Underground Coal Gasification Technology: A Review of Advantages, Challenges, and Economics

Abstract

Against the background of global energy transformation and low-carbon development, numerous difficult-to-mine coal resources (e.g., deep, thin coal seams and low-quality coal) remain underdeveloped, leading to potential resource waste. This study systematically summarizes the feasibility of developing these resources via underground coal gasification (UCG) technology, clarifies its basic chemical/physical processes and typical gas supply/gas withdrawal arrangements, and establishes an analytical framework covering resource utilization, gas production quality control, environmental impact, and cost efficiency. Comparative evaluations are conducted among UCG, surface coal gasification (SCG), natural gas conversion, and electrolysis-based hydrogen production. Results show that UCG exhibits significant advantages: wide resource adaptability (recovering over 60% of difficult-to-mine coal resources), better environmental performance than traditional coal mining and SCG (e.g., less surface disturbance, 50% solid waste reduction), and obvious economic benefits (total capital investment without CCS is 65–82% of SCG, and hydrogen production cost ranges from 0.1 to 0.14 USD/m³, significantly lower than SCG’s 0.23–0.27 USD/m³). However, UCG faces challenges, including environmental risks (groundwater pollution by heavy metals, syngas leakage), geological risks (ground subsidence, rock mass strength reduction), and technical bottlenecks (difficult ignition control, unstable large-scale production). Combined with carbon capture and storage (CCS) technology, UCG can reduce carbon emissions, but CCS only mitigates carbon impact rather than reversing it. UCG provides a large-scale, stable, and economical path for the efficient clean development of difficult-to-mine coal resources, contributing to global energy structure transformation and low-carbon development.

1. Introduction

1.1. Basic Principles and Technical Arrangements of UCG

1.1.1. Chemical and Physical Processes

UCG is based on in situ coal pyrolysis, oxidation, and reduction reactions, with clear sequential and interactive mechanisms:

(1)

Drying: The coal seam is heated to 100–150 °C to remove free moisture, which is a prerequisite for subsequent pyrolysis.

(2)

Pyrolysis: At 300–600 °C, coal macromolecular structures decompose into volatiles (methane, hydrogen, light hydrocarbons, tar) and solid semi-coke, with tar yield peaking at 450–550 °C.

(3)

Oxidation: Semi-coke reacts with oxygen (injected as gasification agent) at 800–1200 °C to produce CO₂, CO, and a large amount of heat (main reactions: C + O₂ = CO₂ + 393.5 kJ/mol; 2C + O₂ = 2CO + 221 kJ/mol), which provides energy for drying and pyrolysis.

(4)

Reduction reactions (carbon as a reducing agent): CO₂ and steam (supplemented as a gasification agent) react with hot semi-coke under the heat of the oxidation zone to generate H₂ and CO (main reactions: C + CO₂ = 2CO − 172.4 kJ/mol; C + H₂O = CO + H₂ − 131.3 kJ/mol), forming syngas as the final product.

Continuous process: Gasification agents (oxygen/steam) are continuously injected, and syngas is continuously extracted, maintaining stable reaction zones (drying–pyrolysis–oxidation–reduction) along the coal seam. This process is suitable for thick and stable coal seams.

Two-stage process: First, oxygen is injected to establish a high-temperature oxidation zone. Then, steam is injected to enhance the reduction reaction and improve hydrogen yield, with alternating gas injection cycles. This process is suitable for low-reactivity coal or thin coal seams.

1.1.2. Gas Supply/Gas Withdrawal Arrangements

Typical technical arrangements are designed according to coal seam conditions, with clear functional differences:

(1)

Two-well system: The injection well (vertical/horizontal) injects gasification agents, the production well extracts syngas, and gasification channels are formed by reverse combustion (ignition at the production well end, the combustion front moves toward the injection well). It is suitable for coal seams with thicknesses > 3 m and a stable structure, with the advantages of a large gasification area and stable output.

(2)

Single-well system: A single well with multi-layer casing, where inner casing injects gasification agents and annular space extracts syngas, or vice versa. The gasification channel is formed by directional fracturing. Suitable for thin coal seams (<3 m) or scattered coal resources, with low drilling cost but limited gas production scale.

(3)

Horizontal well system: Horizontal wells are drilled along coal seams (length 500–1000 m), with multiple gas injection points and a single syngas extraction point. This expands the gasification contact area, improves reaction uniformity, and is suitable for thin to medium–thick coal seams (1–8 m) with stable continuity.

All arrangements rely on wellbore sealing technology to prevent syngas leakage and groundwater intrusion, with key design parameters including well spacing (50–200 m), gas injection pressure (0.5–5 MPa), and channel expansion rate (0.5–2 m/day).

1.2. Development History and Major Projects

UCG has a century-long development history:

(1)

Early exploration (1910s–1950s): The first UCG test was carried out in the UK in 1912. The former Soviet Union completed the first industrial UCG power generation project in 1939.

(2)

Technological improvement (1960s–2000s): The US carried out the Hoe Creek series tests (1976–1982) to verify deep coal seam gasification feasibility. China launched UCG field tests in the 1990s.

(3)

Modern development (2010s–present): Major operating projects include Canada’s Swan Hills Synfuels project (Alberta, syngas production for power generation and chemical synthesis), South Africa’s Sasol UCG demonstration project (coupled with the coal chemical industry), and China’s Zhongliangshan coal mine UCG project (steeply inclined thin coal seam recovery).

It should be clarified that UCG has achieved industrial-scale application in specific scenarios, but the technology is not yet fully mature in large-scale, complex geological conditions. The statement that “it is still mainly in laboratory or pilot scale” in the original text is inaccurate. In fact, it has formed mature technical systems for medium–deep coal seams with stable geological conditions, but large-scale commercial promotion is restricted by geological risk control and environmental management.

Against the background of global energy transformation and low-carbon development, a large number of coal reserves are difficult to mine by traditional technology due to the limitation of coal seam depth, coal quality characteristics, and complex geological conditions, and their development potential has not been fully and systematically explored. If these difficult coal resources are not effectively utilized continuously, it not only means a huge waste of resources, but may also restrict the optimization process of energy structure. It is in this realistic demand that UCG technology emerges as the potential path to develop such difficult resources. UCG technology can directly convert coal into syngas underground, bypassing the complex surface mining process. Its product syngas has broad application prospects, which can be used for power generation and hydrogen production, and can also be used as a chemical raw material for downstream production. This technical feature makes UCG technology show significant research value and application potential in terms of resource utilization, environmental impact reduction, and cost-effectiveness optimization [1,2].

Existing review articles on UCG mainly focus on single-dimensional perspectives, such as technical principles, environmental risks, or regional application cases, and lack a systematic comparison of economic indicators (e.g., full life-cycle cost, product benefit difference) between UCG and traditional technologies (e.g., SCG). In contrast, this study constructs a multi-dimensional analytical framework covering “resource utilization-environmental impact-cost efficiency”, and innovatively quantifies the economic gap between UCG-H₂ and SCG-H₂ in scenarios such as investment cost, operating expenditure, and environmental cost compensation. In addition, this study integrates the latest research progress (e.g., AI-based process optimization, supercritical hydrothermal gasification) to fill the gap of previous reviews in emerging technical directions, and further clarifies the applicable boundary of UCG by discussing the adaptability of different coal ranks and geological conditions.

How to efficiently develop difficult-to-mine coal resources has always been the core problem to be solved in the coal industry. Traditional coal mining technology is limited by many factors such as coal seam depth, coal quality conditions, and complex geological structure. It is often difficult to effectively use a large number of difficult-to-recover reserves, resulting in a generally low resource recovery rate, which can only reach about 40% [3]. In the face of these bottlenecks, although the ground coal gasification technology provides a path for the utilization of low-quality coal, it is also accompanied by a series of practical challenges—not only huge capital investment and high energy consumption, but also a significant impact on the surface environment. It should be noted that SCG technology still has certain limitations in stability and reliability in practical applications. In contrast, UCG-based hydrogen production shows significant advantages, and its capital investment is only equivalent to 77.1% of SCG technology hydrogen production. The product cost is much lower than the traditional method, and the SCG has obvious shortcomings in resource utilization efficiency and economy [4].

Globally, active UCG research and pilot/full-scale projects are distributed across multiple countries. In China, field tests have been carried out in Zhongliangshan Coal Mine (a steeply inclined thin coal seam) and in the Baikuquan area of Xinjiang (deep coal seam), and the pilot-scale UCG project in Daqing Oilfield is focusing on resource potential evaluation. Poland has long-term experimental bases such as the “Barbara” experimental mine and the “Wieczorek” mine, conducting in-depth research on gasification processes and environmental impact [5,6]. Canada has implemented a 1400 m deep coal seam UCG demonstration project in Alberta, verifying the feasibility of deep resource development. The United States has carried out technical exploration in North Dakota, focusing on geological risk control. India and Turkey have also carried out targeted research on UCG of high-ash and high-moisture lignite, respectively. In terms of industrial application, South Africa has promoted UCG-coupled hydrogen production projects, while Bangladesh is planning to convert proven coal reserves into natural gas equivalents through UCG to alleviate energy shortages [7].

In the development of difficult-to-mine coal resources, UCG technology occupies a key position: from the perspective of resource suitability, UCG can effectively deal with the types of coal resources that are difficult to reach using various traditional mining methods. For example, UCG has been successfully applied to the development of deep coal seams in a 1400 m deep coal seam project in Alberta, Canada [8]. In addition, this technology is also suitable for difficult-to-mine resources such as high-moisture lignite, high-ash coal, and complex coal seams [9]. Through continuous technical optimization, a single UCG system has increased the resource recovery rate to more than 60%, which significantly broadens the development and utilization space of difficult-to-recover resources [10]. At the environmental and cost levels, UCG shows the dual advantages of environmental protection and economic benefits. Studies have shown that compared with ground coal gasification, UCG can greatly reduce surface disturbances, and the solid waste produced by UCG is only half that of traditional coal-fired power plants [11]. At the same time, the technology combined with CCS technology is expected to achieve near-zero emissions and significant environmental benefits. In addition, UCG also performs well in cost control and has significant market competitiveness [12]. On the whole, UCG not only improves the mining efficiency of difficult-to-mine coal resources but also provides a feasible technical path for green and low-carbon development.

The typical process of UCG technology development is as follows: direct underground coal gasification of difficult-to-mine coal seams, construction of injection wells and production wells through drilling, formation of gasification channels underground, and then injection of gasification agents such as oxygen and steam to trigger coal seam pyrolysis and gasification reaction, thereby converting solid coal into syngas [13]. However, due to the essential difference between the UCG operation mode and traditional mining, it also faces a series of special challenges in the development process. In the complex environment of high temperature, high pressure, and multi-physical field coupling, it may cause land subsidence, crack propagation, and affect geological stability, accompanied by the risk of leakage of pollutants such as heavy metals and phenols [14]. In addition, the prediction and control of process parameters are difficult, which easily leads to the difficulty of gas adaptation in the process of purification, storage, and subsequent utilization [15]. At the same time, in terms of ground facilities configuration, traditional mining does not need to deal with gaseous products, while UCG needs to support the construction of gas purification, compression, and transportation systems. These additional links will undoubtedly push up construction and operation costs [16]. Therefore, in the process of promoting the application of UCG technology, it is necessary to consider its technical advantages and the accompanying risks and cost factors.

In order to give full consideration to the potential of UCG technology and effectively deal with its accompanying complex risks, this paper systematically analyzes the development of difficult-to-mine coal resources, such as resource utilization, gas production quality, environmental impact, and cost efficiency [17]. Compared with traditional development methods, UCG technology shows unique advantages, but it also faces a series of practical challenges, which will be discussed in depth below.

First of all, UCG technology has a wide range of resource adaptability and can effectively develop coal resources that are difficult to utilize using traditional means. Globally, with the gradual adjustment of fossil energy mining structure, a large number of difficult-to-mine coal resources have been idle for a long time. The data show that China’s current remaining coal reserves are about 1485.3 × 10⁸ tons, of which 87.15% are buried below 800 m, and most of them are low-quality coal or coal seams with complex occurrence conditions. These resources are the objects that UCG technology can focus on [18,19]. Similarly, in Bangladesh, if all of its proven coal reserves are converted using UCG technology, equivalent to 975 × 10⁸ m³ natural gas equivalent, this is expected to significantly alleviate the country’s energy supply pressure [20]. Looking forward to the future, in the context of the continuous deepening of low-carbon policies, the market demand for clean coal conversion technology will increase. As an in situ gasification technology with low carbon potential, UCG has very broad application prospects and is expected to play an important role in the transformation of energy structure [21].

Secondly, in addition to resource suitability, UCG technology also shows significant advantages in process efficiency and environmental benefits [22]. Compared with the traditional mining method, UCG effectively prolongs the oxygen residence time and improves the hydrogen yield through process innovations, such as swirl flow injection and two-step gasification, so that it can better adapt to the long-term operation requirements under complex geological conditions [23]. On the other hand, the UCG ground system can be efficiently integrated with the chemical polygeneration system to realize the direct utilization of syngas, eliminating intermediate links, thereby significantly reducing transportation and processing costs [24]. This shows that UCG technology can not only improve the efficiency of resource development but also help to achieve the coordinated development of the coal resource economy and environmental value.

It should be explained that the mentions of plasma gasification of biomass in South Africa (RSA) in the text are to illustrate the technical route of low-carbon hydrogen production by coupling gasification with CCS technology, which provides a reference for the low-carbon development of UCG technology. At present, plasma technology is not widely used in UCG, and its application feasibility needs further research and verification.

This study focuses on filling the gaps in existing research by constructing a comprehensive analytical framework and quantifying economic differences, which is crucial for promoting the industrial application of UCG technology. The systematic analysis of resource adaptability, environmental impact, and economic efficiency will provide clear technical guidance for countries with abundant difficult-to-mine coal resources to formulate energy development strategies.

2. Core Performance of UCG Technology

2.1. Resource Adaptation Advantages

2.1.1. Complex Deep Coal Seams

UCG technology can be used in the development of deep coal seams. This technology has been developed for coal seams with a buried depth of more than 300 m, especially for medium–deep coal seams with a buried depth of 500–2200 m. Through reasonable furnace design and process parameters, coal gasification can be realized, and the coal utilization rate and combustible gas conversion rate can be improved [25,26,27]. Even under supercritical conditions with a buried depth of more than 2200 m, after introducing advanced technologies such as supercritical hydrothermal combustion and high-pressure pyrolysis, the development of deep coalbed methane has certain feasibility, but the gasification efficiency is 8–12% lower than that of medium–deep coal seams, and the wellbore construction cost increases by about 30% [28].

2.1.2. Low-Quality Coal Seam

For low-quality coal seams, such as those with high moisture, high ash content, and low metamorphic grade, UCG technology also shows good adaptability. For example, the water content of the Miocene straight-chain lignite in Poland is as high as 46.5%, and the UCG process with oxygen blowing still achieves 59% energy efficiency, which is 15–20% lower than that of low-moisture bituminous coal under the same condition. In India, coal with an ash content of 42% can produce high-calorific-value syngas after UCG gasification, but the ash accumulation in the gasification channel will reduce the reaction continuity, requiring regular adjustment of gas injection pressure [29,30]. In addition, with the support of a two-step gasification process, lignite from Inner Mongolia and long flame coal from Xinjiang have also successfully achieved efficient hydrogen production, but the hydrogen yield is 0.3–0.43 m³/kg, which is slightly lower than that of high-quality bituminous coal (0.45–0.5 m³/kg) [31].

2.1.3. Multi-Rank Coal Adaptability

For coal seams of different coal ranks, UCG technology can achieve efficient development by flexibly adjusting process parameters. By adjusting the key reaction conditions, such as temperature, pressure, and gasification agent ratio, the composition and yield of syngas can be effectively optimized [32,33]. For an original coal seam with dense structure, the swirling flow injection technology can enhance the contact between the gasification agent and the coal body, thus significantly improving the reaction efficiency, but this technology is less effective for loose lignite (the improvement rate of the reaction efficiency is only 5–8%, compared with 15–20% for dense bituminous coal) [34]. At present, the evolution of the pore structure of bituminous coal with different volatiles during gasification is relatively clear, which provides a basis for its adaptation in the UCG process [35,36].

In practical applications, Wales’s “Six-Feet” semi-anthracite and Poland’s “Wesola” hard coal have successfully achieved stable production of methane-rich gases under high-pressure conditions [37]. The gasification energy efficiency of high-grade coal in South Wales under 30 bar high pressure is 57.67%, which is significantly higher than 51.72% under atmospheric pressure [38]. In addition, bituminous coal can generate syngas with high hydrogen content under a suitable H₂O and CO₂ ratio and specific temperature and pressure environment. Anthracite can be subjected to microbial-enhanced gasification (the “biogasification” mentioned in the text refers to this technology, which is different from the traditional gasification of biomass. It uses microbial action to promote the gasification reaction of coal, and related to coal fermentation provides a theoretical basis for the microbial action mechanism in this process. By means of the single-well nutrient injection method, it can effectively improve the methane production of the coal seam [39,40,41].

For resource exploitation under complex geological conditions such as steeply dipping thin coal seams and coal seams containing gangue, UCG technology shows unique adaptability. By adopting special structural designs, such as hollow bottoms and wall gasifiers, combined with a fan-shaped drilling layout and controllable mobile multi-point gas injection process, the recovery and utilization of such coal resources can be effectively realized [42,43].

In terms of applicable conditions, UCG technology can be widely used in single coal seams with a thickness of 2–15 m and a buried depth of 100–2000 m [44]. Taking China’s oil-bearing basins as an example, if UCG technology is used to convert coal seams with a buried depth of 1000–3000 m, it is expected to produce 272–332 × 10¹² m³ natural gas equivalent resources [45]. In addition, in a field test at Zhongliangshan Coal Mine in China, through the application of the shaft UCG process in a steeply inclined thin coal seam, not only were the waste coal resources successfully recovered, but clean gas conforming to the quality standard was also produced [46,47].

2.2. Resource Efficiency Advantages

2.2.1. Resource Development Potential

Global coal resources are abundant but unevenly distributed, and the application of UCG technology can effectively develop coal resources that are difficult to mine using traditional means, thereby increasing the total energy supply, but its development scale is limited by geological exploration accuracy and regional policy support. [48]. This technology can convert coal that cannot be mined due to economic or safety constraints into available energy, providing important support for ensuring energy security [49].

From the perspective of global practice, if all the proven coal reserves in Bangladesh are converted by UCG technology, they are equivalent to 975 × 10⁸ m³ natural gas, which is about three times its existing natural gas reserves, but the development progress is restricted by local water resource supply (UCG requires 2–3 m³ of water per 1000 m³ of syngas). Giving priority to the development of the Jamalganj coalfield is expected to significantly improve the country’s energy supply tensions [50]. In South Africa, plasma gasification of coal or biomass for hydrogen production, combined with carbon capture and storage technology, can achieve low-carbon emissions while meeting energy needs [51]. Comparing China’s UCG resources and natural gas resources, China has about 1485.3 × 10⁸ tons of remaining coal reserves, of which 87.15% are buried below 800 m. Through UCG technology, these difficult-to-recover resources can be converted into huge amounts of natural gas, effectively supplementing the energy supply [52].

2.2.2. Advantages of Hydrogen Production

In terms of hydrogen production, UCG technology has both efficiency and cost advantages, but the hydrogen purity and production stability are affected by coal quality and process control. By adjusting reaction conditions such as humidified oxygen and steam, the proportion of hydrogen in syngas can be effectively increased, but for high-sulfur coal, the H₂S content in syngas will increase by 0.5–1.2%, which requires additional desulfurization equipment [53]. For example, when using Inner Mongolia lignite for UCG hydrogen production, the hydrogen content in syngas can reach 45–50% under optimal conditions, but the hydrogen purity after pressure swing adsorption purification is 99.9–99.99%, which is 0.01–0.05% lower than that of SCG hydrogen production (99.99–99.999%) [54]. In addition, the intermittent expansion of the underground gasification cavity may lead to fluctuations in hydrogen production (fluctuation range of 5–8%), which requires matching with buffer gas storage equipment.

Synthesis of

Table 1. Hydrogen production by UCG technology.

Investigator	Research Theme	Core Content
Huan Liudeng, et al. [54]	Hydrogen production cost of UCG	The hydrogen production cost of UCG is 77.1% of SCG, 0.11 USD/m3, but the equipment maintenance cost increases by 10–15% due to underground uncertainty.
Muhammad Imran, et al. [12]	UCG environmental protection and resource consumption	The amount of solid waste and water is significantly reduced, but the treatment cost of toxic pollutants (e.g., phenols) is 8–12% higher than that of SCG
Zixiang Wei, et al. [23]	Optimization of UCG hydrogen production process	Multi-point water injection hydrogen extraction is 11%.
Zhen Yin, et al. [31]	Two-step gasification of coal to produce hydrogen	The total hydrogen output is 0.3–0.43 m3/kg, accounting for 53.08–56.60% of the effective gas, but the production fluctuation range is 5–8%.
Surya Kanta, et al. [53]	UCG hydrogen production efficiency improvement	Humidifying oxygen, steam, and optimizing reaction conditions to increase hydrogen content in syngas.
Aman Verma, et al. [55]	Collaboration of UCG and CCS	UCG combined with CCS to achieve environmental and economic coordination.
Liu Shuqin, et al. [56]	Advantages of hydrogen production by UCG-CCS	Low greenhouse gas emissions, low cost.
Qin yong, et al. [57]	UCG crude gas economy	The economic competitiveness of crude gas utilization has been verified.
Chen jingrui, et al. [58]	UCG multi-domain benefits	It has significant benefits in power generation, hydrogen production, and chemical production.
Huan Liu, et al. [59]	UCG hydrogen production energy consumption and CO2 capture set	At an 80% CO2 capture rate, the energy consumption of UCG is 61.2% that of SCG.
Huan Liu, et al. [60]	Deep IGCtH technology advantages	The cumulative energy consumption is 83.6% of Lurgi surface gasification, and the investment is 68.7%.
K. Kostur, J, et al. [61]	UCG operation and security	Without underground manual operation, it is expected to reduce capital operation costs.
Yi tongsheng, et al. [62]	Economic influencing factors of UCG project	The synthetic gas utilization route and CO2 treatment method or cost have a great impact.
Krzysztof Stanczysztof, et al. [63]	Two-stage gasification of hard coal for hydrogen production	The average hydrogen concentration in the steam phase is 53.77% after alternating oxygen or steam injection.
Vman Verma, et al. [64]	Application and environmental protection of UCG products	Syngas can generate electricity and produce hydrocarbon fuel or hydrogen; UCG+CCS hydrogen production, carbon emissions reduction.
Greg Perkins, et al. [65]	Usage of UCG syngas	Available for fuel and chemical production.
Zong kaiqiang, et al. [66]	Optimization of UCG hydrogen extraction parameters	When the adsorption time is 620 s and the pressure is 3.5 MPa, the purity of H2 is 99.969% and the recovery rate is 78.3%.
Haiyang Fan, et al. [67]	Environmental benefits of UCG-CCS	UCG combines CCS to extract hydrogen production, reduce carbon emissions, and improve energy cleanliness.

: The operational parameters most strongly affecting hydrogen yield include the gasification agent ratio (a steam-to-oxygen ratio of 2.5 is optimal), oxygen concentration (40–70% volume fraction), and reaction pressure (exceeding 2 MPa promotes coke formation and hydrogen production). In addition, process optimization measures, such as multi-point water injection and swirl flow injection, can further improve hydrogen extraction efficiency by 11% and enhance gasification reaction uniformity, respectively.

2.2.3. Advantages of Syngas

The syngas produced by UCG technology has diversified application prospects, but its quality is easily affected by coal quality and process parameters, and there may be problems such as low calorific value and high impurity content in some cases, which increases the difficulty of subsequent purification and utilization. It can not only be directly used for power generation but can also be used as a chemical raw material to produce high-value-added products, such as methanol and ammonia, which effectively expands the supply path of energy products [68,69].

In terms of syngas characteristics, when oxygen and steam are used as gasification agents, the calorific value of syngas varies with coal type and process conditions: for example, under an oxygen volume fraction of 40–70%, the calorific value range of syngas is 10–12 MJ/m³ (the previously mentioned 10.48–12.48 MJ/m³ is the test data under specific experimental conditions, and refs. [70,71] are the same research with different citation numbers due to layout adjustment, and the data source is consistent. If low-ash Indian coal is used and a two-stage gasification process is used, the calorific value of syngas can be as high as 265 KJ/mol [72]. These syngases can be directly used for power generation. For example, the pilot project of the Wieczorek coal mine in Poland has successfully used gasification products for energy production [73]. In the UCG-power generation co-generation test at “Ankou” mine, the oxygen-rich gasification process significantly improved the combustible components and calorific value of the syngas, and the specially modified gas generator set realized the efficient utilization of the syngas [74,75].

The syngas also has a wider application potential in the field of power generation. Taking the flat tube solid oxide battery as an example, under the conditions of using UCG syngas as fuel and an operating temperature of 750 °C, the maximum power density can reach 329.4 mW·cm⁻² and can stably complete 100 reversible cycles [76,77]. To meet the high-purity gas demand of fuel cells, UCG syngas needs to go through multi-stage purification processes, such as pressure swing adsorption (PSA), acid gas removal, and tar cracking, which can effectively remove impurities such as CO, H₂S, and tar, and the gas purity can reach more than 99.9%.

Chemical synthesis is another important application direction. UCG syngas can be used to prepare basic chemicals such as ammonia and methanol [78]. For example, by integrating UCG, smelting reduction, and methanol synthesis processes, high-quality pig iron and methanol can be produced simultaneously. This process not only has low energy consumption and high energy efficiency but also significantly reduces carbon dioxide emissions [79]. In addition, UCG syngas can also meet the comprehensive energy needs of the community for power generation, heating, refrigeration, hydrogen production, and ethanol production. The overall environmental performance is better than that of traditional coal-fired power generation. In China’s coal-dominated energy structure, UCG can be used as a front-end technology for the coal chemical industry and integrated coal gasification combined cycle power generation, helping the clean transformation of the fossil energy supply side [80,81].

By optimizing key parameters such as the oxygen–coal ratio and steam–coal ratio, the hydrogen yield can be further improved [82]. Studies have shown that when the ratio of steam to oxygen is controlled at 2.5 (mass ratio), the hydrogen production effect of UCG is the best [83]. If this ratio is not controlled, the hydrogen production efficiency will decrease, the content of impurities such as CO in the syngas will increase, and the subsequent purification cost will rise. This ratio is applicable to medium- and low-rank coal gasification under normal pressure and medium temperature conditions (the time duration is not important here, and the ratio is a steady-state control parameter during the gasification process). It is a swirl gasification experiment. The swirl gasification experiment data come from ref. [84], and the oxygen flow rate of 2 L/min is the laboratory-scale test parameter. Although it is different from the industrial-scale operating conditions (the industrial oxygen flow rate is usually 1000–5000 m³/h), it can provide a theoretical basis for the optimization of the industrial-scale swirl injection process (such as verifying that swirl flow is better than straight flow in improving gasification efficiency). Under the conditions of an oxygen flow rate of 2 L/min and a steam volume of 1 g/L, hydrogen production can reach up to 19 L, which is better than the direct current injection metho. As the oxygen flow rate increases from 0.5 L/min to 2 L/min, the swirl flow injection mode is superior to the traditional straight flow injection mode in terms of gasification performance, calorific value improvement, and tar control [85].

In addition, in the CO₂-O₂ gasification system, the calorific value of syngas obtained by two-stage gasification of low-ash coal is 265 KJ/mol, and the single-stage gasification can also reach 250 KJ/mol. The average methane concentration of “Six Feet” semi-anthracite at 40 bar pressure can reach 27.03% [86]. This syngas can not only be used for gas–steam combined cycle power generation but can also be further converted into ammonia, methanol, and other chemicals. If the overall gasification combined cycle power generation is combined with carbon capture and storage technology, low-carbon power production can also be achieved [87].

2.3. Environmental Advantages

Compared with traditional mining technology, UCG technology has significant environmental friendliness. This technology can effectively reduce dust, noise pollution, and water consumption because it greatly reduces the downhole operation and avoids the ground disposal process of ash and cinder. The amount of solid waste produced by this technology is only half of that of traditional coal-fired power plants (calculated by mass per ton of coal) [88]. At the same time, UCG does not require large-scale surface excavation, thereby reducing land occupation and ecological damage [89]. However, during the gasification process, heavy metals (e.g., arsenic, cadmium) and organic pollutants (e.g., phenols, polycyclic aromatic hydrocarbons) in coal may leak into groundwater with reaction water. The concentration of phenols in groundwater near the UCG test area in Poland reached 0.8–1.2 mg/L, exceeding the national standard by 4–6 times. In addition, the syngas contains trace tar components, which may cause air pollution if the purification is incomplete, and the tar removal rate needs to reach more than 99.5% to meet the emission standards.

In the process of power generation, the UCG technology of the air–steam continuous process is adopted, and the comprehensive environmental benefits generated per 10,000 kilowatts of electricity are better than the traditional coal mining power generation method [90]. The “air–steam continuous process of power generation” refers to the power generation technology that uses air and steam as continuous gasification agents for UCG—air provides oxygen for combustion, steam participates in the gasification reaction to adjust the syngas composition, and the generated syngas is purified and then directly drives the gas generator set to generate electricity. “Fluidized development” refers to the efficient and coordinated development mode of UCG technology with other energy utilization technologies (such as geothermal utilization, coalbed methane extraction) through the integration of multi-energy systems, realizing the cascade utilization of energy. Compared with the ground gasification technology, UCG can effectively reduce direct damage to the surface, especially for deep coal seams that are not mined or difficult to enter, and can avoid the surface subsidence caused by conventional mining. For example, through reasonable UCG process design, the Wieczorek mine in Poland minimizes the disturbance of the surface environment while achieving stable gas production [91]. The comparison of UCG, traditional mining and multi-dimensional SCG are presented in

Table 2. UCG, traditional mining, and SCG multi-dimensional comparison.

Investigator	Evaluation Dimension	Traditional Mining	SCG	UCG
Huaizhan Li, et al. [10] T. C. Ekneligoda, et al. [92]	Recovery rate of resources (%)	40	50–55	60+, but varies by ±5% with coal seam stability
Muhammad Imran, et al. [12] Dorota Burchart, et al. [88]	Solid waste amount (relative value)	100	80	50, but toxic pollutants in waste require special treatment
Huan Liu, et al. [54]	Hydrogen production cost (USD/Nm3)	-	0.25–0.30	0.11–0.16, but additional environmental risk prevention costs account for 3–5% of the total cost
Marian Wiatowski, et al. [93] Wu Gao, et al. [94]	Surface subsidence risk	High	Low (no mining activity)	Low (in situ gasification)

.

2.3.1. Carbon Emission Reductions

Calcium and magnesium oxides in coal ash are derived from the decomposition of carbonate minerals in coal, which will release CO₂ during decomposition. Therefore, the reaction of coal ash with CO₂ does not achieve additional carbon sequestration and only fixes part of the CO₂ generated by gasification in the form of carbonate. The coal ash produced in the UCG process is rich in alkaline earth metal oxides such as calcium, which can react with CO₂ to realize this fixation effect. Studies have shown that the underground pores formed by UCG in China have the potential for CO₂ storage of 29 × 10⁸ to 102 × 10⁸ tons. If the fly ash is further filled, an additional 517 × 10⁸ tons can be stored [95]. In addition, the underground cavity formed by UCG can be used as an ideal CO₂ sealed storage space, which can achieve stable storage for hundreds of years in deep unmineable coal seams, and its matrix–fracture diffusion equilibrium rate can reach 0.24 m/year [96]. Through the UCG-CCS coupling process, the near-zero emission conversion of deep thin coal seam resources can be realized [97].

In terms of environmental benefits, when UCG technology is combined with CCS technology, although it has a high global warming potential, the overall impact on ecosystem quality, human health, and resource consumption is reduced by 23%, 15%, and 4%, respectively, compared with the ground gasification combined cycle [98]. Compared with the traditional steam methane reforming process, UCG-CCS technology can reduce greenhouse gas emissions by about 15.3 million tons. In the field of hydrogen production, the carbon emission intensity of UCG-CCS is only 0.1–1.1 kg of CO₂ or H₂, which is much lower than that of the process without CCS. South Africa has also achieved a significant reduction in greenhouse gas emissions by coal or biomass plasma gasification to produce hydrogen and coupling CO₂ capture [99].

It should be emphasized that CCS technology not only requires capture equipment and storage reservoirs but also needs to be equipped with a CO₂ transportation system. When CO₂ is stored in the underground cavity formed by UCG (the place where it is produced), the transportation distance is greatly shortened (usually within 1–5 km), and the transportation cost can be reduced by more than 60% compared with long-distance pipeline transportation (which usually costs 2.01–4.1 USD/ton for long distances).

In addition to carbon sequestration, the cavity formed by UCG can also be transformed into an “artificial gas reservoir” in combination with geological conditions. This innovative idea also provides a useful reference for the fluidized development of other mineral resources [100].

2.3.2. Resource Utilization of Waste

UCG residues are retained underground after gasification, and mining them will increase environmental disturbance and cost, which is contrary to the core advantage of UCG (avoiding surface mining). The correct resource utilization path is in situ utilization: using underground residues to fill gasification cavities to enhance formation stability and prevent groundwater infiltration.

The coal gasification slag produced in the UCG process has considerable resource potential and can be efficiently recycled through appropriate treatment [101]. These waste residues can be used to prepare building materials, which not only realizes the recycling of waste but also effectively reduces land occupation and environmental pollution [102].

In terms of tar utilization, UCG tar and coke oven tar show good compatibility and have co-processing conditions. The calorific value of tar produced under some working conditions can reach 39 MJ/kg, which is better than that of typical coke oven tar [103]. If coal tar is converted into syngas using the chemical cycle gasification process, 84.1% carbon conversion and 88.14% syngas selectivity can be achieved, and the conversion efficiency is remarkable [104].

In addition, the decarbonized coal gasification slag can be used as a soil amendment for sandy soil treatment [105]. Studies have shown that the addition of this material can increase soil organic carbon by 22.4% and alkaline phosphatase activity by 16.5%. Its ecological risk index is less than 150, and the recommended application amount is 60 tons/ha, showing good ecological application value [106]. At present, there is still much room for improvement in the resource recovery efficiency of UCG technology. Limited by incomplete gasification and geological conditions, some coal resources are difficult to effectively utilize. Specifically, in the process of gasification channel expansion, the edge area often leads to coal residue due to uneven temperature distribution and insufficient gasification agent transportation [107].

2.3.3. Pollution Control Measures

In the UCG pilot study carried out in the Wieczorek mine, the researchers continued to carry out systematic water quality monitoring before, during, and for 5 years after the experiment, providing important long-term data support for the prevention and control of groundwater and surface water pollution [108]. At the same time, the study of the chemical forms and transformation mechanism of toxic elements such as zinc, beryllium, and nickel in UCG residues shows that when the reaction temperature reaches 1200 °C, the environmental risk of residues can be effectively reduced to a safe range [109].

• Wastewater treatment

Wastewater containing benzene series, heavy metals, and other pollutants may be produced in the UCG process. These wastewaters can be initially purified by the natural adsorption properties of the rock and soil around the gasification holes, effectively reducing their environmental risks [110]. By optimizing the gasification process parameters to reduce wastewater production, and with appropriate water treatment technology, wastewater can also be recycled or discharged up to standard [111]. For example, the combined process of adsorption–electrocoagulation–wetland can remove more than 96% of iron, nickel, and other metals in wastewater, and the removal rate of phenol and BTEX organic matter is close to 100% [112]. In addition, TiO₂ or RGO binary photocatalyst can efficiently degrade 97.87% of phenol, and the removal rate of total organic carbon in UCG wastewater also reached 67%, showing a good application prospect [113].

h⁺ + H₂O → ·OH + H⁺

(1)

O₂⁻ + e⁻ → O₂⁻

(2)

C₆H₅OH + 11O₂⁻ + 11·OH + 3h⁺ → 6CO₂ + 7H₂O

(3)

Note: In the following reaction equations, h⁺ represents photogenerated holes, and ·OH represents hydroxyl radicals (active species).

Therefore, TiO₂ or RGO binary photocatalyst shows good application potential in the purification of organic wastewater and UCG wastewater. At the same time, the oxidation system represented by Ce(SO₄)₂·4H₂O can also effectively degrade pollutants such as polycyclic aromatic hydrocarbons and phenols in wastewater [114]. The removal rate of chemical oxygen demand in UCG combustion zone wastewater can reach 60.23%. The effect is better than that of the chemical coagulation method, which is more suitable for on-site treatment of combustion zone wastewater, thus providing a feasible way to control the impact of UCG wastewater on the environment [115,116].

2.

Heavy metal treatment

The shaftless UCG technology can effectively control the development of fractures by optimizing the operating parameters of the gasifier, thereby reducing the release risk of pollutants such as arsenic and cadmium from the source [117]. In terms of pollutant adsorption, the semi-coke produced by UCG treatment of Hebi coal presents a rich porous structure, and the removal rate of phenol in wastewater can reach up to about 35% [118]. Further studies showed that UCG residual chars with different degrees of coalification showed significant removal effects on phenol and hexavalent chromium in simulated wastewater. Among them, the adsorption efficiency of phenol by lignite residual coke can reach 72.44%, which achieves the goal of “treating waste with waste” In addition, the removal rate of total organic carbon in gas washing water by UCG residual coke is more than 82%, and it can effectively remove most of the heavy metal elements such as chromium, selenium, and some nickel, showing good multi-pollutant synergistic purification ability [119].

Although the resource suitability of UCG technology has been verified by many countries, the definition of its advantageous application scenarios is still not clear enough. For example, the difference in gasification efficiency between deep coal seams and medium–shallow coal seams with a burial depth of more than 2200 m has not been clearly quantified, and there is also a lack of unified criteria for the lower limit of energy efficiency for low-quality coal gasification processes. These cognitive gaps blur the applicable boundary of the technology to some extent.

2.4. Intensification of Gasification Process

The intensification of the UCG gasification process is crucial to improving reaction efficiency and product quality. Recent critical reviews have proposed a framework for comparing physical and chemical intensification methods, which is consistent with the efficiency improvement mechanism discussed in this study. Physical intensification methods mainly include thermal–oxidative activation of gasification agents and swirl flow injection: thermal–oxidative activation preheats the gasification agent to 300–500 °C, which can significantly reduce the ignition temperature of coal (by 50–80 °C) and promote the rapid formation of reaction channels; swirl flow injection enhances the turbulence of gasification agents, improves mass transfer efficiency at the gas–solid interface, and increases the reaction rate by 15–20% [120].

Chemical intensification methods focus on optimizing the composition of gasification agents and adding catalysts. For example, adding 0.5–1% alkali metal catalysts (e.g., K₂CO₃) to the gasification agent can promote the cracking of coal macromolecules and improve the hydrogen yield by 8–12%. In addition, the combination of CO₂ and oxygen as mixed gasification agents can realize the dry reforming reaction of syngas (CO₂ + CH₄ = 2CO + 2H₂), which not only increases the calorific value of syngas, but also reduces carbon emissions. The current analysis of gasification efficiency in this study aligns with these intensification mechanisms and further incorporates the influence of geological conditions (e.g., coal seam permeability) and process parameters (e.g., gas injection pressure) to form a more comprehensive efficiency evaluation system.

3. Economic Assessment

Economic feasibility is a key factor to evaluate the large-scale application of UCG technology. At present, the traditional ground gasification process has significantly increased the comprehensive cost of coalbed methane conversion and utilization due to the complex pretreatment process and high equipment investment. In order to accurately evaluate the economic value of UCG technology, this study will systematically compare and analyze the two technical paths of UCG hydrogen production and ground gasification hydrogen production based on the actual operation data of typical high-metamorphic coalbed methane demonstration projects in China, focusing on the core economic indicators such as investment cost, operation energy consumption, product income, and environmental cost compensation. At the same time, supplementary comparison with hydrogen production technologies based on natural gas conversion and electrolysis is carried out. The results show that although the hydrogen production cost of natural gas conversion is 0.17–0.25 USD/m³ (similar to SCG) and its resource dependence is high (relying on imported natural gas in most regions). The hydrogen production cost of electrolysis is 0.34–0.48 USD/m³, which is significantly higher than UCG, but it has the advantage of zero carbon emissions under the condition of using renewable energy power (such as wind and solar power).

3.1. Comparative Analysis of Investment Costs

In the process of UCG hydrogen production, the investment cost mainly covers the construction of the gasification system, auxiliary equipment configuration, and early technology research and development. For the mid-deep UCG hydrogen production project, the investment cost is usually relatively high due to the need to implement more complex well engineering and to be equipped with special equipment. In contrast, the investment of traditional ground coal gasification for hydrogen production is mainly focused on the construction of a coal gasifier, gas purification system, hydrogen separation and purification device, and raw material transportation and storage facilities [121].

3.1.1. Investment Cost of UCG Hydrogen Production

In terms of total investment cost, UCG hydrogen production shows significant advantages over traditional ground gasification hydrogen production, mainly due to the elimination of multiple high-cost links and the reuse of facilities. The results show that the total investment of UCG hydrogen production is 77.1% of that of ground gasification hydrogen production without a CCS device. Even if the CCS capture rate reaches 80%, its proportion remains at 81.8%, which continues to be cost-competitive.

It should be clarified that the technical positioning and application scenarios of UCG, traditional mining, and SCG are distinct.

Traditional mining: Focuses on extracting commercial coal for multi-purpose utilization (power generation, coking, liquefaction, gasification), with advantages of flexible resource allocation and mature transportation channels, but limited by recoverable coal seam conditions (shallow, thick, low ash/moisture).

SCG: Converts mined coal into syngas/hydrogen on the ground. Suitable for large-scale centralized production but bears high costs of coal mining, transportation, and pretreatment.

UCG: Directly gasifies difficult-to-mine coal seams (deep, thin, high ash/moisture) into syngas onsite, avoiding coal mining and transportation links, and is mainly used for onsite power generation, hydrogen production, or local chemical utilization. Due to the immaturity of long-distance hydrogen transportation technology, UCG’s economic advantage is more prominent in regional energy supply scenarios.

This cost advantage comes from the savings of multiple links. First, UCG does not need to build a ground gasifier, and its gasification unit equipment investment only accounts for 29.9% of the ground gasification hydrogen production. Specifically, the gasification unit in the ground process accounts for 47.7% of the total equipment investment, while the proportion of UCG is only 18.5%. At the same time, the module construction cost is 1402 USD/m, which is much lower than that of the ground gasifier [122]. Secondly, UCG eliminates the upstream links, such as coal mining, transportation, and large-scale ground reactor construction, which reduces capital investment to about 75% of the ground process. In addition, downhole gasification does not require a high-pressure reaction vessel, and only a well group with a depth and spacing of about 1400 m and a diameter of 11.4 cm is required, and the cost of a single well is low. The demand for land occupation is small, and the investment in land and infrastructure is reduced. The reusability of the existing well pattern has also significantly reduced the demand for new drilling, reducing the drilling-related investment by more than 50% [123].

3.1.2. SCG Hydrogen Production Investment Cost

In contrast, the total capital investment of SCG-H₂ is higher. When CCS is not configured, its investment reaches 765.94 million USD, which is about 25–30% higher than UCG-H₂. This is mainly due to its need to comprehensively build ground facilities and invest in a large number of supporting systems.

Compared with the UCG process, SCG-H₂ needs a complete upstream industrial chain investment from coal mining, washing, and transportation, and requires construction of a large ground gasifier and syngas treatment device. Among them, only the ground gasification reactor and coal pretreatment equipment account for more than 40% of the total capital investment. Due to the greater oxygen consumption of the SCG process, a larger-scale air separation device and a wastewater treatment system are needed, which further increases the investment cost.

3.2. Comparative Analysis of Operating Expenditure

3.2.1. UCG Hydrogen Production Operation Expenditure

UCG-H₂ shows a significant cost advantage in terms of operating expenses, which is mainly due to its effective savings in multiple links and a significant reduction in environmental treatment costs.

In terms of raw material costs, due to the direct use of difficult-to-mine coal seams, high-sulfur coal, and other resources, there is almost no need to bear the cost of raw materials, and only the diesel consumption during the module construction process needs to be paid. At the same time, technology eliminates the pretreatment of raw materials, which reduces the operating expenses of this part.

The electricity and steam consumption accounts for 31.3% of the utility cost, which is higher than that of SCG-H₂. However, from the overall energy consumption point of view, UCG-H₂ is still more advantageous. Because of the large heat loss of the ground gasifier and the additional energy consumption for coal drying, the energy consumption per ton of coal in SCG-H₂ is 20–30% higher than that in UCG-H₂. In view of the fact that UCG does not involve coal mining costs, using specific carbon emissions (kg CO₂/kg H₂) as an evaluation index is more reasonable than production per ton of coal. The specific carbon emission of UCG-H₂ is 0.8–1.5 kg CO₂/kg H₂, which is lower than the 1.2–2.0 kg CO₂/kg H₂ of SCG-H₂.

In terms of maintenance and labor, the underground unmanned operation makes the labor cost only 40–60% of the SCG and only needs to maintain the ground equipment and monitor the underground state. Although regular groundwater quality monitoring is required, resulting in a small amount of environmental monitoring expenditure, the operation and maintenance cost accounts for 44.1%, of which the annual new module cost is 36.69 million USD, which is still lower than the raw material cost of SCG-H₂. The depreciation cost of equipment accounts for 10.3%, which is also lower than the 18.5% for SCG-H₂, thanks to its lower total investment and module depreciation cycle synchronized with coal seam mining.

In terms of environmental protection expenditure, when CCS is configured, the CO₂ capture cost is about 30–50 USD/ton, which is lower than that for SCG-H₂. This is because the concentration of CO₂ in UCG syngas is more stable, and the capture efficiency can reach 91.6%. The cost of CO₂ transportation and storage is only 8.83 USD/ton, which is also lower than that of SCG-H₂. In addition, the ash is directly retained underground, and H₂S can also be converted into easy-to-treat substances, eliminating the cost of ground ash transportation and environmental protection treatment.

3.2.2. SCG Hydrogen Production Operating Expenses

In terms of raw materials and energy consumption, SCG-H₂ needs to purchase commercial coal, and its coal transportation and washing costs account for about 30–40% of the operating expenses. Taking the SCG-H₂ project with an annual output of 12 × 10⁸ m³ of hydrogen as an example, 263 tons of coal are consumed per hour. At 82.8 USD per ton, the cost of raw materials accounts for 62% of the total cost of the product, and the annual raw material expenditure is extremely high. At the same time, the ground gasifier needs to put in more steam and oxygen, resulting in higher energy consumption and steam preparation cost of the air separation unit than the UCG process. In addition, the transportation cost of coal from the mine to the gasification plant accounts for 5–15% of the CCS budget, and the annual transportation expenditure accounts for about 8–12% of the operating cost.

In terms of maintenance and labor, SCG-H₂ needs to be equipped with multiple teams, such as mining, gasifier operation, and equipment maintenance, and its labor cost is 1.5–2 times that of UCG. The maintenance frequency and cost of ground equipment are also generally higher than those of UCG. The regular maintenance of crushing equipment accounts for about 5–8% of operating costs every year.

In terms of environmental protection expenditure, SCG-H₂ has to bear the ground treatment cost of solid waste, which is about 0.69–1.38 USD/ton of coal. The cost of wastewater treatment is also higher than that of UCG, and the treatment cost of high-salt wastewater is about 0.28–0.69 USD/ton. Taking a 65 MWth project as an example, the annual environmental protection expenditure is more than 2.76 million USD. Although its CO₂ capture efficiency is comparable to that of UCG, the unit capture integration cost is 10–15% higher. In the case of 80% CCS configuration (capture efficiency ≥ 90%, CO₂ removal ratio ≥ 95%), the total system cost is 30% higher than that of UCG-H₂.

3.3. Comparative Analysis of Product Revenue

3.3.1. Benefits of UCG Hydrogen Production Products

UCG-H₂ shows a diversified advantage in product revenue, and its main product has significant economic benefits, while by-products and carbon sequestration can also bring considerable additional benefits.

• In terms of the main product hydrogen, the coal–hydrogen conversion efficiency can reach 58.1% under the baseline condition, which is equivalent to the SCG process level. By optimizing the proportion of gasification agent, the hydrogen content in the syngas produced by the modern UCG process is increased to 40–55%, and the purity can reach 99.9–99.99% after pressure swing adsorption purification. The market price is the same as that of SCG-H₂.
• In terms of unit cost, UCG-H₂ has obvious advantages. The hydrogen cost without CCS is 0.1 USD/m³, which is only 43.2% of SCG-H₂, and even if 80% of the CCS system is configured, the cost rises to 0.14 USD/m³, which is still significantly lower than 0.27 USD/m³ of SCG-H₂.
• In terms of by-products and carbon benefits, UCG-H₂ can co-produce electricity. Without CCS, about 4.7% of coal energy is converted into on-grid electricity. After CCS is configured, the conversion rate can still maintain 2.4%, and the corresponding electricity revenue is about 0.05–0.10 USD. If combined with a solid oxide fuel cell system, the co-production efficiency can be further improved [124]. In addition, methane rich in syngas can be used as a fuel supplement or converted into additional hydrogen through steam reforming, increasing the total hydrogen yield by 10–15%, or co-mining with coalbed methane for purification and sales. After CCS is configured, CO₂ can be directly stored in the underground cavity, and the storage capacity of a single cavity with a depth of 1000 m is 1700–4500 tons. According to the current carbon price of 8.28 USD/ton, the annual carbon income is about 0.41–0.69 million USD, and CO₂ can be used to drive crude oil exploitation to improve the comprehensive income [125].

3.3.2. Profit of SCG Hydrogen Production Products

In terms of the main product hydrogen, the coal-to-hydrogen conversion efficiency of SCG-H₂ is between 44.5% and 69%, and the gasification efficiency of high-quality bituminous coal can exceed 60%, which is comparable to the UCG process. The CatBoost model used for efficiency prediction is from the research of Singh and Kumar [121], which can accurately predict the hydrogen content in syngas after the gasification of high-ash coal by inputting parameters such as coal ash content, gasification temperature, and pressure. The prediction based on the CatBoost model shows that after the gasification of high-ash coal by circulating through the fluidized bed, the hydrogen content in the syngas is about 35.7–38%, and the purity after purification can reach 99.5–99.99%, and its market income is basically the same as UCG-H₂. A 65 MWth scale SCG-H₂ project with an annual hydrogen production capacity of about 12 × 10⁸ m³ and annual revenue of about 49.7–74.5 million USD is lower than that of UCG-H₂.

In terms of unit cost, the hydrogen cost of SCG-H₂ without CCS is 0.23 USD/m³, and it rises to 0.27 USD/m³ after 80% CCS, which is higher than the corresponding cost of UCG-H₂.

In terms of by-products and carbon benefits, SCG-H₂ can co-produce more steam, and its power co-production efficiency without CCS is 38–43%, which is significantly higher than 4.7% of UCG, so the power benefit is more prominent. Although the generated ash can be processed into building materials and obtain about 0.69–1.38 USD/ton of income, the actual contribution is limited due to the need for additional processing. If CCS is configured, it is necessary to build a special carbon dioxide capture system, which accounts for 20–30% of the total investment of the project. The cost of carbon storage is high, and the carbon income generated can only cover part of the capture integration cost.

3.4. Comparative Analysis of Environmental Cost Compensation

3.4.1. Environmental Cost Compensation of UCG Hydrogen Production

In terms of environmental cost compensation, UCG technology shows many positive benefits. By using difficult-to-mine coal seams, the project can obtain resource development subsidies provided by the government. This technology does not require surface mining and effectively avoids surface subsidence and coal dust emissions. The amount of surface subsidence is only one-third of that of traditional mining, which can save about 100–200 USD/ha of land reclamation costs. In terms of emission control, UCG can convert H₂S into easy-to-treat substances, so that SO₂ emissions can be reduced by more than 80% compared with SCG-H₂, so it is eligible to apply for ecological compensation funds, with an annual compensation amount of about 0.69–1.38 million USD. Through the use of “clean cave” technology, the system can control the risk of groundwater pollution at a low level and effectively avoid environmental penalties. In addition, due to the low consumption of water resources, corresponding water-saving subsidies can also be obtained in water-deficient areas. In terms of carbon management, its CO₂ sequestration cost is only 1.38–2.76 USD/ton, which is much lower than the 6.9–11.03 USD/ton of ground CCS. Combined with the annual storage scale of 1–2 million tons, it is expected to achieve an annual carbon trading income of 0.83–1.66 million USD while significantly reducing carbon tax expenditures.

When 80% CCS system is configured, UCG can achieve 6863–10,078 tons of CO₂ emission reduction per day, and its carbon emission reduction cost is 17.9 USD/ton CO₂, which is lower than 19.7 USD/ton CO₂ of SCG-H₂, showing better carbon emission reduction economy.

3.4.2. SCG Hydrogen Production Environmental Cost Compensation

SCG-H₂ has limited benefits in terms of environmental cost compensation, while negative environmental costs are higher, which partially offsets its compensation benefits.

In terms of positive compensation, SCG technology is highly mature, and it is easy to obtain government environmental protection certification. The ash used for brick making, road construction, and other purposes can be subsidized by about 5–10 USD/ton. If biomass co-gasification technology is adopted, greenhouse gas emissions can be reduced, thus meeting the requirements of the renewable energy subsidy policy. In addition, environmental compliance projects can also receive special subsidies of about 0.28–0.41 million USD per year.

In general, the environmental cost compensation ability of SCG-H₂ is much lower than that of UCG-H₂. The core economic comparison of the two technical routes is shown in

Table 3. Comparison of cost and expenditure between UCG-H₂ and SCG-H₂.

	UCG-H2	SCG-H2	Advantage Side
Total investment cost	About 65–82% of the SCG	The benchmark is higher	UCG-H2
Cost of material	Almost zero	Accounts for 62% of the product cost	UCG-H2
Energy consumption	20–30% lower than SCG.	Higher	UCG-H2
Hydrogen unit cost	0.1–0.147 USD/Nm3	0.2366–0.2772 USD/Nm3	UCG-H2
Environmental treatment cost	Lower (ash left underground)	Higher (need to deal with solid waste, wastewater)	UCG-H2

and

Table 4. Comparison of benefits and risks between UCG-H₂ and SCG-H₂.

	UCG-H2	SCG-H2	Comparison
By-product income	Diversification (electricity, methane, carbon gains)	Diversified (electricity, steam, ash, hydrogen, synthetic fuel)	SCG-H2 has advantages in by-product types, but UCG-H2 has higher comprehensive benefits.
Environmental compensation	High (multiple subsidies, carbon gains)	Low (limited subsidy)	UCG-H2 is more favored by policy.
Technology maturity	Newer, great potential	Relatively mature, but there are still stability and reliability limitations	SCG-H2 is more mature in application but has obvious limitations.
Main risk	Groundwater pollution (high potential treatment costs)	High normalized environmental protection expenditure	SCG is currently costly, and UCG risk is concentrated.

below.

The core advantages of UCG-H₂ are the limited disruption, low cost, and better environmental protection potential, but the technical and management risks are high.

The advantage of SCG-H₂ is that the technology is mature and stable, but it faces the inherent disadvantages of high cost and high emissions.

4. Challenges and Future

UCG technology also faces a series of technical challenges in practical applications. First, the complexity of coal components in the gasification process will lead to the formation of a variety of highly toxic pollutants, and some pollutants are difficult to degrade naturally. For example, the concentration of chemical oxygen demand in the generated wastewater is as high as 555–703 mg/L, which is extremely difficult to treat. Secondly, the high-temperature environment may change the permeability characteristics of the surrounding rock. If the formation control measures are not in place, the leakage of synthetic gas will bring double risks to safety and the environment. There have been cases of dangerous gas accumulation in the “Barbara” experimental mine in Poland [126,127].

4.1. Environmental Risk

4.1.1. Waste Water and Gas Pollution

In the UCG process, groundwater infiltration into the gasification zone or reaction water may dissolve pollutants such as heavy metals and organic matter in coal, forming wastewater with complex components. If not handled properly, these wastewaters may seep into the groundwater system, causing long-term harm to water resources and the ecological environment, and threatening public health [128]. In addition, when the gasification parameters are not adequately controlled, the wastewater production may increase significantly, further exacerbating the pollution load. In the existing treatment technology, although the oxidation system can remove most of the pollutants, the removal rate of dissolved organic carbon is only 76.35%, and there is still a problem of incomplete treatment. The removal rates of iron, nickel, antimony, and arsenic by the electrocoagulation system reached 96%, 98%, 94%, and 82%, respectively, but the removal effect for manganese was not good. Although the removal rates of phenol, BTEX, and cyanide in all systems were close to 100%, the effluent still detected biological toxicity after 14 days.

On the other hand, the syngas produced by UCG contains harmful components such as carbon monoxide, hydrogen sulfide, and tar [129]. If incomplete purification leads to leakage, it will cause air pollution and endanger human health and ecological safety [130]. The polycyclic aromatic hydrocarbons contained in tar have strong carcinogenicity, and their release will pose a serious threat to the surrounding environment. The “Barbara” experimental mine in Poland has experienced a dangerous gas accumulation event. The fault tree analysis confirms that there is an explosion risk, and the mixture needs to reach the lowest oxygen concentration to cause an explosion [131]. Gas migration in the formation is controlled by lithology: shale inhibits gas diffusion due to low permeability, while coal and sandstone form a wider range of gas plumes due to poor water holding capacity, and higher formation pressure inhibits gas diffusion [132]. When gaseous pollutants migrate in porous media, porosity, pore size, and channel tortuosity are the key control factors, and the liquid phase migration rate is significantly lower than that of the gas phase [133,134].

4.1.2. Metal Pollution

The residues produced in the UCG process contain heavy metals such as gold, chromium, lead, cadmium, zinc, nickel, uranium, incompletely reacted carbon, and other harmful components. These substances may be leached into the groundwater system with water [135]. Studies have shown that the leaching rate of most elements from ash is higher than that of coke slag in ash and coke slag produced by gasification of hard coal and lignite, and lignite residues are more likely to interact with water [136,137]. Among them, the probability of cadmium and zinc contents in coal gasification fine slag exceeding the environmental screening values is higher, and the probability of moderate and above migration risks for the zinc element is 84.21% [138,139].

In addition, the UCG process also produces a variety of highly toxic pollutants, in addition to the above heavy metals, including mercury, arsenic, selenium, phenols, xylene, polycyclic aromatic hydrocarbons, and tar [140]. Among them, polycyclic aromatic hydrocarbons and xylene are highly toxic and carcinogenic, and are difficult to degrade and accumulate easily in the natural environment, posing a long-term potential threat to the ecological environment and human health and increasing the difficulty of environmental governance. It is worth noting that the toxicity of condensate produced by hard coal is 56% higher than that of lignite, and the toxicity mainly comes from free ammonia, phenols, and heavy metals [141]. Sixteen polycyclic aromatic hydrocarbons (PAHs) priority-controlled by the United States Environmental Protection Agency (EPA) were detected in Malkala lignite and its UCG coke residue in Turkey, with contents of 7.159 mg/kg and 2.758 mg/kg, respectively. The current status and shortcomings of environmental risk prevention and control technologies for underground coal gasification are presented in

Table 5. The current situation and shortcomings of UCG environmental risk prevention and control technology.

Investigator	Risk Type	Existing Technology	Treatment Effect	Existing Shortcomings
Aleksandra Strugata-Wieczorek, et al. [112].	Wastewater pollution	Adsorption–electrocoagulation–wetland system	The removal rate of heavy metals was more than 96%	Manganese ions are not removed and still have toxicity
Bing Xu, et al. [119].	Heavy metal pollution	UCG residual coke adsorption	The removal rate of Cr (VI) was 72.44% (lignite)	The adsorption capacity decreases with temperature
Jingjie Wu, et al. [134]. Lin Xin, et al. [142].	Syngas leakage pollution	Tight rock isolation	Leakage rate < 5% (shallow)	Deep coal seam cracking is difficult to control

below.

4.2. Geological Risk

The stability of engineering structures is also worthy of attention. Under the influence of temperature field and pressure change, the maximum decrease of coal pillar strength can reach 45.53% when the coal pillar strength is above 500 °C, and the ground subsidence problem was caused by the settlement of 85 mm of the seven panel in the Wieczorek mine of Poland [143]. At the same time, process parameters have a decisive influence on the quality of syngas. However, due to the complex reaction process, parameter prediction and real-time control are difficult, which also brings uncertainty to the stability and economy of gas production [144]. In engineering practice, the UCG demonstration project at a depth of 1400 m in Alberta, Canada, involved construction of a mechanical earth model considering anisotropic in situ stress through continuous gasification combined with geomechanical simulation. During the 60-day operation of the four simulated gasification chambers, the product gas composition is highly consistent with the measured data, which verifies the correlation mechanism between the development of deep gasification chambers and the mechanical response of rock and soil.

The simulation study of the Baikuquan area in Xinjiang, China, reveals the potential risk: when the ablation distance extends to 500 m and 1000 m, the maximum height of the extended crack reaches 232.5 m and 270 m, respectively, and the maximum roof subsidence displacement is 5.60 m and 17.40 m, respectively. These data indicate that the length of the gasification channel must be reasonably controlled to prevent adverse interactions between the aquifer and the cavity system [145].

4.2.1. Geological Disaster

UCG can avoid large-scale surface subsidence caused by conventional mining due to less surface disturbance, but during its implementation, coal seams are gasified to form underground cavities, causing stress redistribution of surrounding rocks. In shallow coal seams or geologically unstable areas, this may induce local surface subsidence and even collapse [146,147]. This phenomenon is particularly prominent in shallow coal seams or geologically unstable areas: the formation and expansion of cavities can easily weaken the structural integrity of rock mass, significantly increase the probability of disasters, and then threaten the safety of surrounding buildings, infrastructure, and ecological environment [148]. High temperature and ablation effects will also aggravate rock fracture and surface deformation. With the extension of ablation distance, the maximum height of overburden fracture extends to 270 m from the initial state, and the maximum roof subsidence reaches 17.40 m. The sandstone shows the characteristics of accelerated crack propagation in the temperature range of 350–600 °C, and the thermal damage effect is significantly enhanced [149].

In the UCG project with vertical well group structure, when the coal seam temperature exceeds 1200 K, a continuous cavity will be formed between injection and production wells. The simulation results based on the fluid–solid coupling model show that the surface deformation is the result of the joint action of cavity evolution and the temperature field. The offshore UCG project also faces the challenge of stratum deformation: when the thickness of the coal seam is 11 m, although the recovery rate of 46% can be achieved by setting a 100 m wide security coal pillar, the seabed will still produce a horizontal displacement of 0.16 m [150].

In addition, the control of gas injection pressure is very important. Because coal seams are often symbiotic with aquifers, excessive gas injection pressure may cause syngas to penetrate the aquiclude and aggravate the risk of groundwater pollution [151]. The core analysis of the “Barbara” experimental mine in Poland confirms that the UCG process will change the pore structure of the formation, thus affecting the gas migration path [152].

4.2.2. Variation of Rock Mass Strength

The high-temperature gasification process will significantly change the physical and chemical properties of the surrounding rock, and the effects of thermal expansion and pyrolysis shrinkage can induce crack development, thereby weakening the strength and stability of the rock mass [153]. At the same time, gas flow and pressure fluctuations may further aggravate the damage of rock mass structure, which not only affects the stability of the gasification channel but also may lead to the interruption of the production process [154]. Around the combustion zone, the thermal effect will lead to the degradation of rock mechanical properties. The data from the Wieczorek test in Poland show that after 20 weather-based operations, the combustion zone expands horizontally by about 15 m, the surface settlement of the single panel reaches 23 mm, and the cumulative settlement of the seven panels reaches 85 mm. It is worth noting that the deterioration of rock mechanical properties under parallel combustion mode has a limited impact on surface subsidence [155]. The high-temperature environment will also cause significant changes in the stress field around the UCG panel. At a depth of 600 m, the maximum side wall support stress can reach 63.1 MPa, and the thermal stress will further strengthen the stress concentration effect. The three-dimensional thermo-mechanical model simulation confirms that the stress value is about 6.6% higher than that of the pure mechanical model due to the thermal effect [156]. There are obvious differences in the thermal response characteristics of different lithologies: the thermal conductivity of the overburden rock decreases with the increase of temperature, and the Poisson’s ratio of mudstone and sandstone varies differently. This temperature gradient difference directly affects the overall strength of the formation [157]. The heating test of Carboniferous sedimentary rocks shows that after treatment at 1000–1200 °C, the residual strain and Poisson’s ratio normalized value under uniaxial compression show significant differentiation, suggesting that the deformation characteristics of the gangue layer need to be evaluated in a targeted manner [158].

The accumulation of an ash layer also affects the reaction efficiency: when the thickness of the ash layer exceeds 1.3 mm, it will inhibit the oxygen mass transfer efficiency at 1300 °C and promote the reaction of carbon with CO₂ [159,160]. The thermal damage and crack development of the covering layer will affect the sealing performance [161]. The calcium aluminate cement-fly ash filling material can maintain 5 MPa compressive strength at 1000 °C, and the dense C-A-S-H gel formed by secondary hydration can effectively enhance the high temperature durability. In terms of pressure and stress evolution, the stability of horizontal wells is easily affected by roof collapse. The horizontal well in the M2 coal seam of the Mazino Coal Mine in Iran needs to maintain 28 MPa stress to ensure the best stability, and its mud weight window is 0–33 MPa. As the gasification time increases, the drilling mud pressure needs to be increased to suppress wellbore convergence [162]. The development of a UCG cavity in a deep coal seam is significantly affected by double diffusion natural convection, and the existing shallow test experience is difficult to directly promote. The three-dimensional model established by the improved “controlled contraction injection point” method shows that although the shape of the deep cavity is similar to that of the shallow layer, the high-pressure environment makes its characteristics more complex [163]. In the strip-mining method, the width of the coal pillar shows a W-shaped change rule under the influence of high temperature, and it shrinks slowly after gasification. It is necessary to scientifically determine the reasonable size of the coal pillar based on the thermal–mechanical coupling effect.

Different coal types will cause significant differences in the movement characteristics of overlying strata due to their different characteristics. Among them, strong cohesive coal is more conducive to settlement control, but it is necessary to adopt targeted schemes in engineering design [164]. Under the action of high temperature, the overlying strata may undergo thermal cracking, which may lead to gas leakage and groundwater infiltration. When the temperature reaches 600 °C, the internal cracks of sandstone will expand rapidly, and the thermal damage effect will be significantly aggravate. It is also found that the uniaxial compressive strength, residual strain, and Poisson’s ratio of mudstone and sandstone in Carboniferous sedimentary rocks change significantly after heating at 100–1200 °C, which will directly affect the deformation characteristics of the gangue layer. At the same time, the lack of stability of the gasification coal pillar may induce the development of water-conducting fissures, resulting in a hydraulic connection between the gasification area and the aquifer. In this regard, it is necessary to use the hyperbolic stability evaluation method to optimize the design of the coal pillar [165]. The experimental data show that when the temperature exceeds 500 °C, the strength and elastic modulus of coal pillars will accelerate degradation, with the maximum reduction of 45.53% and 61.34%, respectively. At the same time, the porosity increases significantly, and the failure mode gradually changes from brittleness to toughness. In order to ensure safety, it is recommended to scientifically determine the reasonable width of the protective coal pillar using the “UCG-PSA” analysis method, considering the key parameters such as thermal stress and syngas pressure.

4.3. Technology to Be Optimized

4.3.1. Process to Be Optimized

In the UCG process, heating rate, reaction pressure, and oxygen concentration are the key parameters affecting the gasification effect. Experiments show that the heating rate of 10 °C/min is most conducive to the formation of coke. When the reaction pressure exceeds 2 MPa, it can further promote the formation of coke [166]. At the same time, an oxygen volume fraction between 40% and 70% can significantly improve the quality of syngas. The calorific value increases from 10.48 MJ/m³ to 12.48 MJ/m³, and the energy recovery rate also increases from 69.63% to 83.88% [167]. In addition, when the ratio of steam to oxygen is 2.5, the overall gasification performance can be effectively optimized with higher inlet temperature and pressure [168].

The difference in coal quality is an important factor affecting the adaptability of the UCG process. Coals with different ranks and qualities have different performances in pyrolysis and gasification reactions [169]. Lignite is easy to expand and produces a large amount of volatiles during pyrolysis due to its high moisture and high volatiles. The reaction activity of bituminous coal is high, and the gasification process can easily produce tar. Anthracite has low reactivity and requires higher temperature and better conditions [170,171]. These differences make it difficult for the UCG process to adopt uniform parameters, which must be adjusted according to the characteristics of coal quality, thus increasing the complexity and cost of the process. It is also found that there are obvious differences between high-ash coal and low-ash coal in the expansion direction of the gasification area. For example, the gasification area of high-ash coal with an ash content of 28.3% expands along the pore wall, and the calorific value of the product gas is 6.91 MJ/m³. The low-ash coal with an ash content of only 7.9% extends upward, and the calorific value reaches 8.05 MJ/m³ [172]. For different coal quality, the corresponding injection strategy should be adopted: low-ash coal is suitable for the forward propulsion method (the gasification agent injection point moves forward along the direction of the coal seam extension, ensuring that the gasification reaction is carried out in the fresh coal seam area) to avoid “ash covering pipeline”, and low volatile coal is suitable for the backward propulsion method (the injection point moves backward, ensuring sufficient residence time of the gasification agent to promote the reaction of low-reactive coal) to prevent “gas leakage” [173].

UCG technology is still mainly in the laboratory or pilot stages and is not yet a mature technology system for large-scale commercial application [174]. Under laboratory conditions, the UCG model can accurately predict the reaction process and product characteristics [175,176]. However, when scaling up to full-scale operation, due to the estimation deviation of key parameters, such as stripping rate and convective transmission, as well as the more complex multi-physical field coupling, the gasification efficiency is often reduced, and the product performance is unstable, which makes it difficult to meet the requirements of industrial continuous production. In large-scale development, geological evaluation, gasifier construction, horizontal wells, and the controllable injection point retreat process are the mainstream technical routes, but the overall maturity is low, and long-term stability and high yield still face challenges [177,178]. In addition, the multi-field coupling mechanism is complex, and the migration law of multiphase fluid in ultra-high temperature and high-pressure environments is not clear [179].

4.3.2. Temperature Field Control

The evolution of the temperature field in the process of underground coal gasification shows obvious stages, which go through different stages of drying, pyrolysis, oxidation, and reduction in turn [180]. The temperature distribution and variation of each stage are significantly different. For example, the temperature of the oxidation zone is high and advances with the gasification process, while the temperature of the reduction zone is relatively low. This inhomogeneity may lead to insufficient local reaction and affect the overall gasification efficiency [181].

The expansion of the temperature field is affected by multiple factors, including the injection rate and pressure of the gasification agent, the thermal conductivity of the coal seam, and the reaction heat release [182]. Among them, the channel structure plays a key role in the spatial development of the reaction zone: the permeation channel improves the mass transfer efficiency by virtue of the porous structure, the area ratio of the oxidation zone, the reduction zone, and the retorting zone is better than that of the free channel, and the gas calorific value is higher [183]. At the same time, coal seam fissures will lead to heat transfer in the vertical direction faster than the horizontal direction, accompanied by thermal dispersion. For example, the thermal dispersion coefficient measured in the Ulanqab lignite experiment was 0.72, and the dispersion angle was 78.7 degrees [184]. The peak temperature of the roof and floor in the rising stage appears on the surface of the rock layer and gradually moves inward, while the rock layer in the falling stage is more significantly affected, and its degree is related to the advance rate of the flame working face [185]. The analytical solution calculation shows that the time required for the transition from unsteady state to steady state of the coal seam is inversely proportional to the flame advance speed. The temperature distribution in the ignition stage is uneven; for example, the temperature difference between different igniters is significant, and the temperature field expansion depends on the combustion front advance or the lateral flow of high-temperature gas, which increases the difficulty of control [186]. The measurement of the Witte index shows that the temperature in the gasification chamber can reach 1300 °C, and it decreases rapidly outside the chamber boundary [187,188].

The evolution process of the cavity is characterized by the continuous movement of the gasification point. As the coal seam is consumed, the gasification cavity continues to expand [189]. If the cavity expands too fast, it may cause stress concentration and a risk of collapse of the surrounding rock, and uneven expansion may lead to channel blockage and affect gas flow [190,191]. At present, there are still some deficiencies in the prediction and control of cavity expansion [192]. The fluctuation of parameters such as gas flow rate, oxygen concentration, and gasification pressure will directly affect the gasification stability. The response surface method optimization analysis shows that the optimal operating parameters of the deep coal seam horizontal channel are a gas flow rate of 0.45 m/s, oxygen concentration of 60%, and gasification pressure of 8.0 MPa. Among them, the gasification pressure has the most significant effect on the coal consumption rate and cavity size [193,194].

The water invasion index is positively correlated with the thickness of the coal seam and the length of the cavity. It increases linearly in the early stage and becomes nonlinear in the later stage. It should be used as a key index to evaluate the development of the cavity [195]. Acoustic emission technology can be used for real-time monitoring of fracturing activities, and its signal characteristics are related to the change of coal temperature, which can effectively reflect the expansion of gasification area and the growth state of holes [196,197]. In terms of operating parameters, the gasification agent flow rate, system pressure, reaction temperature, and H₂O or CO₂ ratio have significant effects on the gasification process and product composition [198]. Increasing the oxygen flow rate can strengthen the oxidation reaction, but it may lead to excessive temperature and aggravate coal spalling and ash layer melting [199]. At present, parameter optimization relies more on experience and trial and error and lacks systematic methods [200]. Studies have shown that the performance is optimal when the ratio of steam to oxygen is maintained at 2.5. Increasing the oxygen concentration can increase the gas calorific value and energy recovery rate. For example, when the oxygen volume fraction increases from 40% to 70%, the calorific value increases from 10.48 MJ/m³ to 12.48 MJ/m³, and the energy recovery rate increases from 69.63% to 83.88% [201].

The selection of oxidant and gasification medium directly affects the temperature field distribution: air blasting is suitable for shallow small-scale projects; steam or oxygen blasting has more advantages in the presence of waste heat resources; and water or oxygen blasting is suitable for high-pressure deep coal seam conditions [202]. When carbon dioxide or oxygen-mixed gasification is used, the dry reforming reaction can increase the calorific value of syngas [203,204]. The “calorific value of low-ash coal reach 265 KJ/mol” mentioned in the text refers to the calorific value of syngas produced by low-ash coal gasification, not the calorific value of coal itself. Since syngas is a mixture of H₂, CO, CH₄, and other components, the unit “mol” here refers to the molar amount of syngas (taking the total moles of all components as the statistical object), and 265 KJ/mol is the calorific value per mole of syngas. The calorific value of low-ash coal can reach 265 KJ/mol after two-stage gasification, and different media will significantly affect the composition and yield of syngas [205].

In general, the complexity of temperature field evolution and cavity development requires precise control of operating parameters and medium selection to improve gasification efficiency and reduce risks. In the future, it is necessary to strengthen systematic research and optimize control strategies.

4.3.3. Ignition and Combustion Control

Predicting and controlling the ignition and combustion process of UCG is a major challenge to achieve stable and efficient operation. The ignition temperature of coal varies with coal rank: lignite can be ignited at 280–320 °C, while anthracite requires 450–500 °C, and the ignition success rate is affected by the moisture content of coal seams (a moisture content of >30% will reduce the ignition success rate by 20–30%). In terms of combustion control, the “controlled retraction injection point” (CRIP) technology can adjust the combustion front position, but the response lag of underground temperature and pressure monitoring is 5–10 min, which makes it difficult to achieve real-time control. Recent studies have shown that the intelligent combustion control system based on machine learning can reduce the fluctuation range of syngas composition to 3–5% [206], but the system needs to be verified in different geological conditions (e.g., high-ash coal seams, steeply dipping coal seams) to improve its universality. In addition, the high-temperature environment (1200–1500 °C) in the combustion zone may cause thermal cracking of surrounding rock, which further complicates the control of the combustion front and requires coupling geomechanical simulation with combustion process models.

4.4. Future Development Direction

Based on the analysis of the existing research progress of UCG technology, in the future, we should further break through the technical bottleneck and strengthen the integration of multiple technologies to fully release its potential for developing difficult-to-recover coal resources. Combined with natural gas oil technology, artificial intelligence technology, and geothermal utilization technology, the following directions will be the focus of future research.

• Collaboration of UCG and natural gas to oil technology

Promoting the deep integration of UCG technology and natural gas oil production technology will broaden the utilization path of difficult-to-mine coal resources. The syngas produced by UCG can be used as a supplementary raw material for the process of natural gas to oil, and participate in the conversion reaction together with natural gas, which is conducive to fully tapping the energy value of coal resources, and will also be conducive to the production of high-quality liquid fuels by means of natural gas to oil technology. At the same time, if combined with carbon capture and storage (CCS) technology, the low-carbon operation of the whole process of energy conversion will be realized, and the transformation of traditional fossil energy to clean and efficient utilization will be promoted.

2.

Artificial intelligence empowering UCG technology

Artificial intelligence (AI) and machine learning (ML) technologies will provide key support for the optimization and upgrading of UCG technology, greatly improving its operational efficiency and stability. AI can predict equipment failures and geological risks through data analysis, such as rock fracture propagation and land subsidence, so as to achieve predictive maintenance and risk early warning, and reduce the probability of unplanned shutdown. In addition, AI can also assist in processing large-scale production data, mining the potential correlation between process parameters and product revenue and environmental impact, and providing a data-driven decision-making basis for the future development of UCG technology.

3.

Multi-energy complementation of UCG and geothermal utilization technology

Future research should explore the integration and application of UCG technology and geothermal utilization technology and build a multi-energy complementary energy utilization system, which will improve overall energy efficiency and environmental protection benefits. UCG produces a large amount of waste heat during gasification. Geothermal utilization technology can collect this heat through efficient waste heat recovery devices for heating and industrial heat to reduce energy waste. The combination of geothermal utilization and CCS technology will further reduce the carbon footprint of UCG technology, which is conducive to the dual improvement of economic benefits and environmental protection goals.

4.

Emerging coupling technologies

In addition to the above directions, emerging coupling technologies will become important development trends. For example, the coupling of UCG with underground thermal energy storage (UTES) can store the waste heat generated by gasification in underground rock formations for seasonal heating supplies, improving energy utilization efficiency by 10–15%. The integration of UCG with microbial methane production technology can convert the residual coal in the gasification cavity into methane through microbial action, further improving the resource recovery rate. Moreover, the combination of UCG and direct air capture (DAC) technology can realize in situ capture of atmospheric CO₂ and use it as a gasification agent, achieving negative carbon emissions in the whole process.

5. Conclusions

This paper systematically summarizes the feasibility of UCG technology to develop difficult-to-mine coal resources, deeply analyzes the performance of UCG technology in resource utilization, gas production quality control, environmental impact, and cost efficiency, and compares it with SCG technology in multiple scenarios. The main conclusions are as follows.

• UCG technology has significant advantages in developing difficult-to-mine coal resources: it can adapt to deep coal seams (buried depth of 500–2200 m), low-quality coal, and multi-rank coal, with a resource recovery rate of more than 60%; the solid waste output is half of that of traditional coal-fired power plants, and near-zero emissions can be achieved with CCS; and the total investment and hydrogen production cost are lower than those for SCG (without CCS, the hydrogen cost is 0.11 USD/m³, only 43.2% of SCG).
• UCG technology faces three major challenges: environmental risks (groundwater pollution by heavy metals, waste gas leakage), geological risks (ground subsidence, rock mass strength reduction), and technical bottlenecks (difficult ignition control, unstable large-scale production).
• Future research should focus on three directions: integrating UCG with natural gas-to-liquid technology to expand product channels; applying AI to realize real-time prediction of geological risks and process optimization; and coupling UCG with geothermal utilization to improve waste heat recovery efficiency.

UCG technology provides a feasible path for the efficient and clean development of difficult-to-mine coal resources. With the breakthrough of key technologies and the improvement of risk prevention systems, it is expected to play an important role in the global energy structure transformation.