Basic Research on Coal-Bed Gas Hydrates Storage Transport and Purification
|School||Taiyuan University of Technology|
|Keywords||coal-bed gas hydrates storage and transportation methane purification formation and dissociation shrinking core model reaction-diffusion|
The coal-bed methane was also usually called coal-bed gas which main composition is CH4 that is the kind of high quality clean fuel, the industrial chemicals and new clean energy. Prospective reserves of coal-bed methane are 30~35×1012 m3 approximately in China that occupied one third of the world. At the same time China also is the biggest coal-bed methane discharge country that was about 194×108 m3 which composes about global discharge 1/3. Only 184 underground coal mines have established coal-bed methane gas drainage systems and ground gas transportation systems in 2000 among the 1,000 high gas and coal gas outburst mine which about 8.58×108 m3 coal-bed methane was drainage every year. The national average drainage rate of coal-bed methane was only 23%. The amount of National Underground development coal-bed methane was about 16×108 m3 and the average rate of the development coal-bed methane is only about 10% in 2004 among state-owned high gas outburst coal mine. Most gas discharge of coal mines during the mining process has the low CH4 concentration so it directly discharges as the waste gas as lack of effective treatment and use channels and the management of technical factors. It not only be a waste of energy, but also create serious environmental pollution. Moreover lacks of the gas line is restricts our country coal-bed methane development an important external condition, but the massive center small coal-bed methane field (100~5000×108 m3) in the quantity accounts for 57% and the reserves accounts for 18.3% which the geographical position dispersible that it is impossible to construct the pipe net system. Therefore the coal-bed methane cannot carry on the effective processing to withdraw with the economical storage and transport poses the serious threat to the coal mine safety in production also affects the coal-bed to be mad the industry directly the development. The hydrocarbon of coal-bed methane such as CH4 can form the gas hydrate with the water under 0~10℃, 3~8 MPa. The hydrate has the good stability under the atmospheric pressure when dissociation can release 180~200 volumes CH4. At the same time gas which form the hydrate such as N2 phase equilibrium condition and the hydrocarbon are clearly different. The hydrate formation has the selectivity, the gas storage ability and the relatively temperate gas storage condition causes it possibly to become the coal-bed methane processing, storage and transport optimal feasible way. Therefore the basic experimental researches of the coal-bed methane hydrate formation and its property have the vital scientific significance and the application value.The basic study principles and theories of the hydrate storage/transportation and concentration of coal-bed methane were introduced after years of in-depth study in Taiyuan University of Technology. The relevant laws of storage propriety and gas composition changes were studied under different conditions during the process of formation and decomposition experiment of coal-bed methane hydrate in the paper. A more detailed physical model of the hydrate formation and decomposition was derivate based on reaction-diffusion theory. From theories and numerical calculations the mechanism and influencing factors of hydrate particles formation and decomposition were analyzed. The technical and economic evaluation was conducted with other storage technology in industrial production in the last. Main research work in the following areas:1. Hydrate formation experiments was conducted at the pure water and surfactant solution in the form of spray system. From the analysis of the two systems the hydrate formation of surfactant can not only effectively improve the reaction rate, at the same time can also significantly improve the hydrate storage capability.2. Hydrate formation propriety of surfactant solution was analyzed in the different concentrations and pressure. From the experiments it can be found: the pressure was not obvious effects gas content of the gas hydrate, the gas content and the reaction rate increased with the concentration of additives, but after more than 300 ppm it increase dramatically reduced. 3. Formation kinetics equation of the hydrate particles was established according to the reaction-diffusion of gas-solid reaction kinetics principle. From the analysis of formation reaction rate under certain pressure and different temperature it can be found: Both lower temperature and increased pressure can promote hydration reaction and shorten reaction time from which the changes of response mechanism from reaction to diffusion-controlled as these two factors was analyzed according to theoretical models. The mechanism of diffusion coefficient and reaction rate constant in different temperature and press influence to hydrate formation from drop is concluded and it provides a theoretical basis to the hydrate technology.4. Volumetric specific process of gas hydrate formation was in a spontaneous reaction so the whole process reflects the basic requirements under the reaction conditions and energy minimization. Through the volumetric specific experiment the suitable reaction condition can be determinate for it is a spontaneous course.5. The experiment of hydrate concentration process is carried out to coal-bed gas and 19% tetrahydrofuran (THF) solution at different initial pressure on hydrate formation reactor with spring nozzle. The respective gas concentration of coal-bed gas was monitored on-line with using gas chromatography in gas phase and hydrate phase on reactor during hydrate formation. The experimental results show that THF solution can effectively reduce the phase equilibria pressure of gas mixture hydrate formation. High initial pressure can accelerate the reaction rate but CH4 concentration range has decreased and the concentration difference of respective component between hydrate phase and gas phase also decreased. Therefore, the appropriate reducing reaction pressure has the positive effect to concentration of CH4 from the oxygen-containing coal-bed gas in the THF solution.6. Dissociation experiments was conducted on laboratory to synthesis hydrate at constant temperature in different press and dissociation kinetics equation of the hydrate particles was established according to the reaction- diffusion of gas-solid reaction kinetics principle. From the analysis of decomposition rate under certain pressure and different temperature it can be found: the decomposition rate changes significantly with temperature under 60℃. The pressure on the impact of hydrate dissociation is less than temperature. The mechanism of diffusion coefficient and reaction rate constant in different temperature and press influence to hydrate dissociation is concluded.7. From CNG, LNG and NGH in the production transport and investment comprehensive cost comparison it can be found: NGH have the most cost advantages in the transport distance of 250 to 700 km. This distance is also general inter-provincial transport of moderate to hydrate. The cost of the production with gas storage and transportation is higher than that of compressed natural gas. But due to compressed natural gas transport tanker prices are more expensive and transport of compressed natural gas is high-pressure gas it has poor security and more quality of the tank. So it greatly increases the cost of transport in the form of CNG. At the same time storage and transportation at the hydrate form much lower investment in liquefied form below.