A regenerative thermal oxidizer (RTO) is an air-pollution control device designed to destroy volatile organic compounds (VOCs), hazardous air pollutants (HAP) and odorous compounds from manufacturing and industrial processes. Thermal treatment of VOCs and other air pollutants works by a simple reaction of the harmful air pollutants with oxygen and heat. In this environment, the VOCs are converted into harmless inert byproducts, water vapor (H2O) and usable heat. These harmless byproducts are released to atmosphere or used with an energy-recovery technique to further lower the operational costs. With thermal efficiencies of up to 97 percent and destruction efficiencies that can exceed 99 percent, regenerative thermal oxidizers provide high VOC control efficiencies.
Regenerative thermal oxidizers are available to handle process exhaust streams from 2,000 scfm to large applications with greater than 100,000 scfm. Each oxidizer is engineered and built to meet specific process conditions and the characteristics of the process gas stream. (These include quantity, temperature, particulate, composition of VOCs and location relative to the oxidizer position.) While there are subtle differences in regenerative thermal oxidizer design from one manufacturer to another, a common factor is that no matter which system is installed, preventive maintenance is the key to successful uptime.
Regenerative thermal oxidizers essentially provide a “permit to operate” the profit-generating operations of a facility, so employing proper maintenance practices is critical. Proper maintenance helps facilities meet regulatory requirements and avoid unexpected shutdowns and equipment failures.
Keeping a regenerative thermal oxidizer system operating at optimal levels requires diligent system monitoring by qualified, experienced technicians. The mechanical components of a thermal oxidizer generally have 15 to 20 years of useful life before changes such as material embrittlement and obsolescence begin to create problems. Routine inspection of the critical components of any oxidizer, however, can provide crucial information about common points of failure as your oxidizer begins to age.
There is a delicate balance between scheduled downtime and plant uptime reliability. Major shutdown work is intended to support maintenance efforts (including minor downtime repairs) that are carried out until the next major shutdown. Many industrial facilities schedule operations downtime during the end-of-year holidays or in the summer. This means that during these periods, the calendar quickly gets filled up for both the on-site maintenance teams and the skilled technical experts available from the OEM equipment manufacturers. As a result, whether you are planning for an annual, semi-annual or monthly maintenance event, it is important to coordinate with your aftermarket service supplier to ensure the availability of service technicians and necessary spare parts.
Maintenance planners should have an idea of the tasks that need to be completed. It is helpful to keep a specific checklist of parts to order so that service can be completed. Keep in mind that the lead time for some parts is longer than for others. Basic parts such as thermocouples, UV scanners, lens, spark igniters and pressure transmitters should be kept in stock. Longer lead-time parts like motors, bearings and valves can take three to six weeks to procure. Failure of these components without a spare on-site could prove costly.
Keeping an oxidizer running at peak operational efficiency can reduce downtime and ensure that process operations are in compliance with the facility’s environmental operating permit. Detailed inspection plans can be developed with an aftermarket service partner, but they typically include items such as:
- Weekly inspections to evaluate the gas train, burner, gauges, motor intakes and process ducting.
- Monthly preventive work such as lubricating rotating components, cleaning sensing lines and checking filters.
- Semiannual checks to evaluate instrumentation such as pressure transmitters.
- Semiannual evaluations to check the gas train for leaks and confirm valve sealing.
Annual inspections should include an interior review, during which the oxidizer must be cooled to allow safeties testing, valves cleaning and replacement of damaged components.
Safety is a priority when performing any maintenance on any type of machinery. Manufacturer operating and maintenance (O&M) manuals contain a section on cautions and safety in which several levels of advisory statements are described.
An air-pollution control device, a regenerative thermal oxidizer is used to destroy VOCs, HAPs and odorous compounds from manufacturing and industrial processes.
Inspection and Maintenance Tips
Critical areas of inspection and maintenance include the media, burner, sensing line, UV scanner, actuator, cold-face support, insulation, switching valves, temperature sensors, and fans and bearings.
Ceramic Media Condition. Regenerative thermal oxidizer media comes in several configurations: random, structured or a combination. The choice of media supply is based on the manufacturer’s design and, usually, the application. Structured media, which has gained popularity over the last 15 to 20 years, comes in different styles from monolithic structures to complex engineered layered structures. Random media is common in older systems and is supplied in varying shapes and sizes.
Typical ceramic media failure modes include:
- Settling of the ceramic heat exchange media.
- Plugging of the ceramic heat exchange media.
- Plugging of the cold-face support.
- Chemical attack of the ceramic media.
- Burner exposure.
Routine internal inspections help to prevent total failures of the oxidizer media and allow issues to be addressed before more costly repairs are required. Predictive analysis features (PAFs) that measure and trend media-bed pressure drops and temperature profiles can be implemented to provide a higher degree of real-time analysis. Implementing advanced PAF alarm indicators may help to avoid severe damage and extended unplanned downtimes due to material availability.
Burner Condition. During the startup process, technicians will set up the burner to efficiently fire to achieve and maintain the combustion chamber setpoint. This is critical for the regenerative thermal oxidizer to achieve the designed destruction rate efficiency (DRE).
Burner systems are a critical piece in the emissions from the regenerative thermal oxidizer system, so verification of the combustion air ratios and burner operation need to be addressed during semiannual inspections, at a minimum. Proper maintenance and cleaning are important to ensuring the proper operation of the burner. Crucial components include the:
- Spark ignitor, the proper operation and positioning of which are vital to ensuring the burner will light reliably.
- Burner actuator, the operation of which can be verified during shutdown via visual inspection and testing operation.
- UV scanner for flame detection, which must be kept clean of debris during operation.
- Flame inspection, which will provide clues about operating issues. If visual inspection of the flame indicates a potential problem, the maintenance technicians can refer to the service manual for target flows on combustion air and natural gas.
Cold-Face Support Condition. Another area that requires periodic inspection is below the bed or at the cold-face support. The cold-face support system hold many thousands of pounds of delicate ceramic media while being exposed to potentially high temperatures, especially if periodic bake-out is part of the normal oxidizer operation. The cold-face support system also is exposed to chemical compositions of the off gas.
Torn, sagging, severely oxidized or warped metal is a sign of a serious problem requiring immediate attention. It is important to inspect this area from below the cold face in order to look for signs of structural deterioration, condensate buildup and media plugging.
Insulation Condition. The primary areas of a regenerative thermal oxidizer that are insulated internally are the media chambers and the combustion chamber. In these sections, the insulation typically is a ceramic fiber module. Occasionally, a pinned blanket is used; however, this is not a durable, long-term choice due to the insulation pins being exposed to the combustion chamber temperatures (that above 1500°F [816°C]).
Thermal treatment of VOCs and other air pollutants works by a simple reaction of the air pollutants with oxygen and heat. A typical flow of a two-chamber, poppet-valve regenerative thermal oxidizer is shown.
It is essential that the insulation system is installed correctly when the system is new. If not, the insulation system can cause damage over time. Insulation failure effects can be found easily during external inspections. Surface discoloration (also know as hot spots) is a sure sign of higher-than-normal external shell temperatures and possible damage. Weekly walking inspections around the RTO to identify and label heat marks will help with future reviews that seek to determine whether the heat mark is growing. Severe cases call for an internal inspection.
Burner. Another area to monitor is around the burner. Any obstructions on flat surfaces (burner and sight glass) require hand packing of ceramic materials. Over time, this material can dislodge and cause a direct path of heat to the shell.
If an external inspection shows signs of potential internal problems, an internal inspection of any affected area should be conducted. Appropriate confined-space entry protocol must be followed and, depending upon the location of the hot spot, some dismantling of the heat exchanger or other obstructions maybe required.
Obvious points of failure will be fallen modules or dislodged batten-strip blankets. Typically, insulation ill shrink slightly, creating small gaps. If the gaps between modules become too large, however, hot spots will form and the lining will fail eventually. It is possible to improve the situation by stuffing the gaps with ceramic materials.
It is important to note that the visual appearance sometimes is deceiving. In complex transitions and housings, insulation damage is common, especially with oddly shaped transitions and turns.
Switching Valve Condition. The switching valve system is the primary component that provides the oxidizer’s destruction rate efficiency. Regular visual inspection of switching valves is critical. These are the primary moving parts in a regenerative thermal oxidizer, potentially switching more than 150,000 times annually. If the switching valve is not adjusted correctly or experiences a failure, the oxidizer may not achieve the VOC DRE required by the air permit. Close visual inspection of the switching valves should include all mechanical components as well as pneumatic actuators for valve switching, shafts or couplings.
Thermocouple Maintenance. Many air permits call for yearly thermocouple calibration. For those not needing calibration, there are procedures that can be used to ensure that they are within the manufacturer’s specification. If the thermocouple is starting to drift from the specification, it should be replaced. It is a good idea to have spare thermocouples in your on-site spare parts stock.
In conclusion, preventive maintenance is necessary to the long-term reliability and performance of an RTO. Through a combination of in-house resources and expert partners, you can make sure you are enabling the clean production of your manufacturing or processing operation.