Oxidizers

Chapter 1: What are Oxidizers?

Oxidizers, or incinerators, are equipment used to treat waste gas or plant emissions that contain harmful pollutants by thermally decomposing them into simpler, stable compounds. Oxidizers act like burners or reactors where the stream of preheated waste gas is oxidized at temperatures up to 1832 °F (1,000 °C).

Waste gas, such as volatile organic compounds (VOCs), organic hazardous air pollutants (HAPs), or odors, are combusted into carbon dioxide and water vapor. Waste gasses contain non-organic air pollutants, such as halogenated and sulfuric compounds, and the combustion products include acidic gasses that contribute to smog formation and acid rain. Oxidizers, such as scrubbers, are used to remove the dangerous compounds and release clean air or vapors.

The US Environmental Protection Agency (EPA) regulates the emission of harmful compounds as stipulated under the Clean Air Act (CAA). The agency requires industrial facilities to install pollution controls to meet the specific emission limits stated in the CAA. Industrial plants that emit harmful air compounds are oil refineries, coal-fired and gas power plants, chemical plants, cement plants, steel mills, glass factories, and so on.

Chapter 2: Air Pollutants from Industrial Gas

Air pollutants are substances suspended in the atmosphere that can cause damage to people‘s health, environment, and property. These can be categorized as hazardous air pollutants, criteria air pollutants, and greenhouse gases. Hazardous air pollutants are the most dangerous among the three. Even in low concentrations, they can lead to serious health concerns or even death. Industries have been developing and implementing methods to eliminate hazardous pollutants and convert them into less dangerous compounds.

• Hazardous Air Pollutants (HAPs): Compared to other air pollutants, they are typically present in localized, small concentrations but pose serious harm to health and the environment. HAPs, or toxic air pollutants, are air pollutants known to cause health effects such as cancer, reproductive diseases, and birth defects. They can leach into the environment by being taken up by plants and animals through digestive processes. Currently, about 187 listed hazardous pollutants under the CAA, some of which belong to the category of VOCs.
  
  

  

  
  

  Votatilte organic compounds (VOCs) are organic compounds that vaporize easily at room temperature and atmospheric pressure. They tend to form photochemical smog through reaction with nitrogen oxide. Some VOCs can cause eye irritation, respiratory problems, and cancer. VOCs can be produced biogenically or artificially. Biogenic volatile organic compounds, or BVOCs are produced from the metabolism of plants, animals, and microorganisms, making up a larger fraction of the VOCs in the atmosphere. The formation of BVOCs is largely controlled by ambient temperature and remains balanced in an undisturbed ecosystem. Artificial or anthropogenic VOCs are directly or indirectly made by humans and are highly concentrated in urban regions. The main sources of anthropogenic VOCs are industrial equipment, automobiles, and solvents from both industrial use and household items.

• Criteria Air Pollutants: Criteria air pollutants, or common air pollutants, include ground-level ozone, particulate matter, carbon monoxide, lead, sulfur dioxide, and nitrogen dioxide. These are pollutants that are generally distributed across a particular region. The main effect of criteria air pollutants is that they degrade the air quality of the affected regions, which over time produces negative effects on health and the environment. As mandated by the CAA, EPA sets air quality standards through the National Ambient Air Quality Standards. State and local agencies are required to provide and implement strategies to ensure that emissions in their regions fall within the standardized emission levels.
  
  

  

  
  
• Greenhouse Gases: These are gases that absorb radiant energy from the sun while preventing the energy reflected by the earth‘s surface from escaping. This abnormal heating of the atmosphere is known as the greenhouse effect. Common greenhouse gases are carbon dioxide, methane, ozone, and water vapor. Methane is a greenhouse gas present in large amounts due to emissions from petroleum refining, power generation, and agricultural practices. Compared to carbon dioxide, it has a global warming potential of 34, carbon dioxide is the baseline, with a value of 1.
  
  

  

  
  

GET YOUR COMPANY LISTED BELOW

Chapter 3: Oxidizer Process Description

Thermal oxidizers break down organic or hydrocarbon-based HAPs and VOCs into carbon dioxide and water. The process starts by introducing the waste gas stream into the combustion chamber with air supplied by a high-draft fan. The supplied air colume is controlled at sufficicient levels to burn the combustible compounds. Aside from creating complete combustion, the air is supplied to dilute the waste gas stream to safe levels. For safe operation, the lower explosive limit (LEL) at the combustion chamber must not exceed 25% unless a capable concentration monitoring system is in place, where the maximum LEL can be increased to 50%. Reaching the LEL of the waste gas compounds can result in the explosion of the chamber and the vent system.

For safe operation, the concentration of combustible gas should not exceed 25% of the lower explosive limit (LEL) in the combustion chamber. With upstream monitoring, sometimes excursions up to 50% can be allowed.

After the waste gas stream is diluted, it is ignited in the combustion chamber. A piloted burner or igniter initiates the combustion of the gas. Since the gas is diluted with air, the generated heat is low. If the combustion is not self-sustaining, additional auxiliary fuel is injected into the system to maintain chamber temperatures.

Some types of oxidizers do not need auxiliary fuel for continuous burning. For example, catalytic oxidizers use catalyst media to aid the reaction, a material that accelerates the rate of a chemical reaction without being spent. This results in a lower operating temperature than a thermal oxidizer and less fuel use. In some cases, air-to-air heat exchangers are used to preheat the inlet air with the hot outlet treated exhaust gas to save more energy.

Another way to improve the temperatures inside the chamber without using too much auxiliary fuel is by utilizing the exhaust heat from the chamber. After combustion, the exhaust gasses contain high heat energy, which will be wasted when directly released. Heat exchangers transfer heat from the exhaust air stream to the intake air stream. The preheated air that goes into the chamber requires less heat to ignite. Ceramic media situated within the combustion chamber can also preheat the air. The ceramic media absorbs heat from the previous reaction that took place and transfers it to the incoming stream of gasses.

The exhaust gasses are released into the atmosphere through a stack. A stack is usually constructed to create a natural draft to move air out of the combustion chamber. Along the stack are a series of probes for taking samples. The samples are processed by emissions monitoring systems.

Since the stream of waste gas can also contain acid-forming compounds and particulate matter, additional downstream or stream equipment is needed. To remove acid gasses, scrubbers, particularly wet ones, are a popular choice.

A wet scrubber introduces a scrubbing liquid into the waste gas stream. The contact is made by spraying the scrubbing liquid downward into the waste gas flowing from the bottom of the scrubbing vessel. Common equipment used to remove particulate matter are cyclones and electrostatic precipitators.

In cyclone separators, the gas stream is introduced tangentially to the inner walls of the cyclone. As the stream hits the wall, the gas changes direction and swirls around the chamber. The particles behave differently and are separated by falling instead of changing direction.

Wet electrostatic precipitators (WESP) are a particulate matter control method that uses an intense electric field to charge and capture particles and droplets onto a collection surface. A discharge electrode gives particles in a gas stream a negative charge as the gas flows through the collection section. With a negative charge, the particles are attracted to the grounded surface of the collection electrode. WESPs operate at low pressure drop and have over 90% collection removal efficiency.

The Three Ts of Oxidizer Design

The main factors in oxidizer design are temperature, (residence) time, and turbulence.

Temperature

Air-polluting compounds in waste gasses ignite at different temperatures. The combustion, or fire, chamber is kept at temperatures that exceed 1600 oF (870 °C). Thermocouples monitor the temperature and regulate the gas burner. Process temperatures are excessively high to ensure near destruction (99.9%) of the compounds for the residence time.

Operating temperatures vary according to the type of VOCs or pollutants present, with higher temperatures dependent on the destruction rate efficiency (DRE) and the compound being oxidized. All substances have an autoignition temperature, which must be exceeded in the thermal oxidation process to achieve the required DRE.

Temperature offers the highest increase in removal efficiency. Increasing it provides the best results for improving the DRE, typically to the fourth power.

Residence Time

Residence time means how long the waste gasses will remain in the combustion chamber. The required level for various VOCs must be achieved within the residence time, which is about 0.1 to 1 second. Lower chamber temperatures are possible if the residence time is increased, but the system's throughput normally suffers.

Shorter residence times require higher chamber temperatures. When residence time is decreased without increasing the chamber temperature, the DRE is significantly reduced, resulting in unignited VOCs. Typical VOC destruction efficiencies are 99.9%, with residence times of no more than one second and chamber temperatures of around 1650 to 2190 °F (900–1200 °C).

Turbulence

Turbulence increases the efficiency of the thermal oxidizer and ensures that all VOCs in the waste gas stream are burned. It is the chaotic flow of fluids that increases mixing and mass distribution and prevents gasses from collecting in dead regions. Turbulence is created by inducing a draft of air or water vapor directed tangentially into the combustion chamber. Various designs of thermal oxidizers have combustion chambers with geometries or internals intended to generate turbulence.

Chapter 4: Types of Thermal Oxidizers

Thermal oxidation is one of the techniques for VOC emission control. Other methods include adsorption, absorption, condensation, membrane filtration, and catalytic oxidation. Among these methods, thermal and catalytic oxidation are widely used due to their suitability for gaseous pollutants and high removal efficiency.

Thermal oxidizers mainly rely on the oxidation brought about by combustion. There are three main types of thermal oxidizers: direct-fired, regenerative, and recuperative. They differ in the method of heat utilization and heat recovery. In addition, there are other types, such as flameless thermal oxidizers and enclosed vapor combustion units.

• Direct-fired Thermal Oxidizers (DFTO): Direct-fired thermal oxidizers, also known as afterburners, are the simplest type of thermal oxidizers. They introduce the waste gas stream into the combustion chamber without preheating or heat recovery processes. After entering the firing chamber, the heated air remains for a pre-specified amount of time, referred to as the residence or dwell time.
  

  The firing chamber operates at 1800 oF up to 2200 oF (982–1204 °C) with an airflow rate of 500 cu ft up to 50,000 cu ft (14.2 to 1416 cu m) per minute, with emissions contained until the required thermal destruction rate efficiency (DRE) is reached. Once the DFTO processes the emissions, safe air and water vapor are released from the chamber. DFTOs have an efficiency rate for hydrocarbon destruction of 99% and achieve emission compliance with the least amount of capital investment.

  
  

  

  
  
• Regenerative Thermal Oxidizers (RTO): This is one of the most common types of thermal oxidizers. It uses multiple types and layers of ceramic beds inside the combustion chamber that absorb heat from the exhaust gasses. The ceramic beds are used alternately and undergo heating and regeneration cycles throughout their operation.
  

  The process starts by heating the incoming waste gasses directed by control valves. The intake temperature is then raised from ambient to near combustion temperatures. As most of the heat is absorbed by the incoming gasses, the ceramic bed becomes cooler, resulting in less heat transfer. The control valves then redirect the intake flow to another ceramic bed that has been previously heated. The cool ceramic bed undergoes a heat regeneration phase from the exhaust gasses, preparing it for another heating phase.

  
  

  

  
  

  Regenerative thermal oxidizers have thermal efficiencies of around 95% with destruction removal efficiencies of more than 99%. This results in less auxiliary fuel consumption and less heat released into the atmosphere.

  

  
  
• Thermal Recuperative Oxidizers (TRO): This is another type of thermal oxidizer that uses heat from the exhaust to preheat the incoming waste gasses. In contrast with regenerative thermal oxidizers, thermal recuperative oxidizers use heat exchangers instead of ceramic media. The process starts by elevating the temperature of the incoming waste gas through the heat exchanger. As the air and waste gas mixture is burned, it then passes through the other side of the heat exchanger before being released to the stack. The heat exchanger recovers heat from the exhaust, which in turn raises the temperature of the intake.

  The heat exchangers can be either plate or shell-and-tube heat exchangers. Thermal oxidizers with plate heat exchangers require lower investment and have higher thermal efficiency at lower operating temperatures. At higher operating temperatures, shell-and-tube heat exchangers are preferred. Thermal efficiencies of thermal recuperative oxidizers range from 70 to 80%.

  

  
  
• Flameless Thermal Oxidizers (FTO): This type of thermal oxidizer uses specially designed non-catalytic ceramic beds with good thermal and flow distribution properties. Unlike other thermal oxidizers, air and waste gasses are premixed before being introduced into the combustion chamber. Burners or previous reactions preheat the combustion. When the mixture of air and gasses reach the combustion chamber, they are ignited from the high temperatures. In cases where the exothermic reaction of the air and gasses are not enough, burners and electric heaters are used to heat the ceramic media to operating temperatures.

  

  
  
• Vapor Combustion Units (VCU): Vapor combustion units are enclosed flare systems. VCUs operate the same way as direct-fired thermal oxidizers. The only difference is that the waste gas stream contains little to no oxygen. Thus, the stream is not flammable until it reaches the combustion chamber, where it is mixed with air. The auxiliary fuel is also burned to maintain the temperature inside the combustion chamber.
  
  

  

  
  

Chapter 5: Catalytic Oxidizers

Catalyst oxidizers operate in the same way as thermal oxidizers but with the addition of a catalyst bed. The catalyst further enhances the oxidation of VOCs by increasing the reaction rate. This catalytic process increases the removal efficiency of the oxidizer and allows the combustion chamber to operate at lower temperatures. The downside of this process is the additional maintenance and replacement of the catalyst media due to the effects of degradation and sintering. In addition, some catalysts are deactivated in the presence of certain compounds or catalyst poisons, such as sulfides and halides.

In a catalytic oxidizer, the air and waste gas stream is drawn and preheated either through regenerative or recuperative methods. Like thermal oxidizers, catalytic oxidizers that use regenerative methods (alternating ceramic beds) of heat recovery are called regenerative catalytic oxidizers, while those that use recuperative methods (heat exchangers) are called recuperative catalytic oxidizers. The preheated stream is then ignited and burned in the combustion chamber. Unlike in thermal oxidizers, this initial burning is not the main means of destroying all of the VOCs; rather, this is needed only to increase the temperature to around 392 to 932 ° F (200 to 500°C), enough to initiate the catalytic reaction. The heated gases then pass through the catalyst bed, where they are further broken down to achieve a destruction removal efficiency of above 99%.

The main destruction of VOCs happens when they come into contact with the catalyst. As the stream passes through the catalyst bed, the VOCs are adsorbed on the catalyst. The catalyst's surface of the catalyst has active sites where atoms such as oxygen and hydrogen have high affinity. While on the active site, it is easier for the VOC compound to lose the bonds between its atoms that are attracted to the active sites of the catalyst. New and more stable bonds form, creating the products of the reaction. The formation of these products releases it from its attachment to the catalyst, freeing up the active site. This results in lesser heat required to facilitate oxidation, and, in turn, better destruction efficiency.

The type of catalyst used largely depends on the types of VOCs and contaminants present in the waste gas stream. A catalyst can be selective in that it facilitates a reaction well for certain compounds while being weak for others. That is why in some systems, catalysts are combined to create a synergistic effect to improve the overall performance of the oxidizer. Catalysts can be classified as metal oxides or noble metals.

• Metal Oxides: Metal oxides are generally cheaper but less efficient than noble metal catalysts. They can be single or mixed depending on the required activity and selectivity for removing certain waste gas compositions. The most widely used metal oxide catalyst is manganese oxide which can oxidize ethanol, acetone, propane, propene, ethyl acetate, hexane, benzene, and toluene. Manganese oxide is usually combined with other catalysts, such as cerium, cobalt, and titanium oxides, to improve its selectivity.
• Noble Metals: These catalysts are more common due to their efficiency but much more expensive than metal oxides. They are usually combined with metal oxides, which act as supports or carriers for the noble metal active phase. Noble metal catalysts can also be mixed for better removal efficiency. Common noble metals used are platinum, palladium, and gold.
  
  

  

  
  

  Catalyst systems can also be categorized according to their method of contacting the gas stream. The catalyst must have a shape and distribution that can maximize the contact of the active sites to the VOCs in the stream, especially if the VOC concentration is small and the gas mixture flow rate is high. The methods of enabling catalyst contact are enumerated below.

  • Fixed-bed Monolithic Catalysts: This is the most common method of contacting the gas stream with the catalyst. A monolithic catalyst is a catalyst with active sites supported by either a metallic or ceramic substrate. The substrate has a porous honeycomb structure composed of microscopic parallel channels with thin walls. On the surface of the honeycomb are deposits of the main catalyst, which contact the gas stream as it passes through the microscopic channels. Fixed-bed monolithic catalysts are characterized as having low attrition and low-pressure drop.
    
    

    

    
    
  • Packed-bed Catalysts: In this type, the catalysts are in pellet form packed in a tube or perforated shallow trays where gases pass through. Catalyst structures are available in various shapes, such as spheres, cylinders, cubes, and lobules. Finer catalyst structures are available in the form of particles where their size is in the order of around a millimeter. Particulate catalyst is preferred over the pelletized ones due to their better efficiency, although it's at the expense of a higher pressure drop.

    

    
    
  • Fluidized-bed Catalysts: Fluidized-bed reactors are catalytic systems where particulate catalysts are suspended and swirled by the flow of gases coming from the bottom of the reactor. Initially, the catalyst is supported by a porous plate. This porous plate, known as the distributor, allows the flow of gases to suspend the catalyst. Fluidization is achieved when the gas velocity is enough to counter the weight of the particle. The main advantage of a fluidized-bed reactor is the high heat transfer rate that which allows the processing of VOCs with high heating values without subjecting the catalysts and their structure to high temperatures.

    

    
    

Chapter 6: Oxidizers Selection

When air pollution regulations went into effect, it became difficult to find an effective gaseous emissions control system. With technological advances and process developments, systems for the processing of gaseous emissions have radically improved. Today, there are a wide variety of available options designed to meet regulatory requirements.

Direct Thermal Oxidizers

The considerations for choosing a direct thermal oxidizer include capital outlay, operational costs, and safety. Direct thermal oxidizers are typically used for processes that have:

1. Low inlet volume that is less than 1,000 standard cubic feet (28.3 cu m) per minute
2. High concentrations of VOCs but lower LEL
3. Particulates in the process gas
4. Temperatures over 600 ° F (315 ° C)

Recuperative Thermal Oxidizers

Recuperative thermal oxidizers have high operating costs with higher NOx emissions. They are good for processes that have high VOC streams, a small air flow rate, and batch-type cycling. They are typically used for processes with:

1. Process gas volume from 500 SCFM to 30,000 SCFM
2. VOC concentrations of 10% to 25% of the LEL
3. Particulates in the process gas or after combustion of the VOC
4. Temperatures up to 600 °F (315 °C)

Direct Catalytic Oxidizers

Direct catalytic oxidizers have the same design as direct thermal oxidizers and are used with processes that have:

1. Gas volume range from 500 SCFM to 30,000 SCFM
2. VOC, NOx, or Ammonia in the air stream with concentration ranges of 0% to 15% of the LEL
3. No particulates, heavy metals, or silicone in the process gas
4. Temperatures up to 800 °F (427 °C)

Recuperative Catalytic Oxidizers

The advantages of recuperative catalytic oxidizers are lower fuel usage and cost-effective construction materials. They are not used if there are catalyst poisons in the system, but they are used for processes that have:

1. Process volume ranging from 500 SCFM to 30,000 SCFM
2. Concentrations ranging from 0% to 15% of the LEL
3. No particulates, heavy metals, or silicones
4. Temperatures up to 400 °F (204 °C)

Regenerative Thermal Oxidizers

Regenerative thermal oxidizers have lower capital and operational costs with higher airflow and low VOC concentrations, with thermal effectiveness of 97%. They are used with processes that have:

1. Process volume ranges of 2,000 SCFM to 80,000 SCFM
2. VOC concentrations with ranges of 0% to 15% of the LEL
3. Clean or low particulates
4. Temperatures up to 500 °F (260 °C)

Other Key Factors

1. Air Pollution Control Equipment: A complete understanding of all forms of oxidizer equipment is essential for making the right choice to fit an application. Every manufacturer and producer has proprietary technology specifically suited to their products that needs to be examined to find the right fit for an oxidizer system.
2. Custom Engineering: This is a major selling point for oxidizer producers. They work closely with customers to engineer a solution that meets each customer's individual needs.
3. Cost: Manufacturers have the same concerns for cost as their customers and work closely to develop a system with a total cost that fits a customer’s needs but is still in compliance with environmental regulations. The greatest cost of an oxidizer system comes from the energy necessary to operate it.
4. Controls: The most effective and robust oxidizers have easy-to-operate and automatically controlled systems with advanced technologies for performance efficiency.
5. Maintenance and Support: Manufacturers in the oxidizer industry proudly invest in personnel and services that guarantee customers the efficient and consistent operation of their oxidizer system.

Conclusion:

• Oxidizers, or incinerators, are pieces of equipment used to treat waste gas or plant emissions that contain harmful pollutants by thermally decomposing them into simpler, more stable compounds.
• Air pollutants are substances suspended in the atmosphere that can cause damage to people‘s health, environment, and property. These can be categorized as hazardous air pollutants, criteria air pollutants, and greenhouse gases.
• VOCs are votatile organic compounds that vaporize easily at room temperature and atmospheric pressure. Some VOCs can cause health problems such as eye irritation, respiratory problems, and cancer.
• Thermal oxidizers mainly rely on the oxidation brought about by combustion. There are three main types of thermal oxidizers. These are three main types of thermal oxidizers: direct-fired, regenerative, and recuperative.
• Catalyst oxidizers operate in the same way as thermal oxidizers, but with the addition of a catalyst bed. The catalyst further enhances the oxidation of VOCs by increasing the reaction rate.

GET YOUR COMPANY LISTED BELOW