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].
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.