Waste incineration has been a widespread waste reduction technique for over one hundred years. Bonfires are a simple example but refuse was first dealt with in this way on a large scale following the industrial revolution when street littering in London became intolerable. This was a similar process to that which exists today only there were no environmental controls. The streets became cleaner but, with the incinerators and all London's heavy industry, the skies filled with smog. Even approaching the end of the 20th century emissions have been largely ignored. It is only with the incineration directives that came strict standards which incinerators had to comply to by 1996. It meant that pollutants in incinerators' emissions such as, sulphur oxides or organic carbon, had to be reduced to acceptable levels. So dramatic was the required turnaround that mostly all of the incinerators closed down. Of the 30 UK incinerators in the 1970s, only 4 had the major upgrade necessary to survive implementation of the emission standards. The directive put emission limits with which the incinerators had to comply. This has defined how modern incinerators operate. A modern day mass burn incinerator is not just one highly complicated operation but merely a host of bolt-on processes put end to end. The following diagram shows the recently built (1998) Dudley plant in the West Midlands:

Figure 1: An example of a mass burn waste incinerator (Dudley)

The incinerator before it is operational, first has to be brought up to the running temperature of about 750ºC. This is done by burning oil for a period of (for some plants) over 24 hours. For this reason the process is only suited to constant operation - a plant shutdown will be uneconomic and even ecologically unsound.

Waste will normally arrive in an RCV and be tipped into the holding area (1) where it will be picked up by the grabs (2) and dropped into the feed hoppers (3). The waste will then be mechanically pushed by a hydraulic ram (5) onto the moving grate within incinerator (4). This will allow the refuse to gradually pass through the incinerator over a period of about two and a half hours. Air will be drawn to the combustion chamber in from the holding chamber (6) so as to avoid the escape of malodours to the surrounding community. The ash will be quenched (7) and then recycling can be carried out by extracting the metal content using an electromagnet. The heat from the combustion chamber will be utilised in a multi-pass boiler (8) where the energy will be superheated steam for efficient use in gas turbines for electricity production. The flue gases will then need to go through a clean up operation before they can be discharged to the air. Dry urea is injected directly into the combustion chamber to limit the production of nitrous oxides (NOx). The gases will then go to a scrubber reactor (9) which sprays lime milk in order to treat acid pollutants (SO2 and HCl) and injects active carbon to remove residual organic compounds such as dioxins. Further to this they will pas through a bag house filter which removes particulate matter and take out the heavy metals. The large proportion of poisonous, carcinogenic, and environmentally damaging components have now been removed from the flue gases and the remainder (mostly carbon dioxide and water vapour) is ready to be discharged through the chimney stack. Variations on this theme will include different methods for moving the material through the combustion chamber and supplying the required air.

It can also be economically feasible for larger scale plants to bolt on a MRF to their front end. The waste can then be pre-sorted for recycling and will allow for better predetermination of the waste content going into the incinerator.

It is useful to consider all the inputs and output of the process in order to do a proper assessment. These are shown in Figure 2. The input (municipal waste) will relinquish it's energy content through the combustion process. This energy can be harvested and converted into electricity at an efficiency of about 21%. The waste will be reduced to 24% of it's original weight and only 10% of the original volume (statistics from Dudley waste incinerator). 18% of this is bottom ash which is inert but contains a concentration of heavy metals. 3% more is recovered as ferrous metals. And 3% fly ash is collected - this being a special waste because of the lime that is used to neutralise the acid gases giving it an irritant property. The fly ash has to be sent to a specific landfill for 'special waste' (that which is potentially hazardous), but the bottom ash can be disposed of in regular municipal landfills or if a market can be found then it can be ideal to use it as a substitute raw material in road construction. There is the clear environmental benefit here which is the reduction of need for landfilling. This prolongs the life of our existing landfills which in turn reduces the need to find new sites for landfilling purposes. Landfilling will compact waste to 30% of it's original volume; incineration and ash disposal on the other hand reduces volume to 10% thereby potentially trebling the life of a landfill. Also the bottom ash that is sent to landfill will be inert so will not generate problematic landfill gas. One concern that has to be addressed is that the ash contains a higher concentration of heavy metals. Therefore the landfill will still need to have effective leachate controls and to keep close checks on it to reduce the risk of polluting local water supplies.

Figure 2: Process flow diagram for Waste-to-Energy combustion

When considering the emissions from Waste-to-Energy plants, incineration has been given a bad name because of the heavily polluting old generators. Modern plants have to meet the standards laid down in the directives drawn up to combat air pollution from waste incinerators if they are going to avoid closure. These directives keep emissions to 'tolerable' levels but what tolerable actually means is open to opinion. Incinerators comply with existing legislation but there is concern that the gases emitted still have an adverse effect on human health and the environment.

An encouraging fact is that the incinerator in Dudley created great interest with the local community which caused them to ask when it was going to be switched on. They were told it had already been operating for three months! (Personal communication with Jim Stuart of Dudley Waste Services Ltd.)

When looking at the environmental impact of carbon emissions from the process, it is useful to compare to energy production from biomass. This remarkably similar technology involves incineration and a release of carbon dioxide to the atmosphere. As opposed to fossil fuel this does not have a permanent effect on the environment as it involves a short CO2 cycle. In other words for the CO2 emitted to the atmosphere from the burning of trees, the same amount will be reabsorbed back into replanted trees. Therefore there is no overall impact on the environment. Waste incineration from this environmental perspective is a similar process - the carbon dioxide emitted when the combustible portion of MSW is burnt, for the most part, comes from and is reabsorbed by plants. These plants will in turn be used for food, clothing etc. and once discarded will eventually go to constitute the fuel.

In fact about 62% of municipal waste is biodegradable - the rest is either inert (22%) or derived from oil in the ground (general plastics and that which is used in clothing). This non-ecologically carbon cycle friendly portion is 16% of the household waste stream. Therefore, if the inert fraction is removed from the waste stream and the remainder is burnt electricity generation, about 80% (62/{62+16}) of the emissions from burning waste for have a short carbon cycle. Hence substituting Waste-to-Energy for fossil fuel power generation could be ecologically beneficial by reducing the quantity of fossil fuels burnt. It could cut greenhouse gas emissions by 80%, substantially reducing the environmental burden.

Mass burn incineration and landfill gas power generation can be compared by estimating the possible energy output from the process. If all waste in the UK (90Mt) was incinerated at 24% efficiency, and using a load factor of 65%, 3.7GW of power could be generated - 5% of the UK's installed capacity. This compares favourably to landfill gas (which will only be generating 0.6GW). An increase in process efficiency is a boost in sustainability as there is an increase in energy production per unit of emission to the environment.

All noise is designed to be contained by building sound proofing into the walls of the plant. Noise from the process drops away to background levels typically within tens of metres from a Waste-to-Energy plant.

There is still a visual impact. A waste incinerator is a prominent feature in any landscape. They are however designed to be as tidy and visually unintrusive and appealing as the process and space requirement will allow. In fact there is a plant in Vienna which is designed with visual acceptability as paramount and appears an impressive and attractive landmark. When it comes down to a choice between another landfill and a self contained incinerator, most residents would likely prefer the latter where noise, smell and litter are contained. It does have to realised however that when it comes to the planning stage, the construction of a new plant is likely to receive opposition.