Combustion or incineration technology is used for a very wide range of waste to reduce its volume and hazardous characteristics as well as to generate heat and electricity. It is most widely applied and can be implemented on a large as well as small scale. Waste generally contains organic matter, minerals, met­als, and water. Metals are separated before incineration. During incineration, flue gases are generated that contain energy which can be transformed into heat and electricity. The organic waste burns when it has reached the ignition temperature and come into contact with oxygen. The overall oxidative pro­cess of combustion is highly exothermic, although it occurs in the stages of drying, degassing, pyrolysis, gasification, and combustion; not all of which are exothermic in nature. Initially, heat may be needed to start the process. However, once the chain reaction of the combustion process is started [11], no external heat or additional fuel is required. Although the stages of the com­bustion process are inseparable, furnace design, air distribution, and control system can affect these stages and reduce pollutant emissions.

Typical Reaction Conditions and Products from Pyrolysis, Gasification, Incineration, and Plasma-Based Processes

A typical set of reaction conditions for the combustion process is outlined in Table 6.3 [7, 9]. The table also compares the typical operating conditions for combustion with those of pyrolysis, gasification, and plasma treatment (i. e., other important thermochemical processes). Air in the combustion pro­cess can be replaced by oxygen, and it is the only thermochemical process where air (or oxygen) is in stoichiometric excess. In fully oxidative combus­tion, the flue gas contains water vapor, nitrogen, carbon dioxide, and oxygen. However, depending upon the nature of the waste and the operating condi­tions, smaller amounts of CO, HCl, HF, HBr, HI, NOx, SO2, VOCs, PCDD/F, PCBs, and heavy metal compounds can be a part of flue gas [7]. The pres­ence of these compounds can cause environmental issues, and they should be removed before the flue gas is emitted to the environment. Depending upon the combustion temperature, heavy metals and inorganic matters (salts) can end up in the flue gas or fly ash. The amount and composition of solid residue during combustion depends on the nature of waste, combustion temperature,

and the detailed process design. In MSW incineration, the bottom ash is about 25 to 30% by weight of solid waste input. Fly ash can be 1 to 5% by weight of the waste input [12]. Additional treatment of the bottom ash can allow its use in the concrete and other construction industries. Fly ash needs to be immobi­lized or vitrified to make it environmentally safe for landfill disposal.

For an effective incineration, an excess of air or oxygen (generally 1.2 to

2.5 times stoichiometric requirement) is necessary. This number depends upon the nature of fuel (gas, liquid, or solid) and the incinerator design. In principle, the incinerator generates excess energy from MSW combustion to produce heat and electricity. The incinerator size may vary depending on the availability of waste. In Europe, the average capacity is about 193 k tons per year [7, 11]. A typical schematic of the incineration plant is shown in Figure 6.4. Such a plant includes (a) waste reception, storage, and preparation unit; (b) a combustor; (c) boiler or other type of energy recovery unit; (d) flue gas cleaning and emission monitoring and control system; (f) waste water control and management system (e. g., from site drainage, flue gas treatment, and waste storage); and (g) residue management and discharge system which includes bottom ash management and treatment system as well as solid resi­due discharge and disposal system [13]. The plant design also depends on the nature of the waste being treated (its chemical composition and physical and thermal characteristics), types and quantities of residues which in turn depend on the installation design, its operation, and the waste input.

Process stability and optimization depend significantly upon the variabil­ity in the waste input. Although a narrow variability of input will give more stability and better environmental performance, this may require expensive waste pretreatment and the selective collection of waste. On the other hand, gas cleaning equipment is generally 15 to 35% of total capital investment, so its optimization is also essential [7]. Thus, a prudent optimization of the overall waste management system is often needed.

There are basically three types of incinerators used in commercial opera­tions: grate incinerators, rotary kilns, and fluidized beds [7].