The extra stack, as you say, is a new addition, apparently added in 2014/2015. Since the ship is always (?) forward deployed, I wonder how they got enough yard time, and where, to add whatever the new system is.
Since we are hazarding guesses... I worked in a very large data center and as computers grew smaller and heat per server increased, they needed much more cooling and electricity per square foot of server rack space. And since the LCC is a bit of a floating data center, I will guess that this is not for steam or diesel exhaust, but rather for exhaust from additional air conditioning/ cooling. A guess only.