Underground Coal Gasification Process Research Articles

Application of the thermoporoelasticity model in numerical modelling of underground coal gasification influence on the surrounding medium

Abstract The purpose of this paper was to present the thermoporoelasticity model adapted for application in modelling processes, where phase transition may occur, such as during underground coal gasification (UCG). The mathematical model of the medium (soil/rock with pores filled with liquid/gas) in non-isothermal conditions is based on Biot's poroelasticity model. The poroelasticity model is expanded here by the influence of temperature and adjusted to the case where both liquid and highly compressible fluid are present in pores by using the gas laws. This requires considering temperature-dependent physical quantities such as pore fluid density, heat transfer coefficient and viscosity as functions of temperature. Based on the proposed mathematical model and the finite element method, a numerical model was built for the purpose of computing processes occurring in the vicinity of the UCG generator. The result of the authors’ work is a three-dimensional (3D) model, which was not only modified, but derived straight from the laws of thermodynamics, where fields of displacement, temperature and fluid flow are coupled. The model makes it possible to determine results significant to modelling of the UCG process, the reach of the gaseous phase's presence in pores, subsidence values, temperature distribution and directions and rate of seepage, without losing the simplicity and elegance of Biot's original concept. Next, the results of simulations for a hypothetical deposit to estimate the environmental impact of UCG are presented. After applying specific geometry and parameters, the model can be useful for verifying if the chosen technology of UCG in specific conditions will be safe for the environment and infrastructure.

Modelling of Gas Flow in the Underground Coal Gasification Process and its Interactions with the Rock Environment

Modelling of Gas Flow in the Underground Coal Gasification Process and its Interactions with the Rock Environment

CFD Simulation of Temperature Variation in Carboniferous Rock Strata During UCG

The numerical simulation was based on the computational fluid dynamics formalism in order to identify the change of temperature in rock strata during underground coal gasification (UCG). The calculations simulated the coal gasification process using oxygen and water vapour as a gasification agent in 120 hours. Based on the selected software (Ansys-Fluent) a model of underground coal gasification (UCG) process was developed. The flow of the gasification agent, the description of the turbulence model, the heat-exchange model and the method of simulation of chemical reactions of gasification are presented herein.

Determination of Kinetic Parameters of Coal Pyrolysis to Simulate the Process of Underground Coal Gasification (UCG)

Purpose The aim of the research presented in this paper was to determine the values of the kinetic parameters of coal pyrolysis from two areas of the planned experiment, UCG, i.e. the Barbara Experimental Mine of the Central Mining Institute and the Wieczorek Mine. Methods The thermal decomposition of coal analysis used the thermogravimetric technique. The test was carried out in a temperature range of 298–1173 K in a nitrogen atmosphere for three fixed heating rates, β − 5, 10, and 15 K/min. A selection of sample heating rates of coal and reaction environments were designed to reflect the conditions seen during the process of underground coal gasification. The kinetic parameters were determined by using modified Coats-Redfern, Kissinger and Mianowski-Radko methods. Results The values of the activation energy, E, and the pre-exponential factor, A, were determined for a given model of the first order decomposition reaction of coal. The study successfully compared kinetic parameters of the tested coals. Practical implications Designated kinetic parameters may be used to model the process of pyrolysis and – as preliminary data – for installation design of pilot underground coal gasification. Originality/value The devolatilization of a homogenous lump of coal is a complex issue. Currently, the CFD technique (Computational Fluid Dynamics) is commonly used for the multi-dimensional and multiphase phenomena modelling. The mathematical models, describing the kinetics of the decomposition of coal, proposed in the article can, therefore, be an integral part of models based on numerical fluid mechanics.

Neuro-adaptive sliding mode control for underground coal gasification energy conversion process

ABSTRACT Due to the non-availability of model parameters, the model-base control of a nonlinear and infinite-dimensional underground coal gasification (UCG) process is a challenging task. In this paper, a robust neuro-adaptive sliding mode control (NASMC) is designed for the UCG process to maintain a desired heating value level. The unknown model parameters used in NASMC are estimated using the feed-forward neural network. Moreover, the controller also requires time derivatives of some model parameters, which are estimated by uniform robust exact differentiator. As the relative degree of the output with respect to the input is zero, therefore, to apply NASMC, the relative degree is increased to one. This approach maintains the desired heating value and provides insensitivity to input disturbance and model uncertainties. A comparison is also made between NASMC and an already designed conventional SMC. The simulation results show that NASMC exhibits better performance as compared to the conservative SMC design.

Mechanical performance evolution and size determination of strip coal pillars with an account of thermo-mechanical coupling in underground coal gasification

In this study, underground coal gasification (UCG) technology with the strip mining method was recommended to avoid the instability of surrounding rock in the UCG cavity and possible groundwater pollution, in which mechanical performance and reasonable size of the strip pillar play the crucial role. The respective innovative calculation method considering the thermo-mechanical coupling effect was proposed and applied to size designs of pillars in the UCG projects of the Shanjiaoshu Mine and Mazino coal deposit. The proposed method feasibility was experimentally validated and yielded the following predictions of the coal pillar mechanical performance evolution during the UCG process. Heat-affected zones in the coal pillar and immediate roof gradually increased with the following decline. High temperatures above 250 °C affected the coal and rock masses only at a distance not exceeding 2 m from the surface, but their effect continued even after the UCG termination. High temperatures reduced the load-bearing capacity in the area near the coal pillar surface and induced long-term thermal stresses in coal pillars, which alternatively played a dominant role in affecting the ultimate strength of the coal pillar at different stages. The plastic zone width xp in the pillar is varied and will remain unchanged once it reaches its peak value. The width of strip pillars Wp is greatly influenced by high temperatures; it exhibits a W-shaped variation pattern during the UCG process and slowly drops after gasification. The strip pillar width of UCG can be determined using the maximum value of function Wp(t) considering a certain safety factor. While the wide pillar's total strength and size are slightly affected by the temperature, its width can be obtained from Wp(0) with a safety factor. This study can be used for predicting the mechanical performance evolution and size determination of pillars in the UCG process.

Outlook for the coal industry and new coal production technologies

Outlook for the coal industry and new coal production technologies

Data-Driven Modeling and Design of Multivariable Dynamic Sliding Mode Control for the Underground Coal Gasification Project Thar

The energy output per unit time is an important performance metric to determine the potential of an underground coal gasification (UCG) site for electricity production. The energy output per unit time is a function of heating value and flow rate of syngas, and therefore, it is essential to devise a multivariable closed-loop system to enhance the efficiency of the UCG process. In this work, a model-based, multivariable dynamic sliding mode control (DSMC) has been designed for the cavity simulation model (CAVSIM), parameterized with the operating parameters and coal properties of the UCG Project Thar (UPT) field. The model-based control of CAVSIM is not possible due to its complex and multidimensional dynamics, and thus, a simple linear multivariable model is identified by employing a subspace (N4SID) system identification technique. The regular form of the linear model is formulated to design the model-based DSMC. Moreover, the stability of zero dynamics is shown on the approximate model of CAVSIM. Consequently, the designed controller is implemented on CAVSIM, and simulation results are compared with conventional sliding mode control (SMC). It has been observed that both the controllers have achieved the tracking objectives in the presence of input disturbance and modeling uncertainties. However, DSMC utilizes lesser control energy to achieve the desired objectives. Furthermore, the continuous control inputs in DSMC significantly reduce chattering.

Insights into solid-gas conversion and cavity growth during Underground Coal Gasification (UCG) through Thermo-Hydraulic-Chemical (THC) modelling

This paper presents the application of a thermal-hydraulic-chemical model to simulate (1) a laboratory small-scale borehole combustion experiment using high-ash-content coal and (2) an ex-situ large-scale UCG experiment using low-moisture-content hard coal. The focus was to study temperature development, syngas composition, pore structure alteration of coal, and cavity growth pertinent to the UCG process. Both numerical simulations reproduced the temperature development trends along the gasification channel. Compared with the measured cavity with a horizontal length of 15 cm and a maximum vertical extension of 5.4 cm that was created in the 10-h-long borehole combustion experiment, a similar simulated cavity was formed in the first simulation, which had a horizontal length of 10 cm and a maximum vertical extension of 5.1–5.7 cm. This simulation indicated that the proposed model can be used to estimate the cavity growth with the process of UCG due to solid-gas conversion. In the second simulation, the simulated composition of syngas showed good agreement with the experimental results. It also revealed that fierce gasification and combustion mainly occurred close to the inlet, and thus created an L-shaped cavity in the 20-h-long UCG process. Moreover, the UCG process was distinctly divided into three stages from the perspective of solid loss and pore structure alteration at a representative node owing to thermal expansion, compressional shrinkage, pyrolysis, and gasification and combustion. The second stage of 3–11 h was the main period for combustion and gasification of the UCG process, namely, 70% of the solid mass was converted into syngas at temperatures above 1320 K, and the porosity increased linearly with time and the permeability showed exponential growth. Furthermore, a parametric sensitivity study based on the second simulation indicated that the simulated cavity growth of UCG was sensitive to the kinetics of char combustion, while the assumed pyrolyzed production ratios in the model were not of significance to the cavity growth during UCG. It is claimed that the coupled thermo-hydraulic-chemical model adopted in this paper provides novel insights into the UCG reactor's dynamic behavior, including solid-gas conversion and cavity growth.

Influence of scale and atmosphere on the pyrolysis properties of large-scale bituminous coal

In order to explore the influence of scale and atmosphere on the pyrolysis properties of coal in the underground coal gasification process, bituminous coal samples measuring 5, 8, and 10 cm were separately pyrolyzed in N2 and CO2, and the functional groups and pore structure at various scales were also analyzed. The results indicated that the main influences on large-scale bituminous coal pyrolysis were both the inner and outer temperatures, and the inner temperature was lower than the outer temperature. The production rate of pyrolysis tar in CO2 is higher than that in N2. The pyrene, naphthalene, and benzene concentrations in the pyrolysis tar decreased, but the indene increased with the increase of scale. The specific surface area and pore volume of pyrolysis coke were significantly increased in CO2 with the increase of scale. The average pore diameter of pyrolysis coke showed a decreasing trend with the increase of scale. In a N2 atmosphere, the average pore diameter and the most probable diameter were both greater than those in CO2. The −OH, CH2, and CH3 concentrations in the coke in CO2 were lower than those in N2. The larger the scale, the more −OH was contained in the coke.

Model Predictive Control and Adaptive Kalman Filter Design for an Underground Coal Gasification Process

The control of a nonlinear system such as an underground coal gasification (UCG) process is a challenging task. Several nonlinear design approaches are implemented to improve the tracking performance of the UCG process, however, the nonlinear techniques make implementation complex and computationally inefficient. In this work, a constrained linear model predictive control (MPC) is designed for the UCG process to track the desired trajectory of the heating value, while satisfying actuator constraints pertaining to UCG. The unknown states required for MPC design are reconstructed by using linear adaptive Kalman filter (AKF) and unscented Kalman filter (UKF). The design of MPC and AKF is based on the quasi-linear model of the UCG process. A fair comparison between different control strategies is conducted which include MPC– AKF, MPC– UKF, MPC– gain scheduled modified Utkin observer (GSMUO) and dynamic integral sliding mode control (DISMC)–GSMUO. The quantitative analysis and simulation results show that MPC- AKF outperforms its counterparts by yielding the least tracking error and average control energy. This conclusion holds, even in the presence of an external disturbance, parametric variations, and measurement and process noises. Moreover, MPC- AKF yields 51%, 44% and 46% improvement in absolute relative root-mean-squared error with reference to MPC– UKF, MPC– GSMUO and DISMC–GSMUO, respectively. A quantitative analysis has also been carried for AKF and UKF, which shows that the performance of AKF is more robust against changes in the initial values of measurement and process covariances.

Application of geostatistical methods in spatio-temporal modelling of temperature changes of UCG experimental trial

Underground coal gasification (UCG) technology enables coal to the extracted from underground seams that are too deep or affected by tectonic disturbances. In the underground, coal is transformed into syngas with gasification agents and subsequently extracted to the surface. The paper describes the use of geostatistical methods of modelling for the analysis of the UCG process. Based on the geostatistical theory and utilization of Isatis's geostatistical environment, a spatio-temporal model of temperature changes in the experimental generator during the UCG process in laboratory conditions was constructed. For the construction of this model, data from the experiment were used. From the measured data, temperatures obtained from thermocouples placed in the experimental generator were processed. A spatio-temporal model of temperature changes or temperature slices obtained from this model was used to analyze and subsequently calculate the combustion front movement during the experiment. Individual time slices of temperatures were analyzed in ArcGIS-ArcMap environment. Based on the visualization of these slices, an animation of temperature propagation and movement of temperature zones in time and space was created, which represents a view of the resulting process dynamics. It is clear from the model results that shortly after the experiment, two oxidation zones were created - left and right. From the calculated motion vectors of the front, a cyclic movement in the clockwise direction of the left oxidation zone is evident, with the highest movement velocity occurring at approximately half the duration of the experiment.

In situ sorption phenomena can mitigate potential negative environmental effects of underground coal gasification (UCG) - an experimental study of phenol removal on UCG-derived residues in the aspect of contaminant retardation

The aim of the study was to investigate the sorption interactions between phenol and materials obtained from four different underground coal gasification (UCG) ex-situ simulations. These interactions are significant in terms of the impact of the UCG on the groundwater environment. Sorption parameters were determined for two sample types: raw coal mined from the coal-bed and then subjected to the gasification process; and char residue acquired from the cavity formed as a result of the UCG processes. Laboratory-scale tests were carried out using deionized water and aqueous solutions with increasing concentrations of phenol (from 50 mg/dm3 to 2000 mg/dm3) at 298 K. On the assumption of physical interactions (non-specific physisorption) and due to a nonlinear mass distribution of adsorbed substances as a function of equilibrium concentration, the Freundlich isotherm model was applied to describe adsorption phenomena. The isotherms have good fitting (R2 from 0.5716 to 0.9811). Relatively high percentage phenol removal efficiency was observed for all tested chars (from 17.0% to 99.8% for the 1.0–2.5 mm fraction and from 6.9% to 99.6% for the 10.0–12.5 mm fraction). Additionally, the sorption characteristics was used to evaluate the retardation coefficients. The largest delay in the organic pollutant migration in the environment around a UCG reactor occurs for phenol transport in the layer of the post-process char from ‘Wesoła’ after 40 bar pressure experiment.

A numerical investigation into the environmental impact of underground coal gasification technology based on a coupled thermal-hydro-chemical model

This study aims to improve the understanding of the potential environmental impact of the underground coal gasification (UCG) process based on a numerical investigation of the gaseous and dissolved chemicals’ transport in the strata surrounding the UCG reactor. A coupled thermal-hydro-chemical (THC) model in the framework of COMPASS code is employed and further developed for this purpose. The model is compared against analytical solutions in the verification process and applied against a former UCG trial (Hoe Creek UCG programme) for validation purpose. The numerical model is then applied to investigate the potential environmental impact of UCG in three different geological materials (sandstone, coal, and shale) in two steps: the potential gas propagation during UCG process and the solute transport in UCG decommissioning stage. The results indicate that the gas propagation is limited to a near-cavity area in geological strata with low permeability and high air entry value such as shale, which is mainly attributed to the dry zone generated by the high temperature, while a larger gas plume is observed for materials with poor water retention characteristics, such as coal and sandstone. Besides, the gas propagation is retarded to some extent due to the reducing effect of high pressure on gas diffusion. The propagation of dissolved chemicals via diffusion is limited to near-cavity areas, i.e. 2 m, in three studied materials after 10 years. Based on the improved understanding of the various chemicals’ migration in different materials, by selecting the target coal seam surrounded by strata layers with high air entry value and low permeability, the gas losses from the reactor during the UCG reactor operation stage can be minimized and the potential negative environmental impact of ash leaching and solute propagation in UCG decommissioning stage can be avoided. This work also provides further insight and suggestions into the importance of investigating the adsorption capacities of different geological media to evaluate their performance as a natural cleaning system.

MODELING OF MATERIAL BALANCE FROM THE EXPERIMENTAL UCG

The underground coal gasification is a continually evolving technology, which converts coal to calorific gas. There are many important parameters in this technology, which are difficult to measure. These parameters include the underground cavity growth, amount gasified coal, and the leakage of input and output gaseous components into the surrounding layers during the coal gasification process. Mathematical modeling of this process is one of the possible alternatives for determining these unknown parameters. In this paper, the structure of the mathematical model of laboratory underground coal gasification process from the material balance aspect is presented. The material balance consists of mass components entering and leaving from the UCG process. The paper shows a material balance in the form of a general mass balance and atomic species balance. The material balance was testing by six UCG laboratory experiments, which were realized in two ex-situ reactors.

A quantitative assessment of the environmental sustainability of UCG and CO2 storage

In this study, an innovative numerical model was developed to quantify the environmental sustainability situation of in situ underground coal gasification (UCG) and the CO2 storage process, which is expressed in terms of the environmental sustainability index (ESI). This approach is based on four environmental indicators: rock and soil, groundwater, surface water, and atmosphere. Based on the ESI values, the methodology proposed herein is used to classify the environmental sustainability state of the UCG process and its corresponding threshold limit value. Finally, the developed mathematical model was applied to possible European coal deposits, specifically in a Bulgarian coal basin. Research efforts have focused on the development of a mathematical model for environmental impact assessments to pave the way for full-scale trial and commercial applications.

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.

Design and implementation of multi-variable [formula omitted] robust control for the underground coal gasification project Thar

The energy per unit time is an important performance indicator in determining the performance of an underground coal gasification (UCG) site to produce electricity. In literature, model-based strategies are employed by considering UCG as a single input single output (SISO) system, in which only the heating value of syngas is maintained at the desired level by varying inlet gas flow rate. However, the energy per unit time is also dependent on the flow rate of the produced gas mixture. Therefore, in this work, a model-based multi-variable robust control design, based on H∞ technique is proposed for the UCG process. The actual nonlinear model of UCG is very complex due to its 3D axisymmetric geometry, which makes the model-based control design a formidable task. Thus, a simple linear model with two inputs (flow rate and composition of inlet gas) and two outputs (flow rate and heating value of syngas) is identified by using subspace-based (N4SID) system identification technique. The linear model is then employed to design the H∞ (S/KS mixed sensitivity) multi-variable robust controller. The simulation results show that the designed controller has achieved both robust stability and performance in the presence of modeling inaccuracies and external disturbance. Furthermore, the designed controller is also implemented on the actual nonlinear cavity simulator (CAVSIM) for the UCG process. The controller exhibits an adequate performance by tracking the desired set points for the heating value and flow rate of the syngas.

Aquatic risk assessment based on the results of research on mine waters as a part of a pilot underground coal gasification process

One of the possible applications of available hard coal resources is their underground gasification directly in the deposit. The gas obtained in this way is discharged to the surface for use in chemical synthesis or energy purposes. The environmental risk of the technology is mainly related to the potential negative impact of the process on groundwater and surface water, due to the need to remove process condensates and the drainage of the exploitation area. The risk occurs both during the process and after its completion. This article presents the results of a pilot study carried out in the Wieczorek Mine. The coal gasification experiment was carried out over 6 months (with 2 months of gas extraction) in real conditions in a georeactor located 450 m below the surface and on the surface of the object (cooling and cleaning system for gasification products). Water monitoring was carried out before the experiment, during the experiment and 5 years after the end of the process. The analysis includes the results of underground and surface water monitoring within the range of the potential impact of the pilot plant. The paper also presents the analysis of water-tar condensate collected from the gas cleaning system.

The Effectiveness of Reactive Materials for Contaminant Removal in the Process of Coal Conversion

ABSTRACT The technology of permeable reactive barriers (PRB) is one of the most frequently developed methods for protecting soil and water from pollution. These barriers are zones filled with reactive material in which contaminants are immobilized and/or their concentration is reduced to the limit values during the flow of contaminated groundwater. This article presents a study on the efficiency of the removal of contaminants from the post-processing water from the underground coal gasification (UCG) process. The tests were carried out in a laboratory using a flow-through reactor design. The post-processing water came from a UCG experiment carried out in the Barbara mine, Mikołów, Poland. Activated coal, zeolite, and nano-iron were used as the reactive materials in the experiment. The obtained results were compared to tests carried out with reference water (artificial) with strictly defined characteristics. Research has shown that activated carbon is the most effective material used in the reaction zone for removing organic contaminants from groundwater generated during the coal conversion process. A new feature is the use of PRB in a georeactor zone during the UCG process to limit the potential risk of contamination spreading in the case of uncontrolled and unpredictable operation, in emergency situations related to gas leaks into the environment, during underground fires, and for water polluted by high-toxicity substances.