Reduction techniques and costs

VOLATILE ORGANIC COMPOUNDS (VOCs)

Only the techniques for treating VOCs emitted by stationary sources are dealt with here.

Primary techniques

Action may be undertaken at source. There are many options here, because of the diversity of processes. In the case of solvent use, this action involves the use of substitute products, reduction in consumption, etc.
Where this is impossible, or insufficient to meet the emission limits laid down by regulations, it is necessary to use techniques for processing the waste gases.

Secondary techniques

These techniques fall into two groups :

• destruction techniques, with the possible option of extracting energy from VOCs
• recovery techniques, with the option of recycling compounds.

Techniques for destroying VOCs Incineration. There are three ways of oxidising VOCs and converting them into H2O and CO2: either with recuperative or regenerative types of thermal incinerators at high temperature (600 to 1000°C) or else by using a catalyser at lower temperatures (300 à 600°C). These three types of incinerators each have their particular area of application. The choice is both technical and economic, depending on many parameters such as the quantity of air to be processed, the concentration of VOCs, and the calorific value of VOCs. Since the amount of additional energy consumed is the main operating expense, the aim as far as possible should be energy self-sufficiency, which often requires changes to the process. In the regenerative type of thermal incinerators for example, the gas to be processed is passed through several layers of inert material (e.g. ceramic) in turn, to pre-heat it. Energy recovery is very high, making these systems suitable for large throughputs of low concentrations of VOCs.

VOC recovery techniques
Adsorption of VOCs. VOCs can be adsorbed by activated carbon at temperatures of between 20 and 40°C. When the bed of activated carbon becomes saturated, the VOCs are desorbed using inert hot gas or steam. This technique offers the advantage of being able to re-use the products recovered.
It is generally difficult to separate more than two VOCs for recycling. Specific treatments need to be applied if the aim is to recover and recycle the products regenerated in this process.

The adsorbent material may therefore be a zeolite. This has the advantage that its adsorption capacity alters little with an increase in temperature. However zeolites are more expensive and have poorer adsorption capacity than activated carbon.

There are many other techniques such as bio-purification, recovery by condensation or separation on membranes.