What is Energy from Waste (EfW)?

From Janurary 2019, none of Devon’s black bin bag waste goes to landfill. If it cannot be recycled or composted it goes to a Energy from Waste plant (also known as an Energy Recovery Facility)

Creating energy from waste is the process of generating electricity through treating waste. In Devon there are 2 EfW facilities that burn residual waste at high temperatures, which creates steam to power turbines and create electricty. You might also hear this being called incineration. At our Plymouth plant some of the heat energy generated from burning the waste is also used to heat the Royal Navy Dockyard at Devonport.

Exeter Energy from Waste process

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Energy from Waste Process
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Tipping Hall

Waste collection lorries deliver the waste to the tipping hall.

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Waste Bunker

Waste is stored temporarily in the bunker, where it is mixed until the grab crane moves it to the hopper.

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Grabber

The grabber lifts around 1 tonne of waste with each grab – thats the weight of a baby elephant!

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Feed Hopper

Waste is placed into the feed hopper and then pushed into the combustion chamber by a ram.

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Combustion Chamber

  • The kiln oscillates (moves back and forth) and is tilted to allow the waste to tumble through the cell to ensure that all of it is burnt.
  • The kiln has air injected into it to promote combustion and control temperatures of up to 1100°C.
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Post Combustion Chamber and Boiler

All gases produced by the combustion process must travel through a post combustion chamber to the boiler. The minimum operating temperature in the post-combustion chamber is 850ºC, in order to burn the waste at a high enough heat to limit the production of dioxins. This requirement is maintained under all start up, operating and shutdown conditions. An oil fired booster burner is used where necessary to maintain this temperature i.e to reach temperature again after it’s been shut down for maintenance.

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Boiler

  • The boiler converts the heat energy from the combustion gas to high pressure steam energy.
  • The steam energy passes through the steam turbine to give an electrical output.
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Generation of Heat and Electricity

  • Around 4.5MW per hour of electricity is generated, of which around 3.75MW is exported to the national grid at all times.
  • There is also the potential for up to 13MW of heat to be exported into the local community. A local district heating system is being investigated, but digging up the surrounding building to put in pipes is currently too expensive.
  • Also, if more heat is exported, less electricity will be generated.
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Economiser

The economiser reuses the residual heat from the flue gases to pre-heat the hot water before it goes to the boiler.

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Emissions Control

  • Once the gases have passed through the boiler, they are treated to minimise the level of pollution emitted through the stack.
  • Hydrated lime is used to neutralise the acidic gases and activated carbon is used to remove residual metals, dioxins and other pollutants.
  • After this, the gases are then passed through 42 bag filters, which take out the last impurities and dust particles before being discharged through the stack.
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Stack

  • The emissions from the stack is monitored at all times by a temperature sensor, a pressure transmitter, a flow meter, a dust analyser, multi-component gas analyser and a volatile organic compound (VOC) analyser. This data is transmitted to the computer in the Control Room.
  • Emissions have to comply with the Industrial Emissions Directive and are set by the Environmental Permit issued by the Environment Agency under which the facility operates. The data is available online for the general public to access at all times.
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Bottom Ash

  • Ferrous metals are removed using an electromagnet and will be recycled into new products.
  • Bottom ash (approximately 20% of the input waste) is sent away to be turned into aggregate and used as a construction material for things such as roads.
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Air Pollution Control (APC) Ash

APC Ash (approximately 3% of input waste by weight) is collected by a company that neutralises its hazardous nature and turns the ash into building blocks.