Oxidisers

Oxidisers treat VOCs and other contaminants by thermal or catalytic reaction with oxygen. Thermal oxidisers treat most VOCs at around 750-900 Degrees Celsius, where as catalytic systems can be effective down to 180 Deg C. Dioxins are combusted at 1200 deg C.

Thermal Oxidisers differ in the method used to recover heat. There are two prime categories of heat recovery: primary and secondary.

Primary heat recovery is where the oxidiser itself recovers heat from its own exhaust to minimise its fuel requirements.

Secondary heat recovery is where the fuel consumption of some external system is minimised by supplementing it with heat from the oxidiser.

Primary heat recovery can be achieved by recuperative heat exchange or regenerative heat exchange.

Recuperative Oxidisers

This type of oxidiser utilises mechanical shell and tube type exchangers to pre-heat the incoming air stream with waste heat from the exhaust. This heat exchange mechanism is continuous and hence the operational output is steady. The heat transfer resistance limits the thermal efficiency to around 80% with a practical sized heat exchanger. More typically they operate between 50 and 75% thermal efficiency. Destruction efficiencies depend on residence time, temperature, gas mixing and the chemistry of the contaminant (particularly Dioxins). It is typically >99%.

Regenerative Oxidisers (RTOs)

This is the more popular type of oxidiser in recent years and this is primarily due to the very high thermal efficiencies it can achieve. RTOs use a ceramic media to store and release heat on a dynamic flow cycle. The air flow direction through the unit is changed every 120 seconds. this has the effect of "juggling" the heat within the combustion chamber and minimising the required "top-up" heat from the burner(s). The thermal efficiency of an RTO typically starts at 80% and goes up to 95%. RTO's can therefore operate autothermally at relative low contaminant levels (2000-3000mg/m3 VOC). This means that the y require little or no support fuel to maintain combustion.

Because of the dynamic cycle, the output is however unsteady. The temperature and destruction profiles vary. the emissions can have a spike which may require "smoothing" to meet TWA or odour requirements. this is achieved by using a purge canister(s). This allows each ceramic bed to be purged of its untreated inventory at the point of cycle change, thus smoothing the spike. "puff-cleaners" are a simpler and cheaper version of this concept.

Without a purge system emission destruction is around 98%. This can go beyond 99% with suitable purging equipment.