 |
Thermal Oxidation Technology Review |
Thermal Oxidation is the chemical reaction in which heat and oxygen initiates an exothermic reaction (i.e., generates heat) whereby organic compounds are converted into carbon dioxide and water. In air pollution control, this reaction allows us to destroy VOCs and HAPs up to levels greater than 99%.
The oxidation reaction is summarized as follows: CaH2b + (a + b/2)O2 → aCO 2 + H2O + heat
This conversion reaction is used in the design of thermal oxidizers. The oxidation reaction is typically initiated in the 1450°F temperature range. However, the temperatures may need to be elevated for certain organic compounds, such as those containing chlorine, in order to attain high destruction efficiencies. As can be seen in the above equation, the quantity of heat generated is proportional to the level of organics involved in the reaction. At a certain concentration level of organics (e.g., VOCs and HAPs), the reaction can become self propagating as the reaction will generate enough heat to continually initiate the oxidation reaction.
For thermal oxidizer design, another critical design parameter is retention time. Retention time is the period which a unit of gas is maintained at the critical oxidation temperature. Typical design is for 1 second although this may be increased to ensure more complete oxidation.
Once the temperature and retention time are established and assuring that proper gas flow distribution is maintained the final design criteria becomes that of achieving optimal thermal efficiency. Thermal oxidizers require large inputs of heat, typically through combustion of natural gas, which can be quite costly. Therefore, varying heat recovery technologies are incorporated into oxidizer designs. Direct fired thermal oxidizers, recuperative thermal oxidizers, regenerative thermal oxidizers, and catalytic thermal oxidizers are all variations implemented to minimize heating requirements and operating costs.
Direct Fired Thermal Oxidizers
The direct fired thermal oxidizers use the highest quantities of natural gas as they do not employ any type of heat recovery. This high heat requirement generally dictates that these units are only employed in applications where the concentration of organic compounds is above 50% of the Lower Explosive Limit (LEL). The LEL is the lowest organic concentration at which an air stream can be combustible. This level is different for each organic compound and must be calculated for mixtures.
Recuperative Thermal Oxidizers
Recuperative thermal oxidizers implement heat recovery to lower the heat requirement and thus the operating costs. Recuperative thermal oxidizers typically use shell and tube heat exchangers made of metal to exchange heat between the exhaust of the oxidizer and the incoming inlet stream for preheating. This design typically achieves a heat recovery efficiency of 50% to 70%. This technology is typically implemented in applications where the inlet stream is at 25% of the LEL.
Regenerative Thermal Oxidizers
Regenerative thermal oxidizers also implement heat recovery to lower operating costs. They use ceramic heat exchanger media to achieve heat recovery efficiencies of 85% to 95%. Regenerative thermal oxidizers use at least two beds of ceramic media for heat recovery. One bed of ceramic media will be used as the outlet bed where heat will be deposited from the high temperature, oxidized gas stream prior to being exhausted to atmosphere. After 1 to 2 minutes, the system flow will switch direction and enter through the preheated ceramic bed that was previously used as the outlet. In this manner, the incoming gas stream is preheated minimizing or possibly eliminating any need for natural gas. The oxidized gas stream will exhaust through the ceramic bed that previously was used as the inlet. This cycle will occur continuously to achieve the high levels of heat recovery.
Catalytic Thermal Oxidizers
Catalytic thermal oxidizers use catalyst to lower the activation energy required for oxidation to be initiated. Catalysts made of noble metal oxides of compounds, such as palladium, platinum, and rhodium allow the oxidation reaction to take place at temperatures from 600 °F to 850 °F versus the 1450 °F required without catalyst. In this manner, the energy requirements are significantly reduced.
Each of these oxidation technologies has advantages and disadvantages over one another. When reviewed along with other air pollution control equipment the decision process can become confusing. Fusion Environmental can assist by conveying all the advantages/disadvantages of each technology for a particular application.
For more detailed information on selecting the right technology, you may Contact Us or you may provide us your application specific details through our Information Request Form. Fusion Environmental Corporation will review your particular application and provide a recommendation on the best possible solution.