1 Mechanical Engineering, National University of Ireland Galway, Galway, Ireland .... 25th European Biomass Conference and Exhibition, 12-15 June 2017, ...
25th European Biomass Conference and Exhibition, 12-15 June 2017, Stockholm, Sweden
NEW PSEUDO-COMPONENTS OF HEMICELLULOSE AND LIGNIN K Dussan1,2,3, S Dooley4 and R F D Monaghan* 1,2,3 Mechanical Engineering, National University of Ireland Galway, Galway, Ireland 2 Ryan Institute for Environmental, Marine and Energy Research, Galway, Ireland 3 Centre for Marine & Renewable Energy, Galway, Ireland 4 School of Physics, Trinity College Dublin, Dublin, Ireland
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ABSTRACT: Improved process development and design of technologies such as pyrolysis and gasification of lignocellulosic biomass can be aided greatly by advanced comprehension of their chemistry and thermal reactivity. Hemicellulose and lignin have significant chemical variations within plant species and after biomass treatment, which are outside of the capability of most pyrolysis kinetic models [1]. This study creates a chemical/physical kinetic model that can (i) be readily adapted to numerous types of lignocellulosic biomass, and (ii) offer detailed information of product distribution and composition within a satisfactory level of accuracy. A detailed lumped-mechanistic model, i.e. Ranzi’s kinetic model of biomass pyrolysis [2], was used to adapt the use of new pseudo-components of hemicellulose and lignin, including acetyl side-chains, uronic acids, hexose carbohydrates, and guaiacyl-model compounds. The new pseudo-components and their corresponding stoichiometric product distribution were defined to more accurately reflect chemical features that correlate with different biomass groups and their properties, as determined using different characterisation techniques, such as chemical hydrolysis, Nuclear Magnetic Resonance and thermogravimetric analysis. Through this approach, biochemical characterisation of biomass can be implemented to extend the applicability and to improve the accuracy of semi-empirical kinetic models for biomass pyrolysis. Keywords: pyrolysis; modelling; chemical composition; hemicellulose; lignin; thermochemical conversion.
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Hardwoods contain xylans (15-30%) and glucomannans (1-5%), while softwoods are comprised mostly of galacto- and glucomannans (15-30%), with a minor fraction of arabinoxylans (7-10%) [4]. On the other side, grasses are contained other less common polysaccharides, including xyloglucans (1-5% of hemicellulose) and mixed linkage glucans (10-30% of hemicellulose) [5, 6]. Furthermore, other functionalities are found within the structure of xylans and mannan polysaccharides. The β1,4-linked xylose backbone in xylan contains 4-O-methyl glucuronic acid, glucuronic acid and acetyl groups [6]. Galacto- and gluco-mannan polysaccharides are formed by a β-(1,4)-linked mannose and glucose backbone with galactose and acetyl side groups [9]. Despite this diverse distribution among wood and biomass species, most studies of the characterisation of the pyrolysis behaviour of hemicellulose have only employed xylan-based polysaccharides to represent hemicellulose even in primarily-hexose softwood hemicellulose. Materials such as poly(β-D-xylopyranose) and O-acetyl-4-O-methylglucuronoxylan have been reported to degrade at 475-550 K and 505-600 K in a two-stage process affected by heating rate and acetylation degree [11-15]. On the other hand, hexosan-based polysaccharides, such as β-glucan, galacto- and glucomannan, are thermally degraded at higher temperatures between 500 and 623 K (3-20 K min-1) . The variability of the thermal stability of hexosan polysaccharides is directly associated to their sugar composition, backbone/side groups linkages in their structure and their molecular weight [16][17]. In addition to degradation temperature, the pyrolysis selectivity of different carbohydrates have been shown to vary as a function of its structure. Mannose produces higher amounts of 5-hydroxymethylfurfural (HMF) and water than galactose at low heating rates (0.2 K s-1) [21]. Furthermore, oxygen heterocyclic compounds were reported to be formed to a greater extent from glucomannan than from galactomannan, while yields of sugars and linear ketones and aldehydes were of the same order [16]. Within pentose polysaccharides, arabinoxylan has also been shown to react at temperatures higher than
INTRODUCTION
The reaction kinetics of the pyrolysis of biomass is commonly modelled through numerical approaches that employ “lumped” species and reaction mechanisms to fit experimental data of conversion and product selectivity. Slow pyrolysis data as obtained from thermogravimetric analysis and gas analysis is commonly used to train these model fitting numerical approaches of pyrolysis, using either untreated biomass or biomass components as initial substrate. This modelling method thus relies on the accuracy of training data and the analytical capacity of the experimental apparatus. One of the most important challenges in this field is the conciliate fundamental reaction kinetics with empirical/engineering numerical models to describe appropriately the time-dependant formation of chemical products and in situ changes of the biomass structure [1]. This study presents a review of the state-of-the-art of experimental studies for the understanding and modelling of the pyrolysis of hemicellulose and lignin. These review is used to revisit the definition of chemical functionalities or fragments used as model compounds or pseudo-components of hemicellulose and lignin in the most advanced lumped reaction mechanism of biomass pyrolysis developed by Ranzi et al. [2, 3]. This study utilises the comparison of mass conversion and evolved species fractions among different isolated biomass components, to 1) refine the combinations of the pseudo-components that best represent the reactivity and spectroscopic characteristics of actual biomasses, and 2) to define the lumped chemical reaction pathways and the kinetic parameters which best describe the reactivity of each pseudocomponent.
2 EXPERIMENTAL APPROACHES IN THE STUDY OF HEMICELLULOSE PYROLYSIS Hemicellulose is an heterogeneous fraction in biomass that contains pentose and hexose carbohydrates in different proportions depending on plant species.
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25th European Biomass Conference and Exhibition, 12-15 June 2017, Stockholm, Sweden
those of xylose by 30 K, leading to lower furanic and oxygenated products. On the other hand, hexosan-based polysaccharides have been shown to form a lower amount of furanic products (5-hydroxymethylfurfural) thus favouring higher anhydrous sugar yields [16].
concentration or degree of substitution of the lignin material. 3.2 Pyrolysis and characterization of chemical composition of technical lignins Lignin can be isolated through various chemical treatments, including acid and enzymatic hydrolysis, and solvation in alcohols, organic acids, and organic solvents. These isolated fractions are referred to as technical lignins. However, separation from the biomass structure involves the inherent destruction of labile lignin-lignin bonds, re-condensation of aromatic groups, and the modification of aliphatic and aromatic functionalities. The delignification achieved through the Björkman method [32] leads to the production of the so-called “milled wood lignin” (MWL). This method involves the extensive milling of biomass and subsequent extraction of lignin using 1,4-dioxane at mild temperatures. This type of technical lignin is often considered to preserve intact the native lignin structure. However, milder treatments, such as enzymatic hydrolysis, should preserve the structure to a greater extent since this treatment only targets carbohydrates and is carried out at low temperatures. On the other hand, processes such as Organosolv (with ethanol, acetic and/or formic acid) or alkali pulping increase the hydroxyl groups content and decrease the molecular weights of the lignin compared to MWL [33]. Early experimental studies showed that pyrolysis of technical lignins involved both the cleavage of ether linkages and the dehydration of alkyl chains [34]. Analysis of the residual lignins or pyrolysed biomass has shown that methoxy groups and aryl ether bonds start to cleave at temperatures as low as 520 K [34, 35]. Jakab et al. [37, 38] presented one of the largest experimental studies of pyrolysis of various MWLs, Organosolv and Alkali lignins from hardwoods, softwoods and straws. Char yields obtained after pyrolysis (0.33 K s-1, 373-1223 K) varied between 26 and 40% wt. and were inversely proportional to the total hydroxyl and methoxyl content as determined from the spectroscopic characterisation of the technical lignins [37, 38]. Furthermore, they observed that lignins with higher aliphatic hydroxyl and aromatic methoxyl groups concentrations released larger amounts of formaldehyde and methanol, respectively. The product release rates were similar among different lignins, with major release of monomeric phenolic compounds, such as methyl and ethyl guaiacol, at 680 K. Jegers et al. [42] proposed a simple reaction network based on lumped product compounds and experimental data of the pyrolysis of Kraft lignin, milled wood lignin and pine wood. In this work, methanol yields were higher from MWL than from Kraft lignin which was attributed to methylol groups in the propanoid side chains of the lignins, being more abundant in MWLs [42]. Furthermore, yields of propyl-substituted phenolic products from MWL were higher than from Kraft lignin. This was attributed to the modification of the structure of native lignin brought by condensation reactions, that either lead to a wider range of carbon atoms in the aliphatic chains of lignin units or to the formation of different inter-aromatic linkages, such as C5‒C5’ bonds [42]. Inherent differences in the reactivity of different technical lignins was shown in the study presented by Jiang et al. [44]. While Klason lignins (acid hydrolysis residue) from hardwoods reported char yields around 41
3 EXPERIMENTAL APPROACHES IN THE STUDY OF LIGNIN PYROLYSIS Experimental approaches for the study of the lignin pyrolysis rely on the use of either synthetic model compounds or lignin fractions isolated from lignocellulosic biomass. In the first category, major attention is given to dimers that contain β‒O‒4’ aryl ether linkages, one of the most abundant types of linkages found in native lignin. In softwoods and hardwoods, circa 45 and 60% of linkages between aromatic units correspond to β‒O‒4’ aryl ether [27], thus justifying the selection of this type of bond as a model representation of the native structure of lignin. In this Section, a review of several experimental studies involving synthetic model compounds and isolated lignins is presented. 3.1 Lignin model compounds based unsubstituted and methoxy-substituted dimers containing a β‒O‒4’ aryl ether group Britt et al. [28] reported one of the first experimental studies of synthetic lignin dimers using 1-phenyl-2phenoxyethane, 1-phenyl-2-(2-methoxyphenoxy)ethane, and 1-phenyl-2-(2,6-dimethoxyphenoxy)ethane. In this study, Britt et al. found that major products obtained experimentally from 1-phenyl-2-phenoxyethane, styrene (38% mol) and phenol (59% mol), were formed through two competing pathways, Cβ‒O homolysis and 1,2hydrogen elimination (concerted reaction). The methoxyand dimethoxy-substituted compounds decomposed 4 and 11 times more rapidly than 1-phenyl-2-phenoxyethane. Their major products included styrene, 2-hydroxybenzaldehyde and toluene. However, side mechanisms formed other compounds such as 2-hydroxybenzaldehyde (32% mol) and 2-hydroxyphenol (9% mol) from 1phenyl-2-(2-methoxyphenoxy)ethane, and 2methylphenol (24% mol), 2-hydroxy-3methoxybenzaldehyde (9% mol) and 6-methoxy-2hydroxyphenol (3% mol) from 1-phenyl-2-(2,6-dimethoxyphenoxy)ethane. A later work by Jarvis et al. [29] determined that concerted reactions dominated over homolytic scission pathways under pyrolytic conditions (
