.1 MWe output. The turbine was observed to actually reach 320 MMBTU/hr heat input during emission testing, and this was scaled up slightly to 325 MMBTU/hr. The peak output on distillate was scaled up to 290 MMBTU/hr though the maximum observed rate was 269 MMBTU/hr, but data sheets from the manufacturer predicted potential rates of ~ 275 MMBTU/hr.