Companies in all industries are constantly seeking ways to reduce a plant’s operating costs and carbon footprint. At the same time, they are looking to avoid costly utility-distribution improvements by optimizing the facility’s energy consumption.
Thermal oxidizers require significant amounts of energy to achieve their intended benefit. One of the reasons for this is that treatment of the process exhaust air requires heating it to the highest autoignition temperature of compounds to be oxidized. The combustion chamber temperature can range from 1400 to 1700°F (760 to 927°C) — or higher as needed. Though various configurations exist to improve on energy efficiencies, thermal oxidizers remain a substantial user of natural gas and electricity.
One way to mitigate an oxidizer’s operating costs and carbon footprint is to consider adding a secondary heat exchanger as a means to recoup heat from the oxidizer’s exhaust. Energy recovered can be used to heat fresh air, recirculated air, water, thermal oil or steam. The amount of heat recouped varies with the types of thermal oxidizer. Obviously, the more thermally efficient the oxidizer, the less available heat there is to recover from its exhaust, though there are some important exceptions.
Types of Thermal Oxidizers and Heat Recovery
Non-recuperative direct-fired thermal oxidizers typically are used in situations with high concentrations of volatile organic compounds (VOCs) or where minimal initial capital investment is required. Though this type of thermal oxidizer is low in upfront capital cost, the operating costs can be high. This is especially true in applications where the amount or composition of VOCs provides little supplemental heat to the combustion chamber (low heat of combustion energy relative to the mass of exhaust). Because heat recovery is not included with this type of oxidizer, the high outlet temperature of non-recuperative oxidizers makes them good candidates for secondary energy recovery.
Recuperative thermal oxidizers differ from non-recuperative systems by incorporating a heat exchanger to preheat the process exhaust air prior to entering the combustion chamber. Mass-corrected thermal efficiencies (MCTE) typically range from 60 to 65 percent, and some recuperative thermal oxidizers can provide as high as 70 percent MCTE. While this may seem high, it is short of what can be achieved by regenerative thermal oxidizers. Therefore, recuperative thermal oxidizers may offer good energy recovery opportunities.
Regenerative thermal oxidizers (RTOs) were developed to optimize energy usage. By alternating the process flow past heat-capturing ceramic media, heat is transferred from the regenerative thermal oxidizer’s exhaust air back to the incoming process exhaust air. RTOs readily achieve greater than 95 percent MCTE. At first glance, this might seem to provide only longer-term heat recovery returns on investment. Certain conditions, however, significantly decrease the time needed to justify the cost of heat recovery equipment.
If the VOC concentration entering the combustion chamber is high enough, the operation becomes auto-thermal; that is to say, the combustion of VOCs is self-sustaining. No additional heat is needed from the burner. VOC concentrations above auto-thermal result in excess heat not required to maintain the oxidation process.
In auto-thermal cases, once the chamber temperature increases above the combustion chamber setpoint, a hot-gas-bypass system is activated. Temperature in the chamber is controlled by a variable energy recovery (VER) system, which opens a bypass damper to allow a certain portion of the hot gas from the burner chamber to bypass the ceramic heat-exchange media. This hot gas stream provides a good source of recoverable heat.
Frequently, secondary heat recovery is achieved using either air-to-air (200°F [93°C] and higher exhaust gas temperature) or air-to-liquid (300°F [149°C] and higher exhaust gas temperature) heat-exchange methods.
There are different opportunities to recover heat from hot exhaust air:
- Air can be extracted directly from the stack at high volumes. This method would be used in the absence of a regenerative thermal oxidizer hot-gas-bypass system.
- Exhaust air can be blended with hot bypass air from the VER system.
- Hot bypass air from the VER system can be used on its own.
Air-to-air systems recover heat either directly through recirculation or indirectly using heat exchangers. These systems are a good fit when the distance from the thermal oxidizer to the intended source for the heat is relatively short. Air-to-air heat recovery may be used effectively to preheat combustion air for the oxidizer or the building HVAC, or to heat an oven in other applications.
Liquid heat recovery systems transfer the excess energy from air to fluid and then to the heat-exchange device for hot water or oil. When the temperatures needed are below 300°F, high temperature, high pressure (HTHP) hot water is ideal. At these temperatures, it is easy to make 300°F hot water and economically pump it a long distance through small diameter insulated pipes. When the need is for multiple zones, the main heater can be tapped at several places. Each zone can be independently and precisely controlled by a throttling valve with a separate temperature control loop. These systems generally are limited to producing 330°F (166°C) hot water at 100 psig. Typical systems can be constructed using schedule 40 ASTM A106, A53 pipe, 150 lb flanges, valves and fittings. Local codes and plant requirements may change this, so they must be consulted. A significant advantage of HTHP water is that these systems can be FDA compliant.
When the temperatures are higher than 300°F, liquid heat recovery systems use synthetic oils instead of water as the heat transfer medium. System complexity increases, along with construction and material costs. Most systems use 600 lb flanges and components. Despite their complexity, these systems share the same flexibilities as the HTHP water systems. Recovered heat at high temperature also can be used to generate steam via a waste-heat boiler. These systems are quite popular in chemical and petrochemical plants.
Installation Considerations for Heat Recovery Systems
Whatever heat recovery method you consider, there is the possibility an air-to-air or air-to-liquid heat recovery system can be used with success. When designing and implementing a proper heat recovery system, it is important to consider:
- The differential pressure across the system (for regenerative thermal oxidizers).
- Materials of construction.
- Bypass capability and maintenance access.
Location. The distance between the thermal oxidizer 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. It should be evaluated to determine if the existing fan is suitable or if a new one is needed.
Materials of Construction. Depending on 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.
Heat Recovery Alternatives
A slowly emerging technology that can be used for secondary heat recovery is the organic-rankine cycle (ORC) process. The ORC process — known and applied for a long time in geothermal applications — is gaining more importance in the industrial sector as a waste-heat-to-power-conversion process.
The ORC process is a steam cycle that uses heat from the exhaust gas of the oxidizer or other heat sources to generate electricity. It can recover heat from exhaust gases at the stack or from a VER hot bypass system. During the ORC process, hot exhaust gas flows from the heat source into the ORC module. There, the working medium is evaporated through the thermal energy. The pressurized steam is led into a turbo-generator, where part of its thermal energy — 11 to 20 percent — is converted into electricity. After that, the vapor is condensed through cooling. A pump injects the liquid working-medium back into the evaporator.
The recent development of smaller ORC compact units makes the technology suitable for applications with smaller waste-heat sources (less than 200 kWth). In in installations employing combined heat and power (CHP) technologies as well, the ORC technology can transform waste heat into valuable energy with an overall efficiency of greater than 95 percent. Because the ORC technology is still capital intensive, the number of applications in combination with thermal oxidizers is low. Nonetheless, it presents an interesting future alternative.
In conclusion, companies with thermal oxidizers may be able to optimize the facility’s energy cost by recovering heat from the existing process. The recovery of waste heat from an oxidizer may reduce a plant’s carbon footprint and improve the bottom line. A properly selected and designed heat recovery system will help any processor optimize energy and utility consumption. Consult with an oxidizer supplier to see how much can be saved through secondary heat recovery.