The gases from the combustion process go through a multi stage cleaning process. What comes out of the chimney is largely steam, oxygen, nitrogen and carbon dioxide with tiny amounts of pollutants. These are well within the strict levels set by the Environment Agency in the facility’s environmental permit. Monitors at the base of the chimney continuously record emission levels. You can see the current monitoring information here.
What are the emissions?
Particles, also called particulates, are formed during the combustion process. These are captured in bag filters and form part of the Air Pollution Control Residue (APCR) which is sent for disposal off site in sealed containers.
Carbon monoxide and VOCs as Total Organic Carbon
High temperatures, a good mix of waste and sufficient air are all needed to achieve complete combustion and limit the formation of carbon monoxide and total organic carbon.
The temperature of the furnace is carefully controlled and additional burners can be used to bring up the temperature if it starts to drop.
A good mix of waste is achieved, initially using the grab in the bunker, but also on the moving grate in the furnace which keeps the waste moving throughout combustion.
The air flow into the furnace is controlled using fans alongside and underneath the grate.
Sulphur dioxide, hydrogen chloride and hydrogen fluoride
Many of the things we throw away, including batteries and plastics, contain either sulphur, chlorine or fluorine. These are released during combustion and react with the air to form sulphur dioxide, hydrogen chloride and hydrogen fluoride.
We use hydrated lime and lime slurry to neutralise these gases and convert them into a solid which can be captured in the bag filters. This excess lime, particulates, salts and carbon dust make up the Air Pollution Control Residue, which is sent for disposal off site in sealed containers.
Oxides of nitrogen and ammonia
Whenever anything is burnt in air, oxides of nitrogen are formed. This is because the air we breathe is made up of nitrogen (78%) and oxygen (21%) which combine during combustion. Nitrogen dioxide and nitrogen monoxide are known as oxides of nitrogen or NOx.
To control these in the combustion process, we inject ammonia into the furnace, which reacts with the oxides of nitrogen to produce nitrogen and water.
Bottom ash recycling
The bottom ash left after the waste has been burnt is cooled and carried by conveyor to an on-site recycling facility. Metals are separated for recycling and the remaining ash is processed to make secondary aggregate products for use in road building and construction activities.
Bottom ash is a non-hazardous material and is tested regularly to ensure this, in accordance with the Environmental Services Association Sampling and Testing Protocol. Each month, a random sample is taken from the bottom ash and tested at an independent UKAS-accredited lab. The results of this testing are reported to the Environment Agency in line with the requirements of the facility’s Environmental Permit. The facility entered operations in late March 2017 and testing began in April 2017 – to date, all samples have tested as non-hazardous.