# Applications of Thermal Plasmas for the Environment
## 1 Introduction  
The increasing volumes of a very wide range of waste streams is a growing concern. Thermal processing such as incineration is most commonly used for the treatment of waste streams, whereby the often-incomplete combustion of organic waste can lead to dangerous products in the exhaust gases. The stricter legislation on the processing of waste streams and the limitations of conventional technologies makes plasma technologies increasingly attractive _for sustainable development_. In most industrialized countries, the order of priority for waste disposal methods is governed by the concept of the so-called waste pyramid \"Ladder of Lansink\", proposed by the Dutch politician [PERSON] in 1979): (1) waste prevention; (2) reuse (of the waste as it is); (3) recycling (reuse of compounds without loss of quality) and downcycling (material reuse with loss of quality); (4) energy recovery (by incineration, gasification or pyrolysis); (5) incineration and/or destruction without energy recovery; and (6) landfill.  
The states of matter are characterized by their average energy per particle (atom or molecule). Solid matter has the lowest, _plasma the highest_. This high energy (temperature) implies that plasma is chemically very active (fast chemical conversion or modification with relatively few particles). Nuclear fusion research in view of a new sustainable energy source is the most ambitious application; it has provided many new insights into the plasma medium and is a very important stimulus for plasma research. The present paper deals with the treatment of waste using thermal plasmas.  
Non-thermal as well as thermal plasmas are used for the processing of materials and waste. Thermal (hot) plasmas are characterized by their high energy density and by the equal temperatures of the electrons and the heavy particles, i.e., thermal plasmas are in local thermodynamic equilibrium. Non-thermal plasmas (also called cold plasmas), on the other hand, are non-equilibrium ionized gases, which are characterized by lower energydensities and by the large difference between the electron temperature and the temperature of the heavy particles; therefore, they are suited for the treatment of heat-sensitive materials and for low-temperature plasma chemistry. A detailed review comprising various types of non-thermal plasmas with the mechanisms of their generation and with their applications is given in [1].  
Thermal plasmas [2] are also characterized by a high energy transfer rate, short reaction times for chemical reactions in the plasma, and a wide choice of plasma media. These properties make thermal plasmas suitable for a variety of industrial applications [3; 4], such as cutting, welding, spraying, furnaces for metallurgy with DC arcs and graphite electrodes, metal smelting and refining, metal recovery and purification from metal-containing waste, treatment of low radioactive waste, and environmentally friendly treatment of waste with plasma torches (the subject of this article). The first applications of thermal plasmas were in the 1980s, and since then their number has increased significantly [3]. Basic research in the 1990s led to major advances in understanding the fundamental phenomena involved and to a renewed interest in the use of thermal plasmas in materials processing and the environmentally friendly treatment of waste streams [3]. The characteristics of thermal plasma waste treatment are outlined in Section 2. A few applications of thermal plasma waste treatment are given in Section 3. Some environmental aspects are discussed in Section 4. Conclusions and outlook are presented in Section 5.