Introduction

Control devices for gaseous pollutants system selection and design must be based on some particular knowledge about the exhaust gases to be treated. Information on gas flow rates and temperatures is required to correctly size the collector for the projected process operating conditions. The overall and maximum gas flow rates must be as precise as feasible since most gaseous pollutant management systems have a restricted range of acceptable gas velocities.

Particulate materials trapped in the gas stream can significantly reduce the collector’s efficiency and dependability. Beds of collecting medium (e.g., fixed adsorption beds, catalyst beds) or pre-collector heat exchangers are used in gaseous pollutant management systems.

Particulate matter can build up in these spaces, obstructing appropriate gas movement. Particulate matter that is relatively big (i.e., > 3 micrometers) or sticky has a particularly negative impact. A pre-collector may be required if the gaseous contaminant control system is susceptible to particle-related issues.

Commercially, there are five basic approaches for capturing and/or destroying gaseous pollutants:

• Adsorption 
• Absorption 
• Condensation
• Combustion 
• Automotive Emission control

Adsorption

Adsorption is the process of gaseous pollutants interacting with the surface of a solid adsorbent. Pellets in a thick bed, tiny beads in a fluidized bed, or fibers pressed onto a flat surface are adsorbents in various physical forms.

Physical and chemical adsorption processes are the two types of adsorption mechanisms. The difference between physical and chemical adsorption is how the gas or vapor molecule is retained to the adsorbent surface.

Intermolecular cohesion holds the gas or vapor molecule to the solid surface via physical adsorption. Physical adsorption may be easily reversed by using heat or lowering the pressure.

A chemical reaction occurs between the adsorbent and the gaseous pollutant in chemical adsorption. This is a difficult reaction to reverse.

Organic chemicals are routinely captured and concentrated by physical adsorption. Acid gases such as hydrogen fluoride, hydrogen sulfide, and hydrogen chloride are routinely controlled by chemical adsorption. Mercury gas is also controlled by chemical adsorption.

Adsorption processes are concentration-dependent, but it doesn’t imply they can’t operate at small concentrations (e.g., such as 1 to 100 ppm levels). Both physical and chemical adsorption techniques are most effective at high pollutant concentrations. This is due to the concentration driving power available to drive contaminant diffusion to the adsorbent’s surface. 

When the gaseous temperature is low, almost all adsorption techniques function well. The gas temperature is typically kept below 120 degrees Fahrenheit in physical adsorption. Because of the power of the chemical bond produced during adsorption, chemical adsorption may be carried out at greater temperatures. The temperature range of many chemical adsorption processes is 100°F to 400°F.

Absorption 

Gaseous contaminants that are dissolvable in water may be removed using absorbers. This method is one of the most important for removing acid gas molecules and water-soluble organic compounds.

The contaminated gas or vapor is absorbed from the exhaust gases when it comes into contact with the liquid. The rate of pollutant capture increases as the interaction between the liquid and the pollutant-laden gas increases. Consequently, factors such as 1. turbulent blending of the contaminant gas stream with the liquid and 2. increased aqueous liquid surface area promote absorption.

The capacity to collect and maintain the pollutant in solution long enough to complete the essential processes is the only limitation to the widespread use of absorbers based on irreversible chemical reactions. It’s also crucial to keep the right balance of dissolved and suspended materials in the liquid to avoid considerable build-up of materials that exceed their solubility limits and precipitates in sprinklers or other wetted areas of the absorption vessel.

Even when pollutant concentrations are low, absorption mechanisms maintain high efficiency (i.e., less than 100 ppm). The absorbers must be built with very effective gas-liquid interaction to maximize the mass transfer conditions in low concentration situations. Biological systems are often built to withstand moderate-to-low concentrations.

When the gas and liquid temperatures are both low, all absorption mechanisms work optimally. Under chilly circumstances, gas and vapor phase pollutants are most soluble. In most circumstances, the cooling supplied by the evaporation of water existing in the recirculated absorber stream liquid is sufficient to lower the gas temperature to the point where adequate absorption can occur.

Condensation

Condensation systems are primarily used to recover organic compounds in process industries’ effluent gas streams at moderate to high concentrations. they are divided into three groups depending on their broad working temperature range.

• Direct and indirect water-based condensers (40°F to 80°F)
• Condensers for refrigeration (–50°F to –150°F)
• Cryogenic condensers (–100 to –320 degrees Fahrenheit)

Condensers that use cooling water in close interaction or indirect contact vessels are the most popular. Refrigeration and cryogenic systems are generally utilized to recover high-value pollutants efficiently.

Condensation system applicability is determined using vapor pressure data to manage the impurities. It lowers the contaminant concentration in the gas stream to a level equal to the compound’s vapor pressure at the condenser’s operating temperature.

Condensation systems are most commonly employed to handle excessive levels of organic pollutants. Condensations work best when the temperature is below freezing. Pre-cooling is required if the incoming gas stream is hot to enable effective condensation of organic molecules.

Condensers work by heat transferred from the gas stream being treated to the system’s cooling medium. Particulate debris that collects on heat transfer surfaces can diminish the system’s performance. As a result, particle matter must occasionally be removed before entering the condenser.

Combustion

VOCs and other gaseous hydrocarbon contaminants can be converted to carbon dioxide and water via a method known as combustion or incineration—chemically known as fast oxidation. VOCs and hydrocarbon vapors are normally incinerated in a special incinerator known as an afterburner. The afterburner must generate enough turbulence and burning duration and maintain a high enough temperature to accomplish full combustion. 

Because it minimizes the needed burning duration and temperature, sufficient turbulence or mixing is a vital element in combustion when the waste gas is a combustible combination that does not require the addition of air or fuel, a procedure known as direct flame incineration can be utilized.

An afterburner is generally built of a steel shell with a refractory substance like firebrick lining. The refractory lining serves as a heat insulator and protects the shell. Gaseous organic pollutants may be virtually entirely oxidized with adequate time and high enough temperatures, with incineration efficiency reaching 100%. Certain substances, such as platinum, can aid in the combustion process. These compounds, known as catalysts, allow full oxidation of flammable gases at low temperatures.

Afterburners are used to limit the quantity of photochemically reactive molecules discharged into the air, regulate smells, and eliminate dangerous compounds. They’re used in many industrial settings where VOC vapors are released due to combustion or solvent evaporation.

Automotive Emission Control

Automobiles were discovered as one of the leading air pollution causes in metropolitan areas in the 1960s. California was the first state to set passenger automobile emission rules in 1965. Other nations have recognized the automobile as a major source of pollution, and many have enacted various rules and testing techniques with varying degrees of rigor. Different regulatory approaches and air quality goals, and worries about the contradictory aim of better fuel efficiency have resulted in invariances.

Automobile exhaust pollution regulation is one of the world’s most pressing environmental issues. Automobile exhaust pollution is controlled by alternate fuels, catalytic converters, and emission reduction. The emission control system in vehicles limits the internal-combustion engine’s noxious gas emissions.

Pollution emissions from automobiles are normally minimal, but as the number of vehicles on the road grows, so do pollution levels in the environment. The transportation sector is responsible for around 35% of CO, 30% of HC, and 25% of NOx emissions into the environment. The environment and human health are both harmed by these contaminants. 

The air-fuel ratio has a significant impact on vehicle emissions. Engine upgrades, fuel processing, fuel additives, exhaust gas recirculation (EGR), positive crankcase ventilation (PCV), and catalytic converters are all examples of exhaust gas emission reduction strategies.

A catalytic converter is a device that alters higher levels of hazardous exhaust gas pollutants into lower levels of toxic exhaust gas pollutants. Different types of catalysts, such as noble metal and base metal catalysts, are utilized to treat car exhaust gases. The catalytic converter was shown to be successful and consistent in decreasing noxious tailpipe emissions, and it was designed for use in trucks, buses, automobiles, motorbikes, and other construction vehicles.