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Published October 22, 2015 08:37 by Water Network Research, Official research team of The Water Network

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What Is Non-Thermal Plasma?

Non-thermal plasma uses a reactor that consists of two electrodes (one electrode is in the form of a metal pipe, and the other electrode is a metal wire that runs down the middle of the pipe) separated by a void space that is lined with a dielectric material and is filled with glass beads. This type of reactor is called Dielectric-Barrier Discharge (DBD). See Figure 1.

DBD Type Non-Thermal Plasma Reactor Emissions flow inside of the pipe. A phenomenon occurs when the voltage through the beads exceeds the insulating effect of the beads and millions of micro-discharges occur. The duration of these discharges is measured in nano-seconds. The individual discharges cannot be seen with the human eye, but the overall effect produces a silent glow. This effect will only occur when the power source is alternating current (AC). DBD cannot be induced with direct current (DC) power because the capacitive coupling of the dielectric necessitates an AC field. In this environment, in addition to electrons flying about, atoms are being separated from their molecules to become free radicals. Since free radicals are highly reactive, they quickly recombine with other atoms and/or molecules to form new compounds.

Figure 1 DBD Type Non-Thermal Plasma Reactor

Using oxygen as an example, the normal state of oxygen is a molecule containing two oxygen atoms. Thus, it is written as O2. In a DBD field, the oxygen molecules splits into two atoms of oxygen, O+ and O+. The elemental oxygen radical, being very reactive, will form ozone, O3, when the radical oxygen -4- atom reacts with a normal molecule of oxygen (O2). The oxygen radicals also react with other compounds. For example, oxygen radicals react with carbon monoxide (CO) to form carbon dioxide (CO2), sulfur dioxide (SO2) to form sulfur trioxide (SO3), and nitrogen oxide (NOx) to form nitric acid (HNO3) in the presence of moisture. Ozone will also react with small (2.5 micron) carbon particles (soot) to form carbon dioxide, and reacts with elemental mercury (Hg) to form mercury oxide (HgO). Oxidizing elemental mercury changes it from a vapor to a solid phase. Sulfur trioxide hydrolyzes into sulfuric acid, when exposed to moisture. Depending on the concentrations of nitrogen and sulfur compounds, this process is capable of producing significant amounts of mineral acids.

How can non-thermal plasma be used for odor control in waste water treatment plants?

Plasmas have been demonstrated to be able to decompose volatile organic compounds (VOCs) and thus odour molecules very efficiently for low decontamination levels (less than 1 gCorg/Nm3 ; i.e., 100 ppm or lower. Many VOCs, e.g., aldehydes, fatty acids, alkanes, formic acid, amines, esters, or thiols have a strong and unpleasant odour (so-called OVOCs - Odour Active Volatile Organic Compounds). In non-thermal plasmas the chemical processes are based on non-thermal activation of particles via collisions. The quality and quantity of collisions is determined by the density of species and the kinetic parameters (e.g., mean velocity, collision frequency). During electrical breakdown of the gas free electrons with high kinetic energies are produced via ionizing collisions.

Electrons undergo further electron-molecule collisions with quite different reaction rates determined by the corresponding energy thresholds and gas temperature. A multitude of inorganic reactions follows due to the formation of radicals. In air plasmas at low gas temperatures reactive oxygen species like ozone (O3), O and OH as well as HO2 as strong oxidizing agents are formed. Many molecules are readily attacked by free radicals. In many situations, e.g., water treatment plants, sulphur containing molecules like hydrogen sulphide (H2S), dimethyl sulphide (DMS, (CH3)2S) or other thiols are responsible for bad smell of exhaust gases. All three pollutants could be completely removed when a sufficient energy density was deposited in the plasma. The efficiency of the process has been found to increase with decreasing the initial concentration of sulphide compounds, while the energy yield (mass of removed compounds per kWh) remained almost unchanged. SO2 was the only identified by-product of H2S decomposition, but the sulphur balance suggested the formation of undetected SO3. The by-products analysed during the degradation of DMS and C2H5SH enabled to propose a reaction mechanism, starting with radical attack and breaking of C-S bonds.

List of companies to involved in plasma controlled system can be found HERE

Sources:

US EPA report on Using Non-Thermal Plasma to Control Air Pollutants Available online at http://www3.epa.gov/ttn/catc/