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

.