The use of plasma to treat waste is a relatively new concept. Plasma is gen­erated when gaseous molecules are forced into high-energy collisions with charged electrons resulting in the generation of charged particles. There are fundamentally two types of plasmas: high-temperature or fusion plasmas or low-temperature or gas discharge plasmas [60]. The low-temperature plas­mas can be further divided into thermal plasmas in which a quasi-equilib­rium state occurs (characterized by high electron density and temperature between 2,000 and 30,000°C) and cold plasmas characterized by a nonequi­librium state [61]. Thermal and gas plasmas are most widely used for waste treatment. As shown in Table 6.3, the plasma process operates at a higher temperature than other thermochemical processes. Plasma technology also

TABLE 6.5

Some Typical Literature Studies on Pyrolysis of Waste Materials

Type of Waste Authors Comments

uses less than required stoichiometric oxygen and generally operates at low residence time [62-64].

Haberlein and Murphy [61] indicated that the most important advantages of plasma technology are (a) high energy density and high temperatures, and (b) use of electricity as the energy source. The first advantage allows
(i) rapid heating and reactor startup, (ii) high heat and kinetic rates, (iii) smaller installation size (due to smaller residence time), and (iv) process­ing of materials with high melting or boiling point. The second advantage allows increased process controllability and flexibility due to decoupling of heat generation from the oxygen potential and lower off-gas flow rates resulting in lower gas cleaning costs. Because electricity can be expensive, plasma technology is most desirable for waste streams that contain most organic materials with high heating value and for the waste that generates valuable coproducts such as synthesis gas, hydrogen, or electricity. Plasma technology is also valuable for treating waste materials containing inor­ganic solids, because these materials can either be recovered or reduced in volume or can be oxidized and immobilized in a vitrified nonleaching slag. In general, plasma technology is capable of processing a wide variety of waste materials.

Waste treatment by plasma technology can be divided into three catego­ries: plasma pyrolysis, plasma gasification, and plasma compaction and vitrification of solid wastes. For solid wastes with high organic content, a combination of these three categories is often used [7].