White smoke coming from stack

Isn't white typically associated with steam?

Steam certainly looks white and if that is the issue, an abundance of entrained moisture venting through their system, then a centrifugal gas/liquid separator would be appropriate to make that plume disappear. I do not know enough about ESPs or their process to rule-out other particulate entrained within their emissions. While a separator might address the problem, always preferred if the cause of the problem can be determined before "fixing it".

My guess is that you've got a tube leak into the boiler furnace.

My thoughts exactly matched with TBP.
However, from the picture it seems you have two boilers.
If yes, what about the other?
Is it under shutdown?
If no, then why white smoke is not coming out from its stack?

That doesn't look like steam to me. Visible water vapor usually is separated from the stack and then disappears after a short while. Stack emissions are hard to read from a picture though. Many years ago I was trained to read opacity and from that picture looks bad, like maybe 25-30%

Are you injecting some kind of ammonia solution upstream of electrostatic precipitator for corrosion control ? Some months ago, there was a posting on E tips about problems related to amm sulphide caking up in the ESP. Could these whitish fumes be amm sulphide / amm sulfite? Ammonia reacting with H2S or SO2 resulting from combustion of sulphur in coal ?