Secondary reduction techniques reduce SO2 emissions by treating pollutants that have already been released into flue gases, unlike primary techniques, which reduce emissions at source.

Among the different secondary reduction techniques, regenerative processes are distinct from other (non-regenerative) processes. With regenerative processes, the SO2 is extracted, concentrated and recovered in the form of elementary sulphur or sulphuric acid.

Non-regenerative processes cannot produce sulphuric acid or sulphur, although other by-products, such as gypsum, can be recovered from some of these processes.

Installing a flue gas treatment process is not a simple matter. It requires a unit for receiving, storing and distributing the reagent, a reactor bringing the reagent into contact with the sulphur-bearing effluent, an efficient dust extraction system (fabric filters or electrostatic filters) and a system for collecting and storing the residues. Some processes also require liquid effluent treatment systems. With regenerative processes, a sulphur (or sulphuric acid) production unit (Claus unit) has to be installed downstream from the scrubbing tower.

Incorporating these processes into an existing facility is costly and difficult.


Non regenerative processes

The main reagents used to absorb SO2 in non regenerative processes are lime, limestone, soda ash or ammonia. When used in the presence of oxygen, quicklime (CaO), slaked lime (Ca(OH)2) and limestone (CaCO3) will form sulphites and sulphates.

Non regenerative processes may be dry, semi-dry or wet processes.


Dry processes

With dry processes, the SO2 is neutralised by injecting powdered lime, limestone or sodium bicarbonate into the flue gases. The reaction by-products (sulphates and calcium sulphites) are then filtered out.

For efficient neutralisation, the effluent has to be in contact with the reagent for some time. Reagents with a large specific surface area are therefore the most effective. They can be injected into the combustion chamber or directly into the flue gas.

Dry, crushed sodium bicarbonate can also be used as a reagent. This has the advantage of also reacting with other compounds present, such as NOX.

Average rates of sulphur removal with this process range from 20% to 50%, sometimes reaching 70% under optimum conditions with large quantities of reagent. However, high rates of sulphur dioxide reduction entail significantly increased emissions of particulate matter. These have to be removed with appropriately designed equipment.

This process is not suitable if there are significant and rapid variations in the flow of flue gases or their SO2 concentrations.


Semi-dry processes

In semi-dry processes, the reagent is injected in semi-liquid form, as lime slurry or a solution of sodium carbonate. The absorption reaction takes place simultaneously with the evaporation of the water content of the reagent. It has the advantage of ensuring better contact between the reagent and the gas to be cleaned. The reaction by-products (calcium or sodium sulphites or sulphates) have to be filtered out. An efficient extraction system (fabric filters or electrostatic filters) must therefore be installed to remove dust before the effluent is released into the atmosphere. The reaction by-products are then recovered, treated and either recycled or disposed of.

Sulphur removal is more efficient than with dry processes, using the same amount of reagent. Cleaning efficiency depends on the type and quantity of reagent used, the initial SO2 content and the temperature and O2 content of the flue gas. Sulphur removal rates with this process average 80%.

This process is not suitable if there are significant and rapid variations in the flow of flue gases or their SO2 concentrations.


Wet processes

With wet processes, the SO2 formed during combustion is trapped by washing the flue gases. Different reagents may be used, such as limestone, lime, sodium carbonate, seawater, ammonia. The most common processes involve lime or limestone.

Specially designed scrubbing towers are used. The reaction residues are liquid and must be treated before they are released. This involves several transformation stages.

Wet washing is the most effective of the lime and limestone-based emission reduction processes, with sulphur removal rates of 95 to 98%.

These processes are considered to be very expensive and are only installed in large combustion plants (> 300 MW).


Regenerative processes

These are wet processes, meaning that the reagent is used in liquid form or in suspension in the washing water. Different reagents may be used to extract the SO2.

The Wellman Lord process, for example, absorbs the SO2 in a sodium sulphite solution to form sodium bisulphate. The SO2 is then desorbed and concentrated by condensing the water. The gaseous effluent is recovered and transferred to a sulphur manufacturing plant. Variations on this type of process may involve an aqueous solution of amines, ammonia or magnesium hydroxide.

The SO2 may also be adsorbed onto activated carbon (Sulfacid process) or, in the presence of oxygen, oxidised into SO3. Water, preferably demineralised, is sprayed onto the bed of activated carbon. Sulphuric acid is then recovered from the base of the bed. Depending on the quantity of sulphuric acid produced, it may be used internally at the plant or sold.

This technique is very demanding in terms of flue gas characteristics: low dust content, temperature range of 50° to 80°C, SO2 content of up to 2% of the gas volume, O2 content 5 times higher than the SO2 content, etc.

A highly efficient dust extraction system (fabric filters or electrostatic filters) must be fitted downstream of the SO2 removal system.

Soda ash can also be used as an adsorbent but the material and economic constraints involved are such that this process can only be used at petroleum processing sites that have already installed sulphur producing plants (Claus units) for other purposes.

Regenerative processes are among the most efficient, with sulphur removal rates of 95 to 98%. However, they require costly and voluminous equipment downstream from the combustion plant, which limits their use to large combustion plants (> 300 MW).