Underground » Ventilation, Gas Drainage and Monitoring
The primary objective of this project was to carry out pilot scale tests of gel systems developed in ACARP project C14021 for assessing the effectiveness of the control of simulated pillar and roof fires.
In this project, the following major tasks have been completed:
· Carried out a review of the gel technology practiced in the Chinese coal industry;
· Developed approaches for gel preparation in pilot scale;
· Developed approaches for applying polymer hydrogels to simulated pillar and roof ;
· Carried out tests for controlling simulated pillar and roof fires using hydrogels ;
· Developed, in pilot scale nature, guidelines for gel selection for particular types of heatings and operating procedures for field applications.
High viscosity sodium carboxymethylcellulose (CMC) cross-liked by Al3+ was selected for the pilot-scale tests. The approach found to be most effective in applying polymer hydrogels to the simulated pillar or roof face was first spraying with a 4% CMC gelling mixture using a gravity feed spray gun to fill cracks and holes on exposed faces, and then injecting a gel-forming mixture with 2% CMC into the coal bed close to the face at multiple points to form a continuous gel barrier. The surface barrier formed by spraying gel on the face prevented the injected gel-forming mixture from leaking out of the coal bed before gelation. Moreover, the surface barrier could be used to buy time for drilling injection holes and injecting gel materials as it prevented smoke and toxic gases from leaking out. This would lead to a safer working environment for drilling injection holes and injecting gels. The approach of combined early spraying with later injection can maximise air-blocking area for a given volume of gel.
The effectiveness of CMC-Al3+ gel system was successfully demonstrated for the control of simulated pillar and roof fires. Coal bed temperature change and the variation of gas concentrations in coal bed indicated that the gel barrier was effective not only for preventing oxygen ingress to the hot spot but also for sealing the leakage of the toxic/combustible gases. The hydrogen level after applying gel can be higher than the lower explosive limit of hydrogen for a short period of time and then quickly dropped to very low levels. This hydrogen generation was caused by insufficient oxygen supply after applying gels and presence of coal moisture rather than the use of gel.
Guidelines for the selection of gel systems suitable for particular types of heatings and the operating procedures for delivering gel forming mixtures to heatings occurred in goaf, pillar and coal roof layer through injection and/or spray have been developed. Though the CMC-Al3+ gel system can be readily applied in operating coal mines, the guidelines and operating procedures are of the pilot-scale nature, not standard operating procedures used in coal mines, and would require substantial field-based testing and modifications for mine site applications.
The pilot scale test work conducted in this project has demonstrated the effectiveness of CMC-Al3+ gel system. Applications of the gel technique at mine sites will require further developments of field-based approaches and equipment.