What is Incineration?

Each year the UK produces around 100 millions tonnes of Municipal Solid Waste (MSW), most of which is sent to landfill. MSW is waste collected by a municipality within a given area, and is mostly made up of household waste but can also include some commercial wastes.

The EU Landfill Directive (1999/31/EC) stipulates that we must find alternative ways to treat our waste, ultimately, in order to reduce the amounts of waste we send to landfill. This is because the landfilling of waste is having a negative effect on our environment as: (a) the biodegradable portion of MSW (green waste, food waste, paper/card waste) sent to landfill being broken down by bacteria and releasing methane into the atmosphere, a powerful greenhouse gas known to contribute to climate change, and; (b) we are unnecessarily throwing away and wasting our valuable existing materials and resources, forcing us to extract and process new raw materials in order to produce virgin material products. Mass burn incineration with energy recovery is one of the alternative options available to us.

Incineration is an alternative waste treatment technology that involves the combustion of typically unprepared (raw or residual) MSW, with the recovery of energy in the form of electricity and/or heat generation (DEFRA). Ultimately, the aim of waste incineration is to reduce the impacts residual waste has on the environment when sent to landfill.

Waste Incineration Technologies

Moving Grate

The moving grate furnace system is the most commonly used combustion system for high through-put MSW processing in the UK (DEFRA). Waste is continuously fed into one end of the furnace, where it is moved along by a mechanically activated grate, and ash is continuously discharged at the other end. The process conditions are controlled in order to optimise the waste combustion, and to ensure the complete combustion of the feed (DEFRA). A quench is used at the end of the process in order to rapidly cool down the remaining non-combustibles (ash).

Moving grate incineration systems used to treat MSW can be divided into three main sub categories. These include:

• The Stepped Inclined Grate – bars, rockers and/ or vibration is used in order to move waste down each of the grates.
• The Roller Grate – this system uses a series of adjacent rollers which rotate in the direction of the waste movement.
• Inclined Counter-Rotating Grates – grate bars rotate backwards to agitate the waste and prevent it tumbling down the forward inclined grate until burn out is complete (DEFRA).

Fixed Grate

The fixed grate furnace system involves three main stages with the waste being moved along in series. The first stage of this system is the drying out and initial combustion stage. This is followed by a secondary combustion stage, until the final stage which is for final carbon burn out.

Fluidised Bed

The fluidised bed technique used to combust MSW involves the pre-sorting of waste materials in order to remove any heavy and inert objects prior to combustion. This initial pre-sorting of waste is followed by the mechanical processing of waste materials in order to reduce their particle size and facilitate combustion. The bed of the furnace is ‘fluidised’ by air being vertically blown through the material at a high flow rate (DEFRA).

Fluidised bed incineration systems used to treat MSW can be divided into two main sub categories. These include:

• Bubbling Fluidised Bed – the air flow rate is high enough to combust the waste, but not high enough to carry solids out of the combustion chamber.
• Circulating Fluidised Bed - the air flow rate is much higher than the bubbling fluidised bed, therefore particles are carried out of the combustion chamber by flue gas, and solids return back to bed.

Rotary Kiln

The rotary kiln furnace system used to combust MSW involves a two stage process. The first stage of the process involves waste being fed into a downwards inclined rotating kiln where it undergoes primary combustion. The rotation moves the waste through the kiln with a tumbling action which exposes the waste to heat and oxygen (DEFRA). This stage is followed by waste entering a secondary combustion chamber where further combustion of MSW takes place.

Potential Advantages

• Waste incineration reduces the amount of residual MSW sent to landfill.
• Waste incineration technologies comply with the EU Waste Incineration Directive (2000/76/EC) as they achieve low emission levels.
• Non-combustible material (ash) from the incineration process can be recycled and used in the construction and road building industries.
• Incineration plants can be located nearby to residential areas. This supports the proximity principle which suggest waste should be disposed of as near as possible to its place of generation.
• Waste incineration can reduce the volume of waste to 10% of its initial value.
• Waste incineration releases energy from the waste due to its calorific value, which is used to generate electricity and/or heat.

Potential Disadvantages

• Unless incineration only deals with truly residual waste (what is left once maximum recycling and composting has happened) it will undermine recycling and composting.
• Once constructed, an incineration plant requires a continuous feed of waste for, on average, a period of around 30 years. This obliges waste managers to bring waste to specific incinerators even though the cost may be higher than alternative treatments such as recycling and composting.
• Increased technical experience is required to operate an incineration plant.
• Air pollution controls in waste incineration plants are extremely expensive.

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