Using existing Wet Flue Gas Desulfurization to Control Emissions from Additional Units
|By Robert Nicolo, Director of AQCS, and Marco Mendoza, AQCS Process Engineer, Mitsubishi Hitachi Power Systems Americas|
|Robert Nicolo||Marco Mendoza|
Increasingly stringent emissions regulations are driving utilities to shut down aging coal-fired units rather than bearing the burden of installing emission controls. For multiple unit plants with an in-place WFGD system, there is another solution: routing the flue gas from nearby, untreated units to the existing WFGD. Overall plant emissions can be reduced at a relatively low cost by upgrading and using the existing WFGD.
When deciding if an existing WFGD can be used to treat additional flue gas, a few things must be considered.
Depending on the location of the additional units relative to the WFGD, upgrades to the ID fan from the untreated units or a booster fan may be required. The added pressure drop from the absorber and absorber inlet duct may also affect the performance of the absorber’s original, dedicated ID fan.
The tie-in location should be carefully considered to allow sufficient mixing and ensure that no flow perturbations are formed near the absorber inlet. However, due to the nature of retrofit design, this is not always possible and tie-in locations will be selected based on space available. In that case, CFD modeling should be used to evaluate potential flow problems and the need for new vaning or mixing devices.
The increased gas flow increases the absorber velocity which will affect its performance. The blended flue gas may have a new SO2 concentration and the increased gas flow lowers the absorber’s available liquid-to-gas ratio, effectively reducing the absorber’s SO2 removal efficiency. The type of absorber (open spray tower, tray, DCFS, etc.), reagent, nozzle type and other absorber specifics should be carefully considered. Increasing the absorber recycle pumps’ capacity to increase the absorber L/G will ensure proper SO2 removal and absorber performance. Depending on the existing pumps’ design, upgrades may include changing out motors and gear boxes; often, upper spray level gear boxes and motors can be moved to lower spray levels and new equipment purchased for the upper levels only. Additionally, spray nozzles can be upgraded to increase recycle slurry spray rates and optimize head on the recycle pumps. The increases in flow and head may require that recycle lines are replaced with larger piping. In the worst case, spray levels would be replaced with configurations with higher spray rates. Note that the increase in the slurry spray rate may not increase proportionately with the increase in flue gas flow as the blended units SO2 concentration may be lower than the original design SO2 concentration.
Mist eliminator performance will be affected with the higher velocities. WFGD have been traditionally designed for velocities of 10 – 12 ft/s. However, today, ME can operate up to 16 – 18 ft/s with low liquid entrainment. Upgrades could include replacing existing ME with new high velocity designs as well as adding another stage of ME. Blanking plates can be replaced with high velocity, flat ME to fill in the area around the existing ME. The ME wash headers should also be added or upgraded to provide sufficient spray coverage. If headroom is limited in the absorber, upstream preliminary ME could be considered.
Other absorber auxiliary systems should be carefully analyzed and their maximum capacities understood. Oxidation air blowers designed with a high stoichiometry may not require upgrading or modification for the absorber to work satisfactorily. Additional SO2 from adjacent units will lower the overall oxidation stoichiometry, but may still be conservative for adequate forced oxidation.
To maintain the same reagent stoichiometry, the capacity of the reagent feed system may need to be increased. This might be accomplished by increasing the flow rate from the slurry tanks to the absorber or increasing the slurry concentration in the tank.
The existing absorber bleed system may require operational changes to ensure the density in the reaction tank is consistent with the original design. This will ensure that the dewatering system can remain unchanged and reduce the overall cost of the WFGD upgrade. The absorber bleed system may require mechanical upgrades if the increase in overall SO2 is substantial and the bleed system cannot keep up with the gypsum production.
The additional flow and higher exit velocity will also affect the stack flow distribution. Liquid collection devices may be required to ensure that moisture droplets are not entrained and discharged from the chimney.
With MATS compliance just around the corner, upgrading an existing WFGD can be a viable solution for life extension of marginal, older coal-fired units. The increase in performance will support additional PM removal, oxidized mercury collection and HCI reduction. It must be analyzed on a case-by case basis. Depending on site specifics, a viable emissions control solution may only require a few low-cost upgrades.
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