Study on Pyrolysis Behaviors of Various Plant Fibers

*Ke Zhang, Quanxing Zheng, Zhongya Guo, Lili Fu, Qi Zhang and Bing Wang*

## **Abstract**

Pyrolysis is an effective way to convert plant fibers into high-value-added chemicals and bioenergy. The pyrolysis behavior of plant fibers varies with their compositions. A high-performance anion-exchange chromatography integrated pulse amperometric method was established to detect the composition of arabinose, galactose, glucose, xylose, and mannose in plant fiber hydrolysate. The contents of cellulose, hemicellulose, and lignin in six plant fibers were calculated. Furthermore, the pyrolysis kinetic parameters of the plant fibers and their pyrolysis product distribution depending on chemical compositions were analyzed. The pyrolysis of flax fiber with high cellulose content (92.19%) tended to generate ketones, accounting for about 37.3% of the total product distribution, while coniferous and broadleaf fiber with high hemicellulose contents (13.23 and 15.07%, respectively) was more likely to generate aldehydes and hydrocarbons. Furthermore, the result of pyrolysis of a grass fiber demonstrated the interactions between its chemical components, which had been captured during pyrolysis from the perspective of pyrolysis product distribution that inhibits the pyrolysis to generate CO2, and promoted the generation of furan, phenols, and toluene, to different degrees. The research results are expected to provide basic data and theoretical support for obtaining high-value-added chemicals and biomass energy through the pyrolysis of plant fibers.

**Keywords:** plant fiber, biomass chemical component, pyrolysis characteristic, pyrolysis products, pyrolysis behaviors

### **1. Introduction**

Plant fiber is a kind of natural composite, which is mainly composed of cellulose, hemicellulose, and lignin [1]. The three polysaccharides are quite different in the composition of monosaccharides. Cellulose is a linear polymer composed of dehydrated glucose linked by β-1,4-glycosidic bonds. Hemicellulose is much more complex than cellulose. It is a heteropolysaccharide with some branches composed of pentose (xylose and arabinose), hexose (glucose, mannose, and galactose) and hexoic acid (4-O-methyl-D-glucuronic acid, D-glucuronic acid, and D-galacturonic acid). These functional groups can be assembled into various hemicellulose polysaccharides with different structures from linear to highly branched, such as β-1,4-D-xylan, arabinose xylan, mannan, dextran, galactose, and galactomannan. The detailed sugar

compositions and chemical structures of these hemicellulosic polysaccharides vary according to plant species [1, 2]. Lignin is an aromatic polymer with highly branched chains, which is composed of phenylpropane derivative monomers (such as coumarin, coniferol, and sinapinol). The structure of lignin is complex and its molecular weight is large [1]. Due to the complex connection between cellulose, hemicellulose, and lignin in plant fibers, it is a challenge to complete the separation of the three components. At present, the "nitric acid-ethanol method," the "12% hydrochloric acid hydrolysis method," and the "72% sulfuric acid method" is used to measure the cellulose content, hemicellulose or pentosan content, and lignin content, respectively. However, these methods are cumbersome and the structural composition of hemicellulose could not be distinguished effectively. The National Renewable Energy Laboratory (NREL) of the United States has developed a method to systematically analyze the contents of cellulose, hemicellulose, and lignin in fibers by two-stage acid hydrolysis [3]. This method was improved by us to use high concentration sulfuric acid to convert plant fibers into oligosaccharides at low temperatures firstly, and then using dilute acid to further convert oligosaccharides into monosaccharides at high temperatures. The contents of cellulose and hemicellulose are determined based on analyzing monosaccharides by high-performance liquid chromatography. The contents of acid-insoluble lignin and acid-soluble lignin are determined by weighing the filter residue and detecting acid hydrolysate with UV. It is simple and easy to operate and is widely used by international research institutions [3, 4], and it is also economically viable for scaling up.

Moreover, pyrolysis of plant fibers is a multistep reaction process due to their complex multi-components. At present, calculation of pyrolysis kinetic parameters and analysis of pyrolysis product distribution from the perspective of cellulose, hemicellulose, lignin, and other component groups are effective ways to deeply understand plant fibers' pyrolysis behavior. The pyrolysis and combustion kinetics of various fibers are essential for the multipurpose utilization of biomass materials, because pyrolysis is an effective way to convert plant fibers into high-value-added chemicals and bioenergy [5–9]. Compared with model method, model-free kinetics, as Friedman method [10– 14], Vyazovkin method [10, 13, 15], Ozawa method [16, 17], Kissinger-Akahira-Sunose (KAS) method [13, 15, 18], Flynn-Wall-Ozawa (FWO) method [13, 15, 18], and distributed activation energy model (DAEM) [11, 19–22], could reduce the errors introduced by the model fit. According to Friedman method [11, 13, 16], the pyrolysis and combustion processes of different plant fibers by using multiple heating rate programs were used to obtain reliable kinetics, in order to reveal their pyrolysis and combustion properties. In addition, the pyrolytic characteristics and tendentious pyrolysis reaction path of each polysaccharide (cellulose, hemicellulose, or lignin) could be demonstrated from the perspective of the pyrolytic product distribution of plant fibers with different chemical compositions. Interactions among these different chemical compositions during pyrolysis were also first noticed and speculated by analyzing the pyrolysis product of the actual whole and the sum of pyrolysis product by the accumulation of individual chemical composition. These pyrolysis properties investigated above would be useful references for the production of high-value-added products and biomass energy from plant fibers.
