Working Principle of Thermal Oxidizers and its Applications

What is a thermal Oxidizer?

• A thermal Oxidizer is an essential part of chemical or thermal power plants to control pollution.
• It is also called the thermal incinerator
• Some waste gases (air pollutants) generated from chemical plants are harmful and need to be converted to other gases to avoid air pollution
• In thermal oxidizers, waste gases are decomposed at a high temperature. These gases can be exhausted into the atmosphere.
• The combustion process in thermal oxidizer destroys harmful ammonia, hydrocarbons (CxHyOz), volatile organic compounds (VOC), Hydrogen Sulphide (H2s), and other chemical compounds such as smoke particles and odor
• Thermal oxidization is called post-combustion treatment, to convert CO, and hydrocarbon (volatile HAP emissions) with combustion to carbon dioxide (CO₂) and water (H2O)
• They may be called afterburners, fume incinerators, or tail gas incinerators
• Thermal oxidizers are similar to catalytic oxidizers where a catalyst is used to accelerate the oxidation reaction.
• The thermal oxidizer is also called a thermal incinerator, direct-fired oxidizer, or afterburner

Working Principle of Thermal Oxidizer

1.  A pollutant containing VOCs with air is forced into an oxidizer using the system fan. A burner is used to increase the temperature of fumes. The air can be preheated directly or indirectly by entering to oxidizer
2. The flow of air passes through a heat recovery system (an air-to-air heat exchanger) to preheat the air before entering through a separate burner or combustion chamber. Thermal oxidizers operate with an integral heat exchanger for fuel savings to reduce operating cost
3.  The air is heated at a high temperature with strong turbulence to ensure the complete destruction of  VOC.
4.  Operating temperatures of gases are more than 1400°F, with dwell times around  1 second. This ensures the proper combustion of  VOCs into products (CO₂ and H₂O)
5. The hot and clean air passes through the hot pass of the heat recovery (heat exchanger)
6. Low temperature and clean air are allowed to be exhausted into the atmosphere.

Major Components of Incinerator

Thermal oxidizers comprise the following major components:

• Combustion chamber
  • Combustion of waste gases is carried out in the combustor
  • The inner surface has a high-temperature refractory lining
• Waste /Air Pollutants Feed system
  • This consists of inlet piping with isolation and control valves, flame arrester, disentrainment, or water liquid-seal drums.
• Air inlets
  • For the combustion process of waste gases, the air is supplied using blowers
  • The correct amount of air must be supplied to an incinerator in order to achieve complete combustion of off-gases
  • Sufficient turbulence or vortex in airflow may help fast combustion of waste gases
• Waste burner
  • Burners are provided to assist combustion of waste gases in the incinerator
  • The burner consists of an air inlet and fuel pipes with pilot
  • Fuel piping trains to burner and pilot, including the inlet piping with isolation and control valves
• Observation ports
  • It is to check the performance and monitoring of the incinerator
• Stack
  • The gases from the incinerator are exhausted using the stack
• Secondary recovery System
  • Some thermal Oxidizers include a secondary recovery system at exhaust for toxic components such as chlorine, sulfur, and particulate matter
• Choke plate
  • A choke plate can be added to increase the residence time of air pollutants for proper combustion in the incinerator
• Flame safeguard, temperature, over the fire, fuel, and air controls.

Type of Thermal Oxidizers

• Regenerative Thermal Oxidizer (RTO)
• Thermal Oxidizers without heat recovery
• Thermal Oxidizers with heat recovery for processing of hot oil, water and steam
• Thermal Oxidizers with preheating of water gas steam (Recuperative Oxidizer)
• Catalytic Thermal Oxidizer (CTO)

Regenerative Thermal Oxidizer (RTO)

• This is one of the widely used air pollution control technology in many industries
• They are versatile and extremely efficient as heat recovery efficiency can reach up to 97%. This is possible with heat storage using dense ceramic stoneware. Regenerative Thermal Oxidizers are an ideal choice for low VOC concentrations and during long continuous operations.
• Regenerative Thermal Oxidizers burn volatile organic compounds (VOCs) and other hazardous air pollutants (HAPs), which are created through main combustion (chemical processes) in thermal power plants as industrial exhaust steams
• To achieve this extremely high heat, about 1500°F is used to destroy the exhaust dangerous pollutants like VOC
• Regenerative Thermal Oxidizer is generally designed for large volumes, and low VOC concentration air pollution applications using ceramic media
• Recuperative thermal oxidizers use metallic shells and tube heat exchangers with primary and secondary heat.
• In this configuration, among other distinctions, the outgoing clean process stream of the secondary heat exchanger process is routed to another part of the plant for its use or back to the process itself
• It has lower operating costs compared to other thermal oxidizers for a high airflow rate with low volatile organic compound (VOC) or fume streams
• Rather than allowing only the clean hot air to be exhausted  into the atmosphere, the RTO unit captures up to 97% of the heat prior to exhausting it to the atmosphere.

 

Direct Fired Thermal Oxidizers (Afterburners)

• A direct-fired thermal oxidizer (DFTO)r is the simplest form of thermal oxidation in which a process stream is added with a burner. Residence time is provided in order to get the desired destruction removal efficiency of VOCs
• It is also called afterburners. The capital cost is less if such thermal oxidizers are correctly installed
• Afterburners have many industrial applications where a high concentration of VOCs is considered as a fuel source rather than natural gas, oil, or some other form of fuel for complete combustion for the desired operating temperature.

• Direct Fired Thermal Oxidizers (DFTO) can use natural draft or forced draft burners to maintain a specific destruction temperature and residence time to ensure thorough destruction efficiency (DE) up to 99%
• This type of oxidizer is ideal for those who want low emissions, reduced flame visibility, lower heat and noise, and smokeless combustion.

Thermal Recuperative Oxidizers

• This thermal oxidizer is sometimes used to burn waste gases with recuperative methods
• The combustion chamber is connected with  a heat exchanger  which has two parts, primary and secondary exchanger
• A primary heat exchanger is used to preheat the incoming unclean air by recuperating heat from the exiting hot clean air.  This is achieved by a shell and tube Or plate  type heat exchanger
• As the incoming air passes on one side of the metal tube or plate, hot clean air from the combustion chamber passes on the other side of the tube or plate and heat is transferred to the incoming air through the process of conduction using the metal as the medium of heat transfer
•   In the secondary heat exchanger, a similar concept is used for heat transfer, but the air heated by the outgoing hot clean process stream is returned to another part of the plant which may be taken back to the process oven or furnace.

 

 

Catalytic Oxidizers (CATOX)

• Air pollution treatment is carried out in The catalytic thermal Oxidizers for a wide range of temperatures custom-designed to treat an assortment of VOCs
• Catalytic oxidation occurs through a chemical reaction between the VOC hydrocarbon molecules and a precious or base metal catalyst bed that is internal to the oxidizer system
• A catalyst accelerates the rate of a chemical reaction during combustion in oxidizers, allowing the reaction to occur in a normal temperature range of 500°F – 700°F

 

• A catalytic thermal oxidizer ( by Konokogs Inc) is presented below

 

Catalytic Recuperative Oxidizers

• A catalytic recuperative oxidizer is a type of Oxidizer that is proficient at destroying volatile organic compounds (VOCs), odorous compounds, and other air pollutants.
• This oxidizer processes hot gases that are heated and forced into a heat exchanger toward the oxidizer. The waste gases are processed and it is heated to the catalyst operating temperature at the burner
• As this heated gas passes through the catalyst the heat releasing action takes place and the pollutants can be converted to heat, water vapor, and carbon dioxide.

Key factors for the Design of Thermal Oxidizer

• The Temperature of Flue Gases: The temperature of oxidizers should be high enough to ignite the organic constituents (VOCs) in the waste stream. The maximum temperature of the combustor is maintained from 1200° F to 1600°F
• Residence time: There should be sufficient time for the combustion reaction to occur.
• Turbulence: Strong mixing of the combustion air with the waste gas is the essential combustion chamber
• Saving fuel by heat recovery:
  • To reduce the amount of fuel for oxidation, thermal oxidizers  need to have some form of heat recovery
  • The rate of waste heat recovery generally increases with a decrease in inlet concentration of VOC or HAP
  • Waste heat recovery can be either recuperative or regenerative
  • In a recuperative thermal oxidizer, heat is recovered by passing the high-temperature exhaust gases through a non-contact (indirect) air-to-air heat exchanger, to heat the incoming cold air to the oxidizer
  • In regenerative heat thermal oxidizer, both hot exhaust gases and cool gases are alternatively passed through a fixed bed of a ceramics channel.
• Capital and Operating costs
  • shipping, installation, and maintenance cost
  • Fuel, electricity, and compressed charges

Other Options to reduce pollution without Thermal Oxidizer

The following options can be used to control pollution

• Installation of Low NOx Burner in the furnace
• Secondary heat recovery systems: waste heat boilers, steam generators, hot oil heaters, and other custom heat recovery modules
• Continuous Emission Monitoring (CEM) equipment
• Acid flue gas scrubbers
• Engineered turnkey solutions

Scope of CFD Modeling 

• For combustion modeling, we need to understand  the following subject
  • Basic of combustion
  • Turbulent non-premixed combustion
  • Multiphase turbulent combustion
• CFD modeling can be used to optimize the performance of thermal oxidizer
  • Mixing time of fuel of air
  • turbulence level
  • Temperature
• Velocity and temperature contours are given for thermal oxidizers

 

• The temperature of waste gases is increased due to the high-temperature gases of the burner. This ensures the complete burning of waste gases.