The gel was developed by CSIRO's Sheng Xue and Shenggen Hu under an ACARP grant to tackle one of coal mining's major safety issues.

The mitigation of underground heatings or spontaneous combustion is still an issue in Australian mines since current techniques for controlling and extinguishing spontaneous combustion are either effective only in limited situations or expensive, Xue said.

Techniques currently used include mainly inert gas and foam, injection of water-based fire suppressant (such as water slurries, aqueous solutions of CaCl2 and MgCl2) and inorganic sealant (cement or bentonite based sealants).

The problem with these, Xue said, is once spontaneous combustion in a coal mine is well developed, inert gas technique becomes less effective.

“Foam is a mixture of liquid (water), gas (such as air), and a surfactant, and it is not considered suitable for controlling and extinguishing spontaneous combustion mainly because the bubbles in foam cannot withstand pressure and hence could not penetrate the cracks in coal," he said.

“In general, water-based fire suppressants cannot sustain air-blocking for a sufficiently long time due to water leaking.

“Cement or bentonite based sealants are usually expensive and cannot be cost-effectively applied to areas needing a large volume of sealants.

“In addition, the results with cement or bentonite based sealants were not good due to formation of cracks in the sealants."

With this in mind Xue launched the Controlling Underground Heating Using Innovative Fire Suppressant Injection project which aimed to develop long-term effective and low-cost polymer gel fire suppressants.

To begin with, a set of selection criteria for suitable gel fire suppressants was developed, a number of the gels meeting the selection criteria were examined and laboratory tested, and the best gels suitable for the fire suppressant were identified.

Xue and Hu developed suitable approaches for gelation time control and forming compositions, and evaluated the gel strength and general stability of selected gel systems. The thermal stability and performance of the gels were also evaluated.

The developed gels include a mixture of polymer, cross-linker, pH modifier and water. Once the gels are properly prepared, they can be pumped directly into the areas of heatings via a delivery system.

“The developed gels are found to be favourable for fire suppression in coal mines due to its low cost, easy preparation and relatively high stability,” Xue said.

“Small-scale fire suppression tests demonstrated that gel fire suppressants are effective to control and extinguish spontaneous heatings through air blocking and cooling.

“Through the application of fire-suppressant injection, the Australian operations will benefit from the effective and speedy control of heatings or spontaneous combustion. There is also no doubt the gels can be [used] in the mine operations of other countries to mitigate the heatings or spontaneous combustion."

The project’s report is currently with industry monitors for review, but Xue says he would like to seek ACARP support for a project extension to develop a mobile gel delivery system and undertake field demonstration of the gels and the system.