Environmental and energy performance of cotton stalk residues for sustainable bioenergy production

Abstract

This study evaluates the energy and environmental performance of cotton stalk residues as a process-level solid bioenergy resource using a gate-to-gate analytical framework. Briquettes were produced under varying particle sizes (≤ 0.7–3 mm), compaction pressures (3–16 MPa), and moisture contents (6–15%). The characterization included density, total specific energy consumption (SEC_T), calorific value, ash content, thermogravimetric behavior, and combustion emissions (CO₂, CO, SO₂, and NO_x). To quantify the trade-off between densification energy demand and emission-related outputs, two composite indicators, the Energy Performance Index (EPI) and the Environmental Impact Index for Energy (EIIE) were applied and interpreted strictly within a process-level context. The results showed that high briquette density (up to 1180 kg·m⁻³), a gross calorific value of 18.64 MJ·kg⁻¹, and low ash content (3.4%) can be achieved under suitable compaction conditions, while combustion emissions were dominated by CO₂ with comparatively low levels of other pollutants. Optimal energy and environmental performance at the process level was observed at moderate moisture contents and coarser particle sizes, reflecting a balance between mechanical energy input and fuel quality. Overall, the findings indicate that cotton stalk residues can be converted into operationally efficient solid biofuels with favorable process-level emission characteristics, supporting their valorization in waste-to-energy applications. The study further presents a novel process-level integrated assessment that quantitatively links densification energy demand, fuel energy output, and combustion-related emissions through composite performance indicators (EPI and EIIE).

Introduction

Agricultural biomass residues represent an important renewable resource for low-carbon energy production while mitigating environmental impacts associated with conventional waste disposal. Cotton stalks are generated in large quantities after each harvest season and remain among the most underutilized residues in cotton-producing regions. Common disposal practices, such as open field burning, contribute to air pollution, greenhouse gas emissions, and the loss of recoverable biomass resources¹. Converting cotton stalk residues into solid biofuels through briquetting provides an effective waste-to-energy pathway by reducing volume, improving handling, and enhancing fuel quality. Raw cotton stalks are limited by low bulk density, fibrous structure, and high moisture retention, which increase transportation and processing costs^2,3. Briquetting overcomes these constraints by producing dense, mechanically stable fuels with improved combustion characteristics, with reported calorific values exceeding 3800 kcal·kg⁻¹^4–6.

Recent research highlights that meaningful assessment of agricultural biofuels requires integrated consideration of mechanical behavior, thermal characteristics, and combustion emissions. In this study, environmental performance is defined at the process level, focusing on operational emissions and energy efficiency during densification and combustion. This performance is quantified using the Environmental Impact Index for Energy (EIIE). Optimization of particle size, moisture content, and compaction pressure has been shown to improve briquette durability, reduce energy demand, and limit pollutant emissions^7,8.

Mechanical behavior during densification governs particle rearrangement, frictional resistance, and deformation, thereby directly affecting briquette integrity and total specific energy consumption (SEC_T)^9,10. Thermal properties, including ash content, ignition behavior, burnout characteristics, and gaseous emissions, further determine fuel performance and environmental acceptability¹¹. Understanding the coupled influence of processing parameters on both densification energy demand and combustion behavior is therefore essential for process optimization.

Although several studies have investigated individual aspects of agricultural residue briquetting, relatively few have integrated mechanical energy consumption with combustion emission performance for cotton stalk residues within a unified waste-to-energy framework^12–14. Recent high-impact studies on biomass briquetting have primarily focused on density optimization, calorific value, or emission characterization in isolation, without explicitly linking densification energy demand to emission-related performance metrics within a unified quantitative framework^15–17. Consequently, the combined assessment of energy input, fuel energy output, and emission behavior remains insufficiently explored for cotton stalk briquettes. This gap is addressed in the present study through a novel process-level integrated assessment that quantitatively links densification energy demand, fuel energy output, and combustion-related emissions, allowing a more comprehensive interpretation of mechanical, thermal, energy, and environmental performance beyond single-parameter evaluation.

To enable such integrated evaluation, composite performance indicators are employed. The Energy Performance Index (EPI) relates useful thermal energy output to the mechanical energy input required for briquetting, while the Environmental Impact Index for Energy (EIIE) integrates energy content, specific energy consumption during compaction, and pollutant emissions. By combining these indices, briquetting conditions can be consistently compared and ranked while simultaneously accounting for energy efficiency and emission behavior, which cannot be captured by conventional single-parameter indicators. Thus, the objective of this study is to evaluate the mechanical, thermal, energy, and emission performance of cotton stalk briquettes produced under different particle sizes, moisture contents, and compaction pressures, using a process-level integrated framework, to support their valorization as renewable solid biofuels within waste-to-energy systems.

Materials and methods

This study followed a structured methodology to evaluate cotton stalk residues as a potential feedstock for waste-to-energy applications. The experimental procedures included raw material preparation, mechanical testing, briquette formation, and thermal analysis under standardized conditions, as illustrated in Fig. 1.

Fig. 1.

Preparation, pre-processing steps and the mechanical and thermal tests of cotton stalk residues for waste-to-energy conversion.

Cotton stalk sample preparation

Cotton stalk residues used in this study were obtained from the Agricultural Research Center farm. The material was left in the field for seven days for natural drying, then processed through a rotary mower (1.2 m width, 1800 rpm) to facilitate uniform size reduction. After mowing, the material was baled using a reciprocating baler (Rate: 90 strokes·min⁻¹, chamber size: 460 × 360 mm) to ease handling and transport. Subsequently, the bales were shredded using a Vermeer drum-type shredder (flywheel diameter: 500 mm, cutter speed: 2400 rpm) for coarse disintegration. The shredded material was then further ground using a Wiley Cutting Mill (model: Thomas Scientific, USA) operating at 990 rpm and equipped with eight hammers, producing finely ground biomass suitable for compaction.

To ensure a comprehensive analysis of particle size influence, as requested by preliminary assessments, the ground material was sieved into three particle size fractions < 0.7 mm, 0.7–1.4 mm, and 1.4–3.0 mm^9,10. Samples were conditioned to three moisture levels: 6%, 10%, and 20% (dry basis). Moisture adjustment was conducted via controlled water spraying or ambient drying, followed by sealing the samples in airtight containers for 24 h to ensure uniform distribution¹⁸.

Life cycle thinking framework and system boundary

This study adopts a life cycle thinking approach to evaluate the environmental and energy performance of cotton stalk residues when valorized as solid biofuel briquettes. Rather than conducting a full ISO-based life cycle assessment, the analysis is deliberately limited to a gate-to-gate, process-level evaluation that focuses on the most influential stages of the biomass utilization chain directly affecting energy efficiency, emissions, and production cost.

The study explicitly addresses the operational stages of cotton stalk valorization, including mechanical preprocessing, densification, and laboratory-scale combustion. These stages represent the most energy- and emission-intensive phases of the process. Upstream agricultural inputs, infrastructure-related impacts, and downstream ash management are intentionally excluded. Cotton stalks are treated as an agricultural by-product with no additional cultivation burden, and the study emphasizes practical process-level performance rather than comprehensive system-wide sustainability. By limiting the system boundary to these stages, the analysis provides insight into operational energy demand and emission performance without implying a full cradle-to-grave life cycle assessment. Consequently, all results should be interpreted strictly as process-level indicators. To account for the implications of this boundary definition, a qualitative sensitivity rationale was considered. The exclusion of upstream agricultural stages is not expected to influence the comparative evaluation of briquetting conditions, as these stages represent fixed background processes independent of particle size, moisture content, or compaction pressure. Similarly, downstream stages such as ash handling and infrastructure-related impacts would contribute marginally relative to the dominant mechanical energy demand during densification and the quantified combustion emissions. Including these stages would increase absolute environmental burdens, but the relative trends, rankings, and optimization conclusions derived from the Energy Performance Index (EPI) and Environmental Impact Index for Energy (EIIE) are expected to remain unchanged. This boundary selection reflects a pragmatic life cycle thinking perspective, emphasizing energy- and emission-relevant operational stages within agricultural residue-based bioenergy processing chains and ensuring that environmental and energy performance assessments remain strictly process-level.

Mechanical properties of ground cotton stalk

Dynamic friction coefficient (µ_d)

The dynamic friction coefficient between ground cotton stalk and structural surfaces was measured using a friction test setup (Fig. 2) involving a horizontal belt and variable contact surfaces, following the method described by Mohsenin¹⁰.

Fig. 2.

Setup for measuring the dynamic friction force: (1) Sliding surface, (2) Plate carriage, (3) Calibration masses, (4) Hollow cylindrical, (5) Powder material, (6) Roller wheels, (7) Hook, (8) Hand wheel, (9) Test stand, (10) Small pulley, (11) Wire, (12) Force gauge, and (13). Sliding ruler.

The coefficient was calculated based on the force required to initiate motion between the biomass and test surface at constant velocity using Eq. (1).

1

Where µ_d is the dynamic coefficient of friction, F_f is the friction force (F_f =F_T -F_E), N; F_T is force needed to get a filled cylindrical container moving, N; F_E is force needed to get an empty cylindrical container moving, N; and N_l is the normal load pressing the sample to the contact surface, N. Experimental procedures were carried out under three distinct moisture contents (6%, 10%, and 20% on a dry basis), with applied vertical loads ranging between 2 and 9 N.

The experiments were performed on two surface types of stainless steel and plain steel. The apparatus consisted of a horizontally mobile carriage equipped with roller wheels, on which a hollow cylindrical container (3.2 cm inner diameter) was mounted. The carriage dimensions were 26 × 13.5 × 4.5 cm. The sample-filled container was placed 1 mm above a metal sheet (30 × 50 cm) to ensure uniform surface contact. A digital force gauge (MARK-10, Model M3-10, USA) was employed to measure the horizontal force necessary to initiate sliding. The friction force was determined by subtracting the force needed to move the empty container from that needed to move the filled one. This coefficient is essential in evaluating the flowability of biomass during feeding and compaction operations in densification systems. Reducing friction can improve material handling and decrease mechanical energy losses in waste-to-energy applications.

Internal friction coefficient

To evaluate the internal friction coefficient of ground cotton stalk material, a direct shear cell device was employed. This apparatus consisted of two cylindrical segments, an upper and a lower part enclosed by a surrounding structure. Test material was placed into a hollow cylindrical container with dimensions of 50 mm in outer diameter, 40 mm in inner diameter, and a height of 24 mm. During testing, the container was subjected to both vertical loading and horizontal shearing to simulate frictional interaction. The full configuration of the apparatus is shown in Fig. 3.

Fig. 3.

Apparatus used for determining internal friction properties of ground cotton stalk: (A) Fully assembled dual-cylinder shear apparatus; (B) Disassembled view showing components: (1) Horizontal pulling mechanism connected to a digital force gauge via wire, (2) Upper open-ended cylinder, (3) Calibrated weights for exerting vertical (normal) loads, (4) Central sample container, and (5) Lower cylinder with a closed base.

Measurements were conducted under different moisture levels of 6%, 10%, and 20% on a dry basis. Normal forces applied during testing ranged between 2 N and 9 N. The frictional response was recorded as the shear force (F), while the corresponding normal load (W) was applied vertically. The relationship between the measured friction force and normal force was analyzed using a linear regression model, from which the internal coefficient of friction was derived. This relationship follows the form expressed in Eq. 2¹⁰.

2

Where F represents the friction force, N; W is the normal force to the surface of contact, N; m is the slope of the line (internal coefficient of friction) and b is the cohesion force, N.

Compression behavior and densification energy

The compaction behavior of ground cotton stalk was evaluated using a custom-designed briquetting system composed of three main parts: a vertical compression ram, a cylindrical die chamber, and a base plate (Fig. 4). A hydraulic universal testing machine (AUTOGRAPH) with a 1000 kN capacity was used to apply axial force and measure real-time compression data. The die had an internal diameter of 63 mm and a height of 170 mm, with a radial clearance of 0.2 mm to allow air escape during loading.

Fig. 4.

Compression setup for ground cotton stalk. (A) Assembled briquetting die; (B) Individual components of the die system; (C) Computerized compression system: (1) Piston rod; (2) Compression ram; (3) Die cavity; (4) Base plate; (5) Ground cotton stalk sample.

Samples of three particle sizes (< 0.7, 0.7–1.4, and 1.4–3.0 mm) at two moisture contents (6% and 15% on a dry basis) were used. For each test, a sample mass of 70 g of material was measured manually using an analytical and loaded into the die. Compression was applied at a constant crosshead speed of 60 mm·min⁻¹, and target forces of 10, 20, 30, 40, and 50 kN were tested. Axial force and ram displacement were recorded continuously.

The particle density (ρ_P) was determined by the direct method in accordance with Mohsenin¹⁰, as shown in Eq. 3.

3

Where is the particle density, kg·m⁻³; P_m is the briquette particle mass, kg; V_P is the briquette particle volume = , m³; d is the briquette diameter, m; and L is the briquette length, m. The ρ_P relationship during the cotton stalk compaction was obtained by recording instantaneous axial load values, die diameter, and material deformation during compression using the hydraulic press control system.

To calculate the total energy consumption (SEC_T) associated with biomass densification, the mechanical operations previously performed on the cotton stalk residues namely rotary mowing, baling, shredding with a Vermeer drum cutter, and grinding using a Wiley Cutting Mill were considered as pre-processing steps. These operations, carried out after field drying, were essential to produce ground material suitable for compaction and thermal analysis. The total specific energy consumption (SEC_T) was estimated by summing the compaction energy derived from the force–displacement curve and the specific energy of pre-processing stages. To evaluate the overall energy demand during biomass densification, the SEC_T was estimated as the sum of energy inputs from pre-processing operations (SEC_P) and the mechanical energy required for compaction. The compaction energy component was calculated by integrating the force–displacement curve, reflecting the actual work input during densification, and is a critical metric for process efficiency. The SEC_T was determined using a modified version of the method described by Shinners and Friede¹⁹, expressed in Eq. (4).

4

Where SEC_T is the total specific energy consumption of compaction, kJ.kg⁻¹; F is the applied axial force (load), N; y is the material deformation, mm; and SEC_p is the specific energy consumed during pre-processing, kJ.kg⁻¹, which calculated as the sum of energy consumed during rotary cutting, baling, shredding, and milling, as expressed in Eq. (5).

5

Where EC_r is the consumed energy during rotary cutting, kJ·kg⁻¹; EC_b is the consumed energy during baling, kJ·kg⁻¹; EC_s is the consumed energy during shredding, kJ·kg⁻¹; and EC_m is the consumed energy during milling, kJ·kg⁻¹.

Each individual component (EC_r, EC_b, EC_s, and EC_m) was calculated using Eq. (6)²⁰ based on measured fuel consumption for each respective pre-processing operation.

6

Where EC is the consumed energy, kJ·kg⁻¹; V_f is the rate of fuel consumption, L.h^-1; CV is the calorific value of the fuel, ≈ 35 MJ·L⁻¹; and Mp is the machine productivity, kg·h⁻¹. The energy consumed during sieving (EC_g) was excluded from detailed SEC_p calculations due to its minimal energy consumption and reliance on low-power electric motors. However, its contribution was approximated as an average value of 2.5 kJ·kg⁻¹, based on reported energy demands for laboratory-scale sieve analysis of biomass materials²¹.

The compression ratio (CR) was used to quantify volume reduction because of compaction. It was defined as the ratio of the particle density of the compacted briquette to the bulk density of the raw loose material, as expressed in Eq. 7²².

7

Where CR is compression ratio; ρ_p is particle density of cotton stalk briquettes, kg·m⁻³; and ρ_b is bulk density of loose ground cotton stalk, kg·m⁻³.

Thermal properties of cotton stalk briquette

The thermal properties of cotton stalk briquettes were evaluated to assess their suitability as solid biofuel in waste-to-energy applications. Tests included calorific value, ash content, flue gas emissions, and thermogravimetric analysis, providing insights into combustion quality, energy efficiency, and environmental impact. All samples were stored in airtight containers at room temperature to maintain moisture and prevent degradation.

Calorific value of cotton stalk briquette (CV)

The calorific value (CV) of cotton stalk briquettes was measured to evaluate their energy potential as solid biofuel. The test followed ASTM D2015-96 using a bomb calorimeter (Model CAB003; Fisons Scientific Equipment, UK) calibrated with benzoic acid tablets. Approximately 1 g of finely ground sample was combusted in a high-pressure oxygen atmosphere, and the resulting temperature rise (ΔT) was recorded. The calorific value, representing the total thermal energy released including the latent heat of vaporization, was then calculated using Eq. (8)²³.

8

Where CV is calorific value of material, MJ·kg⁻¹; W_eq is equivalent weight of the system, 0.0105 MJ·^oC⁻¹; ΔT is change in temperature, ^oC; and W_s is weight of sample, kg.

Ash content of cotton stalk briquette (AC)

The ash content (AC) of cotton stalk briquettes was measured to determine the inorganic residue remaining after complete combustion, an indicator of combustion efficiency and fuel cleanliness. Following ASTM D3174-97, about 1 g of finely ground briquette was placed in a pre-weighed porcelain crucible and combusted in a muffle furnace (Thermo Scientific) at 575 ± 25 °C for at least four hours. After incineration, the crucibles were cooled in a desiccator and reweighed until a constant mass was obtained. A controlled multi-stage heating protocol was used to ensure complete oxidation of volatiles, involving gradual temperature increases from ambient to 575 °C. The ash content was then calculated as a percentage of the original dry sample mass using Eq. 9²⁴.

9

Where AC is the ash content, %. Lower ash content is generally associated with higher combustion efficiency and reduced post-combustion handling requirements, making it a desirable trait for biomass fuels intended for thermal energy recovery.

Flue gas emissions of cotton stalk briquette

Flue gas emissions were analyzed to evaluate the environmental impact of cotton stalk briquette combustion. Tests were performed in a controlled, custom-designed stove (Fig. 5) using a multi-gas analyzer (MRU DELTA 1600 V; Germany) placed 1 m downstream of the boiler outlet. Major emissions measured included O₂ (%), CO₂ (%), CO (ppm), NO and NOₓ (ppm), and SO₂ (ppm). The probe was positioned at the flue center for representative sampling, with three readings taken at one-minute intervals per trial.

Fig. 5.

The briquetting stove. (1) fuel chamber, (2) viewing door, (3) air inlet, (4) exhaust pipe, and (5) emission measurement probe.

The combustion chamber was cleaned between tests to avoid interference. Combustion tests were conducted under controlled conditions with an airflow rate of 25 L·min⁻¹, an excess air ratio of 1.2, and a combustion temperature range maintained between 550 and 650 °C. These parameters were continuously monitored to ensure stable and representative combustion conditions for all briquette samples.

Thermo gravimetric analysis (TGA) of cotton stalk briquette

Thermal stability and combustion behavior of the cotton stalk briquettes were further examined through thermogravimetric analysis (TGA), which provides insight into mass loss patterns and decomposition stages under controlled heating. The analysis was carried out using a DTG-60 H thermobalance (Shimadzu; Japan), equipped with a precision-controlled furnace and a nitrogen-purged atmosphere. ~10 mg of finely ground briquette sample was placed in an alumina crucible and heated from ambient temperature to 800 °C at a constant rate of 10 °C·min⁻¹. A continuous nitrogen flow of 50 ml·min⁻¹ was maintained to ensure inert conditions throughout the test. The real-time mass loss was recorded to identify distinct phases of thermal degradation, including moisture evaporation, volatile release, and fixed carbon combustion²⁵. This analysis supports the understanding of the combustion kinetics and thermal decomposition profile of the briquetted biomass, which are essential for optimizing its use as a reliable and efficient solid fuel in waste-to-energy systems.

Energy and environmental performance indicator

Based on a life cycle thinking framework and the defined system boundary, composite performance indicators were developed to integrate energy input, fuel output, and combustion-related emissions into a unified evaluation of cotton stalk briquettes.

Energy Performance Index (EPI)

To complement the thermal characterization of cotton stalk briquettes, an energy performance index (EPI) was calculated to evaluate the overall energy efficiency of the densification-to-combustion process. This index relates the calorific value (CV) of the briquettes to the total specific energy consumption (SEC_T) required during compaction, offering a practical assessment of energy recovery efficiency. The EPI was calculated from Eq. 10.

10

Where EPI is energy performance index; CV is calorific value, kJ·kg⁻¹; and SEC_T is the total specific energy, kJ.kg^-1.

Environmental Impact Index for Energy (EIIE)

To evaluate the relative process-level environmental performance of cotton stalk briquettes as a renewable fuel, an Environmental Impact Index for Energy (EIIE) was developed. This composite metric integrates energy output, mechanical input and combustion-related emissions for comparative evaluation of different densification conditions within a defined operational system boundary. EIIE is defined by Eq. 11.

11

Where EIIE is the environmental impact index for energy; CV is calorific value, kJ·kg⁻¹; SEC_T is specific energy consumption, kJ.kg⁻¹; and EI is the emission index calculated as a screening-level weighted sum of measured flue gas concentrations (Eq. 12).

12

Emission concentrations are expressed in ppm, and α, β, and γ are dimensionless weighting factors applied to maintain internal consistency and enable relative comparison among experimental conditions. The selected values (α = 1.0, β = 1.5, and γ = 2.0) reflect the comparative relevance of CO, NOₓ, and SO₂ under identical combustion settings. The numerical values assigned to the weighting factors (α = 1.0, β = 1.5, and γ = 2.0) are based on a relative severity rationale commonly applied in screening-level environmental indicators rather than formal life cycle normalization. These weights reflect comparative regulatory relevance, health impact potential, and atmospheric persistence of the measured pollutants under identical combustion conditions. SO₂ was assigned the highest weight due to its strong acidification potential and regulatory stringency, followed by NOₓ because of its role in smog formation and secondary particulate matter, while CO was assigned a baseline weight as an indicator of incomplete combustion rather than a primary environmental stressor. Similar relative weighting approaches have been employed in process-level emission indices and operational environmental performance assessments to support internal comparison and ranking of operating conditions without implying absolute environmental burdens. This weighting approach is intended to support internal ranking of briquetting conditions and does not represent an absolute environmental impact assessment or full life cycle normalization^26,27. In addition to their process-level comparative relevance, the SEC_T, EPI, and EIIE metrics. SEC_T represents the mechanical energy required for densification and serves as a proxy for production cost per kilogram of briquettes. The EPI reflects energy productivity by linking fuel energy yield to the mechanical energy invested during compaction, indicating the operational efficiency of the densification process. Similarly, the EIIE integrates calorific value, energy input, and emissions into a single comparative indicator that captures environmental trade-offs relevant to sustainable competitive bioenergy systems.

Statistical analysis

Statistical analyses were performed to evaluate the effects of moisture content, particle size, and applied compression force on the mechanical and thermal behavior of ground cotton stalk residues. Both one-way and two-way analyses of variance (ANOVA) were employed to investigate the influence of individual factors and their possible interactions on the measured parameters. Where significant differences were detected (p < 0.05), Tukey’s honestly significant difference (HSD) test was conducted for pairwise comparisons of treatment means, particularly for compressibility index and specific energy consumption values. The level of statistical significance was consistently reported in the results section to support the interpretation of trends observed in internal and rolling friction behavior, compaction efficiency, energy requirements, and flue gas emission characteristics.

Results

Particle size distribution and bulk density

Table 1 presents the particle size distribution and bulk density of ground cotton stalk material. The distribution was strongly skewed toward finer fractions, which influences inter-particle bonding and energy consumption during compaction. Particles smaller than 0.7 mm constituted the majority (48.3%), followed by intermediate particles (0.7–1.4 mm) at 41.1%, while coarse particles (1.4–3.0 mm) represented only 10.6% of the total mass.

Table 1.

Particle size distribution and bulk density of ground cotton stalk.

Particle size, mm	< 0.7	0.7–1.4	1.4–3.4
Particle size percentage, %	48.3	41.1	10.6
Bulk density, kg·m− 3		287

The dominance of fine and intermediate particles suggests favorable packing characteristics and surface interactions during processing. Finer particles typically reduce inter-particle resistance and promote smoother flow, while intermediate sizes offer a balance between flowability and mechanical interlocking. Such a composition is beneficial for densification and enhances compaction consistency, which are crucial for improving the mechanical strength and energy yield of briquetted biomass in waste-to-energy systems. The bulk density of the ground material was recorded at 287 kg·m⁻³, representing an average value measured for the combined particle size distribution and supporting its suitability for efficient compaction.

Mechanical properties of cotton stalk

Dynamic frictional behavior of ground cotton stalk

Figure 6 presents the frictional behavior of ground cotton stalk against stainless steel (SS) and carbon steel (St) surfaces under varying moisture contents and normal loads. Results are displayed as friction force (Fig. 6a) and dynamic friction coefficient (µ_d, Fig. 6b), providing insight into tribological performance under practical handling conditions.

Fig. 6.

Effect of load and moisture content on friction force and dynamic friction coefficient of ground cotton stalk.

Both friction force and µ_d increased with higher moisture content and applied load. Steel showed greater friction than stainless steel due to its rougher texture and higher water retention. At 6% MC, µ_d was 0.60 (SS) and 0.83 (St), rising to 0.86 and 1.08 at 20% MC, indicating moisture-induced adhesion and increased contact area. This behavior reflects water film formation and fiber softening, especially on rough steel surfaces. Two-way ANOVA confirmed significant effects of moisture and surface type (p < 0.01) and their interaction (p < 0.05). Tukey’s test showed higher values at 20% MC (p < 0.05), with St consistently giving greater resistance.

Internal coefficient of friction for ground cotton stalk

Figure 7 illustrates the internal frictional behavior of ground cotton stalk under varying normal loads at moisture contents of 6%, 10%, and 20% (dry basis). Figure 7a displays the linear relationship between normal force (W) and shear force (F), while Fig. 7b presents the derived parameters: internal coefficient of friction (m) and cohesion (b). Across all moisture levels, a clear linear increase in friction force was observed with rising normal load. At 6% moisture content, friction force ranged from 0.98 N (2 N normal load) to 4.68 N (9 N normal load). Increasing the moisture to 10% resulted in 1.15–5.36 N, while at 20% MC, the maximum reached 5.83 N.

Fig. 7.

Internal friction behavior of ground cotton stalk under varying normal loads and moisture contents.

The linear model F = mW + b (Eq. 2) effectively described the data, enabling accurate estimation of internal friction and cohesion. As shown in Fig. 7b, the coefficient of internal friction increased from 0.57 at 6% MC to 0.77 at 20% MC an approximate 35% rise. In contrast, cohesion values declined from 1.40 N at 6% MC to 0.60 N at 20% MC, representing a 57% reduction.

Statistical analysis using one-way ANOVA confirmed a significant effect of moisture content on both m and b (p < 0.01). Tukey’s HSD test revealed significant differences across all moisture levels (p < 0.05), with the internal friction coefficient at 20% MC significantly higher than at 6% and 10%, and the corresponding cohesion significantly lower. The linear regression models used to derive these parameters achieved coefficients of determination (R²) above 0.96, validating the robustness of the applied model.

Compression–density relationship

Figure 8 presents the variation in particle density (ρ_P) of cotton stalk briquettes as a function of particle size, moisture content, and applied compression load. The results reveal a consistent increase in particle density with higher compaction pressures across all treatments, confirming a strong correlation between external force and biomass consolidation. Among the three particle sizes examined (< 0.7, 0.7–1.4, and 1.4–3.0 mm), the finest particles (< 0.7 mm) consistently produced the densest briquettes. At a compression force of 50 kN and 6% moisture content, samples with < 0.7 mm particles achieved an average particle density of 1152 ± 21 kg·m⁻³, significantly higher (p < 0.01) than those composed of 3.0 mm particles, which averaged 974 ± 26 kg·m⁻³.

Fig. 8.

Effect of pressure, moisture, and particle size on particle density of briquette.

Statistical analysis using two-way ANOVA confirmed that both particle size and moisture content had significant effects on particle density (p < 0.05), with an interaction effect also detected (p = 0.03). According to the Tukey test, the increase in density between 6% and 15% moisture content was more pronounced in finer particles. At 30 kN, increasing the moisture content from 6% to 15% led to a 12.7% increase in ρ_P for < 0.7 mm particles, while the same increase was only 6.5% for 1.4–3.0 mm particles.

In general, higher compaction pressures, finer particle sizes, and moderate moisture contents promoted greater packing efficiency and structural integrity in the briquettes. Beyond 15% moisture, density gains became marginal, suggesting a mechanical limit of compaction. Optimal densification was observed for samples below 0.7 mm particle size and 15% moisture content compressed at 50 kN.

Total specific energy consumption (SEC_T)

Figure 9 presents the compression behavior and total specific energy consumption (SEC_T) of cotton stalk briquettes under varying particle sizes and moisture contents. The load–deformation curve (Fig. 9a) characterizes the mechanical response during axial compaction. Initial deformation increased slightly from 55 mm at 10 kN to 57 mm at 20 kN, followed by a sharper rise, reaching 50 kN at 58 mm. This trend reflects the typical densification stages, including particle rearrangement at low resistance, followed by increased compaction with higher internal friction and deformation.

Fig. 9.

(a) Load-deformation, and (b) Total specific energy consumption under varying pressure, moisture, and particle size.

Figure 9b shows the variation of SEC_T with compaction pressure, particle size, and moisture content. A strong linear trend was observed between applied pressure and SEC_T, with values increasing from approximately 208.1 kJ·kg⁻¹ at 3 MPa to 242.4 kJ·kg⁻¹ at 16 MPa. This confirms that compaction pressure is the dominant factor influencing energy demand. Particle size also had a measurable effect: finer particles (< 0.7 mm) required slightly higher energy than medium (0.7–1.4 mm) and coarse (1.4–3.0 mm) particles at all pressure levels.

Moisture content affected SEC_T as well. Briquettes with 6% MC showed slightly higher energy consumption than those at 15% MC, regardless of particle size. For example, at 10 MPa, SEC_T for fine particles was 225.51 kJ·kg⁻¹ at 6% MC and 225.46 kJ·kg⁻¹ at 15% MC. Two-way ANOVA confirmed significant effects of pressure and particle size (p < 0.01), whereas the interaction between particle size and moisture content was not significant (p > 0.05). The data indicates that increasing pressure consistently elevates SEC_T, while moderate moisture and coarser particle sizes slightly reduce the total energy requirement.

Compression ratio (CR)

The compression ratio (CR) quantifies the volume reduction of biomass during densification and serves as an indicator of material compressibility and packing behavior. Figure 10 presents the variation in CR of cotton stalk briquettes at different particle sizes (0.7 mm, 1.4 mm, and 3.0 mm) and two moisture content levels (6% and 15% dry basis). The results clearly show that both moisture content and particle size significantly affected CR values.

Fig. 10.

Compression ratio of ground cotton stalk as a function of particle size, pressure, and moisture content.

At 6% moisture, CR ranged between 2.06 and 2.32, whereas at 15% moisture, it increased to 2.42–2.79. The finest fraction (< 0.7 mm) exhibited the highest CR values at both moisture levels, reaching a maximum of 2.79 at 15% MC, while the coarsest fraction (3.0 mm) showed the lowest values. Two-way ANOVA confirmed that moisture content and particle size had significant effects on CR (p < 0.01), with a notable interaction effect (p < 0.05).

Tukey’s post hoc test indicated that the differences in CR between 0.7 mm and 1.4 mm particle sizes were not significant at 6% MC but became significant at 15% MC. In general, CR values increased with finer particle sizes and higher moisture contents, reflecting improved compactibility under these conditions.

Thermal properties of briquette cotton stalks

Calorific value (CV)

The gross calorific value (CV) represents a key indicator of the energy potential of briquetted cotton stalks. The measurements conducted under dry conditions and without binders yielded a mean value of 16.48 ± 0.17 MJ·kg⁻¹, indicating a relatively high energy density among agricultural residues. The test followed the ASTM D2015-96 standard using a bomb calorimeter, ensuring the inclusion of latent heat of vaporization in the total energy released.

This result demonstrates that the briquetted cotton stalks possess consistent and reproducible thermal properties suitable for solid biofuel applications. The uniformity of calorific values across the tested samples reflects effective densification and homogeneity of the material. No significant variation (p > 0.05) was observed among replicate samples, confirming the stability of thermal performance. The calorific value achieved in this study surpasses the threshold of 15 MJ·kg⁻¹, which is generally considered adequate for efficient combustion in small- and medium-scale thermal energy systems. Combined with mechanical compactness, these results indicate that briquettes can provide stable heat output and efficient energy recovery during combustion.

Ash content (AC)

The ash content (AC) of briquetted cotton stalks was determined to evaluate combustion cleanliness and post-combustion residue formation. The mean ash content was 3.4 ± 0.2%, based on oven-dry samples tested according to ASTM D3174-97. The combustion of the samples in a muffle furnace at 575 ± 25 °C for at least four hours ensured complete oxidation of volatile compounds before the measurement of residual inorganic matter.

The results indicate a consistently low ash yield across all tested samples, confirming stable combustion behavior and minimal mineral residue. Statistical analysis showed no significant variation (p > 0.05) between replicates. This low AC value suggests that cotton stalk briquettes produce clean combustion with minimal fouling, reducing ash-related operational issues in thermal conversion systems.

Flue gas emissions of cotton stalk

Figure 11 presents the time-resolved variation of major flue gas components during the combustion of briquetted cotton stalks over a continuous 720-second period. The monitored gases included oxygen (O₂), carbon dioxide (CO₂), carbon monoxide (CO), sulfur dioxide (SO₂), nitric oxide (NO), and total nitrogen oxides (NOₓ).

Fig. 11.

Flue gas emission values of cotton stalk briquette.

The temporal evolution of these gases provides clear insight into the combustion stages and oxidation dynamics of the biomass fuel. During the ignition phase, oxygen concentrations decreased sharply from 4.5% to 2.3%, indicating rapid oxygen consumption associated with the oxidation of volatile matter released from the fuel surface. Simultaneously, CO concentrations increased from approximately 100 ppm to 310 ppm, while CO₂ levels peaked at 10.2%, signifying intense oxidative activity and high reactivity of volatile compounds. These trends are typical for the early combustion of herbaceous biomass, which is characterized by a high proportion of easily vaporized organic fractions.

As the process transitioned to the main combustion phase, between approximately 60 and 480 s, the gas concentrations exhibited relative stability, reflecting steady-state oxidation of fixed carbon. Oxygen levels gradually recovered from 2.3% to 4.4%, indicating an improved balance between oxygen supply and fuel oxidation rate. CO₂ concentrations remained within a narrow range of 8.5% to 9.0%, while CO concentrations decreased from 310 ppm to 185 ppm, confirming a gradual enhancement in combustion completeness as the fuel bed stabilized. Statistical analysis revealed a significant negative correlation between CO and O₂ (p < 0.05, R² = 0.88), confirming the inverse relationship between oxygen availability and incomplete combustion products. Sulfur dioxide emissions remained relatively moderate and exhibited a consistent decline over time. Initial SO₂ levels of approximately 40 ppm dropped progressively to 20 ppm by the end of the combustion period, with no observable secondary peaks during the burnout stage. This steady decline indicates that sulfur oxidation primarily occurred during the initial heating and devolatilization phases, and that subsequent sulfur release from mineral-bound forms was limited.

In the final burnout stage, spanning roughly 540 to 720 s, the gas composition shifted markedly toward oxygen-rich conditions. Oxygen concentrations increased sharply from 5.5% to 17.5%, reflecting the depletion of combustible material and the cessation of significant oxidation reactions. At the same time, CO₂ concentrations decreased from 7.0% to 0.9%, and CO concentrations declined from 140 ppm to 40 ppm, indicating the near completion of carbon oxidation. Regression analysis for this final phase again revealed a strong inverse correlation between O₂ and CO (R² > 0.91, p < 0.01), confirming the high efficiency of combustion termination.

Nitrogen oxide emissions displayed relatively stable behavior throughout the entire combustion cycle. NO concentrations decreased from 85 ppm to 72 ppm, while total NOₓ values dropped from 53 ppm to 46 ppm. The close correspondence between these two measures suggests limited secondary oxidation of NO to NO₂, implying moderate combustion temperatures and effective oxygen control. Although flue gas temperature was not directly monitored in this study, the observed emission trends particularly the stability of NOₓ levels and the strong inverse O₂–CO relationship indirectly reflect controlled combustion conditions and moderate thermal behavior.

Overall, the flue gas data clearly define three combustion stages that follow a continuous sequence of ignition, steady oxidation, and burnout. Each phase was characterized by distinct oxygen and carbon dynamics, reflecting the progressive conversion of volatile and fixed carbon fractions within the fuel matrix. At the end of the combustion period, the final gas composition was approximately 40 ppm CO, 0.9% CO₂, 20 ppm SO₂, 46 ppm NOₓ, and 17.5% O₂. These final readings confirm that the combustion of briquetted cotton stalks approached completion with minimal residual emissions and negligible post-combustion reactivity. The recorded values also indicate consistent and stable combustion behavior, validating the suitability of the briquettes as clean-burning and efficient biofuel.

Thermogravimetric analysis (TGA)

Figure 12 illustrates the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of briquetted cotton stalks obtained under an inert nitrogen atmosphere at a heating rate of 10 °C·min⁻¹. The TG curve represents mass loss as a function of temperature, while the DTG curve highlights the rate of thermal decomposition, allowing a detailed understanding of the pyrolysis behavior of the material. The initial mass of each sample was approximately 10 ± 0.1 mg, and the temperature range studied extended from room temperature up to 800 °C.

Fig. 12.

Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) of cotton stalk briquette.

The thermogravimetric behavior of the cotton stalk briquette exhibited four distinct stages of thermal transformation. The first stage, occurring between 30 and 150 °C, corresponds to the evaporation of surface and physically bound moisture. In this region, a minor mass loss of approximately 5–7% was recorded, with the DTG curve showing a small shoulder peak near 95 °C, representing the end of the dehydration process. The second stage, observed between 150 and 385 °C, represents the active devolatilization phase associated with the initial breakdown of hemicellulose and partial cellulose decomposition. A pronounced weight reduction was detected, accounting for about 50.7% of the total mass loss. The DTG peak in this region appeared at 286 °C, indicating the maximum rate of thermal degradation. This region corresponds to the most intense release of volatile gases, including CO, CO₂, CH₄, and light hydrocarbons.

The third stage, ranging from 385 to 500 °C, is attributed mainly to the degradation of cellulose and the initial decomposition of lignin structures. The mass loss in this stage was approximately 12%, with a DTG peak centered at 368 °C. The slope of the TG curve in this region became less steep, reflecting the gradual transition from volatile evolution to char formation.The fourth and final stages occurred between 500 and 800 °C, corresponding primarily to the slow degradation of lignin and the carbonization of the remaining fixed carbon. The rate of mass loss was considerably lower in this zone, representing around 8% of the total. The DTG curve exhibited a broad and shallow shoulder between 520 and 610 °C, indicating the slow decomposition of more thermally stable aromatic structures. Beyond 700 °C, the TG curve flattened, signifying the completion of the pyrolysis process. At the end of the experiment, the residual mass (char yield) was approximately 22.8 ± 0.5% of the original sample weight. The high proportion of residual char suggests that the briquetted cotton stalk possesses substantial lignin content and a robust carbonaceous matrix, contributing to good structural stability during heating. The analysis of the derivative curve confirmed that the main degradation events were centered between 250 and 400 °C, representing the most thermally reactive region of the fuel. The high intensity of the DTG peaks in this temperature range indicates efficient volatile release and rapid conversion kinetics, which are favorable for thermal conversion applications such as gasification and combustion.

The thermal decomposition profile obtained from the TGA and DTG curves clearly demonstrates the progressive transformation of cotton stalk briquettes from moisture removal to volatile evolution, and finally to char stabilization. The distinct multistage behavior observed reflects the complex composition of the biomass and the sequential breakdown of hemicellulose, cellulose, and lignin components. The final residue remained stable up to 800 °C, indicating high thermal resistance and potential for use in thermochemical conversion systems with controlled energy release.

To assess the energy efficiency and environmental sustainability of the densification process, the Energy Performance Index (EPI) and the Environmental Impact Index (EIIE) were calculated for all cotton stalk briquette treatments.

Energy Performance Index (EPI)

Table 2 presents the Energy Performance Index (EPI) values for cotton stalk briquettes, which ranged from 89.6 at 3 MPa and 15% MC to 76.9 at 16 MPa and < 0.7 mm particle size. The EPI exhibited a distinct inverse relationship with compaction pressure, whereby increasing the applied pressure consistently reduced the index across all treatments. This trend reflects the elevated energy input required at higher compact levels, which diminishes the net energy efficiency of the densified biomass.

Table 2.

Energy Performance Index (EPI) values of cotton stalk briquette.

Pressure, MPa	1.4–3.0 mm		0.7–1.4 mm		< 0.7 mm
	6% MC	15% MC	6% MC	15% MC	6% MC	15% MC
3	89.5	89.6	89.4	89.4	89.4	89.4
6	86.0	86.1	86.0	86.0	85.9	85.9
10	82.8	83.0	82.8	82.9	82.7	82.7
13	79.8	80.1	79.8	79.9	79.7	79.7
16	77.0	77.4	77.0	77.1	76.9	76.9

At the lowest pressure of 3 MPa, EPI values were uniformly high across all particle sizes and MC, ranging narrowly between 89.4 and 89.6, indicating minimal variation in energy performance. However, as pressure increased to 16 MPa, EPI declined to 77.0–76.9, with the lowest values consistently observed for fine particles, suggesting reduced energy return on input at high densification levels.

Particle size had a subtle but consistent influence on EPI. Finer particles (< 0.7 mm) produced slightly lower EPI values compared to medium (0.7–1.4 mm) and coarse (1.4–3.0 mm) sizes under the same pressure and moisture conditions. For instance, at 13 MPa and 6% MC, EPI values were 79.8, 79.8, and 79.7 for coarse, medium, and fine particles, respectively. MC also influenced the EPI, although its effect was comparatively modest. In general, briquettes formed at 15% moisture exhibited marginally higher EPI values than those at 6% MC. At 10 MPa and coarse particle size, EPI was 83.0 at 15% MC compared to 82.8 at 6% MC.

Statistical analysis using two-way ANOVA revealed that both pressure and particle size had significant effects on EPI (p < 0.01), while the interaction between MC and particle size was not significant (p > 0.05). The highest EPI value (89.6) was recorded at 3 MPa, 1.4–3.0 mm particle size, and 15% MC, signifying the optimal configuration for maximizing energy return relative to mechanical input. Conversely, the lowest EPI (76.9) occurred at 16 MPa and < 0.7 mm particle size, reflecting the performance penalty associated with aggressive densification of fine biomass.

Environmental Impact Index (EIIE)

As shown in Table 3, the environmental impact index for energy (EIIE) ranged from 0.2636 to 0.3071, indicating a clear influence of compaction pressure, particle size, and MC on the environmental performance of cotton stalk briquettes. Statistical analysis using two-way ANOVA revealed that compaction pressure had a highly significant effect (p < 0.001) on EIIE, while MC and particle size showed moderately significant effects (p < 0.05). No significant interaction effects were observed, indicating that these variables influence EIIE independently.

Table 3.

Environmental Impact Index for Energy (EIIE) of cotton stalk briquettes.

Pressure, MPa	1.4–3.0 mm		0.7–1.4 mm		< 0.7 mm
	6% MC	15% MC	6% MC	15% MC	6% MC	15% MC
3	0.3070	0.3071	0.3065	0.3066	0.3065	0.3066
6	0.2949	0.2953	0.2948	0.2950	0.2944	0.2945
10	0.2840	0.2846	0.2838	0.2841	0.2834	0.2835
13	0.2737	0.2747	0.2736	0.2738	0.2732	0.2732
16	0.2642	0.2655	0.2641	0.2644	0.2636	0.2637

A strong inverse relationship between compact pressure and EIIE was observed across all treatments. As the compaction pressure increased from 3 MPa to 16 MPa, the EIIE consistently decreased for all particle sizes and moisture levels. For example, briquettes with 1.4–3.0 mm particle size and 15% MC showed a decline in EIIE from 0.3071 at 3 MPa to 0.2655 at 16 MPa, representing a reduction of nearly 13.5%. This trend indicates that although higher densification improves structural stability and reduces porosity, it requires more mechanical energy input, which in turn offsets the net environmental and energy benefits. The results further revealed that particle size exerted a measurable effect, particularly under lower compaction pressures. Coarser particles (1.4–3.0 mm) consistently yielded higher EIIE values compared to finer fractions, suggesting that larger particle geometry enhances environmental performance under moderate processing conditions. At 10 MPa and 6% MC, the EIIE for coarse particles reached 0.2840, compared with 0.2834 for fine particles (< 0.7 mm). This difference is mainly due to the lower surface area and reduced interlocking friction of coarse particles, which decreases specific energy consumption (SEC_T) and allows more efficient compaction with less energy loss.

Moisture content also influenced EIIE, though to a lesser extent. Across all particle sizes, briquettes compacted at 15% MC exhibited slightly higher EIIE values than those at 6% MC. For example, at 6 MPa and medium particle size (0.7–1.4 mm), the EIIE increased slightly from 0.2606 at 6% MC to 0.2608 at 15% MC. While these differences were statistically significant (p < 0.05), their practical effect was modest. Nonetheless, the observed pattern reflects the plasticizing effect of moderate moisture, which improves particle rearrangement and bonding, thereby reducing resistance to compression and enhancing densification uniformity.

The favorable EIIE values obtained in this study were also attributed to reduced emission intensities during combustion, particularly the low average concentrations of key pollutants. The average measured flue gas emissions were CO ≈ 221 ppm, NOₓ ≈ 50 ppm, and SO₂ ≈ 26 ppm, all of which fall within internationally recommended environmental thresholds for clean biomass combustion. These low emission levels significantly reduced the emission index (EI), thereby increasing the EIIE. Comparative evaluation of data within the same experimental matrix confirmed that higher compaction pressures (> 13 MPa) and finer particle sizes (< 0.7 mm) resulted in lower EIIE values because of increased frictional resistance and localized heat buildup during densification. Conversely, moderate pressures (3–6 MPa) and coarse particles (1.4–3.0 mm) achieved optimal values due to reduced compression work and improved structural integrity of the briquettes.

According to Table 3, the highest EIIE value (0.3071) was achieved at 3 MPa compaction pressure, 1.4–3.0 mm particle size, and 15% MC. This combination represents the most favorable balance between energy yield, compaction efficiency, and environmental performance. The data indicates that optimal briquetting conditions can achieve both energetic and ecological efficiency, reducing emissions while maintaining high energy density. These indices provide insight into the operational stages of biomass utilization, reflecting energy efficiency and environmental performance, and highlight the relevance of a life cycle thinking perspective in evaluating sustainable bioenergy production.

In summary, the results confirm that the EIIE serves as a robust indicator for evaluating the sustainability of biomass densification processes. The trends observed across different treatments highlight that moderate pressure, coarse particle size, and 15% MC offer the best compromise between energy consumption, emission reduction, and product quality in cotton stalk briquettes.

Integration of energy and environmental indices within a life cycle perspective

To provide an integrated interpretation of the process-level environmental and energy performance of cotton stalk briquetting, the calculated energy and environmental performance indicators were collectively evaluated within a life cycle thinking (process-level) perspective (Fig. 13). Rather than considering each metric independently, this approach allows the identification of key stages within the biomass valorization chain where energy is primarily consumed and where environmental and energetic benefits are realized.

Fig. 13.

Process-level framework for cotton stalk briquette valorization and associated energy and environmental indicators.

The present analysis is limited to operational stages, including mechanical preprocessing, densification, and laboratory-scale combustion, which represent the most energy- and emission-intensive phases. Upstream cultivation, infrastructure, and downstream ash management are excluded. Consequently, all energy and environmental indicators reported here, including SEC_T, EPI, and EIIE, are interpreted strictly at the process level only, providing insights into operational performance without implying a full cradle-to-grave life cycle assessment. The specific energy consumption during compression (SEC_T) highlights the energy-intensive nature of mechanical densification, particularly under high compaction pressures and fine particle sizes. This stage represents the main operational energy hotspot within the system, as mechanical work is directly converted into compaction energy. In contrast, the Energy Performance Index (EPI) reflects the efficiency with which this invested energy contributes to improved fuel quality, linking densification energy requirements to the resulting energetic performance of the briquettes.

Similarly, the Environmental Impact Index for Energy (EIIE) provides insight into the operational environmental effectiveness of the briquetting process by combining energy yield with emission-related indicators measured during combustion. The observed trends indicate that optimized processing conditions can simultaneously reduce emission intensity and maintain adequate energy efficiency, demonstrating that densification parameters play a critical role in governing the operational environmental footprint of biomass-based solid fuels at the operational level only.

Moisture content emerges as a regulating factor across the valorization chain, influencing both energy consumption during densification and emission behavior during combustion. Moderate moisture levels improve particle bonding and densification efficiency without disproportionately increasing energy demand, thereby enhancing both EPI and EIIE. This interaction underscores the importance of process optimization in balancing energy inputs and environmental outputs within the defined system boundary.

In addition to the composite indices, the gross calorific value represents a key indicator for interpreting the process-level energy balance of cotton stalk briquettes. While SEC_T quantifies the energy invested during mechanical processing, the calorific value reflects the recoverable energy during the end-use phase. From a process-level life cycle thinking perspective, the balance between energy input during densification and energy output during combustion determines the operational effectiveness of biomass valorization. The relatively high calorific value obtained in this study indicates that the energy consumed during processing can be effectively offset during the use phase, reinforcing the suitability of cotton stalk briquettes as an operationally efficient bioenergy carrier.

Thus, the combined evaluation of SEC_T, EPI, EIIE, and calorific value enables a structured understanding of energy consumption and environmental benefits across the main operational stages of cotton stalk utilization. This integrated assessment demonstrates how experimentally derived performance indicators can support process-level life cycle thinking, without representing a full ISO-compliant life cycle assessment.

Discussion

This study provides a comprehensive assessment of cotton stalk residues as a renewable solid biofuel through an integrated evaluation of mechanical densification, tribological behavior, thermal characteristics, energy efficiency, and environmental performance within a clearly defined operational system boundary. The analysis reflects a process-level life cycle thinking perspective, where energy consumption, material transformation, and environmental outcomes are interpreted across sequential operational stages, from post-harvest residue availability to final energy recovery. Stages dominated by energy and material inputs (collection, preprocessing, densification) are distinguished from stages focused on energy recovery and environmental benefits (combustion), enabling a holistic assessment of operational performance. It is emphasized that this life cycle-oriented interpretation is strictly limited to mechanical preprocessing, densification, and laboratory-scale combustion; upstream agricultural inputs, infrastructure, and downstream ash management are excluded. Accordingly, all energy and environmental indicators (SEC_T, EPI, EIIE) are interpreted at the process-level, providing realistic insights into operational sustainability without implying a full cradle-to-grave LCA.

Particle size distribution was a key mechanical determinant within the operational boundary. Fine (< 0.7 mm) and intermediate (0.7–1.4 mm) fractions accounted for 48.3% and 41.1% of the total mass, whereas coarser particles (1.4–3.0 mm) represented only 10.6%. Fine particles, with higher surface area, enhance interparticle bonding and reduce void spaces, promoting higher briquette density and mechanical integrity^28,29. However, the same increase in surface area elevates frictional resistance during compaction, increasing mechanical energy input. Coarser particles, though less abundant, facilitate stress redistribution and reduce internal friction, lowering specific energy consumption during densification. This trade-off illustrates a process-level energy balance: particle size optimization must consider both density and cumulative energy input while maintaining acceptable fuel quality.

Moisture content (MC) critically regulates both surface and internal interactions during densification. The dynamic friction coefficient increased with MC on stainless steel (SS) and carbon steel (St) surfaces, ranging from 0.60 to 0.86 on SS and from 0.83 to 1.08 on St as MC increased from 6% to 20%. These trends reflect moisture-induced adhesion, fiber softening, and increased real contact area, particularly on rougher carbon steel surfaces. Internal friction within the biomass matrix increased with MC, whereas cohesion decreased, showing that water alters the balance between solid–solid contacts and lubricated interfaces^7,30,31. From a process-level perspective, moderate MC (~ 10–15%) reduces internal resistance and improves particle deformation, lowering net compaction energy. Excessive MC weakens cohesion, increases elastic recovery, and raises energy demand without additional benefit during combustion. Controlled MC within the optimal range thus enhances operational efficiency and energy conservation^32,33. Compaction pressure exerted a dominant influence on the compression–density relationship and represents the most energy-intensive stage within the operational life cycle. Briquette density increased monotonically with applied pressure, reflecting progressive particle rearrangement, deformation, and void elimination. Fine particles achieved higher densities, especially at low MC, due to efficient packing of interstitial spaces. Specific energy consumption (SEC_T) increased sharply with pressure, reaching 242.41 kJ·kg⁻¹ at the highest load, particularly for dry and fine fractions, highlighting diminishing returns: marginal density gains at high pressures disproportionately increase energy input, reducing net energy efficiency. Moderate MC (~ 15%) slightly decreased SEC_T by enhancing plasticity and reducing frictional resistance. The marginal differences in SEC_T observed between 6% and 15% moisture content can be attributed to competing mechanical effects of moisture during densification. While moderate moisture improves particle plasticity and rearrangement, reducing frictional resistance, it simultaneously increases adhesive interactions and real contact area between particles. Within this moisture range, these opposing mechanisms partially offset each other, resulting in only small net variations in total specific energy consumption. Compression ratio (CR) values confirmed that finer particles and moderate MC improve compatibility but emphasized that densification benefits must be balanced against energy expenditure³⁴.

Thermal characteristics and calorific properties represent the energy recovery stage and contextualize upstream energy investments. Cotton stalk briquettes exhibited high calorific value (~ 16.48 MJ·kg⁻¹) and low ash content (~ 3.4%), comparable to or exceeding rice husk and wheat straw^4,5,7. Comparable calorific values have been reported for commercial wood briquettes, typically ranging from ~ 16–18 MJ·kg⁻¹ with low ash contents (≤ 5%) depending on feedstock and processing conditions, and for agricultural residues such as corn stover briquettes with heating values often around 15–17 MJ·kg⁻¹ under similar densification processes. These benchmarks from both wood‑based and agricultural biomass fuels provide context for the competitive energy density and favorable combustion quality of cotton stalk briquettes^5,6. From a process-level energy perspective, calorific value defines the maximum recoverable energy, setting the upper limit for net energy gain. Densification concentrates energy per unit volume, improving storage, transport, and combustion efficiency. Thermogravimetric analysis revealed multi-stage decomposition, with rapid devolatilization between 150 and 370 °C and a relatively high char yield (~ 23%), indicative of substantial lignin content^35,36. This thermal stability supports efficient combustion and potential application in pyrolysis or gasification, extending the functional life cycle of the material.

Flue gas emission profiles provide insights into the environmental dimension. Stable, low emissions of SO₂ (< 40 ppm) and NO_x (< 55 ppm), together with an inverse correlation between CO and O₂, indicate efficient and complete combustion^26,37. Low emissions reduce environmental burden and minimize the need for additional emission-control technologies. For context, the measured emission levels were benchmarked against commonly reported limits and emission profiles of conventional solid fuels. The observed SO₂ and NO_x values are generally below health-based reference levels reported in the World Health Organization air quality guidelines and are comparable to or lower than emission factors typically associated with traditional biomass fuels such as firewood or agricultural waste, which show measurable NO_x and CO emissions during combustion^38,39. In addition to WHO benchmarks, regulatory thresholds under the European Ecodesign framework for solid fuel combustion products (e.g., boilers and stoves) specify maximum seasonal space heating emissions of particulate matter, organic gaseous compounds, carbon monoxide, and nitrogen oxides under standardized conditions, providing further context for the environmental performance of stalk briquette combustion⁴⁰. This comparison highlights the relative environmental compatibility of cotton stalk briquettes under optimized combustion conditions. Energy Performance Index (EPI), integrating mechanical input and thermal output, ranged from 76.9 to 89.6, with optimal values at low compaction pressures (3 MPa), coarse particle sizes (1.4–3.0 mm), and moderate MC (15%). These conditions represent a favorable balance, where sufficient densification is achieved without excessive mechanical energy consumption. Fine particles, despite improving density and uniformity, lowered EPI due to higher frictional losses, whereas moderate MC slightly enhanced EPI^34,35,41,42.

The Environmental Impact Index for Energy (EIIE) complements EPI by incorporating emission effects. EIIE trends mirrored those of EPI, with optimal values at low pressures, coarser particles, and moderate MC^43,44. Higher pressures and fine particles increased internal friction and energy losses without proportional gains in fuel quality, lowering EIIE. Consistently low emissions of CO, NO_x, and SO₂ contributed to favorable EIIE outcomes, confirming the environmental compatibility of cotton stalk briquettes^9,26,27.

Mechanistically, particle size, MC, and pressure collectively govern stress distribution, frictional resistance, and bonding during densification. Fine particles increase contact points and bonding potential but elevate frictional energy losses. Moisture acts as a plasticizer facilitating deformation and bonding but reduces cohesion when excessive. Coarse particles enable easier rearrangement and lower energy demand at the cost of slightly reduced density. Optimizing these parameters balances energy and material inputs during processing with energy recovery and low emissions during utilization^45,46. Thermal stability and combustion behavior of optimally densified briquettes confirm that upstream processing conditions strongly influence downstream performance^29,36,47.

From an applied perspective, maintaining compaction pressures ≤ 6 MPa, particle sizes 0.7–3.0 mm, and MC ~ 15% provides the best compromise between mechanical strength, energy efficiency, and environmental performance. These conditions reduce SEC_T, enhance EPI and EIIE, and yield briquettes with predictable thermal and emission behavior suitable for decentralized and industrial energy systems^33,39,48. Optimization minimizes unnecessary energy input while maximizing recoverable energy, aligning cotton stalk briquette production with circular economy principles and low-carbon strategies.

Overall, cotton stalk briquette production can be interpreted through a process-level life cycle thinking framework, where energy consumption, material transformation, and environmental impacts are jointly considered across operational stages. Optimized particle size distribution, controlled MC, and moderate compaction pressures reduce energy demand while preserving high calorific value and low emissions. Combined use of EPI and EIIE provides a robust framework for evaluating energetic and environmental performance without implying a full LCA. Cotton stalk briquettes therefore represent a viable, low-emission, and energy-efficient biofuel option contributing to renewable energy development and sustainable agricultural residue management^49–51. Energy-intensive steps such as grinding and densification are the main consumption phases, whereas combustion constitutes the primary recovery stage. Joint evaluation through SEC_T, EPI, and EIIE demonstrates that optimized processing conditions reduce cumulative energy demand, maximize energy recovery, and minimize environmental burdens, supporting sustainable biomass valorization within agricultural production systems.

From an industrial perspective, the observed trade-off between density gains and declining EPI/EIIE has direct implications for scalability and cost-effectiveness. Higher compaction pressures and fine particle fractions, while improving briquette density, increase mechanical energy input and reduce net energy efficiency, which may elevate operational costs in large-scale production. Industrial implementation should therefore target moderate pressures (≤ 6 MPa) and controlled particle size distributions (0.7–3.0 mm) to achieve a balance between fuel quality, energy performance, and cost-efficiency, ensuring that the process remains economically viable while maintaining favorable thermal and emission characteristics^5,6,12.

Conclusions

Cotton stalk residues can be effectively valorized as low-emission solid biofuel, with key factors such as moisture content, particle size, and compaction pressure significantly influencing briquette quality, energy efficiency, and combustion stability. Under optimized conditions, briquettes exhibited high calorific value, low ash content, and stable combustion behavior, confirming their suitability for clean energy generation at operational scales. These findings indicate that moderate compaction pressures and controlled particle size distributions can enhance energy recovery while minimizing energy input during densification, supporting cost-effective and scalable production. Importantly, this study demonstrates the potential for integrating cotton stalk briquettes into agricultural and rural energy systems, contributing to circular bioeconomy objectives and promoting renewable energy utilization in farming communities. The study was limited to laboratory-scale tests, with a gate-to-gate boundary, and did not include durability assessments, which should be considered when interpreting the results.

Summary points

• Cotton stalk briquettes represent a renewable and low-emission energy source.

• Moderate compaction and controlled particle size improve operational efficiency.

• The findings support scalable industrial deployment and renewable energy adoption in agricultural systems.

Future research should investigate the sensitivity of energy and environmental performance to emission weighting factors, develop advanced emission control strategies, optimize processing parameters, and evaluate large-scale industrial implementation. In addition, scale-up studies, long-term storage stability, economic and techno-economic analyses, and integration into decentralized energy systems should be addressed to support commercial feasibility and practical deployment of cotton stalk briquettes.

Author contributions

Conceptualization, M.M.I., A.A. and M.G.; Methodology, M.M.I., A.A. and M.G.; Investigation, A.A. and M.G.; Data curation, M.M.I. and M.G.; Formal analysis, M.M.I. and A.A.; Visualization, M.M.I. and M.G.; Writing—original draft preparation, M.M.I., A.A. and M.G.; Writing—review and editing, M.M.I., A.A. and M.G. All authors have read and agreed to the published version of the manuscript.

Data availability

The datasets generated and/or analyzed of the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval and consent to participate

All authors are approval in participation in this research.

Consent for publication

All authors are consent of publication of the research paper.