Thermal Oxidizers
What are Thermal Oxidizers?
Thermal oxidizers are used to reduce air pollution emissions from a variety of industrial processes by treating the polluted exhaust gas prior to discharging the exhaust gas to atmosphere. To destroy the contaminants in a polluted exhaust gas, a thermal oxidizer uses the principle of "thermal oxidation". Thermal oxidation is a combustion process in which the contaminants within the polluted exhaust gas react with oxygen in a temperature controlled environment to create an oxidation reaction. This chemical reaction is what reduces the pollutants in the exhaust gas to an innocuous emission (CO2, water vapor, and heat) to atmosphere.
Thermal Oxidizer Types:
There are basically four types of thermal oxidizer. Direct fired thermal oxidizer (some times known as an afterburner), recuperative thermal oxidizer, regenerative thermal oxidizer (RTO), and flameless thermal oxidizer (FTO). All of these thermal oxidizers operate on the same principle mentioned above which is thermally promoting an oxidation reaction between the pollutant compound in the exhaust gas and the available oxygen.
Direct Fired Thermal Oxidizer (Afterburner) –
A direct fired thermal oxidizer or afterburner is the simplest type of thermal oxidizer. This type of thermal oxidizer does not include any type of heat recovery system to recover a portion of the waste heat generated by the combustion process. As shown in Figure 1, a direct fired thermal oxidizer system typically includes a burner, combustion chamber, blower, air/fuel train, instrumentation & controls, and an exhaust stack.
Figure 1
Typically a direct fired thermal oxidizer is less expensive than other types of thermal oxidizers. However, this type of thermal oxidizer will have higher operating cost (fuel use) for waste streams that are endothermic.
- Recuperative Thermal Oxidizer
A recuperative thermal oxidizer is basically a direct fired thermal oxidizer with heat recovery added at the discharge end of the combustion chamber. This type of thermal oxidizer system incorporates down stream heat recovery equipment such as gas to gas heat exchangers, waste heat recovery boilers, gas to liquid heat exchangers, etc… to maximize the thermal oxidizer system overall efficiency. The down stream heat recovery equipment utilizes the available heat produced within the thermal oxidizer combustion chamber to reduce auxiliary fuel consumption, to produce steam, to heat a process air stream, or to heat a liquid stream. A recuperative thermal oxidizer system can have a single heat recovery device as shown in Figure 2 or multiple heat recovery devices as shown in Figure 3.
Figure 2
(Single Heat Recovery)
Figure 3
(Multiple Heat Recovery)
In addition to the recuperative thermal oxidizer systems shown above, a recuperative thermal oxidizer can incorporate combustion air preheat to conserve fuel or other heat recover equipment such as hot oil heater, hot water economizer, or asphalt heater.
Recuperative thermal oxidizers can be used to treat multiple gaseous and liquid waste streams that are endothermic and/or exothermic.
Regenerative Thermal Oxidizer (RTO)
The regenerative thermal oxidizer (RTO) is most suitable for industrial & commercial applications that produce clean gaseous waste streams with high volume flow rates and low VOC concentrations. As shown in Figure 4, the regenerative thermal oxidizer uses multiple beds of heat resistance media to store and recover heat from the process gas by direct contact.
Figure 4
( 2 Bed RTO)
This type of oxidizer operates on a timed cycle. The basic operation involves one bed of media absorbing heat from a hot gas stream while the other bed of media is giving up heat to the incoming waste gas. A series of hot dampers re-directs the gas flows as needed to reverse the roles of each bed of media.
- Flameless Thermal Oxidizer (FTO)
The Flameless Thermal Oxidizer (FTO) is a patented technology offered by Thermatrix, Inc. and is another type of thermal oxidizer used for destruction of pollutants in a process waste gas.
The FTO typically includes a refractory lined oxidation vessel with a centered dip tube, hot ceramic bed, and support burner as shown in Figure 5. This technology achieves thermal oxidation of the VOC's using the heated packed ceramic bed. The vapors are oxidized when they come in contact with the heated bed of ceramic pieces and there is never a flame in the oxidation vessel.
Figure 5
(Typical FTO)
Each type of thermal oxidizer listed above has industrial application limitations because of their inherent design features. Careful consideration of the industrial application, type of waste gas, operating costs, safety, and emission requirements play an important role in deciding what thermal oxidizer technology to select.
Because of their ability to handle both liquid and gaseous waste streams, their ability handle waste streams from a wide range of industrial applications, their ability to handle dirty waste streams, and their ability to achieve very high pollutant destruction removal efficiency (DRE), the direct fired thermal oxidizer and the recuperative thermal oxidizer provide the highest level of reliability and application flexibility of the four thermal oxidizer types listed.
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