Thermal oxidizers reduce air pollution by heating contaminated air until it reaches temperatures high enough to break down the hazardous compounds, leaving carbon dioxide and water vapor as byproducts. Many industrial processes result in the production of air pollutants such as volatile organic compounds, hydrocarbon, solvent fumes and halogenated and hazardous air pollutants. As these pollutants can cause serious environmental and biological damage, it is vital that they be managed and disposed of properly.
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Applications of Thermal Oxidizers
Thermal oxidizers offer a means of disposal for toxic air pollutants and are used in several industries including polymer and resin manufacturing, food processing, printing, pharmaceuticals, painting, roofing manufacture and many more. Thermal oxidizers are a large piece of equipment and are therefore most widely used in industrial settings that yield high emissions. It is important to consider the requirements of a given application prior to purchase to ensure optimal oxidation occurs. As thermal oxidizers function by burning or heating the contaminants, temperatures are particularly important and range from 1000 to 1800 ° F on average. The solvent load and volume must also be taken into consideration as well as the required airflow. A properly selected thermal oxidizer should boast destruction efficiency rates between 90% and 99%. The higher this percentage the lesser the pollutants released into the atmosphere.
As concerns over emissions mount, the EPA encourages and often requires the use of air pollution control mechanisms such as thermal oxidizers which are a popular choice in industrial settings with high process air emissions. Thermal oxidizers can be categorized as direct flame or non-flame. The former is the simplest option as it requires a firing box into which the process stream flows. A burner converts the materials as needed. The time needed to complete this goal varies. Non-flame oxidizers are often more popular as they offer ample opportunity for the recovery and reuse of the heat energy. In these systems the tainted air is forced through a heated tube which slowly increases the temperature until the impurities are broken down. The purified air is then exhausted into the atmosphere. Heat recovery systems in this particular type of pollution control equipment may be either recuperative or regenerative. Recuperative models recover about 50% to 75% of the generated heat using a plate or shell and tube heat exchanger which uses the heat from recently cleaned air to heat the dirty incoming air. Regenerative models are much more efficient with up to 95% energy efficiency as they use ceramic heat transfer beds. The energy and cost savings of these types of thermal oxidizers make their application more popular than direct flame options in many industrial settings.
Basics Of Thermal Oxidizers
Since the environment protection agency issued stricter rules and regulations regarding volatile inorganic compounds (VOCs), in 1992, thermal oxidizers have been extensively used in the industrial manufacturing processes. Whenever the exhaust airstream primarily consists of VOCs, such as in distillation vents, reactor vents, operations including ovens, dryers and kilns, and solvent operations, thermal oxidizers are considered as the emission control systems, over the other available options, such as air filters, air scrubbers, electrostatic precipitators, and other oxidizers. However, with the development of newer thermal oxidizers, they are also used when emissions include particulate matter, which usually are a result of incomplete incineration of coke, carbon residue, or hydrocarbons.
Thermal oxidizers are also known by other names, including thermal incinerator oxidizer, direct-flame incinerator or afterburner. However, afterburner is only used when the combustion is incomplete, it is not used for other specialized purposes.
How Thermal Oxidizers Work
In thermal oxidation, the combustible materials present in the exhaust air streams are oxidized in the presence of oxygen by achieving the ignition temperature. When the materials, like VOCs, are completely combusted, it results in carbon dioxide and water. That is what the primary goal of an oxidizer is.
Efficiency of an Oxidizer
Four factors determine if the combustion process will be efficient:
- Residence Time (The Time Required for the Combustion to Occur)
- The Availability of Oxygen
- Turbulence for Mixing of Combustion Air with Waste Gas
The rule of thumb is, higher the temperature an incinerator achieves, the better it works. Second, if the residence time is shorter, the reactor temperature must be higher. To know what is the nominal residence time of a reacting waste gas; divide the volume of the combustion chamber by the volumetric flow rate of the gas. However, to make thermal oxidizers operation feasible, they are designed not to provide residence time more than 1 second, with typical temperatures range of 1200 to 2000 °F. A buyer must know, it is difficult to change the residence time, once an oxidizer is built. While selecting an oxidizer, a buyer should consider these basic design parameters, if the processing of VOCs is required.
Design of a Thermal Incinerator
A basic, straight thermal oxidizer consists of a combustion chamber, without any auxiliary heat recovery unit, which typically is a heat exchanger. The most important part of the oxidizer is the nozzle-stabilized flame, which usually is maintained by waste gas compounds, supplemental air if necessary, and auxiliary fuel. When the waste gas stream passes through the flame, it gets heated to its ignition temperature, and the organic particles present in the stream obliterate, making it safe under regulation to be released to the atmosphere. The different compounds have variable ignition temperature, which commonly is determined empirically. Oxidizers have extensive applications, because they have been proved as one of the most effective methods to destroy VOC, with an efficiency as high as 99.9999%. They are considered over other options, when the exhaust air stream is above 20% of the lowest explosive limit.
Different Types Of Thermal Oxidizers
Since the environment protection agency categorized and brought regulations regarding the volatile organic compounds, the use of oxidizers at a range of industries became mandatory. The industries where thermal oxidizers are used extensively as emission control systems include:
- Chemical Processing
- Processes Including Fabric Coating and Rubber Extrusion
- Manufacturing of Electronic Components
- Metal Coating and Related Work
- Wood Work
- Processing of Formaldehyde and Sterilizers
The emissions by these industries have direct implications on the health of humans and wildlife alike, as exhaust is normally rich in organic compounds, which are commonly reactive. To control their emissions, many types of air pollution control equipment, including air scrubbers, industrial blowers, vacuum cleaners, mist collectors, air filters, and electrostatic precipitators, are used. However, the main equipment that is used for the compounds are thermal oxidizers.
Thermal oxidizers, as the name implies, use thermal energy to destroy these compounds in the exhaust airstreams. After combustion, the products are generally water and carbon dioxide, which are deemed appropriate for release into the atmosphere. To different needs, different types of thermal oxidizer evolved. Here we are discussing them and their use, as well as their advantages.
- Direct Fired Thermal Oxidizer
- It is the most basic form of oxidizer with no heat recovery system. They are used when the air exhaust stream is exothermic, which means having enough volatile compounds in the airstream that can contribute to the fuel itself. When the air stream has enough heating value, it is called self-sustaining oxidizer, however, that rarely happens, thus, auxiliary heat is required by adding fuel. They are readily used when the gas streams have acid producing components and halogens.
- Regenerative Thermal Oxidizer (RTO)
- RTO has a very effective heat recovery system that provides efficiency as high as 95 percent. The heat system has a ceramic packing, most commonly a honeycomb structure, over which the hot exhaust gas passes. The ceramic absorbs heat when the air streams exit the oxidizer, however, to achieve high efficiency, most oxidizers have the bed or packing at multiple places, over which the path of hot air stream is reversed or repeated with use of large valves. Some oxidizers may have 3 beds whose use is rotated so that destruction rate efficiency remains high. These days, two chamber oxidizers are available that can be 98% efficient. Alternatively, a puff chamber approach is in use that cycles the raw gas.
- Recuperative Thermal Oxidizer (RCO)
- A RCO is commonly used when the concentration of volatile compounds is higher in the airstream, and a regenerative thermal oxidizer cannot be used. The heat recovery system in this type is metallic and can recover heat close to 70%, which is significantly lower than regenerative oxidizers. However, if needed higher efficiencies can be achieved, but it may lead to corrosion, because of condensation of acid gases. These types of oxidizers can operate at limited temperatures as metallic heat exchanger has its limits. However, there are engineering methods to optimize the operating temperature, which includes co-current flow, tubular design, and selection of metal and metallurgy. The RCO may be less efficient, however, it suits the processing well when the VOC content is higher in the air stream and can provide a certain degree of heat.
Ventilation Air Methane Thermal Oxidizer
Since climate change has become the central issue of the world environment, many mitigation methods have been developed to contain the release of greenhouse gases, such as methane and carbon dioxide from various sources. Carbon dioxide is a common byproduct of combustion, it is released by almost all manufacturing units, smaller or larger; methane majorly is emitted by mines as ventilation air methane (VAM).
Recently, to combat the carbon dioxide emission problem, Reykjavik Energy, a company in Iceland that has the largest geothermal plant with a capacity of 303 megawatts, has come up with a new, groundbreaking method. The company has developed a way to preserve the greenhouse gas into rocks. In this method, carbon dioxide dissolved in water is pumped into volcanic rock—basaltic rock—that is porous. The dissolved gas undergoes chemical reaction and turns into a carbonate, as it binds with natural elements found in the rock, such as magnesium, calcium, and iron. Technology is shifting the paradigm in a battle against greenhouse gases.
There are no similar emission control systems for methane; however, thermal flow-reversal reactor (TFRR), which in essence is an oxidizer, has emerged as a mitigation method. These types of thermal oxidizers destroy methane that is present in the exhaust air of coal mine shafts. The methane by combustion is turned into water and carbon dioxide, which in comparison to methane is almost 25 times less potent greenhouse gas. The technology is commercially available and has been tested for VAM emission.
- VAM Thermal Oxidizer Technology
- To destruct methane, the oxidizer uses the same technology that has been in use for a number of years for controlling volatile organic compounds (VOC). Depending on the exhaust, a VAM oxidizer can be regenerative thermal oxidation (RTO) or regenerative catalytic oxidizer (RCO). VAM oxidizers have ceramic filled bed systems—just like other oxidizers—that are pr-heated at 1,830 °F, and then mine air is introduced through a system of dampers and valves to direct the airflow. When the methane, present in the exhaust airstream, comes across the preheated system, it burns. The heat-exchange system supports auto-thermal operation, while saving a lot of energy by absorbing the heat released by the combustion.
- Using VAM Oxidizers
- If a mine’s exhaust has methane concentration in the range of 0.5 to 1.9 percent, and the ventilation flow is equal or more than 100,000 scfm, these special oxidizers ideally could be used. However, RCO and RTO systems that can destroy methane, when the concentration is as low as 0.2 percent, are now available. The specialized thermal oxidizers have also been used to produce steam for electric power generation, by tapping the excess heat produced by the system, just as regenerative thermal oxidizers.
- To give push to the technology, several developers have a range of financial contracts, including various lease agreements, royalty payments, profit sharing, and sale of a turnkey facility. As there are not any regulatory drivers, the application is quite limited; however, the technology is driven by the United Nations Framework Convention on Climate Change (UNFCCC), which is a proponent for the reduction of greenhouse gases.