In agreement with prior experimental work, thermodynamics predicts differences in the outcome of biomass pyrolysis conducted in sealed, constant-volume systems as opposed to constant-pressure systems. In particular, much higher values of the fixed-carbon yield can be expected in constant-volume systems. Avicel cellulose is known to give very low char and fixed-carbon yields; thus, char and fixed-carbon yields from cellulose have been given primary attention in this work. Our tubing bomb results reveal (i) fixed-carbon yields that realize the theoretical limiting values when the vessel is pre-pressurized to a modest pressure with N-2 gas, and (ii) a gas product composed of steam (water) and CO2, with traces of CO and virtually no tars. Above a small range of modest temperatures and pressures, the char endures a molten phase and becomes a hard coke. The ash content of the char/coke reflects the composition of the glass wool and kao wool materials used to hold the Avicel powder in place.