As the first generation of oxidizer systems nears the end of their service life, many can-making plants face repair or replacement of their existing air pollution control systems.
Like many others in the industry, a Silgan can-making plant in the Midwest had been using a recuperative thermal oxidizer with direct heat recovery for control of emissions from its sheet-coating lines. After more than a decade of service, the oxidizer needed repairs to continue to meet strict compliance limits. As an alternative to rebuilding the existing oxidizer, Silgan Containers Corp., Woodland Hills, Calif., evaluated whether a new oxidizer might better suit current process needs.
Historically, recuperative thermal oxidizers with direct heat recovery have been a popular choice in can-making facilities -- especially those with oven zones operating above 350oF (177oC).
In the past, recuperative thermal oxidizers had a capital cost advantage over regenerative thermal oxidizers and offered more flexible volatile organic compounds (VOCs) loading limitations. Their one disadvantage has been in supplemental fuel usage. Recuperative thermal oxidizers top out at 70 percent internal heat recovery; by contrast, regenerative thermal oxidizers are able to achieve more than 95 percent.
For canmakers, this drawback was minimized with the use of additional heat recovery. Hot, purified air from the oxidizer is routed directly back to the oven zones and not lost to the atmosphere. This has reduced the operating cost “penalty” of the recuperative thermal oxidizer and, in the past, had swung the balance almost exclusively toward specifying recuperative systems for VOC loads above 10 percent lower explosive limit (LEL).
So exclusively that, when hearing that Milwaukee-based Anguil Environmental Systems had recommended a regenerative thermal oxidizer for its Midwest coating facility, Silgan responded almost incredulously: “They recommended what? This is clearly not an RTO [regenerative thermal oxidizer] application.”
A disadvantage of all recuperative thermal oxidizer designs is that the steel in the heat exchanger is exposed to high burner chamber temperatures -- typically up to 1,600oF (871oC). Silgan’s system had a history of requiring ongoing heat exchanger maintenance, which drove up costs and impacted throughput. The engineering team at Silgan wanted to effect a permanent fix for the aging system, replace it with an equivalent system, or replace it with alternative equipment.
After evaluating several options, the engineering team at Silgan selected the regenerative thermal oxidizer that Anguil had recommended. The team cited the capital cost advantage and operating cost savings as key reasons for the change. As a custom-built abatement system, the oxidizer was designed specifically for Silgan’s application with high loadings and concentrations. Ultimately, Anguil designed, manufactured and installed a 40,000 scfm regenerative thermal oxidizer with heat recovery, hot-gas bypass and an oven purge system.
With the new regenerative thermal oxidizer, the solvent-laden process gas enters the abatement system through an inlet manifold. Flow-control poppet valves direct the process gas into one of two energy-recovery chambers, where it is preheated. Then, the process gas and contaminants are progressively heated in an inlet ceramic bed as they move toward the combustion chamber.
The VOCs are oxidized in the combustion chamber, releasing thermal energy in the ceramic bed that is in the outlet flow direction from the combustion chamber. The outlet ceramic bed is heated and the gas is cooled so that the outlet gas temperature is only slightly higher than the process inlet temperature. Flow-control poppet valves routinely alternate the airflow direction into the ceramic beds to maximize energy recovery within the oxidizer. The VOC oxidation and high energy recovery within these oxidizers reduces the auxiliary fuel requirement and operating costs. For example, with a regenerative thermal oxidizer at 95 percent thermal energy recovery, the outlet temperature may be only 70oF (40oC) higher than the inlet process gas temperature. The oxidizer can reach self-sustaining operation (no auxiliary fuel required) at typical operating concentrations.
The process emissions at the Silgan facility as well as the temperature of the oven zones presented some challenges and opportunities.
With process LEL levels as high as 14 percent, Silgan’s engineering team was concern about the high temperatures in the regenerative thermal oxidizer. A hot-side bypass valve was provided to allow excess oxidizer reaction-chamber heat to be vented directly into the exhaust or the back to the oven inlet manifold during periods when the inlet VOC loading provides more heat than is necessary to maintain the setpoint temperature. This primary heat recovery saves thousands of dollars in operating costs because the ovens require less fuel to reach the desired temperature. With the Anguil design, there is no loss of residence time at temperature, ensuring destruction and eliminating the concern of overheating the unit. VOC destruction efficiency is guaranteed whether the bypass is open or not.
Following installation of the regenerative thermal oxidizer, Silgan found another potentially energy-reducing strategy it is currently investigating: using a secondary heat exchanger to recover additional heat from the regenerative thermal oxidizer exhaust stack. Initial estimates show that an extra 6.5 million BTU/hr could be recovered utilizing a heat exchanger in the oxidizer stack. Fresh air (at an average outdoor temperature of 46oF [8oC]) makes a single pass through a 50-percent-effective heat exchanger and is heated to approximately 350oF (177oC). This recovered heat can be used for other processes or comfort heating during the winter months, which could translate into significant savings.
The regenerative thermal oxidizer also is equipped with a high temperature bake-out system, which is similar to the self-cleaning option in an oven. This feature removes organic buildup on the cold face of the heat exchange media. In the bake-out mode, the regenerative thermal oxidizer is taken offline from the process. At a reduced airflow, the outlet temperature is allowed to reach an elevated temperature before the flow direction is switched. This hot air vaporizes organic particulate, essentially clearing the media bed of any obstruction. The flow direction then is switched and the opposite cold face is cleaned. Standard Anguil systems perform bake-out at 650oF (343oC). Silgan specified stainless steel media supports and poppet valves so bake-out temperatures could reach 800oF (427oC), ensuring a more complete cleaning. Scheduled regenerative thermal oxidizer bake-outs reduce the pressure drop across the heat-recovery beds. Anguil included transmitters to monitor media-bed pressure drop and provide both continuous recording of this data as well as an indication to the operators when a bake-out is recommended.
Dan Gallo, Silgan’s area manager of manufacturing, was pleased with the outcome. “We selected Anguil because of its technical excellence and commitment to service,” he said. “Not only has the company been able to troubleshoot its own equipment, but Anguil has also provided operating solutions for oxidizers made by other manufacturers. We are pleased with their dedication to excellence and are happy to have Anguil as a business partner.”