Limestone Consumption for Removal of Sulfur from coal

do a mass balance

Agreed. Mass balance.

Calcium Carbonate (limestone) → Calcium Oxide.

Calcium Oxide + Sulphur from the coal + a bit of oxygen from the air → Calcium Sulphate.

The rest is arithmetic, based upon the molecular weights of the above materials.

How do you prevent oxidation of the sulfur to produce some oxides of sulfur? One assumes this is not done in a closed system. The high sulfur content coal is awfully high in S.

Regarding the high sulfur content coal. I agree. I worked in the coal research area for 18 years and never came across a coal with that high a sulfur level.

A mass balance will tell you nothing! Now a mole balance will indeed tell you something. When dealing with reactions you look at molar quantities, not mass quantities.

Also one does not treat sulfur in the coal you treat its combustion byproduct. Coal is a very complex network of various molecules intertwined in one another. The sulfur in the coal is not elemental sulfur which the above equation suggests, sulfur in coal is in the form of sulfur compounds like thiophenes. The exact makeup of coal is indeterminent so a coal feedstock is typically characterized by an ultimate analysis which is based on the elements; carbon, hydrogen, sulfur, nitrogen, oxygen, and others (moisture and ash).

So given an elemental speciation and quantity of coal one can determine how much mass of each element is in the coal. From here this can be converted to molar quantities by dividing by the molecular weight of the component.

During combustion of coal, X moles of sulfur will be converted to X moles of sulfur dioxide (assume only SO₂ is formed since negligible amounts of SO₃ are formed as well).

So if the X moles exceeds standard emission levels in your area, you have to SCRUB the stack gas to remove excess sulfur dioxide below the standard. This can be accomplished with a limestone slurry (CaCO₃ and water) flowing countercurrently (downward) to the stack gas in an absorption tower OR direct injection of wet or dry limestone into the furnace.

The conclusion is,

You would need at least X moles of limestone and 0.5X moles of water to accomplish the conversion of X moles of S (or SO₂ however you look at it). Just convert moles back to mass quantities and you're set. The following is the underlying reaction:

CaCO₃ + SO₂ + 0.5H₂O ---> CaSO₃*(0.5H₂O) + CO₂

Note this is only part of the story since one needs to account for the heating value of coal used since the regulation is posted as grams of SO2 per higher heating value of fuel fed.

-David