Types of emission
Three substances which will not be affected by flue gas cleaning are produced during combustion.
CO: The level of carbon monoxide indicates the quality of combustion. If the gas phase combustion is complete, there will be no CO. The EU and other authorities have laid down regulations on periodic average values. The emission levels from our plants are typically 5-10 times below the limit values.
CO2 + H20 : Carbon dioxide (weak greenhouse gas) and H20 (water) are the end products of the combustion process.
The total amount of greenhouse gas (in CO2 equivalents) generated by combustion is less than 10% of the amount released by dumping the same quantity of waste in landfills.
NO and NO2, (NOx) : Nitrogen oxide which will oxidise to form the strong greenhouse gas NO2. A BWV plant emits less NOx than typical w-t-e plants because of the furnace design and by controlling the elements in the combustion process (pyrolisis, gasification and combustion).
Emission of nitrogen oxide can be further reduced by means of:
- The SNCR-process: injection of NH3 or urea into the flue gas, or
- The SCR-process: injection of NH3 and use of a catalyst.
Any other substances are retained through the flue gas cleaning.
The number of different substances depends on the composition of the waste.
HCl and SO2 develop when the waste contains sulphur and chloride. When the flue gas comes into contact with strong bases in wet or dry form, solid compounds are formed which can be removed by means of flue gas cleaning.
Dioxin and furan are extremely toxic and hard degradable substances which are not accepted in nature.
Active carbon completely absorbs dioxin and furan, which can subsequently be burned or placed in a special dump. The catalyst in an SCR DeNOx process will destroy the dioxins and the furans.
PAH (poly-aromatic hydrogens) are toxic gases which do not decompose with poor combustion. Like CO, the amount of PAH reflects the quality of combustion. It is thus subject to limitation in the form of:
TOC (total organic carbon), which is the sum of combustible organic compounds. The figure is normally a weak echo of the CO value. Active carbon used for removal of dioxin and furan has a very high capacity of absorbing other substances such as PAH, CFC and gaseous metal compounds, e.g. mercury.
Particle and metal emission
During cooling down of the flue gas, gaseous metal compounds are condensed on the fly ash particles. Comparable types of waste will thus normally show a relatively stable relationship between the quantity of particles and the quantity of metal.
Particle filters in the flue gas purification system maintain the level of particle emission at the level required by current legislation.
The level of particle emissions permitted by law is so low that the level applying to metal emissions is automatically observed as well.
