At the present time, the western world produces about 10 tonnes of waste per person annually in the form of household waste, industrial waste and waste from activities such as energy production, agriculture, mining and sewage disposal. Incineration is the best way to dispose of much of this material in an environmentally-friendly manner. For example, clinical waste is still buried in many-developing countries, only to be dug up by dogs with hazardous consequences: clearly, those countries have an urgent need for simple cheap incinerators. Incineration technology is still evolving rapidly throughout the world and further innovations can be anticipated in the next few years.
An important aspect of incineration combustion systems involves treatment of the gaseous, liquid and solid effluent in order to minimize their environmental effects. In addition, energy can be derived from many wastes and energy recovery is therefore an important part of incinerator design.
A typical incinerator consists of three components: the furnace chamber, the heat recovery boiler, and the flue gas treatment plant.
In a municipal solid waste (MSW) incinerator, waste material is usually burned in a refractory-lined furnace chamber as a moving bed on a grate with underfire air. The waste material is very inhomogeneous, and various techniques are used to mix the burning material on the grate as it progresses along the chamber. For example, a reciprocating grate or steps allowing the material to fall and disperse may be used. Other mixing techniques used include rotary kilns and fluidized beds.
Material introduced to the bed must dry first before it can be ignited. This drying stage is significant since municipal waste typically contains 30% water. This water content is the main factor influencing the calorific value of the waste. As the waste heats up, it loses volatiles which burn above the bed. When ignition temperature is reached, the material in the bed burns in the surrounding oxygen, emitting Carbon Dioxide and gradually pyrolyzing to a char. Once the oxygen is used up, the carbon dioxide is reduced to Carbon Monoxide, which burns in the freeboard along with Pyrolysis products. Eventually, the char burns to an ash which drops into the ash pit. Usually the ash contains less than 5% carbon, and indeed the permitted carbon in ash may be controlled by legislation. Again it is emphasized that all these bed combustion processes take place while the bed is being mixed; at present, there is no good mathematical model of the total process and empirical correlations are used for bed design.
The overall air/fuel ratio in the furnace chamber is kept lean to ensure that there is sufficient air to minimize residual carbon in the ash, and secondary air is injected above the bed to ensure that solid and gaseous hydrocarbons are fully oxidized before they are passed to the flue gas treatment plant. Due to rapid variations in the local composition of waste, optimization of air flow distribution poses a very difficult control problem and empirical techniques are usually used.
The heat recovery boiler consists of two stages. In the first stage, the gases are cooled by the radiation of heat to the walls whilst in the second, gases are further cooled by convective heat transfer to tubes located in the flow. In the radiant section, water in the wall tubes is boiled and passed to the boiler drum where the saturated steam and the water are separated. If superheated steam is required, then the first convection sections are used as superheater.
The main problems encountered in heat recovery boilers are related to corrosion since flue gases from the incineration can contain corrosive components, such as hydrochloric acid from the burning of PVC plastics. To minimize corrosion, wall temperatures must be kept relatively low and impingement of flame or particles on the wall must be avoided to prevent reducing conditions at the metal tube walls.
To minimize the emission of pollutants, the first stage of flue gas treatment is achieved in the furnace and boiler by ensuring that there is sufficient excess air and residence time to complete the combustion process.
The removal of pollutants such as particles and acid gases from the incinerator flue gases is accomplished in a flue gas treatment plant. This follows well-established chemical engineering design principles, and consists of components such as venturi and tower Scrubbers, Heat Exchangers, Electrostatic and bag Filters, Fans and Pumps.
Although dioxins can be destroyed in an incinerator, they can be formed in the flue gas treatment plant. The minimization of dioxins may be achieved by ensuring that the optimum temperature/composition/time history of the flue gases is satisfied. Permitted levels of emission of all pollutants is tightly controlled by legislation.
As already mentioned, incinerators provide a vital stepping-stone in improving energy efficiency. A key point is that energy in the form of electricity is worth about eight times more than the same amount of energy as steam heat. Hence, it is important that the proportion of waste converted to electricity is maximized.
Since much of the heat produced by an incinerator during high summer is wasted, there are opportunities for air conditioning systems which use hot water to provide chilling during this period.
Nasserzadeh, V., Swithenbank, J. and Jones, B. (1993) "Effect of high-speed secondary air jets on the overall performance of a large MSW incinerator with a vertical shaft". Combust. Sci. & Tech. 92:389-422.
Nasserzadeh, V., Swithenbank, J., et al. (1993) "Three-dimensional mathematical modelling of the Sheffield Clinical Incinerator, using computational fluid dynamics and experimental data". J. Inst. Energy. 66:169-179.