Pyrolysis of Amaranth Inflorescence Wastes: Bioenergy Potential, Biochar and Hydrocarbon Rich Bio-Oil Production

Abstract

Many agro-industrial companies grow amaranth for the subsequent production of amaranth oil, flour, cereals, flakes, and bran. After the grain is extracted, waste in the form of inflorescences remains, which can be used to obtain useful new products. This work investigated the use of pyrolysis to recycle amaranth inflorescence wastes (AIW). Thermochemical conversion experiments in an inert medium were carried out in a laboratory setup at 550 °C and a heating rate of 10 °C/min. It was found that the AIW pyrolysis produced 37.1 wt.% bio-oil, 35.8 wt.% pyrogas and 27.1 wt.% biochar. The oil fraction of the obtained bio-oil contains 41.8% of hydrocarbons. Thermogravimetric analysis of AIW was performed in the temperature range from 40 to 1000 °C at heating rates of 10, 15, and 20 °C/min in argon medium (75 mL/min). The kinetic parameters were determined by the model-free Friedman, Ozawa-Flynn-Wall, and Kissinger-Akahira-Sunose methods. The average activation energy values are in the range of 208.44–216.17 kJ/mol, and they were used to calculate the thermodynamic parameters. The results indicate that the pyrolysis application will allow efficient conversion of AIW into value-added products.

1. Introduction

Amaranth is of great importance for world food security, especially for developing countries in Africa and Asia [1,2]. These plants are pseudocereals that were very important for ancient civilizations [3]. Currently, interest in this culture is growing for a number of reasons. First, amaranth can grow in a wide range of weather conditions and is drought-tolerant. Secondly, the growing demand for a healthy diet encourages the use of this plant. Amaranth is recognized as a rich and inexpensive source of dietary fiber, minerals, vitamins, proteins, and antioxidants [2,4]. One of the most common cereal species is Amaranthus cruentus [5]. After the grain is extracted, waste in the form of inflorescences remains, which can be used to obtain useful new products.

Pyrolysis is a technology widely used for waste disposal [6,7,8]. The uniqueness of this process lies in the simultaneous production of gaseous, liquid and solid products. The sphere of use of these products is quite wide, including the chemical industry [9], energy [10,11], and agro-industrial complex [12,13]. The process of biomass pyrolysis can be carried out in a decentralized manner, which is especially important for agriculture [14]. It is necessary to assess the energy potential of the resulting pyrolysis products in order to assess the possibility of creating a local non-volatile enterprise. It should be noted that biooil is of particular interest since its composition is very complex and largely depends on the feedstock [15]. It is important to find such a source of biomass, which initially, without the use of catalysts during thermochemical conversion and joint pyrolysis with polymers, contains a significant amount of hydrocarbons.

Currently, thermogravimetric analysis (TGA) is used for a detailed assessment of the pyrolysis process [16,17,18]. TGA data are used to study the kinetics of the thermochemical conversion. This allows a comprehensive study of pyrolysis reactions, revealing the characteristic mechanism and predicting the degree of complexity of the reaction, which is necessary for designing pyrolysis apparatuses and evaluating the possibilities of using products [19]. The kinetics of the amaranth inflorescence wastes (AIW) pyrolysis process was studied using the model-free methods of Friedman, OFW (Ozawa-Flynn-Wall) and KAS (Kissinger-Akahira-Sunose), since they have shown themselves to be effective in assessing the kinetics of biomass pyrolysis reactions [6,20,21,22,23,24,25,26]. Currently, numerous studies are being carried out on the use of biomass as a raw material for thermal decomposition [27,28]; however, there are few works on the pyrolysis of amaranth [8,29,30,31], and no study has yet been reported on the thermal decomposition characteristics of the inflorescences of this plant.

This study is aimed at solving the following problems: (a) determining the material balance of the pyrolysis process of a new type of plant waste; (b) study of the composition and quality of the resulting pyrolysis products to assess their subsequent use; (c) analysis of the features of thermal decomposition of waste according to TGA data at heating rates of 10, 15 and 20 °C/min in an inert atmosphere; (d) determination of kinetic triplets for the main stage of pyrolysis—isolation of volatile components using model-free methods; and (e) determination of thermodynamic functions for subsequent design, optimization and scaling of the parameters of the pyrolysis reactor. Thus, the cultivation of amaranth and the subsequent pyrolysis of the remaining waste will improve not only food, but also energy security, which is especially important for countries with adverse climatic conditions.

2. Materials and Methods

2.1. Amaranth Inflorescence Wastes

AIW samples were taken from a farm after harvest (Russia). Inflorescences were dried at room temperature. All samples of AIW were manually cut into small pieces with the help of blades and then finely powdered using a mixer-cum-grinder. All powdered samples were kept in airtight containers for use in further experiments.

2.2. Physicochemical Characterization

The proximate analysis of all samples was performed to estimate the volatile matter, ash content, moisture, and fixed carbon using appropriate ASTM protocols (E1755-01, 2020; ASTM E1756-08, 2020; E871-82, 2019; E872-82, 2019; D1762-84, 2021). The percentages of carbon, hydrogen, nitrogen, and sulfur were determined using the CHNS analyzer (Euro EA 3000, Eurovector, S.p.A., Milan, Italy) and oxygen was calculated by difference. The higher heating value (HHV) was calculated according to the formula presented in the work [32]. The content of macro- and microelements was studied using the energy-dispersive fluorescence X-ray spectrometer (EDX-800HS2, Shimadzu, Kyoto, Japan) by a semi-quantitative method. Gas chromatography–mass spectrometry of the pyrolysis liquid were carried out on spectrometer (GCMS-QP2010, Shimadzu, Japan) on HP-5MS column (0.25 μm, 30 m). The evolved gas was analyzed by a gas chromatograph Chromatec-Crystal 5000.2 (Chromatec, Yoshkar-Ola, Russia) using GOST 32507−2013 and ASTM D 5134-98, 2008. The gas samples were delivered to the given machine from the autoclave’s gas output through special heat-resistant tubing. The gas separation was carried out in capillary column with a length of 30 m and two absorption chambers. Chromatography was run in following temperature mode: 90 degrees for 4 min, from 90 °C to 250 °C with the heating rate of 10 °C/min. The gas carrier was helium and the stream velocity was 2.5 mL/min.

2.3. Pyrolysis Experimental Procedure

Pyrolysis was carried out on a laboratory setup described in [8]. The initial temperature was 25 °C, the heating rate was 10 °C/min, and the temperature of the pyrolysis process itself was 550 °C. The material balance was determined according to the method presented in [8,33]. Each experiment was repeated at least three times.

2.4. Thermogravimetric Analysis (TGA)

A thermogravimetric analyzer (STA 449 F1 Jupiter, Netzsch, Selb, Germany) was applied to record the mass loss as a function of temperature during the pyrolysis of AIW. The sample was placed in a crucible and heated from 40 to 1000 °C at three different heating rates (10, 15, 20 °C/min) with argon flowing at 75 mL/min. To ensure the repeatability of the experiment with an error of 1.5%, the experimental conditions were repeated at least three times. The results showed that the TG and DTG curves were almost identical, which consequently gave very low standard deviations for the calculated kinetic parameters obtained. The results presented here are a set of those experiments that satisfy the above conditions.

2.5. Kinetic Analysis

The mechanism of pyrolysis is characterized by a rather complex set of competitive and parallel reactions, which is also complicated by the variability of the lignocellulosic composition of the biomass [34]. The global pyrolysis reaction is expressed by the following equation [35]:

$$\mathrm{Biomass} \left(\mathrm{solid}\right)\stackrel{k\left(T\right)}{\to }\mathrm{Volatiles} \left(\mathrm{condencable}+\mathrm{noncondencable}\right)+\mathrm{Char},$$

(1)

where $k\left(T\right)$ is reaction rate constant, which is expressed by the Arrhenius equation:

$$k\left(T\right)=A{e}^{-E_{\alpha }/RT},$$

(2)

where E_α is the activation energy (kJ/mol); T is temperature (K); R is the universal gas constant (8.314 J/mol∙K); and A is the pre-exponential factor (1/s).

The biomass conversion rate α is defined as the mass fraction of the degraded sample. It can be calculated for each point of TGA according to the equation [36,37,38]:

$$\alpha =\frac{m_0-m}{m_0-m_f},$$

(3)

where m₀ is the initial sample weight of the sample (mg); m is actual weight to each point of analysis (mg); and m_f is the final weight of the sample after pyrolysis (mg).

Generalized fundamental expression for non-isothermal TGA experiments at linear heating rate:

$$\beta =\frac{dT}{dt}=\frac{dT}{d\alpha }\frac{d\alpha }{dt},$$

(4)

Or

$$\frac{d\alpha }{dt}=\frac{A}{\beta }\cdot \exp \left(\frac{-E_{\alpha }}{RT}\right)f\left(\alpha \right),$$

(5)

where f(α) is function of conversion.

The analytical form of the function f(α) depends on the thermal decomposition mechanism. Integration of Equation (5) makes it possible to analyze the kinetic data obtained by the TGA method. Integration can be performed using model-free (isoconversion) methods [38,39], which estimate the activation energy (Eα) when changing the degree of conversion α. These methods are also called “multi-curve” since they require the use of several kinetic curves for analysis [40].

2.6. Model-Free Methods

In this work, the modeless methods of Friedman, OFW, and KAS were used to analyze the kinetic parameters [6,20].

The Friedman’s method is a differential method which is expressed by the equation:

$$\ln \left[{\beta }_i{\left(\frac{d\alpha }{dT}\right)}_{\alpha ,i}\right]=\ln \left[A_{\alpha }f\left(\alpha \right)\right]-\frac{E_{\alpha }}{RT},$$

(6)

where the subscript i is given heating rate value, and subscript α is given conversion degree.

The OFW method is an integral method which is expressed by the equation:

$$\ln \left({\beta }_i\right)=\ln \left(\frac{A_{\alpha }{E}_{\alpha }}{R_g\left(\alpha \right)}\right)-5.331-1.052\frac{E_{\alpha }}{R{T}_{\alpha i}},$$

(7)

The KAS method used for kinetic determination is given in equation:

$$\ln \left(\frac{{\beta }_i}{T_{{}_{\alpha i}}^2}\right)=\ln \left(\frac{A_{\alpha }R}{E_{\alpha } g\left(\alpha \right)}\right)-\frac{E_{\alpha }}{R{T}_{\alpha i}},$$

(8)

where g(α) is constant with given conversion value.

2.7. Reaction Model Determination for AIW Pyrolysis

The master-plot method is used to predict solid state mechanisms in the thermal decomposition of biomass. The master graph is built either in a differential or in a differential-integral form [37]. Various models are fitted to solid-phase kinetic data based on such reaction mechanisms as nucleation, geometric shape, diffusion, and reaction order [19,40]. The theoretical master plots do not depend on the heating rate, but strictly depend on the kinetic model used to model the reaction [41]. To construct a differential graph, a comparison is used at the control point α = 0.5 [42].

$$\frac{\frac{d\alpha }{d\theta }}{{\left(\frac{d\alpha }{d\theta }\right)}_{0.5}}=\frac{f\left(\alpha \right)}{f\left(0.5\right)},$$

(9)

where $\frac{f\left(\alpha \right)}{f\left(0.5\right)}$– theoretically determined from the function, the expressions for which are given in [43]; θ denotes the reaction time taken to attain a particular α at infinite temperature. The left side of expression (9) is the experimental curve calculated using the following equation:

$$\frac{\frac{d\alpha }{d\theta }}{{\left(\frac{d\alpha }{d\theta }\right)}_{0.5}}=\frac{\frac{d\alpha }{dt}}{{\left(\frac{d\alpha }{dt}\right)}_{0.5}}\cdot \frac{\exp \left(\frac{E}{RT}\right)}{\exp \left(\frac{E}{R{T}_{0.5}}\right)},$$

(10)

where $T_{0.5}$ is the reaction temperature at α = 0.5.

2.8. Thermodynamic Parameters

Estimating thermodynamic parameters is a useful tool for understanding biomass pyrolysis, determining the feasibility of a thermal decomposition process, and calculating energy performance. Enthalpy change ∆H (kJ/mol), Gibbs free energy ∆G (kJ/mol), and entropy change ∆S (J/mol∙K) were calculated according to the equations derived from the activation complex theory (Eyring Theory) using the following formulas [44,45,46]:

$$\Delta H=E_{\alpha }-R\cdot T_{peak},$$

(11)

$$\Delta G=E_{\alpha }+\left(R\cdot T_{peak}\cdot \ln \frac{K_B\cdot T_{peak}}{h\cdot A}\right),$$

(12)

$$\Delta S=\frac{\Delta H-\Delta G}{T_{peak}},$$

(13)

where $T_{peak}$ is the temperature corresponding to the maximum mass loss rate, °C; $K_B$ is Boltzmann constant (1.38 ∙ 10⁻²³ J/K); h is Planck’s constant (6.626 ∙ 10⁻³⁴ J∙s).

3. Results and Discussion

3.1. Results of Proximate and Ultimate Analyses

To assess the possibility of using AIW as a bioenergy raw material, the main physical and chemical characteristics were considered (

Table 1. The results of the proximate and ultimate analyses of AIW sample.

Analysis	Values
Proximate (wt.%)—based on air-dried basis:
Moisture	7.42 ± 0.02
Volatile matter	74.65 ± 0.30
Ash	8.76 ± 0.01
Fixed carbon	9.17 ± 0.06
HHV, MJ/kg	17.87
Ultimate (wt.%)—based on dry basis:
Carbon	41.83 ± 0.26
Hydrogen	6.81 ± 0.08
Nitrogen	4.71 ± 0.13
Oxygen	37.89 ± 0.17

).

Humidity and ash content in the AIW sample corresponds to the range of values typical for commercial biomass fuels (humidity up to 25.6 wt.%, ash content up to 9.8 wt.%) [47]. The test sample has a high content of volatile substances; therefore, it is suitable for various thermochemical processes due to its high flammability. The obtained value of volatile substances is comparable with the values obtained for other agricultural wastes suitable for energy use [47,48]. In addition, this means that the AIW sample is more reactive than traditional energy sources such as coal. The HHV of the sample corresponds to the commercial fuel olive stone (17.88 MJ/kg), energy crops–thistle (17.75 MJ/kg) [47], as well as such biomass as: apple tree branches (17.82 MJ/kg), feijoa leaves (17.81 MJ/kg), hazelnut tree leaves (17.87 MJ/kg), kiwi branches (17.81 MJ/kg), and olive stone (17.88 MJ/kg) [49].

3.2. Pyrolysis Products Yields and Their Quality

The pyrolysis products of AIW are shown in Figure 1. The presented values are consistent with the data obtained from the pyrolysis of rice husks [34], switchgrass [50], algal waste [51], and poultry litter [52]. The maximum mass fraction of 37.1 wt.% is characteristic of the pyrolysis liquid. In connection with the subsequent use of pyrolysis liquid for energy purposes, it was separated into oil and aqueous fractions. It is important to use a homogeneous fuel to ensure timely ignition, as well as efficient atomization in the combustion zone and maintaining flame stability in combustion devices [53].

The aqueous fraction of the pyrolysis liquid contains 85.72% water, 10.4% acetic acid, and 3.88% unidentified components. Water is the main component in the liquid, which is explained by the humidity of the AIW sample, dehydration reactions at temperatures below 550 °C, and the occurrence of secondary cracking reactions of oxygen-containing macromolecular compounds at high temperatures [54]. The oil fraction has a diverse and rich composition. Approximately 70.85% of the relative content of the total peak area was identified (GC-MS analysis). The identified compounds were classified into the following main chemical categories: hydrocarbons, phenols, alcohols, ketones, ethers, and N-containing heterocycles. Components with a peak area ≥ 1% are presented in

Table 2. The main components of the oil fraction (peak area ≥ 1%).

N°	Area, %	Name	Formula	Mw, g/mol
1	12.36	Tetratetracontane	C44H90	619.8
2	9.70	Tetracontane	C40H82	563.1
3	8.20	1-Octacosanol	C28H58O	410.8
4	5.44	2,6,10,14,18,22-Tetracosahexaene, 2,6,10,15,19,23-hexamethyl-, (all-E)-	C30H50	410.7
5	2.19	Octacosanoic acid, methyl ester	C29H58O2	438.8
6	2.00	Phenol	C6H5OH	94.11
7	1.80	Pentadecane	C15H32	212.41
8	1.50	Triacontanoic acid, methyl ester	C31H62O2	466.82
9	1.43	Tetracosane	C24H50	338.7
10	1.40	Phenol, 2-methoxy-	C7H8O2	124.12
11	1.24	Pyridine, 3-methyl-	C6H7N	93.13
12	1.11	Octadecane	C18H38	254.49

. Saturated hydrocarbons tetratetracontane and tetracontane are present in large quantities. It is known that the oil fraction from red amaranth seeds is a rich source of squalene, so the content of 2,6,10,14,18,22-Tetracosahexaene, 2,6,10,15,19,23-hexamethyl-, (all-E) equals 5.44% [55]. All identified compounds (including peak area ≤ 1%) are grouped and shown in Figure 2. The oil fraction contains 41.8% hydrocarbons, which characterizes it as a high-quality fuel.

It should also be noted that there are no organic acids in the oil fraction; they are present only in the composition of esters. Accordingly, the pH value is high and the liquid is characterized by an alkaline reaction, which is also important for the design of power plants.

The concentrations of the detected pyro-gas components, converted to nitrogen-free composition, are shown in

Table 3. Pyro—gas composition.

Component	CO2	CO	CH4	C2H6	CxHy	C2H4	H2
Concentration, %	47.30	47.14	3.51	1.17	0.75	0.12	0.01

. It was found that the predominant components in AIW pyrolysis are CO₂ and CO. The total concentration of these gases reaches 94.44%. The combustible part of the pyrolysis gas includes 52.7% of the components, which is consistent with the data of other authors [52,56,57].

The main physicochemical characteristics of AIW biochar (

Table 4. The results of the proximate and ultimate analyses for biomass biochars.

Analysis	Biomass
	AIW	Maize Stalk [58]	Lantana Camara [58]	Pine Needles [58]	Black Gram [58]
Proximate (wt.%)
Volatile matter	21.34 ± 0.03	20.67	22.56	27.62	23.56
Ash	20.49 ± 0.01	19.7	15.7	13.5	23.3
Moisture	4.4 ± 0.19	11.5	6.13	8.05	12.41
Fixed carbon	53.77 ± 0.0,9	48.13	55.61	50.83	40.73
HHV *, MJ/kg	20.92	23.7	25.87	22.33	21.06
Ultimate (wt.%)
carbon	56.56 ± 0.17	61.9	70.5	65.8	56.7
hydrogen	3.09 ± 0.05	3.56	2.69	2.13	3.14
nitrogen	4.12 ± 0.01	1.17	0.86	0.78	1.24
oxygen	15.75 ± 0.14	13.67	10.25	17.79	15.62

* calculated.

) correspond to biochars obtained by pyrolysis (process temperature 500 °C) of different biomass [58]. The elemental composition of AIW biochar is typical, in which the content of carbon is in the range of 50–87.2%, hydrogen 0.7–4.5%, nitrogen 0.08–6.94%, and oxygen 6–30% [59]. Figure 3 shows the microelement composition of the ash of the solid carbonaceous residue. The predominant components of the ash were K and Ca, and their total content was 81.8% of the total mass.

Thus, the studied biochar can serve as a direct source of potassium, which is the most important element—a biophile, the removal of which with the harvest of agricultural crops is always greater than that of phosphorus and nitrogen. An analysis of the literature showed that elevated values of K, Mg, and Ca in the solid pyrolysis product make it possible to use it for liming and neutralizing acidic soils [60,61].

3.3. Thermal Degradation Analysis

The results of pyrolysis of AIW samples at heating rates of 10, 15, and 20 °C/min in an argon atmosphere are shown in Figure 4. The TG curves are the change in weight loss with temperature, and the DTG curves are the rate of weight loss with temperature. According to the shape of the curves, it can be judged that the thermal degradation of the studied AIW sample occurs similarly to the general trend of biomass pyrolysis.

Based on the analysis of the obtained TG data, the AIW pyrolysis process can be divided into 3 main stages (

Table 5. Main stages of thermal decomposition.

N°	Pyrolysis Stage	Heating Rate (°C/min)	Starting Temperature (°C)	Ending Temperature (°C)	Temperature Peak (°C)
I	Moisture evaporation	10 15 20	40 40 40	191.26 190.76 191.77	103.1 115.4 126.7
II	Devolatilization	10 15 20	191.26 190.76 191.77	529.5 544.48 558.95	317.7 322.6 328.5
III	Degradation of char and minerals	10 15 20	529.5 544.48 558.95	1000 1000 1000	668.2 685.6 690.6

).

The first stage in the temperature range from 40 °C to 190 °C corresponded to the process of evaporation of physically bound moisture from samples of AIW. It is also possible to release light volatile components at this stage [46]. The average weight loss at this stage was 9.25 wt.% for three heating rates (

Table 6. Mass loss characteristics of AIW obtained from TGA analysis.

Heating Rate (°C/min)	Mass Loss, wt.%			Residual Mass, wt.%
	Moisture Evaporation	Devolatilization	Degradation of Char and Minerals
10	9.14	58.65	5.79	26.24
15	9.33	59.17	5.89	25.61
20	9.28	61.08	4.88	24.76
Average, %	9.25	59.63	5.52	25.54

). The first stage has a small peak characterized by an endothermic reaction, which is associated with the absorption of heat in the process of moisture evaporation [44].

The main stage, corresponding to the main pyrolysis, occurred in the temperature range from 190 °C to the temperature range of 530–560 °C for three heating rates and was accompanied by the main loss of organic matter mass. During this stage, there was an active decomposition of the biomass components and the release of volatile substances associated with the thermal destruction of hemicellulose, cellulose, and lignin [62,63]. The average weight loss during the release of volatiles was 59.63 wt.%. As can be seen from Figure 4, rapid weight loss begins above a temperature of 190 °C, which is associated with the rapid breakdown of thermally unstable components of hemicellulose and extractives [37,64]. Hemicellulose consists of short chain heteropolysaccharides and is an amorphous and branched structure [8,19,33,37,39,41,65,66]. Furthermore, with an increase in the pyrolysis temperature, cellulose is involved in the degradation process, which is characterized by a higher decomposition temperature (315–400 °C) due to the presence of a long polymer of glucose units and a large number of hydrogen bonds in its composition [67]. Cellulose, due to its chemical structure, is more resistant to thermal degradation; its decomposition is typical for the temperature range of 270–350 °C [68,69].

On the DTG-curves (Figure 4) at the stage of devolatilization, one can note the maximum temperature peak, which has values of 317.7, 322.6, and 328.5 °C for the three heating rates. This peak is characterized by an endothermic reaction. In addition, a small temperature exothermic peak is found at 402.7–422.1 °C, which can be associated with the beginning of the decomposition of lignin in the test sample. The literature data indicate that the onset of lignin decomposition for various types of biomass occurs in the temperature range of 280–550 °C [16]. The mechanism of lignin pyrolysis is more complex than that of cellulose and hemicellulose; it includes reactions of free radicals [70,71]. Due to the fact that lignin has the highest thermal stability, it decomposes slowly throughout the thermal degradation up to a temperature of 900 °C [16].

The third stage, which is typical for the temperature range of 529.5 °C and up to 1000 °C for three heating rates, is associated with the process of degradation of char and minerals. At this stage, after the completion of the release of volatiles and the main thermal destruction, the process of enrichment with carbon and the formation of the structure of carbonaceous matter continue. Although small, inorganic minerals in biomass can have a significant effect on the pyrolysis process. In this regard, the process of thermal degradation of mineral components is primarily associated with the decomposition of CaCO₃ in the temperature range from 780 to 1000 °C [72]. In addition, pyrolysis products can interact with inorganic elements in the residual carbonaceous matter [73]. In this case, the mineral components act as catalysts in the reactions of gas formation from pyrolysis products [72]. The residual fraction as a result of AIW pyrolysis was 25.5 wt.% for the three heating rates. As a result of the experiments, it was revealed that the nature of the TG and DTG curves of the studied samples is similar to the biomass of herbaceous plants, which were reported in [16,17,74].

3.4. Kinetic Analysis

In this work, a kinetic analysis was carried out for the main stage of pyrolysis—devolatilization—since at this stage, the maximum mass loss occurs [75]. AIW kinetic parameters were determined using three model-free methods: Friedman, KAS, and OFW, based on TGA data. Figure 5 shows the results of linear regression in the range of conversions from 0.1 to 0.9 of the kinetic analysis of the total thermal decomposition reactions of the AIW samples. Straight line slope data obtained from each model-free method were used to calculate the Eα values presented in

Table 7. Values of activated energy according to different methods.

α	Friedman		KAS		OFW
	Eα (kJ/mol)	Log A (1/s)	Eα (kJ/mol)	Log A (1/s)	Eα (kJ/mol)	Log A (1/s)
0.1	164.48	13.22	185.27	15.72	185.42	15.68
0.2	152.52	11.44	156.78	12.26	157.00	12.23
0.3	184.77	14.16	167.31	12.93	167.50	12.90
0.4	212.88	16.47	189.57	14.76	189.74	14.72
0.5	231.03	17.80	209.08	16.31	209.23	16.26
0.6	250.60	19.10	225.76	17.52	225.91	17.48
0.7	291.94	21.85	253.73	19.48	253.88	19.44
0.8	244.29	16.61	265.25	19.30	265.43	19.26
0.9	213.02	13.17	223.21	14.75	223.43	14.70
Average	216.17	15.98	208.44	15.89	208.61	15.85

.

The dynamics of Eα values obtained by the Friedman, KAS, and OFW methods highlight the complexity of the AIW sample kinetics. Eα gradually increases until reaching its maximum at a conversion rate of 0.8 for the OFW and KAS methods, and α conversion rate of 0.7 for the Friedman method. A similar trend in Eα values was found during pyrolysis of such biomass as bark of Ficus natalensis [7], water hyacinth [76], elephant grass [77], and mustard stalk [78].

The pre-exponential factor A characterizes the frequency of collisions of reacting molecules. This indicator makes it possible to explain the chemistry of reactions, which is important for optimizing the pyrolysis process [36]. Almost all obtained A values are in the range from 10⁴ to 10⁹, which indicates a low reactivity of the test sample and the occurrence of a surface reaction, as well as a tight junctional complex (closed complex) [36,39].

The Eα values are in the range of 152.52–291.94 kJ/mol (Friedman), 156.78–265.25 kJ/mol (KAS), and 157.00–265.43 kJ/mol (OFW). The value of Eα shows a measure of the minimum energy required to start a chemical reaction, as well as a potential measure of reactivity [79,80]. According to the literature data, the KAS and OFW methods are less accurate than the Friedman method [39,79], since it does not contain assumptions and approximations [39,81]. It should be noted that the Eα values calculated by the KAS, OFW, and Friedman methods for the AIW sample agree with each other. The average value of Eα obtained by the Friedman method is only 3.7% higher than that calculated by the OFW and KAS methods. Comparative analysis of Eα values for different types of biomass is presented in

Table 8. Comparison of biomasses activation energy.

Fuel	Heating Rate (K/min)	Used Methods	Activation Energy (kJ/mol)	Reference
AIW	10, 15, and 20	Friedman, KAS, OFW	216.17 208.44 208.61	Present Study
Cotton stalk	10–40	KAS, OFW	223–230 213–240	[82]
Sugarcane leaves	5–40	Friedman, KAS, OFW	239.58 226.75 226.97	[21]
Prosopis juliflora fuelwood	2–25	Friedman, KAS, OFW	219.3 204.0 203.2	[22]
Phyllanthus emblica seeds	10–50	Friedman, KAS, OFW	189.95 184.77 195.10	[23]
Camphor branch	2.5, 5, and 10	Ozawa	190	[83]
Microalgae Chlorella vulgaris	10–40	Kissinger, Friedman, OFW, KAS, Vyazovkin, DAEM	135.6–337.1	[24]
Digested biomass wastes	10, 15, and 20	Friedman, KAS	202.55 202.21	[75]
Sorghum bicolor	2, 5, and 8	Friedman and KAS	226.6	[84]
Pea waste	10–40	KAS, OFW	212.71 211.55	[85]
Basswood waste	20–40	KAS, OFW	197.2 207.9	[86]

.

Figure 6 compares the theoretical differential plots of f(α))/f(0.5) versus α with the experimental plot of (dα/dθ)/(dα/dθ) 0.5 versus α for a heating rate of 10 °C/min to draw a conclusion about the reaction mechanism of solid-phase pyrolysis.

In the range of α from 0.1 to 0.5, the AIW degradation mechanism refers to the one-dimensional diffusion (D1) process, i.e., heat transfer in the sample occurs by diffusion. When α values are greater than 0.5, the AIW degradation mechanism tends to random nucleation with one nucleus in a single particle (F1). In the range of α from 0.7 to 0.9, the mechanism is reduced to random nucleation with two nuclei in the individual particle (F2). The F1 and F2 degradation mechanisms are initiated from random points that act as growth centers for the development of the degradation reaction [87]. Similar results were obtained for other types of biomass [88]. A slight discrepancy between the experimental curves of the master plot for the studied AIW samples can be explained by the deviation of the ideal conditions adopted in the kinetic models from the actual pyrolysis conditions.

3.5. Thermodynamic Analysis

To design, optimize, and scale the parameters of the pyrolysis reactor, it is also necessary to know the thermodynamic properties of the feedstock used. Thermodynamic parameters were determined for a heating rate of 10 °C/min (Figure 7). In biomass pyrolysis, ΔH is the total energy required for biomass decomposition into solid, liquid, and gaseous products [45,89,90]. The ΔH values for the studied AIW sample were in the range of 152.28–287.03 kJ/mol according to the Friedman method, 151.86–260.34 kJ/mol according to the KAS method, and 152.05–260.52 kJ/mol according to the FWO method. Positive values of ΔH indicate the endothermic nature of biomass pyrolysis, which implies the need for energy from an external heat source [6]. The difference between the average values of Eα and ΔH is insignificant, approximately 5 kJ/mol (for all methods), which indicates that the studied AIW sample is suitable for pyrolysis [43,87,88,90,91,92,93,94].

The change in ΔG makes it possible to judge the energy available in biomass [6,49,95,96]. The ΔG values for the studied AIW sample ranged from 147.90 to 150.78 kJ/mol by the Friedman method, 148.38–150.19 kJ/mol by the KAS method, and 148.38–150.18 kJ/mol by the OFW method. For most known biomasses and their mixtures, the ΔG values are positive [87,90,95,96,97,98]. The resulting average value of ΔG is 149.61 kJ/mol of the AIW sample. It is comparable to the ΔG values for pseudo-hemicelluloses of cocoa shell (143.19 kJ/mol) [99], torrefied biomass of Acacia nilotica T-250 (159.97 kJ/mol) [100], and red macroalgae Gelidium floridanum for stage 1 (147.25 kJ/mol) [44], but higher than mustard stalk (128 kJ/mol) [78]. The data obtained indicate the high energy potential of AIW.

Entropy is a function of the state of a thermodynamic system, which characterizes the direction of spontaneous processes and is a measure of their irreversibility. The change in ΔS serves as a measure of the change in the order of a thermodynamic system. The entropy of the system is the higher the greater the degree of disorder of the system. Thus, if the process goes in the direction of increasing the disorder of the system, then ΔS is a positive value. To increase the degree of order in the system, it is necessary to expend energy [90,93]. The ΔS values of amaranth samples range from 2.15 to 235.45 J/mol∙K by the Friedman method and 1.42-189.78 J/mol∙K by the KAS and OFW methods. Throughout the conversion process, the ΔS values were positive for the three model-free methods, indicating a high reactivity of the biomass and a rapid formation of the activated complex. It should be noted that the degree of disorder in the resulting products was quite high, and this is typical of the pyrolysis process [48,100]. The mean ΔS of the AIW sample was 105.41 J/mol∙K by the Friedman method, 91.15 J/mol∙K by the KAS method, and 91.46 J/mol∙K by the OFW method. The ΔS value is comparable with the values obtained for mixtures of sugarcane bagasse, water hyacinth Eichhornia crassipes and yellow oleander Thevetia Peruviana [101].

4. Conclusions

In this work, a study was made of the pyrolysis of a new type of plant waste using TGA and experiments in a laboratory installation for thermochemical processing. The physicochemical parameters of the studied raw materials correspond to the range of values typical for commercial biomass fuels. The test sample has a high content of volatile substances and high reactivity. The maximum specific gravity in the pyrolysis products of 37.1% corresponds to the pyrolysis liquid. The maximum mass fraction in the pyrolysis products of 37.1 wt.% corresponds to the pyrolysis liquid. At the same time, the oil fraction contains 41.8% hydrocarbons, which characterizes it as a high-quality fuel. Analysis of the features of thermal decomposition of waste was determined at heating rates of 10, 15, and 20 °C/min in an inert atmosphere. The main stage of thermochemical degradation is devolatilization. The kinetic parameters for this stage were determined using the model-free methods of Friedman, OFW, and KAS. The one-dimensional diffusion model (D1), then random nucleation with two nuclei in the individual particle (F1), and random nucleation with two nuclei on the individual particle (F2) were recommended to describe the mechanism of AIW thermal destruction. The average activation energy values are in the range of 208.44–216.17 kJ/mol, and they were used to calculate the thermodynamic parameters. The results indicate that the pyrolysis application will allow the efficient conversion of AIW into value-added products.