Life Cycle Assessment And Optimization Analysis Of Different Liquefied Natural Gas (LNG) Usage Scenarios

Coal is used as the basic energy source in China, however, the exploitation and utilization of coal has led to serious environmental impacts. Compared to coal and oil, the efficiency of natural gas is higher, which also has less pollution under the condition of the same calorific or output. Compared to other types of natural gas, Liquefied Natural Gas (LNG) has easy transportation and storage, higher safety performance, and large calorific value. So the application scope of LNG is more and more extensive around the world. Although LNG is a kind of clean energy, its gasification and transportation stages, expecially when LNG is used in different ways, the variations of CO2, NOx, and SO2 emissions can cause different degrees of effects on the environment. At first, the paper applies Life Cycle Assessment (LCA) method to compare with the environmental impacts of the coal, oil and LNG in the whole life cycle, and then this paper in depth evaluates the environmental impacts of different life cycle stages of LNG and different LNG usage scenarios. Global warming potential (GWP), acidification potential (AP), eutrification potential (EP) and photochemical oxidant potential (POCP) are chosen as the environmental impact indicators. We can use sensitivity analysis in the Gabi 5.0 software to confirm the effect degree for the final environmental impact about the input energy and material, as well as exporting polluants. We combine the environmrntal impact of LNG with urban planning to optimize LNG allocation proportion. The research contents of this article are as follows:(1) At first, the environmental impacts of coal, oil and LNG are compared and analysed from mining to usage stages. The results show that coal has the largest impacts with regard to the four environemtnal impact categories, and oil has higher environmental impacts of AP, EP and POCP than LNG, except for GWP.(2) The life cycle inventories of three LNG usage scenarios are constructed:for hydrogen production (S1); for electricity generation (S2); and, for vehicle fuel (S3), including all the input raw materials, energy and output pollutants of the gasification, transportation and usage stages. The LCA on the above three scenarios are then conducted including the gasification, transportation and usage stages. The results indicate:In the S1, the gasification stage has higher normalization results of GWP, AP, EP and POCP than the usage stage; In the S2, the usage stage has higher normalization results of GWP, AP, EP and POCP than the gasification stage; In the S3, to the normalization result of GWP, the usage stage is higher than the gasification stage, while to the normalization results of AP, EP and POCP, it is the opposite; and, the transportation stage has the lowest normalization results of GWP, AP, EP and POCP.(3) The LCA comparisons of the three LNG usage scenarios are conducted. The results indicate that the largest environmental impact category for the three scenarios is GWP. For GWP, S2 and S3 have the highest contributions, and for AP, EP and POCP, S2 has the highest contributions. The sensitivity analysis shows that the electricity production and supply has the largest contribution to the four environmental impact categories, and the other larger contribution factors are carbon dioxide emissions and nitrogen oxide emissions.(4) We optimize the LNG allocation proportion, combining the environmental impact with urban planning. When LNG is allocated 25% for hydrogen production,73% for electricity generation and 2% for vehicle fuel, the impact of GWP is minimum, and the total cost is minimum.