Carbonization Reactions in Biomass Pyrolysis Processes: History
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Zhong Hu
,
Lin Wei

Biochar is a carbon-rich solid produced during the thermochemical processes of various biomass feedstocks. Biomass pyrolysis is one of the most common processes to produce biochar.

  • biochar
  • biomass
  • pyrolysis

1. Reactivity of Cellulose in Pyrolysis

Cellulose is the most abundant organic compound on earth. Cellulose, with the chemical formula (C6H10O5)n, is a polysaccharide composed of linear chains of hundreds to tens of thousands of 1,4-β-D-glucopyranose units. The schematic diagram of the long linear chain molecule structure of cellulose is shown in Figure 1.
Figure 1. Schematic structure of cellulose (C6H10O5)n.
By weight, cellulose accounts for 40–50% of biomass. It is difficult to understand the complex reaction process and detailed reaction mechanism of biomass pyrolysis only through experimental methods. Studying pyrolysis mechanisms and the product pathway of cellulose is of great significance for producing new carbon materials and exploring innovative applications. When using pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS) to study the pyrolysis mechanism of cellulose, since Py-GC/MS cannot capture the reactions related to free radicals at the molecular level in a short time, it cannot provide detailed information on the pyrolysis mechanism [1][2]. ReaxFF MD simulations are effective methods to reveal the internal reaction mechanisms from a microscopic perspective. The decomposition of cellulose was classified into three categories using ReaxFF MD [3]: (1) Depolymerization reactions; (2) other chain scission reactions; and (3) release of low molecular weight products such as glycolaldehyde, water, formaldehyde, and formic acid. Since the simulated temperature is higher than the experimental temperature and the chemical and electrostatic ambient temperature of crystalline (or amorphous) cellulose does not exist, levoglucosan (LGA) is not observed in the simulation, and the kinetic parameters do not depend on molecular weight or initial conformation. Then, post-processing tools were used to parse the bond information. Specific algorithms were developed to search for LGA among decomposition products, taking into account both individual molecules and LGA end groups. Similar studies have been performed on glycolaldehyde, formaldehyde, formic acid, and hydroxymethyl radical. Some smaller products, such as water, carbon monoxide, carbon dioxide, and hydroxyl radicals, can be identified by their chemical compositions.
It should be pointed out that the experimental techniques, including product-specific Py-GC/MS experiments, cannot directly monitor the temporal evolution trend of specific products during cellulose pyrolysis because the radial reaction time is many orders of magnitude shorter than the experimental techniques allow, resulting in a lower concentration of pyrolyzed products. However, the simplicity of observing the evolution of different products as a function of time and temperature through ReaxFF MD simulations provides a feasible method for computationally probing the evolution of the pyrolysis products for experimental or industrial applications [4].
Zhang, et al. [4] combined large-scale models with GPU-based ReaxFF MD simulations using a canonical ensemble (conservation of substance quantity (N), volume (V), and temperature (T) of species, also known as the NVT ensemble) in a periodic cubic box and a unique cheminformatics-based reaction analysis tool (VARxMD) and studied the cellulose pyrolysis process and revealed the evolution and reaction mechanism of cellulose pyrolysis products. The three major products are hydroxyl-acetone, propyl aldehyde, and glycolaldehyde, in addition to levoglucosan, CO2, CO, H2O, etc., which are closely related to the experimental literature. Both the overall spectral product evolution and the underlying detailed chemical reactions of cellulose pyrolysis have been revealed. For example, the reaction pathway of hydroxyl-acetone is to undergo a series of depolymerization reactions such as homolysis, elimination, and ring opening at 800K to form the unstable compound C6H10O6. The unstable C6H10O6 then undergoes ring formation, rearrangement of unsaturated bonds, and hydroxyl radicals falling off to generate fragment C6H9O5. C6H9O5 decomposes into two species by bond dissociation, namely 2-hydroxyl-malonaldehyde (C3H4O3) and 2-hydroxyl-propyl-aldehyde radical (C3H5O2). 2-hydroxyl-propyl-aldehyde radial rearranges and reacts with the hydrogen radical to generate hydroxyl-aceton C3H6O2. In the reaction pathway of glycolaldehyde, C2H4O3, at 800 K, the generation of C2H4O3 comes from the bond dissociation of different compounds, such as C6H10O6 and C11H19O9 released by depolymerization of cellulose [4]. Since C6H10O6 is the initial reactant for the formation of hydroxyl-acetone and 2-hydroxy-propionaldehyde, the same initial fragment may undergo different reactions in the cellulose pyrolysis system to produce different compounds, and the three main products compete with each other for formation at high temperatures. The weight percentage of the pyrolysis products was found to be a function of temperature. When the temperature is low, the decomposition process predominates. As the temperature increases, the rate of thermal degradation of cellulose is accelerated, accompanied by the appearance of fragments and inorganic gas molecules. The inorganic gas production also increases with temperature between 500 and 1400 K, which is consistent with Py-GC/MS experiments. The pyrolysis products can be grouped roughly into syngas, tar (or bio-oil), and biochar [3]. Syngas and its gas compounds have carbon atoms equal to or less than 4, and bio-oil is a collection of compounds of C5–C9, C10–C19, C20–C29, and C30–C39. The rest (C40+) can be treated as biochar. The gases in syngas are released rapidly and increase monotonically with increasing temperature. When the temperature is increased, biochar is rapidly reduced, so a lower temperature favors a higher yield of biochar. The above results are in good agreement with the experimental data [5]. Analysis of the molecular products formed by cellulose pyrolysis in ReaxFF MD simulations at different time steps and temperatures ranging from 800K to 1400 K revealed that two compounds, namely C2H4O2 and C6H10O5, dominated the system. With the help of VARxMD, it was found that most of C2H4O2 is glycolaldehyde and C6H10O5 is levoglucosan or the precursor of levoglucosan, which is consistent with the experimental data of Py-GC/MS. At 700 K, the precursor of levoglucosan constitutes about 80% of the C6H10O5 compounds, which drops to only 15% at 1400 K. Since levoglucosan is a large molecule for the cellulose model, both levoglucosan and its precursors are considered levoglucosan products. The evolution trend of the large model simulation is closer to that of the Py-GC/MS experiments than the small model [4].

2. Reactivity of Lignin in Pyrolysis

Lignin is a polyphenolic polymer. Three types of phenylpropanoid units are generally considered the main precursors for lignin biosynthesis: coniferyl, sinapyl, and p-coumaryl alcohol (see Figure 2), which structurally give rise to guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units, respectively, and are linked by different C–C bonds and ether bonds, such as α-O-4 bonds and β-O-4 bonds [6]. Lignin is the most recalcitrant of the three components of lignocellulosic biomass. Lignin, the second most abundant natural polymer after cellulose, is pyrolyzed differently from cellulose and hemicellulose due to the differences in chemical structure and characteristics [7]. Generally speaking, the lignin in softwoods is mainly composed of guaiacyl units and contains a small number of p-hydroxyphenyl units; in contrast, the lignin in hardwoods is mainly composed of guaiacyl units and syringyl units and contains a small number of p-hyfroxyphenyl units. The lignin in grasses typically contains all three types of monlignol units, with peripheral groups (i.e., hydroxycinnamic acids) incorporated into its core structure. The lignin macromolecules are mainly linked by C–C and C–O bonds between their phenylpropanoid structural units, among which aryl ether bonds (β-O-4) are the most common and important interunit linkages (see Figure 3).
Figure 2. Schematic basic units of lignin polymer: (a) coniferyl, (b) sinapyl, and (c) p-coumaryl alcohol structures.
Figure 3. Schematic structure of lignin, where the purple wavy lines represent the continuing of the molecule.
It can be seen from Figure 3 that lignin contains a variety of oxygen-containing functional groups, including methoxyl, hydroxyl, carboxyl, and carbonyl groups, etc., which significantly affect the reactivity of lignin. The content of methoxyl groups in lignin is related to the formation of lignin pyrolysis char, i.e., lignin with high methoxyl group contents produces less char during the pyrolysis process.
Many lignin models have been proposed for ReaxFF MD simulations in the last century, including the softwood lignin models by Alder, Freudenberg, Brukraft, Forss, and Glasser; the hardwood lignin model by Nimz; and the pine kraft lignin model by Marton [7][8]. The corresponding details of the four lignin models are listed in Table 1 [7][8].
Table 1. Constituent details of four lignin models [7][8].
Lignin Model Species Constituents Number of C6H3 Units
H G S
Alder Softwood C6400H7200O2320 40 560 40
Freudenberg Softwood C6980H7640O2280 200 480 40
Nimz Hardwood C10854H11940O4062 42 612 396
Marton Kraft lignin C5080H5040O1640S40 120 440 0
The simulation results show that the Alder, Frendenberg, and Nimz lignin models have three pyrolysis stages. The first stage refers to the main process, starting with the formation of initial pyrolysis products and ending with the complete consumption of source lignin molecules. The second stage starts with the cracking of primary pyrolysis products into secondary pyrolysis products and ends with a maximum. The third stage begins with the reduction of secondary pyrolysis products. However, the Marton lignin model only has the first and second stages. Consumptions of all α-O-4 linkages were similar in the softwood and hardwood lignin models. The differences in pyrolysis product evolution and linkage behavior between hardwood, softwood, and kraft lignin can be attributed to the different reactions of linkages and their linked monomers induced by different oxygen-containing substituents [7]. The ring structure evolutions of the four lignin models are almost identical. The similarities and differences in the pyrolysis of the different lignin models suggested that the simulation work can provide new insights into the high-value utilization of lignin.
The study of the ReaxFF MD simulated pyrolysis of lignin model will be helpful for better understanding the reaction behavior and pyrolysis characteristics in the pyrolysis of lignin, so as to optimize the pyrolysis process of lignin, and even the whole biomass feedstock. Furthermore, lignin is heterogeneous and does not have a well-defined primary structure. The phenylpropane units are organized into a 3D amorphous polymeric network with varying degrees of aggregation. Abundant forms of biomass are a promising alternative to fossil fuels [9]. Lignin depolymerization is a difficult process that requires specific reaction conditions [10][11] and selected catalysts [9] to increase the yield of high aromatic monomers and produce value-added fuels and chemicals.

3. Reactivity of Hemicellulose in Pyrolysis

Hemicellulose has a heteropolymeric structure (lower molecular weight than cellulose) that is composed of a variety of sugar monomers, including glucose, galactose, mannose, xylose, arabinose, 4-O-methyl glucuronic acid, and galacturounic acid residues. The composition of hemicellulose from different biomass materials varies, and its structure is more complex than that of cellulose. Xylan is often used as a typical hemicellulose model [12][13], as shown in Figure 4. Previous work has provided important insights into the relationship between the distribution of pyrolysis products and the structural features of xylosyl hemicelluloses [14][15][16][17]. The main differences between xylan and cellulose pyrolysis are as follows: (1) xylan melts and generates bubbles during pyrolysis, producing many unidentified didehydrated pentose, while levoglucosan is the main compound in the dellulose bio-oil; (2) xylan and pyranose molecules formed during the pyrolysis of xylan tend to form char through multi-step dehydration reactions [14][15][16][17][18][19][20][21].
Figure 4. Schematic structure of xylan, where the purple wavy lines represent the continuing of the molecule.
Furthermore, ReaxFF MD simulations showed that cellulose and hemicellulose were the main sources of CO and CO2 production, although there were slightly more CO2 molecules in hemicellulose due to the potential presence of carboxyl and carbonyl groups. Due to the high O content, cellulose and hemicellulose are the main sources of C2H2O2 and CH3CHO. Most of the C2H4O2 molecules come from the degeneration of the cellulose. In addition, CH3OH is mainly derived from cellulose due to the cleavage of the hydroxyl-rich pyran rings. The dissociation of -OCH3 radicals in the lignin also provides the precursors for the formation of CH3OH [22]. Table 2 lists the percentage of each gas produced during the pyrolysis process of the three main components of the biomass (wheat straw) at a temperature of 2000 K [22].
Table 2. Percentage of each gas produced resulting from the three main components of the biomass (wheat straw) during the pyrolysis process at 2000 K.
  CO (%) CO2 (%) CH4 (%) H2O (%)
Cellulose 48.29 47.04 53.33 83.96
Lignin 7.32 4.68 40.00 5.30
Hemicellulose 44.39 48.28 6.67 10.74

Functional groups play an important role in the pyrolysis process and determine the chemical properties of organic compounds. Cellulose, hemicellulose, and lignin in biomass have different kinds and numbers of functional groups, which lead to their different properties. In addition, phenyl rings in lignin and pyran rings in cellulose and hemicellulose also have potential effects on the thermal stability of organics. Therefore, the study of its evolution and behavior is beneficial to understanding the mechanism of biomass pyrolysis [22]. The functional groups are generally divided into six categories: ether groups (R–O–R), hydroxyl groups (R–OH), aldehyde groups (R–CHO), ester groups (R–COO–R), carboxyl groups (R–COOH), and carbonyl groups (R–CO–R). The ether and hydroxyl groups are the most abundant, while the other four functional groups are much less abundant. The hydroxyl groups are concentrated in the cellulose and hemicellulose in the form of alcoholic hydroxyl groups [23]. The ether groups are mainly derived from the pyran rings and glycosidic bonds in the cellulose and hemicellulose, as well as the methoxy or other carbon structures connected to the phenyl rings in the lignin [24]. The aldehyde and carbonyl groups are concentrated in lignin, while hemicellulose provides carboxyl groups. Furthermore, the ester groups are distributed in hemicellulose and lignin.

This entry is adapted from the peer-reviewed paper 10.3390/jcs7090354

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