# Environmental Impact of the Natural Gas Liquefaction Process: An Example from China
## 2 Method
### Life Cycle Inventory  
Inventory analysis is the expression of basic data throughout the life cycle and is the basis of the life cycle impact assessment. The process of establishing the inventory is to establish the system input and output of the corresponding functional unit for each process unit within the determined product system. As the production processes of natural gas liquefaction plants in China are very similar, a natural gas liquefaction plant in the Shanxi Province was used as a model for the process flow. Based on this process flow, the environmental impact of natural gas liquefaction in the eight provinces and in China overall could be assessed. The specific process flow of natural gas liquefaction is shown in Figure 3. According to the functions of each step in liquefaction production, the entire production process was divided into three phases: pretreatment of gas, liquefaction and storage, and off-loading packing. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Province** & **Production Capacity (10\({}^{4}\) m\({}^{3}\)/d)** & **No.** & **Province** & **Production Capacity (10\({}^{4}\) m\({}^{3}\)/d)** \\ \hline
1 & Heilongjiang & 48 & 10 & Xinjiang & 523 \\
2 & Jilin & 80 & 11 & Inner Mongolia & 1854 \\
3 & Liaoning & 10 & 12 & Shaanxi & 1650 \\
4 & Gansu & 55 & 13 & Shanxi & 685 \\
5 & Shandong & 100 & 14 & Ninggia & 630 \\
6 & Hubei & 12 & 15 & Henan & 225 \\
7 & Qinghai & 65 & 16 & Sichuan & 405 \\
8 & Yunnan & 6 & 17 & Hebei & 407 \\
9 & Guangdong & 60 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Provinces of natural gas liquefaction production and the capacity of each province in China. Figure 2: Proportion of the liquefied natural gas production in different provinces in China. According to the evaluation principles of LCA, the system boundary for LCA in the natural gas liquefaction process includes the entire process, from the input of raw gas to the output of LNG. According to the output of pollutants in each step of the process flow, we determined the life-cycle pollutant emission map of the natural gas liquefaction plant, as shown in Figure 4. It should be noted that the pollutant emissions during natural gas liquefaction also include other substances, such as N\({}_{2}\), which are not shown in Figure 4. This is because this part of the emission is minimal, and its impact on the environment is low. Therefore, in the calculation process, they were ignored. In addition, from the perspective of energy input, electricity is an important part of energy consumption in natural gas liquefaction. However, the environmental impact caused by it accounts for less than 1% of the total environmental impact, so it was omitted too. It can be seen from Figure 4 that, in phase A, the main pollutant emissions include dust, CO\({}_{2}\), CO, CH\({}_{4}\), SO\({}_{2}\), H\({}_{2}\)S, and NH\({}_{4}+\). In phase B, the main pollutant emissions include CO\({}_{2}\) and CO. In phase C, the main pollutant emission includes CH\({}_{4}\). In addition, based on the gas composition report of natural gas sources in the Shanxi Province, we categorized the pollutant emission inventory of the LNG production process in the Shanxi Province per unit of output (Table 2). Figure 4: Emissions of pollutants from natural gas liquefaction plants throughout their life cycle. Phase A is pretreatment of feed gas, Phase B is liquefaction, and Phase C is storage and off-loading package. Figure 3: Flow chart of the natural gas liquefaction process. In fact, the amount of pollutants discharged by each province in the natural gas liquefaction stage was determined by the gas composition of the province's raw material gas. Therefore, in accordance with the liquefaction process of Shanxi Province, we summarized the pollutant emission inventories of LNG production in Xinjiang, Inner Mongolia, Shaanxi, Ningxia, Henan, Sichuan, and Hebei under unit output based on the gas composition reports of the eight provinces of China.