Frequently Asked Questions

On Dec. 24, 2018, at approximately 4:15 a.m., U. S. Steel’s Clairton Plant experienced a significant fire which required the immediate shutdown of the No. 2 control room and No. 5 control room, which house the equipment and controls necessary to clean the coke oven gases. The fire was successfully extinguished by plant and external fire departments by approximately 9:30 a.m. and no injuries were reported.

Our coking facilities convert coal into coke which is used in the making of liquid iron for the steelmaking process. The coking process produces coke oven gas (COG) as a by-product. No. 2 control room is the facility where clean coke oven gas is separated from the other gases that are generated during the coking process. After this separation, the other gases go through the remaining processes of No. 2 control room and No. 5 control room for additional gas cleaning steps.

However, the fire caused significant damage to the No. 2 control room building roof structure, and the related equipment used to clean coke oven gases, known as vacuum machines. This prevented us from restarting those processes.

U. S. Steel is employing all available resources around the clock to make the necessary repairs to the impacted areas and equipment.   By employing all available resources and working around the clock, we are able to commit to have the desulfurization process at the Clairton Plant operational by April 15, 2019, when 100% of the coke oven gas generated will be desulfurized.  This is one month ahead of our original estimated completion date of May 15. This progress is made possible by the dedicated efforts of U. S. Steel employees and building tradesmen and women.


The control room that was affected by the fire is about the size of a football field.  Portions of its roof and supporting structure were significantly damaged and presented a safety risk.  As soon as that area was cleared for entry on Tuesday, Jan. 8, a comprehensive safety and fire investigation team that included an independent fire investigator began their work.   It has been determined that the fire originated from a mechanical failure in the C-521 vacuum machine area, however, what caused that failure is still being investigated by the experts.

The damaged equipment impacted a process that performed desulfurization.  This is the process that removes sulfur from coke oven gas. After the removal of the sulfur, we recycle the remaining Coke Oven Gas (COG) to fuel our processes at our Clairton, Edgar Thomson and Irvin plants=U. S. Steel is now utilizing a blend of coke oven gas and natural gas in our process units.  Many mitigation techniques are also being used to minimize the environmental impact of the No. 2 and No. 5 control rooms being down, including flaring the coke oven gas at the Irvin facility for better dispersion. 

We are also decreasing coke production by further increasing coking times on all batteries. This will result in a net of about 8,000 tons of coke produced per day, another 15% decrease in production over previous increases in coking times already made, with an overall decrease in production of around 30%. Details of these specifications can be found the ACHD’s revised enforcement order here.  This is an evolving process in which we continue to adjust the techniques to maximize the mitigation results. To view our weekly updates provided to the ACHD click the "updates" tab.

The Liberty Borough Monitor, located at South Allegheny High School, tracks PM2.5 daily National Ambient Air Quality Standards (NAAQS) and hourly SO2 NAAQS. Since the incident, the Liberty Borough monitor has recorded exceedances during seven individual hours over five days during the first 16 days immediately following the fire that were above the 1-Hr SO2 standard of 75 ppb. The exceedances over the 75 ppb occurred on:

12/26/2018 – 10 a.m. – 79 ppb

12/26/2018 – 11 a.m. – 80 pp

12/28/2018 – 10 a.m. – 145 ppb

1/2/2019 – 9 p.m. – 81 ppb

1/3/2019 – 11 p.m. – 85 ppb

1/8/2019 – 12 midnight – 80 ppb

1/8/2019 – 4 a.m. – 76 ppb

3/28/2019  3 a.m. 82 ppb


The last Liberty monitor exceedance of the hourly standard was on March, 28, 2019. There have been no SO2 NAAQs exceedances at the Liberty monitor since that date.

The Department also regularly monitors SO2 at the North Braddock monitor.  Since the incident there were two hours in which the North Braddock monitor recorded exceedances of the hourly SO2 standard:  January 7 at the 11pm and February 4 at the 10 pm hour.  The last North Braddock monitor exceedance of the hourly standard was on February 4, 2019 at the 10 pm hour. This exceedance occurred on a day when there was an air quality alert in approximately twenty counties across Pennsylvania – as the Department explained in its public statements.  There have been no SO2 NAAQS exceedances at the Braddock monitor since February 4, 2019.

In addition, in full cooperation with the Department, U. S. Steel agreed to providing sufficient funding for additional temporary air quality monitors to determine air quality impacts and whether the mitigation efforts have been successful.  The Department installed two additional monitors to measure SO2 in the ambient air at Clairton and West Mifflin. These locations were selected by the Department to assess air quality impacts from U. S. Steel’s redirection of the combustion of coke oven gas.  The Clairton monitor began monitoring SO2 on January 24, 2019.  There have been no SO2 NAAQS exceedances at the Clairton monitor. The West Mifflin monitor began monitoring SO2 on February 21, 2019.  This monitor was strategically located downwind of the flares at U. S. Steel’s Irvin plant where most of the coke oven gas was redirected and is being combusted.  There have been no SO2 NAAQS exceedances at the West Mifflin monitor.

On February 2, 3, and 4 the Liberty air quality PM2.5 monitor recorded 24-hour average exceedances of the PM2.5 NAAQS above the standard of 35µg/m3.  These exceedances occurred on days when there was an air quality alert impacting approximately twenty counties across Pennsylvania, including western Ohio and central and eastern Pennsylvania.

Clairton maintains a by-product recovery plant, where we recover the various by-products from the coke oven gas stream during the gas cleaning process.

Nos. 1, 2 and 5 control rooms are part of the by-product recovery process.

No. 1 control room pulls the coke oven gas away from the coke oven batteries and compresses it to push it through the rest of the gas cleaning process. Ammonia is removed from the coke oven gas at the No. 1 control room.

No. 2 control room separates the coke oven gas into two gas streams. One is a clean gas stream that is used as fuel to the coke oven batteries, boilers, and other sources of energy at our facilities. The second gas stream is then processed for additional by-product removal. This gas stream contains the sulfur compounds that are removed in the No. 5 control room.

We are committed to our community, our shared environment and our role as a good corporate citizen.

Upon extinguishing the fire, we devoted all available resources and personnel toward mitigation efforts that could be implemented immediately. Hot idling means to shut down and stop operating coke making, and it is not possible to shut down a coke making facility rapidly as hot idling is a deliberate and lengthy process (due to the extreme temperatures of the ovens and need to safely control explosive gas). In this instance, diverting resources to begin a hot idle would have redirected efforts away from remedying the problem and created additional environmental and economic impacts.

Long-term, a hot idle at Clairton could mean the permanent closure of some of the impacted batteries, and other batteries would sustain damage that would adversely impact the facility’s operation and environmental performance for several years. Such a hot idle action would likely have an adverse impact on the operating levels of all of our Mon Valley facilities and could result in workforce reductions.