WFGD Mass Transfer

The WFGD Absorber shown on the left in the slide above, represents the basic design for WFGD gas absorption, and the fundamental design approach of mass-transfer for Flue Gas Desulfurization.? There are many ways system suppliers provide for gas absorption within the absorber towers, and I tend to think of these designs, for the most part, as variations on a theme.? One design stands somewhat apart from these, and that is the Chiyoda gas absorber.

For absorber towers, the gas enters the absorber and is contacted with gypsum slurry droplets for the most established process design - Limestone Forced Oxidation LSFO.? This leads to the acronym LSFO WFGD - Limestone Forced Oxidation, Wet Flue Gas Desulfurization.? Limestone is the reagent.? Gypsum is the by-product.??

The slide above shows such an absorber design.? Fine droplets are created through the action of large slurry pumps, which recirculate the slurry from the absorber sump, pushing the slurry up to, and through, the spray headers and nozzles.? Whether the flue gas enters the absorber from the bottom, side or top of the absorber; or the spray is co-current or counter-current to the gas flow; the contact is between the fine slurried spray droplets, and the flue gas.??

For the Chiyoda design, the contact between gas and slurry occurs via the sparging of the flue gas into and through a pool of gypsum slurry.??

For the tower, the contacting power, if you will, comes from the large recirculation pumps.? For the Chiyoda design, it comes from the large fans.??

One key to understanding how the absorber works, is to consider not just gas absorption, but also diffusion with reaction.??

For example, if you were to run flue gas into an absorber tower, and recirculate water only, you would quickly reach a very acidic pH value.? You could no longer absorb the SO2 in the flue gas.? It is only through the dissolution of the limestone into the slightly acidic gypsum slurry, for reaction with the absorbed SO2, that the absorbed, or diffused, sulfur dioxide can be reacted with the dissolved Ca++.? This allows the precipitation of the gypsum, allowing further dissolution of SO2.

The flue gas is contacted within the absorber by the recirculating solvent, and as a result, the constituents in the flue gas are absorbed by the solvent.? For limestone-based systems, generating a gypsum by-product, the solvent is in fact a two-phase system, a slurried mixture of water and gypsum.?

The absorber is a mass transfer device, but so much more.? It dissolves the SO2 into the gypsum slurry.? It provides good contact between the flue gas and the gypsum slurry.? It provides continuous sub-saturation of the gypsum slurry with respect to SO2 via limestone dissolution.? It not only allows absorption of the SO2 into the liquid, but dissolution, followed by precipitation of the gypsum.? And not just precipitation of the gypsum, but crystallization, continuing to grow crystals in the absorber, until the slurry is finally discharged from the absorber sump.??

Gas - Liquid - Dissolution - Precipitation - Crystallization - Solid.? All in one big contacting device.? And not just SO2 - but let’s not get ahead of ourselves!

Above, we discussed in some detail the WFGD design which is the standard bearer of the industry - Limestone Forced Oxidation LSFO.??

For seawater scrubbers, of which at least a few are planned for India, there is no solid reagent and no solid waste product.? Alkaline seawater is used to contact the flue gas, and acidified water is returned to the sea.? The type of coal being burned, and the SO2 efficiency needed, determines the amount of seawater that must be used to sufficiently neutralize the flue gas acidity.? The allowable pH of the acidified seawater, and the SO2 efficiency required, determines to a great degree the power required for sufficient mass transfer.??

Some limited research on my part has revealed that GE, Andritz, Ducon, and Mitsubiushi are among those who have won orders for WFGD in India, and these are all spray/tray type designs. Whether you have the flue gas enter from the bottom, side, or top, and flow down, up or sideways, or some form of packing, or unique spray nozzle, the Concept is Contact.? The same is true for Chiyoda.? Mass transfer is a must for the absorber to do its job.