The Minnesota Pollution Control Agency proposes to issue an air permit, with comment being taken until April 21, to the Hibbing Public Utilities Commission (HPUC) and Laurentian Energy Authority LLC for their facility in Hibbing in St. Louis County.
This is a major amendment application received in November 2014. The amendment is for the installation of a 90 MMBtu/hr natural gas auxiliary burner on the existing wood‐fired Boiler (emission unit 007). The auxiliary burner will allow for the use of natural gas as a supplemental fuel during startup and shutdown activities.
Natural gas will also be used as a supplemental fuel on occasion to stabilize combustion of the wood during wet periods when the wood fuel received has higher moisture content.
The proposed burner will be a low‐NOX design. The addition of the auxiliary burner does not increase the rated capacity of the boiler and only allows for combustion of natural gas up to 90 MMBtu/hr of the maximum nameplate capacity of 230 MMBtu/hr.
The HPUC is also requesting that the permitted allowable SO2 emission rate for each of the coal boilers (EU 001, EU 002, and EU 003) be lowered from 1.58 lb/MMbtu heat input to 0.90 lb/MMbtu heat input. The original SO2 emission rate was based on a higher sulfur content coal than what is currently being used at the facility. The lower allowable emission rate allows HPUC to demonstrate compliance with the 1‐hour SO2 national ambient air quality standards (NAAQS).
HPUC operates a co‐generation facility for the city of Hibbing. The facility generates electricity that is sold to the grid and steam for space heating of businesses, schools, and residences. The HPUC plant is located in downtown Hibbing and was originally constructed in 1919. The emission units at the source consist of three coal/natural gas‐fired boilers, a wood‐fired boiler, an ash‐handling system, as well as the two natural gas‐fired boilers located a few blocks away at Hibbing High School. The high school boilers are connected to the HPUC steam distribution system.
The six boilers are labeled Boiler No. 1A, Boiler No. 2A, Boiler No. 3A, High School Boiler No. 1, High School Boiler No. 2, and Wood‐Fired Boiler. Other air emission sources at the facility include a railcar/truck coal unloading station and an ash transfer system.
Boilers 1A, 2A, 3A, and the Wood‐Fired Boiler are rated at approximately 17.8 MWe, 17.8 MWe, 21.4 MWe, and 19.4 MWe generating capacity respectively. These boilers generate steam that is fed to a single steam header that can then be routed to any of the three steam/turbine generators (rated at 9.4 MW, 18.4 MW, and 6.4 MW). Steam is extracted from a turbine stage to feed the district heating system, which takes precedence over the generation of electricity. The process of generating electricity and providing steam for heat or other industrial processes is accomplished via a topping cycle cogeneration process.
Hibbing Public Utilities Commission started operation of the Wood‐Fired Boiler (EU 007) beginning in January 2007.