Ericwong,
Sounds to me like you simply need an air preheater upgrade with modern heat transfer profiles with a higher heat transfer than the DU and NF6 that you are probably using.
www.airpreheatercompany.com have a selection of new profiles that would increase your heat transfer and other...
Just a quick note on using SBS injection with Ljungstrom air preheaters. If injection is ahead of the air preheater considerations have to be made to the design of the air preheater heat transfer surface to allow for improved cleaning.