Thermal Oxidizer and Catalytic Oxidizer Selection
Selecting the appropriate air pollution control equipment for a particular industry can be challenging to say the least. Though companies are experts in their field, they are usually not experts in regard to air pollution control equipment. That's where a qualified supplier of air pollution control equipment comes in. A qualified staff with extensive experience applying, designing, installing and thermal &, catalytic oxidizers will steer a buyer clear of common selection errors.
Recuperative thermal oxidizers provide high VOC destruction efficiency, but are very costly to operate due to the high operating temperature (1400F) and low thermal efficiency of this design. The fuel usage can be 5-6 times that of a comparable catalytic oxidizer and 7-10 times the cost of operating a regenerative thermal oxidizer. A thermal oxidizer utilizes a burner and a heat exchanger to heat the contaminated airstream and break the hydrocarbon bonds, recombining the carbon, hydrogen and oxygen to produce water vapor and carbon dioxide.
Regenerative thermal oxidizers (RTOs) also destroy VOC's and fumes by oxidizing the emissions at high temperature. The difference between the regenerative thermal oxidizer and thermal oxidizer are, most importantly, the thermal efficiency. A regenerative thermal oxidizer has a nominal 95% thermal effective heat exchanger. The conventional thermal oxidizer has a nominal 70% effective heat exchanger. In the most simplified terms, the regenerative oxidizer will lose 5% of its energy input (fuel) up the exhaust stack (100%-95%= 5%). The thermal recuperative oxidizer will lose 30%. The simplified comparison shows that, on average, a regenerative oxidizer will use 1/6th (5% divided by 30%) the fuel of a thermal oxidizer.
Regenerative thermal oxidizers also have one other limitation that is not an issue with recuperative thermal oxidizers or catalytic oxidizers. Most regenerative systems utilize a two chamber design. This design provides over 98% VOC destruction. The energy efficiency of the RTO is a result of its flow-reversing design. This reversal of flow that occurs, about every 3 minutes, causes a puff of dirty air to exit the exhaust stack. When the flow control valves switch position a small portion of the contaminated air becomes trapped momentarily, then is back flushed out to the exhaust stack. With visible emissions, such as heavy condensable solvents, or long chain hydrocarbons, the back flushed air will be visible as it exits the oxidizer exhaust stack. As a rule of thumb, if the process exhausting to the RTO has a visible exhaust, it will have a visible puff during every valve switch. While the oxidizer still maintains an overall 98+% VOC destruction efficiency, the puff can be a nuisance issue if a facility is near a residential area. Special options are available to capture this puff of emissions. Kono Kogs has retrofit systems with this option, and many regenerative thermal oxidizers are being configured with this feature as well.
Destruction of pollutants gives off heat (exothermic reaction). This heat offsets the amount of fuel needed to heat the pollutant airstream. In some conditions the amount of pollutants will be high enough that the oxidizer can operate fuel free (no fuel usage). This is a good indication of the thermal efficiency of an oxidizer. A recuperative thermal oxidizer with a 70% effective heat exchanger will operate fuel free at solvent concentrations of 20% LEL or greater. A regenerative thermal oxidizer will operate fuel free at as low as 4% LEL.
For many applications, catalytic oxidizers accomplish the same VOC destruction performance as thermal oxidizers, at lower temperature and therefore lower fuel costs. Catalytic oxidizers are relatively energy efficient with low operating costs and they do not cause extreme thermal stress to equipment because they operate at a low temperature. Most catalytic oxidizers operate at 650F or less. This lower temperature combined with a 70-75% heat exchanger effectiveness allow catalytic oxidizers to operate fuel free at emission concentrations of 8% LEL or greater.
In general, catalytic oxidizers require about 20% of the fuel used by a recuperative thermal oxidizer. And Regenerative thermal oxidizers (RTOs) require only 60% of the fuel used by a catalytic oxidizer. RTOs are becoming the system of choice for most applications because of their reliability and low operating costs. At low flow rates the difference in fuel cost, in dollars, is not significant between a catalytic oxidizer and a regenerative thermal oxidizer, but as flow increases above 7,000 scfm the fuel cost differences can become significant.
We should not forget about electric costs. Many regenerative thermal oxidizers have higher flow resistance (pressure drop across the oxidizer) than catalytic and recuperative thermal oxidizers. This means the fan power required to force air through (or draw it through) the oxidizer can be higher than that required for catalytic./thermal recuperative systems, resulting in higher electric costs. At high airflows and light VOC concentrations the regenerative thermal oxidizers still provide significantly lower overall operating costs than catalytic or recuperative thermal oxidizers because of the RTOs fuel efficiency. In fact, a recuperative thermal oxidizer will likely always be more expensive to operate than a regenerative thermal oxidizer.
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The air pollution equipment technology that is appropriate for a given application is dependent upon the type and volume of emissions produced by the industrial process. Experienced oxidizer suppliers will help new buyers determine which air pollution control equipment will best destroy their emissions at an affordable total cost of ownership. They will provide references in your industry for the applicable technology (thermal oxidizers, catalytic oxidizers or regenerative thermal oxidizers), cost estimates for equipment, installation, maintenance, and commissioning, and provide operating cost estimates for all applicable oxidizer technologies. With this information, an objective comparison of oxidizer options can be made.