Primary techniques are applied to reduce emissions of SO2 at source, unlike secondary techniques, which treat pollutants after they are released into flue gases.

 

Sulphur content in fuel

Using low-sulphur or sulphur-free fuel can significantly reduce emissions of SO2. Substituting natural gas for coal can reduce SO2 emissions by over 99%.

The table below shows SO2 concentrations in flue gases, by type of fuel and without sulphur removal measures.

IV.3.3 Tableau teneur soufre

 

Combustion on sulphur-removing fluidised beds

This combustion technique involves injecting combustion air through a layer of solid fuel. A fluidised bed may be fixed, circulating, bubbling or dense.

To reduce the formation of sulphur dioxide, an alkaline reagent is added to the fuel.

Combustion takes place at a controlled temperature in the presence of the adsorbent. The most commonly used reagents are quicklime, slaked lime and limestone.

Depending on the fuel used, the adsorption reaction requires a Ca/S ratio of 1.5 to 7. The reaction by-products (sulphates and calcium sulphite) are not usually recyclable and are removed from the fluidised bed along with other impurities.

Under ideal conditions, with circulating fluidised beds and a Ca/S ratio of more than 2, this technique can remove sulphur at a rate of more than 90%.

Combustion on a fluidised bed also reduces NOx formation. However, this technique can increase emissions of N2O, which is a powerful greenhouse gas.