1. Introduction
Climate change has become a critical issue that restricts global sustainable development [
1]. The greenhouse gas (GHG) emissions from fossil fuel consumption are world-widely considered as a crucial source of climate change [
2]. Reducing energy consumption (EC) is believed effective to save energy, protect the environment, and achieve economic sustainability [
3,
4]. It is in line with the concept of global sustainability and has been gradually ratified by global decision-makers [
5,
6]. It has become one of the most critical topics in global economics [
7].
The industrial sector provides an indispensable material foundation for the survival and development of human beings. It is a powerful driving force for future social development. However, the industrial sector is considered energy-intensive [
8,
9,
10], accounting for 37% of total global EC in 2017 [
11] and more than 50% of global end-use EC in 2018 [
12]. According to the International Energy Outlook (IEO) 2019, the EC of the industrial sector will increase by more than 30% from 2018 to 2050 [
12]. The industrial sector includes three distinct industry types: energy-intensive manufacturing, nonenergy-intensive manufacturing, and nonmanufacturing [
13]. The ongoing long-term trend of increasing production in energy-intensive manufacturing drives the most rapid growth of EC in the industrial sector [
11]. The chemical industry is one of the representatives and the largest energy consumer among energy-intensive industries. The share of energy use in the chemical industry accounts for 19% and 14% of the total delivered EC in the industrial sector of the Organization for Economic Cooperation and Development (OECD) countries and the non-OECD countries, respectively [
13]. The share is estimated to rise to 20% in both regions in 2040 based on the IEO 2016 reference case [
13]. Therefore, how to adjust the balance between economic development and the environment is a common issue for all countries to address the challenges related to energy, resources, and environment [
14,
15].
China’s rapidly growing population and economy have driven the country to be the top energy producer and consumer [
16] and CO
2 emitter [
2] in the world. BP Energy Outlook 2018 estimates that China will still consume around one-quarter of world energy in 2040 despite the slowing growth of energy demand [
17]. Moreover, as one of the largest chemical manufacturing countries in the world [
18,
19], China’s share of industrial energy consumption will only decrease from 29% in 2018 to 24% in 2050, according to International Energy Outlook 2019 [
12]. The study of the EC of the sector, especially the energy-intensive chemical industry in China, has important implications for the industrial upgrading and transformation of energy structure in the industrial sector worldwide.
Urea is not only a critical chemical fertilizer but also a widely used industrial raw materials. The urea industry is one of the representative chemical industrial sectors in the world. Urea production in China began in 1958 [
20]. China is the world’s largest producer and consumer of urea, producing 61.9 million tons of urea and consuming over 55% of total urea produced along with the Southwest Asian region in 2016 [
21,
22]. With the growing urea demand, the EC of the urea industry has increased. It is significant to analyze the EC in the production process, estimate the situation of energy use, and find the balance between the industrial economy and environmental improvement to achieve sustainable development.
The life cycle assessment (LCA) is a “cradle-to-grave” or “gate-to-gate” evaluation of the environmental costs associated with a given product [
23,
24]. It differs from traditional evaluation methods such as the single-factor energy efficiency evaluation method [
25,
26] and the total-factor energy efficiency evaluation method [
27,
28,
29], in which the energy efficiency assessments are incomplete. The LCA offers a holistic view of environmental interactions that covers a range of activities from the extraction of raw materials to the production and distribution of energy, through the use, reuse, and final disposal of a product [
24]. It is regarded as a common decision-support tool for both policymakers and industry experts in assessing the impacts of a product or process [
24,
25,
26,
27,
28,
29,
30,
31]. A combination of EC and GHG emissions analysis with the comparison of life cycle performance of production is conducive to the study of energy-saving and emissions reduction [
32].
Most studies focus on the breakthrough of a specific production technology [
33,
34], or only consider the EC in the production stage [
35], resulting in the lack of the evaluations of the life cycle energy consumption (LcEC). In this study, the LCA and the production process of the urea industry are carried out to establish a life cycle framework of industrial urea, which includes three stages: raw material production stage, production stage, and waste-treatment stage. Moreover, by using the inventory data from seven different real industrial urea operations, the LcEC of the urea industry from raw material extraction to disposal is evaluated the first time in this study. The GHG emissions generated throughout the LcEC of urea production are also estimated. This paper provides a systematic, valid, and realistic judgment on the EC and GHGs impacts of the chemical industry, which can be used as a scientific basis for future development strategies and policies to promote sustainability in the industrial sector.
4. Conclusions
In this study, the LCA is applied to establish a life cycle framework of urea production, which divides the life cycle into three stages: the material preparation stage, synthesis stage, and waste-treatment stage. Based on the inventory data onto 7 real urea industries, LcEC of urea production, and LcGHG emissions generated by the process of EC are studied in this paper. The results show that the average LcEC is about 30.1 GJ/t urea. The EC
RMP, EC
PP, and EC
WD is about 0.388 GJ/t urea, 24.8 GJ/t urea, and 4.92 GJ/t urea, accounting for 1.3%, 82.4%, and 16.3% of average LcEC, respectively (
Figure 2). Coal plays the primary energy in the urea production, which supports 94.4% of LcEC (
Figure 3), and 77.9% of EC
RMP, 95.5% of EC
PP, and 90.6% of EC
WD (
Figure 4). Therefore, the synthesis stage is the dominant energy consumer, in which the supplying of steam consumes 62.0% of EC
pp, where 99.3% comes from coal consumption (
Figure 5). It reveals that the proportion of coal consumption in the life cycle of the urea industry is higher than that of coal consumption generally in China. Besides, due to the life cycle performance of EC in the urea industry, LcGHG emissions present a similar trend with LcEC (
Figure 6). In detail, CO
2 equivalent emissions of the material preparation stage, synthesis stage, and waste-treatment stage account for 82.9%, 90.5%, and 89.0% of GHG emissions of each stage (
Figure 7). Finally, the steam-consuming produces 1.35 t eq.CO
2/t urea GHG emissions, accounting for 61.8% of the total GHG emissions from the synthesis stage; in detail, the generation of CO
2 equivalent emissions by steam account for 93.6% of GHG emissions (
Figure 8).
Urea production technologies and processes have been widely studied, and the results have been visible progress. The performance of EC and GHG emissions in the life cycle of urea production illustrates that whatever process or technique used in the urea factory nowadays, the reduction of coal consumption will still be a crucial task for the urea industry. Improving the energy efficiency of steam-consuming equipment or replacing coal with green energy is an effective way to reduce coal consumption. Besides, the promotion of the application of green energy, such as renewable energy, will contribute to the reform of industrial energy consumption structure, reduce the consumption of primary energy, and relieve GHG emissions. This is a powerful driving force for the realization of sustainable industrial development in the future.