Development of Binderless Waste-Derived Briquettes: Effect of Plastic Content on Combustion Performance and Kinetics

Peter Ebhodaghe Akhator ^1,2,*, Ufuoma Georgina Unueroh ³

1. Department of Mechanical Engineering, University of Benin, Benin City, Nigeria

2. Department of Chemical Engineering, University of Johannesburg, Johannesburg, South Africa

3. Department of Materials and Metallurgical Engineering, University of Benin, Benin City, Nigeria

* Correspondence: Peter Ebhodaghe Akhator

Academic Editor: Gasim Hayder

Special Issue: Bioenergy and Waste-to-Energy

Received: February 16, 2026 | Accepted: May 07, 2026 | Published: May 15, 2026

Journal of Energy and Power Technology 2026, Volume 8, Issue 2, doi:10.21926/jept.2602009

Recommended citation: Akhator PE, Unueroh UG. Development of Binderless Waste-Derived Briquettes: Effect of Plastic Content on Combustion Performance and Kinetics. Journal of Energy and Power Technology 2026; 8(2): 009; doi:10.21926/jept.2602009.

© 2026 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

Briquettes are becoming an important means of realizing the potential of biomass and various solid waste streams as a renewable energy source. A densification process produced them called briquetting, which involves compressing various biomass materials either singly [1,2] or in combination [3,4,5,6] into a uniform and high-density structure. It is a physical method of converting irregular and crushed biomass into a smaller, more uniform form by applying mechanical and plastic deformation under well-controlled temperature, pressure, and moisture conditions. Briquettes have many advantages over raw biomass, such as higher density and calorific value, reduced emissions of greenhouse gases, including CO₂, N₂O, and SO₂, and a substantial reduction in toxic pollutants [2,7]. They are denser and have lower volume, which enhances their handling, storage, and transportation efficiency [8,9] and helps combat deforestation and ensure environmental sustainability [10].

Biomass briquettes are increasingly popular as an excellent substitute for traditional fuels such as firewood and charcoal for domestic heating, cooking, and industrial activities in urban and rural societies worldwide [11,12,13]. Plastic waste is an emerging, promising raw material for the production of briquettes, due to its high calorific value. For example, Cheremisinoff [14] reported that plastic waste has a calorific value of about 34 MJ/kg, higher than that of wood (about 20 MJ/kg) and paper waste (about 17 MJ/kg). Incorporating plastic waste into briquettes could increase their energy content and thermal performance, while alleviating environmental concerns related to plastic buildup, as plastic is non-biodegradable [5,9,15].

Several studies examined the incorporation of waste plastics into biomass or other solid wastes to enhance briquette characteristics [3,5,9,16,17]. Consistently, these studies indicated that the addition of plastic enhances the calorific value and mechanical strength of the briquettes and lowers the ash content. Despite these promising energy and mechanical performance indicators, plastic-biomass briquettes are yet to be widely adopted [18,19]. This is probably due to inadequate knowledge of their combustion behaviour and kinetics. It is important to address these gaps to optimize briquette design, improve operational efficiency, and promote consumer acceptance of waste-plastic-based briquettes.

Thermogravimetric analysis (TGA) is a common technique for studying the thermal degradation and energy release properties of solid fuels, including briquettes. Studies that have used thermogravimetric methods have typically indicated positive combustion behaviour and clear kinetic characteristics, highlighting the possibility of briquettes as a sustainable fuel source [2,7,20,21]. Indicatively, Jia [20] examined briquettes obtained from different types of biomass, including masson pine, Chinese fir, willow, slash pine, and poplar, and recorded activation energies between 56 and 542 kJ/mol by the Coats-Redfern method. Equally, Yiga et al. [2] examined carbonized rice husk briquettes prepared by low-pressure methods and cassava flour binder and found that the activation energies were 75-83 kJ/mol. Qi et al. [7] demonstrated that briquetting enhanced combustion efficiency and decreased pollutant emissions such as SO₂ and NO, with high goodness-of-fit measures (>0.99) using the Coats-Redfern kinetic analysis. Bongomin et al. [22] performed elaborate analyses of the thermal decomposition of five biomass wastes, coffee husk, groundnut shell, macadamia nutshell, rice husk, and tea waste, using TGA coupled with the Coats-Redfern method. Their findings showed that macadamia shells were the most reactive to thermal degradation, and coffee husk was the most difficult to degrade, with thermodynamic parameters indicating a high enthalpy change but a low Gibbs free energy.

Moreover, Liu et al. [21] examined the combustion and kinetic behaviour of briquettes prepared from corn straw at various heating rates. They reported excellent combustion performance, with activation energies of approximately 109-114 kJ/mol, based on Flynn-Wall-Ozawa (FWO), Friedman, Starink, and Kissinger-Akahira-Sunose (KAS) methods. Nevertheless, research directly focused on briquettes made from mixtures of plastics and sawdust is limited, and systematic research on the impact of different plastic concentrations on their combustion characteristics and kinetics is lacking. Filling this knowledge gap is crucial for optimising such briquette formulations, enhancing combustion performance, and advancing their eco-friendly utilization.

In this study, binderless briquettes were made using high-pressure compaction (~17 MPa) of mixtures of sawdust and waste PET plastics, and their combustion performance and kinetics were evaluated. Additionally, characterisations based on physicochemical analysis, SEM, FTIR, durability, and energetic parameters were provided. The insights from this study are intended to inform the development and optimization of briquette production and their combustion systems, to achieve cleaner, more efficient energy solutions in the future.

2. Materials and Methods

2.1 Raw Materials Collection and Preparation

Sawdust (emanating from the processing of different tree species) and discarded polyethylene terephthalate (PET) plastic bottles are the primary materials utilized in this study for briquette production. The PET bottles were collected from waste receptacles in Benin City, Edo State, Nigeria. The tops and labels of the bottles were removed to ensure uniformity before heating them at 230°C to facilitate melting. The molten PET was held at that temperature for 5 minutes before being allowed to cool for 10 minutes, and thereafter pulverized into powder using a locally fabricated pulverizer. The powder was sieved using a lab sieve with a 50 μm mesh to obtain a uniform grain size (see Figure 1a). The sawdust was obtained from the sawmill's dumpsite in Benin City, Nigeria. It was sieved using a lab sieve with a 50 μm mesh to obtain a uniform particle size (see Figure 1b).

Figure 1 Prepared biomass samples used in this study (a) sieved PET powder, (b) sieved sawdust.

2.2 Briquettes Production

The PET powder (PP) and sieved sawdust (SD) were mixed at four mass ratios as illustrated in Table 1 to yield four types of briquettes: BR-A, BR-B, BR-C, and BR-D. No binder was used in this study.

Table 1 The briquette types and their mixing ratios.

One hundred grams of each blend was carefully placed in a plastic bucket and thoroughly stirred by hand with a wooden spatula until a uniform, consistent blend was achieved. The homogeneous mixture was transferred into a high-pressure hydraulic press briquetting machine (~17 MPa), where it was compressed to form compact, densified briquettes, as shown in Figure 2. The briquetting machine produces 50 briquettes per hour. The briquettes, which are 4.5 cm in diameter, have heights ranging from 1.5 cm to 3.7 cm.

Figure 2 Produced PET-wood waste briquettes.

2.3 Analysis of Physicochemical Properties, Ash Composition, and Heavy Metals Concentration

The carbon (C), hydrogen (H), nitrogen (N), and sulphur (S) contents of both the raw materials and briquettes were determined using an elemental analyser (Exeter, EA-1110) following ASTM D5373-02 standards, while the oxygen content was calculated using Equation (1).

\[ O\%=100-(N+C+H+S+ASH)\% \tag{1} \]

The proximate properties of the biomass samples were determined using a thermogravimetric analyzer (NETZSCH-STA449F5) with a sample amount of 12 ± 0.5 mg. following the ASTM E1131-08 standard. The analyser was run until constant weights were recorded. This residual weight is the ash content. Percentage ash content was calculated using Equation (2).

\[ \%Ash=\frac{mass\,of\,residual\,ash}{initial\,mass\,of\,sample} \tag{2} \]

An oxygen bomb calorimeter (CAL3K-A) was used to measure the samples’ higher heating values (HHV). Each experiment was repeated three times.

The ash composition of the briquettes was analyzed using an energy-dispersive X-ray fluorescence (XRF) spectroscopy (Rigaku RIX-3000, Japan). The concentrations of heavy metals in the briquettes were determined using an atomic absorption spectrometer (GBC, Avanta-PM) and compared with European standards. Table 2 shows the ash composition of the briquettes.

Table 2 Ash composition of produced briquettes.

2.4 Mechanical Characterization of the Briquettes

To evaluate the structural integrity of the briquettes, they were lifted to a height of 2 meters and then dropped onto a sturdy concrete floor to simulate a standardized, controlled method to assess their mechanical strength and durability under impact conditions similar to those encountered in real-world handling stresses, such as transportation, handling, and storage. The mechanical strength was calculated by dividing the briquette’s weight after dropping by its weight before dropping. Additionally, the briquettes’ particle density was determined using standard methods based on the mass-to-volume ratio. Each characterization experiment was performed three times.

2.5 Morphology Studies and FTIR Spectrometry

The morphologies of the briquettes were analyzed using Scanning electron microscopy (SEM) microphotographs, obtained with a Thermo Scientific Phenom ProX scanning electron microscope, PhenomWorld, Eindhoven, Netherlands. The briquette samples were placed on the aluminum holder stub using sticky carbon tape, then vacuum-dried and scanned at an acceleration voltage of 15 kV. The surface functional groups of the samples were analyzed by Fourier transform infrared (FTIR) spectroscopy on a ThermoFisher Scientific iS50 FTIR spectrometer, which scans from 4000 to 400 cm^-1 at a resolution of 4 cm^-1 and a scanning speed of 2 mm/sec. The briquettes were ground into fine powders before the FTIR analysis.

2.6 Analysis of Combustion Properties of the Briquettes

The combustion properties of the produced briquettes were examined using a thermogravimetric analyzer (NETZSCH-STA449F5). The analyzer operates from room temperature to 1600°C and can operate with nitrogen, oxygen, or air. The briquettes were ground and sieved through a laboratory test sieve to particle sizes ranging from 50 to 70 μm. About 10 ± 0.5 mg of each briquette type was weighed, placed into a platinum crucible and inserted into the thermogravimetric analyser. The samples were heated from ambient temperature to 900°C at 10°C/min in an oxygen atmosphere at 100 mL/min. The resulting data were used to generate TG and DTG curves for all samples.

Several key combustion parameters, including the ignition temperature (T_i), peak temperature (T_p), and burnout temperature (T_b), were extracted from the TG and DTG curves to evaluate the overall combustion performance of the briquettes. T_i is typically determined using the tangent method on the TG-DTG curves [23]. Furthermore, three combustion indices, namely, the ignition index (D_i), burnout index (D_b), and combustion intensity index (H_f), were evaluated during the combustion analysis. These indices were calculated using Equations (3)-(5) [23,24].

\[ D_i=\frac{DTG_{max}}{t_it_p} \tag{3} \]

\[ D_b=\frac{DTG_{max}}{t_bt_p\Delta t_{1/2}} \tag{4} \]

\[ H_f=T_p\times10^{-3}\ln\left(\frac{\Delta T_{1/2}}{DTG_{max}}\right) \tag{5} \]

where DTG_max = highest weight loss rate (wt.%/°C), t_i = time taken to ignite (min), t_b = time taken to burnout (min), ∆T_1/2 = temperature range at half of DTG_max value (°C), ∆t_1/2 = time range at half of DTG_max value (min), t_p = time to reach peak temperature (min).

2.7 Combustion Kinetic Modeling

Kinetic analysis is very important in the study of the combustion process of solid fuels, including briquettes. In this study, the Coats-Redfern method was used to estimate kinetic parameters from TGA data. It was selected for its strength, ease, and efficiency in non-isothermal conditions characteristic of biomass thermal degradation [25]. It is especially appropriate for analyzing data obtained at a single heating rate, which aligns with our experimental design. The reaction rate constant (k) is dependent on absolute temperature (T), which is expressed by the Arrhenius equation [26]:

\[ k(T)=A\exp\left(-\frac{E_a}{RT}\right) \tag{6} \]

T indicates the temperature (K), A signifies the pre-exponential factor (min^-1), E_a indicates the activation energy (kJ/mol), and R is the universal gas constant (0.008314 kJ/mol K). The solid-gas reaction rate is defined as follows [26]:

\[ \frac{dx}{dt}=k(T)\cdot g(x) \tag{7} \]

$\frac{dx}{dt}$ represents the rate of degradation, which is a linear function of the temperature rate constant. x represents the briquettes’ weight loss ratio during the combustion, t signifies the time for the combustion, and g(x) is the differential form of the reaction model, which depends on the reaction mechanism. The weight loss ratio (x) can be obtained from thermogravimetric analysis data, as follows [27]:

\[ x=\frac{m_0-m_t}{m_0-m_f} \tag{8} \]

m₀, m_t, and m_f are the initial, instantaneous, and residual mass of samples during the combustion process, respectively.

Combining Equations (6) and (7) at the heating rate (β = dT/dt) and rearranging gives Equation (9):

\[ \frac{dx}{dT}=\frac{A}{\beta}\exp\left(-\frac{E_a}{RT}\right)g(x) \tag{9} \]

The integral expression of g(x) is presented in Equation (10).

\[ f(x)=\int_0^x\frac{dx}{g(x)}=\frac{A}{\beta}\int_{T_0}^T\exp\left(-\frac{E_a}{RT}\right)dT \tag{10} \]

A = pre-exponential factor (min^-1), E_a = activation energy (kJ/mol), k(T) = reaction rate constant, R = universal gas constant, T = reaction temperature (K), x = conversion rate, and g(x) = kinetic model, f(x) = the integral form of the kinetic model.

Kinetic parameters, such as the activation energy, can be determined by integrating Equation (10) and approximating the exponential term. For the Coats-Redfern method, after approximation and rearrangement, the solution is expressed as Equation (11) [27]:

\[ \ln\frac{f(x)}{T^2}=\ln\frac{AR}{\beta E_a}-\frac{E_a}{RT} \tag{11} \]

In this study, the briquettes’ combustion process was approximated as a first-order kinetic reaction; hence, according to [27],

\[ g(x)=1-x \tag{12} \]

Substituting in Equation (10) and integrating, Equation (11) translates to:

\[ \ln\left[-\frac{\ln(1-x)}{T^2}\right]=\ln\frac{AR}{\beta E_a}-\frac{E_a}{RT} \tag{13} \]

Straight lines were fitted from the plots of $\ln\left[-\frac{\ln(1-x)}{T^2}\right]$ against $\frac{1}{T}$, and the activation energy (E_a) was estimated from the slope $\left(-\frac{E_a}{R}\right)$ of the plots.

2.8 Water Boiling Test (WBT)

Version 4.2.3 of the Water Boiling Test (WBT) protocol was used to ascertain the thermal performance of the developed briquettes practically. An upgraded cookstove was used, and the performance evaluation involved boiling 5 litres of water in a flat-bottomed, cylindrical aluminium cooking pot with a maximum capacity of 7 litres. The tests were carried out using the briquette samples, like typical household cooking methods. Specific operational parameters, such as thermal efficiency, time to boil, specific fuel consumption, burning rate, and firepower, were evaluated for all three WBT phases: cold-start high-power phase, hot-start high-power phase, and simmer low-power phase [28]. During high-power (cold start) conditions, 5 litres of water were heated to its boiling point in the 7-litre pot using the cookstove at room temperature. For the high-power (hot start) phase, 5 litres of water were heated to boiling point temperature in a 7-litre pot with the cookstove pre-heated, and during the simmer phase, the water temperature was kept at about 3°C below boiling point for 45 minutes [28]. Kerosene was used for igniting the briquettes during the test.

2.9 Energetic Characterization

The energetic densities (energy per unit volume) of the developed briquettes were determined by multiplying their particle density (PD) with their higher heating value (HHV), as shown in Equation (14) [2].

\[ ED=PD\times HHV \tag{14} \]

where ED is the energetic density (MJ/m³), PD is the particle density (kg/m³), and HHV is the higher heating value (MJ/kg).

The Fuel Value Index (FVI) of the developed briquettes was determined using Equation (15) [2].

\[ FVI=\frac{ED}{Ash} \tag{15} \]

where FVI is fuel value index (MJ/m³%), and Ash is the Ash content of the briquettes (%).

3. Results and Discussion

3.1 Physicochemical Properties of Raw Materials and Briquettes

Table 3 presents the physicochemical properties of both the raw materials and the briquettes (BR-A to BR-D) produced in this study. PET plastic is known to contain a high proportion of volatile matter (VM) due to its manufacturing process and chemical composition [29,30]. Consequently, the VM content of PET waste was substantially higher than that of sawdust, thereby directly impacting the overall VM levels in the briquettes. Specifically, the VM percentage increased with increasing plastic content, rising from approximately 83.19% in BR-B to 85.19% in BR-D, as indicated in Table 3. These observations are consistent with findings from previous studies [3,17].

Table 3 Physicochemical properties of the produced briquettes.

FC was observed to be the highest in BR-A, probably due to the high amount of FC in SD. It was, However, it was observed to increase from 6.84% in BR-B to 8.05% in BR-D as the PET content increased. All samples had lower FC than BR-A, probably because the raw sawdust contained higher FC. Table 3 also summarises the elemental composition analysis. The carbon content and heating value of PET waste were higher than those of the sawdust, which was probably attributable to the chemical composition and structure of plastics [14]. This had a significant impact on the carbon content and HHV of the plastic-containing briquettes, as indicated in Table 3. The ratio of plastic in the briquette blends was positively correlated with the carbon content and heating value in the samples BR-B to BR-D, with carbon content increasing from 48.55 to 57.87% and heating value rising from 20 to 23.65 MJ/kg. These findings are consistent with previous studies [5,17,31]. The hydrogen and oxygen content of the briquettes was comparable among samples, as indicated in Table 3. No significant amounts of nitrogen were detected, and no sulphur was detected at all, which means that these briquettes are not expected to generate significant amounts of nitrogen oxides (NO_x) during the combustion or co-combustion process [5,32].

3.2 Fuel Characteristics of the Produced Briquettes

As indicated in Table 3, the atomic ratios of the hydrogen to carbon (H/C) and oxygen to carbon (O/C) in the briquettes were between 0.086 and 0.118 and 0.828 to 0.596, respectively. These ratios were plotted on the Van Krevelen diagram (Figure 3), which graphically illustrated the evolution of the composition and fuel properties of the briquettes [33]. Reduced ratios usually indicate improved fuel quality, as reflected by the proximity to the origin of the diagram [33,34]. The briquette produced from sawdust alone (BR-A) was initially located in the upper right-hand corner, as is typical of biomass. Upon mixing with PET plastics, the samples moved toward the origin, indicating greater levels of coalification, higher energy density, and better solid-fuel prospects [35]. The diagram also indicates that the rise in plastic content shifts briquettes from the lignite area (BR-B) to the bituminous coal area (BR-C and BR-D). Comprehensively, the addition of PET plastic increases the coalification and consequently the quality of the fuel of the briquettes.

Figure 3 Van Krevelen diagram of produced briquettes.

From Table 3, the O/C ratios of the briquettes decreased from 0.828 in BR-A to 0.596 in BR-D, indicating significant de-oxygenation, which leads to an increase in higher energy density [4,36]. Hence, BR-D shows the highest HHV. The lower O/C ratio in BR-D (0.596) indicates it is the most hydrophobic of the briquettes [37,38]. It will be the least likely to absorb moisture from the air, making it the most stable for long-term storage in humid environments without crumbling or losing quality. The H/C ratios dropped from 0.11 to 0.08 from BR-A to BR-D, as seen in Table 3. These remarkably low ratios suggest that the briquettes will burn with very little flame and minimal smoke. They will provide a steady, glowing heat rather than a rapid, volatile-driven fire [38,39].

3.3 Mechanical Characterisation of the Briquettes

Table 4 presents the drop strengths and densities of the briquettes produced in this study. The data reveal that the drop strength and density of the briquettes increase with increasing plastic content. Specifically, the drop strength increases from 77.8% in BR-A (sawdust only) to 82.9% in BR-D (highest plastic content), implying improved mechanical durability and resistance to breakage during handling and transportation. The bulk density also shows a significant upward trend, increasing from 648.12 kg/m³ in BR-A to 715.83 kg/m³ in BR-D, which indicates enhanced compactness and energy density of the briquettes. Yiga et al. [2] observed a rise in density and mechanical strength for briquettes produced from carbonised rice husk.

Table 4 Drop strength and density of produced briquettes.

3.4 Heavy Metal Concentration

Table 5 presents the concentrations of heavy metals in the produced briquettes. The results confirm that the concentrations of heavy metals in the briquettes comply with the standards set by the European Pellet Council (EPC) [40] for densified solid fuels. However, the cadmium content in BR-D was slightly above the set limit.

Table 5 Heavy metal concentrations of the produced briquettes.

3.5 FTIR Spectroscopy Results

Figure 4 shows the FTIR spectra of the developed briquettes.

Figure 4 FTIR spectra of developed briquettes.

The FTIR spectra of the briquettes provide a detailed view of their chemical composition by showing characteristic absorbance bands associated with specific functional groups and molecular structures. For all samples, there’s a broad band near 3400 cm^-1, which comes from O-H stretching vibrations, mainly due to moisture or hydroxyl groups present in the biomass. Interestingly, as the PET content increases, this band decreases in intensity, possibly indicating fewer hydroxyl groups in PET than in the biomass. In the 2900-2800 cm^-1 range, signals related to aliphatic C-H stretching vibrations are observed, which are common in components like cellulose, hemicellulose, lignin, and the hydrocarbon chains in PET. These signals remain fairly consistent across all spectra, with minor variations that suggest changes in aliphatic content as PET content increases. A key feature appears around 1700 cm^-1 in the spectra of briquettes with higher PET levels (BR-B, BR-C, BR-D). This peak corresponds to ester carbonyl groups (C=O), which are a hallmark of PET. Its increasing intensity with higher PET ratios confirms the presence of ester functionalities typical of PET plastics.

Furthermore, in spectra with more PET, especially those with higher ratios, there are noticeable peaks in the 1600-1500 cm^-1 range. These are associated with aromatic C=C stretching vibrations, reflecting the aromatic rings within PET molecules. This peak is less prominent in the pure sawdust sample (BR-A), which mainly contains lignin and other biomass components. Finally, the bands between 1200 and 1000 cm^-1 are linked to C-O stretching vibrations in esters, ethers, and carbohydrates. PET’s ester groups add or strengthen these peaks, highlighting its chemical influence on the briquettes. According to Carnaje et al. [41], the detection of both aliphatic and aromatic hydrocarbons suggests the briquettes contain fats and oils, which can help improve their ignition properties.

3.6 Surface Morphologies of the Briquettes

Figure 5 presents SEM micrographs showing the morphology and structure of the produced briquettes. The SEM micrographs reveal notable morphological differences between the briquettes, which are probably attributable to the increasing content of PET plastics, acting as an effective binder and densifying agent.

Figure 5 SEM micrographs of produced briquettes (a) BR-A, (b) BR-B, (c) BR-C, and (d) BR-D.

The micrograph (a) shows that the briquette made entirely of sawdust (BR-A) has a very porous and loose structure. The sawdust particles are clearly visible, irregular in shape, and many large gaps seperate them. This suggests that, without any binding agent, the particles don’t stick together very well.

From micrographs (b) to (d), it can be observed that the structure of briquettes with more PET, progressing from BR-B to BR-D, becomes noticeably denser, more tightly packed, and more uniform. The images reveal that the sawdust fibres are being compressed and wrapped in a smoother material, probably the melted and re-solidified PET plastic. This layer fills the gaps and helps bond the particles much more strongly. Overall, increasing the PET content from BR-A to BR-D results in a clear transition from a porous, loosely held-together structure to a denser, layered, and more cohesive one. This enhanced density and bonding make the briquettes more durable and less prone to breaking apart or disintegrating. The layered, fibrous composition also promotes more controlled and stable burning, which could reduce emissions and improve fuel efficiency.

In particular, briquettes with higher PET, like BR-C and BR-D, are likely to be stronger during handling and storage, and to burn more evenly and efficiently. This means they might burn longer and produce cleaner, more consistent emissions. The SEM images clearly show that adding PET improves internal cohesion and reduces porosity, thereby enhancing physical and thermal performance. On the other hand, pure biomass briquettes are more porous and fragile, which negatively affects their durability. Overall, as the PET content increases, the structure improves, making these PET-sawdust composites a promising option for sustainable, high-performance fuels.

3.7 Thermogravimetric Analysis

Figure 6 presents the non-isothermal combustion behaviour of the briquettes (BR-A to BR-D) recorded using a thermogravimetric analyser (TGA). Table 6 presents key combustion parameters, including the ignition temperature (T_i), peak temperature (T_p), and burnout temperature (T_b). As shown in Figure 6, the briquettes initially experienced weight loss predominantly due to the evaporation of moisture and some volatiles, consistent with previous studies [7,21]. This initial weight loss occurred over approximately 26 to 344°C for BR-A, 28 to 324°C for BR-B, 26 to 330°C for BR-C, and 26 to 319°C for BR-D. The initial weight reduction was approximately 9.79% for BR-A, 10.25% for BR-B, 9.07% for BR-C, and 6.69% for BR-D.

Figure 6 TG-DTG curves of the produced briquettes (BR-A to BR-D). T_i: ignition temperature (°C), T_p: peak temperature (°C), T_b: burnout temperature (°C), T₁: initial temperature at the medium point of maximum combustion rate, T₂: final temperature at the medium point of maximum combustion rate.

The evaporation of moisture led to the formation of cracks on the briquettes’ surfaces, which subsequently facilitated the release of volatile compounds [42].

Table 6 Combustion parameters for the various briquette types.

3.7.1 Effects of Plastic Concentration on T_i, T_p and T_b

The ignition temperature (T_i) for briquettes BR-A through BR-D is shown in Figure 7a. The data reveal that as the PET content increases, the ignition temperature tends to decrease. In particular, BR-D has the lowest ignition temperature at 319°C. The ΔT values show how much lower the ignition temperature is than that of BR-A, which contains no PET. For BR-D, this reduction is quite significant, about 25°C lower. This suggests that adding PET makes the briquettes easier to ignite. That’s likely because PET is combustible and requires less energy to start burning. In solid fuels, higher amounts of volatile matter, fixed carbon, and oxygen generally facilitate ignition. During the initial stages of combustion, volatile substances are released from the fuel’s surface, which then help promote burning. The fixed carbon, which burns more steadily, can keep the fire going once it’s started, especially when fueled by those earlier-released volatiles. According to Table 3, BR-D has the highest volatile matter content at 85.19%. When these volatiles are released, they increase the briquette's porosity, creating a larger reactive surface area. This makes it easier for oxygen to interact with the fuel, speeding combustion, reducing resistance, and lowering the overall ignition temperature.

Figure 7 (a) Ignition temperature, (b) peak temperature, and (c) burnout temperature of the briquette types.

The presence of certain catalytic oxides like CaO, Fe₂O₃, and K₂O helps make the solid fuels easier to ignite, lowering the ignition temperature [43,44]. Looking at the ash composition of the briquettes, shown in Table 2, BR-D’s ash contains a good amount of CaO (26.55%), Fe₂O₃ (24.50%), and K₂O (12.70%), with only a small amount of TiO₂ (2.91%). These ingredients likely work together to help BR-D ignite at the lowest temperature observed. On the other hand, the ash from BR-A is mostly composed of titanium compounds, with TiO₂ making up 40.25%. Since titanium compounds are known to slow down biomass combustion [45]. This probably explains why BR-A takes longer to ignite, at around 344°C.

The ashes of BR-B and BR-C also contain significant amounts of CaO, Fe₂O₃, and K₂O. Their catalytic effects likely help these briquettes ignite more easily, which is why their ignition temperatures are lower than those of BR-A.

Figure 7b shows the peak temperatures (T_p) during combustion for briquettes from BR-A to BR-D. As can be observed, briquettes that contain plastics have lower peak temperatures than BR-A, which is made entirely from sawdust. According to Table 3, the greater the PET content in the briquettes, the higher the volatile matter (VM) content. Since VM is released early during burning, it generates heat that helps preheat the remaining carbon, making it easier to ignite. This process explains why plastic-rich briquettes have a lower peak temperature. BR-D, which has the most plastic, reaches the lowest peak temperature of 377°C. The difference in peak temperatures between BR-A and BR-D is quite significant (ranging from 2 to 49°C), showing a strong link between plastic content and peak temperature.

Figure 7c shows the burnout temperatures (T_b), which indicate how much heat is needed to completely burn out the briquettes. These temperatures increase as PET content rises, especially for BR-D. The higher residual heat suggests that PET plastic contributes to more stable, longer-lasting combustion due to its thermal stability and ability to form char or other residues that require higher temperatures to fully burn out [46]. The amount of char residue also increases with PET content: about 4.45% for BR-A, 6.01% for BR-B, 8.32% for BR-C, and 24.83% for BR-D. Generally, more char residue is associated with a higher heating value (HHV) [46,47], which aligns with the trend seen in the residual weights and energy content across these briquettes. Similar relationships between char and HHV have been reported in other studies, like Liu et al. [21], who examined briquettes made from corn straw.

Overall, adding PET plastic alters the thermal behaviour of the briquettes by making them easier to ignite and promoting more complete burning at higher residual heat levels. This suggests that PET acts as a kind of combustion enhancer, enabling quicker ignition and more efficient, cleaner burning. These effects are important for designing briquettes tailored for specific uses, especially where fast ignition and steady, long-lasting combustion with minimal emissions are desired.

3.7.2 Effect of Plastic Concentration on Combustion Performance Indices

Table 7 highlights the combustion performance indices of the produced briquette types (BR-A to BR-D), computed from TG-DTG curves. The results include important combustion parameters, such as the ignition index (D_i), burnout index (D_b), and burning rate intensity (H_f), which are defined by Equations (3)-(5). These parameters help understand the ease of ignition, combustion stability, and overall combustion performance of each briquette.

Table 7 Key combustion performance indices for the briquettes.

The ignition index indicates a material's ability to release volatiles, which directly affects ignition and combustion stability [48,49]. An increase in D_i means an increase in volatile release capacity and, therefore, an increase in ignition capacity [50]. The effect of VM content on ignition performance is evident in the D_i values; the BR-D, with a VM content of 85.19%, has the largest D_i value of about 0.0466. This implies that BR-D will ignite more easily than others. Better ignition characteristics are beneficial in quick start-up in cooking applications, reducing waiting time. As shown in Table 7, the greater the plastic content, the larger D_i.

The burnout process is defined by D_b, which is a factor that relies on T_b and T_p. Reduced T_b values permit faster burnout, since unburned compounds are burned more rapidly. Conversely, higher burnout temperatures imply longer process times that require high temperatures and prolonged residence times [51,52]. Table 6 indicates that the burnout temperature of BR-A to BR-D increases with the percentage of plastics in the mixture; therefore, the burnout index of BR-A to BR-D rises with the percentage of plastics in the mixture, with BR-D having the highest T_b (790°C) and D_b (4.005E-03), indicating high combustion reactivity. The H_f index is used to determine the rate and intensity of combustion, and the lower the H_f, the more efficient the combustion [53]. The H_f index of BR-A is approximately 1.564, and that of BR-D is approximately 1.132, which means that BR-D may generate a more constant heat production as opposed to burning intensely. The same trends have been noted in comparative studies of coal and biomass combustion [20,52,54]. Based on these results, it can be concluded that incorporating plastics into the briquette mixture enhances their combustion characteristics and performance, indicating good prospects for energy applications.

3.8 Kinetic Analysis

The combustion kinetics of the four briquettes (BR-A to BR-D) were determined using data from the thermogravimetric analysis. An Arrhenius-type kinetic model was employed to analyse the TG-DTG data. As shown in Figure 6, the DTG curves for the four briquettes, measured at a constant heating rate of 10°C/min, exhibit single, bell-shaped peaks without shoulders across the temperature range from ignition to burnout. This suggests that the combustion process for all four briquettes proceeded as a single-step reaction, consistent with established methods for identifying single-step kinetics [54,55]. The Coats-Redfern method was applied to analyse the combustion region, assuming the process follows a first-order kinetic reaction. While we acknowledge the concerns raised by the ICTAC Kinetics Committee regarding the limitations of single-heating-rate methods, such as their potential difficulty in accurately determining kinetic triplets due to the complex nature of biomass combustion, the Coats-Redfern method was chosen based on practical considerations. The Coats-Redfern technique is useful for initial assessments when approximate kinetic parameters surffice, enabling meaningful comparisons among different biomass types under similar conditions. Moreover, conducting experiments at various heating rates requires significant time, resources, and specialised equipment, which is quite taxing. Additionally, analysing data at a single heating rate mirrors a real-world briquette combustion scenario, where processes typically occur under constant heating conditions, thus increasing the relevance of the findings from this study. Despite its limitations, the Coats-Redfern method has been demonstrated to be reliable in numerous studies when applied with an awareness of its constraints [22,25,56,57,58]. Figure 8 presents the results of fitting the conversion rate data (x) as a function of reaction temperature (T) for briquettes BR-A to BR-D. The correlation coefficients ranged from 0.9598 to 0.9803.

Figure 8 Kinetic plots for the produced briquettes.

Using the Coats-Redfern method at a heating rate of 10°C/min, the activation energies (E_a) for the four briquettes were calculated and are summarised in Table 8.

Table 8 Activation energies of the produced briquettes.

The results indicate that the E_a values for briquettes with plastic (BR-B to BR-D) range from 18.67 to 21.74 kJ/mol, and are lower than that of BR-A, without plastic (29.31 kJ/mol). The observed trend suggests that increasing the PET plastic content reduces the activation energy required for the briquettes’ combustion process. The key reason for the decrease in activation energy is the strong synergistic physical and chemical interactions between the decomposing biomass and plastic materials, which promote increased carbon conversion efficiency and a higher yield of volatile products compared to the combustion of biomass or plastic alone [59,60,61,62]. For instance, kinetic studies by Mishra et al. [61] demonstrated that blending Samanea saman seeds (SS) with PET plastic at a 3:1 ratio required a lower activation energy to initiate degradation than either component alone. Oyedun et al. [62] reported that increasing the plastic content in mixtures of sawdust and HDPE reduced the activation energy required to initiate the decomposition reaction. Similarly, Burra and Gupta [59] observed that blending polycarbonate with pinewood reduced the activation energy of the polycarbonate by about 50 kJ/mol, compared to the degradation of the individual polymer. The addition of PET plastic improved the reactivity of the briquettes during thermal decomposition, a trend that aligns well with observations from our study.

3.9 Water Boiling Test Performance Metrics

Table 9 presents the performance of briquettes BR-A to BR-D during the various WBT phases: cold start, hot start, and simmer, focusing on important parameters such as time to boil, burning rate, fuel consumption, energy efficiency, and firepower.

Table 9 WBT performance metrics variables for the developed briquettes.

For the cold start high power phase, the time to boil reduced from BR-A to BR-D, implying enhanced heating performance. The burning rate and fuel consumption were lower, suggesting better combustion, and the thermal efficiency increased with the proportion of PET plastic, reaching above 54% in BR-D. Similar trends were observed in hot start, with reduced boiling time and improved efficiency. BR-D consistently exhibited the best performance, consuming less fuel and energy per liter, and having greater thermal efficiency. The burning rate and fuel consumption were much lower during simmering, indicating energy conservation. The thermal efficiency continues to increase to more than 73% in BR-D. The firepower was slightly reduced, which was appropriate to keep the heat at a constant low level.

In general, BR-D consistently outperforms the other briquettes across all the stages: it uses less fuel, has higher thermal efficiencies, and boils more quickly. This indicates that BR-D is more efficient in energy consumption, cheaper, and can heat quickly and simmer for a long time, hence it is more ideal for clean cooking [39,63].

3.10 Energetic Properties of Developed Briquettes

Table 10 presents the energetic properties of the developed briquettes BR-A to BR-D. The data show a clear trend of increasing energetic density and Fuel Value Index (FVI) from BR-A to BR-D.

Table 10 Energetic properties of the developed briquettes.

These parameters are essential indicators of the energy storage capacity, combustion efficiency, and general applicability of the briquettes across various applications. Energetic density increases to 16,929.38 MJ/m³ in BR-D as compared to 12,962.40 MJ/m³ in BR-A. An increase in the energetic density means that more energy is stored in a given volume. This increase enhances the efficiency of storage and transport since less volume is needed to produce the same energy output [39]. It also indicates better packing and densification of the briquettes, which may be because of the presence of PET particles or to superior production methods. The FVI rises from 2,365.40 MJ/m³% in BR-A to 6,461.59 MJ/m³% in BR-D. The Fuel Value Index (FVI) essentially measures how much energy a briquette contains based on its volume. The higher the FVI, the more efficient the briquette is at burning and the greater its potential to produce heat. Briquettes with a higher FVI are especially suitable for applications that require a lot of heat or long-lasting heat output [63]. The changes in energy properties observed are likely connected to physical changes in the briquettes’ structure (as shown in Figure 5). These include becoming denser, having fewer pores, and forming stronger bonds between particles. All of these factors help improve how well the briquettes burn and how much energy they retain during combustion.

4. Conclusions

Thermogravimetric analysis was done to explore how different amounts of PET plastics affect the combustion behavior and kinetics of waste-derived briquettes made from blends of sawdust and waste PET plastics. To assess the efficiency and energy consumption of these briquettes, a water boiling test (WBT) was conducted. Four types of briquettes were produced by high-pressure compaction with different mass ratios of the raw materials without binders. The SEM micrographs showed that adding PET improved the internal bonding between particles and reduced porosity, making the briquettes stronger, more durable, and more stable during burning. Overall, the presence of PET significantly influenced their thermal properties by lowering the ignition temperature, decreasing the energy required to initiate combustion, and increasing the temperature at which they are completely burned out. For example, BR-D had the lowest ignition index (0.0466% min^-2 °C^-1), the lowest activation energy (18.67 kJ/mol), and the highest burnout index (4.005E-03% °C^-1 min^-3), which implies fast ignition and robust combustion, although it surprisingly had the highest char residue. In all stages of the WBT, BR-D was always superior to the other briquettes in terms of less fuel, higher thermal efficiencies, and faster boiling of water. These characteristics make BR-D especially appropriate for quick heating and prolonged simmering, which are ideal for clean cooking. Furthermore, BR-D showed the best energy potential with the highest energetic density (16.93 GJ/m³) and fuel value index (6.46 GJ/m³%).

These results are relevant to the development of waste-to-energy conversion technologies and demonstrate that a mixture of sawdust and PET can be used to optimize combustion and increase energy yield during briquette burning. Future research will involve a life-cycle assessment (LCA) to assess the environmental impact of the briquette production.

Acknowledgments

The authors would like to appreciate the financial contributions of the World Bank Centre of Excellence in Reproductive Health Initiative (CERHI), University of Benin, towards the success of this project.

Author Contributions

Peter Ebhodaghe Akhator: Conceptualisation, Methodology, Data curation, Formal analysis, Supervision, Writing (original draft, review and editing), Resources. Ufuoma Georgina Unueroh: Conceptualisation, Methodology, Data curation, Formal analysis, Writing (original draft, review and editing), Resources.

Competing Interests

The authors declare that the study was carried out without any commercial or financial relationships that could present a potential conflict of interest.

Data Availability Statement

Data will be made available on request.

AI-Assisted Technologies Statement

During the preparation of this work, the authors used an artificial intelligence tool (OpenAI) to improve the language and readability. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.