Underground Coal Gasification Model Research Articles

Simulation of underground coal gasification ignition in deep coal seam based on transitional diffusion mechanism: Influence of inlet temperature and O2

Underground coal gasification (UCG) in deep coal seam was attracting more and more attentions all over the world due to its characteristics of safety, clean and low carbon. In the study, the ignition of UCG in deep coal seam was simulated by employing computational fluid dynamics (CFD). A new method setting gas diffusion in coal seam as transitional diffusion (TD) was put forward, and the TD and Fickian diffusion (FD) mechanism were compared. It was elucidated that TD mechanism was more accurate for UCG model. The default FD mechanism in coal seam overestimated the reaction rate of coal seam. With TD mechanism, coal seam in turn went through external heating controlled (EHC) region and hybrid heating controlled (HHC) region during the ignition of UCG. The demarcation temperature between EHC and HHC regions was 720 K. Then, various inlet temperatures and mass fractions of O2 were attempted during the simulation of UCG ignition. The time spent by the ignition was shortest with 1100 K inlet temperature and it was ∼180 s. The less the inlet mass fraction of O2 was used, the slower combustion rate was. CO2 was the most potential indicator to get access to the underground condition.

Underground coal gasification modelling in deep coal seams and its implications to carbon storage in a climate-conscious world

As a clean coal technology, underground coal gasification (UCG) boasts a prospect in tapping worldwide deep coal seams, with a tremendous potential to be linked with following carbon storage and sequestration. Therefore, UCG ought to play a vital role as a sustainable technological alternative in a climate-conscious world. However, the world has very limited experience in deep UCG development because most of the tests were on shallow coals. This paper aims to bridge the knowledge gap to shed light on the development of UCG cavities in deep coal seams. A large-scale 3D UCG model was constructed with an improved Controlled Retraction Injection Point (CRIP) method, which is more suitable to deep UCG. In addition, the effect of double-diffusive natural convection was included. Modelling results show that the improved method contributed to a favourable development of UCG cavities in deep coal seams with a good gas production. Compared to a shallow UCG, pore variations exhibited a similar pattern but with distinct characteristics owing to the effects of a high pressure. Furthermore, double-diffusive natural convection was revealed to play an important role in the development of cavities and the general UCG performance. The relevant findings will help enhance the knowledge base of UCG operations in world deep coal seams, promote the clean utilization of world coal resources, and if linked to carbon dioxide storage and sequestration, contribute to the reduction of carbon dioxide emissions in a climate-conscious world.

Removal of pollutants from aqueous solutions by coals and residual cokes obtained from simulated underground coal gasification experiments

Groundwater pollution caused by underground coal gasification (UCG) as well as its remediation has attracted significant public attention. In this paper, model gasification experiments on three types of coals with different coalification degrees were carried out using a UCG model test system. The residual coal and coke samples were characterized via SEM, low temperature N2 adsorption, and FT-IR. Thereafter, their adsorption of phenol and Cr(VI) from simulated wastewater as well as the purification of gas washing water were investigated. Compared with the raw coals, the UCG residual cokes presented a higher specific surface area (SBET) of 44.99–381.51 m2/g and total pore volume (Vt) of 0.040–0.228 cm3/g; Cokes obtained from brown coal gasification had the highest SBET and Vt values. Parts of O-containing groups were preserved after the gasification. Phenol and Cr(VI) were removed by all types of coals and residual cokes. The removal rate of phenol increased with SBET and Vt, and cokes presented higher phenol adsorption capacities than those of raw coals. The O-containing groups significantly influenced and favored the removal of Cr(VI). Brown coal and its UCG coke had the highest adsorption capacities, with phenol removal rates of 17.71% and 72.44%, and presented the highest adsorption capacities, with Cr(VI) removal rates of 29.02% and 21.77%, respectively. When UCG residual cokes were used, the removal rates of total organic carbon (TOC) in both types of gas washing water exceeded 82%. Almost all the Cr and Se and some of the Ni were removed from gas washing water.

Analysis of syngas production and reaction zones in hydrogen oriented underground coal gasification

Underground Coal Gasification (UCG) is an emerging technology that utilizes unmined coals. In UCG process, coal is burned and gasified underground. Produced syngas is collected through drilled wells to be processed or used directly. Syngas comprises mainly CO2, CO, H2 and CH4 and its hydrogen content majorly depends on gasification agents and properties of coal gasified. It is assumed that the combustible components of syngas, such as CO and H2, are products of heterogeneous gasification reactions and oxygen is almost solely consumed by homogeneous combustion reactions of these combustible components. These assumptions are widely used to simplify dynamic UCG models. Here, a lab-scale hydrogen oriented underground coal gasification experiment was performed for Turkish lignite. Using the experimental data, a two-dimensional underground coal gasification computational fluid dynamics (CFD) model was built to investigate cavity formation, reaction zones, reaction rates and the syngas properties. By developing the CFD model, valuable insights regarding which reactions are dominant in which region are gained and UCG process is better understood. Two-stage gasification method was employed in the experiment. Experiments were performed using lignite having high volatile, high ash and moderate moisture contents in a prism-shaped ex-situ reactor. Oxygen and steam were successfully supplied as gasification agents. In the steam-gasification stage with a supply rate of 5 m3/h, hydrogen content and calorific value of syngas were measured up to 40 (vol.) % and 8 MJ/Nm3, respectively. Experimental and CFD model results were compared to validate the mathematical model and assumptions used in UCG syngas production process.

Thermal-hydro-chemical-mechanical alteration of coal pores in underground coal gasification

Underground coal gasification (UCG) is being considered an economically and environmentally sustainable clean coal technology which converts coal in-situ into combustible gases. It involves a number of complex physiochemical processes, e.g., drying and evaporation, thermal expansion, pyrolysis, compressional shrinkage, gasification and combustion. As a result, coal pores constantly undergo drastic changes in the course of UCG. This work aims to shed light upon the complex changes of coal structure in the context of UCG through modelling. A 3-D UCG model was constructed using very fine grids, and several important indicators regarding pore variations were studied, i.e., weight loss of coal, solid concentrations, porosity and permeability. The modelling results were comprehensively compared with experimental data, and a good agreement in trend was achieved. It shows that coal pores alteration is associated with the various phenomena occurred in UCG, and four distinct characteristics can be categorized in the indicator curves. Furthermore, coal pores variation with respect to coal gasification and combustion, not covered in the experimental works, was also revealed. The findings would serve as a helpful guide for process design and optimization.

Microbial degradation of organic pollutants in groundwater related to underground coal gasification

AbstractGroundwater pollution is regarded as one of the most serious environmental risks related to underground coal gasification (UCG). In this paper, two kinds of high efficient phenol degrading strains were isolated from activated sludges, which were obtained from coking wastewater and domestic sewage, and were named as JC and WX correspondingly. The isolated bacteria were identified by a combination method of physiological and biochemical analyses and 16SrDNA sequencing. The total organic carbon (TOC) and organic pollutants in gas washing water produced from the UCG model test of lignite were measured by TOC analyzer and gas chromatography‐mass spectrometer (GC‐MS), and the microbial degradation effect of gas washing water was finally studied by the isolated bacteria. The results reveal that JC and WX bacteria were pseudomonas aeruginosa and achromobacter xylosoxidans, respectively, and that phenolic compounds were the main organic pollutants in gas washing water, taking 95.01 percent of the total organics. The removal of TOC exceeded 72.9%, the degradation efficiency of total organic pollutants (TOM) achieved 79.03% and the degradation efficiency of phenolic compounds could be 98.76% by JC and 99.04% by WX, respectively. Most benzene series (BTEX) could not be degraded by the screened two kinds of bacteria, whereas the concentrations of BETX were increased. Some short‐chain hydrocarbon compounds with low concentrations were also detected in wastewater after microbial degradation.

Effect of Fixed and Removable Gas-Injection Patterns on the Expansion of Reaction Zones during Underground Coal Gasification

The expansion pattern of the three reaction zones, including oxidation, reduction, and dry distillation zones in the underground coal gasification (UCG) process, is recognized to be different using various gas-injection patterns, on account of which the corresponding stability and syngas components are notably distinguished. The current work carried out UCG model tests in a removable gas-injection point (receded five times). During the stable gasification process, the temperature field map and changes in syngas composition were monitored to determine the area proportions among the three zones and their expansion rates. The differences in the expansion of the three zones between fixed and removable gas-injection points were then explained according to cavity growth. The results showed that the UCG process using a removable gas-injection point exhibited a better performance than that using a fixed point in some aspects, including gasification stability, syngas calorific value, area ratio of the reduction zo...

Heavy oil mobilization from underground coal gasification in a contiguous coal seam

Underground coal gasification (UCG) is a process to convert coal in-situ to combustible gases which are of versatile end use from power generation to chemical synthesis. It extracts the energy of coals which are normally too deep/thin to recover with conventional technologies, thus increasing enormously the recoverable coal reserves worldwide. It is generally advised to operate UCG in a coal seam with a thickness no less than 2 m as excessive heat loss to the surrounding environment significantly undermines the gross thermal efficiency. However, UCG can still be attractive in a thin seam when a heavy oil reservoir adjoins. This paper aims to shed light upon the feasibility of mobilizing heavy oil with UCG in a contiguous thin coal seam. A 3-D UCG model was integrated into a heavy oil reservoir model. Modelling results showed that, with a creative layout of process wells, it is possible to grow a UCG fire in favour of mobilizing neighbouring heavy oil. UCG can mobilize significant oil due to a remarkable reduction in oil viscosity in a short time. The characteristics of production wells as well as the expansion of a thermal cavity were also studied. Furthermore, the effects of coal thickness, oil well deployment and solution gas in live oil were investigated with respect to thermal cavity propagation and oil production, which can serve as a guide in designing future reservoir development with UCG.

Mathematical Modeling of the Underground Coal Gasification Process in One Gasification Cycle

Underground coal gasification (UCG) has its own reaction characteristics with the growth of the combustion cavity because the coal seam is fixed while the gasification face moves. The theoretical understanding of UCG requires continuous and deep research. In order to explain the UCG mechanism by means of simulation calculation, based on the analysis of the reaction process of UCG, radial and axial reaction models are established. The comprehensive heat transfer coefficient of the coal seam is determined by comparing the calculation results with the field test results. The differential method is used to calculate variations in the composition of the outlet gas, the temperature field distribution in the channel, the thickness of the radial reaction zone, and the influence of the oxygen fraction on the length of the oxidation zone. The results indicate that in one gasification cycle, the changes of gas composition is close to that of the field test, which initially increases and then decreases with increasin...

Optimization of synthesis gas heating values and tar by-product yield in underground coal gasification

Increasing global energy demand is a central challenge of the 21st century. Apart from renewable energy concepts, sustainable bridging and buffering technologies are needed which simultaneously offer long-term energy supply guarantees. Underground coal gasification (UCG) shows a promising potential to meet these requirements of future energy markets by in situ conversion of coal to a valuable synthesis gas that can be used in various industrial applications. Currently conducted in more than 14 countries worldwide, a main focus of international UCG efforts lies in improved technical control to reach the desired high gas qualities and simultaneously reduce the tar by-product yield as a potential source of groundwater contamination. Referring to an innovative thermodynamic UCG model parameterized with exemplary base-case data from the UCG field trials Hanna-I, Centralia-Partial Seam CRIP (PSC) and Pricetown, we investigated pyrolysis temperature, operating pressure, water influx and gasification agent injection, optimizing synthesis gas heating values and reduced tar production. General best-fit scenarios were performed assuming an idealized UCG reactor. Besides these best-fit simulations, selected worst-case scenarios considering gas losses were tested. Overall, the model results indicate that operational process improvement potentials vary over wide ranges in the order of few percent to more than 50% compared to the examined base cases. In view of selected available literature data, near-zero tar production rates and simultaneous Lower Heating Value (LHV) improvements in the range of ∼2–40% are feasible.

Monitoring and evaluation of simulated underground coal gasification in an ex-situ experimental artificial coal seam system

In this study, to better simulate underground coal gasification (UCG), an artificial coal seam was constructed to use as a simulated underground gasifier, which comprised coal blocks excavated from the coal seam. This study reports the process and results of three independently designed experiments using coaxial-hole and linking-hole UCG models: (a) a coaxial model using a coaxial pipeline as a gasification channel, (b) a coaxial model using the coaxial pipeline combined with a bottom cross-hole, and (c) a linking-hole model using a horizontal V-shaped cross-hole. In the present work, the fracturing activities and cavity growth inside the reactor were monitored with acoustic emission (AE) technologies. During the process, the temperature profiles, gas production rate, and gas content were measured successively. The results show that AE activities monitored during UCG process are significantly affected by operational variables such as feed gas rate, feed gas content, and linking-hole types. Moreover, the amount of coal consumed during UCG process were estimated using both of the stoichiometric approach and balance computation of carbon (C) based on the product gas contents. A maximum error of less than 10% was observed in these methods, in which the gas leakage was also considered. This demonstrates that the estimated results using the proposed stoichiometric approach could be useful for evaluating energy recovery during UCG.

<i>Ex Situ</i> UCG Model Experiments with Oxygen Enriched Air in an Artificial Coal Seam

Underground Coal Gasification (UCG) demands precise evaluation of the combustion area in the coal seam. Especially, the monitoring of fracture activity in the coal seam and around rock is important not only for efficient gas production but also for estimation of subsidence and gas leakage to the surface. For this objective, laboratory experiments were conducted using the simulated UCG models. This paper also investigated gas energy for coal consumption, the production gas quantity and heat value, the application of oxygen element balance in the gasification reaction process, and the gas composition obtained in this study. During burning of the coal, temperatures inside the coal, contents of product gases and acoustic emission (AE) activities were monitored successively under the control of feeding gas (air/oxygen and steam) flow rate. Comparison of the temperature variation and accumulated AE event curves revealed a close correlation between them. The local change of temperature inside the coal induced fractures with AE. The AE activity was related closely to the local changes of temperature inside the model. The evaluation of gas energy recovery calculated from the obtained product gas provided a fair evaluation for the coal consumed, and the quantity of gas product and calorific value obtained from the UCG process.

Effect of various coal constitutive models on coupled thermo-mechanical modeling of underground coal gasification

Underground coal gasification (UCG) is an advanced method that can address the high energy demand in future by adding new resources to the proved reserves. The high temperature nature of the gasification reactions along with the formation of cavities within a coal seam necessitates a comprehensive analysis of mechanical and thermomechanical impacts on the UCG process. On the other hand, geomechanics plays a significant role in the efficiency of the UCG process by varying the reaction rates, porosity, and permeability in a flow model. More importantly, geomechanical modeling of UCG is necessary to mitigate the associated environmental hazards such as land subsidence and groundwater contamination.In this study, the results from a reservoir simulator are coupled with a geomechanical module to solve for the displacement and stress variations. A controlled retracting injection point (CRIP) method is applied in the reservoir simulator to model reactions and geochemistry. By performing sensitivity analyses using the linear elastic, hyperbolic, and elasto-plastic constitutive models, the two compelling factors of high temperature and cavity evolution, which have the highest contributions to the geomechanical responses of the UCG process, are comprehensively investigated. At the end, a stress rebalancing technique is imposed on the linear elastic model to mimic the elasto-plastic solution by capturing the shear softening and the post-failure behavior of coal.

Physicochemical properties of Hebi semi-coke from underground coal gasification and its adsorption for phenol

Abstract Groundwater pollution caused by underground coal gasification (UCG) as well as its purification is a great concern for scientific and engineering community. In the present study, a UCG model test of Hebi coal was carried out by air/steam two-stage gasification method. The physicochemical properties of UCG residual semi-coke samples were characterized by SEM, nitrogen adsorption at 77 K and FTIR, respectively. Adsorption experiments of phenol onto semi-coke samples were also performed. The results can be summarized as follows: (a) obvious cracks and small voids were observed in semi-coke samples; (b) compared with raw coal, semi-coke presented a larger BET surface area of 21.6 m2/g, larger pore volume of 0.087 cm3/g and wider pore size distribution with micro pores in size of 1.5–2 nm; (c) surface functional groups such as hydroxyl and carbonyl were partially preserved after gasification; (d) and the maximum removal rate of phenol approximately reached 35%. The dispersion forces between π electrons of aromatic nucleus in semi-coke and phenol ring and the formation of electron donor-acceptor complex make it work for phenol adsorption.

Monitoring of coal fracturing in underground coal gasification by acoustic emission techniques

During the underground coal gasification (UCG) process, fracturing and cracks occur inside the gasification zone and surrounding rocks as the underground coal cavity evolves. Although fracturing activity and crack extension directly affect gasification efficiency and have environmental impacts, little research to date has focused on their effects. This study discusses the application of acoustic emission (AE) analysis for the evaluation of distinctly designed UCG models and operational parameters and describes the gasification process based on its results. We studied the cavity growth, fracturing mechanism, and the effects of various design and operational variables, such as linking-hole type, gas feed rate, and gasification agent. We found that the AE activity was closely related to the temperature change occurring inside the coal, with AE generation apparently resulting from crack initiation and extension around the coal gasification area, which occurs as a result of thermal stress. UCG modeling showed that the location of AE sources reflects the size of the gasification area and the cavity growth. In addition, the quantitative information on the located AE sources can be obtained. The introduction of a process control system into UCG modeling along with AE monitoring allowed for the real-time monitoring of the fracturing and cavity evolution inside a combustion reactor. Together, these processes have the potential to significantly reduce field risk in UCG by enabling the timely adjustment of operational parameters. Thus, AE monitoring is useful for maintaining a safe and efficient UCG process.

Evaluation of Coal Combustion Zone and Gas Energy Recovery for Underground Coal Gasification (UCG) Process

During underground coal gasification (UCG) operations, evaluation of coal gasification cavity evolution and precise control of the underground reactor are important for efficient gasification. It is also essential to estimate the energy recovery of a UCG system and the whole gasification process to ensure an effective combustion and gasification rate. An experimental simulation of UCG using an artificial coal seam comprising a compacted broken coal block was conducted using ex situ UCG models. The main goal of the experiments was the establishment of evaluation methods for the gasification zone and energy recovery during UCG. To investigate the distribution and extent of fracture activity, and to evaluate the propagation of the combustion area in the UCG reactor, we used acoustic emissions (AE) monitoring. This was combined with traditional measurements of temperature variation and product gas content. This paper presents the results of AE analysis of the fracturing activities and damage mechanisms of the...

Fault Reactivation Can Generate Hydraulic Short Circuits in Underground Coal Gasification—New Insights from Regional-Scale Thermo-Mechanical 3D Modeling

Underground coal gasification (UCG) has the potential to increase worldwide coal reserves by utilization of coal deposits not mineable by conventional methods. This involves combusting coal in situ to produce a synthesis gas, applicable for electricity generation and chemical feedstock production. Three-dimensional (3D) thermo-mechanical models already significantly contribute to UCG design by process optimization and mitigation of the environmental footprint. We developed the first 3D UCG model based on real structural geological data to investigate the impacts of using isothermal and non-isothermal simulations, two different pillar widths and four varying regional stress regimes on the spatial changes in temperature and permeability, ground surface subsidence and fault reactivation. Our simulation results demonstrate that non-isothermal processes have to be considered in these assessments due to thermally-induced stresses. Furthermore, we demonstrate that permeability increase is limited to the close reactor vicinity, although the presence of previously undetected faults can introduce formation of hydraulic short circuits between single UCG channels over large distances. This requires particular consideration of potentially present sub-seismic faults in the exploration and site selection stages, since the required pillar widths may be easily underestimated in presence of faults with different orientations with respect to the regional stress regime.

Innovative thermodynamic underground coal gasification model for coupled synthesis gas quality and tar production analyses

Underground Coal Gasification (UCG) technology is steadily improving due to high scientific and industrial efforts in currently over 14 countries worldwide. A fundamental UCG objective refers to syngas production for multiple end-uses, accompanied by environmental impact mitigation focusing contaminant reduction. In terms of this topic, the control of groundwater quality endangering tars has been a key problem rarely addressed in UCG publications so far. Considering UCG main sub-processes, operating parameters and tar spectrum knowledge grounded upon established thermodynamic equilibrium principles, an innovative and flexible model approach for coupled gas quality-tar production balances is presented here. The model is validated against literature data of the Hanna-I and Centralia-Partial Seam CRIP (PSC) field trials. For both trials good matching results were found. Main gas compounds and Lower Heating Values (LHVs) results are close to reported data partly reaching less than 10% deviation (relative error range for main compounds 4.32–18.6%, LHV 6.60–21.7%). Tar literature trend-modeling comparisons down to the single pollutant scale are addressed for the first time considering published data. Results here successfully reflect main qualitative tar tendencies, while current quantitative prognoses are on a satisfactory level, and expected to be further improved with the availability of more comprehensive in-situ data.

The environmental effect of underground coal gasification semi‐coke on confined groundwater

From the health and environmental point of view, the control of soluble metal elements and organic compounds in underground coal gasification (UCG) by‐product is of great importance for groundwater protection. Unlike the leaching of gasification ash as well as leaching in normal pressure, the leaching of trace elements and organics from UCG semi‐coke into confined groundwater has received little concern. In this research, UCG model test of Inner Mongolia lignite was performed, the obtained UCG semi‐coke was then soaked by applying stainless steel leaching column at 0.5, 1.0, 1.5 and 2.0 MPa with nitrogen as pressurizing agent, the concentrations of trace element and total organic carbons (TOC values) in lixiviums were finally determined by applying ICP‐MS and TOC analyzer to investigate the slight leaching in confined groundwater. The results indicate that Cd, Co, Cr, Ni, Pb and V are undetected in all lixivium samples, and compared with soaking under normal pressure, the concentrations of As, Ba, Mn and Sr are lower in confined water. TOC values in lixivium samples approximately approach to the background value of groundwater, indicating that no extra organic compounds could be leached and TOC values also show the similar changing trend as As, Ba, Mn and Sr behave under normal and high pressures. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 1584–1589, 2016

Modelling Underground Coal Gasification—A Review

The technical feasibility of underground coal gasification (UCG) has been established through many field trials and laboratory-scale experiments over the past decades. However, the UCG is site specific and the commercialization of UCG is being hindered due to the lack of complete information for a specific site of operation. Since conducting UCG trials and data extraction are costly and difficult, modeling has been an important part of UCG study to predict the effect of various physical and operating parameters on the performance of the process. Over the years, various models have been developed in order to improve the understanding of the UCG process. This article reviews the approaches, key concepts, assumptions, and limitations of various forward gasification UCG models for cavity growth and product gas recovery. However, emphasis is given to the most important models, such as packed bed models, the channel model, and the coal slab model. In addition, because of the integral part of the main models, various sub-models such as drying and pyrolysis are also included in this review. The aim of this study is to provide an overview of the various simulation methodologies and sub-models in order to enhance the understanding of the critical aspects of the UCG process.