Application of pressure swing adsorption technology in coalbed methane

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Application of Pressure Swing Adsorption Technology in Coal Bed Methane Tang Xiaodong??〔1〕? Meng Yingfeng Abstract This paper briefly introduces the principle, characteristics, applications, and current development status of pressure swing adsorption (PSA) technology at home and abroad. In light of the particularities of coalbed methane development in China, technical approaches are proposed for applying PSA technology to horizontal drilling for coalbed methane in order to improve pressure sealing and enhance the recovery rate of coalbed methane from low-permeability coal seams, as well as for the purification of coalbed methane, the recovery of light hydrocarbons, and their storage and transportation. Further analysis shows that the application of PSA technology in coalbed methane development offers technical advancement and economic advantages. Finally, several suggestions are put forward for the promotion and application of PSA technology in China’s coalbed methane development [1]. Department of Chemical Engineering, Southwest Petroleum Institute, Lecturer, 637001 Nanchong City, Sichuan Province. 1 Introduction China possesses extremely rich coalbed methane resources; it is estimated that the reserves of coalbed methane at depths of up to 2000 meters amount to 30–35 trillion cubic meters. Considering a recoverable factor of 50%, the recoverable resources amount to 15–17.5 trillion cubic meters, which is equivalent to 13.19–15.39 billion tons of crude oil. At the current rate of crude oil extraction, these resources could sustain China’s needs for over 100 years. Therefore, the prospects for coalbed methane development in our country are enormous. At present, the main challenges to the development of coalbed methane in China are technical issues, such as low permeability of coal seams, low production per well, and the inability to achieve industrial-scale production. These factors prevent China from adopting the successful strategies used in the United States for coalbed methane development; instead, it is necessary to develop technologies suitable for the development of China’s coalbed methane industry. This paper addresses the specific characteristics of coalbed methane development in China by proposing the use of PSA technology in horizontal drilling for coalbed methane, in order to improve the recovery rate of coalbed methane as well as its purification, light hydrocarbon recovery, and storage and transportation, thereby promoting technological advancement in China’s coalbed methane industry. 2 Introduction to PSA technology PSA technology is a gas separation technique that was developed in the 1960s. It relies on fixed-bed adsorption, and by continuously changing the thermodynamic parameters (pressure) that affect the equilibrium of the system, an adsorption and desorption regeneration cycle is established. This approach combines the advantages of fixed-bed adsorption with those of a cyclic process. It is widely used in the gas separation industry of petroleum, natural gas, and chemicals, for example in the separation, recovery, and purification of gases such as H2, O2, N2, He, and Ar, as well as components like CH4, C2H4, C2H2, nCnH2n+2, H2O, CO2, and CO. By 1986, more than 500 large-scale PSA units were in operation both domestically and internationally, with the maximum processing capacity reaching 100,000 m?3/h. The advantages of the PSA separation process are: (1) high product purity, for example, pure hydrogen with a purity of 99.999% can be obtained ； (2) It operates at room temperature and low pressure (0.05–3.00 MPa), requires no external heating source, and has a simple design ； (3) It can operate in a single stage; several components in the feed gas can be removed in one step, and there is no need for pretreatment of moisture and CO2 in the feed gas ； (4) The adsorbent has a long service life, requires low standards regarding the quality of the feed gas, and the installation is easy to operate with great flexibility ； (5) High degree of automation, low operating costs, and significant energy savings and consumption reduction. Research on PSA technology in our country began in the early 1970s, with the Southwest Institute of Chemical Engineering taking the lead in developing this technology. To date, more than 350 PSA units have been developed both domestically and internationally, for use in nine different applications. Table 1 lists some of the PSA technologies developed by this institute that can be used for coalbed methane development. 3 Applications of PSA technology in coalbed methane development 3.1 Application of PSA technology in coalbed methane drilling Given that China’s coalbed methane reservoirs are of low permeability and pressure-confined type. Some have also proposed using horizontal well technology in the large-scale development of coalbed methane. The main advantage of horizontal wells is that they are perpendicular to the maximum permeability direction of the coal seam, thereby increasing the production of coalbed methane. However, when drilling horizontal wells, it is necessary to consider the issue of drilling fluid contamination of coal seams; otherwise, the consequences will be detrimental. In the oil and gas industry, there have been cases where drilling fluids severely contaminated oil and gas reservoirs. Table 1 lists the PSA technologies that can be used for coalbed methane development: PSA-N2, PSA-CO2/R, PSA-CO2/R, PSA-CH4. The raw materials used include air, various CO2-containing mixtures, natural gas, and coal mine gas. The processing capacity ranges from 30–6000 m³/h for PSA-N2, 1500–50,000 m³/h for PSA-CO2/R, 200–10,000 m³/h for PSA-CO2/R, and 500–20,000 m³/h for PSA-CH4. The purity of the purified gas varies as follows: N2 ≥ 99%, CO2 < 0.2%, CO + CO2 < 100 ppm, CH4: 50–95%. The operating temperatures are at room temperature for all these systems. The operating pressures range from 0.3 MPa for PSA-N2, 0.7–2.0 MPa for PSA-CO2/R, 0.3–0.8 MPa for PSA-CO2/R, and 0.4–0.8 MPa for PSA-CH4. Applications in coalbed methane development include drilling for coalbed methane, improving the recovery rate of coalbed methane, purifying coalbed methane, and recovering light hydrocarbons through coalbed methane purification. Lessons from the decommissioning of oil and gas wells. Therefore, to prevent coal seam contamination, air drilling technology can be employed. Air drilling is a low-pressure drilling technique that uses air as the circulating medium. Compared with drilling using drilling fluid, it offers advantages such as lower costs, a mechanical drilling speed 3 to 4 times higher, a longer drill bit life, the ability to effectively develop low-pressure, low-permeability reservoirs, and good reservoir protection. However, air drilling poses insurmountable risks, as a fire at the bottom of the well may occur when encountering coal seams. A mixture of coalbed methane and compressed air can cause a fire at the bottom of the well, which can melt drill bits, drill collars, drill pipes, and other tools at that location, leading to accidents. Therefore, to prevent the hazards of air drilling, air atomization drilling and natural gas drilling can be employed. Air atomization drilling reduces the risk of fire at the bottom of the well, but it increases the costs associated with foaming agents, preservatives, water, etc., shortens the life of the drill bit, and reduces the drilling speed. Using natural gas to drill coalbed methane wells can prevent fires at the well bottom, but it is not economical, and there are issues related to the gas supply and safety. Currently, the United States and Canada have installed a type of hollow-fiber polymer membrane in skid-mounted nitrogen production units (NPU) designed for oil fields; the nitrogen produced is used in horizontal drilling, vertical drilling, and negative-pressure drilling. It is much cheaper than using air atomization or natural gas for drilling, and it overcomes all the disadvantages of drilling with drilling fluid, air atomization, or natural gas. When using a mobile PS-N?2 unit to produce nitrogen for the drilling of coalbed methane horizontal wells, its cost is only 1/8 of that of natural gas and 1/2 of that of nitrogen production via air cryogenic method. Furthermore, when the PSA-N?2 unit is used in the drilling of coalbed methane horizontal wells, it also produces oxygen-enriched air with an oxygen content of ≥40%. Supplying this air to the diesel engines used in drilling enables oxygen-enriched combustion of diesel within the cylinders, thereby increasing the power of the diesel engines, reducing fuel consumption as well as the emission of harmful substances such as CO, HC, and soot. This improves the performance of the diesel engines used in drilling, increases the mechanical drilling speed, and reduces drilling costs. For example, four-stroke non-turbocharged diesel engines and four-stroke turbocharged diesel engines use 35% oxygen-enriched air, resulting in a 51% increase in indicated power; their mechanical efficiencies improve by 6–14.4% and 3.7–8.5%, respectively, while fuel consumption decreases by 5.6–12.6% and 3.6–7.8%, respectively. 3?2 Application of PSA technology in improving coalbed methane recovery rates. China’s coalbed methane reservoirs are of the pressure-confined type, with low permeability of the coal seams; this results in much greater resistance to the desorption and flow of coalbed methane within the coal pores compared to coalbed methane reservoirs that are of the hydraulic-confined type. To improve the recovery rate of coalbed methane in our country, the Enhanced Coalbed Methane Recovery method (ECBM) can be employed; this is a new technology developed by Amoco to increase the recovery rate of coalbed methane. ECBM injects N2, CO2, and flue gas into the coal seam, reducing the partial pressure of methane in the coal pores, which facilitates the desorption of CH4 from the coal matrix ； Injection gases with a greater affinity for coal than CH4 (such as CO and N2) will cause the displacement and desorption of CH4 (i.e., competitive adsorption) ； Injecting gas also increases the driving force (pressure energy) for coalbed methane to flow toward the gas well, which helps pressure-confined coalbed methane overcome the flow resistance in low-permeability coal seams. The N?2 injection tests conducted indoors by Amoco have achieved a 100% CH?4 recovery rate ； The N2 injection test using a 5-point well pattern in the northwestern part of the San Juan Basin can also triple the production of coalbed methane. This indicates that ECBM increases the energy of the production layer, accelerating the desorption and migration of coalbed methane, thereby improving the coalbed methane recovery rate. To extract coalbed methane using ECBM, an adequate supply of N2, CO2, or flue gas is required. N2 can be obtained from the air, and there are many sources available as CO2 resources (see Table 2); however, some of these CO2 sources need to be concentrated and purified before they can be used in ECBM. The common methods for recovering CO2 are solvent absorption methods, such as the Catacarb method, Benfield method, G-V method, MEA method, ADIP method, Fluor method, etc. The problems with these methods include long process flows, complex equipment, high energy consumption, and the fact that many absorption solvents are corrosive to the equipment. When using PSA technology to recover CO2 from various CO2-rich gas sources, it achieves significant energy savings and reduced consumption, thereby completely overcoming the disadvantages of the solvent method. For example, when the PSA method is used to replace the Benfield method, MEA method, low-temperature methanol method, room-temperature methanol method, Fluor method, and Selexol method for carbon removal, their overall energy consumption is only 4–2%, 4–7%, 7–2%, 2%, 47%, and 40–1% of the original values, respectively. Table 2 Various CO2 sources and their CO2 contents. CO2 source CO2% Petroleum-derived dissolved gas Trace to 90 Natural gas 5–80 Natural CO2 wells 80–99.5 Flue gas 10–16 Synthetic ammonia industry 98–99 Petrochemical processes 98–99 A comparison of the costs of various gases used in ECBM is shown in Table 3. Table 3 shows that the N2 injection technique results in the lowest cost for improving coalbed methane recovery rates; the cost of producing N2 using the PSA method is only half that of the cryogenic method, and its economic production capacity abroad has reached 1.2 million m³/day. Therefore, the PSA-N?2 device is fully viable for N₂ injection development in coalbed methane. However, while the ECBM method improves the recovery rate of coalbed methane, it also increases the content of N2 and CO2 in the extracted coalbed methane; therefore, these gases must be removed to meet the requirements for pipeline transportation and end-users. Table 3 Comparison of costs of various gas sources used in ECBM. Gas source cost (relative value): N2 (PSA method): 1.0; N2 (cryogenic method): 2.0; CO2: 2.0–6.0; Flue gas: 4.0–6.0, 3.3. Application of PSA technology in coalbed methane purification: When coalbed methane contains CO2 and water, solvent absorption methods are generally used for carbon and water removal, while solid adsorption methods (temperature swing adsorption, TSA) are employed for water removal. Decarburization by the solvent method is described above. The main methods currently available for the dehydration of coalbed methane include the TEG method, DEG method, molecular sieve method (TSA), and CaCl2 aqueous solution method. These methods have issues such as high investment and operating costs, batch operation, and waste liquid discharge to varying degrees; the PSA method for dehydration can eliminate these drawbacks. In fact, the PSA method can simultaneously remove CO2 and water from coalbed methane. Using PSA‑CH4, PSA‑C+H2/R, and PSA‑CO2/R systems to purify coalbed methane is superior to deep cooling for nitrogen removal and solvent absorption for carbon removal. Reinjecting the removed N2 and CO2 into the coal seam can reduce the production costs of ECBM. When coalbed methane contains H2S, CO2, and water simultaneously, a desulfurization unit can be added before the PSA-based decarburization equipment. Given the low permeability of coal seams in China and the low production volume per well, it is preferable to use a solid desulfurizer (SDA) system. This desulfurizer can selectively remove H2S and RSH. It features simple process equipment, no need for utility facilities, and is designed for single-use; its sulfur capacity is around 12%. The cost to remove 1 kg of H2S is 50 yuan ； The desulfurized waste desulfurizer requires no treatment; it can be discarded directly in open areas or farmlands without harming the environment, and it can even promote plant growth ； It is suitable for feed gas with H2S content in the range of 0.1–5 g/m3; the processing capacity of the unit should be less than 100,000 m3/day. It is an effective method for desulfurizing natural gas with low sulfur content. Recently, the Sichuan Luzhou Natural Gas Research Institute has developed three types of iron oxide desulfurizers for normal temperature use: CT8-4, CT8-4A, and CT8-4B. These desulfurizers outperform the Sulfatreat desulfurizer in terms of activity, sulfur capacity, and cost. Therefore, it is entirely feasible to purify coalbed methane using the SDA and PSA methods. 3?4 The application of PSA technology in the recovery of light hydrocarbons from coalbed methane: Many coal seams in China contain abundant C1+3 resources. For example (Yu Liangchen et al., 1981), the C?+?3 content in the coking-coal fertilizers from the Beipiao mining area ranges from 0.098 to 35.57% (V); in the lean coking coal mining areas of Zhongliangshan, Tianfu, and Nantong in Sichuan, this content ranges from 0.058 to 31.8% (V). In the Tanjiachong coal seam of the Liwangmiao anthracite mining area in Hunan, the average concentration of C??2–8? is 4.814% (V). In the Xu Er natural gas from the Zhongba gas field in Sichuan, the C1+C3 components account for only 2.7% (V); therefore, light hydrocarbon recovery from it also offers considerable economic benefits. Therefore, recovering light hydrocarbons from coalbed methane holds high economic value. Currently, turbine expansion refrigeration for the recovery of light hydrocarbons is the main method in the deep processing of natural gas. Using it directly to recover the C3+ components from coalbed methane would require the removal of 70–95% of the C1+C2 components in coalbed methane, which is extremely uneconomical. Using the PSA-C?+?3/R unit to remove C?1+C?2 from coalbed methane not only reduces investment and operating costs but also significantly increases the production capacity of the unit. There are two process routes for using the PSA method to recover light hydrocarbons from coalbed methane, and the principle flows are shown in Figures 1 and 2. In Figure 1, coalbed methane first enters the SDA and PSA-CO2/T (or PSA-CH4) units for desulfurization, decarbonization, and dehydration (nitrogen removal is also carried out when coalbed methane is extracted using ECBM); it then enters the PSA-C1+C2/R unit to remove C1+C2. Finally, conventional light hydrocarbon recovery processes are used to recover liquefied petroleum gas (LPG) and light oil, thereby increasing the processing capacity of the facility. For example, after purification, a certain coalbed methane (with composition shown in Table 4) is fed into a PSA-C?+?2/R unit to remove C?1 and C?2 (with a removal rate of 90% for each of these compounds). This allows the C?+?3 content in the coalbed methane to increase from 16% and 33% to 66% and 85%, thereby increasing the processing capacity of the unit by more than three times. The principle flow shown in Figure 2 differs from that in Figure 1 in that a compressor and a separator are used in place of the conventional light hydrocarbon recovery units, thereby simplifying the process flow and reducing equipment investment. Figure 1 Principle flow diagram 1 of the PSA system + conventional hydrocarbon conversion recovery process: SDA ； 2: PSA-CO₂/R (or PSA-CH₄) ； 3: PSA-C?+?2/R: 4: Conventional light hydrocarbon recovery process. Figure 2 Principle flowchart 1 of the PSA method for light hydrocarbon recovery: SDA ； 2: PSA-CO₂/R (or PSA-CH₄) ； 3: PSA-C?＋?2/R: 4: Compressor ； 5: Primary separator ； 6: Table 4 – Composition (%) of inlet and outlet gases of the PSA-C+?2/R unit in the secondary separator. Components: C?1, C?2, C?3+C?4, C?1+C?5, N?2, H?2S, CO?2. Coalbed methane: 74–82%, 5–10%, 12–79%, 3–54%, 0–10%, 0–03%, 3–62%. Inlet: 77–68%, 5–29%, 13–26%, 3–67%, 0–10%, 0%, 0%. Outlet: 30–68%, 2–09%, 52–35%, 14–50%, 0–38%, 0%, 0%. 3.5 Application of PSA technology in the storage and transportation of coalbed methane. For large-scale coalbed methane development, pipeline transportation can be used; however, for small-scale developments where individual wells produce low volumes of gas, the wells are located far from gas transmission systems, and there are no local users, conventional natural gas pipeline transportation and liquefaction are not suitable as methods for storage and transportation. In recent years, following compressed natural gas (CNG) and liquefied natural gas (LNG) as vehicle fuels, a new method of storing natural gas – adsorbed natural gas (ANG) – for use in vehicles has gradually attracted attention. ANG technology is a technique that involves filling storage tanks with specialized adsorbents for natural gas, which have a high specific surface area. Thanks to their large internal surface area and abundant microporous structure (pore size < 30 nm), these adsorbents enable the adsorption and storage of natural gas at normal temperatures and low pressures (3–6 MPa). Compared with LNG, ANG has advantages such as lower investment and no evaporation losses ； Compared to CNG, its investment and operating costs are reduced by 50%; there is greater flexibility in terms of tank shape and material selection, it is lightweight, operates at lower pressures, is easy to use, and is safe and reliable. The ANG technology has been successfully developed in China by the Petroleum University. Therefore, for small-scale coalbed methane development, ANG can be used as a means for storing and transporting coalbed methane. However, when ANG is used for storing and transporting coalbed methane, the presence of the C?+?3 components inhibits the adsorption capacity of the adsorbent for CH4, and they should be removed prior to filling. Using the PSA-C?+?2/R unit to purify coalbed methane ensures that its purity meets the requirements of ANG. By combining the purification process with the light hydrocarbons recovery process, it is possible to reduce both the costs associated with purifying ANG feed gas and the operating costs of light hydrocarbon recovery. Additionally, this approach can increase the processing capacity of conventional light hydrocarbon recovery units. The principle flow diagram is shown in Figure 3. Figure 3: Principle flow diagram for using PSA method in the purification of ANG feed gas and light hydrocarbon recovery 1?SDA ； 2?PSA-CO?2/R9 or PSA-CH?4) ； 3?PSA-C?+?3/R ； 4?Compressor 5?ANG 6?Traditional light hydrocarbon recovery unit ； 7? Primary separator ； 8? Secondary separator. Process one and Process two. Process 1 shows that coalbed methane first passes through an SDA and a PSA-CO2/R unit to remove impurities such as H2S, CO2, and water; the purified gas then goes through a PSA-C3/R unit to yield dry gas (C1+C2) and desorbed gas rich in C3 components. The dry gas is compressed to 6?0 MPa by a single-stage compressor and stored in cylinders filled with adsorbents, thereby producing ANG. The ANG cylinders are transported to users by vehicle and can be used directly in cars or for household use. The desorbed gas rich in C3+ components can be separated using conventional light hydrocarbon recovery units to yield LPG and light oil ； LPG can be used as a household fuel and an automotive fuel, while light oil can serve as a component in gasoline blending or for the production of solvents of grades 30, 60, and 120. The difference between Process 2 and Process 1 is that the desorbed gas rich in C+?3 components, which comes out of PSA-C?+?2/R, is pressurized by a compressor; thereafter, residual C?1+C?2 is removed in a primary separator, and finally LPG and light oil are obtained in a secondary separator. 4 Conclusion The progress of China’s coalbed methane industry depends on technological advancements suited to the characteristics of coalbed methane development in China. This paper addresses the specific characteristics of coalbed methane development in China by proposing the use of PSA technology in the drilling of horizontal wells for coalbed methane, its combination with ECBM to improve the recovery rate of coalbed methane from low-permeability coal seams, as well as its application in the purification of coalbed methane, the recovery of light hydrocarbons, and their storage and transportation. Further analysis shows that PSA, as a mature chemical separation technology, combined with oil and gas development technologies, holds great potential for application in coalbed methane development. It boasts advanced technology and economic advantages, but to put it into practical use, the author believes three aspects of work still need to be done. First, in-depth technical and economic evaluations should be conducted on the application of existing PSA technologies in China for coalbed methane development. Secondly, the coal, oil and gas, and chemical industries should strengthen cooperation to conduct comprehensive research on integrating mature technologies such as PSA-N2, PSA-CO2/R, PSA-CH4, and PSA-CO2/R for the development of coalbed methane. Finally, PSA-N2, PSA-CO2/R, PSA-CH4, and PSA-C+2/R units with different processing capacities should be developed to meet the needs of coalbed methane development. ▲ This post was last edited by bht11 on 2007-6-9 16:02]