Acid rain is caused by sulfur dioxide and nitrogen oxides reacting with water and oxygen in the atmosphere to form acids. The primary sources of these pollutants are fossil fuel combustion by power plants and vehicles. Acid rain harms aquatic ecosystems by lowering pH levels and damaging fish and plant life. It also erodes buildings and speeds up the decay of materials. Engineering solutions to reduce acid rain include flue gas desulfurization systems that use limestone to remove sulfur dioxide from power plant emissions and catalytic converters in vehicles to reduce nitrogen oxides.

1. 1.0 INTRODUCTION
“Acid Rain,” or more precisely acid precipitation, is the word used to describe
rainfall that has a pH level of less than 5.6. Acid rain is formed when oxides of nitrogen
and sulfite combine with moisture in the atmosphere to make nitric acid and sulfuric
acids. These acids can be carried away far from its origin.
The two primary sources of acid rain are sulfur dioxide (SO2), and oxides of
nitrogen (NOx). Any precipitation with a pH level less than 5.6 is considered to be acid
rainfall. The difference between regular precipitation and acid precipitation is the pH
level. pH is a symbol indicating how acidic or basic a solution is in ratios of relative
concentration of hydrogen ions in a solution. Not only does the acidicity of acid
precipitation depend on emission levels, but also on the chemical mixtures in which
sulfur dioxide and nitrogen oxides interact in the atmosphere each having varying
degrees of success.
Acid rain is rain consisting of water droplets that are unusually acidic because of
atmospheric pollution most notably the excessive amounts of sulfur and nitrogen
released by cars and industrial processes. Acid rain is also called acid deposition because
this term includes other forms of acidic precipitation such as snow.
Acidic deposition occurs in two ways which is wet and dry. Wet deposition is any
form of precipitation that removes acids from the atmosphere and deposits them on the
Earth’s surface. Dry deposition polluting particles and gases stick to the ground via dust
and smoke in the absence of precipitation. This form of deposition is dangerous however
because precipitation can eventually wash pollutants into streams, lakes, and rivers.
Acidity itself is determined based on the pH level of the water droplets. PH is the
scale measuring the amount of acid in the water and liquid. The pH scale ranges from 0
to 14 with lower pH being more acidic while a high pH is alkaline; seven is neutral.
Normal rain water is slightly acidic and has a pH range of 5.3-6.0. Acid deposition is
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2. anything below that scale. It is also important to note that the pH scale is logarithmic
and each whole number on the scale represents a 10-fold change.
1.1FORMATION OF AN ACID RAIN
Natural unpolluted rain is not pure water, it is a dilute solution of carbonic acid,
which forms when atmospheric carbon dioxide dissolves in water. This acid dissociates
in water to release only enough H+ ions to lower the pH of precipitation from 7 to about
5.6; thus, the acidic precipitation which arises from man’s pollution of the atmosphere
has a pH below 5.6. (A pH of 7 represents neutrality. Each unit decrease of pH
corresponds to a ten-fold increase in acidity.)
Acid rain is formed when pollutant compounds, primarily the oxides of sulphur
and nitrogen, react with oxygen and moisture in complex reactions in the atmosphere to
form acids. Although the details of the chemical reactions which take place in the
atmosphere are not completely understood, some of the oxides of sulphur and nitrogen
are converted to sulphuric and nitric acids, respectively. These are strong acids and they
dissociate completely in water releasing hydrogen ions to solution; thus, they can lower
the pH of precipitation significantly. One of the most acidic rainfalls yet recorded fell in
Scotland in 1974 and was measured at 2.4 on the pH scale -- roughly the pH of vinegar
(dilute acetic acid) and over one thousand times as acidic as natural rain.
Formation of Reactants
Burning of Fossil Fuels: S(in compounds) + O2(g) -> SO2(g)
Burning of Zinc Sulfide: 2ZnS(s) + 3O2(g) - > 2ZnO(s) + 2SO2(g)
The other major acidic oxide that contributes to the formation of acid rain is nitrogen
dioxide. Nitric oxide is formed in high localised temperatures created by lightning strikes
and naturally reacts in the atmosphere to produce nitrogen dioxide. Nitrogen dioxide is
also produced in the high temperatures of combustion chambers of power stations and
motor vehicles.
Chemical Equations for the Formation:
Formation of Nitrogen Dioxide: N2(g) + 2O2(g) -> 2NO2(g)
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3. Both sulfur dioxide and nitrogen dioxide are acidic oxides and react with water to form
acids.
Sulfur dioxide reacts with water to form sulfurous acid.
SO2(g) + H2O(l) -> H2SO3(aq)
Substances in the upper atmosphere then catalyse the reaction between sulfurous acid
and oxygen to form sulfuric acid.
2H2SO3(aq) + O2(g) -> 2H2SO4(aq)
Similarly, nitrogen dioxide reacts with water to form a mixture of nitric acid and nitrous
acid.
2NO2(g) + H2O(l) -> HNO3(aq) + HNO2(aq)
Substances in the atmosphere then catalyse the reaction between nitrous acid and
oxygen causing the formation of more nitric acid.
2HNO2(aq) + O2(g) -> 2HNO3(aq)
Both sulfuric acid and nitric acid are soluble in water and are the major acids present in
acid rain. As this forms and falls onto the Earth's surface, these strong acids are also
brought to the surface causing harmful effects on the built and the natural environment.
1.2ACID DEPOSITION
Acid deposition, also called acid rain, is rain or gases that have been polluted by
high amounts of chemicals and acids in the atmosphere. It can result from decaying
plants and animals or natural cataclysms, such as volcanoes, but the major cause of acid
rain is the releasing of chemicals by humans. The main gases that lead to acid rain are
sulfur dioxide and nitrogen dioxide. When they come into contact with water and oxygen
they turn into acids. Acid Deposition can be in the form of precipitation, which is called
wet deposition, or it could be in the form of gases and microscopic particles floating the
air, which is called dry deposition.
Scientists can measure how much acid is in rain or a body of water by using the pH
scale. There are 14 numbers on it, ranging from 0 through 14. If a lake has a low pH,
that tells us that there is a high amount of acid in the lake. If a lake has a pH 8 or
above, it is alkaline, which means there is not a lot of acid in it. When a body of water
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4. has a pH of 7, it is neutral, since it is in the middle. New York State's rain pH level is
between 4 and 4.5. That is 30 times more acidic than the normal level.
One of the central sources of sulfur dioxide and nitrogen oxide come from power
plants. When power plants generate electricity, they are burning the fossil fuel, coal.
Coal is sometimes dubbed as the dirty fuel source because when it is burned, it lets out
sulfur, nitrogen, and other gases. The more coal we use, the more sulfur and nitrogen
we are admitting into our atmosphere. Fumes and emissions from cars and other
vehicles are also another source of sulfur dioxide and nitrogen oxide.
1.3 Environmental Effects of Acid Rain
More is known about the effects of acid rain on some sectors of the environment
than on others. A good deal is known about the effects of acidification on aquatic
ecosystems but much less is known about the effects on terrestrial ecosystems,
agricultural crops, or human health.
i)
Aquatic Ecosystems
Aquatic organisms vary greatly in their ability to tolerate fluctuations in the
acidity of their environment. Some species are very sensitive to acidification and,
as the pH of lakes, rivers and groundwaters decreases, the least-tolerant species
disappear first, followed by less sensitive species as the pH continues to drop.
Studies have shown that the number and diversity of fish species decrease in
lakes when the pH drops below 6.0. Other organisms such as algal become less
diverse in lakes as the pH drops below 6.0 and the survival of rooted plants is
generally diminished in acidified lakes, while the growth of benthic (bottomgrowing) mosses and attached algae is usually enhanced. As the pH falls, the
number of invertebrates in the water column and in the sediments decreases, the
rate of decomposition of organic matter decreases, and fungi begin to replace
bacteria as the dominant decomposer organisms. These developments can lead
to a reduction in nutrient cycling in a lake, and this in turn can result in reduced
productivity.
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5. The loss of species from an ecosystem reduces its diversity and may
make the whole community progressively more unstable. Thus, even an acidresistant species may be lost, if its natural prey is acid-sensitive and disappears
from the environment.Limited information now suggests that aquatic biological
communities can recover fairly quickly from acidification (in years rather than
decades) once the level of acid loading is reduced. The artificial "liming" of
acidified waters has had some success in decreasing acidification and reestablishing some fish populations. However, the procedure is expensive and
liming does not precisely reverse the acidification process.
ii)
Terrestrial Ecosystems
Terrestrial ecosystems are inherently extremely complex; so many factors
influence the growth and development of land-based ecosystems that it is
difficult to isolate and characterize the effects of acid rain alone. Some facts are
known, however. Acid rain can: damage foliage; accelerate the erosion of the
waxy covering of leaves which may lead to the loss of water or which may
reduce a plant’s ability to resist the attack of disease-causing organisms; inhibit
the germination of seeds and the growth of seedlings; decrease the respiration
of organisms living in the soil, which may in turn affect the availability of some
nutrients; increase the leaching of nutrient ions from the soil; and enhance the
solubilization of aluminum in the soil, which can have negative effects on
biological processes. On the other hand, it is not known to what extent the illeffects listed above might be counterbalanced by the nutrient input which could
be derived from the sulphur and (especially) the nitrogen compounds which are
found in acidic precipitation. Acidic precipitation has not yet been shown to
damage agricultural crops directly, but air pollution in general does inhibit the
growth of some commercial species. A related issue concerns the damaging
effect of ozone on sensitive agricultural crops, which is now well documented.
Ozone is a major component of photochemical smog, of which nitrogen dioxide is
a precursor. Thus, a reduction in NOx emissions could have the two-fold
beneficial effect of reducing both acid rain and ozone pollution.
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6. iii)
Human Health
Although direct effects of acid rain on human health have yet to be
unambiguously demonstrated, some health authorities feel it may be injurious to
some people. However, evidence that sulphate air pollution affects human health
is now widely accepted by medical authorities.
Extremely small particulates, formed in the atmosphere by the oxidation of
sulphur dioxide, are capable of penetrating deeply into the human respiratory
system. Acidic particulates can cause chronic bronchitis or emphysema, with the
resultant difficulty in breathing leading to increased strain and, possibly,
eventually to heart disease. Oxides of nitrogen can suppress the action of
pulmonary scavenger cells whose function it is to purify the lungs by removing
insoluble particulates. This effect could also lead to increased susceptibility to
respiratory ailments.
iv)
Man-Made Structures
Acidic precipitation can contribute to the processes of materials erosion. Thus,
buildings, roads, paint, sculptures and other man-made structures can be
aesthetically and functionally damaged. The question is, what portion of the
observed corrosion and deterioration is due to the effect of acid rain? At the
present time, no useful estimate can be made. The most that can be said, in
advance of the extensive research needed in this area, is that air pollution has an
effect on materials deterioration and acid deposition is one component of that
complex situation.
1.4ENGINEERING SOLUTIONS
 CONTROL METHODS FOR SO2
1. GAS ABSORPTION & STRIPPING: This is the standard method for removing any
component from a gas stream, but it is only good as long as we can find a liquid solvent
in which the gaseous component we want to remove is much more soluble than the
other components of the gas stream.
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7. The overall process is very straightforward. The feed gas enters an absorber (a vertical
column) where the gas stream flow up and the liquid stream flows down. Normally, a
packing material is used inside the column in order to provide a larger surface area for
the liquid and gases to come in to contact. Once the gas has been stripped of the target
component, it is released into the atmosphere or used for some other purpose. The
liquid solvent which now contains the element we wanted to remove, passes to a
stripper. Inside the stripper, the solubility of the gas in the solvent is greatly reduced.
This allows the gas to come out of the solution. Later, this gas is cooled and sent to
storage for future use an d the stripped solvent is returned to the absorber.
2. LIMESTONE WET SCRUBBERS: This method is used primarily with sources who
produce SO2 through coal or oil burning. In the limestone wet scrubber, the solid ash
particulates are first removed. Then, the flue gas flows to a tower where it travels
countercurrent to a scrubbing slurry comprised of water and limestone particles. To
increase the efficiency of scrubbing, some designs use a packing amterial with a very
high open area in the tower, a spraying and mist eliminator system, or custom-made
bubbler designs.
Inside the tower, the SO2 dissolves in the slurry and reacts with the limestone to
produce CO2 and solid CaSO3. The CO2 enters the gas stream, while the CaSO3 is
oxidized to CaSO4. This oxidation can occur in variety of places. For example, it is partly
accomplished by the excess oxygen already present in the flue gases. It can also be
done in an effluent holding tank or an additional oxidizing vessel.
Once the process is complete, the slurry is recirculated from the holding tank. An
additional stream is sent to a settler and filter to remove solids. Finally , freshly ground
limestone is added to an effluent hold tank. The scrubber will operate near the adiabatic
saturation temperature of the entering flue gas. The cleaned gas is usually heated to
restore plume buoyancy and to prevent acid corrosion of duct work. Then, the gas is
released to the stack. The water in the waste slurry is reduced by thickening. The filter
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8. cake is usually mixed with dry fly ash from the plant to further reduce the water in the
waste stream. The ultimate destination for the final waste stream is a landfill.
Some problems with limestone scrubbers are corrosion of stacks and duct work due to
chemical content of exhaust gases, solid deposition, scaling and plugging caused by
calcium sulfate, and plugging of mist eliminator equipment.
Alternative wet systems include: using quicklime as an alternative to limestone in wet
throwaway processes (i.e. processes that use a reagent only once and then throw it
away) and double alkali systems which avoids the solid deposition, scaling and plugging
problems caused by limestone.
3. DRY SYSTEMS: Overall, dry systems have fewer corrosion and scaling problems
associated with them. Typically, dry systems inject dry alkaline particles into the gas
stream, where they react with the gas to remove SO2. The particles containing the SO2
are then collected in the particle collection device that is used to collect fly ash. Dry
systems eliminate problems with disposal of wet sludge associated with wet scrubbers,
while increasing the amount of dry solids to be disposed of. Sine it is generally more
difficult to dispose of the sludge, dry systems are often considered a good alternative.
4. WET/DRY SYSTEMS: These systems are a combination of the feature of both wet and
dry systems. The type of wet-dry system that is most often used is the spray dryer.
Spray dryers are mainly used in process industries (i.e. industries that produce products
like dried milk, instant coffee, laundry detergents, etc.) In theses spray dryers, water
containing dissolved or suspended solids is dispersed as droplets into a hot gas stream.
The gas stream, usually contaminated with SO2 enters the chamber either through the
side or the top and generally exits through the bottom. The temperature of the gas is
much greater than that of the water so the water droplets will evaporate quickly. The
particles that are formed from the evaporation process are dry before they reach the
walls or the bottom of the reactor. Therefore, the particles for a fine powder that is
relatively easy to be removed. Generally, the powder is cooled before being removed.
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9. One benefit of this process is that since we are able to control the size of the water
droplets and the concentration in the feed stream, we are able to control the size of the
particles that are formed. In the end, all of these specifications give us a powder with a
size distribution that could not be obtained any other way.
 CONTROL METHODS FOR Nox
CONTROL TECHNIQUES FOR NOx REDUCTION
There are only two way to reduce NOx emissions:
•Modifying combustion processes to prevent NOx formation
•Treating combustion gases after flame to convert NOx to N2
COMBUSTION MODIFICATION : This is the most widely used approach to NOx control.
Combustion modification involves mixing part of the combustion air with the fuel and
burning as much of the fuel that the air will allow. Then, some of the heat from the
flames is transferred to whatever is being heated. Next, the remaining air is added and
combustion is finished. This is known as two-stage combustion or reburning
One of the major advantages to this technique is that it is cheap. The disadvantages are
that it requires a larger firebox without a higher combustion rate. Also, it is difficult to
get complete burning of the fuel in the second stage. Therefore, the amount of
unburned fuel and/or carbon monoxide in the exhaust gas increases.
POST-FLAME TREATMENT
Many of these processes require the addition of a reducing agent to the combustion gas
stream to take oxygen away from NO. In automobile engines, a platinum-rhodium
catalyst is used. The reaction is:
2NO + 2CO + p-r catalyst-----------> N2 + 2CO2
On the other hand, for power plants and other large furnaces, there are many choices of
reducing agents. However, the most popular is ammonia. The desired conversion
reaction is:
6NO + 4NH3 -------------> 5N2 + 6H2O
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10. However, there is always some oxygen present. This oxygen causes reactions like the
following:
4NO + 4NH3 +O2 ---------> 4N2 + 6H2O
If the above reaction occurs, the NO2 is reduced by the following reaction:
2NO2 + 4NH3 +O2 ---------> 3N2 + 6H2O
All of these reactions are expensive to carry out. They can occur either over a zeolite
catalyst or in a gas stream in a part of a furnace where the temperature is between
1600 and 1800 degrees Fahrenheit. If the temperature is greater than 1800 degrees,
the NO content increases rather than decreases, which exactly what we DON'T want.
The dominant reaction is:
NH3+O2 ---------> NO + 3/2H2O
1.5 CONCLUSION
“Acid rain”, or more precisely acid precipitation, is a broad term referring to a
mixture of wet and dry deposition from the atmosphere containing a pH level of less
than 5.6 that is unusually acidic. Wet deposition refers to acidic rain, for and snow while
dry deposition refers to the acid chemicals become dust or smoke and falls to the
ground, buildings, cars and trees then washed away rainstorms, leading to acidic
mixture. This acid precipitation is caused by emissions of sulphur dioxide and nitrogen
oxide, which react with the water molecules in the atmosphere to produce acids. Acid
rain causes the acidification of lakes and streams and accelerates the decay of building
materials and paints, buildings, statues and sculptures. It is best to reduce the use of
any related to the emission of sulphur and nitrogen to control the acidic level of acid
rain.
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