RTOs are widely used by manufacturers to control airborne pollutants in waste exhaust streams. In a typical twin bed regenerative thermal oxidiser, contaminated process gases enter through an inlet manifold and are directed toward an energy recovery chamber for preheating. From here, the VOC-laden gases are drawn along the first ceramic regeneration bed, and progressively heated as they move toward the combustion chamber.
The elevated temperature approaches the auto-ignition temperature of most VOCs present in the process air stream as it moves through the successive ceramic regeneration layers toward the combustion chamber.
Inside The RTO
Inside the RTO’s insulated combustion chamber, a small amount of heat is added to the warmed gas stream as it passes through the combustion chamber, ensuring its uniform heating and maximising the oxidation of volatile compounds within it during the system’s residence time.
After the oxidation of the VOCs in the combustion chamber is complete, outgoing gases release their heat energy into the second ceramic regeneration bed before they are released into the atmosphere. This makes the second bed warmer than the first so that a flow control valve switch can be used to alternate the direction of flow and increase the oxidiser’s fuel efficiency. If the concentration of VOCs in the process gas stream is high enough, the energy from their combustion then allows for the self-sustained operation of the RTO.
For applications in which high hydrocarbon efficiency is required, a third regeneration bed can be added to the regenerative thermal oxidiser system. The addition of the third bed allows the inlet bed to be purged before it is switched to the outlet bed. Tower type RTOs are the most common 3-bed RTOs used for general industrial applications.
Operating temperatures for regenerative thermal oxidisers are comparable to those of direct flame and recuperative thermal oxidiser types—typically between 750ºC (1800ºF) and 1200ºC (2200ºF). As with these other types of thermal oxidisers, the VOC destruction efficiency for RTOs depends largely on their design characteristics, including residence time, the temperatures reached during combustion and the degree of mixing of combustible organics within them.
Because the heat energy captured by the ceramic media in a regenerative thermal oxidiser originates from the combustion of the VOCs in the target emission air stream, their overall efficiency can be reduced significantly with gas flows containing lower concentrations of VOCs.
Thermal Oxidiser Destruction Efficiency
Thermal efficiency is an important consideration in thermal oxidiser design through its impact on overall operational costs. Thermal oxidation units generally require extremely high temperatures that are achieved through the combustion of natural gas, and can, therefore, become costly over time.
Rotor concentrators are often used to increase the thermal efficiency of thermal oxidation systems by reducing the amount of air flowing through the oxidiser system, thereby increasing the concentration of VOCs in the incoming process exhaust stream. The rotor concentrator is a continuously rotating wheel coated with an adsorbing agent. As process exhaust flows through the wheel, VOCs and other organic pollutants are adsorbed on the wheel’s surface, while clean air is released to the atmosphere. Later, exposing the wheel to a desorption gas cleans the contaminants off, producing a small, highly concentrated stream of volatile organics ready for input to the thermal oxidation unit.
Thermal oxidisers generally have destruction efficiencies ranging from 90% to greater than 95%. Sometimes destruction efficiencies are expressed in milligrams per cubic metre of VOCs. Many types of thermal oxidation systems are also effective at destroying or removing particulate matter and odours from process exhaust streams.
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