Often the installation of an RTO/RCO is focused on the air emission control issue without full consideration of the potential for secondary energy recovery. There are a multitude of options to recover heat from the RTO/RCO exhaust and provide substantial plant energy savings.
Companies with regenerative thermal oxidizers (RTOs) may be able to optimize their facility energy costs by recovering heat from the RTO/RCO unit
To fully optimize recovered energy, utilization decisions must be made up front (along with the RTO design features).
Some potential options include:
- Augmenting the general ventilation heating and/or cooling capabilities of the plant (recovered energy can be used to supplement or replace plant hot makeup air requirements in winter months or be directed to cooling systems in the summer)
- Direct process heat and/or cooling replacement (recovered energy allows for more efficient use of ovens, dip tanks, dryers, heater boxes, coolers, etc.)
- Combined general HVAC and process heat replacement
- Supplemental heating for existing combustion sources (to provide preheat to incoming or makeup air for boilers, dryers, hot water generators or steam systems)
Many factors come into play with the RTO when determining what is the most efficient and cost-effective solution. Three options proven to provide significant energy savings include Direct RTO Stack Exhaust Heat, RTO Hot Gas Bypass Exhaust, and RTO Stack with Secondary Heat Exchanger:
Direct RTO Stack Exhaust Heat
This method of heat recovery channels the RTO/RCO stack exhaust directly back to a boiler, heater, furnace, dryer or other process that requires combustion air or heated make up air. However, the following factors need to be considered in the economic evaluation of the installation:
- Location of the RTO/RCO stack relative to the process connection
- Quantity of oxygen left in the stack exhaust
- Stack exhaust temperature and moisture content
NESTEC has broad experience with these systems. One solution we worked on was designed for a RTO/RCO stack exhaust to recirculate within interior ductwork. This system maintains velocity and temperature in the ductwork and minimizes potential condensable buildup and associated fires without the need for ambient air stream heating coils.
Depending on the ambient conditions and associated utilization, the system provides $450,000 to $680,000 per year in equivalent natural gas savings.
RTO Hot Gas Bypass Exhaust
Process exhausts with VOC/HAP loadings greater than 3% of the lower explosive limit (LEL) offer a potential for high temperature exhaust air (+1500 oF) from an RTO hot gas bypass system. These systems bleed hot gas from the RTO combustion chamber without affecting the heat-exchange media, thus maintaining the RTO self-sustaining mode of operation and providing high temperature gas for secondary energy recovery.
Hot gas bypass is therefore useful for secondary heat recovery without reducing the RTO thermal energy recovery (TER) efficiency. This type of system can be utilized in several different approaches. Hot bypass gas can be:
- Directed to a waste heat boiler for steam or hot oil generation
- Blended with ambient air to the desired:
- Temperature, or
- Required oxygen content of other process combustion make up air
- Directed to a secondary air to liquid heat exchanger (or blended with ambient air to the most economical heat exchanger temperature)
- Directed to an Absorption chiller (in a low-pressure system, the absorption fluid is evaporated, removing heat from the chilled water; a heat source such as steam, exhaust gas or hot water is used to regenerate the Absorption solution)
Nestec has installed several of these systems. One particular case directed the RTO hot gas bypass exhaust to a waste heat boiler, providing $2,200,000 per year in equivalent natural gas savings.
NESTEC RTO with hot gas bypass to a waste heat boiler
RTO Stack with Secondary Heat Exchanger
This method of heat recovery utilizes the RTO/RCO stack exhaust along with a secondary heat exchanger. A variety of heat exchangers can be employed, including:
- Air to Air, to:
- Preheat process make up air (or other source combustion air requirements)
- Air to liquid, to:
- Preheat boiler make up water
- Transfer a heated liquid:
- To multiple sources
- Long distance
One system NESTEC installed utilizing this option was designed with secondary air-to-air heat exchangers to preheat process make up. The resulting solution provides $220,000 per year in equivalent natural gas savings.
In conclusion, companies with regenerative thermal oxidizers (RTOs) may be able to optimize their facility energy costs by recovering heat from the RTO/RCO unit. The recovery of waste heat from an RTO can also reduce carbon footprints and dramatically improve the bottom line. A properly selected and designed heat-recovery system will optimize energy and utility consumption.
Several other energy saving features are also available with NESTEC designs, many of which can be applied to existing RTO systems.
If you want to maximize your RTO energy conservation and reduce your energy bill, contact NESTEC for a free process analysis.
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