Reduction techniques and costs

Sulphur oxides (SOx)

In certain cases, it will be technically possible to implement several solutions for reducing the same pollutant at a given industrial facility. In other cases, only one technique will be effective in maintaining emissions at the desired level.

Sulphur oxide emissions to air mainly come from the combustion of fossil fuels containing variable proportions of sulphur (0.5% to 3% for oil and 0.5% to 10% for coal). Natural gas contains practically no sulphur.

Sulphur removal techniques may be applied at three levels, before, during or after combustion.

Action before combustion (focusing on the fuel)

As SO2 emissions are directly related to the sulphur content of the fuel used, the first solution is, where possible, to use fuel with a lower sulphur content. For example, it is now possible to find heavy fuel oils with less than 0.5% sulphur content. The effectiveness of such a measure is a direct result of a lower sulphur content of the fuel. However, these fuels are more expensive and given the stringent levels of emissions laid down by regulations, these measures are not always suitable.

Action during and after combustion

This type of sulphur removal is based on the injection of a (calcic) alkali to reduce the sulphur oxides formed during combustion. The main parameters on which the success of this type of treatment largely depend are :

• nature of the calcic chemical and the Ca/S ratio
• temperature
• specific surfaces and granular structure of the reagent

Action during combustion (in the furnace)

Combustion on sulphur-removing fluidised beds (gas/solid reaction)
This depends on the coal being suspended during combustion and the injection of an alkaline chemical at the same time to ensure a reduction in the sulphur oxides by a neutralising reaction. This technique yields a good gas/solid exchange. Certain facilities recirculate the fuel and the reagent which provides increased yield from the reaction.
Effectiveness 80-90%. Ca/S: typically 3-4 and 1.5-2 for circulating beds.
This technique also reduces the formation of NOx because of the lower temperature obtained by keeping the fuel fluidised.

The injection of alkaline chemicals into the flame (gas/solid reaction)
Here the reagent is injected directly into the combustion furnace. Effectiveness: 60%. Ca/S: 3-4

These two techniques, which require relatively low temperatures, help to reduce the formation of NOx in a proportion of 40-50%.

Action after combustion (focusing on the combustion gases)

This type of treatment is based mainly on neutralising reactions between SO2 and the basic substances (CaCO3 Ca(OH)2, CaO, MgCO3, Na2CO3, Na2SO3, NH3, etc.).

There are three different types of treatment: wet, semi-dry and dry processes.

Wet processes (gas/liquid reaction)
After removing dust from the gases of combustion, they are brought into contact with a solution containing the reagent. The solution is pulverised to increase the efficiency of the reaction with the neutralising agent. Effectiveness: 95% Ca/S: 1-1.5

Semi-dry processes (gas/liquid reaction)
The neutralising solution is sprayed into the waste gas. The hot fumes cause the water to evaporate from the solution, and the result of the reaction is in the form of a dry dust. Effectiveness: 80-90% Ca/S: 1.5

Dry processes (gas/solid reaction)
The neutralising reagent is injected directly into the flow of waste gases. The products of the reaction are separated out on a bag filter. During this filtration, the neutralising reaction continues. Effectiveness: 40-60% Ca/S: 2-3

Principle of the dry bicarbonate process