Coil coaters with varying solvent loadings from prime and finish ovens can use a regenerative thermal oxidizer (RTO) to comply with emission regulations. High fuel efficiency can be maintained over a range of solvents with a bypass system that evens out the Btus stored in the ceramic media. A secondary heat recovery system provides hot water for pretreatment.
RTOs offer a highly energy efficient method of oxidizing organic emissions (VOCs). However, when VOC emissions vary greatly, as in the case of coil coating, the energy storing capability of the RTO can be a handicap. If the design thermal energy regain (TER) of the unit is based on high-solvent operation, the unit will operate in a self-sustaining mode at that condition. At low-solvent conditions, the operating cost of the unit will increase due to the additional fuel required to maintain operation. The solution to the problem is to design the unit with a TER based on low solvent levels and bypass excess heat released during high-solvent operations around the ceramic media.
Consolidated Systems, Columbia, South Carolina, is a coil coater that contracts with a number of manufacturers to coat their products. However, Consolidated does manufacture some of its own products, including truck and trailer siding, roofing and metal studs. The finisher has the capability to coat steel, aluminum and galvalume sheets from 24 to 60 inches wide.
Consolidated applies various paints, mostly solvent-borne, using a roll-coating process. In addition, it can apply polyester, siliconized polyester and fluorocarbon protective coatings to protect products from attack by various compounds. The coated materials are cured in a flotation oven.
In a flotation oven, air jets support the metal on a cushion of air as it passes through the dryer, which prevents marring of the coated surface. Depending on the product, the solvent loading in the exhaust stream will vary from about one to more than two gpm. The flow rate ranges from 15,000 to 30,000 scfm.
Flexibility needs met. To meet its flexibility needs, Consolidated selected a customized RTO designed by Ross Air Systems, Somerville, New Jersey. The three-chamber unit appears similar to a conventional RTO, except a portion of the oxidized air stream can be vented from the oxidation zone, bypassing the ceramic heat storage media.
When the unit operates at designed solvent loadings, the bypass remains closed. The Btu value in the airstream, coupled with the design TER of 85 pct, permits the unit to operate without additional fuel. As the solvent level increases above the designed solvent loading, the bypass damper opens. This action stabilizes the amount of heat transported and stored in the ceramic beds, permitting essentially constant thermal operation. The bypassed heat mixes with the air existing in the beds and is vented to the atmosphere.
Since it was installed, the unit has required virtually no maintenance. The high-energy use of the RTO, plus the recovery of additional heat, results in essentially zero fuel costs for Consolidated. The unit has also consistently achieved destruction efficiencies greater than those promised, providing Consolidated with a margin of safety.
How RTOs function. An RTO consists of a combustion chamber plus two or more heat transfer beds filled with ceramic packing. Solvent-laden emissions from the process are directed to the combustion chamber where they are heated with an auxiliary flame and oxidized. Hot gas leaving the combustion chamber flows through one of the heat transfer beds and releases its heat to the ceramic packing. When the packing is sufficiently hot, the hot gas leaving the combustion chamber is diverted to another heat transfer bed.
Solvent-laden emissions from the process are then directed through the hot regeneration chamber where they are preheated before entering the combustion chamber. This preheating reduces, and can sometimes eliminate, the need for auxiliary fuel in the combustion chamber.
Each heat transfer bed alternates between capturing the heat contained in the gas leaving the combustion chamber and releasing the heat to the gas entering the combustion chamber. In the case where an odd number of heat recovery chambers are used, before a heat transfer bed switches from releasing heat to capturing heat, a small amount of solvent-free exhaust gas is used to purge the contents of the bed back into the combustion chamber to prevent unoxidized solvent from entering the exhaust stack.