Rising natural gas fuel costs and a desire to create a smaller carbon footprint have led plant operators and air pollution control equipment suppliers to seek alterative technologies to reduce operating costs associated with emissions control. Although the regenerative thermal oxidizer (RTO) generally is considered the most energy-efficient volatile organic compound (VOC) control technology available, it can be expensive to operate when handling large airflows with low VOC concentrations.
Recent maximum achievable control technology (MACT) standards promulgated by the Environmental Protection Agency (EPA) have allowed for emission reduction as low as 90 percent for hazardous air pollutants (HAPs) on some applications (e.g., wood products MACT). This has opened the door for alternative technologies that typically achieve higher destruction and removal (DRE) efficiencies -- sometimes in excess of 98 percent.
The biofilter and the regenerative catalytic oxidizer (RCO) are two common alternatives to a regenerative thermal oxidizer currently used for these applications. Both the technologies can be suitable for a range of applications; however, marked differences exist between the two technologies. This article will describe benefits of using regenerative catalytic oxidizers and compare the performance of RCOs with biofilters in some existing installations.
What Is It?By definition, a regenerative catalytic oxidizer is a regenerative thermal oxidizer with a catalyst added to the top layer of the heat exchange media, allowing it to operate at a lower temperature. Both regenerative styles of oxidizers require supplemental fuel and electrical energy for operation, but the reduced operating temperature in a regenerative catalytic oxidizer provides savings in fuel costs. Also, improvements in heat exchange media designs have allowed for lower pressure drop for both technologies, resulting in lower fan horsepower requirements.
Another advantage of some designs of regenerative catalytic oxidizers is that they can be designed to operate in dual mode (thermal or catalytic) and be capable of switching while online between the two modes. Called thermal catalytic regenerative oxidizers, systems with this capability provide the benefits of both regenerative oxidizer designs. They provide the flexibility to increase the operating temperature of the regenerative catalytic oxidizer if destruction and removal (DRE) efficiencies increase or the catalyst degenerates.
Testing conducted on a number of regenerative catalytic oxidizer installations showed the lowest operating temperature at which the units could be run and still achieve the guaranteed destruction efficiency of 95 percent (table 1). All of the units were designed to operate at 800°F (427°C). After starting up and operating on line at 800°F, the retention chamber temperature was incrementally reduced to determine the point at which the VOC/HAP conversion efficiency dropped below the required minimum of 95 percent. The results are shown in table 2.
Operating Cost ComparisonAlthough a biofilter -- a pollution control method that using organic material and microbiological activity to capture and degrade VOCs and other process pollutants -- could provide savings in operating costs and reduce greenhouse gas production when compared to a regenerative thermal oxidizer, a more meaningful comparison is between a biofilter and a regenerative catalytic oxidizer. The lower operating temperature and system pressure drop of a regenerative catalytic oxidizer result in operating costs that are equal to or, in some cases, lower than a comparably sized biofilter.
To illustrate the comparative cost of operation for the three pollution control systems, a typical plywood dryer unit was selected with 80,000 acfm airflow. Inlet temperature of 300°F (149°C) and a VOC/HAP loading of 105 lb/hr (90 ppm) at 15,000 BTU/lb were assumed. Table 3 provides a comparison of the three technologies.
As table 3 shows, the actual total cost of operation for the regenerative catalytic oxidizer in this sample application is lower than the biofilter system. This often can be the case for applications where the VOC loadings are high enough to meet the energy requirement of the regenerative catalytic operation for fuel-free operation, but low enough to make the regenerative thermal oxidizer more expensive to operate.
To illustrate the other extreme, a worst case scenario would be an application where VOCs were not present in the process exhaust stream. In this case, the supplemental fuel required would be the maximum amount. For the regenerative catalytic unit described in table 3, this would equate to 1.13 MMBTUH of additional fuel required. At $7/MMBTU and 8,000 hours operation, this would mean an additional $63,280 per year in operating cost.
In conclusion, the regenerative thermal oxidizer has been the dominant technology in treating emissions in the wood products industry for many years for good reason. It is able to handle particulate, achieve a high level of destruction efficiency and provide relatively low cost of operation. The recent changes in compliance regulations, higher energy costs and an increased emphasis on greenhouse gas reduction have lead alternatives such as regenerative catalytic oxidizers and biofilters.
Regenerative catalytic oxidizers offer many of the same advantages of their thermal counterparts. The technology is familiar and well tested in the industry. The lower operating temperatures for regenerative catalytic oxidizers have resulted in systems that require no auxiliary fuel, producing operating cost savings and a smaller carbon footprint.