Transportation and disposal of ash in thermal power station burning high sulphur coal — Coal is the primary fuel for a thermal power plant. Few power plants burn imported coal and pet coke which contains higher quantity of sulfur. While burning this type of fuel, lime will be injected to absorb the sulfur dioxide liberated while burning the coal in order to reduce the quantity of sulfur dioxide emitted in to the atmosphere along with the flue gas. The ash generated contains Calcium sulfate and unburnt CaO.
A substantial part of India’s coal is being imported from Indonesia and other countries to bridge the gap between the domestic demand and supply. The imported coal has higher calorific value but also a higher content of sulfur. While burning, more than 95 % of this sulfur gets converted into sulfur dioxide. As per the recent amendment to the Indian environment protection act, emission norms for power stations have become more stringent.
The latest regulation has considerably reduced the allowable emission of sulfur dioxide. In order to meet these figures, power stations have to be able to absorb sulfur dioxide from flue gas. Sulfur dioxide is acidic, therefore the sorbent slurries or other materials used to remove the sulfur dioxide from the flue gases are alkaline like lime. There are different methods of controlling sulfur in the pipestack. The most common are the following:
- In the case of Pulverized Fuel (PF) boilers, Flue Gas Desulfurizer (FGD) could be used.
- In the case of PF boilers in coastal areas, sea water can be used.
- For Fluidized bed combustion (FBC) boilers, dry limestone can be injected into the furnace.
Sulfur Dioxide Removal
In the case of PF boilers, the common method to remove sulfur dioxide from the flue gas is wet scrubbing with alkaline sorbent like limestone, lime or sea water. Dry scrubbing can also be used, however, the wet process is more efficient. During wet scrubbing, the gas first passes through the ESP for removing the ash before passing through the sulfur dioxide absorber. In the case of dry injection or spray drying, the flue gas first passes through the SO2 absorber and then the ash removal.
Most FGD systems employ two stages: One for the removal of fly ash and the other for removal of SO2. Yet, the removal of both the ash and SO2 in one single scrubbing vessel is also feasible. However, these systems have severe maintenance issues and low efficiency. In the case of FBC boilers, the lime will be injected into the furnace as milled limestone during combustion.
It then reacts with SO2 to form sulfates which become part of the ash. There will also be some quantity of unburnt lime in the furnace which will be removed with the ash. The reaction in wet scrubbing using limestone slurry produces CaSO3 (calcium sulfite) and can be expressed as:
Ca(OH)2 + SO2 → CaSO3 + H2O
The CaSO3 (calcium sulfite) is further oxidized to CaSO4 · 2H2O which is Gypsum.
CaSO3 + H2O · ½O2 → CaSO4 · H2O
A natural alkaline for sulphur absorption is seawater: It absorbs the SO2, which, when oxygen is added, reacts to form sulfate ions and free hydrogen. The surplus hydrogen reacts with the carbonates contained in the sea water to release CO2 gas:
SO2 + H2O+ ½O2 → SO4 + 2H
HCO3 + H → H2O+ CO2
In case of PF boilers with FGD after the ESP, fly ash will have the same composition as normal ash and can be handled accordingly. In applications with a dry FGD before the ESP, the fly ash will contain CaSO4 and unburnt lime, which makes special care necessary.
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