AIR POLLUTION AND CONTROL METHODS UNIT 4 - CONTROL OF GASEOUS POLLUTANTS.pptx

Gives brief idea about types of gaseous pollutants and how to control them

1. UNIT 4 CONTROL OF GASEOUS POLLUTANTS By V. Nikhila Bhavani Assistant Professor VJIT, HYD

2. SYLLABUS Adsorption, Absorption, Combustion So x Control technology- Natural dispersion by dilution, Using alternate fuels, removal of sulphur from fuels(Desulfurization) NO x Control technology- NO x control by modification of operating and design conditions- Low Excess air combustion, Decreasing Combustion air temperature, Two stage combustion, Flue gas recirculation.

3. ABSORPTION

4. PACKED TOWER

5. PLATE TOWER

6. SPRAY TOWER

7. ADSORPTION

8. NON-REGENERATIVE BED

9. COMBUSTION

10. NO X CONTROL TECHNOLOGY

11. Nox emissions from flue gases can be reduced by Dilution in atmosphere by increasing stack height Modification of operating and design conditions Treatment of flue gases

12. MODIFICATION OF OPERATING AND DESIGN CONDITIONS Low excess air combustion: The presence of excess air, effects both the temperature and oxygen concentration of gases in the post combustion zone. Normal boiler units operate in the presence of 10-20% excess air to ensure complete combustion of the fuel but this excess air provides enough oxygen for reacting with nitrogen. The results for pulverized coal firing in an experimental unit are shown in figure

13. In this specific case, NOX decreased from 60 ppm at 25% excess air to as low as 175ppm at 1.4% excess air. But this results in lowering of carbon combustion efficiency. It was observed that combustion efficiency falls from 99.5% to 25% excess air to around 92% to 2% excess air. This occurs in spite of the fact that the flame temperature increases as the inlet air concentration approaches the stoichiometric value. The rapid drop-off in oxygen concentration in this case dominates over the temperature effect.

14. ii) Decreasing combustion air temperature In many industries waste heat is available to help preheat the air entering a combustion process. Although this process leads to appreciable energy savings, the added energy increases the flame temperature. Thus No x emissions increase. Data from full sized boiler tests indicate a three fold increase in No x emissions when combustion air is preheated from 25°C to 300°C

15. Since temperature has a major influence on nitric oxide formation, an effective method of control is cooling of primary flame zone by heat transfer to surrounding surfaces. iii) Two Stage Combustion: This is one of the most effective control method for the suppression of the formation of nitrogen oxides. In the first stage the fuel and air are burned at near- stoichiometric conditions. All the fuel is fired with only 85-95% of total air requirements at the bottom of the furnace followed by secondary air injection higher up in the furnace to complete the combustion.

16. Thus in the primary zone incomplete combustion of the fuel takes place resulting in reduced concentrations of Nox in furnace gases. Heat removal and gas dilution between zones cause the temperature of the gases after the primary zone to decrease so that the final stage of combustion process occurs at a lower temperature. Reduction in the emission of Nox by 38% for coal and oil firing and by 50% for natural gas combustion has been observed under these conditions.

17. iv) Flue gas recirculation: The recirculation of coal flue gas into the combustion chamber is found to be quite effective in reducing NOX formation from stationary sources. A portion of he cooled flue-gas is injected back into the combustion zone. This additional gas acts as a thermal sink and reduces the overall combustion temperature. In addition the oxygen concentration is lowered as the flue gases going out have a higher N/O ratio i.e., lower oxygen availability.

18. Both these effects favour a reduction in NOX emissions. One disadvantage is the increased cost of duct work, since large volumes of gas are mixed with the primary air prior to combustion. Effect of recirculation in oil-fired furnaces us shown in figure

19. SO X CONTROL TECHNOLOGY