**1. Introduction**

The use of biomass for the production of energy and valuable chemicals is gaining attention because it is renewable, clean, cheap, and readily available. There are several types of biomass (such as wood, starch, agricultural residues, energy crops, and industrial and municipal solid wastes) suitable for energy conversion after subjecting the biomass to pretreatment processes like pelleting, and the pretreatment method is often determined by the conversion pathway intended for the biomass; however, the need to press biomass into pellets arises from its heterogeneous nature and low energy density, which makes its conversion in energy production systems very problematic [1]. This means that pelleting of biomass is basically a technique used to improve biomass characteristics in the form of a pellet with regular shape, along with higher density, strength, and durability, as well as excellent combustion characteristics and low ash content, which are factors used to define good-quality biomass pellets [2,3]. The improved quality of pelletized biomass makes it suitable as a fuel for use in household

heating boiler systems, cooking, and electricity generation. Most biomass pellets are usually made from wood such as Norway spruce; the global fuel pellet market experienced expeditious growth in recent times, and it is anticipated to have even faster growth in the near future [4]. Even though almost all biomass can be pelletized, not all are likely to form durable pellets because of variations in characteristics; hence, di fferent types of biomass are sometimes blended for the purpose of improving quality. Additionally, because of issues related to dust formation and self-ignition, additives such as starch may be added to the biomass during pelleting in order to increase the overall quality of the pellets [2,5]. However, the production of durable biomass pellets is always challenged by a host of factors, including a lack of fundamental understanding of the bonding mechanism of major components during pelleting, type of materials to be blended with the biomass, how these materials affect the mechanism of bonding, and di fferent pellet quality parameters. Other factors impacting the production of durable biomass pellets include types of biomass for pelleting, moisture content of the biomass, organic and elemental constituents of the biomass, particle size and distribution, and pellet press compression force and temperature [6–9]. Most of these factors were studied by other researchers, ye<sup>t</sup> di fferences in the mechanism of bonding between pure and blended biomass pellets relevant to quality still need to be investigated. As previously mentioned, wood is a type of biomass most commonly used in the production of fuel pellets and remains a major source of energy in most countries. On the other hand, starch is perceived as a good additive to wood for the production of durable biomass pellets. However, to the best of the authors' knowledge, the production of biomass pellets from pure starch and other materials (such as wood) blended with starch, with the aim to investigate the mechanism of bonding relevant to quality, is sparsely studied. To lay the groundwork for a better understanding of what was investigated in this study, the section below presents a brief synopsis of the chemistry of wood and starch.

### *1.1. Overview of the Chemistry of Wood and Starch*

It was reported that wood such as Norway spruce remains the most common material for the production of fuel pellets in Sweden, and that starch has excellent applications in a handful of industrial sectors such as biofuel, food, pulp, and paper [5,10]. The properties of these two materials are controlled by complex interactions between their physical and chemical structure. For instance, the structural characteristics of wood are such that its cells are made up of varying proportions of three major substances (cellulose, hemicellulose, and lignin), whose structures are bound by functional groups that are responsible for the behavior of wood in many conversion processes. Depending on the type of wood, the weight percentages of cellulose, hemicellulose, and lignin in wood range from 40% and 50% for cellulose, from 20% to 28% for hemicellulose, and from 25% to 30% for lignin [3,7,11,12]. The wood structure is complex and anisotropic with optimized hierarchical levels that span from the macro to the micro, molecular, and even nano scale. The structure of the cellulose component of wood is linked by β-(1,4)-glycosidic bonds with a high degree of polymerization, while that of hemicellulose is partially substituted by acetyl groups with a lower degree of polymerization in comparison to cellulose. The substituted acetyl groups in the hemicellulose structure means that the hydroxyl groups (–OH) at carbon positions C2 and C3 are partly substituted by *O*-acetyl groups, one of the adhesive degradation products of hemicellulose responsible for natural bonding [13,14]. Lignin is complex in nature and contains aromatic rings that are responsible for its glue e ffects; the thermosetting properties of lignin are exhibited at temperatures above or equal to 100 ◦C, and the adhesive nature of thermally melted lignin significantly contributes to the strength and durability of pellets made from lignocellulosic biomass [15–17].

Starch, on the other hand, is a soft, white, and tasteless powder whose usefulness as an additive in improving bonding properties of biomass pellets cannot be overemphasized. This is because of the chemical structure of its two major monomer units known as amylose and amylopectin [18]. Just like wood, the structures of these two monomer units of starch are also bound by functional groups that confer specific properties. The functionality of starch depends on these groups and the

average molecular weight of its amylose and amylopectin contents [19]. Both of these constituents of starch consist of chains of α-(1,4)-glycosidic bonds that are linked by d-glucose residues connected via α-(1,6)-glycosidic linkages, thereby forming polymer branches [19,20]. More often than not, the relative weight percentages of amylose and amylopectin in starch range from 18% to 33% for amylose, and from 72% to 82% for amylopectin [19]. Morphological features of starch include perfectly spherical particles with many void spaces exhibiting both internal and external surface areas that are determined by the shape and size of its particles, which induces higher molecular mobility when starch is heated (gelatinization) [19,21].

The structures of the major components of both wood and starch can be found in References [12,22].
