The analyzes of the releases of selected gaseous components were carried out for 15 samples of bituminous coal exposed to 20 MPa, simulating the pressure of the rock mass at the depth of 800 m and at the temperature of 20, 30, 40 and 50°C, simulating the primary pressure of the rock mass. temperature in the coal seams studied. The results obtained demonstrated that the desorption of carbon monoxide increases rapidly with the increase in the primary temperature of the rock mass and that the quantity of carbon dioxide released varies according to the samples (Fig. 2).
Figure 3 presents these values classified according to the increase in the primary temperature of the rock mass.
In general, carbon monoxide levels at low primary temperatures (26–36°C) were low and increased above 41°C. The exception was reported for samples 4, 5, 9, and 15, which exhibited high concentrations of carbon monoxide despite the fact that the temperature of the rock mass occurring in these veins was low and ranged from 21 to 23° vs.
The correlation between the desorbed carbon monoxide content of a coal sample and the respective moisture content and the value of the auto-ignition index was also observed (Fig. 4).
As can be seen in Fig. 4, significant amounts of carbon monoxide were desorbed from the previously mentioned samples 4, 5, 9 and 15, characterized by the highest values of the autoignition index (autoignition groups IV and V) (Table 1). Interestingly, samples 12, 13 and 14, with relatively low values of the auto-ignition index, also demonstrated high concentrations of carbon monoxide. In Table 2, the amounts of carbon monoxide recalculated by mass of dry ashless coal substance are presented.
Values of total moisture content in the coal samples tested (Table 1) ranged from 1.2% w/w for sample 8 to 19.1%w/w for sample 4. It has been reported previously that the sorption capacity of coal may decrease with high ash content33. In the study presented in this article, however, the ash content was low, in the range of 1.6% w/w (sample 3) to 10.1% w/w (sample 5) and therefore no d direct impact of this parameter on carbon monoxide. desorption was observed.
The heat of oxidation depends on the mass of coal that undergoes the oxidation process. The content of ashless dry coal substance in the examined samples, zcsw, was 72.6% w/w for sample 4 to 96.4% w/w for sample 3 (Table 1). Figure 5 shows the volumes of carbon monoxide released for temperature conditions characteristic of particular seams and recalculated to the mass of dry ashless coal substance.
The higher the auto-ignition group, the more carbon monoxide emission is still observed after the above recalculation and taking into account the ash and moisture content of the coal samples tested. Another correlation, not related to the auto-ignition group of the coal, but to the primary temperature of the rock mass, was also reported for samples 12, 13 and 14.
Research studies on the adsorption/desorption of gases on/from bituminous coals reported in the literature most often refer to the combination of carbon dioxide and methane34. Based on knowledge of the structure of bituminous coals, and in particular the nature of the coal surface as well as studies of sorption processes, the authors presented interpretations of the research results with reference to changes occurring during the self-heating of coal. However, the conditions of the experiments used in these studies poorly reproduce the conditions of a coal mine, to mention only the mass and the granulation of the coal and the vacuum degassing of the samples.
Natural desorption of gases from bituminous coal results in the release of methane into the atmosphere along with smaller amounts of ethane, propane and carbon dioxide35,36,37. Extensive studies in this area have been carried out by Xu et al.38.39 and Chen et al.40. Exposure of coal to air results in parallel processes of coal oxidation and desorption of carbon monoxide and dioxide41. The product of early coal oxidation, detected in the highest concentrations and at low temperatures, both under underground conditions and in the laboratory, is carbon monoxide. Trace amounts of other gases make them difficult to detect at this stage42. In the event of natural desorption, the increase in the carbon monoxide content takes place. Fan et al.43 on the basis of the experimental study on the sorption of primary carbon monoxide in coal seams concluded that its adsorption mechanism is similar to the adsorption of methane. The applicability of the method initially dedicated to the measurement of the quantity of methane, also to carbon monoxide, has been demonstrated. The study results also confirmed that primary carbon monoxide is present in the Xichuan mine, and the amount of this gas changes with location, reaching the value of 487.79 × 10–6 m3CO/Mg and is not related to the carbon self-heating process. Three explanations for the presence of carbon monoxide have been given: (i) release during coal mining, (ii) release through cracks in rock strata caused by coal mining and (iii) the oxidation of residual carbon.43.
Zhang et al.44 measured carbon monoxide emission under non-isothermal conditions using an apparatus of their own design and construction, and coal type, grain size, and carbon oxygen content as variables adopted in the experimental procedure. They found that applying coal with larger particle diameters resulted in lower carbon monoxide emissions, especially at higher temperatures.
Xu et al.45 also studied the process of self-heating of coal at low temperatures. In their experiment, concentrations of gaseous products as well as the temperature inside the research vessel were measured when the coal samples were subjected to heating. Based on the data analysis, four stages of the oxidation process were identified: (i) preliminary heating without oxidation reaction or volatilization, (ii) volatilization without oxidation, (iii) unevenly accelerated oxidation with parallel vaporization , and (iv) late heating step, when the entire volume of oxygen is consumed in the oxidation reaction. Silk17 also confirmed the correlation between coal particle size and intensity of carbon monoxide release: the finer the coal sample was ground (1-3 mm) with a ball mill, the greater the amount of carbon monoxide. high. The concentration of carbon monoxide released was also lower for the coal samples with higher moisture content.
Xi et al.46 found that the presence of certain oxidation products can facilitate the identification of particular phases and intensity of the carbon self-heating process. The order in which the products appear varied between the types of coal. However, the mechanism of these phenomena, and in particular the effects of various factors on the initiation and development of the self-heating process, has been reported to be still unknown.
In the research work presented in this article, the share of grains greater than 0.7 mm constituted more than 70% of the coal sample; therefore, analyzes performed with porosimeters would not reflect coal mine conditions. The experimental bench used in the study allowed the sampling of desorbed gas at ambient pressure and without dilution effect derived from air.
Based on the research results presented in this article, it can be concluded that the most moisture-rich coal samples (samples 4, 5, 9, and 15, Table 1) emitted the greatest amounts of carbon monoxide. However, a multidimensional approach is needed to accurately estimate emissions of carbon monoxide and other gases; the coal auto-ignition group or auto-ignition index, as well as the primary temperature of the rock mass occurring at the given seam depth must also be taken into account. A complex analysis of these dependencies is shown in Fig. 6.
As mentioned previously, the reported amounts of desorbed carbon monoxide were high for samples from the high carbon auto-ignition groups (IV and V): samples 4, 5, 9 and 15, respectively acquired from the veins 212, 215, 206 and 207, at shallow depth, where the primary temperature of the rock mass only slightly exceeds 20°C.
Another strong correlation was observed for the primary temperature: samples 12, 13 and 14, although autoignition groups I and II, also showed high amounts of desorbed carbon monoxide, which resulted from relatively primary temperature elevations of the rock mass, 41, 43 and 48°C, respectively. At such primary temperatures, coals that are theoretically not very prone to auto-ignition, taking into account their auto-ignition groups, release under mine conditions quantities of carbon monoxide comparable to coals particularly prone to the combustion process. self-ignition.
In summary, based on the experimental results presented in this article, the coal samples studied can be divided into the following groups:
Coals emitting less than 0.001 m3CO/mgcsw; with a primary temperature of the rock mass below 40°C and self-ignition groups II and III (samples 1, 2, 3, 6, 7, 10 and 11).
Coals emitting at more than 0.001 m3CO/mgcsw; with a primary mass temperature above 40°C and self-ignition groups I and II (samples 12, 13 and 14).
Coals emitting at more than 0.003 m3CO/mgcsw; with a primary temperature of the rock mass below 25°C and auto-ignition groups of IV and V (samples 4, 5, 9 and 15).
The primary temperature of the rock mass surrounding the longwall significantly affects the amounts of carbon monoxide released, and therefore testing of coal at laboratory temperature leads to erroneous conclusions. The same coal at laboratory temperature will emit much lower concentrations of carbon monoxide than at the primary temperature of the rock mass. The higher the temperature of the rock mass, the greater the error in the correct assessment of fire risk. Since concentrations of carbon monoxide and other gases are commonly used as indicators of the progress of coal self-heating, gas desorption and factors affecting its intensity must be considered, especially when coal mining takes place at deeper and deeper depths.