Oxidative Torrefaction of Sunflower Husk Pellets in the Kaolin Layer

The aim of this work was to experimentally study the oxidative torrefaction of sunflower husk pellets in a kaolin layer. The kaolin layer was used to limit oxygen access to the biomass to prevent it from self-ignition. The changes in mass yield, volatiles, ash content, high calorific value (HHV), hydrophobicity, and morphology of the biomass were investigated at torrefaction temperatures 240, 260, and 280 °C, process durations 30–60 min, and kaolin layer heights 3–5 cm. The Van Krevelen diagram and the CHO index based on atomic C, H, and O data were also used for explanation. As a result of oxidative torrefaction in the kaolin layer, the atomic O/C and H/C ratios decreased and the sunflower husk position shifted towards peat. During torrefaction at 260 °C for 1 h (kaolin layer height of 3 cm) the O/C decreased by 26.9% and the H/C by 18.8%. Reducing the height of kaolin layer, as well as increasing the temperature and time of torrefaction, led to a decrease in mass yield and an increase in ash content. Torrefaction increased the FC/VM ratio from 50 to 225%, which improved the fuel properties of the biomass. A 16.5% increase in HHV was achieved compared to untreated sunflower husk. As the torrefaction intensity increased, drier fuel was formed, the moisture absorption of which decreased from 213 to 25.8%.


Introduction
Sunflower (Helianthus annuus L.) is a large herbaceous crop, widely cultivated throughout the world with a total production of about 48 million tons. Sunflower husk is byproduct of the sunflower oil industry and makes up 45-60% of the seed weight [1]. Typically, sunflower husk is used as animal feed, but the annually growing production capacities does not allow utilizing large volume of the husk as feed additive, therefore, oil and fat industry plants need to the development other processing methods. It is known that raw sunflower husk has some disadvantages such as low heating value and low bulk density, high moisture, and volatile matter contents; its energy density is lower than that of fossil fuel [2,3]. Research have shown [4][5][6][7] that for effective use of biomass as energy, feedstock in thermochemical conversion processes requires its pretreatment. Torrefaction combined with granulation can convert biomass into the attractive fuel with high energy density, lower biodegradability, and improved mechanical durability that also provides better economic viability for transport and handling. To reduce energy consumption for pelletizing, it is better to use raw biomass than torrefied biomass [8][9][10][11][12][13].
Torrefaction is moderate thermochemical treatment of biomass at temperatures of 200-300 °C in the oxidizing and non-oxidizing atmosphere [14,15]. At the present time, most studies of sunflower husk torrefaction focused on the use of non-oxidizing atmosphere: torrefaction of sunflower residues in a tabletop heated reactor [16], torrefaction of sunflower seed shell in a tube furnace [17], torrefaction of sunflower seed shells in a thermogravimetric analyzer [18], torrefaction in a vertical reactor with external heating by combustion products [8], torrefaction in a multi-hearth furnace [19]. Non-oxidative torrefaction is rather energyintensive and expensive process due to the use of inert gas (nitrogen) [20], which also dilutes the gaseous products, which makes it difficult for their condensation and combustion after torrefaction [21].
In the case of oxidative torrefaction, using air as a carrier gas reduces the cost of inert gas consumption and maintenance [18,22]. Active oxygen from the air stream is burned together with volatile matters and, thus, additional thermal energy is generated, which can be useful for conversion of torrefaction into the self-sustaining thermal process [21]. Oxidative torrefaction also provides higher content of fixed carbon in biomass than non-oxidative torrefaction [23]. Currently there are few studies in the literature about oxidative torrefaction of sunflower husk. Perhaps, this is due to the fact that oxidative torrefaction occurs in temperature range close to the onset of exothermic processes in biomass during its heating [24], and sunflower husk contains a significant amount of xylan [19]. It is known that the thermal decomposition of xylan produces furfural, which is easily ignited and can serve as a catalyst that causes a sharp increase in temperature and self-heating of biomass [25]. In the literature, there are mainly articles on oxidative torrefaction of sunflower husk in fluidized bed [6,[24][25][26]. Studies have shown that excess oxygen content in gas phase leads to significant decrease in mass and energy yield of biomass [27]. A moderate amount of oxygen can contribute to torrefaction process by increasing released internal energy and energy efficiency of the process, but effective removal of excess heat from the reaction zone is required. There is also difficulty of ensuring the required residence time of biomass in a fluidized bed reactor operating in an ideal mixing mode [24]. Thus, in connection with some advantages of oxidative torrefaction over non-oxidative torrefaction, it becomes necessary to study more detail the methods of torrefaction of sunflower husk with limited oxygen content.
The oxygen concentration can be controlled by increasing or decreasing its diffusion into the biomass through the layer of mineral matter [28,29]. According to this technology, oxygen diffuses from the environment through the mineral layer towards biomass and takes part in torrefaction process. Due to immobility of all components in the reactor, it is possible to use biomass in the form of pellets without risk of their destruction. Depending on the changing torrefaction conditions (temperature, residence time, layer height) and type of mineral filler, properties of torrefied products can vary in a quite wide range [21]. This torrefaction technology was studied only on wood biomass [30]. Therefore, there is insufficient data in the literature on the effect of oxidative torrefaction in the mineral layer on the fuel properties of sunflower husk.
Kaolin is suitable to use as mineral filler, because it is cheap, environmentally friendly material and retains its heatresistant properties at high temperatures [31]. It is assumed that kaolin should not have a catalytic effect, since it does not subject to special treatment (activation, dehydroxylation, etc.) [32,33] and torrefaction is carried out at temperatures below 400 °C [34]. Therefore, the aim of this work is to study oxidative torrefaction of sunflower husk pellets in the kaolin layer and investigate whether kaolin without the use of special additives effects on the suppression of oxidation reactions. Torrefaction temperature, torrefaction time, and height of kaolin layer over the range of 240-280 °C, 30-60 min, and 3-5 cm, respectively were varied. Also, such properties of sunflower husk as mass yield (MY), higher heating value (HHV), energy yield (EY), hydrophobicity, and morphology were studied.

Materials
The object under study were sunflower husk pellets produced by JSC Kazan Oil Extraction Plant, which is one of the leading Russian oilseeds (rapeseed, sunflower) processing companies. The content of cellulose was 46.4%, hemicellulose 31.6%, lignin 17.1%, and extractive substances 4.9% (according to technical specifications 9147-003-77,194,055-15, Russia). In accordance with these specifications, pellets from sunflower husks had a mass fraction of crude fat 3-5% (dry basis), the mass fraction of crude cellulose 55-70% (dry basis). Sunflower pellets had a diameter of 8 mm and a length of about 20 mm. Before the experiment; pellets were dried at 105 °C for 1 h in the drying chamber (ShSL-43/250 V, Russia). After this dried sunflower, pellets were stored in sealed plastic bags for further experiments. Density of the untreated pellets was 1000-1400 kg/ m 3 . Kaolin with thermal and chemical resistance was used as a mineral layer [31]. Before torrefaction kaolin was dried at 200 °C for 2 h. In this study, torrefied sunflower husk is denoted as SA-B-C, where the parameter A indicates torrefaction temperature, B denotes torrefaction time and C denotes height of kaolin layer. For example, S240-30-3 indicates torrefied sunflower husk which was obtained by torrefaction at 240 °C for 30 min inside of 3 cm kaolin layer.

Material Characteristics
CHNS contents were determined using elemental analyzer (EuroEA3000, Eurovector SpA, Italy). The tests were repeated at least twice to ensure the repeatability of test results, which was within ± 0.4%. Oxygen content was determined by deducting from 100% of CHNS and ash contents. Moisture, ash content and volatile matter content per dry weight of sunflower husk samples were determined according to BS EN ISO [36].
Mass yield measures the amount of solid product releases in the torrefaction process. The mass yield was calculated as the ratio of torrefied biomass mass to the initial mass of untreated biomass: Higher heating value of biomass samples was calculated according to the correlation proposed by Bychkov et al. [37].
where C, H, and N are the content of carbon, hydrogen, and nitrogen (wt.%), respectively.
Energy density ratio was calculated as [38]: To compare the hydrophobicity of fuels, sunflower husk pellets were immersed in a vessel with distilled water in a mesh strainer at room temperature for two hours. Further samples were air-dried for an hour, prior to the determination of its moisture content. Water absorption of pellets (W abs ) was calculated by the formula: where m 0 is the sample mass before the test (g); m 1 is the sample mass after the test (g).
SEM analysis was carried out to study changes in structure of the samples due to the torrefaction using a SEM Energy density ratio (EDR) = HHV torrefied biomass HHV untreated biomass HITACHI TM-1000 with a magnification of up to 10,000 and a resolution of 30 nm.

Torrefaction Process
Torrefaction of sunflower husk pellets was carried out inside of 100 ml glass beaker filled with the kaolin layer. The beaker was heated in a 1.6 kW muffle furnace (PMLS-2/1200, Russia). In this case, the beaker was not sealed during the torrefaction process. The samples under study were weighed on laboratory scales (VIBRA NT/NTR-220CE, Japan). Oxygen from the furnace chamber diffused through the kaolin layer towards biomass and took part in torrefaction process [28]. Initially, a small part of kaolin was poured into the bottom of the glass beaker, and then pellets (about 5 g) were put down, and then were covered with remaining kaolin. Sunflower husk pellets did not touch each other inside the kaolin layer. The muffle furnace was heated to the desired temperature for 30-40 min. Torrefaction experiments were performed at different torrefaction temperatures (240, 260, 280 °C), residence time (30, 60 min), and height of kaolin layer (3, 4, 5 cm). Torrefaction experiment at each operating condition was repeated three times to ensure experimental repeatability. Biomass with kaolin layer after torrefaction was cooled to room temperature in a desiccator. After cooling, torrefied sunflower pellets were easily cleaned from the kaolin, weighed, and stored in sealed plastic bags for further analysis.

Fuel Properties of Biomass
According to technology, kaolin layer was used to limit oxygen access to the sunflower husk samples from the environment [29]. At the same time, varying height of kaolin layer made it possible to control diffusion flow of oxygen to biomass, as a result of which the properties of the torrefied biomass changed. Moreover, properties of untreated and torrefied sunflower husk depended on torrefaction temperature and torrefaction time.
The results of proximate and ultimate analysis of sunflower husk samples are obtained. It can be observed from Table 1 that torrefied sunflower husk pellets had significantly lower moisture content (1.2-1.3%) compared to untreated samples (7.0%). Torrefied sunflower husk pellets had a larger ash residue than untreated biomass. Ash content slightly increased depending on increase in temperature and torrefaction time.
The increase of kaolin layer height had the opposite effect due to a decrease in the access of oxygen to the biomass [30]. Untreated sunflower husk contained 77.6% volatiles and 12.5% fixed carbon. With increase in temperature and torrefaction time, a tendency towards decrease in volatiles and increase in fixed carbon was observed and kaolin layer height had the opposite effect. The increase of ash content and fixed carbon leads to a clear carbonization phenomenon due to thermal destruction of cellulose in biomass [39,40]. FC/VM (fuel ratio) increased as the volatiles decreased due to the torrefaction. This ratio in untreated biomass is only 0.16 that is too low for being a promising solid fuel.
However, torrefaction improved this ratio to 0.52 (for S280-60-3) which made it a better fuel quality.
As shown in Table 2, after torrefaction carbon content increased while hydrogen and oxygen contents decreased. The lower content of oxygen and hydrogen in the torrefied sunflower husk samples may indicate about removal of light volatiles, physically bound moisture and the decrease of the hemicellulose content [41]. The decrease in oxygen content is favorable for fuel quality since higher oxygen content makes the biomass more hydrophilic and minimizes its storage time. Nitrogen content was not high and sulfur was not detected in the samples, which gives importance to biomass, as nitrogen reduces HHV and sulfur causes equipment corrosion and pollution. It can be noted that a comparison of the elemental composition of biomass obtained by torrefaction inside bentonite and talc [30] and torrefaction inside kaolin showed that the elemental composition differed insignificantly. This indicates that the kaolin itself has not affected the torrefaction reactions and its main purpose was to change the oxygen diffusive flux to the biomass from the environment. As a result of torrefaction, the higher heating value of sunflower husk samples increased significantly. It can be noted that HHV was affected by all three parameters of torrefaction (temperature, layer height, and torrefaction time). The maximum HHV of torrefied biomass (22.06 MJ/ kg) was obtained for S280-60-4. This is probably due to the fact that there was maximum carbon content under the given torrefaction conditions in biomass. The increase in HHV during oxidative torrefaction of sunflower husk in a fluidized bed in flue gases was 19.1% [24,25], and in the case of using a kaolin layer the increase in HHV was 16.5% (Table 2). In [8] after oxidative torrefaction the HHV increased by 15.1%. Thus, the use of the kaolin layer in oxidative torrefaction improves the energy properties of sunflower husk pellets and is comparable with the use of the fluidized bed.
Energy density ratio could be used to determine the densification of torrefied biomass energy content. Table 2 shows the EDR values at three heights of kaolin layer: 3, 4, and 5 cm for 30 and 60 min. In most cases, there was a tendency to changing the EDR with increasing the kaolin layer height. The effect of kaolin layer height increased with temperature at 240 and 260 °C. On the other hand, increase in temperature to 280 °C had the opposite effect. Note that the effect of torrefaction time on energy density ratio was not typical. Thus, energy density ratio increased when torrefaction time increased from 30 to 60 min.
Additional information of changes in the chemical composition of torrefied sunflower husk samples can be observed in the Van Krevelen diagram (Fig. 1), which shows the obtained profiles of the H/C ratio depending on the O/C ratio. Oxidative torrefaction in the kaolin layer led to an obvious decrease in the O/C and H/C ratio. The figure also shows changes leading to the movement of fuel towards the position of the peat (S260-60-3, S280-60-3 and S280-60-4 samples). It can be assumed that torrefaction may provide an opportunity for biomass substitution in the co-firing systems [17]. Figure 2 shows the CHO index values of sunflower husk samples. It is observed that the CHO index values ranged from − 0.45 to − 0.20, which means these biomass samples have a relatively lower amount of oxygen and higher amount of relative hydrogen content. Increasing the temperature and torrefaction time reduced the amount of oxygen and hydrogen and increased the relative amount of carbon, which, in turn, gave a good CHO index. It is also observed that the different torrefaction conditions can affect the value of CHO index in a rather broad range. The best CHO index values (− 0.40 and − 0.45) were obtained for samples S280-60-3 and S280-60-4. This is also confirmed by the Van Krevelen diagram and the HHV values.

Mass Yield
Influence of various parameters on mass yield when using kaolin as the mineral layer was analyzed. Since the main purpose of the mineral layer is to limit the access of oxygen to biomass from the environment thereby repressing oxidation reactions and do not allow mass yield to decrease excessively [21].
The mass yield was obtained at three values of torrefaction temperature: 240, 260 and 290 °C. Experimental results show that the mass yield increased with the height of mineral layer. At the low height of mineral layer, mass yield  was affected by oxygen diffusion, and at the higher heightconcentration of water vapor inside the glass beaker [21]. Figure 3 shows that at lower temperature (240 °C), when realized kinetic regime of torrefaction [42] influence of layer height on the mass yield was not significant. Leontiev et al. [28] mentioned that the similar results were for woody biomass; as the comparison showed, at 240 °C and 60 min, mass yield was consistent with the results when using bentonite with sodium bicarbonate as inhibitor. The decomposition of hemicelluloses and removal of low molecular weight aromatic compounds mainly occurred at this temperature [43,44].
During torrefaction temperature increased to 260 °C the decrease in mass yield became more obvious. This is probably due to the fact that at temperatures above 250 °C the rate of decomposition of hemicellulose is high, and the reactions of depolymerization and devolatilization in hemicellulose become considerable [39]. The increase in mass yield with height of kaolin layer was observed. The influence of kaolin layer height was insignificant due to the rate limit of hemicellulose decomposition in sunf lower husk by chemical kinetics [21]. In the case of temperature increasing to 280 °C (the diffusive regime of torrefaction) process was mainly characterized by depolymerization of lignin, cellulose, and hemicellulose [45] that was led to a maximum decrease in mass yield (77,1%). The increase in the height of kaolin layer significantly affected mass yield.
The increase in torrefaction time also affected the mass yield at constant temperatures. Longer torrefaction time caused the decrease in mass yield due to raised heat release [30]. The difference in mass yield from 30 to 60 min at 240 °C was almost negligible. At 260 °C and 280 °C, the mass yield decreased more visibly with an increase in the torrefaction time. Thus, the decrease in height of kaolin layer as well as the increase in temperature and torrefaction time resulted in the reduction of the mass yield. Comparison with results of mass yield during torrefaction of wood pellets inside bentonite layer [28], all other conditions being equal, showed similar results, and the difference was 2-5%.

Hydrophobicity
As a result of torrefaction, changes take place in the organic structure of the biomass. As a rule, under more severe conditions of torrefaction, the structure of torrefied biomass increasingly resembles the structure of common solid fuel (for example, the hydrocarbon structure of coal), acquiring also the hydrophobic properties typical of nonpolar hydrocarbons [16]. Figure 4 shows dependence of moisture absorption on the kaolin layer height at different temperatures and torrefaction times. A general tendency towards a decrease in the content of absorbed moisture can be noted with an increase in torrefaction temperature and torrefaction time. On the other hand, content of absorbed moisture increased with the height of kaolin layer. The smallest amount of absorbed moisture for each temperature and torrefaction time was obtained at the lowest kaolin layer height (3 cm). Note that untreated sunflower husk sample had the highest water absorption (213%). Torrefaction significantly increased the hydrophobicity of sunflower husk. The minimal content of absorbed moisture (25.8%) was found for S280-60-3, which was 88% reduction in hydrophilicity. Obviously, a drier and less hydrophilic solid product was formed under more severe torrefaction conditions. This simplifies the requirements for the storage and transportation conditions of torrefied biomass and preserves its energy value.
This result is consistent with Aslam et al. [39], where the hydrophobicity of torrefied rice husks was investigated , and a decrease in the biomass tendency to swell in water was observed with an increase in torrefaction temperature from its initial value of 308% to 92%.

Morphological Features
SEM micrographs of sunflower husk samples before and after torrefaction at three values of temperature: 240, 260, and 280 °C were obtained (Fig. 5). Sunflower husk is a woody plant tissue, homogeneous in physical structure [46]. As a rule, due to thermal effect there are physical changes in the structure and cell tissue of biomass. At the same time, there is no generally accepted agreement in the literature on morphological changes in biomass during torrefaction [17]. Earlier, in the process of preliminary grinding and granulation, the fiber structure of the original sunflower husk was significantly destroyed. As a result of the high pressure during pressing, the pieces of fibers began to stick together due to the newly formed intermolecular bonds. In the micrograph, the untreated material had the generally compact structure with a fibrillar organization of smooth, homogeneous parts and some roughness on the outer surface (a). It is possible to notice the internal structure with some porous ovals (b).
The fiber structure was retained in the torrefied sunflower husk samples, cracks and irregularities can be observed on its surface. In the range of 200-300 °C, melting of cell wall cellulose occurred. Torrefied sample at 240 °C had visible destruction of the external structure and exfoliation of microfibril fibers (c). Destruction to the biomass structure became more significant as the torrefaction temperature increased. The large exfoliation of microfibrils and curling of their edges were clearly visible at 260 °C (d). On the sample torrefied at 280 °C (e), some microporous structure was visible on leaf-shaped surfaces with well-defined porous ovals formed by the removal of volatiles and moisture from the interior of cells. These new pores differed from those observed in micrographs of untreated biomass as they were more rounded. An increase in the torrefaction time led to a deeper release of volatiles, a thinning of the cell walls, the development of cracks. Bilgic et al. [17] compared the SEM-images of untreated and torrefied at 300 °C sunflower husks, and they pointed out that torrefaction led to fragility и fractal structure of the samples due to formed cracks. This increased the specific surface area and porosity of samples, which can positively affect the rate and efficiency of fuel combustion [47]. Table 3 shows the main components of the costs arising in the production of pellets from sunflower husks [48]. As an example, an enterprise with a capacity of 5,000 tons per year is considered. With torrefaction, the costs for energy, amortization are higher, respectively, the price of a ton of torrefied pellets is also higher. However, taking into account the higher HHV, it turns out that the production of such pellets is not less profitable.

The Multiple Linear Regression Analysis
Statistical data processing was performed using multiple linear regression analysis. We investigated the effect on HHV (y1), hydrophobicity (y2), mass yield ( Tables 4 and 5. All models were built by the method of exclusion of the least significant factor. The following equations were obtained: HHV will increase with increasing carbon content in the biomass. Hydrophobicity will increase with decreasing torrefaction temperature and increasing kaolin layer height and VM content. Mass yield will be higher with higher moisture content and higher VM content. The FC:VM ratio will increase with increasing duration of torrefaction and increasing ash content. This study showed that oxidative torrefaction can lead to some improvement in the fuel properties of sunflower husks. However, the torrefaction conditions should be optimized.

Conclusion
Oxidative torrefaction of sunflower husk pellets in kaolin layer was studied for the first time. Kaolin effectively reduced oxygen diffusion into the sunflower husk without the use of special additives and inhibited oxidation reactions. Reducing the kaolin layer height as well as increasing the temperature and torrefaction time resulted in a decrease in the mass yield, and the minimum mass yield (77.1%) was obtained at 280 °C, torrefaction time 60 min, and kaolin layer height 3 cm. There was a tendency to decrease the amount of volatiles and increase the amount of fixed carbon. FC/VM of untreated biomass was equal to 0.16; after torrefaction, the FC/VM value became 0.24-0.52, which indicates a significant improvement in fuel quality. The atomic O/C and H/C ratios decreased and the sunflower husk position shifted towards peat. The maximum HHV of the torrefied biomass was 22.06 MJ/kg, which corresponds to an increase in HHV of 16.5% compared to untreated sunflower husk. Torrefaction also improved the physical properties of biomass by increasing their hydrophobicity by 88%. The specific surface area and porosity of the samples increased while maintaining the fibrous structure of the sunflower husk, which can positively affect the rate and efficiency of fuel combustion.