This article throws light upon the four main processes to control gaseous pollutants from stack gases. The processes are: 1. Combustion 2. Absorption 3. Adsorption4. Closed Circuit and Recovery Systems.
Process # 1. Combustion:
Combustion process like flame combustion or catalytic combustion can be utilized to greatest advantage when the gases or vapours to be controlled are organic in nature. Equipment employing the principle of flame combustion includes.
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(i) Fume and vapour incinerators
(ii) After-burners
(iii) Flares, either with steam injection or venturi flare.
The use of after-burners or incinerators has met with varying success depending on the kind of after-burner used and the type of incinerator. Flare design should provide for smokeless combustion of gases of variable composition and a wide range of flow rates.
Venturi flares mix air with the gas in the proper ratio prior to ignition to achieve smokeless burning. Steam injection flares mix steam with the stack gases as they reach the stack.
Catalytic combustion process is used with success for the control of effluent gases, fumes and odours from refineries, burning waste cracking gases, phenolic-resin curing ovens. Coffee roasting process, foundry core baking ovens and chemical plants. Gases and fumes containing excessive amount of particulates matter reduce the effectiveness of catalytic combustion units due to a coating that forms on the catalyst.
Process # 2. Absorption:
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In this process, effluent gases are passed through absorbers (scrubbers) which contain liquid absorbents that remove one or more of the pollutants in the gas stream.
The efficiency of this process depends on:
i. Amount of surface contact between gas and liquid.
ii. Contact time.
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iii. Concentration of the absorbing medium.
iv. Speed of reaction between the absorbent and the gases.
Absorbents are being used to remove SO2, H2S, SO3, F, and oxides of nitrogen. The absorbents may be either reactive or non-reactive with the pollutants removed by them. Some of the reactive absorbents are regenerative (i.e., they may be treated and reused), while others are of non-regenerative type.
The common absorbing solution used for removing different gaseous pollutants from gas streams:
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Gaseous pollutant – Common absorbents used in solution form
SO2 – Di-methyl-aniline, mixture of xylidine and water (1: 1), ammonium sulphite, basic aluminium sulphate, ethanol amines, sodium sulphite, ammonium sulphite and bi-sulphate, water, alkaline water, a suspension of Ca (OH)2, calcium sulphite and calcium sulphate, barium thionates and sulphites.
H2S – NaOH and phenol mix (mole ratio 3: 2), tripotassium phosphate, sodium alamine or potassium di-methyl glycine, ethanol amines, soda ash solution containing suspended iron oxide or hydroxide, soda ash alone, sodium thioarsenate ammoniacal liquor from coke ovens.
HF – Water, sodium hydroxide.
Oxides of Nitrogen – Water, aqueous nitric acid.
Process # 3. Adsorption:
In the process the effluent gases are passed through absorbers which contain solids of porous structure. The commonly used adsorbs include activated carbon, silica gel, activated alumina, lithium chloride, activated bauxite, etc. Active carbon appears to be the adsorbent most suitable for recovering organic solvent vapours.
The steps necessary for effective removal of gaseous pollutants by adsorbents are:
i. Contact of the gaseous or vaporous pollutant with the solid adsorbent.
ii. Separation (desorption) of the adsorbed gaseous pollutant from the solid adsorbent by regeneration or replacement of the adsorbents.
iii. Recovery of the gases for final disposal
The efficiency of removal of gases by adsorbents depends on:
i. The physical and chemical characteristics of the adsorbent.
ii. The concentration and nature of gas to be adsorbed.
Process # 4. Closed Circuit and Recovery Systems:
Gaseous like SO2, Oxides of Nitrogen, and Hydrocarbons can be recovered from the waste gas streams if they are present in sufficient concentrations. The most usual method at smelters is to use the SO2 stream as the raw material for the manufacture of H2SO4.
Similarly the vapour recovery methods used in refineries are useful when the concentration of hydrocarbons in the effluent stream is high and relatively uncontaminated.
Oxides of N from wastes gas streams in a HNO3 plant is recovered using commercial zeolite. Oxides of N absorbed in the bed are recovered as enriched oxides of N and HNO3.by regenerating the bed at elevated temperature with hot air or steam.
In another process known as wet lime process for removing SO2 from power plants, pulverized limestone is injected in to the boiler furnace, where the heat drives CO2, converting the CaCO3 to the reactive oxide form. The oxide then reacts with the SO2 to form solid sulphites and sulphaetes.