gypsum crystal habit modifiers- wet FGD system 1

[davefitz]

for wet FGD systems on coal fired plants that produce marketable gypsum:

Kindly advise if anyone has a concise and documented explanation of why hydrated calcium sulphate ( gypsum) crystals form large particles when firing high sulphur coal at full load which are easily de-watered, but the crystal habit changes to small, planar crystals which cannot be dewatered under the following conditions:

-fires a coal which has much lower sulphur content than the design coal
- has upset increases in Al and Fe due to ESP mishaps allowing excess flyash to be captured by the FGD slurry - possible FeF or AlF blinding or crystal habit modification?
- high hardness in the FGD makeup water , which includes high sediments of FeO and quartz fines

Some proposed fixes that were suggested were to lower the slurry's solids density by 33%, and/or increase the slurry's calcium to sulphate ratio. However, fiddling around with major process variables without any knowledge or explanation as to what the h*&^ is occurring is not good practice.

Kindly advise a reference that documents an solution and its explantaion, if available.

 

[unclesyd]

Not an expert by any means but having been involved in the manufacture of Adipic Acid (HOAD) I was exposed to it's use in FGd to improve the environment for crystallized of Calcium Sulphate. I checked with colleagues as to who was the expert in process and we couldn't come up with a name of anyone currently involved with the Adipc sales for Ascend Performance Chemicals. Along with Ascend there are several other suppliers of HOAD for addition to FGD systems

There is a lot of information written about his particular process. I'll see what I can do next week, hopefully some contacts.

 

[davefitz]

I was under the opinion that adipic acid is used to reduce the surface tension of the slurry droplets so as to improve mass trnasfer of SO2 across the droplet interface, so as to improve SO2 removal. Also, that is may act as a surfactant and increase foaming. But if you can confirm that it also acts as a " crystal habit modifier",please advise.

 

[unclesyd]

My information came from a project meeting on an Adipic Acid (HOAD)expansion. I have to admit that crystallisation was not one of better subjects, so I wasn't paying that much attention and only remember little of the talking points at the meting.
I took no notes, but as I recall the subject was discussed parallel to discussions of some PH changes from HOAD.
I'm trying to contact some people that should know about this or possibly lead me to someone who does.

 

[unclesyd]

I finally caught up with an old colleague who is now in chemical sales business. I was unaware that the trend is from HOAD to AGS (Adipic, Glutaric, and Succinic Acids)for use in FGD system. The reason is that HOAD is tough to get in solution and AGS is a liquid ready to use. He said that AGS or HOAD does improve the crystallization of the Sulfate as stated in attached brochure.

http://my.engineering.com/action/file/download?file_guid=345158

 

[davefitz]

Thanks, UncleSy!

I'll forward this info the the FGD mfr and ask for their review of its effectiveness.

In the meantime, we are switching to hi sulphur coal,which will probably solve the issue in the interim.

 

[unclesyd]

I'll get back with my contact as we didn't have much time to talk as he was going to meeting. I asked him for a more definitive reference, aside from this brochure, to the use of dibasic acids for FGD systems