Secondary Heat Recovery

The oxidizer design and process operating conditions will determine how much excess heat is available for secondary heat recovery.  Heat recovery can be accomplished in two different ways:  Extracting heat from the stack or extracting heat directly from the combustion chamber. Dryer exhaust air temperature, solvent concentration and heat exchanger efficiency determine the oxidizer stack air temperature.  As oxidizer efficiency is increased, the stack temperature will decrease.  Destruction efficiency requirements determine the combustion chamber temperature. When concentrations rise above the minimum energy required for self-sustaining operation, excess heat is generated in the combustion chamber.  This excess heat presents a great opportunity for heat recovery.  Whereas the oxidizer stack temperature will be 100 to 400oF (55 to 222oC) above the inlet temperature, air from the combustion chamber will be at 1,400 to 1,600oF. Depending on the need, various forms of heat recovery options are available.

Figure 4. With direct air heat recovery, the heated air from the oxidizer is ducted directly to the process. This approach is impractical where space is limited or the oxidizer is located a great distance from the heat user.

Direct Air. Direct air heat recovery is an arrangement where heated air from the oxidizer is ducted directly to the process (figure 4).  Coupled to a convection dryer, this air can be utilized as preheated makeup air.  This system is simple and requires little auxiliary equipment: modulating dampers, fans and pressure control loops.  One drawback is that direct air recovery requires a large amount of high temperature ductwork.  This system often proves impractical when space is limited or the oxidizer is located a great distance from the dryer.

Figure 5. Heat recovery with an air-to-air heat exchanger should be considered if the products of combustion from the oxidizer may contaminate the process.

Air-to-Air. Similar to direct air heat recovery, heat recovery with an air-to-air heat exchanger (figure 5) should be considered if products of combustion from the oxidizer may contaminate the drying process.  It is important to carefully analyze the dryer operating conditions whenever considering a blended air heating system.  Dryer air temperature is directly related to the volume of makeup air.  If air volume and temperature requirements cannot be balanced, an auxiliary heating system such as a gas fired burner may be required to supplement the heat recovery system.

Figure 6. An oil- or steam-to-air heat exchanger located in the dryer’s recirculating airstream allows the air temperature to be controlled independently from the makeup air volume.

Air-to-Oil. Another recovery method is an air-to-oil heat exchanger.  The exhaust air from the oxidizer passes through a heat exchanger, heating a thermal oil (figure 6).  This system offers more operating flexibility for the dryer than the direct air or air-to-air systems. With either of the air-based systems, the dryer must take a volume of makeup air that is proportional to the heat requirements of that zone, and the more heat required, the more makeup air that zone must take. This often is contrary to the optimum dryer operating condition.  The zones with high evaporation rates (the early zones of the dryer) usually operate at lower temperatures than the latter zones that require much lower exhaust rates.  The air-to-oil heat exchanger is better suited to these requirements. An oil-to-air heat exchanger located in the dryer’s recirculating airstream allows the air temperature to be controlled independently from the makeup air volume.  The hot oil circulation system is more compact but more complex and expensive than a ductwork system.

Air-to-Steam. Air-to-steam heat recovery systems are similar in design and operation to the air-to-oil system (figure 6). Hot oil systems can operate at higher temperatures than steam, but steam offers higher heat transfer rates. Typically, steam is preferred in a facility already using steam. Otherwise, a hot oil circulation system will be used.

Absorption Chiller. A less common heat recovery alternative is using waste heat to provide chilled water. Usually, the cost and complexity of these systems are prohibitively high when compared to a conventional chiller system.

Heat Recovery Considerations

It is important to understand that as the efficiency of the primary heat recovery systems is increased, the amount of heat available for secondary recovery decreases.  A well-designed primary heat recovery system may eliminate the need for any secondary system.  The design goal for any new system should be to first optimize the primary heat recovery system.  

Secondary heat recovery requires the interconnection of two separate operating systems.  It is important to thoroughly analyze the operating cycles of both the dryer and oxidizer through their entire range of operation.  The dryer provides fuel to the oxidizer; the oxidizer provides heat to the dryer. If supply and demand are not balanced for all operating conditions, an auxiliary heat source may be required at the dryer.  Generally, it is not recommended to over-fire the oxidizer to provide heat to the dryer.

This analysis gets even more complex when considering a tertiary heat recovery system. With primary and secondary recovery, the process requiring the heat operates on the same schedule as the process generating the heat.  The tertiary recovery system may be providing plant heating or cooling, or energy to another process.  Supply and demand are rarely balanced, so tertiary systems normally are considered supplemental.

Designing an efficient heat recovery system is not difficult.  Waste heat sources are matched with potential users, and the appropriate heat transfer and control system is designed.  However, determining the feasibility of a heat recovery system is difficult.  Heat recovery feasibility is a financial decision.  Energy savings must be balanced against capital, operating and maintenance costs over the life of the system.  Accurately quantifying these variables can be extremely difficult.  A thorough analysis of potential heat sources and users is necessary.  The skill and experience of the system designer are critical.  Fluctuating utility prices and changing regulations complicate payback calculations.  The technology of heat recovery is well developed.  The art lies in the application.