Regenerative thermal oxidizers offer a highly energy efficient way to burn organic emissions. But when the emissions contain high amounts of combustible vapors, the energy efficiency becomes a handicap. The heat of combustion from the vapors produces more heat than required, causing overheating and reduced equipment life. A plant whose emissions vary from high to low levels of organics is often forced to use thermal oxidizers that consume more energy than a regenerative thermal oxidizer.
Chesapeake Finished Metals, Elk Ridge, Maryland, is a coil coating job shop that produces a host of products. Depending on the product produced, the solvent loading in the exhaust from the coil coating ovens ranges from a low of one pct LEL (lower explosive limit) to as high as 25 pct LEL. Flow rate is also highly variable, ranging from 12,000 to 46,000 scfm.
To comply with environmental regulations while controlling energy costs, Chesapeake used a regenerative thermal oxidizer to destroy solvents in the oven exhaust. The regenerative thermal oxidizer (RTO) initially worked well, as along as solvent loadings remained below three to five pct. But at higher loadings, the heat of combustion generated by the solvents was more than required to preheat the exhaust from the coil coating lines.
Temperatures in the heat transfer beds and the combustion section frequently exceeded the design parameters of the unit. Over time, the excess temperature caused serious structural weaknesses in the unit, reduced reliability and increased maintenance cost. The poor reliability, as well as leaks caused by the structural damage, threatened to reduce the unit's solvent destroying efficiency below the required environmental compliance levels. Chesapeake had no choice but to replace its RTO.
When Chesapeake evaluated replacement equipment, it considered catalytic and recuperative oxidizers. Catalytic oxidizers, while thermally efficient, need to be designed for a relatively narrow range of solvent types and concentrations. Being a job shop, Chesapeake needed an oxidizer with greater flexibility.
Recuperative oxidizers were sufficiently flexible to handle the company's wide range of solvents and solvent loading. However, at the low loadings usually experienced, the thermal efficiency (70 to 80 pct) of a recuperative unit would cause a major increase in the plant's energy costs.
To meet flexibility and efficiency needs, Chesapeake selected a customized RTO designed by Huntington Environmental Systems. Having five heat transfer beds, it appears similar to a conventional regenerative unit with one major exception. A portion of the hot gas from the combustion chamber and a portion of the solvent-laden exhaust from the coil coating ovens can by-pass the heat transfer beds.
When the unit operates at low solvent loadings, the by-passes remain closed. Operating at its maximum thermal efficiency (about 85 pct), the RTO requires little or no auxiliary fuel, even at solvent loadings as low as two pct LEL. At those levels, the solvent's heat of combustion is sufficient to keep the combustion chamber hot enough to oxidize at least 99 pct of the incoming solvent.
When the oven exhaust contains higher solvent loadings, the temperature in the combustion chamber and the oxidizer's exhaust stack are controlled by modulating flow through the by-passes. Using proprietary control algorithms, the system by-passes a portion of the exhaust from the RTO combustion chamber based on the final stack temperature. It also by-passes a portion of the exhaust from the coil coating ovens based on the temperature in the combustion chamber. With these controls, high energy efficiency is maintained, solvent oxidation is virtually complete, and the unit does not overheat.
The RTO also has two mechanical features that further reduce operating costs and improve reliability. The valves that direct flow to the heat transfer beds (three per bed, 15 total) are all operated by cams mounted on a single shaft. Driven by a 1.5 hp motor, the shaft makes one complete turn for each full cycle of the oxidizer. Because a simple cam rather than multiple hydraulic valve actuators is used, maintenance on these high-temperature valves is virtually eliminated. To further assure low maintenance costs and high system reliability, the control valves that modulate the by-passes are electrically actuated, high-performance butterfly valves.
To further increase the thermal efficiency of the coil coating operation, the RTO exhaust is used to preheat the makeup air used in the coil coating ovens. This is possible because, unlike conventional RTO systems that use an induced draft fan to pull gas through the unit, the RTO uses forced draft fans to operate at a positive pressure. Under positive pressure, the exhaust gas can be used in a heat exchanger to preheat the makeup air entering the ovens.
Since installation in late 1995, the RTO has required virtually no maintenance. With the high energy efficiency of the RTO plus the recovery of additional heat to preheat makeup air for the ovens, energy costs for thermal oxidation are virtually zero. The unit has also consistently achieved greater than 99 pct destruction of solvent emissions. All of these results have been achieved at solvent loadings ranging from one to 25 pct LEL without damage to the equipment.