In high-methane mines of the JSW S.A. ventilation means of fighting methane hazard are insufficient what has caused applying wide-reaching demethanation. Demethanation constitutes an independent from mining heading network of ducts composed of pipelines and demethanation holes. Methane flows to the demethanation network form the surrounding rocks and coal seams, thought the inflow in holes is forced by ty under-pressure created by the methane drainage stations, most often located on the surface.
There are following methods of rock mass degasification:
- Dog heading demethanation,
- Advanced demethanation – led before starting exploitation in the non-unstressed rock mass, where the original stress occurs,
- Exploitation demethanation – led simultaneously with seam exploitation in the rock mass where natural deposit balance was disturbed by exploitation and the demethanation itself is led in rocks with changeable in time stress state,
- Captivating methane from sealed space of old goafs.
In 2009, in JSW S.A. mines 62% of methane intake came from exploitation demethanation, 36% form sealed old goafs and 2% from dog headings. Advanced demethanation is seldom used in the JSW S.A. mines due to poor permeability of coals causing little efficiency of this method.
Rock mass demethanation by means of drainage holes is based on making bunch of holes directly from a heading or niches and connecting them in an appropriate manner with demethanation pipeline. In order to reduce air inflow to the drainage hole it is sealed with cemented pipe. However, despite this fact there is still some air sucked into the hole. During the exploitation additional gaps create, resulting from exploitation stress work, which cause air inflow to the hole increase and in consequence lower methane concentration in captured mixture. Methane-aerial mixture with gas concentration of 50%-60% is captured by means of demethanation. Such manner of methane drainage causes constant mixture expense and methane concentration change with dependence on:
- Barometric pressure fluctuations,
- Gas inflow from drainage holes and from behind sealing dams,
- Holes and down pipelines network leakproofness.
Rock mass demethanation is an effective way of fighting methane hazard, ensuring methane inflow decrease to the work spaces.
Methane drainage effectiveness in the JSW S.A., depending on the mine is 36÷45%.
Rock mass advancing demethanation in Polish conditions due to poor permeability of coal seams is ineffective. Therefore, exploitation heading demethanation technology is based on capturing methane from zones of greater permeability in longwall environment during exploitation. The ways of demethanation used in Polish mines are adjusted to a previously designed exploitation and longwall ventilation system. There are two methods of exploitation heading demethanation:
- By drainage holes drilled from ventilation heading to relaxed zone in roof and/or floor of exploited seam. The method is the basic method, yet both the place of making holes and their parameters are dependent on longwall exploitation and ventilation system,
- By drainage roadways made in seam in the relaxed zone over the exploited seam, called overlying drainage method (Hirchbach) or under the exploited seam, called underlying drainage method.