This article takes an in-depth look at Thermal oxidizers.
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Thermal oxidizers, also known as thermal incinerators, are combustion devices that combust pollutants like Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) into carbon dioxide and water. Thermal oxidizers are used in air pollution control by treating the pollutants before release to the earth’s atmosphere.
Thermal oxidizers are similar in operation to catalytic oxidizers, which use a catalyst to facilitate an oxidation reaction.
Some important design factors include:
In the process, VOCs are heated to a set point temperature thereby oxidizing it and breaking down the VOCs into carbon dioxide and water, which are both exhaustible to the atmosphere.
Thermal oxidizers usually have a mechanism for heat recovery in order to minimize fuel usage in the oxidation process.
Generally, the more the VOC/HAP concentration decreases, the more the percentage of heat recovery increases for a given thermal oxidizer design.
The heat recovery can be classified to be either recuperative or regenerative:
These types of heat recovery form the basis of the widely used types of thermal oxidizers. These types will be elaborated later in this article.
With thermal oxidizers -- just like many other high temperature combustion processes -- there are several chemical reactions occurring simultaneously, with main outputs being carbon dioxide and water vapour.
However, in cases where sulfur is present in the fuel or the pollutant, sulfur dioxide will also be formed.
Moreover, 0.5% to 2% of total sulfur can proceed to form sulfur trioxide. The sulfur trioxide ultimately forms a vapour phase of sulfuric acid as the gas stream cools below 600°F.
Where fluorine or chlorine is present in the organic compound being burnt, acids like hydrofluoric acid and hydrochloric acid will be formed.
Chlorine gas can form in the chamber if there are limited hydrogen atoms per molecule.
If nitrogen atoms are present in the organic compound being oxidized, nitrogen oxides can form or chemical reduction can happen to yield molecular nitrogen.
The quantity of nitrogen oxides formed in the thermal oxodizer’s burner flame is usually higher than the quantity of nitrogen oxides from the organic compounds.
For the combustion reactions to occur in full:
Therefore, where oxygen and turbulence levels are sufficient, the destruction efficiency is primarily affected by:
Each of these are described in more detail below.
For oxidation to occur efficiently, the temperature needed is about 200°F to 500°F over and above the auto-ignition temperature of the organic compounds that are most difficult to oxidize.
Some common organic compounds and their auto-ignition temperatures are found below.
By implication, the minimum oxidation temperature is around 1300°F. In as much as many organic vapours are capable of being oxidized below 1300°F, the equilibrium conditions between oxygen, carbon dioxide and carbon monoxide favour the production of carbon monoxide the more the temperature decreases below 1300°F. As the temperature is increased beyond the minimum oxidation temperature, there is an increase in reaction rates and a need for lower residence times to accomplish effective destruction. Thus, most thermal oxidizers operate at temperatures between 1300°F and 1800°F.
The ratio of the refractory-lined combustion chamber volume to the volumetric flow rate of combustion products via the chamber can be used to calculate the residence time of gases.
Therefore, t = V/Q where t is the residence time (sec); V is the volume of the chamber (ft3); and Q is the volumetric flow rate (ft3/sec).
Factors such as the rate of reaction at the prevailing temperature, and the mixture of the waste stream and the hot combustion gases coming from the burners affect the residence time necessary for oxidation reactions in the combustion chamber. These residence times are oftenly between 0.3 seconds and 2 seconds.
The organic vapour composition influences the determination of the minimum oxidation temperature. As previously mentioned, efficient oxidation is generally between 200°F to 500°F beyond the auto-ignition temperature of the most difficult organic compounds to oxidize in the gas stream.
The amount of fuel requirement for oxidation is reduced by the organic vapour in the process stream which becomes another source of energy. At their lower explosive limit organic vapours contain about 50 Btu/scf. However, thermal oxidizers oftenly operate below 25%-50% of the lower explosive limit of the organic vapour concentration. Thus, the waste gas stream can be carried to the combustio device safely. By implication, the waste gas stream can support the oxidation process using a maximum of 12-25 Btu/scf.
The 3 most common and widely documented types of thermal oxidizers are:
The Direct fired thermal oxidizer (DFTO) is also known as a non-recuperative thermal oxidizer. It has a basic design with a burner and a retention (combustion) chamber being the main components and are also commonly referred to as afterburners or incinerators or direct flame thermal oxidizers.
The DFTOs are mainly used in applications where there is no necessity for primary air-to-air heat exchangers. They are mostly used in applications with high solvent concentration levels, high energy demand, low air volumes and secondary heat recovery needs. DFTOs are generally used in processes with:
DFTOs use combustion to destroy VOCs and HAPs created during chemical processes in industrial exhaust streams. The thermal oxidation as a chemical process is accomplished by raising the exhaust stream temperature to between in 1250°F and 1800°F. In this temperature range the molecules’ chemical bonds are broken and the VOCs and HAPs in the exhaust stream are combusted to give combinations of carbon dioxide, water and thermal energy. The DFTOs can achieve a 99% hydrocarbon destruction rate even in high pollutant exhaust streams.
The Direct fired thermal oxidizer’s combustion chamber serves to:
DFTOs are thus best suited for exhaust streams with high concentration of HAPs, VOCs, and odors. Considerations such as operational costs, capital expenditure, and safety are important in the application of a Direct fired thermal oxidizer. As an example, the heat recovery cost offsets the low inlet volume operational savings.
The benefits of a Direct fired thermal oxidizer include:
The Direct fired thermal oxidizer is commonly applied on:
Recuperative thermal oxidizers have an air-to-air heat exchanger that has a combustion chamber. They are agile and can handle a variety of VOC concentrations and process flow rates.
A recuperative thermal oxidizer is in essence a direct fired thermal oxidizer with a primary and/or secondary source of heat recovery. Shell and tube or plate type heat exchangers are used in conjunction with thermal recuperative oxidizers. The heat exchanger’s purpose is to preheat the VOC-laden air (incoming process air). The incoming process air is pushed into the system inlet using a high-pressure fan system. The exhaust stream is then directed into the tube sides of the shell and tube heat exchanger. The VOC-laden air passes through the combustion chamber and is evenly heated and thoroughly cycled. An exothermic reaction occurs as the stream is heated to a very high temperature. The particles break down and the VOCs and HAPs in the exhaust stream are combusted to give combinations of carbon dioxide, water and thermal energy. The process gas temperature is raised by the primary heat recovered before the process gas goes into the combustion chamber. This results in the oxidizer burner system utilizing lesser fuel.
After the combustion process, the hot clean stream passes back through the heat exchanger. The cooled clean exhaust stream is released to the atmosphere void or with minimal VOCs. The VOCs and HAPs destruction efficiency (DRE) is typically greater than 99%. The Recuperative thermal oxidizers are generally regarded to have sustainably better DRE capabilities than Regenerative thermal oxidizers.
Recuperative thermal oxidizers are usually used in processes with:
The heat exchanger in thermal oxidizers serves to transfer heat between fluids. For recuperative thermal oxidizers, the shell and tube exchanger is the most popular option and is mainly made up of a shell casing with tubes inside. The heat transfer in the shell and tube exchanger is accomplished by running one fluid through the tubes and another fluid over the tubes inside the shell. Thus, the heat is transferred from one fluid to the other. The heat is then useful as energy for the recuperative thermal oxidizer or other needs of the plant.
The benefits of Recuperative thermal oxidizer include:
Various comparisons can be made between the recuperative thermal oxidizer and the regenerative thermal oxidizer. The regenerative thermal oxidizer will be elaborated in the succeeding section.
Regenerative thermal oxidizers use a heat exchange media made of ceramic instead of an air-to-air heat exchanger made of stainless steel. They are used to destroy VOCs and HAPs created from chemical processes and industrial exhaust streams. Regenerative thermal oxidizers use very high heat, approximately 1500°F, to clean the pollutants. They are mostly intended for large volumes and low VOC and HAP concentrations in the air pollutants.
The technology used in regenerative thermal oxidizers is based on the use of ceramic media as the heat exchanger. This design is in contrast to that of recuperative thermal oxidizers which use shell and tube heat exchanger with primary and/or secondary source of heat. The regenerative thermal oxidizer technology yields high air flow at low operating costs for low VOC fume streams. 95% of the clean hot air is captured by the technology instead of allowing the heat to escape to the atmosphere.
Regenerative thermal oxidizers are usually used in processes with:
Regenerative thermal oxidizer systems have the advantage of low capital and operating cost for larger airflow processes with low VOC concentrations, less than 8% of the lower explosive limit. The thermal effectiveness can be as high as 97%.
The regenerative thermal oxidizer process has process steps which include:
The regenerative thermal oxidizer commonly exists in two sizing configurations, which are two-canister and three-canister.
Two-canister: The two-canister regenerative thermal oxidizers require low capital expenditures with better maintainability and a DRE as high as 99%. The VOC-laden exhaust stream of a two-canister regenerative thermal oxidizer is channeled using a high pressure fan into the first heat exchange bed. Subsequently, the exhaust stream begins the heating process as it passes through the media. Next, the exhaust stream goes into the combustion chamber. At this stage the stream is heated by the burners to the optimal temperature for combustion. This completes the oxidation process and after this the stream, which is now clean, filters into the second bed of heat exchange where it is cooled.
The clean stream then passes through another bed of media where the stream temperature is brought down and the media temperature is brought up. In the end, the clean cool stream is ejected into the atmosphere.
Three-canister: The three-canister regenerative thermal oxidizer is best suited for aqueous and vapour-tolerant applications. Odors and organic compounds are almost completely destroyed by the high DRE of three-canister regenerative thermal oxidizer which can go as high as 99%. The regenerative thermal oxidizer accomplishes the process by converting pollutants in the stream to carbon dioxide and water vapour. Thermal energy is recovered in the process and aids in reducing the operating cost of the equipment. Just as with the two-canister regenerative thermal oxidizer, the three-canister regenerative thermal oxidizer has the VOC-laden exhaust stream entering the bed of the heat exchanger by the use of a high pressure fan system. The stream then passes directly through the media and gets heated before it goes into the combustion chamber. The steam is further heated to reach the optimal combustion temperature in the combustion chamber by the use of burners and this completes the oxidation process. Subsequently, the clean stream is fed into the heat recovery chamber and then goes via the media bed which heats the media and cools the air. Finally, any remaining VOCs in the clean stream are then trapped in the final chamber of the three-canister regenerative thermal oxidizer. This is accomplished by using clean air to purge the stream. The two-canister regenerative thermal oxidizer does not have this final step and this makes the three-canister regenerative thermal oxidizer more efficient and able to reach a higher DRE.
The key differences between the two-canister regenerative thermal oxidizer and the three-canister regenerative thermal oxidizer would be that the former has a lower capital expenditure, better maintainability and a DRE up to 99% while the latter is better suited for bake-out processes with a DRE higher than 99% although they are physically bulky.
The regenerative thermal oxidizer has numerous benefits in as much it has its drawbacks which include:
The benefits of the regenerative thermal oxidizer which outweighs its drawbacks include:
As thermal oxidizers are the primary method used in industries for VOC and HAP destruction, catalytic oxidizers are the more fuel efficient option with lower operating costs. Various differences can be noted between catalytic oxidizers and thermal oxidizers and some of them can be seen in the table below.
Catalytic oxidation is accomplished by a chemical reaction between a base metal catalyst bed in the oxidizer system with the VOC/HAP hydrocarbon molecules. The catalyst serves as a substance that accelerates the rate of the chemical reaction. The reaction can thus occur under the normal temperature range of 550°F – 650°F.
A catalytic oxidizer is an equipment for pollution control which is used to treat VOC-laden industrial exhaust streams. The principle of operation involves raising the exhaust stream temperature to a point that breaks the chemical bonds holding the VOC molecules together. This is accomplished across the catalyst media’s precious metals and the breaking down of the VOC molecules is the oxidation process. These VOCs from the process exhaust stream convert, in the oxidation process, to carbon dioxide, water and thermal energy. The operating temperature in a catalytic oxidizer is significantly lower than that of thermal oxidation. The catalytic oxidizer system can be self-sustaining when the process stream VOC loading level is added. Minimal supplemental fuel is required to support the operation and this contributes to the reduction of operating costs. The supplemental fuel requirements are even lower when there is a high organic vapour content in the contaminated gas.
The layer of catalyst, known as a bed, enables the oxidation reaction at a lower temperature than that of the gas phase thermal oxidation. In many cases catalytic oxidizers can operate without supplemental fuel which is only needed at start-up. There is no need for refractory-lined combustion chambers due to the low gas temperatures and this reduces the weight of the unit. The catalytic oxidation happens at the surface of the catalyst and the steps in the process are:
By implication, reduced oxidation efficiency is a result of problems associated with the catalyst surface. Thus, with sufficient oxygen and turbulence levels, catalytic destruction efficiency is primarily affected by oxidation temperature, catalyst properties,VOC concentration and composition, space velocity, and catalyst deactivation.
Catalysts typically are noble metals e.g. palladium, platinum and rhodium, and base metal oxides e.g. manganese trioxide, chromium trioxide and vanadium pentoxide. The noble metals are prevalently used due to their wide operating temperature range, high activity, resistance to deactivation, and thermal durability. Base metal catalysts have higher pressure drops as compared to catalysts on monolithic structures. The choice of catalyst depends on the VOCs to be treated. The catalyst chosen must be appropriate for the desired oxidation reaction and should be resistant to deactivation by either the VOC or other materials in the gas stream. The heterogeneous catalyst unit is typically composed of three components, namely: the carrier; the substrate; and the catalyst material.
The substrate is a thin solid material on which the catalyst material and the carrier are deposited. It is mostly made up of stainless steel or ceramic with the most common substrate form used in VOC oxidation applications being the monolithic honeycomb. The monolithic honeycomb gives the maximum surface area, minimizes pressure drop, and accommodates gas streams better. In some cases, ceramic pellets are used as substrate, however, they have a drawback of providing less surface area per unit volume and are prone to plugging by the gas stream’s particles.
The carrier, also known as the washcoat, is an inorganic material with a high surface area and contains a complex pore structure which the catalyst materials are deposited into. On top of providing the high surface area, the carrier maintains the selectivity, activity and durability of the finished catalyst. The most commonly used carrier is alumina, however, it has drawbacks of being highly reactive with sulfur trioxide to form compounds that alter the carrier’s internal surface. This alteration results in catalyst deactivation.
In cases where sulfur-bearing compounds are noted in the gas stream, titanium dioxide and silicon dioxide are the preferred carrier option. The dispersion of the catalyst material within the carrier is accomplished by soaking the catalyst material in an aqueous solution of a catalyst’s precursor salt. The carrier is then deposited onto the substrate once catalyzed.
The temperature’s effect on the oxidation catalyst’s activity is best described by the light-off curve. This is a conversion efficiency vs temperature S-shaped graph which has three regions, which are: reactions limited by kinetics; light-off; and reaction limited by mass transfer regions. There is a distinct light-off curve for each VOC, however, in general the light-off curve is illustrated below.
The reaction is kinetically limited at low temperatures. Any conversion that occurs in this region depends on oxygen and VOC molecules interaction at the catalyst surface. The more the temperature increases the more the reaction rate increases abruptly because of the heat of reaction and this is the light-off region. The light-off region has an increased conversion efficiency over a narrow temperature range. When the reaction gets to the mass transfer limited region the ability of the reactants to reach the catalyst sites becomes the only limitation to the reaction. This is usually the desired region to operate catalytic oxidizer and is normally between 400°F to 1000°F. The catalytic oxidation’s upper temperature limit is the sintering temperature of the carrier material.
The space velocity affects the amount of catalyst which the catalytic oxidizer needs. Space velocity is the ratio of the volumetric flow rate to the catalyst bed volume at standard conditions.
SV = Qstd/BV
where SV is the space velocity (hr-1); BV is the volume of the catalyst bed (ft3); and Qstd is the volumetric flow rate under standard conditions (ft3/hr). The reactivity of the catalyst is directly proportional to the space velocity and inversely proportional to the catalyst volume required for VOC destruction. For each operating temperature, a decreased space velocity yields an increased destruction efficiency.
The VOC composition determines the minimum inlet temperature to the catalytic oxidizer. The minimum inlet temperature is determined from the light-off curve. Just like thermal oxidizers, the gas stream’s organic vapour acts as the energy source,nevertheless, with different consequences in the catalytic oxidizers. The inlet temperature of the catalytic oxidizer should be maintained above the light-off temperature. Thus, an increase in the catalyst’s bed gas stream temperature results from the heat released from the VOC oxidation. Oxidation of a 1% of the lower explosive limit of VOC concentration can yield about a 27°F increase in gas stream temperature. Due to the risk the high temperature poses on the catalyst the inlet concentrations to catalytic oxidizers are usually limited to 25% or less of the lower explosive limit.
Catalyst deactivation is the decrease in destruction efficiency due to operating problems of the catalyst that cause a rapid or slow loss of catalyst activity. Sintering of the carrier due to excessive temperature is an example of a problem that can cause deactivation and other causes include masking, fouling and poisoning. Fouling happens when the particulate matter deposits on the catalyst surface and blocks the access of the organic compounds. Masking happens when some materials with a high absorptive affinity such as sulfur and halogen compounds reduce the active sites on the catalytic surface and organic compounds will have limited surface area. Poisoning happens when irreversible reactions occur between some metals and the catalyst thereby making the catalyst inactive.
Since a catalyst, by definition, is a substance that speeds up a chemical reaction without itself being consumed, the catalytic oxidation process operates at a lower temperature with lesser fuel consumption and minimizes nitrogen oxides (NOx) formation. Reduction of carbon monoxide (CO) and NOx is important since these compounds are strictly regulated just like VOCs.
A catalytic oxidizer can be operated with a quicker warm-up time with shorter run durations and with no adverse effects on the equipment's life cycle. Catalytic systems can lower annual overall operating costs better than direct fire thermal oxidizers, recuperative thermal oxidizers and regenerative thermal oxidizers.
The benefits of a catalytic oxidizer include:
The catalysts can be used in the recuperative thermal oxidizers and regenerative thermal oxidizers. These will be briefly described below..
The catalytic recuperative thermal oxidizer is best suited for destroying VOCs, odors and other air pollutants. In the process, the heated gas is pushed towards the combustion chamber via the heat exchanger. The heated gas passes through the catalyst and the heat releasing action happens simultaneously. The pollutants are thus converted into carbon dioxide, water vapour and thermal heat which is recoverable.
The catalytic regenerative thermal oxidizer is best suited for VOC oxidation technology that requires low operating cost with applications that have a low VOC or HAP concentration. The combination of a catalytic oxidizer, which runs at low operating temperature, and the heat recovery characteristics of a regenerative thermal oxidizer makes the catalytic regenerative thermal oxidizer the best option for many applications with low VOC concentrations.
Various options for thermal oxidizers exist among other combustion devices that combust pollutants like Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) into carbon dioxide and water. Thermal oxidizers are useful in air pollution control by treating the pollutants before release to the earth’s atmosphere. Choice of each combustion device needs to be done cognizant of the merits and demerits of each device.
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