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Sulphur Dioxide 


Sulphur Dioxide is a gaseous pollutant with the chemical formula SO₂. It is toxic, colourless and has a pungent smell. 


Sulphur Dioxide is predominantly produced as a result of the combustion of fuels containing sulphur and sulphur compounds such as coal, biomass, oil, natural gas and biogas. 

Environmental Impact 

Sulphur Dioxide is a major air pollutant. It can have significant effects on human, plant and animal life. It is a major source of acid rain through the production of sulphuric acid (H₂SO₄) in the environment. Acid rain can have a considerable impact on plant life and ecosystems where it deposits, in extreme cases areas can become desertified. 


Control of sulphur dioxide is by two main routes: the removal of sulphur from the fuel before combustion (pre-combustion) or the removal of sulphur dioxide itself from flue gases after combustion (post-combustion). 
An example of pre-combustion treatment is the reduction of hydrogen sulphide (H₂S) from biogas. One method for doing this is by dosing the waste material producing the gas with ferric salts . 
An example of post-combustion treatment is the use of a wet scrubber utilising calcium carbonate (CaCO₃). In this method sulphur dioxide in the waste gas reacts with the calcium carbonate to produce calcium sulphate (CaSO₄) which can be removed and is often sold as a product. 


Alkali Environmental perform periodic measurements of Sulphur Dioxide in ducted emissions from point sources to the standard reference method BS EN 14791. The reported results carry UKAS and MCERTS accreditation where the method can be performed to the standard. 
These measurements are taken using a manual sampling train whereby a single result is obtained for a gas sample taken from the duct. Often reported results are corrected for oxygen concentration to avoid the affect of dilution air on the result. 

Emission Limits 

Some examples of common sulphur dioxide emission limits: 
15mg/m³ at 15% O₂ – Biogas Engines (Medium Combustion Plant Directive) 
35mg/m³ at 3% O₂ – Biogas Boilers (1-5MWth - Medium Combustion Plant Directive) 
200mg/m³ at 3% O₂ – Biogas Boilers (>5MWth - Medium Combustion Plant Directive) 
200mg/m³ at 6% O₂ – Biomass Combustion (Medium Combustion Plant Directive) 
350mg/m³ at 5% O₂ – Biogas Engines 
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