| Oil shale resources of the world are rich, the Oil shale deposits more than100have beendiscovered by37countries around the world. If they are converted into reserves of shale oil,the weight of shale oil could reach more than410billion tons, and it’s three times thanrecoverable reserves of the world natural crude oil. Many countries have paid close attentionto develop the oil shale because of the broad development prospects and important alternativeenergy. China is rich in oil shale resources, if the identified reserves are converted into shaleoil, the weight could reach50.049billion tons, oil shale and gas will be an importantcomplement and be expected to be a substitute, when the supply of conventional oil and gasresources are insufficient.The underground water sealing and frozen wall need to be build in the mining area, whencarried out in situ heating of oil shale mining. The method of conventional artificial freezingis to use refrigeration brine solution and refrigeration units, it need to consume a lot ofelectrical energy. But, the northeast of China have rich oil shale reserves, and cold naturalsource because of low temperature in long winter. For these cases, the experimental platformof underground mining of oil shale in situ freeze wall joint refrigeration system wasestablished in this paper. By using three kinds of experimental operation mode including thenatural air-cooled refrigeration in winter, joint freezing and separate refrigeration unit coolingto study the effects of the frozen underground wall and the economic analysis of the jointfreezing, including the following aspects:First of all, the numerical model of underground freezing and heat transfer using singlerow frozen pipes was established based on the analysis of the impact of the underground heaton underground freezing and heat transfer. Freezing temperature fields under different coolingconditions were analyzed using ANSYS analysis software, and the influence law of cryogeniccoolant temperature and flow rate on underground temperature field was obtained. The simulation results show that the lower coolant temperature and the higher traffic would resultin the shorter cross lap time of freeze lap, but there are optimal combination between the crosslap time and the coolant temperature and flow. The cross lap time of freeze lap and theinvestment costs of operation and equipment could be both effectively reduced. The rationalcoolant temperature and coolant flow were provided in this paper.Secondly, considering northeast climate characteristics of China and combined withsimulation results, the multifunctional experimental platform of the underground frozen wallfor mining of oil shale in situ, it could meet separate refrigeration unit, the joint freeze ofrefrigeration units and air-cooled refrigeration unit, and refrigeration unit cooling freeze,simultaneously. The freezing time of separate refrigeration unit was720h, the averagecooling extent in1m radius range of soil under the mainland was4℃effected by the frozenpipes, the temperature drop rapidly in the pre-cooling, the cooling rate was slow graduallywith the gradual increase in ambient temperature. By analysis on the operating data ofair-cooled refrigeration unit, it showed that the cooling effect was better when the ambienttemperature was below-10℃, and its outlet temperature was about-5℃. The coolingpower would further increased with the decrease of the ambient temperature. Cooling powerwas twice than it was at-10℃when the ambient temperature was-20℃. The freezing timeof joint refrigeration system was360h, the results of alternate cooling operation mode thatusing air-cooled refrigeration unit to freeze when the ambient temperature was below-10℃and using refrigeration unit to freeze when the ambient temperature risen to above-10℃showed that the cooling power of air-cooled refrigeration unit was about20kW, it couldbasically meet the needs of frozen for freeze holes, when the ambient temperature was below-10℃. The cooling rate of underground temperature field was relatively stable, and the overallcooling rate was consistent with using alone air-cooled unit to freeze.The freezing time ofseparate refrigeration units was1980h, the cooling power of refrigeration units decreasedwith the rise of ambient temperature. The temperature increased by10℃, the cooling powerwould be down about10%, at the same time,the cooling fluid temperature also increasedfrom-18.5℃to-13.5℃and the average cooling power was42.5kW. The drop curve ofunderground temperature was concave, the soil temperature between adjacent intermediate freeze hole declined fastest and the soil temperature inside the freeze ring decreased slightlyfaster outside the corresponding position. The soil temperature before frozen decreasedrapidly and the temperature declined slowly after frozen until the frozen end. The diameterof permafrost body was above1.0m, and the soil temperature between adjacent intermediatefreeze hole has dropped to below0℃and reached frozen state.The curve which showed the decline of temperature measured in field dropped changedwith time was consistent with the curve of numerical simulation, by comparative analysis onthe changes in ground temperature field and the simulation results. When using air-cooledrefrigeration and Joint refrigeration, the maximum temperature difference between simulationand experiment didn’t exceed15%and the average temperature difference was8.2%. Whenusing refrigeration units, the maximum temperature difference between simulation andexperiment didn’t exceed10%and the average temperature difference was6.4%, these dataverified the reliability of numerical simulation.Finally, the results showed that in cold areas, using joint refrigeration system to freezecompared with conventional and artificial freezing could save operating costs more than20%yearly and the cost of investment in air-cooled units would be available to recover in3to4years or shorter time, by comparative analysis of economic on Joint refrigeration system andconventional and artificial freezing. The result showed that the cost of the HDPE coaxial tubeheat exchanger was30.9percent of the cost of steel tube heat exchanger in the same frozen,and the HDPE coaxial tube heat exchanger had advantages in construction costs, constructionspeed and construction.convenience, by the cost comparison analysis on HDPE and steelcoaxial tube heat exchanger and steel tube heat exchanger.TO sum up, the cold natural source could be underutilized in high temperature in-situ oilshale mining. The energy consumption, ammonia and freon released by refrigeration unitswas reduced by using joint refrigeration system. Not only reduces the cost, but also to protectthe environment, so as to promote technology in situ oil shale mining for further development. |
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