Characteristics of hemicellulose, cellulose and lignin pyrolysis
Introduction
With the depletion of fossil fuel and the concern of environmental protection, the utilization of biomass resources has attracted increasing worldwide interest. Pyrolysis, as one of the promising thermochemical conversion routes, plays a vital role in biomass conversion. However, pyrolysis is an extremely complex process; it generally goes through a series of reactions and can be influenced by many factors [1], [2], [3], [4]. It is thus essential to study the fundamentals of biomass pyrolysis. Hemicellulose, cellulose and lignin are the three main components of biomass and they in general cover respectively 20–40, 40–60, and 10–25 wt.% for lignocellulosic biomass [5]. Previous studies showed that biomass pyrolysis can be divided into four individual stages: moisture evolution, hemicellulose decomposition, cellulose decomposition and lignin decomposition [6], [7]. It was also suggested that the pyrolysis of any biomass can be considered as the superposition of the three main components [2], [7], [8], [9]. Knowledge of the pyrolysis characteristics of the three main components is the basis and thus essentially important for a better understanding to biomass thermal chemical conversion.
So far, numerous studies based on the main components have been carried out, most of them were focused on developing kinetics models for predicting behavior of biomass pyrolysis [2], [6], [8], [10]. Our previous study on the three components [11] focused on the prediction of biomass pyrolysis behavior in TGA from the fractions of the three components. The structure property of the three components was found to influence greatly the pyrolysis characteristics [7], [12]. Li et al. [13] using FTIR analyzed the formation characteristics of gas compounds from cellulose pyrolysis at various conditions, involving different heating rates, residence times, and gas flows. Ferdous, et al. [14] investigated the gas product property from lignin pyrolysis at changing temperatures and heating rates. Bassilakis et al. [15] studied the gas product releasing from d-glucose, chlorogenic acid and xylan pyrolysis using FTIR quantitatively. The organics mixtures were classified into methanol, formaldehyde, formic acid, acetaldehyde, acetic acid and others, but it was difficult to distinguish these components using FTIR. Evans and Milne [16] carried out the pyrolysis of wood and the main constituents (cellulose, xylan and lignin) using molecular-beam with mass spectrometry, the intermediates of the gas products were determined and a potential reaction pathway was suggested. Still, the characteristics of different gas products formation from pyrolyzing the three biomass components, the energy consumption occurring in the process, and their relationships with the chemical structure of biomass components were not discussed in-depth in the previous studies.
In the present study, the pyrolysis of hemicellulose, cellulose and lignin in TGA and packed bed, together with the energy consumption and gas product releasing behaviors were investigated in detail. The objective of this study is to gain a comprehensive understanding to the three components pyrolysis with focuses on the gas product releasing properties, thus facilitate to establish an universe model to simulate biomass pyrolysis based the three model compounds. It is favorable for the development of advanced biomass pyrolysis process.
Section snippets
Materials
The three main components (cellulose, hemicellulose, and lignin) were purchased from commercial chemical shop (Sigma-Aldrich Chemie GmbH). Cellulose is in powder fibrous form, and lignin is alkali lignin in brown powders. A commercial hemicellulose can hardly be purchased whereas xylan, although it might have different physical and chemical properties, has been widely used as a representative component of hemicellulose in pyrolysis processes [8], [9], [17]. Here, xylan, in yellow powder form,
Chemical structure of three components
The chemical structure of the three components was analyzed using FTIR through pelleting the sample with KBr powder, the method was described in detail in our previous publication [20]. The IR spectra of cellulose, hemicellulose and lignin are shown in Fig. 1. The typical functional groups and the IR signal with the possible compounds are listed in Table 1 for a reference [12], [20], [21]. It can be observed that the three components of biomass are most likely consisted of alkene, esters,
Conclusions
The pyrolysis characteristics of hemicellulose, cellulose and lignin were investigated using TGA-DSC and packed bed coupled respectively with FTIR and Micro-GC as on-line gas monitors. Great differences of the pyrolysis behavior among the three main components were drawn from the experimental results. Hemicellulose was easy to be degraded, and its pyrolysis was focused at 220–315 °C. The pyrolysis of cellulose was mainly happened at 315–400 °C, while that of lignin covered a whole temperature
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