The energy efficiency of pollution control equipment has risen considerably. Thermal efficiencies up to 97 percent can be achieved in regenerative thermal (RTO) and regenerative catalytic (RCO) oxidizers. Recuperative thermal or recuperative catalytic units can provide up to 80 percent thermal efficiency. High efficiency natural gas-fired burners with high turndown rates can further reduce fuel costs. The use of high-efficiency electric motors also can yield substantial savings. Using AC variable-frequency drives provides a company with both operational flexibility and lower electrical consumption.
Upgrading or retrofitting an existing oxidizer that does not incorporate these features can net large energy savings and typically can be justified with a financial payback. For example, a regenerative thermal oxidizer originally designed with random packed media for heat recovery can be modified to use high efficiency, modern structured media. Because structured media requires less system pressure drop, less fan horsepower is needed, reducing electrical costs. Upgraded media also increase thermal heat recovery, providing additional natural gas savings. Electric motors can be replaced with high-efficiency models. A system using a volume-control damper can be refitted with a variable-frequency drive for more economical airflow. In addition, high velocity gas-fired burners with improved turndown ratios and temperature distribution can be added.
If an even greater level of efficiency is desired, secondary add-on energy-saving systems can be incorporated into a new unit or retrofit into an existing system. Secondary recovery systems use the heat energy that the oxidizer would normally exhaust directly into the atmosphere. The easiest way to capture heat from the exhaust stack is with an air-to-air heat exchanger. The heat exchanger can be designed for minimal pressure drop so as not to affect the operation of the oxidizer while returning temperature controlled fresh air for a range of uses. This heated fresh air can be used for process makeup air (ovens/dryers, kilns, curing zones, etc.), building comfort heating or, in some cases, can completely replace the need for natural-gas-fired burners 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. The hot water created can be returned to the process for use (air preheat, condensation control, etc.) or used for building comfort heating. This coil also could be used as a preheat section to preheat cool water for a steam generator. Thermal oil is 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 plants use chilled water for various reasons (air-conditioning, chill rolls, condensers, etc.). While the initial capital cost is higher than a conventional chilled water system, the “free” energy provided from the oxidizer exhaust to run the system can make it an economical choice.
In summary, as the cost of energy continues to rise, companies 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 or adding a bed of catalyst on top of the existing ceramic media in a regenerative thermal oxidizer can generate significant energy savings. Replacing an older system with one of today’s high efficiency systems now offers 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 your plant and process, as well as your balance sheet.