Low-carbon roadmap of chemical production: A case study of ethylene in China

Highlights

• A comprehensive analysis of life cycle CO₂ emissions of ethylene was performed.

• Four strategies of carbon emission reduction have been discussed.

• Life cycle CO₂ emissions of ethylene production in the last 17 years have been analysed.

• Scenario analysis was performed to achieve a low-carbon production of ethylene industry in China.

• Policy measures were propounded to achieve a sustainable development in chemical industry.

Abstract

The increasing emissions of carbon dioxide (CO₂) are primarily driven by the rapid expansion of energy-intensive sectors such as the chemical industry. This work selects ethylene, one of the most important chemicals, as a model study to represent the low-carbon roadmap of chemical production. Four strategies improving the efficiency of fossil resource usage, developing the technology for carbon capture and storage (CCS), CO₂ chemical conversion, and converting biomass resources into chemicals, are used to reduce CO₂ emissions. A comprehensive analysis of the life cycle CO₂ emissions of different ethylene production routes has been performed to compare their emission reduction potential. The results indicate that the BMTO (biomass to olefins via methanol-to-olefins) pathway releases the least CO₂ (− 1.3 t CO₂/t ethylene), while the CFTO (coal to olefins via Fischer-Tropsch synthesis) possesses the highest CO₂ emissions. Combining CCS with BMTO results in CO₂ emissions of – 8.2 t per t ethylene. Furthermore, we analysed the annual production and CO₂ emissions of ethylene in the last 17 years and integrated this real-time change with different pathways. The CO₂ emissions have decreased by 29.4% per t ethylene from 2000 to 2016 in China. However, the total amount of CO₂ emissions continuously increases in ethylene production industry. Given that China has promised to hit peak CO₂ emissions by 2030, a scenario analysis was performed. To achieve this goal, the ratios of BMTO, CO2MTO (CO₂ to olefins via methanol-to-olefins) or BETE (ethanol to ethylene pathway originating from biomass) pathways should increase by 1.0%, 1.2% and 1.1% annually from 2020, respectively. Then more than 500 million metric tons of CO₂ will be eliminated from 2020 to 2040. The results highlight the pivotal role that regulation and policy administration can play in controlling CO₂ emissions by increasing average technological level and turning to low-carbon routes in the chemical industry in China.

Introduction

Carbon dioxide (CO₂) has become the focus of world attention due to its impact on climate change [1], [2]. It has been reported that atmospheric CO₂ concentration has increased from approximately 280 ppm in 1750, at the beginning of the industrial era, to a level of 404 ppm in February of 2017 [3], [4]. The chemical industry, as one of the most energy-intensive sectors, is responsible for 16% of direct global CO₂ emissions [5]. Without decisive action, energy-related CO₂ emissions will keep increasing due to rapid development. For example, an annual average CO₂ growth rate for the chemical industry is 3.37% in China during the period 1980–2010 [6]. Therefore, a low-carbon roadmap for the chemical industry is essential to make the right decisions for reducing CO₂ emissions.

Given that the chemical industry is diverse and complex, the analysis has to be restricted to individual products [7]. Herein, ethylene is taken as an example because ethylene is one of the most important chemicals and its production process contributes 30% of energy to the chemical industry [8], [9]. Ethylene can be used to produce polyethylene, polystyrene, polyethylene terephthalate and polyvinyl chloride [10]. In China, the ethylene industry has expanded dramatically from an annual production of 4.7 million metric tons in 2000 to 19.5 million metric tons in 2015 [11], [12]. However, a gap between supply and demand of ethylene still exists. Annual production of ethylene only reached 51.9% of the ethylene equivalent consumption in 2015 [12]. Therefore, the demand for ethylene will continue to show an increasing trend in the short term. To tackle high environmental impact and booming expansion associated with ethylene production site, Chinese government and researchers have paid significant attention to upgrading and restructuring of ethylene plants [13], [14].

Ethylene is conventionally produced by steam cracking (SC), the feedstocks for which can be a broad range of hydrocarbon feedstocks [15]. Ethylene is predominantly from SC of mixed petroleum (PSC) in Europe and Asia, while North America and the Middle East adopt light hydrocarbons as feedstocks for SC [7], [15]. This process is crucially dependent on petroleum and emits considerable CO₂, which conflicts with the growing pressure for fossil fuel reduction and climate change mitigation. Therefore, alternative routes and feedstocks are explored and developed to adapt to the sustainable development of the olefins industry [15]. For example, China, with large coal reserves, focuses on the conversion of coal-based methanol-to-olefins (CMTO) in an attempt to reduce reliance on imported petroleum [16]. By techno-economic analysis (TEA) and life cycle assessment (LCA), Xiang et al. [17] found that CMTO is economically competitive and independent of petroleum, however, controversies have accompanied this route because CMTO has higher energy consumption and generates more greenhouse gas (GHG) emissions than the oil route [18], [19]. Furthermore, Chen et al. [20] found CMTO leads to about 2.5 times GHG emissions of PSC and NG-based methanol-to-olefins (NMTO), and the eco-efficiency of NMTO is the highest among the three production routes.

Different from China, the U.S.A. has developed SC of natural gas liquids (NGLs) to ethylene due to its vast shale gas reserves. He et al. [21] conducted a techno-economic-environmental analysis of SC of shale gas (SSC) to olefin in the U.S.A. and found that shale regions of the U.S.A. could supply feedstocks for ethylene for more than 130 years. SSC has a little higher GHG emissions in comparison with SC of conventional natural gas (NSC) [8], [21].

SC and methanol-to-olefins (MTO) based on natural gas, shale gas, and coal can reduce dependence on petroleum. However, these technologies still emit CO₂. CO₂ emissions in CMTO pathway are even higher than that of the PSC pathway. Carbon capture and storage (CCS) was used to reduce CO₂ emissions. By LCA, Xiang et al. [19], [22] found that CMTO process with CCS is competitive in product cost as well as low GHG emissions. However, the eco-efficiency of integration of CCS with CMTO is lower than PSC and NMTO routes [20].

Switching from fossil sources to the renewable resources such as CO₂ and biomass, particularly bio-waste, is attractive since it can establish a sustainable and low carbon centre to produce chemicals [23], [24]. Some researchers have developed work associated with ethylene production adopting biomass as raw material. Hong et al. [9] performed a LCA to estimate corn- and cassava-based ethylene production. Liptow et al. [25] evaluated the environmental burden of biomass-based ethylene production routes via gasification and fermentation. Ghanta et al. [26] estimated environmental impacts of ethylene production from naphtha, ethane, and ethanol. These work focused on varietal environmental impacts of ethylene production, which makes a limited focus on CO₂ or GHG emissions.

By LCA and TEA, the previous work assessed CO₂ emissions of specific pathways for ethylene production. However, it is not appropriate to directly compare CO₂ emissions among these pathways due to different system boundary, functional unit, location, allocation method and study time, as shown in Table 1. Given this, Ren et al. [27], [28] have made an extensive comparison of different olefins production routes in terms of energy use and CO₂ emissions by a simplified method of LCA. Some processes, such as transportation, are excluded in the study. Inaccuracy will be inevitable without regard to the uniform functional unit and appropriate system boundary. Also, some technologies, such as catalytic pyrolytic process (CPP), Fischer-Tropsch-to-olefins (FTO), and MTO of CO₂ have rarely been evaluated for energy consumption and CO₂ emissions. Therefore, a comprehensive analysis of CO₂ emissions of different pathways is imperative for guiding process development and promoting a CO₂ emissions reduction in the ethylene industry.

Furthermore, with a rapid increase of annual ethylene production in China, work has also been devoted to the ethylene industry to reduce energy consumption and CO₂ emissions. By replacing with the advanced equipment, implementing delicacy management, enlarging the scale of steam crackers, adjusting ratios of feedstocks and other technology, energy consumption has been reduced from 32.7 GJ/t ethylene in 2000 to 23.6 GJ/t ethylene in 2016 (energy consumption was calculated by authors in Section 4.2). It still has potential to reduce energy consumption and CO₂ emissions of ethylene production by optimizing production structure, replacing with advanced technology and attached technologies, such as air preheating technology of cracking furnace, enhanced heat transfer technology, etc. [29]. However, few studies integrate this real-time change with a comparison of different pathways.

The focus of this study is to provide a comprehensive analysis of the life cycle CO₂ emissions of different ethylene production pathways to tackle deteriorating climate change and rapid ethylene development. This study first describes and analyses alternative production approaches of ethylene, then provides a comparative LCA of the selected routes for evaluating CO₂ emissions of each option. Scenario analysis is performed combining discussion of CO₂ variation with technology, policy, and historical data, to identify the significant potential for reducing CO₂ emissions, and a practical solution facing enormous demand of ethylene and pressure of climate change is proposed.

The study is structured as follows. A brief description of alternative production methods and system boundary of ethylene is presented in Section 2. Details of life cycle methodology and life cycle inventory are presented in Section 3. Section 4 describes results of the carbon emissions of the model and the key findings of this study. Then, we come to the conclusions in Section 5. Finally, some policy measures are proposed.