Pine burns very well, if there is little or no bark as long as it is not punky. The bark on most evergreens has evolved to protect the tree in a forest fire. It has it's own retardant. If split well, this does not hamper it but do not leave whole unsplit pieces with good bark still on it. This should not really be an issue for 15 yr old wood.
I once put 1 pc of unsplit fairly fresh cut hemlock with a huge number of limb stubs, maybe 6-7" diameter in the evaporator just to get rid of it. That piece was still there an hour later, and after re fueling around it every 7 minutes. Never again, it messed up my boil rate for over an hour. That was before I had any blower for combustion. I don't know if that might help.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.