Process intensification of biomass fast pyrolysis via autothermal operation

Abstract

<p>Heat transfer is the bottleneck to fast pyrolysis of biomass. Although the enthalpy for pyrolysis of biomass is relatively small, operation at temperatures around 500°C constrains heat carrier selection to inert gases and granular media that can sustain only modest thermal fluxes in practical pyrolysis systems. With heat transfer controlling the rate of pyrolysis, reactor capacity only scales as the square of reactor diameter and does not benefit from economies of scale in building larger reactors. We have eliminated this heat transfer bottleneck by replacing it with partial oxidation of pyrolysis products to provide the enthalpy for pyrolysis in a fluidized bed reactor, a process that can be described as autothermal pyrolysis.</p> <p>The oxygen-to-biomass equivalence ratio depends upon the kind of biomass being pyrolyzed and the level of parasitic heat losses from the reactor, but under conditions that simulate adiabatic operation, equivalence ratios are approximately 0.06 and 0.10 for corn stover and red oak biomass, respectively. Despite the presence of oxidation reactions, autothermal operation resulted in no significant loss in bio-oil yields from corn stover (59 wt.%) or red oak (64 wt.%) compared to conventional pyrolysis conditions</p> <p>In both studies, carbon balances indicate that less valuable pyrolysis products (char and aqueous, bio-oil light ends) are consumed via partial oxidative reactions to provide the enthalpy for pyrolysis. For corn stover, the carbon yields of char and bio-oil light ends decreased by 18.5% and 4.7%, respectively, whereas red oak carbon yields of char and bio-oil light ends decreased by 25.0% and 21.3%, respectively. The most valuable pyrolysis product (organic-rich, bio-oil heavy ends) were the least affected in both studies. For corn stover, carbon yields of bio-oil heavy ends increased by 0.9% whereas red oak carbon yields decreased by 8.0%.</p> <p>Autothermal operation also resulted in significant process intensification of this 8.9 cm diameter reactor, increasing corn stover throughput from 7.8 to 21.9 kg hr-1 and increasing red oak throughput from 4.8 to 15.4 kg hr-1. Autothermal operation has the potential to intensify biomass fast pyrolysis, while simplifying reactor design and operation, to improve the commercial prospects for pyrolysis.</p>