Thermal oxidizers are pollution-control units designed to prevent process-generated volatile organic compounds (VOCs) from entering the environment. Different technologies offer tradeoffs among initial cost, operating expense, complexity, emissions, fuel efficiency and destruction efficiency. This one-page reference provides information on selecting thermal oxidizers.
Thermal oxidizer designs vary widely, but can be generally split into three main categories: direct-fired, regenerative and catalytic. A thermal oxidizer sustains the proper conditions for oxidization of the combustible materials in three ways: by maintaining an operating temperature sufficiently above the autoignition point of the gas, by providing enough time for combustion and by introducing excess oxygen to complete the oxidation reactions.
Direct-fired thermal oxidizers
Direct-fired thermal oxidizers use a burner to heat a chamber to proper oxidation temperatures for the required destruction efficiency. The chamber must be designed to maintain an adequate residence time and provide sufficient velocity for turbulent mixing. If the process gas has sufficient heat content, it can be used as the fuel gas for the burner. Otherwise, supplemental fuel is required. A recuperative thermal oxidizer is a variation that incorporates heat recovery. Direct-fired thermal oxidizers offer high destruction efficiencies (up to 99.99%) and can provide low NOx and CO emissions.
FIGURE 1. Regenerative thermal oxidizers capture the heat from oxidation, and retain it with ceramic media
Regenerative oxidizers
Regenerative thermal oxidizers are used for applications where the combustible concentration is below 3% of the lower explosivity limit. This type employs ceramic media to capture heat from oxidation to reach thermal efficiencies of up to 98%. Thermal energy is retained by the ceramic media and is then used to heat and oxidize the process gas as it enters.
To accomplish this, the system uses multiple beds and alternates the inlet and outlet of the oxidation chamber. A two-bed system would cycle about every two minutes, allowing heat to be captured by the ceramic media on the outlet and heating the process gas from residual heat in the inlet bed. Once the system cycles, the direction of flow is reversed, allowing the temperature to be regenerated on the beds. Regenerative thermal oxidizers can operate on little to no fuel and achieve 98.5% destruction efficiency with low NOx and CO emissions, even with lean process gases. Adding a purge step to the cycle requires an additional bed, but increases destruction efficiency to 99.5%.
Catalytic thermal oxidizers
A catalytic thermal oxidizer utilizes a catalytic bed to promote oxidation, lowering the temperature required to oxidize the process gas. Due to the lower temperature, a catalytic thermal oxidizer uses less fuel than a direct-fired model and can even be designed to be self-sustaining through the use of a heat exchanger to pre-heat the process gas. This type of system is limited by the combustible concentration of the process gas and is limited to components that will not poison the catalyst. For the correct applications, a catalytic thermal oxidizer can offer high destruction efficiency and low NOx and CO emissions.
Selection guidance
When selecting a combustion system, emissions and destruction efficiency have become the primary criteria. In general, the decision factors should prioritize process-gas composition, followed by emissions, and then fuel efficiency and capital costs.
Handling challenging components. Corrosive components, such as H2S and halogenated chemicals, demand systems capable of safely disposing of them. Once concentrations of any of these compounds reaches a certain level, typically the safest and most effective way to destroy them is through specially designed direct-fired thermal oxidizers. Catalytic thermal oxidizers and regenerative thermal oxidizers are not suitable, because they are too sensitive to the presence of these chemicals.
Destruction efficiency. A simple enclosed combustor (flare) will achieve about 98% destruction efficiency. Up to 99.5% destruction efficiency can be achieved with a temperature-controlled combustor, a regenerative thermal oxidizer or a catalytic thermal oxidizer. Above that, a direct-fired thermal oxidizer or an ultra-low-emissions combustor is required.
NOx emissions. Although they offer great destruction efficiency, direct-fired thermal oxidizers do not improve much over simpler combustors in NOx generation. Several low-NOx burner designs can improve NOx emissions for direct-fired thermal oxidizers. Ammonia-injection systems can lower NOx, but can be expensive. Regenerative and catalytic thermal oxidizers offer lower NOx emissions.
Fuel efficiency. Regenerative thermal oxidizers offer the best fuel efficiencies, recovering up to 98% of thermal energy. These systems are ideal for low-concentration and high-flowrate applications. Catalytic thermal oxidizers can also operate at high thermal efficiencies by incorporating heat exchangers to pre-heat the process gas before it passes through the catalyst. Recuperative thermal oxidizers can be used to pre-heat process gas to raise fuel efficiency, or to recover heat for use in another part of the plant.
Editor’s note: This “Facts at your Fingertips” is based on material from Vij, A.D., Enclosed Combustion Equipment and Technology, Chem. Eng., January 2018, pp. 46–49.