In reference to your charcoal consumption rate, in my experience, a 5-gallon (20 liter) bucket of 1/2" dense char (oak) is roughly equivalent to 1 gallon of gasoline.
Spruce is a very “light” wood, and has half the BTUs of hardwood… So, you’re probably looking at 10 gallons in volume (40 liters) to get the equivalent of 1 gallon of gasoline.
So, in cases like this, it all about carbon density. We had phenomenally long run times with pyrolyzed pellets (super dense), and lasted twice as long as the pyrolyzed pine wood chips we tried.
You can see a sortable list of the energy content of wood species on my blog:
Hope this helps…