The Illinois Environmental Protection Agency is taking comment until Nov. 6 on an application from United States Steel Corp.–Granite City Works for revisions to the air pollution control construction permit for its Cogeneration Boiler.
This boiler is designed to fire a combination of blast furnace gas (BFG) and natural gas to supply steam for the Granite City Works. The requested revisions address US Steel’s increased need for natural gas to make up more of the fuel for this boiler than it originally anticipated. The Illinois EPA has reviewed US Steel’s application and made a preliminary determination that the application meets applicable requirements.
The Granite City Works is an integrated iron and steel mill producing flat rolled steel products. Some of the steam that is needed for the operation of this facility is supplied by the new Cogeneration Boiler, which began operation in October 2009. This boiler is a “cogeneration” unit because the high-pressure steam produced by this boiler is used both to generate electricity for the facility and in manufacturing processes at the facility.
BFG, the primary fuel, is a byproduct of the two existing blast furnaces at the Granite City Works. Blast furnace gas is a low-heat content gaseous fuel that is composed primarily of nitrogen, carbon monoxide, carbon dioxide and hydrogen. The Cogen Boiler also burns natural gas.
The Cogen Boiler replaced the steam generating capability of ten old, smaller boilers at the Granite City Works, Boilers 1 through 10, which were permanently shut down after the shakedown of the Cogen Boiler was completed. These boilers also used BFG as their primary fuel. They did not produce high-pressure steam as needed for cogeneration of electricity.
Along with the Cogen Boiler, US Steel also constructed a second flare for BFG. Flares are installed with blast furnaces to be able to safely dispose of surplus BFG from the furnaces that cannot be used as fuel. BFG flares must use natural gas or other high-Btu gas in pilot burners to maintain stable combustion of the low-heat content BFG. The second flare (BFG Flare 2) provided US Steel with more capacity for flaring surplus BFG. This additional capacity was needed for periods when the Cogen Boiler would not be in operation, such as during periodic maintenance. The situation in these periods would be the same as if all ten of the old boilers were out of service at the same time.
Historically, before these ten boilers were permanently shut down, at most only a few of them were out of service at any time. In those circumstances, the existing flare for BFG (now BFG Flare 1) had sufficient capacity to handle the surplus BFG that could not be used as fuel.
In conjunction with the construction of the Cogen Boiler, US Steel also constructed a new cooling tower. A cooling tower is often needed to condense any return steam from the turbine generator or heating/process operations. The new cooling tower serves this role for the Cogen Boiler.
The 2008 construction permit has been revised four times to provide for temporary increases in natural gas usage by the Cogen Boiler to address events at the Granite City Works that were not foreseen by US Steel when it was developing the Cogen project. US Steel is now seeking permanent revisions to the permit to allow natural gas to make up more of the fuel burned in the Cogen Boiler. In conjunction with these changes, US Steel is also requesting other changes to address issues with the original permit that have been identified since the permit was originally issued.
US Steel has concluded that the limit on natural gas usage that it proposed in its original application, as reflected in the original permit, was not realistic. The Cogen Boiler needs to be allowed to use more natural gas to be able address the variety of events and operating scenarios that may occur at the facility. In particular, outages of another boiler at the facility necessitate use of more natural gas by the Cogen Boiler to ensure the stable operation of the Cogen Boiler and to maintain the safety of the employees at the facility. Reduced operating levels and unplanned and unexpected outages of the blast furnace(s) also mean that more natural gas must be used by the Cogen Boiler.