**1. Introduction**

Biomass is a renewable fuel and carbon natural because it consumes CO2 from the atmosphere during growth. CO2 as a primary greenhouse gas (GHG) is widely believed to be a major contributor to climate change. Reducing CO2 emissions is the main advantage of utilizing biomass. Biomass can be converted to gas or liquid fuels (using gasification, anaerobic digestion, pyrolysis, fermentation and transesterification), heat and power, and chemicals. Heat and power generation for industrial and utility-scale applications primarily use direct combustion or co-firing (replacing a portion of the coal with biomass in coal-fired plants) [1]. Recently, the United Kingdom started using wood pellets in co-firing and in dedicated biomass power plants to meet the European Commission's 2020 climate and energy plan, which primarily is a reduction in GHG and an increase of renewable energy usage in total energy consumption [2]. In the U.S, biomass (wood and waste) was used to generate about 1.5% of the total power output in 2016 [3]. To increase domestic biomass utilization, the development of technologies to reduce its cost and increase its utilization efficiency is required.

Developing large-scale (>50 MW) biomass power plants and co-firing may allow for power generation at high efficiencies and relatively low costs [1]. In addition, reducing biomass feedstock processing and transportation costs might help to further lower power generation cost. Biomass is generally considered a low quality fuel mainly due to its lower energy density which is attributed to a high moisture content, less carbon, more oxygen, lower density and lower heating value [4]. These characteristics contribute to inefficiencies associated with transportation, handling, storage and conversion of biomass in an efficient and economic manner. High transportation cost results in less biomass being collected and utilized. Biomass combustion yields a lower flame temperature which results in decreased thermal efficiency [5]. Gasification of biomass results in a lower quality syngas with a high tar concentration [6]. For a pulverized coal (PC) boiler, used by the majority of coal power plants, or an entrained-flow gasifier (used in integrated gasification combined cycle (IGCC)), the average coal particle size is required to be less than 100 μm (0.1 mm) [7,8]. Reducing biomass particle size to below 0.2 mm without pretreatment is difficult and costly because biomass is fibrous and compressible [9]. Torrefaction and low temperature pyrolysis carbonization processes have been proposed as pretreatment methods that could potentially address these issues [5,10–15].

Torrefaction has been extensively investigated in recent years and was reviewed by Madanayake [16], Chen [15], Chew [14] and Van der Stelt [13]. It has been successfully tested and demonstrated in pilot scale and (semi)commercial facilities [17]. Torrefaction is a mild pyrolysis which takes place at 200 to 300 ◦C in an inert or non-oxidative environment with the aim of producing torrefied biomass (solid) as the primary product [13,15,18,19]. It focuses on improving physicochemical properties of biomass including increased energy density, improved grindability, and higher hydrophobicity [5,18,20,21]. The heating value of torrefied woody biomass can be increased by 37% compared to the untreated wood [16,22]. For wood treated at 240 ◦C for 0.5 h and ground, the percentage of particles less than 415 μm (0.415 mm) and 150 μm (0.150 mm) were double that of untreated wood [22]. Torrefaction research has been focused on experimental studies of process conditions (temperature, gas environment and time), comparison of various biomass species and their major components, characterization of the products, and applying the torrefied biomass in densification, co-firing in coal power plant, gasification and ironmaking [15,16]. Using torrefied biomass pellets could improve gasification in terms of both energy efficiency and syngas quality because of the removal of oxygenated volatile compounds [15,23]. In addition to non-oxidative or conventional torrefaction, researchers have recently investigated oxidative torrefaction, wet torrefaction, and steam torrefaction to develop alternative technologies to upgrade biomass [15].

Low temperature pyrolysis with the aim of producing biochar (solid) as the primary product occurs in the range of 300 to 500 ◦C [10,12,19]. It significantly improves fuel combustion properties with increased thermal conversion efficiency [5,24]. It generates biochar with higher carbon content (FC-fixed carbon) and fuel ratio (FC/VM-volatile matter) compared to torrefied biomass [19]. Chars produced by torrefaction at 300 ◦C or pyrolysis below 500 ◦C have fuel properties such as fuel ratio, burnout and ignition temperature that fall between a high-volatile and a low-volatile bituminous coal [19]. Biochar produced at 350 ◦C had higher combustion rate than the torrefied woody biomass at 275 ◦C [12]. Treatment temperature is the key difference defining torrefaction and low temperature pyrolysis. The temperature ranges of the reported studies were 300–500 ◦C [10], 200–400 ◦C [12], 200–330 ◦C [5], and 250–300/400–500 ◦C [19]. Fundamental studies spanning the temperature ranges of both of these processes (200 to 500 ◦C) are lacking, therefore, the primary focus of this study is to address this deficiency.

Computer modeling is the primary approach along with experimentation to help with fuel conversion process design, optimization and scale-up [25–27]. Experimental studies are necessary to understand the processes and to obtain experimental data needed to develop and validate the

models. To completely describe the complex and dynamic process by a model, a biomass particle with controlled, well documented characteristics is used to follow the evolution of the individual particles through pyrolysis [25], combustion [26] and gasification [27]. A similar experimental method was applied for this pretreatment study. The most commonly reported experimental studies have investigated small amounts of milled small particles (for example <500 μm (<0.500 mm), 15 g) [22,24,28,29] or larger amounts of chips/blocks (for example 10 mm × 20 mm × 3 mm, 80 g) [10,12,20].To address the lack of fundamental data, this study utilized well defined wood particle samples to obtain torrefaction and low temperature pyrolysis data to thoroughly understand the processes and aid future modeling studies. Physical (appearance, weight, size, grindability) and chemical properties (chemical composition and proximate analysis), and heating value of the untreated and treated woods were investigated. The variations in shrinkage in the three sample reference directions are included because the shrinkage affects heat transfer to the particle and gas flow within the particle [25]. The shrinkage as a function of the mass loss was addressed in this study because the shrinkage is caused by loss of water and a portion of the mass of the biomass due to decomposition to volatiles. These parameters are important for modeling studies.

Wood and wood residues are one of the major sources of biomass [30]. Wood is essentially a series of elongated tubular fibers or cells aligned with the axis or longitudinal direction of the tree trunk and cemented together [31,32]. Each cell wall is composed of various quantities of three polymers: cellulose, hemicellulose, and lignin [32]. Cellulose is the basic structural component of all wood cell walls and primarily responsible for the strength of wood. Hemicellulose acts as a matrix for the cellulose and increases the packing density of the cell walls. Lignin acts as a glue which holds wood fibers together. Thermal treatments of wood cause the thermal degradation of the three cell wall polymers and result in physical and chemical changes of the wood. The physical changes in size, density, and grindability impact supply, handling and conversion of wood. The changes in biochemical composition significantly affect the wood properties and its conversion process. The high heating values of lignin are reported to be higher than the cellulose and hemicellulose due to higher degree of oxidation of the cellulose and hemicellulose [4]. The air-steam gasification conversions of cellulose, hemicellulose and lignin on a carbon basis are 97.9%, 92.2%, and 52.8%, respectively [33]. The product gas composition from cellulose in mol % is 35.5% CO, 27.0% CO2, and 28.7% H2 and from hemicellulose and lignin are approximately 25% CO, 36% CO2, and 32% H2. Pure cellulose produces a lower tar concentration in the syngas compared to beech and willow, which may be due to the hemicellulose and lignin content [6]. Study of the cell wall composition of the untreated and treated wood samples will aid in understanding the cause of treated wood property changes. The biochemical composition of biomass can be characterized using a thermogravimetric analyzer (TGA) [34,35]. Chen et al. qualitatively studied the composition of torrefied biomass (<150 μm, bamboo, banyan and willow) at 250, 275 and 300 ◦C using a TGA [28]. Mafu et al. quantitatively analyzed torrefied biomass (<500 μm (<0.500 mm), softwood chips, hardwood chips and sweet sorghum bagasse) at 260 ◦C using the 'food industry' chemical method [29]. For simplicity, this study utilized the TGA method.

Thermal pretreatments affect the biomass properties (such as shrinkage and grindablity) on a microscopic scale as well as on a macroscopic scale. Study of microstructural transformations will allow insight into the structural features and mechanisms responsible for the property changes. Microstructural transformations that occur during thermal degradation of the biomass are commonly observed using a scanning electron microscope (SEM) [36]. For this study, the same analysis locations in the three sample directions were followed during treatments so that change could be correlated to the different structures in each direction; a more detailed SEM analysis approach than typically reported [28,37]. The objectives of this study are to investigate a single wood particles over a range encompassing temperatures of torrefaction and low temperature pyrolysis; analyze the cell wall compositional changes for the treated woods using TGA; characterize physical and chemical property changes of the treated wood; and to examine the morphological changes of the wood samples during the thermal treatment process using SEM. These results improve the understanding of the property changes of the biomass during pretreatment and will help to develop models for process simulation and the treated biomass applications.
