Non-Thermal Plasma

WHAT IS PLASMA?

Plasma is a quasi-neutral gas of charged and neutral particles which exhibits collective behavior. It is often referred to as the “fourth state of matter” aside from gas, liquid, and solid, and plasma is the most plentiful form of matter (~99%) in the universe. Plasma contains charged species such as electrons and ions, highly reactive and unstable radicals, electronically excited atoms/molecules, besides neutral ground-state atoms/molecules. Lightning and Aurora Borealis are common plasma examples found on Earth.  

Figure 1. Formation of plasma.

Figure 1. Formation of plasma.

Plasma-Assisted Chemical Synthesis

Electric discharge applied to gases generates a variety of short-lived but highly reactive species such as radicals, atoms, and vibrationally excited molecules which react more readily at lower temperatures. This feature enables diverse and alternate chemical reaction pathways, making reactions feasible at lower temperatures, not possible in traditional thermal or catalytic reaction activation.  

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Plasma-assisted methane reforming for syngas production

Syngas is a valuable chemical that is used to produce a host of different chemicals and fuels including methanol, heavy alcohols (e.g., ethanol), acetyls, formaldehyde, methyl tertiary-butyl ether (MTBE), and Fischer-Tropsch liquid fuels (Figure 5).  For some synthesis processes, a low H2:CO ratio syngas is preferred. 

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Direct plasma-assisted synthesis of methanol and acetic acid

Acetic acid (CH3COOH), methanol (CH3OH), and other oxygenates are important industrial chemical products. Currently, acetic acid is most produced by carbonylation of methanol, which is the reaction of carbon monoxide (CO) with methanol.

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Plasma Surface Treatment

A particularly useful form of non-thermal plasma is radio frequency (RF) plasma, which excites the electrons separately from the positive nucleus, generating an equally reactive plasma while consuming a fraction of the energy.

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Plasma-assisted ammonia synthesis from flue gas

Ammonia (NH3) is a critically important industrial chemical mainly used to manufacture fertilizers. Presently, ammonia is produced primarily via the highly energy-intensive Haber-Bosch (H-B) process, which requires extreme operating conditions of 450–600 °C and 150–300 atm and must be executed at very large scales in order to maximize efficiency. It also utilizes H2 and N2 as feedstocks, each of which must first be produced in separate energy-intensive processes.

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More Information About Plasma

How is Plasma Formed?

In principle, plasma can be formed by providing energy to the substance (e.g., heating). With energy, the phase of the substance will change from solid to gas. If sufficient energy is further added to the gas, a molecular gas will gradually dissociate into an atomic gas due to the thermal kinetic energy of the particles exceeding the molecular binding energy (Figure 1). As a result, ionized gas or plasma is formed. Other energy forms such as the application of electric field to gases, chemical reactions such as in flames, fusion reactions in stars, adiabatic gas compression, etc. can also form plasma. 

Types of Plasma

Plasmas are usually classified as non-equilibrium and equilibrium plasma. In equilibrium plasma, the neutral and heavy species (atoms, molecules) are at a lower temperature closer to the surrounding temperatures (atmosphere, furnace, etc.). However, the lighter electrons are at a much higher temperature of ~10,000 K to 40,000 K. This means that the electrons have a higher average speed (energies) than if they were at surrounding temperatures. Non-equilibrium plasmas are usually achieved by the application of electric fields that accelerate the electrons to high energies. Figure 2 shows a non-equilibrium plasma reactor sustained by AC electrical discharge at ACT. In equilibrium plasmas, the temperatures of the neutral species and electrons are closer to each other. These types of plasmas are observed in stars, high current intense arc welding etc. 

Figure 2. A plasma Reactor at ACT.

Figure 2. A plasma Reactor at ACT

What is Plasma Used For?

    • Chemical synthesis using renewable energy
    • Plasma-assisted combustion
    • Greenhouse gas mitigation
    • Semiconductor manufacturing
    • Surface treatment
    • Biological applications
    • Food engineering
    • Toxic air treatment
    • Nanomaterial synthesis
    • Electrochemistry etc. and the applications will continue to grow in the coming years. 

Plasma-related technology development

ACT has been heavily involved in two plasma-related technology developments:

    • Plasma-Assisted Chemical Synthesis 
    • Plasma Surface Treatment 

Learn More
J.A. Bittencourt, Fundamentals of Plasma Physics, 3rd Edition, Springer (2004).
F. Chen, Introduction to Plasma Physics and Controlled Fusion, 2nd Edition, Vol. 1, Plenum Press, New York and London (1984).

 

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