The Grand Challenge

Fossil fuels (oil, coal and natural gas) utilization for power generation and chemical production have led to a profound global economic growth since the turn of the 20^th century. However, the massive use of fossil fuels resulted in an increase in the concentration of carbon dioxide in the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) recognized that this exponential CO2 concentration in the atmosphere is the dominant factor for global warming and climate change. Figure 1 shows the continuous increments of CO2 level since the industrial revolution until today (blue line), which was found to relate to the global mean surface temperature increase (red line). It can be seen that the current CO2 level is exceeded 400 ppm, which is an alarming level that was only reached around four million years ago, when the global temperatures were 2 - 4°C warmer and sea levels were 10 - 25 meters higher than they are today. Continuous emission of greenhouse gases will therefore put the Earth's ecosystems on a trajectory towards rapid climate change that is catastrophic and irreversible.

Figure 1. CO2 level since the industrial revolution until today (blue line), the global mean surface temperature (red line) ^[1].

As a response to this alarming crisis, international efforts such as the United Nations Framework Convention on Climate Change (UN, 1992) and the Paris Agreement (UN, 2015) have set clear goals to limit greenhouse gases emissions to a certain level. For instance, the Paris Agreement, which was signed by 197 countries, has stated the aims of “Holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels”. A recent study ^[2] reveals that to achieve this goals, the CO2 level in the atmosphere would need to be

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reduced from the current level of ~410 to 353 ppm. The CO2 level in the atmosphere was last 350 ppm in the year 1988, and the global Earth surface temperature was then +0.5°C relative to the preindustrial period. This implies that there are urgent needs for innovative technologies that enable removing CO2 from the atmosphere as well as providing alternative ways for supplying human needs from energy and products in a more sustainable manner.

According to the 2018 Energy Outlook issued by the international energy agency IEA ^[3], the global energy demand projected to rise by 25% by 2040, and hence fossil fuels will most likely remain the backbone of the global energy system for the coming decades. Therefore, urgent decarbonization solutions are needed to mitigate CO2 emissions from fossil fuel utilization.

Several options can be used to mitigate CO2 emissions from fossil fuel utilization that include 1) improving the process efficiency, 2) switch to renewable energy sources, 3) replacement of fossil fuel with a low carbon intensity sources (e.g. coal by natural gas or hydrogen) and 4) applying Carbon Capture, Utilization and Storage (CCUS).

The International Energy Agency (IEA) has developed pathways scenarios for the global energy system consistent with the Paris Agreement in limiting the temperature increase to 2.0°C, while securing sufficient energy supply to the society. Figure 2 presents the different pathways and their overall forecasted contributions in CO2 emission reduction. As it can be seen, the energy efficiency and CCS together represent more than a half of the total emission reduction. Energy efficiency is the most important factor in this pathway options as it reduces the overall demand, however, it cannot lead to deep decarbonization without support from other pathways.

Figure 2. The different pathways for the global energy system consistent with the Paris Agreement and its overall contribution in CO2 emission reduction ^[3].

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CCUS is a crucial element in most mitigation scenarios to meet the global warming targets ^[4]. CCUS technologies can be applied to large stationary point sources where a capture system can extract CO2 directly from a gas stream, such as power generation plants as well as most industrial sectors (as shown in Figure 3). In fact, CCUS is the prime option for the hard-to-abate energy intensive industries (such as cement and steel), in achieving deep emissions reductions.

Figure 3. Global CO2 emissions by different sector, 2018 and the potential of CCUS in the different sectors ^[5].

Four main categories have been explored for CCUS technologies: 1) post-combustion, 2) pre-combustion, 3) oxy-pre-combustion, and 4) chemical looping process ^[6]. Figure 4 illustrates the different CO2 capture technologies applied to power plant. The post-combustion option achieved by CO2 separation from the flue-gaseous after the combustion process, and it can be applied to currently installed fossil fuel-based power plants. Pre-combustion capture involves the separation of CO2 and H2, resulting in a hydrogen-rich fuel that can be used in many different applications. The oxy-fuel process uses pure oxygen from an air separation unit (ASU) for combusting the fuel instead of using air, therefore, it resulted in a pure CO2 stream

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after combustion that is ready from compression, transportation, storage or utilization. As it can be seen all these three options require gas separation units that is very costly and will decrease the global efficiency of the plant that has a large effect on the economics ^[7]. Chemical looping process avoids the need for gas separation unit and therefore the energy penalty for CO2 capture is very low as compared to other techniques. The focus of this PhD thesis is on the chemical looping processes.

Figure 4. Various CO2 capture technologies applied to a power plant ^[8].

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1.2 Chemical Looping Process: a Promise Meets Practical Challenges