Question

Industrial standards and methods followed for reducing air pollution

Answer 1

Standards

Ambient Standards

The ambient standards, also known as the National Ambient Air Quality Standards (NAAQS), is set by the Environmental Protection Agency (EPA). They issue standards and guidelines and regulate air pollution. Standards ensure clean air, identifying pollutants that could potentially damage environment, enforce laws which will prevent further degradation of air quality. The NAAQS regulates the six criteria air pollutants: sulfur dioxide (SO₂), particulate matter (PM₁₀), carbon monoxide (CO), ozone (O₃), nitrogen dioxide (NO₂), and lead (Pb). The EPA uses the Federal Reference Method (FRM) and Federal Equivalent Method (FEM) systems measure the amount of pollutants in the air are within the limits.

Emission Standards:

Given by EPA, that control the amount of pollutants like greenhouse gases, such as carbon dioxide (CO₂), oxides of nitrogen and oxides of sulfur. The standards set are established in two phases to stay up to date, Similar to the ambient standards, individuals states may also tighten regulations to their liking. Emission standards also regulate the amount of pollutants released by heavy industry and for electricity. Setting up different performance levels for different industries. Precise data collection ensures technological improvement

Methods:

The following Methods may be used to prevent volatile compounds and burnt hydrocarbons escaping from industries

Adsorption:

physical adsorption, in this activated carbon or anyother composite material is used mechanism is by Van der Waals force, material once saturated can be washed with solvents like methanol, ethanol or simply by heating or using water and material can be recovered, also cost effective. Chemisorptions: uses a chemical adsorption by chemical interation or reaction, chemical may be or may not be recovered, little costlier process . Fluidized bed systems, though more expensive to build and operate, yield high contacting with low pressure loss and regeneration can be accomplished within the system. The fixed beds are less expensive and provide longer packing life, but provide less contacting per unit length and require a larger pressure loss; because they are regenerated individually. Moving beds have properties between fixed and fluidized beds. Regeneration can be achieved by contact with a hot, inert gas, contact with a low pressure gas stream and pressure reduction over the bed. Steam desorption is the most commonly used process for regeneration.

Incineration:

Incineration or combustion or oxidation of pure hydrocarbons produces carbon dioxide and water. Sulfur and nitrogen compounds produce acid gases. oxidation must be done at high temperature, unless catalysts are involved. For example by using catalytic converters, thermal oxidation of the by-products of the incomplete engine combustion can be safely accomplished at temperatures much lower than would be required without the aid of catalysis.

Condensation:

Condensation and gas absorption are most commonly used for highly concentrated volatile compound streams that are advantageous to recover and the relatively large expense is warranted. It employs a drop in temperature and/ or increase in pressure to cause the VOCs in the emission stream to condense. The cleaned air stream is separated from the condensate containing target pollutants. Condensation is used to recover gasoline and fuel vapors at gasoline loading terminals and in gasoline dispensing facilities.

Gas Absorption:

Gas absorption involves the absorption of a gas into a liquid. Water can be used for recovery of water-soluble compounds such as acetone and low molecular weight alcohols, which can later be separated from water using distillation. Additives are often used affecting the surface tension, reducing interfacial resistance and increasing the apparent solubility. Gas absorption techniques are used for the recovery of a variety of chemicals in the coke manufacturing industry. They are often called scrubbers.

Particulate Control

Particulate matter escape can be prevented by gravitational settling, centrifugal impaction, inertial impaction, direct interception, diffusion and the electrostatic attraction. Various hi technology equipment’s and processes must have to be arranged by industries to carry out these processes

Physical barrears:

Fabric Filters:

Fabric filtration is one of the most common techniques to collect particulate matter from industrial waste gases. The use of fabric filters is based on the principle of filtration, which is a reliable, efficient and economic methods to remove particulate matter from the gases. The air pollution control equipment using fabric filters are known as bag houses.

Bag Houses

A bag house or a bag filter consists of numerous vertically hanging, tubular bags, 4 to 18 inches in diameter and 10 to 40 feet long. They are suspended with their open ends attached to a manifold. The number of bags can vary from a few hundreds to a thousand or more depending upon the size of the bag house. Bag houses are constructed as single or compartmental units. In both cases, the bags are housed in a shell made of rigid metal material. Occasionally, it is necessary to include insulation with the shell when treating high temperature flue gas. This is done to prevent moisture or acid mist from condensing in the unit, causing corrosion and rapid deterioration of the bag house.

Hoppers are used to store the collected dust temporarily before it is disposed in a landfill or reused in the process. Dust should be removed as soon as possible to avoid packing which would make removal very difficult. They are usually designed with a 60 degrees slope to allow dust to flow freely from the top of the hopper to the bottom discharge opening. Sometimes devices such as strike plates, poke holes, vibrators and rappers are added to promote easy and quick discharge. Access doors or ports are also provided. Access ports provide for easier cleaning, inspection and maintenance of the hopper.
          A discharge device is necessary for emptying the hopper. Discharge devices can be manual (slide gates, hinged doors and drawers) or automatic trickle valves, rotary airlock valves, screw conveyors or pneumatic conveyors)

Filter Media

Woven and felted materials are used to make bag filters. Woven filters are used with low energy cleaning methods such as shaking and reverse air. Felted fabrics are usually used with low energy cleaning systems such as pulse jet cleaning.
While selecting the filter medium for bag houses, the characteristics and properties of the carrier gas and dust particles should be considered. The properties to be noted include:
a) Carrier gas temperature
b) Carrier gas composition
c) Gas flow rate
d) Size and shape of dust particles and its concentration

The abrasion resistance, chemical resistance, tensile strength and permeability and the cost of the fabric should be considered. The fibers used for fabric filters can vary depending on the industrial application. Some filters are made from natural fibers such as cotton or wool. These fibers are relatively inexpensive, but have temperature limitations (< 212 F) and only average abrasion resistance. Cotton is readily available making it very popular for low temperature simple applications. Wool withstands moisture very well and can be made into thick felts easily.
          Synthetic fibers such as nylon, orlon and polyester have slightly higher temperature limitations and chemical resistance. Synthetic fibers are more expensive than natural fibers. Polypropylene is the most inexpensive synthetic fiber and is used in industrial applications such as foundries, coal crushers and food industries. Nylon is the most abrasive resistant synthetic fiber making it useful for applications filtering abrasive dusts. Different types of fibers with varying characteristics are available in the market.

Fabric Treatment

Fabrics are usually pre-treated, to improve their mechanical and dimensional stability. They can be treated with silicone to give them better cake release properties. Natural fibers (wool and cotton) are usually preshrunk to eliminate bag shrinkage during operation. Both synthetic and natural fabrics usually undergo processes such as calendering, napping, singeing, glazing or coating. These processes increase the fabric life and improve dimensional stability and ease of bag cleaning.
a) Calendering:
                    This is the high pressure pressing of the fabric by rollers to flatten, smooth, or decorate the material. Calendering pushes the surface fibers down on to the body of the filter medium. This is done to increase surface life, dimensional stability and to give a more uniform surface to bag fabric.
b) Napping:
                    This is the scraping of the filter surface across metal points or burrs on a revolving cylinder. Napping raises the surface fibers, that provides a number of sites for particle collection by interception or diffusion. Fabrics used for collecting sticky or oily dusts are occasionally napped to provide good collection and bag cleaning ease.
c) Singeing:
                    This is done by passing the filter material over an open flame, removing any straggly surface fibers. This provides a more uniform surface.
d) Glazing:
                    This is the high pressure pressing of the fiber at elevated temperatures. The fibers are fused to the body of the filter medium. Glazing improves the mechanical stability of the filter and helps reduce bag shrinkage that occurs from prolonged use.
e) Coating:
                     Coating or resin treating involves immersing the filter material in natural or synthetic resin such as polyvinyl chloride, cellulose acetate or urea - phenol. This is done to lubricate the woven fibers or to provide high temperature durability or chemical resistance for various fabric material.

Control of Oxides of Nitrogen

A number of inexpensive methods to reduce NOx emissions released from various combustion equipment are being developed by the U.S. Environmental Protection Agency's Office of Research and Development (ORD) These methods aim at eliminating operating problems, increasing the equipment life, and preventing emissions of other pollutants.

General Methods of Control

NOx control can be achieved by:

Fuel denitrogenation
Combustion modification
Modification of operating conditions
Tail-end control equipment

	Selective Catalytic Reduction
	Selective Non-Catalytic Reduction
	Electron Beam Radiation
	Staged Combustion

The most promising methods of reducing NOx emissions currently are classified into three groups. The methods for each group are:

Before burning:
    Fuel denitrogenation

During burning:
    Staged combustion
    Catalytic combustion

In exhaust Gas:
    Flue gas treatment
    Catalytic emission control

Fuel Denitrogenation

One approach in reducing nitrogen oxide emission is to remove a large part of the nitrogen contained in the fuels. Nitrogen is removed from liquid fuels by mixing the fuels with hydrogen gas, heating the mixture and using a catalyst to cause nitrogen in the fuel and gaseous hydrogen to unite. This produces ammonia and cleaner fuel. This technology can reduce the nitrogen contained in both naturally occurring and synthetic fuels.

Combustion Modification

Combustion control uses one of the following strategies:

	Reduce peak temperatures of the flame zone. the methods are :           Increase the rate of flame cooling           Decrease the adiabatic flame temperature by dilution.
	Reduce residence time in the flame zone. For this we, change the shape of the flame zone
	Reduce Oxygen concentration in the flame one. This can be accomplished by: decreasing the excess air controlled mixing of fuel and air using a fuel rich primary flame zone.

Modification of Operating Conditions

The operating conditions can be modified to achieve significant reductions in the rate of thermal NOx production. The various methods are:

	Low-excess firing
	Off-stoichiometric combustion ( staged combustion )
	Flue gas recirculation
	Reduced air preheat
	Reduced firing rates
	Water Injection

The flue gas treatment can be used to control the oxides of nitrogen in the following manner:

NOx emissions can also be removed by removing them from the exhaust gases that are released from burners.
In one process, ammonia is added to the flue gas prior to the gas passing over a catalyst. The catalyst enables the ammonia to react chemically with the NOx converting it to molecular nitrogen and water. This system promises as high as 90% removal of nitrogen oxides from the flue gases.
In a second process, both NOx and SOx are removed. The combustion gases are moved across a bed of copper oxide, which reacts, with the sulfur oxide to form copper sulfate. The copper sulfate acts as a catalyst for reducing NOx to ammonia. Approximately 90% of the NOx and SOx can be removed from the flue gases through this process.

Water injection is used for the NOx removal as follows:

One of the methods of reducing NOx emissions from oil-fired combustion systems is to mix water with the oil before it is sprayed into the burner. Water decreases the combustion temperature and can reduce NOx emissions from burning light weight oils by as much as 15%.
A significant added advantage in using these emulsions is that they reduce the emission of particulate matter. When water is mixed in the oil, each oil droplet sprayed into the firebox has several tiny water droplets inside. The heat existing in the firebox makes these water droplets flash into steam and explode the oil droplet. Increasing the surface area of the oil, enables it to burn faster and more completely. A reduction in particulate emissions can be achieved regardless of whether light or heavy oils are being burned.

Tail End Control Equipment

Combustion modification and modification of operating conditions provide significant reductions in NOx, but not enough to meet regulations. For further reduction in emissions, tail-end control equipment is required.
Some of the control processes are:

	Selective Catalytic Reduction
	Selective non catalytic Reduction
	Electron Beam Radiation
	Staged Combustion

Selective catalytic reduction can be used in NOx control in the following manner:

Another method of reducing the pollutant emissions is to use a catalyst to achieve oxidation of fuel rather than high temperature. Catalytic combustors like natural gas, propane and vaporized distillate oil, for gas turbines have reduced NOx emissions to well below 10 ppm. Fuel and air are mixed to the desired ratio and introduced into a chamber containing the ceramic or metal catalyst.

In this process, the nitrogen oxides in the flue gases are reduced to nitrogen
During this process, only the NOx species are reduced. NH₃ is used as a reducing gas.
The catalyst is a combination of titanium and vanadium oxides.

The reactions are given below :

     4 NO + 4 NH₃ + O₂   ----->   4N₂ + 6H₂O

     2NO₂ + 4 NH3+ O₂   ----->   3N₂ + 6H₂O

Selective catalytic reduction catalyst is best at around 300 too 400 ^oC.
Typical efficiencies are around 80 %.

The selective noncatalytic reduction process involves the following:

At Higher temperatures (900-1000 ^oC), NH3 will reduce NOx to nitrogen without a catalyst.
At NH3: NOx molar ratios of 1:1, to 2:1, about 40 - 60% reduction is obtained.
SNR is cheaper than SCR is terms of operation cost and capital cost.
Tight temperature controls are needed. At lower temperatures, non reacted ammonia is emitted. At higher
temperatures the ammonia is oxidized to NO.

The electron beam radiation process is as follows:

This treatment process is under development, and is not widely used. Work is underway to determine the
feasibility of electron beam radiation for neutralizing hazardous wastes and air toxics.

Irradiation of flue gases containing NOx or SOx produce nitrate and sulfate ions.
The addition of water and ammonia produces NH4NO3, and (NH4)2SO4
The solids are removed from the gas, and are sold as fertilizers.

The staged combustion process to control NOx can be explained as under:

Staged combustion processes significantly reduce NOx emissions. In the initial stage of combustion, the air supplied to the burners is less than the amount required to completely burn the fuel. During this stage, fuel bound nitrogen is released but cannot be oxidized, so it forms stable molecules of harmless molecular nitrogen (N2). Other components of the fuel are also released without being fully oxidized. These include carbon particles and carbon monoxide. By adding a second stage, in the air-fuel mixture, the carbon and carbon monoxide can be burned, converting them to carbon dioxide.
        Modifying existing coal furnaces to achieve a staged combustion process has resulted in a 30% to 50% reduction in NOx emissions. Besides reducing NOx emissions, limiting the air during the combustion process increases the efficiency of converting fuel to usable heat.
         This is a cheap approach. However, it requires a larger firebox for the same combustion rate and it is difficult to get complete burning of the fuel in the second stage, so that the amount of unburned fuel and/or carbon monoxide in the exhaust gas is increased.
        Some practical examples of the application of this technology are:

1. Pulverized Coal Burner:

A coal burner design based on staged combustion may reduce NOx by as much as 85%. The burner produces a fuel-rich primary combustion zone and controls the fuel-air mixing. These conditions lead to preferential conversion of the nitrogen in the coal to molecular nitrogen (N2). In conventional burners, this fuel nitrogen is the primary source of NOx. Additional air is introduced from the periphery of the burner to complete combustion in a secondary zone. The design also results in low levels of carbonaceous emissions consistent with high energy efficiency.

2. Residential Oil Furnaces:

A primary innovation in the residential oil furnace was used to remove a controlled amount of heat from the fire box and thus reduce the formation of thermal NOx by almost 65%. Oil consumption was also reduced by an average of 15%. Thus fuel savings have been achieved while simultaneously protecting the environment.

3. Small-scale industrial boilers:

In boilers used for light industries and for heating large buildings, reducing the amount of oxygen available during the initial combustion stage has been demonstrated to be a viable technique to reduce NOx emissions from these boilers. However, this results in incomplete combustion so that the amount of carbon particles emitted in the exhaust increases.
The development of a burner for these boilers will limit the NOx emissions while maintaining the high efficiency of the boiler and preventing the formation of the carbon particulate.

4. Catalytic emission control in motor vehicle exhaust:

A special 3-way catalytic converter consists of a catalyst that causes nitric oxide
to oxidize the carbon monoxide and hydrocarbons. In this process, molecular nitrogen, carbon dioxide and water vapor are released.
        In order to make this reaction work efficiently, the proportions of NO, CO and HC entering the catalytic converter must be carefully controlled. This is done by regulating the ratio of air and fuel in the combustion chamber. Too much fuel results in increased CO and HC emissions. Too much oxygen results in increased emissions of nitrogen oxides. An oxygen sensor in the exhaust manifold allows control, while an active feedback device adjusts the mixture of and fuel in the carburetor or fuel injection system.