OMany alternative solutions and technologies exist for reducing and cleaning VOC emissions created by production processes. The treatment technology is often chosen on the basis of its costs, the premises where it will be used, the air volumes treated, the emission concentrations and, where possible, the reuse of the emissions.
The VOC abatement technologies available are based on various mechanisms. It is important to understand the principal mechanisms behind the methods used for the elimination of VOC gases, and their differences.
Oxidation methods
Thermal oxidation
In thermal oxidation VOC gases are oxidised at a high temperature (>750°C) to produce carbon dioxide (CO2) and water (H2O). There are two main methods for thermal oxidation: recuperative and regenerative.
The recuperative method recovers heat from the gases leaving the incineration chamber. The captured heat is used for heating up the VOC emissions coming into the oxidation chamber. Recuperative heat exchangers enable 60–80% recovery of thermal energy. A regenerative thermal oxidizer (RTO) in based on heat-absorbing materials to store heat captured from gas. These materials are located in separate chambers connected by an oxidation chamber, where the hazardous compounds are oxidised. The heat captured by the materials is used for heating the incoming gas in a process where valves are used for changing the direction of the gas flow. The gas coming in the first chamber is heated close to oxidation temperature. It then flows to the oxidation chamber. The outgoing gas heats the second bed of heat-absorbing material and finally leaves through the outlet flue. The thermal efficiency of the process is over 90%.
Catalytic oxidation
In catalytic oxidation, catalysts are used for oxidizing VOC gases at a temperature that is about 500°C lower than that required for thermal oxidation. Both precious and base metal catalysts can be used. The low temperature of catalytic oxidation means that hazardous secondary pollutants, such as NOx and CO, are not produced. Similarly to thermal incineration, both recuperative and regenerative (RCO) technologies can be used in catalytic oxidation.
Catalytic oxidation vs thermal oxidation
Catalytic oxidation
- Low operating temperature saves energy
- Minimizes CO2 emissions
- No secondary pollutants such as NOx and CO
- Quick start-up time
- Smaller unit
- Lighter unit
- Longer lifetime
- Tolerates low oxygen levels
- Much lower auto thermal point (>0.6 g/Nm³)
- Catalysts can become poisoned
- Pressure drop over the catalyst bed
Thermal oxidation
- Higher energy consumption
- Higher CO2 emissions
- Higher thermal stress to structures due to higher operating temperature
- Secondary pollutants such as NOx and CO resulting from higher operating temperature
- Longer start-up time due to higher operating T and heat capacity
- Larger and heavier unit
- Higher maintenance costs
- Needs minimum 8% of excess oxygen in gas
- Higher auto thermal point (>1.2 g/Nm³)
- No risk of poisoning
Comparison between RCO and RTO
| Air flow rate | 25,000 | °C |
| Gas temperature | 25,0 | Nm³/h |
| Operation time | 6000 | h/a |
| VOC Content | 0,8 | g/Nm³ |
| VOC net heat value | 30,0 | kJ/g |
| Advanced technology | RCO RTO | Units | |
|---|---|---|---|
| Thermal efficiency | 94,0% | 95,0% | |
| Oxidation temperature | 320 | 800 | °C |
| Auto thermal point | 0,76 | 1,67 | g/Nm³ |
| Outlet temperature | 43,6 | 63,8 | °C |
| Heating power | 181 | KW | |
| Heating energy | 1088 | MWh/a | |
RCO requires 1088 MWh less heating energy than RTO per year. This produces annual savings of EUR 65,280 (0.06€/kWh).
Flare stack
VOC gases are burnt off either in an open or closed flare system. A flare can be used for cleaning many kinds of emissions, but fuel consumption for this technology is high especially at low concentrations.
Adsorption
Adsorption methods direct VOC gases into solid matter (such as activated carbon). The VOC gases are adsorbed by the porous surface. Adsorption can be used for cleaning large volumes of air with a relatively low VOC content.
Absorption
Absorption is a method based on the separation of soluble gas components from the gas flow by dispersing it with a solvent fluid. The fluid used for absorption is usually water or a mild solvent that does not cause VOC emissions itself. The effluent fluid produced by this process is waste, but it can be post-treated to separate the absorbed chemicals in a clean and concentrated form.
Condensation
Condensation means the changing of gas into liquid. Condensation enables the separation of one or more hazardous substance from gas by changing its physical state. This happens when hot gas cools down, reaching a temperature close to its boiling point or, in this case, close to its condensation point. The condensation method is mostly used for managing high-concentration (> 5000 ppmv) VOC and HAP emissions.
Biofilters
Biofiltration is based on the natural ability of micro-organisms to decompose chemical compounds. In this process, VOC emissions are a source of nutrition for bacteria. Micro-organisms oxidise organic components in a humid environment, producing carbon dioxide and water.
If you would like to read more about VOC emissions and their regulation within the EU, how VOC emissions
affect our environment and how to choose an optimal technology for VOC removal,
download a free guide: How to reduce VOC emissions
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