Understanding the primary and secondary slow pyrolysis mechanisms of holocellulose, lignin and wood with laser-induced fluorescence

Highlights

• Primary and secondary pyrolysis reactions investigated in a fixed-bed reactor.

• LIF applied to on-line characterization of aromatic species in pyrolysis volatiles.

• Pyrolysis of wood and cellulose and lignin as its two main macromolecules.

• Heterogeneous secondary reactions detected in slow pyrolysis at low temperatures.

• Lignin identified as the main PAHs formation source in wood pyrolysis.

Abstract

To understand the complex reaction mechanisms involved in biomass pyrolysis, volatile products are characterized on-line by laser-induced fluorescence (LIF), together with on-line measurements of permanent gases by GC-TCD (Gas Chromatograph-Thermal Conductivity Detector) and temperature evolutions in the bed. The focus is to determine the components that emit fluorescence and reactions involved in producing them from wood and from its two main macromolecular components, holocellulose and lignin. A technical-scale fixed-bed reactor is used to identify primary and secondary reactions involved in pyrolysis. The excitation wavelength used for the LIF measurements is 266 nm and the detected species are aromatic compounds (including one-ring phenolics and two-, three- or four-ring polycyclic aromatic hydrocarbons (PAHs)) and species containing carbonyl groups. Holocellulose volatiles show fluorescence that is attributed to the formation of carbonyl compounds and two-ring PAHs during heterogeneous secondary char-forming reactions, which also enhance the production of CO₂. Volatiles from lignin show first fluorescence typical of one-ring phenolics and small (two–three rings) PAHs. Then, due to the enhancement of heterogeneous secondary reactions, fluorescence signal typical of bigger PAHs (three–four rings) is detected. These aromatic species are produced in parallel to gas species like CH₄. The fluorescence that can be observed in pyrolysis of wood comes mainly from the lignin fraction, undergoing also heterogeneous secondary reactions resulting in the formation of bigger PAHs, although a contribution from cellulose is also present.

Introduction

Pyrolysis of biomass is a relevant step in any thermochemical conversion process, such as combustion and gasification, but it is also an important process itself for the production of char (charcoal or biochar) and bio-oil. However, despite its relevance, many aspects of the process still remain a source of controversy. The reaction mechanism is still largely unknown with respect to its details and widely applicable models to predict the product composition or the enthalpy of the reaction are not yet available [1], [2]. There are several factors that increase the complexity in understanding the process, as the complexity and heterogeneity of the feedstocks, the wide range of possible experimental conditions that can be applied and the difficulty, or even impossibility, to characterize some products with conventional techniques such as Gas Chromatography (GC). An example of this is the characterization of the water insoluble fraction called pyrolytic lignin produced in pyrolysis of lignocellulosic materials [3]. The combination of several analytical techniques is needed to characterize the products of pyrolysis, which makes it quite time consuming and on-line characterization of the transitory process is very challenging.

In the present work, two on-line measurements are combined to get a deeper understanding of the pyrolysis process. On-line characterization of volatiles is carried out, detecting permanent gases with GC-TCD and condensable volatiles emitting fluorescence with laser-induced fluorescence (LIF). Experiments are performed for wood and its two main macromolecular components, holocellulose and lignin, together with temperature measurements in several positions inside the bed of a technical-scale fixed-bed reactor, to try to understand the behavior of biomass based on the behavior of its main components [4] in conditions close to industrial applications.

LIF has been widely used in combustion for detection of oxygenated compounds, polycyclic aromatic hydrocarbons (PAHs) or particulates (together with Laser-Induced Incandescence (LII)). However, its use in pyrolysis is scarcer. It has been applied to single particle experiments [5], [6], detecting mainly PAHs, for the detection of CO and formaldehyde at reactor level in fast pyrolysis [7] and for the characterization of bio-oils produced from fast pyrolysis [8]. The authors of the present paper have also applied LIF with an excitation wavelength of 266 nm to the characterization of volatiles in technical-scale fixed-bed pyrolysis of different wood species at different conditions [9]. The aim of the present study is to get a deeper understanding of the pyrolysis mechanism by analyzing the possible origins of the measured fluorescence and linking its evolution to other produced species, as the permanent gases produced in the pyrolysis process.