Underground » Detection and Prevention of Fires and Explosions
Most cases of frictional ignition in Australia are caused by cutter picks striking a high silica material (conglomerate roof, sandstone roof or material associated with geological structures) in gassy conditions. Keeping picks sharp when cutting this material, spraying the cutting mechanism with water and keeping the face well ventilated significantly reduce the risk of frictional ignition. The occurrence of frictional ignition is a significant hazard to worker safety and resource utilisation in the mining industry, causing a number of mining disasters in Australia and overseas. Despite the wealth of information that exists on frictional ignition, researchers have yet to determine the energy requirements for methane ignition and what practices can minimise frictional energy during roadway development and longwall operations.
Project Objectives
The objectives of this project are to reduce the hazard of frictional ignitions by establishing methods to quantify the risk, develop training and technology transfer methods, and determine the gaps in the knowledge of fictional ignition. The project work includes developing:
- A database of occurrence information and conditions of frictional incidents
- A risk assessment framework to assess the frictional ignition risk
- An indexing method to determine the propensity of stone for incendivity
- A booklet on frictional ignitions targeting mine supervisory staff, which includes a review of relevant research
- A training curriculum and course content for prevention of frictional ignition
- A report detailing latest findings on factors influencing the creation of energy through the cutting process in mines
- Abridged situation report outlining gaps in the information as well as technological shortcomings to adequately address the frictional ignition hazard.
Findings
There have been 50 frictional ignitions of gas in underground coal mines in Australia from September 1988 to August 1999. Forty occurred in New South Wales and 10 in Queensland. While frictional ignitions occurred at 16 different mines, 24 events occurred at only three mines - Munmorah State (10), South Bulga (7) and Southern (7).
Most frictional ignitions (32) have occurred in the Newcastle-Hunter coalfields, but 10 have occurred in the Bowen Basin and five have been experienced in the Southern District of New South Wales. In New South Wales, more ignitions occur in development (29) than in longwall extraction (5), while in Queensland, both aspects of mining have the same number of ignitions.
In most cases, the ignition source is cutter picks striking a high silica material in gassy conditions. In the Newcastle area, this is usually the conglomerate roof. In most other areas, this is the sandstone roof. In Queensland, the incendive material is usually associated with a structure such as stone rolls (4), dyke (1), floater (1), a seam split (1) or a fault (1).
Since 1993, most ignitions have been associated with methane blowers in areas that have been generally well ventilated. While most of these blowers are the result of structures occurring naturally in coal, at least one (Southern 1998) has been associated with a drill hole previously used for methane drainage. Geological discontinuities such as faults, folds, stone rolls and dykes, as well as splits in the seam, are likely to promote fracturing from which blowers develop.
A number of ignitions have been associated with poorly ventilated development sections. Several of the more recent ignitions have been associated with place changing practices.
Using accepted risk assessment methods and collaborating with authorities in the field, an analysis of the factors leading to a frictional ignition was completed. Using the principle of a fiery triangle (fuel + heat + oxygen) without which ignition cannot occur, subordinate fault trees and a questionnaire that can be used by mines to determine the risk were developed. The framework was assessed during exercises at two mines. The risk framework allows mine employees to determine the potential for frictional ignition, establish whether the mine has enough information to properly assess the risk, and assess whether existing controls are sufficient.
Due to difficulties igniting methane during testing, an alternative indexing method was used - the heat given off by the pick striking the rock. The results from the tests indicate that the relative incendivity of rock types can be determined by quantifying the radiant energy produced by a strike. Sufficient differences in radiant energy enabled the samples to be ranked according to their incendivity.
The research has provided sufficient information for mine management to draw up safety plans that could reduce the incidence of ignitions significantly.
Where To From Here
The interaction of different rocks with picks requires further investigation to determine the characteristics, including grain size and strength of the rock, that lead to an increased or decreased generation of frictional heat. The existing indexing method and equipment should be used to build up a base set of information against which other rocks can be assessed.
Further research is also required on the mechanism of frictional ignitions to allow the risk framework developed from this research to be quantified.
In addition, a review of the gaps in the technology found:
- The mechanism of transferring the heat from the hotspot to the methane mixture is still under question and the airflow dynamics might have a bigger influence on the potential for an ignition to occur than was originally thought.
- The influence of nascent carbon in the fuel mix in and around the point of heating should also be investigated further as it is evident from the literature that this could have a significant effect in increasing the sensitivity of the reaction as well as increasing the reaction speed.
Further Information
A short booklet containing the essential aspects of frictional ignition and a training video are available from SIMTARS (phone 07 3810 6333) at cost. The images used in the video and those used in the booklet are similar to ensure visual continuity.