Underground Coal Gasification Process Research Articles

Experimental Study of Reverse Underground Coal Gasification

Underground coal gasification (UCG) produces less pollution and is safer than traditional coal mining. In order to investigate the effects of different gasifying agents or comprehensive analyses of the characteristics of the gas components in the three zones for the reverse underground coal gasification process, a model test was carried out. The results showed that the oxygen concentration of a gasifying agent is recommended to be higher than 21%, which will lead to more combustible gases and a higher calorific value of gas. Higher flow rates and oxygen content generally afforded more desirable gas compositions and calorific values, with the latter as high as 1430.19 kcal/Nm3. For the enriched oxygen gasifying agent in the reverse gasification process, the flow increase from 10 to 20 Nm3/h affords a rapid increase in the growth rate of the flame front, from 1.80 to 4.88 m/day, which is much faster than that for the air gasifying agent. Increasing the gas injection rate and oxygen concentration will increase the growth rate of the flame front. This affects the distribution of the three zones and further leads to different characteristics of the gas components.

Determination of random pore model parameters for underground coal gasification simulation

Gasification technologies represent the most viable options of thermochemical processing of solid fuels. They are also characterized by lower emissions and higher efficiency when compared to conventional combustion systems. Particular attention has been paid to underground coal gasification offering the possibility of utilization for energy purposes coal resources otherwise inaccessible for economic or safety reasons. The disadvantage of this process is, however, the difficult control both in terms of technological and environmental aspects. The underground coal gasification process requires investigation of numerous heterogeneous reactions and transport processes, influenced by various process parameters, such as the temperature, type and flow rate of a gasification agent and geological conditions of the georeactor. In the paper a new, alternative way of the determination of kinetics of coal gasification by the Random Pore Model application is proposed. The procedure for determination of model parameters is presented. The structural parameter was estimated on the basis of measurements of char porous structure parameters. The reactivity measurements made for selected Polish coals were applied in determination of kinetic constants. The results of gasification process simulations for determined parameters and Random Pore Model are also given.

A coupled thermal-mechanical numerical model of underground coal gasification (UCG) including spontaneous coal combustion and its effects

Underground coal gasification (UCG) is a promising option for extracting energy from coal in unworked or hard to access areas of the subsurface. From a geotechnical perspective, UCG involves various complex phenomena resulting from the elevated temperatures induced within the rock surrounding the UCG burn. This paper presents a coupled thermal-mechanical numerical model developed to represent a UCG trial in Wieczorek, Poland. Temperature dependent mechanical properties were assigned according to results obtained from laboratory experiments and data available in the literature. The coal burning process was simulated by modifying the energy balance equation with an additional term related to the calorific value of coal as a source. This source term was described using a time decay function to reflect the fact that the energy release from coal gradually decreases with time. The mechanical degradation of coal due to burning was simulated by removing the burned zone from the calculation after a specific time, which depended on zone size and type of coal. In this study, it was found that the maximum temperature at the burning zone was always <1000 °C, which agrees with previous research carried out for other UCG trials. The size of the burning zone was predicted to spread about 15 m laterally after 20 days of burning. Ground subsidence was evaluated for single and multiple (parallel) panel simulations; subsidence at the top of the numerical mesh, corresponding to a depth of 395 m below the surface, ranged from 23 mm for a single panel to 85 mm for seven panels. The degradation of mechanical properties of the rock surrounding the burned zone due to heating was found to have a marginal effect on the ground subsidence when parallel burning was carried out. The numerical modelling results obtained from this study may provide guidance for the design and operation of UCG processes.

Robust multi-objective control design for underground coal gasification energy conversion process

ABSTRACTThe efficiency of an underground coal gasification (UCG) energy conversion process can be increased by maintaining a desired heating value of the product gases. In literature this task has been accomplished by adopting nonlinear model-based control strategies. To exploit the flexibility of the linear control design techniques, a linear model of the UCG process has been developed, which retains the dynamics of the nonlinear model around the operating point of interest. To account for external disturbance and modelling inaccuracies, an output-based robust multi-objective H2/H∞ control law integrated with pole placement has been proposed. The problem is solved by formulating linear matrix inequality (LMI) constraints for performance and robustness. The simulation results show that the designed controller gives adequate performance in the presence of modelling inaccuracies and external disturbance. Moreover, it has been shown that performance of the designed controller is better as compared to the standard PI controller.

The effects of thermal treatments on the subcritical crack growth of Pingdingshan sandstone at elevated high temperatures

Increasing the deeper understanding of the thermal damages and failure mechanisms of sandstone undergoing thermal treatments at different temperatures is a key concern for deep-mining and underground coal gasification processes. In this research study, a scanning electron microscope (SEM) apparatus, JSM-5410LV, which was equipped with a built-in digital electro-hydraulic servo loading system, was applied to carry out a series of three-point bending tests on Pingdingshan sandstone following heat treatments at elevated and high temperatures ranging from 25 to 600 °C. The subcritical crack initiation load, peak load, and elastic modulus were found to increase with the increases in the thermal treatment temperatures until a maximum was achieved at 125 °C, after which decreases were observed. However, it should be noted that there were sudden drops observed for the specimens after the thermal treatment temperature reached 150 °C due to the thermal diffusivity of the cement. The subcritical crack growth length was theoretically calculated, and a digital speckle correlation method (DSCM) was applied to verify the initial load and subcritical crack growth length. It was found that the fracture toughness fluctuated significantly when the thermal treatment temperature ranged from 25 to 125 °C, and reached a peak of 47.45 MPa mm0.5. It was also observed that, as the temperature was raised from 175 to 600 °C, the fracture toughness gradually decreased. The subcritical crack growth mode was determined to be intra-granular cracking following the thermal treatments below 125 °C, while a mixture of intra-granular and trans-granular cracking occurred in the specimens which experienced thermal treatments of 175 °C. The relationship between heat treatment temperature and subcritical crack growth was derived, which could be used to develop geothermal energy extraction from the critical temperature resources.

Coal Seam Gasification in Faulting Zones (Heat and Mass Balance Study)

In this article, the mass and heat balance calculations of underground coal gasification process for thin coal seams in faulting zones of Lvivskyi coal basin (Ukraine) are defined. The purpose of the research is to establish regularities of heat and mass balance changes in faulting zones influence due to usage air and oxygen-enriched blast. A comprehensive methodology that included analytical calculations is implemented in the work. The output parameters of coal gasification products for the Lvivvyhillia coal mines are detailed. The heat balance is performed on the basis of the mass balance of underground coal gasification analytical results and is described in detail. Interpretations based on the conducted research and investigation are also presented. Conclusions regarding the implementation of the offered method are made on the basis of undertaken investigations. According to conducted research the technology of underground coal gasification can be carry out in the faulting zone of stable geodynamic and tectonic activity. The obtained results with sufficient accuracy in practical application will allow consume coal reserves in the faulting zones using environmentally friendly conversion technology to obtain power and chemical generator gas, chemicals and heat.

Investigation of the pollution risk of residues from a laboratory-scale underground coal gasification of Malkara (Turkey) lignite

The aim of this study was to examine the risk of heavy metal, ammonia and sulfate transition from underground coal gasification (UCG) chars and process water of Malkara lignite to the groundwater around the UCG cavity. The residues were collected during laboratory-scale underground coal gasification studies of Malkara lignite in ex situ gasification simulator. In order to compare gasification with combustion in terms of water pollution, ash samples were obtained by combustion of Malkara lignite. To determine the leaching behavior of the char and ash samples, two leaching tests (EPA TCLP and EN 12457-2 methods) were applied. The concentration of heavy metals in char, ash and process water samples was determined by ICP-OES. Additionally, ammonia and sulfate analyses were conducted for process water. Based on the results of this study, it has been found that the concentration of some elements, such as B, Sb and Al in char and ash eluates exceeded the maximum level of drinking water standards. However, there is no landfill concern about char samples according to EU Landfill Directive. A significant increase in B concentration was observed in ash samples. Considering B and some other elements combustion residues were identified with much higher water contamination ability, compared to gasification residues. Ammonia concentration was observed extremely high in process water of the gasification tests. UCG technique may be more environmentally applicable technology compared to combustion for Malkara lignite reserves. However, further detailed studies are necessary in the early stage of the UCG field test design and application.

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.

Sliding Mode Control of Underground Coal Gasification Energy Conversion Process

The control of highly complex and nonlinear underground coal gasification (UCG) energy conversion process is a challenging job. As the process occurs under the surface of the earth, so it is either impossible or very expensive to measure all the important parameters, which further complicates the control design. The input of the UCG process is the flow rate of the injected air and the heating value of the product gas is the output. In this paper, a sliding mode control (SMC) algorithm is designed for a simplified model of an actual UCG process in order to maintain a desired constant heating value. The relative degree of the sliding variable is zero, because the input is readily available in it. As the heating value is the only measurement available, the trivial control design is not possible. Therefore, the time derivative of the control is selected as the system input, and then the relative degree becomes one and the conventional SMC may be implemented. This approach let us maintain the output at the desired level and provides insensitivity with respect to different types of uncertainties. The stability of the zero dynamics is proved, which ensures that the overall system is stable. The simulation results demonstrate the robustness of the SMC design against the input disturbance and the modeling inaccuracies.

Sequential Transformation Behavior of Iron-Bearing Minerals during Underground Coal Gasification

Detailed mineralogical information from underground coal gasification (UCG) is essential to better understand the chemical reactions far below the surface. It is of great scientific significance to study the mineral transformation and identify the typical minerals in certain process conditions, because it may help to ensure the stable operation of gasification processes and improve the utilization efficiency of coal seams. The transformation of iron-bearing minerals has the typical characteristics during the UCG process and is expected to indicate the process parameters. In this paper, UCG progress was subdivided into pyrolysis, reduction and oxidation stages, and the progressive coal conversion products were prepared. Two types of lignite with different iron contents, Ulankarma and Ulanqab coals, were used in this study. The minerals in the coal transformation products were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation performed using the phase diagram of FactSage 7.1 was used to help to understand the transformation of minerals. The results indicate that the transformation behavior of iron-bearing minerals in the two lignites are similar during the pyrolysis process, in which pyrite (FeS2) in the raw coal is gradually converted into pyrrhotite (Fe1−xS). In the reduction stage, pyrrhotite is transformed into magnetite (Fe3O4) and then changes to FeO. The reaction of FeO and Al2O3 in the low iron coal produces hercynite above 1000 °C because of the difference in the contents of Si and Al, while in the high iron coal, FeO reacts with SiO2 to generate augite (Fe2Si2O6). When the temperature increases to 1400 °C, both hercynite and augite are converted to the thermodynamically-stable sekaninaite.

A new model for evaluation of cavity shape and volume during Underground Coal Gasification process

Coal seams are converted to syngas by advanced thermo-chemical processes through Underground Coal Gasification (UCG) method. Inability to predict the shape and volume of the underground cavity is an important scientific gap in UCG method which is the main subject of this paper. For this purpose, firstly, a series of equations are introduced to predict the cavity growth dimensions over time. Subsequently, these equations are extended in numerical simulation of the Computational Fluid Dynamics (CFD), incorporating the commercial COMSOL software. According to the simulation, the amount of oxidant necessary to convert a certain amount of coal (in the heterogeneous phase) is calculated. The model results indicated that the shape and volume of cavity could be predicted at the onset of the gasification process. The numerical results agreed well with the field data.

Integrated analysis of geofiltrational parameters in the context of underground coal gasification relying upon calculations and modeling

Objective of the paper is to substantiate and implement the integrated approach aimed at the studies of filtration and mass-transfer processes within roof rocks of coal seams in the context of their underground gasification. Integrity of the studies is substantiated by the use of analytical calculations as well as physical and numerical modeling. Zones with different permeability have been determined and values of specific water inflow have been identified basing upon the performed numerical modeling and involving multicomponent transformations within roof formation. The research data actuality is in the studies of spatial and temporal dynamics of rock technogenic rock permeability above gasification channel depending upon their geological structure which will favour the substantiation of efficient engineering solutions to control a process of underground coal gasification under difficult hydrogeological conditions.

Sensing underground coal gasification by ground penetrating radar

The paper describes the results of research on the applicability of the ground penetrating radar (GPR) method for remote sensing and monitoring of the underground coal gasification (UCG) processes. The gasification of coal in a bed entails various technological problems and poses risks to the environment. Therefore, in parallel with research on coal gasification technologies, it is necessary to develop techniques for remote sensing of the process environment. One such technique may be the radar method, which allows imaging of regions of mass loss (voids, fissures) in coal during and after carrying out a gasification process in the bed. The paper describes two research experiments. The first one was carried out on a large-scale model constructed on the surface. It simulated a coal seam in natural geological conditions. A second experiment was performed in a shallow coal deposit maintained in a disused mine and kept accessible for research purposes. Tests performed in the laboratory and in situ conditions showed that the method provides valuable data for assessing and monitoring gasification surfaces in the UCG processes. The advantage of the GPR method is its high resolution and the possibility of determining the spatial shape of various zones and forms created in the coal by the gasification process.

Energy and exergy analysis of alternating injection of oxygen and steam in the low emission underground gasification of deep thin coal

Recent studies have shown that by coupling the underground coal gasification (UCG) with the carbon capture and storage (CCS), the coal energy can be economically extracted with a low carbon footprint. To investigate the effect of UCG and CCS process parameters on the feasibility of the UCG-CCS process, we utilize a validated mathematical model, previously published by the same authors, that can predict the composition of the UCG product, temperature profile, and coal conversion rate for alternating injection of air and steam for unmineable deep thin coal layers. We use the results of the model to conduct an energy and exergy analysis of the UCG process. We study the effect of various process parameters on the efficiency of the UCG process, the zero-emission recovery factor of coal, and the total CO2 emission of the process. Moreover, we compare the alternating injection of air/steam with the injection of an air and steam mixture.Exergy analysis shows that the alternating injection of air/steam describes a practical process for UCG at low pressure. However, injecting a mixture of steam and oxygen results in a practical recovery factor of coal higher than the alternating injection process. Additionally, we show that the zero-emission conversion of unmineable deep thin coal resources in a coupled UCG-CCS process, that is not practical with the current state of technology, can be realized by increasing the energy efficiency of the carbon dioxide capture process.

Recent developments and current position of underground coal gasification

Underground coal gasification is a conversion and extraction process, for the production of useful synthetic product gas from an in-situ coal seam, to use in power generation, heat production or as a chemical feedstock. While many variants of the underground coal gasification process have been considered and over 75 trials performed throughout the world, the recent work has tended to focus on the control of the process, its environmental impact on underground and surface conditions and its potential for carbon capture and storage. Academic research has produced a set of mathematical models of underground coal gasification, and the European Union-supported programme has addressed the production of a decarbonised product gas for carbon capture and storage. In recent years, significant progress has been made into the modelling of tar formation, spalling, flows within the cavity and the control of minor gasification components, like BTEX and phenols, from underground coal gasification cavities (BTEX refers to the chemicals benzene, toluene, ethylbenzene and xylene). The paper reviews the most recent underground coal gasification field trial and modelling experience and refers to the pubic concern and caution by regulators that arise when a commercial or pilot-scale project seeks approval. It will propose solutions for the next generation of underground coal gasification projects. These include the need to access deeper coal seams and the use of new techniques for modelling the process.

Computational Studies for Underground Coal Gasification (UCG) Process

Underground coal gasification (UCG) is a well proven technology in order to access the coal lying either too deep underground, or is otherwise too costly to be extracted using the conventional mining methods. UCG product gas is commonly used as a chemical feedstock or as fuel for power generation. During the UCG process, a cavity is formed in the coal seam during its conversion to gaseous products. The cavity grows in a three-dimensional fashion as the gasification proceeds. The UCG process is indeed a result of several complex interactions of various geo-thermo-mechanical processes such as the fluid flow, heat and mass transfer, chemical reactions, water influx, thermo-mechanical failure, and other geological aspects. The rate of the growth of this cavity and its shape will have a significant impact on the gas flow patterns, chemical kinetics, temperature distributions, and finally the quality of the product gas. It has been observed that there is insufficient information available in the literature to provide clear insight into these issues. It leaves us with a great opportunity to investigate and explore the UCG process, both from the experimental as well as theoretical perspectives. In the development and exploration of new research, experiment is undoubtedly very important. However, due to the excessive cost involvement with experimentation it is not always recommended for the complicated process like UCG. Recently, with the advent of the high performance computational facilities it is quite possible to make alternative experimentation numerically of many physically involved problems using certain computational tools like CFD (computational fluid dynamics). In order to gain a comprehensive understanding of the underlying physical phenomena, modeling strategies have frequently been utilized for the UCG process. Keeping in view the above, the various modeling strategies commonly deployed for carrying out mathematical modeling of UCG process are described here in a concise manner. The available strategies are categorized in several groups and their salient features are discussed in order to have a good understanding of the underlying physical phenomena. This would likely to be a valuable documentation in order to understand the physical process of UCG and will pave to formulate new and involved modeling and simulation techniques for computationally modeling the UCG process.

An overview of the geological controls in underground coal gasification

Coal’s reign will extend well into this millennium as the global demand for coal is expected to increase on average by 2-1% per year through 2019. Enhanced utilization of the domestic coal resource through clean coal technologies is necessary to meet the energy needs while achieving reduced emissions. Underground coal gasification (UCG) is one of such potential technologies. Geology of the area plays decisive role throughout the life of a UCG project and imperative for every phase of the project cycle starting from planning, site selection, design to cessation of operations and restoration of the site. Impermeable over/underlying strata with low porosity and less deformation are most suitable for UCG processes as they act as seal between the coal seam and the surrounding aquifers while limiting the degree of subsidence. Inrush of excess water into the gasification chamber reduces the efficacy of the process and may even quench the reactions in progress. Presence of fresh water aquifer in the vicinity of target coal seam should be abandoned in order to avoid groundwater contamination. UCG is not a proven technology that is still evolving and there are risks that need to be monitored and managed. Effective shutdown programme should intend at minimising the post-burn contaminant generation by flushing out potential organic and inorganic contaminants from the underground strata and treating contaminants, and to restore ground water quality to near baseline conditions.

Policy and Regulatory Issues for Underground Coal Gasification in India

Underground coal gasification (UCG) is in its nascent stage of development. Most of the projects are in the nature of pilot projects. UCG technology requires acceptance in general commercial framework as it matures with the progress of time. Policy and regulatory framework, therefore, is considered here only in the expectation that UCG technology may finally be rolled out sooner than later. India is actively pursuing consultations with major countries which have recorded successes in implementing UCG technology in varying measures. In this background, the discussion on policy and regulatory framework is essentially an effort to capture the broad outline of the understanding of the UCG process in a regulatory construct as compared with other regulatory regimes of similar nature.

&lt;i&gt;Ex Situ&lt;/i&gt; 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.

Kinetics of heterogeneous reactions with coal in context of underground coal gasification

The kinetics of the reactions involving coal is generally determined in laboratory using thermo-gravimetric analyzer (TGA) or drop tube furnace. These instruments suffer from the main limitation that they can handle only small sample sizes and therefore it becomes difficult to bring out the diffusion and conduction related limitations encountered by the large particles associated with the a processes like underground coal gasification (UCG). To address these issues, we present a systematic approach to be followed in laboratory to obtain data on both kinetics and diffusional effects for all the important reactions in UCG. The relatively slower reaction of CO2 gasification of char may be studied well in TGA, whereas fast reactions i.e. char combustion, can only be studied in a specially designed apparatus. A correlation for the effectiveness factor is proposed which allows one to conveniently include particle level reaction-diffusion processes in the global model for the UCG process. The kinetic parameters determined in this study have been used in a UCG process model simulation. A comparison between model predictions and observations from laboratory scale UCG experiments on the same coal shows an excellent match for both exit gas quality and cavity growth. These model predictions demonstrate the applicability of kinetic procedures and parameters determined in this study.