Less expensive to purchase, install, and operate compared to multiple chamber units, this single rotary valve thermal oxidizer helps maintain this grinding wheel manufacturer's good neighbor policy.
Even as air quality improves, the U.S. continues to tighten its criteria for what constitutes 'good air.'Government regulations now control even short-term deviations from air quality standards lasting only a few days a year, and communities have increased their sensitivity to infrequent emissions of low-concentrations of odors.
Tyrolit North America, Inc. (Westborough, MA), manufacturer of abrasive grinding and cutting wheels, experienced this situation in early 1997. While in compliance with emission standards for VOCs (volatile organic compounds), about two to three times a year the company received complaints about odors emitting from their kilns. While the complaints were considered infrequent, Tyrolit sought to improve relations with its neighbors by eliminating odors.
The second largest producer of grinding wheels in the U.S., Tyrolit manufactures thousands of designs for cutting and grinding steel. Fabricated from silicon carbide and/or aluminum oxide with either vitrified and resinous processes, the grinding wheels are produced in a wide variety of sizes, porosity, and abrasive particle size.
Production of the vitrified wheels begins with mixing the abrasive with organic fillers that shape the wheel and provide the required porosity. Filler materials include walnut shells as well as solid hydrocarbon compounds similar to naphthalene. The abrasive/filler mixture is formed into wheels and the wheels are packed in sand for firing in a kiln. The sand provides uniform temperature distribution and prevents wheel distortion.
Large containers of wheels packed in sand then enter preheating ovens and kilns where they are gradually heated to temperatures in excess of 1,000°F. Tyrolit uses batch kilns as well as continuous kilns that provide counter-current air flow to conserve energy. While the maximum kiln temperatures are sufficient to fully oxidize the organic filler, some organics evaporate and/or partially oxidize before the wheels reach maximum temperature. The evaporated and partially oxidized organic vapors can cause occasional odor problems.
In late 1997, Tyrolit purchased a thermal oxidization system on the exhaust gas from the kilns producing vitrified wheels. The turnkey abatement system included the ductwork required to collect gas from more than 30 emission points on their batch kilns, tunnel kilns, and pre-heat ovens.
The ducts direct the process emissions to two identical single can, rotary valve regenerative thermal oxidizers (RTOs). Designed to operate in parallel or individually, the oxidizers use a distinctive technique to direct emissions between the heating and regenerating sections of the oxidizer.
An RTO uses ceramic monolithic to store and then release heat from hot, oxidized emissions. Hot oxidized gas from the oxidizer's burner chamber flows through a series of packed ceramic monolithic beds before discharge to atmosphere. The ceramic beds absorb heat from the hot gas until the beds approach gas-ignition temperature. The hot beds are then used to preheat emissions entering the burner chamber, thus reducing the amount of external energy required. An equal number of ceramic beds are then used to absorb heat from gas exiting the burner chamber. Depending on the heat of combustion of the emissions and the system design, RTOs can operate with little or no external fuel.
In practice, RTOs typically use three ceramic beds--one absorbing heat from outlet gas, one releasing heat to inlet gas, and one either in transition or purging. Lower-cost two-bed RTOs are also available, but they eliminate the transition cycle and purge sequence of the third bed. The result is reduced destruction efficiency and an increase in pressure fluctuations.
Indexing Diverter Valve Designs
While old-style convention regenerative oxidizers are often designed with two or more ceramic beds, each additional bed increases construction and maintenance costs. This increased cost is caused primarily by the larger number of valves needed to direct emissions between the beds. The three valves required for each bed must each be large enough to accommodate full gas flow while maintaining a low-pressure drop and withstanding the high temperature of combusted gas.
An RTO was selected for the Tyrolit application that uses 12 beds of structured ceramic packing and a single, indexing diverter valve, the Reeco® RL. As emissions from the kilns enter the bottom of the oxidizer, the indexing valve directs the gas upward through five adjacent sections of the packing for pre-heating. After the gas passes through the packing, it enters the burner chamber where, if necessary, an auxiliary fuel-burner boosts the temperature to 1,800°F to assure complete combustion of the organics. The gas then passes down through five adjacent (but different) sections of the packing to release its heat. Periodically, the rotary valve indexes, such that one of the five pre-heat chambers becomes idled, the previously idled chamber is used to absorb heat, one of the heat-absorbing sections is purged, and the previously purged heat-absorbing sections enters the pre-heat zone--always maintaining five sections on inlet and five sections on outlet.
By using a single indexing valve instead of multiple butter fly or poppet valves, fabrication and installation cost of the unit is decreased and reliability increases. Generally, the cost of the Reeco ® RL is about the same as a two chamber unit but has a destruction efficiency greater than a three chamber unit.
Each of the RTOs at Tyrolit is designed to handle 25,500 CFM at a thermal efficiency of 95% and is guaranteed to completely oxidize at least >95.5% of the VOCs. The VOC-level in the emissions from Tyrolite's vitrified wheel kilns averages 40-100 ppm, but can peak to 950 ppm.
Based on the successful performance of these units, a third RL thermal oxidation system will be installed to handle emissions from Tyrolit's production of resin-based grinding wheels. As with the oxidizers on thevitrified wheel kilns, the new system will be designed to eliminate odors as well as VOCs.