Oxidizers

Chapter 1: What is an Oxidizer?

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, many stable compounds. Oxidizers act like burners or reactors where the stream of preheated waste gas oxidized at temperatures up to 1,000°C. At these temperatures, the waste gas containing air pollutants such as volatile organic compounds (VOCs) or organic hazardous air pollutants (HAPs) or odors are combusted into carbon dioxide and water vapor. For waste gases that contain non-organic air pollutants such as halogenated and sulfuric compounds, the products of combustion include acidic gases. Acid gases contribute to the formation of smog and acid rain. Acid gas removal systems such as scrubbers are installed to mitigate the problem.

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

Air pollutants are substances suspended in the atmosphere that can cause damage to peoples‘ 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, there are about 187 listed hazardous pollutants under the CAA, some of which belong to the category of VOCs.
  
  

  

  
  

  VOCs are organic compounds that vaporize easily at room temperature and atmospheric pressure. They have a tendency to form photochemical smog through reaction with nitrogen oxide. In itself, 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 which make up for the larger fraction of the VOCs present in the atmosphere. The formation of BVOCs is largely controlled by ambient temperature and is balanced in an undisturbed ecosystem. Artificial or anthropogenic VOCs are directly or indirectly made by humans which are highly concentrated in urban regions. 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 produce 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 assure 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 reflected energy 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 in which carbon dioxide is the baseline with a value of 1.
  
  

  

  
  

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Chapter 3: Oxidizer Process Description

Thermal oxidizers are used to 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 volume of supplied air is controlled at levels sufficient 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.

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 from the reaction is low. In cases where the combustion is not self-sustaining, additional auxiliary fuel is injected into the system to maintain chamber temperatures. There are types of oxidizers that do not need auxiliary fuel for continuous burning. These self-sustaining oxidizers, known as catalytic oxidizers, use catalyst media to aid the reaction. A catalyst is a material that accelerates the rate of a chemical reaction without being spent. This results in a lower operating temperature than that of a thermal oxidizer.

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 gases contain high heat energy which will be wasted when directly released. Heat exchangers are used to 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. Air can also be preheated by using ceramic media situated within the combustion chamber. The ceramic media absorbs heat from the previous reaction that took place and transfers it to the incoming stream of gases.

The exhaust gases 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. For removing acid gases, scrubbers, particularly wet scrubbers, are a popular choice. A wet scrubber works by introducing a scrubbing liquid into the stream of waste gas. The contact is made by spraying downward the scrubbing liquid into the waste gas flowing from the bottom of the scrubbing vessel. To remove particulate matter, the common equipment used are cyclones and electrostatic precipitators. In a cyclone separator, the gas stream is introduced tangentially to the inner walls of the cyclone. As the stream hits the wall, the gas changes its direction and swirls around the chamber. The particles, meanwhile, behave differently and are separated by falling down instead of changing direction. Electrostatic precipitators, on the other hand, use charged screens or electrodes. As the particles suspended in the gas pass the first screen, they pick up the charge from the electrode which is a negative charge. Upon reaching the second screen, the negatively charged particles are attracted to the screen which has a positive charge.

Factors in Oxidizer Design

The three main factors in designing oxidizers are temperature, residence time, and turbulence. Common air-polluting compounds found in waste gases ignite at different temperatures as shown in the table below.

The values above are the autoignition temperatures only. Process temperatures of thermal oxidizers need to be at a much higher level to ensure near destruction (99.9%) of the compounds for a given residence time. Operating temperatures of thermal oxidizers can reach 1,500°C depending on the type of VOCs or pollutants present.

Residence time is the duration of transit of the waste gases through the combustion chamber. The required level of 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; however, the throughput of the system suffers. The shorter the residence time, the higher the required chamber temperature. If the residence time is decreased without increasing the chamber temperature, the destruction efficiency is reduced which results in more unignited VOCs. Typical VOC destruction efficiencies are 99.9% with a residence time of no more than one second and chamber temperatures of around 900 to 1,200°C.

Turbulence is another factor to increase the efficiency of the thermal oxidizer which is created to ensure that all VOCs in the waste gas stream are burned. Turbulence is the chaotic flow of fluids that promotes better mixing and mass distribution within the chamber. This prevents any dead regions where gases are not burned efficiently. Turbulence is created by forcing or inducing a draft of air or water vapor directed tangentially into the combustion chamber. Other thermal oxidizers have combustion chambers with geometries or internals intended to generate turbulence.

Temperature offers the highest increase in removal efficiency. Increasing the purification temperature provides the best method for improving DRE and typically increases it to the fourth power.

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. These are direct-fired, regenerative, and recuperative. They differ in the method of heat utilization and heat recovery. Other types exist 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 which involves introducing the waste gas stream into the combustion chamber without preheating or heat recovery processes. This type is sufficient to achieve emission compliance with the least capital investment.
  
  

  

  
  
• Regenerative Thermal Oxidizers (RTO): This is one of the most common types of thermal oxidizers which uses multiple types and layers of ceramic beds inside the combustion chamber that absorb heat from the exhaust gases. The ceramic beds are used alternately and undergo heating and regeneration cycles throughout their operation. The process starts by heating the incoming waste gases 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 gases, 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 gases which prepares 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 gases. 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 oxidizers use specially designed non-catalytic ceramic beds that have good thermal and flow distribution properties. Unlike other thermal oxidizers, air and waste gases are premixed before being introduced in the combustion chamber. The combustion is preheated by burners or the previous reactions. When the mixture of air and gases reach the combustion chamber, they are ignited due to high temperature. In cases where the exothermic reaction of the air and gases 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 basically enclosed flare systems. VCU operates 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. Burning of auxiliary fuel is also done 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 stream of air and waste gas is drawn and preheated either through regenerative or recuperative methods. Similar to 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 200 to 500°C, enough to initiate the catalytic reaction. The heated gases then pass through the catalyst bed where it is further broken down achieving a destruction removal efficiency of above 99%.

The main destruction of VOCs happens when it comes into contact with the catalyst. As the stream passes through the catalyst bed, the VOCs are adsorbed on the catalyst. The 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 which 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 which 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 is able to 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 its better efficiency but 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 which allows the processing of VOCs with high heating values without subjecting the catalysts and their structure to high temperatures.

    

    
    

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 peoples‘ health, environment, and property. These can be categorized as hazardous air pollutants, criteria air pollutants, and greenhouse gases.
• VOCs are organic compounds that vaporize easily at room temperature and atmospheric pressure. In itself, 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 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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