Stationary source emissions control

Transcription

1 Air Quality 4 th ed Chapter 10- Control of Emissions from Stationary Sources Stationary source emissions control Approaches Tall stacks Fuel use policies Pollution prevention Gas cleaning 1

2 Tall stacks Reduce ground level concentrations Historically favored by industry Regulatory limits on use to meet air quality goals Fuel-use policies Use fuels with lower environmental impacts Natural gas- fuel of choice Coal has significant environmental impacts Nuclear power Low environmental impact Safety concerns Intensively used in greater Chicago area, France, other countries 2

3 Pollution prevention Substitution Process equipment changes Operating practices Maintenance Process changes Energy conservation Gas cleaning technology End of the pipe approach Historically favored by regulators Generally fugitive emissions are not controlled 3

4 Gas cleaning technology Particle or dust collection Removal of gas-phase substances Cleaning performance Can be quantified Where E = collection efficiency A = concentration of influent B = emission reduction required or emission rate 4

5 Cleaning performance Can be quantified from collection efficiency Let us assume A = 600 lb/hour B = 30 lbs/hour Cleaning performance Particle collection efficiency Gas removal efficiency 5

6 Generalized fractional collection efficiency curve Fractional collection efficiencies of dust collectors 6

7 Particle collection systems Cyclones Filtration Electrostatic precipitation Scrubbers Cyclones Collect particles by inertial separation Two basic types Simple cyclone (low efficiency) Multi-cyclone ( medium to high efficiency) 7

8 Cyclones Simple cyclones Used for nuisance dusts Low capital costs Low operating costs Cyclones High efficiency cyclones Used as pre-cleaners Parallel units Higher gas volumes 8

9 Simple cyclones used on grain elevators in Galveston, Tx Multiple cyclone 9

10 Fabric filtration High collection efficiencies (99.5+) Referred to as baghouses High capital costs High operating costs Fabric filtration Cleaning efficiency increases with soiling High resistance to airflow Problems with gas temperatures & abrasion Bags must be periodically cleaned. Bags must be periodically replaced. 10

11 Fabric filtration-schematic of baghouse Fabric filtration-baghouse used in the steel industry 11

12 Electrostatic precipitators Impart electrical change to particles Particles deposited on collection plates Collection efficiencies of 95-99% Dust collector of choice on high S coal Electrostatic precipitators 12

13 Electrostatic precipitators High collection efficiency Used on coal-fired power plants Work relatively poorly on low S coal Work poorly on low resistivity dusts Low resistance to airflow Wet scrubbers Several types Open-spray tower Venturi scrubber 13

14 Venturi scrubber used in the steel industry Venturi scrubber-schematic 14

15 Control of gas-phase emissions Thermal oxidation Adsorption Absorption Biological treatment Thermal oxidation Flammable limits Lower explosion limit(lel) Upper explosion limit(uel) 15

16 Thermal oxidation Waste streams > LEL Use flare oxidation Waste streams < LEL Use thermal oxidation Use catalytic oxidation Flare tower in refinery industry 16

17 Adsorption Attract gases/vapors to solid surface Physical phenomenon Works well on non-polar HCs Often used for solvent recovery Can be used to control malodors Adsorption Typically use activated carbon Use deep carbon beds Activated carbon must be regenerated Has limited adsorption capacity 17

18 Adsorption system Absorption Reaction of gas with solvent Generally called scrubbing Expensive gas cleaning 18

19 Scrubbers Typically use countercurrent systems Gas flows up Scrubbing liquid flows down Use packing materials to increase collection efficiency Fixed bed scrubber 19

20 Packing material Scrubber concerns Corrosive gases Scaling/plugging Waste disposal 20

21 Control of SO x Coal beneficiation Fluidized bed combustion(fbc) Flue gas desulfurization(fgd) Flue-gas desulfurization(fgd) Use to scrub SO 2 by coal-burning utilities Use limestone/lime scrubbers Achieve 90+% collection efficiency 21

22 FGD Wet scrubbing Use limestone slurry Must treat wastes FGD Limestone scrubbing 22

23 FGD Limestone scrubbing(continued) Lime scrubbing FGD system 23

24 Fluidized-bed combustion Biological treatment Bioscrubbers Trickle bed reactors Biofilters 24

25 Control of NO x Low NO x burner Control mixing of fuel and air Staged combustion Fuel- rich followed by fuel-lean Reduce O 2 or excess O 2 to cool Catalytic reduction Control of NO x Catalytic reduction titanium/vanadium 25