LNG接收站储罐冷却过程中蒸发气体的零排放技术

LNG接收站的大型储罐在投用前需要逐步冷却至-162℃,冷却前储罐内充满氮气,冷却过程中将产生大量高含氮气的蒸发气体(BOG),LNG接收站工艺系统无法对其回收利用,只能直接排放至火炬;同时LNG储罐冷却中后期产生BOG的流量极大,超出了接收站BOG的回收处理能力,大量BOG被排放至火炬,造成大量浪费。为此,珠海LNG接收站通过调研国内已投用LNG储罐的冷却方式,并对其预冷过程进行研究,创新性地提出了储罐冷却前下排式氮气置换法和"BOG+LNG"储罐冷却工艺,降低了LNG储罐冷却过程中BOG的氮气含量,提高了LNG接收站冷能利用效率,同时也降低了BOG的产生量,使之能更好地匹配于LNG接收站BOG的回收处理能力。现场实验结果表明:(1)下排式氮气置换法能够在LNG储罐冷却前将罐内氮气置换至合格要求;(2)"BOG+LNG"储罐冷却工艺能够有效降低LNG储罐冷却过程中BOG的产生量,使之不超过LNG接收站的回收处理能力,实现了LNG储罐冷却过程中BOG的零排放。该方法可作为LNG储罐投产试车的借鉴和参考。

BOG zero emission technology in the process of tank cooling in LNG terminals

Large-scale tanks in LNG terminals should be cooled gradually down to －162 ℃ before they are put into service. Before the cooling operation, tanks are full of nitrogen. In the process of cooling, therefore, a large amount of boil-off gas (BOG) with high nitrogen content is generated, but it can only be discharged directly to the flare because the technological system of an LNG terminal is incapable of recycling it. Besides, the BOG flow rates in the middle and late stages of LNG tank cooling are extremely high, being beyond the recycling capacity of the terminal, so a large amount of BOG is discharged to the flare. As a result, a large amount of waste is generated. To solve this problem, the Zhuhai LNG Terminal investigated the cooling modes of in-service LNG tanks in China and studied their precooling processes and innovatively proposed a down-discharge nitrogen replacement method before tank cooling and a "BOG+LNG" tank cooling technology. As a result, nitrogen content of BOG in the cooling process of LNG tanks is decreased and cold energy utilization efficiency of LNG terminals is improved. Meanwhile, BOG generated in the process of LNG tank cooling is decreased to well match the BOG recycling capacity of LNG terminals. On-site experimental results indicated that by virtue of down-discharge nitrogen replacement , the nitrogen inside the tank is replaced to a qualified level. With the "BOG+LNG" tank cooling technology, BOG generated in the process of LNG tank cooling is reduced effectively, without exceeding the recycling capacity of the terminal. Thus, BOG zero emission is realized in the process of LNG tank cooling. This method can provide a reference for operation and production of LNG tanks.