Producing Biochars With Consistent Properties From Forestry Residues: Using Spatial Variations of Reactor Temperature History to Choose Pyrolysis Conditions

One desirable quality of reactors used for the thermochemical conversion of biomass is that they be “feedstock-blind”: capable of producing consistent products even when the feedstocks or the feedstock properties vary. To achieve a satisfactory level of reproducibility , we must be able to accurately control the operation of a reactor so that each biomass load will be processed with the temperature program required to produce biochar with the desired properties. Biomass pyrolysis involves a complicated series of sequential and parallel reactions that include dehydration , depolymerization and decomposition of biomass components (hemicellulose , cellulose , lignin) , devolatilization , gas cracking , condensation , etc. Therefore , the distribution of the pyrolysis products , as well as the chemical and physical properties of the produced biochar , will depend on the complete temperature history of the biomass particles. For slow pyrolysis of biomass , differences in feedstock properties (moisture , composition , heat capacity , heat of reaction) , particle sizes , or flow rates of inert gases change the heat transfer rates and temperature distributions within the reactor.  As a result , feedstock and operating differences will have significant effects on the temperature histories of biomass particles. Past research has consistently shown that biochar properties (like H/C or O/C ratios) vary with highest pyrolysis temperature and reaction time at that temperature. In a recent study[1] , we have shown that biomass heating rates and particle sizes also affect the chemical and physical properties of biochars. We need to understand the spatial temperature variations within a reactor in order to be able to make biochars with reproducible and tunable properties. In this study , wood and residues (branches , leaves , bark) from a managed slash pine and eucalyptus operation were pyrolyzed in a fixed-bed reactor under nitrogen over a range of temperatures and particle sizes.  The lab scale (1 L) reactor was custom-built to enable consistent placement of multiple thermocouples to monitor spatial temperature distributions during slow pyrolysis. Overall yields , H/C and O/C ratios , and pore structure characteristics of the produced biochars were evaluated with respect to the internal reactor temperature variability and temperature histories. Finally , we present a theoretical analysis demonstrating how reaction engineering principles and experimental data can be used to develop temperature programs that produce biochars with desired properties from a variety of feedstocks. [1] Hao Sun , W.C. Hockaday , C.A. Masiello and K. Zygourakis , Industrial and Engineering Chemistry Research , 51 (9) , 3587–3597 (2012).