(147e) 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.