VOCs thermal oxidation

In this article, we aim to explore the combustion phenomenon by analysing its fundamental elements and how they affect the thermal oxidation of VOCs.
In detail, we will try to answer three questions:

• What is combustion?
• What elements combine to achieve complete combustion? And how do they affect the abatement efficiency of VOCs?
• Let’s start by addressing the first one.

Combustion is a chemical reaction in which a fuel is oxidised by an oxidizer (usually oxygen contained in the air), resulting in the release of heat (exothermic reaction) and electromagnetic radiation (including light).
This phenomenon can be illustrated by the so-called “fire triangle”, which indicates the three elements that must be present simultaneously for combustion to occur:

1. the fuel (solid, liquid or gaseous);
2. the oxidizer (air containing oxygen or other substances that release oxygen);
3. the ignition energy (heat, temperature).

In the context of pollution control, combustion refers to the reaction of oxygen with one or more compounds typically containing carbon and hydrogen, and sometimes other atoms such as sulphur, nitrogen or halogenates.

This phenomenon occurs when the initial bonds of a carbon-based chemical are broken and form new bonds with oxygen, usually resulting in carbon dioxide and water (while, for the other atoms, the corresponding acid is formed). This reaction is represented by the following chemical formula: VOC + O₂ → CO₂ + H₂O + NO₂ + HCl + SO₂.

When discussing VOCs abatement, it is essential to consider another aspect, known as the “Three Ts of combustion”. This expression refers to three elements that play a crucial role in abatement effectiveness; they are: temperature, turbulence and residence time.
Before analysing these three factors, it is helpful to take a small step back to understand more about thermal oxidizers and their role. These machines use the principle of thermal oxidation (at high temperatures) to degrade the Volatile Organic Compounds present in the air stream. Within the combustion chamber, harmful pollutants are burned and transformed into harmless substances such as carbon dioxide (CO2) and water vapour (H2O).
The aim is, therefore, to reduce VOCs as much as possible while simultaneously ensuring thermal efficiency and cost-effectiveness. For this to occur, all variables that influence abatement efficiency must be considered when designing afterburners. It is at this point that the three Ts we mentioned earlier come into play.
For complete combustion, it is necessary to ensure that:

• oxygen and pollutant remain in the chamber for the appropriate amount of time and at the correct temperature;
• there is the right amount of oxygen properly mixed with the pollutant in the combustion chamber (turbulence).

Let’s analyze these three important elements together!

The residence time of pollutants in the combustion chamber varies depending on the type of VOC. Generally speaking, we can say that the average time is 0.5 to 1 second but, in the presence of complex and halogenated hydrocarbons, it increases to 2 seconds or more, as they are more difficult to destroy and oxidise.

With the term ‘temperature’, we refer to the temperature inside the combustion chamber.

It is a key variable in the abatement of VOCs because if it is too low the bonds cannot break (and thus the compounds are not abated).
Like residence time, temperature is also influenced by the type of VOC to be treated because, depending on the structure, the bonds of the compounds can break more easily at high or low temperatures.

Turbulence is the parameter for assessing how well mixed and moving the molecules are within the effluent. The degree of turbulence between oxygen and pollutants to be abated is very important. Typically, the Reynolds number is the criterion that defines turbulence. In an oxidizer, it is calculated as:

Re = ( (oxidizer internal diameter) x (effluent velocity) x (effluent density) ) / (effluent viscosity)

To ensure complete turbulence, the Reynolds number must be greater than 10,000.
As we can see, the Reynolds number parameters are also somewhat related to each other and to temperature too. In this context, great importance is placed on the velocity of the effluent at high temperatures, which typically must exceed 6 m/s whenever possible.

At this point, it should be clear that each type of pollutant requires different design considerations, as it may be necessary to modify turbulence, temperature and residence time in order to achieve optimal abatement effectiveness and efficiency.
To these three parameters, however, we need to add a fourth: the concentration of oxygen in the effluent.
Combustion is, in fact, a chemical reaction of oxidation of fuel molecules by an oxidizing agent, which is typically oxygen. The reaction is never stoichiometric and, in order to achieve near-100% efficiency, it is necessary to work with an excess air level beyond the theoretical amount. For gaseous fuels, this excess air (especially oxygen) equals 3%.
In VOCs thermal oxidizer, the burner is considered an auxiliary device to combustion: it is used for ignition and temperature maintenance in the absence of pollutants, which usually act as the main fuel in the combustion chamber due to the self-sustaining phenomenon. However, for these types of machines, it is not possible (or is very difficult) to have oxygen concentrations at the stack below 17-21%.
In certain oxidizers, especially of the Direct Fired type, it is more likely to encounter oxygen concentration conditions close to the excess air values mentioned earlier precisely because of their application and configuration.
As a result of the above, it is easy to understand that all these factors must be perfectly balanced in order to find the right mix for VOCs abatement. For this reason, VOCs thermal oxidizer must be designed by specialised individuals with experience in this field.
Our team will be happy to design the optimal solution based on your production needs!
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