Control Of Gaseous Pollutants:

(a)Control of Sulphur Oxides: 

Introduction: mechanism of formation of sulphur dioxides : 

Sulfur dioxide (SO2) is a gas that is formed when sulfur-containing fuels, such as coal and oil, are burned. The chemical reaction that occurs during combustion is:

S + O2 --> SO2

In addition to being formed during combustion, sulfur dioxide can also be formed naturally through volcanic eruptions and the weathering of rocks that contain sulfur. Once in the atmosphere, sulfur dioxide can react with other compounds to form other sulfur oxides, such as sulfur trioxide (SO3) and sulfates (SO4). These reactions can occur both in the atmosphere and in the human lungs.

Sulfur dioxide is also a major component of acid rain, which is caused by the reaction of sulfur dioxide and nitrogen oxides with water vapor in the atmosphere to form sulfuric acid and nitric acid. These acids then fall to the ground as acid rain, which can harm plants, animals, and aquatic life, as well as damage buildings and other structures.

Control Methods : 

There are several methods that can be used to control the emissions of sulfur oxides (SOx), including:

Fuel Switching: This involves using fuels that contain less sulfur, such as natural gas, instead of coal or oil.

Flue gas desulfurization (FGD): This is a process that removes sulfur dioxide from the flue gas produced by power plants. The most common type of FGD is called "wet scrubbing," which involves spraying a slurry of lime or limestone into the flue gas to remove the sulfur dioxide. 

Low-sulfur Fuels: This involves using fuels that contain less sulfur, such as low-sulfur coal or diesel fuel.

Emission Control Technologies: These include catalytic reduction, selective non-catalytic reduction, and selective catalytic reduction.

Combustion Modification: This includes techniques like Process Optimization and Oxy-combustion which can reduce the sulfur dioxide emissions from power plants.

Emission Trading: Emission trading systems, also known as cap and trade, set a limit on the total amount of emissions that can be produced by a certain industry or region, and then companies are issued permits or allowances for their emissions. These permits can be bought and sold, providing an economic incentive for companies to reduce their emissions.

Laws and Regulations: Governments have implemented laws and regulations to limit the amount of sulfur dioxide emissions from industry and power generation. The most common regulation is the National Ambient Air Quality Standards (NAAQS) which regulates the maximum levels of pollutants in the air.

Control methods : 

Pre Combustion :Low Sulphur Fuel Firing

Low sulfur fuel firing is a pre-combustion control method for reducing sulfur emissions from fuel. It involves burning a fuel that has a lower sulfur content than traditional fuels. This reduces the amount of sulfur that is released into the air when the fuel is burned. It is one of the most effective ways to reduce sulfur emissions from fuel combustion and is widely used in power plants and industrial boilers.

Desulphurization of Fuel  : 

Fuel desulfurization is a pre-combustion control method for reducing sulfur emissions from fuel. It involves removing sulfur from the fuel before it is burned. This can be done through a variety of methods, including physical, chemical, and biological processes. Some common methods of fuel desulfurization include distillation, hydrotreatment, and biotechnology.

Physical methods include distillation, which separates different components of the fuel based on their boiling points, and solvent extraction, which uses a liquid to remove sulfur from the fuel.

Chemical methods include hydrotreatment, which uses hydrogen to remove sulfur from the fuel, and catalytic cracking, which breaks down the fuel molecules to remove sulfur.

Biological methods include biotechnology, which uses microorganisms to remove sulfur from the fuel.

These methods can effectively remove a high percentage of sulfur from the fuel, making it a good option to reduce sulfur emissions. However, it can be more expensive than other methods and may not be suitable for all types of fuels.

Post combustion :Tall stack Dispersion

Tall stack dispersion is a post-combustion control method for reducing sulfur emissions. It involves releasing the flue gas from industrial facilities, such as power plants, through tall stacks or chimneys. The taller the stack, the more dilution of pollutants in the air. The pollutants, including sulfur compounds, are dispersed over a greater area, reducing the concentration of pollutants at ground level.

Tall stack dispersion is a passive method of reducing emissions and does not involve any treatment of the flue gas. It is relatively simple and inexpensive to implement, but it only works well in areas with low population density and a high wind speed. Additionally, it can not eliminate all the pollution, just reduce the concentration of pollutants at ground level.

Flue Gas Desulphurization :Non Regenerative process :

Flue gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide (SO2) from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes. Non-regenerative processes for FGD include wet scrubbers and dry sorbent injection. Wet scrubbers use a liquid reagent, such as limestone or lime, to react with and remove SO2 from the flue gas.

Dry sorbent injection involves injecting a dry powder, such as activated carbon or trona, into the flue gas to capture the SO2. Both of these methods are effective at removing SO2 from flue gas, but they do not regenerate the reagent and it needs to be replaced or disposed of after use.

Regenerative process : 

Regenerative processes for flue gas desulfurization (FGD) involve using a solid or liquid reagent that can be regenerated and reused. Some examples of regenerative processes include:

Lime/limestone FGD: In this process, a slurry of lime or limestone is used to absorb SO2 from the flue gas. The calcium sulfite produced can be oxidized to gypsum, which can be sold as a by-product. 

Ammonia FGD: In this process, ammonia is used to absorb SO2 from the flue gas. The resulting ammonium sulfite can be easily converted back to ammonia by heating.

Solid sorbent FGD: In this process, a solid sorbent, such as activated carbon or zeolites, is used to capture SO2 from the flue gas. The sorbent can be regenerated by heating it in the presence of oxygen.

Redox FGD: This process uses a chemical reaction to convert the SO2 in the flue gas into a solid that can be easily removed. This process can be regenerated by heating the solid and allowing it to release sulfur dioxide.

Regenerative process is more efficient in terms of material usage and costs. But they are also more complex to operate and require more space. 

Dry process : carbon adsorption & spray dryer scrubbing 

Dry process flue gas desulfurization (FGD) refers to techniques that remove sulfur dioxide (SO2) from flue gas without the use of a liquid reagent. Two common dry process methods are carbon adsorption and spray dryer scrubbing.

Carbon Adsorption: In this process, activated carbon is used to adsorb SO2 from the flue gas. The carbon can be regenerated by heating it in the presence of oxygen, which releases the adsorbed SO2.

Spray Dryer Scrubbing: Spray dryer scrubbing uses a dry powder reagent, such as sodium bicarbonate or trona, to react with and remove SO2 from the flue gas. The resulting solid product can be collected and disposed of or potentially used as a by-product. Spray dryer scrubbing also uses a spray dryer to dry the reagent before it is injected into the flue gas.

Both of these dry process methods are effective at removing SO2 from flue gas, but they have some limitations such as high energy consumption and high operating costs. Additionally, they have lower SO2 removal efficiency compared to wet FGD processes.

(b) Control of Nitrogen Oxides:

Introduction : mechanism of formation of nitrogen oxides 

Nitrogen oxides (NOx) are a group of gases that are composed of nitrogen and oxygen. They are formed through a process called thermal NOx formation, which occurs when nitrogen and oxygen in the air are heated to high temperatures during combustion. This can occur in engines, boilers, and other types of industrial equipment that burn fossil fuels. The specific reaction that leads to the formation of NOx is the combination of nitrogen and oxygen molecules at high temperatures, resulting in the formation of nitrogen monoxide (NO) and nitrogen dioxide (NO2) gases. These gases can have harmful effects on human health and the environment. 

Control methods: Precombustion or combustion control methods Post combustion

Precombustion control methods aim to reduce the formation of NOx before it is produced during the combustion process. These methods can include:

Fuel switching: Using a fuel with a lower nitrogen content, such as natural gas instead of coal, can reduce the amount of nitrogen available for NOx formation.

Low-nox Burners: These burners are designed to reduce the temperature of the combustion process, which limits the formation of NOx.

Selective catalytic reduction (SCR): This method involves injecting a reducing agent, such as ammonia, into the combustion process. The reducing agent reacts with the NOx to form nitrogen and water vapor.

Post-combustion control methods aim to reduce the amount of NOx that is produced during combustion by removing it from the exhaust gases. These methods can include:

Selective non-catalytic reduction (SNCR): This method involves injecting a reducing agent, such as ammonia, into the exhaust gases. The reducing agent reacts with the NOx to form nitrogen and water vapor.

Catalytic Reduction: This method uses a catalyst to convert NOx into nitrogen and water vapour 

Adsorption: This method uses materials that adsorb NOx from the exhaust gases.

Scrubbing : This method uses a solution to scrub the exhaust gases and remove NOx.

All these methods have different efficiency and applicability based on the source of NOx and the specific condition of the process 

Catalytic Decomposition for control of NOX 

Catalytic decomposition is a post-combustion control method for NOx reduction that uses a catalyst to convert NOx into nitrogen and water vapor. This process is also known as catalytic conversion.

The catalytic decomposition process takes place in a catalytic converter, which is typically located in the exhaust system of a combustion process. The converter is filled with a catalyst, such as platinum or palladium, that facilitates the chemical reactions that break down the NOx.

The catalytic converter works by passing the exhaust gases through a bed of catalysts at a high temperature, typically between 400-800°C. The NOx in the exhaust gases then comes in contact with the catalysts and undergoes a series of reactions, converting the NOx into nitrogen (N2) and water vapor (H2O) which are released into the atmosphere.

Catalytic decomposition is a highly effective method for controlling NOx emissions, and is widely used in the automotive industry to reduce emissions from vehicles. It is also used in the power generation and industrial sectors to reduce NOx emissions from boilers and other types of combustion equipment. 

Catalytic reduction :Selective non catalytic reduction and selective catalytic reduction  for control of NOX

Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) are two methods used to control NOx emissions in post-combustion systems.

SNCR is a method that reduces NOx by injecting a reducing agent, such as ammonia or urea, into the exhaust gases. The reducing agent reacts with the NOx to form nitrogen and water vapor. SNCR is typically used in industrial boilers and other types of equipment that burn fossil fuels. However, SNCR is less efficient than SCR and it is most effective at lower temperatures.

SCR is a method that uses a catalyst to convert NOx into nitrogen and water vapor. An ammonia-based reducing agent is injected into the exhaust gases and it is passed through a catalytic reactor, where it reacts with the NOx to form nitrogen and water vapor. SCR is typically used in power plants and other large-scale combustion systems. SCR is more efficient than SNCR and it is most effective at higher temperatures.

Both methods are widely used in power plants and other large-scale combustion systems to reduce NOx emissions. However, the choice of which method to use depends on the specific application and the emissions reduction goals.

Adsorption is a post-combustion control method that uses materials that adsorb NOx from the exhaust gases. This process involves passing the exhaust gases through a bed of adsorbent material, such as activated carbon, zeolites, or metal oxides. The NOx molecules in the exhaust gases are attracted to the surface of the adsorbent material and adhere to it. Once the adsorbent material is saturated with NOx, it can be regenerated by heating or through other methods, releasing the NOx molecules and allowing the adsorbent material to be reused.

Absorption is a similar process to adsorption, but it is a liquid-based method. In this method, a liquid absorbent, such as an aqueous solution of an amine, is used to remove NOx from the exhaust gases. The NOx molecules in the exhaust gases dissolve in the liquid absorbent and can be removed from the exhaust stream. The NOx-laden absorbent is then treated to release the NOx, which can be further treated or stored.

Both Adsorption and Absorption are considered as a reliable method of NOx control in many industrial processes, especially in power plants, chemical plants and other heavy industries. However, the efficiency of these methods depends on the specific conditions of the process and the type of adsorbent or absorbent used. 

(c) Control of VOCs Introduction , sources and control mechanisms 

Volatile Organic Compounds (VOCs) are a group of chemicals that have a high vapor pressure at room temperature and can easily evaporate into the air. They are found in a wide range of industrial and consumer products, including paints, solvents, cleaning supplies, and fuels. VOCs can have a negative impact on human health and the environment, as they can contribute to the formation of ground-level ozone and smog, and can also have adverse effects on the nervous system, liver and kidney, and some of them are known as Carcinogens.

To control the emissions of VOCs, various techniques are used. Some methods focus on reducing the amount of VOCs released during the production process, while others focus on capturing and treating the VOCs before they are released into the air.

The control methods can be divided into:

Source control: This involves reducing the amount of VOCs used in a process or using alternative materials that do not emit VOCs.

Process control: This involves modifying the process to reduce the amount of VOCs released.

Capture and Treatment: This involves using methods such as adsorption, condensation, or oxidation to remove VOCs from the air.

Regulation: This involves implementing regulations to limit the amount of VOCs that can be released into the air, and enforcing compliance with these regulations.

All these methods are used together to control the emissions of VOCs and reduce their negative impact on human health and the environment. It is important to note that the choice of method will depend on the specific VOCs and conditions of the process. 

VOCs (volatile organic compounds) are a group of chemicals that have a high vapor pressure and low water solubility. They are emitted as gases from certain solids or liquids and are found in many products and industrial processes. Some common sources of VOCs include:

Industrial processes: VOCs are emitted from various industrial processes, such as chemical manufacturing, paint and coating manufacturing, petrochemical production and printing.

Automobile Exhaust: VOCs are released from the fuel and lubricant used in internal combustion engines.

Consumer Products: Many common household and personal care products, such as cleaning supplies, pesticides, and personal care products, contain VOCs.

Building Materials: VOCs are emitted from the use of certain building materials, such as insulation, flooring, and adhesives.

Agriculture: VOCs are emitted from the use of pesticides and fertilizers in agriculture.

Control mechanisms for VOCs include:

Source Reduction: This involves reducing or eliminating the use of VOCs in products and processes.

Vapor Recovery Systems: These systems capture VOCs before they are released into the air.

Air Pollution Control Equipment: This equipment, such as carbon adsorbers, catalytic oxidizers, and thermal oxidizers, are used to destroy VOCs in the air.

Emission Regulations: Governmental agencies set limits on the amount of VOCs that can be released into the air by industry and individuals.

Best Management practices: Implementing best management practices such as proper storage, handling, and application of VOC-containing products can help reduce emissions.

It is important to note that the most effective method of controlling VOC emissions is through source reduction and prevention, however, in some cases, a combination of control methods may be necessary to achieve desired emission levels.