cross posting to sci.engr.mech
Dear Pete Mills:
This is physical chemistry, more than just chemistry.
I'm not sure why the type of fuel, other than a slight change in the amount of water produced per kilogram of fuel burnt. Hopefully you'll get some better answers here...
As to the excess air, the resulting mix of excess air plus combustion products, has a lower relative humidity (which is lower percent water at the temperature considered), and hence a lower dewpoint. Of course the excess air itself is going to lower the bulk temperature somewhat...
David A. Smith