The Alaska Department of Environmental Conservation is taking comment until April 3 on an air permit change needed to adjust NOx emissions factors for a newly-built power plant of the Alaska Energy & Electric Cooperative (AEEC).
The permitting covers the Soldotna Combustion Turbine Plant (SCT). This is a new power plant located 2.2 miles east-northeast of Soldotna, Alaska, within the boundaries of existing AEEC properties. It provides a peaking and emergency back-up for the electricity AEEC purchases from Chugach Electric Association.
The plant contains two new 49-MW General Electric LM6000 combustion turbine generators (CTG) with dry-low NOx emissions, a 50 million British thermal units per hour (MMBtu/hr) natural gas-fired inlet air heater, two 2-MMBtu/hr natural gas-fired heaters, a 605 brake horsepower (bhp) Kohler 400REZX emergency LPG generator, a 144 bhp diesel firewater pump, and several small emission units.
AEEC will use the inlet air heater to preheat the CTG inlet air when ambient temperature falls below 20 degrees Fahrenheit (ºF). The inlet air heating equipment will provide a hot water/glycol solution that will transfer heat to the inlet air and provide sufficient heating to meet the minimum inlet temperature for the CTGs. AEEC will use the two natural gas fuel heaters to heat the fuel gas. As of July 1, 2014, AEEC had only installed one CTG (emissions source EU 1) and the emergency diesel firewater pump (EU 7) out of the seven emission units authorized. AEEC installed EU 1 and EU 7 in June 2011.
Source tests performed in July 2014 showed that the turbine NOx emission rate was higher than the vendor guaranteed emission rate of 24.32 lb/hr when the turbine is operated at loads below 25 MW, but at loads above 25 MW the NOx emissions do not exceed 17.38 lb/hr. The NOx emissions under the plant’s air permit was based on the vendor-provided NOx emission rates of 24.32 lb/hr. Therefore, the stationary source NOx potential-to-emit (PTE) was underestimated using the vendor-provided data. With only EU 1 operating, the potential NOx emissions of the stationary source will not exceed the PSD threshold. However, when the second turbine (EU 2) comes on line, the potential NOx emissions of the stationary source will exceed the PSD threshold.
A PSD avoidance limit is not necessary when only one turbine (EU 1) is operating. However, when AEEC installs and operates the second turbine (EU 2), a NOx emission limit is necessary to keep the stationary source NOx emissions below the 250 ton per year (tpy) PSD threshold. The emission calculations involved are based on the following assumptions:
- The CTGs (EUs 1 and 2) NOx emissions will be limited such that the stationary source NOx emissions remain under 250 tpy.
- The LPG-fired generator (EU 6) will operate no more than 500 hours per year (hr/yr).
- The natural gas-fired heaters (EUs 3, 4, and 5), the diesel-fired firewater pump (EU 7) have unlimited operating hours, and
- The small emission units at the stationary source have unlimited operating hours.
Based on these assumptions, the department has determined the turbine NOx emissions must be less than 231 tpy for the stationary source to stay below PSD thresholds.