Cold Energy Recovery And Its Applications Research Of Natural Gas In The City Gate Station

For a long time, China's coal-dominated energy structure caused serious environmental pollution. In order to improve the energy structure and ease the pressure on the environment, the application of natural gas in China who is little pollution, rich reserve and good combustion performance has more and more widely, this accelerated the development of China's domestic natural gas resources and the introduction of the pace of overseas LNG, Which also contributed to the development of long distance gas pipelines.Considering the economics of natural gas transmission process, most pipelines are using high-pressure transmission, after these natural gas which transports by the high-pressure pipelines arrives the city gate, all must be transferred to medium-pressure through the regulator station before entering the medium pressure pipe networks for users. The traditional method is to use the throttle regulator buck expansion, the exergy loss in the process of throttling is relatively large, makes a lot of energy being wasted. If uses the turbo-expander to adjust the pressure, the pressure decreases and the temperature drops in the process of adjusting the pressure, producing a large number of high-quality pressure and cold energy. This paper first proposed the cold energy of high-pressure natural gas that produced in the process of adjusting the pressure can recycle for air conditioning system, in order to achieve energy savings.First, this paper analyzed cold exergy and pressure exergy of high-pressure natural gas from the thermodynamic point that produced in the process of adjusting the pressure. For a certain components of natural gas, the ambient temperature T0 and the pressure dropping gradients have a certain impact on cold exergy. With the ambient temperature T0 rise, cold exergy will increase; the pressure dropping gradients rise, cold exergy will also increase. To find the cold exergy of high-pressure natural gas that produced in the process of adjusting the pressure, we must find the temperature that the pressure of high-pressure natural gas is step-down. Through comparative analysis of two calculation methods, verify the accuracy of the thermodynamic calculation method, and come to a certain extent, it has better versatility and practicality than check enthalpy-entropy diagram method. In addition, there are some thermodynamic analysis and comparison to the throttle and the turbo-expander, analysis showed that the exergy loss of the turbo-expander is smaller, it has better thermodynamic perfection. After analysis and calculation the cold energy that one distribution station can recovery each year, proposed a suitable recovery system process, analyzed the necessity to set a variety of devices and storage systems in the cold energy recovery system. At the same time, calculated the air conditioning load of the office building of the distribution station, proposed the flow that the cold energy from high-pressure natural gas can be recovered for air conditioning system, and explained the types and characteristics of heat-exchange equipments in the cold energy utilization system, designed multi-stage heat exchange system that based on plate-fin heat exchanger, plate heat exchanger and the cooler, in addition, identified the flow parameters of the cold energy recovery system.Analyzed heat transfer characteristics and the heat transfer process specifically to every heat exchange equipment in the system, and established the calculation flow chart to carry on the design calculation to various heat interchangers, and identified the process parameters of heat transfer equipment. In addition, there has an economic comparison to natural gas direct-fired air conditioning system and high pressure natural gas cold energy recovering air-conditioning system, the results showed when cold energy produced in the process of high pressure natural gas dropping pressure can satisfy the load of office building's air-conditioning system at the gate station, although the initial investment of the cold energy recovering air-conditioning system is slightly higher than the natural gas direct-fired unit, the running cost is much less, and payback period is relatively short, high pressure natural gas cold energy using for office building's air-conditioning system at the gate station is a kind of ideal way, provided theoretical basis for its practical application.