Potential of CBM as an Energy Vector in Active Mines and Abandoned Mines in Russia and Europe

Abstract

The energy sector is in the spotlight today for its contribution to global warming and its dependence on global geopolitics. Even though many countries have reduced their use of coal, the COVID-19 crisis, the drop in temperatures in Central Asia, and the war between Russia and Ukraine have shown that coal continues to play an important role in this sector today. As long as we continue to depend energetically on coal, it is necessary to create the basis for the successful extraction and industrial use of coal mine methane (CMM), for example, as an unconventional energy resource. Early degassing technology is a technique that allows for the extraction of the methane contained within the coal seams. The application of this technology would reduce emissions, improve mine safety, and even increase their profitability. However, this technology has been understudied and is still not implemented on a large scale today. Moreover, mines with this technology generally burn the extracted methane in flares, losing a potential unconventional fuel. This study, therefore, presents different scenarios of the use of coalbed methane (CBM), with the aim of generating an impact on pollutant emissions from coal mines. To this end, a model has been designed to evaluate the economic efficiency of degasification. In addition, an emissions analysis was carried out. The results showed that the use of this technology has a negative impact on the economy of mines, which can be completely reversed with the use of CBM as fuel. Furthermore, it is observed that degasification, in addition to reducing the number of accidents in coal mining, reduces emissions by 30–40%.

1. Introduction

Nowadays, the entire world is developing new energy systems capable of solving environmental problems caused by the use of fossil fuels and traditional technologies [1]. This scenario has a greater impact on countries whose energy system is based on coal and natural gas, such as China and Russia [2]. China is the largest contributor of CO₂ emissions with 27% of total emissions and Russia ranks fourth with 4.7% of emissions [3]. Consequently, seams essentially improve the coal mine technologies to minimize the environmental impact, as the efforts of other countries, such as the EU, do not have a large impact on total emissions. The development of greenest coal mines will contribute to achieving the environmental goals of countries such as China, which aims to be carbon neutral by 2060 [4].

Despite the decrease in the use of coal for energy production in the last years, the global share of the total energy supplied by this source in 2019 was 26.8%, with 162.4 EJ [5]. During this year, Russia’s coal production was 430 Mt, making it the sixth largest producer in the world [6,7]. The influence of coal in this country is also demonstrated by the presence of many companies dedicated to this fuel. At the end of 2015, there were 245 coal mining and processing enterprises in the country; 70 of them are mines, 119 opencast mines, and 56 processing plants [8]. In the rest of Europe, coal continues to be important, but is losing strength due to the increased use of other energy sources. In 2020, the total coal production in the EU was 296 Mt, with Germany (101 Mt) and Poland (96 Mt) as the main producers (Figure 1) [6]. This amount has been slightly increased during 2021 due to changes in energy geopolitics resulting from the war between Russia and Ukraine. In 2015, there were about 128 coal mines and 207 coal power plants in different countries of the EU. The largest number of coal mines is in Poland (35), followed by Spain (26), Germany and Bulgaria (12 each). Nowadays, around 50 of these mines have been closed across Spain, Czechia, Germany, Hungary, Poland, Romania, Italy, Slovakia, Slovenia, and the United Kingdom [9].

Coal mines present a huge impact on the surrounding environment, polluting water [10], producing greenhouse gas emissions [4], and air pollution [11,12]. Moreover, coal extraction leads to collateral environmental problems as the methane contained in the coal seams escapes into the atmosphere. Coal mine methane (CMM) is the gas emitted related to mining activities either from the coal seam itself or from other gassy formations underground. These emissions occur in both underground and opencast mines, whether abandoned or active. Due to their design, underground mines are a source of methane storage, which can be captured to avoid emissions and be used as an energy source [13]. Methane captured during the coal production is named coalbed methane (CBM). The process of extracting CBM is called early degasification technology and allows to increase safety in mines operations, improve the mining efficiency, and reduce greenhouse gas emissions. Degasification is applied in other countries such as Australia and especially China [14]. Experience with this technique reveals that the most relevant parameters for its application are the extraction periods, the difference between coal layers, the location of undergoing extraction, the method used and the stress state of the coal beta [15]. As established by Haifeng Wang et al., CBM extraction is classified according to the stage at which extraction occurs (pre-mining, during coal-seam mining, and after coal-seam mining), the type of coal seam (adjacent coal-seam CBM extraction and adjacent rock seam CBM extraction), the method used (ground well CBM extraction, eddying borehole CBM extraction, crossing borehole CBM extraction, etc.) or the stress state of the coal seams (original coal-seam CBM extraction and pressure-relief coal-seam CBM extraction) [14,16].

The extraction of the CBM or degasification is generally carried out to reduce accidents due to explosive atmospheres. For this, there are different techniques such as the injection of foams or nitrogen, being the last the most used [17]. In this way, the volume of CBM is displaced and gas is extracted, predominantly methane, which, as previously indicated, is usually burned in a chimney [18]. Early degassing technology, however, aims to act before starting the activity in the mine. In fact, this activity could start between 5 and 6 months before normal operation. It is also applicable to abandoned mines [19].

The main alternatives to early degasification are mainly venting and fracturing the rock with foams [20]. The first technique is the most widely used in coal mining [21]. This depends on a rigorous control of the methane content in the mine gallery to reduce its content below 4%, requiring evacuation from 2% for safety [21]. It is a very mature and reliable technology, although it cannot reduce accidents 100%, and what is more important, it does not prevent methane and carbon dioxide emissions into the atmosphere. Ventilation is essential whatever technique is used [22]. Froth rock fracturing is typically carried out during normal mining operation. This can produce movements in the rock and therefore leaks into the mine. The early degasification aims to reduce the probability of accidents considerably and is also capable of retaining greenhouse gases. If this unconventional gas is used as an energy source, the positive impact on the environment is significant.

Besides the environmental issues, the relevance of solving the problem of the extraction and widespread use of coal methane in Russia is due to the following factors, low level of mining safety (continuous accidents with significant casualties) [23,24], the impossibility of providing modern mining rates without the use of effective degasification with the methane extraction from massif containing coal gas [25], the Decree of the Government of Russia No. 410 of 1 July 2005 on increasing fines for methane emissions by 1000 times [26], and unfulfilled international agreements of the Russian Federation in the field of environmental protection. Mine safety could be one of the most important parameters to boost the degasification technologies as, nowadays, the problems derived from the formation of explosive atmospheres still pose serious accidents and constitute a problem regarding worker safety in mines [27]. Correct management of the safety aspect helps in the stability and efficiency work environment [28], but even applying these measures, accidents in coal mines still happen [29]. The effect of reduce the methane contain in mines is clear since it drives to a safer work environment, so its impact is well known. Nevertheless, this fact must be considering as one of the most important positive effects of the early degasification technology.

As overmentioned, early degasification technology is able to solve environmental problems as it allows the use of CBM as non-conventional fuel [29]. A recent analysis of the coal Russian mines shows that the use of degasification technology does not reach 40% and only 35% of the methane is extracted in the same plants where it is degasified [30]. In some mines, this percentage is used as a fuel for the combustion chambers to produce heat, but also there are some isolated cases where the methane extracted is used as an energy source to produce electricity (Kuzbass mine), considerably improving environmental and economic factors [31]. Then, the use of the early degasification methodology is not widely extended in Russia and the current technology generates low-quality gas. A better understanding of the physics that controls the CBM accumulation can provide better guidance for the early degasification engineering [31,32,33,34].

Most mines are heavily dependent on fossil fuels such as heavy fuel oil (HFO), diesel, or coal to produce electricity for self-consumption. Covering this dependence with renewable energies is currently impracticable due to the intermittency of this type of energy. Underground coal mines require a constant 24/7 power supply. In addition, many mines in Russia are isolated and in remote locations, which complicates their electrification. However, some studies have carried out environmental and economic analyses on the implementation of renewable energy in coal mines, which show substantial improvements [35]. The literature review reveals that little attention has been paid to economic and environmental feasibility analyses of degasification technology, and even less so in Russian mines and abandoned mines of UE [35].

The extraction of CMM avoids its emission into the atmosphere. Therefore, its use should result in a positive emissions balance. This is observed with other technologies, such as the production of biogas from a slurry, which, according to RED II, implies an emission reduction of 200% [36]. However, a more detailed environmental study needs to be carried out, including different models of use.

In this sense, this study aims to provide a new approach to sustainable mining using CBM as an energy vector. To this end, this research will contribute to the development of early degasification technology in three main areas. Three scenarios of CMM use will be defined: (1) a first scenario in which this technology is not applied, a second scenario in which CMM is extracted and flared, and a second scenario in which CBM will be used for energy purposes. An emissions analysis and a new economic model based on the economic efficiency of the majority operations affecting each process will be applied to each scenario. From the third scenario, the feasibility of using CBM as a fuel for road transport in coal mines will be analyzed (2). Finally, the potential use of this technology in Russian and EU mines, both active and abandoned, will be analyzed (3).

2. Methodology

2.1. Scenario’s Definition

The present study establishes three different scenarios for degasification. Figure 2 represents the scenario analysis procedure. The first one releases methane to the atmosphere as it is the traditional procedure. In this case, the CMM is released during normal mine operation. This scenario is expected to have a higher impact as 1 ton of methane produces as much greenhouse effect in the atmosphere as 21 tons of CO₂. In scenario 2, early degasification technology is applied. This scenario includes a torch that burns the CBM, so methane emissions are substituted by combustion product emissions (mainly carbon monoxide and dioxide). To properly assess degasification technology in terms of environmental impact, it must be taken into consideration as well that the environmental efficiency of CBM extraction is determined by direct methane emissions and the greenhouse emissions produced by CBM combustion.

The last scenario considers extraction and use of methane as fuel gas. Therefore, the energetic recovery of methane is considered. CBM will be used as an energy resource considering the energy and transport demands of the mine. The variety of mines found in Russia and Europe makes the analysis difficult, so a typical mine with a production of 4000 t/day will be studied [28]. Given that 50.5 kWh are consumed to extract one ton of coal, it is assumed that the studied mine demands approximately 200 MWh/day [35]. This value is assumed constant throughout the year, discarding peak loads from the analysis. In addition, the potential use of CBM as a fuel for heavy road transport will be considered. Generally, coal is transported by maritime or rail routes. However, it can be assumed that 20% of this transport takes place by road [37]. Therefore, considering that a heavy-duty truck loads approximately 60 t of coal per day, 67 trucks will be needed.

In view of the above, comparison between different scenarios allows for determining the environmental consequences of the early degasification implementation and the use of CBM as an energetic resource. This comparison is carried out by two procedures: environmental assessment and economical assessment. The study focuses on underground mines as they are still a source of methane storage today. Abandoned mines, especially in Europe, will also be taken into consideration.

2.2. Environmental Assessment of Degasification Technology

As a first step in the environmental evaluation, the present study focuses on gas emission assessment based on the gas emissions produced in this type of mines. The emissions are assessed by considering the greenhouse power of the different gases.

Therefore, the main pollution factors that affect the atmosphere in the different scenarios are:

• Scenario 1: The direct CMM emissions;
• Scenario 2: The combustion products after the burning CBM and the harmful gases of the equipment used during the early degasification;
• Scenario 3: Again, the gases of the equipment used during the degasification and combustion products are emitted. However, if methane is used as fuel gas, there will be a reduction in combustion emissions from other sources.

Scenario 1 emissions are directly calculated as no reaction takes place and therefore, these values are normally well known in a mine. Methane emissions have been estimated considering that the mine emits the same amount of methane as is extracted by early degasification, but naturally, through production wells and leakages. The greenhouse effect impact directly derived from methane is 21 times higher than that of CO₂ [38].

To properly address scenarios 2 and 3, several factors have to be considered besides the amount of CBM. The quality of the extracted methane depends on each mine, specifically on the resource conditions, but normally, at a depth between 400 and 500 m, the CBM steam composition is about 90–98% of methane, 0.06–1.8% of heavy hydrocarbons, and approximately 1% of hydrogen, without sulfur compounds [39]. The gas extracted composition is mainly methane, so the combustion reaction is:

CH₄ + 2O₂ → CO_{2 +} 2H₂O

The main products of this combustion are water steam and CO₂, together with CO (due to incomplete combustion reactions) and NO₂ [40].

Considering these reaction products, the total GHG emissions can be obtained by addressing the different gas products. Those products are calculated according to the following equation:

M_CO,y = C_CO·B_M·(1 − q_M)

(1)

where:

• M_CO,y is the carbon monoxide efficiency during the combustion of the extracted gas, tons;
• B_M is methane consumption in the combustion. In other words, the extracted methane, m³;
• q_M is the methane composition of CBM steam (estimated at 90%);
• C_CO is the calorific potential associated with incomplete combustion calculated by Equation (2), exposed below.

C_CO = q₃·R·Q^r_i

(2)

Being:

• q₃ is the chemical incompleteness of the combustion (estimated at 2%) [40];
• R is the loss heat coefficient regarding the chemical incompleteness, which is considered to be 0.5 [40];
• Q^r_i is the methane calorific value (0.0359 GJ/m³) [41].

Additionally, the nitrous oxides emissions (specifically NO₂) are addressed through the following equation:

C_CO = q₃·R·Q^r_i

(3)

where:

• K_NO2 is a parameter that characterizes the number of nitrogen oxides generated per 1 GJ of heat (0.08 kg/GJ) [41];
• β is a coefficient that considers the degree of reduction of nitrogen oxide emissions as a result of the application of technical solutions (estimated at 0).

Once the emissions of carbon monoxide and nitrogen oxides have been calculated, the CO₂ emissions during methane combustion can be estimated based on the natural gas emission factor, which is 2.16 kg CO₂/m³ of natural gas [42].

In scenario 3, the GHG emissions from methane combustion and from equipment associated with to the early degasification are the same as in scenario 2. The difference between scenario 3 and 2, is that due to the use of methane as an energy source, CO₂ emissions from the generation of energy from other sources are offset.

The calculation of these parameters will allow to evaluate and quantify the reduction of gas emissions.

2.3. Economical Assessment of Early Degasification Technology

The gas accumulation problem directly affects mine stability and economy. There are numerous factors that produce a gas reservoir in mines [43], but nowadays it is possible to predict CBM production, which makes the early degasification technology easier to implement in current mines [44].

In recent years, CMM extraction has been developed intensively due to the use of methane extraction technologies [45] in several countries, such as United States, Australia, and China [17,46,47]. In Russia, some of the existing mines are starting to work with these technologies in deep wells, allowing them to reduce the gas pressure and the capacity to accumulate gas in the most dangerous areas in the mine [48]. Furthermore, its implementation is still hampered due to the lack of comprehension about its advantages, specifically regarding economic aspects. Consequently, a simplified method to calculate the economical improvements due to this technology implementation is explained below.

The aim of this new method is to calculate the economic effect of implementing early methane extraction technology. This factor can be estimated according to the equation described below.

$$E=E_1+E_2-E_3-E_4$$

(4)

where:

• E is the economic effect of early degasification;
• E₁ is the economic effect of increasing the rate of development work;
• E₂ is the economic effect of increasing the load work in the mine;
• E₃ is the economic effect of the extracted methane utilization (methane sales);
• E₄ is the economic effect of reducing methane emissions to the atmosphere.

Each one of these factors can be estimated according to the following procedure:

The next equation allows obtaining the economic effect of increasing the rate of preparatory work (E₁):

$$E_1=\left(1-\frac{V_1}{V_2}\right)K_s{C}_D{D}_1{\gamma }_{fc}\frac{V_{st}}{100V_m}$$

(5)

where:

• V₁/V₂ is the average rate of development working in the mine and in the area of early degassing preparation (%);
• Ks coefficient of the discrepancy between the decrease in the number of existing development faces and sections and the rate of growth in the speed of workings (%);
• C_D is the production cost of mining 1 ton of coal before the introduction of new technology (USD/t);
• D₁ is the annual mining production in the previous period (t);
• γ_fc. is the proportion of the conditionally fixed costs of development work over the general cost of the mine (%);
• V_st is the volume of mining work in the studied area (m³);
• V_m is the volume of the whole mine (m³).

The economic effect of increasing the load work in mine (E₂) can be calculated as:

$$E_2=\left[0.1+0.7\left(q-1.1\right)\right]{\gamma }_{fc}^{*}{C}_D{D}_{1c}\frac{N}{100}$$

(6)

where:

• q is the coefficient of growth of the load work (%), which includes the cost of fueling the early degasification reaction process;
• γ* fc is the proportion between the fixed cost of work intensification and the cost of 1 ton of coal (%);
• D_1s the daily load before implementation (t/day);
• N the number of days for mining this area.

The economic effect of using the extracted methane (E₃) is represented by the following expression:

$$E_3=Q_M{C}_D\frac{S_{coal}}{1000S_{methane}}$$

(7)

where:

• Q_M is the amount of extracted methane (m³);
• S_coal is the calorific value of 1 kg of coal (MJ);
• S_methane is the calorific value of 1 m³ of methane expressed in (MJ).

The economic effect of reducing methane emissions to the atmosphere (E₄) is determined by the following equation:

$$E_4={\displaystyle \sum }{\Delta }_i{\theta }_i$$

(8)

where:

• Δ_i is the reduction of certain types of emissions by replacing coal with methane (m³);
• θ_i is the amount of payment per unit of corresponding emissions (USD/m³).

3. Results and Discussion

For the GHG emissions calculations, it is considered an average production forecast of each early degasification well of 1 m³/min [34]. To compare the different scenarios, the period of methane extraction from the mine is assumed to be one year, during which the wells are working without any stoppage. The total volume of methane emitted or extracted by the mine in the case study is then 525,000 m³.

Evaluating the first scenario, with an absolute methane density of 0.657 kg/m³ at standard conditions (pressure of 1 atm and temperature of 298 K) [49], a total of 345 tons of methane are obtained from the mine. Knowing that the greenhouse impact of methane is 21 times greater than that of CO₂, the emission of 525,000 m³ of methane is equivalent to 7243 tons of CO₂. In other words, one ton of CO₂ corresponds to 71.5 m³ of methane [34].

For the second and third scenarios, emissions of all gases from methane combustion must be considered. Firstly, based on equations 1 and 2, the CO emissions, due to incomplete combustion reactions, are calculated. Secondly, based on equation 3, the NO₂ emissions are calculated. The carbon monoxide and the nitrogen dioxide emissions, from the combustion of 525,000 m³, are: M_co,y = 18.84 tons and M_NO2,M = 1.51 tons. The maximum carbon monoxide and nitrogen emissions are 0.6 and 0.05 g/s, respectively. After calculating these emissions from incomplete combustion, the resulting methane can be approximated to natural gas. Knowing that one m3 of natural gas equals 2.16 kg of CO₂, the carbon dioxide emissions from the combustion of methane extracted from an early degasification well over one year is 1134 tons. Comparing the emissions of the first scenario and the second scenario, the reduction of greenhouse emissions seems clear when using this combustion technology. At the same time, comparing scenario 2 and 3, it is clearly observed that despite generating the same greenhouse emissions, the offsetting of emissions due to the use of methane energy makes this technology even more beneficial.

Besides the combustion gases, this study intends to obtain the emissions related to the diesel equipment involved in scenarios 2 and 3. The “Provisional Methodological Guide for the calculation of emissions from unorganized sources in the construction materials industry” [50] explains how to calculate the harmful gases originated by early degasification equipment.

According to this study, the maximum pollutant emissions take place during the hydraulic segregation of the coal layer, being the diesel consumption approximately 360 kg/h, for real pumps used in this process (UN1 pump units 630 × 700). The processing of layer degasification needs about 45 h [21]. Based on the calculated fuel consumption and the corresponding specific emissions, the emissions of the equipment during the construction and the treatment of the well are in

Table 1. Emissions of the equipment during the construction and the treatment of the well.

Harmful Substance	Specific Emissions per 1 g of Fuel (g)	Emissions of Harmful Substances
		(g/s)	Gross (t)
	Data	Data	Data
Carbon monoxide	0.0300	3.00	0.49
Nitrogen oxides (mainly NO2)	0.0400	4.00	0.65
Sulfur dioxide (SO2)	0.0200	2.00	0.32
Hydrocarbon	0.0300	3.00	0.49
Soot	0.0155	1.55	0.25
TOTAL		13.55	2.20

. The total emissions of harmful substances from the equipment during this process are 2.20 tons for each well per year.

The other process that generates significant pollution due to its diesel consumption during early degasification is the mixing of methane in the plant prior to combustion. During this process, the diesel consumption is 40 kg/h during approximately 100 h [21]. These data are for the real case of Peko USP-50 mixing plant. The emissions of the equipment during the mixing of the gas for each well per year are in

Table 2. Emissions of the equipment during the mixing of the gas.

Harmful Substance	Specific Emissions per 1 g of Fuel (g)	Emissions of Harmful Substances
		(g/s)	Gross (t)
	Data	Data	Data
Carbon monoxide	0.0300	0.33	0.12
Nitrogen oxides (mainly NO2)	0.0400	0.44	0.16
Sulfur dioxide (SO2)	0.0200	0.22	0.18
Hydrocarbon	0.0300	0.33	0.12
Soot	0.0155	0.17	0.06
TOTAL		1.51	0.54

.

Additionally,

Table 3. Relation between the different contaminators and their emission source.

Pollutant	Title 2
	Peko Mixing Plant USP-50	UN1 Pump Units-630 × 700	Torches
Carbon monoxide	0.12	0.49	18.84
Nitrogen oxides (mainly NO2)	0.16	0.65	1.50
Sulfur dioxide (SO2)	0.08	0.32	-
Hydrocarbon	0.12	0.49	-
Soot	0.06	0.25	-
TOTAL	0.54	2.20	20.34

shows the relation between the different contaminators and their emission source.

The estimated gross emissions of pollutants, from diesel consumption and incomplete methane combustion, into the atmosphere, are 23.08 tons for each well per year, estimating an extraction flow rate of 1 m³/min.

In addition, as a result of the burning of methane extracted from the well in the scenario, 2134 tons of CO₂ are formed. To assess this value, it should be considered when a mine with a gas capacity of methane of 50 m³ per ton of coal produced produces one million tons of coal per year. In this case, only the methane emission is equivalent to almost 690 thousand tons of CO₂. Therefore, calculations show that the proportion of emissions that use early methane extraction technology (scenarios 2 and 3) in air pollution is negligible and does not exceed 1%.

If a mechanism for the trading of greenhouse gas (GHG) emissions quotas is implemented, it is necessary to take into account the magnitude of their emission reduction. It should be emphasized that coal methane from operating mines falls into this category, but not all coal methane. Naturally, the method of using methane also affects the equivalent value of carbon. However, even with the burning of conventional methane, a significant reduction in GHG emissions is achieved. Therefore, 1 ton of CO₂ is formed during the combustion of 463 m3 of CH₄, which, when released into the atmosphere in scenario 1, is equivalent to 6.39 tons of CO₂. Consequently, the burning of every 1000 m³ of methane is equivalent to a reduction in CO₂ emissions of 11.6 tons (2.2 tons for scenarios 2 and 3 and 13.8 tons for scenario 1).

When methane is used to generate heat or electrical energy, the additional effect is determined by the difference in specific CO₂ emissions per unit of power generation (Figure 3). Then, when methane is used in a boiler room (in the coal-to-gas conversion variant), CO₂ emissions with the same heat production will decrease by 1.74 times. By transferring, for example, a DKVR-10/13 boiler, the absolute carbon dioxide emissions will decrease by almost 15 thousand tons per year [51].

For the conditions of the Lenin mine (Karaganda basin), the value of the early extraction of methane through the wells amounted to about 20 million of m³, which corresponds to a reduction of CO₂ emissions by around 230 thousand tons. With the cost of a ton of carbon equivalent from 5 to 7 USD, the economic effect, even in the case of flaring, will be about 1400 thousand USD, which exceeds the cost of implementing this technology, but taking into account the time factor, it is more than 75%.

Therefore, the use of methane early extraction technology will reduce greenhouse emissions into the atmosphere. Additionally, air quality will improve, involving a great enhancement of human health [52]. The application of this technology must be during the entire mine life and also during the postmining activities [53].

Continuing with the environmental analysis, a comparison is made between the emissions from methane extracted from active and abandoned underground mines in Russia and Europe. Russia continues to be one of the largest consumers and producers of coal today. On the other hand, Europe has opted for the disappearance of this fuel. Currently, the number of underground coal mines in Russia is about 70, while in Europe, there are about 128 mines, many of them abandoned due to the decline in the use of coal in recent years.

Based on the methane flow rate estimation of 1 m³/min per early degasification well, the total methane that would be extracted from mines in Russia and Europe can be known. To ensure the extraction of methane from the underground mine, normally, 3 to 8 degasification wells are required, depending on the mining and geological conditions [31]. For this study, the average number of early degasification wells present in each mine is estimated as 4, and methane extraction is considered to be the same in both active and abandoned mines.

For scenario 1, in which methane is released into the atmosphere, the CO₂ equivalent emissions emitted in Russia is approximately 2.03 million tons. In the same scenario, CO₂ emissions for Europe correspond to 3.71 million tons. Looking at scenarios 2 and 3, the reduction of greenhouse gas emissions due to the combustion of the extracted methane is again clearly visible.

Table 4. Emissions in underground coal mines in Russia and Europe due to the combustion of methane extracted by early degasification (scenarios 2 and 3).

Greenhouse Gases	Emissions in Russia	Greenhouse Gases
Carbon monoxide (CO) in tons	5.28	9.65
Nitrogen oxides (mainly NO2) in tons	0.42	0.77
Carbon dioxide (CO2) in tons	317.52	580.61

shows the harmful emissions from the combustion of methane that would be extracted in underground mines in Russia and Europe.

The economical assessment was carried out for each scenario. The greater the economic impact, the greater the investment. As scenario 1 does not consider degasification, there will not be any economic effect of early degasification. Scenario 2 establishes methane burning in a torch, which means that degasification will take place; however, the methane will not be used for producing energy, therefore, not reducing degasification costs from selling benefits. On the other hand, scenario 3 considers the implementation of early degasification, together with methane-selling production. The obtained data for each economic factor and scenario can be seen in

Table 5. Comparison between economic degasification indicator and methane production.

Indicator Name	Scenario 1	Scenario 2	Scenario 3
E1 (million USD)	-	3.30	3.30
E2 (million USD)	-	0.40	0.40
E3 (million USD)	-	0.00	1.50
E4 (million USD)	-	0.18	0.23
TOTAL	0.00	3.88	2.43

:

In order to properly calculate the effect of using methane as an energy source, E₃ has been estimated, considering that the coal bed methane price was approximately 0.1 USD/m³. Although in both scenarios 2 and 3, methane is burned, the economic effect of reducing methane emissions is different (E₄). In scenario 2, the emissions are produced due to methane burning, which means producing carbon dioxide and carbon monoxide, which, as previously mentioned, present a lower contribution to the greenhouse effect than methane. The same situation takes place in scenario 3, but with a significant difference: as methane is burned for energy production, the combustion emissions are now deductible from the global emission amount.

The data presented in Table 4 clearly confirm the advantages of early degasification compared to methane extraction from the coal layers. It should be noted that the evaluation was carried out under the condition that methane extraction volumes from a well were reached at the level of 15 mills m³ in the production option and 1.5 mils m³ in the early degasification production. However, if we consider scenario 1, it might be reasonable to think that the best option is not to extract methane, as no associated economic effect is present. Nevertheless, degasification is becoming a mandatory operation, which means that if not implemented, it can have associated penalty fees and taxes [54]. If degasification becomes mandatory, scenario 1 will have the same costs as scenario 2, plus the penalty fees. Figure 4 shows the three scenarios’ economic costs, considering 0.5% of penalty fees.

The importance of accident prevention in mines is reflected in the literature, exiting numerous studies in which different prediction methods to avoid accidents are presented [55,56,57]. Moreover, mine accidents have a stretch relationship with coal prices, affecting them directly in a negative way [58]. Unfortunately, these economic efficiency criteria do not completely consider the risk factor for human life and the probability of catastrophic accidents during methane explosions in coal mines. These factors also constitute a great economic burden to mining companies [20], so the improvement of the security involved in applying early degasification will always lead to an economic improvement in this aspect; however, it is difficult to measure.

To extract 15 mills m³ of methane with a useful life of 5 years, the average production rate should be approximately 6 m³/min, which in coal seams is observed only in individual wells and is associated with mining and geological characteristics. At the same time, methane elimination provides the opportunity to increase workload between 50% and 80%, which implies economic growth [59].

Despite the successful results, the interest of the mining companies in this technology is still not enough. This situation may improve if the mining legislation improves in Russia, being necessary to gradually ban the development of underground extraction when the volume of methane in coal seams is more than 9 m³/t. This value allows for improving safety, economic factors, and greenhouse emissions, forcing companies to use early degasification. To avoid the “knockdown” effect for current companies, the proposed measure can be applied gradually, for example, over a period of 15 years. The implementation of this measure will ensure, after 15 years of the introduction of safety technologies in the underground mines in Russia, increasing the profitability and the energy use of CMM, and finally, the environmental situation regarding greenhouse mining emissions will considerably improve.

European and Russian Potential for CBM Utilization

Following the analysis of the scenarios, this sub-section presents a study of CBM potential as an energy vector. For this purpose, two study areas have been selected, focusing on Russia and Europe. Russia is currently the sixth largest coal producer in the world, while Europe has opted for the disappearance of this fuel. A degasification efficiency of 65% for active mines and 45% for abandoned mines will be considered. Based on the production data used in the environmental and economic analysis, the total CBM production of both regions is estimated. This production will be 4953 MWh/day for Europe and 9758 MWh/day for Russia. The value is higher for Russia, given the potential of this resource in the country. In fact, 283 Mt of coal is produced annually in Europe, while in Russia, the value reaches 435 Mt.

From this annual production and knowing that extracting one ton of coal requires approximately 50.5 kWh, the energy demand of the coal mines in both regions was obtained. As can be seen in Figure 5, the energy potential of the CBM does not cover the energy demand of the mines in any case. However, the demand for land transport is 100% covered, and around 85% of the MBC can be used for energy production, as shown in Figure 6. For the calculation, 100 km of the daily route has been taken for each of the trucks, considering that a natural gas truck consumes about 0.38 m3/km. However, it is important to note that since CBM is a non-conventional fuel, there would be significant savings in GHG emissions. If we assume a 100% saving in accordance with RED II [36], we would be talking about 30% less emissions from European mines and 35% for Russian mines.

4. Conclusions

The application of early degasification technology significantly improves environmental and economic factors allowing mining companies to reduce unnecessary costs and be more sustainable. By forcing companies to fix a maximum volume of methane in coal seams of 6 m3/t, as is proposed in this article, it will be possible to achieve the economic and environmental goals needed nowadays in the mining sector. The implementation of these measures could reduce emissions from coal mining by 30%. To develop a complete and precise environmental assessment, it could be interesting to use a Life Cycle Assessment (EM-LCA) to analyze the environmental impacts in more detail.

This technology also has an impact on the mining economy. The most optimal scenario from an economic point of view is scenario 1, as there is no need to invest in new technologies. This may be contradictory to the environmental analysis, so governmental action is required to encourage their use. Applying degasification technology, scenario 3 requires the same costs as scenario 2, although it is more beneficial as revenue is obtained from the use of CBM. Despite this, less than 35% of CMM is currently utilized in Russian mines. However, only Europe has taken support measures for the implementation of this technology, and the appropriate steps to promote it have been initiated by means of the Proposal for a Regulation of the European Parliament and of the Council, dated 15th December 2022, on methane emissions reduction in the energy sector and amending Regulation (EU) 2019/942. Currently, other countries still have no regulations governing the degasification of coal mines.

The environmental and economic feasibility analysis is positive for early degasification technology, but the following bottlenecks have been identified for the implementation of this technology:

(a)

There is a need to improve CBM extraction and cleaning technologies to increase combustion efficiency and enable its use in sectors such as transport;

(b)

Actions for the application of such technologies should be encouraged. Legislation can help countries such as Russia to reduce pollution from their mines by 35%;

(c)

A scenario in which emissions are penalized would mean that scenario 1 would have the same costs as scenario 2, thus reducing emissions by 11.6 tons of CO₂ per year per mine.

Additionally, the application of early degasification has a direct impact on safety in mines. Safety improvement is a key point in the current Russian mine sector and will allow for the reduction of fatal and serious accidents regarding the formation of explosive atmospheres in underground mines. This trend will also produce a decrease in the mining procedure cost by reducing monetary compensations and down times.

The present model allowed us to draw the effects of early degasification, which, as mentioned above, are significant. However, further works should focus on the improvement of the model, as it should be tested with a case study and data from a real mine operation, along with considering uncertainty and more accuracy.