Underground Coal Gasification Process Research Articles

The Prediction of Possibilities of CO Poisoning and Explosion during Syngas Leakage in the UCG Process

Underground coal gasification (UCG) is an industrial process that converts coal into product gas (syngas). This technology makes it possible to obtain energy without mining coal and post-processing. The UCG process is considered a safe technology with various advantages over conventional mining techniques. However, a small amount of potentially dangerous syngas may escape from the UCG cavity, and it is necessary to pay attention to the safety of the process in this respect. This article analyses the impact of a syngas leak from UCG on a possible CO poisoning and explosion hazard in the vulnerable zones. Measured data from experiments and proposed mathematical models were used for the analysis of the UCG process and simulation studies. It is mainly a mathematical model mixing of gases, which evaluated the danger of explosion and CO poisoning in sensitive zones. This model predicts the composition of the syngas mixture with air because this mixture can be dangerous at a certain concentration. Simulation studies focused mainly on CO poisoning in vulnerable areas based on the measured data from laboratory experiments. Simulation studies have shown that the critical value of CO poisoning can achieve with a certain composition of syngas and its escape into vulnerable areas. The results of the studies here indicate a possible hazard. However, by monitoring and control of UCG process, this risk can be minimized. Based on the obtained results, the control of the supply of fresh air to the endangered area during the monitoring of the syngas composition was proposed to prevent the occurrence of possible poisoning.

Insights into ground response during underground coal gasification through thermo‐mechanical modeling

AbstractThis paper presents a coupled thermo‐mechanical model to investigate the ground response during underground coal gasification (UCG). The model incorporated the temporal and spatial development of temperature, the gradual growth of the cavity, and temperature‐dependent material properties. Model verification was made against two benchmarks to acquire the confidence for the predictive purpose. The first exercise demonstrated the correctness of the model implemented in COMPASS. The second exercise showed that using the ash‐filled cavity to represent null or empty zones is a good option in the numerical modeling and provided highly comparable results to other models. Based on the Hanna UCG trial, different cases were simulated to investigate the effects of the cavity size in the coal seam and the thermal expansion coefficient of the caprock and base rock on key features that take place during the process of UCG. A maximum temperature in the range of 1200–1500 ℃ was induced by the gasification of coal, and a cavity with a maximum length of 13.5 m was formed after 30 days of simulation. Meanwhile, small vertical displacement in the range of ‐5–12 mm took place near the cavity because of the thermal expansion of the geologic materials and the reduction of the overall weight with the creation of the cavity. In addition, it was found the thermal expansion coefficients can influence the thermo‐mechanical response of geologic materials, but the effects were insignificant when its order of magnitude was smaller than 10‐6 K‐1.

Simulation of underground coal gasification based on a coupled thermal-hydraulic-chemical model

This paper presents a coupled thermal-hydraulic-chemical numerical model to study the temperature development and solid–gas conversion during underground coal gasification (UCG). Theoretical formulations are based on the porous medium approach, involving the gas flow, solid mass loss, and heat flow, and the chemical reactions driving the phase change and transformation of gaseous species are considered by the kinetics model. A modified empirical relationship is proposed for the increase of porosity due to combustion and gasification, and the corresponding variation of permeability is expressed by a semi-empirical relationship. A numerical solution is obtained via the coupling of the transport model and the kinetics model for the sink/source terms due to chemical reactions. Verification tests have been conducted and compared with alternative numerical solutions to build confidence in the accuracy of the implementation of the model. Evaluation of the model against literature benchmarks including temperature development and syngas composition variation indicates that the coupled model can predict the UCG process.

Characteristics of Water Contaminants from Underground Coal Gasification (UCG) Process—Effect of Coal Properties and Gasification Pressure

One of the most important issues during UCG process is wastewater production and treatment. Condensed gasification wastewater is contaminated by many hazardous compounds. The composition of the generated UCG-derived wastewater may vary depending on the type of gasified coal and conditions of the gasification process. The main purpose of this study was a qualitative and quantitative characterization of the UCG wastewater produced during four different UCG experiments. Experiments were conducted using semi-anthracite and bituminous coal samples at two distinct pressures, i.e., 20 and 40 bar. The conducted studies revealed significant relationships between the physicochemical composition of the wastewater and the coal properties as well as the gasification pressure. The strongest impact is noticeable in the case of organic pollutants, especially phenols, BTEX and PAH’s. The most abundant group of pollutants were phenols. Conducted studies showed significantly higher concentration levels for bituminous coal: 29.25–49.5 mg/L whereas for semi-anthracite effluents these concentrations were in much lower range 2.1–29.7 mg/L. The opposite situation occurs for BTEX, higher concentrations were in wastewater from semi-anthracite gasification: 5483.1–1496.7 µg/L, while in samples from bituminous coal gasification average BTEX concentrations were: 2514.3–1354.4 µg/L. A similar relationship occurs for the PAH’s concentrations. The higher values were in case of wastewater from semi-anthracite coal experiments and were in range 362–1658 µg/L while from bituminous coal gasification PAH’s values are in lower ranges 407–1090 µg/L. The studies conducted have shown that concentrations of phenols, BTEX and PAH’s decrease with increasing pressure. Pearson’s correlation analysis was performed to enhance the interpretation of the obtained experimental data and showed a very strong relationship between three parameters: phenols, volatile phenols and CODcr.

Determination of the Extent of the Rock Destruction Zones around a Gasification Channel on the Basis of Strength Tests of Sandstone and Claystone Samples Heated at High Temperatures up to 1200 °C and Exposed to Water

This article presents the results of laboratory tests regarding the influence of high temperatures on changes in the strength and structural parameters of rocks that are present in the immediate vicinity of a gasification channel. Sandstone and claystone samples were heated at 300 °C, 600 °C, 900 °C and 1200 °C. Additionally, the heated samples were placed in water for 24 h. The results of the laboratory tests were used in the numerical simulation using RS2 software. The main goal of modeling was to determine the extent of the rock destruction zone around the gasification channel for dry and wet rock masses. In the numerical simulations, three widths of the gasification channel and three ranges of high-temperature impact were modeled. On the basis of the obtained results, it was found that the extent of rock destruction, both in the roof and in the floor, is greater by several percent for a wet rock mass. For the first time, this research presents the effect of water on heated rock samples in terms of the underground coal gasification process. The results of laboratory tests and numerical simulations clearly indicate a reduction in strength, deformation and structural parameters for the temperature of 1200 °C.

Syngas Underground Coal Gasification (UCG) Testing of In-Situ Type Lignite Coal and Fracture Type Coal

The advancement and utilization of technology require energy, namely electricity and fuel energy. The increasing cost of fossil energy, the scarcity of non-renewable energy sources and the increasing energy demand requires diversification of energy resources. One of the new renewable energy sources currently being developed in Indonesia is Underground Coal Gasification (UCG). UCG is a new technology that utilizes unmined coal. In the UCG process, coal is burned underground and the syngas produced is collected through bore wells for processing or direct use. The resulting syngas is analyzed to see the effect of in-situ type of lignite coal and fractured type of coal on syngas production using the UCG method. Tests carried out on fracture-type lignite coal for 120 minutes with a sample weight of 1.3 kg obtained a CH4 gas concentration of 0.24%, which is relatively low compared to the in-situ type lignite coal sample CH4 gas concentration of 1.13%. The CO2 concentration was 54.46% in the fracture sample, and the In-Situ type sample was 52.19%. The O2 content with a value of 19.43% for the Fracture Type sample and 4.94% for the In-Situ type sample. Fracture Type and In-situ Lignite Coal produced fewer syngas products due to UCG testing than high-rank coals such as sub-bituminous and bituminous coal.

Effect of Lignite Properties on Its Suitability for the Implementation of Underground Coal Gasification (UCG) in Selected Deposits

Two experimental simulations of underground coal gasification (UCG) processes, using large bulk samples of lignites, were conducted in a surface laboratory setup. Two different lignite samples were used for the oxygen-blown experiments, i.e., “Velenje” meta-lignite (Slovenia) and “Oltenia” ortho-lignite (Romania). The average moisture content of the samples was 31.6wt.% and 45.6wt.% for the Velenje and Oltenia samples, respectively. The main aim of the study was to assess the suitability of the tested lignites for the underground coal gasification process. The gas composition and its production rates, as well as the temperatures in the artificial seams, were continuously monitored during the experiments. The average calorific value of gas produced during the Velenje lignite experiment (6.4 MJ/Nm3) was much higher compared to the result obtained for the experiment with Oltenia lignite (4.8 MJ/Nm3). The Velenje lignite test was also characterized by significantly higher energy efficiency, i.e., 44.6%, compared to the gasification of Oltenia lignite (33.4%). The gasification experiments carried out showed that the physicochemical properties of the lignite used considerably affect the in situ gasification process. Research also indicates that UCG can be considered as a viable option for the extraction of lignite deposits; however, lignites with a lower moisture content and higher energy density are preferred, due to their much higher process efficiency.

Effect of Oxygen Flow Rate on Combustion Time and Temperature of Underground Coal Gasification

Underground Coal Gasification (UCG) is a process of converting coal in the ground into synthetic gas that has economic value. In the UCG process which will be carried out in the UCG prototype assisted by the presence of oxygen as a gasification agent, which this gasification agent will help the process of burning coal in the ground. The flow rate of oxygen in the process of UCG affecting the coal combustion temperature and effective flame from burning coal. The highest temperature at a flow rate of 5 l/min is 240oC, at an oxygen flow rate of 3 l/min the highest temperature is 143oC and at an oxygen flow rate of 2 l/min the highest temperature is 135oC and time effective flame at a flow rate of 5 l / min ie 80 minutes, effective burning time on the speed of the flow rate of 3 l / min ie for 120 minutes and time effective flame at a flow rate of 2 l / min ie for 165 minutes. This study proves that the greater the oxygen flow rate is used as the gasification agent at UCG process the lignite coal combustion temperatures will be high and effective flame coal combustion process will be more brief.

Regression Models Utilization to the Underground Temperature Determination at Coal Energy Conversion

The underground coal gasification represents a technology capable of obtaining synthetic coal gas from hard-reached coal deposits and coal beds with tectonic faults. This technology is also less expensive than conventional coal mining. The cavity is formed in the coal seam by converting coal to synthetic gas during the underground coal gasification process. The cavity growth rate and the gasification queue’s moving velocity are affected by controllable variables, i.e., the operation pressure, the gasification agent, and the laboratory coal seam geometry. These variables can be continuously measured by standard measuring devices and techniques as opposed to the underground temperature. This paper researches the possibility of the regression models utilization for temperature data prediction for this reason. Several regression models were proposed that were differed in their structures, i.e., the number and type of selected controllable variables as independent variables. The goal was to find such a regression model structure, where the underground temperature is predicted with the greatest possible accuracy. The regression model structures’ proposal was realized on data obtained from two laboratory measurements realized in the ex situ reactor. The obtained temperature data can be used for visualization of the cavity growth in the gasified coal seam.

Sequential transformation study of minerals during underground coal gasification: Promising to indirectly reflect the actual reaction condition

AbstractThe mineralogical characteristics of underground coal gasification (UCG) residuals are closely related to the gasification conditions. The study of the mineral composition and transformation of ash and slag under different gasification conditions has important scientific significance for understanding the real gasification conditions, improving the utilization efficiency of coal, and identifying and controlling the environmental impact. In this paper, the sequential transformation of minerals under different temperature and atmosphere are studied by X‐ray diffraction (XRD) and FactSage software, and the elemental composition of the surface of the UCG ash and slag is characterized by SEM‐EDX. The results show that the main minerals in the semicoke are quartz (SiO2), illite (K1.5Al4(Si6.5Al1.5)O20(OH)4), and sanidine (KAlSi3O8). In the 900–1300°C reduction, ash, quartz, sanidine, hercynite (FeAl2O4), anorthite (CaAl2Si2O8), and mullite (Al6Si2O13) are the main minerals. The minerals in the oxidation residuals (1100–1500°C) are quartz, anorthite, hematite (Fe2O3), hercynite, cordierite (Fe2Al4Si5O18), and cristobalite. The reaction temperature has a significant effect on the changes in mineralogy. The reaction temperature is low during the pyrolysis process, and most changes involve only structural changes. With the reaction temperature increases, a series of reactions occur between different minerals. The reaction atmosphere also has a significant effect on the mineralogy. The research on the transformation of minerals in UCG residues can indirectly reflect the actual process of UCG, which can provide suggestions for the stable operation of the UCG process.

Effect Of Equivalent Ratio (ER) On the Flow and Combustion Characteristics in A Typical Underground Coal Gasification (UCG) Cavity

Underground Coal Gasification (UCG) is thought to be the most favourable clean coal technology option from geological-engineering-environmental viewpoint (less polluting and high efficiency) for extracting energy from coal without digging it out or burning it on the surface. UCG process requires only injecting oxidizing agent (O2 or air with steam) as raw material, into the buried coal seam, at an effective ratio which regulates the performance of gasification. This study aims to evaluate the influence of equivalent ratio (ER) on the flow and combustion characteristics in a typical half tear-drop shape of UCG cavity which is generally formed during the UCG process. A flow modeling software, Ansys FLUENT is used to construct a 3-D model and to solve problems in the cavity. The boundary conditions are- (i) a mass-flow-inlet passing oxidizer (in this case, air) into the cavity, (ii) a fuel-inlet where the coal volatiles are originated and (iii) a pressure-outlet for flowing the product Syngas out of the cavity. A steady-state simulation has been run using k-? turbulence model. The mass flow rate of air varied according to an equivalent ratio (ER) of 0.16, 0.33, 0.49 and 0.82, while the fuel flow rate was fixed. The optimal condition of ER has been identified through observing flow and combustion characteristics, which looked apparently stable at ER 0.33. In general, the flow circulation mainly takes place around the ash-rubble pile. A high temperature zone is found at the air-releasing point of the injection pipe into the ash-rubble pile. This study could practically be useful to identify one of the vital controlling factors of gasification performance (i.e., ER impact on product gas flow characteristics) which might become a cost-effective solution in advance of commencement of any physical operation.

Experimental Studies of the Effect of Design and Technological Solutions on the Intensification of an Underground Coal Gasification Process

This paper represents the results of experimental studies of physical modeling of the underground coal gasification process in terms of implementation of design and technological solutions aimed at intensification of a gasification process of thin coal seams. A series of experimental studies were performed in terms of a stand unit with the provided criteria of similarity to field conditions as well as kinetics of thermochemical processes occurring within a gas generator. Hard coal (high volatile bituminous coal) was selected as the raw material to be gasified, as that coal grade prevails in Ukrainian energy balance since it is represented by rather great reserves. Five blow types were tested during the research (air, air–steam, oxygen–steam, oxygen–enriched, and carbon dioxide and oxygen). As a result, the effect of tightness of a gas generator on the quantitative and qualitative parameters of coal gasification while varying the blow by reagents and changing the pressure in a reaction channel has been identified. Special attention was paid to the design solutions involving blow supply immediately into the combustion face of a gas generator. The experimental results demonstrate maximum efficiency of the applied gas generator design involving flexible pipelines and activator in the reaction channel and a blow direction onto the reaction channel face combined with blow stream reversing which will make it possible to improve caloricity of the generator gas up to 18% (i.e., from 8.4 to 12.8 MJ/m3 depending upon a blow type). Consideration of the obtained results of physical modelling can be used with sufficient accuracy to establish modern enterprises based on the underground coal seam gasification; this will help develop more efficiently the substandard coal reserves to generate heat energy as well as power-producing and chemical raw material. The research conclusions can provide technical reference for developing a new generation of underground coal gasification technology.

Changes in properties of tar obtained during underground coal gasification process

In this study, the composition of tars collected during a six-day underground coal gasification (UCG) test at the experimental mine ‘Barbara’ in Poland in 2013 was examined. During the test, tar samples were taken every day from the liquid product separator and analysed by the methods used for testing properties of typical coke oven (coal) tar. The obtained results were compared with each other and with the data for coal tar. As gasification progressed, a decreasing trend in the water content and an increasing trend in the ash content were observed. The tars tested were characterized by large changes in the residue after coking and content of parts insoluble in toluene and by smaller fluctuations in the content of parts insoluble in quinoline. All tested samples were characterized by very high distillation losses, while for samples starting from the third day of gasification, a clear decrease in losses was visible. A chromatographic analysis showed that there were no major differences in composition between the tested tars and that none of the tar had a dominant component such as naphthalene in coal tar. The content of polycyclic aromatic hydrocarbons (PAHs) in UCG tars is several times lower than that in coal tar. No light monoaromatic hydrocarbons (benzene, toluene, ethylbenzene and xylenes—BTEX) were found in the analysed tars, which results from the fact that these compounds, due to their high volatility, did not separate from the process gas in the liquid product separator.

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