Coal self-ignition is basically a physical-chemical process in which temperature of coal substance increases in consequence of heat accumulation from normal temperature of surrounding environment up to the temperature of ignition.

Coal reacts with atmospheric oxygen, creating heat energy and releasing oxidizing products. Water, carbon dioxide and carbon monoxide release in the first stage of oxidation. Coal substance then releases gaseous hydrocarbons (first saturated, then insaturated and then hydrogen). If sufficient amount of oxygen is supplied, coal substance heats and blazes up in the focus of self-ignition.

On the basis of analyses and research of self-ignition in the mines of the OKD joint-stock company, the causes can be divided into the following, most frequent groups:

- residual coal left in caved areas by objective reasons (e.g. presence of a non-profitable coal seam in the direct overburden of exploited seam; tectonic faults) or by different technological reasons,

- slow advance of longwalls,

- mine air flow through caved areas,

- untight closures of gobs,

- fracturing of rock massif by rock bursts,

- formation of cuts above support during transition of driving operations from rock massif into coal seam and vice versa

- ending of a long mine working in a tectonic fault or in a place with exposed contact between the seam and so-called "varied layers" without its subsequent decontamination or tight enclosure.

- mine air flow through rock massif as a consequence of untight closing dams (flow between seams), etc.

From the viewpoint of mine and staff safety, self-ignition is an unwelcome phenomenon. It is dangerous as it can, under certain circumstances, change into open fire that can, in turn, propagate through mine workings very fast. Fumes from such a fire contain toxic gases (carbon monoxide).

The prevailing part of new self-ignitions has been documented in the saddle seams No. 37, 39 and 40 of the Karvina strata. Data has been adapted from ?1?.

The aim of anti-ignition measures is to prevent access of oxygen to coal. This can be accomplished by various methods, such as regulation of stope ventilation, backfilling of caved areas with nitrogen or nitrogen foam, utilization of fast-setting and filling materials, faster advance, water flooding, utilization of inhibitors and, not least, flushing of ash mixture into caved areas of stopes.

Principle of this method lies in the flushing of caved rocks of an operating stope with ash suspension and, if needful, flotation waste.

In spite of the fact that this method is relatively efficient, it cannot be applied in all cases. There are three known methods of flushing of caved areas of stopes:

flushing using lost piping,

flushing using tow piping,

flushing using flushing boreholes.

The flushing of caved areas of stope using flushing boreholes as a part of the anti-ignition prevention has been applied in many cases. The self-ignition in the Dul CSA mine of the OKD is a good example.

In this mine, the mixture was transported into the assumed site of self-ignition in the caved area of the stope by flushing boreholes ?2?. Fig. 1 shows the flushing of the focus of the self-ignition from the gallery behind the stope by five cased boreholes of the diameter of 75 mm and the length of 6 to 8 m. Total amount of flushed ash mixture reached about 440 cubic metres and the desired effect, i.e. efficient extinguishment of the self-ignition, was accomplished.

This method of flushing the caved area by flushing boreholes is only efficient, if the focus of self-ignition is accessible from one of the galleries, which are driven in an acceptable distance. Drilling in the caved area of the stope is problematic as it is often necessary to flush the near area by a setting mixture and drill it again in order to maintain desired stability of the boreholes.

Hitherto mentioned methods of flushing of the caving area can effectively hit the focus of self-ignition on the ground or in its immediate vicinity. If a self-ignition occurs in a higher part of the stope caving zone, their effectiveness is substantially reduced. However, statistics shows that such self-ignitions represent approximately 40 per cent of the total number of cases ?3?. Transport of flushing mixtures into upper parts of the caving zone from overlying galleries would be a solution, but this is usually impossible due to the liquidation of mine workings.