Thermal oxidisers use high temperature to convert contaminants to carbon dioxide and water vapour. Normally the reaction takes place at a temperature ranging from 750°C to 850°C. At these temperatures, with good mixing and sufficient oxygen, the destruction efficiency in the combustion chamber is almost 100%.

There are three types of thermal oxidiser:

* DIRECT FIRED OXIDISERS,

* RECUPERATIVE OXIDISERS,

* REGENERATIVE OXIDISERS,

Whilst a direct fired oxidiser contains no primary heat recovery, for most processes, the cost of the fuel required to heat a process stream to the combustion temperature necessitates the inclusion of some sort of heat recovery.

Both recuperative and regenerative technologies include heat recovery and some basic information will be given below. 

The choice of which type of thermal oxidiser to use is generally one of economics. We will analyse capital and running costs in relation to the process inlet conditions, the required removal efficiency and the variability of the process.

A recuperative thermal oxidiser uses an integral primary shell and tube heat exchanger to preheat the incoming contaminated air using the high temperature air that is exhausted from the oxidiser

Where the level of VOC is significant, the heat released during the oxidation of the VOC will also be recovered, improving the cost effectiveness of the system.

A primary heat exchanger can recover up to 70% of the heat input by the burner or released during the oxidation process by pre-heating the inlet exhaust stream, reducing the support fuel required by the burner to maintain the required oxidation temperature.

They are a simple, cost effective, means of destroying VOC where the inlet concentration is relatively high or particularly where heat can be usefully recovered for other processes.

Secondary heat exchangers can also be utilised to recover heat as a utility for other energy requirements.

The principle of the RTO is simple. The contaminated stream is introduced into the oxidiser via a hot ceramic bed where it picks up heat prior to combustion. The hot volatile gases then enter the combustion chamber where they are oxidised (typically at 800ºC) in an exothermic reaction which releases additional energy into the system. If there is sufficient heat generated from the internal heat recovery and from the exothermic reaction, then the burner will switch off (No Flame Operation). In this autothermal condition, the combustion chamber temperature will be maintained without support fuel.

The gas is held at the designed temperature in the combustion chamber for the required residence time. The cleaned gas exits through a second cooler bed of ceramic media where it gives up its heat, leaving the oxidiser at as little as only 40-500ºC hotter than it entered. At the end of this cycle the oxidiser outlet bed is hot and the inlet bed is now cooler. The cycle is therefore reversed periodically in order to maintain equilibrium and to keep the regained heat within the oxidiser system.

The switching of direction gives a brief emission of untreated air to atmosphere, but this is not problematical in many applications because emission limits are calculated on a thirty minute average figure.

With high solvent levels and in particular, on odour control applications, this occasional output may not be acceptable and the exhaust pulsing has to be reduced to almost zero. Here we utilise a purge system and an additional canister, making the canisters "odd" in number. The 'odd' canister is purged prior to change from inlet to outlet mode. The purge system effectively sweeps out any untreated air and puts it back into the system to prevent it from being released to atmosphere. This allows us to achieve an elevated destruction efficiency.

The capital cost of a two canister RTO is obviously lower than that of a three canister RTO and it can meet the destruction efficiency required by many industrial processes. For higher destruction efficiencies, however, in the order of 99% removal or greater, three or 'odd' canister units are normally selected.