2.4.3. Thermogravimetric Analysis (TGA)

Combusting biomass at elevated temperatures creates chemical modifications that a ffect the properties and performance of the biomass in any thermal pretreatment or conversion process, and the extent of chemical modification depends on the temperature level and the duration of conditions of thermal exposure [33,34]. In thermal analysis of biomass materials, physical and chemical changes associated with the material are usually determined as a function of temperature. TGA analysis of pure and blended samples of NSP and PSP was conducted for the following reasons: (1) to determine and quantify major organic constituents based on the degradation temperature of each component; (2) to determine di fferences in the temperature range of chemical modifications relevant to bonding and pellet quality; (3) to determine the combustion characteristics/efficiency of the pellets relevant to quality. Since the pellet samples were made from two di fferent materials (Norway spruce and pea starch), TGA analysis was undertaken to specifically establish how, if any, di fferences in modification temperature can facilitate particle bonding. A plot of weight loss against temperature is usually obtained from TGA analysis and represents the thermal behavior of a sample and the media through which components of the sample can be quantified based on the degradation temperature of each component [7,35]. Therefore, the major organic components of NSP (100%) and the NSP/PSP (50%/50%) blend were quantified from the plot obtained from this analysis, done in accordance with (after moisture evaporation) di fferences in the thermal decomposition temperatures of each component, as the pellet samples lost weight as temperature increased. The major organic components of biomass are commonly di fferentiated and identified using TGA due to di fferences in the structure of the components [33,36–38]. The procedure for TGA analysis is described below.

Thermal analyses of pure and blended samples of NSP and PSP were performed with a Perkin Elmer TGA 4000 instrument in which samples were firstly placed in the combustion chamber of the

instrument and ignited via software programming. The samples were heated from about 28 to 760 ◦C at a heating rate of 20 ◦C·min−<sup>1</sup> under a nitrogen gas flow rate of 19.8 mL·min−<sup>1</sup> in order to maintain a non-reactive atmosphere as the analysis progressed. The weight loss of the samples was typically monitored as the samples were heated, cooled, and isothermally held.

### 2.4.4. Di fferential Scanning Calorimetric Analysis (DSC)

According to Back [39], su fficient bonding areas are activated when biomass components are plasticized above their transition temperatures (Tg). The comparative assessment of combustion profiles helps to determine the series of stages that characterize the thermal behavior of biomass [40]. DSC analysis was used as a complementary technique to TGA to determine the Tg or softening temperature of the pellet samples as a function of heat flow, as well as establish phase changes caused by thermal exposure as result of blending. The intention was to compare the Tg of the pure and blended pellet samples and determine if this would have an impact on how bonding occurs in pelleting of di fferent biomass materials. Polymer viscosity significantly deceases to show obvious flow characteristics that are relevant to bonding at Tg higher than 50 ◦C [7,41]. As previously mentioned, the major organic and elemental components of PSP (100%) used in this study were determined by DSC according to the method described by Moorthy et al. [42].

A Perkin Elmer DSC 6000 was used to determine Tg caused by thermal degradation at specific heat flow during thermal analysis of pure and blended samples of NSP and PSP. The samples were heated to a temperature of approximately 440 ◦C at a heating and nitrogen gas flow rates similar to those given for TGA analysis. A computer was used to monitor the temperature and regulate the rate at which temperature changed for a given amount of heat.

### 2.4.5. Scanning Electron Microscopic Analysis (SEM)

To understand sample composition and morphological characteristics relevant to bonding at a micro scale, as well as view structural changes that are vital factors of pellets quality in terms of strength, a high-resolution SEM that o ffers surface morphology of samples at high magnifications was used. The intention was to use images generated by the SEM to determine visible di fferences in particle bonding mechanism between the pure and blended pellet samples that were relevant to quality through the identification of quality features from the images.

The morphological characterization of pure and blended samples of NSP and PSP was conducted with a JEOL (JSM-6390LV) model SEM instrument fitted with an EDX analyzer, which was used for the analysis of major elemental components. Samples were placed on an aluminum holder stub using a double-sided sticky carbon tape and sputter-coated with gold (Au) using an Eiko IB3 Ion Coater. Sputter-coating was necessary to increase conductivity and prevent charging e ffects that may lead to blurred images, which may hamper detailed interpretation of results. Thereafter, samples were mounted on stub holders in the sample chamber of the SEM for morphological examination.
