Upload
eashwar-ranganathan
View
7
Download
1
Tags:
Embed Size (px)
DESCRIPTION
ESE3201
Citation preview
Air Pollutants and Their EffectsTie the corresponding items togetherCO ~ Irritating to lungs
~ Contributing to tropospheric/ground level O3NOx ~ Aerosol
~ Mainly from transportation
VOC ~ Mainly from fossil fuel combustion in stationary sources
~ Erosion of buildings
PM ~ In the form of solid or liquid~ Highly water soluble
SOx ~ smoke
Air Pollutants: Carbon Monoxide (CO) Colorless, odorless, tasteless gas
Produced during incomplete combustion
77% of total CO: emitted from transportation
Form carboxyhemoglobin (COHb) in the bloodstream
An indoor air pollutant
CO & Health EffectsTypical concentration Roadway: 550 ppm Cigarette smoke: 2400 ppm Smoky bar: 2030 ppm (~ 1 hr ambient standard) Smokers [COHb]: ~ 510%
1.7% ~ 35 ppm for 1 hour 2.5%: tone signal 10%: headache 30%: fatigue, impaired judgment 60%: lost of consciousness
[COHb] in blood and symptons
CO: Effects of Exposure
Source: Seinfeld, J.H. 1986, Atmospheric Chemistry and Physics of Air Pollution, published by John Wiley & Sons, Inc., New York
Dimethyl Sulfide (CH3SCH3, 80110 ppt)
Sulfur Containing Pollutants
Carbonyl Sulfide (OCS, 500 ppt)
Hydrogen Sulfide (H2S, 450840 ppt)
Carbon Disulfide (CS2, 35120 ppt)
Carbonyl Sulfide (COS, 500 ppt)
SO2, 1500 ppt)
Wetland
Global Sulfur Emissions EstimatesSource SO2 SO42- Total Sulfur (Tg(S) yr-1)*
Fossil-fuel burning 70 2.2 7177 & industryBiomass burning 2.8 0.1 2.23.0 (1.4/1.1)Oceans (DMS) -- 40320 1525 (8.4/11.6)aWetlands (H2S,DMS, and CS2) -- -- 0.012 (0.8/0.2)Plants + soils -- 24 0.250.78 (0.3/0.2)bVolcanoes 78 24 9.311.8 (7.6/3.0)Anthropogenic (Total) 7380Natural 2540
Total 98120
*(w/o sea salta and soil dustb)Numbers in ( ) are fluxes from Northern Hemisphere/South Hemisphere. Source: Berresheim et al., 95
Anthropogenic Sulfur Emissions
Source: Dignon and Hameed (1989), Hameed and Dignon (1992), and Spiro et al. (1992)
Oxides of Sulfur (SOx) ~ 90% of the anthropogenic-related SOx emitting from
fossil fuel combustion
Transformation of SO2(g) to sulfuric acid (H2SO4):
- Reaction with hydroxyl radicals:
SO2 + OH HOSO2
HOSO2 + O2 SO3 + HO2
SO3 + H2O H2SO4
Oxides of Sulfur (SOx) Transformation of SO2(g) to sulfuric acid (H2SO4) (ctd.):
- Reaction with O2 (needs large activation energy):
2SO2(g) + O2 2SO3(g)
SO3(g) + H2O(pte) H2SO4(pte)
- Soluble in water:
SO2(g) + H2O(pte) H2SO4(pte) . (not balanced)
Neutralization reactions of SO2(g) to (NH4)2SO4:
H2SO4(pte) + 2NH3(g) (NH4)2SO4(pte)
SOx: Effects Precursor of sulfate particles Most sulfate particles in urban air: 0.20.9 m
- p visibility- easily penetrating deep in human lungs- synergistic effects
Highly water soluble: erosion of building materials
(1) Sulfur oxides:
(2) Others (e.g., H2S):
Control of Sulfur-Containing Pollutants
Pre-combustion control; pollution preventionRemoval of SO2 from rich waste stream lean waste stream Combustion Flue gas desulfurization (FGD):
- Wet-throwaway- Double alkali- Dry throwaway- Wet-dry throwaway- Adsorption/absorption
Removal of S from natural gas and petroleum
Unit of sulfur content emitted:
Pre-combustion control: Fuel switching: p 30-90% emission; blending; temporary Compliance coal or low-sulfur coal:
Control of SO2: Pre-combustion Control
% or mass of SO2 emitted per heat energy delivered
- Meeting NSPS without control: [emission] < 1.2 lb/MBtu (0.5 g/MJ) (NSPS: new source performance standard)
- Organic bound: chemical/biological treatment- Inorganic form: washing to remove pyrite (FeS2),
p ash content
Control of Rich SO2 in Waste GasesResource Recovery
for smelting ores:SO2 + 0.5O2 vanadium catalyst SO3;SO3 + H2O o H2SO4
Cost-effective if (a) SO2 concentration is high enough(b) the chemical conversion leads to marketable
products (H2SO4)
Combustion control: Fluidized-bed combustion (FBC):
Control of Lean SO2 in Waste Streams
CaCO3 + SO2 + 0.5 O2 o CaSO4 + CO2 ~ 90% p of S
Advanced Pressurized Fluidized-Bed Combustion (PFBC)Source: http://www.engineering-4e.com/systems.htm
Flue gas desulfurization (FGD)- Often used for coal/oil power plantsProcesses: - Wet-throwaway: limestone, hydrated lime/quicklime- Double alkali (DA): sodium (bi)carbonate (Na2CO3, NaHCO3)- Dry-throwaway: Boiler injection (CaCO3/Ca(OH)2); Boiler/flue gas injection (Na2CO3 or NaHCO3)
- Wet-dry: spray dryers (SD)- Regenerative adsorption/absorption: Regenerableadsorbents or absorbents
Control of Lean SO2 in Waste Streams
Control of Lean SO2 in Waste Streams
CaCO3 + SO2 + 2H2O o CaSO32H2O ~ 90% p of SCaO + SO2 + 2H2O o CaSO32H2O ~ 95% p of S
1. Expensive2. Corrosion, scaling, plugging problems3. n amounts of water used4. Sludge treatment: CaSO3 CaSO4 thickening
vacuum filtration
Disadvantages of wet scrubber:
(CaCO3 CaO (porous materials) + CO2)heat
Wet-throwaway process (limestone), Wet Scrubber:
Wet-throwaway processes (Quicklime/hydrated lime):- An alternative to limestone in wet-throwaway processes
- CaO (burned lime/quicklime) hold tank hydrated to Ca(OH)2- More reactive than CaCO3 because of higher surface area
- More expensive because of additional steps
Control of Lean SO2 in Waste Streams
Eashwar Ranganathan
Na2CO3 + SO2 o Na2SO3 + CO2- To solve the problems of wet scrubbing
- Solubility of sodium salts >> solubility of calcium salts
liquid w/o solid particles regenerated with CaO/CaCO3
Na2SO3 + CaCO3 + 0.5O2 o CaSO4 + Na2CO3 (in hold tank)CaSO4 can be precipitates
Control of Lean SO2 in Waste StreamsDouble alkali scrubber
Control of Lean SO2 in Waste StreamsDouble alkali scrubber
(Source: Air Pollution Control Engineering by Noel De Nevers, McGraw Hill, 95)
Control of Lean SO2 in Waste StreamsScrubber
Taken from www.induscoenviro.com/img/scrubber1.jpg
Control of Lean SO2 in Waste StreamsScrubber
Taken from www.mikropul.com/products/wscrub...turi.gif
Control of Lean SO2 in Waste StreamsScrubber
Taken from www.q-net.net.au/~legion/scrubber.jpg
Control of Lean SO2 in Waste Streams
CaCO3 CaO CaSO3 CaSO4
Disadvantages of dry system1. More expensive by requiring excess of lime/limestone2. Increase in solid waste3. Less efficient than wet-throwaway (DA)4. Mainly based on empirical tests
dried at high temp. SO2 O2
Dry-throwaway process:
Spray dryers (SD): dispersion from high-pressure gas atomizing nozzle for heat-sensitive products (e.g., SO2-containing flue gases) more like wet-lime scrubber partly like dry sorbent injection
Control of Lean SO2 in Waste Streams
Picture taken from class.fst.ohio-state.edu/Dairy_T...-141.gif
Control of Lean SO2 in Waste Streams Spray dryers (SD):
Taken from www.niro.com/NIRO/cmsresources.n...ber2.gif
Control of Lean SO2 in Waste Streams Regenerative Systems: Adsorbents/absorbents can be regenerated Concurrent removal of SOx and NOx: still under study
ReagentsCaCO3 Ca(OH)2 Na2CO3 NaHCO3 Regenerablelimestone hydrated lime sodium sodium bi- adsorbents/
Process (quicklime) carbonate carbonate absorbents----------------------------------------------------------------------------------------------------Wet-throwaway Limestone / Lime scrubbing
DA DA DA
Dry-throwaway Boiler Boiler lime Boiler or flue Boiler or fluelimestone injection injection injectioninjection
Wet-dry SD SD SD
Regenerative Many kinds, producing SO2 or S or H2SO4. Some control both SO2and NOx.
Control of Lean SO2 in Waste Streams
Removal of sulfur from natural gas streams:
H2S(g) H2S (l) H+ + HS- o add alkali remove H+
Control of Sulfur in Natural Gas & Petroleum
Removal of sulfur from hydrocarbon fuels:
H2S + O2 o S (inert, harmless) + H2O(Undesirable oxidation: H2S + 3/2 O2 o SO2 + H2O)HC-containing S + H2 catalysts (Ni/Co) HCs + H2S
Reduction Elemental Oxidation Oxidationform 1st step 2nd step
----------------------------------------------------------------------------------------------------High pressure+ reaction with O2, (1) atmospherehigh temperature+ at (1) high temperatures (2) catalytic reactor hydrogen gas + catalyst. (2) low temperaturesBiological processes (low pressuresand temperatures).
NH3 N2 NO NO2H2S S SO2 SO3
Reactions of Nitrogen and Sulfur: ComparisonElementary oxidation & reduction
Reaction( with water) Reaction (with NH4+ or other cations)----------------------------------------------------------------------------------------------------Rate depends on atmospheric Rate depends on [atmospheric cations]moisture content
HNO3 nitrate particles
H2SO4 sulfate particle
Reactions of Nitrogen and Sulfur: ComparisonAdditional Reactions
Nitrogen Containing Pollutants
N2O- Colorless- Almost completely from natural sources- Long residence time | 120 years- Important Green-house gas
NH3- Emitted from natural & anthropogenic sources
N2O, NOx, HNO3, and NH3
NO2 and Ozone NO2
irritating the lungs, causing bronchitis and pneumonia, and lower resistance to respiratory infections
NOx + OHx HNO3
hQNOx + volatile organic compounds photochemical oxidants
Photochemical smog pollutantsO3: chest constriction & irritation of mucous membranes
Others: PAN (peroxyacetylnitrate), HCHO, acrolein, etc. causing irritation of eyes and respiratory systems (coughing),
Thermal NOx Prompt NOx Fuel NOx
(Source: Air Pollution Control Engineering by Noel De Nevers, McGraw Hill,95)
Control NOx: Formation
During the 1st part of combustion Mainly converted from original form in fuel Carbon bearing radicals
Control NOx: Prompt NOx
CH + N2 HCN + N
N + O2 NO + O
HCN + O2 NO
HCN + NO N2
Most of the fuel nitrogen is converted in the flame to HCN, subsequently converted to NH or NH2.
Control NOx: Fuel NOx
NH + O2 NO + H2O, or NH + NO N2 + H2ONH2 + O2 NO + H2O, or NH2 + NO N2 + H2O
[NOx] from fuel depends on [NO]/[O2] ratio in the flame zone At high-temp. zone of flame, if [O2] p [NO] p Usually, 20-50% of fuel nitrogen is converted to NOx,
depending on (1) furnace conditions, and (2) chemical nature of nitrogen in fuels.
During combustion:- Emitted [NOx] n with
Control NOx During Combustion
n peak temperature, n time at high temperature, n [O2] at high temperature
- Peak temperature depends on (1) fuel and oxidizer used, (2) flame size, (3) degree of fuel-air premix, and (4) amounts of fuel-air preheat
During combustion:Control NOx During Combustion
- Peak temperature
(Source: Air Pollution Control Engineering by Noel De Nevers, McGraw Hill, 2000)
Low NOx burner / Two-stage combustion:1st stage: low excess air
Control NOx During Combustion
p [O2] and max. temp. p [NOx] formed
2nd stage: - w/ excess air; - the max. temp. from the 1st stage is low enough to p [NO] formed
- Often add extra fuel (e.g., CH4) without N
Modify Combustion / Combustor Design
Control NOx During Combustion
Disadvantages: (1) n fuel consumption, (2) n [CO] emissions, and (3) larger burner.
Low NOx burner / Two-stage combustion (ctd.):Modify Combustion / Combustor Design
Control NOx During Combustion
prevent fuel-rich p thermal NOx and prompt NOx
[NOx] formed v fuel nitrogen
Low NOx burner / Flue gas recirculation (FGR):
When fuel gas is employed: through mixing + excess air
Modify Combustion / Combustor Design
(Source: Air Pollution Control Engineering by Noel De Nevers, McGraw Hill, 2000)
If oil is employed: prompt NOx
From mobile emission sources:Adding reducing agents:2NO + 2CO pt-rh catalysts N2 + 2CO2
Control NOx Post-Combustion
3-way catalytic converter
Taken from images.gasgoo.com/MiMwMDRfMDA0Iz...rter.jpg
Chemical Treatment of Combustion Exhaust Gases
Air-to-fuel ratio & Catalytic Converter PerformanceC
onve
rsio
n ef
ficie
ncy
(%)
Air-to-fuel ratio
(Source: Introduction to Environmental Engineering and Science by G.M. Masters, Prentice Hall, 2008)
Control of NOx Post-Combustion
14.8:1 14.9:1
(Source: Introduction to Environmental Engineering and Science by G.M. Masters, Prentice Hall, 97)
Control of NOx Post-CombustionAir-to-fuel ratio for vehicle emission control
From stationary emission sources:(1) Selective catalytic reduction (SCR) w/ reducing reagents:
Control NOx Post-Combustion
6NO + 4NH3 o 5N2 + 6H2O4NO + 4NH3 + O2 o 4N2 + 6H2O2NO2 + 4NH3 + O2 o 3N2 + 6H2O
Note: @ temp. > 1800oF, NH3 + O2 o NO + 1.5H2Odominates over other reactions
Chemical Treatment of Combustion Exhaust Gases
Control NOx Post-Combustion
(3) Scrubbing w/ solution of NaOH & KMnO4- Expensive electrochemical regeneration of KMnO4- More suitable for small-scale processes
From stationary emission sources (ctd.):(2) Selective noncatalytic reduction (SNCR)
Chemical Treatment of Combustion Exhaust Gases
Preventing artificial dilution of exhausts before release: - depending on dilution of excess air;- standard limit/regulation based on O2% in the exhaust
(Max. 6-7% of O2, by volume)
Other units: mg/m3, lb/106 Btu, g/GJ, Pg/kcal
Regulating NOx in Exhausts
Most abundant photochemical oxidants, in addition to formaldehyde (HCHO), peroxybenzoyl nitrate (PBzN), peroxyacetyl nitrate (PAN), and acrolein (CH2CHCOH)
Health effects of photochemical pollutants
O3: chest constriction & irritation of mucous membranes
Others (PAN, HCHO, acrolein, etc.): irritation of eyes and respiratory systems (coughing),
Photochemical Reactions: Ground Level O3
NO-NO2-O3 photochemical reaction sequenceN2 + O2 2NO2NO + O2 2NO2NO2 + hv NO + OO + O2 + M O3 + MO3 + NO NO2 + O2
Photochemical Reactions: Ground Level O3
When RCOO + NO RCO + NO2(RCOO & RCO : VOC radicals)
Concentrations measured in Los Angeles, CA, USA, Jul. 19, 1965
Source: U.S. HEW, 1970, Air Quality Criteria for Carbon Monoxide, AP-62, National Air Pollution Control Administration, Washington, DC
Air Pollutants and Ground Level O3
Why and how do VOC emissions aggravate ground level [O3]?
Photochemical Reactions: Ground Level O3
Tropospheric chemical reactions! To reduce [O3]: p both VOCs and NOx