Generic Methods for Coke Oven Gas Desulphurisation by Wilfried ...

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198THE COKE OVEN MANAGERS’ASSOCIATIONThe available processes suitable for desulphurisation of coke ovengas can be subdivided into three generic methods, namely• dry oxidative processes,• wet oxidative processes and• neutralization or absorption/stripping processes.In Fig. 3 you can see the most common processes for each of thegeneric methods.Dry Oxidative ProcessesThe Iron Oxide Process was used for the first time in the early 17 thcentury and the process has hardly changed since then. For theabsorption of hydrogen sulphide ferric hydroxide mass is utilised.An iron oxide process based plant mainly consists of cleaning,depositing and emptying towers with a certain number of baskets or ofboxes with layers of ferric hydroxide.As you can see from Fig. 5, oxygen for sulphur forming is required(approx. 1 to 1.5 % vol. of oxygen). That means oxygen has to beadded to the coke oven gas. Due to formation of water by the process,the coke oven gas should be slightly overheated before entering thetowers/boxes.By this process elemental sulphur is produced and forms a layer onthe iron oxide mass and lowers the activity of the mass. Therefore afterloading of the mass to a certain extent, this basket tower/box has to bechanged to a tower/box with fresh mass. In the beginning in using thisprocess big boxes were used, but later cylindrical towers have beenused because of operating the process at higher pressure and because ofless necessary space.Another way to adsorb hydrogen sulphide is the use of the ZincOxide Process.
COKE OVEN GAS DESULPHURISATION 199This process, however, is utilised to absorb only small amounts ofhydrogen sulphide because of the consumption of zinc oxide mass. Inaddition it has to be taken into account, that the coke oven gas has to beheated up to the adsorption temperature of approx. 300 °C.Wet Oxidative ProcessesThe particular feature of these wet oxidative processes fordesulphurisation is the direct production of elemental sulphur.All wet oxidative processes (Fig. 3) utilise a reduction-oxidationcatalyst to facilitate the forming of hydrogen sulphide to elementalsulphur. In Europe and northern America the most applied process isthe Stretford Process.A Stretford desulphurisation plant mainly consists of theabsorber/scrubber and the oxidizer, where the elemental sulphur isformed as froth/foam. This foam is fed to the slurry tank, to thecentrifuge and finally to the autoclave to improve the sulphur quality.The following chemical agents are required for the Stretfordprocess:• Sodium hydroxide as absorbents• Sodium vanadate for sulphur forming• ADA (anthraquinone disulfonic acid) as catalyst foroxidation of sodium vanadateIn Figure 8 you can find the mainly occurring chemical reactions.Side reactions with hydrogen cyanite in the presence of elementalsulphur result in forming of a high amount of thiocyanate (SCN - ) andalso thiosulfate (S 2 O 3 -- ) is formed; this will inactivate the scrubbingsolution and will lead together with the sluiced out vanadium to highlycontaminated waste water, which cannot be sent to a biological wastewater treatment plant. The waste water also contains quinone andhydrochinone compounds from ADA. All this results in a dramaticalincrease of COD in the sluiced out waste water and requires a separatetreatment plant.
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Generic Methods for Coke Oven Gas Desulphurisation by Wilfried ...

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