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Title: Jordanian-German


1
Jordanian-German Winter Academy 2006
Combustions Emissions By Eng. Samar
Jaber February, 2006
2
  • - Combustion of standard fossil fuels results in
    nine emissions, carbon dioxide, nitrogen, oxygen,
    water, nitrogen oxide, carbon monoxide , sulfur
    oxides, volatile organic compounds (VOCs) , and
    particulate matter.
  • - We will describe the formation and control of
    each pollutant

3
What is NOx ?
  • - A Nitrogen Oxide, or NOx, is the generic term
    for a group of highly reactive gases, all of
    which contain nitrogen and oxygen in varying
    amounts. Many of the nitrogen oxides are
    colorless and odorless. However, one common
    pollutant, nitrogen dioxide (NO2) along with
    particles in the air can often be seen as
  • a reddish-brown layer
  • over many urban areas.

4
Where does NOx come from?
  • Nitrogen oxides form when fuel is burned at high
    temperatures, as in a combustion process.
  • In an internal combustion engine, a mixture of
    air and fuel is burned. When the mixture is tuned
    so as to consume every molecule of reactant (in
    this case fuel and oxygen) it is said to be
    "running at stoichiometry". With this burns,
    combustion temperatures reach a high enough level
    to actually burn some of the nitrogen in the
    air, yielding various oxides of nitrogen, the
    results of which can be seen over major cities
    such as Los Angeles , California in the summer in
    the form of brown clouds of smog.
  • .

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  • In general, the contribution of mobile sources to
    the total NOx level ranges from 60 to 80 percent
  • For stationary sources, it ranges between 20 and
    40 percent.

7
Primary Sources of NOx Formation in Combustion
Processes
8
  • Thermal NOx refers to NOx formed through high
    temperature oxidation of the diatomic nitrogen
    found in combustion air. The formation rate is
    primary function of temperature and the residence
    time of nitrogen at temperature.
  • N2 O ? NO N
  • N O2 ? NO O
  • N OH ? NO H
  • Prompt NOx This is a fast reaction between the
    N2, O2 and hydrocarbon (CH) fragments

9
  • Fuel NOx The major source of NOx production
    from burning fuels such as certain coals and oil,
    is the conversion of fuel bound nitrogen to NOx
    during combustion. During combustion, the
    nitrogen bound in the fuel is released as a free
    radical and ultimately forms free N2, or NO.
  • Fuel NOx can contribute as much a s 50 of total
    emissions when combusting oil and as much as 80
    when combusting coal.

10
  • The most significant factors affecting NOx
    formation are flame temperature , the amount of
    nitrogen in the fuel , excess air level and
    combustion air temperature.

11
  • The Formation of NOx

12
Carbon Monoxide Emissions
  • Carbon monoxide is a pollutant that is readily
    absorbed in the body and can impair the
    oxygen-carrying capacity of the hemoglobin.
    Impairment of the body's hemoglobin results in
    less oxygen to the brain, heart, and tissues.
    Even short-term over exposure to carbon monoxide
    can be critical, or fatal, to people with heart
    and lung diseases. It may also cause headaches
    and dizziness in healthy people.
  • During combustion, carbon in the fuel oxidizes
    through a series of reactions to form carbon
    dioxide (CO2). However, 100 percent conversion of
    carbon to CO2 is rarely achieved in practice and
    some carbon only oxidizes to the intermediate
    step, carbon monoxide.

13
Sulfur Compounds (Sox)
  • The primary reason sulfur compounds, are
    classified as a pollutant is because they react
    with water vapor (in the flue gas and atmosphere)
    to form sulfuric acid mist. Airborne sulfuric
    acid has been found in fog, smog, acid rain, and
    snow. Sulfuric acid has also been found in lakes,
    rivers, and soil.
  • The acid is extremely corrosive and harmful to
    the environment.

14
  • The combustion of fuels containing sulfur
    (primarily oils and coals) results in pollutants
    occurring in the forms of SO2 (sulfur dioxide)
    and SO3 (sulfur trioxide), together referred to
    as SOx (sulfur oxides). The level of SOx emitted
    depends directly on the sulfur content of the
    fuel.
  • Typically, about 95 of the sulfur in the fuel
    will be emitted as SO2, 1-5 as SO3, and 1-3 as
    sulfate particulate. Sulfate particulate is not
    considered part of the total SOx emissions.

15
Volatile Organic Compounds (VOCs)/Hydrocarbons
(HC)
  • VOCs are compounds containing combinations of
    carbon, hydrogen, and sometimes oxygen. VOCs
    vaporize easily once emitted into the air and are
    of concern because of their role in ground level
    ozone formation.
  • Formation of VOCs result from poor or
    incomplete combustion .
  • VOC's are vapors released from gasoline,
    paints, solvents, pesticides, and other
    chemicals.

16
  • Formation of VOCs in commercial and industrial
    boilers primarily result from poor or incomplete
    combustion due to improper burner set-up and
    adjustment.

17
  • In the combustion chamber the unburned
    hydrocarbon emissions have several different
    sources
  • 1. During compression and combustion, the
    increasing cylinder pressure forces some of the
    gas in the cylinder into crevices, or narrow
    volumes connected to the combustion chamber The
    volumes between the piston, rings and cylinder
    wall are the largest of these.

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  • 2. Most of this gas is unburned fuel-air mixture
    much of it escapes the primary combustion process
    because the entrance to these crevices is too
    narrow for the flame to enter.

20
  • 3. This gas which leaves these crevices in the
    expansion and exhaust processes, is one of the
    unburned hydro carbon emissions.

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Particulate Matter (PM)
  • Emissions of PM from combustion sources consist
    of many different types of compounds, including
    nitrates, sulfates, carbons, oxides, and any
    uncombusted elements in the fuel.
  • Particulate pollutants can be corrosive, toxic to
    plants and animals, and harmful to humans.
  • PM emissions are primarily dependent on the grade
    of fuel fired in the boiler. Generally, PM levels
    from natural gas are significantly lower than
    those of oils.

23
  • PM emissions generally are classified into two
    categories, PM and PM10. PM10 is a particulate
    matter with a diameter less than 10 microns. All
    PM can pose a health problem. However, the
    greatest concern is with PM10, because of its
    ability to bypass the body's natural filtering
    system.
  • When burning heavy oils, particulate levels
    mainly depend on four fuel constituents sulfur,
    ash, carbon residue, and asphalenes. These
    constituents exist in fuel oils, particularly
    residual oils, and have a major effect on
    particulate emissions. By knowing the fuel
    constituent levels, the particulate emissions for
    the oil can be estimated.

24
Health and Environmental Impacts
  • The main reason that NOx is considered an
    environmental problem is because it initiates
    reactions that result in the production of ozone
    and acid rain. Ozone and acid rain can damage
    fabric, cause rubber to crack, reduce visibility,
    damage buildings, harm forests and lakes, and
    cause health problems.
  • By controlling NOx levels, along with the other
    pollutants, the levels of acid rain and ozone can
    be reduced.

25
Smog
  • Smog is a kind of air pollution, originally
    named for the mixture of smoke and fog in the
    air. Classic smog results from large amounts of
    coal burning in an area and is caused by a
    mixture of smoke and sulfur dioxide.
  • In the 1950s a new type of smog, known as
    Photochemical Smog, was first described. This is
    a noxious mixture of air pollutants
    includingNitrogen Oxides, Tropospheric Ozone and
    Volatile Organic Compounds (VOCs)
  • All of these chemicals are usually highly
    reactive and oxidizing. Due to this fact,
    photochemical smog is considered to be a problem
    of modern industrialization.

26
  • Photochemical smog is a concern in most major
    urban centres but, because it travels with the
    wind, it can affect sparsely populated areas as
    well.
  • Smog is caused by a reaction between sunlight
    and emissions mainly from human activity.

27
Acid Rain
  • NOx and sulfur dioxide react with other
    substances in the air to form acids which fall to
    earth as rain, fog, snow or dry particles. Some
    may be carried by wind for hundreds of miles.

28
  • Acid rain damages causes deterioration of
    cars, buildings and historical monuments and
    causes lakes and streams to become acidic and
    unsuitable for many fish. Acid rain also reduces
    how far and how clearly we can see through the
    air, an effect called visibility reduction.

29
Particles
  • NOx reacts with ammonia, moisture, and other
    compounds to form nitric acid and related
    particles. Human health concerns include effects
    on breathing and the respiratory system, damage
    to lung tissue, and premature death. Small
    particles penetrate deeply into sensitive parts
    of the lungs and can cause or worsen respiratory
    disease such as emphysema and bronchitis, and
    aggravate existing heart disease.

30
Water Quality Deterioration
  • Increased nitrogen loading in water bodies,
    particularly coastal estuaries, upsets the
    chemical balance of nutrients used by aquatic
    plants and animals. Additional nitrogen
    accelerates "eutrophication," which leads to
    oxygen depletion and reduces fish and shellfish
    populations.

31
Global Warming
  • One member of the NOx is a greenhouse gas. It
    accumulates in the atmosphere with other
    greenhouse gasses causing a gradual rise in the
    earth's temperature. This will lead to increased
    risks to human health, a rise in the sea level,
    and other adverse changes to plant and animal
    habitat.

32
Toxic Chemicals
  • In the air, NOx reacts readily with common
    organic chemicals and even ozone, to form a wide
    variety of toxic products, some of which may
    cause biological mutations.

33
Visibility Impairment
  • Nitrate particles and nitrogen dioxide can
    block the transmission of light, reducing
    visibility in urban areas and on a regional scale
    in our national parks.

34
NOx Control Technologies
  • Post combustion methods address NOx emissions
    after formation
  • Combustion control techniques prevent the
    formation of NOx during the combustion process.
  • Post combustion methods tend to be more expensive
    than combustion control techniques

35
Post combustion control methods
36
Selective Non-catalytic Reduction
  • Selective non-catalytic reduction involves the
    injection of a NOx reducing agent, such as
    ammonia or urea. The ammonia or urea breaks down
    the NOx in the exhaust gases into water and
    atmospheric nitrogen. Selective non-catalytic
    reduction reduces NOx up to 70.

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Selective Catalytic Reduction
  • - Selective catalytic reduction involves the
    injection of ammonia in the boiler exhaust gases
    in the presence of a catalyst.
  • - The catalyst allows the ammonia to reduce
    NOx levels at lower exhaust temperatures than
    selective non-catalytic reduction. Unlike
    selective non-catalytic reduction, where the
    exhaust gases must be approximately 1400-1600 F,
    selective catalytic reduction can be utilized
    where exhaust gasses are between 500 F and 1200
    F, depending on the catalyst used.
  • - Selective catalytic reduction can result in
    NOx reductions up to 90.

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Combustion Control Techniques
41
Low Excess Air (LEA) Firing
  • One of the factors influencing NOx formation is
    the excess air levels. High excess air levels
    (gt45) may result in increased NOx formation
    because the excess nitrogen and oxygen in the
    combustion air entering the flame will combine to
    form thermal NOx.
  • Low excess air firing can be used on most
    boilers and generally results in overall NOx
    reductions of 5-10 when firing natural gas.

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Low Nitrogen Fuel Oil
  • When firing fuel oils, NOx formed by fuel-bound
    nitrogen can account for 20-50 of the total NOx
    level.
  • One method to reduce NOx levels from boilers
    firing distillate oils is through
  • the use of low nitrogen fuel oil

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Reburning
  • Reburning is a method of NOx control that uses
    hydrocarbon radicals to convert nitrogen oxide
    (NO) to nitrogen (N2) and carbon dioxide (CO2).
  • Reburning can be applied to boilers that cannot
    use standard low NOx combustion modification
    techniques due to the need to maintain high
    furnace temperatures, such as wet bottom boilers.
  • In many cases, reburning can be more
    economical than post combustion NOx controls that
    would otherwise be used in these instances.

46
Reburning is accomplished by diverting a
portion of a boiler's fuel, typically 10-20, to
a point above the primary combustion zone where
it is injected to create a fuel rich "reburn
zone." The remaining combustion air is then
injected above the reburn zone to provide the
necessary burnout air.
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Water/Steam Injection
  • By injecting water or steam into the flame,
    flame temperatures are reduced, thereby lowering
    thermal NOx formation and overall NOx levels.
  • Water or steam injection can reduce NOx up to
    80 (when firing natural gas) and can result in
    lower reductions when firing oils.
  • Many times water or steam injection is used in
    conjunction with other NOx control methods such
    as burner modifications or flue gas recirculation.

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Flue Gas Recirculation
  • FGR entails recalculating a portion of
    relatively cool exhaust gases back into the
    combustion process in order to lower the flame
    temperature and reduce NOx formation. It is
    currently the most effective and popular low NOx
    technology for fire tube and water tube boilers.
  • In many applications, it does not require any
    additional reduction equipment to comply with
    regulations.

51
  • FGR technology can be classified into two
    types
  •  
  • External FGR utilizes an external fan to
    recirculate the flue gases back into the flame.
    External piping routes the exhaust gases from the
    stack to the burner. A valve controls the
    recirculation rate, based on boiler input.
  • Induced FGR utilizes the combustion air fan to
    recirculate the flue gases back into the flame. A
    portion of the flue gases are routed by duct work
    or internally to the combustion air fan, where
    they are premixed with the combustion air and
    introduced into the flame through the burner.

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Dilution with cold inerts
54
  • Example
  • Consider the Nitric Oxide formation in the
    post flame gases of a stoichiometric propane-air
    mixture at atmospheric pressure. Assuming
    adiabatic conditions, how does the initial rate
    of NO formation (ppm/s) from the Zoldivich
    mechanism compare for no dilution and 25
    dilution by N2 (moles of N2 added equals 0.25 the
    number of moles of air). The reactants and N2
    diluents are initially at 298 K.

55
  • Solution

56
For the case of No Dilution
57
With 25 N2 Dilution
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  • Each method results in a different
  • degree of NOx control.
  • For example, when firing natural gas, low
    excess air
  • firing typically reduces NOx by 10, FGR by
    75,
  • and selective catalytic reduction by 90.
  • Selecting the best low NOx control package
    should be made with total boiler performance in
    mind

62
  • Some NOx control technologies used to reduce NOx
    levels by lowering flame temperatures by
    modifying air/fuel mixing patterns. The lower
    flame temperature and decreased mixing intensity
    can result in higher CO levels.
  • An induced FGR package can lower NOx levels by
    reducing flame temperature without increasing CO
    levels. CO levels remain constant or are lowered
    because the flue gas is introduced into the flame
    in early stages of combustion and the air fuel
    mixing is intensified. Intensified mixing offsets
    the decrease in flame temperature and results in
    CO levels that are lower than achieved without
    FGR.

63
CO Control Technologies
  • High flame temperatures and intimate air/fuel
    mixing are essential for low CO emissions.
  • Some NOx control technologies reduce NOx levels
    by lowering flame temperatures by modifying
    air/fuel mixing patterns. The lower flame
    temperature and decreased mixing intensity can
    result in higher CO levels.

64
  • An induced flue gas recirculation package can
    lower NOx levels by reducing flame temperature
    without increasing CO levels. CO levels remain
    constant or are lowered because the flue gas is
    introduced into the flame in early stages of
    combustion and the air fuel mixing is
    intensified.
  • Intensified mixing offsets the decrease in flame
    temperature and results in CO levels that are
    lower than achieved without FGR. But, the level
    of CO depends on the burner design. Not all flue
    gas recirculation applications result in lower CO
    levels.

65
SOx Control Technologies
  • Methods of SOx reduction include switching to low
    sulfur fuel, desulfurizing the fuel, and
    utilizing a flue gas desulfurization (FGD)
    system.
  • Fuel desulfurization, which primarily applies to
    coal, involves removing sulfur from the fuel
    prior to burning. Flue gas desulfurization
    involves the utilization of scrubbers to remove
    SOx emissions from the flue gases.

66
  • For users of industrial boilers, utilizing low
    sulfur fuels is the most cost effective method of
    SOx reduction. Because SOx emissions primarily
    depend on the sulfur content of the fuel, burning
    fuels containing a minimal amount of sulfur
    (distillate oil) can achieve SOx reductions,
    without the need to install and maintain
    expensive equipment.

67
VOCs Control Technologies
  • To control VOC emissions from commercial and
    industrial boilers, no auxiliary equipment is
    needed properly maintaining the burner/boiler
    package will keep VOC emissions at a minimum.
    Proper maintenance includes keeping the air/fuel
    ratio at the manufacturer's specified setting,
    having the proper air and fuel pressures at the
    burner, and maintaining the atomizing air
    pressure on oil burners at the correct levels. An
    improperly maintained boiler/burner package can
    result in VOC levels over 100 times the normal
    levels

68
PM Control Technologies
  • The emission levels of particulate matter can
    be lowered by switching from a residual to a
    distillate oil or by switching from a distillate
    oil to a natural gas. Additionally, through
    proper burner set-up, adjustment and maintenance,
    particulate emissions can be minimized, but not
    to the extent accomplished by switching fuels

69
  • Thank you
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