Companies in every sector of industry are continually searching for areas in their process to save energy, thus reducing costs and maximizing profits. The low hanging fruit for energy optimization and savings can be found in the systems and machinery requiring the most utilities to operate. Air pollution control equipment and, for the purpose of this article, regenerative thermal oxidizers (RTOs) are some of the biggest consumers of utilities. Reusing exhaust heat from these units provides facilities an opportunity to recover lost dollars and reduce their carbon footprint.
Basic Operation of an RTO
A regenerative thermal oxidizer consists of a purification chamber located above a single or multiple energy-recovery chambers. These energy-recovery chambers are filled with ceramic heat-exchange media. The hydrocarbon-laden air enters the inlet heater and is directed to one of the energy-recovery chambers through a directional valve. The air passes through the heat-exchange media, absorbing heat from the hot inlet ceramic bed. It then enters the purification chamber at a temperature very close to the oxidation temperature — typically 1400 to 1550°F (760 to 843°C).
What Types of Systems Can Be Used to Recover Heat?
Many systems may be utilized for heat recovery. They include air-to-air systems, waste-heat boilers and air-to-liquid systems.
Air-to-Air Heat Recovery System. One of the most common recovery systems from a thermal oxidizer is a standard air-to-air heat exchanger. Air-to-air heat exchangers can either be a tube and shell or plate.
Waste Heat Boiler System. If the process stream contains a very high inlet solvent loading necessitating a VER bypass valve to avoid a high temperature situation, a waste heat boiler may be considered. Waste heat boilers have numerous sizes and orientations and generally can fit most footprint requirements.
Air-to-Liquid Systems. Another common option for heat recovery is using the hot oxidizer air to heat a flowing liquid such as thermal oil, glycol or water. The hot air from the oxidizer passes over liquid-filled tubes to increase the temperature. In some instances, the hot oxidizer air is passed through the tubes to heat the liquid that is on the outside. A benefit of a liquid system is the use of piping as opposed to ductwork. Piping is generally cheaper than ducting and more cost efficient to keep heated.
If the incoming gas contains a sufficient concentration of solvents, the energy content of those organics provides the necessary heat to raise the temperature of the exhaust stream to the combustion setpoint. As the VOC concentration increases, the burner modulates back, first to minimum fire, then switching off at a setpoint between 1650 and 1750°F (899 and 954°C). At this point, the regenerative thermal oxidizer is self-sustaining with only the VOC loading as a fuel source.
Once oxidized, the purified air leaves the unit by passing through the cold outlet heat-exchange media in an adjacent chamber or pie section. The heat in the air is transferred temporarily to the heat-exchange media. The clean air is discharged through a stack to the atmosphere. The ideal scenario is that the temperature change from inlet to outlet ranges from 60 to 120°F (15 to 49°C), which indicates good thermal efficiency and low natural gas consumption. Due to numerous process parameters and air constituents, however, the temperature of the air exhausted can be significantly higher than the temperature of the air when enters the regenerative thermal oxidizer. These high exhaust temperatures represent energy that could potentially be captured and reused.
In some instances, further increases in VOC concentration cause the burner chamber temperature to increase, activating a hot bypass system. If the burner chamber temperature increases above a setpoint, a variable energy-recovery (VER) system opens a bypass damper to control the temperature. As the hot gas bypass damper modulates, a portion of the hot gas from the burner chamber bypasses the oxidizer's heat-exchange beds, thus controlling the burner chamber temperature. The hot bypass gas flow is mixed with the normal oxidizer exhaust flow stream before discharging through the exhaust stack duct. This occurrence is key when evaluating a system for heat-recovery options and will be discussed later in this article.
Heat Recovery and Regenerative Thermal Oxidizers
Regenerative thermal oxidizers, even though effective and otherwise fuel efficient, are a substantial user of natural gas and electricity. They do not have to be a total drain on the bottom line, however. The capture of waste heat contained in the essentially clean, high temperature exhaust air can allows it to be re-utilized in the process or in other beneficial ways. Waste-heat recovery from an RTO can be realized when installing a new system or retrofitted after a system has been installed. The impetus for an end user to consider this type of heat-recovery system is the bottom line: How much heat can be recovered, and how quickly will this equipment be paid off, so the plant can truly realize the savings?
What Are the Options for Recovering Heat from Industrial Processes?
Generally, one of three options is used to recover heat from an RTO. As it relates to achieving the highest rate of return, they can be graded as good, better or best.
Direct Stack Heat. A good method of heat recovery is direct stack heat. This method is used in the absence of any sort of hot bypass system (described earlier in this article). Air can be extracted directly from the stack at a very high volume. It is a lower grade heat source because most of the heat has been shed to the heat-exchange media that it has traveled through after oxidation. This provides a good rate of return but is the least effective method.
Blended RTO Exhaust with Hot Bypass Heat Displacement. A second method of heat recovery is blended regenerative thermal oxidizer exhaust and hot bypass heat displacement. Acquiring heat from the exhaust duct of the RTO after it has blended with heat displaced directly from the combustion chamber through the hot bypass damper provides a high volume of air, with a much higher temperature. This provides a good rate of return.
Direct Heat Displacement from the VER Hot Bypass Damper. In my opinion, the best method of heat recovery is direct heat displacement from the VER hot bypass damper. In an oxidizer where the system reaches self-sustain mode, and the process has a high concentration of VOCs, a hot bypass damper will open to shed excess heat to prevent a high temperature fault in the RTO. This air does not go through the heat-exchange media but exits via the hot bypass damper through a duct directly into the stack. This is high grade heat providing the best opportunity for quick payback.
Where Should Heat Recovered from Industrial Processes Be Deployed?
To fully optimize recovered heat, the decision regarding where it should be sent must be made. Some potential options include:
- General ventilation heating needs in the plant. It can be used to supplement or replace hot makeup air requirements in winter months.
- Direct process heat replacement. It can be used for ovens, dip tanks, dryers, heater boxes, etc.
- Combined general HVAC and process heat replacement.
- Supplemental heating for existing combustion sources. For instance, it can be used to preheat incoming or makeup air for as boilers, hot water generators or steam systems.
Many factors come into play when determining what is the most effective and cost-efficient solution.
Design Considerations and Challenges
Numerous aspects must be considered to design and implement a proper heat-recovery system. A few items and challenges to take into consideration are:
- Differential pressure across the system.
- Materials of construction.
- Bypass capability and maintenance access.
Location. The distance between the RTO and where the recovered heat will go is critical. Transferring heat a large distance via insulated duct may be costly.
Differential Pressure Across the System. The addition of heat recovery can increase pressure requirements for the system’s fan. Therefore, it will need to be evaluated to determine if an existing fan is suitable or if a new one is needed.
Materials of Construction. Depending upon the process stream, chemical changes to abated exhaust air may be destructive to milder steels. Heat-recovery components should be evaluated and selected to avoid corrosion or a chemical attack.
Bypass Capability and Maintenance Access. Access to the heat-recovery system should be taken into consideration when choosing a system and its location at the facility. Maintenance access to the heat-recovery system should be available without shutting down the thermal oxidizer.
In conclusion, companies with regenerative thermal oxidizers may be able to optimize their facility’s energy cost by recovering heat from the unit. The recovery of waste heat from an RTO may reduce carbon footprint and improve the bottom line. A properly selected and designed heat-recovery system will at the least help any manufacturer optimize energy and utility consumption. Consult with an RTO supplier to see how much can be saved through secondary heat recovery.