Analysis of Thermal and Catalytic Pyrolysis Processes in Belém: A Socioeconomic Perspective

Abstract

This study aims to assess the by-products generated through the thermal and catalytic pyrolysis of the organic matter and paper fractions of municipal solid waste (MSW) in different socioeconomic regions, through the yields of reaction products (bio-oil, biochar, H₂O, and gas), acid value and chemical composition of bio-oils, and characterization of biochar, on a laboratory scale. The organic matter and paper segregated from the gravimetric composition of the total waste sample were subjected to drying, crushing, and sieving pre-treatment. The experiments were carried out at 450 °C and 1.0 atmosphere, and at 400 °C and 475 °C and 1.0 atmosphere, using a basic catalyst, Ca(OH)₂, at 10.0% by mass, in discontinuous mode. The bio-oil was characterized by acidity value and the chemical functions present in the bio-oil identified by FT-IR, NMR, and composition by GC-MS. The biochar was characterized by SEM/EDS and XRD. The bio-oil yield increased with the addition of the catalyst and the pyrolysis temperature. For catalytic pyrolysis, bio-char and gas yields increased slightly with the Ca(OH)₂ content, while bio-oil and H₂O phases remained constant. The GC-MS of the liquid reaction products identified the presence of hydrocarbons and oxygenates, as well as nitrogen-containing compounds, including amides and amines. The acidity of the bio-oil decreased with the addition of the basic catalyst in the process. The concentration of hydrocarbons in the bio-oil appeared with the addition of the catalyst in the catalytic pyrolysis process as the catalytic deoxygenation of fatty acid molecules occurred, through decarboxylation/decarbonylation, producing aliphatic and aromatic hydrocarbons, introducing the basic catalyst into the thermal process.

1. Introduction

The generation of solid waste directly correlates with the way of life, population growth, and consumption patterns and is, without a doubt, one of the biggest challenges that governments have faced. According to [1], in 2011, cities around the world generated 1.3 billion tons of solid waste annually, a volume that is expected to reach 2.2 billion tons by 2025. It is estimated that municipalities that are part of developing countries currently spend 20 to 50% of their budgets on the management and disposal of solid waste, with little or no value derived from them [2].

The impacts of unmanaged urban solid waste go beyond cities, affecting ecosystems such as the oceans, where millions of tons of plastic were discarded in 2010 according to [3]. In 2022, Brazil generated 77.1 million tons of urban solid waste (MSW), collecting 71.72 million tons. Of these, 43.8 million were sent to landfills, and 27.97 million were inappropriately disposed of. Recycling activities processed 1.12 million tons, and composting 127.5 thousand tons, but these numbers are still insufficient compared to other countries, resulting in significant financial losses for the efficient management of MSW.

In 2021, only 35.8% of the urban solid waste (1,773,927 tons) in the Northern Region of Brazil was disposed of properly, while 64.4% (3,209,013 tons) was managed inadequately [4]. The gradual increase in solid waste makes it difficult to transport, store, dispose of, and manage these materials efficiently [5]. According to Sustainable Development Goal 11 (SDG 11) [6], adequate management of urban waste is critically significant for advancing sustainability and ensuring the preservation of natural resources for future generations [7].

Urban solid waste (MSW) has high economic potential, influenced by the efficiency of waste management [8]. Solid waste is heterogeneous and may be made up of elements that are difficult to degrade/treat and assimilate into the environment, causing risks to environmental protection and consequences for public health [9,10]. However, until the adequate disposal and treatment of solid waste is achieved, other steps are necessary, following applicable guidelines through the development of solid waste management systems [11]. To implement an efficient MSW management system, it is essential to know the per capita generation and gravimetric composition, topics that have been the focus of recent studies, such as those by [12,13,14], which address the sectorization of collection routes, gravimetric characterization and energy valuation of MSW treatments [5].

Several research studies indicate that the generation of urban solid waste (MSW) is influenced by socioeconomic class, as per the studies by the authors [15,16,17]. Higher-class regions produce more waste, with greater diversity and quantity of recyclable materials, while lower-class areas generate less waste, with organic materials predominating. Waste disposal and treatment are more efficient in higher-income regions, while inadequate management in lower-income areas can worsen environmental and public health problems [18,19].

Carneiro [20] stood out as one of the pioneers in exploring the economic potential of MSW generated in the municipalities of Belém, which will host COP 30 in 2025, and Ananindeua in the state of Pará/Brazil. Their research allowed the identification of an average change of 13% in the amount of recycled material between 2000 and 2006, influenced by the social profile of the population Silva et al. [12]. These significant contributions provide valuable insights for the development of comprehensive strategies in integrated MSW management, considering economic, environmental, and social aspects [21]. Due to the challenges in the treatment and final disposal of solid waste, alternatives such as thermochemical technologies, including combustion, gasification, and pyrolysis, have been studied. Pyrolysis, a thermal process of converting biomass into energy, occurs in an inert environment and can generate high-value-added products, being a promising option for renewable energy sources. This process, carried out with or without a catalyst, decomposes biomass in the absence of oxygen and converts municipal solid waste (MSW) into three by-products: bio-oil, biochar (coal), and biogas, with the proportion of each depending on the operational conditions, such as temperature, heating rate, and vapor residence time [22,23,24,25,26,27].

The main objective of MSW pyrolysis is to recover energy, since the products resulting from a pyrolysis process have, in most cases, properties similar to those of fuels. The pyrolysis process has the potential to convert (MSW) into a usable source of energy for homes, as pointed out by [28]. Furthermore, the energy products generated during pyrolysis can be used to optimize the operational efficiency of larger-scale plants that use this technology.

The composition and properties of oils and biochars obtained through pyrolysis can make them suitable as raw materials for various industrial sectors. Considering the heterogeneous nature of (MSW) and the variation in its composition according to location, this section offers a brief overview of the products resulting from MSW pyrolysis. This also allows you to distinguish between the different types of products generated by the process. MSW contains a significant proportion of organic waste, mainly composed of food waste, wood, and garden waste. Food waste contains several carbon-rich substances, which makes it a valuable source for fuel production [29].

The pyrolysis of different types of organic waste has been widely studied and documented in the literature by several researchers, including [30,31,32,33,34,35,36]. Furthermore, other studies have specifically focused on the pyrolysis of solid waste, such as paper fraction, as investigated by [37,38,39,40,41,42,43,44,45,46]. MSW in homes is generally stored in a single location. In many cases, municipal authorities carry out collection without separating the different components present in MSW. Therefore, it is essential to develop a pyrolysis process capable of treating mixed MSW. Wastes whose composition resembles the organic fraction are the most suitable for pyrolysis experiments, contributing to a better understanding of the real composition and yield of the products generated. Due to the complexity of this organic fraction, most pyrolysis studies have focused on individual components. However, MSW components do not act independently during the pyrolysis process, making it crucial to examine the actual behavior of mixed MSW. Several researchers have studied the pyrolysis of MSW, combining several fractions to better understand this behavior, among them [47,48,49,50,51,52,53,54,55].

Some authors have investigated the pyrolysis and thermal–catalytic cracking of MSW, such as flash pyrolysis [39,47,56,57], fixed bed reactors [50,58,59,60,61,62,63], and fluidized bed reactors [64], and experiments were carried out on micro [65], laboratory [39,47,53,57,62,63,66,67,68], and pilot scales [58]. The largest number of research studies on catalysts focuses on zeolites [69], which are acid catalysts, and they form products mainly in the gasoline range, as seen in [65]. The basic ones are widely used in reactions to obtain biofuels due to the high levels of conversion achieved in this process, with most molecules remaining in the boiling range of diesel fuel, allowing reaction rates to be obtained higher than those obtained with the same number of catalysts in the acid process [68]. Among the basic catalysts used in the pyrolysis of MSW fractions are CaO studies [39], calcined calcite (CaO) [47], calcium hydroxide CaOH₂ [70], and calcined dolomite (MgO·CaO) [47,53].

It is important to highlight that catalytic pyrolysis is one of the most important processes in the refining industry, especially when it comes to the process of obtaining better quality gasoline and higher octane (through the optimization of aromatic and olefin contents [22,23,24,25,26]. To date, no research has investigated the effect of temperature and percentage of CaOH₂-type catalyst on the MSW fraction (organic matter and paper) and its implications on the morphology of biochar and in the crystalline structure, as well as in the yield of reaction products, chemical composition, and acidity of bio-oils obtained by pyrolysis and catalytic pyrolysis, in different socioeconomic regions in the same municipality.

Considering the great impact caused by waste, it is essential to have a broad discussion on its treatment and destination, addressing technological, economic, and environmental aspects, taking into account the various existing technological alternatives, and considering, above all, the regionalities of each location, the economic valorization of materials, and their energy valorization when feasible. Thus, based on the scenario presented and aiming to contribute to the development of technology to promote the sustainability of the MSW treatment sector, in summary, the work proposes to investigate an alternative use of urban solid waste (MSW), through pyrolysis, with the production of materials of commercial interest, as opposed to the conventional forms of disposal.

2. Methodology

For the development of the research, the methodological procedures were structured in stages to investigate all the relevant points of the proposal according to the flowchart presented in Figure 1.

This research followed the methodology of collection, transportation, segregation, treatment, and disposal of urban solid waste adopted by the studies of [71,72], focusing on the 21 neighborhoods of Belém/Pará, served by the company TERRAPLENA LTDA/Belém, PA, Brazil, responsible for Lot 1 (Figure 2), according to the Municipal Basic Sanitation Plan of Belém [73]. The study area, with 37 itineraries, reflects the scope of collection, water and sewage treatment services, and others related to basic sanitation. Analyzing the specific sanitation conditions and the socioeconomic diversity of the neighborhoods was essential to evaluate the efficiency of the services and direct public policies, ensuring an equitable distribution of resources and improvements in the living conditions of the population.

To characterize urban solid waste (MSW) in the 21 neighborhoods served by TERRAPLENA LTDA, a work plan was developed that included quantitative and qualitative analysis stages. Initially, visits were made to the neighborhoods to understand the waste flow and estimate the volume collected. Samples were then collected, transported, and subjected to gravimetric and laboratory analysis. Based on the 37 itineraries, nine sectors were formed considering socioeconomic characteristics and geographic proximity, facilitating gravimetric analysis and waste management in the company’s coverage area in Belém (Figure 3).

The neighborhoods were classified according to family income following a methodology adapted from the IBGE [74] (

Table 1. Socioeconomic classification in the municipality of Belém/Pará/Brazil based on minimum salary [74].

Socioeconomic Classification
Classes	Family Income (Minimum/Basic Salary)
A	over 20 salaries
B	from 10 to 20 salaries
C	from 04 to 10 salaries
D	from 02 to 04 salaries
E	up to 02 salaries

), which divides the population into five classes (A, B, C, D, and E) based on the minimum wage. In Belém, there are no neighborhoods in classes A and B, and approximately 61% of the neighborhoods belong to class E.

With the organization of sectors, we were able to establish the formation of regions that represent larger areas. These regions were considered as units for sampling solid waste intended for laboratory analysis. Thus, we determined the composition of three regions, as shown in

Table 2. Determination of the grouping of sectors into regions.

Region	Sectors	Neighborhoods
1	1, 2, and 3	Aurá, Águas Lindas, Curió-Utinga, Guanabara, Castanheira, Souza, and Marco
2	4, 5, and 6	Canudos, Terra Firme, Guamá, Condor, Jurunas, and Fátima
3	7, 8, and 9	Umarizal, São Brás, Cremação, Batista Campos, Nazaré, Reduto, Campina, and Cidade Velha

.

2.1. Gravimetric Composition of Urban Solid Waste

The gravimetric composition of the waste was determined using the STATIDISK 13.0 software based on a sample of approximately 100 kg. The capacity of the collection truck (15 m³ per route), a significance level of 5%, confidence of 95%, and margin of error of 10% were considered. These parameters ensured an accurate and statistically valid analysis according to Assunção et al. [71].

The gravimetric analysis of the waste was performed by fractions (paper, Cardboard, Tetrapak, Rigid Plastic, Malleable Plastic, glass, metal, organic matter, Textile, Sanitary Waste, and others), allowing a detailed understanding of the composition of the waste generated. This method, more specific than general characterizations, facilitated waste management, starting with the measurement of the total mass to ensure the representativeness of the sample, according to the methodology of Assunção et al. [71].

The type of collection performed was the door-to-door method and followed the internal schedule of the company TERRAPLENA. The waste was sent to the Federal University of Pará, where gravimetric characterization was performed immediately after collection, preserving its original composition, including the degree of moisture. The process followed the ABNT solid waste sampling guidelines [75].

The samples were collected in plastic bags at random points along the compactor truck route, ensuring representative data on the composition of household waste. Sampling at random points avoided distortions by reflecting the diversity of conditions along the route, and prioritizing residential areas allowed for capturing a variety of waste, essential for understanding disposal practices and guiding waste management [76].

The waste was then weighed and placed on a surface waterproofed with 6 × 6 m tarps. The waste was then manually segregated and classified into different fractions: paper, Cardboard, Tetrapak, Rigid Plastic, Malleable Plastic, glass, metal, organic matter, Fabrics, Sanitary Waste, and Rejects/others. Each fraction was weighed on a digital scale (Welmy, São Paulo-Brazil, Model: W200/50).

Pré-Tratamento das Amostras e Determinações Laboratoriais

Municipal solid waste (MSW), due to inadequate disposal and exposure to the environment, had a high moisture content, damaging the pyrolysis process. To remedy this, the organic fraction underwent a drying process in a thermal oven with air recirculation and analog temperature control (Model De Leo Ltda, 127 V, Porto Alegre-RS/BRAZIL) at 105 °C for 24 h. After drying, the MSW was crushed using a TRAPP TRF 600 model knife mill using sieves of different diameters (0.8 mm for the organic fraction and 5 mm for the others). The crushed material was measured on a scale model WELMY CLASS 3 W200/S (maximum capacity of 200 kg and minimum precision of 1.0 kg). The separation of the crushed material was carried out using a PRODUTEST Telastem, São Paulo, SP, Brazil sifting system for analyses of LTDA. Finally, the pre-treated organic fraction was stored in a freezer at 0 °C to avoid physicochemical and microbiological degradation.

2.2. Experimental Procedure

2.2.1. Pyrolysis Process Experimental Apparatus

To carry out the experiments, a laboratory-scale pyrolysis unit was used, consisting of a cylindrical borosilicate reactor with a volumetric capacity of approximately 200 mL. This reactor was inserted into a cylindrical oven equipped with a collar-type ceramic resistance of 800 W of power. The resistance was connected to a digital temperature and heating rate controller (THERMA, model TH90DP202-000, Piedade, São Paulo, Brazil), which includes a temperature sensor type K (Ecil, model QK.2, Piedade, São Paulo, Brazil). A nitrogen cylinder, with a two-stage pressure regulating valve (CEMPER, model CS-54), was connected to the reactor in addition to a gas flow meter (N2) (Omel, model 189–162, São Paulo, Brazil), calibrated for air (1 atm, 21 °C) and with a flow range of 0–200 mL/min, as described in detail by Assunção et al. [71] and De Castro et al. [77]. Figure 4 illustrates the configuration of the pyrolysis laboratory unit.

2.2.2. Experimental Procedures

In the laboratory-scale investigation, thermal and catalytic pyrolysis experiments were carried out to verify the influence of variations in catalyst percentage on the yields and characteristics of the products obtained, at temperatures of 400 °C, 450 °C, and 475 °C, aiming to evaluate the influence of process parameters, yield, and characteristics of the products obtained, all carried out at a heating rate of 10 °C/min.

Table 3. Thermal and catalytic pyrolysis experiments on the laboratory scale.

Experiments	Material	Catalyst Mass Ca(OH)2 (%)	Temperature (°C)	Retention Time (min.)
Region (1)	F.O. + Paper	0	450	1 h 20
Region (2)	F.O. + Paper	0	450	1 h 20
Region (3)	F.O. + Paper	0	450	1 h 20
Region (1)	F.O. + Paper	10	400	1 h 20
Region (2)	F.O. + Paper	10	400	1 h 20
Region (3)	F.O. + Paper	10	400	1 h 20
Region (1)	F.O. + Paper	10	475	1 h 20
Region (2)	F.O. + Paper	10	475	1 h 20
Region (3)	F.O. + Paper	10	475	1 h 20

Legend: F.O.: organic fraction.

shows the experimental conditions used in the pyrolysis processes of urban solid waste (organic fraction and paper) in the absence and presence of Ca(OH)₂ catalyst.

The experiments were carried out in the semi-continuous mode, at temperatures of 450 °C and 475 °C at 1.0 atm, with 10% Ca(OH)₂ catalyst, in order to evaluate the influence of this final temperature variation on the yield and the physical–chemical characteristics of the liquid product obtained (bio-oil). The masses of the MSW fractions (organic and paper) were initially weighed on a semi-analytical balance (QUIMIS, Q—500L210C, Diadema, São Paulo, Brazil), being approximately 40 g for the thermal experiments and 30 g for the catalytic pyrolysis.

Then, the samples were placed in the 200 mL borosilicate glass reactor, which was inserted into the jacketed cylindrical furnace. Using the temperature control system, the reaction time, heating rate, and final process temperature (set-point) were programmed based on Equation (3), resulting in different process times for each pre-defined temperature. A time of 10 min was established to keep each final operating temperature constant. The experimental apparatus was assembled by connecting the cooling condenser to the reactor, with the refrigerator fluid maintained at 20 °C. From room temperature (25 °C), the slow pyrolysis process began with a heating rate of 10 °C/min, monitoring and collecting the operational parameters of the process, such as elevation temperature (heating ramp), time, and temperature of product formation.

2.3. Physicochemical and Chemical Composition of Bio-Oil

2.3.1. Physicochemical Characterization of Bio-Oil and Aqueous Phase

The bio-oil and the aqueous phase were characterized for acidity according to the AOCS Cd 3d-63 method as described in Almeida et al. [25] and Castro et al. [77].

2.3.2. Chemical Composition of Bio-Oil and Aqueous Phase

The chemical composition of the bio-oil and the aqueous phase was analyzed by GC-MS, following the method described by Almeida et al. [25] and Castro et al. [77]. Concentrations were expressed in areas without internal standards. An FTIR analysis was performed with a Shimadzu Prestige 21 spectrometer, Kyoto, Japan, using KBr plates for liquid film uniformity, with a resolution of 16 cm⁻¹ and a scanning range of 400 to 4000 cm⁻¹. NMR spectra were obtained on a VARIAN UNITY 300 spectrometer, Palo Alto, CA, USA (Varian), (300 MHz) using deuterated chloroform as solvent and TMS as reference. The parameters for hydrogen NMR included 128 transients, a 7.16 µs pulse, and a 1.666 s relaxation; for carbon, 3940 transients, an 8.7 µs pulse, and a 0 s relaxation.

2.4. Characterization of Biochar

2.4.1. SEM and EDS Analysis

The morphological characterization of the biochars, obtained by thermal pyrolysis and catalytic pyrolysis with the addition of 10.0% by mass of the Ca(OH)₂ catalyst to organic matter and paper, was carried out using scanning electron microscopy. A Vega 3 model microscope (Tescan GmbH, Brno, Czech Republic), available at the Scanning Electron Microscopy Laboratory of the Military Institute of Engineering (LME-IME), was used. The magnifications used were 500×, 1.0 k×, and 5.0 k×. The samples were covered with a thin layer of gold using a Sputter Coater (Leica Biosystems, Nußloch, Germany, Model: Bal-zers SCD 050). Elemental analysis and mapping were performed by energy-dispersive X-ray spectroscopy (Oxford Instruments, Abingdon, UK, Model: Aztec 4.3).

2.4.2. X-Ray Diffraction Analysis (XRD)

The crystallographic characterization of the biochars obtained by thermal and catalytic pyrolysis with 10.0% (by mass) of Ca(OH)₂ of organic matter and paper was carried out by X-ray diffraction with an X’Pert Pro diffractometer from the manufacturer PANalytical Empyrean, Almelo, in the Netherlands in the laboratory river of the Military Institute of Engineering. The following analysis conditions were used: scanning range from 10° to 90°, with a pass of 0.05° and time of 150 s, using a cobalt tube, power of 40 kv, and current of 40 mA.

2.5. Yields from Bench-Scale Thermal and Catalytic Pyrolysis Experiments

Applying the principle of mass conservation in a batch-mode pyrolysis/catalytic reactor operating as an open thermodynamic system results in the following equations.

$$\dot{m_{in,pyrolysis/catalytic}}-\dot{m_{out,pyrolysis/catalytic}}={\displaystyle \frac{d{m}_{Feed}}{d\tau }}$$

(1)

$$\dot{m_{in,pyrolysis/catalytic}}=0$$

(2)

$$-\dot{m_{out,pyrolysis/catalytic}}={\displaystyle \frac{d{m}_{Feed}}{d\tau }}$$

(3)

$$-\dot{m_{out,pyrolysis/catalytic}}=\dot{m_{vapors,pyrolysis/catalytic}}$$

(4)

where $\dot{m_{in,pyrolysis/catalytic}}$ is the mass flow rate entering the glass reactor, $\dot{m_{out,pyrolysis/catalytic}}$ is the mass flow rate leaving the glass reactor, ${\displaystyle \frac{d{m}_{Feed}}{d\tau }}$ is the time rate variation of feed mass inside the glass reactor, and $\dot{m_{vapors,pyrolysis/catalytic}}$ is the mass flow rate of pyrolysis/catalytic cracking vapors leaving the glass reactor and entering the condenser. Applying an overall steady-state mass balance within the condenser yields Equation (5).

$$\dot{m_{vapors,pyrolysis/catalyst}}=\dot{m_{gas}}+\dot{m_{bio-oil}}$$

(5)

where $\dot{m_{gas}}$ is the mass flow rate of non-condensable gases leaving the condenser, computed by difference, and $\dot{m_{boi-oil}}$ is the mass flow rate of bio-oil collected inside the separation funnel. The mass of the solid remaining in the reactor is $\dot{m_{solid}}$. Performing a steady-state global mass balance within the control volume consisting of glass reactors, condenser, and separation funnel yields Equation (6).

$$\dot{m_{Feed}}=\dot{m_{solid}}+m_{gas}+\dot{m_{bio-oil}}$$

(6)

The yields of liquid products (oily, aqueous, and others), carbonaceous materials (coke), and non-condensable gases were calculated as percentages of the initial sample mass (MSW or MSW + catalyst) in each bench-scale experiment. The liquid and solid yields were determined using Equations (7) and (8), while the gas yield was calculated by difference, ensuring that the total yield equaled 100%. Process performance was assessed based on the bio-oil, bio-char, and gas yields.

$$Y_{bio-oil}\left[\%\right]={\displaystyle \frac{M_{bio-oil}}{M_{Feed}}}\times 100$$

(7)

$$Y_{solids}\left[\%\right]={\displaystyle \frac{M_{solids}}{M_{Feed}}}\times 100$$

(8)

$$Y_{gas}\left[\%\right]=100-(Y_{bio-oil}+Y_{solids})$$

(9)

3. Results

3.1. Scanning Electron Microscopy Analysis (SEM)

To carry out the scanning electron microscopy analysis, it was necessary to coat the samples with gold and palladium. Figure 5 presents the magnifications in magnitude of [500× and 1.0 k×] of the biochars from the experiments carried out by thermal pyrolysis at 450 °C for the different socioeconomic regions.

Analysis by SEM of the biochar obtained from the pyrolysis of MSW, mainly the organic fraction and paper, processed at a temperature of 450 °C, revealed a series of important morphological characteristics. The temperature of 450 °C is moderate for pyrolysis, resulting in biochar that showed specific structural properties, influenced by the incomplete thermal decomposition of organic matter. The biochar obtained at 450 °C presented a porous surface, with the formation of macropores, which are pores smaller than 50 nm, although in smaller quantities compared to the biochar produced at higher temperatures. The organic fraction and paper contain vegetable fibers, cellulose, and lignin which, when pyrolyzed, generate carbonaceous structures that maintain some porosity. The porous morphology is visible in the SEM images, with pores of varying sizes, which can improve the adsorption capacity of biochar for contaminants. The presence of microcracks in the biochar, observed in both the SEM images, suggested that the material could be more fragile and brittle, which would represent a disadvantage for uses that require greater structural strength, such as in the construction of filters or barriers. However, these cracks increased the specific surface area, which could compensate for the fragility in terms of adsorption efficiency.

Although the organic fraction and paper were predominantly composed of organic material, inorganic particles could still be found, such as traces of metals or minerals, which were not volatilized during pyrolysis. The SEM captured these particles as bright or dense spots embedded in the carbon matrix. These inorganic components influenced the adsorption properties of biochar depending on its chemical composition. The biochar was mainly composed of CO, and the inorganic portion contained mainly minerals such as Ca, Mg, K, P, and inorganic carbonates depending on the raw material used [70].

Both images illustrated for the different socioeconomic regions showed a charred surface (black color) and granules (white color) of different sizes spread across the surface, as reported by [71], being similar to the SEM images of CaCO₃ (calcite) reported by [71,78,79] and SEM images of biochar obtained by MSW pyrolysis reported by [80]. The MSW biochar particles showed irregular shape and size.

The images obtained from SEM of the catalytic pyrolysis experiments showed similarities between the temperatures of 400 °C (Figure 6) and 475 °C (Figure 7) regardless of the different socioeconomic regions.

The biochar produced with the addition of Ca(OH)₂ presented a highly porous surface but with an irregular morphology. The presence of the catalyst favored the formation of macropores, even at relatively low temperatures, such as 400 °C and 475 °C. The SEM images revealed a network of interconnected pores, which could increase the specific surface area, improving the adsorption properties of the biochar. Electron microscopy often revealed clusters of inorganic particles, mainly calcium (Ca) compounds, such as CaCO₃, formed by the reaction of Ca(OH)₂ during pyrolysis. These particles were visible as bright spots or dense structures in the SEM images, generally located on the surface or within the porous structure of the biochar. These agglomerates can change the physical and chemical properties of the material, influencing its interaction with pollutants.

Fissures and cracks, common in biochar, also appear in catalytic pyrolysis with Ca(OH)₂. These microcracks increase the specific surface area, facilitating the adsorption of contaminants, but can also indicate structural fragility, resulting from the contraction of the material during cooling after pyrolysis. The surface morphology of biochar after the pyrolysis of MSW is presented in Figure 8, which displays an image obtained by SEM using a backscattered electron detector. According to the principles of this detector, the “light” cubic-shaped components correspond to elements with higher atomic numbers in relation to the surrounding area. In turn, the “dark” structures represent the carbonaceous components of the biochar.

3.2. Energy-Dispersive Spectroscopy Analysis—EDS

The results of the elemental analysis performed by EDS at one point for biochars obtained by the pyrolysis of the fraction (organic matter and paper) of MSW at 450 °C, 1.0 atmosphere are illustrated in

Table 4. Mass percentages and atomic mass of the biochars obtained by the pyrolysis of the fraction (organic matter and paper) of MSW at 450 °C, 1.0 atmosphere on a laboratory scale.

Mass Percentages and Atomic Masses of Biochars Obtained by Thermal Pyrolysis at 450 °C
Chemical Elements	(R1) 450 °C		(R2) 450 °C		(R3) 450 °C
	Mass [wt.%]	SD	Mass [wt.%]	SD	Mass [wt.%]	SD
C	70.0	0.2	60.4	0.1	62.0	0.2
O	13.6	0.1	26.3	0.1	22.2	0.2
K	6.7	0.0	1.6	0.0	2.5	0.0
Cl	5.5	0.0	1.6	0.0	3.0	0.0
Ca	1.5	0.0	8.4	0.0	7.2	0.0
Na	1.5	0.0	1.5	0.0	1.6	0.0
S	0.3	0.0	-	-	-	0.0
Mg	0.3	0.0	0.3	0.0	0.3	0.0
Al	0.3	0.0	0.1	0.0	0.1	0.0
P	0.2	0.0	0.2	0.0	0.4	0.0
Fe	0.1	0.0	0.1	0.0	0.1	0.0
Si	0.1	0.0	-	-	0.5	0.0
Cu	-	-	-	-	0.1	0.0

SD = Standard Deviation.

and by pyrolysis catalyst (organic matter and paper) of MSW at 400 °C and 475 °C, 1.0 atmosphere, with 10.0% (by mass) Ca(OH)₂ as catalyst, on a laboratory scale, are illustrated in

Table 5. Percentages by mass and atomic mass of the biochars obtained by the catalytic pyrolysis of the fraction (organic matter and paper) of MSW at 400 °C, 1.0 atmosphere, with 10.0% (by mass) of Ca(OH)₂ as catalyst, on a laboratory scale.

Mass Percentages and Atomic Masses of Biochars Obtained by Catalytic Pyrolysis at 400 °C
Chemical Elements	(R1) 400 °C		(R2) 400 °C		(R3) 400 °C
	Mass [wt.%]	SD	Mass [wt.%]	SD	Mass [wt.%]	SD
C	62.5	0.1	60.5	0.1	56.7	0.1
O	18.9	0.1	24.5	0.1	28.8	0.1
K	2.9	0.0	2.4	0.0	1.1	0.0
Cl	4.1	0.0	3.4	0.0	1.1	0.0
Ca	6.8	0.0	6.2	0.0	9.5	0.0
Na	2.3	0.0	2.3	0.0	1.4	0.0
S	0.2	0.0	-	0.0	-	0.0
Mg	0.4	0.0	0.2	0.0	0.2	0.0
Al	0.2	0.0	-	0.0	0.6	0.0
P	0.6	0.0	-	0.0	0.3	0.0
Fe	0.7	0.0	0.4	0.0	-	0.0
Si	0.4	0.0	-	0.0	0.2	0.0

SD = Standard Deviation.

and

Table 6. Percentages by mass and atomic mass of the biochars obtained by the catalytic pyrolysis of the fraction (organic matter and paper) of MSW at 475 °C, 1.0 atmosphere, with 10.0% (by mass) of Ca(OH)₂ as catalyst, on a laboratory scale.

Mass Percentages and Atomic Masses of Biochars Obtained by Catalytic Pyrolysis at 475 °C
Chemical Elements	(R1) 475 °C		(R2) 475 °C		(R3) 475 °C
	Mass [wt.%]	SD	Mass [wt.%]	SD	Mass [wt.%]	SD
C	77.6	0.1	70.4	0.1	45.0	0.1
O	18.2	0.1	19.6	0.1	32.4	0.1
K	1.5	0.0	1.7	0.0	2.6	0.0
Cl	0.5	0.0	1.7	0.0	2.6	0.0
Ca	0.8	0.0	1.3	0.0	14.2	0.0
Na	0.9	0.0	1.2	0.0	1.5	0.0
S	-	-	-	-	0.1	0.0
Mg	0.4	0.0	0.1	0.0	0.4	0.0
Al	-	-	-	-	0.1	0.0
P	-	-	0.2	0.0	0.5	0.0
Fe	0.1	0.0	-	-	0.4	0.0
Si	-	-	0.1	0.0	0.2	0.0
Cu	-	-	0.1	0.0	0.1	0.0

SD = Standard Deviation.

, respectively.

The results obtained for the thermal pyrolysis process at 450 °C in different socioeconomic regions showed carbon as the predominant element, since the pyrolysis process carbonizes organic matter and paper, concentrating the carbon content in the biochar. Generally, biochars from MSW are mainly composed of CO, while the inorganic portion mainly contains minerals such as Ca, K, P, and inorganic carbonates depending on the raw material [81]. The presence of oxygen was significant, although in smaller quantities than carbon, due to the partial volatilization of oxygenated compounds during pyrolysis. The carbon to oxygen ratio tends to be higher due to the removal of volatile oxygenated compounds during pyrolysis. The high level of carbonization in MSW-derived biochars makes them rich in carbon, which makes them resistant to degradation in the environment [39]. This recalcitrance of MSW-derived biochars can help combat increased carbon emissions from the soil when these materials are used for carbon sequestration [81].

The presence of Mg is found in some organic fractions or in the paper fraction, and magnesium is a common component of minerals or additives present in MSW. Si can come from different sources in MSW, such as particles of earth, glass, sand, or the mineral impurities found in organic waste. In papers, it may also be present due to the use of silicate-based additives during manufacturing.

The presence of Al may appear in biochar if it is present in organic matter or paper, generally as impurities or as part of packaging or additives used in paper processing. Iron can be found in small quantities as a contaminant in organic matter or paper, especially if the MSW contains metallic particles or iron-containing minerals. Regarding K, it is common in the organic fraction of MSW, as it is an essential nutrient in plants and can be present in food residues or plant remains. It is also important in the formation of ash in biochar. The presence of Na can come from food waste, paper treated with chlorides or salts, or even organic waste that contains mineral salts.

In catalytic pyrolysis with 10% Ca(OH)₂, carried out at temperatures of 400 °C and 475 °C in different socioeconomic regions, the results obtained were similar. The greatest formation was observed for the elements carbon (C) and oxygen (O), with carbon being the most abundant component and representing the fundamental structure of biochar derived from carbonized biomass. The presence of oxygen can be attributed to the deoxygenation reactions promoted by Ca(OH)₂, in addition to the fact that biochars produced at lower temperatures tend to contain oxygen in their composition. Furthermore, calcium hydroxide (Ca(OH)₂), used as a catalyst, also contains oxygen in its structure, which can be detected in EDS analysis. Ca(OH)₂ can be converted into calcium oxide (CaO) and calcium carbonate (CaCO₃), both containing oxygen and contributing to the detection of this element.

3.3. Crystallographic Analysis by X-Ray Diffractometry

The XRD analysis of the biochar obtained by the pyrolysis of the fraction (organic matter and paper) of MSW at 450 °C, 1.0 atmosphere is illustrated in Figure 9. The phases identified in different amounts for all the samples are described as follows: graphite (main peak at 0.335 nm, 30.9° 2θ CoKα), calcite (main peak at 0.213 nm, 50.5° 2θ CoKα), and aragonite (main peak at 0.253 nm, 42.6° 2θ CoKα).

The XRD analysis of the biochar obtained by the catalytic pyrolysis of the fraction (organic matter and paper) of MSW at 400 °C, 1.0 atmosphere, with 10% (by mass) of Ca(OH)₂ is illustrated in Figure 10. The phases identified in different amounts for all the samples are described as follows: quartz (main peak at 0.334 nm, 31.0° 2θ CoKα), sylvite (main peak at 0.315 nm, 32.9° 2θ CoKα), and calcite (main peak at 0.303 nm, 34.3° 2θ CoKα). The presence of quartz (SiO₂) is probably due to the presence of small sand particles inside the MSW. Organic carbon is mainly in the amorphous phase, which is the majority phase typical of this type of material. The symbol ‘*’ indicates the quartz peak with a strong preferential orientation effect due to micro preparation.

The XRD analysis of the biochar obtained by the catalytic pyrolysis of the fraction (organic matter and paper) of MSW at 475 °C, 1.0 atmosphere, with 10% (by mass) of Ca(OH)₂ is illustrated in Figure 11. The phases identified in different amounts for all the samples are described as follows: quartz (main peak at 0.335 nm, 30.9° 2θ CoKα), sylvite (main peak at 0.315 nm, 32.9° 2θ CoKα), calcite (main peak at 0.303 nm, 34.3° 2θ CoKα), and aragonite (main peak at 0.209 nm, 50.2° 2θ CoKα). The presence of quartz (SiO₂) is probably due to the presence of small sand particles inside the MSW. Organic carbon is mainly in the amorphous phase, which is the majority phase typical of this type of material.

3.4. Pyrolysis of MSW Fraction

3.4.1. Process Conditions, Mass Balances, and Yields of Reaction Products

From the MSW pre-treatment processes, pyrolysis processes were applied on a laboratory scale to evaluate the yields and specifications of the products formed, investigating the influence of socioeconomic regions. According to Figure 12, the pyrolysis reaction products are as follows: the generation of bio-oil (a), aqueous phase (liquid phase) (b), and (c) biochar.

Table 7. Process parameters, mass balances, and yields of reaction products at 450 °C, 1.0 atmosphere on a laboratory scale for socioeconomic regions.

Process Parameters	Region 1	Region 2	Region 3
	450 [°C]	450 [°C]	450 [°C]
	0.0 (wt.)	0.0 (wt.)	0.0 (wt.)
Mass of urban solid wastes [g]	40.1	40.11	40.57
Cracking time [min]	20	20	20
Initial cracking temperature [°C]	334	363	364
Mass of solids (coke) [g]	16.11	16.61	19.28
Mass of bio-oil [g]	9.45	9.19	6.54
Mass of H2O [g]	6.54	7.42	6.68
Mass of gas [g]	8.00	6.89	8.07
Yield of bio-oil [%]	23.57	22.91	16.12
Yield of H2O [%]	16.31	18.49	16.46
Yield of solids [%]	40.17	41.41	45.52
Yield of gas [%]	19.95	17.21	19.89

summarizes the mass yields of the products (liquids, solids, water, and gas) obtained from the organic and paper fractions of MSW, processed at a final temperature of 450 °C and 1.0 atm, on a laboratory scale. The thermal experiments, conducted at a heating rate of 10 °C/min, encompassed various socioeconomic regions. The objective of the thermal and catalytic pyrolysis was to produce hydrocarbon mixtures and other compounds with potential as fuel. The highest bio-oil yield (23.57% wt.) was recorded in Region 1, classified socioeconomically as E, whereas Region 3, classified as C, showed a lower yield of 16.12% wt. This variation highlights the influence of socioeconomic factors, where higher income levels correlate with reduced organic waste fractions, leading to lower bio-oil production [18].

The biochar yields exhibited an inverse relationship to the condensed product yields, with the highest biochar yield recorded in Region 3 at 45.52% (wt.). The aqueous fraction ranged from 18.49% (wt.) in Region 2 to 16.46% (wt.) in Region 3, while the gaseous fraction peaked in Region 1 at 19.95% (wt.). Among the three experiments, biochar formation consistently showed the highest yields. The pyrolysis process demonstrated that temperature significantly influences product distribution: higher temperatures favor bio-oil and synthesis gas production, while lower temperatures enhance biochar formation. Optimal biochar production from MSW typically occurs within a temperature range of 300–600 °C.

In the catalytic pyrolysis experiments, the final temperatures of 400 °C (

Table 8. Process parameters, mass balances, and yields of reaction products by pyrolysis and catalytic cracking of urban solid wastes at 400 °C, 1.0 atm, 10.0% (wt.) of Ca(OH)₂, on laboratory scale.

Process Parameters	Region 1	Region 2	Region 3
	400 [°C]	400 [°C]	400 [°C]
	10.0 (wt.)	10.0 (wt.)	10.0 (wt.)
Mass of urban solid wastes [g]	30.0	30.0	30.03
Mass of Ca(OH)2 [g]	3.0	3.0	3.0
Cracking time [min]	20	20	20
Initial cracking temperature [°C]	351	327	328
Mass of solids (coke) [g]	15.2	15.13	14.12
Mass of bio-oil [g]	7.23	4.49	3.88
Mass of H2O [g]	2.90	2.33	6.39
Mass of gas [g]	4.67	8.05	5.65
Yield of bio-oil [%]	24.10	14.97	12.92
Yield of H2O [%]	9.66	7.77	21.27
Yield of solids [%]	50.67	50.43	47.01
Yield of gas [%]	15.57	26.83	18.81

) and 475 °C (

Table 9. Process parameters, mass balances, and yields of reaction products by pyrolysis and catalytic cracking of urban solid wastes at 475 °C, 1.0 atm, 10.0% (wt.) of Ca(OH)₂, on laboratory scale.

Process Parameters	Region 1	Region 2	Region 3
	475 [°C]	475 [°C]	475 [°C]
	10.0 (wt.)	10.0 (wt.)	10.0 (wt.)
Mass of urban solid wastes [g]	30.0	30.08	30.06
Mass of Ca(OH)2 [g]	3.0	3.02	3.00
Cracking time [min]	20	20	20
Initial cracking temperature [°C]	364	381	340
Mass of solids (coke) [g]	13.87	10.66	11.18
Mass of bio-oil [g]	8.48	5.68	6.45
Mass of H2O [g]	1.53	4.28	4.95
Mass of gas [g]	6.10	9.47	7.48
Yield of bio-oil [%]	28.28	18.88	21.45
Yield of H2O [%]	5.11	14.22	16.46
Yield of solids [%]	46.24	35.43	37.19
Yield of gas [%]	20.35	31.48	24.88

) were used at a constant pressure of 1.0 atmosphere, with the addition of 10% Ca(OH)₂ catalyst on a laboratory scale. Figure 13 illustrates the by-product yields from all the pyrolysis experiments in this study. The process was conducted at a heating rate of 10 °C/min across various socioeconomic regions. The highest bio-oil yield was observed at 450 °C in Region 1, reaching 28.28% by mass (wt.), while the lowest yield, 18.88% by mass (wt.), occurred in Region 2, which has the highest socioeconomic classification. The reduced bio-oil yield in the higher-income regions is likely linked to lower organic waste fractions, reflecting differences in consumption and disposal patterns. Higher-income households often purchase processed foods, resulting in less organic waste, and are more likely to eat outside, further reducing food preparation residues. Conversely, lower-income families tend to prepare meals at home, contributing to the higher organic waste content in their discards [8,18,82,83,84].

The addition of the basic catalyst Ca(OH)₂ was found to increase bio-oil yield only at a temperature of 475 °C. As noted by Lu et al. (2009) [85], introducing catalysts in the cellulose pyrolysis process promotes both the conversion of polymers into volatile compounds and the secondary pyrolysis of these volatiles. The initial effect increases the total peak area, while the latter reduces it. This suggests that Ca(OH)₂ significantly enhances the secondary conversion of the volatile compounds produced during the pyrolysis process.

In all the reviewed studies, an increase in bio-oil yield was observed between 450 °C and 600 °C, but this yield tends to decrease once the temperature exceeds 600 °C. Conversely, the bio-char yield follows an inverse trend, peaking between 400 °C and 600 °C before declining, while the gas yield continues to rise across the entire temperature range. A specific study by [84] on MSW pyrolysis found that bio-char yield peaks at 400 °C and decreases beyond this point, while bio-oil yield increases with higher temperatures, reaching its maximum at 700 °C. The gas yield consistently rises, aligning with the general trends observed in pyrolysis product yields relative to temperature, also referenced by [71]. These findings are consistent with the other literature on MSW pyrolysis [42,43,44,45,46].

3.4.2. Physicochemical and Compositional Characterization of Bio-Oil

Acidity of Bio-Oil

Table 10. Effect of temperature on the acid index of bio-oils and aqueous phase obtained by thermal pyrolysis at 450 °C and catalytic pyrolysis of the MSW fraction at 400 and 475 °C, 1.0 atm, 10.0% (by mass) of Ca(OH)₂ on a laboratory scale.

Physicochemical Property	Temperature
	R1	R2	R3	R1	R2	R3	R1	R2	R3
Acid Index	450 °C	450 °C	450 °C	400 °C Ca(OH)2	400 °C Ca(OH)2	400 °C Ca(OH)2	475 °C Ca(OH)2	475 °C Ca(OH)2	475 °C Ca(OH)2
I.ABio-Oil [mg KOH/g]	116.8	115.1	115.3	34.45	34.41	40.0	36.36	36.31	37.20
I.AAqueous Phase [mg KOH/g]	69.01	65.55	76.23	42.3	42.1	42.0	43.4	4.2	40.0

shows the effect of temperature on the acidity of the bio-oil and the aqueous phase obtained by thermal pyrolysis at 450 °C and the catalytic pyrolysis of the MSW fraction at 400 and 475 °C, 1.0 atm, 10.0% (in mass) of Ca(OH)₂ on a laboratory scale.

The acidity index of the bio-oil and the aqueous phase obtained by the catalytic pyrolysis with Ca(OH)₂ as a catalyst was considerably reduced compared to the products generated by the thermal pyrolysis. This is due to the neutralizing effect of the catalyst on organic acids, such as carboxylic acids, formed during the thermal decomposition of the waste. In the thermal pyrolysis of waste containing organic components, such as the organic fraction and paper, a series of thermal degradation reactions occur, such as the breaking of the bonds of cellulose (present in paper) and hemicellulose, resulting in the formation of volatile products rich in oxygen, such as carboxylic acids (mainly acetic and formic acid), aldehydes, and phenols.

Meanwhile lignin (present in the paper), in turn, generates aromatic compounds, such as phenols, which also contribute to the acidity of the bio-oil. These carboxylic acids, due to their high concentration, are responsible for the high acidity level of the bio-oil. This strongly indicated acidity of bio-oil constitutes a disadvantage that limits its direct use as fuel in the transport sector. It can cause corrosion problems in the mechanical components of engines. To avoid these problems caused by the high acidity of bio-oils, the use of catalytic cracking is recommended [32].

With the addition of Ca(OH)₂, the neutralization of carboxylic acids occurs quickly, directly reducing the acidity index, in addition to catalytic deoxygenation that facilitates the removal of oxygen in the form of CO₂ and H₂O, reducing the amount of oxygenated compounds that could form new acids.

Fourier Transform in Infrared of Bio-Oil

As the infrared spectra of the different socioeconomic regions were similar, it was decided to present a representative spectrum of the bio-oil from each experiment carried out in Region 1 (Figure 14). The qualitative analysis by FT-IR of the chemical functions present in bio-oils, obtained by the thermal pyrolysis of the MSW fraction (organic matter and paper) at 450 °C and by catalytic pyrolysis at 400 °C and 475 °C, 1.0 atm, with 10% by mass of Ca(OH)₂, on a laboratory scale, is illustrated in Figure 13. It is important to highlight that all the spectra of the analyzed samples exhibit vibrations characteristic of unsaturated hydrocarbons.

The bands in the range of 2922–2852 cm⁻¹ in the FTIR spectrum generally correspond to the stretching vibrations of the C-H bonds in the methyl (CH₃) and methylene (-CH₂-) groups present in aliphatic hydrocarbons. These bands indicate the presence of saturated hydrocarbon chains (alkanes) in the analyzed sample. The band at 1702 cm⁻¹ in the FTIR spectrum is typically attributed to the stretching vibration of the C=O (carbonyl) bond in compounds such as ketones, aldehydes, carboxylic acids, and esters. This band indicates the presence of carbonyl groups in the sample, which are common in many organic compounds, such as bio-oils.

The band at 1457 cm⁻¹ in the FTIR spectrum is generally associated with the bending vibrations (or deformation) of the C-H bonds in the methyl (-CH₃) and methylene (-CH₂-) groups in aliphatic hydrocarbons. This band may also indicate the presence of aromatic groups, related to deformation vibrations in the benzene ring. It is common in organic compounds that contain these structures, such as hydrocarbons and aromatic compounds present in bio-oils. The band at 1408 cm⁻¹ in the spectrum is generally attributed to the symmetric bending vibration of C-H bonds in methyl groups (-CH₃). It may also be associated with the deformation vibration of the O-H bond in carboxylic acids. Furthermore, this band may indicate the presence of salts of carboxylic acids, such as carboxylates, which exhibit absorption in this region.

NMR of Bio-Oil

The ¹³C and ¹H NMR spectra of the thermal and catalytic MSW bio-oil samples from Region 1 are illustrated in Figure 15.

In the ¹³C NMR spectra of the bio-oil from the experiment referring to the thermal pyrolysis at 450 °C, the presence of chemical shifts relative to carbons in the range of 178.7 to 178.5 ppm was noted, which is generally attributed to carbonyl group carbons (C=O), as in carboxylic acids, esters, and amides. The NMR spectra of the sample show signs characteristics of olefinic hydrocarbons with chemical shifts of carbons with double bonds observed at 115.5 and 139.2 ppm in the ¹³C NMR spectrum and linear aliphatic hydrocarbons with chemical shifts typical of long-chain carbons CH₂ (methylene) and CH₃ (methyl group) between 14 and 31.9 ppm in the ¹³C NMR spectrum. The same can be observed in the 1H NMR spectrum with peaks (chemical shift) at 0.87 to 1.26 ppm, confirming the presence of aliphatic hydrocarbons, while peaks between 1.62 and 2.16 and close to 4.83 ppm confirm the presence of hydrocarbons and olefins, corroborated by the infrared spectra presented. The chemical shift of 7.27 ppm is characteristic of protons linked to aromatic rings, as in benzene and its derivatives. This value indicates that the protons are unprotected due to the circulation of electrons in aromatic systems, which increases the chemical shift.

Gas Chromatography Analysis of Bio-Oil

Table 11. Effect of temperature on the chemical composition, expressed in hydrocarbons, oxygenated and nitrogenated, of the bio-oils obtained by the thermal pyrolysis at 450 °C and the catalytic pyrolysis of the MSW fraction at 400 and 475 °C, 1.0 atm, 10.0% (by mass) Ca(OH)₂ on a laboratory scale.

Temperature [°C]	Concentration [%area.]
	Hydrocarbons	Oxygenated	Nitrogenated	Chlorinated
450 (R1)	44.34	42.09	9.23	4.34
450 (R2)	49.34	39.09	7.23	4.34
450 (R3)	40.07	42.91	12.40	4.34
400 (R1)	71.32	22.85	5.82	-
400 (R2)	68.16	25.48	6.36	-
400 (R3)	69.70	23.83	6.47	-
475 (R1)	65.69	28.48	5.82	-
475 (R2)	67.53	28.63	5.84	-
475 (R3)	66.77	26.40	6.82	-

and Figure 16 illustrate the effect of temperature on the chemical composition, expressed in hydrocarbons, oxygenated and nitrogenous, of the bio-oils obtained by the thermal pyrolysis at 450 °C and the catalytic pyrolysis of the MSW fraction at 400 and 475 °C, 1.0 atm, 10.0% (by mass) of Ca(OH)₂, evaluating socioeconomic regions 1, 2, and 3, on a laboratory scale. The chemical functions (alcohols, carboxylic, oxygenated, and nitrogenous acids), sum of peak areas, CAS numbers, and retention times of all the molecules identified in the bio-oil by GC-MS are illustrated in Supplementary Tables S1–S9.

From the GC-MS analysis of the bio-oil obtained by the pyrolysis of urban solid waste at 450 °C, the chemical compounds identified were aliphatic hydrocarbons (alkanes, alkenes, cycloalkanes, and cycloalkenes) and aromatic hydrocarbons (benzenes, indenes, and naphthalenes) with total percentage concentration in the area of 44.34, 49.34, and 40.07% for R1, R2, and R3, respectively. The oxygenated compounds (esters, ethers, phenols, carboxylic acids, ketones, and aldehydes) presented a percentage in the area of 42.09, 39.04, and 42.91% for R1, R2, and R3, respectively; in addition, organic compounds containing nitrogen were identified at 9.22%, 7.22%, and 12.40% and chlorine 4.34% for R1, R2, and R3 in percentages of area. The results found are in line with the literature [39,63,86,87,88].

As a result, the difference found between regions R1, R2, and R3 was less than 4.0%, which can be explained due to the gravimetric composition of the solid waste collected and/or the socioeconomic classification of each region.

From the GC-MS analysis of the bio-oil obtained by the pyrolysis of urban solid waste at 400 °C, using the catalyst, the chemical compounds identified were aliphatic hydrocarbons (alkanes, alkenes, cycloalkanes, and cycloalkenes) and aromatic hydrocarbons (benzenes, indenes, and naphthalenes) with total concentration percentage in the area of 71.32%, 68.16%, and 69.70% for R1, R2, and R3, respectively. The oxygenated compounds (esters, carboxylic acids, alcohols, ketones, and phenols) presented a percentage in the area of 22.85%, 25.48%, and 23.83% for R1, R2, and R3, respectively; in addition, organic compounds containing nitrogen were identified in percentages in the area of 5.82%, 6.36%, and 6.47% for R1, R2, and R3, respectively. The results found are in line with the literature [39,63,86,87,88]. As a result, a difference of less than 3% in the hydrocarbon composition between regions R1, R2, and R3, taking R1 as a reference, stands out, highlighting the difference found between regions R1 and R3, which was less than 1.0%. This can be explained due to the addition of the Ca(OH)₂ catalyst.

It is also worth highlighting the increase in the concentration [% area] of hydrocarbons in relation to the experiments without the catalyst at 450 °C, and the decrease in oxygenated, nitrogenated, and chlorinated compounds. In view of this, it appears that the Ca(OH)₂ catalyst reduces the formation of these compounds in bio-oils produced regardless of the socioeconomic classification of the region.

From the GC-MS analysis of the bio-oil obtained by the pyrolysis of urban solid waste at 475 °C, using the catalyst, the chemical compounds identified were aliphatic hydrocarbons (alkanes, alkenes, cycloalkanes, and cycloalkenes) and aromatic hydrocarbons (benzenes, indenes, and naphthalenes) with total percentage concentration in the area of 65.69%, 67.53%, and 66.77% for R1, R2, and R3, respectively. The oxygenated compounds (esters, carboxylic acids, alcohols, ketones, and phenols) presented a percentage in area of 28.48%, 26.63%, and 26.40% for R1, R2, and R3, respectively; in addition, organic compounds were identified having nitrogen in percentages in the area of 5.82%, 5.83%, and 6.82% for R1, R2, and R3, respectively. The results found are in line with the literature [39,63,86,87,88]. As a result, a difference of less than 2.0% in the composition of hydrocarbons between the regions R1, R2, and R3, taking R2 as a reference, was observed. In addition, a decrease in the production of oxygenates and nitrogenates was observed in relation to the experiments without the catalyst; that is, regardless of the temperature used and the socioeconomic classification of each region, the application of the Ca(OH)₂ catalyst favored the formation of hydrocarbons. According to the quantification and identification of the compounds present in the bio-oil samples, the presence of major compounds was found to be as follows: Benzene, (1-methylethyl)—D-Limonene for the R1 and R2 regions; o-xylene for the regions R1, R2, and R3 in experiments at 450 °C; Dodecanoic acid, 9-Octadecenoic acid, ethyl ester, and o-xylene for region R1; Phenol, Phenol, 4-ethyl-2-methoxy-e Phenol for region R2 and 2-Heptadecanone for region R3 in experiments at 400 °C with Ca(OH)₂ catalyst; 2-Methyl-Z,Z-3,13-octadecadienol and 10-Octadecenoic acid, methyl ester, bis(2-ethylhexyl) o-Cymene ester for the R1 region; 9-Octadecene, 1-methoxy-, (E)- and Hexadecanoic acid, methyl ester, 10-Octadecenoic acid, methyl ester for the R2 region; and Pentadecane and 2-Heptadecanone for the R3 region. The majority of compounds can be applied by the chemical and petrochemical industries for the production of inputs, pharmaceuticals, and polymers, in addition to combustible liquids, due to the percentage concentration in area below 30% in all the experiments carried out [39,63,86,87,88].

In view of this, it appears that the application of the Ca(OH)₂ catalyst favors the formation of hydrocarbons by reducing the concentration in area of oxygenated, nitrogenated, and chlorinated in bio-oils regardless of the temperature used in the process and the socioeconomic classification of each region.

4. Conclusions

The thermal and catalytic pyrolysis resulted in a biochar with porous morphology and enriched with calcium compounds, as evidenced by the SEM analysis. Although carbonization is partial, the porous properties and the presence of calcium particles make biochar promising for applications in environmental remediation, such as the adsorption of heavy metals and the neutralization of acids. However, the structural fragility and heterogeneous distribution of the catalyst can limit its efficiency under certain conditions. To optimize the use of this biochar, it would be important to consider adjustments to the pyrolysis process or a higher temperature to increase complete carbonization and improve the uniformity of the material.

In relation to XRD, catalytic pyrolysis with Ca(OH)₂ resulted in the formation of crystalline inorganic compounds, such as quartz, sylvite, and calcite. The carbon in the organic fraction of MSW is predominantly in amorphous form, which is characteristic of biochars produced at moderate temperatures. The presence of quartz confirms the contamination of the MSW by mineral materials (sand), while sylvite and calcite indicate the interaction of Ca(OH)₂ with the components present in the waste during the process. This suggests that the pyrolysis process was efficient in transforming organic matter and paper into biochar, while the minerals present can influence the properties of the final product.

The socioeconomic profile of the regions has a direct influence on the composition of urban solid waste, impacting the yields of pyrolysis by-products. In regions with lower income, the greater fraction of organic waste favors the production of bio-oil. In regions with higher income, where waste is less organic, there is an increase in biochar production. Temperature is another crucial factor in the distribution of pyrolysis products. The balance between biochar and bio-oil production occurs at around 450 °C. Lower temperatures tend to favor the formation of biochar, while higher temperatures promote greater yields of bio-oil and synthesis gas.

The addition of Ca(OH)₂ as a catalyst was more effective at higher temperatures (475 °C), where it contributed to increasing the bio-oil yield by intensifying the secondary pyrolysis of volatile compounds. The choice of temperature and catalyst is, therefore, crucial to optimize bio-oil production in the pyrolysis process.

The addition of Ca(OH)₂ in catalytic pyrolysis not only reduces the acidity index of the bio-oil and the aqueous phase, but also promotes deoxygenation, improving the quality of the bio-oil. These effects make the catalytic process essential for obtaining bio-oils with properties more favorable for use as fuel, reducing corrosion problems and increasing product stability.

The FT-IR spectrum confirms the presence of saturated and unsaturated hydrocarbons, in addition to carboxylic groups, which indicate the formation of compounds typical of the thermal decomposition of organic waste and paper.

The presence of aromatic groups and carboxylic acid salts also suggests that the bio-oil contains a complex mixture of organic compounds, many of which can be useful as fuels, but whose acidity may need additional treatment to avoid corrosion problems in practical applications.

The use of catalysts, such as Ca(OH)₂, appears to influence the formation of carboxylates and neutralize part of the acids formed, improving the final quality of the bio-oil. The NMR identified the presence of saturated and unsaturated hydrocarbons suggesting that the bio-oil has the potential to be used as a fuel source, with aliphatic and olefinic components suitable for this purpose.

The GC-MS analysis of the bio-oils obtained by the pyrolysis of urban solid waste (MSW) reveals that the application of the Ca(OH)₂ catalyst favored the formation of hydrocarbons regardless of the temperature (400 °C, 450 °C, and 475 °C) and the socioeconomic classification of the regions studied. An increase in the concentration of hydrocarbons and a significant reduction in oxygenated, nitrogenous, and chlorinated compounds was observed, which are less desirable due to their lower energy efficiency and corrosion problems.

Furthermore, the variation between the chemical compositions of different regions was less than 4%, suggesting that the socioeconomic classification has little impact on the quality of the bio-oil when using the catalyst. The major compounds identified, such as benzene, d-limonene, fatty acids, and esters, can be applied in the chemical, petrochemical, and fuel industries, reinforcing the economic value of bio-oils. In conclusion, the addition of Ca(OH)₂ is beneficial to the pyrolysis process, improving the production of hydrocarbons and minimizing unwanted compounds regardless of the socioeconomic origin of the MSW.