Regenerative thermal oxidizers (RTOs) are routinely used for controlling volatile organic compound (VOC) emissions from various sources. With more than 20 years of process use and hundreds of installations, this technology generally is considered to be quite mature. In spite of this vast installed base, examples of misapplication of the technology are not uncommon. It is important to realize that each application is unique, and regenerative oxidizer design features must be considered carefully when applying the technology.
Case in point: a large glass-mat production facility located in the Midwest. The facility produces glass-fiber mat used mainly in the production of asphalt roofing shingles. To abate the emissions from the glass-mat curing line, the facility decided to install a regenerative thermal oxidizer system. The facility initially estimated a total flow requirement of 60,000 scfm (table 1).
Due to high particulate loading from the glass-mat curing line, the facility had agreed to install a prefilter upstream of the oxidizer. The oxidizer design needed to take into account the high moisture content of the stream along with the high inlet temperature and a formaldehyde emissions limit of 4 ppmw.
Based on the design parameters, a two-canister regenerative thermal oxidizer with pneumatic poppet valves was selected. It consisted of shallow beds and used 0.5" random packing in a portion of the bed.
Once installed, however, the oxidizer with pneumatic valves had numerous problems with lines freezing in the cold weather. Operation was unreliable, causing unscheduled shutdowns and startup problems. It was designed with high velocities through the beds, making it marginal for the specified flow. And, when the facility tried to increase the flow through the oxidizer beyond the designed flow, the higher velocity forced some media out of the stack. It was never able to achieve the guaranteed VOC destruction and formaldehyde emissions limits.
Design RevisitedAfter the two-canister oxidizer was installed, the facility requirements changed, so the oxidizer also would need to be used to abate additional sources. Due to the problems with the installed oxidizer and the increased flow requiring abatement, the company first decided to have another oxidizer supplier modify the existing oxidizer system. But after evaluations by several oxidizer manufacturers, the glass-mat manufacturer decided this approach was not feasible. Instead, the company decided to replace the existing regenerative thermal oxidizer with a larger, more efficient system (table 2).
Another requirement specified by the company was the ability to purge the mat oven line simultaneously with the regenerative thermal oxidizer through a common fan and stack. The oxidizer control system and safeties must be designed in accordance with NFPA requirements. Thus, the oxidizer system would be required to purge the oven lines as well as independently start up using fresh air.
In addition to those design challenges, the facility had limited space to install the new unit and required a fast delivery schedule. The oxidizer system was required at the jobsite within 11 weeks of placing the order, and the facility was willing to pay a premium for expedited delivery.
After evaluating the bids from various vendors for the new oxidizer, the glass-mat manufacturer selected Pro-Environmental Inc. (PEI), Rancho Cucamonga, Calif., due to its understanding of the process, its reputation and its proposed engineered solution.
PEI chose to provide a five-canister oxidizer design for this application. The system incorporates a modular design in order to provide flexibility for future expansion. The materials of construction were carefully selected keeping in mind the high moisture content of the process stream and its VOC content. Some of the design parameters for the new oxidizer are provided in table 3:
PEI also incorporated the oven-purging requirements into its design (figure 1). The supplied system is capable of purging the ovens using the supplied forced-draft fan. During the oven-purging operation, the oxidizer is isolated from the process, such that the process air is diverted to the stack. Furthermore, the oxidizer is capable of starting up from a cold start using the purge air fan isolated from the process when the process ovens are being purged.
The supplied PLC was pre-programmed to signal the oxidizer ready status and oven's purge status, as well as automatically switch from the purge/startup mode to process mode and accept the process air for destruction of the VOCs. PEI engineers worked closely with the plant engineers to make sure the control objectives were achieved without disrupting either the mat production or the oxidizer operation.
PEI worked closely with its suppliers and vendors to ensure that the glass-mat manufacturer's demanding delivery schedule was met. The oxidizer system and all of its components were delivered to the site within 11 weeks of placing the order. System installation, commissioning and startup took and additional four weeks.
The oxidizer has now been in operation for more than 18 months, successfully meeting all of the facilities operational and air emission compliance needs.
For more information on oxidizers from Pro-Environmental Inc., Rancho Cucamonga, Calif., call (909) 989-3010; e-mail firstname.lastname@example.org; or visit www.pro-env.com.