Controlling Underground Heatings Using Innovative Fire-Suppressant Injection - Proof of Concept Study

One of the major safety issues in underground coal mines is spontaneous heating and combustion resulted from the natural oxidation of coal and other carbonaceous materials.  Underground heating can result in production delays and, if uncontrolled, loss of life, property and coal resource. The mitigation of the heatings is still an issue, since current techniques for controlling and extinguishing the heatings are not always effective and can be expensive.

The objective of this project was to develop long-term effective and low-cost polymer gel fire-suppressants that could be deployed for controlling and extinguishing coal heatings in underground coal mines. These fire-suppressing gels can be applied to the areas undergoing coal heatings at a controllable gelation time and impermeable gel barriers can be formed in adjacent areas to block ingress of air. In addition to air blocking, the gels could quench or cool the heatings with their high water content.

In this ACARP project, the following major tasks have been completed:

· Development of a set of selection criteria for suitable gel fire-suppressants

· Identification of a number of gel systems against the selection criteria

· Development of suitable approaches for gelation time control

· Development of suitable gel forming compositions of selected gel systems

· Evaluation of the gel strength and general stability of selected gel systems

· Evaluation of the thermal stability of selected gel systems

· Evaluation of the performance of gel fire suppressants for controlling and extinguishing heatings.

Based on the selection criteria, a gel system suitable for controlling and extinguishing heatings needs to meet the requirements of easy preparation, no or low toxicity, adjustable gelation time, low sensitivity to dissolved salts, controllable viscosity, long gel life, relatively high thermal stability, incombustible and low cost.

A number of gel systems have been developed and examined against the selection criteria. Initial studies show that four gel systems: anionic polyacrylamide (HPAM)-Al³⁺, sodium carboxymethylcellulose (CMC)-Al³⁺, acrylamide-acrylate copolymer-Al³⁺ and xanthan-Al³⁺ met the selection criteria. Detailed investigations on these four gel systems were undertaken.

Gel forming methods with controlled gelation time for the selected four gel systems have been developed. A typical gel consists of about 1-3.0% polymer, 500-3000 ppm cross-linker, 0.2-2% pH modifier with the remainder being water (up to 98%). A preservative in the dosage level of 0.1-0.2% may be added if a biopolymer is used. The chemical specifications, such as molecular weight and hydrolysis degree, of suitable gel forming polymers have been identified.

The gelation time of selected gel systems can be controlled with the proper choice of initial pH, aluminium salts and pH modifiers. Aluminium citrate and dihydroxyaluminium sodium carbonate (DASC) were found to be the best sources of Al³⁺ for polymer cross-linking. Glucono delta lactone (GDL) and fumaric acid were found to be two of a number of suitable pH modifiers.

A polymer concentration of 0.5% to 2% will generally produce a tonguing gel, while 2% to 3% can give a semi-rigid or rigid gel. A lightly tonguing gel or an intermediate strength gel is as effective as a rigid gel to block access of air to the area undergoing heating. The optimum gelling composition for a required gel strength may be determined by beaker scale tests for a given set of materials and water.

The use of saline water as a solvent to prepare a gelling mixture would generally reduce the viscosity of the mixture and the gel strength. When the total hardness of brine water is in excess of 1000 ppm, a water softening agent, such as soda ash, should be used to remove the calcium in the water so that potential polymer precipitation or short gel life can be avoided.

All polymer gels will shrink slowly due to water evaporation when the gel surface is exposed to the air. Polymer gel surfaces remain elastic and do not crack during gel dehydration, which makes gel fire-suppressants more effective than other sealants, such as water glass and cement, that would develop surface cracks once dehydrated. A polymer gel injected into a coal seam at a temperature lower than 50⁰C can maintain gelled status for up to one year. The interaction between injected gel and the coal seam does not cause any noticeable problems. However adsorptive fine particles, such as clay and coal dust in the size range of less than 500mm, will adsorb polymer, leading to reduced effective polymer concentration.

The gelled state of selected gels at a temperature lower than 90⁰C would still enable them to seal air passages to heating areas. It is, therefore, possible that the gel barrier or envelope for blocking access of air can be formed around the areas of heatings.

This study has shown that the HPAM-Al³⁺ gel is the most favourable gel system for suppression of underground coal heatings due to its low cost, easy preparation and relatively high thermal stability. Small scale fire-suppression tests with HPAM - Al³⁺ gel demonstrated that the gel fire suppressants are effective to control and extinguish coal heatings through air blocking and cooling.

Although the work in this project has demonstrated the feasibility of gel fire-suppressants, applications of gel fire-suppressants at mine sites require the development of appropriate delivery systems, operational procedures and guidelines. It is therefore recommended that further field-based research work be conducted. The main components may include:

· Develop an underground-based mobile gel preparation and injection system

· Develop a surface-based gel preparation and injection system

· Develop injection approaches that would maximize air-blocking area for a given volume of gel

· Undertake field tests at known heating sites exposed during open cut mining of old workings

· Develop operational procedures and guidelines for the gel injection technique.