Underground Coal Gasification System Research Articles

Visualization of Movement and Expansion of Coal Reaction Zone by Acoustic Emission Monitoring in Underground Coal Gasification System

Underground coal gasification (UCG) is the process of directly recovering energy as combustible gases such as hydrogen and carbon monoxide by combusting unmined coal resources in situ. The UCG process is an invisible phenomenon, in which fracturing activity at high temperature (>1000 °C) in coal seams expands the gasification zone and increases the combustible components of the product gas. However, excessive expansion of the gasification zone may cause environmental problems such as gas leakage, deformation of the surrounding ground, and groundwater pollution. Therefore, visualization of the gasification zone of UCG is required for both improving gasification efficiency and developing UCG systems with low environmental impact. In this study, the large-scale model UCG experiments conducted on a laboratory scale (size: 625 mm × 650 mm × 2792 mm (H × W × L)) were carried out to discuss the visualization of the gasification reaction zone of coal in UCG by Acoustic Emission (AE) technique with uniaxial and triaxial acceleration transducers. As the results of temperature monitoring and AE source location analysis show, AE sources are located near the high-temperature zone (>1000 °C). In addition, the located AE sources move and expand with the movement and expansion of the high-temperature zone. AE measurement can be a useful technique for monitoring the progress of the UCG reaction zone. AE measurement with triaxial sensors is also useful to predict a high-temperature zone though the measurable range, which has to be considered.

Development and assessment of an integrated underground gasification system for cleaner outputs

In this study, an underground coal gasification-based integrated multigenerational system is developed to generate multiple useful outputs, including electricity, heating, cooling, hydrogen, ethanol and sulfuric acid, for achieving cleaner applications and meeting the community needs. An application of this conceptually developed system is considered for a community as identified in Kutahya, Turkey. The proposed system is developed, modelled, and analyzed using thermodynamic-based, sensitive modeling tools and software packages. The effects of operating conditions such as oxygen, steam, as well as lignite feed rate, reference temperature, and gasifier temperature are investigated to evaluate the performance of the integrated underground coal gasification system. This research further aims to analyze the influence of varying system parameters and operational conditions on ethanol and hydrogen generation and increase energetic and exergetic efficiencies while reducing harmful product gas emissions. In this regard, many parametric studies of the underground coal-powered multigeneration system are conducted. According to the results, the integrated underground lignite gasification combined system's overall system energetic and exergetic efficiencies are found to be 68.8% and 66.56%, respectively.

Study on Generating Acoustic Emission with Heating Coal for Application as Monitoring System for Underground Coal Gasification

Underground Coal Gasification (UCG) is a technology to recover combustible gases by drilling holes and in-situ gasification. The establishment of a safe UCG system requires monitoring of the gasification reaction area because the gasification process that occurs in the underground is an invisible event. It is considered that Acoustic Emission (AE) monitoring is an effective technique for monitoring the underground gasification area since many fracturing activities occur in the coal seam due to coal heating. Therefore, the temperature condition of AE generations is investigated using the coal heating experiments and examines the applicability of AE measurement as a monitoring method for UCG. Measurements of temperature change and AE events with the thermocouples and the acceleration transducers during the coal specimen were heated. As a result, many AE events are found when the temperature of coal is changed and detected in the sensor where its location was near the heat source. Additionally, AE sources are expanded widely with the expansion of the high-temperature region based on the analysis results of the AE source location. It is possible to monitor the temperature change of coal and the expansion of the high-temperature region by applying AE measurement monitoring during UCG.

Experimental study on evaluation of underground coal gasification with a horizontal hole using two different coals

Underground coal gasification (UCG) is a technique to extract coal energy with heat energy and combustible gases through chemical reactions in the underground gasifier. In this study, an application of a coaxial UCG system with a horizontal hole is discussed by means of the model UCG experiments with a large-scale simulated coal seam having dimensions of 550 × 600 × 2740 mm. The two types of coal having 30.18 MJ/kg of calorific value with 7.9% of ash (type 1) and 22.66 MJ/kg of calorific value with 28.3% of ash (type 2) were used for the experiments to evaluate the effect of coal quality on temperature distribution of the gasification area and product gas quality. The oxygen-enriched air was used. The injection rate of the gasification agents was elevated during the experiments to analyze the effect on the product gas. The results show that the gasification area is expanded along the wall of a coaxial hole, not upwards for the type 2 coal with high ash content. The average calorific value of product gas for types 1 and 2 is 8.05 MJ/m3 and 6.91 MJ/m3 respectively, while an increase of injection flow rate produces an improvement of the calorific value for both types of coal. Additionally, it is suggested that the reacted carbon and the product gas volume can be estimated with the volume of oxygen injected regardless of the coal quality if the gasification efficiency and the reaction temperature are similar. These results help to estimate several important parameters, e.g. reacted coal amount, recovered gas volume, and recovered energy from the coal when the actual field implementation is designed.

Unsteady Heat Conduction Study During Underground Coal Gasiflcation For Constructing an In-situ Pyrolysis Project.(Dept.M)

Underground coal gasification (UCG) Is a potential coal utilization technology, receiving renewed interest around the world. In the last .five years there has been Increasing Interest In tile world energy and mining Industry for tile development of a new operating underground coal gasification systems to generate synthetic gas from underground coal seams. In this paper, the problem of unsteady heat conduction In the underground coal and rock layers during underground gasification Is considered. The heating of coal results In releasing considerable amount of gases mainly hydrogen and methane which would be collected and pumped through pipelines for use in different Industrial applications. In the present analysis, a spherical type heat source having a surface temperature of (1000OC, 1500OC, 1750OC and 2000OC) is used for heating the coal layer for achieving tile pyrolysis process. The differential equations describing the unsteady heat conduction in both coal and rock layers are solved numerically using the discretization method (control volume formulation). A computer program Is constructed for calculating the temperature fields and the gasified volumes of mineral coal as well as the heating velocity in the coal seams. In the experimental part, a model of an underground coal layer Is constructed. To validate the numerical model, an electrical spherical heat source Is Inserted directly into the coal layer for heating. The temperature distribution In coal layer is measured In different locations using the Thermocouples. The results show that the temperature profiles and the maximum attainable temperature in the coal seams depend on the time and the surface temperature of the heating reactor. Furthermore, the gasified volume of coal increase with the increase of both the surface temperature of the heating source and the running time. It is found also that the heating velocity in the underground coal. Lies between the mean value (2 to 10-3 K/min.) and increase with the time to reach a maximum value, after this it decrease to approach zero at the steady-state conditions. This study indicates that geometry, position, surface temperature and distribution of heating sources are very important parameters which should be carefully chosen for realizing an efficient pyrolysis of underground coal and avoiding thereby the combustion of a part of the coal seams. The analysis indicated that the best temperature is between (700-800oC) at which the high efficiency of pyrolysis process, the analysis might be useful in constructing an in-situ pyrolysis project for utilizing the underground coal seams.

Underground coal gasification – A numerical approach to study the formation of syngas and its reactive transport in the surrounding strata

In this work, a method to study the formation of syngas during the underground coal gasification (UCG) process and its reactive transport in the surrounding strata is proposed. It combines a thermodynamic equilibrium stoichiometric model of the cavity reactions with a coupled thermo-hydraulic-chemical–mechanical (THCM) framework of COMPASS code for the transport of UCG products away from the cavity. With the input information of coal properties obtained from the South Wales coalfield, gasification reagents (air and steam) and thermodynamic conditions (initial temperature and pressure), the thermodynamic equilibrium model developed provides the maximum yield of gasification products and temperature from a UCG system. Gasification results giving the syngas composition with the highest percentage of methane and carbon dioxide, are then used as the chemical (gas) and thermal boundary conditions for the coupled thermo-chemical model of the THCM framework to analyse the variations of temperature and gas concentrations, in strata surrounding the UCG reactor. For that purpose, a set of numerical simulations considering three porous media (coal, shale and sandstone) with different physico-chemical properties is conducted. The gasification results demonstrate that increasing the amount of steam injected in the UCG reactor decreases the temperature of the system as well as the concentration of carbon monoxide and nitrogen, while benefiting the production of hydrogen, methane and carbon dioxide. The numerical simulations performed using the THCM model indicate that multicomponent gas diffusion and advection are competing transport mechanisms in porous media with intrinsic permeability higher than 1 mD (sandstone), while the gas diffusion becomes a dominant transport process in porous media with an intrinsic permeability lower than 1 mD (coal and shale). Moreover, the simulation results of reactive transport of methane and carbon dioxide in different porous media demonstrate the significance of considering the adsorption effect in the gas transport in the overall UCG process. In particular, the retardation of the gas front due to gas sorption is the most pronounced in coal, followed by shale and then sandstone. In conclusion, the model presented in this study demonstrates its potential application in managing the environmental practices, reducing pollution risk and securing greater public and regulatory support for UCG technology.

Control of Combustion/Gasification Area with Compact Underground Coal Gasification System

Control of Combustion/Gasification Area with Compact Underground Coal Gasification System

Evaluation of a Compact Coaxial Underground Coal Gasification System Inside an Artificial Coal Seam

The Underground Coal Gasification (UCG) system is a clean technology for obtaining energy from coal. The coaxial UCG system is supposed to be compact and flexible in order to adapt to complicated geological conditions caused by the existence of faults and folds in the ground. In this study, the application of a coaxial UCG system with a horizontal well is discussed, by means of an ex situ model UCG experiment in a large-scale simulated coal seam with dimensions of 550 × 600 × 2740 mm. A horizontal well with a 45-mm diameter and a 2600-mm length was used as an injection/production well. During the experiment, changes in temperature field and product gas compositions were observed when changing the outlet position of the injection pipe. It was found that the UCG reactor is unstable and expands continuously due to fracturing activity caused by coal crack initiation and extension under the influence of thermal stress. Therefore, acoustic emission (AE) is considered an effective tool to monitor fracturing activities and visualize the gasification zone of coal. The results gathered from monitoring of AEs agree with the measured data of temperatures; the source location of AE was detected around the region where temperature increased. The average calorific value of the produced gas was 6.85 MJ/Nm3, and the gasification efficiency, defined as the conversion efficiency of the gasified coal to syngas, was 65.43%, in the whole experimental process. The study results suggest that the recovered coal energy from a coaxial UCG system is comparable to that of a conventional UCG system. Therefore, a coaxial UCG system may be a feasible option to utilize abandoned underground coal resources without mining.

Laboratory scale studies on CO2 oxy-fuel combustion in the context of underground coal gasification

Oxidizing/gasifying agents play an essential role in the economy of underground coal gasification (UCG). The selection of feed gases to UCG depends on the prevailing conditions and inherent properties of a coal seam. Steam based UCG operation would lead to transportation and operational difficulty for efficient gasification. Alternatively, CO2 gas is a potential gasifying medium for coal gasification and this option would ensure the reutilization of waste CO2 gas in a UCG operation. Thus, in the present study, borehole coal combustion and gasification experiments are carried out to simulate CO2 enhanced UCG system using a low ash coal originating from the Northeast region of India. UCG experiments are simulated in a laboratory scale using CO2/O2 gases and the results are compared with pure oxygen and oxygen enriched air based UCG operation. The composition and calorific value of syngas are analysed for various gasifying agents. The CO2/O2 borehole experimental studies show that a CO enriched product gas (∼40 vol.%) can be generated under dried coal seam conditions. Thermo-gravimetric analysis (TGA) studies show that the rate of pyrolysis is higher under a CO2 atmosphere as compared to N2 atmosphere. Further, the TGA results show that the generated CO2 pyrolysis products led to substantial char gasification even at low temperatures. The reactivity of tar with CO2 enhanced the calorific value of the product gas due to dry reforming reactions. Also, the presence of suitable inorganic species in the coal progressed the CO2 gasification through ash catalysis.

Effect of Injection Flow Rate on Product Gas Quality in Underground Coal Gasification (UCG) Based on Laboratory Scale Experiment: Development of Co-Axial UCG System

Underground coal gasification (UCG) is a technique to recover coal energy without mining by converting coal into a valuable gas. Model UCG experiments on a laboratory scale were carried out under a low flow rate (6~12 L/min) and a high flow rate (15~30 L/min) with a constant oxygen concentration. During the experiments, the coal temperature was higher and the fracturing events were more active under the high flow rate. Additionally, the gasification efficiency, which means the conversion efficiency of the gasified coal to the product gas, was 71.22% in the low flow rate and 82.42% in the high flow rate. These results suggest that the energy recovery rate with the UCG process can be improved by the increase of the reaction temperature and the promotion of the gasification area.

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

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

CO2 enhanced in-situ oxy-coal gasification based carbon-neutral conventional power generating systems

UCG (Underground coal gasification) is an economic and a viable in-situ clean coal technology in exploiting deep underground coal resources. Sustaining a combustion front in the in-situ gasification of high ash coal is difficult if steam acts as a gasifying medium. In addition, the use of superheated steam causes the transportation problem for deep coal seam. Conversely, these problems could be avoided if CO2 is used as the gasifying medium. In order to implement a CCS ready system in existing plants, the present study investigates the feasibility of integration of CO2/O2 gasification of in-situ coal with conventional power generating systems such as steam turbine and combined cycle power plants performing in a CO2/O2 mode of combustion. The effect of operating pressure and the ratio of CO2/O2 input to an UCG system on the net thermal efficiency of the IGCC system are evaluated. A detailed energy analysis of the CO2 enhanced IUGCC (UCG integrated with the combined cycle) shows a high net thermal efficiency of 41% with CCS (carbon capture and storage) at the UCG operating pressure of 27 bar. IUGST (Integration of low pressure UCG with a steam turbine cycle) system shows a net thermal efficiency of 27% with CCS.

Evaluation of Structural Changes in the Coal Specimen Heating Process and UCG Model Experiments for Developing Efficient UCG Systems

In the underground coal gasification (UCG) process, cavity growth with crack extension inside the coal seam is an important phenomenon that directly influences gasification efficiency. An efficient and environmentally friendly UCG system also relies upon the precise control and evaluation of the gasification zone. This paper presents details of laboratory studies undertaken to evaluate structural changes that occur inside the coal under thermal stress and to evaluate underground coal-oxygen gasification simulated in an ex-situ reactor. The effects of feed temperature, the direction of the stratified plane, and the inherent microcracks on the coal fracture and crack extension were investigated using some heating experiments performed using plate-shaped and cylindrical coal specimens. To monitor the failure process and to measure the microcrack distribution inside the coal specimen before and after heating, acoustic emission (AE) analysis and X-ray CT were applied. We also introduce a laboratory-scale UCG model experiment conducted with set design and operating parameters. The temperature profiles, AE activities, product gas concentration as well as the gasifier weight lossess were measured successively during gasification. The product gas mainly comprised combustible components such as CO, CH4, and H2 (27.5, 5.5, and 17.2 vol% respectively), which produced a high average calorific value (9.1 MJ/m3).

Energy analysis of underground coal gasification with CO2 capture and auxiliary power production

Underground coal gasification is a method of converting in situ coal into synthesis gas through the same chemical reactions that occur in surface gasifiers. If underground coal gasification systems are combined with carbon capture and storage methods, CO2 emissions can be greatly reduced. Carbon capture and storage methods require considerable amount of energy to separate CO2 and prepare it for transport and storage, which is typically considered a parasitic loss. This study investigates utilizing waste heat rejected during processing of syngas from underground coal gasification using an auxiliary power plant, to supply the required energy associated with amine-based CO2 capture and compression processes necessary for transport and sequestration. The system investigated is the Newman Spinney underground coal gasification test plant coupled with a basic Rankine cycle. It is shown that the energy requirement for CO2 processing can be met through the use of an auxiliary plant. The basic model illustrates that the power produced by the auxiliary plant is 2.1 times greater than that required by CO2 compression, while supplying the required heat for amine-based CO2 capture. Parametric analyses are performed to investigate the impact of air injection rate, syngas cooling and CO2 capture and compression requirements on system performance.

Control of combustion area using electrical resistivity method for underground coal gasification

Underground coal gasification (UCG) is one of the clean technologies to collect heat energy and gases (hydrogen, methane, etc.) in an underground coal seam. It is necessary to further developing environmentally friendly UCG system construction. One of the most important UCG’s problems is underground control of combustion area for efficient gas production, estimation of subsidence and gas leakage to the surface. For this objective, laboratory experiments were conducted according to the UCG model to identify the process of combustion cavity development by monitoring the electrical resistivity activity on the coal samples to setup fundamental data for the technology engineering to evaluate combustion area. While burning coal specimens, that had been sampled from various coal deposits, electrical resistivity was monitored. Symmetric four electrodes system (ABMN) of direct and low-frequency current electric resistance method was used for laboratory resistivity measurement of rock samples. Made research and the results suggest that front-end of electro conductivity activity during heating and combusting of coal specimen depended on heating temperature. Combusting coal electro conductivity has complicated multistage type of change. Electrical resistivity method is expected to be a useful geophysical tool to for evaluation of combustion volume and its migration in the coal seam.

Underground coal-air gasification based solid oxide fuel cell system

Underground coal gasification (UCG) is a potential clean coal technology which converts coal into combustible gas in situ. The syngas produced from the UCG process using dry air contains more steam, tar and higher hydrocarbons compared to the conventional gasifiers. The focus of the present work is to use solid oxide fuel cells (SOFCs) to produce electrical power efficiently. To this end, a UCG system, based on air alone as the gasifying medium, is integrated with the SOFC system taking advantage of the high temperature exhaust from the latter to reform the syngas for producing hydrogen. Additional power is produced in the conventional way from a steam turbine making use of the unutilized fuel in SOFC. A detailed energy analysis of this system shows more than 4% thermal efficiency gain over an electricity generating system based entirely on a steam turbine cycle.

Relations between the basic factors and characteristics in the underground degasification of coal beds

On the basis of study data and many years` experience with underground gas generators at different coal fields, the influence of determining geological and technological factors on the characteristics of underground coal gasification (UCG) may be evaluated. Taking the relations between these factors into account permits the choice of optimal UCG systems and parameters.

石炭地下ガス化による水素製造に関する基礎研究

Underground coal gasification was examined to be a new system for hydrogen production. For this purpose, a packed bed of coal in a quartz glass tube was used to simulate experimentally an underground coal gasification system. Carbonization of coal and char gasification reactions were investigated for discussion of the process of hydrogen production with H2O as a gasifier.Results are summarized as follows.(1) Combustion reaction rate of the packed bed of coal was obtained from the propagation speed of flame front. Apparent activation energy for combustion reaction of coal in oxygen was 33 kJ/mol, which implies that mass transfer process may affect the combustion reaction. This means that the reaction rate and combustion temperature can be controlled by the flow rate of oxygen.(2) Production of hydrogen increased with combustion temperature and partial pressure of steam in the range of experiment. In the present study, 60 percent increment of hydrogen was attained in comparison with experiment of 02 as a gasifier.(3) Reaction, CO+H2O H2+CO2 was the most important for the increase of hydrogen production in a case of 600°C of combustion temperature, and both reactions, C+H2O H2+CO and CO +H2O-H2+CO2 contributed to the hydrogen production in the case of combustion temperatures higher than 750°C.