Review of Explosion Protection of UK Coal Mine Methane Extraction Systems
Methane drainage is used when ventilation alone cannot dilute the concentration of the gas to a safe level. Pumps extract the methane from underground bore-holes and transport it to the surface where it is either vented or utilised as a fuel for steam generation or electrical generation using gas engines. Methane is a greenhouse gas 21 times more potent than carbon dioxide and subject to the Kyoto Protocol. To reduce the burden of greenhouse gas emissions vented from mines, flaring of drained methane has been introduced. This introduces a potential ignition source into the methane drainage system.
At the request of HM Inspectorate of Mines (HMIM), the Health and Safety Laboratory (HSL) has reviewed the methane drainage arrangements of UK deep mines that have introduced methane flaring or utilise mine methane in other ways. Particular attention has been drawn in this study to the existing safety arrangements that prevent transmission of flame from the area of utilisation to the underground workings and to comment on the choice of device.
Advanced Fire Detector for Coal Mines
An analysis of mine fire data showed that mine personnel discovered 73% of fires and smoke events that lead to withdrawals. All types of environmental monitors discovered 13% and the current FIDESCO fire monitoring system, based on the CO electrochemical sensor and the products of combustion (POC) semiconductor sensor, only accounted for 4% of discovered events.
There are now less personnel working underground in a typical mine than in previous years, thus reducing the effectiveness of this means of detection. A recent survey by UK Coal showed that only 59% of FIDESCOs were fully utilised because of its drawbacks, including susceptibility to interferents resulting in false alarms, and instrumental drift. In addition, replacement POC sensors for the current FIDES monitors are becoming increasingly difficult to source.
This joint project (HSE, HSL, UK Coal Mining Ltd and TES Bretby Ltd) was initiated to investigate the performance of proposed solutions to the sensor shortage and to explore the potential of an 'intelligent' monitoring system with an array of sensors. The system should have the sensitivity of existing FIDESCO but not be susceptible to interferents like diesel exhaust, methane and coal dust.
Tests have been carried out by HSL and TES on possible replacement semiconductor gas sensors for FIDESCO. None investigated showed the ideal combination of high sensitivity to CO with low sensitivity to methane found in the current FIDES sensor. Three series of fire tests using smouldering conveyor belt, coal, wood and oil/grease, with and without diesel exhaust as an interferent, were performed in the surface training gallery at UK Coal's Welbeck Colliery. The sensors examined were alternative semiconductor gas sensors; optical (single and dual wavelength) and ionisation smoke detectors (supplied by Kidde plc); and a nitrogen dioxide sensor. Analysis of the data is continuing to determine the optimum combination of sensors. The combination of optical smoke and nitrogen dioxide sensors currently looks promising. The ultimate objective of the project is to produce a prototype (non-IS) fire detector in collaboration with a manufacturer to supersede the FIDESCO monitor.
Evaluation of Bagged Barriers
A new type of stone-dust passive barrier system, originally developed in South Africa, is currently in use in South African and Australian coal mines. The stone-dust instead of being spread on wooden shelves, as in the German and Polish stone-dust barriers, is contained in bags suspended from the roof of the mine roadway. It is claimed that the system is as least as effective as the conventional passive barrier systems, stone-dust or water, with the added advantages of being easier to install and move as well as operating at lower explosion pressures.
UK Coal is seeking approval to use the bagged stone-dust barrier system in UK coal mines. To inform the approval process, HMIM requested assistance from HSL in evaluating the effectiveness of the bagged barrier system. The objectives were:
The project has now been completed and the main findings were:
Taking into account modern mining practices it is considered the bagged barrier systems proposed under the UK scheme will give a level of explosion protection as high, if not higher in certain circumstances, than is currently provided by the Polish stone-dust barriers. The bagged barrier systems also have the advantage of greater flexibility in where they can be installed, easier installation and simpler maintenance.
UPTUN - Upgrading Methods for Fire Safety in Existing Tunnels
Mines Rescue Service Ltd (MRSL) is contributing to this project under the European Commission's (EC) Fifth Framework programme with research on two tasks:
Task 1 was a state of the art review, targeting six specific areas:
Task 2 was development of an evacuation support system. The major area of work has involved the investigation of a purpose-designed active audible/visible beacon system to provide guidance through smoke. The research considered the provision of visible, audible and tactile directional cues, the latter two being useful where there is very poor visibility. Psycho-acoustic, visual perception and ergonomic factors have been taken fully into consideration. To assist in direction finding through heavy smoke, sound localisation techniques have also been considered. The work has resulted in a prototype, proof-of-principle system that will have potential application in supporting evacuation in a variety of tunnels and building structures. The system provides a dual environmental monitoring and evacuation support function and makes novel use of contactless power transfer and telemetry techniques. The project is due to be completed in mid-2004.
Alarm and Evacuation Systems
MRSL completed its work on European Coal and Steel Community (ECSC) sponsored project PR 062 that examined a variety of issues associated with an overall objective of enhancing mine emergency response, involving:
The work complemented HSE-funded research, described in the Mine Environment section below, concerning climatic chamber wearing trials of filter self rescuers (FSR) and self contained self rescuers (SCSR) under a range of hot and humid conditions, together with the simulation of high temperature breathing characteristic of SCSRs and FSRs.
MRSL completed its contribution to ECSC project PR 094 to research and develop the application of water mist firefighting technology. Water mist systems were shown, unequivocally, to have several advantages over other gaseous or sprinkler fire-fighting systems. Water mist systems typically produce water droplets with a volumetric mean diameter of 30 to 300 μm and have flow rates of at least an order of magnitude less than a conventional sprinkler system. A range of nozzle designs, pressure delivery systems and air supply/application methods have been investigated to determine an optimum droplet size range with respect to carrying distance over a range of ventilation parameters. Several system designs were developed and shown to be highly effective against all classes of fire. The development effort concentrated on portable firefighting schemes, but application in fixed systems is equally valid. The additional quenching from using water additives was also shown to be significant.
A subsidiary component concerned fire detection, involved: