Selecting the proper air-pollution control equipment for your process means focusing on the removal of specific pollutants while also ensuring that production goals are met
Air-pollution control (APC) equipment is used to prevent pollutants from being released into the atmosphere and is required for regulatory compliance. However, there are many APC options depending on the specific needs of an organization. Proper maintenance and operation of an APC system are crucial and will impact upstream equipment efficiency, production output and compliance. This article focuses on providing an overview of APC systems, explains their basic operating principles and describes the application for each.
There are a number of factors to consider when choosing APC equipment. The best equipment for you must meet your exhaust-emission needs, which depends on the process gas to be treated. The selection of equipment typically involves evaluation of other factors, including initial costs, longterm costs, maintenance and longevity. Understanding which factors are most important to you when meeting your APC needs can help guide the decision-making process.
The APC equipment best suited for your site depends on the pollutants to be mitigated. Here, those pollutants are categorized into the following groups, which are described in further detail throughout the article:
- Acid gas
- Mercury and heavy metals
- Dioxins and furans
- Oxides of nitrogen (NOx)
- Particulate matter (PM)
- Volatile organic compounds (VOCs)
- Hazardous air pollutants (HAPs)
Acid-gas control (SOx, HCl, HF)
Choosing the right acid-gas control equipment can have a significant impact on operation cost and is crucial to a plant’s operating efficiency. Acid-gas control equipment can target a number of pollutants, including sulfur oxides (SOx), hydrochloric acid (HCl), and hydrofluoric acid (HF). The best equipment will target the necessary pollutants, meet efficiency needs, require the least maintenance and, if necessary, easily integrate with existing equipment.
Circulating dry scrubbers.While focusing on acid gas, a circulating dry scrubber really is a multi-pollutant removal system. It includes a reaction tower, reagent storage silo, waste silo, baghouse, recycle-air slide conveyors and fluegas recirculation line. It typically utilizes hydrated lime (Ca(OH)2) as the reagent.
The process begins when the gas enters the reaction tower and goes through a fluidized reagent (Figure 1). The reaction converts the acid into particulate matter that is transported by the gas stream to the baghouse, where it is separated from the gas stream. The collected “dust” is recycled and reintroduced into the reaction tower to optimize reagent efficiency. Fresh reagent flow is continuously adjusted to meet the emission output setpoint. Waste product is removed from the baghouse for disposal to balance the amount of reagent circulating in the system.
FIGURE 1. A circulating dry scrubber can serve as a multi-pollutant removal system, utilizing hydrated lime as the reagent
There are a number of advantages of the circulating dry scrubber. It can provide 99% or higher SO2 removal efficiency. It removes SO3, HCl, HF, Pb, dioxins, furans and Hg. Under the right conditions, it can offer up to 60% NOx removal, so if only 50% reduction is required, additional equipment may not be necessary.
The circulating dry scrubber’s high recycle rate allows for low consumption of new reagent. Keep in mind that if you do not need to run at 99% SO2 removal to meet requirements, then the amount of reagent that is needed will be accordingly lower. The circulating dry scrubber has a lower power consumption than typical wet scrubbers and has no wastewater stream. Compared to other technologies, it tends to require less maintenance and has very few moving parts. It also does not utilize a slurry, so there are no related slurry-handling issues.
Wet scrubbers. Wet scrubbers (Figure 2) are available in a number of designs. As the name indicates, the wet scrubber utilizes liquid to perform the scrubbing function. The liquid can be water or another type of aqueous solution. How the wet scrubber works will depend on the type of wet scrubber and the type of contaminant being removed.
FIGURE 2. There are a number of wet-scrubber configurations that can be customized to suit specific process needs
Wet scrubber types include the following: open-spray tower; packed-bed scrubber; and Venturi scrubber. The open-spray tower involves a chemical interaction between the scrubbing liquid droplet (mostly caustic solution or lime slurry) with the pollutant. A packed-bed scrubber focuses on providing packing for increased surface area for the liquid (water, aqueous solution, caustic and so on) to contact the gases. It cannot handle a high dust load. The Venturi scrubber focuses on particulate-matter capture via high-velocity droplet atomization. Caustic can be added for acid reduction. The Venturi scrubber is discussed in more detail in the section on particulate-matter collection.
Wet scrubbers can effectively remove an assortment of contaminants from process gas streams. The typical wet scrubber is dedicated to acid-gas control, including SO2 and HCl. In addition to acid gases, wet scrubbers may also remove particulate matter and certain organic gases.
A benefit of a wet scrubber is that it can remove both particulate matter and gases simultaneously. Wet scrubbers also tend to take up less space. However, as mentioned in the previous section, wet scrubbers have a higher power consumption than circulating dry scrubbers and generate wastewater or other waste liquid streams.
Dry-sorbent injection (DSI). DSI systems can reduce SOx, HCl, HF and more in two basic steps. First, a dry sorbent is injected into the fluegas, where it reacts with the acid gases. The compound created from neutralization is a particle. Second, a particulate-matter control device collects the unused and converted reagent, as well as particulate matter in the gas stream downstream of the dry sorbent injection system.
DSI systems (Figure 3) have a smaller footprint than the circulating dry scrubber and wet scrubber and easily integrate with an existing baghouse or electrostatic precipitator (ESP). The systems also tend to have much lower capital costs. However, the systems tend to require a higher reagent usage, which may result in higher operating costs when compared with other systems. Dry-sorbent injection systems can be an option in smaller plants where the installation of other scrubber types would not be cost effective.
FIGURE 3. Dry-sorbent injection systems are more compact than typical scrubbing units and can easily integrate into existing plant footprints
Mercury, heavy metals and more
For pollutants such as dioxins, furans and mercury and other heavy metals, it is essential to ensure system efficiency, cost effectiveness and safety — for this purpose, activated-carbon systems are usually an effective APC choice.
Activated-carbon injection (ACI). ACI and DSI systems are actually very similar systems, except that the DSI system injects an alkaline reagent (hydrated lime, sodium bicarbonate or trona) and the ACI system injects powdered activated carbon (an absorbent). Note that activated carbon is not actually a reagent. It is very porous and does not chemically react like a lime-based reagent.
ACI is utilized for absorbing emissions from fluegases. In an ACI system, the activated carbon is pneumatically injected from a storage silo into the fluegas ductwork. The activated carbon then adsorbs mercury from the fluegas and is collected with the process gas’ dust in the plant’s particle-collection device. ACI may remove some hydrocarbons depending on the type and process-gas conditions.
The efficiency of the activated-carbon injection system depends on the gas conditions and heavily depends on the temperature. An ACI system is capable of mitigating 99% of the mercury. It is an effective and cost-efficient method of reducing mercury and heavy metals from most boilers. An ACI system can be utilized in a variety of industries, including cement, coal-fired power and biomass or biofuels. It is possible for a system to operate for extended periods of time with little maintenance.
NOx control
NOx — one of the most heavily regulated pollutants from industry — is typically controlled using reduction-based APC technologies.
Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Selective catalytic reduction systems generally use ammonia or urea as a reducing agent with a catalyst. The catalyst allows for the NOx to be removed at a lower gas-stream temperature. The SCR system will include an injection system for the reagent and a catalyst. The injection system can easily be added to an existing system, but the catalyst requires more space and a tie-in with the existing ducting system. The SCR system is temperature dependent, but not as highly temperature dependent as SNCR. It also needs regular maintenance to ensure proper performance.
As the name indicates, unlike selective catalytic reduction, selective non-catalytic reduction does not use a catalyst. SNCR systems are easier to install in an existing system, and while they do not require a catalyst, they are very temperature sensitive.
FIGURE 4. High temperatures are essential for selective non-catalytic reduction systems to work effectively
The SNCR can be an ammonia or urea injection system. For the reaction to take place, the temperature must be extremely high (Figure 4). Liquid ammonia is injected into the NOx-containing process gas and the ammonia removes the oxygen from the NOx and makes it nitrogen and water vapor. There are many different ways of performing the ammonia injection:
4NO + 4NH3 + O2 → 4N2 + 6H2O
2NO + 4NH3 + O2 → 3N2 + 6H2O
NO + NO2 + 2NH3 → 2N2 + 3H2O
With both selective catalytic and non-catalytic reduction, there will also be different safety regulations that will come into play depending on the ammonia type and concentration. This will include regulations regarding the handling of ammonia, as well as requirements for the containment area in case of an ammonia leak.
SCR and SNCR are both post-combustion technologies and commonly used NOx-reduction technologies. The SCR provides higher NOx-removal efficiencies, typically providing well over 80% NOx reduction. This is accomplished within a lower temperature range than is required in SNCR systems. U.S. Environmental Protection Agency (EPA) regulations support or require NOx-control installations to achieve the lowest emissions level possible.
Particulate matter collection
Particulate matter includes solids or liquid droplets that are so small that they can be inhaled and cause serious health problems. Particle pollution includes: PM10 (particles with diameters that are 10 μm and smaller); and PM2.5 (particles with diameters that are 2.5 μm and smaller). On February 7, 2024, the EPA proposed to strengthen the National Ambient Air Quality Standards for Particulate Matter (PM2.5) from a level of 12.0 μm/m3 to 9.0 μm/m3.
Fabric-filter dust collector (baghouse). A fabric-filter dust collector (also commonly referred to as a baghouse) collects particulate matter from the fluegas stream (Figure 5). Baghouses are essentially like a giant vacuum cleaner, except that they contain hanging filter bags, and the dust is collected on the bags’ surface. Located on top of the bags is a cleaning system that blows pressurized air in short pulses into the bags, forcing the dust to fall into the hoppers underneath.
FIGURE 5. A fabric filter or baghouse dust collector can remove extremely high quantities of particulate matter from a gas stream
Fabric filters can handle extremely high quantities of dust. A properly designed system can be extremely efficient. Thanks to the self-cleaning system, baghouses can continuously operate for years and years. Fabric filters cannot, however, deal with very moist or sticky dusts, tars or oily matter.
Wet electrostatic precipitator (WESP). A WESP uses an induced electrostatic charge to remove particulate-matter emissions in exhaust-gas streams, in situations where the gas stream is already wet, such as downstream of a wet scrubber. The WESP applies a negative voltage of several thousand volts to an electrode grid, creating an electric corona discharge that ionizes the particles and droplets in the moisture-saturated gas stream (Figure 6).
FIGURE 6. A wet electrostatic precipitator uses an electric charge to mitigate particulate-matter content in exhaust gas
The ionized particles and droplets then migrate toward the grounded plates or tubes, where they collect. This use of electrostatic force means the system has no moving parts. The collected particles and droplets are washed down via wash cycles and discharged through the water-deluge system.
A WESP removes aerosols, mists, ultra-fine particulate matter (sub-micron), organic particulate matter and sticky particulate matter. This includes particulate matter that is moist, sticky, tarry and oily. A WESP has no barriers to gas flow, resulting in a low pressure drop. Unlike the baghouse, a WESP is unable to handle large inlet loadings of particulate matter. In certain applications, a WESP is an excellent APC system to be placed before a regenerative thermal oxidizer, as it protects the ceramic media bed.
Venturi scrubber. A Venturi scrubber is another type of wet scrubber used for particulate-matter collection. The design of the Venturi scrubber typically allows for the vertical downflow of gas through the Venturi throat (Figure 7). The gas makes contact with a scrubbing liquid, and the resulting liquid droplets trap dust particles. This mixture enters a cyclonic separator, allowing for the separation of particulate matter, resulting in the exit of clean gas.
FIGURE 7. Venturi scrubbers have been used for over a century to remove particulate matter from gas streams
Venturi scrubbers are an old technology that has been used for over 100 years. They are optimized for particulate-matter capture and can be especially effective for dust that is sticky or moist. A downside to consider is that Venturi scrubbers can potentially create very large pressure drops in the gas stream, resulting in high energy consumption for downstream fan equipment.
VOCs and HAPs
For VOCs and HAPs, an oxidizer technology is usually most suitable.
Regenerative thermal oxidizers (RTO). RTOs are used to destroy VOCs and HAPs, including odor-causing compounds. RTOs are highly effective and are the most commonly used oxidizer for this purpose. Most regenerative thermal oxidizers have a destruction removal efficiency of 95 to 99%, with certain models achieving greater than 99%.
The RTO’s destruction efficiency relies on “The Three Ts:” high temperature, time and turbulence. This means that the process must occur at the correct high temperature, for the correct duration and have the proper airflow to mix. RTOs are efficient at regenerating thermal energy to reduce operating costs. Keep in mind that the RTO’s operating process is dependent on a temperature that typically ranges between 1,400 and 1,700°F.
FIGURE 8. Regenerative thermal oxidizers can achieve thermal efficiencies as high as 95–97%
The thermal regeneration, which drives the fuel savings of the burner, occurs in the heat-recovery chamber. This chamber is filled with ceramic media, suited for the gas constituents, to absorb the hot gases from the combustion chamber (Figure 8). While one ceramic media bed is heating up with the exhaust gas, the other ceramic media bed is preheating the process gas to reduce the fuel needed to achieve the target destruction temperature (Figure 9). The process gas flow direction is changed as denoted between Cycle 1 and Cycle 2. The typical thermal efficiency of an RTO is between 95 and 97% on a mass-corrected basis.
FIGURE 9. Thermal regenerative technologies usually employ two chambers in tandem to help improve heat recovery
Regenerative catalytic oxidizer (RCO). A regenerative thermal oxidizer can become a regenerative catalytic oxidizer with the addition of a catalytic ceramic-media layer. The added catalyst allows for VOC destruction at a much lower temperature. The lower temperature leads to lower energy consumption when compared to an RTO.
RCOs are very efficient at destroying VOCs and HAPs, including odors. RCOs tend to be more energy efficient than standard RTO units and have a destruction efficiency that can be greater than 99%. RCOs may also be the better economic choice in situations where the VOC concentration is low or burner fuel costs are high. Keep in mind that RCOs cannot be used when certain pollutants are present in the gas that will destroy the catalyst material.
Direct-fired thermal oxidizer (DFTO). A direct-fired oxidizer is a thermal oxidizer utilized to destroy VOCs and HAPs that is not regenerative. Like the RTO, a direct-fired oxidizer uses thermal oxidation to destroy pollutants. Unlike a regenerative thermal oxidizer, a direct-fired oxidizer solely utilizes a burner to heat the pollutants without a ceramic media bed to capture the heat post combustion chamber or preheat the process gas before the combustion chamber. DFTOs are best for exhaust streams with high concentrations of pollutants, low flows, or that are not used continuously. ■
Edited by Mary Page Bailey
All images provided by the author
Author
Robert Bobeck currently serves as the director of air pollution control products at LDX Solutions (60 Chastain Center Blvd, Ste 60, Kennesaw, GA 30144; Phone: +1 (770) 429-5575; Website: www.ldxsolutions.com). He has over 20 years of experience in the field of air pollution control and process development. He holds a B.S. degree in mechanical engineering, an M.S. degree in automotive engineering, an MBA and a Ph.D. in process engineering.