Energy Production Features of Miscanthus Pellets Blended with Pine Sawdust

The primary objective of this study was to evaluate the qualities of Miscanthus pellets blended with pine sawdust at various ratios ( Miscanthus /pine sawdust—0:100, 25:75, 50:50, 75:25, and 100:0) and relate them to energy generation potential under typical production conditions of the widely used bioenergy production technologies according to literature. Samples of each material were milled to the required sizes and blended in the proportions mentioned above. Water was added (6%) to each mixture to achieve a uniform moisture content of 10% on wet basis. The mixtures were then subjected to pressure agglomeration in the form of mechanical compression using a single pellet press so that homogeneously sized fuel pellets were obtained. Thereafter, the pure and blended pellet samples were examined using a range of analytical techniques to reveal any alterations in characteristics important to the utilization of the pellets as a green energy source. The results showed that, although temperature variations generally caused an estimated 6% moisture loss on a wet basis during pelleting with positive influence on the features of the pellets, the quality of the pellets in terms of ash composition (2–4%), hardness (41–46 kg/pellet), and heating value (20–21 MJ/kg) was in general more desirable for the blended pellets than for pure Miscanthus pellet. Structural analysis also revealed low levels of hydrophobic groups in the blends relative to pure Miscanthus , which were consistent with the fractions of pine sawdust and were also the reason for the pellets’ increased hardness.


Introduction
Sweden continues to be one of the nations that has declared the transition to net-zero emissions an objective, and it wants to accomplish this goal by the year 2045.The interest in alternative energy sources has grown over the past several decades because of this proclamation.A tried-and-true technique for lowering greenhouse gas (GHG) emissions in conformity with the principles of sustainable development is the production of energy from lignocellulosic biomass [1].According to the Swedish Energy Agency [2], bioenergy, mostly from forest-based resources, make up around 25% of Sweden's whole energy supply.Although they have less widespread usage than other resources like wood, perennial herbaceous energy plants like Miscanthus have the same technological capacity for producing bioenergy.Due to the fact that Miscanthus has a high rate of land use and is tolerant of marginal areas, there is thought to be little competition for Miscanthus on lands used for food production [3].In Europe, for instance, Miscanthus has been used for a variety of purposes, including bioenergy (combustion for heat and electricity generation or gasification), building materials (such as light concrete, wall and wind-protection covering, loam walls, insulation, roofing), car parts (steering wheels, oil binder), horticulture (pots, culture substrates, mulch, and bedding for fruits and vegetables), and animal husbandry (horse bedding) [4].
Given the foregoing, it is obvious that Miscanthus has piqued scholars' curiosity over time.Numerous research investigations have previously been carried out to encourage its growth and evaluate and enhance its potential and performance in energy production systems.Notwithstanding, most of the research on Miscanthus has come from a variety of initiatives and programmes that aim to both address the barriers to the widespread use of Miscanthus-based energy and materials while also promoting the environmental advantages of using this perennial energy crop.Although both raw and pelleted biomass may be used for energy generation, research have indicated that pressure agglomeration-related pelleting followed by combustion is the most advantageous method for generating energy from biomass materials like Miscanthus [5][6][7].The use of pelleted biomass has a number of advantages, such as consistency in physico-chemical properties (which increases combustion stability and decreases the risk of slag formation); gravity feeding within thermal energy systems (due to homogeneous size and structure); ease of handling, storage, and transportation over long distances (due to higher density); and increased economic output in relation to lower transportation and storage costs [8][9][10].Nevertheless, due to variations in properties, not all biomass materials can be compressed into pellets, which explains why Miscanthus-based pellets frequently lack the necessary durability and may not always exhibit the benefits listed above due to poor inter-particle bonding brought on by these materials' high ash and low lignin levels [11][12][13].The amount of cellulose, hemicellulose, and lignin in a biomass is crucial for maximizing energy production strategies [11,14,15].During pelleting, the strength of attraction forces will often decrease in accordance with the proportion of ash and lignin in the material, which in turn will affect the durability of the pellet, as low durability pellets will result in a high degree of disintegration that can have adverse negative impact on feeding and combustion systems [8][9][10].Therefore, blending with other materials that are less ashcontaining and rich in lignin is a popular technique used to solve the problem of poor quality of pellets formed from the compression process of materials such as Miscanthus.However, to be deemed a viable solution, the material of choice must be affordable, safe for the environment, and easily accessible, as well as help improve the ash-melting behaviour as shown in a recent study on mixing Miscanthus with perennial wild plant species [16].
The development of alternative and novel biomass utilization technologies that can ensure the economic viability of biomass processing, as well as the availability of considerable quantities of biomass, is essential for the successful transition toward a society that is carbon-neutral [17].From this vantage point, wood residues appear to be a viable choice since they satisfy the criteria already outlined and look like a fair solution to the problem of lack of durability in pellets manufactured from high-ash and low-lignin materials like Miscanthus.
Research on Miscanthus pelleting and use is widely available in the literature; however, many of the studies fall short of addressing quality issues from the perspective of energy generation.A few of the studies that clearly assessed the use of Miscanthus as a source of energy include the study cited above in which Miscanthus was blended with various perennial biomass materials with the aim of improving its energy production potential.Other studies are succinctly described in subsequent paragraphs.Even though Miscanthus has the potential to be used as an energy source, since it has been reported to have similar characteristics to the woody biomass [18], the crucial quality factors that ultimately determine the techno-economic viability of using it as an energy source have not been sufficiently studied [19].For instance, Choi et al. [20] examined the influence of chemical composition on the pelleting process of Miscanthus, particularly focusing on the optimum pressure and compression ratio required for the production of high density pellets.The quality of pellets made from different types of biomass, including Miscanthus, was also investigated by Stolarski et al. [21] who determined that blending residues from lignocellulosic biomass with energy crops like Miscanthus will enhance binding tendencies of particles for better quality pellets in terms of durability and lower ash content.They also alluded that in order to definitively establish the impact of quality of blended pellets on energy production efficiency, the characteristics of the pellets must be thoroughly investigated from a technoeconomic, environmental, and energy production perspective.Miscanthus pellets' suitability as a solid fuel and a biocarbon-carrier for fertilisers was also evaluated by Szufa et al. [22].The quality of copra meal-blended Miscanthus pellets was also studied by Szyszlak-Bargłowicz et al. [5] in relation to energy consumption of the pelleting process.The properties of pellets derived from different biomass materials including Miscanthus and their combustion behaviour were also examined by Kasurinen et al. [23] who focused on the toxicological consequences brought on by burning the pellets in residential-scale boiler systems.They determined that burning Miscanthus pellets generated more particle emission and relative toxicity than burning pellets produced from other biomass.They further claimed that the characteristics of Miscanthus pellets differed from those of pellets made from other materials, such as wood, and that this was the cause of the toxicological consequences.This highlights the issues with the quality of pellets made from Miscanthus and the need for research on quality-improving strategies that limits the issues encountered when using Miscanthusderived pellets as an energy source.Specifically, due to its elevated levels of ash and hydrophobic components, it is typically problematic to use Miscanthus in its pure form and impractical to compress it into pellets that can meet the highest quality standards in terms of features [24,25].The overall objective of this study, therefore, was to assess the characteristics of Miscanthus pellets blended with pine sawdust at various ratios (Miscanthus/pine sawdust-0:100, 25:75, 50:50, 75:25, and 100:0) and relate them to the potential for energy generation under typical production conditions of the widely used bioenergy production technologies following information available in the literature.

Research Materials and Preparation
The research materials used in this study were Miscanthus and wood residues in the form of pine sawdust.While the latter was obtained from a local sawmill in Karlstad, Sweden, the former was collected from a research farm in Hokkaido University in Japan.The materials were initially airdried at room temperature and milled to suitable sizes (< 1 mm) required for the pelleting process using a motor-driven laboratory Wiley mill (Model ED-5 mill).Shortly after milling, the materials were blended in various proportions as presented in Table 1, with approximately 6% of water added to each blend to achieve a final and uniform moisture content of 10% on wet basis (w.b).Water was added to the mixtures to improve bonding between particles and to make mechanical compression into pellets less complicated.However, after the addition of water, the blends were mechanically mixed for about 15 min using a lab-scale RW 20 IKA-WERK Janke & Kunkel mixer to ensure an even distribution of each material.The blended materials were subsequently placed in small airtight plastic vials and stored at room temperature for 24 h to allow good water penetration and retention between particles prior to pelleting.

The Pelleting Process
The pelleting of Miscanthus and its various blends with pine sawdust was prepared using a single pellet press available at the Environmental and Energy Systems Division of Karlstad University in Sweden.The press had a steel cylinder that was about 137 mm in height, 120 mm wide, and an 8.2 mm cylinder hole that allows an 8.0 mm piston to compress the material against a detachable bottom plate.Nylon rods that were around 8.2 mm in diameter were positioned above and below the materials to be compressed.Detailed description of the pellet press, its components and mode of operation, and its schematic representation can be found in [26,27].However, the procedure was such that around 1.2 g of finely ground particles of pure Miscanthus and its various blends with pine sawdust was subjected to pressure agglomeration using the single pellet press described above.The die temperature of the pellet press was set at 100°C for the entire pellet production process to allow for consistency in heat supply, while a maximum pressure of 16 kN was applied at a compression velocity of 30 mm min −1 and a holding time of 10 s.The produced pellets were mechanically ejected from the die at a velocity of 5 mm min −1 .A total of 105 pellets were produced for the analysis performed in this study with focus on the main features of the pellets and the best blend (in terms of quality) relevant to energy production in the commonly used thermochemical processes such as those listed in Table 2.

Pellets' Quality Analysis and Determination of Key Features
The produced pellets were analysed following known standard methods like the EN 14961-2 [30] for the analysis of non-industrial use of wood pellets.In addition to determining the best Miscanthus-pine sawdust blend, analysis of the pellets was necessary to establish critical features that may affect energy production efficiency under standard production conditions such as those presented in Table 2.The key features of the pellets considered in this study were the heating value, elemental composition, and combustion behaviour, as well as morphological and structural properties.The methods used to measure these attributes are described in the following subsections.Table 3 compares the weight fractions of the most significant organic and inorganic components of Miscanthus that are pertinent to its utilization as an energy source.
Table 1 The various mixing ratios of Miscanthus and pine sawdust pellets *The range was carefully selected to ensure comprehensive coverage of all possible mixtures using Miscanthus and pine sawdust.A 25% interval was deemed appropriate as it may provide significant variations between the ranges while remaining manageable in terms of sample sizes and project scope To enable the development of acceptable statistical inferences, all experiments were randomized and replicated in line with the approach described in [31].

Determination of Thermophysical Properties
The thermophysical features of the pellets were determined according to the procedures outlined in [26,32].Density, hardness, moisture, and ash contents were the properties investigated.While density was manually measured (in g/ cm 3 ) as the ratio of the mass to the volume of the pellet samples, hardness was determined in kilogramme (kg) using a KAHL motor-driven hardness tester (K3175-0011, Reinbek, Germany), with a spring (3.5 mm diameter) installed for the 0-100 kg range.The hardness was analysed according to the European standard (EN) method for solid biofuel analyses [33].Ash and moisture contents were determined from the thermogravimetric analysis (TGA) plot in Fig. 2 following the expression in Eqs. 1 and 2: where MC represents moisture content, w i depicts the initial weight in grammes (g) at 30°C (taken as weight before combustion), and w f denotes the final weight (g) after the evaporation of moisture below 90°C (taken as weight after combustion).
where AC denotes ash content, and w a and w s are weights of ash and sample after and before combustion, respectively.
It is important to note that TGA is a method that documents a sample's initial and final weights at specific ( 1) temperatures.Hence, moisture evaporation, which typically takes place between 50 and 140°C, generally correlates to the initial stage of the breakdown process of biomass under thermal circumstances [34].Equation 2 made it possible to calculate the ash contents of the pellets in a similar manner as the MC expression in Eq. 1.In this instance, the weight that remained after ignition and full oxidation of the organic matter contents of the pellets at 630°C was taken as the final weight of the pellets after combustion.Similar outcomes were obtained from three other runs of the TGA trials, and an average value was calculated and presented in both the MC and AC assessments.

Chemical Properties and Higher Heating Value Determination
The chemical properties of the pellets were determined in terms of the mass fractions of major elements.This was achieved using an elemental analyser specific to the determination of carbon, hydrogen, and nitrogen with the amount of oxygen calculated by difference The specific type of elemental analyser used was a Thermo Fisher Scientific Flash 2000 Organic Elemental Analyser (OEA) equipped with a Thermo Scientific MAS 200R automated Autosampler, and the ASTM standard method D5291 [35] was followed during this analysis.As part of the analytical process, roughly 5 mg of each sample was packed in tin capsules, which were then inserted into the instrument's combustion chamber using the autosampler mentioned above.The combustion gases were separated by a gas chromatography (GC) column after combustion, and the resulting gases were then swept over a copper-filled layer before being detected by a thermal conductivity detector (TCD).Higher heating value (HHV) is one of the most crucial features in the energy evaluation of fuels and biofuels [15].Experimental determination of HHV is often hampered by expensive experimental approaches and undesired experimental flaws.For these reasons, several correlation models believed to be simpler, quicker, and less costly have been developed to evaluate HHV.However, the proportions of the elemental components of biomass determine the HHV of the biomass [18,36].In this study, the heating values of both the pure and blended pellets were determined from the weight fractions of C, H, and O, in accordance with the following equation [37]: where C and H depict the contents of carbon and hydrogen, while O represents oxygen content of the pellets.The estimation of HHV was necessary to ascertain the amount of energy available for conversion since the pellets were considered for use as feedstock in combined heat and power (CHP) plants.

Molecular Structure Analysis
The Fourier transform infrared spectroscopy (FTIR) is a quick, simple, and accurate method to evaluate structural differences from blending two or more materials [26].The molecular structures of the pure and pine sawdust-blended Miscanthus pellets were determined by qualitative and quantitative identification of key functional groups.The Fourier transform infrared spectroscopy in attenuated total reflectance (FTIR-ATR) mode was used for this purpose and the procedure described in [26] was followed during the analysis.The analysis was necessary to understand variations in structural properties of the pellets and to predict the effects of these structural variations on the quality of the pellets and how these could affect energy production efficiency under standard conditions.The exact FTIR-ATR instrument utilized was the Perkin Elmer Spectrum Two FT-IR Spectrometer designed with a Smart Golden Gate Diamond ATR accessory.Three measurements were performed for each pellet sample and the temperature-adjustable ATR accessory was equilibrated at 30°C throughout the measurements.Using a metal rod and continuous mechanical pressure, samples were forced against the diamond surface to ensure perfect contact.The analysis was conducted at a resolution of 4 cm −1 , with 100 background (air) scans and about 30 sample scans used to obtain all spectra.

Assessment of the Combustion Performances of the Pellets
The performance of the pellets under thermal conditions was investigated via the use of a Perkin Elmer TGA 4000 thermogravimetric analyser (TGA) and derivative (3) HHV(MJ∕kg) = −1.3675+ 0.3137C + 0.7009H + 0.0318O thermogravimetry (DTG) following the recommendations of the International Confederation for Thermal Analysis and Calorimetry (ICTAC) [38].This was conducted to evaluate the combustion characteristics and thermal behaviour of the pellets at specific temperatures.Due to the sample holder's size restrictions, the pellets were manually cut into small pieces using a razor blade, and a small mass of about 5 mg of each sample was combusted during the TGA experiment.The pellet samples were heated from room temperature to a maximum temperature of 630 °C under a nitrogen gas environment and at a heating rate of 15 °C/min.Derivative of the thermogravimetry (DTG) was also performed with the same instrument to determine the exact temperature at which devolatilization of the pellets occurred.The nitrogen gas flow rate was 19.8 ml −1 min.The graphical curves obtained after TGA and DTG were represented as a function of temperature in this study.

Morphological and Microstructural Analysis
The morphological and microstructural features of the pellets that could not be viewed by the naked eye were examined under a scanning electron microscope (SEM).This was performed to observe structural alterations resulting from blending and to determine morphological features of relevance to energy production.A Zeiss Auriga field emission gun scanning electron microscope (FEG-SEM) was used to image the samples in secondary electron mode using an inlens secondary electron detector.The procedure was such that a nominal amount of sample was carefully and manually cut from the surface of each pellet, which were then placed on a conductive carbon tape and stuck onto an aluminium stub.This was followed by coating with a thin carbon layer at 100 Å using an Emitech Carbon Coater before morphological examination in the SEM chamber.The surface textures of each pellet sample were scanned with a focused beam of electrons and images revealing macro-and microstructural features were captured.An electron high tension of 5kV was maintained throughout imaging.

Statistical Analysis
To assess the importance of variations in treatments, we employed the general linear model (GLM) to conduct an analysis of variance (ANOVA) on measured physical and chemical characteristics.Tukey's HSD test was applied to examine the differences among the means of the individual treatments and their interactions (n = 3).IBM SPSS Statistics for Windows, Version 27.0 (Released 2020.Armonk, NY: IBM Corp) was utilized for data management and analysis.All statistical tests were conducted with a significance level of p < 0.05 as the threshold for significance.

Physical Properties
The physical properties of biomass pellets are mostly affected by the pelleting process conditions and the characteristics of the biomass particles [39].To produce a suitable energy product, these factors must be combined in a proper manner.The pure samples were used as the reference pellets to the blended pellets to establish any alterations in physical properties and how these changes could affect the energy production potential of the pellets under standard production conditions (Table 2).The major physical properties of the pure and pine sawdust-blended Miscanthus pellets investigated in this study and their units of measurement as well as their statistical data are presented in Table 4.
As can be observed in Table 4, while some of the pellets show similarities (as indicated by the superscript letter 'a' within each row) in thermophysical properties, others displayed statistically significant differences (indicated by the superscript letter 'b').In other words, the pellets showed major differences in other physical attributes but no discernible differences in density.For instance, the hardness values for M0, M25, and M50 show considerable differences in comparison to those of M75 and M100 pellets.Hardness decreased with increasing proportion of Miscanthus in the blends as the blend containing 25% Miscanthus (M25) exhibits greater hardness value in comparison to other blends (M50 and M75), which may be due to higher lignin content offered by its fraction of pine sawdust; lignin contributes to the hardness of pellets made from lignocellulosic materials [40].Meanwhile, although high density values may not necessarily mean high quality, the values obtained for density are well within the threshold required for energy production, since high density materials make gravity feeding facile in conversion systems and guarantees optimum energy production efficiency [41].Lignin-based pellets with high hardness values are also less prone to surface indentation or penetration and mechanical failures that are often associated with fracture, chipping, or disintegration [40,42,43].There are also significant differences in terms of moisture content between the pellets with higher ratios of Miscanthus (M75 and M100) and those with lower percentages (M0, M25, and M50).Notwithstanding, the pellets generally exhibited low moisture contents and lost around 6% of their contents of moisture (from a uniform moisture content of 10%) during the pelleting and cooling processes.Although some amount of water was added to the mixtures prior to pelleting as previously described, all the pellets' low moisture contents may have been caused by some water being lost as a result of temperature increases during pelleting, but the values are still within the range needed for energy production.Veritably, it should be mentioned that the majority of thermal energy production systems typically allow a certain amount of feedstock moisture content of up to at least 10% to stimulate water-gas shift reactions, which is a critical step in the energy production process [41,44].The pellets also show variation in ash composition, which increases in the blends in accordance with the increasing ratio of Miscanthus with M25 exhibiting the lowest value (around 2%).Ash content and its quantity are often a function of the amount of inorganic compounds (such as the compounds of sodium, potassium, magnesium, and silicon) in biomass and should always be less than 4% by mass to avoid technical glitches connected to corrosion, fouling, and the formation of slag [45].

Chemical Composition and Heating Value
Elemental composition has a considerable impact on the quality and fuel attributes of biomass pellets [46].The chemical compositions of both the pure and blended pellets were investigated in terms of key elemental constituents that are relevant to energy production.The elements were used in estimating the heating value of the pellets as described in a previous section.Table 5 shows the percentage composition of the major elements and heating values of the pellets.
The main elements in both the pure and blended pellets are carbon, hydrogen, and oxygen (Table 5), with M100 having a substantially lower carbon content than M0 probably because of lower quantities of aromatic biopolymer components like lignin.This supports the fact that herbaceous biomass such as Miscanthus has lower carbon content than woody biomass as reported by [43].
Additionally, almost half of the elemental constituents of the pellets are carbon, with M25 exhibiting the highest percentage, possibly due to greater structural biopolymer components such as lignin, cellulose, and hemicellulose.This is because the amount of biopolymer components in biomass are intimately correlated with carbon levels [43].However, the higher carbon content of the M25 pellet also explains why the blend displayed a greater hardness value than other blends (M50 and M75).There is equally slight variation in the hydrogen and oxygen contents of the pellets, with M75 exhibiting the highest percentages of both elements (8% for hydrogen and ca.47% for oxygen) in comparison to M25 and M50 pellets.Besides, it is vital to mention that the percentages determined for each element may be affected by the vagaries of agricultural practices where the materials used in the study (miscanthus and pine sawdust) were obtained prior to analysis.That said, the percentage compositions are similar to those reported in the literature [18,46,47] and fall within acceptable limits required for energy production.In fact, the oxidation of carbon and hydrogen during combustion is what generates the heat released during energy production, and the oxidation process is aided by the fuel's oxygen content; furthermore, since carbon, hydrogen, and oxygen constitute the major organic portion of biomass, the elements allow the biomass to release stored energy when bonds are broken under thermal energy production conditions [25,48,49].An essential factor in estimating the amount of energy present in a given volume of biomass is the higher heating value (HHV) [18], which, as previously mentioned, also takes into account the elemental composition of the biomass.The HHV of pure pine sawdust (M0) was higher than that of pure Miscanthus (M100) as seen from Table 6.The blends, particularly M25 and M50, showed HHV values that were greater than their pure materials (M0 and M100).This corroborates the idea that mixing woody biomass, such as pine sawdust, with herbaceous biomass increases the fuel's ability to generate heat [44].Based on this analysis, therefore, the blend, M25, showed superiority in terms of HHV with approximately 21 MJ/kg of energy chemically bound in it.Its greater HHV was due mainly to its higher content of carbon and lower ash content.This implies that, in contrast to the M50 and M75 blends, the M25 blend will probably perform optimally under energy production settings due to its heating value only, which might enable the best possible heat transmission with little to no sintering-related difficulties.

Chemical Structure Elucidation
FTIR analysis was primarily used to clarify differences in the chemical structures of the pellets that may have resulted from blending and to predict the potential effects of these structural changes on the properties of the pellets as well as on energy production efficiency based on literature information.This was done in line with the identification of key functional groups.Figure 1 shows the infrared (IR) spectra of pure and pine sawdust-blended Miscanthus pellets obtained after FTIR analysis.Again, the pure samples were used as references while spectra interpretation was limited to regions with notable changes.
The FTIR spectra of the pellets in Fig. 1 clearly show some similarities, but this does not necessarily mean they share the same properties.A closer look reveals differences in spectral features that indicate structural changes between the pellets across the entire spectra, particularly between 1734 and 1127 cm −1 , which are regions linked to groups such as C=O.Generally, the strong and broad peak at 3366 cm −1 is often associated with the stretching vibration of the -OH group, an active bonding group responsible for intra-and inter-molecular hydrogen bonding and which determines the type of attraction forces between particles during pellet formation [9].The transmittance peak at 2946 cm −1 indicates low concentrations of the antisymmetric stretching vibrations of hydrophobic groups such as CH 2 (C-H bonds within a methyl group) that were most likely the reason for the high compression strengths (in terms of hardness) of the pellets.Bonding quality between biomass particles is significantly impacted by the presence of hydrophobic groups during pelleting [9].The peak at 1734 cm −1 is attributed to the stretching vibrations of groups like C=O on the acetyl group of hemicellulose.The peaks at 1635 and 1516 cm −1 are the vibration peaks of the aromatic group C=C found in lignin, the only aromatic component of biomass [9,12].Furthermore, the transmittance peaks at 1516, 1274, and 1127 cm −1 are the characteristic peaks of the cellulose fingerprint region.The peak at 1374 cm −1 is linked to the vibration peak of C−C and C−O in hemicellulose.Meanwhile, the high intensity peak at 1034 cm −1 and the size of the peaks within this region for all samples suggest that the structure of M0 (pure pine sawdust pellet) contains more C−O groups than the other samples whose peak intensities decreased with increasing amount of Miscanthus in the blends.This agrees with Beer-Lambert's law (peak intensity is proportional to concentration).However, between the blends, the M25 pellet appears to contain higher proportions of the groups identified above.Nonetheless, with the various spectral peaks and their associated functional groups, it possible to hypothesize that both the pure and blended pellet samples are characterized by oxygen-containing functional groups that can be construed to mean the production of good quality pellets in terms of hardness because strong electrostatic attraction results from the presence of polar functional groups like the oxygen-containing functional groups [9].In relation to energy production, this means that pure and pine sawdust-blended miscanthus pellets generally contain a variety of polar and non-polar functional groups that can facilitate the balance of chain reactions that occur under combustion conditions.Meanwhile, it is important to point out that biomass materials have intricate structural makeup.This structural complexity makes it difficult to comprehend the relationship between biomass molecular structure and energy production under thermal circumstances, which calls for more research.

Combustion Behaviour of the Pellets
TGA tracks changes in weight caused by a sample's oxidation, breakdown, and dehydration over time at various temperatures.Due to the distinctive sequence of physicochemical reactions happening across temperature ranges and heating rates, characteristic thermogravimetric curves are provided for individual materials.These distinctive qualities are connected to the sample's molecular composition.Thus, the following subsections present the thermal characteristic curves and combustion parameters of the pellets obtained from TGA.From the TGA curves in Fig. 2, the pellets generally displayed three stages of weight loss (A, B, and C) with the first occurring between 50 and 87 °C (step A) due to moisture evaporation.The second weight loss stage could be observed in the temperature range 220 and 390°C (step B) and can be attributed to degradation from the release of huge amounts of volatiles (devolatilization) hence the rapid drop in the curves at the temperature range mentioned above.The third and final weight loss stage between 390 and 630 °C (step C) occurred due to the combustion of leftover char.However, the pellets with higher proportions of Miscanthus (M50 and M75) generated more residual ash at the end of the TGA experiment and did so in accordance with the percentage of miscanthus in the blend.On the other hand, the pellet containing a lower percentage of Miscanthus (M25) generated the least amount of residual ash (around 2%).This supports the data presented in Table 5 and aligns with a previous study [26].It also agrees with the study performed by Stolarski et al. [21] who found that pellets made from Miscanthus exhibited greater ash content relative to other biomass materials like wood.Nonetheless, the rapid weight loss observed in step B also signalled the beginning of the formation of aromatics that are often associated with thermal degradation and condensation reactions, which can continue over a wide temperature range [26].In view of this, the M25 pellet seem to achieve better thermal stability and reactivity in step C following its release of higher quantities of volatiles owing, perhaps, to greater amounts of cellulose and lignin, as well as the breakdown and condensation of ether bonds in lignin.The higher the content of cellulose and lignin in lignocellulosic materials, the greater will be the thermal stability and reactivity of the material [9].It could also be noted that the maximum degradation temperature of the pellets was generally observed at approximately 630 °C with the generation of varying degrees of unburned residual material (ash) as previously described.The highly reactive nature of Miscanthus, possibly as a result of its increased cellulose concentration, was linked to this maximal breakdown temperature.This is in line with the material's rapid thermal degradation pattern, which according to [5] normally takes place between 180 and 600 °C.The rate of thermal breakdown of Miscanthus impacts the combustion behaviour of other materials it is blended with [5,8].

Combustion Profiles of the Pellets from DTG
Derivative thermogravimetry (DTG) provides the rate of degradation and shows the exact point at which the weight loss of a sample is most noticeable.Since the thermogram curves of the pellets from TGA occurred close together, DTG was used to simplify the interpretation of the weight versus temperature curves.The DTG curves for pure and pine sawdust-blended miscanthus pellets are shown in Fig. 3.
The DTG profiles of the pellets (Fig. 3) extend over a wide temperature range (50-630 °C) within which the TGA was performed and clearly shows that weight loss was highest between 300 and 400 °C (maximum rate of degradation) for all samples as a result of the release of volatiles arising from the decomposition of cellulose and hemicellulose contents of the pellets (step B in Fig. 3).Generally, the thermal degradation characteristic peaks (A-C) were assumed to be connected to the degradation of biopolymer components like cellulose, hemicellulose, and lignin at 300, 368, and 390 °C respectively.However, it could be seen from stage B of the DTG plot in Fig. 3 that pure pine sawdust (M0) generated the highest volatiles because of its greater amount of reactive functional groups like the oxygen-containing groups emanating from higher cellulose and hemicellulose contents.This is supported by the functional group data presented in Fig. 1.For pure miscanthus (M100) and its blends (M25, M50, M75), the generation of volatiles are in accordance with the proportion of pine sawdust in the blends (stage B, Fig. 3), as the blend with the higher percentage of pine sawdust (M25) generated the most volatiles (judging by its degradation rate) in comparison to M50 and M75, which exhibited much more reduced devolatilization peaks due to the presence of fewer oxygenated groups.The thermal behaviour of biomass is usually swayed by the content and distribution of components of the biomass, which often vary depending on the nature and source of the biomass [41].The lower rate of devolatilization exhibited by pure miscanthus (M100) and its generation of more residual ash (Table 5) in comparison to its blends (M25, M50, and M75) is an indication of poorer thermal stability and reactivity as well as more content of inorganic elements such as Na, K, Mg, and Si [7].

Micro-and Macrostructural Properties
The SEM images of Miscanthus and pine sawdust pellets and their blends are shown in Fig. 4a-e.
The surface morphologies of the pellets show slight differences that are mostly connected to the bright colour particles seen in the SEM images, which are more pronounced in the image of pure Miscanthus pellet (Fig. 4e) and indicate the presence of alkali silicates like sodium, potassium, and magnesium silicates (Na 2 SiO 3 , K 2 SiO 3 , MgSiO 3 .XH 2 O) that are the main components of ash [26].These bright colour particles can also be observed as tiny spots that are distributed around the images of the blends (Fig. 4b-d), with the concentration of the spots greater in the images of M50 and M75 (Fig. 4 c and d) relative to that of M25 (Fig. 4b).This is an indication of higher ash concentration and corroborates the ash content data presented in Table 4.It also agrees with the studies of Von Cossel et al. [16].This equally suggests that the M25 pellet may contain lesser amounts of the undesirable ash-related compounds mentioned above and confirms that this blend is of superior quality compared to other blends (M50 and M75) on the basis of ash composition.This aligns with the study conducted by Kasurinen et al. [23] and agrees with the target of the production of less toxic substances that are often associated with the use of higher proportions of Miscanthus during energy production.The SEM image of pure pine sawdust pellet (M0, Fig. 4a) also displayed very minimal spots that further confirms its low ash content, a factor often used to define good quality pellets [46].Nevertheless, surface diagnosis of the pellets generally shows compact fibre structures with no noticeable pores, which can be construed to mean strong inter-particle bonding and the production of dense and physically stable pellets.
The morphological features described above are vital because they indicate how well the pellets can resist external forces after a prolonged use and how well they can reduce dust emissions during handling, as well as improve combustion efficiency.However, this analysis showed that the pellets did not display any significant differences in morphological features beyond the tiny bright colour dots that were interpreted as ash distribution.The M25 pellet showed less distribution of the ash-related tiny dots in its SEM image relative to the other blended pellets, which as previously mentioned is an indication of reduced ash concentration.Therefore, the combustion of M25 pellet would likely generate lesser quantities of non-combustible waste at the end of a combustion cycle when used as feedstock in CHP plants.The necessity of routine CHP plant maintenance would also be lessened in this situation.

Potential Advantages of Combining Miscanthus with Woody Biomass
Miscanthus is generally characterized by a higher productivity rate when compared to other biomass materials like wood; paradoxically, it is considered a low-grade material not particularly suitable for energy production in its raw form due to its loosely connected fibres, high ash, and relatively low lignin contents (Table 6), whereas the opposite is the case for woody materials such as pine sawdust [8,11,40,42,43].Because of their homogeneous, low ash, chlorine, and nitrogen contents as well as their easy-to-use nature, woodbased fuels have been the most common solid biofuels utilized for energy generation [23].This growing dominance in the use of wood-based biomass as fuels for energy production is, however, being constrained by several factors, including the high costs of sustainable forestry and competition from other industries.As a result, alternative biomass fuels, like herbaceous biomass, which grow more quickly and are more economically viable, are now considered for energy production [3].Although these substitutes to the dominant woody materials require little soil and may be grown successfully on fallow land, it is usually impractical to use them to produce pellets that satisfy the highest standards in terms of quality, especially for the reasons stated previously [8].Moreover, the toxicological characteristics of their combustion-related emissions and the resulting negative health consequences are further causes for worry due to their elevated amounts of ash and other chemical elements such as those mentioned above.However, very little is known about these toxicological issues and the accompanying health effects of their combustion [23].Therefore, a way thought to mitigate this toxicological risk is to consider blending herbaceous materials like Miscanthus with high grade materials such as wood and compressing the combination into pellets to enhance the properties and limit any combustion-related issues.Since the pine sawdust-blended Miscanthus pellets investigated in this study shared similar properties, the proportion of blending with other materials may be considered insignificant.Nevertheless, additional advancements to the combustion technologies presently in use may also offer a solution to the toxicological problems associated with generating energy from pellets produced from herbaceous plants.
Although the pine sawdust-blended Miscanthus pellets investigated in this study generally exhibited properties that can allow them to compete favourably as a source of biofuel, they also displayed intricate structural makeup that requires additional research to better understand the relationship between their molecular structures and energy production under combustion conditions.Their combustion chemistry also needs to be further investigated for extended knowledge on their degradation patterns and how these impact energy production under standard conditions.The investigation must make use of a range of measurement techniques, including experimental, computational, and theoretical thermo-kinetic analyses performed at different heating rates.

Other Benefits of Using Miscanthus as a Multifunctional Energy Crop
Miscanthus may be a key player in the sustainable growth of biofuels since it is an energy crop with minimal care needs, a high dry matter yield, and a high energy content material.Using Miscanthus as a source of energy prevents the burning of fossil fuels and the resulting GHG emissions.Therefore, adaption of Miscanthus pellets would directly contribute to UN Goal 7, affordable and clean energy, and Goal 13, climate action due to a significant reduction in GHG emissions.In addition to energy and climate benefits, Miscanthus plant development offers ecosystem services such as improving structural resources for agricultural landscapes, providing shelter, and increasing the diversity of insects that would improve pollination activity over the crops placed in the surrounding areas of Miscanthus plantation [46].Lastly, upscaling Miscanthus production and infrastructure to ensure its conversion to energy are also crucial from a societal perspective in terms of boosting population growth and job prospects.This is particularly true given the recent increase in the unemployment rate in Sweden's Värmland County.The Värmland County in Sweden is a very sparsely inhabited area, and as the population ages, more land may be abandoned.As a result, integrating energy crops into the landscape may boost the economics of rural communities and provide a greater return than reforestation and afforestation on unused and marginal croplands.

Conclusions
The analysis and results of this work prove that the most beneficial way to the use of Miscanthus as a source of energy in CHP plants is by blending it with a low ash-containing and high heating value material like pine sawdust and subjecting the blend to pressure agglomeration aimed at the production of densified pellets with more uniform structural composition.In general, however, the study demonstrates that the biomass blending approach offers a viable alternative to fulfil the quality and performance standards of blended pellets, as opposed to the conversion of a single pelleted biomass material.

Fig. 1 Figures 2
Fig.1The FTIR spectra of pure and pine sawdust-blended Miscanthus pellets showing the peak intensities that corresponds to key functional groups at various wavenumbers

Fig. 2 Fig. 3
Fig. 2 TGA curves showing combustion profiles of pure and pine sawdust-blended Miscanthus pellets

Table 5
The key elemental components and heating values of pure and pine sawdustblended Miscanthus pellets *Mean value ± standard error, n =3.Within the row, values with letters in superscript are significantly different (p < 0.05) based on an ANOVA and Tukey's HSD mean comparison tests