Air Pollution Control Equipment
The term "air pollution" is used so frequently that many of us believe we have a complete understanding of its meaning. Scientists and environmentalists have made so many predictions of the negative effects...
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This article will provide industry insights on oxidizers.
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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 in which the stream of preheated waste gas is oxidized at temperatures up to 1832°F (1,000°C).
Waste gas containing volatile organic compounds (VOCs), organic hazardous air pollutants (HAPs), or odors is combusted into carbon dioxide and water vapor. Waste gasses can also contain non-organic air pollutants, such as halogenated and sulfuric compounds. The combustion products include acid gasses that can contribute to smog formation and acid rain. Oxidizers, like scrubbers, remove 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 EPA requires industrial facilities to install pollution controls to meet the specific emission limits stated in the CAA. Industrial plants that emit harmful air compounds include oil refineries, coal-fired and gas power plants, chemical plants, cement plants, steel mills, and glass factories.
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.
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 often supplied by a forced draft fan. The supplied air is controlled at sufficient 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 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.
A pilot burner or igniter initiates the combustion of the fuel gas in the thermal oxidizer burner. The thermal oxidizer is heated to the specified operating temperature. Depending on the level of combustibles in the waste gas stream, heat will be released by the waste gas entering the thermal oxidizer. If the combustion is not self-sustaining, additional auxiliary fuel is injected into the system to maintain chamber temperatures.
The amount of auxiliary fuel can vary for different types of oxidizers. 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.
Another way to maintain temperatures inside the chamber without using too much auxiliary fuel is by using 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 within the combustion chamber can also preheat the air, such as with a regenerative thermal oxidizer or RTO (see later description). 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 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 stream of waste gas contains acid-forming compounds and particulate matter, additional downstream or stream equipment might be needed. To remove acid gasses, scrubbers, particularly wet ones, are popular.
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. As the stream hits the wall, the gas changes direction and swirls around the chamber. The particles are separated due to centrifugal force, falling into 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 main factors in oxidizer design are temperature, (residence) time, and turbulence.
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 can vary according to the type and concentration of VOCs, with higher temperatures leading to higher destruction/removal efficiency (DRE) of the compound(s) being oxidized. All substances have an autoignition temperature, which must be exceeded in the thermal oxidation process to achieve the required DRE.
Residence time refers to 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 balance between capital and operating cost must be considered.
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 increases the efficiency of the thermal oxidizer and ensures that all VOCs in the waste gas stream are burned. A chaotic flow of fluids increases mixing and mass distribution and prevents gasses from collecting in dead regions. Many designs of thermal oxidizers have combustion chambers with geometries intended to generate turbulence.
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 their methods of heat utilization and heat recovery. Additional types include flameless thermal oxidizers and enclosed vapor combustion units.
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.
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.
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%.
Catalyst oxidizers operate in the same way as thermal oxidizers but with the addition of a catalyst bed. The catalyst increases the reaction rate of the VOCs for a given temperature relative to thermal oxidation. As a result, the catalytic process achieves comparable removal efficiency to the thermal oxidizer but 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 can be preheated through either 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 heated in the combustion chamber. Unlike in thermal oxidizers, this initial heating 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 (200–500°C), enough to initiate the catalytic reaction. The heated gasses then pass through the catalyst bed, where they are further broken down to achieve a destruction removal efficiency of above 95%.
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 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 the attachment to the catalyst, freeing up the active site. This results in lesser heat required to facilitate oxidation.
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.
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.
When air pollution regulations were enacted, finding an effective gaseous emissions control system became difficult. However, with technological advances and process developments, systems for processing gaseous emissions have radically improved. Today, various options have been designed to meet regulatory requirements.
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:
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:
Direct catalytic oxidizers have the same design as direct thermal oxidizers but incorporate catalysts to reduce operating temperature. They are used with processes that have:
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 with:
Regenerative thermal oxidizers have lower capital and operational costs, with higher airflow, low VOC concentrations, and thermal effectiveness of 92% to 95%. They are used with processes that have:
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