DESULFURISATION OF COAL AND OIL

Abhilasha

COAL Coal can contain up to 10% sulfur by weight, although values of

1-4% are more typical depending on the region where the coal was extracted.

Sulfurous gases (primarily sulfur dioxide) produced by the combustion of fossil fuels are the major cause of acid rain and contribute to other pollution related health problems.

Ever since the 1990 revisions to the Clean Air Act, there have been tight restrictions on the amount of sulfur that can be released into the atmosphere as a result of the combustion of fossil fuels.

There are three ways to reduce sulfur emissions from burning coal: 1) the first is through crushing/washing the coal which

physically separates some of the sulfur from the coal. 2)The second method is through the use of "scrubbers"

which remove most of the sulfur from the combustion gases before they are released into the atmosphere.

3) Lastly, plants can burn younger coals that contain less sulfur, such as the coal found in Alaska (average .3% sulfur by weight).

TYPES OF COAL Anthracite: This is the highest ranked, hardest, oldest,

and least common type of coal.It possesses a high energy content, high percentage of carbon (>85%), energy content of 25 million BTU /ton (11,300 BTU/lb).

Bituminous: This is the second rank of coal, softer and younger than anthracite, and containing a lower percentage of carbon (45-85%) and therefore more moisture and volatiles, energy content of 24 million BTU /ton (10,900 BTU//b) in the US.

Subbituminous: This is the third rank of coal, possessing 35-45% carbon and more moisture than bituminous coal, energy content of 17-18 million BTU /ton (7,700-8,300 BTU/lb) in the US.

Lignite: This is the softest, youngest, and wettest rank of coal, often referred to as "brown coal" with a carbon content of only 25-35% and correspondingly lower energy content., energy content of 13 million BTU /ton (5,900 BTU/lb) in the US.

Control of sulfur EmissionsMethods of control: 1) Precombustion Controls: Reduce

emission potential of fuel by switching fuel, using lower sulfur content fuels, and replacing gasoline by ethanol or methanol.

2) Combustion control : Improving the combustion process, by using fluidised bed boilers, and using gas turbines and stirling engines in I.C. Engines .

3) Post Combustion Controls : Capturing the emissions before release in ambience, by Desulfurising the flue gase, and using catalytic converters in exhaust pipes.

Precombustion Measures

Precleaning: sulfur is binded to coal in two ways: 1) Organic , need chemical and biological

treatment . The removal of organic sulphur from coal has been investigated by using model organic substrates, most commonly dibenzothiophene (DBT).

2) Inorganic, in form of inorganic pyrite (FeS2), seperated by physical treatment,

e.g. washing , Aerobic, acidophilic chemolithotrophes like certain of the Thiobacillus species, have been studied in relation to the desulphurisation of the inorganic sulphur in coal.

Those microbes have long been known to oxidise sulphur during the leaching of metals like copper, nickel, zinc and uranium from low grade sulphide ores.

Using heap-leaching approach to microbial desulphurisation at the mine itself, which is a technique commonly employed for metals. This is cheap and simple solution, in practice it is difficult to maintain optimum conditions for the process.

The investigated micro-organisms are mesophiles and the rapid temperature increases experienced coupled with the lengthy period of contact time required, at around 4–5 days, form major limiting factors.

The use of extreme thermophile microbes, like Sulfolobus sp. gives a faster rate of reaction.

Pyrite has specific gravity 3.6 times greater than coal, thus the density difference allows to separate them by washing.

It helps in reducing sulfur as well as ash content.

Reducing the sulphur present in the initial fuel is around five times more expensive than removing the pollutant from the flue gas.

Combustion controls Fluidised bed

combustion: Crushed coal is mixed

with limestone is held in suspension by fast raising air injected from bottom of the bed.

SO2 formed during combustion react with limestone to form calcium sulfate, which falls to the bottom of furnace and removed.

Removal rate of sulfur is 90%.

Post combustion measures

Flue gas desulfurisation: May be Wet or Dry, A slurry of CaCO3 and siliceous compound is sprayed into flue gases.

SO2 is absorbed by the slurry, producing calcium sulfite or calcium sulfate ppt. , precipitate is then removed as sludge.

As a result there is formation of inert calcium sulfite dihydrate.

CaO + H2O -----> Ca(OH)2

SO2 + H2O <----> H2SO3

H2SO3 + Ca(OH)2 -----> CaSO3.2 H2O

Lime slurries have efficiency upto 95%.

Dry scrubbers use lime in order to desulfurise the coal, that adds the expenses to it. But it is more efficient.

CaO+SO2 + 2H2O -----> CaSO3.2 H2O

Drawbacks of Scrubbers:

1) They are subjected corrosion, scaling, and

plugging problem, which may reduce overall

efficiency.

2) A 1000 MW plant uses up 1000 gallon water

while burning 3% of coal.

Desulfurisation of oil

HydrodesulfurizationHydrodesulfurization (HDS) is a catalytic

chemical process widely used to remove sulfur (S) from natural gas and from refined petroleum products such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils.

The purpose of removing the sulfur is to reduce the sulfur dioxide (SO2) emissions that result from using those fuels in automotive vehicles, aircraft, railroad locomotives, ships, gas or oil burning power plants, residential and industrial furnaces, and other forms of fuel combustion.

Procedure Hydrogenation is a class of chemical reactions that

results in the addition of hydrogen. Hydrogenolysis is a type of hydrogenation that results in the cleavage of the C-X chemical bond, where C is a carbon atom and X is a sulfur, nitrogen or oxygen atom. The net result of a hydrogenolysis reaction is the formation of C-H and H-X chemical bonds. Thus, hydrodesulfurization is a hydrogenolysis reaction. Using ethanethiol (C2H5SH), a sulfur compound present in some petroleum refining streams, as an example, the hydrodesulfurization reaction can be expressed as:

Ethanethiol + Hydrogen ⇒ Ethane + Hydrogen Sulfide

C2H5SH + H2 ⇒ C2H6 + H2S

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