Pyrolysis Valorization of Vegetable Wastes: Thermal, Kinetic, Thermodynamics, and Pyrogas Analyses

Abstract

In comparison to other methods, valorising food waste through pyrolysis appears to be the most promising because it is environmentally friendly, fast, and has a low infrastructure footprint. On the other hand, understanding the pyrolytic kinetic behaviour of feedstocks is critical to the design of pyrolysers. As a result, the pyrolytic degradation of some common kitchen vegetable waste, such as tomato, cucumber, carrot, and their blend, has been investigated in this study using a thermogravimetric analyser. The most prevalent model fitting method, Coats–Redfern, was used for the kinetic analysis, and the various mechanisms have been investigated. Some high-quality fitting mechanisms were identified and used to estimate the thermodynamic properties. As the generation of pyrolysis gases for chemical/energy production is important to the overall process applicability, TGA-coupled mass spectrometry was used to analyse the pyrogas for individual and blend samples. By comparing the devolatilization properties of the blend with single feedstocks, the presence of chemical interactions/synergistic effects between the vegetable samples in the blend was validated. The model, based on a first-order reaction mechanism, was found to be the best-fitting model for predicting the pyrolysis kinetics. The calculated thermodynamic properties (ΔH (enthalpy change $\approx$ E (activation energy))) demonstrated that pyrolysis of the chosen feedstocks is technically feasible. According to the TGA–MS analysis, blending had a considerable impact on the pyrogas, resulting in CO₂ composition reductions of 17.10%, 9.11%, and 16.79%, respectively, in the cases of tomato, cucumber, and carrot. Overall, this study demonstrates the viability of the pyrolysis of kitchen vegetable waste as a waste management alternative, as well as an effective and sustainable source of pyrogas.

1. Introduction

Because of the rapid rate of urbanization and industrialization, massive amounts of solid waste are generated all over the world. A lack of landfills, environmental concerns, and public understanding all add to the concern about appropriate sustainable waste management. People are consuming more and producing vast amounts of garbage in order to reach higher living standards, and enterprises are expanding to accommodate these demands [1]. As a result, these unsustainable habits put additional strain on the ecosystem, which is already under stress, necessitating immediate action to undo the impact and safeguard the world for generations to come.

Thermochemical waste-to-energy (WTE) conversion approaches are gaining popularity due to the numerous advantages of converting trash into heat, fuel, energy, and chemicals [2,3,4,5,6]. Biomass waste in particular is an inexpensive and abundant source of energy [7]. As a result, biomass conversion to energy justifies even greater attention. To gain the benefits of WTE conversion, a variety of pathways can be applied. For example, pyrolysis is a feasible technology that needs further development before being industrialized and employed on a larger scale [8]. This will make it easier for people throughout the world to implement and use principles such as sustainable development as well as the circular economy.

Biochar is a carbon-rich material that results from the pyrolysis process of converting waste to value-added products [9]. It is utilized in the form of adsorbents in water treatment and in agriculture to improve water retention and soil fertility [10]. Introducing biochar to soil can increase crop yields in many ways, including improving the cation exchange capacity of the soil, improving water retaining capability, upgrading soil fertility/quality, and augmenting soil microbe development, and neutralizing extreme pH soils [11,12,13]. As a result, it is critical to comprehend the processes that go into making biochar and other products; this can be performed via thermogravimetric analysis (TGA) [14]. In addition, prior understanding of the kinetics of pyrolysis degradation is also crucial for pyrolyser design, which can be obtained using TGA [15].

Tomatoes are the second most popular vegetable in the world, behind potatoes, and carrots and cucumbers are among the top ten most popular veggies. FAOSTAT estimates global tomato production at 182 million tons (MT), gherkin and cucumber production at 75 MT, and combined carrot/turnip production at nearly 40 MT [16]. Huge amounts of vegetable waste are produced annually as a result of the vast consumption of carrots, cucumbers, and tomatoes around the world, including ‘out-of-time’ storage.

In general, the pyrolysis of food waste is severely limited by its massive amount of water, which negatively affects the energy balance of the process. It is therefore essential to adopt some pretreatment processes to reduce moisture content and hence the energy requirements for the pyrolysis process [17]. Solar and artificial dryers are among the options for drying food waste, as drying can significantly reduce waste volume by reducing moisture levels and thereby provide more and better waste management and storage options [18]. Despite the chosen veggies’ decent moisture levels, they have a high-level volatile composition, making them potential energy sources. As a result, the potential of carrot, cucumber, and tomato as pyrolysis feedstocks has been studied in this investigation.

The thermal properties and pyrolytic behaviour of a range of vegetable wastes have been studied by researchers. The pyrolysis of potato peel (Solanum tuberosum) waste was explored by Liang et al. (2015) [19]. According to the study, potato peel biochar has a high adsorption potential and can thus be utilized as a pyrolysis feedstock to manufacture a variety of industrial goods. Kumar et al. (2020) investigated the torrefaction responses of lettuce (Lactuca sativa), cauliflower (Brassica oleracea), and coriander (Coriandrum sativum) [20]. According to the study, chars made from the aforementioned veggie wastes possess excellent heating properties and can thus be applied as solid fuels. Chen et al. (2015) investigated the pyrolysis and gasification properties in Chinese cabbage (Brassica rapa subsp. pekinensis) [21]. Chinese cabbage, according to the study, could be used for both pyrolysis and gasification feedstocks. For their pyrolysis studies, Mary et al. (2016) employed cauliflower leaves (Brassica oleracea) and pea pod (Pisum sativum) waste as feedstock [22]. The research found that biochars can augment the carbon and water holding capacity of soils, making them effective soil conditioners. Garlic (Allium sativum) skin and onion (Allium cepa) waste were studied by Reddy and Rhim (2018) [23]. The study concluded that garlic and onion peels can be used as a long-term source of biopolymer generation. Maia and de Morais (2016) investigated red pepper (Capsicum annuum) [24]. According to the research, red pepper char possesses a decent heating value and so can be used as a fuel. The pyrolysis behaviour of peas (Pisum sativum) was studied by Müsellim et al. (2018) [25]. The findings confirmed that pea waste is a long-term source of bioenergy. Sriram (2018) looked at the pyrolysis thermal properties of a banana blossom petal (Musa balbisiana) [26]. The findings of the study confirmed that the vegetable waste may be used to generate bioenergy.

The aforementioned studies infer that the vegetable wastes can be pyrolysed to generate biochar. As a result, the pyrolysis of some common vegetable wastes, including tomato, cucumber, and carrot in terms of biochar production, has been investigated in this work. Another rationale for choosing wastes is that there has been very little research conducted on these food wastes [9]. Information on pyrolysis kinetics is essential for the design and fabrication of pyrolysis reactors, as well as the optimization of pyrolysis critical operating parameters such as temperature and heating rate. As a result, the TGA of the pyrolytic degradation of tomato, cucumber, and carrot wastes, which has received little attention to date, is examined in this study [9]. Large volumes of feedstock would be required to generate specialty commodity products from lignocellulosic biomass by pyrolysis, which would necessitate the use of mixed biomass rather than a single type of biomass [27]. Furthermore, blending of feedstocks has a major effect on the pyrolysis products distribution [28]. Hence, the current research also looks into the pyrolytic behaviour of ternary (tomato + cucumber + carrot) vegetable waste blends. Since the generation of pyrogas for energy production is critical to the overall process applicability, the evolved gases have been analysed using a TG analyser integrated with a mass spectrometer. Coats–Redfern (CR), a single-heating rate model, has been used in this work to calculate the pyrolytic disintegration kinetics of the feedstocks (individual and ternary (blend) vegetable wastes).

The outcomes of this study will aid in the design and fabrication of pyrolysis reactors as well as the estimation of the process’s heat and mass balance data [29].

2. Materials and Methodology

2.1. Feedstocks and Their Characterisation

Tomatoes (Solanum lycopersicum), cucumbers (Cucumis sativus), and carrots (Daucus carota) are the three vegetable wastes studied in this study. These raw materials were evaluated as a single component system (tomato, cucumber, and carrot) as well as a ternary component system (carrots, cucumbers, and tomatoes). The ternary blend was made by mixing equal amount (by weight) of individual components.

Because the initial moisture levels of tomatoes, cucumbers, and carrots were found to be 94.00%, 96.00%, and 90.00%, respectively, the feedstocks were oven-dried for one full day at 140 °C [30] to decrease their moisture level to well below 15%, thereby allowing the feedstocks to meet the criteria for pyrolysis investigations [31]. The dry material was then broken into smaller chunks and processed into finer particles as needed for the proximate, ultimate, and TG analyses. Following D7582-12 (ASTM standard), the proximate tests were performed on a TA SDT 650 analyser. Test runs were performed at least three times in order to achieve three consistent results that were within ±5% of each other. The ultimate or elemental tests were conducted using an elemental analyser (EuroEA-3000 model) in accordance with ASTM D 3176–8. The values are based on the average of three measurements for each sample falling within a 6% margin of error. The findings of the proximate and ultimate analysis are presented in Section 3.1.

The gross calorific values (GCV) of foods are computed using the following correlation developed by Channiwala and Parikh [32] based on the results of ultimate and proximate analyses:

$$\mathrm{GCV}\left(\frac{\mathrm{kJ}}{\mathrm{kg}}\right)=349.1\mathrm{C}+100.5\mathrm{S}+1178.3\mathrm{H}-103.4\mathrm{O}-15.1\mathrm{N}-21.1\mathrm{Ash}$$

(1)

where C, S, H, O, and N are compositions (%) of carbon, sulphur, hydrogen, oxygen, and nitrogen elements, respectively.

The aforementioned relationship is utilized as the composition of the components and is well below the allowed values indicated by the researchers, namely C: 0.00–92.00%, O: 0.00–50.00%, H: 0.43–25.00%, N: 0.00–5.60%, Ash: 0.00–71.00%, and GCV: 4745–55,345 kJ/kg.

The net calorific value (NCV) is determined applying the below equation:

$$\mathrm{NCV}(\frac{\mathrm{kJ}}{\mathrm{kg}})=\mathrm{GCV}-h_{st}\left(0.09\mathrm{H}+0.01M\right)$$

(2)

where $h_{st}$ denotes enthalpy of steam (2260 kJ/kg), and $M$ denotes moisture (%).

2.2. Pyrolysis Behaviour of Feedstock Samples

The TGA was performed on the particles that passed through 125 µ sieve (Haver & Boecker (Germany)). Under a N₂ environment (100 mL/min), the test was conducted at 10 °C/min heating rate in a non-isotheral condition, i.e., from room temperature to 800 °C (Discovery TGA 550, TA Instruments). The TGA studies were carried out at a low heating rate (i.e., 10 °C/min) because the intention of the study was to make biochar. Slow heating rates offer the advantage of allowing for efficient heat transfer between particles, which results in effective particle cracking, breakdown, and effective biochar production. About 7–17 mg of samples were utilized for the TGA runs. To ensure consistency, the TG analysis was repeated three times. To monitor the developing gases, a mass spectrometer was coupled to the thermogravimetric analyser.

2.3. Kinetic Analysis Using CR Model

The CR model is defined as follows:

$$\ln \left[\frac{g(\propto )}{T^2}\right]=\ln \left[\frac{AR}{\beta E}\right]-\frac{E}{RT}$$

(3)

Between $\ln \left[\frac{g(\propto )}{T^2}\right]$ and $\frac{1}{T}$, there is a linear relationship that can be used to derive the pre-exponential factor and activation energy. In general, the CR model mechanisms are based on power law (PL), reaction orders, geometrical contraction, diffusion, and nucleation.

Table 1. CR model’s reaction mechanisms used in this work.

Denotations	Various CR Model Mechanisms	$\mathbf{Integral}\mathbf{Form}\mathit{g} (\propto )$	Equation
P2	2-Power law	${\alpha }^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$	(4)
P3	3-Power law	${\alpha }^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}$	(5)
P4	4-Power law	${\alpha }^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.}$	(6)
P2/3	2/3-Power law	${\alpha }^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$	(7)
F1	I-order reaction	$-\ln \left(1-\propto \right)$	(8)
F2	II-order reaction	${\left(1-\propto \right)}^{-1}-1$	(9)
F3	III-order reaction	$[{\left(1-\propto \right)}^{-2}-1]/2$	(10)
R2	Contracting Area	$1-\left(1-\alpha \right)\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$	(11)
R3	Contracting Volume	$1-\left(1-\alpha \right)\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$	(12)
D1	1D diffusional	${\propto }^2$	(13)
D2	2D diffusional	$\left(1-\propto \right)\ln \left(1-\propto \right)+\propto$	(14)
D3	3D diffusional	$1-{\left(1-\propto \right)}^{1/3{ }^2}$	(15)
D4	Ginstling–Brounstein diffusional	$\left[1-\left(\frac{2}{3}\right)\propto \right]-{\left(1-\propto \right)}^{2/3}$	(16)
A2	Avrami–Erofeev	$-\ln {\left(1-\propto \right)}^{1/2}$	(17)
A3	Avrami–Erofeev	$-\ln {\left(1-\propto \right)}^{1/3}$	(18)
A4	Avrami–Erofeev	$-\ln {\left(1-\propto \right)}^{1/4}$	(19)

[33] lists the various mechanisms of the CR model that were used in this work.

2.4. Thermodynamic Properties

The thermodynamic properties of Gibbs free energy (G), enthalpy (H), and entropy (S) were determined using the kinetic parameters of the better fitting mechanisms. The following equations were applied to calculate the thermodynamic parameters:

$$\Delta H=E-R{T}_p$$

(20)

$$\Delta G=E+R{T}_p \ln (\frac{k_b{T}_p}{hA})$$

(21)

$$\Delta S=\frac{\Delta H-\Delta G}{T_p}$$

(22)

where R signifies the ideal gas constant (8.314 J mol⁻¹K⁻¹), $T_p$ implies the peak decomposition temperature (K), $k_b$ designates the Boltzmann constant (1.38 × 10⁻²³ J K⁻¹), and $h$ denotes the Planck constant (6.63 × 10⁻³⁴ Js).

2.5. Pyrolysis Gas Analysis

The pyrolysis gas analysis provides knowledge on feedstock thermal behaviour and product distribution. The National Institute of Standards and Technology (NIST) database is used to retrieve mass-to-charge ratios of ionized molecules used in mass spectrometers (MS) to detect gas components. Due to the impossibility of detecting all gases in the pyrogas mixture, only the H₂, CH₄, H₂O, and CO₂ gas components were investigated using the equation below.

$$Biomass waste\stackrel{\Delta }{\to }{\mathrm{Char}}_{\left(\mathrm{s}\right)}+{\mathrm{Tar}}_{\left(\mathrm{l}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}+{\mathrm{H}}_{2\left(\mathrm{g}\right)}+{\mathrm{CO}}_{\left(\mathrm{g}\right)}+{\mathrm{CO}}_{2\left(\mathrm{g}\right)}+{\mathrm{CH}}_{4\left(\mathrm{g}\right)}$$

(23)

The atomic mass units (amu) 2 (H₂), 15, 16 (CH₄), 17, 18 (H₂O), and 44 (CO₂) were employed to examine the gas components H₂, CH₄, H₂O, and CO₂. The amu 28 is normally used to analyse gases such as N₂, CO, C₂H₄, and C₂H₆. Since the current gas analysis was conducted in a N₂ atmosphere, this large nitrogen concentration would distort the results of the amu 28 data. Hence, the data for CO, C₂H₄, and C₂H₆ were not presented in this study [34,35].

Radojevic et al. [36] suggested a new approach for determining the volume of components (gas) in syngas mixture. Using the following equation, the volumetric share of gas components is determined:

$$v_i\left[\frac{{\mathrm{m}}^3}{\mathrm{kg}}\right]=\frac{1}{\beta } \frac{v_c}{m} \underset{T1}{\overset{T2}{{\displaystyle \int }}}\frac{I{C}_i}{I{C}_c} dT$$

(24)

where $v_i$ denotes the volumetric composition of gas component (m³ kg⁻¹) ‘i’, $\beta$ denotes the heating rate (K/min) of the pyrolysis process, $v_c$ denotes the volumetric rate of N₂ carrier gas (mL/min), m indicates the amount of the sample used (mg), $I{C}_i$ denotes the ith components peak ion current (A), $I{C}_c$ is the ion current of the carrier (N₂) gas (A), and $T1$ and $T2$ are the lower and upper limits of the integral function, reflecting the temperature range employed in the calculation. The logarithm of ith component ion current gives the $T1$ value, while $T2$ is determined from the peak ion current of the ith component.

3. Results and Discussion

3.1. Feedstocks Characterisation

The findings of the proximate tests of the veggie wastes are provided in

Table 2. Proximate test results of the vegetable wastes.

Vegetable Waste	Components * (%)
	Moisture	Volatile Content	Fixed Carbon d	Ash
Tomato	10.00	72.00	13.60	4.41
Cucumber	8.14	64.04	25.95	1.87
Carrot	6.59	79.87	8.33	5.22

* Air-dried basis. ^d calculated by difference.

.

All the samples have a moisture level of less than 10%. These vegetable waste samples clearly meet the pyrolysis standards, which demand that all feedstocks must possess a moisture level of less than 15% [37]. There is a lot of volatile matter in all the samples (64–80%). Biomass fuels with decent volatile composition and low ash content are thought to be optimal for thermochemical conversion processes such as pyrolysis, torrefaction, and gasification. High levels of organic volatile matter suggest that significant amounts of inorganic vapours could be produced during conversion. Low ignition temperatures are also indicated by using feedstocks with a high composition of volatile materials. The amount of volatile and fixed carbon content in a fuel determines its calorific value. Tomato and cucumber samples had a high fixed carbon content (13–26%), implying moderate heating values. Carrots, on the contrary, have a small, fixed carbon composition (8%) signifying that these may exhibit a low heating value. It is interesting to see that all the samples had a very low ash content.

Table 3. The findings of the ultimate analysis of the vegetable wastes.

Vegetable Waste	Components * (%)
	Carbon	Hydrogen	Nitrogen	Oxygen d	Sulphur
Tomato	49.68	7.35	3.72	39.25	0.00
Cucumber	56.69	6.52	3.91	32.88	0.00
Carrot	41.54	5.39	1.37	51.7	0.00

* Dry-ash free basis. ^d calculated by difference.

depicts the findings of the ultimate analysis of the vegetable wastes.

Table 3 summarizes the findings of the ultimate analysis of the three veggies. The elemental analysis of the samples reveals that the carbon contents of the tomato and cucumber samples are higher (50% and 57%, respectively) than the oxygen–hydrogen compositions (tomato, 47%; cucumber, 38%). However, in the case of carrots, the oxygen–hydrogen content (57%) is higher than the carbon content (42%). Bonds between carbon and oxygen and carbon and hydrogen, in general, have less energy than bonds between carbon and carbon. This implies that the tomato and cucumbers may have a higher energy density than carrot. NOx emissions from these samples are unlikely to cause considerable environmental harm due to the lower quantities of nitrogen (≤4) in the samples. Due to the lack of sulphur, pyrolysis emissions from these samples will have negligible environmental impact.

The calculated GCVs and NCVs of the samples are presented in

Table 4. The GCV and NCV of vegetable waste samples.

Vegetable Waste	GCV (MJ/kg)	NCV (MJ/kg)
Tomato	19.13	17.64
Cucumber	21.58	20.21
Carrot	13.36	12.21

.

Tomato and cucumber samples have moderate heating values. The carrot samples, on the other hand, had a low NCV. The low NCV of carrots may be attributed to their low fixed carbon content (8%). The moderate heating values of the tomato and cucumber samples imply that their pyrolysis products will have moderate heating values as well.

3.2. Pyrolysis Behaviour of Feedstock Samples

The TGA and DTG profiles of vegetable waste samples are shown in Figure 1 and Figure 2.

The DTG curves of the samples indicate that the first endotherm occurs between room temperature and 160 °C and is ascribed to moisture evaporation as well as some low-boiling organics [38]; however, the mass loss in this region is negligible due to the drying pretreatment and is found to be in the range of 5–9%. The pyrolytic breakdown of hemicellulose is primarily responsible for the second DTG peak, which can be found in the range of 275–280 °C. Several other research studies in the scientific literature have found that hemicelluloses degrade at temperatures between 150 and 300 °C [39,40,41,42]. At 320–325 °C, the third and last main peak is observed. This peak is primarily due to cellulose, which degrades at quite moderate temperatures, say 300–400 °C [39,41,43]. Lignin, the other component of biomass, begins to degrade at 400 °C and continues till 800 °C, with some peaks occurring at 500 °C throughout the pyrolysis stage. The TGA findings of the current work are compared with the results of a few other works. Kumar et al. (2020) [20] investigated the TGA of cauliflower, coriander, and lettuce leftovers. According to the researchers, the release of organics and lignocellulose degradation began at 200 °C for coriander and lettuce residues and at 290 °C for cauliflower residues. For coriander and lettuce wastes, lignin breakdown extended until 700 °C. However, for cauliflower residue, lignin breakdown continued only until 500 °C. Sriram and Swaminathan (2018) [26] investigated the pyrolytic behaviour of Musa balbisiana by employing TGA. The researchers found that lignin degradation began at 160 °C and continued until 900 °C as a separate pyrolytic process. Because the degradation of cellulose and hemicellulose is complex, it was inferred that there is a thermal overlap between cellulosic materials and lignin at lower temperatures. In a similar approach, Chen et al. (2015) [21] conducted TGA to study the pyrolysis decomposition properties of Chinese cabbage. It was found that cellulose decomposition took place between 300 °C and 400 °C, hemicellulose degradation took place between 210 °C and 400 °C, and lignin decomposition took place throughout a wide temperature range till 800 °C, with the majority of the weight loss taking place between 280 and 400 °C, which is consistent with the findings of the current work.

It is clearly obvious from the derivative curves, specifically the devolatilization region (150–540 °C), that there exist chemical interactions/synergistic effects between the vegetable samples when they are mixed/blended.

Further, in general, the composition of biomass constituents, extractives, hemicellulose, cellulose, and lignin, influence the yield of solid residue (biochar). After decomposition, the yields of biochar are found to be 39.81%, 39.02%, 41.43%, and 35.29% for tomato, cucumber, carrot, and ternary blend, respectively. These biochar yields comprise the ash and fixed carbon content of the samples (Table 2), as well as the char produced during the pyrolysis process. High biochar yields of the vegetable wastes can be ascribed to their decent cellulose content. The lignocellulosic composition of cellulose, hemicellulose, and lignin was reported by Szyma’nska-Chargot et al. (2017) [44]. For tomato, cucumber, and carrot, the composition of cellulose has been reported to be 8.60, 16.13, and 10.01 g/100 g dry pomace, respectively, whereas the composition of hemi-cellulose has been observed to be 5.33, 4.33, and 5.73 g/100 g dry pomace, respectively. The lignin content of tomato, cucumber, and carrot has been found to be 5.85, 4.51, and 2.50 g/100 g dry pomace, respectively.

3.3. Kinetics Analysis: CR Model

The slope of $\ln \left[\frac{g(\propto )}{T^2}\right]$ vs. $\frac{1}{T}$ is employed to calculate the activation energy (E). Only the best-fitting reaction models have been examined, and the kinetic factors acquired from them are presented in

Table 5. Kinetic properties of the veggie wastes evaluated applying various reaction mechanisms of the CR model.

Vegetable	Mechanisms	A (s−1)	E (kJ mol−1)	R2
Tomato	F1	2.06	39.09	0.90
	D2	1.19 × 102	68.59	0.81
	D3	2.59 × 102	77.53	0.87
	D4	5.45 × 101	71.40	0.83
Cucumber	F1	2.95	40.25	0.90
	D2	2.57 × 102	71.41	0.82
	D3	5.34 × 102	80.08	0.87
	D4	1.16 × 102	74.15	0.84
Carrot	F1	1.92	38.57	0.90
	D2	1.18 × 102	68.11	0.81
	D3	2.44 × 102	76.80	0.87
	D4	5.34 × 101	70.86	0.83
Ternary blend	F1	3.03 × 10−1	28.97	0.90
	D2	3.79	49.43	0.80
	D3	7.95	57.78	0.87
	D4	1.71	52.05	0.83

.

The best-fitting model, as can be noticed, is the F1 model. This designates that F1 is more influential in the devolatilization zone of the veggie wastes. Among the individual vegetable wastes, cucumber displayed a slightly higher E value while the E values of tomato and carrot were close to each other. With respect to ternary blend, the E values were lower than the E values of individual vegetable waste samples. For all the samples, the A values were between 10⁻¹ and 10² (s⁻¹), implying that the pyrolytic decomposition of the feedstock would go through degradation more easily with fewer impediments.

The implications of the present study are compared with a few other similar works. Jeguirim et al. (2014) [45] studied the pyrolytic kinetics of olive waste, date palm trunk, pine sawdust, and Posidonia oceania employing various reaction mechanisms/equations of the CR model. Two reaction zones, devolatilization and char formation, were considered in the study. In the case of pine sawdust, for the devolatilization step, the surface reaction R2 was dominant while for the char formation reaction F3 was influential. For olive waste, the devolatilization stage was well described by D1, whereas the char formation step was controlled by the F3 mechanism. The authors observed that for the date palm trunk devolatilization reaction, the D2 was effective; however, for the char formation step, the F3 was prominent. In the case of Posidonia oceania, the devolatilization degradation was well described by both the D1 and D2 reaction models at the same time, and the char formation stage best fits with F1 as well as R2 mechanisms. Lei et al. (2019) [46] investigated the pyrolysis degradation kinetics of hemicellulose separated from Camellia oleifera shell biomass. The researchers perceived that the pyrolysis of hemicellulose can be well described by the D1 mechanism. In another study, Guo and Lua (2001) [47] studied the pyrolytic kinetics of palm shells at two different temperature regimes: low and high. For the low-temperature regime, the F1 was prominent, while for the high-temperature regime the R3 was dominant.

3.4. Thermodynamic Properties

Table 6. Thermodynamic characteristics of vegetable wastes as determined by the CR model’s various reaction mechanisms.

Vegetable Waste	Mechanisms	ΔH (kJ mol−1)	ΔG (kJ mol−1)	ΔS (kJ mol−1 K−1)
Tomato	F1	34.11	185.85	−0.2530
	D2	63.60	195.13	−0.2193
	D3	72.54	200.19	−0.2128
	D4	66.42	201.84	−0.2258
Cucumber	F1	35.26	185.23	−0.2500
	D2	66.42	194.11	−0.2129
	D3	75.09	199.14	−0.2068
	D4	69.16	200.81	−0.2195
Carrot	F1	33.57	186.22	−0.2536
	D2	63.11	195.16	−0.2194
	D3	71.80	200.21	−0.2133
	D4	65.85	201.88	−0.2260
Ternary blend	F1	24.00	162.16	−0.2310
	D2	44.45	192.73	−0.2479
	D3	52.81	197.40	−0.2418
	D4	47.08	199.30	−0.2545

shows the thermodynamic properties of the veggie wastes estimated by applying different CR reaction mechanisms.

$\Delta H$ is a thermodynamic state function that indicates whether a process absorbs or transmits heat. The table shows that $\Delta H$ values range between 24 and 76 kJ mol⁻¹. All models have positive values, which supports the contention that energy must come from an outer source to achieve the transformation. The results suggest that the pyrolysis of these vegetable wastes requires only a small amount of external energy. All values are consistent and positive, ranging between 162 and 202 kJ mol⁻¹. The positive values indicate that the pyrolysis process is endergonic, in which the standard change in free energy is positive. A negative value for each sample indicates the formation of activated complexes, wherein a small value indicates that the samples are slow to react.

In a similar study, Lei et al. (2019) [46] determined the thermodynamic parameters using kinetic parameters of the different CR models’ reaction mechanisms. The estimated $\Delta H$ values ranged from 46–165 kJ mol⁻¹, while the $\Delta G$ values were between 114–152 kJ mol⁻¹. The researchers interpreted that when the activation energy values were smaller, the creation of an activated complex was smooth. The $\Delta S$ values were predominantly negative and were low. From the $\Delta S$ values, it was reported that the degree of the system is proximate to its thermodynamic equilibrium. Kumar et al. (2021) [48] also used various CR reaction models to estimate the thermodynamic values from the kinetic parameters. The researchers classified the pyrolysis of dry kitchen wastes into three stages: stage I, stage II, and stage III. The $\Delta H$ values of the first stage, second stage, and third stage were in the range of 2–53 kJ mol⁻¹, 32–99 kJ mol⁻¹, and 90 kJ mol⁻¹, respectively, while the $\Delta G$ values of the first stage, second stage, and third stage were in the range 69–117 kJ mol⁻¹, 104–174 kJ mol⁻¹, and 85–250 kJ mol⁻¹, respectively. Similar to the present study, only negative values were obtained for all the reaction models.

3.5. Pyrogas Analysis Findings

3.5.1. TGA–MS Profiles

Figure 3, Figure 4, Figure 5 and Figure 6 show the TGA–MS contours of tomato, cucumber, carrot, and ternary blend.

Figure 3, Figure 4, Figure 5 and Figure 6 show that the peak releases of H₂O and CO₂ occur relatively close together, while the peak release of CH₄ occurs a little later, then by the evolution of H₂. Gases were released in the following order: H₂O, CO₂, CH₄, and H₂, which is matching with the findings of Ma et al. [49]. Normally, the release of the aforementioned gases is an intricate occurrence because some gases discharge as a result of thermal decomposition of feedstocks, others are adsorbed, and the remainder are liberated as a result of secondary reactions between the product gas components [50,51].

3.5.2. Pyrolysis Stages

In general, the thermal and derivative curves are divided into four regions based on the slope of the curves: zeroth stage (150 °C), first stage (between 150 and 250 °C), second stage (from 250 to 500 °C), and third stage (below 500 °C). The zeroth stage relates to the evaporation stage, during which no gas components are produced, and hence there is no pyrolysis. Gas components such as H₂O, CH₄, C₂H₄, and C₂H₆ are liberated in the first stage [52]. At this stage, no permanent gases such as H₂, O₂, CO, and CO₂ are produced. Except for H₂, most gas products are synthesized in the second stage. In this stage, biomass is primarily pyrolysed. While in the last stage, i.e., third stage, biomass pyrolysis ensues, resulting in CO₂ and H₂ production. This stage also includes the interaction between CO₂ and H₂. The recognized gas components and their decomposition regions have been presented as Supplementary Table S1.

It was found that the temperature of the H₂O peak is in the 202–331 °C range, the temperature of the CO₂ peak is between 273 and 329 °C, the temperature of the CH₄ endotherm lies in the 269–524 °C range, and the temperature of the H₂ peak is between 582 and 655 °C. The noted temperature ranges show that the liberation of H₂O occurs during the first two stages of pyrolysis, the evolution of CO₂ occurs during the pyrolysis second stage, the discharge of CH₄ occurs at the second and third levels of decomposition, and the release of H₂ occurs during the last stage of pyrolysis.

3.5.3. Effect of Blending on the Pyrogas Components Yield

Figure 7 shows the differences in pyrogas component composition between individual vegetable wastes and the ternary blend.

Among the individual vegetable waste samples, cucumber delivered the highest H₂ and CH₄ compositions (1.23 m³/kg of biomass), followed by carrot (1.08 m³/kg of biomass) and tomato (1.07 m³/kg of biomass). Cucumber produced the least CO₂ composition (1.84 m³/kg of biomass), while cucumber and carrot produced CO₂ compositions that were nearly identical (2.02 m³/kg of biomass and 2.01 m³/kg of biomass, respectively). The differences in the composition of pyrogas components in these samples could be attributed to differences in extractive and lignocellulosic compositions, as well as the nature of the biomass components’ linkages.

It should be emphasized that the effect of blending on H₂ and CH₄ composition is negligible because the composition of H₂ and CH₄ in individual and mix vegetable waste samples is remarkably similar. However, blending has a major impact on H₂O and CO₂ compositions, as the blend sample shows a considerable increase in H₂O liberation and a significant reduction in CO₂. In the case of tomato, cucumber, and carrot, ternary blending resulted in CO₂ reductions of 17.10%, 9.11%, and 16.79%, respectively. The difference in the pyrogas components’ composition between the blend and individual vegetable samples suggests that the veggie samples in the blend have a synergistic effect. Further work shall focus on studying the actual synergistic effect by conducting pyrolysis experiments as suggested by Chen et al. (2016) [53] and Wu et al. (2014) [54].

4. Conclusions

In addition to being rich in nutrients and organic matter, food wastes can be used as a source of energy and value-added commodities. In comparison to other approaches, valorising food waste via pyrolysis appears to be an attractive option with its environmental friendliness, speed, and low infrastructure footprint. An initial low-cost waste energy source, e.g., flue gas or solar, drying pretreatment stage is recommended to save energy costs for drying due to using the high value energy used for the pyrolysis process. A thorough understanding of the pyrolysis kinetics of feedstocks is important to the design and construction of pyrolysis reactors. As a result, in this work, the pyrolytic properties of certain typical household vegetable wastes, such as tomato, cucumber, carrot, and their blend, have been evaluated under non-isothermal conditions using a thermogravimetric analyser. The thermochemical analysis (low ash content (5.5%), high volatile content (64.0–80.0%), modest carbon content (41.0–57.0%), and low HHV (12.0–20.5 MJ/kg)) of the chosen wastes confirm that they can be utilized as an excellent feedstock for the production of energy and chemicals. Like any other biomass, the thermal degradation behaviour of the waste samples showed three distinct zones. The Coats–Redfern model was used to explore the kinetic mechanism of the samples utilizing sixteen solid-state reaction processes. The models, based on first-order (F1) reaction rates, were determined to be the best-fit models among the mechanisms. The thermodynamic characteristics found indicated that pyrolysis of the vegetable wastes is a highly promising option to generate specialty products. The vast range of A values and positive values of ΔG and ΔH revealed that all samples were characterised by complex and difficult multi-phasic degradation reactions at various conversions. The pyrogas for individual and blend samples was examined using TGA coupled mass spectrometry because the generation of pyrolysis gases for chemical/energy generation is crucial to the overall process viability. The devolatilization behaviour of the blend confirms the existence of chemical interactions/synergistic effects between the vegetable samples in the blend. The TGA–MS analysis indicated that the blending had a significant impact on the pyrogas yields with CO₂ composition reductions of 17.10%, 9.11%, and 16.79% in the cases of tomato, cucumber, and carrot, respectively. The inferences of this study are likely to aid research into the thermal conversion of food waste. The future scope of work shall focus on investigating the thermal characteristics and pyrolytic kinetics of other cooked food wastes, such as grains, meat, fruits, and so on.