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Gaseous Control Techniques , Lecture 10, Fuel Technology2, (Week 14).pptx

  1. Techniques for controlling gaseous emission  Recovery Techniques 1: Absorption 2: Adsorption Techniques  Destruction techniques 1: Thermal Combustion 2: catalytic Combustion
  2. Absorption  The removal of one or more selected components from a gas mixture by absorption is probably the most important operation in the control of gaseous pollutant emissions.  Absorption is a process in which a gaseous pollutant is dissolved in a liquid.  Water is the most commonly used absorbent liquid. As the gas stream passes through the liquid, the liquid absorbs the gas, in much the same way that sugar is absorbed in a glass of water when stirred.  Absorption is commonly used to recover products or to purify gas streams that have high concentrations of organic compounds.  Absorption equipment is designed to get as much mixing between the gas and liquid as possible.
  3.  Absorbers are often referred to as scrubbers, and there are various types of absorption equipment.  The principal types of gas absorption equipment include 1. spray towers, 2. packed columns, 3. spray chambers, and 4. venture scrubbers.  The packed column is by far the most commonly used for the absorption of gaseous pollutants.
  4.  The packed column absorber has a column filled with an inert (nonreactive) substance, such as plastic or ceramic, which increases the liquid surface area for the liquid/gas interface.  The inert material helps to maximize the absorption capability of the column. In addition, the introduction of the gas and liquid at opposite ends of the column causes mixing to be more efficient because of the counter-current flow through the column. In general, absorbers can achieve removal efficiencies grater than 95 percent.  One potential problem with absorption is the generation of waste-water, which converts an air pollution problem to a water pollution problem.
  5. 1. Absorbent: the liquid, usually water mixed with neutralizing agents, into which the contaminant is absorbed 2. Solute: the gaseous contaminant being absorbed, such as SO2, H2S, and so forth 3. Carrier gas : the inert portion of the gas stream, usually flue gas, from which the contaminant is to be removed 4. Interface : the area where the gas phase and the absorbent contact each other 5. Solubility : the capability of a gas to be dissolved in a liquid
  6. Adsorption Adsorption is used when 1. The pollutant gas is incombustible or difficult to burn 2. The pollutant is sufficiently valuable to warrant recovery 3. The pollutant is in very dilute concentration in the exhaust system .  The technique is based on the reaction of gases on the solid adsorbents.  The adsorption may be physical or chemical.  In this method gas is passed through a bed of adsorbents packed in the specially designed towers to allow the maximum contact between the two
  7.  Physical adsorption depends on the temperature and pressure conditions. Adsorption is promoted by increase in pressure and decrease in temperature  Chemical adsorption depends on the reactivity of the gases and their bond forming capacity with the surface of the adsorbent, which provides surface for the reaction.  Adsorbent can be regenerated for continuous reuse.  In some cases if is not economical to regenerate, it better to dispose the pollutant together with the adsorbent
  8.  Adsorption is mostly pollutant specific.  e.g – Activated carbon, silica gel and diatomaceous earth are suitable for adsorption of water vapours from a gas phase. It can also adsorb SO2 and NH3.  Activated carbon is most suited for removal of organic gases from gas stream.
  9.  Carbon adsorption systems are either regenerative or non-regenerative.  Regenerative system usually contains more than one carbon bed. As one bed actively removes pollutants, another bed is being regenerated for future use.  Non-regenerative systems have thinner beds of activated carbon. In a non-regenerative adsorber, the spent carbon is disposed of when it becomes saturated with the pollutant.
  10. Combustion  In many cases it is not possible to remove the required amount of specific pollutant from an exhaust stream by techniques such as absorption or adsorption.  The other technique available is Combustion  Combustion refers to rapid oxidation of substances (usually referred as fuels) with evolution of heat. To summarize, Combustion is defined as rapid, hightemperature gas- phaseoxidation.  Simply, the contaminant (a carbon-hydrogen substance) is burned with air and converted to carbon dioxide and water vapor.
  11.  Combustion process involves three distinct components. 1. Fuel : -A solid, liquid or gaseous substance with energy rich C-C or C-H bonds among others, which are broken up during combustion 2. Oxidant:- A substance which aids in combustion process by breaking the chemical bonds allowing the release of heat 3. Diluent:- A substance that does not take part in the combustion process but acts as carrier of the fuel or the oxidants. Most common diluents is Nitrogen present in the air.
  12.  Combustible gases are burned in open air, which produces flare  The flare is usually employed to remove hydrocarbons and organic vapours, odorous compounds in refineries and chemical works.  It can also burn gases such as NH3, HCN or other toxic or dangerous gases.  If aromatic hydrocarbons are present, they burn with Smokey flame. This can be avoided by injecting a steam into the flame, which reacts and forms hydrogen and CO both burn smokelessly.  However such steam-injected flare are little noisy
  13. END