The grand challenges in carbon capture, utilization, and storage

Introduction

If we were in a global war against climate change, we would carry out large-scale carbon capture, utilization, and storage (CCUS) (Smit et al., 2014). Some argue that if we fight the war against climate change via CCUS, this implies that we are promoting the continued use of fossil fuels instead of replacing fossil fuels by renewable energy such as solar and wind. At present, the contribution of fossil fuels in our energy supply is over 80%, while the renewable is only 10% (International Energy Agency (IEA), 2013). Most, if not all, of the energy scenarios predict an increase in the share of the renewables, but in absolute numbers the fossil fuels will continue to provide most of our energy needs in the foreseeable future. This is simply because the growth in renewable energy will not be able to keep up with our increasing energy demand associated with a growing world population. In such a scenario, a war against climate change without CCUS implies that we have to dramatically reduce our current energy consumption, and hence, accept a dramatic reduction in GDP. Thus, the adaptation of large-scale CCUS might be inevitable to mitigate ever-increasing CO₂ emissions with given population growth predictions.

At present, there are still very few signs of starting a war against climate change soon. The consequence is that we may significantly overshoot CO₂ levels in the atmosphere before any serious action is taken. In such a scenario, it is very likely that we also need to deploy technologies that can achieve negative emissions (i.e., direct CO₂ capture from air). In this scenario, the price of carbon will be so high that any technology that uses CO₂ as a source of carbon will have such an economic advantage that CO₂ will replace fossil fuels for those applications (e.g., plastics and soaps). Thus, CO₂ utilization next to storage will be an integral component of carbon management.

Carbon Capture

For carbon capture (Wilcox, 2012), it is important to distinguish between stationary sources (power plants, factories, etc.) and mobile sources (cars, airplanes, etc.) of CO₂. At present, there are no practical solutions for on-board capture CO₂ directly from mobile sources; therefore, we focus in the remainder on capturing CO₂ from stationary sources.

Coal-Fired Power Plants

Removing CO₂ from gases emitted from stationary sources can be done, using quite old technology. Most natural-gas contains more CO₂ than is allowed to put in pipelines. Hence, gas companies use the amine scrubbing process developed by Bottoms (1930) to separate CO₂ from methane. A very similar process can be used to remove CO₂ from flue gas. There is very little doubt in the engineering community that this amine absorption technology can be scaled-up and implemented to capture flue gases. The problem, however, is that the regeneration of the amine solution and the subsequent compression of CO₂ for transport and geological storage is very energy intensive. As a consequence, a power plant with carbon capture will not only be more expensive to build, but also will have reduction in efficiency as high as 35% (Herzog et al., 1993). Research is therefore focused on increasing the efficiency of the absorption process and on finding alternatives (e.g., solid adsorption or membranes).

Building a new power plant in which carbon capture would be added from the very beginning would give more possibilities to optimize the efficiency of combined power generation and carbon capture. One such example is oxy-combustion in which coal is burned with pure oxygen, and CO₂ is captured simply by condensing the water. In oxy-combustion, however, one needs to separate oxygen from the air, which is also an energy intensive separation. Other alternatives include, chemical looping, and integrated gasification combined cycle (IGCC), in which coal is converted into syngas, and the carbon capture process involves separation from H₂.

Carbon Capture from Dilute Sources

Thermodynamics tells us that the lower the concentration of CO₂, the more expensive it is to capture a ton of CO₂ (House et al., 2011; Wilcox, 2012). Hence, most carbon capture effort has been initially focused on flue gas streams from coal-fired power plants, which contain about 15% CO₂. As coal was the cheapest source of fossil fuel for many years, such an objective made perfect sense. However, the scenarios did not forecast the availability of large amounts of cheap natural gas in recent years. Replacing coal-fired power plants by gas-fired power plants will significantly reduce CO₂ emissions, but the scenario of burning all natural-gas reserves without CCS is only marginally less disastrous compared to burning all coal without CCS.

In the recent IPCC report (Stocker et al., 2013), the importance of negative emissions has been established. According to the most recent IPCC report, however, we are currently emitting more CO₂ than predicted by the most pessimistic IPCC scenario. Therefore, it is more than likely that we will overshoot the target CO₂ levels. In such a scenario, one may need technologies to activity mitigate the effects of climate change (Shepherd, 2009), which involves land management practices, accelerated weathering, albedo modification, etc., and also technologies to reduce carbon levels from the atmosphere (Keith, 2009; Lackner et al., 2012).

At present, one can achieve negative emissions by co-firing excess biomass in a coal-fired power plant and capturing the CO₂ together with the CO₂ from the coal. This Bio-energy CCS (BECCS) scheme has an environmental advantage that the flue gas of the biomass is cleaned using the existing infrastructure of the coal-fired power plant (Gough and Upham, 2011). However, there is not enough disposable biomass that this can be applied on a large-scale.

Utilization

In the context of flue gases, CO₂ is seen as a waste product. However, there are many applications, where CO₂ is utilized or considered as a valuable commodity.

Enhanced Oil Recovery

At present, CO₂ is most valuable for enhanced oil recovery (EOR). In 2008, in the United States, about 80% of CO₂ for EOR is obtained from natural resources and the rest is from anthropogenic sources such as coal gasification or gas processing (Advanced Resources International (ARI), 2010). The permanent storage of CO₂ in depleted oil fields is definitely one of the attractive carbon storage options. The fact that there is a market for CO₂ is an important incentive to develop more efficient carbon capture technologies, such that anthropogenic CO₂ can compete with CO₂ from natural reservoirs.

There are two practical issues. Firstly, the total amount of CO₂ that can be used for EOR is much less than the total emissions of CO₂, which implies that EOR can only be a partial solution. For example, CO₂ used in EOR operations was limited to only 60 million tons (Advanced Resources International (ARI), 2010). Secondly, one may wonder whether EOR gives a net CO₂ reduction. The argument is that because of EOR we produce more oil, and hence, further increase anthropogenic CO₂ emissions. However, a better argument is to compare one barrel of oil produced with EOR compared to one barrel of oil that is produced without EOR. In this comparison, one barrel of oil with EOR gives a lower CO₂ emission as a fraction of the CO₂ used to recover the oil stays in the reservoir. However, here we did assume that the other barrel of oil does remain in the reservoir and that EOR does not increase the demand for oil.

CO₂ to Chemicals

If we look at the current chemical industry, about 7% of all oil is used as feedstock for carbon in products ranging from plastics to soaps. Replacing oil by a renewable feedstock is an important long-term challenge of the chemical industry. The viability of using CO₂ as a chemical feedstock is considerably improved if the price of carbon is sufficiently high such that CO₂ can replace oil. While the carbon-free energy sources required for CO₂-to-chemical technologies (e.g., solar and wind) are still expensive for such CO₂ utilization schemes, the research and development of those CO₂ conversion pathways should be developed now to prepare for our rapidly changing future.

CO₂ to Fuels

The challenge with upgrading CO₂ to a fuel is that it requires energy. As it does not make any sense to use fossil fuels for this process, we assume that we will use renewable energy. The first argument is, if we have renewable energies we should primarily use this for generating electricity. However, this leaves us with two problems: storage of energy and transportation fuels.

An important advantage of fossil fuels is their high energy density. Renewable energies such as wind or solar require large-scale energy storage to ensure that electricity can be produced at time in which there is no wind or sun. Sometimes, this energy storage can be as simple as pumping water, but not all countries have this option. For example, Denmark has an excess of wind energy during the winter, but too little during summer. The idea is to use methanol to store the excess energy in the winter and use a conventional power plant with carbon capture in the summer. In this cycle, efficient conversion of CO₂ into a fossil fuel like methanol is an essential step.

Incorporating CO₂ into Construction and Building Materials

The cement industry produces about 7% of CO₂ emissions and is the second largest emitter of CO₂ after coal-fired power plants (International Energy Agency Greenhouse Gas R&D Programme (IEA-GHG), 2002). Replacing 10% of building materials with carbonate minerals is expected to reduce CO₂ emissions by 1.6 Gt/year, which is about 5% of the global CO₂ emissions as of 2011 (Sridhar and Hill, 2011). However, it is important to determine the correct composition of carbonate minerals to be included in the concrete matrix to reduce issues related to mechanical strength of the materials.

Storage

Given the enormous amounts of CO₂ we are emitting, it is difficult to imagine any form of carbon storage other than injecting into geological formations. Appropriate geological formations such as deep saline aquifers, depleted oil and gas fields, unmineable coal seams, and silicate formations (e.g., basalt) can accommodate up to 11,000 Gt CO₂ (Dooley et al., 2006), which is much greater compared to the annual CO₂ emissions, which are to the order of 30 Gt of CO₂/year. In addition, from our experience with EOR we know how to transport and inject CO₂ in geological formations. The challenge is, however, the scale. At present, only about 50 Mt CO₂ has been stored today and 13 Mt CO₂/year is expected by 2016 given plans in place for additional projects (Levina et al., 2013).

However, the scientific challenge is to ensure that the CO₂ remains safely in these storage sites for thousands of years. Development of technologies for monitoring, verification, and assessment (MVA) to entire that the CO₂ remains trapped underground is essential. While the process of injecting CO₂ is well understood, the cost of monitoring the fate of injected CO₂ over many years may be too prohibitive unless, the cost of deploying MVA technologies is substantially reduced. In addition, key questions related to long-term safety such as induced seismicity and the potential for forming fractures need to be addressed, which constitute an important aspect of risk assessments of geologic storage.

Ideally, mineralizing CO₂ into the form of a carbonate (e.g., limestone, magnesite) will reduce the amount of mobile CO₂ that needs to be monitored. However, the kinetics of this natural process may be on the order of geological timescales. An active area of research is to enhance this mineralization process (Gadikota et al., 2014).

Outlook

Given all the uncertainties, we argue that the first, and arguably, the most important challenge is to ensure that research is carried out on all aspects of CCUS. To reduce CO₂ levels in the atmosphere, land management and bio-energy with CCS (BECCS) may be low-hanging fruit. We can further envision capture technologies that are highly optimized for point sources ranging from coal-fired and gas-fired power plants to cement plants as well as direct air capture. The parasitic energy consumption should be lowered and the long-term recyclability of the CO₂ capture medium should be achieved. Cutting-edge research in carbon storage and utilization (i) will improve our understanding of the long-term effects of large-scale injection of CO₂ in geological formation, (ii) will enable us to develop alternatives for geological storage such as carbon mineralization, and (iii) will even allow for the development of the innovative chemistry to convert CO₂ into synthetic fuels and chemicals.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This publication is based on work supported by the National Science Foundation – Research Coordination Network on Carbon Capture, Utilization and Storage (NSF RCN-CCUS).

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