Ex-situ Model Experiment for Development of Compact Underground Coal Gasification (UCG) System

Transcription

1 Modern Environmental Science and Engineering (ISSN ) September 2016, Volume 2, No. 9, pp Doi: /mese( )/ /004 Academic Star Publishing Company, Ex-situ Model Experiment for Development of Compact Underground Coal Gasification (UCG) System Akihiro Hamanaka 1, Fa-qiang Su 1, Ken-ichi Itakura 1, Kohki Satoh 1, Kazuhiro Takahashi 1, Gota Deguchi 2, Jun-ichi Kodama 3 and Koutarou Ohga 3 1. Muroran Institute of Technology, Muroran, Japan 2. Underground Resources Innovation Network, Sapporo, Japan 3. Hokkaido University, Sapporo, Japan Abstract: Underground Coal Gasification (UCG) is a technique to recover coal energy in form of gas in surface by burned and gasified in an underground coal seam abandoned for eir technical or economic reasons. The product gas consists of several combustible gases: carbon monoxide, hydrogen, methane, and or hydrocarbon gases. Recently, this technology is focused as a clean coal technology to recover unused coal resources without environmental impacts. However, it might be difficult to adopt conventional UCG systems in Japan because of geological conditions that are complicated by existence of faults and folds. Therefore, an alternative UCG system, compact, safe, and highly efficient have to be introduced. In this study, UCG model experiments were conducted for two UCG system, linking method (a conventional UCG system) and co-axial method (an alternative UCG system), by using artificial coal seam whose size was m made by crushed coal. Three types of model experiments, test 1 (linking method), test 2 (linking method), and test 3 (co-axial method), were carried out. In test 1 and test 2, average calorific value of product gas was 6.79 MJ/Nm 3 and 8.25 MJ/Nm 3, respectively, whereas in test 3, value was 4.85 MJ/Nm 3. Additionally, zone of co-axial method is smaller than that of linking method. These results demonstrate that co-axial UCG system needs more consideration to improve quality of product gas and zone. Key words: underground coal,, unused coal, co-axial UCG system 1. Introduction Coal is a fossil fuel used all over world as a primary energy. This resource has advantages compared with or fossil fuels: much reserves, uneven distribution in various parts of world, and low price. On or hands, environmental issues such as air pollution and global warming are suspected attributable to carbon dioxide (CO 2 ) generated when coal is burned. As a result, development and diffusion of clean coal technologies (CCT) are promoted to mitigate CO 2 emission from a use of coal: Corresponding author: Akihiro Hamanaka, Ph.D., Research Associate, research areas/interests: mining, underground coal. hamanaka@mmm.muroran-it.ac.jp. technical development of improving use efficiency and carbon capture storage (CCS). Japan uses 170 million tons of coal annually. Annual coal production is only 1.3 million tons and most of coal is imported from abroad countries. Most of coal mining in Japan was closed by 2001 because of complicated geological conditions for mining development and high price of domestic coal. However, re are still abundant unused coal resources not able to recover due to se technical and economic reasons in underground. The amount of coal resources is estimated 30 billion tons in Japan, and 15 billion tons of coal present in Hokkaido. A technology of Under Coal Gasification (UCG) has a great potential to recover energy from se coal resources.

2 600 Ex-situ Model Experiment for Development of Compact Underground Coal Gasification (UCG) System Many advantages are expected in technology of UCG: utilizing unused coal, lower capital/operating costs, no surface disposal ash, and possibility of combinationn of CCS. Development of UCG is studied in all countries of world such as Australia, China, India, Poland, South Africa, New Zealand, Canada and USA [1-4]. However, UCG systems studied in se countries are usually large-scale development, and not suited in Japan which are local resident near development area and has complicated geologicall conditions. Therefore, a small-scale UCG system with compact, safe, and high efficient, should be developed in orderr to utilize unused coal. 2. Underground Coal Gasification Underground coal (UCG) is a technique to extract energy from coal without mining. UCG operation consists of three steps mainly. In first step, an injection and a production well are drilled to coal seam, n a well linking is established between wells as channel. Second step is to ignite coal seam in-situ and inject agents such as air, oxygen, and steam to recover a synsis gas from a production well. Final step is to return environment back to its original state [5]. For UCG process, carbon monoxide (CO), hydrogen (H 2 ), methane (CH 4 ) can be recovered as main combustible gases. These gases are formed by chemical reactions under high temperature in undergroundd gasifier. A channel can be divided roughly into three zones: oxidization zone, reduction zone, and drying and pyrolysis zone. Fig. 1 shows division of channel into three channels. A role of oxidization zone is to heat and increase temperature of coal seam to enhance chemical reactions in following zones. CO 2 and H 2 O are reduced to CO and H 2 as main chemical reactions in reduction zone. CH 4 is formed under catalytic effect of coal powders or metallic oxides. In drying and pyrolysis zone, various kinds of gases are formed Fig. 1 Division of channel into threee channels [5]. such as CH 4, H 2 O, CO, CO 2, H 2, and C 2 H 6 attributable to pyrolysis of coal seam. Calorific value of synsis gas recovered by UCG is usually low (3~4 MJ/ /Nm 3 ) when air is injected as a agent, meaning that usages of gas are limited because of low calorific value. For this reason, technologies of separation and recovery or improvement techniques of product gas quality should be discussed in terms of an efficient utilization of gas recovered by UCG. 3. UCG system An old technique of UCG system is to gasify coal in channel made by a well linking between injection and production well. Fig. 2 shows concept of a conventional UCG system (forward combustion). Generally, forward and reverse combustion approaches are used in UCG process. In former, coal seam is ignited in an injection borehole and zone moves to a production borehole. By contrast, in latter, coal seam is ignited in a production hole and zone moves to an injection borehole. Controlled retractable injection point (CRIP) method used reverse combustion is a common UCG system in recent year. This method improves produces higher quality gas as a result of lower heat losss [6]. overalll efficiency of UCG process because this method

3 Ex-situ Model Experiment for Development of Compact Underground Coal Gasification (UCG) System 601 having complicated geological conditions. Besides, this system gives small impacts to surrounding environments. However, zone in a co-axial system is smaller than that of a conventional UCG system because it is limited around a well. 4. UCG Model Experiment Fig. 2 Conventional UCG system. The conventional UCG system is suitable for thin, no contamination, and deep coal seams. An alternative UCG system, refore, has to be developed in Japan because geological conditions are complicated such as existence of faults and folds. Additionally, effects to surrounding environment have to be also considered because local communities exist near developing site. Given se backgrounds, we develop a co-axial UCG system with compact, safe, and high efficient. Fig. 3 shows a concept of a co-axial UCG system. This system uses only well drilling and a double pipe. Gasification agents are injected from inner pipe to promote reaction and a production gas is recovered from outer pipe. A co-axial UCG system is applicable for coal seams An ex-situ UCG model experiment was conductedd by using an artificial coal seam in order to collect various data for commercial viability or pilot-scale testt of UCG. Threee types of model experiments, test 1 (linking method), test 2 (linking method), and test 3 (co-axial method), were carried out. An artificial coal seam whose size was m was made by crushed coal (Bibai 5 layer) supplied Table 1. The artificial coal seam has threee wells of 45 mmm in diameter as gas injection or production, and distance between wells is 1. 2 m. The bottoms of wells are connected with by sunago-coal mine. The typical proximate analyses of coal in Bibai 5 layer showss a linking hole, and a cylindrical tube of 34 mm in diameter and 2.4 m in length, which is made of a stainless-steel wire-mesh, is buried in linking hole. External walls of artificial coal seam were covered with refractory cement to prevent heat release and gas leakage. Temperature and acousticc emission (AE) weree monitored to visualize inner part of coal seam. The distributions of each sensor are shown Figs. 4 and 5. In ignitionn phase, coal in production welll was ignited to conduct reverse combustion with a heated charcoal and oxygen. During experiment, mixture of air and oxygen is used as a agent (Fig. 6). The total time to inject agents in each experiment is 47 hours, 45 hours, and 32 hours, respectively. At end of experiment, CO 2 or N 2 gas are injected in order to extinguish a fire. The amount of product gas was measured with ultrasonic flowmeter and compositions of gas was continuously monitored with gas chromatographh Fig. 3 Co-axial UCG system.

4 602 Ex-situ u Model Expe eriment for De evelopment of o Compact Underground U Coal Gasification (UCG) System S (Inficon, Miicro GC 30000A). Fig. 7 shows s overall scheme of UCG U model exxperiment. 5. Results and Discusssion In test 1 (injection welll: No. 2, prodduction well: No. 1) and test 2 (injection weell: No. 3, prooduction welll: No. 1), gasifi fication zone expanded e aroound injecction well though coal was ignited in prroduction welll. Table 1 Proxximate analysees of coal in Biibai 5 layer. Calorrific Fixed Total sulfur Moisture Ashh Volatiles valuue carbon (% %) (MJ/kkg) Fig.. 5 Scheme off AE sensors. Fig.. 6 Gasificatioon agents in total experimentt. Fig. 4 Schem me of rmocoouples. Fig. 7 Overaall scheme of UCG U model exp periment.

5 Ex-situ Model Experiment for Development of Compact Underground Coal Gasification (UCG) System 603 Therefore, se testss were simulated forward combustion. Yong et al. (2014) pointed out difficulty of laboratory experiments on reverse combustion attributable to coal characteristics and dimensions of experiments [7]. Tests 1 and 2, refore, switched from reverse to forward combustion because length of a channel was not enough for velocity moving a fire. Due to se events, test 3 was conducted by using No.1 well which coal was not reacted. Fig. 8(a)-(c) show analysis results of AE source locations obtained by application of least squares iteration technique [8]. Red, green, and blue spheres represent AE sources occurred early, middle, and later period during experiment, respectively. As AE can be detected when cracks were generated due to rmal stress, visualization of inner part of gasifier can be achieved by monitoring AE. These results show that zone in each test was expanded around injection well. Furrmore, it can be (a) Test 1 (b) Test 2 (c) Test 3 Fig. 8 AE source locations. understood from result of test 1 that ignited fire moved to production well to injection well. In test 2 and test 3, it can be understood that zone expanded around injection well due to highh densities of AE events. Fig. 9(a)-(c) show results of temperature profiles in each test. In test 1, temperature in T21 and T41 which are located in base of injection well is increased and reached over 1,200 degree in beginning stage of test. After that, temperaturee is increased and reached over 1,000 degree in T22 and T23. T42 and T43 show maximum temperature in beginning stage, and n decreased rapidly. Thesee results indicate that zone moved to an upward direction along No.2 well. In test 2, temperature in T36, T46, T51 and T52 which are located in base of injection well is increased and reached over 1,200 degree in beginningg stage of test. Then, temperature in T35 and T45 is increased to 600 degree. From se results, it can be expected thatt zone was expanded from central position of No.3 well to outside along linking hole though zone was not expandedd sufficiently to horizontal direction because increase of temperature in T35 and T45 was relatively low. In test 3, No.1 well was used as a co-axial welll because coal around No.3 well which planned to use in test 3 was already reacted in test 2. Furrmore, coal was ignited 20 cm above from bottom of No..1 well after 8 hours from starting of test 3 since lower part of No.1 welll was sealed with melted steel pipe due to high temperature. At first, temperature in T13 is increased and showed 1,2000 degree. After that, temperature is increased in T12, T31, and T11 in order of upper to lower. These resultss show that zone moved from ignitionn point to upward first, and moved to downwardd gradually after that. The monitoring results of main compositions of product gas (CO, CO 2, CH 4, H 2 ) are indicated in Fig. 10( a)-(c). The gas concentration of test 1 and 2 shows

6 604 Offshore Drilling Operations and Fishery Monitoring Programs: A Quantitative Impact Assessment Experience in Sourn Bahia, Norast of Brazil (a) Test 1 (a) Test 1 (b) Test 2 (b) Test 2 (c) Test 3 Fig. 9 Temperature profiles in each test. similar tendency. The concentration of CO is increased to 50-60% after 10 and 20 hours from starting of test in test 1 and 2, respectively. Then, concentration of CO 2 is increased whereas that of CO is decreased gradually. CH 4 and H 2 keep a stable concentration during each test. The monitoring results of test 3 show different with that of test 1 and 2. The concentration of CO is reached at maximum value a few hours later after ignition though value is lower than that of test 1 and 2. CH 4 is rarely detected during test 3 though H 2 keep a stable concentration as with test 1 and 2. (c) Test 3 Fig. 10 Main compositions and calorific value of a product gas. The calorific value in test 1 and 2 shows similar tendency with transition of concentration of CO. The value is increased and reached maximum value (10-12 MJ/Nm 3 ) after 10 and 20 hours in test 1 and 2, respectively. Then, value is decreased gradually with elapsed time. Average calorific value and compositions of product gas is showed in Table 2. The average calorificc value of product gas in test 2 is higher than that of test 1; 6.79 MJ/Nm 3 in test 1 and 8.25

7 Offshore Drilling Operations and Fishery Monitoring Programs: A Quantitative Impact Assessment Experience in Sourn Bahia, Norast of Brazil 605 Table 2 Average calorific value and compositions of product gas. Calorific value (MJ/Nm 3 ) H 2 CO CH 4 CO 2 Test Test Test MJ/Nm 3 in test 2, respectively. This is due to higher O 2 concentration of agents in test 2. The concentration of combustible gas such as CH 4 and H 2 was increased because O 2 concentration of agents in test 2 was 3-5% higher than that of test 1, meaning that zone was expanded attributable to promoting oxidation reaction. The calorific value in test 3 is lower than that of test 1 and 2; 8 MJ/Nm 3 in maximum value and 4.85 MJ/Nm 3 in average value. The concentration of CO and CH 4 in test 3 is much lower than that of test 1 and 2 though re are little differences in H 2 concentration. Furrmore, process is almost finished after 20 hours elapsed from starting test 3 whereas process continues over 40 hours in test 1 and 2. This is due to difficulty to expand zone in co-axial UCG system. According to Fig. 8 and 9, it can be understood that zone is limited near co-axial well and smaller than that of linking method (test 1 and 2). Therefore, method to expand zone have to be considered to improve efficiency of co-axial UCG system as a future task. 6. Conclusion UCG has a great potential to recover unused coal resources which are not able to recover by conventional mining technique due to technical and economic reasons. Co-axial UCG system with compact, safe, and highly efficient is a useful technique to adopt under geological conditions that are complicated by existence of faults and folds. The duration time of process for co-axial method is shorter than that of linking method. Additionally, calorific value of production gas is lower in co-axial method. According to results of AE source locations and temperature profiles, it can be understood that zone of co-axial method is limited near co-axial well and smaller than that of linking method. It is necessary to study improvement techniques to expand zone in co-axial UCG system such as improving permeability of coal artificially and increasing pressure of gasifier as future studies. References [1] A. Y. Klimenko, Early ideas in underground coal and ir evolution, Energies 2 (2009) [2] E. Shafirovich and A. Varma, Underground coal : A brief review of current status, Ind. Eng. Chem. Res. 48 (2009) (17) [3] L. Yang, X. Zhang, S. Liu, L. Yu and W. Zhang, Field test of large-scale hydrogen manufacturing from underground coal (UCG), Int. J. Hydrog. Energy 33 (2008) [4] M. Wiatowski, K. Kapusta, J. Świądrowski, K. Cybulski, M. Ludwik-Pardała, J. Grabowski and K. Stańczyk, Technological aspects of underground coal in experimental Barbara Mine, Fuel 159 (2015) [5] J. Kačur, M. Durdán, M. Laciak and P. Flegner, Impact analysis of oxidant in process of underground, Measurement 51 (2014) [6] A. W. Bhutto, A. A. Bazmi and G. Zahedi, Underground coal : From fundamentals to applications, Prog. Energ. Combust 39 (2013) (1) [7] C. Yong, L. Jie, W. Zhangqing, Z. Xiaochun, F. Chenzi, L. Dongyu and W. Xuan, Forward and reverse combustion of coal with production of high-quality syngas in a simulated pilot system for in situ, Appl. Energ. 131 (2014) [8] F. Q. Su, T. Nakanowataru, K. Itakura, K. Ohga and G. Deguchi, Evaluation of structural changes in coal specimen heating process and UCG model experiments for developing efficient UCG systems, Energies 6 (2013)