AIR QUALITY AND POLLUTION (TKA 3301) LECTURE NOTES 9- Criteria Pollutants (NOx, SOx, O3)
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Transcript of AIR QUALITY AND POLLUTION (TKA 3301) LECTURE NOTES 9- Criteria Pollutants (NOx, SOx, O3)
Criteria PollutantsCriteria Pollutants(NO(NOx,x, SO SOxx, O, O33) )
Criteria PollutantsCriteria Pollutants(NO(NOx,x, SO SOxx, O, O33) )
Dr. Marzuki Hj. IsmailDr. Marzuki Hj. Ismail
Semester I 2009/10 Semester I 2009/10
ENTECHENTECH
Dr. Marzuki Hj. IsmailDr. Marzuki Hj. Ismail
Semester I 2009/10 Semester I 2009/10
ENTECHENTECH
NITROGEN OXIDES (NOX)
Topic Covered
1. Types of Nitrogen Oxides (NOx)
2. Sources of NOx
3. Fate of NOx in the atmosphere –NO2 photolytic
cycle
4. Effects of NOx
5. Control of NOx
I – TYPES OF NITROGEN OXIDES
Nitrogen Oxides (NOx) normally found in the atmosphere
include:
1. Nitrous oxide (N2O)
2. Nitric oxide (NO)
3. Nitrogen dioxide (NO2)
1. Nitrous oxide (N2O)
A stable gas anesthetic characteristic (laughing gas)
Typical ambient concentration below threshold concentration for a biological effect.
No significant anthropogenic sources.
Types Of Nitrogen Oxides (Cont’d)
2. Nitric oxide (NO): NO is a colorless gas with typical ambient concentration <0.5
ppm. At such concentrations, no biological toxicity.
NO is a precursor to NO2 formation and an active compound in
O3 formation.
2 major sources of NO in combustion processes:
High temperature reaction between N2 and O2 generating
NO as a principle byproduct (thermal NO) Organically bound nitrogen in fuel is converted to NO (fuel-
NO) Major sources of NO:
Automobiles Power plants Industrial furnaces
Types Of Nitrogen Oxides (Cont’d)3. Nitrogen dioxide (NO2)
NO2 is a highly reactive, reddish-brown gas present in urban air.
NO2 is a strong oxidizing agent; plays a major role in atmospheric
reactions that produce ground level ozone (O3).
NO2 is of greatest concern due to its low solubility, hence can
penetrate deep into the respiratory tract (the alveoli). Natural sources: micro biological processes in soil and
atmospheric oxidation of ammonia. Man made sources (more important): high temperature
combustion of fuels in automobiles, power plants and industries which generates NO. NO is then oxidized to NO2 in the
atmosphere.
2NO + O2 2NO2
Nitrogen tetraoxide is also formed. Nitrogen dioxide (NO2) absorbs light photochemical smog.
II – SOURCES OF NITROGEN OXIDES
1. Natural sources:
Soil bacteria releases nitrous oxide (N2O) to atmosphere.
In upper troposphere and stratosphere:
N2O + O 2NO (nitric oxide)
Atomic oxygen (O) results from dissociation of ozone (O3)
Nitric oxide further reacts with atomic oxygen to form
nitrogen dioxide:
NO + O3 NO2 + O2
II – Sources Of Nitrogen Oxides (Cont’d)
2. Anthropogenic sources: Main source (~ 96%) of nitrogen oxides: combustion process. At high temperatures (T>1600 K), nitrogen & oxygen react to form
nitric oxide:
N2 + O2 2NO (endothermic reaction-absorbs heat)
The rate of NO formation is highly influenced by temperature. At lower temperatures, NO reacts with O3 or O2 to form NO2:
2NO + O2 2NO2
Very little NO2 is formed at high temperatures during combustion
because of its instability. Ultimately NO2(g) is converted to either NO2
- or NO3- in the form of
particulates. The particulates are washed out by precipitation resulting in the
formation of nitric acid (HNO3).
Nitric acid in the atmosphere is a form “acid rain” normally found downwind of industrialized area.
III – FATE OF NOx (NO2 PHOTOLYTIC CYCLE)
Reactions in the absence of VOCs
Nitrogen oxide absorbs ultraviolet & decomposes followed by ozone formation
NO2 + UV radiation NO + O
O2 + O O3
Ozone reacts with nitric oxide to form nitrogen dioxide & oxygen
O3 + NO NO2 + O2
Result: No accumulation of ozone
Atomic oxygen react with hydrocarbons to form alkyl-
peroxyl radicals (RO2)
O + VOCs RO2
III – Fate of NOx - NO2 Photolytic Cycle (Cont’d)
Alkylperoxyl radicals (RO2) can oxidize NO to produce
alkyl-oxyl (RO) radicals & NO2
RO2 + NO RO + NO2
This removes NO from the cycle thus the reaction
that removes O3 from the system is eliminated (NO
not available for reaction with O3)
Net effect is: O3 build up
Alkylperoxyl radicals (RO2) can react with O2 & NO2 to
produce peroxyacetyl nitrates (PAN)
The end product is photochemical smog (reactive photochemical oxidants) consisting of several air pollutants such as ozone, PAN, aldehydes (RCOH),
ketones (RCOR’), alkyl nitrates (CnH2n+1NO3) & CO.
III – Fate of NOx - NO2 Photolytic Cycle (Cont’d)
Photochemical oxidants are secondary pollutants; also known as PAN.
Formed through series of reactions initiated by the absorption of a photon by an atom, molecules, free radical or ion.
Ozone (O3) is the main photochemical oxidant.
O3 formation is through the nitrogen dioxide
photolytic cycle.
IV – EFFECTS OF NOx
Health effects:
NO2 is a pulmonary irritant affecting the upper respiratory system.
Sensitive individuals are those already suffering from asthma, respiratory disorders & lung disease.
Environmental effects:
NO2 contributes to acid rain and eutrophication (premature aging
of enclosed water bodies).
Degradation of vegetation – bleaching or death of plant tissue, loss of leaves, & decreased growth rate.
Degradation of materials – NO2 forms salts that increase
corrosion; fades fabric; degrades rubber.
Impairs visibility – NO and NO2 react with water vapor to form
aerosol droplets which limit visibility
IV – Effects of NOx (Cont’d)
Exposure to NO2 Effects
0.7 to 5 ppm for 10 h Can develop abnormalities in pulmonary airway resistance.
Higher concentration Can irritate lungs; cause bronchitis and pneumonia and lower resistance
to respiratory resistance.
Elevated concentration Can cause inflammation of the lungs.For 5 to 72 h
PneumoniaPneumoniaBronchitisBronchitis
V – CONTROL OF NOx
Three broad approaches:
1. Combustion modification
2. Flue gas treatment
Combustion modification
3 strategies are usually adopted:
i. Reduce peak temperature in the flame zone
ii. Reduce gas residence time in the flame ozone
iii. Reduce oxygen concentration in the flame zone
Changes to the combustion process can be achieved
by modifying operating conditions on existing furnaces
or by installing new low NOx burners or furnaces.
V – Control of NOx (Cont’d)
Modification methods:
Water/steam injection reduces flame temperature, hence
less generation of NOx
Flue gas recirculation (FGR) reduces flame temperature as well as oxygen concentration
Low excess air firing (LEA) and off-stoichiometric combustion (OSC)/staged combustion have been found to
reduce NOx emission from 19% to 60%.
Flue gas treatment
Higher removal efficiencies.
Includes:
1. Selective catalytic reduction (SCR)
2. Non-catalytic reduction, adsorption.
V – Control of NOx (Cont’d) Selective Catalytic Reduction (SCR):
Most advanced and most effective method
Removal efficiency >90%
Method is widely used in Japan and Europe
NOx species are reduced by ammonia, ultimately to nitrogen over a
heterogeneous catalyst in the presence of oxygen
Catalyst is a mixture of titanium and vanadium oxides.
Principles reaction:
4NO + 4NH3 4N2 + 6H2O
2NO2 + 4NH3 + O2 3N2 + 6H2O
Problems with SCR:
Formation of highly corrosive ammonium sulfate [(NH4)2SO4] and
ammonium bisulfate (NH4HSO4)
ACID RAIN
Unpolluted rain has pH ~ 5.6. it is naturally acidic due to
carbonate buffer system.
Some places in USA recorded very low pH.
In Malaysia, low pHs have been recorded in industrialized
areas such as Perai, Penang and Pasir Gudang, Johor.
Chemical reactions in the atmosphere convert SO2, NOx
and VOCs to acid compounds and associated oxidants.
TKA 3301:SULFUR OXIDES (SOx)
TOPICS COVERED
I. Dominant forms of SOx
II. Sources of SOx
III. Formation of SOx
IV. Sulfur cycle in the atmosphere
V. Control of anthropogenic SOx
I – DOMINANT FORMS OF SOx
Dominant oxides: Sulfur dioxide (SO2) and sulfur
trioxide (SO3).
SO2 is colorless, non flammable, non explosive,
poisonous to both plants and animals.
> 3 ppm, SO2 has a pungent irritating smell.
II – SOURCES OF SOx
SOX – primary or secondary pollutant? sulfur oxides may be both primary and secondary pollutants. Sources of primary pollutants: power plants, industry,
volcanoes, oceans emitting SO2, SO3, SO42-.
Sources of secondary pollutants: biological decay processes, industrial plants emitting H2S which is oxidized to SO2.
Natural & anthropogenic sources Natural sources
Volcanoes
Oceans: decay processes (H2S; SO2; SO4)
Anthropogenic sources Power plants
Industries: SO2
III – FORMATION OF SOx
Natural sources Oxidation of hydrogen sulfide by ozone
Reaction: H2S + O3 H2O + SO2
Anthropogenic sources Combustion of fuels containing sulfur generates sulfur dioxide
Reaction: S + O2 SO2
Sulfur dioxide is then converted to sulfate either by catalytic oxidation or photochemical oxidation.
Catalytic oxidation Most effective if water droplets containing Fe3+, or Mn2+ or
NH3 are present
Reaction: 2SO2 + 2H2O + O2 2H2SO4
III – Formation of SOx
Photochemical oxidation Occurs in low humidity condition First step: photoexcitation of SO2
Reaction: SO2 + hv SO2*
Second step: the excited molecule reacts with oxygen to form sulfur trioxide
Reaction: SO2* + O2 SO3 + O
Sulfur trioxide (hygroscopic) reacts with moisture and forms sulfuric acid
Reaction: SO2 + H2O H2SO4
The above reaction is a major source of acid rain. The ultimate fate of SO2 in the atmosphere is conversion to
sulfate (SO42-) salts which will be removed by sedimentation
or by washout with rain.
IV – SULFUR CYCLE
Reaction
In the atmosphere, sulfur dioxide (SO2) is converted to
sulfur trioxide (SO3) or sulfuric acid (H2SO4) and its salt
(SO42-).
SO2 + O SO3
SO3 + H2O H2SO4
Sulfur oxides in combination with particulates and
moisture produce damaging effects.
Sulfur oxide is a major source of acid deposition.
V – CONTROL OF SOx
Most important source of SO2: combustion of fossil fuels.
To reduce SO2 emission, therefore we need to control
emission from major sources which burn fossil fuels such as power plants.
Other sources: sulfuric acid plants; smelters 4 possible methods to reduce SO2 emission:
Change to low sulfur fuels: natural gas; liquified natural gas (LNG) –imported/containerized; low sulfur oil (Malaysian oil-sweet crude – 0.2 to 0.5%); low sulfur coal (<1%)
Use desulfurized coal or oil Use tall stacks for better dispersion Use fuel gas desulfurization system
V – Control of SOx
Desulfurization processes for coal and oil Coal:
Sulfur is present in two forms:
1. Organic (chemically bound hence more complex and costly to remove ex by coal gasification process).
2. Inorganic (iron pyrite-FeS2 – in particle form
can be removed by gravity washing). Oil:
Desulfurization of oil (ex hydrosulfurization process) is also commercially feasible although very expensive.
Clean Coal & Remove SO2
Coal washing removes 25% to 40% of the sulfur. Only the pyritic sulfur is washed out. Organic sulfur doesn’t wash out
V – Control of SOx
Absorption: Liquid scrubbing: bringing dirty effluent gas into contact
with the scrubbing liquid and separating the cleaned gas from the contaminated liquid
Mass transfer process in which a pollutant from a gas phase is transferred to a liquid phase.
Applicable to pollutants having high solubility in liquid phase
Water is commonly used as the absorbing medium. Process is applicable to NH3, Cl2, and SO2.
The control equipment is commonly termed as a scrubber. Scrubbers remove several pollutants simultaneously including particulates.
V – Control of SOx
Mass transfer takes place in 3 steps: Diffusion of pollutant gas from the gaseous phase to the liquid
phase Transfer across the gas/liquid interface Diffusion of dissolved gas away from the interface into the
liquid.
Lime and limestone scrubbing: SO2 is removed in lime (Ca(OH)2 or limestone (CaCO3) flue gas
desulfurization FGD system. Overall reactions:
SO2 + CaCO3 CaSO3 + CO2
SO2 + Ca(OH)2 CaSO3 + H2O
CaSO3 + 1/2O2 CaSO4
(O2 from flue gas; CaSO4 is calcium sulfate)
OZONE (O3)
Air pollutants Primary air pollutants
Materials that when released pose health risks in their unmodified forms
Secondary air pollutants Primary pollutants interact with one
another, sunlight, or natural gases to produce new, harmful compounds
Recommended Malaysian Air Quality Guidelines (Ambient Standards)
ppm g/m3 ppm g/m3
CO8-hr avg1-hr avg
930
1035
935
1040
NO2
Annual 0.17 320 0.053 100
O3
8-hr avg1-hr avg
0.060.10
120200
0.080.12
157235
Pb Quarterly avg - 1.5 - 1.5
Malaysia U.S.
Recommended Malaysian Air Quality Guidelines (Ambient Standards)
ppm g/m3 ppm g/m3
TSPAnnual
24-hr avg--
90260
--
--
PM10
Annual24-hr avg
--
50150
--
50150
SO2
Annual1-hr avg
10-min avg
0.040.13 0.19
105350 500
0.03--
80--
Photochemical Smog
hydrocarbons + NOx + sunlight → photochemical smog (oxidants)
primary oxidants produced: ozone (O3) formaldehyd
e peroxyacetyl
nitrate (PAN)
Photochemical Smog Photochemical smog – secondary pollutants
formed by reaction of nitrogen oxides and HC with sunlight
Includes ozone (O3) destroys chlorophyll, injures lung tissue ground-level ozone is “bad ozone”
Photochemical Smog
Photochemical Smog
Ozone: Health Effects
Increased incidents of respiratory distress.
Repeated exposures to ozone: Increased susceptibility to respiratory
infection Lung inflammation Aggravation of pre-existing respiratory
diseases such as asthma. Decreases in lung function and increased
respiratory symptoms such as chest pain and cough.
Ozone: Environmental Effects
Ozone also affects vegetation and ecosystems reductions in agricultural and
commercial forest yields ($0.5 billion/yr in US alone)
reduced growth and survivability of tree seedlings
increased plant susceptibility to disease, pests, and other environmental stresses (e.g., harsh weather).
http://www.ncl.ac.uk/airweb/ozone/greece.jpg
Ozone Revised Standards
In 1997, the 1-hour ozone standard of 0.12 parts per million (ppm) was replaced with a new 8-hour 0.08 ppm standard.
Areas that do not meet the new 8-hour standard will not be designated "nonattainment" until this year.
Open top chambers constructed in the study area
Plan view of chambers arrangements
.