Many industrial processes produce or use volatile organic compounds (VOCs) or hazardous air pollutants (HAPs). VOCs and HAPs are hydrocarbons — necessary components for industries such as printing, painting or coating. VOCs also are emitted from many processing facilities during the manufacture of chemicals, pharmaceuticals and foods, among others.
Pollution-control equipment is installed on these processes to prevent the VOCs and other emissions from reacting with nitrogen oxides (NOX) in the presence of heat and sunlight and forming ground-level ozone. This is the basis for continuous Clean Air Act legislation and enforcement by state and federal regulatory agencies.
Today’s environmental concerns have led many industries to search for more effective pollution-control methods. While many technologies are available to simply control VOCs and other emissions, oxidation technologies often are capable of exceeding clean air regulations. When applied appropriately, oxidation technologies such as catalytic and thermal treatment provide continuous performance with the lowest treatment cost.
Many factors impact the choice of air pollution-control equipment and the ability to meet or exceed environmental goals such as best available control technology (BACT) and maximum available control technology (MACT). Application details about the process operation such as uptime, maximum VOC loading, maximum airflow, energy usage concerns, permitted emission rates and other process operation data are critical to equipment design.
An RTO utilizes a ceramic heat-exchange media to recover heat as it exits the combustion chamber. Every few minutes, the flow of this exhaust gas is reversed through the action of a valve system. The heated ceramic media then is used to preheat the incoming process air.
Considerations for Determining the Correct Oxidizer Technology
In determining the most appropriate technology to control a process stream, it is necessary to characterize the airstream by the following conditions:
- VOC loadings.
- Required VOC destruction rate efficiency (DRE).
Airflow. Airflow is the volumetric flow rate of the process gas, corrected to standardized conditions of temperature and pressure, as measured in standard cubic feet per minute (scfm). This represents a fixed number of moles of gas, regardless of composition and actual flow conditions.
Process Temperature. This can range from ambient air temperature (the temperature of the air that surrounds us) to process temperatures greater than 500°F (260°C).
VOC Loadings. The type and quantity of VOCs in the exhaust airstream must be identified in both the worst-case (highest concentration) and normal operation scenarios. VOC concentrations can play a significant role in the operating expenses: Items such as electrical and natural gas usage, consumable parts and equipment maintenance are affected. For example, the operating cost for a thermal oxidizer might be higher than a regenerative thermal oxidizer due to the natural gas usage required.
VOC concentration also is evaluated for safe operation of the system. Most of the time, the VOC concentration is converted to a percentage of the lower flammable limit (LFL) in the exhaust stream to be treated. The LFL is the lowest concentration (percentage) of a gas or vapor in air capable of producing a flash of fire in the presence of an ignition source (arc, flame, heat).
Required VOC Destruction Rate Efficiency (DRE). Typically, greater than 95 percent destruction is required. Some applications may require greater than 99 percent, depending on the type of VOC and local regulations. Characterization of the VOC-laden stream and the required environmental emission limits such as BACT and MACT assist in selecting the appropriate air pollution-control technology to achieve the necessary DRE.
Contaminants. Particulate buildup can present an issue with some oxidizer technologies. In most cases, particulates can be eliminated by placing a filter in front of the oxidizer system. In some instances, however, the particulate is formed inside the combustion chamber of the oxidizer. This adds several more layers of consideration when selecting the correct control device. Silicone, for example, can build up on heat exchanger tubes, burners and fans, and it creates maintenance nightmares if the wrong technology is chosen. Silicone also can mask catalyst, which makes a catalytic oxidizer an incorrect technology choice (at times) for an application that can contains silicone.
The main advantages of catalytic oxidizers include lower auxiliary fuel usage and more cost-effective construction materials when compared to the typical heat exchanger that is required for the higher temperatures found in recuperative thermal oxidizers.
Oxidizer Technology Options
Fully oxidizing VOCs requires a combination of time, temperature and turbulence. Thermal oxidizers convert (destroy) hydrocarbons (VOCs) to CO2 and H2O through the use of heat.
Four types of oxidizers are used for VOC control:
- Regenerative thermal oxidizer.
- Direct-fired thermal oxidizer.
- Recuperative thermal oxidizer.
- Catalytic oxidizer.
Regenerative Thermal Oxidizer. A regenerative thermal oxidizer (RTO) is the most common applied oxidizer technology due to its high thermal efficiency. An RTO utilizes a ceramic heat-exchange media to recover heat as it exits the combustion chamber. Every few minutes, the flow of this exhaust gas is reversed through the action of a valve system, and the heated ceramic media then is used to preheat the incoming process air. This repeated cycling allows for the regeneration of the heat in the ceramic media, leading to high thermal efficiency.
Direct-fired thermal oxidizers (DFTO) are basic in design. The main components are a burner and a combustion (retention) chamber.
Regenerative thermal oxidizers typically are utilized for processes that have:
- Process gas volume ranging from 500 to 80,000 scfm for one system.
- VOC concentrations ranging from 0 to 15 percent of the lower explosive limit (LEL).
- Clean or low particulate in the process gas or after combustion of the VOC.
- Process gas temperatures up to 500°F (260°C).
The advantages of a regenerative thermal oxidizer system revolve mainly around the capital and operational cost advantages for processes with larger airflow and low VOC concentrations (less than 8 percent LEL). The thermal effectiveness can be up to 97 percent with a regenerative thermal oxidizer.
Direct-fired Thermal Oxidizer. Direct-fired thermal oxidizers (DFTO) are very basic in design. The main components consist of a burner and a combustion (retention) chamber. DTFOs typically are utilized for processes that have:
- Low inlet volume. The cutoff typically is less than 1,000 scfm.
- Very high concentration of VOCs, typically greater than 25 percent of the LEL.
- Presence of particulate in the process gas.
- High process gas temperature, typically greater than 600°F (315°C).
The guidelines for applying a direct-fired thermal oxidizer revolve around some key criteria: capital expenditure, operational costs and safety. For example, low inlet volume operational savings usually do not justify the cost of the heat recovery that is necessary. Likewise, high VOC concentrations can cause issues with safety and control in oxidizers that utilize heat recovery. Additionally, if you have concerns about the generation of burner-related emissions (NOX and CO) due to high burner input requirements, this may not be the best technology.
Recuperative Thermal Oxidizer. A recuperative thermal oxidizer utilizes an air-to-air heat exchanger to preheat the incoming process air using the clean, hot air from the oxidizer combustion chamber. This primary heat recovery raises the temperature of the process gas before entering the combustion chamber, resulting in lower fuel requirements for the oxidizer burner system. Thermal recuperative oxidizers utilize plate or shell-and-tube type heat exchangers.
A recuperative thermal oxidizer utilizes an air-to-air heat exchanger to preheat the incoming process air using the clean, hot air from the oxidizer combustion chamber.
Recuperative thermal oxidizers are typically utilized for processes that have:
- Process gas volume ranging from 500 to 30,000 scfm.
- VOC concentrations ranging from 10 to 25 percent of the LEL.
- Presence of particulate in the process gas or after combustion of the VOC.
- Process gas temperatures up to 600°F (315°C).
This technology does have some aspects that warrant consideration, including a typically higher operating cost, as well as higher NOX emissions than those produced using a recuperative catalytic oxidizer or a regenerative thermal oxidizer. It is a good choice for any technology process that has a high continuous VOC stream, a small airflow rate or batch-type cycling. Also, a secondary system added to a recuperative oxidizer can raise efficiencies and reduce operational costs.
Operation schedules should be considered with recuperative thermal oxidizers because the stainless steel alloys utilized in the heat exchangers have finite life spans if the system is going to be turned on and off each day.
Catalytic Oxidizer. Catalytic oxidizers are similar in design and operation to thermal oxidizers, except that a catalyst material is placed within the combustion chamber. A catalyst lowers the activation energy required to start the combustion reaction, thereby lowering the operational temperature (and operational costs) of the oxidizer.
A recuperative catalytic oxidizer utilizes a heat exchanger to preheat the incoming process air, resulting in lower fuel requirements for the oxidizer burner system.
Recuperative catalytic oxidizers are typically utilized for processes that have:
- Process gas volume ranging from 500 to 30,000 scfm.
- VOC concentrations ranging from 0 to 15 percent of the LEL.
- No particulate, heavy metals or silicone in the process gas or after combustion of the VOC.
- Process gas temperatures up to 400°F (204°C).
Catalytic oxidizers are similar in design and operation to thermal oxidizers, except that a catalyst material is placed within the combustion chamber.
The main advantages of this system include lower auxiliary fuel usage and more cost-effective construction materials when compared to the typical heat exchanger that is required for the higher temperatures found in recuperative thermal oxidizers. Catalytic oxidizers usually are not selected if there are any types of catalyst poisons present in the system.
In conclusion, understanding the technologies that are available for air-pollution control is a complicated endeavor; however, it often is necessary for complying with environmental regulations. Choosing an experienced supplier capable of evaluating and supplying a range of oxidizer technologies will ensure the appropriate technology is selected.