THERMAL OXIDISERS

Description

Thermal oxidisers are used to incinerate pollutants such as VOCs and odours in gas streams. The contaminated gas is mixed with air or oxygen in a high-temperature furnace. The internal temperature of the thermal oxidiser is typically 800 C, which is generally 200-400 C above the auto-ignition temperature of the pollutants.

Thermal oxidisers are designed to hold the pollutants at the required temperature for the required residence time to ensure a clean burn. VOCs and odours are converted to mainly CO2 and water; the production of NOx species can be minimised by keeping the residence time low. If SO2 is produced, further treatment may be required.

Apart from differences in the designs of burners, there are generally two types of thermal oxidiser. Recuperative thermal oxidisers pass the exhaust gas through heat exchangers to preheat the incoming gas. This can produce significant fuel savings. In regenerative thermal oxidisers, the exhaust gas is directed through one of a pair of ceramic beds. When the first bed reaches a sufficient temperature the gas flow reverses. The incoming gas is then preheated by this bed and the exhaust gas offloads its heat into the second bed. Much of the incineration occurs within these ceramic beds. In many cases, a secondary heat recovery system can be incorporated so that hot clean air is passed back to the process.

In most cases a fuel, such as gas, is required to aid the combustion process unless the calorific value of the waste gas stream is high.

Application

Thermal oxidisers have applications in VOC destruction in the plastics, computer, motor, engineering, coating and petrochemical industries. The food and animal products industries are a major area where thermal oxidisers are used to eliminate odours.

Recuperative thermal oxidisers are generally suitable for high concentrations of VOCs in lower flow rates. Regenerative thermal oxidisers are designed for high flow rates (above 10,000 m3/h, for example) and lower concentrations of VOCs.

Sizing

The sizing of thermal oxidisers is dependant on the exhaust gas flow rate, the composition and temperature of the polluted gas and any secondary heat recovery requirements. A typical thermal oxidiser sized to decontaminate a gas stream of 15,000 m3/hour would require a fan rated at about 50 kW.