Spray Dryer is a cost-effective option for FGD waste treatment
Power
plants with wet FGD in the U.S. have two choices --either the treat and
discharge path, where the plant must meet the assigned limits for mercury,
arsenic, selenium and nitrates/nitrites as N, or a voluntary incentive program
using evaporation technology. These voluntary incentives program limitations may
be met with more traditional evaporation technology, yet the salt dryer results
in the wastewater stream being entirely evaporated into the flue gas, leaving
behind only FGD solids and previously dissolved salts such as calcium and
magnesium chloride. These solids are collected in a downstream particulate
collection device PCD.
B&W's
research indicated the advantages of the spray dryer (aka salt dryer, when used
in a wet FGD application) far outweighed those of the circulating fluid bed salt
dryer. The circulating fluid bed salt dryer often required adding insoluble
material, which increased cost; the circulating fluid bed salt dryer required a
large recirculating load of hygroscopic salts and experienced elevated pressure
pumping issues as a result of the relatively high suspended solids in wet FGD
wastewater. Additionally, there is an elevated risk of corrosion resulting from
contact between damp particles high in chlorides with the circulating fluid bed
salt dryer vessel carbon steel walls. B&W also looked at the waste-to-energy
applications, which have very high chloride fuel and high spray dryer outlet
temperatures similar to a salt dryer. These applications also favor spray drying
for SO2 control.
Depending on the amount of water to be evaporated and the size of the boiler,
anywhere from 6 percent to 12 percent of the flue gas generated in the boiler
will be used to evaporate the wet FGD wastewater. For instance, a 900MW unit
burning low-chloride coal such as Powder River Basin coal with its typically low
chlorine content, and a WFGD system capable of higher chloride loadings, may
only necessitate blowdown of 30gpm - 50 gpm, which would require 2 percent to 3
percent of the flue gas to be bypassed. However, if this same unit was burning
eastern fuel with high chloride levels, blowdown of 170gpm - 200 gpm or more may
be necessary, requiring 10 percent to 12 percent flue gas bypass. An operator
would want to limit this to no more than 12 percent in most circumstances.
A
popular misunderstanding is that this level of bypass around the air heater
leads to a significant impact on efficiency. However, we normally expect a 12
percent bypass to impact boiler efficiency by approximately 0.3 percent.
The
solids/salt combination that exits the salt dryer at 300F may well be handled in
the existing PCD. But if not, a separate fabric filter is needed if the
additional solids loading to the PCD can't be tolerated for some reason. This
also would require a booster fan and possibly solids handling equipment. Another
advantage of including a separate fabric filter is that fly ash collected in the
existing PCD will continue to be sellable, since it will not be contaminated
with high-chloride salts from the WFGD. This may warrant the installation of a
separate fabric filter even if the existing PCD can tolerate the increased
solids flow.