(567c) Pyrolysis Temperature History Controls Reaction Kinetics and Biochar Properties

Brewer, C. E., New Mexico State University
Gai, L., Rice University
Zygourakis, K., Rice University

Biomass pyrolysis involves a complicated series of reactions including dehydration, depolymerization and decomposition of biomass components (hemicellulose, cellulose, lignin), devolatilization, gas cracking, condensation, etc. The kinetics of these reactions control pyrolysis product distribution and properties. Previous studies have shown that the properties of biochars produced from the same feedstock are controlled by operating conditions that include highest heat treatment temperature, heating rate, particle size and heat treatment time at the highest temperature[1]. Since feedstock properties can also vary considerably (even for the same type of biomass), the development of reactor systems that can produce biochars with reproducible and tunable properties will require the development of systematic procedures that combine reaction engineering modeling with carefully planned experiments to describe the dynamics of slow pyrolysis.

This study presents an approach that combines experimental results and reaction models to relate the properties of the final biochar product to the concept of pyrolysis temperature history. More specifically, the property targeted was biochar O/C ratio, which has been shown to relate to biochar stability. We used a computer-controlled small-scale reactor (1.5 mL capacity) to produce a collection of biochars from different forestry residues under nitrogen and slow pyrolysis conditions. The custom-built computer software, that combined feedforward and feedback control, allowed us to accurately program the temperature history of biomass samples. Runs with different particle sizes were carried out for each feedstock and the overall yields, H/C and O/C ratios, and pore structure characteristics of the produced biochars were measured. Finally, we combined the biochar temperature history data with reaction kinetics parameters estimated from thermogravimetric analysis (TGA) experiments to develop and evaluate phenomenological reaction models for the pyrolysis process, using models from the literature as a starting point. The combination of temperature histories and reaction kinetics can accurately describe the dynamics of pyrolysis reactions—something that cannot be done by specifying maximum treatment temperature, treatment time, or even heat treatment intensity (an integral of reaction temperature over time). This study represents an important first step towards the development of procedures that will determine what information about a new biomass feedstock is needed to prescribe an optimal pyrolysis temperature history that will produce a biochar with targeted properties for a given feedstock and particle size.

[1] Hao Sun, W.C. Hockaday, C.A. Masiello and K. Zygourakis, Industrial and Engineering Chemistry Research, 51 (9), 3587–3597 (2012).