A Comprehensive Environmental Life Cycle Assessment of the Use of Hydrochar Pellets in Combined Heat and Power Plants

: Hydrothermal carbonization (HTC) has been seen as a potentially beneﬁcial process for converting wet biomass into value-added products. It is, however, necessary to overcome the challenges associated with handling the powdered form of hydrochar—a solid product of the HTC process—by controlling the formation of dust and facilitating smoother transportation and distribution in a potentially wide marketplace. In this paper, the authors investigate the environmental consequences of di ﬀ erent alternatives for using hydrochar pellets produced from mixed sludges from pulp and paper mills in Sweden, using the environmental life cycle assessment (E-LCA). Two scenarios for possible end-uses of hydrochar in combined heat and power (CHP) plants as a source of energy (heat and electricity) were assessed. In these scenarios, hydrochar pellets were assumed to be combusted in CHP plants, thereby avoiding the use of combustible solid wastes (Scenario A) and coal (Scenario B), respectively, to recover energy in the form of electricity and heat. The environmental damages to Human Health, Ecosystem Quality, Climate Change, and Resources are evaluated based on 1 tonne of dry sludge as the functional unit. The results from this analysis illustrate that Scenario B, in which hydrochar replaces coal, o ﬀ ers the greatest reduction in all the environmental damage characterizations, except the Resources category. The displacement of energy-based coal due to hydrochar combustion contributed most signiﬁcantly to the environmental damages wrought by the system—ranging from 52% in Resources to 93% in Ecosystem Quality. Overall, the results highlight that the application of hydrochar pellets for energy recovery to o ﬀ set waste- and coal-based energy sources has great environmental beneﬁts. The favorability of sludge hydrochar over solid wastes as fuel for CHP plants may be counter-intuitive at ﬁrst, since HTC is an energy-intensive process, but when accounting for the necessity of dependence on imports of wastes for instance, the hydrochar pellet may well emerge as a good option for CHPs in Sweden.


Introduction
Incineration plants for energy recovery may have the supply of heat as their sole reason for existence, in which case, they are referred to as heat-only boilers. At the other end of the spectrum of energy supply, there are power plants supplying only electricity. Any waste heat that is generated may at best find use within the plant, but is not sold to external end-users. These are what are referred to, in thermodynamic parlance, as condensing power plants. In between these two extremes lie the promoting products and alternative processes, and educating in general. The E-LCA approach has been numerously applied on waste management systems [15][16][17].
Although there are several studies suggesting that hydrothermal carbonization (HTC) is an environmentally beneficial process, a systems-level evaluation of the environmental impacts associated with HTC and subsequent hydrochar combustion has not been well-documented in the scientific literature. The environmental footprint of the HTC of food wastes followed by the combustion of the hydrochar product in powder form, for energy production, was determined by Berge et al. [18] by adopting the E-LCA approach. In another work [19], the energy use impact and associated climate change effects of producing hydrochar briquettes co-formed with coal fines for use as a fuel in power plants has been investigated using E-LCA as the tool. In said study, the source of the hydrochar was loblolly pine and the process adopted was fast HTC.
For soil application of hydrochar, Mohammadi et al. [20] studied the environmental implications of the production of hydrochar from pulp and paper mill biosludge for application in forest ecosystems. A detailed system analysis is imperative when one wishes to more objectively identify and quantify environmental costs and benefits in the entire life cycle of a product. Considering that using HTC in this manner is in its infancy, a major obstacle associated with conducting such a comprehensive analysis of HTC is the lack of relevant data.
In this paper, the use of E-LCA to comparatively analyze the environmental impacts and damage consequences associated with the utilization of hydrochar pellets in CHPs as a source of energy for heat and electricity generation, has enabled the authors to contribute to some of the said purposes of the tool referred to above. Hydrochar pellets are produced from pulp and paper mill sludge and are assumed to substitute coal and solid wastes in CHPs. This study is part of the continuum to earlier ones by the authors, in which alternate approaches to handling mixed sludges from pulp and paper mills were studied and analyzed for their environmental footprints.

Goal and Scope Definition
The aim of this E-LCA is to quantify the environmental impacts (and damage) of the HTC of pulp and paper mill sludge and the subsequent combustion of the hydrochar product for energy production in CHP plants, replacing two different fuels (coal and solid wastes). The mixed sludge was assumed to be composed of fiber sludge and biosludge in equal proportion. This study also attempts to identify the most impactful processes and understand how parameters related to sludge waste carbonization followed by hydrochar combustion affect the environmental consequences of two alternative systems. The selected fuels in this assessment are commonly used in European CHP plants.
The mass-based functional unit (FU) is commonly adopted in E-LCA analyses of waste treatment systems [15,21,22]. In the current study, 1 tonne (t) of dry matter sludge in each CHP scenario has been chosen as the FU.

System Boundaries and Scenarios
The system includes all processes starting from sludge gathering right up to the end-use/s of the hydrochar pellets and the incinerated ash (the by-product generated), with all the conversion/transformation processes in between these two ( Figure 1). In other words, the assessment encompasses the "cradle" (the collection of the sludge) to the end-uses, which may be labelled as "graves" or "cradles" depending on the perspective adopted. The two scenarios which were considered in the analysis have been described hereunder in detail. Sludge incineration without energy recovery was taken into account as the conventional mode of handling against which the two pellet scenarios were compared.
Scenario A, where hydrochar pellets substitute solid waste, investigates the impacts of hydrochar production using mixed sludges from pulp and paper mills, in an HTC reactor, and using the resulting Sustainability 2020, 12, 9026 4 of 15 solid fuel in a heat-and electricity-producing power unit. The collected sludge is dewatered and then, transported to the HTC plant where it is converted into hydrochar and gas ( Figure 1). The hydrochar is pelletized and delivered to the CHP units to be combusted and to offset solid waste-based energy sources (electricity and heat) by avoiding the solid waste combustion process. The exhaust gases, owing to their high carbon dioxide content, are not captured for combustion. Scenario B, in which hydrochar pellets substitute coal, also uses mixed sludges from pulp and paper mills to generate hydrochar via the HTC process ( Figure 1). The exhaust gases are assumed to be released to the atmosphere. After pelletizing hydrochar, it is then transported to CHP plants to replace coal in heat and electricity production.
In both the scenarios, the process water including solid residues (organic compounds) is treated in a wastewater treatment (WWT) process, where the solid residues are separated from the liquid part and returned to the HTC reactor, and added to the incoming feedstock of the next cycle of the process. The cleaned water from WWT process is then discharged to the hydrosphere. The residual bottom ash from the hydrochar combustion from both the CHP units is delivered to landfill sites. To evaluate the environmental footprint of co-products, the system expansion approach was adopted in this work.
Sustainability 2020, 12, x FOR PEER REVIEW 4 of 15 dewatered and then, transported to the HTC plant where it is converted into hydrochar and gas ( Figure 1). The hydrochar is pelletized and delivered to the CHP units to be combusted and to offset solid waste-based energy sources (electricity and heat) by avoiding the solid waste combustion process. The exhaust gases, owing to their high carbon dioxide content, are not captured for combustion. Scenario B, in which hydrochar pellets substitute coal, also uses mixed sludges from pulp and paper mills to generate hydrochar via the HTC process ( Figure 1). The exhaust gases are assumed to be released to the atmosphere. After pelletizing hydrochar, it is then transported to CHP plants to replace coal in heat and electricity production.
In both the scenarios, the process water including solid residues (organic compounds) is treated in a wastewater treatment (WWT) process, where the solid residues are separated from the liquid part and returned to the HTC reactor, and added to the incoming feedstock of the next cycle of the process. The cleaned water from WWT process is then discharged to the hydrosphere. The residual bottom ash from the hydrochar combustion from both the CHP units is delivered to landfill sites. To evaluate the environmental footprint of co-products, the system expansion approach was adopted in this work. Life cycle diagrams for using HC pellet in CHP plants. The dashed lines represent avoided processes (the acronyms HC, WWT, and CHP represent hydrochar, wastewater treatment, and combined heat and power, respectively). The boxes shaded in grey represent the environmental sinks. The labelled flows are: X1,2 and X1,3-dewatered sludge; X1,4 and X5,4-process water; X4,1-solid residues; X2,5-slurry HC; X4,7-treated water; X2,6-gases; X5,8-HC powder; X8,9, X9,10, and X9,12-HC pellet; X10,11 and X11,12-ash; X11,7-heavy metals; X13,10-solid waste; X14,12-coal.

Inventory Analysis
The life cycle inventory included data from the relevant literature, HTC process experiments, and the ecoinvent 3.1 [23] and ELCD databases [24] under Swedish conditions for all the scenarios.
Inventory data for the HTC process were gathered from hydrochar experiments carried out by Ahlroth, Bialik, and Jensen [10] and Fuglesang et al. [25] in Sweden. The experiment evaluated the HTC treatment of two different types of sludge from the pulp and paper industry in Innventia's Parr reactor [26] at 200 • C, for 3 h with a hydrochar yield of 68.5% and 60 kg exhaust gas, which is released to the air. The exhaust gas is composed of CO 2 (over 90%) [27], and thereby not combustible. In the experiments, fiber sludge and biosludge were considered, and the average data (elemental and energy analyses) of these two sludges and their corresponding hydrochars were used in the current study. The slurry hydrochar from the HTC stage was dewatered using a filter press and dried to a moisture content of 8%. The latent energy required to evaporate the moisture away was calculated using its enthalpy of vaporization (2.5 MJ per kg of water). The total thermal energy needed for material drying was calculated assuming an energy loss factor of 25% [28]. However, this energy requirement was met by the extra (waste) heat available in pulp and paper mills, thanks to the incineration of organic waste materials such as wood residues.
The produced hydrochar was then assumed to be pelletized by a medium-size pelletizing machine with a product mass flow of around 40 kg per h and electricity requirement of approximately 0.05 kWh per kg pelleted hydrochar [29]. The densification process for hydrochar was modeled considering a material loss factor of 10% based on the experiments conducted at Karlstad University (data not shown). The electricity required for the dewatering, HTC, and densification processes was sourced from the Swedish electricity grid.
The next stage of this process is hydrochar pellet combustion for heat and power generation. Because of a lack of data associated with the energy required for hydrochar combustion, and the distribution of heavy metal elements during this process, hydrochar combustion is assumed to behave in the same way as sludge incineration does; thus, the incineration module of mixed sludges [30] was employed for both scenarios A and B to model hydrochar combustion in CHP plants including emissions and the flue gas treatment process. Electricity and heat produced from the hydrochar combustion were calculated based on the energy recovery efficiencies reported in [31], where HTC was used to treat olive mill wastes and was followed by combustion for energy recovery. The in-plant electricity use in the CHP is 4.5% of the generated output. A 10% heat loss in the district heating network was considered based on a transmission distance of 5 km. The use of heat from CHP and HTC processes was assumed to avoid the use of district heating fuels in both scenarios. The gaseous emissions associated with the incineration [32] and HTC facilities [33] were taken into account based on the published experimental studies.
Much uncertainty still exists about the effects of HTC and combustion on heavy metals in the sludge as well as the partial solubilization of the ash-forming components during the HTC process. Therefore, it is difficult to model and requires consideration of many factors such as reaction time and temperature, process water pH, and characteristics of the sludge. However, some experimental works [25,34] reported that heavy metals mostly remain in the solid residues produced during the thermochemical treatment process. In this assessment, we assumed that all the heavy metals are removed with the solid residues and hydrochar on the one hand, and the ash generated on the other, during the HTC and combustion processes, respectively. Likewise, the ash-forming components were considered to remain integrated within the hydrochar. We estimated the leachability of heavy metals in the ash, which is landfilled according to Latosińska and Gawdzik [35]. We assumed that the construction and maintenance of the infrastructure are depreciated over a long lifetime; hence, these were not included in the E-LCA. The assumptions and life cycle inventory for both the scenarios are illustrated in Table 1. Of the aforenamed four steps, the first two are mandatory, while the last two are optional. In this analysis though, all the four steps have been adopted. Inventory flows are characterized and grouped into 15 impact categories: carcinogens (CA), non-carcinogens (NCA), respiratory inorganics (RI), ionizing radiation (IR), ozone layer depletion potential (ODP), respiratory organics (RO), aquatic ecotoxicity (AET), terrestrial ecotoxicity (TE), terrestrial acidification/nutrification (TA/N), land occupation (LO), aquatic acidification (AA), aquatic eutrophication (AE), global warming (GWP), Sustainability 2020, 12, 9026 7 of 15 non-renewable energy (NRE) use, and mineral extraction (ME). These impacts are subsequently further characterized (second-level) and grouped into four damage categories: human health (HH) in Damage Adjusted Years of Life (DALY), ecosystem quality (EQ) in Potentially Disappeared Fraction (PDF m 2 yr), climate change (CC) in kg CO 2 eq., and resources (RE) in MJ primarily [38]. Figure 2 shows the damage categories and their corresponding impacts. Default normalization factors (refer to Table 2) are used, and equi-weighting is adopted to express the final result as a single score [38].
Sustainability 2020, 12, x FOR PEER REVIEW 7 of 15 ecotoxicity (AET), terrestrial ecotoxicity (TE), terrestrial acidification/nutrification (TA/N), land occupation (LO), aquatic acidification (AA), aquatic eutrophication (AE), global warming (GWP), non-renewable energy (NRE) use, and mineral extraction (ME). These impacts are subsequently further characterized (second-level) and grouped into four damage categories: human health (HH) in Damage Adjusted Years of Life (DALY), ecosystem quality (EQ) in Potentially Disappeared Fraction (PDF m 2 yr), climate change (CC) in kg CO2 eq., and resources (RE) in MJ primarily [38]. Figure 2 shows the damage categories and their corresponding impacts. Default normalization factors (refer to Table 2) are used, and equi-weighting is adopted to express the final result as a single score [38].

Environmental Performance of Two Scenarios
The E-LCA results of the environmental impacts (midpoint values) and the environmental damages (endpoint values) for the two hydrochar pellet pathways are presented in Tables 3 and 4, respectively.
A positive impact/damage potential shows a burden to the environment (negative environmental effects), while a negative potential shows a reduction in environmental impacts (positive environmental effects). For both the scenarios, emission abatement was greater than the

Environmental Performance of Two Scenarios
The E-LCA results of the environmental impacts (midpoint values) and the environmental damages (endpoint values) for the two hydrochar pellet pathways are presented in Tables 3 and 4, respectively. A positive impact/damage potential shows a burden to the environment (negative environmental effects), while a negative potential shows a reduction in environmental impacts (positive environmental effects). For both the scenarios, emission abatement was greater than the emissions per se, resulting in a negative value for most of the environmental impacts. However, the results indicate that some impacts such as CA, AE, and TE have a greater cost to environment in Scenario A, while other impacts like NCA, IR, and ODP were greater in Scenario B. In terms of damage categories, Scenario B, in which hydrochar pellets replace coal in CHP plants, offered better environmental performance in all the Sustainability 2020, 12, 9026 8 of 15 categories except resources compared to Scenario A, in which hydrochar pellets substitute solid waste and result in saving the raw materials, e.g., aluminum, copper, iron, and nickel, embedded in municipal and industrial waste. Figure 3 presents the comparison between the two cases: differences are large, varying from 10% for resources to 83% for climate change. This high difference in climate change is because the avoided process of coal combustion in Scenario B significantly emits more GHGs, vis-à-vis the avoided process of solid waste combustion in Scenario A.
The two hydrochar systems, where hydrochar pellets substitute solid waste (Scenario A) and coal (Scenario B) in CHP plants, considerably improved the environmental consequences of management of sludge relative to the baseline scenario (i.e., incineration of sludge) with respect to the different impacts and their corresponding damages. However, the environmental performance of the application of hydrochar from sludge for energy purposes is influenced by its usage in CHP plants running with different fuels. The observed differences between the hydrochar scenarios are mostly due to the avoidance of environmental impacts thanks to the displacement of the use of solid wastes and coal as fuels. The avoidance of environmental impacts is thus credited to each system. Similar impact-mitigation results of hydrochar systems were found in other studies [18,31], where the HTC of food wastes and the subsequent combustion of the generated hydrochar for energy production were evaluated via the E-LCA method. In the case of hydrochar production from pulp and paper mill sludge for use in CHP plants, no studies have been published. However, the potential of hydrochar pellets from sludge for use as fuels has been reported by a few researchers [36,39].

Contribution Analysis
Results from the contribution analysis are presented in Figure 4. This figure illustrates the contributions (in percentage) of the different processes, including sludge dewatering, hydrochar combustion, densification of produced hydrochar, and landfilling of incinerated ash. The percentage contribution of each of these processes is according to its individual contribution to the corresponding damage potential. It is seen that the avoided use of conventional (currently used) fuels (solid waste and coal) was the dominant process in all the environmental damages in both scenarios. The contribution of this process (energy displaced in Figure 4) ranged from 52% in improvement of RE to 95% reduction in EQ damage in Scenario B. These results are in agreement with the findings from Eriksson et al. [40]. Berge, Li, Flora, and Ro [18] reported that hydrochar combustion contributed most significantly to the overall environmental impacts of the system, with most of the impacts-GWP, TA, and TE especially-being attributed to hydrochar combustion. This, courtesy of the substitution of lignite-sourced electricity in the wider system, contributed to over 90% of the offsets. Benavente, Fullana, and Berge [31] have also shown that GWP, TA, TE, ME, and human toxicity, benefited immensely thanks to such a substitution.
In our analysis, the production and densification of sludge hydrochar also contribute significantly to the environmental damage-to RE (about 22%) and CC (27% in Scenario A and 13% in Scenario B). This is mainly due to the electricity required to run the HTC process and operate the pelletizing equipment. Sludge dewatering and wastewater treatment contributed 20% to RE, owing to the electricity-intensiveness of these two processes. However, the impacts associated with sludge dewatering played a relatively smaller role in the HH (5%), EQ (2%), and CC (11% and 5% in scenarios A and B) environmental damage characterization values. It must be mentioned at this juncture that the environmental impacts/damages due to the HTC process may change when it is scaled up, because of the changes in requirements for auxiliary equipment, which will be called for.
The avoided sludge incineration contributed slightly to reductions in HH, EQ, CC, and RS damages by 3%, 1%, 2%, and 5%, respectively. Landfilling of the incinerated ash is responsible for around 7% of HH and 4% of EQ damages. This is largely due to the mobility of heavy metals in ash. For both the scenarios, transportation had a minor contribution, about 1%, to all the damages.

Contribution Analysis
Results from the contribution analysis are presented in Figure 4. This figure illustrates the contributions (in percentage) of the different processes, including sludge dewatering, hydrochar combustion, densification of produced hydrochar, and landfilling of incinerated ash. The percentage contribution of each of these processes is according to its individual contribution to the corresponding damage potential. It is seen that the avoided use of conventional (currently used) fuels (solid waste and coal) was the dominant process in all the environmental damages in both scenarios. The contribution of this process (energy displaced in Figure 4) ranged from 52% in improvement of RE to 95% reduction in EQ damage in Scenario B. These results are in agreement with the findings from Eriksson et al. [40]. Berge, Li, Flora, and Ro [18] reported that hydrochar combustion contributed most significantly to the overall environmental impacts of the system, with most of the impacts-GWP, TA, and TE especially-being attributed to hydrochar combustion. This, courtesy of the substitution of lignite-sourced electricity in the wider system, contributed to over 90% of the offsets. Benavente, Fullana, and Berge [31] have also shown that GWP, TA, TE, ME, and human toxicity, benefited immensely thanks to such a substitution.
In our analysis, the production and densification of sludge hydrochar also contribute significantly to the environmental damage-to RE (about 22%) and CC (27% in Scenario A and 13% in Scenario B). This is mainly due to the electricity required to run the HTC process and operate the pelletizing equipment. Sludge dewatering and wastewater treatment contributed 20% to RE, owing to the electricity-intensiveness of these two processes. However, the impacts associated with sludge dewatering played a relatively smaller role in the HH (5%), EQ (2%), and CC (11% and 5% in scenarios A and B) environmental damage characterization values. It must be mentioned at this juncture that the environmental impacts/damages due to the HTC process may change when it is scaled up, because of the changes in requirements for auxiliary equipment, which will be called for.
The avoided sludge incineration contributed slightly to reductions in HH, EQ, CC, and RS damages by 3%, 1%, 2%, and 5%, respectively. Landfilling of the incinerated ash is responsible for around 7% of HH and 4% of EQ damages. This is largely due to the mobility of heavy metals in ash. For both the scenarios, transportation had a minor contribution, about 1%, to all the damages.

Single-Score Analysis
For an effective interpretation of the results from the comparative analysis, impact categories were normalized, weighted, and converted to a single score. Figure 5 shows the single score results based on one tonne of dry sludge. According to the results, both the systems analyzed in this study are environmentally preferable to the baseline scenario of combustion of mixed sludges. However, Scenario B, in which the sludge is converted to hydrochar pellets to replace coal, has a lower environmental footprint than Scenario A. This discrepancy could be attributed to the better performance of Scenario B in most of the damage categories relative to Scenario A, in which hydrochar pellets from sludge substitute solid waste in CHP units. Therefore, when hydrochar is used as biofuel, the environmental impacts are very much sensitive to the type of fuel replaced by the hydrochar and also the existing waste management system that the HTC process substitutes. [19,41].
Scenario A has better results only in the resources damage category, due to the greater reduction in NRE use and mainly ME impact because of avoiding the incineration of raw materials embedded in solid waste. Therefore, if the reduction in the use of non-renewable fuel sources and minerals is the main motivation, the use of hydrochar pellets to replace solid waste can be the preferred alternative for handling mixed sludges. However, the application of sludge hydrochar instead of coal is suggested if a comprehensive outlook and optimizing environmental and health costs are focused on.

Single-Score Analysis
For an effective interpretation of the results from the comparative analysis, impact categories were normalized, weighted, and converted to a single score. Figure 5 shows the single score results based on one tonne of dry sludge. According to the results, both the systems analyzed in this study are environmentally preferable to the baseline scenario of combustion of mixed sludges. However, Scenario B, in which the sludge is converted to hydrochar pellets to replace coal, has a lower environmental footprint than Scenario A. This discrepancy could be attributed to the better performance of Scenario B in most of the damage categories relative to Scenario A, in which hydrochar pellets from sludge substitute solid waste in CHP units. Therefore, when hydrochar is used as biofuel, the environmental impacts are very much sensitive to the type of fuel replaced by the hydrochar and also the existing waste management system that the HTC process substitutes. [19,41].
Scenario A has better results only in the resources damage category, due to the greater reduction in NRE use and mainly ME impact because of avoiding the incineration of raw materials embedded in solid waste. Therefore, if the reduction in the use of non-renewable fuel sources and minerals is the main motivation, the use of hydrochar pellets to replace solid waste can be the preferred alternative for handling mixed sludges. However, the application of sludge hydrochar instead of coal is suggested if a comprehensive outlook and optimizing environmental and health costs are focused on.

Implementation of Hydrochar Pellet Systems
Coal accounts for a very small share in the Swedish power sector, but it dominates the power mixes in many other countries. Substituting it with hydrochar will be one of the many strategies which could be adopted to decarbonize the global energy sector. Sweden plans to achieve 100% renewable electricity generation by 2040, followed by zero net GHG emissions in 2045 [5]. Biomass is one of the most important fuels in Swedish district heat and electricity generation with about 46% [42] and 9% [5] contributions, respectively. Therefore, the improvement of energy recovery from biomass sources is expected to play a key role in the attainment of Sweden's energy-related targets. Hence, it is beneficial to explore efficient ways to upgrade pulp and paper mill sludge into high valueadded bioenergy. This raises the question-which form of biomass delivers the greatest energy benefits? Previous studies have indicated that the fuel quality of raw biomass can significantly be increased by the carbonization process and the resultant hydrochar has the potential to be used as a source of energy in firing units to replace conventional coal or other sources partially and reduce the environmental impacts [18,43]. By pelletizing hydrochars, fuel quality can be improved [11], and in addition to a decrease in the generation of nuisance dust, benefits related to transport, storage, and combustion are achieved. Liu, Quek, and Balasubramanian [6] concluded that the enhanced mechanical durability and considerably improved combustion characteristics make hydrochar pellets more suitable as solid fuels vis-à-vis raw biomass pellets. Therefore, carbonization of sludge feedstock, combined with pelletization, has the potential to obtain high-quality biomass fuel and to provide a better alternative for the utilization of raw sludge as a source of energy.
Our results showed that hydrochar also has the potential to replace solid waste in CHP units and deliver significant environmental benefits. The favorability of hydrochar pellets over solid wastes as a fuel for CHP plants may be counter-intuitive at first, due to the energy-demanding processes of HTC and pelletizing, but when accounting for the necessity of dependence on imports of wastes for instance, one moves towards substantiating the claim that sludge hydrochar may well emerge as a good option for CHPs in Sweden. It can be also seen from the literature that energy generation from solid wastes causes considerable environmental impacts-natural resources, global warming, and toxic emissions [17]. Eriksson, Finnveden, Ekvall, and Björklund [40] reported that waste incineration is usually the preferred option when incineration replaces landfilling. However, it cannot be denied that incineration is still an inferior option compared to material recovery, when

Implementation of Hydrochar Pellet Systems
Coal accounts for a very small share in the Swedish power sector, but it dominates the power mixes in many other countries. Substituting it with hydrochar will be one of the many strategies which could be adopted to decarbonize the global energy sector. Sweden plans to achieve 100% renewable electricity generation by 2040, followed by zero net GHG emissions in 2045 [5]. Biomass is one of the most important fuels in Swedish district heat and electricity generation with about 46% [42] and 9% [5] contributions, respectively. Therefore, the improvement of energy recovery from biomass sources is expected to play a key role in the attainment of Sweden's energy-related targets. Hence, it is beneficial to explore efficient ways to upgrade pulp and paper mill sludge into high value-added bioenergy. This raises the question-which form of biomass delivers the greatest energy benefits? Previous studies have indicated that the fuel quality of raw biomass can significantly be increased by the carbonization process and the resultant hydrochar has the potential to be used as a source of energy in firing units to replace conventional coal or other sources partially and reduce the environmental impacts [18,43]. By pelletizing hydrochars, fuel quality can be improved [11], and in addition to a decrease in the generation of nuisance dust, benefits related to transport, storage, and combustion are achieved. Liu, Quek, and Balasubramanian [6] concluded that the enhanced mechanical durability and considerably improved combustion characteristics make hydrochar pellets more suitable as solid fuels vis-à-vis raw biomass pellets. Therefore, carbonization of sludge feedstock, combined with pelletization, has the potential to obtain high-quality biomass fuel and to provide a better alternative for the utilization of raw sludge as a source of energy.
Our results showed that hydrochar also has the potential to replace solid waste in CHP units and deliver significant environmental benefits. The favorability of hydrochar pellets over solid wastes as a fuel for CHP plants may be counter-intuitive at first, due to the energy-demanding processes of HTC and pelletizing, but when accounting for the necessity of dependence on imports of wastes for instance, one moves towards substantiating the claim that sludge hydrochar may well emerge as a good option for CHPs in Sweden. It can be also seen from the literature that energy generation from solid wastes causes considerable environmental impacts-natural resources, global warming, and toxic emissions [17]. Eriksson, Finnveden, Ekvall, and Björklund [40] reported that waste incineration is usually the preferred option when incineration replaces landfilling. However, it cannot be denied that incineration is still an inferior option compared to material recovery, when viewed from a life cycle perspective. Hence, it is imperative that the potential for material recovery needs to be considered in future research in this field.
In addition to energy-related applications, hydrochar production from pulp and paper mill sludge and its use as a soil amendment can be a good option for waste-stream management strategies. Hydrochar pellets have been shown as a possible partial substitute for peat in forestry nursery substrates [44]. The environmental response of the addition of sludge hydrochar in forest ecosystems can lead to a great reduction in GWP, TE, acidification, and eutrophication impacts compared to incineration and landfilling of sludge [20]. To date, the economic feasibility and social impact of using hydrochar as a fuel source or soil conditioner have not garnered as much attention as the environmental benefits. Only a few authors have investigated the economics of hydrochar for energy generation, mostly focusing on food waste as feedstock [45,46], and in the case of hydrochar produced from pulp and paper mill sludge, no study has been conducted yet, to the knowledge of the authors of this paper.
Financial returns and social benefits contribute significantly in the promotion of renewable energy technologies [47,48]. Therefore, the authors would like to recommend an economic analysis in addition to the E-LCA before a more convincing selection can be made.

Conclusions
In this paper, the authors have studied the application of hydrochar pellets from pulp and paper mill sludges in CHP plants to substitute different fuel materials with the aim of identifying opportunities for improving its environmental performance. The results show that carbonization of the sludge feedstock, combined with pelletization to generate heat and power, can offer significant environmental benefits compared with incineration of the sludge. Between the scenarios, Scenario B, in which hydrochar substitutes coal, has significant environmental advantages (Human Health, Ecosystem Quality, and Climate Change damage categories) over Scenario A, where hydrochar substitutes solid waste. Scenario A was slightly better in terms of reduction in damage in the Resources endpoint category, due to the greater abatement in NRE and ME impacts. The results of the endpoint damage analysis prove that Scenario B delivers the highest environmental savings. This indicates the importance of heat and electricity generation from hydrochar pellets specifically when they are utilized to offset coal-based power sources. Decisions about using hydrochar-derived heat or electricity to replace the average Swedish energy mix (applied to other countries using coal-based energy sources) call for the investigation of specific environmental priorities. As a systems analysis study, some assumptions on emissions from HTC reactor and hydrochar combustion, energy recovery efficiency, and the fate of heavy metals were made in this assessment. Hence, to support decisions for implementation, it is recommended that future research works focus on reducing uncertainties, for instance by the use of site-specific emission and energy efficiency measurements, as well as methods to improve the fuel quality of the hydrochar pellets.