1. Introduction
With the increasing emphasis on environmental protection across the world, the advantages of natural gas in the clean production of energy have been gradually highlighted owing to its green environmental protection characteristics, and the prospects for its development are very promising [
1,
2]. The 13th Five-Year Plan for Energy Development, issued by the National Energy Administration of China, has predicted that natural gas will account for more than 10% of the energy consumption structure, and natural gas consumption will double by the year 2020. By the year 2030, the proportion of natural gas consumption will increase to 15%, consumers of natural gas may span across 500 billion square meters, and there will be a large scope for industrial development [
3,
4,
5]. In China, the coal-to-gas project has led to a sharp rise in the natural gas supply and demand. Furthermore, it has increased the demand for liquefied natural gas (LNG) [
6].
In fact, LNG has marked advantages over gaseous natural gas. After liquefaction, the volume of natural gas is greatly reduced to about 6% of the volume of the same amount of gaseous natural gas. From the perspective of storage and transportation, LNG is more efficient and convenient [
7]. In addition, in the absence of a gas pipeline, LNG is transported by tankers to places it is difficult for the pipeline to reach in “point supply” mode, and local people can also use natural gas. Therefore, the use of LNG has begun to become widespread and the demand for it is gradually increasing; thus, the production of LNG has also witnessed an expansion [
8].
Life cycle assessment (LCA) is an environmental impact assessment method set in accordance with the international ISO 14040 standard (1997, Environmental Management-Life cycle assessment-Principles and framework). It is an environmental impact assessment for the entire cycle of a product, from raw material procurement, to processing, manufacturing, sales, use, and disposal or recycling, and has a wide range of applications. In the field of architecture, Erlanfsson et al. [
9] put forward a number of significant characteristics of the LCA methodology for buildings, constructions, and operation services, and then checked whether it was in accordance with “today’s practice and development needs”. In the field of transportation, Ally et al. [
10] conducted a Life Cycle Assessment of Perth’s fuel cell bus trials to determine the overall environmental footprint and energy requirements by studying all stages of the entire transportation system, including hydrogen infrastructure, bus manufacturing, operations, and end-of-life disposal. In the field of energy, Shi et al. [
11] provided a comparative LCA of remanufactured liquefied natural gas (LNG) and diesel engines, and identified the processes that contribute most to energy consumption and environmental impacts during the life cycles of the two engines.
As far as the liquefaction of natural gas is concerned, the process is very complicated and involves many important phases. At different phases, industrial pollutants, such as carbon oxides, sulfides and nitrogen oxides, are generated, and these industrial pollutants have an adverse impact on the environment. As a result, the study of environmental issues related to LNG has gradually become important. In terms of LNG-related technologies, Kanbur et al. [
12] proposed an LNG cold-utilized micro-cogeneration system which combines a Stirling engine and a micro gas turbine. Thermodynamic efficiencies and power generation rates have been studied in thermodynamic analyses, whereas carbon dioxide emission rates and the relevant emission reductions have been studied in environmental analyses. He et al. [
13] studied the energy waste in the regasification process of the LNG re-liquefaction terminal and proposed that the use of LNG cold energy may significantly improve the energy and exergy efficiencies of the system. Concerning the use of LNG, Langshaw et al. [
14] evaluated the environmental and economic performance of LNG as a transition fuel to replace heavy-duty truck diesel, and proposed that the reduction in greenhouse gases by LNG heavy goods vehicles can only be achieved with an improvement in vehicle efficiency. Shi et al. [
15] used the LCA method to quantify the energy-saving and environmental emissions of a remanufactured LNG engine and a newly manufactured diesel engine; both engines were compared in terms of material usage, cumulative energy requirements, and environmental emissions over their entire life cycle. Beyer et al. [
16] evaluated the environmental hazards of the Hammerfest LNG wastewater discharge. Xiao et al. [
17] reported that the main environmental impact of LNG projects is the wastewater, waste gas, and waste products generated during liquefaction. Through an analysis of the LNG production process, the main pollution discharge phases were identified, and the environmental protection measures that the project may undertake were discussed.
In summary, although there are many related studies, there is little research addressing the issue of industrial pollutant emissions during the natural gas liquefaction process, and the resulting environmental impact issues have not been quantitatively analyzed. Simultaneously, considering the complexity of the natural gas liquefaction process, there is no comprehensive and systematic evaluation of the current academic research. Therefore, this article chose the LCA method to comprehensively and carefully analyze the environmental impacts of wastewater, waste gas, and waste emissions during the natural gas liquefaction process in China and, based on this, proposed policy recommendations.
4. Conclusions
This article uses the LCA method to study the problem of industrial pollutant emissions during the natural gas liquefaction process in China. The production data were collected for 86 LNG liquefaction plants in China and the gas quality reports were collected for eight major natural gas liquefaction production provinces (Xinjiang, Inner Mongolia, Shanxi, Shanxi, Ningxia, Henan, Sichuan, and Hebei). Using a natural gas liquefaction plant in Shanxi Province as the process flow reference, we performed a series of data processing and calculations. We first conducted a quantitative study on the environmental impact of natural gas liquefaction. Specifically, the environmental impact potential of natural gas liquefaction at the unit output level of the listed eight provinces, and the environmental impact potential of natural gas liquefaction at the annual output level, were calculated and compared. Based on this, China’s GWP, AP, EP, POCP, DP, and total EI potential were calculated, and the following research conclusions were drawn.
First, 93.60% of China’s natural gas liquefaction output was concentrated in eight provinces: Xinjiang, Inner Mongolia, Shanxi, Shanxi, Ningxia, Henan, Sichuan, and Hebei. Among them, Inner Mongolia and Shaanxi had the largest number of liquefaction plants, and the sum of LNG production in the two provinces exceeded 50% of the total national output. The reason for this may be attributed to the relatively low construction cost of the LNG liquefaction plant in Inner Mongolia, the small initial investment, and local policies that are conducive to the construction of these plants. Shaanxi Province had a large amount of natural gas extraction. Building a liquefaction plant nearby may reduce transportation and time costs.
Second, in terms of the unit LNG production, the GWP, AP, EP, POCP, and DP of each province explained the gas composition of LNG production gas sources in each province. Among them, Inner Mongolia, Ningxia, and Shaanxi contained the largest greenhouse gas emissions, and Sichuan had the lowest greenhouse gas emissions. Gas sources in Xinjiang, Hebei, and Henan contained the greatest amount of acidified substances, and the remaining five provinces had low amounts. Xinjiang, Hebei, and Henan collectively had the most gas eutrophication in the gas source, and the remaining five provinces had low amounts of gas eutrophication. Shaanxi, Ningxia, and Inner Mongolia had the highest levels of photochemical smog, whereas Sichuan had the lowest levels of photochemical smog. Gas sources in Sichuan, Shaanxi, and Inner Mongolia contained the greatest amount of dust, whereas Shanxi had the least amount of dust.
Third, in terms of annual LNG production, the GWP, AP, EP, POCP, DP, and total EI potential in Xinjiang, Inner Mongolia, Shanxi, Shanxi, Ningxia, Henan, Sichuan, Hebei, and China indicated different environmental issues in various provinces and in China overall. Specifically, in terms of greenhouse gas emissions, Inner Mongolia and Shaanxi had the largest emissions, and the resulting greenhouse effect may be the most severe. In terms of acidifying substance emissions, Hebei and Xinjiang had the greatest emissions and may, therefore, experience the greatest acidification effect. In terms of eutrophication emissions, Xinjiang, Hebei, and Inner Mongolia had the greatest emissions, and the eutrophication effect of the water bodies was a major concern. In terms of photochemical smog emissions, Inner Mongolia and Shaanxi had the greatest emissions, and the resulting photochemical pollution effects were the most evident concerns. In terms of dust emissions, Inner Mongolia and Shaanxi had the greatest emissions, and the dust pollution effect was the most severe. It was evident from the total environmental impact potential that Inner Mongolia and Shaanxi were the most affected by severe environmental pollution during the natural gas liquefaction process.
Based on the above research conclusions, this article makes the following three suggestions.
First, China’s coal-to-gas project makes use of natural gas instead of coal, and this can indeed reduce the effect of environmental pollution. However, when assessing the actual benefits, the environmental pollution caused by the liquefaction of natural gas should be considered. A reduction in pollution caused by natural gas liquefaction should be achieved so that the coal-to-gas project can be effectively implemented.
Second, if various pollutants are discharged during natural gas liquefaction, they must be properly treated, recycled, and reused, which may reduce environmental pollution. Take methane as an example—in the natural gas liquefaction process, methane is both an energy input and a pollutant that is emitted. Therefore, some liquefaction plants in Inner Mongolia have begun to optimize the technology link, considering the recovery and reuse of methane, thereby reducing production costs. In addition, the resource utilization of pollutants may bring additional income to natural gas liquefaction plants or reduce the production costs for enterprises using these pollutants.
Third, from the perspective of the five types of environmental impact potentials, there are many differences in the components of natural gas sources in different provinces. Additionally, according to these different components, natural gas liquefaction plants should be targeted to optimize production technologies and reduce environmental pollution. For example, the plants in Inner Mongolia and Shaanxi should pay attention to the reduction in and recovery of greenhouse gases. Moreover, the plants in Xinjiang should consider reducing and reusing acidified pollutants.
Despite these findings, this article has the following limitation. Due to the difficulty of data collection, this article cannot yet quantify the environmental impact of the entire LNG production process. However, one of the major problems of the LNG production is the leakage of methane in the pipelines. Therefore, in further work, the authors plan to extend the boundary of the research and analyze the environmental impact of the entire LNG production process.