Why are these levels so low?
Check out these ash samples to see why.
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Unit 2 Has 40-50% Carbon in the Ash,  while Unit 1 has only 20-30%. “Native” Hg Capture happens inside the De-energized Precipitators and inside the Pulse Jet Baghouses
Sheldon 1 - De-energized precip ash is 20-30% carbon, LOI
(loss-of-ignition) = 20-30%
Sheldon 2 - De-energized precip ash is 40-50% carbon.

*   Explain briefly why unit 2 (actually 2 is higher) is higher

   Mid-90's - Sheldon 1 burner fronts modified to preheat coal before entry (scroll burners).  With the scroll burners, the coal is pre-heated which allows more efficient combustion inside the cyclone, thus less LOI.