Introduction
Climate change, of which greenhouse gas emission is the main driver, is one of the most urgent challenges humanity is currently facing. As depicted in Figure 1, the atmospheric CO2 concentration has been rising rapidly since the start of the measurements in March 1958, with an average increase of approximately 2 ppm per year over the past decade (Tans, 2019). In 2016, the atmospheric CO2 concentration stayed above the symbolic 400 ppm mark all year round for the first time, corresponding to a 30% increase compared to the pre-industrial (before circa 1750) levels of 270 ppm (Betts et al., 2016). The Paris Agreement under the United Nations Framework Convention on Climate Change adopted in 2015 illustrates the worldwide commitment to reduce greenhouse gas emissions to mitigate global warming, but this will require an order-of-magnitude increase in public and private investments in research and development between 2019 and 2030 (Rockström et al., 2017).
Strategies to reduce CO2 emissions can be divided in four categories that focus on either avoidance of CO2 emissions or binding the emitted CO2 in a natural or non-natural sink. The first category is improving energy efficiency, which currently provides the greatest return on investment and has already been successfully applied in many industrial contexts. Although this approach still has potential, annual improvements of 1-2% will not be sufficient to meet the climate targets. The second category, using non- or low-carbon energy sources, (e.g., solar, wind, geothermal), is at large scale still challenging due to the fluctuating nature of the energy supply and the slow rate at which the electricity production is becoming more renewable. Carbon Capture and Sequestration (CCS), i.e., a series of technologies combining CO2 capture from large point sources such as power plants, transportation to a storage site, and sequestration into a (natural) sink, is the third category, but its potential is currently rather limited due to technical and economic hurdles (Spigarelli and Kawatra, 2013; Leung et al., 2014). The fourth category is Carbon Capture and Utilization (CCU), in which CO2 is converted to (high-value) products. This category can be considered as a special case of the third category with the utilization part acting as a non-natural sink (Whipple and Kenis, 2010; Kuhl et al., 2014; Schouten et al., 2014).
CO2 is a thermodynamically very stable molecule and thus a substantial input of energy combined with effective reaction conditions and active catalysts are required for its conversion, c.f. Figure 2. To obtain the desired overall negative CO2 balance, the energy required for its conversion should originate from non- or low carbon energy sources. Hence, the development of CO2 conversion processes has focused on minimizing the required energy input by using the non or low-carbon energy sources in the most efficient way possible. According to a recent study (Voltachem, 2016), the development of new products through the application of innovative technologies powered by renewable energy is one of the main drivers for “electrification” of the chemical industry, i.e., replacing thermal and chemical energy by electrical energy. Other main drivers are economic benefits and improved sustainability through the reduction of feedstocks, by-products, waste, energy use, solvents, and CO2.
Among all the proposed methods for converting CO2, which have as common advantage the ease of integration of non- or low carbon energy sources, electrochemical methods are considered to be the most promising (Endrődi et al., 2017), as several advantages have been claimed compared to the other methods: 1) they can be conducted at ambient conditions (allowing for rapid changes in the production rate as the availability of the renewable energy changes), 2) by a careful selection of the electrocatalyst, electrolyte and operating conditions, it is possible to drive the electrochemical conversion of CO2 toward the desired products, 3) the chemical consumption can be minimized by recycling the electrolytes, 4) the reaction systems are compact, modular and hence scale-up is relatively straightforward, and 5) the electrons are directly used for product formation. However, there are clear challenges for electrification, such as the overall high cost of electricity, the large investment costs, the often poor selectivities and low conversions related to low reaction rates (resulting in large reactor volumes needed for a world-scale plant), the technical and economic feasibility of turning plants on and off safely on short notice, etc. This implies that there is a lot of skepticism whether electrification of the chemical industry is actually feasible (Van Geem et al., 2019; Gani et al., 2020) or whether it is another hype like the numerous ones that have been presented in the last two decades (Banholzer, 2012; Banholzer and Jones, 2013).
The goal of this study is to explore whether the electrochemical conversion of CO2 can be a viable alternative production route of ethylene, which is the key building block of the chemical industry and representative for products with a reasonably high added value. First, a short overview is given of the CO2 reduction process and the performance trends with the focus on ethylene formation. Next, a techno-economic model is developed for a CO2 conversion plant integrated with CO2 capture from a blast-furnace flue gas stream. With this model, the economic competitiveness of this alternative ethylene production route is compared against the current state of the art for ethylene production, i.e., naphtha-based steam cracking, under both current and future conditions. Finally, the CO2 avoidance potential of the process is assessed based on a Life Cycle Analysis, adopting a cradle-to-gate boundary.