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| >> Flue
Gas Desulphurisation >> Merits of Double Alkali Lime & Coustic
Scrubbing
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Flue Gas Desulphurisation
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Parameter |
Lime/Limestone Scrubbing |
Caustic Scrubbing |
Double Alkali Process |
| Chemical used | Lime/Limestone | NaOH | NaOH/Na2CO3 & Lime(CaO) |
| Expected Chemical Consumption | 1.3 kg/kg of SO2 removed for limestone | NaOH : 1.2 kg/kg of SO2 removed | NaOH : 0.09-0.12 kg/kg SO2 removed. CaO : 0.9-1.1 kg/kg of SO2 removed |
| Water consumption | High due to excess lime carry over in sludge which is lost in cake | High as water is lost in bleed. | Low as no carry over of lime in sludge and treated bleed is recycled |
| Optimum use of chemicals | Higher than stoichiometric as bleed contains unreacted lime | Higher than stoichiometric as bleed contains unreacted caustic | Most Optimum as system operates in acidic or at neutral pH |
| Power consumption | High due to high recirculation rates and high pressure drop | Low due to low recirculation rates & low pressure drop | Low due to low recirculation rates & low pressure drop. |
Parameter
|
Lime/Limestone
Scrubbing
|
Caustic
Scrubbing
|
Double
Alkali Process
|
| Achievable Efficiency of SO2 removal | 85-90% | 95-99% + | 95-99% + |
| System availability | 80-87% | 95-98% + | 95-98% + |
| Operating pH | 8.5-10.5 | 8.5-10.5. | 6.9-7.0 |
| Quantity of recirculating liquid (L/G Ratio) | 10 to 14 m3/1000 m3 of gas for 85-90% removal efficiency | 2.6-3.0 & 4.5-6.0 m3/1000 m3 of gas for 95 & 99+ % removal efficiency | 2.6-3.0 & 4.5-6.0 m3/1000 m3 of gas for 95 & 99+ % removal efficiency |
| Pressure drop | High | Low | Low |
Parameter
|
Lime/Limestone
Scrubbing
|
Caustic
Scrubbing
|
Double
Alkali Process
|
| Consistency in maintaining outlet emission levels. | Low | Very high | Very high |
| Capacity to handle surges of high SO2 load at inlet without major effect on outlet SO2 emission | Low due to low reactivity of lime/limestone | High | High (due to operation of scrubbing system in concentrated mode resulting in high buffer capacity of scrubbing liquid) |
| Scaling in scrubber | Extremely high | Low | Practically Nil as it operates in acidic mode |
| Requirement of Spares for maintenance | High-due to erosion & plugging | Low | Low |
| Size of scrubber | High | Low | Low due to higher mass transfer coefficient |
Parameter
|
Lime/Limestone
Scrubbing
|
Caustic
Scrubbing
|
Double
Alkali Process
|
| Quantity of solids | Very high due to excess lime carry over in sludge | Not applicable as bleed needs elaborate special treatment before it is disposed | Low as no excess carry over of lime |
| Quantity of solids | Poor due to presence of excess lime carry over in bleed liquor | Not applicable | Good as no carry over of lime in bleed liquor |