Jordanian-German

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Jordanian-German Winter Academy 2006
Combustions Emissions By Eng. Samar
Jaber February, 2006
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• - 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

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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.

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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.

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Primary Sources of NOx Formation in Combustion
Processes
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• 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

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• 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.

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

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• The Formation of NOx

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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.

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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.

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• 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.

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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.

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

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• 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.

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• 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.

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• 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.

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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.

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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.

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• 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.

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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.

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• 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.

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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.

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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.

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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.

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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.

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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.

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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

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Post combustion control methods
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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
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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.

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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.

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• FGR technology can be classified into two
  types
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• 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
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• 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.

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• Solution

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For the case of No Dilution
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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

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• 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.

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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.

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• 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.

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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.

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• 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.

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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

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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

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• Thank you