# Applications of Thermal Plasmas for the Environment
## 2 Thermal Plasma Waste Treatment  
Thermal plasmas [2] offer unique advantages such as high temperatures in the range of 5000 to 50,000 K, high energy density, high energy transfer rate and extremely low reaction times for chemical reactions. Different types of plasma sources are used in various plasma processing technologies. Arc discharges create a high-density, high-temperature region between the electrodes. If the gas flow in the electrode gap is high enough, the plasma jet extends beyond one of the electrodes, transporting the plasma energy to the reaction region. A plasma torch, also known as a plasmatron, is a device that generates a directed stream of thermal plasma from the nozzle. An overview is given in [2]. The primary source of electricity for plasma torches can be direct current (DC), mains frequency (50 Hz) alternating current (AC), radio frequency (RF) alternating current, or microwaves. Other features of plasma torches include are stabilization mechanism, plasma gas, flow type, and electrode geometry and cooling. Thermal plasmas used in waste treatment and material gasification systems are commonly produced in DC or AC electric arc discharges operating in a non- transferred mode (both electrodes are parts of the torch, and the plasma jet exits the torch through an exit nozzle). The main problem of arc discharges is erosion of electrodes and nozzles. The lifetime of inductively coupled RF discharges and microwave discharges, which operate without electrodes, is higher, but the complexity and cost of power supplies are the main limitation for high-power systems. Thermal plasmas (produced in plasma torches) offer an alternative and superior solution for the treatment of waste streams. Organic components are converted into a calorific syngas (a mixture of mainly hydrogen and carbon monoxide), while the inorganic components can be converted into a slag directly in the process. Due to the unique property of strongly intensifying the energy content of the process gas, plasma torches offer very clear advantages over traditional combustion which relies on the energy content of the waste as a heat source. In the plasma torch, the process heat is supplied directly by heat transfer through the electric arc discharge. The use of electrical energy also enables the allothermic mode of operation, reduces the required oxygen supply (which is normally injected as air along with a lot of nitrogen, increasing the volume of the waste gas to be treated) and allows better control of the chemical processes. Another advantage is the ability to produce a syngas free of nitrogen by providing exactly the amount of gasifier (e.g., CO\({}_{2}\)) to allow for carbon volatilization or solid carbon production. The feed rate of the (pre-treated) waste into the reactor depends on the rate of heating and decomposition of the material and chemical reaction rates in the gases produced. The residence time of the gaseous products of the decomposition of the waste must be high enough for complete dissociation of the gas molecules produced and for the chemical reactions leading to the formation of syngas molecules. A number of chemical reactions take place after the mixing of the plasma with gases, created by volatilization of the treated material. The reaction rate is enhanced by the high temperature and the presence of radicals and ions. Proper mixing of the plasma with the reactor atmosphere and efficient heat transfer to the surface of the material to be treated are necessary. The heating of the material and its volatilization are macroscopic processes with characteristic times well above the time constants of the reactions at high temperature. Hence, the gasification rate is determined primarily by the rate of heating and volatilization of the material. The conditions in the reactor volume must result in complete mixing of all components, and the resulting temperature must be high enough to ensure the proper reaction products composition.