**4. Conclusions**

Spontaneous combustion of coal is a highly dangerous phenomenon that occurs during mining exploitation. It leads to major economic losses for mining enterprises and poses a threat to the working crew. The products of coal combustion are also highly damaging to the natural environment. This is because they penetrate into the mining atmosphere along with the ventilation stream, and then to the surface into the natural environment through the ventilation system. As a result, it is necessary to undertake various steps to reduce the risk of these phenomena in mines.

The method developed and presented in the paper for determining the zone with a particularly high risk of spontaneous coal combustion, in this case for longwalls ventilated with the Y-type system, serves this purpose. The determination of such a zone may serve as the basis for taking effective preventive measures. They involve choosing the exploitation speed, isolating this zone, introducing inert gases, sealing the goaves, etc. For these measures to be effective, it is necessary to identify the sites (areas) where spontaneous coal combustion may occur.

It must also be stressed that mining exploitation also generates other hazards that determine, for example, the application of different ventilation systems. Generally speaking, the Y-type ventilation system under analysis is highly favourable in the case of methane threats, and slightly less favourable

for the threat related to spontaneous coal combustion. Therefore, the tests conducted are of a particular significance in the process of limiting this threat.

The method developed, thanks to the application of advanced spatial models and the use of actual measurement data from the analysed region, allows for early identification of areas in which spontaneous coal combustion may occur.

The results obtained clearly indicate that the demarcated risk zones of spontaneous coal combustion are significantly higher for this ventilation system than for the U-type system.

The comprehensive analysis also indicates that the type of roof rocks forming the caving has a significant impact on the size and location of the zone with a particularly high risk of spontaneous coal combustion. The di fferent tensile strength of these rocks leads to changes in the porosity and permeability of the caving, which in turn has a significant impact on the ventilation parameters of the air flowing through the caving. No such tests have been conducted so far, and the results obtained indicate the significant changes in the location and extent of the risk zone along with the changing values of this strength.

The results obtained also enhance knowledge about the ventilation of underground exploitation regions and should become an important source of information for the ventilation service teams in mines. In particular this concerns the essential di fferences in this process for the U-type and Y-type ventilation systems, as unequivocally indicated by the results obtained.

These results also demonstrate the grea<sup>t</sup> impact exerted on the ventilation process in mine headings by the goaves with caving, which—due to their porosity—must be taken into consideration in this process.

The methodology developed and presented in the paper is of a universal nature and may be successfully applied to multivariate analyses of the spontaneous combustion hazard, as well as on the mining landfill sites.

The authors hope that the results obtained and the methodology developed will find broader application for the support of preventive measures in terms of limiting the risk of spontaneous coal combustion.

**Author Contributions:** Conceptualization, M.T. and J.B.; methodology, J.B. and M.T.; software, M.T.; validation, J.B. and M.T.; formal analysis, J.B. and M.T.; investigation, J.B. and M.T.; resources, J.B. and M.T.; data curation, J.B. and M.T.; writing of the original draft preparation, J.B. and M.T.; writing of review and editing, J.B. and M.T.; visualization, M.T.; supervision, J.B.; project administration, J.B.; funding acquisition, M.T. and J.B.

**Funding:** This research was funded by SUBB, gran<sup>t</sup> numbers 06/030/BKM\_18/0041, 06/030/BKM\_19/0048.

**Conflicts of Interest:** The authors declare no conflict of interest.
