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Nitrogen Injection To Flush Coal Seam Gas Out Of Coal: An Experimental Study

Nitrogen Injection To Flush Coal Seam Gas Out Of Coal: An Experimental Study Arch. Min. Sci., Vol. 60 (2015), No 4, p. 1013­1028 Eleronic version (in color) of this paper is available: http://mining.archives.pl DOI 10.1515/amsc-2015-0067 LEI ZHANG*1, NAJ AZIZ**, TING REN**, JAN NEMCIK**, SHIHAO TU*1 WPROWADZANIE AZOTU DO ZLÓ WGLA W CELU WYPLUKIWANIA GAZÓW Z POKLADU ­ BADANIA EKSPERYMENTALNE Several mines operating in the Bulli seam of the Sydney Basin in NSW, Australia are experiencing difficulties in reducing gas content within the available drainage lead time in various seions of the coal deposit. Increased density of drainage boreholes has proven to be ineffeive, particularly in seions of the coal seam rich in CO2. Plus with the increasing worldwide concern on green house gas reduion and clean energy utilisation, significant attention is paid to develop a more praical and economical method of enhancing the gas recovery from coal seams. A technology based on N2 injeion was proposed to flush the Coal Seam Gas (CSG) out of coal and enhance the gas drainage process. In this study, laboratory tests on CO2 and CH4 gas recovery from coal by N2 injeion are described and results show that N2 flushing has a significant impa on the CO2 and CH4 desorption and removal from coal. During the flushing stage, it was found that N2 flushing plays a more effeive role in reducing adsorbed CH4 than CO2. Comparatively, during the desorption stage, the study shows gas desorption after N2 flushing plays a more effeive role in reducing adsorbed CO2 than CH4. Keywords: N2 injeion, coal seam gas, coal, gas composition, gas volume W kilku kopalniach eksploatujcych zloe Bulli w zaglbiu wglowym Sydney w Nowej Poludniowej Walii w Australii pojawil si problem redukcji zawartoci gazu kopalnianego w zloach zawartego w rónych czciach zloa, w okrelonym czasie. Zwikszenie gstoci wykonywania odwiertów drenaowych okazalo si by metod nieskuteczn, zwlaszcza w czciach zloa bogatego w CO2. Inne kwestie to wzrastajca w wiecie wiadomo koniecznoci redukcji gazów cieplarnianych i wykorzystania czystej energii, std te podejmowane wysilki na rzecz opracowania praktycznych i ekonomicznych metod odzyskiwania gazu ze zló wgla. W pracy przedstawiono technologi opart na wprowadzaniu azotu do zloa w celu wyplukania gazu zawartego w wglu, poprawiajc skuteczno ich odzyskiwania. W prowadzonych pracach badano skuteczno odzysku CO2 i metanu ze zloa wgla po wprowadzeniu * ** SCHOOL OF MINES, CHINA UNIVERSITY OF MINING AND TECHNOLOGY, XUZHOU CITY, JIANGSU 221116, CHINA. KEY LABORATORY OF DEEP COAL RESOURCE MINING, MINISTRY OF EDUCATION OF CHINA, XUZHOU 221116, CHINA. SCHOOL OF CIVIL, MINING & ENVIRONMENTAL ENGINEERING, FACULTY OF ENGINEERING AND INFORMATION SCIENCES, UNIVERSITY OF WOLLONGONG, NSW 2522, AUSTRALIA. CORRESPONDING AUTHORS, E-mail: lz811@uowmail.edu.au; papertsh@126.com do niego azotu. Wyniki bada wskazuj, e wyplukiwanie azotem w powanym stopniu wplywa na proces desorpcji CO2 i CH4 i ich usuwania z wgla. Na etapie wprowadzania azotu, stwierdzono e wyplukiwanie azotem w wikszym stopniu wspomaga usuwanie adsorbowanego CH4 ni CO2. Dla porównania, w trakcie desorpcji, wykazano, e desorpcja gazów po wprowadzeniu do zloa azotu znacznie skuteczniej redukuje ilo adsorbowanego CO2 ni CH4. Slowa kluczowe: wprowadzanie azotu do zloa, gaz zawarty w wglu, wgiel, sklad gazu, objto gazu 1. Introduion There is growing interest in gas injeion to enhance Coal Seam Gas (CSG) recovery. The utilisation of N2 injeion has been found to help CSG recovery (Reeves & Oudinot, 2004, 2005; Florentin et al., 2010; Kiyama et al., 2011; Packham et al., 2012; Zhang, 2013). In the reservoir and economic analysis study of Tiffany unit N2 ­ ECBM pilot (Reeves & Oudinot, 2005), it was found that incremental methane recovery of approximately 10-20% of the original gas in place was achieved with N2 injeion. The future N2 injeion was forecast to add another 25-40% to the total recovery of original gas in place. The future N2 injeion at Tiffany was also forecast to be economic. Packham et al. (2012) reported the results from a field trial condued with Surface to In-Seam (SIS) pre-drainage wells and concluded that the enhanced drainage could provide the means for both accelerating methane drainage and reducing residual gas content. Packham et al. (2011) provided the background to this field trial including details of the reservoir charaeristics, well geometry and installations. They also described how history matching of the reservoir and simulation of the effes of nitrogen injeion indicated that accelerated drainage was likely. In concept, the principle of N2 injeion to Enhance Coalbed Methane recovery (N2-ECBM) can be described as follows: N2 is injeed into a coal reservoir, it displaces the gaseous CSG from the cleat system, decreasing the CSG partial pressure and creating a compositional disequilibrium between the gaseous and adsorbed phases. These combined influences cause the CO2 or CH4 to desorb and diffuse into the cleat system, becoming the "stripped gas" from the matrix. The CSG then migrates to and is produced from produion wells (Reeves & Oudinot, 2004). Gas injeion into coal seams can also cause physical changes to coal and hence coal permeability changes. Kiyama et al. (2011) found that the core coal permeability decreases after supercritical CO2 injeion, showing that adsorption-induced swelling has a significant impa on coal permeability. Subsequent N2 flooding tests following CO2 injeion showed slow strain recovery, suggesting that N2 displaces the adsorbed CO2 in the coal matrix and the permeability of the coal core also recovered to a certain degree after N2 injeion. All these indicates that N2 gas injeion can be used to enhance the gas drainage of CSG and gas injeion will cause a significant impa on coal behaviour and further influence the gas transport in coal when carrying out gas drainage. This research program is a systematic proje to investigate the comprehensive gas flow charaeristics and hard-to-drain problem in the coal seam, the studies include the coal sorption capacity in terms of the temperature and moisture influences (Zhang et al., 2014b), coal particle size influence (Zhang et al., 2014a), coal sorption theory (Zhang et al., 2014c) and permeability influence (Zhang et al., 2014d) have already been carried out and the relevant results have been published. An experiment was condued to further understand the mechanism of N2 gas flushing to enhance the recovery of CSG, such as CO2 and CH4. The relationships between flushing time and N2 as well as CO2 and CH4 concentration, N2 charging volume and CO2 and CH4 recovery volume and flushing process were analysed in this experimental study. The term of gas composition means gas concentration or gas percentage of the gas mixture in this paper and the strain behaviour of coal is not discussed in this study, as it is a topic which is beyond the scope of this paper. 2. Geological background The experimental study of N2 injeion to flush CSG was carried out on coal samples obtained from a typical hard-to-drain area (MG 22, 8-11 c/t) of Metropolitan Colliery in NSW, Australia. The current operating seam is the Bulli seam, which is stratigraphically the uppermost seam in the Illawarra coal measures, of the Sydney Basin, which belong to Permian and Triassic era (Faiz et al., 2007; Aziz et al., 2013). The Bulli coal is high quality coking coal with volatile matters ranging from 18-23% (air dried), the ash level varies between 8-10% (air dried), and the sulphur content is low at around 0.3%, the mineral content averages around 4% (Aziz et al., 2013). The vitrinite content is moderate at 45%, the inertinite content is about 50% and vitrinite refleance at around 1.3 (Saghafi & Roberts, 2008; Aziz et al., 2013). The permeability of the coal varies between 0.5 to 6.0 mD, determined both in situ and in the laboratory (Lingard et al., 1982; Sereshki, 2005; Black, 2012; Zhang, 2013). In situ gas contents of coal in the Illawarra Coal Measures range from less than 1 to 20 m3/t with the highest contents occurring at depths between 600 and 800 m. The desorbed gas often comprises CH4, CO2, N2, C2H6 and other higher hydrocarbons (Faiz et al., 2007). The two most abundant gases are CO2 and CH4, accounting for greater than 90% of the total gas in most areas of the Sydney Basin. Thermal history modelling indicates that most of the hydrocarbon gases were generated as a result of coalification during the Jurassic and Early Cretaceous (Faiz et al., 2003); additional CH4 was apparently generated from post-Cretaceous microbial aivity (Smith & Pallasser, 1996; Faiz et al., 2003). Faiz et al. (2007) stated that isotope carbon-13 (13C) values for CO2 from coal seams of the Illawarra Coal Measures vary between ­25 and +15 (IAEA international standard defining Vienna Peedee Belemnite, VPDB), indicating various sources. These sources include thermogenic gas from coal/microbial oxidation of hydrocarbons (13C ­25 ±5), magmatic aivity (13C ­7 ±3) and residual CO2 after microbial reduion of CO2 to CH4 (0 to +15). Most of the 13C values ranging between ­5 and ­10, suggesting mainly magmatic sources, which was probably associated with the main episodes of igneous aivity in the Permian, Jurassic and Tertiary (Faiz et al., 2007). The variations of CO2 and CH4 are mainly related to the geological struure and depth. The variations in the gas composition have no clear relationship with coal composition or rank but show well-defined relationships with geological struure and stratigraphy. High proportions of CH4 occur in the synclinal struures, whereas the CO2 content increases towards struural highs. Extensive areas of pure CO2 gas occur on anticlines and domes. In struural lows, high CO2 concentrations are found near some dykes and related faults (Faiz & Hutton, 1995). This feature appears to also exist within the typical hard-to-drain area of this study. Many Australian underground coal mines are mining in areas that require the use of gas drainage to reduce coal seam gas content to below a prescribed Threshold Limit Value (TLV). The TLV represents the maximum allowable gas content, relative to gas composition, considered safe for mine operations (Black, 2012). Mine operators are required to ensure seam gas content has been reduced below the applicable TLV prior to mining. In a number of cases, these mines encounter areas where the gas is hard to drain from the coal, ahead of mining (Black, 2012). Fig. 1 shows the gas content and composition analysis of the coal within the typical hard-todrain area (MG 22, 8-11 c/t) of Metropolitan Colliery with 94 sample test results. The scatter of typical hard-to-drain area is concentrated almost entirely in the CO2 rich area. Among the 94 samples, 63 samples are "Fail" samples, accounting for 67.0%, which direly indicates the area is a typically hard-to-drain area. The average values of CO2 in both "Pass" and "fail" samples are 87.6% and 84.5% respeively. Different faors including low permeability, high CO2 concentration and geological variations have caused the hard-to-drain problems in certain parts of Bulli seam. Results from N2 injeion tests may provide invaluable knowledge for field trials of this innovative technology that could potentially lead to much enhanced gas recovery from hard-to-drain or low permeability seams. Fig. 1. Bulli seam outburst threshold limits (Typical hard-to-drain area) 3. Methods and experimental procedures 3.1. Testing apparatus and coal samples The combined set up of a Multi Funion Outburst Research Rig (MFORR) and Gas Chromatograph (GC) used in this test is shown in Fig. 2. The MFORR has various key components. These include the main apparatus support frame and a precision drill, a high pressure chamber which contains a load cell for measuring the load applied to the samples of coal, a pressure transducer for measuring the pressure inside the chamber, several flow meters set in series for measuring the gas flow rate, two strain gauges for measuring volumetric changes of the coal sample vertically and horizontally, a universal socket for loading a sample of coal vertically into the gas pressure chamber, a data acquisition system and a GC for the analysis of the gases discharged from the chamber. A four column GC is used to test gas for CO2, CH4 and N2, which is discharged from the gas chamber in the experiment. Fig. 2. A combination set up of MFORR and GC (modified from Florentin et al., 2010) The sample for the flushing test was colleed from the prescribed hard-to-drain area of the Bulli seam. The standard core samples were prepared with dimension of 54 mm in diameter and 50 mm in height. A 2 mm diameter hole was drilled in the middle of the cored coal. Prior to testing, two strain gauges were glued horizontally and vertically to the sample and both ends of the prepared specimen were sealed with a rubber layer. Fig. 3 shows a snapshot of the sample. Fig. 3. Coal samples for N2 flushing test 3.2. Experimental procedures 3.2.1. Stage 1 ­ Coal sorption process In stage 1, the gas chamber was sealed with the prepared coal sample inside, before the gas sorption process, the system was vacuumed to ­100 kPa (relative pressure) to remove the air inside the chamber and degas the coal samples. The whole system was maintained in a nonleakage condition operated properly by valves through the entire test. During all the three stages of the experiments, the laboratory temperature was kept at 25°C. The coal sample was then loaded axially to 3 MPa (equal to the axial load of 730 kg) initially and then the chamber was injeed with CO2 or CH4 to 3 MPa. CSG gas was injeed to allow the gas to diffuse and adsorbed in coal, until the coal reached gas sorption equilibrium at around 2 MPa pressure. As the MFORR apparatus could not test the sorption capacity of coal, the sorption capacities with CO2 and CH4 were estimated through independent coal isotherm testing. The gravimetric method with only a sample cell, also referred to as the indire gravimetric method, was first reported by Lama and Bartosiewicz (1982), and later by Aziz and Li (1999) and Sereshki (2005). Aually, coal sorption isotherm apparatus in the University of Wollongong is the combination of the gravimetric and volumetric methods, it utilises the gravimetric principle to calculate the total gas amount in the bomb and the volumetric principle to calculate the gas amount in the void space. 3.2.2. Stage 2 ­ N2 injeion to flush CO2 and CH4 process Prior to the commencement of the N2 injeion test, the GC was calibrated to allow accurate measuring of the gas composition of CO2, CH4 and N2 from the low to high range. N2 gas flushing was carried out separately after the coal sample was saturated with CO2 or CH4 at the prescribed 2 MPa. The gas inside the chamber was tested by the GC to make sure that gas composition of either CO2 or CH4 was pure (99.9%), and the whole system was not contaminated by air. At 2 MPa pressure, N2 gas was then introduced to the gas chamber, charged through the central hole of the coal sample to allow N2 gas to penetrate and permeate the coal sample along the radius and flow into the chamber. The direions of N2 gas injeion and flushing through the coal is indicated by blue arrow as shown in the sample in Fig. 3. The released gas was systematically discharged from the side hole of the chamber at 6 min intervals, going through a measuring system and a line of gas flowmeters (0-2 L/min and 0-15 L/min measurement range). The gas was colleed in a 1 L capacity sample bag, which was direly conneed to the GC to test gas composition. 3.2.3. Stage 3 ­ Desorption process after N2 injeion In stage 3, desorption test was carried out, following the N2 injeion test, when the CO2 or CH4 gas composition was around 3%. The N2 injeion valve was closed. Gas pressure inside the chamber began to gradually drop as the remaining gas volume in the chamber was gradually removed. The released gas was colleed in a 1 L storage capacity sample bag and analysed in the GC to test gas composition. The desorption process was suspended when the chamber pressure dropped to atmosphere pressure level. It should be noted that CO2 and CH4 flushing tests were carried out separately, but the experimental procedures were kept the same for comparison purposes. 4. Results and discussions 4.1. Stage 1 ­ Coal sorption process Stage 1 is basically a coal sorption process, and prior to the N2 flushing test. The coal samples were initially saturated with CO2 and CH4 at 2 MPa. This sorption tests were carried out uniquely by an indire gravimetric method of determining the gas content of gas in coal. Four hard-to-drain coal samples were tested for the sorption capacity. Fig. 4 shows the comparative results of the adsorption isotherms for both CO2 and CH4 gas. Fig. 4. Coal sorption isotherms of hard-to-drain coal samples The Langmuir equation shown in Equation 1 was used to model the gas adsorption testing results. Langmuir parameters were calculated for each isotherm and shown in Table 1. na VL P P PL (1) where na is adsorbed gas content (gas volume per unit mass of coal), P is gas pressure, and VL and PL are experimental coefficients. The coefficient VL represents the maximum gas storage capacity of the coal and is termed the `Langmuir volume'. The coefficient PL is the `Langmuir pressure' and represents the gas pressure at which coal adsorbs a volume of gas equal to half of its maximum capacity (Harpalani et al., 2006). TABLE 1 Langmuir parameters for the tested samples in terms of CO2 and CH4 (hard-to-drain area) Langmuir parameters Drainage area GME 2126 Hard-to-drain GME 2127 Hard-to-drain GME 2128 Hard-to-drain GME 2130 Hard-to-drain Langmuir volume for CO2 (cc/g) Langmuir pressure for CO2 (kPa) Langmuir volume for CH4 (cc/g) Langmuir pressure for CH4 (kPa) 4.2. Stage 2 ­ N2 injeion to flush CO2 and CH4 process At 2 MPa pressure, N2 gas was injeed through the central hole of the coal sample to allow N2 gas to penetrate and permeate the coal sample along the radius and flow into the chamber. The gas composition change inside the chamber was continuously monitored and the chamber pressure was maintained constant at 2 MPa during the whole N2 injeion process. As shown in Fig. 5, during the N2 flushing process, the CO2 and CH4 binary gas composition in the chamber gradually decreased and N2 percentage increased, which indicates that CSG continues to be flushed out by N2. The whole flushing test takes more than 13 h (800 min) for CO2 shown in Fig. 5 and 8 h (500 min) for CH4, shown in Fig. 5 . At the lower CSG concentration stage, it appears that the flushing process is becoming harder as coal continues to desorb relatively higher CO2 and CH4 gas and the injeed N2 gas assists this adsorbed gas to desorb into the chamber. This phenomenon is especially apparent for the CO2 flushing test, as coal still sorbed more CO2 at low gas pressure, compared with CH4. This finding generally agreed with the study of Florentin et al., (2010), who carried out similar test and found CSG can be flushed out with N2 injeion in the experimental test. Fig. 5. Gas composition during N2 injeion As each step of the test, gas was discharged through the sample bag of 1L capacity, hence the volume of discharged gas in the chamber can be calculated Fig. 6 shows the volume of the various gases being discharged out of the pressure chamber over the whole test period. With the volume of N2 gas injeed into the chamber increasing, the total volume of CO2 and CH4 flushed out of system was accumulating. In the end, the total gases consumed during the flushing stage was estimated to be 100.9 L of N2, liberating 33.1 L of CO2 out of the system (Fig. 6 ). While, it was estimated that 61.0 L of N2 were consumed in the flushing test, liberating 22.0 L of CH4 (Fig. 6 ). Test results indicate that a greater volume of N2 gas is needed to flush CO2 than CH4 gas out of coal, especially during the later stage of flushing. The total gas volume here includes both free gas in the chamber and adsorbed gas by the coal. Fig. 6. Gas volume during N2 injeion Fig. 7 shows a comparison of colleed gas volume in the flushing stage for CO2 and CH4. It can be observed that more N2 is consumed than the recovered CO2 or CH4. The ratio of colleed volume of N2:CO2 is around 3.05 and the ratio of colleed volume of N2:CH4 is around 2.77. It indicates that more N2 is needed to flush the same amount of CO2 than CH4. Fig. 7. Comparison of colleed gas volume in Stage 2 According to the tested coal sorption isotherm of this typical hard-to-drain coal in stage 1, the average values of Langmuir parameters are followed, VL = 32.2 cc/g, PL = 798.5 kPa for CO2 and VL = 18.9 cc/g, PL = 1064.55 kPa for CH4. Thus, by combining all the parameters and using the Langmuir equation, when coal is saturated at 2 MPa, the adsorbed gas content is 23.01 cc/g for CO2 and 12.33 cc/g for CH4. It should be noted that this calculation is based on the assumption that the coal sample was fully saturated. For the flushed 160 g of coal sample, the adsorbed volume of CO2 was 3.68 L and 1.97 L for CH4. It is believed that all the adsorbed gas is flushed out during Stage 2 and Stage 3. As the gas composition of CO2 or CH4 was very low at the end of the flushing stage, all the gas coming out in the next desorption stage (Stage 3) is assumed to be adsorbed gas, which is 2.3 L for CO2 and 1.1 L for CH4. Hence, the total adsorbed gas volume flushed in Stage 2 is 1.38 L for CO2 and 0.87 L for CH4. Based on the experimental data the following equation is adopted to calculate the gas content in coal during the flushing stage: vt v0 i 1 v c (2) where: vt is the gas content in coal during the flushing stage; v0 is the gas content in coal at the time 0 (starting point of flushing stage); and ­1 are the gas composition in the chamber at the time t and t ­1 during the flushing stage; v is the total gas content drop in coal in the flushing stage; c is the total gas composition drop in the chamber in the flushing stage, all the gas referred here is CO2 or CH4. This above proposed calculation model is based on that the value of gas content in coal changes simultaneously with the change of gas composition or gas partial pressure, and the changing relationship between them is linear. Fig. 8 shows the gas content change in coal during the flushing stage based on the above calculation, in total 1.38 L adsorbed CO2 and 0.87 L of adsorbed CH4 are flushed out of coal, helping reduce coal gas content of CO2 from 23.01 cc/g to 14.385 cc/g and from 12.33 cc/g to 6.89 cc/g for CH4. The reduion of 8.625 cc/g CO2 gas content accounts for 37.5% of the total adsorbed CO2 gas content while the reduion of 5.44 cc/g accounts for 44.1% of the total adsorbed CH4 gas content, which indicates N2 flushing plays a more effeive role in reducing adsorbed CH4 than CO2. Hence, it is obvious that longer flushing time is needed to flush out CO2 than CH4 at the same equilibrium pressure (2 MPa) level. 4.3. Stage 3 ­ Desorption test after N2 injeion In stage 3, a desorption test was carried out following the N2 injeion test when the CO2 or CH4 gas composition was around 3%. The N2 injeion valve was closed. Gas pressure inside the chamber began to drop as the remaining gas volume in the chamber was gradually removed. Fig. 9 shows the pressure drop (relative pressure) linearly in the desorption process, Fig. 9 for the CO2 test and Fig. 9 for the CH4 test. Fig. 8. Comparison of gas content in coal in Stage 2 Fig. 9. Gas pressure drop during desorption Fig. 10 shows the change of gas composition in the desorption process, the gas composition of CO2 or CH4 increases and at the same time the N2 gas composition decreases. Specifically, in the CO2 test, the CO2 percentage starts to increase from 3.4% to 9.4% over a period of around 3 h (200 min) (Fig. 10 ), while in the CH4 test, the CH4 percentage starts to increase from 2.8% to 6.0% over a period of around 2 h (110 min) (Fig. 10 ). More CO2 or CH4 gas desorbs from the coal than N2 in this process indicating greater sorption capacity of CO2 or CH4 than N2. Further measured data after overnight desorption pointed out in Fig. 10 also confirm this conclusion, with CO2 reaching 37.2% and CH4 reaching 12.2%, N2 decreasing to 62.8% and 87.8%, respeively. It should be noted that the pressure in the chamber was reduced to normal atmospheric level (101.320 kPa, absolute pressure). Fig. 10. Gas composition during desorption Fig. 11 shows the colleed gas volume for each gas in the desorption process, as time proceeded, the total amount of gas volume for each gas increased. As there is a high composition of CSG (CO2 or CH4) in the chamber after the flushing test, much more N2 is colleed than CO2 or CH4. At the end of the CO2 flushing test a total of 37.7 L of N2 and 2.3 L of CO2 were colleed, while a total 20.9 L of N2 and 1.1 L of CH4 were colleed in the CH4 flushing test. Fig. 11. Gas volume during coal desorption Fig. 12 shows the comparison of colleed gas volume in the desorption stage for CO2 and CH4. It was found that more N2 volume is colleed than CO2 or CH4 was recovered. The ratio of colleed volume of N2:CO2 is around 16.40 and the ratio of colleed volume of N2:CH4 is around 19.0, which is relatively larger than the CO2 flushing test. Fig. 12. Comparison of colleed gas volume in Stage 3 All the adsorbed gas is flushed out during the Stage 2 and Stage 3 and as the gas composition of CO2 or CH4 is very low at the end of the flushing stage, all the gas coming out in the stage 3 is assumed to be adsorbed gas, which is 2.30 L for CO2 and 1.10 L for CH4. Based on the experimental data the following equation is adopted to calculate the gas content during the desorption stage: vt v0 i 1 v c (3) where: vt is the gas content in coal during the desorption stage; v0 is the gas content in coal at the time 0 (starting point of desorption stage); and ­1 are the gas composition in the chamber at the time t and t ­1 during the desorption stage; v is the total gas content drop in coal in the desorption stage; c is the total gas composition increase in the chamber in the desorption stage, all the gas referred here is CO2 or CH4. This calculation model is also proposed based on the principles claimed in the Stage 2. Packham et al. (2012) reported the continued injeion of nitrogen would create conditions where the methane content of the coal could be reduced to negligible levels. Fig. 13 shows the gas content change in coal during the desorption stage. A total of 2.30 L of adsorbed CO2 and 1.10 L of adsorbed CH4 are desorbed from coal, to help reduce the remaining coal gas content, which is 14.385 cc/g for CO2 and 6.89 cc/g for CH4.The reduion accounts for 62.5% of the total adsorbed CO2 gas content and 55.8% of the total adsorbed CH4 gas content, respeively. It indicates gas desorption with gas pressure drop after N2 flushing plays a more effeive role in reducing adsorbed CO2 than CH4. Fig. 13. Comparison of gas content in coal in Stage 3 5. Conclusions Laboratory N2 injeion tests show that CSG (CO2 and CH4) can be flushed out by N2 injeion. During the N2 flushing process, the CO2 and CH4 percentage of the chamber gas gradually decreases and the N2 percentage increases, and with the N2 flushing test approaching, the colleed total gas volume of both CSG and N2 increases. It is found that at low CO2 or CH4 composition stage, it is hard to use N2 to achieve effeive flushing. After the flushing test, a certain amount of CO2 or CH4 is still adsorbed inside the coal. In the desorption process, the CO2 or CH4 percentage change starts to increase, indicating more CO2 and CH4 gas desorbs from the coal than N2. In the N2 injeion stage, the ratio of N2:CO2 colleed volume is around 3.05 and the ratio is around 2.77 for N2:CH4. In the gas desorption stage, the ratio of N2:CO2 colleed volume is around 16.40 and the ratio is around 19.0 for N2:CH4. During the flushing stage, N2 injeion helps to reduce the adsorbed gas content. The reduion of 8.625 cc/g CO2 gas content accounts for 37.5% of the total adsorbed CO2 gas content while the reduion of 5.44 cc/g accounts for 44.1% of the total adsorbed CH4 gas content, which indicates N2 flushing plays a more effeive role in reducing adsorbed CH4 than CO2. Comparatively, during the desorption stage, a total of 2.30 L of adsorbed CO2 and 1.10 L of adsorbed CH4 are desorbed from coal. The reduion accounts for 62.5% of the total adsorbed CO2 gas content and 55.8% of the total adsorbed CH4 gas content, respeively. It indicates gas desorption after N2 flushing plays a more effeive role in reducing adsorbed CO2 than CH4. The result clearly shows that N2 gas flushing has a significant effe on the CO2 and CH4 desorption and removal from coal. Thus it is important to develop a nitrogen injeion technique in field trials, to enhance gas recovery in tight (hard-to-drain) and low permeable seams in future. Acknowledgment This research is supported by the Fundamental Research Funds for the Central Universities (2015QNA42), the Scholarship from University of Wollongong and China Scholarship Council, the National Natural Science Foundation of China (Grant No. 51374200), and The Priority Academic Programme Development of Higher Education Institutions in Jiangsu Province (Grant No. SZBF2011-6-B35). The authors wish to thank the staff and management of BHP Billiton-Illawarra Coal for providing coal samples used in this study. Thanks are also due to the technical staff at the University of Wollongong especially Col Devenish for experiment assistance. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Archives of Mining Sciences de Gruyter

Nitrogen Injection To Flush Coal Seam Gas Out Of Coal: An Experimental Study

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Abstract

Arch. Min. Sci., Vol. 60 (2015), No 4, p. 1013­1028 Eleronic version (in color) of this paper is available: http://mining.archives.pl DOI 10.1515/amsc-2015-0067 LEI ZHANG*1, NAJ AZIZ**, TING REN**, JAN NEMCIK**, SHIHAO TU*1 WPROWADZANIE AZOTU DO ZLÓ WGLA W CELU WYPLUKIWANIA GAZÓW Z POKLADU ­ BADANIA EKSPERYMENTALNE Several mines operating in the Bulli seam of the Sydney Basin in NSW, Australia are experiencing difficulties in reducing gas content within the available drainage lead time in various seions of the coal deposit. Increased density of drainage boreholes has proven to be ineffeive, particularly in seions of the coal seam rich in CO2. Plus with the increasing worldwide concern on green house gas reduion and clean energy utilisation, significant attention is paid to develop a more praical and economical method of enhancing the gas recovery from coal seams. A technology based on N2 injeion was proposed to flush the Coal Seam Gas (CSG) out of coal and enhance the gas drainage process. In this study, laboratory tests on CO2 and CH4 gas recovery from coal by N2 injeion are described and results show that N2 flushing has a significant impa on the CO2 and CH4 desorption and removal from coal. During the flushing stage, it was found that N2 flushing plays a more effeive role in reducing adsorbed CH4 than CO2. Comparatively, during the desorption stage, the study shows gas desorption after N2 flushing plays a more effeive role in reducing adsorbed CO2 than CH4. Keywords: N2 injeion, coal seam gas, coal, gas composition, gas volume W kilku kopalniach eksploatujcych zloe Bulli w zaglbiu wglowym Sydney w Nowej Poludniowej Walii w Australii pojawil si problem redukcji zawartoci gazu kopalnianego w zloach zawartego w rónych czciach zloa, w okrelonym czasie. Zwikszenie gstoci wykonywania odwiertów drenaowych okazalo si by metod nieskuteczn, zwlaszcza w czciach zloa bogatego w CO2. Inne kwestie to wzrastajca w wiecie wiadomo koniecznoci redukcji gazów cieplarnianych i wykorzystania czystej energii, std te podejmowane wysilki na rzecz opracowania praktycznych i ekonomicznych metod odzyskiwania gazu ze zló wgla. W pracy przedstawiono technologi opart na wprowadzaniu azotu do zloa w celu wyplukania gazu zawartego w wglu, poprawiajc skuteczno ich odzyskiwania. W prowadzonych pracach badano skuteczno odzysku CO2 i metanu ze zloa wgla po wprowadzeniu * ** SCHOOL OF MINES, CHINA UNIVERSITY OF MINING AND TECHNOLOGY, XUZHOU CITY, JIANGSU 221116, CHINA. KEY LABORATORY OF DEEP COAL RESOURCE MINING, MINISTRY OF EDUCATION OF CHINA, XUZHOU 221116, CHINA. SCHOOL OF CIVIL, MINING & ENVIRONMENTAL ENGINEERING, FACULTY OF ENGINEERING AND INFORMATION SCIENCES, UNIVERSITY OF WOLLONGONG, NSW 2522, AUSTRALIA. CORRESPONDING AUTHORS, E-mail: lz811@uowmail.edu.au; papertsh@126.com do niego azotu. Wyniki bada wskazuj, e wyplukiwanie azotem w powanym stopniu wplywa na proces desorpcji CO2 i CH4 i ich usuwania z wgla. Na etapie wprowadzania azotu, stwierdzono e wyplukiwanie azotem w wikszym stopniu wspomaga usuwanie adsorbowanego CH4 ni CO2. Dla porównania, w trakcie desorpcji, wykazano, e desorpcja gazów po wprowadzeniu do zloa azotu znacznie skuteczniej redukuje ilo adsorbowanego CO2 ni CH4. Slowa kluczowe: wprowadzanie azotu do zloa, gaz zawarty w wglu, wgiel, sklad gazu, objto gazu 1. Introduion There is growing interest in gas injeion to enhance Coal Seam Gas (CSG) recovery. The utilisation of N2 injeion has been found to help CSG recovery (Reeves & Oudinot, 2004, 2005; Florentin et al., 2010; Kiyama et al., 2011; Packham et al., 2012; Zhang, 2013). In the reservoir and economic analysis study of Tiffany unit N2 ­ ECBM pilot (Reeves & Oudinot, 2005), it was found that incremental methane recovery of approximately 10-20% of the original gas in place was achieved with N2 injeion. The future N2 injeion was forecast to add another 25-40% to the total recovery of original gas in place. The future N2 injeion at Tiffany was also forecast to be economic. Packham et al. (2012) reported the results from a field trial condued with Surface to In-Seam (SIS) pre-drainage wells and concluded that the enhanced drainage could provide the means for both accelerating methane drainage and reducing residual gas content. Packham et al. (2011) provided the background to this field trial including details of the reservoir charaeristics, well geometry and installations. They also described how history matching of the reservoir and simulation of the effes of nitrogen injeion indicated that accelerated drainage was likely. In concept, the principle of N2 injeion to Enhance Coalbed Methane recovery (N2-ECBM) can be described as follows: N2 is injeed into a coal reservoir, it displaces the gaseous CSG from the cleat system, decreasing the CSG partial pressure and creating a compositional disequilibrium between the gaseous and adsorbed phases. These combined influences cause the CO2 or CH4 to desorb and diffuse into the cleat system, becoming the "stripped gas" from the matrix. The CSG then migrates to and is produced from produion wells (Reeves & Oudinot, 2004). Gas injeion into coal seams can also cause physical changes to coal and hence coal permeability changes. Kiyama et al. (2011) found that the core coal permeability decreases after supercritical CO2 injeion, showing that adsorption-induced swelling has a significant impa on coal permeability. Subsequent N2 flooding tests following CO2 injeion showed slow strain recovery, suggesting that N2 displaces the adsorbed CO2 in the coal matrix and the permeability of the coal core also recovered to a certain degree after N2 injeion. All these indicates that N2 gas injeion can be used to enhance the gas drainage of CSG and gas injeion will cause a significant impa on coal behaviour and further influence the gas transport in coal when carrying out gas drainage. This research program is a systematic proje to investigate the comprehensive gas flow charaeristics and hard-to-drain problem in the coal seam, the studies include the coal sorption capacity in terms of the temperature and moisture influences (Zhang et al., 2014b), coal particle size influence (Zhang et al., 2014a), coal sorption theory (Zhang et al., 2014c) and permeability influence (Zhang et al., 2014d) have already been carried out and the relevant results have been published. An experiment was condued to further understand the mechanism of N2 gas flushing to enhance the recovery of CSG, such as CO2 and CH4. The relationships between flushing time and N2 as well as CO2 and CH4 concentration, N2 charging volume and CO2 and CH4 recovery volume and flushing process were analysed in this experimental study. The term of gas composition means gas concentration or gas percentage of the gas mixture in this paper and the strain behaviour of coal is not discussed in this study, as it is a topic which is beyond the scope of this paper. 2. Geological background The experimental study of N2 injeion to flush CSG was carried out on coal samples obtained from a typical hard-to-drain area (MG 22, 8-11 c/t) of Metropolitan Colliery in NSW, Australia. The current operating seam is the Bulli seam, which is stratigraphically the uppermost seam in the Illawarra coal measures, of the Sydney Basin, which belong to Permian and Triassic era (Faiz et al., 2007; Aziz et al., 2013). The Bulli coal is high quality coking coal with volatile matters ranging from 18-23% (air dried), the ash level varies between 8-10% (air dried), and the sulphur content is low at around 0.3%, the mineral content averages around 4% (Aziz et al., 2013). The vitrinite content is moderate at 45%, the inertinite content is about 50% and vitrinite refleance at around 1.3 (Saghafi & Roberts, 2008; Aziz et al., 2013). The permeability of the coal varies between 0.5 to 6.0 mD, determined both in situ and in the laboratory (Lingard et al., 1982; Sereshki, 2005; Black, 2012; Zhang, 2013). In situ gas contents of coal in the Illawarra Coal Measures range from less than 1 to 20 m3/t with the highest contents occurring at depths between 600 and 800 m. The desorbed gas often comprises CH4, CO2, N2, C2H6 and other higher hydrocarbons (Faiz et al., 2007). The two most abundant gases are CO2 and CH4, accounting for greater than 90% of the total gas in most areas of the Sydney Basin. Thermal history modelling indicates that most of the hydrocarbon gases were generated as a result of coalification during the Jurassic and Early Cretaceous (Faiz et al., 2003); additional CH4 was apparently generated from post-Cretaceous microbial aivity (Smith & Pallasser, 1996; Faiz et al., 2003). Faiz et al. (2007) stated that isotope carbon-13 (13C) values for CO2 from coal seams of the Illawarra Coal Measures vary between ­25 and +15 (IAEA international standard defining Vienna Peedee Belemnite, VPDB), indicating various sources. These sources include thermogenic gas from coal/microbial oxidation of hydrocarbons (13C ­25 ±5), magmatic aivity (13C ­7 ±3) and residual CO2 after microbial reduion of CO2 to CH4 (0 to +15). Most of the 13C values ranging between ­5 and ­10, suggesting mainly magmatic sources, which was probably associated with the main episodes of igneous aivity in the Permian, Jurassic and Tertiary (Faiz et al., 2007). The variations of CO2 and CH4 are mainly related to the geological struure and depth. The variations in the gas composition have no clear relationship with coal composition or rank but show well-defined relationships with geological struure and stratigraphy. High proportions of CH4 occur in the synclinal struures, whereas the CO2 content increases towards struural highs. Extensive areas of pure CO2 gas occur on anticlines and domes. In struural lows, high CO2 concentrations are found near some dykes and related faults (Faiz & Hutton, 1995). This feature appears to also exist within the typical hard-to-drain area of this study. Many Australian underground coal mines are mining in areas that require the use of gas drainage to reduce coal seam gas content to below a prescribed Threshold Limit Value (TLV). The TLV represents the maximum allowable gas content, relative to gas composition, considered safe for mine operations (Black, 2012). Mine operators are required to ensure seam gas content has been reduced below the applicable TLV prior to mining. In a number of cases, these mines encounter areas where the gas is hard to drain from the coal, ahead of mining (Black, 2012). Fig. 1 shows the gas content and composition analysis of the coal within the typical hard-todrain area (MG 22, 8-11 c/t) of Metropolitan Colliery with 94 sample test results. The scatter of typical hard-to-drain area is concentrated almost entirely in the CO2 rich area. Among the 94 samples, 63 samples are "Fail" samples, accounting for 67.0%, which direly indicates the area is a typically hard-to-drain area. The average values of CO2 in both "Pass" and "fail" samples are 87.6% and 84.5% respeively. Different faors including low permeability, high CO2 concentration and geological variations have caused the hard-to-drain problems in certain parts of Bulli seam. Results from N2 injeion tests may provide invaluable knowledge for field trials of this innovative technology that could potentially lead to much enhanced gas recovery from hard-to-drain or low permeability seams. Fig. 1. Bulli seam outburst threshold limits (Typical hard-to-drain area) 3. Methods and experimental procedures 3.1. Testing apparatus and coal samples The combined set up of a Multi Funion Outburst Research Rig (MFORR) and Gas Chromatograph (GC) used in this test is shown in Fig. 2. The MFORR has various key components. These include the main apparatus support frame and a precision drill, a high pressure chamber which contains a load cell for measuring the load applied to the samples of coal, a pressure transducer for measuring the pressure inside the chamber, several flow meters set in series for measuring the gas flow rate, two strain gauges for measuring volumetric changes of the coal sample vertically and horizontally, a universal socket for loading a sample of coal vertically into the gas pressure chamber, a data acquisition system and a GC for the analysis of the gases discharged from the chamber. A four column GC is used to test gas for CO2, CH4 and N2, which is discharged from the gas chamber in the experiment. Fig. 2. A combination set up of MFORR and GC (modified from Florentin et al., 2010) The sample for the flushing test was colleed from the prescribed hard-to-drain area of the Bulli seam. The standard core samples were prepared with dimension of 54 mm in diameter and 50 mm in height. A 2 mm diameter hole was drilled in the middle of the cored coal. Prior to testing, two strain gauges were glued horizontally and vertically to the sample and both ends of the prepared specimen were sealed with a rubber layer. Fig. 3 shows a snapshot of the sample. Fig. 3. Coal samples for N2 flushing test 3.2. Experimental procedures 3.2.1. Stage 1 ­ Coal sorption process In stage 1, the gas chamber was sealed with the prepared coal sample inside, before the gas sorption process, the system was vacuumed to ­100 kPa (relative pressure) to remove the air inside the chamber and degas the coal samples. The whole system was maintained in a nonleakage condition operated properly by valves through the entire test. During all the three stages of the experiments, the laboratory temperature was kept at 25°C. The coal sample was then loaded axially to 3 MPa (equal to the axial load of 730 kg) initially and then the chamber was injeed with CO2 or CH4 to 3 MPa. CSG gas was injeed to allow the gas to diffuse and adsorbed in coal, until the coal reached gas sorption equilibrium at around 2 MPa pressure. As the MFORR apparatus could not test the sorption capacity of coal, the sorption capacities with CO2 and CH4 were estimated through independent coal isotherm testing. The gravimetric method with only a sample cell, also referred to as the indire gravimetric method, was first reported by Lama and Bartosiewicz (1982), and later by Aziz and Li (1999) and Sereshki (2005). Aually, coal sorption isotherm apparatus in the University of Wollongong is the combination of the gravimetric and volumetric methods, it utilises the gravimetric principle to calculate the total gas amount in the bomb and the volumetric principle to calculate the gas amount in the void space. 3.2.2. Stage 2 ­ N2 injeion to flush CO2 and CH4 process Prior to the commencement of the N2 injeion test, the GC was calibrated to allow accurate measuring of the gas composition of CO2, CH4 and N2 from the low to high range. N2 gas flushing was carried out separately after the coal sample was saturated with CO2 or CH4 at the prescribed 2 MPa. The gas inside the chamber was tested by the GC to make sure that gas composition of either CO2 or CH4 was pure (99.9%), and the whole system was not contaminated by air. At 2 MPa pressure, N2 gas was then introduced to the gas chamber, charged through the central hole of the coal sample to allow N2 gas to penetrate and permeate the coal sample along the radius and flow into the chamber. The direions of N2 gas injeion and flushing through the coal is indicated by blue arrow as shown in the sample in Fig. 3. The released gas was systematically discharged from the side hole of the chamber at 6 min intervals, going through a measuring system and a line of gas flowmeters (0-2 L/min and 0-15 L/min measurement range). The gas was colleed in a 1 L capacity sample bag, which was direly conneed to the GC to test gas composition. 3.2.3. Stage 3 ­ Desorption process after N2 injeion In stage 3, desorption test was carried out, following the N2 injeion test, when the CO2 or CH4 gas composition was around 3%. The N2 injeion valve was closed. Gas pressure inside the chamber began to gradually drop as the remaining gas volume in the chamber was gradually removed. The released gas was colleed in a 1 L storage capacity sample bag and analysed in the GC to test gas composition. The desorption process was suspended when the chamber pressure dropped to atmosphere pressure level. It should be noted that CO2 and CH4 flushing tests were carried out separately, but the experimental procedures were kept the same for comparison purposes. 4. Results and discussions 4.1. Stage 1 ­ Coal sorption process Stage 1 is basically a coal sorption process, and prior to the N2 flushing test. The coal samples were initially saturated with CO2 and CH4 at 2 MPa. This sorption tests were carried out uniquely by an indire gravimetric method of determining the gas content of gas in coal. Four hard-to-drain coal samples were tested for the sorption capacity. Fig. 4 shows the comparative results of the adsorption isotherms for both CO2 and CH4 gas. Fig. 4. Coal sorption isotherms of hard-to-drain coal samples The Langmuir equation shown in Equation 1 was used to model the gas adsorption testing results. Langmuir parameters were calculated for each isotherm and shown in Table 1. na VL P P PL (1) where na is adsorbed gas content (gas volume per unit mass of coal), P is gas pressure, and VL and PL are experimental coefficients. The coefficient VL represents the maximum gas storage capacity of the coal and is termed the `Langmuir volume'. The coefficient PL is the `Langmuir pressure' and represents the gas pressure at which coal adsorbs a volume of gas equal to half of its maximum capacity (Harpalani et al., 2006). TABLE 1 Langmuir parameters for the tested samples in terms of CO2 and CH4 (hard-to-drain area) Langmuir parameters Drainage area GME 2126 Hard-to-drain GME 2127 Hard-to-drain GME 2128 Hard-to-drain GME 2130 Hard-to-drain Langmuir volume for CO2 (cc/g) Langmuir pressure for CO2 (kPa) Langmuir volume for CH4 (cc/g) Langmuir pressure for CH4 (kPa) 4.2. Stage 2 ­ N2 injeion to flush CO2 and CH4 process At 2 MPa pressure, N2 gas was injeed through the central hole of the coal sample to allow N2 gas to penetrate and permeate the coal sample along the radius and flow into the chamber. The gas composition change inside the chamber was continuously monitored and the chamber pressure was maintained constant at 2 MPa during the whole N2 injeion process. As shown in Fig. 5, during the N2 flushing process, the CO2 and CH4 binary gas composition in the chamber gradually decreased and N2 percentage increased, which indicates that CSG continues to be flushed out by N2. The whole flushing test takes more than 13 h (800 min) for CO2 shown in Fig. 5 and 8 h (500 min) for CH4, shown in Fig. 5 . At the lower CSG concentration stage, it appears that the flushing process is becoming harder as coal continues to desorb relatively higher CO2 and CH4 gas and the injeed N2 gas assists this adsorbed gas to desorb into the chamber. This phenomenon is especially apparent for the CO2 flushing test, as coal still sorbed more CO2 at low gas pressure, compared with CH4. This finding generally agreed with the study of Florentin et al., (2010), who carried out similar test and found CSG can be flushed out with N2 injeion in the experimental test. Fig. 5. Gas composition during N2 injeion As each step of the test, gas was discharged through the sample bag of 1L capacity, hence the volume of discharged gas in the chamber can be calculated Fig. 6 shows the volume of the various gases being discharged out of the pressure chamber over the whole test period. With the volume of N2 gas injeed into the chamber increasing, the total volume of CO2 and CH4 flushed out of system was accumulating. In the end, the total gases consumed during the flushing stage was estimated to be 100.9 L of N2, liberating 33.1 L of CO2 out of the system (Fig. 6 ). While, it was estimated that 61.0 L of N2 were consumed in the flushing test, liberating 22.0 L of CH4 (Fig. 6 ). Test results indicate that a greater volume of N2 gas is needed to flush CO2 than CH4 gas out of coal, especially during the later stage of flushing. The total gas volume here includes both free gas in the chamber and adsorbed gas by the coal. Fig. 6. Gas volume during N2 injeion Fig. 7 shows a comparison of colleed gas volume in the flushing stage for CO2 and CH4. It can be observed that more N2 is consumed than the recovered CO2 or CH4. The ratio of colleed volume of N2:CO2 is around 3.05 and the ratio of colleed volume of N2:CH4 is around 2.77. It indicates that more N2 is needed to flush the same amount of CO2 than CH4. Fig. 7. Comparison of colleed gas volume in Stage 2 According to the tested coal sorption isotherm of this typical hard-to-drain coal in stage 1, the average values of Langmuir parameters are followed, VL = 32.2 cc/g, PL = 798.5 kPa for CO2 and VL = 18.9 cc/g, PL = 1064.55 kPa for CH4. Thus, by combining all the parameters and using the Langmuir equation, when coal is saturated at 2 MPa, the adsorbed gas content is 23.01 cc/g for CO2 and 12.33 cc/g for CH4. It should be noted that this calculation is based on the assumption that the coal sample was fully saturated. For the flushed 160 g of coal sample, the adsorbed volume of CO2 was 3.68 L and 1.97 L for CH4. It is believed that all the adsorbed gas is flushed out during Stage 2 and Stage 3. As the gas composition of CO2 or CH4 was very low at the end of the flushing stage, all the gas coming out in the next desorption stage (Stage 3) is assumed to be adsorbed gas, which is 2.3 L for CO2 and 1.1 L for CH4. Hence, the total adsorbed gas volume flushed in Stage 2 is 1.38 L for CO2 and 0.87 L for CH4. Based on the experimental data the following equation is adopted to calculate the gas content in coal during the flushing stage: vt v0 i 1 v c (2) where: vt is the gas content in coal during the flushing stage; v0 is the gas content in coal at the time 0 (starting point of flushing stage); and ­1 are the gas composition in the chamber at the time t and t ­1 during the flushing stage; v is the total gas content drop in coal in the flushing stage; c is the total gas composition drop in the chamber in the flushing stage, all the gas referred here is CO2 or CH4. This above proposed calculation model is based on that the value of gas content in coal changes simultaneously with the change of gas composition or gas partial pressure, and the changing relationship between them is linear. Fig. 8 shows the gas content change in coal during the flushing stage based on the above calculation, in total 1.38 L adsorbed CO2 and 0.87 L of adsorbed CH4 are flushed out of coal, helping reduce coal gas content of CO2 from 23.01 cc/g to 14.385 cc/g and from 12.33 cc/g to 6.89 cc/g for CH4. The reduion of 8.625 cc/g CO2 gas content accounts for 37.5% of the total adsorbed CO2 gas content while the reduion of 5.44 cc/g accounts for 44.1% of the total adsorbed CH4 gas content, which indicates N2 flushing plays a more effeive role in reducing adsorbed CH4 than CO2. Hence, it is obvious that longer flushing time is needed to flush out CO2 than CH4 at the same equilibrium pressure (2 MPa) level. 4.3. Stage 3 ­ Desorption test after N2 injeion In stage 3, a desorption test was carried out following the N2 injeion test when the CO2 or CH4 gas composition was around 3%. The N2 injeion valve was closed. Gas pressure inside the chamber began to drop as the remaining gas volume in the chamber was gradually removed. Fig. 9 shows the pressure drop (relative pressure) linearly in the desorption process, Fig. 9 for the CO2 test and Fig. 9 for the CH4 test. Fig. 8. Comparison of gas content in coal in Stage 2 Fig. 9. Gas pressure drop during desorption Fig. 10 shows the change of gas composition in the desorption process, the gas composition of CO2 or CH4 increases and at the same time the N2 gas composition decreases. Specifically, in the CO2 test, the CO2 percentage starts to increase from 3.4% to 9.4% over a period of around 3 h (200 min) (Fig. 10 ), while in the CH4 test, the CH4 percentage starts to increase from 2.8% to 6.0% over a period of around 2 h (110 min) (Fig. 10 ). More CO2 or CH4 gas desorbs from the coal than N2 in this process indicating greater sorption capacity of CO2 or CH4 than N2. Further measured data after overnight desorption pointed out in Fig. 10 also confirm this conclusion, with CO2 reaching 37.2% and CH4 reaching 12.2%, N2 decreasing to 62.8% and 87.8%, respeively. It should be noted that the pressure in the chamber was reduced to normal atmospheric level (101.320 kPa, absolute pressure). Fig. 10. Gas composition during desorption Fig. 11 shows the colleed gas volume for each gas in the desorption process, as time proceeded, the total amount of gas volume for each gas increased. As there is a high composition of CSG (CO2 or CH4) in the chamber after the flushing test, much more N2 is colleed than CO2 or CH4. At the end of the CO2 flushing test a total of 37.7 L of N2 and 2.3 L of CO2 were colleed, while a total 20.9 L of N2 and 1.1 L of CH4 were colleed in the CH4 flushing test. Fig. 11. Gas volume during coal desorption Fig. 12 shows the comparison of colleed gas volume in the desorption stage for CO2 and CH4. It was found that more N2 volume is colleed than CO2 or CH4 was recovered. The ratio of colleed volume of N2:CO2 is around 16.40 and the ratio of colleed volume of N2:CH4 is around 19.0, which is relatively larger than the CO2 flushing test. Fig. 12. Comparison of colleed gas volume in Stage 3 All the adsorbed gas is flushed out during the Stage 2 and Stage 3 and as the gas composition of CO2 or CH4 is very low at the end of the flushing stage, all the gas coming out in the stage 3 is assumed to be adsorbed gas, which is 2.30 L for CO2 and 1.10 L for CH4. Based on the experimental data the following equation is adopted to calculate the gas content during the desorption stage: vt v0 i 1 v c (3) where: vt is the gas content in coal during the desorption stage; v0 is the gas content in coal at the time 0 (starting point of desorption stage); and ­1 are the gas composition in the chamber at the time t and t ­1 during the desorption stage; v is the total gas content drop in coal in the desorption stage; c is the total gas composition increase in the chamber in the desorption stage, all the gas referred here is CO2 or CH4. This calculation model is also proposed based on the principles claimed in the Stage 2. Packham et al. (2012) reported the continued injeion of nitrogen would create conditions where the methane content of the coal could be reduced to negligible levels. Fig. 13 shows the gas content change in coal during the desorption stage. A total of 2.30 L of adsorbed CO2 and 1.10 L of adsorbed CH4 are desorbed from coal, to help reduce the remaining coal gas content, which is 14.385 cc/g for CO2 and 6.89 cc/g for CH4.The reduion accounts for 62.5% of the total adsorbed CO2 gas content and 55.8% of the total adsorbed CH4 gas content, respeively. It indicates gas desorption with gas pressure drop after N2 flushing plays a more effeive role in reducing adsorbed CO2 than CH4. Fig. 13. Comparison of gas content in coal in Stage 3 5. Conclusions Laboratory N2 injeion tests show that CSG (CO2 and CH4) can be flushed out by N2 injeion. During the N2 flushing process, the CO2 and CH4 percentage of the chamber gas gradually decreases and the N2 percentage increases, and with the N2 flushing test approaching, the colleed total gas volume of both CSG and N2 increases. It is found that at low CO2 or CH4 composition stage, it is hard to use N2 to achieve effeive flushing. After the flushing test, a certain amount of CO2 or CH4 is still adsorbed inside the coal. In the desorption process, the CO2 or CH4 percentage change starts to increase, indicating more CO2 and CH4 gas desorbs from the coal than N2. In the N2 injeion stage, the ratio of N2:CO2 colleed volume is around 3.05 and the ratio is around 2.77 for N2:CH4. In the gas desorption stage, the ratio of N2:CO2 colleed volume is around 16.40 and the ratio is around 19.0 for N2:CH4. During the flushing stage, N2 injeion helps to reduce the adsorbed gas content. The reduion of 8.625 cc/g CO2 gas content accounts for 37.5% of the total adsorbed CO2 gas content while the reduion of 5.44 cc/g accounts for 44.1% of the total adsorbed CH4 gas content, which indicates N2 flushing plays a more effeive role in reducing adsorbed CH4 than CO2. Comparatively, during the desorption stage, a total of 2.30 L of adsorbed CO2 and 1.10 L of adsorbed CH4 are desorbed from coal. The reduion accounts for 62.5% of the total adsorbed CO2 gas content and 55.8% of the total adsorbed CH4 gas content, respeively. It indicates gas desorption after N2 flushing plays a more effeive role in reducing adsorbed CO2 than CH4. The result clearly shows that N2 gas flushing has a significant effe on the CO2 and CH4 desorption and removal from coal. Thus it is important to develop a nitrogen injeion technique in field trials, to enhance gas recovery in tight (hard-to-drain) and low permeable seams in future. Acknowledgment This research is supported by the Fundamental Research Funds for the Central Universities (2015QNA42), the Scholarship from University of Wollongong and China Scholarship Council, the National Natural Science Foundation of China (Grant No. 51374200), and The Priority Academic Programme Development of Higher Education Institutions in Jiangsu Province (Grant No. SZBF2011-6-B35). The authors wish to thank the staff and management of BHP Billiton-Illawarra Coal for providing coal samples used in this study. Thanks are also due to the technical staff at the University of Wollongong especially Col Devenish for experiment assistance.

Journal

Archives of Mining Sciencesde Gruyter

Published: Dec 1, 2015

References