Flue gas cleaning technology

Depending on the size of the plant, the energy recovery system and the type of waste, the flue gas stream contains a certain amount of pollutants,such as particulates,halogenides,NOx,SOx,etc.These substances need to be removed/neutralised from the flue gases before releasing them into the atmosphere.Several options of flue gas cleaning are available,but the most commonly used ones can be summarised as follows:

Cyclone

Mainly used in fluidized bed incinerators because of high amounts of sand and dust carry-over:It can only be used for basic de-dusting. A particulate emission of 150 mg/Nm3,as an average,can be obtained. 

Electrostatic precipitator (electro-filter)

This is one step further than a cyclone,but insufficient to meet the present emission limits on particulate emission in the CE.A particulate emission of only 100 – 50 mg/Nm3 c an be obtained.

Dry scrubbing

Acids are neutralised by injection of (dry) sodiumbicarbonate (or lime for poorer efficiencies).All particulates 9and hence Dioxins) are arrested in a baghouse.The newly developed teflon felt,used in the bags,shows arresting efficiencies of 99,8 % for particulates and 97 % on halogenides and other acids. 

Wet scrubbing

Wet scrubbing is used when a high chemical efficiency is required.By using sodiumhydroxide,the reaction between acids and sodiumhydroxide is stoechiometric and therefore the chlorine contents in the flue gas may be as high as 30%.However,arresting submicron particulates may sometimes cause a problem : an average emission level of only 30mg/Nm3 can be expected.

Rotoclone

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Sem-wet (also called semi-dry) scrubbing

cheaper gas cooling (by water) and is used a lot in big installations like in domestic waste processing plant. Big installations (7 ton/h or more) are running much more steady-state because of the continuous waste feeding system. The lime which is injected must be correctly dosed, otherwise the bag house will stack, this phenomena is called clogging of the bag house. Due to the small size of the plant and possible fluctuations in the processnot continuous waste feeding, semi-wet is not recommended for small-medium sized plant.