Oxidizers

Chapter 1: What are Oxidizers?

Oxidizers, also known as incinerators, are devices used to process waste gases or plant emissions containing harmful pollutants by thermally breaking them down into simpler, stable compounds. They function like burners or reactors, where the preheated waste gas is oxidized at temperatures reaching up to 1832°F (1,000°C).

Waste gases containing volatile organic compounds (VOCs), organic hazardous air pollutants (HAPs), or odors are burned to produce carbon dioxide and water vapor. These gases may also include non-organic pollutants such as halogenated and sulfuric compounds. The combustion process generates acid gases, which can contribute to smog and acid rain. Similar to scrubbers, oxidizers eliminate harmful compounds and release cleaner air or vapors.

The US Environmental Protection Agency (EPA) regulates the emission of harmful compounds under the Clean Air Act (CAA). The EPA mandates that industrial facilities install pollution control systems to comply with the emission limits outlined in the CAA. Industries that must adhere to these regulations include oil refineries, coal-fired and gas power plants, chemical plants, cement plants, steel mills, and glass factories.

Chapter 2: What are the air pollutants produced by industrial gas?

Air pollutants are substances suspended in the atmosphere that can harm human health, the environment, and property. They are categorized into three types: hazardous air pollutants, criteria air pollutants, and greenhouse gases. Among these, hazardous air pollutants are the most dangerous, as even at low concentrations, they can cause severe health issues or even death. To address this, industries have been developing and implementing technologies to remove hazardous pollutants and transform them into less harmful 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 are 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, there are 187 hazardous pollutants listed under the CAA, some of which belong to the category of VOCs.

  

  
  
• Volatile organic compounds (VOCs): are organic compounds that vaporize easily at room temperature and atmospheric pressure. They tend to form photochemical smog through a 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 must provide and implement strategies to ensure that emissions in their regions fall within the standardized emission levels.

  

  
  
• Greenhouse Gasses: These are gasses 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 gasses 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, the baseline with a value of 1, methane has a global warming potential of 25.

  

  
  

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Chapter 3: What is the process of using an oxidizer?

Thermal oxidizers decompose organic or hydrocarbon-based hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) into carbon dioxide and water. The process begins by directing the waste gas stream into the combustion chamber, where air is typically introduced via a forced draft fan. This air is controlled to ensure that the combustible compounds are fully burned. Additionally, the air helps to dilute the waste gas stream to safe levels.

To ensure safe operation, the concentration of combustible gas in the combustion chamber should not exceed 25% of the lower explosive limit (LEL). However, with proper upstream monitoring, excursions up to 50% of the LEL may be permitted.

A pilot burner or igniter starts the combustion of the fuel gas in the thermal oxidizer burner. The thermal oxidizer is then heated to its specified operating temperature. Depending on the amount of combustible material in the waste gas stream, the incoming waste gas may release enough heat to sustain the process. If the combustion is not self-sustaining, additional auxiliary fuel is introduced to maintain the required chamber temperatures.

The amount of auxiliary fuel required can differ among various types of oxidizers. For instance, catalytic oxidizers utilize catalyst media to facilitate the reaction. A catalyst speeds up a chemical reaction without being consumed in the process. This allows catalytic oxidizers to operate at lower temperatures compared to thermal oxidizers, resulting in reduced fuel consumption.

Another method to maintain chamber temperatures while minimizing the use of auxiliary fuel is to utilize the heat from the exhaust gases. After combustion, the exhaust gases carry significant heat energy that would otherwise be wasted if released directly. Heat exchangers capture and transfer this heat from the exhaust air stream to the incoming air stream. This preheated air requires less energy to ignite when entering the chamber. Additionally, ceramic media within the combustion chamber, as seen in regenerative thermal oxidizers (RTOs), can also preheat the air. The ceramic media absorbs heat from previous reactions and transfers it to the incoming gas stream.

The exhaust gasses are released into the atmosphere through a stack. A stack is usually constructed to create a means of discharging hot flue gas to an atmospheric chamber. Along the stack are a series of probes for taking samples. The samples are processed by emissions monitoring systems.

If the waste gas stream contains acid-forming compounds and particulate matter, additional downstream equipment may be necessary. Wet scrubbers are commonly used to remove acid gases.

A wet scrubber introduces a scrubbing liquid into the waste gas stream. The contact is made by spraying the scrubbing liquid down 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. This causes the gas to change direction and swirl around the chamber. Centrifugal force then causes the particles to be separated from the gas, with the particles collecting at the bottom of the conical section of the separator.

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 key factors in oxidizer design are temperature, residence time, and turbulence.

Temperature

Air-polluting compounds in waste gasses ignite at different temperatures. To accommodate this, the combustion, or oxidation, chamber is kept at temperatures exceeding 1600°F (870°C). Thermocouples monitor the temperature as controls modulate the gas burner. The process temperatures are high enough to ensure high levels of VOC destruction.

Operating temperatures in oxidizers can vary depending on the type and concentration of VOCs. Higher temperatures generally result in greater destruction and removal efficiency (DRE) of the oxidized compounds. To achieve the desired DRE, the thermal oxidation process must exceed the autoignition temperature of the substances involved.

Residence Time

Residence time refers to the duration that waste gases remain in the combustion chamber. For various VOCs, the required destruction and removal efficiency (DRE) must be achieved within a residence time of approximately 0.1 to 1 second. While increasing residence time can allow for lower chamber temperatures, it is important to balance this with considerations of capital and operating costs.

Shorter residence times necessitate higher chamber temperatures. If residence time is reduced without raising the chamber temperature, the destruction and removal efficiency (DRE) is significantly compromised, leading to unburned VOCs. Typically, VOC destruction efficiencies reach 99.9% with residence times of no more than one second and chamber temperatures ranging from 1650 to 2190°F (900 to 1200°C).

Turbulence

Turbulence enhances the efficiency of a thermal oxidizer by ensuring that all VOCs in the waste gas stream are fully burned. Chaotic fluid flow improves mixing and mass distribution, preventing gases from accumulating in stagnant areas. Many thermal oxidizer designs incorporate specific chamber geometries to promote turbulence and optimize the combustion process.

Chapter 4: What are the different types of thermal oxidizers?

Thermal oxidation is a widely used method for controlling VOC emissions. Alternative methods include adsorption, absorption, condensation, membrane filtration, and catalytic oxidation. Thermal and catalytic oxidation are especially preferred for their high effectiveness in treating gaseous pollutants and achieving high removal efficiencies.

Thermal oxidizers primarily rely on combustion for the oxidation process. The three main types are direct-fired, regenerative, and recuperative thermal oxidizers, each differing in their methods of heat utilization and recovery. Additionally, there are flameless thermal oxidizers and enclosed vapor combustion units, which provide alternative approaches for managing and treating waste gases.

• Direct-Fired Thermal Oxidizers (DFTO): Direct-fired thermal oxidizers, also known as afterburners, are the simplest type of thermal oxidizer. They introduce the waste gas stream into the combustion chamber without preheating or heat recovery. 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 typically operates from 1400°F to 2200°F (760–1204°C) with an airflow rate of 500 cu ft up to 50,000 cu ft (14.2 to 1416 cu m) per minute, with the DFTO designed for the necessary residence time to achieve the required DRE. 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 as high as 99.9999% and achieve emission compliance with the least amount of capital investment.

  

  
  
• Regenerative Thermal Oxidizers (RTO): RTOs are 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 cooling cycles throughout their operation.

  The process starts by heating the incoming waste gasses across ceramic heat recovery media. The gas 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 heating phase from the exhaust gasses, preparing it for another heating phase.

  

  
  

  Regenerative thermal oxidizers have thermal efficiencies of around 92-95%, with destruction removal efficiencies of more than 95%. 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 metallic 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. However, at higher operating temperatures, shell-and-tube heat exchangers are preferred. Thermal efficiencies of thermal recuperative oxidizers range from 50% 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: What are catalytic oxidizers, and how do they function?

Catalytic oxidizers function similarly to thermal oxidizers but include a catalyst bed that accelerates the reaction rate of VOCs at a given temperature. This allows catalytic oxidizers to achieve removal efficiencies comparable to thermal oxidizers, but at lower temperatures. However, this process comes with the drawback of requiring additional maintenance and replacement of the catalyst media, which can degrade or sinter over time. Additionally, some catalysts can become 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 are drawn in and can be preheated using either regenerative or recuperative methods. Regenerative catalytic oxidizers use alternating ceramic beds for heat recovery, while recuperative catalytic oxidizers utilize heat exchangers. After preheating, the stream is ignited and heated in the combustion chamber. Unlike thermal oxidizers, the primary function of this initial heating is not to destroy all VOCs but to raise the temperature to approximately 392 to 932°F (200–500°C) to initiate the catalytic reaction. The heated gases then pass through the catalyst bed, where they are further broken down, achieving a destruction removal efficiency of over 95%.

The primary destruction of VOCs in a catalytic oxidizer occurs when they contact the catalyst. As the stream flows through the catalyst bed, VOCs are adsorbed onto the catalyst's surface, which contains active sites with a high affinity for atoms like oxygen and hydrogen. At these active sites, VOC compounds more easily break the bonds between their atoms, as these bonds are attracted to the catalyst. New, more stable bonds form, resulting in the reaction products. This process releases the VOCs from the catalyst, freeing up the active sites. Consequently, less heat is needed to facilitate oxidation compared to thermal oxidizers.

The choice of catalyst in a catalytic oxidizer depends largely on the specific VOCs and contaminants present in the waste gas stream. Catalysts can be selective, efficiently facilitating reactions for certain compounds while being less effective for others. To enhance overall performance, some systems use combinations of catalysts to achieve a synergistic effect. Catalysts are generally classified into two categories: metal oxides and 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 are 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 and palladium.

  

  
  

  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 has 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 into a tube or shallow perforated trays where gasses pass through. Catalyst structures are available in various shapes, such as spheres, cylinders, cubes, and lobules. Finer catalyst structures are particles around a millimeter large. Particulate catalysts are preferred over pelletized ones due to their better efficiency, though 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 gasses 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 gasses 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, which allows the processing of VOCs with high heating values without subjecting the catalysts and their structure to high temperatures.

  

  
  

Chapter 6: How do you select the appropriate oxidizer?

When air pollution regulations were first introduced, identifying an effective system for controlling gaseous emissions was challenging. However, advancements in technology and process development have significantly improved these systems. Today, there are a variety of options available that are designed to meet regulatory requirements.

Direct Thermal Oxidizers

When choosing a direct thermal oxidizer, considerations include capital investment, operational costs, and safety. Direct thermal oxidizers are typically used for processes that involve:

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

Recuperative Thermal Oxidizers

Recuperative thermal oxidizers have higher capital costs than DFTOs but lower fuel consumption. They are good for processes with high VOC levels, small air flow rates, and batch-type cycling. They are typically used with processes that have:

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 share a similar design with direct thermal oxidizers but use catalysts to lower the operating temperature. They are employed in processes that involve:

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

Recuperative Catalytic Oxidizers

The advantages of recuperative catalytic oxidizers include reduced fuel consumption and cost-effective construction materials. They are not suitable for systems with catalyst poisons but are used in processes involving:

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 offer lower capital and operational costs, higher airflow, low VOC concentrations, and thermal effectiveness ranging from 92% to 95%. They are suitable for processes with:

1. Process volume ranges from 2,000 SCFM to 80,000 SCFM
2. VOC concentrations ranging from 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 for 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 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 and still complies with environmental regulations. The greatest cost of an oxidizer system sometimes comes from the energy necessary to operate it.
4. Controls: The most effective and robust oxidizers have easy-to-operate, 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 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 gasses.
• VOCs are volatile 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: 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.

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