Historically, air pollution control systems had been designed using integrated air-to-air heat exchanger units ranging from 40 percent to 70 percent thermal efficiency. Therefore, these older units capture and reuse only 40 percent to 70 percent of the heat that is required to thoroughly destroy the manufacturing facility's harmful air pollutants. To meet EPA pollutant destruction requirements, catalytic oxidizers typically operate at 650oF (343oC); thermal oxidizers operate at temperatures ranging from 1,400 to 1,800oF (760 to 982oC). Depending upon the air pollution control system design, exhaust stack temperatures can vary between 250 and 1,500oF (121 and 816oC).
The heat required to operate these systems comes from large natural-gas-fired burners. As the cost of energy continues to rise, the importance of operating an energy-efficient system becomes a higher priority. Achieving higher thermal efficiency by design has been possible for many years, but it was typically considered to be cost prohibitive as a capital purchase. However, a financial payback analysis that considers current, higher energy costs may favor a change.
Upgrade Or Update
Modern air pollution control systems not only exceed all U.S. EPA, state and local regulations by destroying in excess of 99 percent of a facility's air pollutants, but they are also energy efficient. Using a low-pressure-drop structured ceramic media for heat exchange, it is possible to achieve as high as 97 percent thermal efficiency with a regenerative thermal oxidizer (RTO). Energy-efficient natural-gas-fired burners that achieve higher turndown rates help further reduce fuel consumption.The use of premium efficiency vs. standard efficiency electric motors can yield large electrical savings, especially if a facility operates on a 24/7 schedule. Incorporating an AC variable-frequency drive (VFD) unit to control and automatically vary supply fan motor speeds for volumetric control can lower electrical consumption.
If your facility's air pollution control system does not have these features, an upgrade or retrofit may net energy and cost savings. For example, an RTO that was originally equipped with random-packed media beds can be upgraded to structured media beds. This both increases thermal efficiency, yielding fuel savings, and reduces the total system pressure drop, saving fan horsepower and ultimately electricity. Original fan motors can be replaced with high efficiency models, and a system with a volume-control damper can be refitted with a variable-frequency drive for more economical airflow control. Burners can be replaced with new, lean-burning high velocity models to improve turndown and temperature distribution.
Additionally, to help a manufacturing facility further reduce its energy costs and operate as efficiently as possible, almost any air pollution control system can be equipped with secondary devices that use the heat energy the system would typically exhaust directly to the atmosphere.
Options For Recovered Heat
A commonly used add-on to capture heat from the exhaust stack is an air-to-air heat exchanger. It can be designed for minimal pressure drop so it will not affect the operation of the oxidizer and can return heated, clean filtered fresh air. This heated fresh air can be used for warehouse or building comfort heating, process makeup air (ovens/dryers, kilns, curing zones, etc.) or, in some cases, can completely replace the gas-fired burners presently used in the manufacturing process.Using the same idea of capturing heat from the exhaust stream, a hot water or thermal oil heat transfer coil can be installed in the oxidizer exhaust stack. Hot water can be used for building comfort heating or can be returned to the process for use (air preheat, condensation control, etc.). This coil also could be used to preheat cool water for a steam generator. Thermal oil could be used as a main process heat source where direct flame heating is not desired. Adding a coil in the exhaust stream can reduce or even remove the heat load required from the thermal oil heating system.
Depending on the stack temperature, the exhaust from the oxidizer could be routed directly to a low pressure steam generator. If the plant uses steam for any reason (carbon bed regeneration, humidity control, etc.), this system could supplement steam production capacity any time the oxidizer is running. In an ideal situation, the steam produced from the oxidizer exhaust would allow the main steam generator to function as a backup system.
Another option is to install an adsorption chiller in the exhaust stream. Many manufacturing facilities use chilled water for various reasons (chill rolls, condensers, air conditioning, etc.). While the initial capital cost is higher than a conventional chilled water system, the “free” energy from the oxidizer exhaust can make it an economical choice.
In summary, as the cost of energy continues to rise, manufacturing facilities should consider all available options to help reduce the energy costs associated with operating their air pollution control systems. Retrofitting a secondary recovery system to an older air pollution control unit can generate energy savings. Replacing an older system with a high efficiency system can offer a short-term payback on a company's capital investment. Either a retrofit or an upgrade to a new system could provide substantial operating savings and have a positive impact on a company's bottom line. PH
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