Click to enlarge A single rotary valve style, regenerative thermal oxidizer was installed at a California pet food manufacturing facility to eliminate process odors in the exhaust.

One of the more difficult air pollution challenges to solve is odor. At times highly subjective, at others painfully obvious, the presence of odor is problematic for the emitter mostly because of the variability of claims by complainants. In fact, cases have occurred in which companies have spent millions of dollars to legitimately, and in popular opinion, effectively solve the odor problem - only to be at the mercy of the public that suspects a phantom odor, perhaps due to being exposed to one for so long. This danger of variability makes it incumbent on the industrial facility that the issue be solved effectively on the first attempt. Piecing together a solution by papering over it will inevitably lead to recurrence. Further complaints, bad press and monetary fines are likely to result.

Because of tighter annual budgets for most companies, the piecemeal approach often occurs due to the tendency to find the lowest cost option instead of the most effective one. Initial capital investments into solutions that are inadequate inevitably fail due to subsequent costs of ongoing operation in the form of consumables or parts replacement. Instead, industrial equipment owners facing an odor problem would do well to consider both the spectrum of available solutions as well as any history in solving a similar odor problem. An effective solution for odor problems lies on that path.

The most common technologies for remediating industrial odor problems include:

• Carbon or other sorbent filtration.
• Maskants or perfumes.
• Tall stack dilution or dispersion.
• Chemical or biological scrubbing.
• Biofiltration.
• Regenerative thermal oxidation.

Of these options, thermal oxidation has often proven to be effective, particularly from the beginning of operation, and there is no need to add consumables or adjust chemistries. Destroying odors with high temperatures is rarely a question of effectiveness. Use of high temperatures - typically in excess of 1,500°F (815°C) - in a thermal oxidizer proves effective. It also often proves to be a slightly more expensive option to operate compared to less effective odor control systems unless the regenerative thermal oxidizer (RTO) is designed with maximum possible heat recovery to minimize use of purchased fuel.  

Click to enlarge In the regenerative thermal oxidizer, the single rotary valve indexes across an open air path and methodically seals it off by reaching the next set position. A continuous air purge captures any scavenging dirty air in the switch and returns it for treatment in the oxidizer.

The typical argument against using thermal oxidation relates to the costs of ownership. However, because of advancements in equipment, a prudent purchasing agent can find a balance of cost efficiency and effectiveness with the right thermal oxidation technology.

Once an oxidizer is in installed, ongoing maintenance costs may dim initial positive experiences of performance. In these cases, the main culprit is the valve sealing system that regenerative thermal oxidizers employ to direct air from chamber to chamber, to take advantage of ceramic heat exchanger beds for maximum fuel efficiency. The more of these flow-reversal valves on an oxidizer, the greater the chance for odor leakage as well as maintenance upkeep and cost.

About 15 years ago, the rotary diverter valve design was introduced for regenerative thermal oxidizers. This single, heavy-duty diverter replaces the multiple valves and actuators found on previous regenerative thermal oxidizer designs. The result is fewer moving parts, less wear, less downtime and lower maintenance costs. Unlike a valve sealing system, the single rotary diverter valve on these oxidizers never truly opens or closes by “making” a seat or seal. Instead, the diverter indexes across an open air path and methodically seals it off by reaching the next set position. A continuous air purge captures any scavenging dirty air in the switch and returns it for treatment in the oxidizer. This operation results in near-zero leakage of odor or dirty air and allows rotary regenerative thermal oxidizers to routinely remove odor to below threshold detection levels in most cases.

Installation of these systems is simplified by a pre-engineered, skid-mounted design that reduces site assembly time and cost. With this approach, nearly all electrical wiring and piping is completed on the unit prior to arriving on site, where just a few utility connections are required.

At Work Removing Odors

Click to enlarge   Tall stacks allow for dilution of odor molecules when sent to the atmosphere, but they are not a stand-alone or complete odor-eliminating solution.

A case study from a pet food manufacturer in California can help illustrate the effectiveness of regenerative thermal oxidizers, specifically those with rotary valve technology, to control odor.

When the pet food manufacturer opened its production plant in California, the company began receiving complaints from nearby residents about an increasingly worsening odor being emitted from the process exhaust. Complaints began coming in at a rapid pace. Threats of fines and sanctions from local government agencies were real possibilities.

Recognizing the need to control the emissions and wanting to remedy the situation for its neighbors, the company invested nearly $4 million to solve the issue. However, despite the investment in odor control systems such as carbon canisters for adsorption and tall stack dilution, the odor lingered.

The company then hired a manufacturer of regenerative thermal oxidizers with a single rotary diverter valve design to address the odor control issues completely. The oxidizer manufacturer had experience with food manufacturing applications and specifically with pet food manufacturing, which gave them experience they could leverage to find an effective, cost-efficient solution.

The oxidizer manufacturer recommended installing a single rotary valve regenerative thermal oxidizer at the end of each of the pet food manufacturer’s process stream ducts. Because the company had confidence in the near-zero leakage single valve technology, the oxidizer manufacturer offered a series of guarantees concerning odor reduction along with the purchase of the systems. If the performance guarantees were not met, the oxidizer maker would be contractually liable for creating a new solution at its own expense.

Initially, the pet food manufacturer had raised concerns about the cost of thermal oxidation technology to control the odor issue. To address those concerns, the oxidizer manufacturer designed a system to maximize fuel efficiency through the use of a proprietary heat exchange ceramic media block internally installed within the regenerative thermal oxidizer. If it were necessary, the oxidizer manufacturer also knew it could incorporate secondary heat recovery equipment at the end of the oxidizer to capture waste energy and allow it to be used in other parts of the pet food manufacturer’s facility.

As soon as the regenerative thermal oxidizer system was commissioned and started for process, plant employees and surrounding neighborhoods noticed a dramatic difference: no more odor. The complaints stopped and the threat of fines was eliminated. The systems have been in use since 2008, and they pet food manufacturer has not reported any system downtime.

The goal in odor control is to effectively eliminate the problem. If the process cannot be changed, the use of odor control equipment will need to be instituted. While initial capital and system maintenance costs for regenerative thermal oxidizers may be more than other odor removal methods, the use of oxidizers is effective in eliminating odors.

Links

• Durr Systems Inc.