Title: Jordanian-German
1Jordanian-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
3What 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.
4Where 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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6- 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.
7Primary 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 12Carbon 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.
13Sulfur 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.
15Volatile 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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19- 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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22Particulate 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.
24Health 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.
25Smog
- 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. -
-
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27Acid 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.
29Particles
-
- 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.
30Water 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.
31Global 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.
32Toxic 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.
33Visibility 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.
34NOx 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
35Post combustion control methods
36Selective 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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38Selective 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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40Combustion Control Techniques
41Low 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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43Low 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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45Reburning
- 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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48Water/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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50Flue 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 - Â
- 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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53Dilution 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 56For the case of No Dilution
57With 25 N2 Dilution
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61- 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.
63CO 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.
65SOx 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.
67VOCs 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
68PM 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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