Basically, Redox is a shorthand for 2 half reactions, two concepts, namely REDuction and OXidation. So, before I go on into the explanation of redox reaction and its applications, let's settle the two foreign concepts first ^.^
OXidation - describes the loss of electrons/ hydrogen or gain of oxygen/ increase in oxidation state by a molecule, atom or ion.
REDuction - describes the gain of electrons / hydrogen or a loss of oxygen / decrease in oxidation state by a molecule, atom or ion.
...to be continued tomorrow...
Sunday, June 7, 2009
Saturday, June 6, 2009
Flue Gas Desulfurization
Flue gas desulfurization(FGD) is the technology used for removing sulfur dioxide (SO2) from the exhaust flue gases in power plants that burn coal or oil to produce steam for the steam turbines that drive their electricity generators.
SO2 is responsible for acid rain formation. Tall flue gas stacks disperse the emissions by diluting the pollutants in ambient air and transporting them to other regions.
As a result of stringent environmental protection regulations regarding SO2 emissions that have been enacted in many countries, it is now being removed from flue gases by many methods, with the following being the most common:
§ Wet scrubbing using a slurry of alkaline sorbent, usually limestone or lime, or seawater to scrub the gases.
§ Spray-dry scrubbing using similar sorbent slurries.
§ Dry sorbent injection systems.
For a typical coal-fired power station, FGD will remove 95 percent or more of the SO2 in the flue gases.
Sulfuric acid mist formation
Fossil fuels contain significant amounts of sulfur. When burned, about 95 percent or more of the sulfur is generally converted to sulfur dioxide (SO2). This happens under normal conditions of temperature and of oxygen present in the flue gas. However, there are some exceptional cases.
For example, when the flue gas has too much oxygen and the SO2 is further oxidized to sulfur trioxide (SO3), which is only one of the ways of forming SO3. Gas temperature is also an important factor. At about 800 °C, formation of SO3 is favoured. Another way that SO3 can be formed is through catalysis by metals in the fuel. This is particularly true for heavy fuel oil, where significant amounts of vanadium are present. No matter what, it does not behave like SO2 in that it forms a liquid aerosol known as sulfuric acid (H2SO4) mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to SO3. Sulfuric acid mist is often the cause of the blue haze that often appears as the flue gas plume dissipates. Increasingly, this problem is being addressed by the use of wet electrostatic precipitators.
Basic principles
Most FGD systems employ two stages: one for fly ash removal and the other for SO2 removal. Attempts have been made to remove both the fly ash and SO2 in one scrubbing vessel. However, these systems experienced severe maintenance problems and low simultaneous removal efficiencies. In wet scrubbing systems the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a wet scrubber, and then into the SO2 absorber. However, in dry injection or spray drying operations, the SO2 is first reacted with the sorbent and then the flue gas passes through a particulate control device.
Another important design consideration associated with wet FGD systems is that the flue gas exiting the absorber is saturated with water and still contains some SO2. (No system is 100% efficient.) Therefore, these gases are highly corrosive to any downstream equipment - i.e., fans, ducts, and stacks. Two methods that minimize corrosion are:
(1) Reheating the gases to above their dew point and
(2) Choosing construction materials and design conditions that allow equipment to withstand the corrosive conditions. The selection of a reheating method or the decision not to reheat (thereby requiring the use of special construction materials) is very controversial topics connected with FGD design. Both alternatives are expensive and must be considered on a by-site basis.
Scrubbing with a basic solid or solution
SO2 is an acid gas and thus the typical sorbent slurries or other materials used to remove the SO2 from the flue gases are alkaline. The reaction taking place in wet scrubbing using a CaCO3 (limestone) slurry produces CaSO3 (calcium sulfite) and can be expressed as:
CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO2 (gas)
When wet scrubbing with a Ca(OH)2 (lime) slurry, the reaction also produces CaSO3 (calcium sulfite) and can be expressed as:
Ca(OH)2 (solid) + SO2 (gas) → CaSO3 (solid) + H2O (liquid)
When wet scrubbing with a Mg(OH)2 (magnesium hydroxide) slurry, the reaction produces MgSO3 (magnesium sulfite) and can be expressed as:
Mg(OH)2 (solid) + SO2 (gas) → MgSO3 (solid) + H2O (liquid)
To partially offset the cost of the FGD installation, in some designs, the CaSO3 (calcium sulfite) is further oxidized to produce marketable CaSO4 · 2H2O (gypsum). This technique is also known as forced oxidation:
CaSO3 (solid) + H2O (liquid) + ½O2 (gas) → CaSO4 (solid) + H2O
A natural alkaline usable to absorb SO2 is seawater. The SO2 is absorbed in the water, and when oxygen is added reacts to form sulfate ions SO4- and free H+. The surplus of H+ is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO2 gas:
SO2 (gas) + H2O + ½O2 (gas)→ SO42- (solid) + 2H+
Wednesday, May 27, 2009
The effects of pollutants on health and environment
i) Poisonous nature of CO
ii) NO2 and SO2 in formation of acid rain and its effects on respiration, buildings and plants.
iii) NO2, CH4 and unburned hydrocarbons in formation of photochemical smog.
Carbon monoxide is colorless and odorless, but very toxic. Carbon monoxide poisoning is the most common type of fatal poisoning in many countries.
Exposures can lead to significant damage to the heart and central nervous system. Carbon monoxide can also have severe effects on the fetus of a pregnant woman.
When in gaseous state, NO2 and SO2 dissolve in rain water to form what we call acid rain.
NO2 + H2O -> NO3H2
SO2 + H2O -> SO3H2
This rain, as its name suggests, has acidic properties. Therefore, when acid rain comes in contact with buildings or surfaces, they will react and resulting in the structures either corroding or being weaker.
When inhaled, the gases will dissolve in the moisture found in our throat or windpipe, resulting in acidic solutions. This will create multiple health problems. As for plants, the concept is similar.
A photochemical smog is the chemical reaction of sunlight, nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere, which leaves airborne particles (called particulate matter) and ground-level ozone.
Nitrogen oxides are released by nitrogen and oxygen in the air reacting together under high temperature such as in the exhaust of fossil fuel-burning engines in cars, trucks, coal power plants, and industrial manufacturing factories.
It can inflame breathing passages, decreasing the lungs' working capacity, and causing shortness of breath, pain when inhaling deeply, wheezing, and coughing. It can cause eye and nose irritation and it dries out the protective membranes of the nose and throat and interferes with the body's ability to fight infection, increasing susceptibility to illness.
ii) NO2 and SO2 in formation of acid rain and its effects on respiration, buildings and plants.
iii) NO2, CH4 and unburned hydrocarbons in formation of photochemical smog.
Carbon monoxide is colorless and odorless, but very toxic. Carbon monoxide poisoning is the most common type of fatal poisoning in many countries.
Exposures can lead to significant damage to the heart and central nervous system. Carbon monoxide can also have severe effects on the fetus of a pregnant woman.
When in gaseous state, NO2 and SO2 dissolve in rain water to form what we call acid rain.
NO2 + H2O -> NO3H2
SO2 + H2O -> SO3H2
This rain, as its name suggests, has acidic properties. Therefore, when acid rain comes in contact with buildings or surfaces, they will react and resulting in the structures either corroding or being weaker.
When inhaled, the gases will dissolve in the moisture found in our throat or windpipe, resulting in acidic solutions. This will create multiple health problems. As for plants, the concept is similar.
A photochemical smog is the chemical reaction of sunlight, nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere, which leaves airborne particles (called particulate matter) and ground-level ozone.
Nitrogen oxides are released by nitrogen and oxygen in the air reacting together under high temperature such as in the exhaust of fossil fuel-burning engines in cars, trucks, coal power plants, and industrial manufacturing factories.
It can inflame breathing passages, decreasing the lungs' working capacity, and causing shortness of breath, pain when inhaling deeply, wheezing, and coughing. It can cause eye and nose irritation and it dries out the protective membranes of the nose and throat and interferes with the body's ability to fight infection, increasing susceptibility to illness.
Sources of pollutants
Carbon monoxide is produced from the partial oxidation of carbon-containing compounds; it forms in preference to the more usual carbon dioxide when there is a reduced availability of oxygen.
Carbon monoxide is a major atmospheric pollutant in urban areas, chiefly from exhaust of internal combution engines, but also from improper burning of various other fuels.
NO is unstable with respect to O2 and N2, although this conversion is very slow at ambient temperatures in the absence of a catalyst. Its synthesis from molecular nitrogen and oxygen requires elevated temperatures, >1000°C. A major natural source is lightning. The use of internal combustion engines has drastically increased the presence of nitric oxide in the environment as well.
Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
2O2 + N2 → 2 NO2
The most important sources of NO2 are internal combustion engines , thermal power stations and, to a lesser extent, pulp mills. Atmospheric nuclear tests are also a source of nitrogen dioxide, which is responsible for the reddish colour of mushroom clouds.
SO2 is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide.
Carbon monoxide is a major atmospheric pollutant in urban areas, chiefly from exhaust of internal combution engines, but also from improper burning of various other fuels.
NO is unstable with respect to O2 and N2, although this conversion is very slow at ambient temperatures in the absence of a catalyst. Its synthesis from molecular nitrogen and oxygen requires elevated temperatures, >1000°C. A major natural source is lightning. The use of internal combustion engines has drastically increased the presence of nitric oxide in the environment as well.
Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
2O2 + N2 → 2 NO2
The most important sources of NO2 are internal combustion engines , thermal power stations and, to a lesser extent, pulp mills. Atmospheric nuclear tests are also a source of nitrogen dioxide, which is responsible for the reddish colour of mushroom clouds.
SO2 is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide.
Ozone Depletion
Just a short intro first, ozone depletion refers to the decrease in ozone levels in the atmosphere :D
the overall cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is dramatically enhanced in the presence of polar stratospheric clouds (PSCs).
the overall cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is dramatically enhanced in the presence of polar stratospheric clouds (PSCs).
Environment - Chlorofluorocarbon and ozone depletion introduction
First, we need to know what is Chlorofluorocarbons (CFC in short). It is a class of synthetic chemicals that are odourless, non-toxic, non-flammable and chemically inert.
When the CFCs are released into the atmosphere, they drift up slowly and, under the influence of ultraviolet radiation from the sun, they react with ozone (O3) to form free chlorine (Cl) atoms and molecular oxygen (O2), thus destroying the ozone layer which protects the Earth's surface from excessive ultraviolet rays from the sun. The chlorine liberated during the breakdown of ozone can react with more ozone, making CFCs very hazardous to the environment. What's worst, CFCs can remain in the atmosphere for more than a CENTURY!
Atmosphere - composition of gases : )
Here is the amazing fact about the composition of our Earth's atmosphere. As we know the atmosphere is made up of many different gases, some of which is presence in such a minute amount that we don't even know of their presence around us!
Lets look at the gases around us (they are ranked according to their volume in the atmosphere):
1st. Nitrogen (N2) with a percentage of 78.084%. (Although we humans have not much use, there's plenty of those fellers jumping around us.)
2nd. Oxygen (O2) with a percentage of 20.946%. (Now THIS is the guy that we depend on the most for us to be able to stay alive, and for me to be sitting in front of my comp designing this resource package.)
3rd. Argon (Ar) with a percentage of 0.934%. (So ya stop giving your pooor computer that what-the-hell-i-thought-the-third-should-be-CO2 stare and believe this, it's ARGon the noble gas!)
4th. Carbon Dioxide (CO2) with a percentage of 0.0383%. (Here comes our first compound gas, and the "bad guys" who can be seen escaping from your nostrils at this very moment, through Mrs. Chua's third eye of course.)
5th. Neon (Ne), 0.001818%. (second noble gas :)
6th. Helium (He), 0.000524%. (third :|
7th. Methane (CH4), 0.0001745%.
8th. Krypton (Kr), 0.000114%. (fourth :(
9th. Hydrogen (H2), 0.000055% (Joel: Hey dat's me!) (Steven: Shut up and carry on with this!)
10th. Nitrous oxide (N2O), 0.00003%
11th. Xenon (Xe), 0.000009% (fifth... Dude, start wondering why are these noble gases taking up so much spaces in our atmosphere -_-|||)
12th. Ozone (O3), 0.0% to 0.000007%. (Finally! Our dear ozone! Where have you been?!)
...
And there's others down the list like nitrogen dioxide, iodine, carbon monoxide and ammonia, not forgetting our {H2O}(g), water vapour, which varies in percentage for different areas on earth.
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