Abstract
Cryogenic generation is one of the most important ways to utilize cold energy during LNG (liquefied natural gas) regasification. This paper fundamentally investigates LNG cryogenic generation with the Stirling cycle method based on previous studies. A basic process of LNG cryogenic generation with the Stirling cycle was presented initially with seawater and LNG as heat source and heat sink. And its thermodynamic analysis was performed to verify the theoretical feasibility of the Stirling cycle method. The generating capacity, the exergy efficiency and the cold energy utilization efficiency of the basic process were also calculated. Subsequently, the influences of evaporation pressure on net work, equipment performance and comprehensive efficiency of cold energy utilization were discussed and the effect of LNG mass flow as well as the ambient temperature was also studied. Finally an improved process of LNG cryogenic generation with Stirling cycle method combined with an air liquefaction process is proposed as feasibility in improvements of the basic process.
1. Introduction
LNG (Liquefied natural gas) has a large temperature difference (about 182 K) between ambient air and water, and thus contains considerable cold energy which can be recovered for electricity generation, air separation, food freezing etc. Using cold energy to generate electricity is more efficient than using waste heat with the same temperature difference [1]. Thus, cryogenic generation is the most effective way to recover the cold energy of LNG. What’s more, many previous studies about more effective way to use LNG cold energy for power generation have been conducted. Wang et al. [2] proposed an ammoniaewater power system with LNG as its heat sink to utilize the low grade waste heat, and optimized the key thermodynamic design parameters. Szargut et al. [3] investigated three variants of cryogenic power plant and studied the influence of the changing ambient temperature on the efficiency of the cryogenic power plant. Dispenza et al. [4,5] proposed an innovative process which uses LNG as the cold source in an improved CHP (combined heat and power) plant, and analyzed the performance based on thermodynamics and economics. Meanwhile, to improve the cold energy recovery efficiency of an LNG cryogenic power plant, some studies have been carried out. Tsatsaronis and Morosuk [6,7] presented a detailed advanced exergetic analysis of a novel cogeneration concept that combines LNG regasification with the generation of electricity. In recent years, researches for the production of electricity form LNG by utilizing its cryogenic energy have developed in China. Liu and Guo [8] proposed a novel cryogenic cycle by using a binary mixture as working fluids and combined with a vapor absorption process for LNG cold energy recovery. Lu et al.[9] proposed a cascading power cycle with LNG directly expanding consisting of a Rankine cycle with ammoniaewater as working fluid and a power cycle of combustion gas to recover cold energy of LNG.
5. Conclusions This paper discusses the Stirling cycle with nitrogen applied for LNG cryogenic power generation. The thermodynamic process of the basic process is analyzed. The parameters of the regasification process and the power cycle, as well as their sensitivities are also studied. More experiments are needed to study the Stirling cycle further.
By calculation, when LNG flow rate is raised to be 60 t/h (vaporization pressure is 3.0 MPa), the net work output of the basic process is 1667 kW. Compared with Senboku Daini cryogenic power station in Japan, the power output of the basic process is increased by 14.97%. The increase of output power can be explained by a series of assumptions, such as isothermal compression, isothermal expansion, etc.
