Underground Coal Gasification Research Articles

Porous Flow of Energy and CO2 Transformation and Storage in Deep Formations: An Overview

The transformation and storage of energy and carbon dioxide in deep reservoirs include underground coal gasification, the underground storage of oil and gas, the underground storage of hydrogen, underground compressed air energy storage, the geological utilization and storage of carbon dioxide, etc [...]

Characteristic Analysis and Risk Control of Syngas Explosion during Underground Coal Gasification.

Gas explosion is one of the main accident risks during underground coal gasification (UCG). There are significant differences in the gas composition and explosive environment between UCG syngas and other gases. Previous research on the explosion characteristics of UCG syngas is not comprehensive enough, especially without considering the influence of the initial temperature on various characteristic parameters. A set of calculation methods for explosion characteristic parameters of UCG syngas based on existing research was proposed, which was applied to analyze explosion characteristics of syngas produced by different gasifying processes in the Huating UCG industrial test. The results showed that with the initial temperature improving, the maximum temperature and upper explosion limit of different gases increased, while the maximum pressure, lower explosion limit, and oxygen content safety limit decreased. However, the explosion thermal effect, pressure rise rate, and explosion characteristic values showed small changes. When the initial temperature increased from 298 to 1473 K, the explosion temperature of different gas explosions increased from 1645-2286 to 2652-3238 K, the maximum pressure dropped from 0.59-0.81 MPa (absolute pressure) to 0.19-0.23 MPa, the lower explosion limit dropped from 12.34-29.79% to 0.58-1.77%, the upper explosion limit increased from 55.68-83.35% to 70.89-93.73%, and the safety limit of oxygen content dropped from 4.86-6.37% to 0.26-0.34%. In addition, the gas calorific value also affected the values of various explosion characteristic parameters, among which the explosive thermal effect, maximum temperature, maximum pressure, pressure rise rate, explosion characteristic value, and safety limit of oxygen content in the syngas were all proportional to the calorific value of gas, while the lower and upper limits of explosion were inversely proportional to it. Based on the above research, syngas explosion-prone stages and causes of each potential risk area in the Huating UCG project were analyzed, the explosion characteristic parameters were determined, and targeted prevention and control measures were proposed accordingly. This study can lay a theoretical foundation for the study of syngas explosion characteristics and risk control for the UCG project.

Insights into bacterial diversity in industrial post-processing water from underground coal gasification (UCG) process

This study represents the first culture-independent profiling of microbial diversity in post-processing wastewater from underground coal gasification (UCG) processes. Three types of post-processing wastewater, named W1, W2 and W3, were obtained from three UCG processes involving two types of coal and two gasification agents, namely oxygen-enriched air and oxygen. Very high concentrations of BTEX (benzene, toluene, ethylbenzene, xylene), polyaromatic hydrocarbons (PAHs), and phenol were detected in the wastewater, classifying it into the fifth toxicity class, indicating very high acute toxicity. The values for the Shannon (H), Ace and Chao1 indices in W2 were the lowest compared to their values in W1 and W3. The dominate phyla were Proteobacteria, contributing 84.64% and 77.92% in W1 and W3, respectively, while Firmicutes dominated in W2 with a contribution of 66.85%. At the class level, Gammaproteobacteria and Alphaproteobacteria were predominant in W1 and W3, while Bacilli and Actinobacteria were predominant in W2. Among Bacilli, the Paenibacillus and Bacillus genera were the most numerous. Our results suggest that the main differentiating factor of the bacterial structure and diversity in the wastewater could be the gasification agent. These findings provide new insights into the shifting patterns of dominant bacteria in post-processing wastewater and illustrate the spread of bacteria in industrial contaminated wastewater.

Characterization of the migration of organic contaminants in laboratory-scale groundwater polluted by underground coal gasification.

Underground coal gasification (UCG) is a promising technology, but the groundwater pollution caused by UCG is a potential risk to the environment. The measured results of the stratum in the combustion cavity resulting from UCG had proven that the combustion cavity would be filled with some UCG residues and caving rocks when UCG was finished. The pollutants in underground water around the combustion cavity include organic pollutants, inorganic pollutants, and ammonia nitrogen, and one of the primary organic pollutants is phenol. The migration and diffusion characteristics of organic pollutants (taking phenol as a representative) in the groundwater of the combustion cavity were investigated by breakthrough experiments and numerical simulations. The results show that the hydraulic conductivity of the coarse UCG residues is much higher than that of fine residues, and the hydraulic conductivity of the UCG residues with the size of - 0.15 mm and 0.15-0.3 mm are 4.68 × 10-6 m/s and 1.91 × 10-4 m/s respectively. The dispersivity λ for the migration of organic pollutants will be influenced significantly by the size of UCG residues in fractures of the combustion cavity, while the distribution coefficient Kd will not. The dispersivity of organic pollutants in the fine UCG residues is more significant than that in the coarse residues, and the λ for the two kinds of residues are 3.868 cm and 1.765 cm, respectively. The shape of the migration path slightly affects the pollutant concentration distribution along the path, but the width of a path has a more pronounced influence on the concentration distribution. In this research, the influence was formulated by a new technical term, MPWIT, which is related to transverse dispersion. Specifically, while the transverse dispersion values account for 20% and 10% of the longitudinal dispersion, respectively, the corresponding MPWIT values are 39.48 mm and 33.96 mm.

Pig slurry - A polydisperse substrate necessary for the biogasification of a lignite bed

The in situ transformation of the carbohydrate substances is presented in the context of methane (CH4) formation by methanogens. In situ coal conversion methods have been characterized due to the beneficial effect of unconventional coal mining. The research was carried out on less mature coals, with a lower carbonisation degree of organic matter, because they are more effective in conversion to bioenergy gas, which can be used for all pro-ecological activities. The original concept of experimental research was presented by combining the properties of a polydisperse substrate, which is pig slurry, and used for brown coal biogasification (anaerobic digestion) in two brown coal bed systems. For this purpose, a laboratory monosubstrate reactor for the fermentation of pig slurry in a lignite bed was developed. The mainaim of the research presented in the article is to assess the amount of agricultural biogas produced in the process of mesophilic methane fermentation with the use of a polydisperse substrate using a brown coal bed placed in a fermentor. The partial goals are:- presentation of analogies of techniques and methods of underground coal gasification (UCG) to biogasification of brown coal;- a set of correlation equations for calculating flow resistance coefficients through granular porous structures;- comparison of experimental values of flow resistance with calculations according to various models.The justification of the main goal and partial goals refers to the development of a new technique and method of obtaining biogas, taking into account the Reynolds number and the own model of the gas flow resistance coefficient.The novelty of the article are: developed own model for determining the correlation dependence of the flow resistance coefficient as a function of the Reynolds number, - the results of research that indicate the practical use of pig slurry in a lignite bed for which intensive biogas production takes place at lignite.- minor deposit in relation to lignite - flat fragments of a deposit. The inhibition phenomenon and the dynamics of the process during the production of biogas in the brown coal bed were also observed in the research. An additional advantage of the innovation in the article is the combination of a polydisperse substrate with a porous bed - the simplicity of the research is the production of biogas on a lignite bed, which is a medium for biogas-producing bacteria.It has been shown that:- the developed lignite biogasification system with the participation of a polydisperse substrate in the fermenter (laboratory vessel) ensures the production of biogas in the absence of thermal insulation of the fermenter;- for a lignite deposit, the biogas stream is stable with increasing temperature; while above the temperature of 38 °C–42 °C, the biogas stream is characterized by a downward trend.Comparing the flows of the biogas stream, the following can be observed:- the intensity of biogas production for lignite (smaller bed), there is a greater phase exchange, because the bed layout provides more space for the adhesion of methanogenic microorganisms, - in turn, in the case of lignite (flat fragments of the deposit), there is less phase exchange, i.e. less biogas production.The performed rheological tests of pig slurry proved the foaming properties of the sub-strate at 25 °C and 500 rpm for a kinematic viscosity of 27.5 mPa. Characteristic foaming properties are manifested by the polydisperse system, which is pig slurry.

Numerical simulation study on chemical ignition process of underground coal gasification

Underground coal gasification (UCG) is a process that produces combustible gas by the in-situ gasification of coal seams. For a steady and efficient gasification project, the ignition of coal seam is a prerequisite. The heating method and reacting process of coal seam play an essential role to the ignition. A coal seam ignition model coupled with heptane combustion was created to simulate the deep UCG ignition stage employing the chemical ignition method. A 1700 K heptane jet flame and a U-shaped pyrolysis gas flame formed the heat source. The flame temperature of pyrolysis gas was around 1300 K. The effective heating length of the flame increased with the heating time, which reached 600 mm at 400 s. The coal seam reaction process was divided into four stages: (1) heptane flame stabilization, (2) coal seam heating, (3) pyrolysis gas release, and (4) pyrolysis gas flame stabilization. It was the third stage when the coal seam ignited. The coal seam would ignite faster with a lower excess air coefficient, and the ignition time ranged from 225 to 275 s when the excess air coefficient was 1.39. The outlet carbon dioxide concentration increased by about 5 %–17 % when the coal seam ignited.

A novel gas injection method with swirl flow in underground gasification for improving gas production and controlling pollution yields

Underground coal gasification (UCG) is a promising technology, which can convert deep coal seams into high calorific value gas products through in-situ controllable combustion, for the traditional coal mining technology. However, the structure of raw coal is dense, and it is difficult for the injected gasification agent to penetrate from the coal surface to the inside of coal in the gasification channel, which leads to worse gasification performance. Different from conventional straight flow injection, swirl flow injection is proposed to improve the effective contact between the gas and the coal surface to enhance the gasification reaction efficiency. In this work, various types of novel swirl nozzles were independently designed. The characteristics of gas production, temperature distribution and tar pollutant are studied with different swirl angles. As the swirl number increases from 0 to 1.41, there is a significant enhancement in the residence time of oxygen in the axial direction and a reduction of the mass transfer between oxygen and the coal wall in the radial direction. As the oxygen flow rate increases from 0.5 to 2 L/min, swirl flow injection exhibits stronger gasification performance than traditional straight flow injection, such as maximum gas calorific value and tar generation control capability.

Simulation of Underground Coal-Gasification Process Using Aspen Plus

In order to study the underground coal-gasification process, Aspen Plus software was used to simulate the lignite underground gasification process, and a variety of unit operation modules were selected and combined with the kinetic equations of coal underground gasification. The model can reflect the complete gasification process of the coal underground gasifier well, and the simulation results are more in line with the experimental results of the lignite underground gasification model test. The changes in the temperature and pressure of oxygen, gasification water, spray water, and syngas in pipelines were studied, and the effects of pipe diameters on pipeline conveying performance were investigated as well. The effects of the oxygen/water ratio, processing capacity, and spray-water volume on the components of syngas and components in different reaction zones were studied. In addition, the change tendency of gasification products under different conditions was researched. The results indicate that: (1) The depth of injection and the formation pressure at that depth need to be taken into account to determine a reasonable injection pressure. (2) The liquid-water injection process should select a lower injection pressure. (3) Increasing the oxygen/water ratio favors H2 production and decreasing the oxygen/water ratio favors CH4 production. (4) The content of CO2 is the highest in the oxidation zone, the lowest in the reduction zone, and then increases a little in the methanation reaction zone for the transform reaction. The content of CO is the lowest in the oxidation zone and the highest in the reduction zone. In the methanation reaction zone, CO partially converts into H2 and CO2, and the content of CO is reduced. (5) The injection of spray water does not affect the components of the gas but will increase the water vapor content in the gas; thus, this changes the molar fraction of the wet gas.

Constructed wetland as a green remediation technology for the treatment of wastewater from underground coal gasification process.

The wastewater from underground coal gasification (UCG) process has extremely complex composition and high concentrations of toxic and refractory compounds including phenolics, aliphatic and aromatic hydrocarbons, ammonia, cyanides, hazardous metals and metalloids. So, the development of biological processes for treating UCG wastewater poses a serious challenge in the sustainable coal industry. The aim of the study was to develop an innovative and efficient wetland construction technology suitable for a treatment of UCG wastewater using available and low-cost media. During the bioremediation process the toxicity of the raw wastewater decreased significantly between 74%-99%. The toxicity units (TU) ranged from values corresponding to very high acute toxic for raw wastewater to non-toxic for effluents from wetland columns after 60 days of the experiment. The toxicity results correlated with the decrease of some organic and inorganic compounds such as phenols, aromatic hydrocarbons, cyanides, metals and ammonia observed during the bioremediation process. The removal percentage of organic compounds like BTEX, PAHs and phenol was around 99% just after 14 days of treatment. A similar removal rate was indicated for cyanide and metals (Zn, Cr, Cd and Pb). Concluded, in order to effectively assess remediation technologies, it is desirable to consider combination of physicochemical parameters with ecotoxicity measurements. The present findings show that wetland remediation technology can be used to clean-up the heavily contaminated waters from the UCG process. Wetland technology as a nature-based solution has the potential to turn coal gasification wastewater into usable recycled water. It is economically and environmentally alternative treatment method.

Thermal treatment effects on rock fracture behaviour under three loading modes: A comparative analysis of granite and mudstone

Temperature and thermal damage have a major impact on the behaviour of fractures in the earth’s crust. For deep earth engineering such as underground coal gasification, geothermal energy extraction, and nuclear waste repositories, developing fracture criteria governing the propagation of fractures under thermal effect is crucial. As a result, the ability of stress, energy, and strain-based fracture criteria to predict rock fracture behavior both before and after thermal treatment is assessed in this paper considering two rock types; granite, and mudstone, which are very different from one another in terms of their mineralogical composition and mechanical properties. The overall findings showed that the energy-based criterion was more effective in predicting fracture parameters both before and after thermal treatment than the stress and strain-based criteria. Specifically, after thermal treatment at 500 °C for the granite, and at any temperature for the mudstone, the strain-based criterion was no longer valid. It was found that the temperature and loading mode have a significant impact on the granite’s transition from brittle to ductile, where this transition can occur at an approximated temperature of 460 °C. Meanwhile, an increase in brittleness with rising temperature was noted in mudstone. Moreover, the Fracture Process Zone (FPZ) size was predicted by the energy-based criterion to be greater than that predicted by stress and strain-based criteria. It was found that the FPZ size is proportional to the brittleness degree of the two rock types, which is considerably impacted by thermal treatment. In addition, this study discusses several critical areas in rock fracture mechanics in the presence of thermal effects including fracture envelopes under mixed mode loading I/II, fracture stress, and initial crack extension angle. All of these are of high importance in assessing the safety and stability of rock structures in the presence of thermal effects.

A coupled thermal-force-chemical-displacement multi-field model for underground coal gasification based on controlled retraction injection point technology and its thermal analysis

Underground coal gasification is an essential means of meeting the growing energy needs of some countries. In this paper, a new numerical simulation method of thermal-force-chemical-displacement multi-field coupling is developed, and in order to better adapt to the actual working conditions, taking Shanjiaoshu Coal Mine as the geological background, the first gasification working face of No. 12 coal seam is simulated based on the controlled retraction injection point technology. This method is used to simulate the heating of coal body by the ignition device at the initial stage of UCG, the chemical-thermal changes triggered by spontaneous combustion of the coal body under the continuous backward movement of the injection point, and the simulation of the formation of the cavity after the coal gasification reaction. Finally, the evolution of temperature and displacement fields in No.12 coal seam after gasification reaction is analyzed. Therefore, this study aims to simulate the actual working conditions more realistically, optimize the UCG technology, and produce more clean energy syngas to improve the energy situation in China.

Experimental studies of the deformation characteristics of mudstones under high temperature exposure

The article presents the results of laboratory thermomechanical tests of argillite samples. It has been confirmed that the deformation-strength properties of the studied type of geomaterials undergo significant changes under high-temperature influences. Features of changes in the values of deformations of argillite samples at various stages of heating were revealed. It is shown that the research results can be used in modeling thermomechanical processes occurring during underground gasification of coal in a reaction channel, to predict the development of deformation of the host rocks of an underground gas generator.

Searching of Phenol-Degrading Bacteria in Raw Wastewater from Underground Coal Gasification Process as Suitable Candidates in Bioaugmenation Approach

Searching of Phenol-Degrading Bacteria in Raw Wastewater from Underground Coal Gasification Process as Suitable Candidates in Bioaugmenation Approach

H2 production enhancement in underground coal gasification with steam addition: Effect of injection conditions

Hydrogen is a clean energy source, and its use helps to mitigate climate change. Underground coal gasification for hydrogen production is a promising method for countries with abundant coal resources and high hydrogen demand. However, the existing coal-to-hydrogen process underground exhibit low production efficiency. This work proposes the use of swirl flow injection to improve hydrogen production efficiency, and also explores the influence of other injection conditions (oxygen flow rate, steam mass flow and steam temperature) on hydrogen production. For straight flow injection, the effect of steam addition on gasification performance depends on the oxygen flow rate, and the highest H2 production is 14.6 L under 1 L/min oxygen and 1 g/L steam. With a swirl flow injection, H2 production increases with steam addition for both 1 L/min and 2 L/min oxygen, which shows a larger H2 production than straight flow injection. The highest H2 production for swirl flow injection is 19 L under 2 L/min oxygen and 1 g/L steam. An increase in steam temperature barely affects the composition of products but apparently increases the amount of products. The fracture effect of high temperature steam might cause a sudden increase of H2 and CH4 at the late stage.

Influences of removable gas injection methods on the temperature field and gas composition of the underground coal gasification process

In this paper, the effects of changing the injection position of a gasification agent on temperature fields and gas compositions are investigated through an experimental model of the underground gasification of coaxial coal. A 3D model of the temperature field is constructed based on the experimental temperature parameters using kriging interpolation; the 900 °C isothermal surface is used as the reference plane to define the oxidation zone, and its plane and volume expansion rate are analyzed. The underground coal gasification experiment is divided into five stages according to the injection location. The average expansion rates of the oxidation zone in each stage are 2.04 cm/h, 1.93 cm/h, 3.44 cm/h, 3.68 cm/h, and 3 cm/h along the X-direction and 0.69 cm/h, 0.09 cm/h, 0.56 cm/h, 1.69 cm/h, and 1.01 cm/h along the Z-direction; the expansion rate along the X-direction is greater than that along the Z-direction. The expansion volumes of the oxidation zone after the five stages increase from 0.0097 m3 to 0.0168 m3, 0.0321 m3, 0.0521 m3 and 0.0657 m3. An analysis of the expanded volume of the oxidation zone compared with the volume of the cavity shows that both features are similar in shape and that the final volume of the cavity is smaller than the final expanded volume of the oxidation zone; the volume of the cavity is approximately 77.3 % of the expanded volume of the oxidation zone. The formation of a cavity requires the temperature to reach the conditions for the oxidation reaction of the coal and the rupture and flaking of the ash layer that accumulates after the reaction by the actions of certain stresses, such as thermal. The composition of the product gas and the strength of the chemical reactions involved are evaluated based on stoichiometric conservation. A short period after a shift in the injection position of the gasifier greatly impacts the chemical equilibrium of the UCG process with a large change in the strength of the chemical reaction, followed by a relatively small change when the gasification process stabilizes.

Economic assessment of clean hydrogen production from fossil fuels in the intermountain-west region, USA

The transition from fossil fuels to carbon-neutral energy sources is necessary to reduce greenhouse gas (GHG) emissions and combat climate change. Hydrogen (H2) provides a promising path to harness fossil fuels to reduce emissions in sectors such as transportation. However, regional economic analyses of various H2 production techniques are still lacking. We selected a well-known fossil fuel-exporting region, the USA's Intermountain-West (I-WEST), to analyze the carbon intensity of H2 production and demonstrate regional tradeoffs. Currently, 78 % of global H2 production comes from natural gas and coal. Therefore, we considered steam methane reforming (SMR), surface coal gasification (SCG) and underground coal gasification (UCG) as H2 production methods in this work. We developed the cost estimation frameworks of SMR, SCG and UCG with and without carbon capture, utilization and sequestration (CCUS). In addition, we identified optimal sites for H2 hubs by considering the proximity to energy sources, energy markets, storage sites and CO2 sequestration sites. We included new production tax credits (PTCs) in the cost estimation to quantify the economic benefit of CCUS. Our results suggest that the UCG has the lowest levelized cost of H2 production due to the elimination of coal production cost. H2 production using the SMR process with 99 % carbon capture is profitable when the PTCs are considered. We also analyzed carbon utilization opportunities where CO2 conversion to formic acid is a promising profitable option. This work quantifies the potential of H2 production from fossil fuels in the I-WEST region, a key parameter for designing energy transition pathways.

Modeling Test of Combustion Cavity Growth during Underground Coal Gasification in the Early Stage of Ignition.

The growth parameters of the underground coal gasification (UCG) combustion cavity are important for the regulation of its gasification process. The irregular cavities formed in the early stages of ignition can affect the stability of the gasification process. In this study, a heat-solid coupling model is used to determine the combustion cavity boundary at the early stage of coal seam ignition to simulate the movement of the combustion cavity boundary by indirectly inheriting the coal seam temperature. It reveals the evolution of the temperature field, stress field, and plasticity zone at the combustion cavity boundary at the early stage of ignition in the UCG process and compares with the ex situ small-scale experiments. The simulation results show that in the early stage of ignition, the temperature transfer to the top of the coal seam and the direction of the gasification agent outlet pipeline is faster, while the transfer rate to the direction of the gasification agent inlet pipeline is slower. The main stresses are mainly distributed in the left and right sides of the combustion cavity and gradually increase directly above. The plastic zone is mainly distributed directly above the combustion cavity and arc-shaped plastic zones. The experimental results show that the temperature directly above the combustion cavity is higher than in the other directions, and the ash layer hinders the temperature transfer to the bottom. Therefore, the combustion cavity has a longer elliptical shape in the upper part, which is consistent with the simulation results. The model better reveals the extension law of the combustion cavity at the early stage of UCG ignition and provides theoretical guidance for the study of combustion cavity formation.

Study on Erosion Wear of Single- and Double-Orifice Throttling Tools for Underground Coal Gasification

In underground coal gasification, as a choke regulating the formation gas lift pressure, the throttling tool can effectively reduce the production cost, the number of ground heating and insertion equipment, and gas consumption. Because in this process, the coal is transformed into composite synthetic gas through a series of technical treatments, the throttling tool is in a working environment with high temperature and pressure. In the process of transportation of combined synthetic gas, the pulverized coal parts produced by incomplete coal combination move with the gas in the throttling tool. The high temperature and high-pressure gas carrying large-diameter pressed coal parts will cause serial erosion and wear to the throttling device, resulting in failure and well-controlled safety risks. Therefore, according to the Joule–Thomson effect, this paper independently designs downhole throttling tools with single- and double-hole structures. According to actual field conditions, the erosion wear of throttling tools with different systems in high-temperature gas–solid two-phase flow was simulated and predicted, and the internal flow field characteristics of throttling means were analyzed. The difference between the wall wear distribution, wall collision position, and wall erosion effect of different structure throttling mechanisms with the change in gas velocity was investigated, which guides the practical use of the subsequent throttling tools.

Achieving climate neutrality in coal mining regions through the underground coal gasification

Coal mining regions face significant challenges in transitioning towards climate neutrality due to their dependence on fossil fuel extraction. This research explores the potential of underground coal gasification (UCG) technology as a pathway towards climate neutrality in these regions. UCG involves converting coal in-situ into syngas, a cleaner-burning fuel, while mitigating greenhouse gas emissions. The study investigates the material-thermal parameters of the gasification process using the MTB SPGV software and analyzes the gas concentration and producer gas yield parameters in the “Stepova” Mine field of SE “Lvivvuhillia”. Additionally, technological solutions for achieving climate neutrality through carbon dioxide (CO2) utilization are proposed, focusing on pre-separation of CO2 from producer gas obtained during UCG. A technological scheme for CO2 utilization is presented, outlining the steps from gas extraction to storage and utilization in underground spaces. The proposed method offers a promising approach to mitigate CO2 emissions and optimize resource utilization in coalmining regions, contributing to global efforts for environmental sustainability and climate action.

Research on Temperature Simulation of Underground Coal Gasification Wellbore

This study aims to integrate the theoretical distribution calculation of temperature and pressure during the gasification process through the derivation of heat conduction, thermal radiation and pressure control theory in the underground coal gasification process, and analyze the temperature of the underground coal gasification process through finite element modeling. Field influencing factors and pressure changes over time. It is of great significance to obtain the relevant parameters of the gasification chamber in the underground coal gasification process.