In the 21st century manufacturing world, process heating systems have become an essential part of the overall manufacturing operation for many industries. Almost every finished consumer product requires some form of heat-treating method, whether it is baking, curing, annealing or drying at some point in the production process. Hence, an array of industrial ovens and furnaces exist to specifically suit each production process.
Although process heating equipment is ubiquitous in the manufacturing landscape, an inherent downside exists to deploying it: the creation and emission of hazardous air pollutants (HAPs) and volatile organic compounds (VOCs). With increasingly stringent regulations around VOCs and HAPs, manufacturers must turn to air pollution control systems to stay compliant. Thermal oxidation is the most commonly used method to control these air pollution emissions. Thermal oxidizers — often referred to as incinerator or combustor-type systems — rely on the process of combustion to oxidize VOCs, carbon monoxide (CO) and volatile HAP emissions down into carbon dioxide (CO2) and water (H2O). They can have high destruction-rate efficiencies (DRE) — levels up to 99.99 percent.
Evaluating the appropriate oxidation system and configuration for a specific process stream requires in-depth analysis of the specific process gas and operating parameters. Among the three basic types — regenerative, direct-fired and recuperative — the most commonly used are the recuperative systems, which also are known as oxidizers with shell-and-tube heat exchangers. They generally are air-to-air exchange systems in which the hot, clean air passes over a series of stainless steel tubes. They transfer heat energy via convection to the incoming colder air passing through the tubes.
Unfortunately, the price of the fuel necessary to run oxidizers can be exorbitant. In order to lower operating costs, a heat exchanger can be used to transfer heat, thereby utilizing heat that would otherwise be wasted. Heat-exchange methods can be the defining feature of a thermal oxidizer because they can control the desired level of efficiency. Therefore, engineering and design parameters must be meticulously calculated.
Several primary and secondary heat-exchange methods can be utilized. A primary heat exchanger is used to preheat the process gas as it enters the thermal oxidizer. The heat is transferred from the hot gas leaving the combustion process to the cold air entering the chamber. A secondary heat-exchange system uses the waste heat from the combustion process to run parallel production processes throughout the facility. They include ovens, furnaces, hot-water generators and general comfort heating. Utilizing the waste heat for the aforementioned processes cuts down on the general operating costs for the facility.
By custom engineering a combination system that incorporates the industrial ovens, thermal oxidizer and heat exchangers, the overall performance of the process heating system can be optimized to save the plant time, money and energy. Oxidation, or combustion, is a chemical process resulting in the release of heat. A comprehensive heat processing system that works in conjunction with air pollution control systems maximizes the waste-heat recovery process and improves overall system efficiency and sustainability. The thermal energy is harnessed and shared between the systems.
A Combination System Case Study
Combination systems can be implemented in essentially any manufacturing facility that requires both process heating and air pollution control systems. A recent project for a metal-coating operation exemplifies how combination systems can be implemented in existing facilities to enhance operational efficiency.
The metal-processing plant asked a thermal oxidizer manufacturer to replace coating line ovens and a stand-alone thermal oxidizer system. After an extensive evaluation of the existing system operation, it was determined that the optimal solution was the use of a combination system layout. It consists of a three-zone prime oven, a four-zone finish oven, a single standalone thermal oxidizer system, and the secondary heat-recovery system. The combination system works to capitalize on every part of the process, ensuring no heat or material goes to waste.
With coil-coating ovens, it is common to design the oven with multiple zones, depending on the process. The different zones within the oven allow for a gradual heating process, which can be a vital component when dealing with certain materials such as paint or metal. In this particular system, the three-zone prime oven cures the primer onto the metal. As the metal is processed through the oven, it reaches temperatures between 500 and 600°F (260 and 316°C). The four-zone finish oven continues to cure the paint onto the metal as the metal passes through each zone, gradually achieving the desired peak metal temperature.
As the painted metal strips are being cured through the prime and finish ovens, the paint solvents evaporate into the oven’s work chamber. The air laden with VOCs is extracted from the oven’s work chamber and routed through the primary heat exchanger of the thermal oxidizer. The primary heat exchanger preheats the oven exhaust to temperatures in excess of 1100°F (593°C) to minimize the oxidizer’s burner fuel consumption. The VOC exothermic reaction contributes in making the oxidizer’s retention chamber act as a self-sustaining agent without any additional burner heat input. When the solvent vapors oxidize and the exothermic reactions take place, the solvent acts as fuel to the oxidizer, further reducing the primary fuel cost of operating the oxidizer. This optimized and efficient design using the primary heat exchanger will ultimately result in savings of approximately 8 million BTU per hour for the facility.
The use of a secondary heat exchanger further establishes an efficient, economic solution. Once the contaminated air in the prime and finish coater room is effectively captured by the thermal oxidizer, it is routed through the secondary heat exchanger. The secondary heat exchanger continues to supply the preheated air back to both the prime and finish ovens, increasing sustainability. The secondary heat exchanger fully utilizes the waste heat from the thermal oxidizer before discharging the waste through exhaust stack. Recirculating the air from the contaminated coater room as the source of heated air supply helps to reduce the thermal oxidizer’s required capacity. It also reduces the oven burner’s fuel consumption by approximately 6 million BTU per hour.
As demonstrated by this metal-coating operation, the heat is continuously recycled between the oxidizer and ovens via two heat exchangers. The combination system creates optimal operational efficiency and allows the facility to capitalize on the thermal energy across the entire process.
Beyond the combination system, additional methods can be engineered — depending upon the production process and operational demands — to recycle the thermal energy to supplement the process even more. For example, to increase energy savings, the heat process system manufacturer proposed the installation of a waste-heat hot-water heater to capture available waste heat from flue gases. This generates hot water needed for cleaning the metal strips. By not having to use a burner and additional fuel to heat the water to the desired temperature, the installation of the waste-heat hot-water heater saves the facility an additional 3 million BTU per hour.
In summation, the optimized system layout for this particular job resulted in a fuel savings of more than 17 million BTU per hour. That equaled a total savings of $320,000 per year on fuel cost alone. This allowed the facility to pay off the investment in less than a five-year period, thus earning an even higher ROI on the combination system than anticipated. The method of coupling the heat processing systems and air pollution control systems, paired with seeking additional opportunities to capture and recycle heat, is a prime example of how thermal engineering can create value through efficiency.