what is energy from waste?
An Energy-from-Waste (EfW) facility, also known as an Energy Recovery Facility (ERF), takes the portion of your household (or similar) waste which is left after you have removed the recyclable materials and, instead of putting it in landfill, uses it to create much-sought after electrical power and/or heat.
The process can be summarised as follows:
WASTE RECEPTION
Waste is brought to site in lorries and is weighed in using the weighbridges. Lorries then travel to the tipping hall entrance, where fast acting automatic doors raise and then lower quickly to prevent odour leakage and the ingress of vermin. Inside the tipping hall, the vehicle reverses to the edge of the one of the five tipping chutes, then deposits its load into the waste bunker. Automated cranes mix the incoming waste to homogenise the waste, to ensure a more consistent heat release, before feeding it into the waste hopper. The tipping hall and bunker area are kept under a slight negative pressure to avoid unpleasant odour emissions – this is done by taking air from this area and feeding it into the combustion chamber to feed the fire with oxygen and burn off the odour.
waste combustion
Hydraulic rams push the waste from the feed hopper into the bottom of the boiler where it is completely combusted on the well-proven inclined forward feed grate with air-cooled grate blocks. The boiler walls are made of tubes filled with water. In the first passes the radiant heat from the waste combustion heats this water, initially pre-heated in the economisers which use the residual heat in the flue gas, until it becomes steam. In this stage, the temperature of the flue gas in the boiler is required to be at least 850 degrees Celsius for 2 seconds to comply with the requirements of the Pollution Prevention and Control Permit from SEPA. To control the NOx emissions from the process, ammonia is injected into the combustion gases at various heights within the boiler to react with the NOx to produce nitrogen (N2) and water.
ENERGY CONVERSION
The steam is superheated within the superheaters by convection using the heat in the flue gas as it leaves the radiant passes of the boiler. This superheated steam (raised by the boiler to 73 bar(a) and 430 Celsius) is then expanded through the steam turbine which is connected to and spins a generator to produce electrical power. The electricity generated is exported either to the grid or to private wire customers. The Westfield ERF has been designed as a combined heat and power plant, which means that a proportion of the steam generated can be bled from the turbine and supplied to heat customers co-located on the wider Westfield site as either steam or hot water (see more on Heat Networks).
Following expansion in the turbine, the steam is cooled in the air-cooled condensers to maximise the efficiency of the water steam cycle. This cooled condensate is mixed with additional make-up water and fed back into the economiser to be reheated and start the cycle again.
FLUE GAS TREATMENT
Prior to release to the atmosphere, the cooled combustion gases are cleaned in the flue gas treatment equipment. Acidic contaminants in the flue gas react with hydrated lime, and heavy metal compounds and organic contaminants, such as dioxins and furans are removed by adsorption onto powdered activated carbon. The system also includes a fluidised bed reactor which removes the pollutants like SO2, SO3, HCl and HF. Fabric filters are used for the separation of solids such as reacted lime and dust from the flue gas via a gas-permeable fabric. Due to the intensive contact of the flue gas and the adsorbents in the filter layer, the removal of pollutants from the flue gas is further improved. The height and diameter of the stack have been designed to ensure that the cleaned flue gases have the velocity required to disperse in an area that will have no or negligible impact on human health or the environment (see our Environmental and Social Impact Assessment).
RESIDUE MANAGEMENT
Ash, metals, and other aggregates that reach the end of the grate are cooled by a quench and then conveyed to the bottom ash storage hall for aggregation and collection by a third-party for treating for re-use and recycling.
The air pollution control residues resulting from the flue gas treatment are collected in dedicated silos ready for collection by a third-party for treating for re-use and recycling.
An overview of the Westfield ERF site is shown adjacent:
