Will hydrated lime remove HCl from my flue gas?


Yes.  Dry Sorbent Injection with hydrated lime is a proven method of removing HCl from flue gas.  Mississippi Lime has successfully completed full scale trials where hydrated lime DSI allowed the site to economically meet the upcoming 2015 MATS requirements for Acid Gas control (<0.002 lb/mmBtu HCl).



For SO3 and HCl mitigation, someone told me hydrated lime would require a higher stoichiometric ratio than trona.  Is that true?


Technically, yes, more moles of hydrated lime are required compared to trona.  This is due to the fact that trona's molecular weight is about three times that of hydrate (226 lbs/mol vs. 74 lbs/mol).


However, no one buys moles of either sorbent.  They buy tons.  On a typical feed rate comparison, hydrated lime compares favorably to trona for SO3 and HCl mitigation.



Will hydrated lime affect my ash sales?


In general, ash sales can be preserved by using hydrated lime for DSI.  Calcium in the ash is beneficial and will not negatively affect ash properties such as TCLP.  Calcium salts are not soluble and acceptable for landfills.



Will hydrated lime remove SO3 at pre-Air Preheater temperatures?


Hydrated lime has been proven effective at pre-APH injection temperatures.  Mississippi Lime led recent trial work and reported on the effectiveness of hydrated lime at higher temperatures.  Some Utilities are injecting hydrate at hot side temperatures based upon this work.  To this date and to our knowledge, no one has experienced issues with using pre-APH injection of hydrate.



Why would I want to inject in front of the APH?


Pre-APH injection offers many benefits.  Early removal of acid gases aids in corrosion control of ductwork.  An earlier injection point offers the Utility more residence time prior to particulate collection that can help with sorbent utilization.  This extra residence time is also beneficial if the site is co-injecting activated carbon for mercury control.


Removal of SO3 prior to the APH may reduce fouling associated with ammonium bisulfate (ABS) a product of the reaction of SO3 and ammonia slip from the SCR.  Reduction of SO3 prior to the APH lowers the flue gas acid dew point and offers the possibility of reducing APH temperature and improving unit heat rate.


The byproduct of hydrated lime and SO3 is calcium sulfate.  This byproduct has not been problematic when formed at pre-APH temperatures.



If I need to remove SO3 and HCl, can you tell me the relative reaction rates?


While both SO3/H2SO4 and HCl are strong acids, neutralization of SO3 proceeds faster.  Pilot and full scale testing has supported the premise that HCl does not get significantly removed until SO3 is at a low (<5ppm) level.  Mississippi Lime and others in the industry are working on how to better understand this in full scale DSI applications.



What is better for acid gas removal: an ESP or Baghouse?


Conventional wisdom is that a unit with a baghouse will use less sorbent to achieve similar acid gas removal to that of an ESP unit.


There are a lot of ESP units using hydrated lime to meet their Air Quality Control requirements.  Units with marginal or undersized ESPs should consider a test with hydrate DSI.  There are several options that can allow a unit with an undersized ESP to experience the benefits of hydrated lime DSI.  The options include:


  • Optimization of injection grid

  • Split injection (pre- and post-ESP) on wet FGD units

  • Pre-APH injection of hydrate to improve utilization


There is a lot of testing in the area currently.  Look for Mississippi Lime at Air Pollution conferences or call us at (800) 437-5463 for the most updated information.



Won't injection of hydrate after the ESP make my stack particulate levels increase?


Full scale tests have shown that post-ESP injection of hydrate can reduce particulate levels at the stack.  Other Utilities have seen similar behavior.  It is always best to test on your specific system.



I have heard that hydrated lime is hard to handle.  Why is that?


Hydrated lime can be challenging in a poorly designed system.  However, in the past 5 years, Mississippi Lime has led the charge for educating Utilities and Equipment design firms on the best practices for hydrated lime conveying.  Mississippi Lime has recommendations for optimum DSI systems that are based on R&D testing and field results from sites doing well -- and not so well-- at hydrated lime DSI.


The main focus is controlling reaction of hydrated lime with CO2 in the conveying air.  This can be accomplished by using a good hydrate air ratio and controlled conveying air temperature.