Fume emissions from Volatile Organic Compounds are becoming more of a concern to an ever-widening circle of industries. Emission control was once viewed as a problem for large factories with belching smokestacks. The reality is that with an ever-increasing population and industrial growth, process emissions have an increasing impact on the air quality of our communities and cities.
Organic solvent emissions can cause unpleasant odors, but more serious solvents like Toluene, Methanol, Ethanol, Xylene and MEK are photo chemically active and can be smog producers. When exposed to sunlight in the presence of nitrogen oxides, the formation of photochemical smog can result.
To minimize this problem, the emissions must be stopped at the source. This will usually be addressed by using an oxidizer to convert the solvent fumes to carbon dioxide and water vapor.
An oxidizer uses a combustion process for this conversion. There are three common process used. One system uses direct combustion, referred to as an afterburner or thermal oxidizer, in which the VOC fumes are brought into direct contact with the burner flame. The second type of oxidizer uses a catalyst to promote oxidation at lower temperatures. The third type is a regenerative thermal, which uses ceramic beds to recover heat. Oxidizers can be small, ranging in size from 100 scfm to 14,000 scfm for an individual process or up to 100,000 scfm or more to control an entire plant.
The regenerative systems are usually applied to larger to systems, 15,000 scfm and more, where their lower operating cost justifies the initial investment. Let’s examine the characteristics of these systems.
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Direct Thermal Oxidizers
Thermal oxidizers employ electric heating elements or a burner usually fueled with natural gas, propane vapor or fuel oil mounted to fire into an insulated chamber. The chamber is designed so that the VOC vapor or odor is introduced into or near the heat source.
The chamber will be of sufficient size so that the vapor or fume "dwells" in the chamber long enough to be converted to carbon dioxide and water vapor. This dwell time is usually between .5 and 1.0 seconds. Thermal oxidizers will operate between 1100°F and 1600°F depending on the solvent type and destruction efficiencies that are required.
Thermal oxidizers can operate at 99% destruction efficiency at 1600°F for most solvent types. Destruction efficiency will be assured as long as operating temperature, dwell time and good mixing turbulence are maintained to complete the conversion process.
Thermal oxidizers are generally not affected by contaminants such as particulate and other elements that will poison a catalytic cell or clog a regenerative bed. Thermal oxidizers, due to their higher operating temperatures, can use larger amounts of fuel to maintain higher operating temperature, especially if the VOC vapors are at a low percentage.
The lower initial lower cost of the thermal oxidizer makes it attractive for intermittent use or smaller scfm requirements. For medium sized thermal oxidizers or continuous processes a heat recuperator can be added and recover 60% to 70% of the exhausted heat.
Regenerative Thermal Oxidizers
Larger thermals use a regenerative process where the oxidizer intake and exhaust is passed through beds, usually filled with ceramic stoneware, to recover heat. When the exhaust bed is heated, a set of dampers switches the exhaust and intakes routing the VOC process exhaust through the heated bed for preheat. The dampers switch the beds, usually every 30 to 90 seconds, to "regenerate" the now cooled bed.
Regenerative thermal oxidizers will operate usually between 95% and 98% thermal efficiency. Operating temperatures will vary between 1100°F and 1600°F depending on the solvent type and destruction efficiencies that are required. Regenerative thermal oxidizers can operate at 99% destruction efficiency at 1600°F for most solvent types.
Regenerative oxidizers pose a substantial investment and higher fan horsepower, but their thermal operating efficiencies at the larger sizes will offset this cost and provide a savings over the life of the unit.
Catalytic Oxidizers
Catalytic oxidizers also use a burner and chamber in addition to a catalyst bed. Initially, the burner preheats the catalytic bed from 600°F to 1100°F, depending on the type of organic vapor fume and the destruction efficiencies required. Once the preheat temperature is reached, the process exhaust is passed through the catalyst.
The fumes are oxidized and their heat value is released to the catalyst. The burner input is reduced to maintain the required catalyst inlet and outlet temperatures. Since the oxidation occurs at lower temperatures, normally 600°F to 800°F, the lower fuel requirements substantially reduce the operating costs.
The catalytic oxidizer generally uses a ceramic or metal substrate in the shape of a honeycomb monolith or pellet. The substrate is coated with a noble metal element to promote oxidation of organic vapor fume. The most widely used practice is to utilize the honeycomb monolith cut into cubes and packaged into stainless steel canisters. The catalytic oxidizer can operate at 98% destruction efficiencies with proper sizing of the bed and appropriate operating temperatures.
The ceramic substrate catalyst can be cleaned and regenerated to lengthen the effective life span of the catalyst. The catalyst also has a substantial scrap value. Materials such as lead, zinc, arsenic, halogens and silicones will poison the catalyst and render it ineffective. If such contaminants are present, a catalytic oxidizer cannot be used.
Catalytic oxidizers are most cost effective for process flows to 14,000 scfm for VOC fumes below 25% of the lower explosive limit. A heat recuperator can also be added to lower the operating costs an additional 60% to 70%. The fuel savings due to the lower operating temperature more than offsets the cost of the catalyst and occasional catalyst cleaning requirements.
Oxidizer Selection
Which type of oxidizer is right for your application and how much will it cost? The answer can only be determined by evaluating each application and determining a selection based on hours of operation, process needs, purpose, and present and anticipated future pollution control requirements and lowest overall operating costs.
