Plasma Processing exploits the properties of the ionised state of matter for material synthesis and transformation through physical and chemical routes. Plasma Processing began its rapid rise to prominence with the discovery of dielectric etching and metal deposition on silicon wafers which enabled semiconductor manufacturing. This technology has permeated many fields, including Chemistry, Metallurgy, Biology, Agriculture etc. No other branch of Physics has produced so much proliferation in applications.
Global warming is a critical problem facing human existence. CO2 emissions from burning hydrocarbon fuel form a layer in the atmosphere, trapping infrared light scattered from the earth, causing the Greenhouse Effect and elevated global temperature. In 2018 October, the Intergovernmental Panel on Climate Change (IPCC) warned that the rise in global atmospheric temperature has to be limited to a maximum of 1.5C. Therefore, carbon emissions should be halved by 2030 and made zero by 2055. The generally accepted approach to achieve net-zero emissions is to replace Carbon-based fuels with solar or other forms of Renewable Energy (RE). However, an emerging alternative concept is to convert CO2 produced by fossil fuel burning into hydrocarbon fuels using solar electricity to create net-zero emissions when burned. This article attempts to show how these two approaches are complementary.
The fundamental problem with RE is the daily and seasonal energy variation. Heavy dependence on RE calls for storage or conversion into valuable products when not balanced by demand. For many technologically advanced countries, RE excess is foreseen by 2030. Chemical fuels have the highest energy density compared to storage options like batteries. Fuels also allow ease of storage, transportation and distribution. Converting RE electricity into methane or methanol and storing them in gas networks offers long-term and mega-scale energy storage potential. The power to gas (P2G) scheme, which among other things, does away with capital investment in electric grid expansion. Transportation of gas is proven to be substantially less expensive than transport of electric energy.
The P2G approach is to recycle CO2 into fuel. CO2 is removed from the flue gas from fossil fuel combustion or the legacy CO2 already generated by past use and stored in the atmosphere or oceans. The fuel regenerated by reacting the CO2 with Hydrogen produced with solar energy can then be burned for our energy needs maintaining net zero-emission. By recapturing and reusing the CO2 emitted, the CO2 cycle is closed, establishing an equilibrium condition. As a result, the net emission will be zero, a compelling example of the Circular Economy where waste is converted to wealth. This mimics the carbon cycle of the Earth system, which has been in near equilibrium for over millions of years. The clear benefit is that it allows us to continue using the existing infrastructure for hydrocarbon energy storage, transport, etc., and the vast hydrocarbon resources.
Plasma chemical application for CO2 recycling starts with splitting CO2 into CO. Due to the unique nature of the dumbbell-shaped CO2 molecule, there is an efficient dissociation pathway in the Plasma mediated CO2 dissociation. The process starts with vibrational excitation of the molecule by electron impact. Next, the excitation is further mediated by vibrational– vibrational (VV) collisions. Then, the so-called ladder-climbing process incrementally populates the higher vibrational levels, ultimately leading to the dissociation of the CO2 molecule(1). Thus, it is possible to dissociate CO2 with high energy efficiency since only 5.5 eV required for breaking the Carbon bond is needed, compared to the higher demand in the case of electronic excitation–dissociation.
This process requires high electron density for high efficiency and throughput and high levels of non-equilibrium to selectively populate certain degrees of freedom, not available in conventional thermal and non-thermal plasmas (1). Therefore, a novel type of Plasma called warm Plasma operating at the parameter boundary of traditional thermal and non-thermal Plasma had been found very promising for CO2 conversion. Gliding Arc plasma sources and Microwave plasma sources create warm plasmas. These non-equilibrium discharges can supply reactive species and offer elevated temperatures of the order of 2000–3000 K. These warm plasmas can provide non-equilibrium conditions and enhance chemical kinetics through higher gas temperatures.
The versatility in process options makes the plasma chemical conversion of CO2 very attractive as a Carbon valorisation technique. It allows the process to be generally amenable to adaptation to different locations and different types of inputs. The attraction of the Plasma –mediated processes are, On-demand capability, High energy efficiency (~60% demonstrated), High power density (45W/cm3 ), Rapid ramping up and down. It also has the merit that no scarce materials such as catalysts are employed.
We must recapture the CO2 emitted from point sources like fossil-fired power plants and cement and steel plants to close the fuel cycle and make it CO2 neutral. This includes dispersed CO2 in the atmosphere deposited from transportation systems and CO2 stored in the ocean at higher concentrations. Plants extracting CO2 from the atmosphere and oceans may be dispersed around the globe to take advantage of solar energy potential. Developing energy-efficient materials to trap and release CO2 is an important research issue (2).
The advantage of CO2–neutral fuels recycled from CO2 and solar Hydrogen over new energy sources like Hydrogen, ammonia and batteries is that infrastructure is readily available.
The outcome of all these developments is that an industry based on CO2 Recycling using conventional chemistry has already emerged. 19 direct air capture (DAC) plants are operating worldwide (3), capturing more than 0.01 Mt CO2/year, and a 1Mt CO2/year capture plant is in advanced development in the United States. Bill Gates is an early entrepreneur who promoted Carbon Engineering to extract CO2 from the air and convert it to fuel. The extraction is done by huge fans sucking air. Hydrogen from water electrolysis is chemically reacted with CO2. The fuel produced with conventional chemical processes cost $4 per gallon. The newest plant commissioned in September 2021 can capture 4 kt CO2/year for storage in basalt formations in Iceland. In the Net Zero Emissions by 2050 Scenario, DAC is scaled up to capture more than 85 Mt CO2/year by 2030 and ~980 Mt CO2/year by 2050 (3). These targets will require many more large-capacity facilities to advance the technology and minimise operation costs (3).
Research in plasma-based recycling of CO2 to produce Carbon-neutral fuels is pursued in many centres. However, pioneering work has been done at the Dutch Institute for Fundamental Energy Research (DIFFER), Eindhoven University of Technology, and the University of Antwerp in Belgium. An extensive review of the plasma-based processes for CO2 conversion by Snoeckx and Bogaerts (1) is a valuable guide to the subject.
References:
1. Plasma technology — a novel solution for CO2 conversion? R Snoeckx and A Bogaerts, Chem. Soc. Rev., 2017, 46, 5805–5863 2. CO 2 -Neutral Fuels, A Goede, R van de Sanden, Volume:47, Europhysics News, May, 2016 3. IEA (2021), Direct Air Capture, IEA, Paris https://www.iea.org/reports/direct-air-capture
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