Slow Pyrolysis as a Method of Treating Household Biowaste for Biochar Production

Abstract

The amount of waste generated by society is constantly increasing. Consequently, there is a need to develop new and better methods of treating it. A significant part of municipal waste is biowaste, which can be treated as a source of valuable resources such as nutrients, organic matter, and energy. The present work aims to determine the properties of the tested household biowaste and the possibility of using it as feedstock in slow pyrolysis to obtain biochar. The slow pyrolysis process of the biowaste was carried out in an electrically heated Horizontal Tube Furnace (HTF) at temperatures of 400 °C, 500 °C, and 600 °C in a nitrogen atmosphere. The analysis showed that depending on the type and composition of the biowaste, its properties are different. All the biowaste tested has a high moisture content (between 63.51% and 81.53%), which means that the biowaste needs to be dried before the slow pyrolysis process. The characteristics of kitchen biowaste are similar to those of food waste studied by other researchers in different regions of the world. In addition, the properties of kitchen biowaste are similar to those of the typical biomasses used to produce biochar via slow pyrolysis, such as wood, almond shells, and rice husks. Both kinds of garden biowaste tested may have been contaminated (soil, rocks) during collection, which affected the high ash content of spring (17.75%) and autumn (43.83%) biowaste. This, in turn, affected all the properties of the garden biowaste, which differed significantly from both the literature data of other garden wastes and from the properties of typical biomass feedstocks used to produce biochar in slow pyrolysis. For all biowaste tested, it was shown that as the pyrolysis temperature increases, the yield of biochar decreases. The maximum mass yield of biochar for kitchen, spring garden, and autumn garden biowaste was 36.64%, 66.53%, and 66.99%, respectively. Comparing the characteristics of biowaste before slow pyrolysis, biochar obtained from kitchen biowaste had a high carbon content, fixed carbon, and a higher HHV. In contrast, biochar obtained from garden biowaste had a lower carbon content and a lower HHV.

1. Introduction

The continuous rise in the volume of waste generated by modern society has created an urgent demand for the development of improved and more efficient waste treatment methods. Within the European Union, annual biowaste generation is estimated to range between 118 and 138 million tons, with over two-thirds originating from municipal sources and the remainder primarily from the food industry [1].

In accordance with the Waste Framework Directive, biowaste is defined as “biodegradable garden and park waste, food and kitchen waste from households, offices, restaurants, wholesale, canteens, caterers and retail premises, and comparable waste from food processing plants” [2]. Based on this definition, biowaste is generally categorized into two main fractions: garden and park waste, and food and kitchen waste [3]. Consequently, household biowaste can be broadly considered a combination of these two components [4].

Aligned with the principles of the circular economy, biowaste is increasingly being viewed as a valuable resource for the recovery of nutrients, organic matter, and energy [5]. Despite its high recycling potential, a significant portion of biowaste generated annually in Europe continues to be disposed of through landfilling and incineration [6].

The environmental consequences of landfilling biowaste are particularly severe. During decomposition in landfills, biodegradable materials release greenhouse gases such as methane, carbon dioxide, and hydrogen sulfide [7]. According to the European Environment Agency (EEA), biodegradable waste—biowaste included—is a major contributor to landfill-related greenhouse gas emissions, accounting for approximately 3% of the EU’s total emissions [8]. This highlights the critical need to ensure the proper collection of unavoidable biowaste and to adopt sustainable treatment strategies [6].

Up to now, various biological and thermochemical methods have been investigated for the treatment of biowaste [7]. Currently, the most widely applied biological processes include composting (aerobic treatment) and anaerobic digestion [8]. The selection of an appropriate treatment method is largely influenced by the composition of the biowaste and the characteristics of the source separation system. Effective source separation is essential for producing high-quality end products [4]. In contrast, biowaste that is not separately collected and is instead mixed with municipal solid waste (MSW) is typically subjected to incineration [5].

Biowaste typically has a high moisture content, often exceeding 60% [1], which has historically limited the use of thermochemical processes such as pyrolysis and gasification for its treatment. Before thermochemical treatment, the biowaste must be dried, which consumes a lot of energy [7]. Meanwhile, different ways of drying waste like solar drying in a greenhouse to reduce the moisture content of kitchen waste are currently being developed [9,10]. Pyrolysis has emerged as a promising and sustainable method for biowaste valorization, capable of producing significant quantities of renewable bioenergy products such as biochar and bio-oil, while simultaneously reducing greenhouse gas emissions and other environmental pollutants [11]. A key advantage of pyrolysis is the ability to transform low-energy-density feedstocks into biofuels with higher energy density [8].

Depending on the process conditions, several types of pyrolysis can be distinguished, which differ in terms of temperature, duration, and end products. Slow pyrolysis involves gradually heating biomass in an oxygen-free environment, usually at temperatures ranging from 300 to 550 °C [12]. The process lasts from several dozen minutes to a few hours. The main product is biochar—a solid, carbon-rich material that can be used as a soil amendment. Fast pyrolysis takes place at higher temperatures (450–550 °C) but in a much shorter time, often less than 2 s [12]. Its primary product is bio-oil, which can be used as an energy source or fuel [13]. Although this method is more energy-efficient, it requires well-dried and finely ground biomass, as well as precise process control, which limits its practical use for moist kitchen waste. Hydrothermal pyrolysis, on the other hand, occurs in the presence of water, at temperatures of 200–350 °C and under high pressure. It is distinguished by its ability to process high-moisture biomass without the need for drying, making it a promising technology for food waste and other wet organic waste [7]. The final product is a liquid bio-oil and a small amount of biochar. However, this technology is more complex and requires sealed high-pressure reactors, which currently restricts its application in larger industrial facilities [14,15].

Given these distinctions, slow pyrolysis was selected for further consideration due to its operational simplicity and the high yield of stable biochar it provides. Its lower temperature and longer reaction time result in a carbon-rich product well suited for applications such as soil improvement or carbon sequestration. Unlike fast pyrolysis, it does not demand strict control parameters or finely processed input, making it more adaptable to various types of biowaste [12]. Although hydrothermal methods offer the advantage of processing wet materials without pre-drying [7], they involve significantly more complex and costly infrastructure due to the need for high-pressure equipment [14,15].

The yield of pyrolysis products depends on the reactor design, feedstock properties, and operational parameters such as temperature, residence time, heating rate, pressure, ambient gas composition, and presence of mineral catalysts [16,17,18]. At low temperatures (<300 °C), heavy tars and higher biochar yields form due to limited biomass bond breakdown, while high temperatures (>550 °C) cause extensive fragmentation, increasing non-condensable gas yield and reducing biochar [16,19]. Bio-oil yield peaks at 500–550 °C and decreases above 550 °C [20]. Longer residence time and slow heating promote biochar formation via secondary reactions, but reduce bio-oil yield by enhancing tar cracking to gas [16,21,22]. The heating rate influences product distribution: slow heating at 400–600 °C favors biochar, while fast heating increases bio-oil yield [21,22]. Particle size affects heating and product yield—smaller particles enhance bio-oil production due to the larger surface area and less secondary reaction, while larger particles increase biochar by trapping volatiles and promoting re-polymerization [16,23].

Particular attention should be given to biochar, a stable, porous, and carbon-rich by-product of biomass pyrolysis [16]. Biochar is typically produced under slow pyrolysis conditions—characterized by low heating rates and moderate temperatures, generally within the range of 400–700 °C [24]. Due to its unique physicochemical properties, biochar has been investigated for a variety of applications. Its most extensively studied uses include soil amendment [25] and water purification [26]. Additionally, biochar can serve as a solid fuel [10] or as a material for energy storage systems [27]. The production and application of biochar derived from organic waste also offer environmental benefits, including the mitigation of greenhouse gas emissions [28], climate change impacts, and the promotion of long-term carbon sequestration [25,26].

Slow pyrolysis has been widely studied for various types of biowaste, including agricultural residues, forestry biomass, energy crops, algal biomass, food industry waste, and municipal solid waste (MSW) [29]. Yang et al. [30] analyzed an integrated pyrolysis-CHP system converting the organic fraction of MSW in Leicester, UK, achieving nearly 60% cogeneration efficiency at 600 °C. Chhabra et al. [31] pyrolyzed ten MSW fractions from Mumbai, determining an optimal temperature range of 170–520 °C for mixed waste. Tokmurzin et al. [32] studied the co-pyrolysis of MSW organics and coal from Kazakhstan, showing that higher temperatures increased gas yield, while lower temperatures favored biochar from organic MSW. Czajczyńska et al. [33] emphasized the versatility of pyrolysis for organic waste, noting that while individual food wastes (e.g., fruit peels, nut shells) have been well-studied, data on mixed household and garden waste is limited due to a preference for composting. Ronsse et al. [34] showed that biochar yield decreased with more intense pyrolysis conditions when using pine wood, wheat straw, algae, and green waste. Kabenge et al. [35] found banana peels to be suitable for slow pyrolysis, with peak decomposition between 450 and 550 °C. Gupta et al. [23] improved yields of biochar and bio-oil from walnut shells by pretreating them with phosphoric acid (0.2–0.8 M), demonstrating the potential uses of biochar for fuel or wastewater treatment, bio-oil as a mixed fuel or chemical source, and pyrolysis gas (rich in CH₄, H₂, and CO) for combustion. Senneca et al. [36] also investigated walnut shell pyrolysis at 600 °C in N₂ and CO₂ atmospheres, focusing on reaction kinetics.

This report focuses on the treatment of kitchen (food) and garden biowaste, collected during the spring and autumn seasons from Polish households, and explores the feasibility of producing biochar from this material through slow pyrolysis technology.

The primary objectives of this study are as follows:

• To characterize the biowaste materials used in the investigation;
• To evaluate the influence of the slow pyrolysis temperature on the yield and physicochemical properties of the resulting biochar;
• To analyze the properties of the produced biochar.

2. Materials and Methods

2.1. Biowaste Samples

The biomass residues analyzed in this study included different types of biowaste collected from residential areas during distinct seasons in the Silesia region of Poland.

The first category was kitchen biowaste, gathered during the summer from apartment buildings. This type of waste originates from food preparation within individual households and consists mainly of organic scraps such as vegetable and fruit residues (including peels, seeds, and inedible parts), leftover bakery products like bread, cooked foods such as pasta, rice, and groats, as well as meat, dairy products, uneaten ready meals, eggshells, coffee grounds, tea leaves, and nut shells (Figure 1).

The second category was garden biowaste collected in spring from single-family houses. This waste primarily consisted of cut grass, reflecting typical seasonal gardening activities such as mowing and planting (Figure 2). However, it may have also contained small amounts of food waste, as households in these homes often dispose of kitchen and garden waste together in the same bin.

The third category was garden biowaste collected in the autumn from single-family houses. This biowaste mainly comprised fallen leaves and small branches, resulting from seasonal yard cleanup activities like leaf raking (Figure 3). It could also include withered plants and some grass. As with the spring collection, this waste might also contain traces of kitchen biowaste due to the shared disposal system for both waste types in single-family residences.

2.2. Characterization of the Biowaste Samples

The biowaste samples after collection (on a wet basis) were used to determine the total moisture content. Next, the prepared samples were completely dried in a laboratory dryer at 105 °C for 24 h. After drying, the biowastes were ground in an IKA-WERKE M20 mill (manufacturer: IKA^®-Werke GmbH & Co. KG, Staufen, Germany), which is used for dry grinding hard and friable substances.

Proximate analysis involves determining the moisture content, volatile matter, fixed carbon, and ash content in biowaste. To assess the total moisture content, wet biowaste samples were analyzed following the PN-EN ISO 18134-1:2023-02 standard [37]. This method involves drying the sample at 105 °C. All other parameters were determined using dry samples. The volatile matter content was measured according to PN-EN ISO 18123:2016-01 [38], which requires heating the sample at 850 °C under anaerobic conditions for 3 min. The ash content was determined based on the PN-Z-15008-03 standard [39], where samples are calcinated to a constant weight at a temperature of 815 ± 10 °C. Fixed carbon was not measured directly, but instead calculated by subtracting the percentages of moisture, volatile matter, and ash from the total mass [40], using formulas from [12,16].

The ultimate analysis determines the elemental composition of biowaste, including carbon, hydrogen, oxygen, nitrogen, sulfur, and chlorine. All measurements were conducted on dry samples. The carbon and hydrogen contents were determined in accordance with ISO 609:1996 [41], which involves combusting the sample at 950 °C in an oxygen-rich environment. The total nitrogen was measured using the Kjeldahl method, with prior mineralization in concentrated sulfuric acid, as specified in PN-G/-04523 [42]. The sulfur content was determined based on PN-ISO 351:1999 [43], involving combustion at 950 °C in the presence of oxygen. The chlorine content was measured following PN-ISO 587:2000 [44], where the biowaste sample, mixed with Eschka reagent, is combusted at 675 °C in an oxidizing environment. The oxygen content was calculated by difference, subtracting the sum of the measured percentages of moisture, ash, carbon, hydrogen, nitrogen, sulfur, and chlorine from 100% [40].

To further characterize the tested biowaste samples, the higher heating value (HHV) and lower heating value (LHV) were additionally determined in a calorimetric bomb [45] according to PN-ISO 1928:2020-05 [46]. In addition, the total organic matter content, total organic carbon content, and pH of the water extract of the biowaste samples were determined following the laboratory guidelines [47]. In addition, a surface analysis of the studied biowaste was performed using scanning electron microscopy (SEM) (JEOL, model JSM-7001F, manufacturer: JEOL Ltd., Tokyo, Japan) [48].

2.3. Slow Pyrolysis of Biowaste

The slow pyrolysis experiments were conducted in a laboratory-scale batch system. A Horizontal Tube Furnace (HTF), electrically heated and equipped with a water-cooled vessel, was used for this purpose. The equipment, manufactured by the Portuguese company Termolab-fornos Eléctricos Lda., Águeda, Portugal, is shown in Figure 4.

It is capable of operating at temperatures of up to 1300 °C. An S-type thermocouple continuously monitors the furnace wall temperature to ensure accurate thermal control. A recrystallized alumina tube, horizontally positioned within the furnace, with an internal diameter of 4 cm and a length of 55 cm, was used as the reaction chamber. Two ceramic crucibles, each containing around 2.5 g of sample, were placed inside the HTF. Pyrolysis was performed at temperatures of 400 °C, 500 °C, and 600 °C, with a heating rate of 33 °C/min and a residence time of 1 h. A nitrogen flow of 2 L/min ensured an inert atmosphere throughout the process.

2.4. Characterization of the Biochar

To characterize the biochar obtained via slow pyrolysis (Figure 5), the proximate analysis, ultimate analysis, and determination of the higher and lower heating values were carried out. All determinations were performed on dry samples and according to the standards described in Section 2.2 on biowaste characterization. It was assumed that the biochar was completely dried after slow pyrolysis; therefore, the moisture content was not determined.

3. Results and Discussion

3.1. Properties of the Biowaste

The properties of the tested biowaste are summarized in

Table 1. Properties of the tested biowastes.

	Kitchen Biowaste	Spring Garden Biowaste	Autumn Garden Biowaste
Proximate analysis (% wt.)
MC (total moisture content) (wet basis)	68.10	81.53	63.51
VM (volatile matter content) (dry basis)	76.55	42.43	43.75
A (ash content) (dry basis)	6.81	17.75	43.83
FC (fixed carbon) * (dry basis)	16.64	39.82	12.43
Ultimate analysis (% wt.) (dry basis)
C (carbon content)	43.36	31.94	29.99
H (hydrogen content)	7.03	4.53	4.02
N (nitrogen content)	3.12	2.84	1.19
S (sulfur content)	0.10	0.07	0.03
O (oxygen content) *	38.87	42.82	20.92
Cl (chlorine content)	0.71	0.05	0.01
Other measurements (dry basis)
HHV (higher heating value), MJ/kg	17.24	10.60	11.16
LHV (lower heating value), MJ/kg	15.69	9.60	10.27
CCs (combustible compounds) (%wt.)	93.19	82.25	56.17
OS (total organic matter content) (%wt.)	90.06	81.77	55.32
MS (total mineral substances content) (%wt.)	9.94	18.23	44.68
Corg (total organic carbon content) (%wt.)	37.47	35.29	23.18
pH (wet basis)	5.18	9.20	8.62

* by difference.

.

3.1.1. Proximate Analysis Results of Tested Biowaste

The high moisture content of the biowaste samples tested requires pre-drying before slow pyrolysis. The analyzed kitchen biowaste had a moisture content of 68.10%, which is lower than the reported values in the literature, where food waste can reach up to 80% [1] or even 92.36% [49]. This discrepancy may arise from the varying compositions of biowaste. For instance, food waste comprising peelings, raw vegetables, and fruits tends to have a higher moisture content compared to waste containing drier components like bread, eggshells, pasta, or rice. Garden waste typically contains 50–60% moisture [1,50].

However, spring garden waste in this study showed a higher moisture content (81.53%), likely due to grass being collected post rain or the inclusion of moist food waste such as vegetable peelings. Autumn garden waste had a moisture content of 63.51%, slightly above the values cited in the literature [1,50].

Kitchen biowaste showed the highest volatile matter content at 76.55%, slightly exceeding the 71.00% reported in the literature [31,49]. Spring and autumn garden biowaste samples showed similar values—42.43% and 43.75%, respectively—considerably lower than the literature values for garden waste (84.50%) [50] and shredded green waste (77.60%) [51].

The kitchen biowaste sample had an ash content of 6.81%, lower than the 10% reported in some studies [31,49]. In contrast, both garden biowaste samples contained significantly more ash than the 1.00% reported for garden waste [50,51]. Spring garden waste contained 17.75% ash, while autumn garden waste had an extremely high ash content of 43.83%, comparable to that of municipal solid waste’s organic fraction (44.30%) [30]. These high values likely indicate contamination from soil or rocks gathered during collection using rakes or spatulas. The autumn sample appears to be more contaminated than the spring one.

The fixed carbon content in kitchen biowaste (16.64%) and autumn garden biowaste (12.43%) is comparable to the literature values of 18.50% for food waste [31,49] and 15.00% for garden waste [50,51]. However, the spring garden waste sample showed a significantly higher fixed carbon content at 39.82%.

3.1.2. Ultimate Analysis Results of Tested Biowaste

Kitchen biowaste had a carbon content of 43.36%, closely matching the literature values of 41.70% [49] and 46.10% [31]. Spring and autumn garden biowaste had lower carbon contents of 31.94% and 29.99%, respectively, which are below the 50.12% reported for garden waste [50] and the 42.85–45.57% range for grass [52,53].

The hydrogen content of the kitchen biowaste was within the upper range of 5.66–7.47% reported in the literature [31,49,51,52]. The spring and autumn garden samples had lower hydrogen contents, similar to that of the municipal solid waste organic fraction (4.70%) [30].

On a dry basis, kitchen biowaste contained 3.12% nitrogen, similar to the 3.49% reported for canteen food waste [49]. Spring and autumn garden waste samples contained 2.84% and 1.19% nitrogen, respectively—much higher than the literature values for garden waste, which range from 0.14% to 0.20% [50,51]. The spring garden waste nitrogen content is comparable to that of lawn grass (2.68%) [52].

The oxygen content in kitchen biowaste (38.87%) is close to the values reported by Rago et al. (37.09%) [49] and Chhabra et al. (36.55%) [31]. Spring garden biowaste had an oxygen content of 42.82%, which falls within the typical range of 39.40% to 45.91% for garden waste [29,51]. However, autumn garden waste had a significantly lower oxygen content of 20.92%, even below the 26.39% reported for municipal yard waste [31].

The sulfur contents were 0.10% for kitchen, 0.07% for spring garden, and 0.03% for autumn garden biowaste—substantially lower than the 0.40% found in the organic fraction of MSW [30]. The values for the garden waste samples align with the 0.03–0.08% range noted in the literature [50,51].

Kitchen biowaste had the highest chlorine content (0.71%), likely due to the presence of table salt (NaCl), which consists of about 60% chloride by weight [54]. Spring and autumn garden biowaste had significantly lower chlorine contents (0.05% and 0.01%, respectively), well below the 0.74% found in grasses [53].

3.1.3. Results of Other Biowaste Measurements

The Higher Heating Value (HHV) of kitchen biowaste was 17.24 MJ/kg, higher than the 15.08 MJ/kg recorded for canteen food waste [49]. Spring and autumn garden waste had HHVs of 10.60 MJ/kg and 11.16 MJ/kg, respectively—much lower than the 18.12 MJ/kg reported for shredded green waste [51].

The LHVs for kitchen, spring garden, and autumn garden biowaste were 15.69 MJ/kg, 9.60 MJ/kg, and 10.27 MJ/kg, respectively. These values, derived from HHVs, are lower due to the exclusion of the latent heat from water vapor condensation during combustion [40].

The combustible compound content was highest in kitchen biowaste (93.19%), followed by spring garden waste (82.25%) and autumn garden waste (56.17%). This indicates that kitchen waste ignites most easily, while autumn garden waste is most resistant. Similarly, the total organic matter content was 90.06% for kitchen, 81.77% for spring garden, and 55.32% for autumn garden waste, suggesting that kitchen waste is the most biodegradable. The organic content also influences waste collection and storage methods [1].

The pH of kitchen biowaste was acidic at 5.18, while spring and autumn garden biowaste samples were alkaline, with values of 9.20 and 8.62, respectively.

3.1.4. Summary of Biowaste Properties

Among the biowaste types analyzed, kitchen biowaste exhibited the most favorable characteristics for biochar production via slow pyrolysis. It recorded the highest values for volatile matter (76.55%), carbon content (43.36%), hydrogen content (7.03%), and the lowest ash content (6.81%). These factors positively influence biochar yield and quality. Additionally, it demonstrated the highest higher heating value (HHV) at 17.24 MJ/kg. However, kitchen biowaste also contained the highest levels of nitrogen (3.12%), chlorine (0.71%), and sulfur (0.10%) among all samples. These elements can negatively affect combustion by increasing emissions of nitrogen oxides (NO_x), sulfur oxides (SO_x), and hydrogen chloride (HCl), and may contribute to corrosion in thermal processing equipment. Despite these concerns, the physicochemical properties of the kitchen biowaste were consistent with the literature values reported globally for food waste [31,49]. These findings support the conclusions of Ilakovac et al. [55], who noted that the composition of household food waste is relatively uniform across different countries.

The spring garden biowaste differed significantly from kitchen biowaste. Compared to the literature values for garden waste, it contained higher levels of moisture (81.53%), ash (17.75%), and fixed carbon (39.82%), while showing lower levels of volatile matter (42.43%), carbon (31.94%), hydrogen (4.53%), and chlorine (0.05%). The contents of oxygen (42.82%), nitrogen (2.84%), and sulfur (0.07%) were similar to the values reported in the literature. However, its HHV was lower, at 10.60 MJ/kg, indicating reduced energy potential [31,50,51].

In contrast, autumn garden biowaste had a moisture content of 63.51% and a notably high ash content of 43.83%, both higher than the typical literature values [50]. It also had elevated nitrogen content (1.19%), but lower levels of volatile matter (43.75%), carbon (29.99%), hydrogen (2.02%), oxygen (20.92%), and chlorine (0.01%). Its HHV was also relatively low at 11.16 MJ/kg. The fixed carbon (12.43%) and sulfur (0.03%) content in autumn garden biowaste were in line with the data from the literature for similar materials [51,52].

For further comparison, the properties of the tested biowaste were evaluated against commonly used biomass materials for pyrolysis-based biochar production, such as wood [9], almond shells [12], and rice husks [56]. It was shown that among the biowaste analyzed, kitchen biowaste has the most similar properties, and thus would be best suited as a feedstock to produce biochar through the slow pyrolysis process.

3.2. Biochar Yield

The mass yield of biochar (Y_m) obtained from the slow pyrolysis of the analyzed biowaste is presented in

Table 2. Yield and properties of biochar obtained from slow pyrolysis of biowaste.

	Kitchen Biochar			Spring Garden Biochar			Autumn Garden Biochar
	400 °C	500 °C	600 °C	400 °C	500 °C	600 °C	400 °C	500 °C	600 °C
Ym (mass yield of biochar) (% wt.) (dry basis)	36.64	32.02	28.71	66.53	58.13	60.07	66.99	63.15	60.68
Proximate analysis (% wt.) (dry basis)
VM (volatile matter content)	20.49	14.88	11.14	12.59	11.65	5.89	15.42	12.10	5.76
A (ash content)	18.57	21.81	21.09	64.69	65.38	72.57	61.71	64.38	71.32
FC (fixed carbon) *	60.94	63.31	67.77	22.73	22.96	21.54	22.87	23.52	22.93
Ultimate analysis (% wt.) (dry basis)
C (carbon content)	58.02	54.81	55.55	25.34	26.60	21.00	27.84	27.45	25.62
H (hydrogen content)	4.00	3.02	2.67	2.41	1.71	1.41	2.80	2.54	1.47
N (nitrogen content)	3.64	3.46	3.01	1.44	1.52	1.05	1.15	1.00	1.02
O (oxygen content) **	15.77	16.90	17.68	6.12	4.79	3.97	6.50	4.63	0.57
Other measurements (dry basis)
HHV (higher heating value), MJ/kg	22.68	20.84	21.90	9.04	8.95	7.51	9.96	9.02	8.40
LHV (lower heating value), MJ/kg	21.80	20.17	21.31	8.51	8.58	7.20	9.35	8.46	8.07
CCs (combustible compounds), (% wt.)	81.43	78.19	78.91	35.31	34.62	27.43	38.29	35.62	28.68

* by difference. ** by difference (without considering the sulfur and chlorine content).

.

Analysis of these results reveals a consistent trend across all tested biowaste types: as the pyrolysis temperature increases, the biochar yield decreases. This trend has also been observed in studies involving green waste [34], and it aligns with theoretical principles. At lower pyrolysis temperatures, carbonization reactions dominate, favoring solid char formation. As the temperature increases, devolatilization reactions intensify, leading to the release of more volatile compounds. This results in the increased production of bio-oil and gas, which were not evaluated in the present study [16].

Among the different biowaste types studied, autumn garden biowaste consistently produced the highest biochar yields at all pyrolysis temperatures, whereas kitchen biowaste produced the lowest. However, these results must be interpreted considering the ash content in the raw materials. The autumn garden biowaste contained 43.85% ash, spring garden biowaste 17.75%, and kitchen biowaste 6.81%. Since ash is non-combustible and remains in the solid fraction post pyrolysis, it contributes to a higher biochar yield on a dry basis. Therefore, the high ash content in autumn garden biowaste partially explains its greater biochar yield.

3.3. Properties of Biochar

The characteristics of the biochars produced from the slow pyrolysis of various biowastes are summarized in Table 2.

Among all the analyzed samples, the biochar obtained from kitchen biowaste at a pyrolysis temperature of 400 °C showed the most favorable fuel-related properties. This biochar contained the highest levels of volatile matter (20.49%), carbon (58.02%), hydrogen (4.00%), and recorded the highest higher heating value (HHV) at 22.68 MJ/kg. It also had the lowest ash content among the samples, at 18.57%. These attributes suggest good potential for energy recovery, indicating that this biochar could be suitable for use as a fuel. However, this biochar also presented some drawbacks. Its relatively high nitrogen content (3.64%) could lead to increased nitrogen oxide (NO_x) emissions during combustion, raising environmental concerns. Additionally, the high content of combustible compounds (81.43%) may pose risks during storage due to the possibility of spontaneous ignition. The yield of this kitchen biochar was 36.64%, which is not only the highest observed for kitchen-derived biochar in this study, but also exceeds the typical yield for biochar produced via slow pyrolysis, which is around 35% [12]. Biochar with such properties could be used for carbon sequestration or soil improvement through enhanced water and nutrient retention.

In contrast, the biochar obtained from spring and autumn garden biowaste demonstrated similar properties to each other across corresponding pyrolysis temperatures, but differed significantly from kitchen biochar. The yields of both garden biochars were nearly double that of kitchen biochar. However, they also contained more than three times the ash content of kitchen biochar, contributing to significantly reduced values of volatile matter, carbon, and hydrogen content. As a result, their HHVs were less than half those observed for kitchen biochar. Furthermore, the garden-derived biochar exhibited a lower carbon content and HHV compared to their respective feedstocks before pyrolysis. Considering that effective biochar should be rich in carbon and have a calorific value in the range of 20–36 MJ/kg [17], these results suggest that the analyzed garden biowastes are not suitable feedstocks for energy-oriented biochar production through slow pyrolysis.

Nevertheless, due to their high ash content, garden biochars could potentially be repurposed as soil amendments to enhance soil fertility, depending on the mineral composition of the ash. It is also likely that contamination from soil, rocks, and other inorganic matter during the collection process—since garden waste is typically raked directly from the ground—negatively affected the quality of the resulting biochar. While such contamination is a known issue, it remains uncertain whether it can be entirely avoided.

3.4. Scanning Electron Microscope (SEM) Analysis

Scanning electron microscope (SEM) analysis was conducted to assess the surface morphology and structure of both the raw biowaste samples and their corresponding biochars. The SEM images (Figure 6) confirmed the heterogeneous nature of all analyzed biowaste types, each composed of particles differing in shape, size, and surface texture. For kitchen biowaste, the particles displayed diverse morphological features. Some had smooth surfaces with clearly defined sharp edges, while others exhibited rough or irregular textures with poorly distinguishable edges. This variation indicates the mixed composition of kitchen waste, likely containing materials of different origins and physical properties. In contrast, spring garden biowaste was characterized by the presence of large, elongated particles with visible edges and internal porous structures. These morphological traits are typical of fibrous plant materials, such as grass blades, and suggest a more uniform plant-based composition. The autumn garden biowaste consisted mainly of relatively small particles with rough, irregular surfaces with visible cavities.

SEM analysis of the biochars derived from these biowaste types also revealed significant heterogeneity, with particle shapes and sizes varying within and between samples. For kitchen biochar, increasing the pyrolysis temperature led to the formation of larger and more porous particles. This enhanced porosity is a result of greater devolatilization at higher temperatures, which removes more volatile matter from the structure. In the case of spring garden biochar, the particle morphology remained consistent regardless of the pyrolysis temperature. The elongated, porous structures observed in the raw spring garden biowaste were largely preserved during pyrolysis, indicating the structural stability of the plant fibers under thermal treatment. For autumn garden biochar, the porosity of the particles increased with the rising pyrolysis temperature. Despite this change, both spring and autumn garden biochars shared a similar porous morphology, which was different from that of kitchen biochar. This suggests that the structural characteristics of garden waste-derived biochar are primarily influenced by the original biomass structure rather than the pyrolysis conditions alone.

4. Conclusions

This study investigated the properties of three types of biowaste collected in Poland: kitchen biowaste, spring garden biowaste, and autumn garden biowaste. The next stage focused on evaluating the potential of these materials for biochar production via slow pyrolysis. The pyrolysis process was performed in an electrically heated Horizontal Tube Furnace (HTF) in a nitrogen atmosphere at temperatures of 400 °C, 500 °C, and 600 °C. Following pyrolysis, the characteristics of the resulting biochar were analyzed.

The main conclusions of this work can be summarized as follows:

• All biowaste samples had a high moisture content, ranging from 63.51% to 81.53%, indicating the need for drying prior to undergoing slow pyrolysis;
• Property analysis and SEM imaging revealed that the biowaste types are heterogeneous and differ significantly from each other. Therefore, those biowastes should be collected and processed separately;
• The kitchen biowaste properties were found to be similar to those of food waste studied in other regions of the world and are comparable to those of the typical biomasses used for biochar production through slow pyrolysis;
• Both garden biowaste types may have been contaminated with materials such as soil or rocks during collection. This likely contributed to the high ash content observed (17.75% in spring garden biowaste and 43.83% in autumn garden biowaste), leading to significant deviations from both the literature values for similar wastes and from standard biomass feedstocks used in pyrolysis;
• For all three types of biowaste, an increase in the pyrolysis temperature resulted in a decrease in the biochar yield. The highest mass yields recorded were 36.64% for kitchen biowaste, 66.53% for spring garden biowaste, and 66.99% for autumn garden biowaste;
• Kitchen biochar, in comparison to its raw biowaste, showed an increase in carbon content, fixed carbon, and a higher HHV. The SEM images also indicated that higher temperatures led to larger, more porous biochar particles. These properties make kitchen biochar suitable for applications such as carbon sequestration or enhancing soil by improving water and nutrient retention;
• In contrast, both garden-derived biochars had a lower carbon content and HHV than the original garden biowastes. Furthermore, their ash content was over three times higher than that of the kitchen biochar.

5. Future Perspectives

To complement the conclusions of this study, future research should focus on optimizing the slow pyrolysis process for kitchen biowaste to increase the yield of biochar. In parallel, further exploration of the potential applications for kitchen-derived biochar is needed to better understand its practical utility. Additionally, an economic assessment of the pyrolysis process is essential, as favorable technical outcomes may not always translate into financial feasibility for large-scale implementation.

Taking into account that the current study was conducted on a laboratory scale, future experiments should be performed on a pilot or semi-industrial scale to validate the findings under more practical conditions. There is also a need for more studies on co-pyrolysis involving various waste combinations (e.g., municipal solid waste (MSW) with coal [57], plastics [58], or other biomass [59]) to improve product quality and yield. Further investigation into pretreatment methods, such as chemical activation or moisture control, could improve the efficiency of the process and tailor biochar properties for specific applications. Moreover, the development of integrated systems like pyrolysis combined with combined heat and power (CHP) units, supported by life cycle and techno-economic analyses under different regional conditions, will be critical in ensuring commercial viability and sustainable implementation [59,60].

In the case of garden biowaste, further investigation is needed to determine whether contamination—such as with soil or stones—can be prevented during collection. If such impurities persist in the garden biowaste gathered by residents, it may be necessary to introduce pre-cleaning protocols before the pyrolysis process. Moreover, due to the high yield and ash content observed in garden-derived biochar, the properties of the ash should be investigated. While the ash may contain nutrients that have a positive effect on plant growth, they may also have a negative effect on soil properties, depending on the type of soil to which it is applied.

In addition to experimental methods, computational approaches to biowaste pyrolysis play a key role in process optimization, particularly regarding energy efficiency and product distribution. Mathematical models, such as thermal reaction kinetics [61,62], computational fluid dynamics (CFD) [63,64], and Aspen Plus simulations [65,66], can predict the behavior of heterogeneous biowaste during pyrolysis, including decomposition rates and the distribution of product fractions (gas, oil, and biochar). Thermogravimetric analysis (TG/DTG) also allows for the determination of kinetic parameters, which are crucial for scaling up the process from the laboratory to industrial level [62,67,68]. Computer simulations also enable the assessment of how temperature, heating rate, and residence time affect the quality and composition of the products, thereby supporting the design of more efficient and sustainable biowaste processing systems.