Pyrolysis of Forestry Waste in a Screw Reactor with Four Sequential Heating Zones: Influence of Isothermal and Nonisothermal Profiles

Abstract

Woody biomass waste (Pinus radiata) has been pyrolyzed in a laboratory-scale continuous pyrolysis plant, formed by two reactors connected in series: one continuous auger reactor, where the pyrolysis process is performed, and a tubular reactor for vapors upgrading, where the thermal treatment of the pyrolysis vapors occurs to promote further cracking. The pyrolysis reactor has four different heating zones that allows programming different temperature profiles. An extensive and detailed thermal study has been performed using temperatures covering the range from 300 °C to 900 °C in regimes of scaling temperature profiles and isothermal profiles. Both the peak temperature and the heating rate affect the pyrolysis fraction yields, as well as products composition. Higher temperatures result in higher gas yield and lower solid and liquid yields. Increasing the temperature also increases the fixed and elemental carbon contents of the charcoal obtained, and decreases its volatile matter and the hydrogen and oxygen contents. Concerning gas fraction, the share of CO and hydrogen rises with the temperature. The increase of temperature gives rise to heavier liquids with more polycyclic compounds and less monocyclic compounds. The changes in the composition of the charcoal and gas products are more prominent between 300 °C and 700 °C. Compared to nonisothermal experiments, isothermal experiments enhance the production of gases, to the detriment of solid and liquids, but do not affect product composition.