Flue gas treatment (FGT)
Benefits of using lime for flue gas treatment
Lime benefits flue gas treatment because it:
- Is a proven technology
- Is effective in all types of scrubbing technologies (dry, semi-dry, wet) making it the most versatile product to use
- Can be used at a wide temperature range, from 50°C to 1,200°C
- Is applicable across a wide range of sectors – such as energy from waste, clinical waste incineration, biomass combustion, and industrial processes like cement and glass manufacturing and non-ferrous metals production
- Has a range of different products available to maximise scrubbing effectiveness
- Provides abatement solutions for a wide range of acidic pollutants in flue gas
- Produces an incinerator fly ash that can be recycled, for example, into the construction industry, upholding the 'circular economy' model
- Is more cost-effective than many alternative alkali reagents
- Can be combined with other mineral powders to provide abatement of more pollutants including heavy metals and micro-pollutants
- Is abundantly available in the UK
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 other kilns including clinical waste incineration, biomass combustion, and many industrial processes. Flue gas usually contains acidic gases, commonly referred to as SOx - sulfur dioxide (SO2), sulfur trioxide (SO3), hydrogen chloride (HCl), and hydrogen fluoride (HF). Lime is used to remove these acidic pollutants from the flue gas as part of environmental management systems to minimise the impact of these activities on air quality.
The use of lime in all of the three main flue gas treatment processes - dry, semi-dry and wet - shows its flexibility and adaptability in its worldwide application for flue gas treatment.
Calcium oxide (CaO - quicklime) or calcium hydroxide (Ca(OH)2 - hydrated lime), and calcium carbonate (CaCO3 - limestone or chalk) can each be used to neutralise the acidic gases and remove sulfur dioxide from flue gases. This helps to ensure that plants comply with both local and international environmental legislation for their emissions. Together with the specialist flue gas treatment equipment technologies, lime is the most cost effective and versatile alkali that can be used for this kind of treatment, with less dosage and less waste production compared with other reagents.
The number of EfW plants in the UK has seen a marked increase since 2013 and this trend is expected to continue until at least 2023 as government policy is to target reduction in landfilling of household and commercial waste, a contributor of environmentally harmful methane gas and to encourage more recycling and use of EfW. This policy development and growth of flue gas treatment (FGT) is also seen in many other countries around the world. 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. Mineral lime reagents are used in abatement in more than 85% of UK FGT sites.
Types of flue gas treatment
A variety of flue gas abatement techniques have now been designed to suit particular applications, some of which are described below.
1. Low temperature dry injection
Hydrated lime is fluidised in air and injected straight into the exhaust ducting. Generally, over 95% of SOx can be removed, over 99% of the HCl, and over 95% of the HF. The most common neutralisation reactions are as follows:
Ca(OH)2 + SO2 → CaSO3 + H2O
Ca(OH)2 + SO2 + 0.5O2 → CaSO4 + H2O
Ca(OH)2 + 2HCl → CaCl2 + 2H2O
Ca(OH)2 + 2HF → CaF2 + 2H2O
Transformed into calcium sulfite, calcium sulfate, calcium chloride, 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.
2. 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 milliseconds (0.003 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 sulphate. 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. This process can reduce reagent consumption if combined with traditional lower temperature alkali scrubbing processes.
The removed reaction products and ash from the combustion process are either processed to make aggregate products for the construction industry or disposed of in landfill sites.
Calcium hydroxide in water (called milk of lime or sometimes liquid 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 SOx 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 instead, removing over 95% of SOx.
In a typical system, the gas to be cleaned enters the bottom of a 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 sulfate.
By re-circulating the slurry and injecting oxygen, hydrated calcium sulfate (gypsum) is formed, which can be sold as a by-product.
CaSO3 0.5H2O + 0.5O2 + 1.5H2O → CaSO4·2H2O
The Safe Handling of Lime document (available here) gives some general guidelines on the handling requirements for lime products.
However, please refer to the supplier's Safety Data Sheets for the complete safety information referring to an individual product being considered.
Information on Energy from Waste plants is available from:
SOx is one of the pollutant gases targeted by the UK Government's Clean Air Strategy 2019 – available here.