When it comes to air pollution control, understanding the technology that is available is a complicated endeavor, but this knowledge is necessary for complying with environmental regulations. At Catalytic Products International, we are experts in the systems used to control volatile organic compounds (VOCs) and hazardous air pollutants (HAPs), and we want to help you understand how you can use these oxidizers for VOC control and to meet all your air pollution control needs.
There are 5 basic, distinct types of oxidizers used for VOC control. Let's look at each of them, along with some simple guidelines to point you in direction of the correct oxidizer for your process.
Thermal Oxidizers
Thermal Oxidizers convert (destroy) hydrocarbons (VOCs) to CO2 and H2O through the use of heat.
Fully oxidizing VOCs requires a combination of: Time, Temperature, and Turbulence
Direct Thermal Oxidizers
Direct thermal oxidizers are very basic in design, with the main components consisting of a burner and a combustion (retention) chamber. Direct thermal oxidizers are typically utilized for processes that have:
- Low inlet volume (the cutoff is typically less than 1,000 standard cubic feet per minute [SCFM])
- Very high concentration of VOCs (greater than 25% of the lower explosive limit [LEL])
- Presence of particulate in the process gas
- High process gas temperature (typically greater than 600° F)
The guidelines for applying a direct 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 which utilize heat recovery.
This may not be the best technology for you if you have concerns about the generation of burner related emissions (NOx & CO) due to high burner input requirements.
Recuperative Thermal Oxidizers
Recuperative thermal oxidizers use 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. Recuperative thermal oxidizers are typically utilized for processes that have:
- Process gas volume ranging from 500 SCFM to 30,000 SCFM
- VOC concentrations ranging from 10% to 25% of the LEL
- Presence of particulate in the process gas or after combustion of the VOC
- Process gas temperatures up to 600° F
This technology does have some aspects that warrant consideration, including what is typically a 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 air flow rate, or batch type cycling. Also, a secondary system added to a recuperative oxidizer can raise efficiencies and reduce operational costs. Operation schedules should also be considered with recuperative thermal oxidizers, as the stainless steel alloys utilized in the heat exchangers do have finite life spans if the system is going to be turned on and off each day.
Catalytic Oxidizers
Catalytic Oxidizers convert (destroy) hydrocarbons (VOCs) to CO2 and H2O through the use of a catalyst.
Fully oxidizing VOCs requires a combination of temperature, turbulence, and catalyst.
A Catalyst is a substance which lowers the activation energy for a given reaction without being consumed by the reaction.
Catalytic oxidizers are similar in design and operation to thermal oxidizers, except that a catalyst material is placed within the combustion chamber. The catalyst lowers the activation energy required to start the combustion reaction, thereby lowering the operation temperature (and operational costs) of the oxidizer.
Direct Catalytic Oxidizers
As with direct thermal oxidizers, direct catalytic oxidizers are very basic in design, consisting of a burner and a combustion (retention) chamber. Direct catalytic oxidizers are typically utilized for processes that have:
- Process gas volume ranging from 500 SCFM to 30,000 SCFM
- VOC, NOx, or Ammonia within the air stream with concentrations ranging from 0% to 15% of the LEL for VOC
- No particulate, heavy metals, or silicone in the process gas or after combustion of the VOC
- Process gas temperatures up to 800° F
Recuperative Catalytic Oxidizers
As with recuperative thermal oxidizers, recuperative catalytic oxidizers utilize 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 SCFM to 30,000 SCFM
- VOC concentrations ranging from 0% to 15% 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
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 are usually not selected if there are any types of catalyst poisons present in the system.
Regenerative Thermal Oxidizers
Regenerative thermal oxidizers convert (destroy) hydrocarbons (VOCs) to CO2 and H2O through the use of heat.
Fully oxidizing VOCs requires a combination of: Time, Temperature, and Turbulence.
Regenerative thermal oxidizers utilize ceramic heat exchange media instead of a stainless steel heat exchanger. The heat exchanger must have its heat regenerated during normal operation, requiring some added complexity in the form of valves which direct the airflow direction changes as needed. Regenerative thermal oxidizers are typically utilized for processes that have:
- Process gas volume ranging from 2,000 SCFM to 80,000 SCFM for one system
- VOC concentrations ranging from 0% to 15% of the LEL
- Clean or low particulate in the process gas or after combustion of the VOC
- Process gas temperatures up to 500° F
The advantages of a regenerative thermal oxidizer system revolve mainly around the capital and operation cost advantage for processes with larger airflow and low VOC concentrations (<8% LEL). The thermal effectiveness can be up to 97% with a regenerative thermal oxidizer.
We hope that this look at oxidizers has been helpful. If you have any questions about VOC control, please contact us.
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