The Separation and Liquefaction of Oxygenated CBM Yang Kejian and Zhang Wu

Transcription

1 The Separation and Liquefaction of Oxygenated CBM Yang Kejian and Zhang Wu Abstract CBM, or Coal Bed Methane, with air released during coal mining production has been a long time issue. It has not been well utilized but emitted into atmosphere in most of our country s coal mining industry. It is, however, a type of valuable energy resource with great potential of profit. This paper describes a patented technology using cryogenic technology to separate CBM from air and liquefy it as side product of coal mining production. The purity of the liquefied CBM reaches 99% or higher, while the methane content of the air emissions is less than 1%. The technology, with simple equipment and low energy consumption, will turn its investment into high profit within a short period of time. 1. Introduction CBM is a type of flammable gas that adsorbed on the coal bed, also called unconventional natural gas, with main component of methane. During coal mining, CBM must be released, or it will be a great risk of explosion that seriously threats miners lives. However, CBM is a type of valuable energy resource with almost no pollution. It is also an important raw material for chemical industries. Therefore it should be collected and properly utilized instead of wasted. China's CBM resource is very rich, but its industrial application has not been well developed. According to related reports, China's annual CBM emission into the atmosphere counts a great number, one-third of total emission of CBM in the world. Not only has it caused serious environment pollution, but also a huge waste of natural resources. The output of CBM of a single mining well is usually not high, so laying pipelines to export its CBM is not economical. CBM from surface mining well is usually with higher methane content, say above 95%. In this case the CBM can be pooled together and export by pipelines, or it can be liquefied to natural gas (LNG) and shipped with tankers. With most of our mining productions, however, the CBM released has low content of methane, about 50% or even lower than 30%. This kind of CBM is called oxygenated CBM. The oxygenated CBM has not been effectively utilized due to the difficulty of its collection and distribution. It is mostly emitted into atmosphere and wasted. If a process method can be developed that separates the CBM (mainly methane) from air and then liquefies it, the pollution problem will be resolved and great profit can be realized. When the purified CBM is liquefied its volume reduces by 600 times. Thus the cost of its distribution will be lower than that of compressed CBM. Comparing with pipeline distribution of small and medium scale productions, liquefied CBM is more flexible and even lower cost. The researchers and engineers of Chinese Academy of Sciences, or CAS, including the authors of this article, have been working on the research and development in cryogenics for more than 40 years. We are the pioneers in the field of cryogenic engineering in China. We have developed China s first helium liquefaction equipment. The temperature reaches -269 C that is much lower than the CBM liquefaction temperature at -160 C. As early as 1960s and 1970s, we participated in China's aerospace industry cryogenic system design and development to meet the needs of simulation testing for satellites development. We received an award of science

2 conference due to our contributions. Since 1990 we have developed two different systems of LNG used as small-scale production, and involved in many projects including design, installation and testing. We are knowledgeable of and experienced in various domestic and international LNG systems. By successfully applying our gas bearing turbo expander technology to LNG and gas separation industry, we were awarded a second prize of scientific and technological progress of Chinese Academy of Sciences and a third prize of scientific and technological progress of China. In recent years, our extensive research and development work has been focused on recycling of energy resources. Based on the analysis and research of CBM, we have presented our proposal of using cryogenic technology to separate and liquefy CBM. We have already done the calculations and design about the process. The result of computer simulation shows that, after the separation and liquefaction, the purity of the natural gas can reach 99% or higher, while the methane content of the air emissions will be lower than 1%. Such a process can be achieved by a set of simple equipments with low energy consumption. Because oxygenated CBM during mining production is mostly wasted now in China, it is truly an inexpensive raw material for LNG production. LNG, with its high efficiency and pollution free, has its good market value these days, especially the time that is lack of energy supplies. An investment of this kind of project will turn into profit as soon as in two to three years. The projects with this technology will certainly create profit for investors and reduce environment pollution. Its economy and society effect will be significant. 2. The Plan of Liquefaction and Separation Like natural gas, the main component of CBM is methane. The critical temperature of methane is 190.7K (about C), and the critical pressure 46.4 bar. In other words, methane becomes liquid under these conditions. If under pressure of atmosphere, the liquefaction happens at C. So the liquefaction of CBM can only be realized at low temperatures. The process of refrigeration and liquefaction for CBM is similar to that of LNG. There are several types of processes, such as cascade refrigeration and liquefaction cycles, mixed refrigerant refrigeration and liquefaction cycles, and expander refrigeration and liquefaction cycles. The flow rate of oxygenated CBM is usually low, and so as its pressure. Clean and nitrogen-rich gas will be generated during the process of liquefaction and separation, which is just right to be used as refrigerant. This paper describes a process of refrigeration and liquefaction cycle with turbo expanders. There are several methods can be considered for separation of oxygenated CBM, those are pressure swing adsorption (PSA), membrane separation, combustion deoxygenating, and low temperature separation. PSA is based on that the components of mixture of gases have different adsorption capacity on the solid adsorbent. For example, the carbon molecular sieve has stronger adsorption to oxygen than to nitrogen and methane. During the process of PSA, adsorbing happens under higher pressure and then desorbing at lower pressure. A portion of the product is used as desorbing rinse. Adsorbing is under higher pressure, while renewable rinse under atmospheric pressure. There are, practically, two columns exchanging each other periodically. The advantages of PSA are low energy consumption, short time of desorbing and simple operation. The disadvantage is its low recovery rate, between 40 and 50%. This is because the gaps exist between adsorbent layers. The gas

3 stored in the gaps is lost during desorbing stage. Besides a portion of the product gas is needed for rinsing. The relationship between recovery rate and purity of the product is contradictive. When recovery rate is up, the purity is low. While purity gets up, the recovery rate drops. Therefore this method is not economic for separation of oxygenated CBM. Membrane separation is another method. It utilizes the selectivity of infiltration of organic polymer to realize separation of certain component from mixture of gases. The method has advantages of simple equipment, small space occupation, continuous operation and no phase changes. When the mixture of gases is compressed, the pressure of a component on one side of the membrane is higher than that of the other side. Under the pressure difference, the gas of the component will flow through the membrane to another side and be collected. The permeability related to the permeability coefficient, the area of the membrane, and the pressure difference. For example, the permeability coefficients of steam, H 2, NH 3 and He are high, so these gases can penetration the membrane faster. The permeability coefficients of CO 2, H 2 S and O 2 are smaller. While that of N 2, CH 4, CO and C 2 H 6 are very small, those are slow permeability gases. The O 2 content in CBM is about 5-10%, so it is necessary to increase the pressure of CBM for removing O 2 from it effectively. As a result, the partial pressure of methane and nitrogen get higher, so as the permeability of these two gases. Thus there is more loss of the product gas. Also, high pressure of CBM will create potential safety problems. Therefore membrane separation is not suitable for separation of CBM. As oxygen removal from CBM by combustion, its product purity is high, with oxygen content of 0.5% or less. The drawback of this method is the complexity of equipment. The combustion process will increase the contents of CO 2, CO and H 2 S within process gases. CBM usually contains less CO 2, only %, some completely free of H 2 S. The CO 2 can be removed during dehydration of molecular sieve. If combustion method is used, not only the investment in equipment increases, energy consumption going higher, but also the operation is complex. This approach, therefore, is not suitable for the purpose. The principle of low temperature separation is that the mixture of gases is liquefied and then the components of the gases are separated according to the evaporation temperature of the components. It is very suitable for the separation of oxygenated CBM. First, it produces LNG with the highest purity, which is very difficult by using other methods mentioned above. Secondly, the process is safe because of its low pressure and low temperature. Under those conditions the possibility of combustion and explosion of methane is reduced to the lowest. Thirdly, such a process is the most economic one comparing with others. To get product of LNG, the raw material of CBM must be cooled to the temperature at which methane liquefies. Removal of oxygen and nitrogen can been done during the cooling process. Thus, there is no any extra energy consumption or loss of product gas such as that used by other methods. 3. Brief Description of Cryogenic Process The pressure of oxygenated CBM is usually low, so increasing its pressure by a compressor is necessary for its following processes such as purification, liquefaction and separation. Purification process depends on the gas composition of the oxygenated CBM. In most cases, oxygenated CBM contains water vapor and small amount of acid gases

4 such as carbon dioxide. A molecular sieve is effective to remove water and carbon dioxide, which is used by the process introduced in this article. A). Compression and Purification Figure 1 As shown in Figure 1, the raw material gas, oxygenated CBM, from the outfall with slightly positive pressure and at atmosphere temperature goes through a gas-liquid separator (1) where the free water is removed. The gas-liquid separator is also playing a role of a buffer tank. Leaving gas-liquid separator, the dry oxygenated CBM flows through a filter (2) and all the dust is filtered out. The clean oxygenated CBM then enters a compressor (3) to be compressed. After compressed it is cooled by a cooler (4) and entering gas-liquid separator (5) for removal of the condensed water. From there the oxygenated CBM enters a dryer with molecular sieve where carbon dioxide is removed and more dehydration is performed. There are two dryers with molecular sieve (6-1) and (6-2). One is working and the other is recycling as a standby. A handoff between the two happens every 12 hours. After the dryer, the feed gas (oxygenated CBM) is filtered again by a filter (7) to remove some possible powder of molecular sieve. When all these pretreatments complete, the oxygenated CBM enters the section of refrigeration, liquefaction and separation. A part of the clean and dry air from the section of liquefaction and separation, which is described later, will be used as renew gas and purge gas for the dryers. Let us see a period when dryer (6-1) works and dryer (6-2) as standby. The air from the section of liquefaction and separation enters heater (8) and get heated, and then it goes to the standby dryer (6-2), where it takes away the water and carbon dioxide adsorbed during the dryer s working cycle. The clean air followings will, without passing through the

5 heater 8, access directly dryer (6-2) that has be recycled, cooling the molecular sieve for its next working cycle. The cooling gas flows out from the top of the dryer. The recycling of the dryer takes 12 hours. After 12 hours working, dryer (6-1) is then switched with dryer (6-2). The same recycling now takes place on dryer 6-1. B). Liquefaction and Separation 原料气 Figure 2 Figure 2 shows the separation process of oxygenated CBM at low temperature. The oxygenated CBM, after the section of compression and purification, enters a cold box (9) where it is cooled inside a heat exchanger by the cryogenic gas after expansion. Thus the temperature reaches the point at that gas and liquid coexist. The oxygenated CBM at such a temperature enters a separator inside the cold box, where heat and mass transfer happens. A part of clean oxygen-rich air, still in gas phase is separated from the separator. This part of air enters the heat exchanger to recovery cold energy. After heat exchanging there, the air gets warmer and will be used as renewable heating and cooling for the purification. A small portion of high purity nitrogen also released from the separator. After decompression, this part of nitrogen merges with refrigeration gas into heat exchanger for recovering cold energy. At the bottom of the separator, liquefied CBM (LNG) with purity of 99.9% is accumulated. The cold for cryogenic distillation and liquefaction is provided by a refrigeration system. The system mainly consists of refrigeration compressor, air-cooling fan, turboexpanders and heat exchangers. Prior to the start, the refrigeration system is completely filled with nitrogen. When it is started, the portion of nitrogen from separation, after the

6 decompression valve, enters the loop of cooling system as supplement of the refrigerant loss during the operation. The extra nitrogen is emitted after its cold recovery. In the loop of cooling system, the nitrogen, as refrigerant, is compressed by compressor (10) and then cooled by a cooling fan (11). It is then compressed further through the boosters of two stage turbo expander (12) and (13). Turbo expander is high-speed rotating machinery. One end of its shaft is an impeller wheel that is pushed by the gas out of the nozzle and drives the shaft rotating at high speed. The pressure and temperature of the blowing gas drop due to its output of power thus cold effect is produced. The other end of the shaft is a boosting wheel rotating with the shaft and compressing the gas through it. Continuing with our process, the refrigeration gas (nitrogen), after further pressurized by the boosters of the two-stage turbo expander (12) and (13), is cooled by a cooling fan (14) before entering the cold box. In the cold box, the temperature of the refrigeration gas is decreased further inside the heat exchanger by the reflux of cold flow. It is then divided into two parts. One part flows into turbo expander (15) as cold source of middle temperature section. Another part reenters the heat exchanger to reduce the temperature further, and then goes into turbo expander (16) for more falling of its temperature and pressure. This part of cold gas merges with the cold nitrogen from the separator and then enters the cold end of the last exchanger. The gas from last heat exchanger will combine with the gas from expander 15. This combined gas is main cold source of other heat exchangers. After heat exchanging, the gas getting warmer and is then compressed, pressurized, pre-cooled, expansive refrigeration. Thus working cycle continues. The process is designed specifically for oxygenated CBM. The liquefaction and separation happens simultaneously under low temperature. It has advantages of high purity of product, safety of operation. LNG as product can be obtained during the separation. Also, the refrigerant is from the process gases as long as the system running, not need to purchase separately. The components of equipment are simple and easy to install. It is an advanced process. 4. Conclusion Oxygenated CBM released during coal mining production is mostly wasted due to is low pressure, low methane content, and mixed with air. It has been a serious environmental pollution issue. The process method proposed and designed in this paper using cryogenic technology to separate CBM from air and then liquefy it is very practical. It facilitates the distribution and utilization of the product, high purity of LNG. The technologies used in this process are all matured. When it is practically adopted by industry, the rich CBM resources will be fully utilized. At the same time the pollution problem will be effectively reduced. The benefits to people in both economy and environment are significant..