The plywood and composite wood products (PCWP) industry has until October 2008, to be in compliance with maximum achievable control technology (MACT) emissions regulations, but the Masonite facility in Laurel, Miss., decided not to wait until the last minute.
The plant produces composite wood panels for various finished products. Green or dry wood residues are ground into particles of varying sizes using flakers, mechanical refiners and hammer mills; classified according to their physical properties and dried to reduce moisture content. Panel board mats are formed of layers of different sized particles, with the larger particles in the core and the finer particles on the outside of the board. The mats are hot pressed to increase their density and to cure the resin.
The primary finishing steps for the reconstituted wood panels include cooling or hot stacking, grading, trimming/cutting and sanding. Secondary finishing steps include spray or roll coatings that are cured in a kiln or oven to produce the material for the final products.
When Masonite sought a regenerative thermal oxidizer (RTO) to reduce oven emissions, it compared both rotary-valve units and the more conventional multiple-chamber butterfly- or poppet-valve systems from a number of suppliers. It chose the Environmental and Energy Systems division of Dürr Systems (www.durrenvironmental.com/en) to install three RL rotary-valve RTO systems.
“The key qualification was being able to achieve 99% destruction efficiency to meet their PCWP MACT requirements,” says Stephen Blocki, vice president, Energy & Environmental Systems Division of Dürr. “The online bakeout was a key factor since the product has condensable emissions and they didn’t want to shut down production to bake out the RTO unit.”
Much like a self-cleaning oven, the patented online bake uses the existing flushing system to provide a bake-down control from the combustion chamber on each heat recovery chamber. Smoke or fumes from the bake-out process are returned to the inlet of the unit, eliminating any bake-out emissions. Except for small amounts of radiation loss, the energy also is returned to the RL RTO unit.
The online process provides the ability to program and schedule a bake-out frequency that virtually eliminates the potential for excessive buildup of combustibles and the associated runaway fires that were typical of the old-style multi-chamber RTO units.
“The unique design of the rotary valve RL RTO unit encapsulates the inlet process air stream within the hotter exhaust air stream,” says Rodney Pennington, P.E., business development for Dürr Systems. “This keeps all of the associated surfaces at a hotter temperature than the process air, which eliminates condensation.”
The RL has fewer parts than typical multi-chamber units, notes Pennington. “The RL rotary valve RTO unit has one electric actuator and one valve, a total of two components, as compared to the nine actuators, nine valves, 18 solenoid valves, 18 speed control valves and a hydraulic system, a total of 54 components with the old style three-chamber RTO unit,” he says. “The fewer the components, the greater the reliability and less chance of incurring downtime.”