Pyrolysis of Khat waste vs. Coal: Experimental and Aspen plus analysis

Abstract

This study explores the potential of Khat waste as a biofuel through a detailed analysis of its physical properties and pyrolytic behavior, employing both laboratory and simulation techniques. Biomass, including Khat waste, is a valuable renewable energy source with potential applications in reducing waste and CO2 emissions, especially in rural areas of developing countries like Ethiopia. The physical properties of the Khat waste were thoroughly examined, including moisture content, ash, volatile matter (VM), fixed carbon (FC), and elemental composition. The apparent density of the pyrolyzed khat waste was found to be 0.9688 g/cm³. The TGA results showed a moisture loss of 6.7 %, volatile matter of 4.7 %, fixed carbon of 6.78 %, and ash content of 5.55 %. Notably, Khat waste exhibited unique thermal behavior with a downward shift in TG curves above 220 °C. DTGA identified three decomposition peaks: moisture evaporation at 100 °C, hemicellulose breakdown at 210 °C, and cellulose degradation at higher temperatures. Fourier Transform Infrared Spectroscopy (FTIR) provided insights into the functional groups present in Khat waste, including alkene functional groups and O-H bonds, which are crucial for biofuel properties. Simulation using Aspen Plus software modeled the pyrolysis process, highlighting how varying heating rates affect volatile matter and fixed carbon content. Increasing heating rates decreased volatile matter and moisture but increased fixed carbon content. These findings offer valuable insights for optimizing Khat waste as a biofuel, emphasizing the need for tailored pyrolysis conditions to enhance biofuel production efficiency.

Highlights

• •Khat has less nitrogen than coal, which likely reduces pollution when burned.
• •Pyrolysis of khat waste results in weight loss at higher temperatures, with final weights varying by heating rate.
• •DTGA graphs for khat show three peaks, each representing a distinct thermal breakdown process.
• •Key chemical groups and their vibrational frequencies in khat are identified, revealing its molecular composition.
• •Comparison between experimental and simulation results shows strong agreement, indicating the reliability of the findings.

1. Introduction

Biomass, sourced from living matter, is a product of solar energy captured through photosynthesis by plants. Biomass is separated into three primary groups of feedstock; wastes (primarily from agricultural and urban solid waste), forest remnants (such as sawdust, timber wood, and bark residues), and crops; biomass exhibits structural and species variations [1]. Given its role in public welfare, the management of biomass resources should not rely solely on individual decisions. In Ethiopia, solid waste management traditionally involves collection and disposal methods [2].

Biomass waste offers two benefits: it reduces the likelihood of igniting forest and other fires while recovering organic content for useful purposes. In addition to lowering the risk of pollution, proper handling of these wastes makes it easier to recover important organic matter. Furthermore, we may improve environmental sustainability even further by addressing CO2 emissions from garbage exposure in open lots or from waste management-related incineration activities [3]. One promising way to lessen the effects of climate change is to use energy recovery techniques to turn biomass wastes into fuel. The biomass component of this study is waste, which comes from the evergreen plant Catha edulis Forsk [[4], [5], [6]]. It is mostly grown in Ethiopia and some parts of Yemen in East Africa. When chewed, the stimulant qualities of khat, which are act on the central nervous system. Taking Khat induces an emotional state that is usually regarded as peaceful and pleasant, according to the World Health Organization (2006) [7]. This improves alertness and attention while lessening tiredness. It also causes increases in heart rate and blood pressure. Growing environmental concerns and rising worldwide energy demands are driving an increase in the use of bioenergy in traditional energy generation. But even with its widespread use, using biomass feedstock to produce biofuels presents difficulties because of its scarcity and low energy density [2,5,8].

However, biofuels have a wide range of uses, such as in the transportation industry and for cooking, and have a great deal of potential for widespread adoption, especially in rural parts of developing nations like Ethiopia. This research, to study the characteristics of waste Khat and bio-fuel by using software simulation under pyrolysis and laboratory analysis [9]. It will consider both the ultimate and proximal analysis of Khat garbage, as well as heating breakdown (weight loss) and physical properties (bulk and apparent density), as well as a functional category of liquid fuel obtained from pyrolysis Khat waste using FTIR [10]. The factors that influence liquid fuel output in the Aspen plus software and pyrolysis process were investigated. By estimating the production outcome of biofuel from raw properties such as proximate and finally analyzing Khat garbage collected from the test utilizing the Aspen Plus tool [11]. In this study, different previous fields of work related to the formation of biofuel through pyrolysis of waste Khat, characterization of biomass, biofuel production by pyrolysis, and different factors to affect biomass pyrolysis are reviewed [12]. This study aims to fill the research gap by specifically evaluating the yield of valuable compounds from khat waste, such as alkaloids and cellulose, which have applications in biofuels, and agriculture pharmaceuticals. Understanding and harnessing these yields not only presents economic opportunities but also promotes environmental sustainability through effective waste utilization. Biomass energy has the potential to address the worldwide energy shortfall triggered by diminishing resources of fossil fuels [13]. Generally, one notable aspect is the yield obtained from Khat waste through recycling processes. Studies have shown that Khat waste can yield valuable compounds such as alkaloids and cellulose, which can be utilized in various industries, including pharmaceuticals, biofuels, and agricultural applications. Understanding and harnessing this yield not only offers economic opportunities but also promotes environmental sustainability by reducing waste and utilizing renewable resources effectively. By incorporating the description of Khat waste yield into the introduction, this study aims to underscore its recycling value and explore innovative approaches to utilize this underutilized resource [14]. As a result, based on the reviewed literature, the characteristics of feed trash biomass products, as well as the characteristics of Khat composition and product, are to be investigated [15,16].

2. Material used and methods

2.1. Materials used

The materials used in this research laboratory are an oven, sieve, mortar, electronic balance, potassium bromide, distill water, FTIR spectroscope, and TGA, furnace, CHN/S analyzer.

Oven: Used for dry waste khat moisture content determination and until it reached for grinding. Sieve: To reduce the size and shapes of material feedstock at less than 1 mm or to screen the desired size. Distil water: Raw material is washed with distilled water and cleaned. FTIR spectroscope: To be aware of the material feed functional group. CHNS: The primary tool for figuring out the elemental makeup of khat waste is the Flash elemental analyzer. Furnace: to burn samples at a set temperature and measure volatile matter and ash content. TGA: A Thermogravimetric investigation assessing the heat-related behavior of khat biomass feedstock before and after undergoing pyrolysis thermal treatments. Electronic balance: Used to measure the mass of feedstock materials. Mouth and nose masks: used to protect against unwanted odors and pollutants. Crucibles: used for handling samples during measuring of ash and volatile matter contents. Hand protective glove: protects the hand from direct contact with the sample and other chemicals. Mortar: grinding for size reduction [[17], [18], [19], [20]].

2.2. Methods

In this investigation, the pyrolysis feed is derived from leftover Khat. The initial step involves cleaning the Khat waste to remove pollutants and dust. After washing, the samples are dried both in the sun and in an oven until they reach a dryness level suitable for grinding. Khat samples are dried in an oven at (105 °C–110 °C) for 24 h. The drying continues until a constant weight is achieved, which indicates that the moisture content has been reduced to less than 10 %. This ensures that the sample is adequately dried for further analysis [17]. The methodology ensures that the Khat feed is properly prepared and analyzed through a series of precise measurements, including drying, moisture, ash, and density measurements as shown (Fig. 1).

Fig. 1.

Experimental and simulation process method of Khat characterization.

A condenser condenses a furnace and warms pyrolysis; its vapor and water are cooled and circulated for condensation via a chiller. The mass of many biomass particles per occupied unit volume is known as bulk density. The density of biomass varies according to how it is treated. The bulk and apparent densities of oven-dryable coffee and chat samples were determined using the ASTM standard method [[14], [15], [16]]. The elemental analyzer determined the composition of the components [21]. The goal of this study is to find the amounts of the elemental Carbon (C), Hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S) in the waste from the Khat plant. The composition of CHNS was investigated using dried coffee and chat samples. An automatic elemental analyzer apparatus (MODEL: Thermo-scientific analyzer) is used to analyze elemental percentages.

Aspen Plus is the name of the simulation and modeling software package. Aspen Plus is basic software that simulates early pyrolysis and predicts the production of char, bio-oi, and syn-gas. The majority of model development components, known as blocks, react to the material's physical and chemical properties. It begins with a drier block operating at 105 °C −120 °C to remove any excess moisture from the feedstock biomass [22]. The dry stream entered the disintegration block, which was designed to behave similarly to a yield reactor. It turns the raw material into typical simulation components utilizing proximate and chemical information (water, ash, cellulose) [4].

2.3. Characterization of Khat (proximate analysis)

Biological investigations are necessary to characterize and classify urban solid waste [23]. Khat waste's physical properties include density, proximate, and elemental. Proximate analysis refers to the removal of weight in a Khat waste sample when burned at particular temperatures. This proximate analysis covers moisture, fixed carbon, ash, and volatile matter [9,11,13].

Density; The mass of many biomass particles per occupied unit volume is known as bulk density. The density of biomass varies according to how it is treated. The overall and appearance densities of Khat samples dried in an oven were calculated. Bulk densities were estimated using both freely settling and tapped density, as well as the quantity of dry Khat samples packed into the cylinder's graduated capacity. The bulk density can be shown in the following formula (Eq. (1)) [24].

$${\rho }_b=\frac{m_{\mathrm{i}}}{\mathrm{V}}$$ (1)

where ρ_b is bulk density, m_i is the initial mass of khat (before dried) and V is the volume of water. The apparent densities of Khat were determined using the following (Eqs. (2), (3), (4), (5))) a conventional method that utilized 25 mL of water. The dried sample was placed in a jar until it occupied roughly one-third of the container's height. Distilled water was then added to fill the remaining volume of the vessel. The volume of Khat was calculated by subtracting the volume of the distilled water from the total volume of the pycnometer [6].

$${\rho }_a=\frac{m_i}{V_{kh}}$$ (2)

$$V_{Kh}=V_P-V_w$$ (3)

$$V_w=\frac{m_w}{{\rho }_w}$$ (4)

$${\rho }_a=\frac{m_i}{{25ml-V}_w}$$ (5)

where: ρ_a, is apparent density, V_kh is the Volume of khat, m_i is the initial mass of khat, V_w volume of water, m_w is the mass of water, V_p is the total volume of pycnometer and ρ_w is the density of water.

Moisture contents: Moisture content refers to the amount of water lost from materials during drying until a constant weight is achieved, influenced by both the physical and chemical properties of materials enabling them to absorb water from the surroundings. Samples were dried at temperatures ranging from 105 to 120 °C [[25], [26], [27]]. After removal from the oven, the dried sample was examined for 3 h, or until readings indicated no significant further loss of mass, as depicted in (Fig. 2).

Fig. 2.

Electronic balance for moisture content calculation.

The mass of khat waste is measured using an electronic balance both before its placement in the oven and upon its removal after drying [11,13] (Fig. 3). illustrates the moisture analysis process and the following formula shown in (Eqs. (6), (7))).

$${\%M}_{cwet}=\left[\frac{{\mathrm{m}}_i-m_f}{m_i}\right]\times 100$$ (6)

$${\%M}_{cdry}=\frac{m_i-m_f}{m_f}\ast 100$$ (7)

where:

• ➢m_i = mass khat sample before dried (initial mass).
• ➢m_f = Mass of Khat after drying.
• ➢%M_cwet = moisture content at wet bases.
• ➢%M_cdry = moisture contents at dry bases.

Fig. 3.

Moisture content determination method.

Ash Contents: The ash content of Khat waste indicates its suitability as a fuel. This content refers to the residue left after removing volatile matter and fixed carbon. The oven-dried Khat sample was carefully weighed and placed into a ceramic crucible before being placed in the furnace. It was then heated at 600 °C for 4 h to determine the ash content accurately. The dried sample was calculated and deposited in a ceramic crucible before being placed in the furnace at 600 °C.

The mass of the Khat was measured both before entering the furnace and after it had been removed following 4 h of heating (Fig. 4). The measuring process is shown in (Fig. 5) and the percentage content of Ash can be calculated by dividing the amount of ash remaining by the initial mass of the Khat (before drying in the oven)as illustrated in (Eq. (8) [11,13].

$${\%\mathrm{A}}_c=\left[\frac{{\mathrm{m}}_{\text{ash}}}{m_{\mathrm{i}}}\right]\times 100$$ (8)

where.

• ❖%Ac. = percentage contents of ash, m_sh = mass of ash contents after removed, m_i = initial mass of Khat waste sample

Fig. 4.

Mass of khat before and after being kept in the furnace at 600 °C for 4 h for Ash content determination.

Fig. 5.

Ash Contents Calculation technique.

Volatile Matter: The liberation of volatile matter from Khat waste involves the release of compounds such as hydrocarbons, aromatic hydrocarbons, and sulfur compounds at high temperatures. This process can be detected and analyzed through pyrolysis, where the Khat waste is heated to 750 °C for 15 min. During pyrolysis, the thermal decomposition of organic materials occurs, producing gases and leaving behind solid residue (char). Analyzing the volatile components released during pyrolysis provides insights into the chemical composition of Khat waste and its potential applications, such as in biofuel production or understanding its environmental impact when combusted.

As indicated in (Fig. 6) 3.002 g of powder was heated at 750 °C for 15 min in a particularly closed crucible before being placed in the furnace as in (Fig. 7).

Fig. 6.

Mass of Khat powder before and after pyrolyzing at 750 Celsius for 15 min for volatile analysis.

Fig. 7.

Volatile analysis method.

The change of mass before and after measured and volatile content is computed and analyzed by this (Eq. (9)) [9,11,13].

$$\%VM=\frac{m_i-m_f}{m_i}\ast 100$$ (9)

where.

• ✓%VM = volatile content in percentage,
• ✓m_i = mass of dried khat before heating and
• ✓m_f = mass of khat from the furnace and after cooling.

E) Fixed Carbon: The mass loss is also defined as fixed carbon (FC) and this fixed carbon is calculated by the following (Eq. (10)).

$$\%FC=100-\left(M_C+{VM}_c+A_C\right)$$ (10)

where:

• ➢M_C - percentages of moisture contents
• ➢VM = percentages of volatile matter
• ➢A_C- percentages of Ash contents
• ➢FC- percentages of fixed carbon contents

2.4. Characterization of Khat (elemental analysis)

Thermo-Scientific (MODEL: Thermo-Scientific –EA1112 FLASH CHNS/O analyzer) is required to analyze the elemental components of the sample. The sample is subjected to analysis under specific conditions, including a gas flow rate of 120 ml/min, a reference flow rate of 100 ml/min, and an oxygen flow rate of 250 ml/min. When oxygen interacts with carbon, nitrogen, sulfur, or hydrogen components, it generates carbon dioxide, nitrogen dioxide, sulfur dioxide, and water [28,29]. These combustion products are separated via a chromatographic column and identified by a thermal conductivity detector, which produces a signal proportional to the specific mixture, enabling the determination of the equivalent composition of an element in the sample [29]. The oxygen content of biomass is calculated using the formula (% H) + (%O) = 100 - (%C + %N + %S). Thus, this equation can be applied to ascertain the oxygen content of khat waste biomass.

2.5. TGA and FTIR spectroscope analysis

A thermogravimetric analyzer (TA instrument model: SDT Q600) was employed to evaluate the thermal characteristics of untreated Khat samples across a temperature range from ambient to 800 °C, employing heating rates of 10 °C/min, 20 °C/min, and 30 °C/min. A microbalance was utilized to measure 10 g of the sample in the pan [15,16,30]. The pan and sample are heated, and the weight is measured using the heating rate cycle [4,21].

Fourier Transform Infrared Spectroscopy (FTIR) is a powerful analytical technique used to identify and quantify various chemical components in a sample based on their absorption of infrared light. Khat samples should be well mixed to reduce variability within the sample and should be finely powdered to ensure uniformity and maximum surface area for consistent infrared absorption.

The necessary steps for preparing and analyzing Khat samples using FTIR spectroscopy: Drying and Grinding: Begin by drying the Khat samples, if needed. Once dried, grind the Khat waste into a fine powder using a ceramic pestle and mortar. For further homogenization, an agate mortar can be used. KBr Mixing: Combine the powdered Khat with potassium bromide (KBr) particles. The typical ratio is around 1:100 (sample to KBr). This mixture is then used to create a pellet or plate suitable for infrared examination [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]]. Compression: Place the Khat-KBr mixture into a pellet die and compress it using a hydraulic press or a similar device to form a thin, uniform disc. Spectrometer Setup: Utilize a high-resolution FTIR spectrometer, such as the Spectrum 65 FT-IR PerkinElmer, for the analysis. Measurement: Place the KBr pellet in the FTIR spectrometer.

2.6. Aspen plus simulation

There are multiple steps involved in setting up the process and modeling the thermodynamic and kinetic behavior of the pyrolysis processes when simulating the pyrolysis of khat waste with Aspen Plus. To simulate the pyrolysis process, use a reactor unit operation (such as a RYield or RGibbs reactor) to indicate the pressure conditions, residence period, and temperature profile-heating rate including separation equipment like distillation columns, flash separators, or filtration units to separate and recover pyrolysis products, depending on the intended outputs (gas, liquid, or solid) [23].

There are different Aspen Plus unit operation model blocks for completion of this simulation as shown (Fig. 8). The first calculation is a water calculation to determine how much moisture is removed from an RStoic or dry block [31]. The next aspen plus unit block is splitters whose block names are spil1 and split 2 are used to split moisture from dried feed and separate ash from vapor respectively. The RYIELD (DCOMPOSE) block is used to convert the feedstock to traditional simulation components (water, ash, cellulose, hemicellulose, and lignin). The basic data input is necessary at various phases of model building. RGIBBS (PYROLYSE) is the Calculation of chemical and phase equilibrium via the minimization of the Gibbs free energy method. Separation of the char from the product vapor for the reduction of the vapor stream temperature to induce the Condensation of liquid products occurred by HEATER (block ID is COOLER) [32]. FLASH is used to separate pyrolysis oil from non-condensable gases and lastly Calculate the elemental composition of feed by specifying the RYIELD mass, determining moisture, and specifying the RYIELD mass analyzed by CALCULATOR (CONVERT) [[25], [26], [27], [28], [29], [30],33]. The data requirements include component specification, global data specification, properties, blocks, and streams. The proximal, ultimate, and sulfur analyses quantify the enthalpy and density of municipal solid waste using the stoichiometric equation [29,30] (Fig. 8). depicts the model for this Aspen Plus simulation [10]. The simulation is based on equations from IGT (1976) [20], [27]- [28].

Fig. 8.

Khat waste simulation model by Aspen plus.

3. Result and discussion

3.1. Characterization of feedstock

(Table 1, Table 2) show the physical properties of the chosen biomass samples. According to the study, the typical values for different types of biomass vary, notably ashes (1.45–18.2 %), VM (67.4–81.3 %), FC (13.1–17.2 %), C (39.4–50.1 %), O (37.06–41.78 %), H (3.59–5.5 %), N (0.27–0.75 %), and S (0.12–0.18 %) [29]. These findings were cross-validated against literature values. The initial moisture content of raw biomass fell within the range of 50–60 %. This raw biomass can undergo sun-drying or oven-drying processes, resulting in moisture contents ranging from 3 % to 10 % weight percent. In the experiment, the mass of khat in the pycnometer was measured at 8.72 g; while the volume of water displaced was 16 mL. Utilizing (Eq. (5)), the apparent density of pyrolysis khat waste was calculated to be 0.9688 g/cm³. The moisture content of khat waste was determined using an oven dryer. Initially, the khat sample weighed 71 g before being placed in the oven. After drying, the mass decreased to 68 g shown as above (Fig. 2). Consequently, the moisture percentage of this sample was determined to be 4.23 %. The fixed carbon percentages were determined by adding together the ash, volatile, and moisture contents, resulting in 14.74 % for khat waste. This discovery aligns with the proximate results of previous studies on this sample [7,9,11]. Ultimate analysis provides more comprehensive data compared to proximate analysis. Using an elemental analyzer, it identifies ash-free organic components such as carbon, oxygen, nitrogen, and hydrogen [34,35].

Table 1.

Proximate analysis results of Khat waste.

Sn	Name of proximate	Percentage
1	Moisture	4.23
2	Ash	4.68
3	Fixed Carbon	14.74
4	Volatile matter	76.35

Table 2.

Elemental results of both experimental and simulation results.

Sn	Name of Ultimate	Laboratory result (%)	Simulation Value %)	Relative error (%)
1	Carbon	49.83	46.5	6.7
2	Oxygen	41.54	39.05	4.9
3	Nitrogen	0.28	0.26	7.1
4	Sulfur	0.02	0.019	5
5	Hydrogen	5.45	5.33	2.2
	Heating Value	Lab result (KJ/kg)	Simulation result (KJ/kg)	Relative error (%)
1	HHV	19.95	18.92	5.1
2	LHV	18.76	18.23	2.8

The Aspen software simulated both elemental values and variations, as shown in (Table 2) below are identical, with a variation of less than 15 percent, indicating a satisfactory agreement between experimental and simulation results like the previous study [36].

The elemental analyzer value for khat waste in this study closely aligns with the elemental composition of other biomass and municipal solid waste [2,25].

3.2. Thermogravimetric and FTIR analysis

Thermogravimetric Analysis (TGA) and Differential Thermogravimetric Analysis (DTG) were conducted on cassava khat waste to study its thermal decomposition under non-isothermal conditions, using a heating rate of 30 °C/min [26,27] (Fig. 9). Below illustrates the weight loss and proximate analysis of raw Khat at 800 °C, with a heat rate of 30 °C/min. The TGA graph shows distinct weight loss stages with corresponding temperature ranges: moisture loss below 200 °C, volatiles release up to 230 °C, and significant decomposition (pyrolysis) of hemicellulose, cellulose, and lignin between 250 and 800 °C. These findings align closely with the literature, indicating moisture loss (6.7 %), volatile matter (4.7 %), fixed carbon (6.78 %), and ash content (5.55 %) [16,24]. The weight loss increases as the de-volatilization temperature of the sample rises as shown in (Fig. 9).

Fig. 9.

TGA and DTGA at 30 °C/min mass flow rate diagram.

As shown in (Fig. 10), TG curves were similar across most samples, except for khat waste, which exhibited a downward shift above 220 °C. High heating rates resulted in increased fixed and ash content, while lower heating rates produced higher volatile matter and moisture content [37]. The DTG curves, presented in (Fig. 9, Fig. 11, Fig. 12), showed decomposition occurring between 250 and 400 °C for all heating rates. This decomposition is primarily driven by lignocellulosic components, which break down between 200 and 450 °C [38].

Fig. 10.

Weight loss diagram at different Heating Rate.

Fig. 11.

TGA and DTGA diagram at 10 °C/min heating rate.

Fig. 12.

TGA and DTGA diagram at 20 °C/min heating rate.

Lignin normally decomposes first at the lowest temperature and continues until around 800 °C. Hemicellulose decomposes within the temperature range of 150–350 °C, while cellulose breaks down between 275 °C and 450 °C. At a heating temperature of 350 °C, the sample residue exhibited the lowest remaining weight, with final weights at 800 °C of 18.245 wt%, 11.89 wt%, and 9.9125 wt% for heating rates of 30 °C/min, 200 °C/min, and 10 °C/min, respectively, similar to previous findings [25].

The Differential Thermogravimetric Analysis (DTGA) of khat waste reveals three prominent peaks, each corresponding to distinct stages of thermal decomposition [29]. Each peak corresponds to conditions of maximum steady rate [8]. The first peak, occurring around 100 °C, signifies rapid moisture evaporation, marking the initial stage of thermal degradation. The second peak, at approximately 210 °C, primarily results from the breakdown of hemicellulose (Hem), a component known for volatilizing at this temperature range. The third peak occurs at higher temperatures and is influenced by the thermal degradation of cellulose (Cell), which exhibits a more rapid volatilization compared to other biomass components [15,16,39]. Certainly, lignin does volatilize at relatively low temperatures; however, its degradation rate remains extremely slow, allowing it to persist even at 750–800 °C. Furthermore, FTIR spectra plots of a subset of biomass samples are displayed in (Fig. 13), which also shows the drop in aliphatic functional groups at various wavenumbers (cm^-1) and transmittance percentages. The spectroscopic research conducted here offers additional insights into the chemical transformations that take place during thermal degradation, hence enhancing the DTGA results on biomass volatilization patterns [38].

Fig. 13.

FTIR-analysis of Khat waste and functional group.

The selected biomass displays reciprocal groups, including [5,6,24].

• ➢O-H stretching at 3390-3420 cm⁻¹
• ➢CH and CH2 stretching at 2900-2930 cm⁻¹
• ➢C=C stretching at 1615-1640 cm⁻¹
• ➢Benzene ring at 1500-1520 cm⁻¹
• ➢Aromatic skeletal vibrations at 1405-1465 cm⁻¹
• ➢Phenolic OH and aliphatic C-H deformation at 1315-1380 cm⁻¹
• ➢C-O-C stretching observed at 1235-1245 cm⁻¹ in alkyl aromatics
• ➢Asymmetric C-O-C stretching occurring at 1155-1165 cm⁻¹
• ➢C-O stretching occurring at 1030-1060 cm⁻¹
• ➢C-H bending as functional group alkene at 917-756.22 cm⁻¹
• ➢N-H stretch amine at 1075.1288 cm⁻¹

In infrared spectroscopy, wave numbers 555.67 cm⁻¹ and 929.63 cm⁻¹ are associated with alkene functional groups. The absorbance of O-H bonds is calculated by dividing the transmittance percentage by 100 % [5,24]. Infrared radiation causes covalent bonds to stretch, vibrate, bend, or rock, primarily detecting cellulosic content through O-H bonds. Functional groups are critical in biofuel production, each affecting biofuel properties differently. Hydroxyl groups (-OH) are essential for bioethanol, enhancing oxygen content for better combustion and lower emissions [17,18]. Ester groups (-COOR) are crucial for biodiesel, providing biodegradability and lubricity. Carbonyl groups (C=O), alkyl groups (-CH₃, -CH₂-), and aromatic rings also play significant roles [34,35]. Carbonyl groups influence volatility and combustion efficiency, alkyl groups affect hydrocarbon chain length and saturation, impacting energy density and viscosity, while aromatic rings enhance energy density and combustion characteristics of bio-derived gasoline [34,35]. Together, these functional groups diversify biofuel options, optimizing performance across transportation and industrial applications.

3.3. Effects of heating rate

The heating rate can impact the sample's pyrolysis in several ways: as the heating rate rises, the system receives a greater instantaneous thermal energy, requiring more time for the sample biomass to equilibrate with the temperature due to heat transfer constraints [[32], [33], [40]]. In (Fig. 14), the proximate analysis outcomes for khat waste produced at pyrolysis temperatures of 800 °C under different heating rates are depicted [41].

Fig. 14.

Effects of Heating Rate on Proximate Contents of Khat biomass.

As the heating rate increased from 10 °C/min to 30 °C/min, the volatile matter in khat waste decreased from 76.82 % to 75.62 %, and moisture levels dropped from 4.5 % to 4.15 %. Volatile matter typically includes water, gases, and other compounds that vaporize at relatively low temperatures. The decrease in volatile matter with an increased heating rate suggests that the rapid heating may cause a faster expulsion of these components. As the heating rate accelerates, there may be less time for volatile compounds to remain in the sample before they are expelled, leading to a reduction in their overall content [42]. Moisture is a specific subset of volatile matter and decreases with higher heating rates for similar reasons. Faster heating likely leads to a more immediate and efficient removal of moisture from the khat waste. Increasing the heating rate raises fixed carbon content, indicating that slower heating rates at high temperatures optimize pyrolysis. Higher heating rates increase fixed carbon content. This is due to the reduced efficiency in volatilizing organic components, leaving behind more carbon. The observed decrease in both volatile matter and moisture with increased heating rates was attributed to the quicker removal of these components due to the more rapid heating process. This suggests that higher heating rates result in more efficient removal of volatile substances, which is an important factor to consider based on the intended application of Khat waste, whether for energy recovery [43].

4. Conclusion

This research comprehensively analyzed khat waste through proximate and ultimate analysis, focusing on moisture, volatile matter, fixed carbon, ash content, and elemental composition. The moisture content was found to be 4.23 %, with an initial mass reduction to 68 g after oven drying. The apparent density of the pyrolysis khat waste was 0.9688 g/cm³. Pyrolysis was conducted at temperatures from 350 °C to 800 °C and heating rates of 10 °C/min to 30 °C/min, resulting in varying residual weights, with the lowest observed at 800 °C. Differential Thermogravimetric Analysis (DTGA) identified three main thermal decomposition stages. FTIR spectroscopy revealed key functional groups including O-H stretching, CH/CH2 stretching, C=C stretching, benzene ring vibrations, aromatic skeletal vibrations, phenolic OH, aliphatic C-H deformation, and C-O-C stretching, aiding in the understanding of khat waste's chemical structure. Both experimental and Aspen Plus simulation results showed strong agreement, with deviations of less than 15 % for elemental composition and caloric value, confirming the reliability of the simulation. Due to its lower nitrogen content compared to coal, khat waste emits fewer pollutants when burned, making it a more environmentally friendly option. This research underscores the potential of khat waste as a valuable resource for biofuel production. By thoroughly analyzing its thermal behavior, chemical composition, and environmental impact, the study reveals that khat waste can be effectively utilized as a renewable energy source through pyrolysis. Its low nitrogen content makes it a cleaner alternative to coal, potentially reducing harmful emissions and contributing to more sustainable energy solutions. The study's findings are based on laboratory-scale experiments. Future research should include pilot-scale tests to evaluate the practical implications and scalability of khat waste pyrolysis. Additionally, comparing khat waste with other biomass types in terms of energy production and environmental impact will offer a comprehensive understanding of its benefits and limitations.

CRediT authorship contribution statement

Geleta Afessa Moreda: Writing – original draft, Validation, Methodology, Formal analysis, Conceptualization. Debela Alema Teklemariyem: Visualization, Validation, Methodology, Formal analysis. Sorome Deresa Tolasa: Visualization, Software, Investigation, Formal analysis. Gamachis Ragasa Gutata: Writing – review & editing, Visualization, Software, Methodology.

Data availability

The data that has been used is confidential.

Declaration of competing 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.

Appendix A. Supplementary data

The following is the Supplementary data to this article: