Regenerative Thermal Oxidizers (RTO) for VOC-pollutant compounds: How does it work?

Thermal oxidation is a technique that complete destroys the VOC through the conversion to CO2 and H20 at a temperature ranging from 700-1000°C. Using catalysts, the reaction temperature can be drastically reduced. Simple applications are flares or afterburners, while more complex system are Recuperative Thermal Oxidizer and the Regenerative Thermal Oxidizer (RTO). The last one uses the heat derived from the exhaust stream to preheat the incoming gas stream prior to entering the combustion zone, through ceramic heat exchange matrix.

RTO coupled with computational fluid dynamics analysis can be a clear overview regarding what is happening inside the oxidation chamber [1] . It is fundamental the oxidation of VOC components (e.g. benzene, toluene and xylene) focusing on transient behaviour of flow, temperature and concentration in steady and unsteady scenarios. Chemical reaction model must includes NO formation due to oxidation of atmospheric nitrogen (Zeldovich mechanism). Temperatures usually involved operate between 650-900°C and the VOCs treated can act as fuel, achieving the complete autothermal condition at certain concentrations (usually 1.5-2 g/m3).

This technology has several advantages, such as high VOC removal rates, high thermal efficiency, low pressure drops and no production of by-products during operation. Drawbacks are acid gas formation in presence of halogenated compounds and NOx.

Stable RTO operation requires uniform flow and temperature fields. The principal parameters influencing these fields include the air volume, VOC concentrations, and valve switching time. Transient simulation method and thermal equilibrium can estimate the internal velocities and temperature distributions of an RTO across multiple cycles [2].

This technique is becoming more important for gas cleaning and energy conservation. The pollutant is oxidized through chambers containing packing elements (monoliths or random) with specific features:

- capable of absorbing heat rapidly

- stable under thermal cycling conditions

- resistant to attack by pollutants

This last point is critical in the presence of halogen components. Replacement of the packing elements usually requires a shutdown of the RTO while the elements cool, and then extraction for replacement.

Ceramic-halogen reactions act with elements inherently present in ceramic media, especially Na, K, and possibly Li, Ti and Fe, resulting in the formation of precipitates (NaCl, KCl) downstream. Providing packing elements that are relatively stable to halo-reactions offering significant advantages for treatment halogen-volatile organic compounds [3].

Another key factor is that in many countries, there are impending new air pollution regulations, which are heightening the industry’s need for viable control solutions. Similar to other industries, Oil & Gas producers are often required by government regulatory agencies to prevent untreated air pollutants from entering the atmosphere. Thermal and catalytic oxidizers are technologies commonly used on a wide variety of applications where VOC and odour abatement are required.

Midstream companies have historically used flares, vapour combustors, direct-fired thermal oxidisers or recuperative systems for emission destruction. The effectiveness and efficiency of the technologies can result in more emissions and higher operating costs. In the case of flares, water is often injected into the device to reduce visible black smoke. More fuel-efficient RTO abatement technology is now being applied to tail gas treatment where it was in the past thought impossible [4].

Another critical aspect is the Safety re-design of existing thermal oxidation system.

Flashbacks from Thermal Oxidizers can cost the chemical process industries millions of dollars in equipment damage and plant down time. NFPA guidelines says that the operator must verify that the gas stream inlet never overcomes 25% of the Lower Flammable Limit - LFL (or 50% with LFL meter).

An example, applicable also to RTO, can be considered for a vertical thermal oxidizer used in a Gulf Coast chemical plant to incinerate vapors from tank-truck cleaning, tank farm and batch distillation operations was redesigned from the safety point of view [5].

The redesign improved the safety of the system through the addition of a number of important features:

- Adding flame arresters, with temperature measured at the flame arrester discharge.

- Installing the automatic dilution air to increase average gas velocity when necessary.

- Dilution air activation by a measured drop in the average gas velocity.

- Equalization of gas flow distribution into the entrance nozzles on the thermal oxidizer.

- Diameters reduction of inlet nozzles ahead of the unit to provide further increases in velocity.

- Balancing pressure drops for each piping section.

[1] Choi et al. - Chemical Engineering Journal; https://www.sciencedirect.com/science/article/pii/S1385894799001187

[2] Hao et al. – Environmental Engineering Research; https://www.researchgate.net/publication/324362112_Numerical_simulation_of_a_regenerative_thermal_oxidizer_for_volatile_organic_compounds_treatment

[3] Reid et al. - US 6605557; https://patents.justia.com/patent/6605557

[4] Kevin Summ - Processing Shale Feedstocks; https://www.digitalrefining.com/article/1000784,Reducing_emission_treatment_costs_for_gas_processors.html#.XpLElcgzY2x

[5] https://rccostello.com/wordpress/hazards/flashbacks-thermal-oxidizers/