Non-thermal plasma is an important technology in the CANMILK project, enabling innovative solutions for methane activation and decomposition in dairy barns. By generating reactive oxygen or hydrogen species, it helps break down methane molecules. The CANMILK project is harnessing this technology to develop a high-tech, yet cost-efficient method for addressing methane emissions at the ppm level. Although its agricultural applications could be transformative, plasma technology also offers a wide array of uses across different industries, including in fluorescent lamps, ozone generators, and a variety of sustainable chemistry applications. In this article, we explore the diverse potential of plasma in sustainable chemistry.
What is Plasma?
Plasma is a (partially) ionised gas produced by applying energy to a gas. This can be done by heating or by applying electricity. Plasma is often referred to as the “fourth state of matter”, alongside solids, liquids and gases. Gas discharge plasmas are created by applying electricity to a gas and are used in many technological applications such as coating deposition, surface modification and etching (e.g. for microchip fabrication), but also in light sources, lasers, plasma displays and medical applications (e.g. wound healing, sterilisation, dental treatment and also cancer therapy). Another emerging application is in sustainable chemistry, for the electrification of chemical reactions.
Plasma’s Role in Chemical Conversion
When electricity is applied to a gas, the gas is electrically decomposed, producing positive ions and electrons, as well as radicals and excited molecules. Plasma is therefore a highly reactive chemical cocktail, very useful for converting stable molecules such as carbon dioxide (CO2), nitrogen (N2) and methane (CH4) into value-added compounds. In fact, these reactions can take place in plasmas at milder temperature and pressure conditions than would be possible in conventional (thermal) processes.
In addition, because plasma reactors are powered by electricity and can be quickly turned on and off, they can use fluctuating renewable electricity to convert these stable molecules into value-added compounds (chemicals or fuels), known as “power-to-X”.
Plasma-chemical conversion has several other advantages as well, such as flexibility in terms of feed gas. For instance, it can be used for:
- CO2 splitting into CO and O2 (where the CO is a useful base chemical for the chemical industry),
- CH4 conversion into higher hydrocarbons (e.g., acetylene (C2H2) and ethylene (C2H4; also very interesting for the chemical industry)) or into H2 and valuable carbon (e.g., carbon-black or carbon nanotubes),
- Dry reforming of methane: Combining CO₂ and CH₄ to produce syngas, a mixture of CO and H₂, which can be further converted into hydrocarbons and oxygenates.
- Using H₂ or H₂O for the production of oxygenates from CO₂, or achieving this through partial oxidation of methane
- Nitrogen Fixation: Converting nitrogen into ammonia (NH₃) or nitrates (NOx), which are important for fertilizer production
- NH3 cracking to produce green H2
Economic and Scalability Benefits of Plasma
Other advantages of plasma for sustainable chemical applications are the relatively low CAPEX (capital expenditure) costs, as plasma reactors are typically cheap (compared to other technologies using, for example, rare earth metals). Finally, plasma reactors can be scaled up, as required for large-scale industrial production of chemicals, simply by placing a number of reactors in parallel, but they can also operate on a small scale, unlike some well-established industrial chemical processes, such as the Haber-Bosch process for NH3 synthesis. This is very useful for storing peak power, even from a few wind turbines or solar panels, and thus for decentralised production of value-added compounds (e.g. fertiliser production by local farmers in underdeveloped countries).
Plasma Reactor Types and Their Applications
Various types of plasma reactors can be used for these gas conversion applications, such as dielectric barrier discharges (DBDs), microwave (MW), gliding arc (GA), atmospheric pressure glow discharges (APGDs), low-current arc plasmas, nanosecond-pulsed plasmas, spark discharges and corona discharges. Each of these plasma reactors has its own characteristics, strengths and limitations.
The Future of Plasma in Sustainable Chemistry
In summary, sustainable chemistry is one of the emerging applications of plasma technology, and will even gain in importance, because of the need for electrification of the chemical industry. However, more research is needed to further improve the performance, in terms of conversion, energy efficiency, and product selectivity, e.g., by smart reactor design and combining catalysts. Detailed plasma diagnostics and multi-scale modeling can help to gain more insights, and thus to improve these applications.
By Annemie Bogaerts, University of Antwerp