Resource Management

SPECIAL SECTION/RESOURCE MANAGEMENT: Energy-Efficient VOC Removal

June 1, 2008
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Figure 1. Air combs are installed in the process exhaust behind a diffuser, which disrupts and homogenizes the flow to assure an even temperature and eliminate any hotspots.


Processes that use heat and/or solvents usually produce airborne volatile organic compounds (VOCs), which, according to local air quality and U.S. Environmental Protection Agency (EPA) standards, must be captured or rendered inert prior to exhaust emission into the atmosphere. In the ceramic industry, affected processes usually incorporate various kilns, ovens and furnaces.

As heightened awareness and sensitivity to environmental impact meets more acute emission compliance standards and guidelines, companies throughout the U.S. are becoming increasingly educated with respect to emissions standards and are striving to meet compliance guidelines. The traditional bulky and expensive VOC removal technologies usually involve thermal combustors that dilute exhaust air to reduce its volatility to non-explosive levels, and then employ high temperature to oxidize the compounds completely.

Two innovative energy-saving approaches can enhance performance and reduce costs associated with VOC removal and compliance. In appropriate applications, 99.7% or better efficiency with respect to VOC removal can be achieved.

Retrofits

It often does not make economic sense to replace existing equipment. Thus, a modification called a thermal catalytic combustor retrofit is designed for the existing system. A non-platinum, spiral fiber catalyst, which has a high surface area and is durable and efficient, is applied to ceramic combs of varying sizes, depending on the system type and flow rate (see Figure 1). These combs are installed in the process exhaust behind a diffuser, which disrupts and homogenizes the flow to assure an even temperature and eliminate any hotspots.

Since this catalyst reacts favorably at very low temperatures, the required temperature within the combustor is typically reduced by 30-40%, which equates to a 50-60% reduction in the energy required to accomplish the same task. Various safety mechanisms are also installed to assure proper temperature range, volatility control, minimum flow rate and pressure drop.

New Systems

When a new system is designed, the VOC content itself is used, in part, to drive the reaction. Generally speaking, for every gram of VOC per cubic meter of suspension gas in typical applications, the reaction temperature increases by 30ºC. Thus, a system can be designed that reduces the flow rate to one that matches the amount of VOC concentration to the catalyst and arrives at an equilibrium, or a state that uses little or no additional energy to maintain the reaction temperature. In many cases, the heat source used to arrive at a desired trigger temperature can be electric (due to the reduced flow rate), further reducing the overall cost of the system.

In many traditional approaches, the creation of NOx creates yet another problem to be addressed. Since the temperatures used by these new systems are quite low, however, no NOx is formed. Many manufacturers require very high temperatures as part of their process. In these cases, VOCs from solvents and other agents are removed as the system heats up to the optimum range of the catalytic system prior to arriving at higher processing temperatures, thus still providing a low-cost VOC removal remedy.

Figure 2. General schematic of the VOC removal system.

Additional Considerations

These systems feature few, if any, moving parts in nearly all applications. A general schematic is shown in Figure 2. In situations where black smoke, unburned carbons, dust or different solid particulates are present, pre-filtering or other treatment of exhaust air must be accomplished to assure proper catalyst function and longevity. A wide variety of systems is available to match specific flow and pollution control requirements.

Savings for some customers can be substantial; larger manufacturers can save thousands of dollars per day in energy costs. In Europe, with its higher energy prices, typical capital investment recovery is 3-4 months. Still, it can be expected that a U.S. application could see its capital investment recovered within the first year of operation.

For more information regarding VOC removal technologies, contact Catalycom-US at 1034 Emerald Bay Rd., South Lake Tahoe, CA 96150; (530) 543-1025; fax (530) 543-1026; e-mail mail@catalycom-us.com; or visit the website at www.catalycom-us.com.

A specially designed baffle was installed to mix the airflow to assure an even temperature throughout the cross-section of the duct.

ONLINE EXTRA: Success at Volkswagen

Volkswagen’s main production plant in Wolfsburg, Germany, combines the thermal combustion of paint fumes with finish curing via heat exchange from an afterburner installed in the painting exhaust flow. Since VW had a great deal of capital invested in the current system, it made little economic sense to scrap it and build a new system.

A KNV system was installed downstream of the afterburner. One concern in this case was that, due to limited space and pre-existing condition, the system needed to be located quite close to the burner. A specially designed baffle was installed to mix the airflow to assure an even temperature throughout the cross-section of the duct. Next, two walls of ceramic aircombs, treated with the necessary catalyst for the intended VOC content, were installed in the duct via a custom-made steel framework.

VW was able to reduce its operating temperature at the burner from 750 to 500ºC, a 30% temperature reduction. This equated to a 43% reduction in energy requirement and an estimated savings of €3500 (about $5400 at current conversion rates) per day at the time of installation for this system alone.

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