Regenerative Thermal Oxidizer

What is a regenerative thermal oxidizer?


A Regenerative Thermal Oxidizer (RTO) is an industrial-grade air pollution control device that use high temperatures to combust and decompose volatile organic compounds (VOCs) into CO2 and water vapor.

Also known as an RTO, a regenerative thermal oxidizer heats the VOC-laden process exhaust gas to temperatures in the range of 1400-1500°F.

RTOs are available in many different shapes and sizes. The two main designs are the 2-tower and 3-tower RTO (may also be called a 2-can or 3-can RTO). The 2-tower design will suit most cases, while the 3-tower design will be used when the highest destruction efficiency is required (in cases of odor complaints, for example).

Regenerative Thermal Oxidizer (RTO) equipment is used to destroy VOCs and other organic hazardous air pollutants. They are not good for treating particulates, inorganic gases, heavy metals, acids, nor other corrosive gases. If these other contaminants are present, an RTO maybe used in combination with a dust collector or wet scrubber, depending on the situation.

Regenerative Thermal Oxidizer effectiveness on:

Volatile Organic Compounds (VOCs)95
Organic Hazardous Air Pollutants (HAPs)90
Odors* (Organic-based)80

*Note: an RTO may not be effective at removing inorganic odor compounds.

Acids, Chlorinated, and Fluorinated compounds35
Particulate matter25
Corrosive fumes, vapors, gases15
Heavy Metals5

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Main components of an RTO


Ductwork and Fan/Blower

RTOs will use either a forced draft or induced draft fan system, depending on the process air temperature. Installing the fan on the coolest side offers the best performance.

Combustion Chamber and Burner

At the top of the towers sits the combustion chamber which connects all the towers together. Air passes up through one tower then passes horizontally through the upper combustion chamber and across the burner then finally passes down through another tower.

Ceramic media towers

RTOs achieve 95% thermal efficiency by absorbing and storing the waste heat from combustion in large towers of ceramic media. By periodically switching the airflow direction, the waste heat stored in the media may be released to heat incoming air.

Switching Air Valves

RTOs periodically cycle each tower between (Cycle 1) admitting dirty process air and releasing heat from ceramic media, (Cycle 2) purging and cleaning, or (Cycle 3) exhausting and absorbing heat from post-combustion air into the ceramic media.

Heat Exchanger and Stack

After passing through the combustion chamber, the air can be extracted and used to heat other process liquids or gases via a heat exchanger. Otherwise the clean air passes down through a ceramic bed, releasing its heat, then is exhaust via a stack.

Operation: How does an RTO work?


The left tower's exhaust air valve is in the open position. The left tower (C3) is warming up by absorbing heat from the outgoing air. It will be hot by the end of this cycle.

The center tower's purge air valve is in the open position. The center tower (C2) was previously filled with dirty inlet air so it's now purging that air back to the inlet fan.

The right tower's inlet air valve is in the open position. The right tower (C1) is hot from the last cycle and is now releasing heat to warm the incoming dirty air.

A flow schematic for a 3-tower regenerative thermal oxidizer (RTO).


A three-tower regenerative thermal oxidizer (RTO).
Used for applications with odors or high DRE requirements.

RTO Design Considerations


Air Volume Flowrate (CFM)

The volume of process air to be treated is a key consideration. A single RTO system can be designed to handle any amount from 1,000 CFM to over 50,000 CFM. Multiple RTOs can be combined to treat any volume of air.

Destruction or Removal Efficiency (DRE)

Factors that impact DRE:

  1. Temperature
  2. Residence time
  3. Purge cycle
    • Two-cycle no purge (2-tower RTO)
    • Three-cycle with purge (3-tower)

Materials of Construction

Material options include:

  1. Stainless steel
  2. Carbon steel, painted
  3. Aluminized steel
  4. Galvanized steel
  5. Insulation and cladding
  6. Dampers

Equipment Configuration & Layout

Key questions include:

  1. Are multiple control devices needed?
    • Due to high air volume -> 2 RTOs in parallel
    • Due to multiple contaminants -> equipment in series
      • Is a scrubber required?   (Scrubber + RTO combo)
  2. Fan location: induced draft or forced draft?
  3. Type of valves: compressed air (pneumatic), hydraulic, poppet, or rotary
  4. Inlet ductwork, outlet ductwork and exhaust stack
  5. Test ports & access platforms

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Industries, Applications, & Case Studies
Automotive Paint Spray Booth
Automotive
  • RTOs are used in automotive, marine, and other vehicle manufacturing for treating VOCs from paint booths, controlling HAPs from paint curing ovens, engine test and quality control, tire manufacturing, and component painting. The high volume, low concentration conditions are ideal for a regenerative thermal oxidizer.

Chemical Processing
  • Manufacturing or processing chemicals generates exhaust air contaminated with toxic gases and VOCs. These vapors need to be captured and treated prior to release to atmosphere. An RTO is the most robust solution for these applications: tank vents, separators, acid gases, chemical production, and ceramic kilns.

Printing
  • Printing operations require VOC abatement, use of Permanent Total Enclosures (PTEs) for fugitive emissions capture, odor elimination, and toxic fumes destruction. An RTO is an effective control device for Flexographic, Web Offset, Rotogravure, and Converting processes to treat water or solvent-based inks and exhausts from dryers/ovens.

Ethanol & Biodiesel
  • The production of ethanol and biodiesel fuels from dried, distilled grains like corn or cellulose plants creates airborne VOCs, NOx, odors, particulate matter (PM), aerosols, and other Hazardous Air Pollutants (HAPs). An RTO is a reliable solution for your kiln and dryer emissions.

Composites & FRP
  • An RTO is an appropriate solution for abating the air pollutants from Carbon Fiber, FRP, FIberglass, and other composite manufacturing. Composite resins contain styrene that needs to be thermally treated before emitting to atmosphere. Other pollutants from ovens and furnaces include Hydrogen Cyanide, Ammonia (NH3), and Nitrous Oxide (NOX).

Food, Meat, Bakery
  • Food production creates odors which often need to be controlled prior to emission. A three-tower RTO is the ideal solution for to controls odors in large-volume exhausts. The 3-chamber regenerative thermal oxidizer offers 99+% control. Smaller exhaust volumes can use a thermal recuperative oxidizer (TRO) with heat exchanger (HEX).

Industrial Ovens
Natural Gas Processing
  • RTOs are a popular air pollution control device for off-gas treatment from Natural Gas processing, including VOCs and HAPs from amine tail-gas treatment systems, sulfur recovery units, nitrogen rejection units (NRU), glycol regenerators, liquefied natural gas (LNG) processes, and storage tank venting.

Packaging & Labeling
  • Applying coatings, graphics, and adhesive labels to flexible packaging or metal containers releases VOCs that need to be captured and destroyed. An RTO is the most robust solution to treat both the application and drying air streams.

Paint Booth
Paint Booths & Hangars
Pharmaceutical
  • Tablet coating, fluid-bed processing, and tray drying in the Pharmaceutical industry offers unique air pollution control challenges as the ethanol, isopropyl alcohol (IPA), and other VOCs often releases into concentrations in excess of 100% LEL. This poses a fire and explosion risk, so dilution air is required for plant safety. An RTO is often used as the final control device.

Semiconductor
  • In the semiconductor industry, RTOs are used for control of VOC emissions from chip manufacturing, LCDs, printed circuit boards (PCBs), and more. RTOs are highly robust, operating 24/7 for 50+ weeks per year with minimal downtime. They can be designed to withstand corrosive acids or other destructive solvents in the exhaust.

Recommended RTO Manufacturers


Catalytic Products International (CPI)

Catalytic Product International (CPI) is one of the leading manufacturers of Regenerative Thermal Oxidizer systems. CPI offers their TRITON brand of regenerative thermal oxidizer (RTO) in a three-chamber, or two-chamber design. CPI has installed RTOs for the following industries: Chemical Processing, Coffee Roasting, Coil Coating, Drum & Container Manufacturing, Flavors & Fragrances, Flexographic Printing, Foam Production, Food & Bakery, Metal Decorating, Renewable Fuels, Rubber manufacturing, Painting and Coating, Printing, Semiconductor, Wastewater Treatment, and Web Coating & Converting.


CPI differentiators:

regenerative thermal oxidizer optional equipment

RTO: Optional Equipment


Natural Gas Injection

Natural Gas Injection, a.k.a. Supplemental Fuel Injection or Flameless Operation, is a method of RTO operation where the combustion chamber is operating without an open flame. At the inlet to the RTO tower, natural gas is injected into the incoming dirty air stream to increase the VOC concentration to ~3-5% of LEL. At this concentration, as long as the combustion chamber is operating at temperature set point (~1500°F), the VOCs will combust fully without an open flame, generating heat to maintain the set point, and the reaction will continue.

The main advantages of natural gas injection are:

  • Minimizing silica (SiO2) formation
  • Reducing natural gas consumption
  • Lowering NOX emission levels.

Specialized Ceramic Media Bed

  1. Random packing
  2. Saddles
  3. Monolith
  4. Structured media
  5. SiO2-Resistant (silica-resistant) structured ceramic media
  6. Corrosion resistant coating

Waste Heat Recovery

For sufficiently large RTOs, the waste heat from the RTO (~200°F) may be captured and used to heat process air or liquids. For example, RTO waste heat can be used to heat drying ovens, generate hot water, heat the plant in the winter, or another 100-200°F application.

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