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.
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.
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.
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.
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.
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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