Thermoecological Cost Analysis of Hydrothermal Carbonization for Valorization of Under-Sieve Fraction from Municipal Solid Wastes ^†

Abstract

Municipal solid waste (MSW) management poses significant challenges due to the generation of organic waste materials, including the under-sieve fraction (USF), which contains high moisture and organic content. Hydrothermal carbonization (HTC) has emerged as a promising technology for converting USF into hydrochar (HC), a valuable energy-rich material with improved combustible properties. Despite the potential of HTC for waste valorization, comprehensive studies on the thermoecological cost (TEC) and environmental implications of applying HTC to USF are limited. In this study, a detailed analysis of the TEC associated with the HTC process applied to USF from MSW was conducted. The TEC assessment was conducted considering varying dilution ratios (DS/W), operational temperatures (180–220 °C), and reaction times (1–8 h) to evaluate the energy efficiency, resource utilization, and environmental impact of the HTC process. Comparative assessments were made with alternative USF treatment methods, such as bio-stabilization, landfilling, and wastewater treatment. The results indicate that the optimal conditions for minimizing TEC are a temperature of 180 °C, a reaction time of 1 h, and a dilution ratio of 0.15, achieving a TEC value of approximately 9.25 GJ per ton of USF. This represents a significant reduction compared to the conventional treatment methods, which showed a TEC of 14.9 GJ/ton of USF. This study provides a comprehensive comparison of HTC with alternative USF treatment methods, such as bio-stabilization and landfilling, highlighting HTC’s superior energy efficiency and environmental sustainability. These findings offer valuable insights into the energy consumption, resource utilization, and environmental impact of HTC, emphasizing its potential for sustainable waste valorization.

1. Introduction

The under-sieve fraction (USF) is a by-product of the mechanical treatment of mixed municipal solid waste (MSW) aimed at producing a combustible stream or solid recovered fuel (SRF). USF has a high moisture content and a significant amount of biodegradable organic matter. Therefore, it can be effectively treated by hydrothermal carbonization (HTC) to produce hydrochar (HC) with enhanced combustible properties. HTC is a thermo-chemical conversion method that transforms organic feedstocks into valuable, energy-rich material [1]. This process pre-treats biomass with high moisture content, making it suitable for various applications. The solid material produced by HTC, known as HC, is rich in carbon. HTC processes yield a solid char with a higher energy density that is easily friable and more hydrophobic than the original substrate [2,3]. This method is promising for utilizing biomass or waste for cleaner production [2,3,4,5]. The application of HTC for the valorization of USF from MSW is a relatively unexplored area in waste management research. Preliminary investigations on various waste feedstock have shown potential for this process [6,7,8,9,10,11]. However, both the comparison of the most appropriate process conditions (temperature, dilution rate, and duration) and the different fate of the generated HC and process water (PW), should be analyzed considering the overall sustainability of the system, to provide clues for future industrial realization. While HTC has been extensively studied for biomass conversion and waste treatment, comprehensive studies specifically focusing on the conversion of USF using HTC technology are lacking. This gap highlights the need for a detailed investigation into the feasibility, efficiency, and environmental implications of applying HTC to USF valorization. Conducting a thermoecological cost (TEC) analysis is essential to evaluate the environmental performance of the proposed HTC process for USF valorization. The TEC analysis offers a holistic approach to assessing the sustainability and environmental impact of fuel production processes by quantifying the cumulative consumption of non-renewable exergy and considering the environmental losses associated with waste generation and disposal [12]. In the context of HTC for USF valorization, the TEC analysis can provide valuable insights into the energy efficiency, resource utilization, and overall environmental footprint of the process. Although research in this area is still relatively new, several studies of sustainability analysis of waste valorization routes employing the HTC process have shown promising results [13]. The study [14] uses a life cycle assessment (LCA) approach to evaluate the environmental impacts associated with the HTC of food wastes and subsequent HC combustion for energy production. Key findings include the significant influence of HTC process water emissions and HC combustion on environmental impact, with a net negative global warming potential (GWP) for all evaluated substituted energy sources except biomass. Other authors [15] evaluated the environmental impacts of HTC for treating olive mill wastes compared to composting, anaerobic digestion, and incineration. Results showed that HTC with energy recovery is environmentally beneficial, but process water management is critical.

Owsianiak et al. [16] assessed the HTC of green waste, food waste, organic fractions of MSW, and digestate using LCA. Results indicate that HC from green waste performs best in several impact categories due to low transportation needs and optimized pumping efficiency, while HC from the organic fraction of MSW has high potential impacts on human health and ecosystems due to toxic elements in ash disposal. Recently, Mendecka et al. [13] carried out an LCA of the HTC process applied to olive pomace. In this study, 12 different HTC valorization concepts of olive pomace were analyzed in detail based on empirical data. The results showed that the substitution of the products from marginal processes with HTC products leads to environmental load savings for all the analyzed impacts (climate change, acidification, freshwater toxicity, and eutrophication). Another recent study [17] presents LCA findings on HC production from Saudi Arabian date palm frond biomass. The study highlights the significant environmental impact of fossil fuel usage in HTC and drying processes, suggesting optimization strategies to reduce this impact. A study [18] modeled an industrial-sized HTC plant for biowaste, integrating all components including heat recovery and process water treatment. The findings suggested that HTC-char is more environmentally friendly than lignite for electricity production. Zhang et al. [19] assessed the environmental impact of electricity generation from sugarcane bagasse HC via microwave-assisted HTC. The results highlighted significant contributions to climate change and fossil depletion due to energy consumption during the process. These studies provide a comprehensive view of the environmental implications of HTC, showcasing its potential benefits and challenges across different feedstocks and operational conditions.

Apart from the conventional LCA of HTC applied to USF, which is reported elsewhere [20,21], this work is focused on the application of the TEC method, for a comprehensive evaluation of the environmental performance of the proposed process. Overall, the combination of limited studies on HTC for USF valorization and the rationale for conducting a TEC analysis underscores the importance of this research in advancing our understanding of sustainable waste management practices and promoting the adoption of innovative technologies for converting organic waste materials into valuable resources.

The primary objective of this research is to evaluate the TEC associated with the HTC process applied to USF from MSW. The study examines various operational parameters, including dilution ratios (DS/W), operational temperatures, and reaction times. By analyzing these parameters, the research aims to:

• Quantify the TEC of HTC under different process conditions.
• Compare the TEC of HTC with alternative USF treatment methods, such as bio-stabilization, landfilling, and wastewater treatment.
• Identify the optimal conditions for minimizing TEC, thereby enhancing energy efficiency and environmental sustainability.

This comprehensive analysis provides valuable insights into the feasibility and environmental performance of HTC for waste valorization, offering practical insights into optimizing waste valorization strategies and promoting the adoption of innovative technologies for organic waste conversion. The remaining part of the paper is organized as follows. Section 2 describes the reference USF hydrothermal carbonization process concept and TEC modelling approaches proposed in this work. Section 3 presents and discusses the results from this study. Finally, Section 4 provides the concluding remarks.

2. Materials and Methods

2.1. Thermoecological Cost (TEC) Analysis

TEC has been defined by Szargut [22,23] as the cumulative consumption of non-renewable exergy connected with the fabrication of a particular product with the additional inclusion of the consumption resulting from the necessity of compensating the environmental losses caused by the rejection of harmful waste substances to the environment. In the basic form, the boundary of the system analyzed by TEC, following cumulative calculus approach, reaches the level of primary resource extraction and includes chain of the production processes leading to the product under consideration.

The balance of TEC of j-th production branch also includes an additional consumption of resources connected with the waste rejection to the environment p_kj. This additional consumption is linked to the maintenance and operation of abatement installations as well as from the necessity of compensation of other losses in the environment. Under these assumptions, the thermo-ecological cost (${\rho }_j$) for a given system boundary can be presented by Equation (1) [12, 22–23]:

$${\rho }_j+{\displaystyle \sum }_i\left(f_{ij}-a_{ij}\right){\rho }_i={\displaystyle \sum }_s{b}_{sj}+{\displaystyle \sum }_k{p}_{kj}{\zeta }_k$$

(1)

where:

• a_ij coefficient of the consumption of the i-th product per unit of the j-th major product, e.g., in kg/kg or kg/MJ,
• f_ij coefficient of the consumption and by production of the i-th product per unit of the j-th major product, e.g., in kg/kg or kg/MJ,
• b_sj exergy of the s-th non-renewable natural resource immediately consumed in the process under consideration per unit of the j-th product, MJ/kg,
• ρ_i specific thermo-ecological cost of the i-th product, e.g., in MJ/kg,
• $p_{kj}$ amount of k-th harmful substance from j-th process, kg,
• ${\zeta }_k$ thermoecological cost of k-th harmful substance, MJ/kg.

The basic TEC concept can be further extended to the whole life cycle comprising the construction, operational and decommissioning phases of the entire production chain. Similarly to LCA, TEC analysis requires the input data inventory analysis which explores all necessary energy and material flows (inputs) as well as products, co-products, emissions, and waste (outputs) exchanging with the system boundary considered and the environment. In this study, inventory analysis was developed using both primary and secondary data. Primary data for the main processes in the foreground system were given by experimental trials [11]. HTC yields were obtained from laboratory scale discontinuous process, performed in a batch reactor. Products and substrates were analyzed by means of its compositions and energy content.

The cumulative exergy demand (CExD) approach proposed in [12] which accounts for the total exergy consumption burdening all of production stages—from the primary resources consumption, through the transportation and semi-finished products fabrication to the final considered useful product—was used to characterize coefficients of consumption in Equation (1). CExD is specified in MJ-equivalents per unit of product. The balance equations to apply this method is explained bellow in Equation (2):

$${b_j}^{*}={\displaystyle \sum }_i(a_{ij}-f_{ij})b_i^{*}+b_j$$

(2)

where:

• ${b_j}^{*}$ unknown index of cumulative exergy burdening the fabrication of j-th useful products;
• $b_i^{*}$ index of exergy cumulative consumption burdening i-th main product consumed in j-th production branch;
• $a_{ij}$ index of specific consumption of i-th product per unit of product j-th, e.g., kg i/kg j;
• b_j index of direct primary exergy consumption in j-th branch, e.g., MJ/kg j;
• f_ij index of specific by-production production of i-th by-product per unit of j-th main product.

The secondary data related cumulative consumption of non-renewable exergy of the background processes, namely transport, electricity and heat generation, and raw material production, as well as waste treatments, developed by Bosch et al. [24], are retrieved from the Ecoinvent database version 3.2 with the cut-off system model. A detailed approach to the calculation of the missing characterization factors of CExD for the liquid phase leaving the HTC process and for the HC is presented in [20].

For consistency, average EU mixes regarding electricity, materials and other resources were considered. The USF as an input element is treated as a waste. Thus, it is assumed as a zero-burden input of the system and so, in Equation (2), only direct primary exergy consumption is considered in this case [25,26]. By completion of the calculations, all quantities accounted for in the inventory were reported per functional unit, namely the treatment of 1 t of USF.

The direct exergy of input waste and HC is calculated by means of an exergetic content of a solid biomass [27,28,29]. The method proposed by Kotas is utilized in this paper, where the calculation of waste and HC exergy content is based on the elemental composition of C, H, O, and N:

$$b_{sj}\left(\mathrm{MJ}/\mathrm{kg}\right)={\beta }_{sj}·LHV\left(\mathrm{MJ}/\mathrm{kg}\right)$$

(3)

where LHV is the lower heating value of the HC, while β_sj is an indicator based on statistical methods applied to produce a correlation equation studied on a large number of organic compounds and fuels. The used equation is given below:

$${\beta }_{sj}=1.0437+0.1896·{\displaystyle \frac{Z_H}{Z_C}}+0.0617·{\displaystyle \frac{Z_O}{Z_C}}+0.0428·{\displaystyle \frac{Z_N}{Z_C}}$$

(4)

2.2. Laboratory Scale Primary Data

The USF was collected at an Italian MBT plant in the metropolitan area of Florence in Sesto Fiorentino (FI), which processes mixed MSW. Initially, the residual MSW is stored in a bunker, from where it is fed to a primary grinder. Next, a magnetic separation is applied to remove metals. The mainstream passes through a rotating sieve drum with openings of 60 mm. Particles larger than 60 mm, after undergoing a second metal removal, are used for solid recovery fuel (SRF) production. The fraction smaller than 60 mm, referred to as USF, was collected and manually sieved through 20 mm openings.

Table 1. The material composition of the under-sieve fraction.

Material	Composition, %
Paper	8.41 ± 1.97
Plastics	5.48 ± 0.41
Glass	1.73 ± 0.33
Wood	1.82 ± 0.54
Textiles	1.72 ± 0.44
Food waste	2.96 ± 0.52
Coffee pods	1.13 ± 0.18
Inerts	1.27 ± 0.52
Metals	0.84 ± 0.30
Fine fraction < 20 mm	74.64 ± 2.52

reports the USF composition.

As presented in Table 1, the considered USF consist mainly of mixed fine fraction. The remaining part with a size larger than 20 mm consisted of paper, plastics, glass, wood, textiles, food waste, coffee pods, inert materials, and metals.

Hydrothermal carbonization was conducted under an experimental set-up consisting of a stainless steel Zipperclave^® Stirred Reactor equipped with a built-in stirrer by Parker Autoclave Engineers, Erie, PA, USA [30]. The reactor volume was 1000 mL, and the equipment was cooled down by an internal cooling coil which allowed an immediate decrease in temperature, halting ongoing reactions. The HTC process was performed on USF considering 2 levels of dilution (0.07 and 0.15), 3 different operative temperatures (180–200–220 °C), 3 residence times (1, 4, and 8 h), and DS/W 142 ratios (0.15 and 0.07), for example: 180_1h_0.15.

The averaged results of ultimate and proximate analyses of USF and HCs as well as yields and product distributions obtained for different process temperature, after 8 h residence time are summarized in

Table 2. The ultimate analysis of USF and HCs (C—carbon, H—hydrogen, N—nitrogen, S—sulfur, O—oxygen).

Name	C, %	H, %	N, %	S, %	O, %
USF	36.0 ± 2.0	5.2 ± 0.4	1.3 ± 0.1	0.2 ± 0.2	18.2 ± 6.6
180_8h_0.15	27.3 ± 0.9	3.3 ± 0.1	0.75 ± 0.04	0.74 ± 0.03	10.5 ± 0.5
200_8h_0.15	32.3 ± 6.5	4.0 ± 1.0	1.0 ± 0.1	0.5 ± 0.4	10.3 ± 0.3
220_8h_0.15	40.5 ± 1.2	4.77 ± 0.06	1.33 ± 0.04	0.30 ± 0.03	7.1 ± 0.9
180_8h_0.07	36.4 ± 4.7	4.9 ± 0.6	0.9 ± 0.1	0.05 ± 0.03	21.5 ± 2.4
200_8h_0.07	34.6 ± 2.4	4.4 ± 0.3	0.93 ± 0.05	0.3 ± 0.3	14.0 ± 1.1
220_8h_0.07	32.5 ± 8.4	3.9 ± 0.9	1.2 ± 0.2	0.3 ± 0.2	7.7 ± 0.8

,

Table 3. The proximate analysis, higher and lower heating values of USF and HCs.

Name	A, %	VM, %	FC, %	HHV, MJ/kg	LHV, MJ/kg
USF	39.2 ± 7.9	51.7 ± 5.7	9.2 ± 4.6	14.6 ± 0.9	13.7 ± 1.1
180_8h_0.15	57.4 ± 1.6	40.4 ± 0.4	2.2 ± 2.0	10.7 ± 0.2	10.0 ± 0.1
200_8h_0.15	52.0 ± 7.5	42.7 ± 3.7	5.3 ± 3.8	14.4 ± 3.1	13.4 ± 2.6
220_8h_0.15	46.1 ± 0.3	42.9 ± 0.2	11.0 ± 0.1	18.5 ± 0.2	17.4 ± 0.2
180_8h_0.07	36.4 ± 7.8	53.6 ± 6.0	10.0 ± 1.8	15.7 ± 2.2	14.6 ± 2.1
200_8h_0.07	45.7 ± 4.2	45.4 ± 2.7	8.9 ± 1.5	15.2 ± 1.6	14.2 ± 1.5
220_8h_0.07	54.5 ± 10.5	35.1 ± 7.5	10.4 ± 3.0	14.9 ± 3.9	14.0 ± 3.7

and

Table 4. The yields and product distribution of the hydrothermal carbonization process.

Name	Mass Yield, %	Solid, %	Liquid, %	Gas + Loss, %
180_8h_0.15	66.34 ± 1.21	8.65 ± 0.16	87.02 ± 1.96	4.32 ± 2.12
200_8h_0.15	57.81 ± 1.71	7.54 ± 0.22	86.54 ± 1.56	5.92 ± 1.34
220_8h_0.15	44.83 ± 0.90	5.85 ± 0.12	86.42 ± 0.44	7.73 ± 0.56
180_8h_0.07	59.81 ± 3.43	3.91 ± 0.22	92.62 ± 1.59	3.47 ± 1.81
200_8h_0.07	56.14 ± 0.74	3.67 ± 0.05	93.10 ± 0.68	3.23 ± 0.73
220_8h_0.07	44.11 ± 0.86	2.89 ± 0.06	93.27 ± 1.02	3.84 ± 0.96

[11].

2.3. USF Hydrothermal Carbonization Process Concept

The results of the HTC tests were used for the preliminary design of the HTC process layout, in consideration of a hypothetical plant which can treat 40,000 t/year of USF from residual MSW. The concept of HTC was developed and modeled regarding design schemes suggested in the literature [13,31]. Figure 1 illustrates the proposed layout of the HTC process for the USF from MSW. Key components include the mixer, reactor, heat exchanger, centrifuge, air dryer, and wastewater treatment plant. The layout demonstrates the flow of materials and energy through the HTC process, highlighting the integration of various subsystems to enhance process efficiency.

In this scheme, USF is first prepared to reduce and homogenize the particle size; then it is mixed with water to reach the desired DSW ratio. Downstream of the mixer, a pump raises the pressure to feed the slurry to the HTC continuous reactor, a regenerative heat exchanger (HE) preheats the slurry, while an external heater raises its temperature to the required level for HTC. From the HTC reactor, the slurry, containing HC and the aqueous products, and gases, formed during the reactions, exits. The exiting slurry is conveyed to a centrifuge, where liquid–solid separation is performed. The HC is transferred to an air dryer and is then pelletized, making it ready to be used as fuel. Before entering the centrifuge, the slurry passes through the regenerative HE mentioned above. External heating is required to raise the air temperature to the desired value. The liquid phase exiting the centrifuge is sent to a wastewater treatment plant (WWTP).

Different scientific works were consulted to derive the appropriate equations to estimate every single consumption on an industrial scale [13,32]. The equipment involved in the HTC process was studied and electricity and thermal consumption were evaluated. The energy balance was prepared for an HTC process, while considering a hypothetical plant with the ability to treat 40,000 t/y of USF and operational time of 8000 h/y. The plant energy consumption and the volume of the reactors were measured using the simplified methods provided by [32]. All inventory data for the LCA supporting the TEC are extracted from [20].

In this study, HTC layout and a standard solution to treat USF are compared to evaluate the benefits and drawbacks in terms of TEC. Figure 2 shows the two studied layouts highlighting the differences in process flow, treatment stages, and their boundary system conditions in terms of TEC. Case 0 represents the current processing method involving biostabilization and landfilling (Figure 2a). Case 1 is the proposed HTC process (Figure 2b).

HTC process was investigated by means of three process temperatures (180–200–220 °C), three residence times (1–4–8 h), and two DB/W ratios (0.07 and 0.15).

3. Results

The results of the TEC analysis for the USF HTC process under various operational conditions are presented. The direct exergy consumption of USF and the exergy content of HC were calculated using Equations (2) and (3), with data presented in Table 2. The exergy content of waste and direct exergy consumption equaled 14,908 MJ/ton of USF.

The current processing (Case 0) of USF involves aerobic biostabilization, landfilling, and a wastewater treatment plant. Figure 3 presents the results of the TEC analysis for Case 0. The left part of this figure displays the total TEC of the current USF treatment process, excluding direct exergy consumption. The right part shows the TEC emissions, identifying the major contributors such as SO₂, PM, and NO_x.

The total TEC of the processed USF is 73.88 MJ/t. The two largest contributions are given by the electricity consumption of the aerobic biostabilization while the negative value is derived from landfill biogas. The total TEC for this process was calculated to be 73.88 MJ/t. Major contributors to TEC emissions included SO₂ (0.84 MJ/t), particulate matter (PM) (0.30 MJ/t), and NO_x (0.13 MJ/t).

Figure 4 and Figure 5 present TEC analysis results for USF HTC treatment (Case 1) three different operative temperatures (180–200–220 °C), and three different operative times (1–4–8 h). The left part of Figure 4 presents the process contributions to TEC for the HTC treatment under a 0.07 dilution ratio. The right part illustrates the TEC emissions for the same conditions, indicating the impact of different process parameters. Like Figure 4, Figure 5 shows the process contributions (left) and TEC emissions (right) for the HTC treatment under a 0.15 dilution ratio, providing a comparison with the 0.07 dilution ratio.

Regarding the TEC consumptions (electrical, thermal, water, chemical compounds, and HC produced) related to the processes analyzed, Figure 4 and Figure 5 show the specific results related to a ton of processed USF. As can be seen, the exergy content of the USF is the highest contributor. The TEC approach, in contrast to other environmental impact assessments, allows us to include this content in the analysis. The HTC process exhibited lower emissions compared to the base case. SO₂ emissions were reduced to negligible levels, thereby minimizing the overall environmental impact. Apart from direct exergy consumption from USF, drying and reaction occurring during HTC have the highest consumption, as shown in Figure 4 and Figure 5. This is mainly because of high thermal and electrical energy consumption.

Figure 6 compares the TEC balance for the conventional treatment process and HTC with different dilution ratios (0.07 and 0.15), operational temperatures, and times, highlighting the most energy-efficient scenarios. All the cases employing the HTC process are more favorable compared to the base case (Case 0). The initial TEC benchmark represented by Case 0, yielded a value of 14.9 GJ/t of USF.

The best result in terms of TEC balance was obtained for the condition 180-1h with a dilution of 0.15, where the final TEC value is equal to around 9.25 GJ per ton of USF. Quantitatively, this indicates a significant reduction of about 38% in exergy consumption per ton of USF with respect to Case 0.

The comparison indicates that a dilution ratio of 0.15 results in improved energy efficiency and resource utilization compared to a dilution ratio of 0.07. Specifically, the TEC balance shows superior performance with a dilution ratio of 0.15, where TEC ranges from 9.25 to 11.7 GJ/t. In contrast, the TEC ranges from 10.1 to 12.8 GJ/t for a dilution ratio of 0.07.

Moreover, under the optimized operational condition of 180-1h with a dilution ratio of 0.07, there is a notable enhancement in energy efficiency. This configuration achieves a substantial reduction in exergy consumption, approximately 32% lower than the base case. These findings underscore the practical benefits of selecting an appropriate dilution ratio, such as 0.15, to minimize energy consumption and enhance overall process efficiency in industrial applications.

The impact of various operational parameters, such as temperature and residence time, on the TEC in hydrothermal carbonization (HTC) is pronounced. Higher operational temperatures, specifically 200 °C and 220 °C, correspond to elevated exergy consumption compared to the lower temperature of 180 °C. Specifically, the TEC values for dilution ratio of 0.15, and 8 h residence time are approximately 9.75 GJ/t for 180 °C, 10.5 GJ/kg for 200 °C, and 11.2 GJ/kg for 220 °C.

Similarly, longer residence times of 4 h and 8 h contribute to increased exergy consumption compared to a shorter residence time of 1 h. The exergy consumption values vary accordingly, with approximately 9.25 GJ/kg for 1 h, 9.87 GJ/kg for 4 h, and 10.2 GJ/kg for 8 h.

These observations highlight the significant influence of operational conditions on TEC in HTC processes. Optimal parameter selection, such as lower temperatures and shorter residence times, can effectively reduce exergy consumption and enhance overall energy efficiency in HTC operations. Higher temperatures enhance the conversion efficiency of organic feedstocks into HC, potentially increasing the exergy content of the produced HC. Longer reaction times in HTC can allow for a more thorough conversion of organic feedstocks into HC, potentially increasing the energy content of the final product. Although higher temperatures and longer times lead to higher exergy consumption, they potentially increase the conversion efficiency of organic feedstocks into HC, which may justify the additional energy expenditure in terms of output quality and quantity. Although the direct comparison of the results with other studies is not possible due to differences in feedstock composition and the methodology used, observed trends are in line with the literature. The literature consistently supported the findings that optimizing operational parameters such as lower temperatures, shorter residence times, and appropriate dilution ratios significantly reduced energy consumption. The study by Benavente et al. [15] has demonstrated the environmental benefits of HTC in treating olive pomace and olive mill wastes, respectively. This study found that HTC combined with incineration for heat and power production is less environmentally impacting than composting and anaerobic digestion. The findings of this study align with these results, showing that HTC reduces the environmental impact compared to traditional USF treatment methods. However, the unique focus on USF from MSW in this study highlights specific advantages and challenges not addressed in previous studies. Previous studies by authors [13] reported a significant reduction in environmental load by substituting marginal processes with HTC products, leading to savings in climate change, acidification, freshwater toxicity, and eutrophication impacts. In our study, the optimal HTC condition resulted in a TEC value of 9.25 MJ/t, which is a significant reduction compared to the base case of 14.9 MJ/t. This quantitative comparison underscores the efficiency of HTC in reducing the overall environmental burden. Studies by Owsianiak et al. [16] and Luca and Fiori [31] also confirm that optimizing these parameters also led to significant reductions in environmental impacts and improvements in energy efficiency in HTC processes. The results suggest that implementing HTC for USF can contribute to more sustainable waste management practices. By converting organic waste into valuable HC, HTC provides a dual benefit of waste reduction and resource recovery. This aligns with the broader goals of circular economy principles, promoting the reuse and recycling of waste materials to minimize environmental impact.

4. Conclusions

This study investigated the TEC analysis of USF valorization by means of the HTC process. Different HTC process conditions (temperature 180, 200, and 220 °C; reaction time 1, 4, and 8 h; dilution ratio DS/W = 0.07 and 0.15), were compared. The research methods employed in this study involved conducting TEC analyses under various HTC operational scenarios to quantify the energy consumption, resource utilization, and environmental performance of the process. By evaluating the TEC indicators and comparing them with traditional USF treatment methods, the study has advanced our understanding of the benefits and challenges associated with implementing HTC technology for waste valorization.

TEC, which, in contrast to other methods of ecological assessment, can bring all environmental impacts into one measure which is the exergy of the consumed natural, non-renewable resources, provided a holistic approach to assessing the sustainability and environmental impact of fuel production processes by quantifying the cumulative consumption of non-renewable exergy and considering environmental losses associated with waste generation and disposal. This approach offers valuable insights into the energy efficiency, resource utilization, and the overall environmental footprint of the HTC process for USF valorization.

The findings of this study demonstrate that the HTC process, under optimal conditions, is significantly more energy-efficient and environmentally sustainable than the current USF processing methods. The optimal HTC conditions (180 °C, 1 h, and 0.15 dilution ratio) resulted in a TEC value of 9.25 GJ per ton of USF, compared to the base case of 14.9 GJ/t. This reduction highlights the significant energy savings and efficiency improvements achievable through HTC.

The lower dilution ratio (0.15) is more favorable because it requires less water, which reduces the burden on water resources and minimizes the energy needed for heating and maintaining process conditions. This makes the process more viable for large-scale applications. The reduction in TEC and emissions such as SO₂, PM, and NO_x highlights the potential of HTC to mitigate the environmental impact of waste treatment processes. This supports the adoption of HTC in waste management strategies aimed at reducing ecological footprints.

Further research is recommended to explore the long-term sustainability and scalability of the HTC process, as well as the treatment and disposal of by-products such as the liquid and gaseous phases. Investigating alternative waste streams and their behavior under HTC conditions can broaden the applicability of this technology and contribute to the development of optimized waste management practices.

Overall, this study provides valuable insights into the energy consumption, resource utilization, and environmental impact of HTC, emphasizing its potential for sustainable waste valorization. The comprehensive analysis and detailed findings offer a solid foundation for future research and practical applications in sustainable waste management.