Choosing the Right VOC Emission Control Technology

Each year, new federal and state environmental regulations require higher destruction and improved capture of volatile organic compounds (VOCs). The coating industry and VOC control equipment manufacturers must develop technologies to meet these regulations, while providing improvements in both capital investment and operating costs. This paper discusses the various emission control options available and recent new developments.

Emission Control Technologies
A variety of auxiliary pollution control approaches are currently available. This is a brief discussion of the different types, comparing some of the features, benefits and potential drawbacks of each.

Solvent Recovery (Adsorption)
Adsorption is defined as the concentration of a substance (adsorbate) into the surface of a solid (adsorbent). The basic principles of adsorption are applied to air emission control when a solvent-laden air stream passes through a high surface area solids material (typically carbon, alumina, silica gel, or molecular sieves) and is "captured" or adsorbed into the surface of the material. Once the micro-pores of the adsorbent material are filled to capacity, the process stream is then directed to another bed of adsorbent, while the original bed is "desorbed". Desorption (also referred to as regeneration) is a secondary process whereby solvent is removed from the adsorbent by passing high temperature steam (or other gases) through it. The highly concentrated solvent air stream subsequently goes through a condenser and finally a separation or distillation column, where the solvent is separated (or recovered) from the air stream condensate.

Characteristics

• High capital investment
• Moderate operating cost
• 95 to 98% clean-up rates
• Maintenance intensive (steam, condensing and distillation systems, activated carbon material)
• High maintenance cost (replacement or regeneration of carbon material)
• Additional refining necessary if using multiple solvents
• Wastewater quality must be monitored for disposal/treatment purposes
• Potential of solvent to be reused or sold

Thermal Oxidation
Thermal oxidizers are typically used to convert hydrocarbons into carbon dioxide (CO2) and water (H2O) through the process of oxidation. This process is carried out by raising the temperature of the process exhaust to break the hydrogen-carbon bonds which allows new bonds to form, creating CO2 and H2O. When this occurs, an exothermic reaction occurs and heat is released.

Oxidizers typically are designed with 0.5 seconds or greater total residence time. Residence time is critical, not for the oxidation reaction, but for proper mixing. Many designs fail to complete the mixing in the time available. To meet a performance specification, more fuel (i.e. temperature) must be burned than would be needed if mixing were incomplete.

Nitrogen Oxide (NOx), a product of combustion, increases as temperature and fuel input increases. NOx output should be considered because it is a primary component in the production of ozone.

Regenerative Thermal Oxidizer
Regenerative systems are thermal oxidizers which operate at high temperatures of 1500° to 1800° F (815° - 980° C). These systems utilize a ceramic stoneware or other heat exchange media. In most designs, the media is mounted in vertical columns. The process air stream is passed through a column of media as it enters the oxidizer. The air stream is subsequently heated further to oxidation temperatures by a burner in the central combustion chamber. The air stream then exits the oxidizer through a second media column. The second column "stores" energy from the hot air stream. By constantly cycling the air stream among the columns, the incoming air stream is heated by the media, which in the previous cycle was absorbing heat from the air stream exiting the central chamber. As this column loses heat to the incoming air stream, it cycles and becomes the receptor of heat repeating the cycle. The theory is quite simple and has proven to be successful in many applications. Regenerative thermal oxidizers can be designed with one, two, three or more columns.

Characteristics

• Moderate to high capital/installation costs
• High thermal efficiencies
• Low operating cost
• Moderate to high cleanup efficiencies (95 to 99%)
• Higher radiation losses due to system surface area
• Best for low concentrations of VOC in the process exhaust with high volume exhaust flows
• Heavy weight requires additional installation support

Recuperative Thermal Oxidizers
There are a number of recuperative thermal oxidizer designs which can be used to destroy VOCs. The thermal oxidizer is a simple design that passes air through an air-to-air heat exchanger to preheat it before entering the burner chamber. In the burner chamber, the process exhaust air is heated to a sufficiently high temperature and held at this temperature with some degree of turbulence to ensure VOC destruction.

Thermal oxidizers clean emissions by burning or oxidizing them at high temperatures. Typical VOC reduction is 99% or outlet emissions less than 20 mg/Nm ³. Carbon monoxide can be either created (by partial oxidation of VOC) or destroyed (by complete oxidation of the CO to CO2) in a thermal oxidizer, depending upon operating temperature. CO production tends to rise with increasing temperature until it reaches a maximum at about 1200° F (650° C). Then, the CO content tends to decrease rapidly with increasing temperature. At 1400° F (760° C), CO emissions from thermal oxidizers are relatively low.

Characteristics

• Moderate-high capital/installation costs
• High operating cost at low solvent loading
• Cleanup rates can be very high (greater than 99%)
• Need high operating temperatures (1380 to 1500° F or 750 - 815° C) to get low CO levels
• NOx formation - especially at operating temperature above 1500° F (815° C)
• High exhaust gas temperatures:
  
  • Secondary energy recovery is possible
  • High temperature exhaust stack construction is required
• Quality materials of construction necessary for longevity

Catalytic Oxidation
Catalytic oxidizers are an alternative to thermal oxidizers for oxidizing gaseous, combustible contaminants into carbon dioxide and water. Their successful operation is limited to a more restricted range of applications than thermal oxidizers; but where applicable, catalytic units offer the potential of significantly lower fuel consumption and operating costs plus reduced CO and NOx emissions. The basic elements of the catalytic unit are a preheat/mixing section, designed to achieve a uniformly preheated and distributed waste stream flow, and the catalyst bed or catalyst matrix, where the majority of the oxidation reactions take place.

The oxidation of most hydrocarbons and carbon monoxide occur rapidly in the range of 300 to 900° F (150 - 480° C) over catalysts. With thermal oxidizers, the oxidation reaction requires a high temperature of 1200 to 1600° F (650 - 815° C) to break the carbon, hydrogen and oxygen bonds.

Besides reduced energy consumption, NOx emissions from catalytic units are very low because of the lower oxidation temperatures being used, as well as the lower burner firing rates. In addition, the oxidizing nature of the catalyst results in very low CO emissions. However, there are trade-offs involved to gain these advantages. Catalytic oxidizers can be subject to masking agents or poisons that inhibit the effectiveness of the catalyst.

Catalytic systems are limited to applications in which the waste stream has negligible particulate loading and/or "poisons" which can reduce the effectiveness of the catalyst. These poisons are primarily silicon and phosphorus which coat the catalyst; halogens such as chlorine which directly attack the active metals converting them to an inactive form; and sulfur which inhibits the activity of some catalysts. The oxidation activity of the catalyst can also be reduced by the loss of active components through attrition, deposition of unreacted VOC (coking) onto the catalyst surfaces, or sintering of the catalyst (collapse of the catalyst structure caused by high temperatures).

Regenerative Catalytic Oxidizers
This oxidizer is similar in design to the regenerative thermal oxidizer. The addition of catalyst to either the media or the top of the media column allows lower operating temperatures. Depending on component design, this unit could be operated as a thermal oxidizer after catalyst degradation.

Characteristics

• Higher capital/installation costs
• High thermal efficiencies
• Low operating costs
• Very low levels of CO emission
• Very little or no NOx formation
• Best for low VOC, high air flow applications

New Developments
This section discusses some recent developments in catalytic and regenerative thermal oxidation technology.

Low Temperature Catalyst Development

Catalytic Oxidizers

Noble metal catalysts require inlet temperatures above 600º F to achieve more than 98% VOC conversion. Given that the maximum exhaust temperature is 200º - 300º F, the exhaust gases from these applications need to be heated to the desired operating temperatures to generate significant supplemental fuel usage. A catalyst that functions at a lower temperature would lower the operating cost by reducing the need to heat the process system.

A catalyst formulation has been developed that reduces the light-off temperature for oxygenated hydrocarbons. Reactors with the new catalyst can operate at temperatures 100º to 200º F lower than reactors containing conventional catalysts.

Regenerative Thermal Oxidizers
RTO systems have emerged as the leading technology because of the high heat recovery that produces an outstanding operating cost advantage in comparison to other technologies while maintaining high flexibility in the types of processes that can be treated. Typically these systems are very large requiring expensive installation work. Development efforts have focused on a compact, modular and cost effective RTO for the low flow VOC control market. Following are some distinguishing features of these RTOs:

• The oxidizers are designed such that flow directing poppet valves are incorporated into the oxidizer shell to minimize the oxidizer footprint and fabrication costs. This feature eliminates the transition ducts that have been associated with previous traditional designs and further minimizes the field installation.
• The oxidizers are modular and designed to ship as completed assemblies with width dimensions that will comply with commercial trucking equipment without special shipping considerations or permits.
• All oxidizers are completely pre-assembled including all control panels, piping, conduit and electrical wiring. This pre-assembly minimizes the installation effort and maximizes quality control over the finished product.
• Oxidizers undergo simulated run conditions, control systems check out and calibration prior to shipment to facilitate shorter start-up times.
• Oxidizers feature state-of-the-art controls including modems to enable the use of telemetry. Through the use of telemetry, product support and oxidizer up-time is maximized while field service requirements are minimized.
• Oxidizers are designed with minimal entrained volume for a design destruction efficiency goal of 98% without capture of the VOCs exhausted during the poppet valve switch.
• All oxidizers are designed to accept optional VOC capture vessels when destruction efficiency requirements exceed 98%. The design will permit oxidizers to be upgraded in the field if the end user requires.