Waste Incineration Principles

Although the design of an incineration facility will vary to some extent according to the nature of the waste material (the feedstock) being incinerated, the essential elements are basically similar. Each of the essential elements of the plant perform various functions in the handling and combustion of wastes, and the removal and the treatment of by-products such as ash, fumes, heat and noise.

The following is based on descriptions in the Royal Commission's report on incineration and the IPC Guidance note on Waste Incineration. A plant consists of:

· Facilities for handling, storing, and mixing feedstock without creating a hazard or a nuisance. Typically, wastes will be driven by truck into a bunker area and unloaded. The feedstock will then be transferred from the bunker into a hopper, which feeds the combustion chamber.

· A furnace or combustion chamber. The furnace is designed to allow the maximum combustion of the feedstock (primary combustion), and also to subject the resulting gases and aerosol of fine particles to further combustion (secondary combustion), reducing the content of soot and other pollutants in the exhaust.

· A heat recovery system for cooling the exhaust gases prior to further pollutant removal. Heat recovery is achieved through water cooling or air cooling the exhaust gases through a heat exchanger. The recovered heat may be re-used as heat or used to generate electrical power.

· A gas cleaning system, typically consisting of a "scrubber" (filters which absorb pollutants) and an electrostatic precipitator, and/or fabric "bag filter", to remove fine particles and some polluting gases (often termed fly ash).

· A fan to draw exhaust gases through the system before being discharged to the atmosphere via the stack (chimney).

· Facilities for the solid waste and water residues are needed. Ash and clinker from the furnace grate (bottom or grate ash) fall into a water-filled quench tank and are removed as a sludge by conveyor to a bunker. Liquid effluents from the exhaust gas cleaning mechanism ("scrubber") and quench tank will have become contaminated through contact with the exhaust gases and ash.

The process will be designed to achieve as thorough a degree of combustion as possible, which will be affected by the time in the furnace, temperature achieved, oxygen availability and nature of the feedstock. Fumes from the first (primary) combustion stage usually go through secondary combustion stage to improve the degree of combustion. Representative retention times are 30 minutes for solid waste primary combustion of the fumes. The design temperature for secondary combustion is usually between 850oC and 1200oC. Pollutants can reform while the exhaust gases are cooling, so this stage us also considered in the design.

It is more difficult achieve conditions which will allow complete combustion if the feedstock is mixed ("heterogeneous") - as is municipal waste or clinical waste. More homogenous (uniform) feedstock (such as sewage sludge or some chemical wastes) will allow combustion conditions to be optimized more easily.

Pollution Control

Pollutants will be present in the exhaust gases from the combustion chamber, either having been present in the original feedstock (such as metals) or having been produced during the combustion or gas-cooling process. Ideally, all organic compounds would break down into carbon dioxide and water if completely burned, but in practice "products of incomplete combustion" (PICSs) such as dioxins are formed. In addition nitrogen oxides are formed both from nitrogen in the waste and in the air supplied to support high temperature combustion.

Pollutants will subsequently be incorporated into the ash produced from the grate and exhaust-cleaning systems, and in liquid effluents used to quench the ash. These effluents need to be treated and disposed of to sewer.

Various types of filter will be present, although the exact types and the configuration can vary.

Scrubbers remove acid gases and some metal content by passing the gases and some of the metal content by passing the gases through chemical regents. A Selective Non-Catalytic Reducer (SNCR) may be used to convert nitrogen oxides into nitrogen by reacting with ammonia or urea (and inhibits dioxin formation); Flue Gas Recirculation (FGR) also reduces nitrogen oxide and dioxin formation. Ceramic filters, fabric filters, electrostatic prewcipitators, and/or wet scrubbers may be used to remove particulates (and any adsorbed pollutants in or on the particulates). Activated carbon, an adsorbent, may be used to filter out volatile substances such as mercury and PICs.

An incineration plant will not normally have all of these devices. The most common pattern is a combination of physical sifting and scrubbing.

Associated Hazards at an incinerator site include the fly ash itself (which contain high levels of toxic substances) and the storage of waste before incineration occurs (e.g. chemical drums might leak).