A Combined Research On LNG Cold Energy Utilization And CO₂Recovery

Liquefied natural gas (LNG) is a kind of mixed cryogenic liquid (-162℃、0.14MPa), which is mainly made of methane through refrigerated liquefied process. The cold energy that contains in the LNG is about840kWh/tLNG. Accordingly, the high quality cold energy contained in the LNG imported by Liaoning Province every year is about75.6×109kWh.The LNG can not be used directly unless regasificated. During regasification, LNG will release huge high quality cold energy to sea water in the traditional process, which wastes cold energy and impacts marine ecological environment. Building high-performance power cycles in cryogenics situations to achieve power generation is one of the best ways of cold energy utilization. Recently, along with the increasingly greenhouse effect, the combined research on LNG cold energy utilization and CO2recovery causes great interest among relevanted researchers globally. Dashiqiao city in Liaoning Province possesses the85%amount of magnesite of our country. Through the smelting process, about1.1CO2is released during every ton’s production of magnesium oxide materials. However, according to the process of fused magnesia, the exhaust gas is released into the atmosphere directly without any filtering. Is there any possibility can we use the LNG cold energy to recover CO2? This paper will find the answer of the combination research on LNG cold energy utilization and CO2recovery.Initially, basing on the research of LNG cold energy characteristics and gasification curve distribution, combining the releasing of cold energy and physical exergy, as well as the characteristics of Organic Rankine cycle (ORC), this paper put forward a kind of technological process, which combines the twin-stage Rankine cycle power generation system using LNG as cold source, exhaust gas as heat source, and capturing CO2and liquid air respectively from exhaust gas. Then, this paper starts the thermodynamic analysis including the computing of net generation capacity, exergy efficiency and cold energy utilization efficiency, researching on LNG gasification pressure, and organic Rankine cycle working fluid selection. Finally, get the conclusion.Research shows that:(1) The process combining LNG and the exhaust gas in magnesite smelting process as the cold and heat source is in accordance with the basic laws of thermodynamics, and the theory is feasible to get the product of liquid carbon dioxide;(2) The cryogenic organic Rankine cycle (ORC) is of high-performance. The net generation capacity is46.34kWh/tLNG, the coefficient of cold energy utilization is44.23%, the product liquid air (-194℃) is233.30kg/tLNG. The liquefaction coefficient of air Zpr=5.71%. The product liquid carbon dioxide is129.29kg/tLNG;(3) The main influence factors of LNG cold energy generating capacity is the gasification pressure. The lower the gasification pressure is, the net cold energy generating capacity is higher. For example, when gasification pressure is increased from0.6MPa to3.0MPa, the net generating capacity is reduced from46.30kWh/tLNG to12.63kWh/tLNG, the coefficient of cold energy utilization is reduced from44.23%to31.38%.