Rubbish incinerators have been a feature of big city waste management since the 1940's throughout much of Europe. Although their popularity for Municipal Solid Waste (MSW) disposal has risen and then waned several times over the generations, the technology of what is now known as "mass burn" rubbish incineration just kept coming back, and has been developed further with each generation.

 

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Over the past 20 years hugely stringent European Union regulations applied largely through the Waste Incineration Directive (WID) have been imposed. These have, under the eye of the state funded environmental regulators working as a policeman, removed all significant concerns of air pollution and low level, but sustained, chemical fall-out. Such pollution might progressively and over a long period make the land around the rubbish incinerator flue toxic for food crop growth, and did do so around some early plants, but such events just do not happen any longer.

Many wondered during the late 1990s whether, after the WID was introduced, mass-burn rubbish incineration would survive. After all, there were a lot of other waste processing technologies emerging each with much lauded benefits such as reduced production of polluting gases. Lower polluting technologies were seen as requiring greatly reduced parasitic scrubbing apparatus sucking power from the energy generated, before it could be distributed for general use through the power grid.

It was assumed that by reducing this unproductive load other processes would cost less to run, and some could even produce new products such as a process gas know as "syngas", and methane, which would surely be an asset that when taken into account on balance sheets would turn a profit which would wonderfully reduce the operating costs of these new technologies for waste disposal.

But in 2010 it can now be seen that although a small proportion of new waste processing technologies coming on stream currently and in the future will be other types of processes, the mass burn rubbish incinerator is still winning out on low risk, low cost and efficiency grounds , time and time again.

How can this be now that many great engineering minds have been set upon producing alternatives to rubbish incinerators over the past fifteen to twenty years?

Well, in truth the reasons for the continuing popularity of this system are not hard to find. First of all, rubbish incineration has a long and successful pedigree in reducing the bulk of MSW (by about 80%-90%) and mass (by about 65%-75%) of MSW, subject to its composition.

Secondly, it can handle a very wide range of heterogeneous solid waste of the types for which municipalities throughout the world have collection responsibilities, without the need for shutting-down to clear out residues other than exceptionally awkward large items which occasionally find their way into waste handling systems.

The waste needs little, if any, pre-sorting or processing before being burned, and although the burn-out attainable without after-treatment such as a dump grate, clinker generator or vitrifying hearth may not be as good as that of other combustors such as a rotary kiln or a fluidised-bed incinerator, it is nevertheless usually good enough for the vast majority of integrated waste management systems.

Most of the solid residues also remain on the grate rather than being entrained in the off-gases, which makes gas cleaning simpler than say fluidised bed type reactors.

Modern methods of rubbish collection should remove all bulky waste but occasionally fail and sometimes unwanted items including "white goods" (fridges, washing machines and the like) which should bypass the incineration process do go through, and mass burn systems are very tolerant of such events.

After all, a large central rubbish incineration plant will be used as the disposal point by many different waste collection companies, and these carriers will regularly be transporting forms of waste which, if an error occurs and it arrives at the incineration plant rather than say a landfill, will jam the flow through any other form of plant. In other forms of plant not only will the result be a jam, but in addition huge damage can result.

Incredibly, operators have reported outsized objects in the waste stream to include incidences of concrete lamp-posts, large coils of wire rope, lawn mowers, vacuum cleaners, steel scaffolding poles, ball bearings, gas cylinders and even cannon balls! One UK pyrolysis plant was reported as running a downtime of several weeks after a fairly small item, namely a car steering wheel, became jammed in the furnace entrance.

Depending upon the contractor/supplier, different grate systems, boiler configurations and residue handling systems may be offered, but the rubbish incinerator concept used most and which has survived the test of time is the reciprocating grate, tail-end rapped boiler. This reportedly gives high on-load availability without the need for frequent shut-downs for cleaning and has simple clinker bunkering.

Residue processing advisedly takes place afterwards to avoid complicating the material flow path and adversely thereby affecting operating availability. Fly ash is collected separately from bottom ash and comprehensive gas scrubbing and treatment is fitted as standard.

Each of the companies specialist offering their own variants have developed pollution reduction techniques such as combustion gas recirculation (after de-dusting) which can reduce NOX levels and improve boiler efficiency. Another example is ammonia injection into the combustion chamber also to reduce NOX levels, which may be used, and still further catalytic systems may also be included, with the result that almost nothing other than steam and carbon dioxide leaves the flue stack.