Burning coal and municipal waste to produce energy forms various acidic gases. The oxides of sulfur, hydrogen chloride and hydrogen fluoride can be removed from the flue gas stream by calcium carbonate (chalk), calcium oxide (burnt or quicklime) or calcium hydroxide (hydrate).

In power stations, a calcium carbonate slurry can be injected into the gas stream. Another method injects finely divided calcium carbonate directly into the boiler to form calcium oxide. This then reacts to remove the acidic gases in a more efficient process.

For the larger waste to energy plants (over 100,000Te of waste), generally calcium oxide is used which is slaked in-situ to form calcium hydroxide. While the small to medium size plants tend to use calcium hydroxide. Units operating a fluidised bed tend to use calcium carbonate directly in the bed.

Wet scrubbing

Calcium carbonate is added to water with the resulting slurry sprayed into a flue gas scrubber. The process is found to be more efficient if a calcium hydroxide/water slurry is used, removing over 95% of sulfur dioxide. 

In a typical system, the gas to be cleaned enters the bottom of cylinder-like tower and flows upward through a shower of lime slurry. The sulfur dioxide is then absorbed into the spray and precipitates as wet calcium sulfite. 

By re-circulating the slurry and injecting oxygen, calcium sulphate (gypsum) is formed which can be sold as a by-product.

CaSO₃ 0.5H₂O + 0.5O₂ + 1.5H₂0 → CaSO₄ . 2H₂O

Semi-dry scrubbing

Calcium hydroxide in water (milk of lime) is atomised at the top of a spray drier chamber into hot flue gases of approximately 220°C. The water in the milk of lime evaporates, cooling the gases (SO2, SO3, together with any HCl/HF present) which dissolve and react with the lime. When all of the water within the spray has evaporated, the solid reaction products, in combination with any unreacted calcium hydroxide, are carried by the gases from the scrubber into the dust collector (usually a bag filter) at a temperature of approximately 120°C.

The semi-dry scrubbing process is capable of removing up to 95% of SO2 and up to 99% of HCl and HF.

Low temperature dry injection

Hydrated lime is fluidised in air and injected straight into the exhaust ducting. Generally, over 99% of the HCl, over 95% of the HF and over 95% of SO2 can be removed. The neutralisation reactions are as follows:

Ca (OH)₂ + 2HCl → CaCl₂ + 2H₂O

Ca (OH)₂ + 2HF → CaF₂ + 2H₂O

Ca (OH)₂ + SO2 → CaSO₃ + H₂O

Ca (OH)₂ + SO2 + 0.5O2 → CaSO₄ +H₂O

Transformed into calcium chloride, calcium sulfite, calcium sulfate and calcium fluoride, the acidic gases are captured on baghouse filters as solids (similar to the semi-dry scrubbing technique). The excess hydrated lime can be re-circulated to improve utilisation.

Apart from the content of available hydrated lime, the reactive surface area is also of importance for removal efficiency. The high degree of fineness of industrial hydrated limes also increases the efficiency in eliminating acid gas components.

High temperature dry injection:

Hydrated lime is injected directly into the kiln at temperatures in excess of 850°C. The hydrated lime decomposes within 30 milli seconds to produce a porous and very reactive form of quicklime. The reaction is as follows:

Ca (OH)₂ → CaO + H₂O

In the presence of oxygen, quicklime reacts with oxides of sulfur at temperatures below 1200°C to form calcium sulfate. The quicklime also reacts with any HCl or HF present.  Overall, the high temperature dry injection technique can remove 50-65% of sulfur dioxide. The main advantages of this technique are that it requires relatively little capital expenditure and can readily be retro-fitted. However, in contrast, it also has relatively high absorbent costs and is only suitable where partial desulfurisation is required.