Improvement of Higher Heating Value and Hygroscopicity Reduction of Torrefied Rice Husk by Torrefaction and Circulating Gas in the System

Abstract

This study aimed to enhance the thermal characteristics of rice husk biomass through torrefaction conducted in a fixed-bed reactor. A novel approach was employed by circulating the gas produced within the system, instead of using traditional nitrogen. The torrefaction process took place at temperatures ranging from 200 to 320 °C, with different residence times of 10, 20, and 30 min for heat exchange. Quantitative analysis of the torrefied biomass revealed several notable improvements. The higher heating value of the biomass increased significantly, reaching 23.69 MJ/kg at a temperature of 320 °C and a residence time of 30 min. This enhancement indicates the effectiveness of torrefaction in increasing the energy content of the biomass. Furthermore, the torrefied biomass exhibited a remarkable reduction in hygroscopicity, with reduction by as much as 92 wt% compared to raw rice husk biomass. This reduction implies that the torrefied biomass is more resistant to moisture absorption, making it more stable and suitable for various applications. The torrefaction process in the fixed-bed reactor yielded a torrefied biomass with a production yield of 76 wt% (RH-320, RT30). This yield showcases the potential of the employed technique for producing a substantial amount of high-quality torrefied biomass. The resulting biomass holds great promise for diverse applications. It can be utilized for industrial steam production, contributing to the efficient use of biomass resources. Moreover, it could serve as an alternative fuel source for biomass power plants, offering a sustainable energy solution. Overall, this study demonstrates the effectiveness of the proposed torrefaction method in enhancing the thermal characteristics of rice husk biomass. The improved energy content and reduced hygroscopicity make torrefied biomass a valuable resource for various industries, promoting the utilization of biomass as a renewable energy source.

1. Introduction

The use of fossil fuels for thermal energy production has been a long-standing practice worldwide, including in Thailand. However, this practice has significantly intensified the greenhouse effect, causing adverse environmental impacts and contributing substantially to climate change [1]. As a result, there is an urgent need to explore and promote renewable energy sources, such as wind energy, solar energy, and biomass energy [2]. This shift is imperative to reduce our reliance on fossil fuels and mitigate their detrimental effects [3].

Biomass is an interesting renewable energy source that is readily available and can be converted into various forms of energy [4], such as biomass pellets, liquid fuel, and gas fuels. In Thailand, biomass is a good option and suitable for renewable energy [5]. With large swaths of land dedicated to agriculture and crop production, biomass can be readily sourced since there are abundant raw materials. For example, leaf scraps, sawdust, bagasse, rice straw, palm residue, cassava residue, corn cobs, rice husks, and industrial waste can be converted into biofuel using the torrefaction process [6,7], which is a thermochemical process applied to biomass [8]. Several studies have demonstrated the use of biomass as a direct replacement for fossil fuels by burning it directly [9] or approaches such as combining biomass with coal for energy production [10]. Given that Thailand is an agricultural country with much agricultural waste, biomass is an interesting alternative to either replace or significantly reduce the use of fossil fuels [11,12].

Typically, biomass improvements are performed in two atmospheres: non-oxidative torrefaction and oxidative torrefaction. Most studies related to biomass torrefaction have focused on the use of a non-oxidative atmosphere [13]. The results have shown that torrefaction effectively improves the quality of biomass in terms of higher heating value or energy density, lower O/C and H/C atomic ratios, reduced moisture content, increased hydrophobicity, and enhanced biomass uniformity.

Nevertheless, raw biomass materials are hygroscopic, meaning that they contain a cell polymer containing hydroxyl groups. These clusters absorb moisture, resulting in water molecules becoming bound through hydrogen bonds [14]. Their high hygroscopicity leads to a low heating value, low dimensional stability, and low durability. These disadvantages reduce biomass efficiency and increase costs associated with collecting, storing, and transporting raw materials [12,15]. The removal of functional groups significantly reduces the hygroscopicity of torrefied biomass [16], and it enhances the resistance of biochar to microbial degradation, improving handling and storage [17].

The properties of raw biomass clearly require improvement to enhance its heating value component. However, this improvement can be made more efficient with the use of technology in the torrefaction process [18]. This process relies on the principle of low heat exchange [6], employing a temperature range of 200–320 °C and technology for producing solid fuels while subjecting volatile substances to the condensation process [19]. These advancements improve the properties of biomass by increasing the heating value of biofuel products [11,12] and enhancing their resistance to hygroscopicity [9]. The decomposition of biomass components (cellulose, hemicellulose, and lignin) under different reaction conditions causes changes in the structure of biomass, affecting its thermal properties and enhancing the quality of solid fuel for combustion [17].

The process of torrefaction provides the resulting biomass product with superior characteristics compared to raw, untreated biomass. The moisture content in raw biomass consists of oxygen and hydrogen. By reducing volatiles and stabilizing the carbon content, biomass can have a higher heating value [20,21]. This process is suitable for improving the properties of rice husks as a potential biomass fuel, which is one of Thailand’s untapped agricultural wastes. However, rice husk biomass has limitations, such as low heating value, high humidity, and low energy density [22], resulting in insufficient heat energy during combustion [23].

Clearly, there are advantages from torrefied biomass in terms of high combustion heat gain for heat utilization and resistance to moisture recovery [24]. This study aimed to enhance the thermal energy and anti-hygroscopic properties of rice husks using torrefaction in a non-oxidizing fixed-bed reactor, utilizing the gas generated during the initial biomass combustion, instead of nitrogen [3]. This approach increases the thermal and hygroscopic properties of biomass while reducing nitrogen usage [19]. The results of this study could provide valuable information for creating guidelines on whether biomass can be utilized or combined with other fuels in power plants to reduce fossil fuel usage.

2. Materials and Methods

2.1. Raw Materials

Rice husks from Thailand’s Phitsanulok Province, with particle sizes of 40–80 mesh, were used in this study. Before torrefaction, they were dried at 105 °C for 24 h. in an oven until the mass was stable [25]. The moisture content of the biomass was measured as 10 wt%, on a dry basis according to ASTM E871-82 [26]. The analysis of the biomass samples is shown in

Table 1. Chemical compositions and energy contents of raw rice husk.

Items	Rice Husk (Samples)	References
		[17]	[29]	[30]	[31]	[32]
Proximate Analysis (wt%)
MC	10.60 ± 0.65	9.0	11.7	-	7.70	6.44
VM	58.80 ± 1.25	68.5	53.1	73.5	60.70	80.45
FC	14.80 ± 0.80	17.4	20.4	14.7	16.60	8.60
AC	15.80 ± 1.05	14.1	14.8	11.8	15.00	10.95
HHV (MJ/kg)	15.65 ± 1.25	15.24	15.70	16.2	16.00	17.41
EMC (wt%)	7.56 ± 1.30	-	-	-	-	-
Ultimate Analysis (wt%)
C	38.90 ± 0.55	43.10	36.74	40.8	38.50	44.04
H	5.50 ± 0.80	4.90	5.51	5.7	5.70	6.55
N	0.33 ± 0.10	0.50	0.28	1.2	0.60	0.24
O	36.50 ± 0.65	37.30	42.55	40.5	32.20	43.94
S	0.80 ± 0.10	0.41	0.55	0.75	0.30	-

. Proximate analysis was performed according to the ASTM D3172 standard [27] and Ultimate Analysis from Micro-Tru Spec CHNS/O. Higher heating values (HHVs) were measured using an IKA bomb calorimeter model C5000 (Cole-Parmer, Mumbai, India), the yield of the products was determined, and the anti-hygroscopic torrefied biomass properties were studies [15,28].

Table 1 presents the results of proximate analysis and ultimate analysis, the higher heating values (HHVs), and the anti-hygroscopic properties of rice husk biomass samples. The values obtained were consistent with previous reports. The proximate analysis revealed a moisture content of 10.6 wt%, volatile content of 58.8 wt%, fixed carbon of 15.8 wt%, higher heating value (HHV) of 15.65 MJ/kg, and equilibrium moisture content (EMC) of 7.56 wt%. The ultimate analysis indicated the following composition: carbon (C) 38.9 wt%, hydrogen (H) 5.50 wt%, nitrogen (N) 0.33 wt%, oxygen (O) 36.5 wt%, and sulfur (S) 0.8 wt%. These values were determined using standard experimental procedures [15]. These results were important for selecting appropriate values in the subsequent steps of the experiment. Additionally, they can be compared with torrefied biomass obtained from the torrefaction process at different temperature ranges to investigate the thermal properties and resistance to hygroscopic yield of the torrefied biomass in further studies.

2.2. Torrefaction Processes

The system shown in Figure 1 was utilized for testing and demonstrating torrefaction in a fixed-bed reactor. The purpose of the experiment was to produce a solid substance known as “torrefied biomass”. Initially, an infrared gas burner was employed to heat a reactor containing 500 g of rice husk biomass. The experiments were conducted at temperatures of 200, 240, 280, and 320 °C. The gas flow rate circulating within the system was maintained at 10 L/min, and the residence time of the biomass in the reactor was set to 10, 20, and 30 min. As the biomass is burned, it transformed into torrefied gas, which then became the circulating gas within the system. This gas flowed through pipes at a temperature of at least 180 °C [16] and was directed to a hydrostatic and electrostatic precipitator condenser operating at approximately 35–40 °C [33]. The condenser facilitated the condensation of gases produced by the torrefied gas, resulting in the formation of liquid substances, such as “tar and wood vinegar”. The non-condensing gases were subsequently fed back into the suction unit of the reactor for one cycle of operation. Throughout this process, thermocouples, Type-K (T1–T4), were used to monitor the temperature of the system, and the process repeated itself until the predetermined experimental time was completed [19]. By circulating the gases through a water exchanger, the temperature of the biomass in the reactor was effectively decreased. The resulting product was recorded both before and after the experiment.

Torrefied biomass, liquid, and gas were produced in this experiment. The experiments were carried out at different temperatures and residence times to study the yield of the torrefaction products. The experiments were repeated at least three times to obtain accurate values and were compared with the rice husk biomass sample.

Three products were obtained from the torrefaction process: torrefied biomass, liquid, and gas. The yield of the products can be calculated as Equation (1), Equation (2), and Equation (3), respectively [21].

$$\mathrm{Torrefied}\mathrm{biomass}\mathrm{yield}\left(\%\right)=\frac{\mathrm{weight}\mathrm{of}\mathrm{torrefied}\mathrm{biomass}\left(\mathrm{g}\right)}{\mathrm{weight}\mathrm{of}\mathrm{raw}\mathrm{material}\left(\mathrm{g}\right)}\times 100$$

(1)

$$\mathrm{Liquid}\mathrm{yield}\left(\%\right)=\frac{\mathrm{weight}\mathrm{of}\mathrm{liquid}\left(\mathrm{g}\right)}{\mathrm{weight}\mathrm{of}\mathrm{raw}\mathrm{material}\left(\mathrm{g}\right)}\times 100$$

(2)

$$\begin{array}{ll}\mathrm{Gas}\mathrm{yield}\left(\%\right)& =100\%-\mathrm{torrefied}\mathrm{biomass}\mathrm{yield}\left(\%\right)\\ & \mathrm{ }-\mathrm{liquid}\mathrm{yield}\left(\%\right)\end{array}$$

(3)

2.3. Properties of Solid Products

The determination of a higher heating value of torrefied biomass was performed using an IKA brand C5000 bomb calorimeter according to ASTM D-1989 [16] and by weighing approximately 1 g of torrefied biomass in a burning cup and placing the burning cup in the bomb calorimeter. The flow of electric current through the wire caused the ignition.

The heat from the combustion was transferred to 2 L of water in the system, and the temperature of the water was increased. The temperature difference was later compared with that of the standard. Then, the amount of heat from combustion was later determined, and a higher heating value (HHV) was recorded.

The study also determined the hygroscopicity of torrefied biomass and compared it with that of rice husk biomass dried at 105 °C for 24 h using a THERMO-HYGROMETER two-position temperature and humidity meter, model HY-303C (Mumbai, India). Initially, 20 g of torrefied biomass and rice husk biomass samples were weighed and used in the experiment. The experimental time was set for 24 h at a temperature of 25 °C and relative humidity of 55%. Subsequently, Equation (4), was used to calculate the equilibrium moisture content (EMC) and reabsorption rate of the experimental samples [11]:

$$\mathrm{EMC}\left(\%\right)=\frac{{\mathrm{M}}_{\mathrm{wet}}-{\mathrm{M}}_{\mathrm{dried}}}{{\mathrm{M}}_{\mathrm{dried}}}\times 100$$

(4)

where M_dried is the weight of the sample after the experiment, and M_wet is the weight of the sample before the experiment. All experiments were weighed once every hour for a total of 24 h. Data from three replicate experiments were collected.

Hygroscopicity is an important characteristic to consider when evaluating the suitability of biomass for various applications, including renewable energy production. In this study, the equilibrium moisture content (EMC) and the hygroscopicity reduction extent (HRE) were used as measures to assess the hydrophobicity of torrefied biomass. The hygroscopicity reduction extent (HRE) is a quantitative measure introduced in this study to evaluate the reduction in hygroscopicity resulting from the torrefaction process. It provides valuable insights into the extent to which torrefied biomass exhibits resistance to moisture absorption. The calculation of HRE is defined by Equation (5), which considers the temperature and residence time used during torrefaction. By quantifying the HRE, researchers can determine the effectiveness of torrefaction in reducing the hygroscopicity of biomass. The lower that the HRE value is, the greater that the reduction in hygroscopicity is, indicating improved hydrophobicity of the torrefied biomass. This property makes torrefied biomass a viable option as an alternative fuel for renewable energy applications [11].

$$\mathrm{HRE}\left(\%\right)=\left(1-\frac{{\mathrm{EMC}}_{\mathrm{Torrefied}}}{{\mathrm{EMC}}_{\mathrm{raw}}}\right)\times 100$$

(5)

where EMC_torrefied is the weight of the torrefied sample after the experiment; and EMC_raw is the weight of the rice husk in the sample after the experiment.

3. Results and Discussion

3.1. Solid Product of Torrefaction

3.1.1. Proximate Analysis and Ultimate Analysis

The results of the proximate analysis conducted on rice husk samples before and after torrefaction are presented in

Table 2. Fuel characteristics of torrefied biomass at different temperatures from torrefaction process.

Samples	Proximate Analysis			Ultimate Analysis
	(wt%, db)			(wt%, db)
	VM	FC	AC	C	H	N	O
Residence time 10 mins (RT10)
RH-T200	57.4 ± 0.90	16.6 ± 0.65	11.8 ± 0.85	45.6 ± 0.60	5.3 ± 0.95	0.30 ± 0.10	35.3 ± 0.85
RH-T240	52.9 ± 1.05	19.8 ± 0.90	12.2 ±1.10	50.2 ± 0.85	5.0 ± 0.55	0.31 ± 0.10	33.8 ± 1.10
RH-T280	48.2 ± 1.25	23.3 ± 1.10	13.1 ± 0.95	53.8 ± 0.95	5.0 ± 0.60	0.30 ± 0.10	29.1 ± 1.15
RH-T320	38.6 ± 0.95	30.3 ± 0.95	14.5 ± 0.80	59.7 ± 0.90	4.8 ± 0.50	0.29 ± 0.10	22.5 ± 0.95
Residence time 20 mins (RT20)
RH-T200	55.1 ± 1.35	19.3 ± 0.90	13.1 ± 1.05	48.2 ± 0.90	5.1 ± 0.35	0.31 ± 0.15	34.6 ± 1.20
RH-T240	48.3 ± 1.50	23.5 ± 1.05	13.8 ± 1.10	52.2 ± 1.00	4.9 ± 0.50	0.30 ± 0.10	31.4 ± 1.10
RH-T280	45.2 ± 1.25	27.1 ± 1.20	14.5 ± 0.85	59.5 ± 1.05	4.9 ± 0.50	0.30 ± 0.10	28.4 ± 1.00
RH-T320	36.1 ± 1.00	31.3 ± 0.80	15.4 ± 0.95	60.8 ± 0.90	4.5 ± 0.45	0.30 ± 0.15	21.7 ± 0.90
Residence time 30 mins (RT30)
RH-T200	53.5 ± 1.10	20.7 ± 1.10	14.2 ± 1.25	49.2 ± 0.65	5.0 ± 0.30	0.30 ± 0.10	34.1 ± 1.25
RH-T240	47.6 ± 1.15	24.4 ± 0.95	14.8 ± 1.15	55.5 ± 0.80	5.0 ± 0.35	0.30 ± 0.10	30.2 ± 1.20
RH-T280	41.2 ± 1.25	28.6 ± 1.10	15.1 ± 0.95	60.1 ± 1.00	4.9 ± 0.50	0.30 ± 0.10	26.0 ± 1.25
RH-T320	32.5 ± 1.20	34.2 ± 1.05	15.5 ± 1.15	63.4 ± 1.10	4.3 ± 0.40	0.29 ± 0.10	20.5 ± 1.10

. It is observed that the volatile content significantly decreased from 58.8 wt% (RH) to 32.5 wt% (RH-T320, RT30), as the torrefaction temperature increased from 200 to 320 °C, with residence times of 10, 20, and 30 min. Conversely, the fixed carbon content increased from 14.8 wt% (RH) to 34.2 wt% (RH-T320, RT30), and the ash content increased from 10.2 wt% (RH) to 14.0 wt% (RH-T320, RT30). The higher torrefaction temperature facilitated the thermal degradation of biomass [21], resulting in the release of more volatile substances.

The ultimate analysis of rice husk samples and torrefied biomass, presented in Table 2, was conducted with residence times of 10, 20, and 30 min and a temperature range of 200 to 320 °C. The analysis revealed that torrefied biomass retained a relatively higher carbon content compared to hydrogen and oxygen. The carbon content gradually increased from 38.9 wt% (RH) to 63.4 wt% (RH-T320, RT30) with the rising torrefaction temperature. Conversely, the oxygen content exhibited a sharp decrease from 36.5 wt% (RH) to 20.5 wt% (RH-T320, RT30). This reduction can be attributed to de-hydroxylation and decarboxylation reactions occurring during the hemicellulose degradation process [23].

3.1.2. Solid Yield and Higher Heating Value (HHV)

Figure 2A–C illustrates the mass loss experienced by rice husk samples during torrefaction, primarily resulting from the removal of water and volatile substances. The torrefaction process utilized a temperature range of 200 to 320 °C and residence times for heat exchange of 10, 20, and 30 min, significantly influencing the yield. The mass loss of the rice husk samples was notable, with a solid yield of 85.0 wt% observed at a torrefaction temperature of 200 °C and a residence time of 30 min. This outcome can be attributed to the temperature being less than the decomposition temperature of hemicellulose, the most unstable component in biomass. At 240 °C and a residence time of 30 min, the solid yield gradually decreased, resulting in a final yield of 82.0 wt%. However, in the higher temperature range of 280 to 320 °C with a residence time of 30 min, a significant mass loss was observed, and the final yield at 320 °C was only 76.0 wt%. This outcome was primarily due to the intensified thermal cracking reaction and the formation of volatile products at higher temperatures. The production of liquid and gaseous products showed a gradual increase with the torrefaction temperature [34].

Figure 2A–C and Table 2 show the experimental results for torrefied biomass from the rice husk biomass torrefaction process relying on gas circulating in a closed system. The experimental temperature range was 200–320 °C, and the residence times of rice husk biomass samples in the reactor were 10, 20, and 30 min. The yield of large amounts of torrefied biomass continued to decrease due to the decomposition and evaporation of compounds in rice husk biomass samples [35], resulting in a higher heating value of the torrefied biomass. However, these conditions increased the amount of carbon in the rice husk biomass during torrefaction [12]. The improvement of the thermal properties of torrefied biomass from the torrefaction process at 320 °C and residence time of 30 min by removing oxygen from 36.5 (RH) to 20.5 (RH-T320, RT30) wt% is shown in Table 2. The torrefaction process improved the higher heating value of the rice husk biomass sample, which increased from 15.65 (RH) to 23.69 (RH-T320, RT30) MJ/kg [32]. Furthermore, other researchers have obtained higher heating values of torrefaction within the range of 16.5–18.5 MJ/kg and at temperature ranges of 200–300 °C [13].

3.1.3. Equilibrium Moisture Content (EMC) Reabsorption Rate of the Experimental Samples

During torrefaction processes, heat exchange occurs and promotes the formation of hydrogen bonds among the constituents of rice husk biomass, such as hemicellulose and cellulose [23]. This interaction leads to increased resistance to hygroscopicity in the torrefied biomass [13]. The experimental results, depicted in Figure 3, demonstrate the impacts of temperature and residence times on heat exchange. Higher temperatures and longer residence times resulted in more significant heat exchange, further enhancing the properties of the biomass.

The equilibrium moisture content (EMC) of the torrefied biomass was found to be within the range of 0.589 to 1.140 wt%. At temperatures ranging from 200 to 300 °C, other research has obtained equilibrium moisture content of torrefied within the range of 2.34–3.60 wt% [12]. This low moisture content indicates that the torrefied biomass possesses excellent anti-hygroscopic properties [36]. With reduced moisture content, the biomass became more stable and less susceptible to moisture absorption. The findings highlight the effectiveness of the torrefaction process in altering the structure and composition of rice husk biomass, ultimately improving its thermal characteristics [32]. The formation of hydrogen bonds and the subsequent reduction in hygroscopicity contributed to the enhanced properties of the torrefied biomass. It is worth noting that further investigations could be conducted to explore the specific mechanisms and chemical changes occurring during torrefaction [36]. Understanding these processes can provide valuable insights into optimizing the torrefaction parameters and improving the overall efficiency of biomass conversion [12]. This low moisture content indicates that the torrefied biomass possesses excellent anti-hygroscopic properties [36]. With reduced moisture content, the biomass became more stable and less susceptible to moisture absorption. The findings emphasize the effectiveness of the torrefaction process in altering the structure and composition of rice husk biomass, ultimately improving its thermal characteristics [32]. The formation of hydrogen bonds and the subsequent reduction in hygroscopicity contributed to the enhanced properties of the torrefied biomass. It is worth noting that further investigations could be conducted to explore the specific mechanisms and chemical changes occurring during torrefaction [36]. Understanding these processes can provide valuable insights into optimizing the torrefaction parameters and improving the overall efficiency of biomass conversion.

Overall, the results indicate that torrefaction is a promising method for enhancing the quality of rice husk biomass. With reduced hygroscopicity and improved thermal characteristics, torrefied biomass holds significant potential for various applications in industries such as energy production and biomass utilization.

Figure 3 provides additional insights into the hygroscopic behavior of the torrefied biomass at higher temperatures, indicating a reduced hygroscopic impact due to slower hydrolysis. The slower hydrolysis process limits the absorption of moisture by the biomass, resulting in improved resistance to moisture uptake [12]. In Table 2, we observe the presence of volatile organic chemicals condensed within the pores of the torrefied carbon. This occurrence can be attributed to tar condensation within the pores, effectively blocking the passage of moist air through the solid biomass. Consequently, the condensed tar helps to prevent water vapor from condensing within the torrefied biomass [34]. These observations further emphasize the positive effects of torrefaction on the moisture-related properties of rice husk biomass. The reduced hygroscopic impact at higher temperatures and the presence of condensed volatile organic chemicals contribute to the biomass’s enhanced resistance to moisture absorption and condensation. By understanding these mechanisms, researchers can gain valuable insights into optimizing torrefaction processes to achieve specific goals, such as tailoring the biomass’s moisture content and improving its stability. This knowledge could be particularly relevant for applications in which low moisture content and resistance to moisture are crucial, such as energy production and storage. Continued research and exploration of the underlying chemical and physical changes during torrefaction will provide a more comprehensive understanding of these phenomena. Moreover, these insights could pave the way for further improvements in biomass conversion technologies and the development of innovative solutions for sustainable energy production.

The torrefaction process was investigated to assess its impact on the hygroscopicity reduction extent (HRE), as quantified by Equation (5). The experimental study focused on high temperatures and different residence times to examine the effects on moisture and volatile matter removal from the biomass. The results revealed that torrefaction led to significant removal of moisture and volatile matter [11].

Figure 4 shows the experimental results at various temperatures and residence times. Note especially examples T200-10 to T200-30, torrefied to reduce hygroscopicity by more than 90 wt%, and T240-10 to T240-30, torrefied to reduce hygroscopicity by 88 wt%, T280-10 to T280-30, torrefied to reduce hygroscopicity by approximately 87 wt%, and finally T320-10 to T320-30, torrefied to reduce hygroscopicity by 85 wt%. The experiments indicated that high process temperatures and long residence times affected the absorption of moisture. However, the hygroscopicity of the torrefied material was very low compared with that of rice husk biomass [13].

Higher temperatures and longer residence times were found to enhance the hygroscopicity reduction, suggesting that these conditions improve the biomass’s resistance to moisture absorption. By optimizing the torrefaction process parameters, it becomes possible to tailor the properties of the resulting torrefied biomass to meet specific requirements. This ability is particularly advantageous in applications in which low moisture content is desired, such as biomass power plants or industrial processes that rely on dry biomass feedstock [37].

Further exploration of the relationship between torrefaction conditions and hygroscopicity reduction can provide valuable insights for advancing biomass conversion technologies. Additionally, understanding the mechanisms underlying the structural changes could pave the way for the development of more efficient and sustainable biomass utilization methods [38].

Overall, these findings contribute to the expanding knowledge base on biomass conversion technologies and shed light on the potential of torrefaction to enhance the properties of biomass for renewable energy production. The quantification of HRE adds an important dimension to the characterization of torrefied biomass [12], empowering researchers and industry professionals to make well-informed decisions regarding its application in energy systems [11].

4. Conclusions

The properties of rice husk biomass were significantly improved through the application of torrefaction in a fixed-bed reactor, using gas instead of nitrogen, at temperatures ranging from 200 to 320 °C and residence times of 10, 20, and 30 min. This experimental study demonstrated several notable findings.

First, the higher heating value of both the rice husks and the torrefied biomass exhibited a significant enhancement, increasing from 15.65 to 23.69 MJ/kg. This outcome indicates that the torrefaction process effectively increased the energy content of the biomass.

Furthermore, the equilibrium moisture content (EMC) of the torrefied biomass ranged from 0.59 to 1.14 wt%, indicating a substantial reduction compared to untorrefied rice husk biomass. Torrefied biomass displayed remarkable resistance to hygroscopicity, with a reduction of 85 to 92 wt% compared to its untreated counterpart. This finding suggests that torrefied biomass is less susceptible to moisture absorption, making it more stable and suitable for long-term storage.

The approximate analysis and elemental analysis conducted in this study yielded results consistent with previous research, validating the accuracy of the findings. These analyses provide valuable information regarding the composition and characteristics of torrefied biomass.

Overall, this study demonstrated the effectiveness of the employed torrefaction method in enhancing the higher heating value properties of rice husk biomass. The improved energy content and reduced hygroscopicity make torrefied biomass a suitable fuel for biomass power plants. These findings could contribute to the development of biomass energy technologies and the utilization of rice husk biomass as a sustainable energy source.