Flash Pyrolysis Kinetics of Extracted Lignocellulosic Biomass Components

Stefan Pielsticker,Reinhold Kneer,Kentaro Umeki,Benjamin Gövert

doi:10.3389/fenrg.2021.737011

Stefan Pielsticker, Reinhold Kneer + Show 2 more

Open Access

https://doi.org/10.3389/fenrg.2021.737011

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Abstract

Biomass is a complex material mainly composed of the three lignocellulosic components: cellulose, hemicellulose and lignin. The different molecular structures of the individual components result in various decomposition mechanisms during the pyrolysis process. To understand the underlying reactions in more detail, the individual components can be extracted from the biomass and can then be investigated separately. In this work, the pyrolysis kinetics of extracted and purified cellulose, hemicellulose and lignin are examined experimentally in a small-scale fluidized bed reactor (FBR) under N2 pyrolysis conditions. The FBR provides high particle heating rates (approx. 104 K/s) at medium temperatures (573–973 K) with unlimited reaction time and thus complements typically used thermogravimetric analyzers (TGA, low heating rate) and drop tube reactors (high temperature and heating rate). Based on the time-dependent gas concentrations of 22 species, the release rates of these species as well as the overall rate of volatiles released are calculated. A single first-order (SFOR) reaction model and a 2-step model combined with Arrhenius kinetics are calibrated for all three components individually. Considering FBR and additional TGA experiments, different reaction regimes with different activation energies could be identified. By using dimensionless pyrolysis numbers, limits due to reaction kinetics and heat transfer could be determined. The evaluation of the overall model performance revealed model predictions within the ±2σ standard deviation band for cellulose and hemicellulose. For lignin, only the 2-step model gave satisfying results. Modifications to the SFOR model (yield restriction to primary pyrolysis peak or the assumption of distributed reactivity) were found to be promising approaches for the description of flash pyrolysis behavior, which will be further investigated in the future.

Highlights

As fixed in the Paris agreement, the majority of the world’s countries have committed themselves to reduce their anthropogenic greenhouse gas emissions in order to reach the goal of at most 1.5°C temperature increase compared to the pre-industrial level
The present study aims to quantify the kinetic behavior of the extracted biomass basic components cellulose, hemicellulose and lignin individually under flash pyrolysis conditions realized by a small-scale fluidized bed reactor
The following sections present the results of the pyrolysis experiments focussing on the final volatile yield, the gas composition, modeling of reaction kinetics via the single first-order reaction (SFOR) model and the 2-step model, and the overall modeling accuracy combining yield and kinetic modeling

Summary

Introduction

As fixed in the Paris agreement, the majority of the world’s countries have committed themselves to reduce their anthropogenic greenhouse gas emissions in order to reach the goal of at most 1.5°C temperature increase compared to the pre-industrial level. Typical fields of application for biomass are the substitution of coal in large-scale power plants and other carbon-based processes such as cement or iron production [all three compatible with technologies for bioenergy with carbon capture and storage (BECCS) integration] and the substitution of liquid fuels such as gasoline or jet fuels in the mobility sector (Intergovernmental Panel on Climate Change, 2018). The process parameters define the product composition: The liquid bio-oil yield can be maximized by applying high heating rates, moderate temperatures and short gas residence times, while higher temperatures and longer residence times favor higher gas yields and slow heating at low temperatures results in higher char yields (Bridgwater, 2012). A fundamental understanding of the underlying chemical reactions during biomass pyrolysis forms the basis of model developments required for the design and optimization of industrial-scale processes (Hameed et al, 2019)

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