Energy from Waste via Incineration

Energy from Waste (EfW) via incineration is the burning of waste under controlled conditions.

• This produces heat, gases and ash.
• The heat released is used for electricity generation and/or heating buildings.
• The gases are passed through filters and the unwanted components removed before being released to the atmosphere
• This process produces two types of solid residues: "Bottom Ash" and "Fly Ash".
• There are approximately 15 EfW plants for municipal waste operational in the UK dealing with a relatively small proportion of the total waste arisings.

Energy from Waste (EfW) via incineration is the name given to the combustion of waste under controlled conditions, to reduce its volume and hazardous properties, and to generate electricity and/or heat. The fuel value (calorific value) of household waste is about one-third that of coal.

The most widely adopted EfW process is called 'mass burn', where Municipal Solid Waste is burned on a moving grate in a furnace with little or no pre-processing. Air is introduced above and beneath the grate in carefully controlled amounts to ensure proper combustion. The hot gases released are directed to a boiler to recover heat. Around 600 kilowatt-hours of electricity per tonne of waste burned can be recovered. The combustion gases are then cleaned before being released to the atmosphere.

The burning of waste gives rise to two solid residues:

• Bottom ash, the main solid residue, which accounts for about 30% by weight of the original waste and about 10% by volume. Bottom ash is an inorganic sterile material with the consistency of sandy gravel containing about 10-15% ferrous metals. After ash is discharged from the grate it is quenched with water before metals are separated by magnets for recycling. The remaining ash can then be disposed of to landfill or can be recycled into construction materials.
• Fly ash, a much finer ash, arises from the particulate removal during the gas cleaning stage of the incineration process. It consists of fine particulates that arises in the flue gas during burning and the reagents such as lime or activated iron and salts that are used to remove pollutants from the gas stream. It represents about 4% by weight of the input waste. The fly ash is treated and landfilled as hazardous waste. The main hazardous property associated with fly ash is its alkaline nature, which derives from the use of lime in the pollution abatement process.

There are currently around 15 EfW plants for municipal waste operational in the UK dealing with a relatively small proportion of the total waste produced. In other parts of Europe EfW has a more dominant role, particularly in the Netherlands, Sweden, Switzerland and Denmark where a significant portion of the Solid Municipal Waste is treated in this way.