Evolution Mechanisms of Gas-Solid Products in Multi-Source Sludge Pyrolysis: Synergistic Regulation by Temperature and Time Parameters

Abstract

Pyrolysis, as an efficient thermochemical conversion technology, demonstrates substantial advantages in achieving reduction and resource recovery of landfill sludge (LS). This work systematically examined the effects of pyrolysis temperature, residence time, and sludge type on the yield and compositional transformation of pyrolysis gases, as well as the yield and structural characteristics of the derived biochar, using LS and four other types of sludge as subjects. The research results indicate that as the pyrolysis temperature increased from 300 to 900 °C, the total gas yield of the LS sample rose markedly from 11.0 to 139.8 L/kg. The biochar obtained at 600 °C possessed the highest specific surface area (26.327 m²/g), with pore sizes primarily concentrated in the range of 10–20 nm. Extending the residence time facilitated the continuous release of gaseous products but exerted minimal influence on the yield of the solid-phase products. The pyrolysis responses varied considerably among different sludge types. Municipal sludge (MS) exhibited the highest gas production yield (197.5 L/kg), whereas LS demonstrated a greater carbon retention rate (73.7%). This work, based on a systematic analysis of product conversion behaviors, elucidated the correlation mechanism between parameter regulation and product performance during the pyrolysis process, thereby offering theoretical foundations and data support for optimizing LS pyrolysis conditions and enhancing product utilization efficiency.

1. Introduction

The large-scale application of water treatment technologies has led to a substantial increase in sludge generation, which has emerged as a critical bottleneck limiting the sustainable development of urban areas [1]. As an economical and convenient terminal disposal method, sludge landfill technology was widely adopted in its early stages. However, the long-term accumulation of large volumes of landfill sludge (LS) not only occupies land resources but also presents considerable environmental risks [2]. Therefore, the development of efficient reprocessing and resource utilization technologies tailored for LS is urgently required.

Conventional sludge disposal methods include landfilling, composting, incineration, and land application. Despite widespread implementation in engineering practice, these technologies are prone to secondary pollution and high energy consumption, undermining their ability to meet contemporary goals for environmentally sound and sustainable treatment and resource recovery of sludge [3,4]. In recent years, pyrolysis technology, as a thermochemical conversion approach, has attracted widespread attention for its capability to achieve sludge reduction and stabilization while producing high-value-added products [5,6]. The sludge pyrolysis process can simultaneously yield three-phase products, including pyrolysis gas, bio-oil, and biochar [7]. Among these, high-calorific-value gas components such as H₂ and CH₄ in the pyrolysis gas can be utilized as clean energy sources, while bio-oil can serve as both a fuel and a feedstock for chemical synthesis [8]. Biochar, characterized by its well-developed porous structure, not only has potential as a functional material for catalyst carriers and adsorbents but also functions as a soil conditioner to enhance soil quality and carbon sequestration capacity [9,10]. All three-phase pyrolysis products exhibit substantial potential for resource recovery; however, their formation behaviors are strongly influenced by pyrolysis conditions. Inappropriate process parameters may suppress the release and accumulation of target components, consequently diminishing the overall efficiency of resource utilization [11].

The properties of raw materials and pyrolysis conditions jointly determine the progression and characteristics of the pyrolysis reaction product. Pyrolysis conditions encompass parameters including pyrolysis temperature, heating rate, type and flow rate of carrier gas, and residence time. Among them, pyrolysis temperature and residence time are regarded as pivotal regulatory factors, influencing the product distribution, structural evolution, and functional properties [12,13]. Generally, elevating the pyrolysis temperature promotes cracking and condensation reactions of organic components, facilitating the release of gaseous products such as H₂, CH₄, and CO, while simultaneously enhancing the aromatization degree and pore structure development of biochar. For example, sludge biochar prepared by Zhang et al. through pyrolysis at 700 °C exhibited a significantly superior degree of aromatization and structural stability compared to that at 300 °C, along with a notable increase in specific surface area [14]. On the other hand, prolonging the residence time enables more complete material reactions, further improving the degree of carbonization and pyrolysis efficiency. Chen et al. investigated the impact of varying residence times on the yield of three-phase products and found that when the residence time was less than 6 min, the biochar yield decreased progressively with time, while the yields of gaseous and liquid products correspondingly increased [15]. Despite substantial research devoted to elucidating the effects of pyrolysis conditions, significant disparities persist in the reaction pathways and product evolution behaviors across different feedstock types. Particularly, within the complex composition of LS systems, the pyrolysis response mechanism remains unclear. Therefore, a systematic investigation into the regulatory effects of pyrolysis temperature and residence time on the characteristics of LS pyrolysis products is highly significant for elucidating the material transformation pathways during its resource utilization, optimizing process parameters, and enabling the efficient production of target products in a manner that supports sustainable waste management and resource recovery.

In this work, typical LS was used as the raw material, and a programmed-temperature pyrolysis strategy was employed to systematically investigate the regulatory effects of different pyrolysis temperatures and residence times on pyrolysis product characteristics, with the aim of elucidating an extrapolatable effect baseline of the pyrolysis process for multi-source sludge and proposing a tunable operating parameter window. The research primarily focuses on the response patterns of gas yield and its components to changes under varying pyrolysis conditions, as well as the yield and structural evolution characteristics of biochar. The outcomes of this work are expected to provide valuable insights into developing sustainable sludge-to-energy pathways that integrate waste minimization and carbon-neutral strategies. Simultaneously, a comparative analysis was performed to elucidate the differences in products generated during the pyrolysis of various types of sludge. This work aims to advance the sustainable utilization of sludge resources and contribute to the development of environmentally resilient urban systems.

2. Materials and Methods

2.1. Materials

The LS used in this work was sourced from the sludge temporary storage facility of a large solid waste landfill base. Municipal sludge (MS) and landfill leachate sludge (LLS) were collected from a wastewater treatment plant and a leachate treatment plant in Shanghai, respectively, and both samples had undergone mechanical dewatering. The LLS was treated with a solidifying agent (magnesium oxychloride cement + polyvinyl acetate) to produce the solidified sludge (SS1), while another solidified sludge (SS2) was obtained from a landfill site in Shanghai. All five samples were dried to a constant weight in an oven at 105 °C, then ground using a crusher and sieved through an 80-mesh screen, and subsequently stored in a desiccator for further use (Table S1).

2.2. Experimental Setup and Operational Procedures

The sludge pyrolysis experiment was performed in a laboratory-scale horizontal tube furnace (Figure 1). The experimental system comprised a quartz reactor, a horizontal tube furnace, a precision gas flow meter, a drying unit, a particulate removal device, gas bags, and a nitrogen cylinder. Specifically, the pretreated 10 g sludge sample was placed in a high-temperature quartz tube and heated under a nitrogen (N₂) atmosphere at a flow rate of 100 mL/min. The gaseous products were dried using silica gel and collected in gas bags, while the residual solid was identified as biochar. The absolute liquid-phase yield remained consistently < 5 wt% (dry basis), and its effect on product distributions and the main conclusions was negligible. The experiments were conducted under varying conditions determined by different pyrolysis temperatures, residence times, and sludge types.

The LS pyrolysis temperature exploration experiment was conducted in a horizontal tube furnace under various target temperature conditions. A two-stage heating protocol was implemented to isolate drying from devolatilization, thereby ensuring reproducible gas and char yields. The initial temperature was set to 30 °C, then increased to 150 °C at a heating rate of 6 °C/min and maintained for 5 min to eliminate residual moisture from the sludge. Subsequently, the heating rate was adjusted to 15 °C/min, and the heating continued until the set pyrolysis temperature was achieved. The temperature was then maintained for 60 min before the heating was stopped, allowing the system to cool naturally to room temperature. The collection of pyrolysis gas began once the temperature reached the set point and ceased when the temperature started to decrease. Based on the different pyrolysis temperatures (300 °C, 450 °C, 600 °C, 750 °C, and 900 °C), the collected biochars were labeled as LS-300, LS-450, LS-600, LS-750, and LS-900. The pyrolysis temperature was set at 600 °C, and experiments were conducted to investigate the effects of different residence times on LS. All other procedures were consistent with those in the LS pyrolysis temperature experiment. The resulting biochars were named LS₃₀, LS₆₀, LS₉₀, LS₁₂₀, and LS₁₅₀, corresponding to the set residence times of 30 min, 60 min, 90 min, 120 min, and 150 min, respectively. The pyrolysis experiment for different sludge types was conducted following the same procedure as described previously, with the pyrolysis temperature set at 600 °C and maintained for 60 min after reaching the set point. The biochar samples produced were named LS-BC, MS-BC, LLS-BC, SS1-BC, and SS2-BC, corresponding to the initial sludge types (LS, MS, LLS, SS1, and SS2).

2.3. Statistical Analysis

All experimental data were presented as the mean ± standard deviation derived from three independent experiments. The significance of the effects of pyrolysis temperature and residence time on the characteristics of pyrolysis products was evaluated using the Pearson correlation coefficient, with statistical significance defined at p < 0.05.

3. Results

3.1. Effects of Pyrolysis Temperature on Product Distribution, Pyrolysis Gas Composition, and Biochar Properties

As shown in Figure 2a,b, the gas yield during the pyrolysis of LS increased markedly with rising pyrolysis temperature. The total gas yield increased from 11.0 L/kg at 300 °C to 139.8 L/kg at 900 °C, indicating that elevated temperature significantly promoted the formation of pyrolysis gas, composed primarily of H₂, CO, CH₄, CO₂, and C₂ hydrocarbons (C₂H₂, C₂H₄, and C₂H₆) [16,17]. Among these components, H₂ and CO₂ were identified as the predominant species, exhibiting the highest sensitivity to temperature variation. The yield of CO remained relatively low and exhibited negligible change within the lower temperature range (300–600 °C), but increased markedly at elevated temperatures (750–900 °C). In comparison, the yields of CH₄ and C₂ consistently remained at lower levels, with minimal variations in response to temperature changes. It is noteworthy that the yield of H₂ increased rapidly from 0.1 L/kg to 47.5 L/kg, and its relative proportion rose significantly at 450 °C. This phenomenon may be attributed to the enhanced cleavage of C-H bonds under high-temperature conditions and the release of H₂ facilitated by the formation of aromatic compounds [18]. Meanwhile, the CO₂ yield increased steadily from 10.252 to 53.815 L/kg, likely due to the decarboxylation of organic matter and the thermal decomposition of esters and carbonates [19]. Compared to CO₂, the yield of CO remained at a relatively low level within the temperature range of 300–600 °C but rose sharply to 8.4 L/kg at 750 °C and further to 30.2 L/kg at 900 °C, accompanied by a corresponding increase in its relative proportion. The relative proportion of CO₂, in contrast, decreased, which may be ascribed to enhanced Boudouard reaction activity in LS at temperatures above 750 °C [15].

Figure 2c illustrates a notable decrease in biochar yield as the pyrolysis temperature increased, dropping from 87.7% to 63.4%. The result demonstrates that elevated temperatures markedly enhanced the deep cracking and gasification reactions of organic matter in LS, resulting in a substantial decrease in the proportion of solid residues [20]. During the low-temperature stage (300–450 °C), the reduction in biochar yield was mainly attributed to the initial pyrolysis of the sludge and the removal of bound water. The medium-temperature stage (600–750 °C) constituted the active range for pyrolysis reactions. As volatile components were released and organic matter underwent extensive thermal cracking, the pyrolyzable content of the biochar declined markedly, further reducing the solid yield [21]. At the elevated temperature of 900 °C, the organic matter in the sludge was completely decomposed, indicating that high temperature significantly promoted the decomposition of inorganic compounds such as carbonates [22].

To delve deeper into the influence mechanism of pyrolysis temperature on the yield of various gas components, Figure 2d presents a Pearson correlation coefficient heatmap between temperature and the main gas products (H₂, CO, CH₄, CO₂, and C₂) while also revealing the potential correlation relationships among different gas components. The results indicate that the H₂ yield was significantly and positively correlated with temperature (p < 0.0001), further validating that its generation mechanism was predominantly governed by structural fracture and reformation under high-temperature conditions. The CO₂ yield also demonstrated a significant positive correlation with temperature (p < 0.01). Although the Boudouard reaction likely consumed a portion of CO₂ under high-temperature conditions, the overall release continued to increase, indicating that its primary sources were closely associated with the thermal cracking of carboxyl groups and the thermal decomposition of inorganic salts. In the correlation analysis among various gas components, a significant positive correlation was found between CH₄ and C₂ (p < 0.05), suggesting that they may have shared similar generation pathways or both originated from the partial cleavage of aliphatic or chain-like organic structures. It is noteworthy that a significant correlation (p < 0.01) was also observed between H₂ and CO₂, reflecting their consistent response trends to temperature variations. Under high-temperature conditions, they all exhibited a significant release trend, thereby showing a statistically synchronous increase in correlation.

The pyrolysis temperature also exerted a significant influence on the structural characteristics of the solid-phase products. Figure 3a–f presents the SEM images of the original LS and the samples (LS-300 to LS-900) at different pyrolysis temperatures. The surface of the original LS sample was dense and structurally intact, with almost no visible pores, exhibiting a typical muddy block morphology. After pyrolysis at 300 °C, the surface of the LS-300 sample became looser, with localized areas beginning to crack, although the pores were not yet distinctly visible. The LS-450 particles gradually fragmented, and sporadic pores emerged, indicating that some organic components had initiated decomposition, leading to the formation of an initial porous structure [14]. At 600 °C, the LS-600 surface underwent significant structural reconstruction, producing numerous irregular pores. The particle morphology became more porous, with clearly defined pore walls, which indicated that the temperature facilitated organized pore development, likely related to thermal decomposition of organic matter and moderate contraction and structural reorganization of the carbon skeleton [23]. Although the LS-750 sample still retained a porous structure, its pores were slightly denser compared to those of LS-600, with localized thickening of the pore walls and an increasing tendency toward surface densification. This may be attributed to the recrystallization or localized sintering of the carbon skeleton under high-temperature conditions. At 900 °C, significant collapse and particle agglomeration were observed on the surface of LS-900, with some pore structures disappearing and numerous dense block structures being formed. This indicates that high temperatures induced mineral melting and structural consolidation, ultimately prompting pore collapse and blockage [24].

As shown in Figure 3g,h, the nitrogen adsorption-desorption isotherms of all samples displayed typical Type IV curves accompanied by H3-type hysteresis loops, suggesting that the samples were primarily mesoporous, with slit-like or lamellar pore morphologies [25]. The original LS and LS-300 exhibited low adsorption capacities, with relatively flat isotherms, limited pore volumes, and discrete pore size distributions, implying that the pore structures were not yet fully developed. As the pyrolysis temperature increased, the specific surface area gradually rose. Among all samples, LS-600 showed the highest specific surface area (26.327 m²/g), with the pore size mainly distributed between 10 and 20 nm, conforming to the characteristics of typical mesopores [26]. However, the adsorption performance of LS-900 slightly declined, indicating that high temperatures may have caused the contraction or closure of the pore structure. Table S2 provides the detailed pore structure parameters (specific surface area, pore volume, and average pore size) of each sample.

Figure 3i presents the Fourier transform infrared (FTIR) spectra of the original LS and the biochars obtained at different pyrolysis temperatures, revealing the evolution trend of functional groups during the pyrolysis process. The hydroxyl peak near 3420 cm⁻¹ gradually attenuated with increasing temperature, which may have been associated with the decomposition of bound water, carboxylic acids, and alcohols [27]. The C-H stretching vibration peaks of aliphatic -CH₂ and -CH₃ groups at approximately 2920 cm⁻¹ exhibited a progressively weakened trend and nearly disappeared above 600 °C, indicating that the saturated alkyl chains were effectively cleaved and volatilized at elevated temperatures [28]. LS exhibited a distinct C=O (carbonyl) stretching vibration absorption peak at 1650 cm⁻¹, suggesting the presence of oxygen-containing functional groups, such as carboxylic acids, ketones, and esters. The intensity of this peak gradually diminished with increasing pyrolysis temperature and almost completely disappeared at 900 °C, implying that the pyrolysis process involved the continuous cleavage and volatilization of oxygen-containing groups, with the corresponding structures being progressively eliminated as the temperature increased [29]. The aromatic C=C absorption peak at 1455 cm⁻¹ remained relatively stable below 600 °C, weakened after 750 °C, and disappeared at 900 °C, exhibiting a trend similar to that of the aromatic C-H bending vibration at 870 cm⁻¹. This may have been attributed to the cracking or structural reconstruction of the aromatic carbon skeleton at higher pyrolysis temperatures, leading to a weakening of the degree of aromatization [30]. The Si-O stretching vibration peak at 1040 cm⁻¹ remained stable over the entire temperature range, indicating the persistent contribution of amorphous minerals and inorganic salts in this region [31]. Additionally, Table S3 summarizes the temperature-dependent variations in the inorganic composition of LS and its derived biochar. The relative contents (XRF, dry basis) of Si, Mg, and Na increase with temperature, consistent with ash concentration accompanying organic matter removal and the enrichment of thermally stable mineral phases. In contrast, K, Fe, S, and related elements decrease progressively, plausibly due to volatilization losses or temperature-induced phase transformations. Collectively, the aforementioned results indicate selective, temperature-driven migration and enrichment of inorganic constituents during pyrolysis [32].

3.2. Effects of Residence Time on Product Distribution, Pyrolysis Gas Composition, and Biochar Properties

Figure 4a shows that the gas yield was 33.1 L/kg at 30 min and then increased rapidly over time, reaching 85.0 L/kg at 90 min, indicating that the pyrolysis reaction was more vigorous during the initial stage due to the higher reactivity of organic matter decomposition. After 90 min, the growth rate of gas production significantly slowed down, and by 150 min, it had only increased to 96.2 L/kg, approaching a plateau. This indicates that the pyrolysis reaction was gradually approaching its endpoint, with the volatile components essentially depleted and the residual carbonaceous structure difficult to further decompose. Among the major gas components, CO₂ and H₂ remained dominant, with their yields increasing from 13.2 L/kg and 13.7 L/kg to 54.2 L/kg and 31.2 L/kg, respectively, suggesting the continuous breakdown of related oxygen-containing functional groups and carbon-hydrogen structures over time. In comparison, the yields of CH₄ and C₂ remained consistently low with minimal fluctuations, implying that they primarily originated from the initial cleavage of aliphatic structures and were insensitive to the extension of residence time. It is noteworthy that the CO yield consistently remained at an extremely low level, which may have been attributed to the pyrolysis temperature of 600 °C being insufficient to drive the formation reaction. In Figure 4b, the relative content distribution of each component showed little change, further indicating that extending the residence time did not significantly alter the reaction mechanism, and the gas composition remained stable [33].

As shown in Figure 4c, as the residence time increased from 30 min to 150 min, the yield of LS biochar gradually decreased from 75.7% to 72.5%, with an overall change of less than 5%. The results indicate that the primary solid-phase cracking reactions were largely completed in the initial stage, and extending the residence time had a limited impact on the further depletion of biochar. The subsequent pyrolysis process was characterized more by the accumulation of gaseous products rather than the significant depletion of biochar [34].

To investigate the regulatory effect of residence time on the release behavior of various gas components, Figure 4d presents a Pearson correlation coefficient heatmap between residence time and the volume yield of major gases. The results show that the residence time was significantly positively correlated with the yields of H₂ and CO₂ (p < 0.05), indicating that the corresponding functional groups continued to decompose as the residence time increased. Meanwhile, CH₄ also exhibited positive correlations with CO₂, C₂, and H₂ (p < 0.05), suggesting that it may have partially originated from the decomposition processes of similar structural units. It is noteworthy that C₂ exhibited a stronger correlation with CO₂ (p < 0.01), indicating a potential coupling between the cleavage of oxygen-containing groups and the fracture process of unsaturated carbon skeletons. Further analysis reveals that the correlation between H₂ and CO₂ was the most significant (p < 0.001), reflecting their highly synchronized release patterns over time.

Figure 5a–f presents the evolution of the microstructural characteristics of LS and its derived biochar at different residence times (30–150 min). With the extension of residence time, LS₃₀ particles began to disperse, and a small number of cracks and fracture structures emerged at the edges. When the duration was further prolonged to 60 min and 90 min, the surface roughness of the LS₆₀ and LS₉₀ samples increased, with noticeable cleavage traces and the formation of numerous irregular pores and lamellar structures. This indicates that the pyrolysis process entered an active phase, in which the cracking of organic matter and the release of gases resulted in the reorganization of the skeletal structure [35]. As the residence time was extended to 120 min and 150 min, the LS-120 and LS-150 particles exhibited pronounced agglomeration, with their pore boundaries gradually becoming less distinct. This suggests that pyrolysis had entered a structurally stable phase, characterized by reduced surface activity, with the material gradually transitioning into a denser state [36].

As shown in Figure 5g, all samples at different residence times exhibited Type IV adsorption curves accompanied by H3 hysteresis loops, with an open mesoporous structure predominated by irregular flake-like fissures [37]. Figure 5h shows that the pore sizes of all samples were predominantly distributed within the range of 10–20 nm, exhibiting a concentrated and stable distribution pattern [38]. Compared to the original LS, the adsorption capacity of the pyrolysis products was significantly enhanced, reflecting that the pyrolysis reaction induced the continuous generation and expansion of pores. From the adsorption trend, the adsorption capacity of the LS₃₀ sample was significantly higher than that of the original LS, while those of LS₆₀ and LS₉₀ were further enhanced, suggesting an adequate number of mesopores and improved connectivity. As indicated by the data in Table S4, the specific surface area (26.793 m²/g) and pore volume (0.104 cm³/g) of LS₉₀ did not differ significantly from those of LS₆₀ (26.327 m²/g, 0.098 cm³/g), indicating that the pore structure had stabilized within the pyrolysis period of 60–90 min, and the marginal effect of structural regulation was diminishing. In comparison, although LS₁₂₀ and LS₁₅₀ maintained a high pore volume, their specific surface area showed a slight decrease, suggesting the occurrence of pore wall thickening or partial pore channel collapse [39].

Figure 5i illustrates the FTIR spectral characteristics of the samples at different residence times. Overall, the absorption peaks of the main functional groups generally exhibited a weakening trend as the residence time was extended. Among these, the absorption peak of the hydroxyl (-OH) stretching vibration near 3420 cm⁻¹ continuously attenuated, indicating that oxygen-containing groups such as bound water, carboxylic acids, and phenolic hydroxyl groups continuously decomposed and volatilized during the pyrolysis process, leading to a gradual reduction in the oxygen content of the system [40]. The aliphatic C-H stretching vibration peak located at 2920 cm⁻¹ essentially disappeared at 30 min, indicating that the saturated alkyl structure was effectively cleaved during the initial stage of pyrolysis, generating volatile small molecular products [41]. The absorption peak at 1650 cm⁻¹ corresponded to the stretching vibration of the carbonyl group (C=O). This peak gradually weakened with increasing residence time, reflecting the poor thermal stability of ketone, aldehyde, and carboxylic acid structures, which were progressively cleaved under continuous pyrolysis conditions [42]. The absorption peaks at 1455 cm⁻¹ and 870 cm⁻¹ corresponded to the stretching vibration of the aromatic C=C skeleton and the out-of-plane bending vibration of the aromatic C-H, respectively. The two characteristic peaks showed minimal changes within 30–60 min, indicating that the aromatic structure possessed a certain degree of thermal stability. However, it significantly weakened from the 90 min mark and became almost imperceptible by 150 min. This suggests that extending the residence time could further disrupt the aromatic carbon skeleton, leading to a reduction in its characteristic infrared absorption [43].

3.3. Effects of Sludge Type on Product Distribution, Pyrolysis Gas Composition, and Biochar Properties

As shown in Figure 6a, the gas yield and component distribution of different types of sludge displayed notable differences under identical pyrolysis conditions. Among them, MS demonstrated the highest overall gas production performance, with H₂, CH₄, CO₂, and C₂ yields reaching 44.4, 47.5, 85.4, and 20.2 L/kg, respectively, which were significantly higher than those of other samples. This gas production advantage may have been attributed to its higher organic matter content and more complex molecular structure, which provided abundant cleavable groups during pyrolysis and enabled diverse reaction pathways, facilitating the concurrent release of multiple gaseous products. In comparison, the gas release from LS was composed predominantly of H₂ (23.9 L/kg) and CO₂ (35.5 L/kg). The relatively low CH₄ and C₂ yields indicate that the pyrolysis reaction was dominated by aromatization and decarboxylation, with a paucity of saturated aliphatic structures, constraining its gas generation potential [44]. Similarly, the overall gas yield of LLS was relatively low, indicating that its organic matter exhibited poor reactivity, and the gas production primarily originated from the thermal decomposition of inorganic salts. In the case of SS1, which was treated with “magnesium oxychloride cement + polyvinyl acetate”, the release of CH₄, C₂, and H₂ showed a slight increase compared to LLS. According to the elemental composition in Table S5, SS1 exhibited significantly higher Mg and Cl contents than LLS. The enhanced gas release may have originated from the synergistic catalytic effect of Mg and Cl elements in the curing agent [45,46]. As for SS2, CO₂ still predominated, with the yields of H₂ and CH₄ being 16.6 L/kg and 2.0 L/kg, respectively, which were at a medium to low level. This outcome may have been related to the prolonged natural stacking of its raw materials and the enhanced structural stability, which consequently led to a reduction in the available sites for pyrolysis reactions and a limitation in gas production capacity.

Figure 6b presents the variations in the relative composition of gas products obtained from different types of sludge under identical pyrolysis conditions. Overall, CO₂ was the most abundant gas component in all samples. In the MS sample, CO₂ accounted for 51.0%, and the proportions of reductive gases such as CH₄, C₂, and H₂ were also relatively high, significantly outperforming those in LS, indicating a richer organic matter content and more diverse pyrolysis pathways. In comparison, the proportion of CO₂ in LLS and SS1 increased to 86.8% and 70.9%, respectively. The proportion of combustible components such as H₂, CH₄, and C₂ significantly decreased, indicating a lower organic content, with the release of inorganic carbon sources predominating during the pyrolysis process. It is noteworthy that although the CO₂ proportion in the SS2 sample was slightly lower than that in SS1, its H₂ content was relatively higher, suggesting that the pyrolysis process of this sample may have been dominated by the cleavage of carbon-hydrogen bonds [47].

As shown in Figure 6c, the LS sample exhibited the highest biochar yield, reaching 73.7%, which was significantly higher than that of other types of sludge, indicating that its organic matter structure was more prone to carbonization and tended to form a stable carbon skeleton structure during the pyrolysis process [48]. In comparison, MS exhibited the lowest biochar yield at only 50.42%, likely due to its feedstock’s high content of volatile components, which underwent intense cracking and gasification reactions during pyrolysis, thereby reducing residual carbon content. Such discrepancies may have been attributed to differences in their pretreatment methods and compositional makeup among the samples. LS refers to long-term stockpiled landfill sludge, which tends to have a stable structure, is structurally stable, and contains a higher proportion of non-volatile components. In contrast, MS is fresh activated sludge that is rich in readily degradable organic matter [41]. In the other samples, the biochar yields of LLS, SS1, and SS2 increased sequentially, with SS2 reaching a yield of 65.90%, second only to LS. This may have been related to its preliminary curing treatment, as the curing process enhanced the thermal stability of the raw materials, promoted the formation of a dense carbon skeleton, and thereby increased the carbon retention rate. Additionally, Table S5 presents the XRF elemental analysis results of different sludge samples and their corresponding biochars. Overall, the pyrolysis process generally promoted the enrichment of thermally stable elements (e.g., Si, Mg, Al) in the residual solids, whereas elements with lower stability or higher volatility (e.g., K, Fe, S, Cl) exhibited varying degrees of migration and loss.

3.4. Perspectives

This work systematically examines the effects of pyrolysis temperature, residence time, and sludge type on the distribution and structural evolution of pyrolysis products and elucidates the synergistic coupling among organic matter cracking, gas release, and biochar pore reconstruction, thereby providing a mechanistic basis for process parameter optimization. However, pyrolysis technology still faces multiple challenges in real-world engineering applications. Firstly, as a multi-source solid waste, the composition of LS is strongly influenced by factors such as geographic region, landfill age, compaction degree, and pretreatment practices, resulting in pronounced heterogeneity. It tends to induce fluctuations in pyrolysis reaction pathways and product properties, restricting the applicability of fixed process parameters in large-scale operations. Secondly, although the experiments were conducted under well-controlled laboratory conditions, issues such as nonuniform thermal fields, residence time fluctuations, and multiphase flow disturbances remain prevalent in industrial pyrolysis processes, hindering the replication of the pyrolysis behavior patterns. Pilot tests on untreated, real sludge with stepwise scale-up from pilot to full scale, together with quantification of key indicators, are essential for establishing reliable process guidance. Subsequent research should integrate TG-MS and in situ XPS, combined with kinetic fitting models, to elucidate the evolution pathways of intermediate products across reaction stages and the mechanisms of product formation. Furthermore, with a focus on the end product, the quality of the solid-phase product, biochar, is enhanced by optimizing pyrolysis process parameters, promoting its practical application in environmental remediation and catalytic materials. Further expand the scope of sample sources and conduct pilot-scale verification close to actual working conditions, comprehensively considering engineering factors such as heat and mass transfer, gas-solid flow, and reaction kinetics, to facilitate the scale-up and stabilization of pyrolysis technology.

4. Conclusions

This work conducted a comprehensive evaluation of the evolution behavior and structural response characteristics of pyrolysis products in multi-type sludge systems represented by LS, with a focus on analyzing the influence mechanisms of temperature, residence time, and feedstock differences on the pyrolysis process. The research findings demonstrate that elevated pyrolysis temperatures significantly accelerated the thermal cracking of volatile components, substantially increasing the yield of gaseous products. The biochar obtained at 600 °C exhibited the optimal pore structure and the largest specific surface area (26.327 m²/g), reflecting the synergistic interaction between pyrolysis reactions and pore development. However, upon further heating, the carbon skeleton was reconstructed, and the pore structure showed a tendency toward collapse and densification. The prolongation of residence time positively influenced the continuous release of gaseous products, particularly H₂ (increasing from 13.7 to 31.2 L/kg) and CO₂ (increasing from 13.2 to 54.2 L/kg). However, the impact on biochar yield was limited, and its structural stability was primarily maintained within the 60–90 min interval, with a slight decrease in specific surface area after 90 min. There were significant differences in the behavior of different sludge types during pyrolysis: MS exhibited a higher potential for gas production, while LS demonstrated a higher carbon retention rate and stronger structural stability, making it more suitable for the biochar resource utilization pathway. In summary, this work elucidates the regulatory roles of key pyrolysis parameters in sludge transformation from multiple perspectives, including gas-solid product behavior, structural evolution, and feedstock variability. It minimizes waste generation, enhances energy recovery, enables long-term carbon sequestration, and produces biochar that delivers sustainable environmental benefits, promoting a closed-loop circular economy and providing process guidance for the application of pyrolysis in the field of sustainability.