Effective control technologies are crucial for mitigating the adverse effects of air pollution. The selection of combustion-based or non-combustion-based technologies requires careful consideration of numerous factors
Air pollution control technologies are critical for reducing the negative impacts of industrial operations on human health and the environment. However, choosing between combustion-based and non-combustion-based technologies can be a challenging decision, because both approaches have their advantages and disadvantages.
The detrimental effects of air pollution, caused by various sources, such as industrial emissions, vehicle exhaust, and biomass burning, have spurred the development of innovative control technologies. These technologies aim to minimize the release of harmful pollutants into the atmosphere and improve air quality.
This article explores some of the different combustion- and non-combustion-based air-pollution control technologies. Depending on the waste-gas pollutants being controlled, the most suitable air-pollution control system will require special design, engineering, installation, and startup considerations.
The choice between combustion-based and non-combustion-based technologies can be a complex decision-making process. Both approaches have their advantages and disadvantages, and the selection depends on various factors, such as the specific pollutants being targeted the characteristics of the emission source, and location-specific regulatory requirements. Factors like energy efficiency, operational costs, waste management, and the potential for secondary pollutant formation also play a significant role in the decision-making process. In some cases, a combination of technologies may be required (Figure 1).
FIGURE 1. The selection of the best technology for air pollution control — or combination of technologies — depends on many factors beyond the pollutant profile, including energy efficiency, secondary pollutant formation, and more
Combustion-based technologies, such as flue gas desulfurization and selective catalytic reduction, utilize controlled burning to convert pollutants into less harmful substances. Non-combustion-based technologies, including electrostatic precipitators, wet scrubbers, and various filtration systems, focus on physical or chemical methods to capture and remove pollutants from the gas stream. The following sections outline some of the most prevalent pollution-control technologies in industry and provide some guidance on their applications.
Non-combustion technologies
As stated previously, non-combustion pollution-control technologies typically will involve a chemical or physical process designed to eliminate polluting species and prevent their associated environmental harm. There are numerous such technologies in use in industry today in a broad range of applications. It is important to note that aerial gaseous streams are not the only source of air pollution — contaminated water and soil can also contribute.
Soil vapor extraction (SVE) systems. SVE systems employ a technique that removes contaminants and hazardous vapors from the soil subsurface. It uses vacuum pressure to pull air through soil that has been saturated with hydrocarbons or volatile organic compounds (VOCs). SVE can be combined with a thermal oxidizer (see below) to further treat any remaining pollutant gases.
Chemical scrubber systems. When chemical scrubber systems are used, there is thorough contact between the gas stream to be treated and water in the presence of a chemical reagent. The treatment capacity of the scrubber depends on the type of waste-gas pollutants found in the process. For instance, if H2S is found in the waste gas, caustic treatment with an oxidation chemical is required. If NH3 is found in the waste gas, sulfuric acid with an oxidation chemical is required. If organic compounds with good to moderate water solubility are found in the waste gas, oxidation chemicals, such as hypochlorite or peroxide, are required.
Air stripper systems. Air stripper units are designed to remove hydrocarbons or chlorinated compounds from water. Recovered groundwater is pumped into the top of the system. As contaminated groundwater enters through the top of the air stripper, millions of air bubbles are forced by the blower to pressure up through the perforated trays, vigorously aerating the water to a froth and removing VOCs as gravity pulls the water down through each tray in the stripping column.
Oil-water separator systems. Oil-water separators are designed to accelerate the natural separation between water and hydrocarbons. Recovered groundwater is pumped into the first chamber of the system, where small oil droplets contained in the mixture coalesce and form larger droplets that rise to the surface. The large oil droplets are then collected for proper disposal.
Carbon-bed systems. Carbon beds work by physically adsorbing pollutants from the air. Adsorption is a process in which pollutant molecules adhere to the surface of the activated carbon. Although it is evaluated first as a Best Available Control Technology (BACT), carbon-bed technology may not always be the best long-term solution, because carbon replacement and usage can be expensive. There are also safety concerns associated with carbon-bed fires if VOCs cause an exothermic reaction. To reduce such risks, proper air circulation should be included in the design of the bed. Depending on the VOCs, other types of concentrators can be used.
Fabric filters. Also known as baghouses, fabric filters are used for the control of particulate matter. These filters consist of numerous fabric bags or tubes through which the gas stream passes. The fabric material captures and retains the particulate matter, allowing the clean gas to pass through. Periodically, the collected particulate matter is removed from the bags using mechanical shaking or pulsing methods. Baghouses are widely employed in industries where high-efficiency dust control is required, including cement manufacturing, metal processing, and coal-fired power plants.
Combustion technologies
The majority of combustion-based pollution-control methods fall into the thermal oxidizer category. There are several varieties of thermal oxidizers, but they all are designed to sustain the optimal conditions for the oxidation of combustible components of the gas stream. This is done by careful control of the operating temperature so that it is sufficiently above the auto-ignition level to provide enough time and excess oxygen to complete the necessary reactions.
Regenerative thermal oxidizer (RTO) systems. RTOs can handle dilute waste gases and achieve a destruction efficiency of 95–99% at operating temperatures of 1,400–1,500°F (Figure 2). Through the use of a ceramic heat exchanger, a thermal efficiency of up to 97% can be achieved. Depending on the VOC loading, a specially designed RTO can be used to accommodate higher solvent loading, with a slightly lower thermal efficiency, and use of a hot bypass.
FIGURE 2. Regenerative thermal oxidizers (RTOs) enable a very high destruction efficiency for pollutants. There are a variety of RTO configurations that provide different benefits in different applications
A special poppet-valve design can achieve 99% destruction without the use of a puff chamber. The “puff” refers to the small volume of unprocessed air that may remain after RTO treatment. In some applications demanding an extremely high destruction efficiency (above 99%), the “puff” may require re-treatment in a dedicated chamber.
On the same production line, there may be varying exhaust-air volumes, as well as varying solvent mixtures, resulting in high flows and volatile organic compound (VOC) concentrations of up to 25% of the lower explosive level (LEL).
Direct-fired thermal oxidizer (DFTO) systems. DFTOs (Figure 3) are ideal for very high solvent emissions (self-sustaining, with VOC concentrations of up to 50% LEL). They can achieve a destruction efficiency of more than 99% but normally do not provide any energy recovery. Depending on the solvent loading, this system could be a high-energy consumer. However, if periodic vent gas can be optimized and controlled as a continuous flow, thermal oxidation equipment also has heat-recovery options. If vent waste-gas streams have a high heating value to sustain combustion, then thermal oxidation technologies would prove to be a better fit, since the gas streams can be repurposed as fuel gas.
FIGURE 3. In certain direct-fired thermal oxidizers, it is possible to recover heat and potentially use waste streams as fuel gas
Multi-stage thermal oxidizers. Multi-stage thermal oxidation systems operate more like a standard thermal oxidizer, with sufficient excess oxygen and temperature to destroy the combustibles from the initial stage while keeping oxygen and temperatures sufficiently low to avoid the reformation of thermal oxides of nitrogen (NOx). A quench media is required to minimize the temperature of subsequent stages, and recycle flue gas (RFG) provides maximum heat recovery.
There are two primary processes for the subsequent stages in a multi-staged thermal oxidizer: one process uses two additional stages (for a total of three stages). The initial stage is the reducing stage. A second stage is a quench stage, employing a quench media to cool the initial stage flue gas to about 1,400°F, and a third oxidation stage uses air for the final oxidation of the combustibles.
The second process has two stages consisting of the initial reducing stage, employing RFG for quench, and a second oxidizing stage whereby RFG and excess air are combined and injected into the flue gas from the initial stage. The two-stage process is usually an improvement over the three-stage process because it reduces the costs of an extra refractory-lined chamber and simplifies control of the staged process.
Catalytic thermal oxidizers. Catalytic systems dilute waste gases with a destruction efficiency of 95 to 99% at an operating temperature of 700 to 800°F. Thermal efficiencies of 50 to 65% can be achieved with the use of a heat exchanger. Catalytic thermal oxidizers can be subject to poisoning, sintering and masking, and the catalyst is often very expensive to replace. This system is not recommended for abating waste-gas emissions.
Recuperative thermal oxidizers. Recuperative systems are suitable for applications with VOC concentrations of 10 to 35% LEL with moderate to high solvent emissions. Recuperative thermal oxidizers can achieve a destruction efficiency of over 99%, and use a shell-and-tube heat exchanger with thermal effectiveness of up to 70%. This system can result in high operating costs if there is a LEL solvent load of less than 15%.
Steam-generating thermal oxidizer (SGTO) systems. SGTO systems are designed so that a standard two-pass firetube boiler with certain modifications can be utilized as an effective air-pollution control system to destroy VOCs. Part of the process emissions are sent through the combustion air blower of the boiler burner. The remainder is passed to the incinerator section at the rear of the boiler. In this manner, the infrared heat released by combustion is utilized in the boiler furnace. Thus, the SGTO is more efficient than the DFTO waste-heat boiler system. It also requires less space.
Selective catalytic reduction (SCR). SCR units are widely used in power plants and industrial facilities to control NOx emissions. SCR systems work by injecting a reducing agent, typically ammonia or urea, into the flue gas stream. As the gas passes over a catalyst, NOx molecules react with the reducing agent, converting them into nitrogen and water vapor, which are less harmful to the environment. SCR systems have shown significant success in reducing NOx emissions and are particularly effective when combined with other pollution-control technologies.
Final thoughts
In conclusion, effective control technologies are crucial for mitigating the adverse effects of air pollution. The selection of combustion-based or non-combustion-based technologies requires careful consideration of numerous factors, and many pollution-control applications may require tailored solutions that align with specific needs and regulatory requirements.
Ultimately, the goal of air pollution control is to minimize the release of harmful pollutants into the atmosphere, improve air quality and protect human health and the environment. The urgency to address air pollution is amplified by the interconnectedness of environmental challenges, with air pollution exacerbating climate change and vice versa. By reducing air pollution, we not only improve air quality but also contribute to the global effort to combat climate change and create a sustainable and habitable world for future inhabitants.
