New insights into the effect of pressure on catalytic hydropyrolysis of biomass

Catalytic hydropyrolysis of beech wood has been conducted in a fluid bed reactor at 450 °C with a sulfided CoMo catalyst followed by a fixed bed hydrodeoxygenation (HDO) reactor with a sulfided NiMo catalyst at hydrogen pressures between 3.0 and 35.8 bar. Using both reactors the condensable organic yield (condensed organic and C4+ in gas) varied between 18.7 and 21.5 wt% dry ash free basis (daf) and was independent of the hydrogen pressure.

At 15.9 bar hydrogen or higher the condensed organic phase was essentially oxygen free (<0.01 wt% dry basis (db)), but decreasing the hydrogen pressure to 3.0 bar increased the oxygen content to 7.8 wt% db. The char and coke yield was close to constant (11.0–12.7 wt% daf) at hydrogen pressures between 15.9 and 35.8 bar, but increased to 15.7 wt% at 3.0 bar hydrogen due to an increase in the polymerization of pyrolysis vapors.

The measured carbon content on the spent catalysts from both the fluid bed and HDO reactor showed that coking of the catalysts increased when the hydrogen pressure was decreased below 15.9 bar. The increased coking at low hydrogen pressure (<15.9 bar) was ascribed to the polymerization of the more reactive oxygenates produced in the fluid bed reactor.