Chemical Looping Combustion: An Innovative Means of Addressing the Climate Crisis and Improving Carbon Capture
Amir Ibrahim
To many, climate change is a largely invisible problem and has little effect on the day-to-day lives of first world citizens. As such, it is easy for corporate magnates and conspiracy theorists to dismiss science in the name of whatever misplaced beliefs they hold. Thankfully, however, climate change outreach has taken greater hold over the years, and many now understand the truth: climate change will have disastrous environmental and geopolitical consequences, and, unless we adopt a serious plan of action now, we are collectively screwed.
Unfortunately, however, there is no magic bullet solution, and reducing fossil fuel emissions won’t be sufficient on its own in the fight against climate change. Even in the most ideal case in which we immediately eliminate all carbon emissions, humanity will likely experience severe climate impact until 2040. For instance, the Intergovernmental Panel on Climate Change (IPCC) estimates that climate change will jeopardize food security throughout the world and push an estimated 30-130 million people into poverty. The timescale of reducing our greenhouse emissions is simply too long, and so to avert such catastrophe in the near future we must go beyond fossil fuel regulations and look to active means of eliminating our carbon footprint.
Currently, there are various preventative measures that we can take to reduce our carbon footprint. Most notably, carbon dioxide sequestration is a direct means of combating greenhouse emissions. Carbon can be sequestered via natural earth processes such as photosynthesis, and, to this end, is it imperative that we maintain our ecosystems and wetlands across the globe.
As of lately, however, much attention has been devoted to man made carbon sequestration technology. Direct air capture (DAC) and carbon capture and storage (CCS) are among the most well-documented technologies. DAC actively intakes air from the atmosphere and purifies it of carbon dioxide, while CCS sequesters carbon directly from power plants and operates on site. There has been significant investment in DAC technology in recent years, and the largest DAC facility is set to open in Iceland in 2023. CCS technology, however, has been met with considerable resistance.
The primary obstacle in the implementation of CCS is energy inefficiency. To operate CCS technology on site, a portion of a power plant's energy output must be redirected into carbon capture, and this in turn leads to horribly inefficient energy production. The energy penalty is one popular metric used to evaluate the efficiency of carbon capture, and it denotes the percentage of outputted energy that must be redirected into CCS. One study conducted by the National University of Singapore calculates various energy penalties using different fuel sources and means of combustion (Figure 1). The study only records the energy penalty associated with capturing carbon dioxide (averages to 17%), however, there is an additional energy penalty associated with the compression and transportation of carbon dioxide, which can potentially double the penalty depending on the method of storage used.
When you consider the fact that corporations will simply burn more fuel to make up for the loss in energy, it is apparent that traditional CCS technology is not scalable and viable in the long-term. For instance, suppose we achieve a 25% energy penalty: realistically, a power plant will burn 33% more fossil fuel to account for the difference and extract the same net energy output. In addition, modern CCS technology is generally around 90% accurate in capturing carbon, so, while CCS is reducing our total emissions, its effectiveness is rather underwhelming when you consider the exorbitant energy costs and added fuel consumption. For these reasons, many climate change activists have dismissed CCS technology, and the field has widely been labeled as a boondoggle.
However, Chemical Looping Combustion (CLC) is one promising technique that has the potential to completely overhaul our current means of energy production and efficiently sequester our carbon emissions on site. First experimentally demonstrated in Sweden in 2003 by Anders Lyngfelt and Tobias Mattisson, research in chemical looping has been steadfast ever since, and many scientists and chemical engineers have heralded it as the paradigm shift we need in carbon capture. The technology is currently straddling the line between experiment and commercial implementation, with several groups in the United Kingdom, Sweden, Norway, and Singapore actively conducting research in the area.
CLC is an entirely new form of combustion which incorporates carbon capture as an inherent component in energy production. Ordinary fuel-combustion power plants output a stream of flue gas which is primarily composed of nitrogen, fluoride gas, and a small percentage of carbon dioxide. Standard CCS technology then filters this gas and arduously picks out the small portion of carbon dioxide (flue gas is composed of ~70% nitrogen and only 8-10% CO2). Rather than undergoing ordinary combustion and emitting the standard nitrogen-diluted flue gas, CLC utilizes a form of pure oxy-combustion which separates the reactants and products of combustion, in turn producing pure carbon dioxide emissions. These pure carbon emissions are then far easier to sequester due to the lack of dilution, in turn leading to greatly mitigated energy penalties and far less energy intensive CCS technology.
The separation of products and reactants in CLC is achieved via the use of metallic oxygen carriers which selectively react with air and transport pure oxygen to the combustion process (Figure 2). Specifically, a dual reactor system is used to separate the substances. First, air is taken into the air reactor and bonded with reduced metals to form metal oxides. The metal oxides are then transported out of the air reactor and into the fuel reactor, where the oxygen dissociates from the metal and then spontaneously combusts with the hydrocarbon fuel, thus producing energy. Owing to the intermediary metallic oxygen carriers, there are no other contaminants in the combustion reactor, therefore the combustion process produces nothing but pure carbon dioxide and water. Once the metal oxides are reduced and give up the oxygen, they can then be cycled back into the air reactor to begin the transportation process again. This recycling of metallic oxygen carriers between both reactors results in a self-sustaining cycle that inherently filters carbon dioxide from the other non-polluting reactants.
In addition, to efficiently react the metallic oxygen carriers with the air and hydrocarbon fuel, both reactors employ a fluidized bed system. A fluidized bed is a mixture of some granule substance that behaves exactly like a liquid when running air through it at a certain critical temperature and pressure. By turning some substance into a fluidized bed, you can achieve the same type of solubility as an aqueous solution, thus efficiently mixing the reactants together. You can even induce other exotic phenomena in a fluidized bed, such as creating bubbles or waves in a particulate substance like sand. This technology also has a myriad of applications such as cryogenics, paint-coating, and catalyzing other chemical reactions.
Utilizing the CLC setup dramatically reduces the energy penalties and costs of CCS. Recently in 2019, a research group from the Norway University of Science and Technology demonstrated a chemical looping setup which achieved energy penalties as low as 1.5% while successfully capturing 100% of carbon dioxide. This improved energy efficiency represents a monumental leap forward in CCS and circumvents much of the economic concerns normally associated with carbon capture technology. Scaling this technology to industry level and fitting it in current power plants would hugely benefit our global energy production and extend the lifetime of our fossil fuel reserves.
CLC is demonstrably more efficient than traditional carbon capture, however, there are still a fair share of roadblocks in its wide scale commercial implementation. While the actual operation of CLC is economically viable, the installation of the technology would require significant initial investment. CLC is an entirely new and novel means of energy production, therefore its installation is not as simple as retrofitting existing power plants. To install CLC, we would need to completely overhaul our existing infrastructure and fundamentally change the energy industry. To achieve this, greater outreach must be employed, and the CLC community will need to conduct vast education to convince the world that this technology is of value.
In addition, there is still ongoing research to further enhance the efficiency of CLC processes. Specifically, many groups are exploring mechanisms to enhance oxygen uncoupling from the metal oxides. Normally, metal oxides in the combustion chamber react directly with fuel, and this process is known as heterogeneous CLC. However, studies indicate that the energy output of CLC can be further enhanced by utilizing a two-step intermediary process where oxygen first uncouples from the metallic carriers then directly reacts with fuel–this process is known as oxygen-uncoupled CLC. Currently, heterogeneous and oxygen-uncoupled CLC take place in tandem without any means of distinguishing the two, so there is a lot of current material research being conducted to determine which metal oxides are most conducive to the oxygen-uncoupling process.
Lastly, CLC has yet to be demonstrated on a truly industrial scale. The largest experimental CLC setup is operating at the Vienna University of Technology, and this setup produces roughly 120 kilowatts of power. This is rather measly when you consider the fact that most power plants operate in the megawatt regime, and combustion power plants in the United States output anywhere between 200-3500 megawatts. However, a sustainable transportation company known as Alstom has recently implemented one of the first industrial CLC setups, and they have been producing over 3 megawatts of energy using this technology. It should be noted that this is a private corporation providing their own means of energy production, and not an industrial power plant serving an entire grid of people. In any case, CLC has been proven to function in the megawatt regime, so, in theory, the technology should be scalable to the level of industrial power plants–the limiting factor now is the aforementioned motivation and economic incentive.
As humanity comes to grips with a radically changing climate and the crises associated with carbon emissions, we must look to all potential means of sustainability. It is likely that we will be reliant on fossil fuels for at least the next hundred years, so it is imperative that we mitigate the effects of fossil fuel combustion on our climate. Chemical looping combustion circumvents most of the traditional concerns regarding carbon capture technology and represents a realistic means of utilizing fossil fuels while simultaneously reducing our emissions, therefore this technology will likely become essential within the coming years and be paramount in the fight against climate change.
Works Cited
Adánez, Juan et al. “Chemical-Looping Combustion: Status and Research Needs.” Proceedings of the Combustion Institute, Elsevier, 23 Sept. 2018, https://www.sciencedirect.com/science/article/pii/S1540748918306072#bib0007.
Douglas W. Duncan and Eric A. Morrissey. “The Concept of Geologic Carbon Sequestration, Fact Sheet 2010-3122.” USGS Publications Warehouse, https://pubs.usgs.gov/fs/2010/3122/.
Khan, Mohammed et al. “Integration of Chemical Looping Combustion for Cost-Effective CO2 Capture from State-of-the-Art Natural Gas Combined Cycles.” Energy Conversion and Management: X, Elsevier, 14 May 2020, https://www.sciencedirect.com/science/article/pii/S2590174520300167#:~:text=Chemical%20looping%20combustion%20(CLC)%20is,to%20natural%20gas%20power%20plants.&text=Large%20energy%20penalty%20due%20to,relative%20to%20modern%20gas%20turbines.&text=Added%20firing%20after%20the%20CLC,to%20only%201.4%25%2Dpoints.&text=Firing%20with%20natural%20gas%20returns,avoidance%20cost%20of%20%2460.3%2Fton.
Leion, Henrick et al. “Using Chemical-Looping with Oxygen Uncoupling (CLOU) for Combustion of Six Different Solid Fuels.” Energy Procedia, Elsevier, 9 Apr. 2009, https://www.sciencedirect.com/science/article/pii/S1876610209000617.
Levin, Kelly, et al. “6 Big Findings from the IPCC 2022 Report on Climate Impacts, Adaptation and Vulnerability.” World Resources Institute, 27 Feb. 2022, https://www.wri.org/insights/ipcc-report-2022-climate-impacts-adaptation-vulnerability.
Lyngfelt, Anders. Chemical Looping Combustion: Status and Development Challenges. https://pubs.acs.org/doi/10.1021/acs.energyfuels.0c01454.
Moseman, Andrew, and Howard Herzog. “How Efficient Is Carbon Capture and Storage?” MIT Climate Portal, https://climate.mit.edu/ask-mit/how-efficient-carbon-capture-and-storage#:~:text=CCS%20projects%20typically%20target%2090,will%20be%20captured%20and%20stored.
Page, S.C. et al. “Carbon Capture and Storage: Fundamental Thermodynamics and Current Technology.” Energy Policy, Elsevier, 5 Dec. 2008, https://www.sciencedirect.com/science/article/pii/S0301421508005776.
Rhode, Emily. “Carbon Capture and Storage (CCS) Pros and Cons.” Treehugger, Treehugger, 13 Aug. 2021, https://www.treehugger.com/carbon-capture-and-storage-ccs-pros-and-cons-5120005.
Vasudevana, Suraj et al. “Energy Penalty Estimates for CO2 Capture: Comparison between Fuel Types and Capture-Combustion Modes.” Energy, Pergamon, 29 Mar. 2016, https://www.sciencedirect.com/science/article/pii/S036054421630216X.
Comments
Post a Comment