# Coal Dust and Methane as a Hazard in Coal Preparation Plants
## 2 Coal Dust Control in Hard Coal Preparation Plants
These devices comply with ATEX Directives. **3. Indexes for Coal Dust and Methane Hazards Estimation in Hard Coal Preparation Plants**  
The analysis of the coal dust hazard in hard coal mines covers the period from 2003 to 2022, which is marked by extremely dynamic organizational changes in the Polish hard coal mining industry. In 2003, there were 41 mines in operation [20], and each operated a preparation plant with a different range of coal beneficiation range. In 2022, there were 20 mines operating 30 mine sites, i.e., practically former mines [21]. There was also a significant decrease in coal production [22] as shown in Table 2 and in Figure 2.  
Information on hazards at workplaces in the coal preparation plants of Polish mines was obtained from annual reports on the state of natural and technical hazards in hard coal  
Figure 1: View of the PASAT sprinkler system. Figure 2: Hard coal production and number of mines in Poland 2003–2022 [22]. mining periodically issued by the Central Mining Institute [13; 14; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. Information on the number of workplaces in coal preparation plants operating in explosive dust atmospheres and the number of workplaces where permissible dust concentrations were exceeded in the years 2003 to 2022 are presented in Table 2.  
In order to observe trends related to changes of the abovementioned hazards in time and in relation to production, it was decided to develop indexes that could allow an estimation of hazards based on the available historical data. Indexes were elaborated for workplaces with explosive dust atmospheres zone 20 (high risk zones) and lower risk zones as the sum of zones 21 and 22. The example mathematical expression for the index IEZ\({}_{20}\) is the following:  
\[IEZ_{20}=\left(\frac{N_{20}}{N_{20}+N_{21+22}}.\frac{1}{P}\right)\cdot 100 \tag{1}\]  
where IEZ\({}_{20}\) is the explosion hazard index for workplaces at high explosion risk. This index measures the proportion of workplaces in explosive dust atmosphere zone 20 (high explosion risk) (\(N_{20}\)) relative to the total number of workplaces in explosive dust atmosphere zones 20, 21, and 22 (cumulative explosion risk), normalized by annual coal production (\(P\)). By multiplying by 100, the index expresses this proportion as a percentage, providing a standardized indicator to assess and compare the relative risk of zone 20 workplaces across different years or production scales. This metric highlights the specific contribution of high-risk areas to overall safety concerns in coal preparation plant operations. Similarly, index IEZ\({}_{21+22}\) indicates workplaces where the number of explosion hazard risk is low and is derived in similar way as IEZ\({}_{20}\). In Figure 3 the total number of workplaces in explosive dust atmosphere zones (sum of zones 20, 21, and 22) was plotted against yearly coal production. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Coal** & **No. of** & **No. of** & **No. of** & **No. of** & **No. of** \\
**Year** & **Production,** & **Workplaces** & **Workplaces in** & **Workplaces in** & **Workplaces** & **Zone 2** \\
**Million Mg** & **in Zone 20** & **Zone 21 + 22** & **in MAC** & **the MalThe plot in Figure 3 demonstrates a strong correlation between the decline in coal production, driven by mine closures, and the reduction in workplaces located in explosion hazard zones within coal preparation plants. The Pearson correlation coefficient for this dataset is 0.945, indicating a very strong relationship. Despite the overall downward trend in the total number of workplaces in explosion hazard zones, the proportion of workplaces in zone 20 (high risk) has been increasing, even as coal production continues to decline (Figure 4). The dynamics of the change in calculated indexes IEL\({}_{20}\) and IEL\({}_{21+22}\) can be observed in Figure 5 where an upward trend is visible for both indexes. Despite the fact that the coal production is decreasing, the relative number of workplaces with dust Explosive Atmospheres is increasing.