An analysis of self-ignition of mine waste dumps in terms of environmental protection in industrial areas in Poland

In the case of the first studied facility—Facility (I), thermo-visual examinations did not demonstrate any signs of thermal activity in the majority of the area. Above all, the new layered sections of the facility are free from thermal phenomena. However, the central section of the dump is characterized by rather intense thermal phenomena. The cone is also thermally active, yet in this section the thermal activity is not very intensive (see Fig. 1). In the section with the strongest thermal activity, i.e. the southern part of the top, the recorded surface temperature exceeded 600 °C (see Fig. 2). Outside the thermally active zone, the surface temperature did not exceed 25–35 °C.

The tests conducted on the premises of Facility II demonstrated the lack of thermal activity in the majority of the area. Only in the central section, on the top of the older part of the dump, the measurements showed the occurrence of thermal anomalies (see Fig. 3). These were minor areas located along the edge of the scarp on the northern and southern sides where the measured surface temperature was above 80 °C. It is worth mentioning that in 2010, the recorded temperatures for this section were at the level of several hundred degrees Celsius. Outside the sections, the temperature did not exceed 25–40 °C. The relatively high temperature in the area without thermal activity may be explained by the fact that in the morning hours it was exposed to the sun.

During the course of the thermo-visual examination of the extractive waste dump in Facility III, no thermal activity was observed in the majority of the area. Only in the central section of the Facility, in the area which is still in operation, the measurements demonstrated the occurrence of thermal anomalies (see Fig. 4). These were minor areas where the local measured surface temperature was above 200 °C. Outside these areas, the temperature did not exceed 20–30 °C.

The simplest method of analyzing the differences and similarities between the studied objects is their visualization in the space of measured parameters. When two or three parameters are measured, such visualization is not problematic. However, in our study there are nine measured parameters. The graphic presentation of a 9-dimensional space is not possible. This is the rationale of applying the HCA which enables to analyze the similarities between the studied objects (three different dumping facilities in different periods of time) as well as the similarities between the parameters in object space. Nevertheless, the HCA does not allow to simultaneously analyze the relationships between the objects and the measured parameters. This problem was solved by the use of a color map of the experimental data, which enabled an in-depth interpretation of the data structure. In addition, the application of the color map facilitates highlighting the differences and similarities among the clusters showed in the dendrograms, and, in consequence, it helps to distinguish the facilities which are characterized by the highest or the lowest values of the measured parameters. Figure 5 demonstrates the dendrogram for 21 objects representing the studied dumping facilities in different periods of time in the space of 9 measured parameters, the dendrogram for the measured parameters in the object space as well as a color map presenting the values of the measured parameters for particular objects.

Based on the dendrogram presented in Fig. 5a, a clear distinction of the examined objects representing the discussed dumping facilities into two clusters—A and B can be observed. Cluster A includes all samples representing Facility II in the whole period of the monitoring as well as samples representing Facility I taken in Quarter 3, 2017 and in Quarters 1–4, 2018 (objects nos. 1 and 3–6). All samples representing Facility III as well as two samples representing Facility I taken in Quarter 4, 2017 and in Quarter 4, 2018, respectively (objects nos. 2 and 6) are collected in Cluster B. In addition, within each of the clusters certain sub-clusters may be distinguished. Within Cluster A, the following sub-groups can be observed:

• sub-group A₁ collecting all samples taken from Facility II during the whole period of the monitoring (objects nos. 8–14);

• sub-group A₂ including samples taken from Facility I in Quarter III, 2017 and in Quarters 1–4, 2018 (objects nos. 1 and 3–6).

• In turn, Cluster B contains the following three sub-groups:

• sub-group B₁ collecting two samples taken from Facility I in Quarter 4, 2017 and 2018, respectively (objects nos. 2 and 6) as well as samples representing Facility III in Quarter 3, 2017, Quarters 1–3, 2018 and Quarter 1, 2019 (objects nos. 15, 17–19 and 21);

• sub-group B₂ encompassing the remaining two samples taken from Facility III in Quarter 4, 2017 and in Quarter 4, 2018 (objects nos. 16 and 20).

• The dendrogram obtained by means of Ward’s linkage method for the measured parameters in the space of 21 objects (see Fig. 5b) enables to distinguish the three principal clusters of parameters listed below:

• class A collecting parameters nos. 2, 3 and 4 (describing the concentrations of acenaphtene, fluorene and phenanthrene);

• class B containing parameters nos. 1 and 9 (describing the concentrations of naphthalene and chrysene);

• class C including the remaining parameters nos. 5, 6, 7 and 8 (describing the concentrations of anthracene, pyrene, fluoranthene and B(a)anthracene).

The PAHs emissions from a burning mine waste dump must be carefully monitored due to their potential toxicity and genotoxicity^38,39. PAHs have two different roots; one is incomplete combustion of organic matter, whereas the other one is their production in the geological formation when organic sediments were chemically transformed into fossil fuels. In our study, the first path, namely spontaneous coal waste combustion is observed^40,41. It is also worth mentioning that PAHs pose significant human health hazards. The exposure to PAHs may result in skin, lung or stomach cancers in the human organism²⁸.

As mentioned before, a considerable drawback of the Hierarchical Clustering Analysis lies in that it does not allow for simultaneous interpretation of the dendrograms describing objects which represent particular samples taken from the extractive waste facility in different periods in the space of measured parameters and parameters in the object space. The lack of the possibility of interpreting the above relationships significantly limits the knowledge of the studied phenomena because the purpose of the analysis is to determine not only the differences among particular samples but also the underlying cause of such differences. Therefore, the dendrogram presenting the examined samples taken from the three selected facilities in different periods of time (see Fig. 5a) was juxtaposed with a color map of the experimental data (see Fig. 5c) demonstrating the values of the measured parameters arranged according to the order of the objects and parameters organization. The juxtaposition enables to determine the reason why the examined samples were distributed in such a way. In addition, the interpretation of the dendrogram for the objects in parameters space complemented with the color map of experimental data allows distinguishing samples which are characterized by the highest values of the measured parameters.

Analyzing the dendrogram presented in Fig. 5a together with the color map of the experimental data, it can be observed that all samples within Cluster A were characterized by relatively lower values of the measured parameters. Moreover, sub-group A₁ identified within Cluster A was characterized by relatively lowest concentrations of acenaphtene, fluorene, phenanthrene and pyrene (parameters nos. 2, 3, 4, and 7) among all of the examined samples taken from the three selected facilities in the whole period of the monitoring. It confirms the observations which were made previously with the use of a thermo-visual camera which demonstrated that there were no thermal phenomena in the majority of the waste dump areas. The observed sparse emissions of gases result from certain thermal anomalies occurring in the central section of Facility II as well as some small areas located along the northern and southern side of the scarp; however, in any of the places the temperature did not exceed 80 °C (see Fig. 3). The thermo-visual examinations of the remaining sections of the facility showed that the surface temperature did not exceed the level of 25–40 °C and it is a result of sun exposure rather than the occurrence of any thermal phenomena.

Sub-group A₂ which includes all samples taken from Facility I in Quarter 3, 2017 and Quarters 1–4, 2018 (objects nos. 1 and 3–6) differs from A₁ objects in terms of relatively higher concentrations of acenaphtene, fluorene, phenanthrene and pyrene (parameters nos. 2, 3, 4, and 7). Additionally, within sub-group A₂, the uniqueness of sample 1 taken in Quarter 3, 2018 (object no. 5) can be observed; it was characterized by the lowest concentration of chryzene (parameter no. 9) of all the examined samples.

Despite the fact that the results of the thermo-visual analysis of Facility III in general did not show any major signs of thermal activity, the Hierarchical Clustering Analysis by means of which the samples were examined in terms of the emission of the 9 parameters indicated that some thermal phenomena take place at this dump.

Furthermore, the analysis of the color map of the experimental data for the objects grouped in Cluster B makes it abundantly clear that thermal activity takes place in the majority of the monitored areas located at Facility III. What is more, for two samples taken from Facility I in Quarter 4, 2017 and Quarter 4, 2018 (objects nos. 2 and 6), thermal activity was observed. For the two objects, the concentrations of acenaphtene and fluorene are the highest among all of the examined samples (parameters nos. 2 and 3); similarly, the concentrations of phenanthrene, pyrene and B(a)anthracene (parameters nos. 4, 7, and 8) are also high. It confirms the observations made by means of a thermo-visual camera which demonstrated that there were no thermal phenomena if one considers the facility in its entirety. However, there are certain areas in the central section of the facility where thermal phenomena occur.

In the case of the samples taken from Facility III in Quarter 3, 2017 as well as in Quarter 2 and Quarter 3, 2018 (objects nos. 15, 18, and 19) within sub-group B₁, the values of all measured parameters are only slightly increased, which can indicate the occurrence of thermal phenomena. Yet, it must be kept in mind that the concentrations are decidedly lower than for the remaining samples taken from Facility III. It may be also an indication that self-heating of the coal extractive waste takes place. The other two samples taken from Facility III in Quarter 1, 2018 and Quarter 1, 2019 (objects nos. 17 and 21) are characterized by relatively highest concentrations of pyrene and B(a)anthracene (parameters nos. 7 and 8) as well as high concentrations of anthracene and fluorene (parameters nos. 5 and 6), which can result from an intense thermal activity.

A similar observation can be made for the two samples taken from Facility III in Quarter 4, 2017 and Quarter 4, 2018 (objects nos. 16 and 20) which are characterized by the highest concentrations of naphthalene, anthracene, fluoranthene and chrysene of all the examined samples (parameters nos. 1, 5, 6 and 9) as well as high concentrations of phenanthrene (parameter no. 4). Although the results of the thermo-visual analysis of the whole area of Facility III did not demonstrate signs of thermal activity in most of its sections, there exist certain spots with intense thermal processes, which is confirmed by relatively high values of all the measured parameters.

Source: Ecology - nature.com