Flue gas treatment
The use of lime for treating flue gases is a proven technology. Flue gas is generated from the thermal treatment process in Energy from Waste plants (EfW) and contains acidic gases such as hydrogen chloride, sulphur dioxide and hydrogen fluoride. The use of lime in the three main flue gas treatment processes of; dry, semi-dry and wet processes shows its flexibility and adaptability in its worldwide application.
Calcium carbonate (chalk), calcium oxide (quicklime) or calcium hydroxide (hydrated lime) can be used to neutralise acidic gases and remove sulphur dioxide from both EfW plants and power stations. This ensures that plants comply with both local and European environmental legislation for air emissions. Together with the new flue gas treatment equipment technologies, lime is the most cost effective alkali that can be used for this kind of treatment, with less dosage and less waste production compared with other reagents.
The number of Energy from Waste plants in the UK is due to rise significantly in the near future, as the cost of landfilling waste is set to increase dramatically. Lime products can therefore provide a cost effective, efficient solution to the treatment of flue gases generated from the energy recovery process, which reduces the waste volumes sent to landfill.
A variety of flue gas abatement techniques have now been designed to suit particular applications, some of which are described below.
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 sulphite, calcium sulphate and calcium fluoride, the acidic gases are captured on bag 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 sulphur at temperatures below 1200°C to form calcium sulphate. The quicklime also reacts with any HCl or HF present. Overall, the high temperature dry injection technique can remove 50-65% of sulphur 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 desulphurisation is required.
The removed reaction products and ash from the combustion process are disposed of in landfill sites.
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.
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 sulphur 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 sulphur dioxide is then absorbed into the spray and precipitates as wet calcium sulphate.
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 + 1.5H2O → CaSO4 . 2H2O