Secondary and even tertiary energy-recovery systems for thermal oxidation systems help reduce energy costs. Securing a more cost-efficient operation is an ongoing objective; however, energy-recovery systems are not simply off-the-shelf, bolt-on packages with a plug-and-play manual.
To ensure a suitable return on investment, energy-recovery options must focus on system and facility integration. Achieving overall process optimization happens when there is a thorough understanding of the complicated relationship between the process and oxidation technologies. Having comprehensive knowledge about the needs of each variable component, combined with the desire to improve overall process operations by adhering to strict system design criteria, will help ensure the ROI.
Is Your RTO a Suitable Candidate for Heat Recovery?
If you are a facilities engineer, most likely you have been tasked with finding energy efficiencies throughout the company as a way to lower operating costs and improve margins. The obvious initial idea is to tackle the high utility consumers, and the first place to look is behind the plant. That is where you find a regenerative thermal oxidizer (RTO). It has been out of sight and mind since installation because it just works, but it does require a substantial amount of natural gas and electricity to do its job.
Though the oxidizer does its job well, it also directly contributes to your overhead costs because it does not manufacture products, does not contribute to your process, does not help keep the lights on and does not help keep your facility warm in the winter (at least not yet). Nonetheless, to run your process at any moment of the day, the regenerative thermal oxidizer system has to be on and operating as designed.
At a minimum, take these basic steps to ensure the oxidizer is running at its peak control efficiency:
- Tune the burner to only use the required natural gas.
- Ensure no fresh air leaks into the system. Leaks require the oxidizer to abate more air than needed.
- Change the heat exchange media to a more efficient style.
- Add additional layers of heat exchange media to reach self-sustain mode more often, which allows you to turn off the burner and save natural gas.
Overall oxidizer control efficiency is the product of two key items:
- Capture efficiency.
- Destruction efficiency.
Capture efficiency includes collection of VOCs as well as fugitive emissions from the entire process system. Destruction efficiency is the level of removal once the VOCs are introduced into the emission-control device.
Properly maintaining an RTO to run at its peak control efficiency saves energy and improves reliability. The system is probably one of your largest energy consumers, however. As such, it has the potential to deliver the biggest payback because its byproduct is heat.
The following guidelines will help determine if your regenerative thermal oxidizer is a suitable candidate for heat recovery:
- 20,000-scfm minimum volume for RTO applications.
- Minimum 200°F (93.3°C) exhaust gas temperature for air-to-air heat recovery.
- Minimum 300°F (149°C) exhaust gas temperature for other types of heat recovery.
If after careful review of all aspects of your process and thermal oxidizer, you are confident your facility will fiscally benefit from the addition of heat recovery, the next step is to choose the system that best fits your application. Having options is good, and while each offers a quantifiable ROI, the objective here is to determine what truly is the best system for your overall process operation and facility needs. Typical solutions include:
- Air-to-air heat recovery.
- Waste-heat boiler system.
- Air-to-liquid-based system.
The following presents three system options for consideration along with real-world case studies.
A custom heat-recovery system was designed for a can manufacturer in Milwaukee. The captured heat was used to heat water for the can production process. An RTO has the potential to deliver the biggest payback because its byproduct is heat.
Air-to-Air Heat Recovery System
A standard air-to-air heat exchanger is one of the most common energy-recovery systems for a thermal oxidizer. These types of heat exchangers can be of a tube-and-shell or plate design. The recovered energy heats fresh ambient air via the cold side of a heat exchanger. Once the fresh air is heated, it can be directed to different parts of the facility or process.
Ideally, the system is installed close to the facility or process to help reduce the cost of insulated ductwork. New technology also makes it possible to modify and retrofit older systems, which helps reduce footprint size and pressure requirements.
Roofing Materials Case Study. A manufacturer of fiberglass matte used for roofing materials required emissions control equipment for its new facility. The equipment needed to handle roughly 66,000 scfm of process air. Because of the high inlet temperature of 391°F (199°C) to the oxidizer, the thermally efficient oxidizer still produced an outlet temperature of 476°F (247°C).
In this application, heat is recovered directly from the exhaust stack and passed through a two-stage filter before entering a plate-style, air-to-air heat exchanger. The recovered heat supplies the process dryers in the summer as well as 100 percent of the entire plant’s heat in the winter. Based on an 8,000-hr/yr production schedule, the facility recovers 11.32 million BTU/hr in the winter and 17.59 million BTU/hr in the summer. The ROI for this large-scale heat-recovery project was eight months.
Waste-Heat Boiler System
Waste-heat boilers generally fit most footprints and have numerous sizes and orientations, thereby providing broad application flexibility. While this system requires a consistent and high temperature recovered-heat stream, a waste-heat boiler can be extremely beneficial because steam can be used in so many processes.
A waste-heat boiler only can be implemented in situations where an oxidizer has a high inlet solvent loading and reaches self-sustain operating mode. In addition, using a waste-heat boiler also necessitates the shedding of combustion chamber heat through a bypass valve to avoid a high temperature situation.
Coating Line Case Study. At a coating process in the New England area, it was possible to retroactively install a waste-heat boiler using heat captured directly from the combustion chamber of an existing RTO. The scope of supply included an insulated ducting system from the hot bypass valve on the combustion chamber of the RTO that was routed to the adjacent new waste-heat boiler. The exhaust from the bypass valve averages between 1700 to 1750°F (927 to 954°C), consistently. Depending on the actual solvent usage in the coating process (normal was 500 lb/hr), a minimum of 1,950 scfm and a maximum of 3,500 scfm of air is sent to the boiler. The boiler configuration allowed the manufacturer to make 3,032 lb/hr of steam by recovering nearly 3 million BTU/hr of waste heat. The ROI was 24 months for the entire project.
This three-tower RTO includes a heat-recovery system. Designed as a secondary heat-recovery system for a steel company, they used fresh air, or air from the coater room, that was heated up first by a heat exchanger fed from the oxidizer’s hot gas bypass. Then, a secondary burner was used to provide heat for the ovens in the plant.
Air-to-Liquid Based Systems
Air-to-liquid heat-recovery systems are a common option for heat recovery. They are rather similar to an air-to-air heat exchanger, but the hot oxidizer air heats a flowing liquid — a thermal oil, glycol or water. Typically, the hot air from the oxidizer passes over liquid-filled tubes to increase the temperature. In some instances, however, the hot oxidizer air passes through the tubes to heat the liquid that is on the outside.
The benefit of a liquid system as opposed to an air-to-air heat-recovery system is the use of piping instead of ductwork. Piping generally is cheaper than ducting and more cost-efficient to keep heated. Of course, like any of these options, considering where to reintroduce this recovered heat back into the process is important. While recovered hot air has more options, thermal oil-, glycol- or water-based systems require specific processes to be effective. All of this should be considered before choosing this type of system.
Web Coater Case Study. A plant that processes imaging materials and foil tapes was using a 35,000-scfm recuperative oxidizer to treat web coater emissions. The company, in consultation with the oxidizer manufacturer, decided to replace the recuperative oxidizer and install a new regenerative thermal oxidizer.
Due to the incoming solvent load, the new regenerative thermal oxidizer was able to achieve self-sustain operating mode. Equipping a hot-bypass valve on the RTO allowed the web coater to recover high quality hot air. A glycol-based fin/tube coil recovery system was installed to return heat to coils located in the web coater boxes via an existing boiler feed arrangement. The average condition was 40,000-scfm at 370°F (188°C) once it reached the glycol-based system. The configuration allowed the customer to supplement 6.0 million BTU/hr.
In this case, the customer purchased an energy performance contract from the oxidizer manufacturer. The contract required no capital costs for the equipment, and all the energy savings paid for the entire system (RTO and heat recovery) in five years.
In conclusion, when looking at the operational potential for secondary or tertiary heat-recovery systems, ensure you are meeting the energy demands of the process and pollution control systems. Once their overall effectiveness and maximized efficiencies are confirmed, only then is it appropriate to investigate alternative energy-recovery projects and the potential ROI.
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