Monitoring of coal fracturing in underground coal gasification by acoustic emission techniques

During the underground coal gasification (UCG) process, fracturing and cracks occur inside the gasification zone and surrounding rocks as the underground coal cavity evolves.Although fracturing activity and crack extension directly affect gasification efficiency and have environmental impacts, little research to date has focused on their effects. This study discusses the application of acoustic emission (AE) analysis for the evaluation of distinctly designed UCG models and operational parameters and describes the gasification process based on its results. We studied the cavity growth, fracturing mechanism, and the effects of various design and operational variables, such as linking-hole type, gas feed rate, and gasification agent. We found that the AE activity was closely related to the temperature change occurring inside the coal, with AE generation apparently resulting from crack initiation and extension around the coal gasification area, which occurs as a result of thermal stress. UCG modeling showed that the location of AE sources reflects the size of the gasification area and the cavity growth. In addition, the quantitative information on the located AE sources can be obtained. The introduction of a process control system into UCG modeling along with AE monitoring allowed for the real-time monitoring of the fracturing and cavity evolution inside a combustion reactor. Together, these processes have the potential to significantly reduce field risk in UCG by enabling the timely adjustment of operational parameters. Thus, AE monitoring is useful for maintaining a safe and efficient UCG process

Monitoring of coal fracturing in underground coal gasification by acoustic emission techniques

During the underground coal gasification (UCG) process, fracturing and cracks occur inside the gasification zone and surrounding rocks as the underground coal cavity evolves. Although fracturing activity and crack extension directly affect gasification efficiency and have environmental impacts, little research to date has focused on their effects. This study discusses the application of acoustic emission (AE) analysis for the evaluation of distinctly designed UCG models and operational parameters and describes the gasification process based on its results. We studied the cavity growth, fracturing mechanism, and the effects of various design and operational variables, such as linking-hole type, gas feed rate, and gasification agent. We found that the AE activity was closely related to the temperature change occurring inside the coal, with AE generation apparently resulting from crack initiation and extension around the coal gasification area, which occurs as a result of thermal stress. UCG modeling showed that the location of AE sources reflects the size of the gasification area and the cavity growth. In addition, the quantitative information on the located AE sources can be obtained. The introduction of a process control system into UCG modeling along with AE monitoring allowed for the real-time monitoring of the fracturing and cavity evolution inside a combustion reactor. Together, these processes have the potential to significantly reduce field risk in UCG by enabling the timely adjustment of operational parameters. Thus, AE monitoring is useful for maintaining a safe and efficient UCG process

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 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

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

石炭の地下ガス化（UCG）システム開発に関する基礎実験

For Underground Coal Gasification (UCG), it is necessary to evaluate the combustion area in the coal seam precisely. We are planning to use acoustic emission/microseismicity (AE/MS) monitoring for estimation of the underground combustion area. Analyzing the AE/MS waveforms from underground rocks, fracture extension around the combustion reactor can be grasped. For this objective, laboratory experiments are conducted for the UCG model. During burning coal block molded cylindrically by mortar, temperatures inside coal and AE activity were monitored. From the experimental results, it was found that many AE events were generated during combustion of coal, and the AE activity was close related to the change of temperature inside coal block. These AE generations seems to be caused by the crack initiation and extension around coal combustion area in the influence of thermal stress. Therefore, AE/MS monitoring is expected to be a useful tool to evaluate the UCG combustion reactor.特集 : 「資源、新エネルギー、環境、防災研究国際セミナー

室内および小規模フィールド実験による石炭地下ガス化 (UCG) のエネルギー回収率評価

Cavity growth occurring with crack extension and coal consumption during UCG processes directly influences the gasification efficiency and the estimated subsidence and gas leakage to the surface. This report presents an evaluation of the gas energy recovery, coal consumption, and gasification cavity estimation using a proposed stoichiometric method to analyze the coal gasification reaction process. We defined the evaluation parameters of rate of energy recovery and investigated the effects of different parameters using UCG trials conducted with coal blocks and coal seams, adopting different Linking-hole methods and operational parameters. Analyses of results obtained from laboratory experiments and small-scale field trials using V-shaped and L-shaped linking holes, and Coaxial-hole UCG models show that the gasification of Linking-hole models yielded average calorific values of product gas as high as 10.26, 11.11 MJ/m3 (lab.), and 14.39 MJ/m3 (field.). In contrast, the Coaxial-hole models under experimental conditions yielded average calorific values of product gas as: 7.38, 4.70 MJ/m3 (lab.) and 6.66 MJ/m3 (field.). The cavity volume obtained with Coaxial models was about half of the volume obtained from Linking-hole models. Results obtained for these UCG systems show that the feed gas and linking-hole types can influence coal consumption and product gas energy. Fissure ratios were also investigated. Results confirmed major factors underpinning gasification efficiency. Linking-hole types strongly influenced the development of the oxidization surface and fracture cracks for subsequent combustion in the gasification zone. Estimated gas energy recovery results support experimental observations within an acceptable error range of about 10%. Moreover, this stoichiometric approach is simple and useful for evaluating the underground cavity during UCG. Based on these results, we proposed a definition of the energy recovery rate, combined with the obtained volumes of gasification cavities that provide a definition of energy recovery and UCG effects. UCGにおいては，炭層内のき裂進展に伴う燃焼空洞の拡大と石炭の消費が重要であり，これがガス化効率や安全性 (地盤沈下，ガス漏洩等) に大きく影響する。本研究では，ガス化効率，回収エネルギーとガス化空洞の評価方法として，化学量論および化学平衡に基づく評価手法を検討した。生成ガス組成と求めたガス化反応式から，石炭の消費量，ガス生産量等を推定する方法である。また，エネルギー回収率を定義し，UCG室内モデル実験及び露天炭鉱の炭層で行った小規模現場実験の結果を評価し，リンキングの方式や注入ガス等のパラメータがガス化効率やガス化空洞の成長に与える影響を検討した。リンキングの方式として，L字，V字，同軸型のUCG実験を行い，ガス化効率の違いと，その原因を明らかにした。すなわち，リンキング型と同軸型モデルを比較すると，リンキング型UCGモデルの方が発熱量が高く，平均発熱量では，前者が10.26/11.11 MJ/m3 (室内) ，14.39 MJ/m3 (現場) であった。一方，同軸型モデル試験では，7.38/4.70 MJ/m3 (室内) と6.66 MJ/m3 (現場) と低い値であった。実験後の空洞体積の直接評価結果でも，リンキング型の方がガス化領域が拡大していることを確認した。リンキング方式の方が，炭層内にき裂を連続的に進展させやすいためと考えられる。また，エネルギー回収率の評価では，実験前後の供試体質量差から求めたエネルギー回収率と比較検討を行った。その結果，両者の誤差は約10％で，検討した手法によりエネルギー回収率や燃焼ガス化領域の石炭消費量を推定できることがわかった。以上の結果より，検討した化学量論法よる回収エネルギー評価手法は簡便で，実用的であることが明らかになった。Special Edition for Coal Energy Technology; Development and Utilization of Coal Energ