A new approach to producing a prospective energy resource based on coalmine methane

The paper describes topical issues of a prospective method for coalmine methane utilization for obtaining an additional valuable energy resource for the regional development of coal-mining areas. It is noted that the development of the extraction of methane resources is very urgent and is of great economic importance for ensuring the energy independence of Ukraine. The experience and technologies of using methane and coalmine gas by global coal-mining companies are analyzed. Modern prospects and opportunities for using coal gas are studied. There is a need to transform the coalmine methane removal system and directions for maximizing the use of its resources in a wide range of concentrations in the composition of gas-air mixtures based on the development of innovative technologies to improve the efficiency and cost-effectiveness of functioning coal-mining enterprises. Attention is focused on the advantages of using gas hydrate technologies for obtaining additional energy resource under conditions of changing coalmine methane concentrations. The specifics of the process of mixed gas hydrate formation from gas mixtures of various geneses have been studied. It has been revealed that it is the coalmine gas-methane composition that determines and forms the basic condition for hydrate formation. The thermobaric conditions for the hydrate formation process at different methane concentrations in gas mixtures of degassing systems have been experimentally determined. The results obtained are the basis for further research on efficiency of creating gas hydrates from coalmine methane and determining its minimum permissible concentration in the gas mixture of degassing systems according to the technological and economic criteria of hydrate formation.


Introduction
Rational use of natural resources is the main prerequisite for ensuring sustainable social and economic development [1,2].In this regard, state regulation of resource consumption as a tool for environmental management is of great importance in most countries of the world, which is evidenced by a number of international agreements adopted within the framework of the United Nations.For Ukraine, such a tool can be not only the transformation of the coal industry, but also the reduction of operational losses of energy resources of coal-mining and coal-processing enterprises by involving in the economic turnover of associated resources of gas-coal deposits.As for today, coal, oil and natural gas continue to be the main fuel and energy resources [3][4][5][6][7].
In recent years, the energy market has undergone significant changes, which are primarily caused by the ever-increasing needs of society for energy, due to economic and technological development.Leading countries are beginning to fight for the right to develop energy resource deposits on neutral Arctic and Antarctic territories.More stringent environmental regulations are becoming an impetus for the transition to cleaner fuel types [8,9].There is a need to search for new alternative energy sources and develop technologies for their mining and use, as well as 1254 (2023) 012068 IOP Publishing doi:10.1088/1755-1315/1254/1/012068 2 to improve the efficiency of existing technologies in the energy sector [10][11][12].Thus, ensuring energy security is becoming an increasingly complex and multifaceted task.
According to existing predictions and reports, energy consumption is growing every year.Thus, in 2021, the global demand for electricity increased by almost 50% [13].Analyzing the World Energy Outlook report for 2022, the situation has almost remained unchanged, demand continues to rise, but at a somewhat slower pace, given today's high prices for energy carriers and market instability due to Russia's aggression on the territory of Ukraine [14].Having faced with market uncertainty and high prices, consumers are withdrawing some of their gas purchases, and industry is reducing production.Despite a strong post-pandemic economic recovery in 2021, the average annual GDP growth rate for the rest of the decade assumed by the Stated Policies Scenario in 2020 has been slightly revised.As a result, energy demand is growing at a slower pace and the fuels used to meet this demand growth are different from previous projections (figure 1) [14].As it can be seen, in the context of the diversification of hydrocarbon sources and the war unleashed by Russia in Ukraine, an unstable situation has developed on the global fuel and energy market.Of particular importance is the issue of determining the energy security prospects of many countries, including Ukraine, by increasing the volumes of domestic gas production or searching for renewable and alternative fuel types [15,16].Despite the significant potential of the major types of renewable energy sources, their practical use today is only a small part in the fuel-energy balance (figure 1).Nevertheless, the interest in the development of production technologies for new types of energy raw materials is increasing significantly.Currently, alternative types of gas fuel include methane from coal deposits and coalmine methane [17]; gas obtained from the processing of solid fuel (hard and brown coal, oil shale, peat) [18,19]; producer gas, other gaseous fuel derived from biological raw materials [20].Special attention should be paid to the development and implementation of gas production technologies by underground coal gasification [21,22].However, according to many scientists, the most promising source of hydrocarbon fuels are gas hydrate deposits found in many countries of the world [23][24][25][26][27][28][29].
On an industrial scale, methane is not produced from gas hydrate deposits in the world today.However, a number of leading countries, such as Japan, the USA, Canada, India, China, Norway, South Korea, Australia, Turkey, Bulgaria and Ukraine, are implementing large-scale research programs and are actively engaged in studying the issue of gas hydrates, both artificial and natural, as well as in the development of hydrate formation and dissociation technologies [30][31][32][33][34][35][36].
Assessment of the economic efficiency of gas hydrate projects requires certain clarifications.This uncertainty is aggravated by the ongoing search for both optimal technologies for the development of gas hydrate deposits, as well as obtaining gas hydrates for their use when transporting hydrocarbon raw materials.
Obviously, natural gas remains the most commercially prepared energy carrier [37].Therefore, consideration of issues related to the development of hydrocarbons [38][39][40] and the industrial utilization of natural reserves of methane -the most demanded fuel in Ukraine -is extremely relevant and is of great economic importance for ensuring the energy independence of our country [41,42].

Prospects and opportunities of using coal gas
Given the interest in non-conventional energy sources, more and more attention has recently been paid to coal seam gas, which can be extracted by implementing innovative coal-mining technologies.The successful experience of gas companies in the United States has even led to the announcement of the creation of a gas subsector dedicated to the methane production from coal seams [43][44][45].By combining the interests of the gas and coal industries, it is possible to ensure and significantly improve the technical, economic, ecological and social conditions of the population in industrial regions.
Over the past 15-20 years, this area has received significant attention and intensive development in the United States, where the average volume of gas production from coal seams is 55 billion m 3 annually, in Canada -more than 9 billion m 3 , in Australia -5.5 billion m 3 , in China -1.2 billion m 3 , Russia -6 million m 3 [46][47][48].
Natural gas from coal seams is 90% or more methane.Methane is the purest hydrocarbon energy source with almost no harmful impurities, such as nitrogen or sulphur compounds.Currently, commercial production of methane from coal seams is carried out only in the United States.Over 9 thousand wells are in operation, of which more than 40% are located in the San Juan Basin.About 10% of wells in this basin provide 75% of its production and 60% of the total annual coalmine methane production in the United States [49,50].
Also, in recent years, technologies for using ventilation gas removed from the mine along with the ventilation air flow have gained significant development.The first major project of this kind in the world is the Australian West Cliff Ventilation Air Methane Project (WestVAMP), which converts a part of the ventilation jet into heat and electricity using the VOCSIDIZER™ energy system from the Megtec Company (Sweden) [51].This technology involves the use of ventilation gas with an extremely low methane concentration of 0.3-1.2%,but requires further improvement due to low economic efficiency.The Natural Resources Company (Canada) developments use reverse-flow reactors (Catalytic Flow Reversal Reactor, CFRR), which are also designed to generate heat by utilizing an off-standard gas mixture with 0.5-1.0%methane content [52].
When mining coal deposits, outgoing gas-air flows occur, which are formed in the process of diluting the released gas with atmospheric air and methane-air mixture sucked from the mined-out area and drilled wells in the coal-rock mass (figure 2) [53].Under the conditions of high methane-bearing capacity of coal deposits, degassing is the main way of ensuring the coalmining safety.Therefore (regardless of other possibilities of using the methane-air mixture), the main condition remains the extraction of gas from the coal-rock mass and the mined-out area, its localization and removal to the surface or to the outgoing jet.
In Ukraine, coal mines emit most of the methane (up to 90%) into the atmosphere in the form of a low-concentration gas-air mixture from mine ventilation units.In 2016, about 517 million m 3 of methane gas was released during coal mining by 50 mines [54], which is equivalent to the production of 3.3 billion kWh or 3.0 billion m 3 of burned natural gas and exceeds the electricity consumption of all coal-mining enterprises in Ukraine.In global practice, mixtures with a methane concentration above 25-50% are most efficiently utilized.Therefore, it can be concluded that the energy potential of coalmine gas-methane during mining operations is extracted from the bowels, and its potential is not fully utilized for the needs of industry and the population.
Thus, in the conditions of the most powerful coal-mining enterprise in Ukraine, PJSC Mine Administration "Pokrovske", a universal combined degassing method is used -underground method, which extracts about 60% of coal methane, and degassing from the surface, extracting 40% of methane.The underground degassing system is represented by a network of degassing pipelines in mine workings, through which, using two vacuum-pump stations on the surface, coalmine methane is extracted through drilled wells from the mined-out area and the coal mass of the extraction sites.The methane concentration in the gas-air mixture during underground degassing varies widely, ranging from 15 to 70%.Surface degassing involves drilling degassing wells every 300 m in extraction panels ahead of the stoping faces in highly stressed longwall faces.The methane concentration in the gas-air mixture with this method is extremely high, since the mass is not de-stressed and ranges between 90-98%.For gas utilization, a cogeneration plant with a capacity of 18 MW has been implemented at the enterprise, but only up to 40% of methane is utilized, because its flow rate and concentrations are variable, making it difficult to utilize it.
In most other coal mines in Ukraine, coal methane is currently extracted to the surface as part of ventilation jets and underground degassing systems.In this case, the methane-air mixture coming to the surface is usually substandard and is released into the atmosphere or burned.Only if the methane content in the gas-air mixture exceeds 25%, it becomes possible to use cogeneration plants.Thus, at the Stepova mine (PJSC "DTEK Pavlohradvuhillia") for a year, about 6 million kW of electricity has been generated from 1.3 million m 3 of extracted coalmine methane using a cogeneration plant [55].
In this regard, the problem of the large scale of coalmine gas utilization is becoming increasingly important, the great difficulties of which are associated with the variability of its flow rates and the different methane content in the extracted mixtures, as well as the limitated possibility of using cogeneration plants to generate electrical or thermal energy [56,57].
Therefore, today it is required to transform the coalmine methane removal system and the directions for maximizing the use of its resources in a wide range of concentrations in the composition of gas-air mixtures (15-98%) based on the development of innovative technologies to improve the efficiency and cost-effectiveness of functioning coal-mining enterprises.

Advantages of using gas hydrate technologies for obtaining additional energy resource
As a result of significant variations in methane concentration in the gas mixtures of degassing wells, its widespread use is becoming more difficult.Therefore, it becomes necessary to create a method for producing methane in the final chain of mining gas-coal deposits, for which the component composition of the outgoing gas would not be a stringent condition.This method, according to the authors, is the conversion of gas into a solid gas-hydrate state, since gas-hydrate technology makes it possible to convert various gases into hydrates, including their mixtures.In this case, only the equilibrium conditions of the hydrate formation process will change [58,59].It is easier and safer to store and transport methane in the state of gas hydrates to industrial and energy companies for its further use as an energy carrier, which is another advantage of gas hydrate technologies [60,61].
Hydrates are formed and stably exist in a wide range of thermobaric parameters.But each individual gas is characterized by certain variants of pressures and temperatures of the hydrate phase stability existence.The main factors determining the conditions for hydrate formation and storage of gas hydrates, first of all, should be considered the composition of gases, their moisture saturation, phase state, composition and state of water, its mineralization, external pressure and temperature [62].The gas composition determines the basic condition for hydrate formation -the higher the molecular weight of an individual gas or mixture of gases, the lower the pressure required for hydrate formation at the same temperature.Natural gases, consisting of a combination of individual components, form mixed gas hydrates.In this case, during hydrate formation, crystals are simultaneously formed, which are characteristic of both methane and other hydrocarbon and associated gases in the mixture.That is, for the conditions of gas-air mixtures of degassing systems in coal mines, it is possible to apply the concept of mixed gas hydrates.
The following characteristic stages are distinguished during hydrate formation: 1) formation of gas hydrate crystallization centers; 2) sorption growth of crystalline hydrates around crystallization centers; 3) mass crystallization or simultaneous increase of a large number of formed crystallization centers.
Further in the research, the possibility of creating methane gas hydrates from degassing gas mixtures of a coal mine with a wide variation of methane concentrations is experimentally studied.

Research methods
The conversion of methane-air mixtures of degassing systems into the gas-hydrate state, when mining gas-coal deposits, would solve a number of difficulties related to their utilization and subsequent use.However, the specificity of the component composition of gas mixtures necessitate the development of optimal parameters for the hydrate formation process.The process of nucleation and formation of gas hydrates from gas mixtures of various geneses is more complex than from individual gases, which manifests itself in the changing value of equilibrium parameters for hydrate formation and its rate.
In order to determine the patterns of the hydrate formation process and the influence of various technological and regime parameters on it, a bench unit has been developed, consisting of an ILKA KTK-3000 Climatic-Thermal Chamber and a number of other measuring equipment.The Climatic-Thermal Chamber allows conducting research in different temperature conditions and with different humidity.Structurally, the Climatic-Thermal Chamber has four main parts: the working volume, in which the NPO-5 hydrate formation reactor is directly located, the automatic control panel, the refrigeration unit and the steam generator.The NPO-5 unit is equipped with a reducer, which makes it possible to regulate the pressure of the gas mixture from 0 to 10 MPa and maintain it constant throughout the entire duration of the experiment [63].
A transparent polymer pipe with a wall thickness of 2.5 mm is used for conducting experimental research and visual observation of the hydrate formation and hydrate accumulation processes.To record the pressure and velocity of the water and gas flowing out of the highpressure nozzles, the unit is equipped with a measuring system consisting of a direct-acting indicating pressure gauge, a thermometer, an anemometer and a PC-based measuring station.To conduct experimental research on the process of hydrate formation from gas mixtures, jetforming nozzles of an original design are used.

Research results
For reasons of clarity of the experiment and the comparison of the obtained further results, the process of hydrate formation from pure methane is studied as a standard at the first stage, the actual volume fraction of which is 99.7%.The hydrate formation curve and process parameters are given in figure 3. The resulting pattern of methane gas hydrate formation (Fig. 3) has a polynomial character with the value of approximation reliability R 2 = 0.99 and is described by the equation y = 0.02x 2 + 0.27x + 2.62.The pressure range varies from 3 to 7.5 MPa.This has experimentally proven the fact that at pressures below 3 MPa, the hydrate formation process using pure methane is difficult and requires a sufficiently long time to start crystallization with the formation of crystalline hydrate nuclei (according to existing data, up to several days).Accordingly, it is inexpedient to conduct research under such pressures.The increase in temperature parameters occurs naturally with increasing pressure and reaches its maximum value at +10°C.
In order to study the influence of thermobaric parameters on the conditions for the formation of gas hydrates, depending on methane concentration in gas mixtures of degassing systems, samples have been taken directly from the Western Donbas coal-mining enterprises.Their compositions are given in table 1.
The thermobaric parameters of the hydrate formation process from gas mixtures of degassing systems have been obtained experimentally (figure 4).Methane concentrations in the selected mixtures range from 47 to 95%, reflecting their real wide variation with degassing methods in  coal mine conditions.The resulting dependences of pressure on temperature change according to a polynomial law with an approximation value of R 2 = 0.97 for each separately selected mixture.At the same time, the highest pressures of hydrate formation are for a mixture with a methane concentration of 95%, and the lowest, respectively, for a mixture with 47% methane in the gas mixture (figure 4).
The patterns of the gas hydrate formation process from mixtures of various genesis, depending on pressure and temperature, are described by the following equations: -at CH 4 = 47% -y = 0.06x 2 -0.06x + 1.37; -at CH 4 = 54% -y = 0.06x 2 -0.07x + 1.70; -at CH 4 = 75% -y = 0.06x 2 -0.06x + 2.25; -at CH 4 = 86% -y = 0.06x 2 -0.02x + 2.53; -at CH 4 = 95% -y = 0.06x 2 -0.04x + 2.92.Also, the formation parameters of mixed gas hydrates are influenced by the presence of carbon dioxide in the mixture, in addition to hydrocarbon gases.It has been found in the previous research that the pressure required to form carbon dioxide hydrates is by 20-30% lower than for methane hydrates, depending on the conditions of hydrate formation.This fact is also observed during the formation of mixed gas hydrates, although the CO 2 share in the mixtures is quite low.
In the future, for the production of gas hydrates from coalmine methane, it is proposed to build a hydrate formation complex near the facilities of degassing systems in the surface mine complex, followed by their transportation by mobile refrigerators to consumers (industry, population, etc.) and settlements located near the coal mine.
To evaluate the economic efficiency of the proposed technological solutions in the future, it is planned to use the methods of economic and mathematical modeling, taking into account the capital investments in equipment, the cost of the final product, the determination of operating costs, the reduction of charges for atmospheric pollution, the level of consumption by the population or industrial enterprises of gas obtained by the gas hydrate method.

Conclusions
The energy market has undergone significant changes in recent years, which are caused by the growing needs of society for energy resources.In the context of diversification of hydrocarbon sources, the issue of ensuring the energy security of many countries of the world and Europe is of great importance.This encourages the search for new alternative energy sources and the development of innovative technologies, in particular, gas hydrate technologies.
The prospects and possibilities of using coal gas in the implementation of innovative technologies for mining gas-coal deposits have been studied.The experience and technologies of using methane and coalmine gas by global coal-mining companies have been analyzed.The experience of many American enterprises and companies has even made it possible to declare the formation of a gas subsector dedicated to the methane production from coal seams.The experience of Australia and Canada on the use of ventilation gas carried out of the mine together with the ventilation air flow has been studied.
The experience of using degassing systems at Ukraine's largest coal enterprise PJSC Mine Administration "Pokrovske", which uses both underground degassing and surface degassing methods, has been analyzed.For gas utilization, a cogeneration plant has been implemented at the enterprise, but only up to 40% of methane is utilized, because its flow rate and concentrations are variable, making it difficult to utilize it.Thus, in most other coal mines in Ukraine today, the methane-air mixture coming to the surface is usually substandard and is released into the atmosphere or burned.Therefore, it becomes necessary to create a method for producing methane in the final chain of mining gas-coal deposits, for which the component composition of the outgoing gas would not be a stringent condition.This method is the implementation of gas hydrate technologies for obtaining the final product.
The main factors influencing the conditions for the hydrate formation process and storage of gas hydrates, as well as their thermobaric parameters have been determined, which should be further taken into account when developing a technological scheme for the utilization of methane mixtures.The specifics of the process of mixed gas hydrate formation from gas mixtures of various geneses have been studied.It has been revealed that it is gas composition that determines the basic condition for hydrate formation.
A number of experimental researches on the process of obtaining gas hydrates from methaneair mixtures of various geneses have been conducted.In addition, the peculiarities of hydrate formation, which are typical for the formation of mixed gas hydrates, have been determined.For reasons of clarity of the experiment, the process of hydrate formation from pure methane has been studied as a standard, the actual volume fraction of which is 99.7%.The hydrate formation pressure varies in the range of 3.0-7.5MPa, since at pressures lower than 3.0 MPa, the process of crystal hydrate nuclei formation takes a long time.
The thermobaric conditions have been determined for the hydrate formation process at various methane concentrations in gas mixtures of degassing systems (47-95%), indicating their wide variation depending on the method of degassing in different coal mines.The dependence of the methane concentration on the hydrate formation pressure has been revealed: the higher

Figure 1 .
Figure 1.Predicted global consumption of fuel and energy resources [14].

Figure 2 .
Figure 2. Scheme of formation of the outgoing gas-air flows [53].

Figure 3 .
Figure 3. Parameters of the methane hydrate formation process.

Figure 4 .
Figure 4. Parameters of hydrate formation of gas mixtures in degassing systems in the Western Donbas mines.

Table 1 .
Compositions of gas mixtures obtained from degassing systems.