The details of the discovery of the spontaneous combustion fire and technologies used in firefighting and gas stabilisation were compiled by the Mine Safety and Health Administration’s Richard Stoltz and William Francart as well as Laine Adair and John Lewis of Genwal Resources. A presentation on the case and the ensuing discussion were heard at the 11th US/North American Mine Ventilation Symposium at Penn State University earlier this year.

Established in 1999, West Ridge extracts from the Lower Sunnyside seam in Carbon County, Utah at the rate of about 2.7 million tons per annum. Originally developed from four drift openings, West Ridge began developing longwall panels for second mining of the seam.

According to the researchers, ground control concerns surfaced as an issue, so the panels were developed with a two-entry system. They also noted that the inbye end of the mains sit lower in elevation due to a dip in the main entries of 10-15%.

Crews detected the fire situation on February 20, 2004 when a bleeder evaluation point showed trace levels of hydrogen and a carbon monoxide level of 160 parts per million (ppm) was measured. The fire’s location was not yet known and flames were not seen, but production was stopped and the operation cleared.

A gas chromatograph was then used to analyse levels, which MSHA’s technical support team used to try to determine the fire’s location, the men said. The testing reflected the approximate location of the fire, the vicinity of the rear of the fourth right longwall gob.

Because the exact location could not be pinpointed and because the operation was known to liberate high methane (70,000 cubic metres per day), the researchers said those involved opted to seal the gob as opposed to “typical” firefighting methods, which they said were not feasible.

There were three plans developed to manage the situation: the sealing of the entire operation; sealing the panel mouth of the longwall gob; and removing the longwall equipment and sealing the first right and inbye ends of first right and fifth right.

The third option was chosen, barring any future instability of the fire in its speculated location.

In order to remove the face equipment, the longwall team advanced 6m so that screening could be installed. Pre-seal ventilation patterns were kept stable through the use of steel pipes, and doorframes were added to the seals as they were constructed, the researchers said. The future was also considered, as the pipes were installed in a way that they could later be completely sealed with caps or valves and the doorframe preparation included the ability to bolt covers on.

A three-pronged attack

In the end, 76 seals were installed in the vicinity of the active gob at West Ridge, in which three approved designs were utilised:

Cementitious-foam seal: This method included the construction of an inbye and outbye stopping wall, 1.2m apart and temporary in nature. The foam was then inserted via injection ports;

Polyurethane-gravel core seal: Again, the mixture was injected to fill the void, this time into two surface-bonded, dry-stacked concrete block walls that were solid or hollow core; and

Inflatable bags: Much like a concrete block seal. However, inflatable bags were utilised beside the rib and at the top of the seal (instead of hitching) to pre-load them.

The three seals were used in conjunction with one another because “no one single seal construction method could be implemented due to construction and seal supply limitations,” the group noted, adding that the construction of the seals commenced February 26 and by March 7 the process was complete. The standard 72-hour waiting period was then observed with workers withdrawn.

“During this time, atmospheric samples were collected from three underground locations from the surface to evaluate the conditions underground,” the researchers said. As those involved expected, the methane levels went up (though it was noted the heating/fire never intensified through the process of sealing). The temporary sealing of the area was completed by closing all openings and closing the fire door of the fifth right.

Ventilation alteration

The company also opted to alter the ventilation of the operation from its standard that took mains air and placed it at the back bleeder and return. In what they hoped would set a pressure differential and reduce oxygen, the workers changed flow to have intake air splits on both sides of the gob, mains and bleeder. Testing after the adjustment showed a pressure drop of 90Pa.

After some initial obstacles at fourth right that reflected a combustible environment, later testing of the seals into the back gob from the bleeder area showed a 348Pa pressure reduction.

From that point, the team handing the West Ridge fire did not conduct methane liberation to inert the source, but rather injected carbon dioxide at fifth right on the outbye side into the gob for more than seven hours (the prior 348Pa reading fell to less than 75Pa). Then, as the installation of permanent seals was being completed outbye, the injection continued for more than 11 hours.

Just 24 days later on March 15, the project was completed. The researchers cite the sealing around the entire gob as the reason the fire was controlled, aided of course by the other factors introduced, including ventilation adjustments and balancing pressures.

“Pressure differentials across sealed areas have often been the most difficult obstacle for inerting gob areas [and] the West Ridge fire was no exception. This project reminds us that accounting for pressure balancing across the gob is critical for ultimate success in extinguishing similar fires,” the researchers said.