Underground Coal Gasification Site Research Articles

A thermo-mechanical simulation for the stability analysis of a horizontal wellbore in underground coal gasification

The stability analysis of horizontal wells is essential for a successful underground coal gasification (UCG) operation. In this paper, a new 3D coupled thermo-mechanical numerical modeling is proposed for analyzing the stability of UCG horizontal wells. In this model, the effect of front abutment stresses, syngas pressure, syngas temperature and thermal stresses is considered to predict the mud weight window and drilling mud pressure during UCG process. The results show that the roof caving in UCG panel has a greatest impact on the stability of horizontal well. Moreover, when the time of coal gasification is increased, the well convergence increases and for more stability it is necessary to increase the drilling mud pressure. This research was carried out on the M2 coal seam in Mazino coal deposit (Iran). The results showed that the mud weight window for horizontal well drilling is between 0 and 33 MPa. The appropriate stress for the maximum stability of the horizontal well, taking all the thermal and mechanical parameters into account, is 28 MPa. The suggested numerical method is a comprehensive and consistent way for analyzing the stability of horizontal wells in UCG sites.

Modeling Methodology for Site Selection Evaluation of Underground Coal Gasification Based on Combination Weighting Method with Game Theory.

The lack of systematic geological work is an essential reason why underground coal gasification (UCG) has not been industrialized for a long time. Building a scientific index system and favorable area evaluation technology for the UCG site selection is the key to breaking through the geological bottleneck. Aiming at the problems of the single index weight determination method, intense subjectivity, and poor reliability of current evaluation models, we put forward an evaluation modeling methodology for the UCG site selection using the combination weighting method with the game theory. The factors of coal resource conditions associated with the potential risk of UCG are systematically analyzed. From the six dimensions of the geological structure, hydrogeology, seam occurrence, coal properties, reserves, and roof lithology, 23 key factors were selected as evaluation indexes to construct a hierarchical model composed of the target layer, category index layer, and index layer. The influence of each index on UCG and its reasonable value range were systematically analyzed. The evaluation index system for UCG site selection was formed. The improved analytic hierarchy process (AHP) was adopted to sequence indices and determine their subjective weight. And the variability, conflict, and information amount of the index data were analyzed by the CRiteria Importance Through Intercriteria Correlation (CRITIC) method to calculate the objective weight. Then, the subjective and objective weights were combined through game theory. On this basis, fuzzy theory was employed to calculate the membership of indices and construct the fuzzy comprehensive judgment matrix. The evaluation model of the UCG site selection was applied to the suitability evaluation of resource conditions of UCG pilot projects at Zhongliangshan (ZLS), Huating (HT), and Shanjiaoshu (SJS) mines in China. The result shows that the resource conditions of HT are the best, followed by ZLS and, finally, SJS, which are consistent with the actual running effects of the three UCG pilot projects. It indicates that the evaluation model can provide a scientific theoretical basis and reliable technical support for the UCG site selection.

Risk assessment methodology for Underground Coal Gasification technology

Geoenergy technologies, including the Underground Coal Gasification (UCG), are currently being considered as possible solutions for reducing emissions of CO2 and other gases to atmosphere and at the same time, provide sustainable sources of low-carbon energy. In this paper, a flexible risk assessment methodology for UCG technology is presented, based on an established methodology for radioactive waste repositories. The assessment methodology can be applied at any stage in a project, between initial planning and final site abandonment. Central to the approach is the analysis of scenarios, which represents a “source-pathway-receptor” combination and its evolution. Here, a Reference Scenario (RS) and several Alternative Scenarios (AS) are developed and analysed using a numerical model. Results of the RS suggest that contaminant concentrations at an evaluation point are far below any level that could reasonably be detected. In some AS, the calculated concentrations showed an increasing trend at the end of the assessment period, potentially approaching levels that conceivably could be detected. However, as such cases are unexpected and pessimistic, their inclusion is to illustrate worst cases that could happen, rather than to give predictions. An illustrative application demonstrates that plausibly the risks of groundwater contamination from a UCG site should be very low if the site is developed and operated appropriately. The outcomes from applying the numerical model are intended to demonstrate how the methodology and the numerical model can be readily adapted to different sites.

Preliminary modeling for module estimation on the underground coal gasification project

Designing the module is one of the initial works of the underground coal gasification (UCG) feasibility study, consisting of the coal area to be gasified as panels and the coal to be left as pillars. Three models have been designed with each panel dimension 380 m in length, 150 m in width and 18 m thick. The finite element method is used in the study and the 2D and 3D geotechnical simulations have been carried out with variations of the pillars. As a result of 2D modeling, the critical strength reduction factor (SRF) is 0.25 with the highest deformation on the surface is 0.04 m (SMA-C) and 0.03 (SM-D) if the pillar is 50 m width. If SRF is increased to 0.37, the deformation on the surface is 0.36 (SMA-C) and is 13.5 (SM-D), respectively. From the 3D modeling results, if it is assumed that the velocity of the UCG reactor hole rate is 0.24 m/day until it reaches the final target length of the reactor hole 380 m, the maximum deformation of the soil surface at the SMA-C and SM-D locations is 0.074 m and 0.096 m, respectively. Determination of the module is important in the feasibility study and evaluation of the UCG site.

Thermo-fluid–mechanical numerical simulations of surface subsidence at the site of underground coal gasification

This work deals with modelling surface subsidence that aims to help industrialize underground coal gasification (UCG). UCG is a long-known, but poorly industrialized, method of energy extraction from coal. Risks of surface subsidence and groundwater pollution are two main hurdles that are affecting the potential industrialization of UCG. The particular challenge is the existence of groundwater because of implications to both its pollution and its influence on surface subsidence. Additionally, the coal combustion and the complex geometry of the UCG reactors impacts surface subsidence. To meet these challenges, the thermal and fluid analyses should be included in the model, and surface subsidence should be modelled in three dimensions to capture the collapsed shape of the UCG reactor. Based on the nature of these challenges and an earlier successful implementation, Itasca's commercial FLAC3D software with intrinsic thermal and fluid models was chosen to model surface subsidence. This study determines that the inclusion of fluid analysis improves the predictions of surface subsidence when compared with measurements at the highly watered Shatsk UCG site. In turn, thermal analysis mildly influences the modelled surface subsidence. The fluid analysis shows that the flow in the upper aquifer influences surface subsidence to a greater extent than the flow in the lower aquifers. High temperatures cause an upward flow in the lower aquifer located above the UCG reactor but does not change the flow pattern in the upper aquifer. The fluid analysis also reveals that if the UCG reactor is filled with water, surface subsidence does not occur.

Outlook for the coal industry and new coal production technologies

Outlook for the coal industry and new coal production technologies

TÜRK LİNYİTLERİNİN YERALTINDA KÖMÜR GAZLAŞTIRMA PERFORMANSLARININ STOKİOMETRİK DENGE MODELİ İLE BELİRLENMESİ

Underground coal gasification (UCG) is a coal conversion process that enables the utilization of coal reserves in-situ, and it is an alternative technique to conventional mining methods. Previous experimental studies showed that UCG is a suitable method for the usage of low-rank coal or lignite reserves, which have the major part in the Turkish coal reserves. In this study, a thermochemical equilibrium model of UCG process is developed to predict syngas composition and to compare UCG-performance of the selected lignite reserves in Turkey. The lignite sites are chosen according to the described UCG site selection criteria. The equilibrium model consists of gasification reactions and water-gas shift reaction and it considers the effect of the drying process. The model is validated using the results of the lab-scale experimental UCG study. The predictions are made for both oxygen and steam-gasification processes. Results show that the lignite reserves which have high moisture content but low carbon content are suitable for hydrogen-rich syngas production and hydrogen production capabilities of all reserves can be enhanced significantly by the additional steam supply as the gasification agent.

Z-number-based selection of suitable underground coal gasification site considering information reliability

ABSTRACTEnjoying the twelfth largest coal reserve in the world, only 1% of Iran’s energy consumption basket is supplied by coal. In the recent decade, new technologies that produce low-cost and clean energy from coal have received much attention. One of these methods is underground coal gasification (UCG). The UCG has a potential for converting the world’s coal resources into energy, liquid fuels, and chemicals. For extraction of this valuable energy, firstly, the best site for UCG should be selected to minimize the implementation complications and to distribute this energy source properly in the country. UCG site selection is affected by many uncertainties including environmental, economic, and operational parameters. Therefore, it is important to use methods capable of taking into account the uncertainty of various parameters in the site selection. In this study, Z-numbers which are a new generation of fuzzy numbers are used to overcome this issue in suitable site selection for the UCG method. Technical considerations in UCG site selection consist of coal properties, surrounding rock properties, groundwater, and surface infrastructures of the region. In this paper, first, 15 criteria influencing UCG method and four available alternatives (four coal mines in Iran) are considered. Then, by using the Z-AHP method, which is a combination of Z-numbers and conventional analytical hierarchy process (AHP), the suitable mine site for UCG is determined. Based on the results, the alternative mine named Mazino is selected as the best place for UCG site with a final weight of 0.3713. The comparison of final weights of criteria also illustrates that the two criteria of “existence of fault” (C11) and “permeability of coal seam and overburden” (C9) are the most important parameters in UCG site selection. The Z-AHP method has the capability to make an appropriate decision in the existence of experts’ disagreement or lack of experts’ confidence.

Galerkin Finite Element Based Modeling of One Dimensional Packed Bed Reactor for Underground Coal Gasification (UCG) Process

Mathematical models of a complex physicochemical underground coal gasification process involve multidimensional non-linear partial differential equations (PDEs). Therefore, researchers always seek a suitable numerical scheme that would give a good balance between accuracy of the solution and computational complexity. In this paper, the non-linear PDEs describing energy and mass balances of both coal and char are solved by Galerkin finite element method (GFEM). The infinite-dimensional spatial domain is transformed into a finite number of elements, whose dynamics are governed by a system of ordinary differential equations (ODEs). Owing to the execution of GFEM, the time domain and space-dependent first-order ODEs for solids (coal and char) and gases are solved numerically to find a solution of the UCG process. The resultant syngas compositions and the calibrated heating value are influenced by the operation parameters and the type of oxidant used. The results are compared with the experimental data obtained from the Thar coal UCG site, and with existing work, based on the finite difference method (FDM) for the one dimensional (1D) model. The simulated results along with the quantitative analysis show the superiority of the GFEM model over the FDM model.

Qualitative hydrogeological assessment of vertical connectivity in aquifers surrounding an underground coal gasification site

Qualitative hydrogeological assessment of vertical connectivity in aquifers surrounding an underground coal gasification site

Geological and Rock Mechanics Perspectives for Underground Coal Gasification in India

The geological resources of coal in India are more than 308 billion tonnes upto a depth of 1200 m, out of which proved reserve has been reported at around 130 billion tonnes. There is an increasing requirement to increase the energy extraction efficiency from coal as the developmental prospects of India increase. Underground coal gasification (UCG) is a potential mechanism which may be utilized for extraction of deep-seated coal reserves. Some previous studies suggest that lignites from Gujarat and Rajasthan, along with tertiary coals from northeastern India can be useful from the point of view of UCG. We discuss some geological literature available for these areas. Coming to the rock mechanics perspectives, during UCG the rock temperature is considerable high. At this temperature, most empirical models of rock mechanics may not be applied. In this situation, the challenges for numerical modelling of UCG sites increases manifold. We discuss some of the important modelling geomechanical issues related to UCG in India.

Optimization and control of one dimensional packed bed model of underground coal gasification

This paper discusses the optimization and control of the one dimensional (1-D) packed bed model of underground coal gasification (UCG) process for an actual UCG site. The optimization is performed to compensate for the uncertainty in coal and char ultimate analysis (caused by repeated measurements of different samples) and in steam to oxygen (O2) ratio at the reaction front. The constrained nonlinear optimization problem is solved by using state of the art sequential quadratic programing (SQP) algorithm to minimize the error between experimental and simulated heating values. The results of the optimized model are validated with actual filed trials which show a good match for heating value of the product gases. The super twisting controller, a second order sliding mode control (SOSMC) algorithm is successfully implemented on the process model, which keeps the heating value of the product gas at the desired level in the presence of the matched disturbance: steam produced from water influx from surrounding aquifers and due to the moisture content of the coal.

English

Underground coal gasification (UCG) generates potential groundwater pollution because it changes local hydrogeological parameters. Groundwater pollution is caused by diffusion and penetration of contaminants generated by UCG processes towards surrounding strata and possible leaching of residue by natural groundwater flow after gasification. A large number of hazardous water-borne contaminants were identified during different UCG operations conducted so far, and in some locations long-term groundwater contaminations were observed. Organic pollutants were detected after UCG process are phenols, benzene with its derivatives, polycyclic aromatic hydrocarbons (PAHs), heterocycles etc. and inorganic pollutants includes ammonia, mercury, zinc, sulphates, cyanides, heavy metals etc. Adsorption function of coal and surrounding strata makes a significant contribution to decrease the contaminants over time and distance from the burn cavity. Possible pollution control measures regarding UCG include identifying unsuitable zone, sitting a hydraulic barrier and pumping contaminated water out for surface disposal. The paper enumerates major pollutants identified in different UCG sites and its mitigation measures during gasification processes, and groundwater remediation after gasification.   Key words: Underground coal gasification, groundwater contamination, water influx.

Thermal–mechanical–numerical analysis of stress distribution in the vicinity of underground coal gasification (UCG) panels

During underground coal gasification (UCG), whereby coal is converted to syngas in situ, a panel (cavity) is formed in the coal seam. Unlike other underground mining methods, the coal and country rocks in the vicinity of a UCG panel are subjected to high temperatures which may be in excess of 1000°C. UCG operation imposes significant geomechanical changes to the strata. In order to understand these changes, numerical modeling can provide a comprehensive and qualitative understanding of the UCG process. In this paper, a new approach, namely, Underground Coal Gasification-Pillar Stability Analysis (UCG-PSA) is presented to estimate the protection pillar width in UCG based on the Controlled Retraction Injection Point (CRIP) configuration. The UCG-PSA approach considers development load, side abutment load, front abutment load, thermal stress along with cavity syngas pressure as a lateral pressure to determine the protection pillar width. Furthermore, the role of the crushed zone around a pillar was considered in the estimation of the pillar strength. A new 3D Thermal–Mechanical (TM) model is developed for UCG based on the CRIP configuration to predict stress distribution in the vicinity of the UCG panel and to verify the UCG-PSA approach results using the FLAC3D software. In this model, a panel is subjected to high syngas pressure and high temperature. Moreover, after each step of gasification the modeling of caved material is performed. The results of the TM model compared with the Mechanical (M) model show that the stress distribution increased at the front of the face and pillar edge due to the thermal stress. In addition, the results show that the maximum side abutment stress at a depth of 600m is 63.1MPa and applied at a distance about 3.5m away from the pillar edge while in the M model it is 59.2MPa. The proposed methodology could provide a consistent and simple way for the stability analysis of pillars in UCG sites.

Cost comparison of syngas production from natural gas conversion and underground coal gasification

Underground coal gasification (UCG) is a promising technology to reduce the cost of producing syngas from coal. Coal is gasified in place, which may make it safer, cleaner and less expensive than using a surface gasifier. UCG provides an efficient approach to mitigate the tension between supplying energy and ensuring sustainable development. However, the coal gasification industry presently is facing competition from the low price of natural gas. The technology needs to be reviewed to assess its competiveness. In this paper, the production cost of syngas from an imaginary commercial-scale UCG plant was broken down and calculated. The produced syngas was assumed to be used as feedstock in liquid fuel production through the Fischer-Tropsch process or methanol synthesis. The syngas had a hydrogen (H2) to carbon monoxide (CO) ratio of 2. On this basis, its cost was compared with the cost of syngas produced from natural gas. The results indicated that the production cost of syngas from natural gas is mainly determined by the price of natural gas, and varied from $24.46 per thousand cubic meters (TCM) to $90.09/TCM, depending on the assumed price range of natural gas. The cost of producing UCG syngas is affected by the coal seam depth and thickness. Using the Harmon lignite bed in North Dakota, USA, as an example, the cost of producing syngas through UCG was between $37.27/TCM and $39.80/TCM. Therefore, the cost of UCG syngas was within the cost range of syngas produced by natural gas conversion. A sensitivity analysis was conducted to investigate how the cost varies with coal depth and thickness. It was found that by utilizing thicker coal seams, syngas production per cavity can be increased, and the number of new wells drilled per year can be reduced, therefore improving the economics of UCG. Results of this study indicate the competitiveness of UCG regarding to natural gas conversion technologies, and can be used to guide UCG site selection and to optimize the operation strategy.

English

The underground coal gasification (UCG) has a potential for converting the world’s coal resources into energy, liquid fuels and chemicals. The UCG process involves the injection of steam and air or oxygen into an underground coal seam and igniting and burning of coal in-situ to produce the combustible gas. Previous studies showed that many criteria affect site selection of UCG. The criteria include coal seam properties, faulting, discontinuity, properties of hanging wall and footwall of coal seam and hydrogeological regime. In this paper, considering proper UCG site selection criteria, coal seam was ranked and selected for the UCG based on the controlled retraction injection point (CRIP) configuration in Mazino coal deposit. The result of this investigation showed that the M2 coal seam has great advantage (considering the seam thickness and reservoir) related to the other coal seams for the gasification by CRIP configuration. Key words: Underground coal gasification (UCG), site selection criteria, Tabas coalfield, clean energy.

Charaterization of the Harmon Lignite for Underground Coal Gasification

The Harmon lignite bed of the Fort Union Formation (Tertiary Age) beneath western North Dakota presents opportunities for applying underground coal gasification (UCG) technology to recover the unmineable coal resources. However, some characteristics of the formation also present barriers. First, the local aquifers coincide with the lignite bed; second, the lithology of surrounding rocks changes greatly within a short distance. These factors set challenges in site screening, feasibility study and assessment of environmental risks. Although extensive investigation work about the Harmon lignite has been conducted, no work has been done for UCG application. In this paper, we review the site selection criteria of UCG, and apply the experience and tools of reservoir characterization in petroleum exploration to investigate the structure and properties of a target site in western North Dakota. Information and data from state and federal geological surveys, water resource commission and oil companies are collected. A 3-dimensional model is built to simulate the Harmon lignite bed, surrounding rocks and aquifers. This “coal reservoir characterization” work provides a clear view of the structure and composition of the lignite-bearing formation, as well as a better understanding of its in situ geology and hydrogeology. Results of this work greatly facilitate the UCG site selection process. Suitability of the potential site for UCG projects is discussed.

Techno-economic assessment of hydrogen production from underground coal gasification (UCG) in Western Canada with carbon capture and sequestration (CCS) for upgrading bitumen from oil sands

This paper examines the techno-economic viability of hydrogen production from underground coal gasification (UCG) in Western Canada, for the servicing of the oil sands bitumen upgrading industry. Hydrogen production for bitumen upgrading is predominantly achieved via steam methane reforming (SMR); which involves significant greenhouse gas (GHG) emissions along with considerable feedstock (natural gas) cost volatility. UCG is a formidable candidate for cost-competitive environmentally sustainable hydrogen production; given its negligible feedstock cost, the enormity of deep coal reserves in Western Canada and the favourable CO2 sequestration characteristics of potential UCG sites in the Western Canadian sedimentary basin (WCSB). Techno-economic models were developed for UCG and SMR with and without CCS, to estimate the cost of hydrogen production including delivery to a bitumen upgrader. In this paper, at base case conditions, a 5% internal rate of return (IRR) differential between UCG and SMR was considered so as to account for the increased investment risk associated with UCG. The cost of UCG hydrogen production without CCS is estimated to be $1.78/kg of H2. With CCS, this increases to range of $2.11–$2.70/kg of H2, depending on the distance of the site for CO2 sequestration from the UCG plant. The SMR hydrogen production cost without CCS is estimated to be $1.73/kg of H2. In similar fashion to UCG, this rises to a range of $2.14 to $2.41/kg of H2 with the consideration of CCS. Lastly, for hydrogen production without CCS, UCG has a superior cost competitiveness in comparison to SMR for an IRR differential less than 4.6%. This competitive threshold rises to 5.4% for hydrogen production with CCS.

Role of Hydrogeology in Rocky Mountain 1 Underground Coal Gasification Test, Hanna Basin, Wyoming: ABSTRACT

Experience has shown that the designs and implementation of Underground Coal Gasification (UCG) operations that are technically sound and environmentally safe require a thorough understanding of the hydrogeology of the UCG site, complemented by an understanding of the potential interactions between the elements of the hydrogeologic system and UCG process. This is significant because UCG is conducted in the saturated zone, consumes large volumes of ground water, and has the potential to adversely affect ground water quality and flow. The textural, mineralogical, chemical, and structural character of the geologic materials constituting the UCG reactor, as well as the occurrence, flow, and quality of fluids moving through that three-dimensional matrix of geologic materials, must be understood. The US Department of Energy and an industry consortium led by the Gas Research Institute recently conducted the Rocky Mountain 1 Test in the Hanna basin of Wyoming. For this test, the hydrogeologic aspects of the site were characterized to an extent unprecedented in UCG testing. This information was then used to develop and evaluate operating strategies intended to prevent or minimize contamination. Such strategies included gasifying at less than hydrostatic pressure to enhance ground water flow toward the gasification modules and to restrict contaminationmore » to the module area. Hydrogeologic information also allowed a more complete evaluation of process-setting interactions. For example, a substantial and widespread drop in elevation heat noted for the ground water in the target coal emphasized the importance of an adequate water supply for UCG, particularly in a long-term commercial operation.« less

Application of Geological Studies to Overburden Collapse at Underground Coal Gasification Experiments: ABSTRACT

Detailed geologic and mineralogic studies were conducted on the Hanna, Wyoming, and Hoe Creek, Wyoming, underground coal gasification sites. These studies demonstrate the importance geologic factors have on controlling overburden collapse into the reactor cavity during and after coal gasification and on subsequent environmental problems. Parameters that control the collapse of overburden material into the reactor cavity include: duration of the burn; maximum span of unsupported roof rock; lateral and vertical homogeneity, permeability and rock strength; and thickness of overburden materials. The duration of the burn, maximum span of unsupported roof rock, and total thickness of overburden rock are, in large part, determined by the technical engineering aspects of the burn The remaining parameters are determined by geological factors including original composition, depositional environment, diagenesis, and structural deformation. The coals and overburden units at both sites are Eocene in age and are virtually flat lying. The three burn cavities studied at the Hoe Creek site are directly overlain by laterally discontinuous, poorly indurated, kaolinite-cemented sandstones, siltstones, and mudstones of channel and overbank origin. At the Hoe Creek I experiment, a small reactor cavity and a correspondingly short maximum span of unsupported roof rock consisting of fine-grained, low permeability overbank deposits resulted in minimal collapse. At the Hoe Creek II experiment, a significant amount of collapse occurred due to an increased span of unsupported roof rock comprised of poorly consolidated, more permeable channel sandstones and a limited amount of overburden mudstones and siltstones. Roof rock collapse extend d to the surface at the Hoe Creek III experiment where the maximum span of unsupported roof rock was largest and where the roof rock consisted of highly permeable, poorly consolidated channel sandstones. The unit comprising the reactor cavity roof rock at the Hanna II experimental site is a laterally continuous lacustrine delta deposit, which primarily consists of sandstones with lesser amounts of interbedded siltstones and claystones. The overall strength, specifically the bridging capacity, of this unit is enhanced by abundant calcite cement. This cement reduced permeability and interstitial waters which probably kept spalling of the roof rock to a minimum. Consequently, roof rock collapse at the Hanna II experiment was much less extensive than at the Hoe Creek II and III experiments. End_of_Article - Last_Page 1336------------