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A Regenerative Thermal Oxidizer (RTO) is a popular, proven type of air pollution control equipment used to destroy Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) found in industrial exhaust streams. Designed for industrial applications with high air flow volumes and low VOC process streams, RTO systems often achieve high thermal recovery efficiencies (95%+) and high destruction efficiencies (99%+ DRE) relative to other types of air pollution control equipment. However, they also possess more moving parts and generally require more attention via regular inspections and maintenance.
In the simplest terms, Regenerative Thermal Oxidizers operate at very high temperatures (typically 1500 degrees F or higher) to clean pollutants from process exhaust. Ceramic media beds in the RTO unit capture up to 95% of the heat from the exiting clean air before exhausting the air to the atmosphere. The ceramic media beds then release this heat to pre-treat incoming dirty air before the air enters the RTO’s combustion chamber.
A standard three-tower RTO design employs one tower to heat to the incoming air, another tower to absorb waste heat from the exiting air, and a third tower to purge air. The purged air is then sent through the combustion chamber to remove remaining VOCs to help ensure clean air is released into the atmosphere.
1. Process stream (waste stream) is drawn into the oxidizer’s Inlet Manifold by the System Fan through a set of inlet ducts
2. Valves direct the stream up through one bed of the Recovery Chamber filled with ceramic media that has been warmed to temperature
3. Process stream is pre-heated as it travels through the Ceramic Media Bed
4. Process stream enters Combustion Chamber heated by Burner(s) to a Chamber Set Point based on application and pollutants (types of VOCs)
5. Pollutants remain in Combustion Chamber for a specified retention time until they are destroyed and converted into water and CO2
6. Process stream enters the other bed of the Recovery Chamber, where the now-purified air releases thermal energy as it passes through the ceramic media bed, warming it
7. Cooled, clean air exits the RTO through Exhaust Stack
8. Flow direction is switched to take advantage of the now warmed-up chamber bed to pre-heat the new incoming waste stream. (Inlet side is now outlet side.)
See how a Multi-Chamber RTO works.
The cost of an oxidizer varies widely and is dependent on variables including system type, size, and location. Most new RTOs cost more than $100,000 (USD), with larger units costing half a million dollars or more. This does not include installation and commissioning, which can add hundreds of thousands of dollars. Key design features that impact capital cost are the exhaust rate and emission point temperature, which affect unit size and ductwork.
Operating costs for a new RTO also vary widely and are dependent on variables including process exhaust temperature, air pollutant type and concentration, system thermal efficiency, system design complexity (i.e., maintenance needs), and electricity and fuel costs. As an RTO ages, many other factors come into play. That’s why it’s important to track key parameters affecting thermal energy recovery (TER), as a drop of just 2% TER can cost tens of thousands of dollars more each year. Also, while peak VOC loads usually govern the design of a new oxidizer, average VOC loads dictate actual operating costs.
Some RTO manufacturers tout that RTOs offer the lowest cost of ownership and the lowest cost of energy costs compared to other air pollution control technologies. This is not always true. Read our “RTO: Fact vs. Fiction” post to learn more about these and other common RTO misconceptions.
Consideration
RTO
Thermal Oxidizer
Thermal Efficiency
95%
70% – can be higher with incorporation of secondary heat recovery due to high exhaust temperatures
Destruction Efficiency Range (DRE)
Typical regenerative incinerator design efficiencies range from 95 to 99% for RTO systems (Ref: EPA)
Typical recuperative incinerator design efficiencies range from 98 to 99.9999% and above (Ref: EPA)
Additional System Requirements to Achieve Up to 99% DRE
Typically a standard two-chamber RTO attains up to 97% DRE. Using a third chamber and valves – or additional dampers, fans, and “puff chambers”, complicated valve sequencing, and added operational complexities – up to 99% DRE can be achieved
No additional components are required to attain 99.9% DRE
Operating Reactor Temperature Range
An RTO uses natural gas to heat entering waste gas to approx. 760°C to 820°C (1400°F to 1500°F), however, it is capable of operating up to 1100°C (2000°F) for those cases where maximum destruction is necessary (Ref: EPA)
Thermal destruction of most organic compounds occurs between 590°C and 650°C (1100°F and 1200°F). Most hazardous waste incinerators are operated at 980°C to 1200°C (1800°F – 2200°F) to ensure nearly complete destruction of the organics in the waste (Ref: EPA)
Effect of Particulate Matter or Condensable Compounds Entering System
Sustained exposure to even small amounts of particulate matter or condensable materials will eventually build up on the “cold face” or inside the ceramic bed, increasing system pressure drop. Raising the pressure drop across the beds will also add additional work load for the system fan (increased energy costs). System “bake-outs” will sometimes temporarily reduce ill effects of these deposits. Over time severe or permanent damage to ceramic media can occur, and there is no way to permanently, completely remove these deposits without replacing the media. Ceramic media replacements are both costly and time consuming, and can be required with little advance notice.
Shell and tube heat exchanger tubes have a much larger and more accessible open area, and are not severely affected by contaminants. Condensable materials or particulate matter can typically be easily identified and removed without need for “bake-out” from the shell and tube heat exchangers during planned shut downs There is never a need for costly and time-consuming ceramic media replacements.
Additional Components Required to Allow for Higher Concentrations of VOCs for Short Duration Interrupt Conditions
Due to their high thermally efficient nature, RTO systems have great difficulty averting “over temp” conditions within the reactor during rapid increases in VOC concentration. Additional equipment to allow them to ride through elevated VOC conditions are often required. Normally, either a “cold side” or “hot gas” bypass comprised of additional duct and valves and actuators are required. Selection of a bypass is normally determined by analyzing the expected VOC compounds and applying the appropriate type. In this case VOCs are variable, making selection difficult. Cold side would likely be safest, since some compounds have low auto-ignition temperatures which could allow them to combust in the media bed. This could prevent the RTO from shedding stored heat in the media during the inlet cycle and “over temp” the RTO.
No additional components are needed to “ride through” a higher VOC concentration. These conditions are more easily avoided in a thermal oxidizer due to its lower thermal efficiency and ability to shed heat more easily.
Mechanical Components Not Common to Both Systems
Poppet valves, poppet valve actuators, poppet valve solenoids, poppet valve seals and gaskets, additional dampers and/or fans required to increase DRE to 99%, NGI components, hydraulic pack or air compressor to power poppet valve actuators, a regenerative compressed air dryer. Some RTO systems also use a highly complex electronic drive and electric motor and gear box to operate the pilot valves.
No additional mechanical components which are not common to both types of systems.
Typical Gas Flow Ranges (Process Air Volume)
Typical gas flow rates for regenerative incinerators are 2.4 to 240 sm3/sec (5,000 to 500,000 scfm) (Ref: EPA)
Typical gas flow rates for recuperative incinerators are 0.24 – 24 standard cubic meters per second (sm3/sec) (500 to 50,000 standard cubic feet per minute scfm) (Ref: EPA)
On-Stream Reliability
Unlike all other oxidation technologies, RTO systems are highly mechanically intensive systems. RTOs incorporate may additional components not common to other oxidizer designs. This coupled with high impact operation, constant and repetitive pressure changes, and complex sequence of operation significantly impact an RTO’s overall reliability.
Thermal oxidizers are the most proven and reliable method available. Thermal oxidizers operate in a significantly more stable state. There are no constant jarring effects, upsets in flow and pressure, or the addition of numerous mechanical devices not common to other oxidizer systems.
Applicable Disadvantages
– High initial cost
– Difficult and expensive installation
– Large size and weight; and
– High maintenance demand for moving parts
(Ref: EPA)
Even with recuperating waste heat energy, incinerator operating costs are relatively high due to supplemental fuel costs. (Ref: EPA)
Sometimes air volumes are so high – as in many painting and coating applications – they would require a huge, cost-prohibitive RTO system. In these cases, when the airflow is at or near ambient temperatures, a Zeolite Concentrator can be added upstream of the oxidizer to condense fuel-abundant VOCs and reduce air volumes, allowing for a smaller, less expensive RTO.
When the dirty waste stream enters the spinning concentrator wheel, its pollutants are attracted to the Zeolite and stick to its surface. The Concentrator’s blowers then push the heated, lower-volume air stream with its condensed VOCs into the RTO.
Concentrators feature a heat exchanger and require blowers and a separate burner – all of which need to be properly maintained to sustain reliable system performance and prevent damage to both the Concentrator and the oxidizer. The addition of a Concentrator also necessitates a much more complex control system.
RTO systems work especially well for industries including:
• Paint booths
• Coating processes
• Automotive
• Ethanol production
• Pharmaceuticals
• Various other manufacturing processes
Could your team benefit from an RTO refresher course? Learn more about our training services.
Learn our tips for choosing the best RTO manufacturer for your needs.