Co-pyrolysis of mixed municipal solid waste components in a semi-pilot scale rotary kiln: synergistic effects on pyrolysis oil and biochar properties

Abstract

Sustainable valorisation of municipal solid waste (MSW) is essential to mitigate landfill pressures, reduce greenhouse gas emissions, and support the circular bioeconomy. This study investigates the sequential co-pyrolysis of wastepaper (WP), textiles (TW), mixed plastics (MP), and mustard husk (MH) in a semi-pilot-scale rotary kiln reactor at 500 °C. The individual feedstocks, binary (WP-MP), ternary (WP-MP-TW; PTP), and quaternary mixtures (PTP-MH; Mix-4) were pyrolysed to evaluate synergistic decomposition behaviour and product upgrading. Thermogravimetric analysis revealed primary devolatilization between 230–550 °C, with mixtures exhibiting shifted decomposition onset due to hydrogen-donor interactions and radical coupling between biomass and polymer-derived intermediates. Mixed plastics yielded the highest pyrolysis oil content (43 wt%) and the lowest char yield (16 wt%), whereas wastepaper produced the highest char (30–40 wt%). Furthermore, co-pyrolysis significantly enhanced product distribution, with PTP and Mix-4 achieving up to 37 wt% pyrolysis oil, 18–28 wt% char, and reduced gas formation compared to pure biomass. The quality of pyrolysis oil improved markedly with blending; calorific value increased from 18.41 MJ kg⁻¹ (WP pyrolysis oil) to 34.67 MJ kg⁻¹ (WP-MP) and 31.32 MJ kg⁻¹ (Mix-4), accompanied by lower water content (1.22–7.00 wt%) and reduced pH (3.2–4.9). GC-MS results confirmed the formation of increased aromatic and aliphatic hydrocarbons in blended feeds, particularly benzene, toluene, xylene, and long-chain alkanes, indicating a synergistic effect of hydrogen transfer and deoxygenation. Finally, char samples exhibited carbon contents of 32.20–76.10 wt% with porous microstructures, supporting their suitability for energy and material applications. The co-pyrolysis enhances the quality of pyrolysis oil and char, offering a scalable pathway for mixed MSW utilisation, reduced preprocessing requirements, and improved resource recovery.

Sustainable valorisation of municipal solid waste (MSW) is essential to mitigate landfill pressures, reduce greenhouse gas emissions, and support the circular bioeconomy.

1. Introduction

The rapid growth of urban populations, industrial activities, and changing consumption patterns has led to a dramatic increase in municipal solid waste (MSW) generation worldwide. MSW generation is expected to rise steadily over the coming decades, placing immense pressure on existing waste management infrastructure and natural ecosystems. Global municipal solid waste (MSW) generation is expected to increase substantially, rising from approximately 2.3 billion tonnes in 2023 to about 3.8 billion tonnes by 2050.¹ Improper handling and disposal of MSW lead to severe environmental consequences, including land degradation, groundwater contamination, air pollution, and the emission of greenhouse gases (GHGs), particularly methane from landfills.² These challenges are especially pronounced in developing and rapidly urbanising countries, where waste segregation, collection efficiency, and treatment capacity often lag waste generation rates. Conventional MSW management strategies such as landfilling and incineration are increasingly regarded as unsustainable. Landfills require large areas of land, pose long-term environmental risks, and result in the loss of potentially valuable resources. Incineration can reduce waste volume and generate energy, but is associated with high capital costs, air pollutant emissions, and public resistance.³ Consequently, there is a growing global emphasis on adopting sustainable waste valorisation strategies that align with circular economy principles by recovering energy and materials from waste streams rather than treating them as liabilities. Municipal solid waste is inherently heterogeneous and typically comprises multiple fractions, such as paper, plastics, textiles, yard waste, and agricultural residues.⁴ In real-world scenarios, complete segregation of these components is often impractical and economically unfeasible. Therefore, understanding the behaviour of multi-component MSW mixtures during thermochemical conversion is essential for designing realistic and scalable waste-to-energy systems. It is noteworthy that, rather than treating MSW as a uniform feedstock, a structured evaluation of interactions among its major components is required to maximise energy recovery and product quality.

Thermochemical conversion technologies, including pyrolysis, gasification, and hydrothermal processing, have emerged as promising routes for MSW valorisation. Among these, pyrolysis has attracted particular attention for its ability to process a wide range of heterogeneous feedstocks under oxygen-free conditions and at moderate temperatures (400–800 °C), producing value-added products such as bio-oil, biochar, and combustible gases.⁵ Pyrolysis offers operational flexibility, comparatively lower emissions than incineration, and the potential for integration with downstream upgrading processes.⁶ However, the pyrolysis of individual MSW components often yields lower efficiency and suboptimal product quality, especially when processing oxygen-rich biomass fractions such as paper, agricultural residues, and food waste. Pyrolysis oil derived from biomass-rich MSW components typically has a high oxygen content (30–45%), acidity, moisture (10–30%), a low heating value (10–30 MJ kg⁻¹), and poor storage stability.⁷ Similarly, biochar produced from single-feed pyrolysis may exhibit inconsistent physicochemical properties, limiting its applicability for energy or material uses.⁸ These limitations have restricted the direct commercial deployment of MSW-derived pyrolysis products and necessitated further process optimisation and feedstock engineering.

Co-pyrolysis is an advanced form of pyrolysis that involves the simultaneous thermal decomposition of two or more feedstocks. The fundamental rationale for co-pyrolysis is to exploit the complementary chemical characteristics of different materials. Oxygen-rich biomass components can be blended with hydrogen-rich polymers such as plastics and synthetic textiles, enabling synergistic interactions during thermal.⁹ These interactions include hydrogen transfer, radical coupling, enhanced cracking of oxygenated compounds, and in situ deoxygenation, which collectively improve bio-oil yield, higher heating value (HHV), and chemical stability while modifying biochar structure and composition. Several studies have demonstrated that co-pyrolysis of biomass with plastics (plastics and rubber) can significantly enhance oil yield and reduce the levels of oxygenated species in the liquid product. Fang et al. (2020) investigated the pyrolysis behaviour of mixed medical waste and found that the product distribution was nearly uniform, with gas, liquid, and solid fractions each accounting for approximately 33% of the total yield at 500 °C.¹⁰ The liquid was a black, viscous tar (8972.82 kcal kg⁻¹), dominated by 60% hydrocarbons and lipids (C₆–C₂₈). However, after acid purification, the char carbon content reached 63.13% with a calorific value of 5454.54 kcal kg⁻¹, indicating strong energy and material recovery potential.¹⁰ Ding et al. (2016) studied the pyrolysis of municipal solid waste in a fluidised bed to produce the liquid fuel.¹¹ Pyrolysis of mixed MSW produced 38–56 wt% liquid, 17–25 wt% char, and more gas at higher temperature.¹¹ Chen et al. (2016) studied the co-pyrolysis of waste newspaper with high-density polyethylene and reported that waste newspaper with HDPE at 400–500 °C showed strong synergy, increasing pyrolysis oil yield by 31.60% while reducing gas and char by 41.80% and 24.40%. The pyrolysis oil exhibited lower water, oxygen, and acidity content, and lower viscosity, with a higher calorific value (26.8–34.8 MJ kg⁻¹).¹² Miandad et al. (2017) studied the pyrolysis of plastic waste into liquid oil and reported that polystyrene yielded the highest liquid oil (80.80%), with lower gas (13%) and char (6.20%). Pyrolysis oils were found to be aromatic-rich, with HHV of 41.4–41.8 MJ kg⁻¹, viscosity of 1.77–1.90 mPa s, and density of 0.91–0.92 g cm⁻³, comparable to diesel fuel. Table 1 lists the literature used to produce pyrolysis oil from waste biomass, plastics, and municipal waste via pyrolysis.

Table 1. List of reported literature on the biomass, plastics and biomass-plastic blends for pyrolysis oil production.

Feedstocks	Biomass to plastic ratio (B/P)	Temperature (°C)	Heating rate (°C min−1)	N2 flow rate mL min−1	Pyrolysis oil yield (wt%)	Char yield (wt%)	Non-condensable gas yield (wt%)	Conversion efficiencya (%)	References
Waste office paper	1 : 0	600	5	60	49.20	35.10	15.70	64.90	19
Textile waste (cotton)	1 : 0	500	10	100	49	18	32	82	20
Wheat straw	1 : 0	500	10	—	41.30	20	37.50	80
PP	0 : 1	520	—	5	62.05	7.31	30.64	92.69	21
PET	0 : 1	650	20	—	53	10.17	33.60	89.83	22
Paper waste and mixed plastics	2 : 3	400	—	450	77.40	28.50	6.47	93.53	23
Paper and mixed plastics waste	2 : 3	500	—	450	76.80	18.10	11.30	88.70	23
Municipal paper-derived cellulose and PET	1 : 3	600	—	150	57	8	35	92	24
Rice straw and polyethylene	1 : 1	600	—	300	57.20	21.50	21.30	78.50	25
Rice straw and polyethylene	1 : 3	600	—	300	74.18	11.66	14.16	88.34	25
Wheat straw and PET	2 : 3	500	10	—	50.10	9.90	40	90.10	26

^aCalculated for liquid and gas fraction; based on conversion efficiency (%) = (100 − biochar yield (%)).

Despite these promising findings, the reported results in the literature vary widely, reflecting differences in feedstock types, blending ratios, reactor configurations, heating rates, and residence times. Another critical limitation of existing research is the predominance of laboratory-scale experiments, often conducted in fixed-bed, micro-reactor, and thermogravimetric systems. Conventional co-pyrolysis studies primarily focus on isolated binary systems. Additionally, conventional reactors such as fixed-bed and fluidised bed exhibit operational limitations, including low specific capacity, longer ramp time and the requirement of feedstock to be finely milled.¹³ Moreover, entrained flow reactors have shorter feedstock reaction time under gravitational acceleration and require dry feedstock to prevent heat consumption due to water evaporation.¹³ Furthermore, the presence of large agglomerates and the expansion of the feedstock can result in the clogging of the reactor, which can increase maintenance costs.¹⁴ Although these studies provide valuable mechanistic insights, it fails to capture the complexities of scale-up, such as heat and mass transfer limitations, vapour–solid interactions, and continuous solid handling. In contrast, rotary kilns exhibit better mixing and heat transfer between feedstock particles within the reactor via surface conduction. Further, rotary reactors are effective in decreasing agglomeration and swelling caused by coking.¹⁴ Compared to other reactors, rotary-bed reactors are utilised for a wide range of sizes and forms of feedstock, have low pretreatment requirements, and are suitable for large-scale continuous operation.¹³ Additionally, rotary kilns are widely recognised for their robustness, ability to process large and heterogeneous feedstocks, enhanced mixing, and industrial relevance. Recent semi-pilot-scale rotary kiln studies have also reported that feedstock composition and additive interactions significantly influence pyrolysis oil yield, hydrocarbon selectivity, and deoxygenation behaviour during waste pyrolysis processes.^15–17 However, systematic co-pyrolysis studies using semi-pilot or pilot-scale rotary kilns remain limited. Furthermore, most existing studies rely on simplified or idealised model compounds rather than realistic heterogeneous municipal and agro-waste fractions. Therefore, the present study establishes a progressive compositional framework to evaluate the evolution of synergistic interactions as waste-stream complexity increases under scalable semi-pilot rotary kiln thermal conditions.

Keeping in mind the above-mentioned research gaps, the present study investigates the co-pyrolysis of MSW components such as wastepaper, mixed plastics, textiles, and mustard husk in a semi-pilot-scale rotary kiln reactor at 500 °C and 20 °C min⁻¹ heating rate. The optimum temperature of 500 °C was chosen based on previously published literature.^14,18 Further, a stepwise blending strategy, progressing from individual components to binary, ternary, and quaternary mixtures, is utilised to systematically elucidate synergistic and antagonistic interactions during thermal degradation. The study integrates thermogravimetric analysis with detailed characterisation of pyrolysis products, including yield distribution, bio-oil composition and properties, and biochar structure and elemental composition. By bridging the gap between laboratory-scale insights and industrially relevant reactor systems, this work aims to advance scalable, efficient, and sustainable MSW valorisation pathways aligned with the circular economy and climate-mitigation objectives. The present study directly supports the Sustainable Development Goals (SDG) by promoting improved waste management and reduced water contamination (SDG 6), advancing affordable and clean energy through waste-derived fuels (SDG 7), mitigating greenhouse gas emissions via diversion of municipal solid waste from landfills (SDG 13), reducing terrestrial and marine pollution through resource recovery (SDG 14), and fostering sustainable land use and ecosystem protection by minimizing uncontrolled waste disposal (SDG 15).

2. Materials and methods

2.1. Sample collection and preparation

Wastepaper, textile waste, and mixed plastics were collected from the waste yard of Manipal University Jaipur, while yard waste and mustard husk were sourced from a nearby village. To minimise compositional variability and improve reproducibility, approximately 5–10 kg of each waste stream was collected, manually sorted, homogenised, and used as a representative bulk sample prior to experimentation. The mixed plastic fraction predominantly consisted of low-density polyethylene (LDPE) and polypropylene (PP), and a very low amount of polystyrene (PS) and high-density polyethylene (HDPE), <2% identified through manual segregation and visual assessment of commonly discarded packaging materials. The wastepaper fraction included printed sheets, magazines, and packaging paper collected from the university waste stream. Textile waste comprised a mixture of cotton, polyester, and wool. All the samples were thoroughly washed with hot water to remove surface contaminants such as dust, air-dried at ambient conditions for 2–3 days, and subsequently oven-dried at 105 °C for 1 h to ensure complete moisture removal. The dried materials were shredded to an average particle size of approximately 50 mm. Each experiment was conducted using 2 kg of feedstock. Binary (1 : 1), ternary (1 : 1 : 1), and quaternary (1 : 1 : 1 : 1; Mix-4) blends were prepared in equal proportions to systematically evaluate synergistic effects on thermal degradation behaviour and product quality.

2.2. Physicochemical characterisation of feedstocks

The physicochemical characterisation of the samples was performed in accordance with the ASTM standard. The proximate analysis was performed according to ASTM D3173, D3174, and D3175, and the ultimate analysis was performed using a CHNS elemental analyser (Variael CUBE, Germany). The higher heating values (HHV) of MSW and pyrolysis were calculated using an oxygen bomb calorimeter (model 1341, Plain Jacket Calorimeter) according to ASTM D240. The CHO index represents the relative proportions of carbon (C), hydrogen (H), and oxygen (O) in a material and typically ranges from −4 to +4. During pyrolysis, the CHO index indicates the degree of deoxygenation and aromatisation. Lower (negative) values reflect diminished oxidative character, whereas higher (positive) values indicate a stronger oxidation tendency, both of which influence the higher heating value (HHV) of the samples. The CHO index was determined using eqn (1).

1

2.3. Thermal stability analysis

The thermal degradation behaviours were analysed using a thermogravimetric analyser (TGA, DTG-60, Shimadzu), under pyrolytic conditions with N₂ gas at a flow rate of 50 mL min⁻¹, from room temperature to 900 °C at a rate of 20 °C min⁻¹, with approximately 5–10 mg of sample. Analysis was conducted under non-isothermal conditions to plot weight-loss versus temperature curves.

2.4. FTIR analysis

Fourier Transform Infrared (Bruker alpha) was used to analyse functional groups in the feedstocks (pyrolysis oil and char). To prepare the sample, the feed material was dried KBr, which was then uniformly added to the holder at a 1 : 100 ratio. Spectral scanning was performed from 400 to 4000 cm⁻¹ at a 4 cm⁻¹ step size, with 40 steps. Each measurement was performed under standardised conditions during factory calibrations to ensure precision, accuracy, and reproducibility.

2.5. Experimental setup and procedure

Pyrolysis was carried out in a semi-batch rotary kiln reactor, using different solid waste materials and their mixtures at 500 °C. Fig. 1 shows a horizontal reactor setup, made of stainless steel 304, 1360 mm long, with an O.D. of 355 mm and a wall thickness of 3 mm. The rotary kiln was coupled with the primary oil collection vessel maintained at 200 °C, with three condensers arranged in series. The reactor was purged with nitrogen before and after charging the feed to ensure an inert atmosphere for pyrolysis, and it was maintained at 5 L min⁻¹ throughout the experiment. To maximise devolatilization of the feedstock under the given conditions, the rotary kiln was operated at 2 rpm and maintained at 500 °C with a heating rate of 20 °C min⁻¹ for 90 min at atmospheric pressure. The reactor temperature was measured at three locations to maintain thermal uniformity during the process. To provide external heat, three 6 kW electrical heaters were used to heat the rotary kiln drum. Additionally, the temperature of the primary collection vessel was controlled by a 2 kW electrical heater. The evolved gases were condensed by a water cooled counter current single-pass, shell-and-tube condenser system. Cooling water entered at near-ambient temperature (25–30 °C) and typically exited at 35–50 °C. An oil collection vessel was positioned at the bottom of each condenser. Pyrolysis was followed by cooling down the reactor to room temperature, and biochar was collected by a vacuum-assisted solid collection system. The reactor and vapour transfer lines were then cleaned with compressed air prior to conducting the next experimental run. The pyrolysis temperature of 500 °C was selected based on previous literature reporting effective co-pyrolysis performance for lignocellulosic biomass and polyolefin-rich plastic waste under intermediate pyrolysis conditions.^14,18 The operating conditions were maintained constant throughout the study to isolate the effect of sequential feedstock blending on product distribution and synergistic interaction behaviour. A gas flowmeter (Raychem G1-6 BS/EN 1359) was connected at the end of the vapour pipeline, along with a scrubber made of compressed knitted stainless-steel mesh. The yields of pyrolysis oil, char, and non-condensable gas fraction were calculated using eqn (2)–(4).

2

3

% Gas yield = 100 − (% Liquid yield + % char yield) 4

Fig. 1. A systematic layout of the rotary kiln pyrolysis reactor.

2.6. Physicochemical characterisation of pyrolysis oil

Moisture content of the pyrolysis oil was determined using a Karl Fischer water analyser (Metrohm 787 KF Titrino) in accordance with ASTM E-203 and ASTM E-1064, with a standard error of ±3.50%. Furthermore, the density of the pyrolysis oil was measured using a density meter (Anton Paar DMA-4500M), and the mean value was reported. The acidity of the pyrolytic oil was measured using an Eutech waterproof pH meter (pH Spear), which was calibrated with standard buffer solutions of pH 4, 7, and 10 prior to analysis.

2.7. GC-MS analysis

The pyrolysis oil collected from condensers was mixed with acetone for analysis. Subsequently, the chemical composition of the pyrolysis oil was analysed and identified using a GC-MS (GC-MS-QP2020 NX, Shimadzu, Japan) with an Rxi-5 MS column (30 m × 0.25 mm ID, 0.25 µm film thickness). The column was maintained at 60 °C and 250 °C for sample injection. Helium was used as the carrier gas with a flow rate of 2 mL min⁻¹ and a split ratio of 20. The oven temperature was initially increased to 120 °C at 5 °C min⁻¹, followed by a ramp to 270 °C at 10 °C min⁻¹. The MS ion source and interface temperatures were maintained at 230 °C and 250 °C, respectively. For pre-processing, 200 mg of pyrolysis oil was dissolved in 10 mL of solvent (acetone/hexane). The solution was then diluted further and dehydrated using 4 g of anhydrous sodium sulphate. Finally, the sample was filtered through a 0.22 µm syringe mesh filter. Chromatogram peaks were identified using the NIST library after peak integration.

2.8. XRF analysis of char

X-ray fluorescence (XRF) spectroscopy was used to identify the inorganic composition of biochar on the Epsilon-1 Panalytical system (Germany). Biochar samples were burned before analysis to obtain ash, which was ground to a homogeneous powder. The samples were irradiated with primary X-rays, and the sample atoms fluoresced in characteristic patterns, allowing the identification and measurement of major and trace elements (Na, Al, K, Ca, Si, Fe, P, and Mg).

2.9. SEM and EDS analysis of char

The surface morphology of the produced char at 500 °C was analysed using SEM. Surface morphology analysis was carried out using a Scanning Electron Microscope (SEM, Zeiss Supra 40) equipped with an Energy Dispersive X-ray Spectrometer (EDS, Oxford INCA Energy 350). The oven-dried char samples were mounted on carbon tape and double-gold sputter-coated to minimise charging effects. SEM analysis was performed under high-vacuum conditions at an accelerating voltage ranging from 5 to 15 kV.

3. Results and discussions

3.1. Physicochemical characterisation of feedstocks

The physicochemical characterisation of the feedstock is listed in Table 2 along with other reported biomass, such as rapeseed stalk,²⁷ PET, PP, and PVC,²⁷ HDPE, LDPE, and PS,²⁸ and wastepaper.¹² The physicochemical properties of the investigated feedstocks clearly demonstrate substantial variability among biomass wastes, plastics, and their composites, which strongly governs their thermochemical conversion behaviour. From Table 2, it was noticed that waste plastics exhibited the highest volatile matter content among all the samples, followed by textiles, mustard husk and wastepaper. Results confirmed that moisture content varies widely among the samples. Biomass-based wastes such as wastepaper (9.65 wt%), mustard husk (11.55 wt%), rapeseed stalk (5.10 wt%), and waste newspaper (6.50 wt%) exhibit higher moisture levels due to their hydrophilic lignocellulosic structure.²⁹ In contrast, plastics and polymer-rich materials (PP, PET, HDPE, LDPE, PS, PVC) show negligible moisture content, reflecting their hydrophobic nature. A lower moisture content is advantageous for pyrolysis, as less energy is required for water evaporation, thereby improving thermal efficiency.³⁰ Ash content was determined to be higher in biomass-derived feedstock, such as wastepaper (13.97 wt%), RS (10.14 wt%), and WS (11.20 wt%). This is attributed to inherent mineral matter such as silica, alkali, and alkaline earth metals.³¹ Plastics exhibit minimal ash (<4.1 wt%), confirming their cleaner inorganic profile. The higher ash content is known to catalyse secondary cracking and char formation, explaining the higher solid yields typically observed for biomass during pyrolysis.³² The volatile matter (VM) provides insight into devolatilization potential. The plastics show extremely high VM (≥95 wt%), with PP, PS, and LDPE approaching 100 wt%, indicating near-complete volatilisation. However, the biomass wastes exhibit moderately high VM (69–78 wt%), while their fixed carbon (FC) content is comparatively higher (7–13 wt%). The higher FC in biomass reflects the presence of lignin and aromatic char-forming structures, which enhance solid residue formation.³² During pyrolysis, moderately high volatile matter (69–78 wt%) enables rapid devolatilization and gas and pyrolysis oil formation, while higher fixed carbon (7–13 wt%) promotes char formation due to lignin-rich, aromatic structures, resulting in greater solid yield and enhanced thermal stability of biomass-derived char.³³

Table 2. Physicochemical characterisation of feedstocks for their bioenergy potential^a.

Sample	Proximate analysis (wt%)				Elemental analysis (wt%)					H/C	O/C	CHO index	HHV (MJ kg−1)	References
	Moisture	Ash	VM	FC	C	H	N	S	O
Waste paper	9.65	13.97	69.47	6.91	40.1	2.75	1.56	0.23	55.35	0.82	1.03	1.24	7.57	Present study
Textile	6.72	5.50	77.56	10.22	60.4	3.55	1.40	—	34.65	0.70	0.43	−0.16	19.30
Mix plastics	1.31	5.01	89.17	3.59	78.23	8.42	0.44	1.06	11.85	1.29	0.11	−1.07	36.58
Mustard husk	11.55	5.13	70.71	12.56	48.01	5.19	0.92	0.46	45.42	1.29	0.70	0.11	15.58
Paper-plastic composite (PTP)	1.73	9.32	78.16	10.78	71.20	5.17	0.99	0.081	22.55	0.87	0.23	−0.41	31.40
RS	5.10	10.14	73.94	10.82	49.63	6.09	0.36	0.01	44.91	1.47	0.67	−0.13	15.63	27
PET	0	0	93.61	6.39	61.73	4.36	0	0	33.91	0.84	0.41	−0.03	21.96
PP	0	0	99.20	0.90	85.14	11.07	0	3.79	0	1.56	0	−1.56	39.13
PVC	0	0	95.05	4.94	30.25	3.17	0	0	—	1.25	0	−1.25	20.35
HDPE	0	4.10	98.10	1.90	82.50	13.10	0.10	0	1.80	1.90	0.01	−1.88	46.49	28
LDPE	0	0.30	100	0	83.30	12.60	0.10	0	0.70	1.81	0.006	−1.80	46.24
PS	0	1.60	100.00	0	87.80	9.50	0	0	0	1.29	0	−1.30	43.41
WS	6.50	11.20	72.80	9.50	39.78	5.50	0.10	—	54.62	1.66	1.03	0.40	14.57	12

^aRapeseed stalk = RS, polyethylene terephthalate = PET, polypropylene = PP, and polyvinyl chloride = PVC, high density polyethylene = HDPE, low density polyethylene = LDPE, and polystyrene = PS, waste newspaper = WS, FC = fixed carbon, VM = volatile matter, C = carbon, H = hydrogen, O = oxygen, N = nitrogen and S = sulphur.

Furthermore, the elemental analysis confirmed that the carbon content ranges from 40–50 wt% for wastepaper, rapeseed stalk, and mustard husk, whereas plastics are highly carbon-rich, with PP, PS, HDPE, and LDPE containing >82 wt% carbon content. In addition, the textile waste and paper-plastic composite (PTP) show intermediate carbon contents (60.40 wt% and 71.20 wt%, respectively), indicating the coexistence of natural and synthetic fibers.³⁴ Also, hydrogen content is substantially higher in plastics (except PET and PVC) (8.40–13.10 wt%) than in biomass (2.70–6.10 wt%). This hydrogen richness enhances liquid hydrocarbon formation and suppresses char via hydrogen donation during pyrolysis.³⁵ The oxygen content exhibits the opposite trend, exceeding 45 wt% in wastepaper, RS, and mustard husk, while remaining below 12 wt% in mixed plastics and nearly zero in polyolefins. The high oxygen content is associated with lower energy density and increased formation of CO, CO₂, and oxygenated vapours.³⁶ Nitrogen and sulphur contents are generally lower across all samples, though textile waste and wastepaper show slightly elevated nitrogen levels (1.40–1.60 wt%), likely originating from dyes, additives and protein-based fibres.³⁷ Sulphur is negligible in most samples, except in mixed plastics and PP, where it reflects the presence of polymer additives. The lower N and S contents are favourable from an emissions perspective, reducing the risk of NO_x and SO_x formation.³⁸

Fig. 2 presents a Van–Krevelen (H/C vs. O/C) comparison of various waste feedstocks and polymeric materials, highlighting their relative aromaticity and energy potential. Biomass-derived wastes such as wastepaper (WP), textile, mustard husk, and rapeseed stalk (RS) cluster at higher O/C ratios (0.40–1.0) and moderate H/C ratios (0.70–1.50), reflecting oxygen-rich functional groups and lower calorific values.³⁹ Among them, textiles exhibit the highest O/C and lowest H/C, indicating greater oxidation and lower fuel quality than WP and agricultural residues. In contrast, virgin and waste plastics (HDPE, LDPE, PP, PS, PVC) exhibit very low O/C ratios (<0.05) and higher H/C ratios (1.20–1.90), confirming their hydrocarbon-rich nature and higher heating values. HDPE and LDPE exhibit the highest H/C ratios, consistent with saturated polyolefin structures, whereas PVC shows a lower H/C ratio due to heteroatom substitution.^40,41 Mixed plastics fall between pure polymers and biomass, reflecting compositional heterogeneity. Remarkably, paper-plastic composites and PET shift toward intermediate O/C with reduced H/C, indicating partial oxygen incorporation and moderated energy content. The final trend along the arrow demonstrates decreasing O/C and increasing H/C with rising heating value, clearly distinguishing plastics from lignocellulosic wastes and supporting feedstock selection for thermochemical conversion.³⁹ The CHO index is a compositional indicator derived from elemental carbon, hydrogen, and oxygen that reflects the degree of oxygenation and fuel quality of a feedstock. The biomass wastes exhibit higher CHO index values (−0.11 to 1.24) due to their oxygen-rich lignocellulosic structure, indicating lower energy density and greater char/gas formation during pyrolysis.³⁰ In contrast, plastics exhibit very low and negative CHO indices (−1.88 to −0.03), confirming their hydrocarbon-dominated nature and high calorific value. Textile waste and paper-plastic composites show intermediate CHO indices (−0.16 to −0.41), demonstrating improved fuel quality compared to pure biomass. The results confirmed that a lower CHO index corresponds to a higher heating value and greater oil yield potential, validating its usefulness for feedstock evaluation in thermochemical conversion.³⁰ Finally, the HHV values strongly correlate with the elemental composition of feeds. Plastics exhibit the highest HHVs, ranging from 36.60 MJ kg⁻¹ for mixed plastics to 46 MJ kg⁻¹ for HDPE and LDPE, values comparable to those of conventional fossil fuels.²⁸ Biomass wastes show much lower HHVs (7.6–15.60 MJ kg⁻¹), constrained by high oxygen and ash contents.¹² Textile waste (19.30 MJ kg⁻¹) and PET (21.96 MJ kg⁻¹) display moderate HHVs, while the paper-plastic composite (31.40 MJ kg⁻¹) demonstrates a substantial improvement over pure biomass. This enhancement confirms the synergistic benefit of incorporating plastics into biomass matrices for energy recovery.²⁷ The physicochemical results clearly indicate that plastics are hydrogen-rich, low-ash, high-energy feedstocks, favouring liquid and gaseous product formation. Biomass wastes are oxygen-rich and ash-laden, which promotes char formation and lowers energy efficiency. However, the composite materials such as PTP effectively bridge this gap by combining the favourable hydrogen content of plastics with biomass-derived structure, resulting in improved HHV and balanced proximate properties.

Fig. 2. Van–Krevelen diagram of the molar ratio of the feedstocks.

3.2. Thermal analysis of feedstocks

Thermogravimetric analysis (TGA) was employed to evaluate the thermal degradation behaviour of individual textile fibres, waste plastics, biomass wastes, and their mixed feedstocks. The mass loss profiles provide insight into decomposition stages, thermal stability, and interactions among different waste components during pyrolysis. Fig. 3(a) shows the TGA curves of polyester, cotton, wool, silk, and viscose, which exhibit distinctly different degradation behaviours due to variations in chemical composition and polymer structure. All textile samples display an initial minor weight loss below 120 °C, attributed to the evaporation of physically adsorbed moisture.³⁹ Natural fibres such as cotton, wool, silk, and viscose exhibit greater moisture loss than polyester due to their hydrophilic functional groups.⁴² Furthermore, cotton and viscose are very cellulose-rich fibres, and they undergo their main thermal degradation between 280 and 380 °C, corresponding to depolymerisation, dehydration, and cleavage of glycosidic linkages. Cotton shows almost complete degradation by 550 °C, leaving negligible char, reflecting the lower aromaticity of cellulose.⁴³ Also, wool and silk are rich in protein-based fibres and exhibit broader degradation profiles. Their main mass loss occurs between 250 and 450 °C, associated with the breakdown of peptide bonds, disulfide linkages, and side-chain functional groups.⁴⁴ The higher char production in wool is attributed to the presence of sulphur-containing amino acids and crosslinked structure, which favour char formation. Polyester exhibits the highest thermal stability, with a sharp degradation step at 420–480 °C, corresponding to ester bond scission and chain unzipping reactions.⁴⁵ Polyester leaves minimal residue, consistent with its thermoplastic nature and limited char-forming tendency. Furthermore, Fig. 3(b) shows the thermal degradation of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS). The plastics exhibit single-step degradation with negligible mass loss below 350 °C, indicating excellent thermal stability at low temperatures, unlike biomass-based materials.⁴⁰ PS decomposes at relatively lower temperatures around 380–430 °C, due to depolymerization via random chain scission and monomer (styrene) formation. The rapid and complete mass loss reflects the aromatic backbone's tendency toward volatilization rather than char formation.⁴⁰ Polyolefins (LDPE, HDPE, and PP) degrade mainly between 420 and 520 °C via β-scission and radical chain reactions.⁴⁰ HDPE shows slightly higher decomposition temperature than LDPE because of its higher crystallinity and linear structure. PP degrades marginally earlier due to tertiary carbon atoms that favour chain scission.^40,41 It is noteworthy to mention that all plastics leave negligible solid residue, confirming their suitability as hydrogen-rich feedstocks for pyrolysis and co-processing with biomass. Furthermore, Fig. 3(c) shows the thermal degradation behaviour of wastepaper (WP) and mustard husk (MH). It was found that the first stage (<150 °C) corresponds to the removal of unbound moisture.²⁸ The second stage, occurring between 220 and 380 °C, represents the active devolatilization region, dominated by the decomposition of hemicellulose and cellulose.⁴⁶ Mustard husk reaches 450 °C, indicating overlapping degradation of cellulose and lignin. The presence of lignin, which slowly decomposes over a wide temperature range, enhances thermal stability and increases char yield.⁴⁷ In the third stage (>500 °C), a gradual mass loss is observed due to lignin cracking and secondary char rearrangements. Mustard husk leaves a higher residual mass than wastepaper, highlighting its superior char-forming ability. These results demonstrate that agricultural residues are more favourable for biochar production than paper-based wastes.⁴⁷ Finally, Fig. 3(d) shows the decomposition profile of binary (paper-plastic), ternary (PTP), and quaternary (Mix-4) waste blends. The mixed systems display broadened degradation regions, combining features of biomass and plastic components. The initial mass loss below 150 °C is governed by moisture from biomass fractions.⁴⁷ The major degradation region (300–500 °C) reflects the simultaneous decomposition of cellulose-rich materials and plastics. Compared to individual components, the mixed samples exhibit shifted DTG peaks, suggesting synergistic interactions.^40,47 The ternary and quaternary mixtures show enhanced mass loss rates and reduced char residues relative to pure biomass, indicating that hydrogen-rich plastics promote secondary cracking of biomass-derived char and volatiles. This hydrogen-donor effect improves thermal conversion efficiency.⁴⁰ Among all blends, the quaternary mix demonstrates the most balanced degradation behaviour, with moderate thermal stability and controlled residue formation. The results suggest that co-pyrolysis of mixed wastes can optimise devolatilization and char quality, making it suitable for integrated waste-to-energy or biochar applications.

Fig. 3. Thermal decomposition profiles of different solid waste (a) textiles, (b) plastics, (c) waste papers (WP), mustard husk (MH) and (d) binary, ternary and quaternary mixture.

Fig. 4(a) shows the DTG curves of mixed plastics, mixed textiles, wool/viscose, and cotton, reflecting differences in polymer chemistry. Cotton and wool/viscose exhibit sharp, intense peaks around 320–380 °C, indicating rapid depolymerisation of cellulose and protein-based structures.⁴⁵ Cotton shows a single dominant peak, consistent with cellulose dehydration and chain scission, whereas wool/viscose displays a slightly broader response due to overlapping degradation of viscose cellulose and wool keratin, including peptide bond cleavage.⁴¹ Mixed textiles present a wider, multi-step DTG profile with a main peak near 450–520 °C, suggesting combined degradation of natural fibers and synthetic components (polyester), leading to delayed mass-loss maxima. Mixed plastics exhibit the highest-temperature, narrowest peak (420–500 °C), characteristic of chain scission in polyolefins (LDPE/PP), with minimal low-temperature loss due to their low moisture content and absence of labile functional groups.⁴⁰ Furthermore, Fig. 4(b) shows the DTG profiles of paper-plastic binary, PTP-Ternary and mix-4, which reveal synergistic and overlapping degradation behaviours in the blended waste systems. The paper-plastic binary shows two pronounced peaks: an early peak at 280–350 °C, attributed to cellulose decomposition from paper, followed by a higher-temperature peak at 430–520 °C, corresponding to polyolefin chain scission, indicating limited interaction between components.⁴⁰ The PTP ternary exhibits broader, less intense peaks, suggesting partial thermal interactions among paper, textiles, and plastics that smooth individual degradation events and enhance apparent thermal stability.⁴¹ Mix-4 displays the sharpest and most intense main peak near 480–520 °C, reflecting combined degradation, enhanced heat transfer, and the catalytic effects of char formed from lignocellulosic fractions.⁴³ Lastly, Fig. 4(c) shows the DTG curves of MH and WP, which show clear differences in decomposition behaviour and thermal stability. WP exhibits a sharp and intense peak around 350–380 °C, indicating rapid degradation dominated by cellulose depolymerisation and volatilisation of paper components, with minimal secondary reactions.⁴⁷ In contrast, MH shows a broader, deeper central peak centred at 280–330 °C, followed by a secondary shoulder at 450–520 °C. This multi-step pattern suggests heterogeneous organic constituents, with early degradation arising from labile fractions and later mass loss corresponding to more thermally stable constituents.⁴⁷ The broader DTG profile of MH reflects complex interactions and progressive breakdown, while WP undergoes more uniform and abrupt decomposition. The DTG profiles of the feedstocks show distributed thermal degradation.

Fig. 4. DTG profile of (a) cotton, wool, viscose, mixed plastic and textile waste, (b) paper-plastic binary, PTP-Ternary and mix-4, and (c) mustard husk and wastepaper.

3.3. Pyrolysis products yield

The pyrolysis products (pyrolysis oil, char, and gas) yields obtained at 500 °C are listed in Fig. 5. It was observed that, among biomass-rich, plastic-rich, and mixed feedstocks, there were variations in chemical composition and thermal degradation pathways. The reported product-yield ranges and corresponding error bars represent the experimental variation obtained from repeated pyrolysis experiments under identical operating conditions. Waste paper and textile wastes exhibit relatively lower pyrolysis oil yields (20–25 wt%) and higher char (28–32 wt%). This behaviour is attributed to their lignocellulosic and fibrous nature, which promotes dehydration, cross-linking, and the formation of a carbonaceous solid residue during pyrolysis.⁴⁸ The gas yield for these materials remains comparatively high (45–48 wt%), indicating extensive devolatilization and secondary cracking of oxygenated vapours.⁴⁸ The pure plastics exhibit a markedly different trend, yielding the highest pyrolysis oil content (40–45 wt%) and the lowest char content (5–7 wt%). The production of higher liquid products is attributed to the random chain scission of polyolefins, which results in the generation of long-chain hydrocarbons that readily condense as pyrolysis oil.⁴⁹ Further, the formation of gas increases due to β-scission and secondary cracking reactions at higher temperatures.⁵⁰ The results (Fig. 5) confirmed that among the agricultural residues, mustard husk yields moderate oil (30–33 wt%) with substantial char (22–25 wt%). The presence of higher char yield obtained from biomass pyrolysis confirms the presence of lignin and mineral matter, which favour the formation of solid residues.⁴⁷ The gas fraction remains significant, consistent with the decomposition of hemicellulose and cellulose components.⁴⁷ The paper-plastic mixture demonstrated improved pyrolysis oil yield (32–35 wt%) and decreased char yield compared to raw biomass, suggesting a synergistic role of hydrogen-rich plastics in stabilising biomass-derived radicals and suppressing char formation.⁴⁹ The PTP mixture exhibits an increase in pyrolysis oil yield (35–38 wt%), suggesting improved hydrogen transfer and more efficient volatilisation due to the presence of plastics.⁵¹ Furthermore, the mix-4 delivers the most balanced product distribution, with higher pyrolysis oil yield (38–40 wt%), moderate char (30 wt%), and reduced non-condensable gas yield. This indicates strong synergistic interactions among multiple waste components, with plastics enhancing liquid production and biomass contributing to char formation.⁵¹ Products derived from different waste mixtures were strongly influenced by the composition of the individual feedstocks. The results demonstrate that co-pyrolysis of biomass and plastics significantly improves pyrolysis oil yield and process flexibility compared to individual feedstocks, highlighting its potential for integrated waste-to-energy and resource recovery applications.

Fig. 5. Pyrolysis product yields for different components and blended samples.

3.4. Physicochemical characterisation of pyrolysis oil

The physicochemical properties of the pyrolysis oils obtained from different waste feedstocks (Table 3) provide critical insight into their fuel quality, chemical nature, and suitability for energy recovery. The obtained results were compared with other reported pyrolysis oils, including rice husk,⁵² waste newspaper (WS), HDPE, WS/HDPE blend,¹² PP⁵³ and commercial diesel.¹² The key parameters, such as HHV, moisture content, density, and pH, clearly reflect the influence of feedstock composition and pyrolysis reaction pathways. The HHV of the pyrolysis oils varies widely, from 12.10 MJ kg⁻¹ for mustard husk to 39.43 MJ kg⁻¹ for mixed-plastics pyrolysis oil. Biomass-derived pyrolysis oils from wastepaper (18.41 MJ kg⁻¹) and mustard husk exhibit relatively lower HHV (12.10 MJ kg⁻¹) due to their high oxygenated compound content, including acids, ketones, phenolics, and sugars.⁵⁴ These oxygen-rich species reduce the pyrolysis oil's energy density by lowering the C/H ratio and increasing polarity.⁵⁵ In contrast, plastic-derived pyrolysis oil displays a significantly higher HHV (Table 3), approaching that of commercial diesel (42–46 MJ kg⁻¹), which can be attributed to its dominance of aliphatic and aromatic hydrocarbons formed via random chain scission and β-scission reactions of polymers.⁵⁶ Further, the pyrolysis oil derived from paper-plastic blends, PTP and Mix-4 (31–35 MJ kg⁻¹), showed an improved HHV compared to raw pyrolysis oils. The improved HHV was attributed to hydrogen transfer and radical-stabilisation mechanisms during co-pyrolysis, which suppressed oxygenated species and promoted hydrocarbon formation.⁵⁷ Additionally, the findings demonstrated a clear synergistic effect when plastics were incorporated, substantially improving the fuel quality of biomass-derived pyrolysis oils. Water content is a crucial parameter influencing pyrolysis oil stability and combustion efficiency. Biomass-derived pyrolysis oils show higher water contents, particularly mustard husk (31 wt%), rice husk (25.20 wt%), and waste newspaper (53.09 wt%). This is primarily caused by dehydration reactions of cellulose and hemicellulose, as well as the release of inherent moisture from biomass.⁵⁸ High water content reduces HHV, increases viscosity instability, and limits direct fuel applications.⁵⁸ In contrast, plastic-derived pyrolysis oil contains very low water (1.22 wt%), reflecting the absence of oxygenated functional groups in polymers. Furthermore, the blended pyrolysis oils (paper-plastic, PTP, and Mix-4) show intermediate water contents (4–9 wt%), confirming that plastics effectively dilute biomass-derived water formation.⁵⁸ This reduction enhances oil stability and indicates greater upgrading potential than pure biomass oils. The density of pyrolysis oils ranges from 0.93 to 1.15 g mL⁻¹, which is generally higher than that of commercial diesel (0.82–0.85 g mL⁻¹). Biomass pyrolysis oils tend to have higher densities (≥1.01 g mL⁻¹) due to the presence of heavy oxygenated compounds and water.⁵⁹ Mustard husk and rice husk pyrolysis oils show the highest densities, consistent with their elevated water and char precursor contents. Also, the plastic pyrolysis oil has a lower density (0.99 g mL⁻¹), closer to that of diesel, reflecting its hydrocarbon-rich composition. Blended oils such as paper-plastic and Mix-4 show reduced densities (0.93–0.95 g mL⁻¹), indicating a shift toward lighter fractions and improved volatility.⁶⁰ This trend supports the role of plastics in improving the physicochemical quality of pyrolysis oils. All biomass-derived and blended pyrolysis oils are acidic, with pH values between 2.5 and 3.8, while plastic pyrolysis oil shows a comparatively higher pH (4.9). The acidity of biomass pyrolysis oils arises from organic acids (acetic and formic acids) formed during cellulose and hemicellulose decomposition. High acidity can lead to corrosion issues and limit direct engine use.^61,62 The inclusion of plastics increases the pH of mixed pyrolysis oils, indicating a reduction in acid concentration due to hydrogenation and dilution. Although still acidic compared to diesel (pH 5–6), blended pyrolysis oils demonstrate improved chemical stability and reduced corrosiveness.⁶¹ The physicochemical results confirm that plastic-derived pyrolysis oils possess superior fuel properties, while biomass-derived pyrolysis oils are constrained by high water content, acidity, and lower energy density. In addition, co-pyrolysis oils from mixed feedstocks successfully bridge this gap by combining the hydrocarbon richness of plastics with biomass. Positive synergistic deviations are highly dependent on the selected pyrolysis temperature and the overlapping devolatilization behaviour of biomass and plastic fractions, which promote hydrogen-transfer and volatile stabilisation reactions during co-pyrolysis. The higher experimental oil yields of PTP (37 wt%) and Mix-4 (38 wt%) were observed, along with the samples' paper-plastic and PTP pyrolysis oils, which showed the most favourable balance between HHV, water content, density, and acidity, making them promising candidates for further upgrading.

Table 3. Physicochemical characterisation of pyrolysis oil obtained from various feedstocks at 500 °C.

Component	HHV (MJ kg−1)	Water content (wt%)	Density at 25 °C (g mL−1)	pH	References
Waste paper	18.41	8.01	1.01	3	Present study
Textile	28	3.21	1.01	2.90
Mix plastics	39.43	1.22	0.99	4.90
Mustard husk	12.10	31	1.15	2.50
Paper plastic	34.67	4.31	0.93	3.80
PTP	32.70	7	0.98	3.50
Mix-4	31.32	9.41	0.95	3.20
Rice husk	24.80	25.2	1.19	2.80	52
Waste newspaper (WS)	16.98	53.09	1.36	2.60	12
HDPE	43.80	—	1.12	7.20	12
WS/HDPE blend	26.78–34.79	0.5–1.20	1.14–1.24	4.5–4.8	12
PP	47.10	0.004	0.77	—	53
Commercial diesel	42–46	0–0.02	0.82–0.85	5–6	12

3.5. GC-MS of pyrolysis oil

Fig. 6 presents the relative peak area (%) distribution of major chemical classes identified in the pyrolysis oil of wastepaper, textiles, plastics, mustard husk, paper-plastic blends, PTP, and Mix-4. The results clearly demonstrate strong feedstock-dependent trends and positive synergistic effects in mixed systems. From Fig. 6, it was observed that wastepaper and mustard husk are dominated by oxygenated compounds, reflecting their lignocellulosic composition.²⁹ The result confirmed that the wastepaper includes aliphatic (25–30%), ketones (18–20%), furans and furfurals (12–15%), and anhydrosugars (15–18%) constitute the major fractions. The presence of anhydrosugars (levoglucosan derivatives) is characteristic of cellulose depolymerisation, while furans originate from the dehydration of hemicellulose.⁶³ The phenolic compounds (5–8%) arise from lignin fragmentation, indicating partial cleavage of ether and C–C linkages.⁶⁴ Alcohols and acids remain minor components, suggesting limited secondary cracking under the applied pyrolysis conditions.⁶⁴ Mustard husk exhibits higher fractions of phenolics (12–15%) and ketones (15–18%) than wastepaper, attributed to its higher lignin and extractive content. The reduced anhydrosugar contribution relative to wastepaper indicates enhanced secondary reactions, where primary cellulose-derived vapours undergo cracking and repolymerization.⁵⁴ These trends explain the relatively higher char formation observed during the pyrolysis of mustard husk.⁵⁴ Textile waste shows a more complex composition due to the coexistence of natural and synthetic fibres. The oxygenated compounds (ketones, acids, and furans) remain significant, and aliphatic (35–40%) increase notably, reflecting polyester and synthetic fibre degradation.⁶⁵ The reduction in anhydrosugars compared to wastepaper confirms dilution of cellulose-derived products and partial hydrogen transfer from synthetic polymers.²⁰ Furthermore, the plastics display a fundamentally different pyrolysis oil composition. The pyrolysis oil is overwhelmingly dominated by aliphatic (55–60%) and aromatics (25–30%), with minimal oxygenated species. This is consistent with random chain scission and β-scission reactions of polyolefins, yielding long-chain hydrocarbons, olefins, and aromatised fragments at elevated temperatures.⁶⁶ The near absence of acids, furans, and anhydrosugars confirms the plastics' negligible oxygen content. Also, alcohols primarily appear as low-molecular-weight fragments formed by secondary radical recombination.⁶⁷ The dominance of aliphatic and aromatic compounds explains the higher heating value and improved fuel nature of plastic-derived oils. However, it also highlights the limited chemical diversity and lack of oxygenated functional groups, which may influence downstream upgrading strategies.³⁶ The paper-plastic mixture demonstrated a clear synergistic behaviour. The results (Fig. 6) confirmed that the percentage of aliphatic compounds increases sharply (50–55%) compared to pure wastepaper, while oxygenated compounds such as anhydrosugars, acids, and furans decrease significantly.⁶⁸ This shift suggests effective hydrogen donation from plastics, which stabilises biomass derived oxygenated radicals and suppresses repolymerization into char. The aromatic content (15–18%) also increases relative to biomass alone, suggesting enhanced secondary cracking and aromatization.⁶⁸ The PTP mixture indicates that aliphatics remain the dominant class (40–45%), while aromatics increase to 20–25%. The phenolic content was found to be persistent (8–10%), reflecting residual lignin-derived contributions, but at a lower level than in pure biomass.²⁷ The reduced acids and furans indicate improved vapour quality, with lower oxygen content and enhanced fuel characteristics. This compositional shift aligns well with the observed increase in oil yield and HHV for PTP.³³ Furthermore, the mix-4 exhibits the most balanced and diversified compositions of pyrolysis oil. The aliphatic (30–35%) and aromatic (20–25%) components coexist with appreciable levels of phenolics (15–18%) and reduced oxygenated species.⁶⁹ Mix-4 showed higher phenolic and ketone contents than pure plastics and higher hydrocarbon content than raw biomass. This confirms strong multicomponent synergistic interactions where plastics act as hydrogen donors and biomass generates aromatic precursors. Remarkably, acids and anhydro sugars are strongly suppressed in Mix-4, indicating effective secondary cracking and stabilisation of biomass-derived vapours. This compositional improvement is particularly important for reducing the corrosivity and instability of pyrolysis oil.⁶⁴ The observed trends can be mechanistically explained by radical interactions during co-pyrolysis. Biomass pyrolysis generates oxygen-centred radicals prone to condensation and char formation. In the presence of plastics, hydrogen-rich radicals generated by polymer scission quench oxygenated radicals, shifting reaction pathways toward aliphatics and aromatics. Simultaneously, mineral matter in biomass may catalyse cracking of plastic-derived vapours, promoting aromatisation. During pyrolysis, cleavage of C–C bonds in a long polymeric chain produces lower-molecular-weight fragments and reactive radicals, which initiate the propagation reaction. These intermediates undergo β-scission, hydrogen transfer, and radical addition reactions. In wastepaper and some textile fractions, cleavage of β-1,4-glycosidic linkages yields smaller oxygenated species.⁷⁰ In the same case of mixed feedstocks, additional secondary reactions, such as dehydrogenation, cyclisation, aromatisation, and condensation, play a significant role, producing a wide spectrum of products including olefins, paraffins, aromatic hydrocarbons, oxygenated compounds, and char/tar.⁷¹ Aromatic hydrocarbons are mainly generated through thermal rearrangement and cyclisation of aliphatic intermediates, as well as from lignin decomposition in wastepaper and mustard husk.^70,71 Cyclisation and aromatisation proceed via hydrogen migration and ring-closure steps, followed by dehydrogenation. Alcohols and ketones predominantly originate from cellulose devolatilization and the breakdown of oxygen-containing functional groups in textiles and wastepaper.^47,72

Fig. 6. Compositions of pyrolysis oil obtained from different components and their mixtures.

(Random scission)

−CH₂–CH₂–CH₂–CH₂− →·˙CH₂–CH₂–CH₂–CH₂ 5

(β-scission)

6

(Hydrogen transfer)

˙CH₂–CH₃ → CH₄ + ˙CH₂ 7

(Addition)

8

(Recombination)

9

(Radical coupling (LDPE–cellulose))

CH₂–CH₃ + ˙O–CH₂–CH(OH)–CH(OH)–CH(OH) → CH₃–CH₂–O–CH₂–CH(OH)–CH(OH)–CH(OH) 10

(Radical coupling and ether formation (PP–cellulose))

CH₂–CH(CH₃) + ˙O–CH₂–CH(OH)–CH(OH)–CH(OH) → CH₃–CH(OH)–CH₂–O–CH₂–CH(OH)–CH(OH)–CH(OH) 11

(Dehydration)

12

From a fuel perspective, higher aliphatic and aromatic contents correlate with higher HHV and improved thermal stability, while reduced acids, furans, and anhydrosugars enhance oil quality.²⁸ In addition, plastic-rich mixtures and multicomponent blends, such as PTP and Mix-4, produce vapours with superior fuel characteristics compared to pure biomass. However, the retention of phenolics in mixed systems offers additional value for chemical recovery applications. From the results (Fig. 6), it was found that the feedstock composition governs vapour chemistry, while co-pyrolysis induces synergistic effects. Biomass wastes favour oxygenated compounds, plastics yield hydrocarbons, and mixed systems effectively bridge these extremes. GC-MS analysis confirmed synergistic interactions during co-pyrolysis, as the PTP and Mix-4 blends exhibited increased hydrocarbon-rich compounds and reduced acids, oxygenated species, indicating enhanced hydrogen-transfer and deoxygenation reactions between plastic and biomass-derived volatiles. Among all the selected feedstocks, Mix-4 shows the most favourable balance between hydrocarbon richness and chemical diversity, highlighting its potential for integrated waste valorisation through thermochemical conversion (Fig. 6).

3.6. Physicochemical characterisation of char

3.6.1. FTIR analysis of char

The FTIR spectra of chars obtained from wastepaper, textiles, mixed plastics, mustard husk, paper-plastic blend, PTP, and Mix-4 at 500 °C reveal substantial chemical restructuring due to thermal decomposition, devolatilization, and aromatisation. A plot of wavenumber versus transmittance is shown in Fig. 7. The Result confirms that the weak and broad absorption in the 3748–3500 cm⁻¹ region corresponds to O–H stretching vibrations of phenolic groups and residual moisture.⁷³ The lower intensity of this band across all samples indicates effective dehydration and breakdown of hydrogen-bonded hydroxyl groups at 500 °C.⁷⁴ Also, biomass-rich chars such as mustard husk and wastepaper retain slightly stronger O–H features, reflecting the partial preservation of oxygenated surface functionalities.⁷⁴ The peak observed near 2910 cm⁻¹, associated with aliphatic C–H stretching (–CH₂ and –CH₃), is significantly diminished or nearly absent in most chars. This confirms extensive cracking of long aliphatic chains during pyrolysis.⁷⁵ Plastic-containing chars show faint residual signals, suggesting minor unconverted hydrocarbon fragments. Furthermore, the bands peak at 2309–2357 cm⁻¹ are attributed to adsorbed CO₂ and to gaseous species trapped within the developing pore structure of the char.⁷⁶ Their presence, particularly in mixed and plastic-derived chars, indicates enhanced porosity and gas adsorption capacity formed at elevated temperature. The absorption near 1710 cm⁻¹ corresponds to C O stretching of carbonyl, carboxyl, or ester groups.⁷⁵ This band is moderately intense in biomass-derived chars, reflecting incomplete decarboxylation and the persistence of oxygenated functional groups. In contrast, plastic-rich and mixed chars show much weaker carbonyl peaks, indicating greater decarboxylation and structural condensation. A prominent band at 1524 cm⁻¹ is assigned to aromatic C C skeletal vibrations, confirming the formation of condensed aromatic structures.⁷⁷ This peak is strongest in mixed and plastic-containing chars, demonstrating advanced aromatisation and higher thermal stability at 500 °C. The band at 882 cm⁻¹, associated with out-of-plane aromatic C–H bending, further supports the development of polycyclic aromatic domains.⁷⁷ Results confirmed that among all the chars, the Mix-4 char exhibits the highest degree of aromaticity, the fewest aliphatic and hydroxyl groups, and the fewest oxygenated functionalities. This indicates a highly condensed and thermally stable carbon matrix. Mixed plastic-paper-plastic chars perform well but exhibit slightly lower structural uniformity. However, the biomass chars retain more polar functional groups, which enhance surface reactivity but reduce thermal stability.

Fig. 7. FTIR of char obtained from different feeds and their mixtures.

3.6.2. Thermal analysis of chars

TGA of chars produced at 500 °C from wastepaper, textile, mixed plastics, mustard husk, paper-plastic blend, PTP, and Mix-4 provide important insights into their thermal stability, carbonisation degree, and compositional differences (Fig. 8). The weight-loss profiles reveal distinct degradation behaviours that reflect the nature of the original feedstock and the effectiveness of carbonisation during pyrolysis. At lower temperatures (<200 °C), all chars exhibit negligible mass loss, indicating minimal moisture and volatile content.⁷⁸ This confirms that pyrolysis at 500 °C effectively removed physically bound water and low-boiling compounds. Biomass-derived chars (wastepaper, mustard husk) show slightly higher initial losses than plastic-rich chars, due to the presence of residual oxygenated surface groups.⁷⁹ Furthermore, in the intermediate temperature range (200–500 °C), gradual weight loss is observed, associated with the decomposition of less stable functional groups such as carboxyl, phenolic, and ether moieties.⁴⁶ Mustard husk and wastepaper chars exhibit more pronounced mass loss in this region, reflecting incomplete aromatisation and a higher proportion of ash-forming biogenic components. In contrast, mixed plastics and Mix-4 chars show relatively stable behaviour, indicating a higher degree of carbon condensation. The major degradation stage (500–800 °C) highlights clear differences in char stability. Mustard husk char undergoes the greatest weight loss, suggesting a lower degree of graphitisation and greater mineral-catalysed degradation. Waste paper and textile chars experience significant mass reduction, consistent with their cellulose and protein-derived carbon structures, which are less thermally stable.⁸⁰ Paper-plastic and PTP chars exhibit intermediate stability, owing to partial plastic-derived carbon structures. Notably, Mix-4 char exhibits the lowest weight loss, retaining the highest residual mass at higher temperatures. This suggests superior thermal stability, ascribed to synergistic interactions between biomass and plastic-derived radicals during co-pyrolysis.⁸⁰ These interactions promote enhanced aromatisation, formation of polycyclic carbon networks, and suppression of volatile evolution.⁷⁷ Mixed plastics char shows high stability but slightly lower residue than Mix-4, likely due to the absence of oxygen-derived cross-linking from biomass. TGA results demonstrate that Mix-4 char is the most thermally stable, followed by mixed plastics and blended chars, whereas single-source biomass chars are comparatively less stable.

Fig. 8. TGA analysis of chars obtained from various feeds at 500 °C.

3.6.3. XRF analysis of char

X-ray fluorescence (XRF) analysis was employed to determine the inorganic and mineral composition of chars produced at 500 °C from wastepaper, textile, mixed plastics, mustard husk, paper-plastic blend, PTP, and Mix-4 as listed in Table 4. The obtained results were compared with other reported chars, such as non-recyclable mixed plastic waste,⁸¹ waste plastic,⁸² recycled paper waste sludge,⁸³ Textile dyeing sludge,⁸⁴ and rice husk.⁸⁵ The results clearly demonstrate that feedstock composition strongly governs ash chemistry. From Table 4, it was observed that CaO is the dominant oxide in most chars, particularly in paper-plastic (29.70 wt%), wastepaper (22.60 wt%), PTP (21.55 wt%), and Mix-4 (19.80 wt%). The high calcium content originates from paper fillers (CaCO₃), additives, and biomass minerals.⁸⁵ During pyrolysis, CaCO₃ partially decomposes to CaO, which becomes concentrated in the char as organic matter is lost.⁸⁵ Furthermore, the presence of CaO is significant, making these chars suitable for gas-cleaning and adsorption applications as it enhances alkalinity, catalytic activity, and CO₂ adsorption capacity.⁸⁶ SiO₂ content is relatively lower in paper and textile-derived chars but higher in mixed plastics, PTP, and Mix-4. This reflects contamination from dust, fillers, and silica-bearing additives.⁸⁶ In comparison, literature-reported rice husk char shows extremely high SiO₂, highlighting the strong feedstock dependency of silica accumulation.⁸⁵ Also, Al₂O₃, MgO, and Fe₂O₃ are present in moderate amounts, particularly in mixed and textile-related chars. These oxides originate from pigments, dyes, catalysts, and soil contamination. Their presence contributes to thermal stability and catalytic cracking behaviour during high-temperature processing.⁸⁶ K₂O is notably high in mustard husk char (4.92 wt%), reflecting the natural accumulation of potassium in biomass. Potassium acts as a strong catalyst during pyrolysis, explaining the relatively lower thermal stability of biomass-based chars.⁸⁷ Conversely, plastic-rich chars contain very little K₂O, contributing to their higher thermal resistance.⁸⁶ P₂O₅ levels are higher in mustard husk- and sludge-based reference samples, which originate from biological phosphates. Phosphorus-containing compounds are important for nutrient recovery and soil amendment applications but may reduce char stability at elevated temperatures.⁸⁶ In addition, Cl is detected mainly in plastic-containing chars (textile, mix plastics, paper-plastic, Mix-4) that originate from PVC residues, dyes, and processing chemicals.⁷⁷ Chlorine is environmentally significant because it can promote corrosion and dioxin formation if chars are combusted, indicating that such chars are better suited for adsorption or material applications rather than direct combustion. Trace oxides such as TiO₂, ZnO, MnO, and BaO arise from pigments, stabilisers, and additives in plastics and textiles. Their persistence in char highlights pyrolysis's ability to immobilise potentially hazardous elements within the solid matrix.⁸⁶ The loss on combustion (LOC) values indicates high carbon retention in all chars, with lower LOC in paper-plastic blends reflecting higher ash concentration. This confirms effective carbonisation at 500 °C while concentrating inorganic constituents. XRF results confirmed that Mix-4 char shows a balanced mineral composition, combining higher CaO with moderate SiO₂ and lower alkali content. This balance enhances thermal stability, adsorption potential, and structural robustness, making Mix-4 the most versatile char. Biomass-based chars are nutrient-rich but less stable, while plastic-rich chars are thermally stable but require chlorine management.

Table 4. XRF analysis of different chars from various feeds at 500 °C.

Compound (wt%)	Waste paper	Textile	Mix-plastics	Mustard husk	Paper-plastics	PTP	Mix-4	Non-recyclable mixed plastic waste81	Waste plastic82	Recycled paper waste sludge83	Textile dyeing sludge84	Rice husk85
Na2O	—	—	0.30	0.00	—	—	0.28	0.21	0.24	0.09	1.03	—
P2O5	0.03	0.02	0.29	0.79	0.12	0.04	0.03	0.14	—	0.11	4.75	2.43
Al2O3	0.22	0.11	2.11	1.14	1.95	0.40	0.19	1.48	1.47	6.24	38.11	1.73
MgO	0.08	0.10	1.07	1.46	0.67	0.15	0.15	0.75	0.26	2.56	1.83	1.11
SiO2	0.34	0.56	2.87	0.55	3.30	1.55	0.65	2.08	1.75	13.59	15.05	85.35
CaO	22.60	4.02	8.44	13.87	29.70	21.55	19.80	12.20	2.43	6.85	8.06	2.03
Cr2O3	—	—	—	0.02	—	—	—	—		—	0.16	—
SO3	0.05	0.10	0.69	3.30	0.86	0.10	0.15	0.48	—	0.01	—	—
K2O	—	0.24	0.11	4.92	0.47	0.03	1.40	0.08	—	0.03	0.40	—
MnO	—		0.01	0.03	—	—	—	0.01	—	—	0.39	0.45
TiO2	0.07	2.09	2.27	0.07	2.60	0.91	0.94	3.69	2.21	1.13	2.05	0.17
Fe2O3	—	1.11	0.40	0.40	3.68	1.72	0.80	0.28	0.20	0.55	18.93	1.13
ZnO	0.19	—	—	0.03		—	—	—	—		1.41	—
BaO	—			0.02		—	—	—	—	—	—	—
Cl		0.55	4.03		5.55	1.17	4.66	—	0.64	—	1.14	—
Loss on combustion	72.0	90.73	78.15	74.32	49.88	71.50	70.19	—	—	68.34	—	—

3.6.4. SEM and EDS analysis of char

SEM micrographs reveal distinct surface morphologies and pore structures for chars derived from different feedstocks, reflecting their intrinsic composition and pyrolysis behaviour (Fig. 9(a–g)). Wastepaper char exhibits a relatively flat, fragmented surface with scattered microparticles embedded in the matrix (Fig. 9(a)). The smooth regions indicate collapsed cellulose fibres after dehydration and depolymerisation, while the dispersed particles correspond to inorganic fillers (mainly Ca-based minerals).⁴⁶ The limited pore development suggests moderate volatilisation and partial structural collapse during pyrolysis. Textile char shows elongated, fibrous structures retaining the original textile morphology (Fig. 9(b)). The roughened fibre surfaces, with cracks and grooves, indicate thermal degradation of polymeric chains (cellulose/polyester) and the release of volatiles. The preservation of fibrous form implies incomplete melting and gradual carbonisation, resulting in moderate surface area but lower pore interconnectivity.⁴⁶ Mixed plastics char displays irregular, agglomerated carbon clusters with relatively smooth, dense surfaces (Fig. 9(c)). This morphology arises from plastic melting and re-solidification before carbonisation. The limited porosity is attributed to the absence of inherent cellular structures in plastics, although the compact carbon matrix contributes to higher thermal stability.⁸⁸ Furthermore, the mustard husk char exhibits a well-defined, honeycomb-like porous structure, inherited from the biomass's natural vascular tissues (Fig. 9(d)). Numerous open channels and tubular pores are visible, formed by extensive devolatilization of hemicellulose and cellulose. This structure may contribute to adsorption applications, but it can reduce mechanical strength at high temperatures.⁸⁸ The paper-plastic blend char shows a heterogeneous morphology, combining dense plastic-derived regions with porous biomass-derived fragments (Fig. 9(e)). Fine mineral particles are uniformly distributed on the surface, indicating ash enrichment and a synergistic interaction between molten plastics and decomposing paper fibres. Also, the PTP char exhibits partially collapsed fibrous structures coated with fine particulates (Fig. 9(f)). This suggests interaction between plastic melt and textile fibres, leading to surface roughening and moderate pore formation.⁸⁹ It was also observed that Mix-4 char demonstrates the most developed and interconnected porous network, with large cavities and thick carbon walls (Fig. 9(g)). This morphology may be associated with interactions between plastic-derived volatiles and biomass-derived carbonaceous structures during co-pyrolysis.⁸⁹ The SEM and FTIR analyses indicated that Mix-4 char exhibited a comparatively rough, porous surface and oxygen-containing functional groups, suggesting its potential applicability for adsorption and catalysis; however, detailed surface-area and performance-based analyses are required for further validation.

Fig. 9. SEM images of chars derived from various feeds at 500 °C, (a) wastepaper, (b) textile, (c) mixed plastics, (d) mustard husk, (e) paper-plastics, (f) PTP, and (g) Mix 4.

Energy-dispersive X-ray spectroscopy (EDS) was employed to determine the elemental composition of chars derived from various biomass, plastic, and hybrid waste feedstocks pyrolysed at 500 °C. The value recorded from the EDS analysis is listed in Table 5 along with other reported chars, such as wheat straw,⁹⁰ rice husk,⁹¹ waste PET bottles,⁹² mixed plastic waste,⁹³ and hybrid biochar (Daniella oliveri leaves and PET bottle).⁹⁴ The EDS results (Table 5) clearly reflect the influence of feedstock chemistry and thermal decomposition behaviour on char composition. Carbon was the dominant element in all chars, indicating effective carbonisation at 500 °C. Textile-derived char exhibited the highest carbon content (82.70 wt%), attributable to the high proportion of synthetic polymers and cellulose-rich fibres that undergo extensive deoxygenation and aromatisation during pyrolysis.⁹⁴ Similarly, chars from mustard husk, mixed plastics, PTP, and Mix-4 contained high carbon fractions (64–73 wt%), indicating enhanced fixed carbon formation due to secondary char-forming reactions such as cross-linking and condensation.⁹⁴ In contrast, waste paper-plastic char showed relatively lower carbon content (42.70 wt%), reflecting dilution by oxygenated paper components and inorganic fillers.⁹² Furthermore, the oxygen content varied significantly, providing insight into char maturity. The lower oxygen contents in textile and plastic-rich chars (12–20 wt%) indicate an effective removal of oxygen-containing functional groups via decarboxylation and dehydration at 500 °C.⁹⁴ Conversely, biomass-rich chars such as wastepaper and wheat straw retained higher oxygen levels (35 wt%), suggesting the persistence of surface hydroxyl, ether, and carbonyl groups.⁹¹ Also, the rice husk char exhibited an exceptionally high oxygen content (62.50 wt%), consistent with its silica-rich structure and incomplete deoxygenation, due to the stabilising effect of mineral matter.⁹¹ The presence of inorganic elements such as Ca, K, Mg, Na, Si, Cl, and S reflects the mineral composition of the original feedstocks. The calcium was notably high in wastepaper, paper-plastic char (14 wt%), likely originating from paper fillers (CaCO₃), which remain concentrated in the solid residue after volatile loss.⁹³ Potassium and magnesium were prominent in mustard husk and hybrid chars, indicating the retention of alkali and alkaline earth metals, which can catalyse char formation and influence surface reactivity.⁹³ Silicon was abundant in rice husk char (10.80 wt%) and hybrid biochar (23.60 wt%), confirming the strong contribution of siliceous ash, which enhances thermal stability but may reduce electrical conductivity.⁹¹ The EDS analysis of chars demonstrates that pyrolysis at 500 °C yields carbon-rich chars with feedstock-dependent heteroatom and mineral distributions. Biomass-derived chars retain more oxygen and ash-forming elements, favouring surface functionality and adsorption potential, while plastic-rich chars exhibited high carbon.

Table 5. EDX analysis of chars derived from various feeds at 500 °C.

Element wt%	Waste paper	Textile	Mustard husk	Mix plastics	Paper-plastics	PTP	Mix-4	Wheat straw90	Rice husk91	Waste PET bottles92	Mixed plastic waste93	Hybrid biochar (Daniella oliveri leaves and PET bottle)94
C	58.50	82.70	73.20	64.90	42.70	71.30	68.60	57.80	19.30	65.60	67.75	33.80
O	36.60	12.30	20.10	15.50	29.90	19.10	20.10	34.69	62.51	20	20.97	2.81
Mg	0	0.70	0.30	0.90	0.90	2	1.10	0.32	0.07	—	3.78	0.76
Si	0	0	0.20	6.50	2	4.30	1.10	3.64	10.76	2.20	6.38	23.60
S	0.30	1.80	0.50	0	1.40	1.60	0	0.04	—	—	—	0.89
Cl	0	0	0.20	0.90	2.60	0.30	3.60	—	—	—	—	0.55
K	0	0	3.30	1.90	0.20	0.40	0.70	—	0.22	—	—	3.12
Ca	4.60	1.90	2.40	3.80	14	1.10	4.70	—	—	0.20	0.32	6.97
Na	0	0	0	1.60	3	0	0	—	—	0.74	—	0.19

4. Conclusions and future research directions

This study shows that sequential co-pyrolysis in a rotary kiln can be an effective approach for the thermochemical valorisation of heterogeneous municipal solid waste. Blending biomass and polymer-rich feedstocks shifts the process from simple waste disposal to targeted value-added product recovery. The positive synergistic interactions observed across blended feedstocks, driven by hydrogen transfer and radical coupling, fundamentally altered thermal degradation chemistry beyond simple additive effects, with direct implications for industrial-scale feedstock design. From a fuel quality perspective, the enhanced HHV, lower moisture content, improved pH, and hydrocarbon enrichment observed through GC-MS highlight the potential of the recovered pyrolysis oil for downstream upgrading and refining applications. Chars exhibited thermally stable carbonaceous structures with surface morphological and functional characteristics that may be relevant for further adsorption- or catalyst-related investigations. However, additional studies involving upgrading performance, surface-area analysis, application testing, techno-economic assessment, and long-term continuous operation are necessary to establish practical industrial feasibility. Gas yields were estimated by mass balance difference; therefore, detailed compositional interpretation of H₂, CO, CO₂, and light hydrocarbons could not be established in the present study. The present study demonstrates the feasibility and advantages of co-pyrolysis of mixed MSW components in a rotary kiln reactor under scalable operating conditions. Although a detailed techno-economic evaluation was beyond the scope of the present experimental study, factors such as energy consumption, incorporating capital expenditure (CAPEX), operating expenditure (OPEX), product recovery, reactor configuration, and upgrading strategies remain important for assessing the industrial scalability of rotary kiln co-pyrolysis systems. Therefore, future investigations should include techno-economic, energy-efficiency, and life-cycle assessment studies under continuous operating conditions. Several research directions can strengthen industrial deployment and environmental impact. Pilot-scale and continuous-flow reactor investigations are required to evaluate long-term thermal stability, throughput, and heat transfer efficiency. The catalytic and in situ upgrading routes, including mineral-rich biomass ash catalysts and engineered catalysts (zeolites and metal oxides), could be explored to further enhance deoxygenation, aromatic hydrocarbon selectivity, and tar suppression while improving fuel stability. Advanced characterisation techniques, such as in situ FTIR, Py-GC-MS, and real-time gas analysis, can elucidate intermediate reactions and radical chemistry, thereby refining mechanistic understanding. Integration with downstream upgrading processes, such as hydro-processing, emulsification, and refinery co-processing, could enable the production of commercial-grade liquid fuels. Additional studies on downstream fuel upgrading, char valorisation, and co-pyrolysis of other heterogeneous urban waste streams could further expand the applicability of rotary kiln-based thermochemical waste valorisation systems. These future directions may support the development of more optimised, industrially relevant pathways for the utilisation of mixed MSW.

Author contributions

Mukesh Bhatt: conceptualisation, methodology, data curation, writing, & editing the original draft, visualisation; Tanushka Florence Panicker: methodology, writing, & editing the original draft, data curation; Abhishek Sharma: supervision, validation, conduct formal analysis, software, fund acquisition, project administration, and resources, Ranjeet Kumar Mishra: validation, conduct formal analysis, edition the original draft, Madhu Ganesh: conceptualisation, investigation, validation, formal analysis, Jyeshtharaj Bhalchandra Joshi: validation, formal analysis, Pratik N Sheth: validation, formal analysis.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be available on request.