The Effect of Residence Time during the Hydrothermal Carbonization Process of Sewage Sludge on the Properties of Hydrochar

Abstract

The optimal process conditions concerning the hydrothermal carbonization of digested sewage sludge are crucial to the economically effective technology needed to produce a solid product, hydrochar, for energy purposes. Accordingly, different residence times, 0.5 h, 1 h and 2 h, were investigated in order to understand the effect of residence time on the process. Furthermore, the physical and chemical properties of hydrochar were investigated and compared to the raw material. For these reasons, analyses describing fuel properties were performed, including ultimate and proximate analyses, HHV, and TGA analysis. The latter method was employed to study the combustion process of solid samples. In addition, the oxide content of different elements within the ash of solid samples was determined using the XRF method to calculate indices related to operational problems during the combustion process. The results confirmed that time did not matter significantly and the physical and chemical properties of hydrochar were very similar to each other. However, the contact angle for 2 h of residence time confirmed that a longer processing time resulted in a more hydrophobic character of hydrochar and enabled more effective dewaterability of hydrothermal slurry. It was also noted that the hydrothermal carbonization process affected the sewage sludge in a positive way. In brief, the results confirmed that the hydrochar was a brittle, moderately hydrophilic, solid carbon-containing product that provided a different combustion performance than the raw sewage sludge.

1. Introduction

Continuous industrial and technological development improves quality of life, but also results in an increasing demand for energy. Consequently, fossil fuels are exploited, and for this reason, research into renewable energy is required [1,2]. Although solar, nuclear, and wind energy offer great hope, biomass energy seems to be the most promising, as it is a renewable energy carrier offering the possibility of reducing net greenhouse gas emissions by replacing fossil fuels [3]. Recently, interest in biomass has increased significantly due to the low-carbon, closed-loop bioeconomy. Specially cultivated crops such as wheat [4], sunflower [5], and algae [6,7]; animal manure and fertilizers [8]; sewage sludge [9]; red mud [10]; and all organic waste (fruit and vegetable residues) after processing through thermal, thermochemical, biochemical, and chemical conversion methods can be main sources of biofuels.

The preparation of raw biomass into energy-dense solid fuel is an important step towards more efficient thermochemical processing. Hydrothermal carbonization (HTC) is a process by which organic wet feedstock is transformed in the presence of water within a temperature range of 180–260 °C and under elevated pressure into solid, liquid, and gaseous products [11,12]. The HTC process parameters affect the structure of material through hydrolysis, dehydration, decarbonylation, decarboxylation, polymerization, and re-condensation reactions [12,13]. Compared to other methods, the HTC process avoids the energy-intensive drying process. In addition, the presence of water during hydrothermal conversion acts as a catalyst [13,14]. In general, the process produces hydrochar, which usually exhibits an increased energy density and higher hydrophobic and higher calorific value depending on the feedstock origin [15,16,17].

These carbonaceous products can be used in a wide range of applications, including as solid biofuels [18,19], sorbents for environmental remediation [20,21], and soil additives to improve fertility [22,23].

One important parameter affecting the properties of sewage sludge under the hydrothermal carbonization process is residence time. According to the literature, the residence time applied for the hydrothermal carbonization of sewage sludge may vary within a range of 15 min to 1440 min in terms of hydrochar production for combustion purposes. For instance, Parshetti et al. studied hydrothermal carbonization of sewage sludge within 15 min of residence [24], Zheng et al. [25] and Gaur et al. [26] within 60 min, and Zhang et al. within 1440 min [27]. Increases in the time of the HTC process caused changes in higher heating value (HHV). In general, the calorific value may slightly increase or decrease with increased residence time [28]. The different magnitudes and varied growth rates of HHV may be due to the heterogeneity of the sludge. Merzari et al. [29] found that time affected the efficiency of the HTC process. As the time was increased, the mass yield decreased and the ash content increased. Wilk et al. [30] also studied the effect of this parameter on the carbon content of the hydrochar and found that the hydrochar produced in 4 h had 7% higher fixed carbon content than the raw sewage sludge.

Hydrothermal carbonization at an industrial scale requires larger reactors and processing units. However, scaling up creates challenges in terms of material costs, manufacturing, and maintenance. Industrial reactors must be able to withstand higher pressures. Additionally, thicker reactor walls are necessary for safety, but significantly increase material costs. Longer processing times and higher temperatures lead to increased energy consumption. Therefore, it is crucial to choose the optimal parameters for the process, mainly temperature and residence time. The choice of temperature significantly affects the reaction rate and product quality. Higher temperatures can speed up reactions, but can also increase energy consumption, increase pressure in the reactor, and, thus, increase the cost of the materials used. Following previous studies, it can be concluded that a temperature of 200 °C is sufficient to achieve adequately pretreated sewage sludge, i.e., with improved dewaterability in comparison to raw sewage sludge. On the other hand, a longer reaction time can improve the quality of the product, but lengthen production cycles. Finding the optimal balance is an essential issue in order to minimize costs while achieving the desired results. However, the energetic use of sewage sludge and hydrochar poses challenges and problems due to the different properties of these materials. Adverse fuel properties, such as high alkali and chlorine content, lead to a number of undesirable reactions in the combustors of power boilers [31,32]. Typical problems during combustion are associated with the ashes produced from biomass combustion. These include corrosion, ash melting deposit formation, and particle formation as well as problems associated with emissions of sulphur oxides (SO_x), nitrogen oxides (NO_x) and hydrochloric acid (HCl). As the demand for biofuels increases, low-quality fuels are also entering the market, further exacerbating these problems. In brief, the ultimate process conditions are essential to alter the hydrochar properties towards fuel application, and as temperature has been proven to be the most prominent factor, further studies are still required to evaluate the effect of residence time for its sustainable application.

Therefore, the main objective of the study was to investigate the effect of time on the properties of the hydrochar conducted at 3 different residence times: 0.5 h, 1 h, and 2 h. To investigate the influence of this parameter, proximate and ultimate analyses, HHV, and thermal analysis were applied to study the combustion process of hydrochar. Operational problems, in particular the slagging and fouling tendency of hydrochar and its hydrophobic character, were determined based on XRF results, and by contact angle. The innovation of this research is the comprehensive and detailed analysis of the physical and chemical properties of hydrochar with particular attention paid to technological problems occurring during the thermal utilization of hydrochar. This has great practical significance for the comprehensive utilization of hydrothermal carbonization concerning sewage sludge.

2. Materials and Methods

2.1. Material

Samples of dewatered sewage sludge (SS) were collected from the Central Sewage Treatment Plant in Gliwice (Poland) during winter. The SS was concentrated and subjected to anaerobic digestion at 37 °C for about thirty days. It was dewatered through two filtration presses, then sampled and stored in plastic containers at 4 °C to slow down the biodegradation process. Some of the raw sewage sludge was dried and prepared for further comparative analyses. The preparation of the sewage sludge prior to the hydrothermal carbonization process consisted of diluting the sewage sludge with distilled water to about 90%, which resulted in an easy mixing solution in the reactor.

2.2. Hydrothermal Carbonization

The hydrothermal carbonization set-up consists of a batch-type mixing reactor Zipperclave^® (Parker Autoclave Engineers) with a capacity of 1000 mL, a heating system with removable heating jacket; a cooling system; and a control panel to adjust temperature, pressure, and mixing speed in the reactor (Figure 1). Detailed information about the set-up can be found in a previous publication [33]. A prepared sample, 750 g of sewage sludge, was placed in the reactor and sequentially sealed in the reactor. The temperature was set at 200 °C and maintained for specific residence times of 0.5 h, 1 h, and 2 h. While the process was running, the raw material was stirred at a rate of 150 rpm. When the time of reaction was over, the heating was turned off and the heating jacket removed, and then the water cooling was switched on to cool down the reactor to room temperature. The mixture was then removed from the reactor, and the individual phases were separated using a filtration apparatus. The resulting hydrochar was dried and stored in plastic containers for further analysis. The labelling of hydrochar included abbreviations describing the hydrothermal carbonization process and length of residence time applied; e.g., HTC_0.5h represented hydrochar derived during a residence time of 0.5 h.

2.3. Analytical Methods

The moisture content (M), ash content (Ash), and volatile matter content (VM) were determined using a 5E-MAC6710 Proximate Analyser—TGA (Changsha Kaiyuan Instruments Co., Ltd., Changsha, China) in accordance with ASTM D7582 [34] and ISO 17246:2010 [35]. The content of carbon (C), nitrogen (N), hydrogen (H), and sulphur (S) was determined using a Truespec CHNS Leco elemental analyser (CHNS628) in accordance with PKN-ISO/TS 12902:2007 [36]. HHV was determined using a Leco AC500 calorimeter (St. Joseph, MI, USA) according to DIN 51 900 [37] and ISO 1928 [38] standards.

The following parameters were also assessed (

Table 1. Parameters used for the chemical and physical description of hydrochar.

Parameter	Equation
Oxygen content, %	O = 100 − C–H − N − S − Ash − M
Dry matter content, %	Dry matter content = 100 − Ash
Fixed carbon content, %	FC = 100 − VM − Ash − M
Mass yield, %	$MY=\frac{mas{s}_{hydrochar}}{mas{s}_{SS}}·100$ where: masshydrochar—is the mass of hydrochar, kg massSS—is the mass of sewage sludge, kg
Energy density ratio	$EDR=\frac{HH{V}_{hydrochar}}{HH{V}_{SS}}$ where: HHVhydrocha—is the HHV of hydrochar, MJ/kg HHVSS—is the HHV of sewage sludge, MJ/kg
Energy yield, %	$EY=MY·EDR$
Polarity index	$PI=\left(O+N\right)/C$

):

• oxygen (O) content by the differential method, taking into account the elemental analysis and the ash and moisture contents of the solid samples
• dry matter content as the difference between the total percentage of the sample and mineral matter (determined ash content)
• fixed carbon content (FC) based on the difference between 100% and volatile matter, ash, and moisture contents
• mass yield (MY)
• energy density ratio (EDR)
• energy yield (EY)
• polarity index (PI) [39,40]

XRF analysis was performed using a WD-XRF ZSX Primus II Rigaku spectrometer (Rh lamp) (Wilmington, MA, USA) to determine the qualitative and quantitative elemental composition of the test samples. Based on these results, characteristic coefficients related to operational problems were determined, namely: ratio of basic oxides to acid oxides (R_B/A), value of the basic components in the ash (R_B), slagging index (R_S), fouling index (F_u), viscosity index (S_R), ash viscosity index (LF), and slagging ability [41].

The Mettler Toledo STAR System TGA/DSC 3 H T 1600 calibrated with indium, zinc and aluminium, heated at 10 K/min, up to 800 °C, under atmospheric air, was used for TGA analysis of the hydrochars. The combustion process was performed by thermogravimetry (TG) and the degree of sample mass loss by derived thermogravimetry (DTG). In order to assess the combustion properties of the fuel samples, the following indices were determined: ignition temperature (T_i), burnout temperature (T_b), ignition index (D_i), burnout index (D_b), combustion comprehensive index (S), and combustion stability index (H_f). T_i was determined by the intersection method [42,43,44]. T_b was found at the point where mass of sample stabilization occurred, whereas, D_i, D_b, S, and H_f were calculated by the methods described in [45,46].

In order to determine whether the hydrophobic character of the hydrochar increases when increasing the time of the process, the contact angle (CA) of the produced solid materials’ surfaces with water was measured. For this purpose, a preliminary preparation of investigated solid powders was necessary to enable the measurement. For wettability measurements, powdered hydrochars and raw sewage sludge were pelletized in a hydraulic press. Demineralized water drops were used as a polar liquid, and the contact angle was determined using the ellipse fitting method with a DSA25E Krüss (Hamburg, Germany) goniometer. CA was analyzed at least nine times, using at least three separate water drops for each sample.

3. Result and Discussion

The ultimate and proximate analysis of the sewage sludge and the resulting hydrochar supported by energy parameters and HTC product distribution is denoted in

Table 2. Chemical and physical properties of SS and hydrochars supported by product distribution of the HTC process.

Parameter	SS	HTC_0.5h	HTC_1h	HTC_2h
PROXIMATE ANALYSIS
Dry organic matter, %	67.4	54.7	54.6	53.2
VM, %	61.8	48.9	48.6	47.0
FC, %	2.0	4.2	4.2	4.9
ENERGY PARAMETERS
HHV, MJ/kg	14.74	14.07	14.22	14.09
LHV, MJ/kg	13.63	13.13	13.30	13.17
MY, %	-	60.31	61.64	63.02
EDR	-	0.95	0.97	0.96
EY, %	-	57.54	59.49	60.21
PRODUCT DISTRIBUTION
solid fraction, %wt.	-	5.73	5.86	5.99
liquid fraction, %wt.	-	91.91	89.33	89.26
gas and losses, %wt.	-	5.24	4.52	4.32

and shown in Figure 2. The final analysis indicated that the time of the HTC process does not significantly affect the content of individual elements, although the process itself slightly alters these proportions. The carbon content of the sludge was 35.02%, while the HTC process reduced this content by 4.74%, 3.80%, and 4.43% for HTC_0.5h, HTC_1h, and HTC_2h, respectively. These results correlate with the results of the study by Gaur et al. [24]. In contrast, the increase for hydrogen oscillated around 16–18%, and for nitrogen around 28–40%. The lower N content of the hydrochar showed that protein hydrolysis was enhanced during HTC of the SS, since N was mainly derived from proteins in the SS. The lower N content of the hydrochar also indicated that some N in the SS was transferred to the aqueous phase during HTC [47].

Sewage sludge is characterized by its high volatile matter content (61.8%) and relatively low ash content (32.6%). For these parameters, the influence of the HTC process is also noticeable. The HTC process for all time ranges resulted in a decrease in volatile matter content (by about 21–24%) and an increase in ash content (by about 38–44%). A specific trend was observed that when the time is increased, the volatile content increases and the ash content decreases.

It was also noted that mass and energy yields increased with an increase of the residence time by about 2–3%. Concerning the distribution of individual phases, an increase in time caused an increase in the amount of hydrochar and decreased the amount of liquid phase, gas and losses. These results vary from those obtained by other researchers, which is probably due to the heterogeneity of the sewage sludge [47,48].

The chemical changes in the carbon-containing material are depicted in a van Krevelen diagram using atomic H/C and O/C ratios (Figure 3) [49]. The mechanism of the hydrothermal carbonization process starts within the hydrolysis of cellulose, hemicellulose, and extractive substance, followed by the demethanation, dehydration, and decarboxylation reactions. The pathways are illustrated in Figure 3. It was observed that the HTC process led to a significant change to the chemical and physical properties of hydrochar: the H/C and O/C ratios decreased, mainly due to dehydration and demethanation reactions. A similar trend was reported by Parshetti et al. [50] and Saetea et al. [51]. The longer residence time of the HTC process also caused changes: the H/C and O/C ratios of the hydrochars approached values similar to those associated with lignite, and further dehydration and demethanation effects probably occurred in the process.

Table 3. Wettability of SS and hydrochars determined by the solid surface contact angle with water.

Parameter	SS	HTC_0.5h	HTC_1h	HTC_2h
Contact angle with water, °	75.7 ± 6.0	75.7 ± 3.7	75.5 ± 3.5	81.8 ± 4.8
Polarity index	0.64	0.42	0.40	0.38

summarizes the contact angle values for raw sewage sludge and for hydrochars derived at different points in the hydrothermal carbonization process. The measurements were collected from the surface of pelletized material in the first second of distilled water drop contact. Figure 4, in turn, depicts an example of the drops obtained for all materials in the first second of the measurement. It can be noticed that the average contact angle value for HTC_0.5h and HTC_1h does not differ significantly from the average CA value for raw sewage sludge. These values are approximately 75° for all materials, thus indicating that these materials are moderately hydrophilic, as a contact angle above 90° allows for the surface to be classified as hydrophobic. The highest contact angle values were recorded for HTC_2, where the average contact angle was approximately 82°, but values closer to 90° were also recorded for individual drops. Therefore, only hydrochar, obtained after 2 h of the hydrothermal carbonization process, can be considered to be almost hydrophobic. It should also be taken into account that preparing a pellet from such a complex material may be problematic, and, hence, deviations from the average are considerable. The value of the contact angle is also significantly influenced by the roughness of the obtained surface [51]. It is worth noting that for HTC_05 and SS, the drops spread on the surface quickly (from approximately 2 s of the measurement), while for the smooth, more uniform surface of the HTC_1h and HTC_2h, the drop remained unchanged for longer on the surface of the pellet. It can be concluded that as the time of the hydrothermal carbonization process increased, the obtained hydrochars were characterized by a more uniform surface, on which the liquid droplet remained longer compared to the raw material. The most favourable contact angle values were, in turn, obtained for HTC_2h, and this material can be considered as close to hydrophobic. In brief, as the process time increases, the contact angle value for hydrochars increases. The results concur with the results obtained by He et al. [52]. The authors observed changes in the contact angle for hydrochar obtained from sludge at 4, 6, 8, 10, and 12 h. It was demonstrated that as the time of the process increased, the nature of the resulting hydrochar became more hydrophobic, and the droplet stayed on its surface for longer. One explanation is that it could be related to the decreasing of the oxygen containing functional groups along with the time of the process, while carbon aromaticity structure increased, thereby rendering hydrochars highly hydrophobic. Generally, the hydrothermal carbonization process increases the hydrophobicity of the obtained hydrochars in comparison to the raw product [49,50,52], which was demonstrated in this experiment, notably for HTC_2h.

According to the ultimate analysis, the polarity index was determined to support and confirm the observations of dewaterability from the separation procedure. Dewaterability refers to the ability of a material to release water or reduce its water content. The polarity index, as outlined in Table 1, suggested that an extended duration was more effective in enhancing the hydrophobic properties of the hydrochars. Conversely, shorter treatment times resulted in hydrochars exhibiting a stronger attraction to water, as longer treatment times yielded hydrochar with improved dewatering capabilities. Śliz et al. [53] obtained similar results. As the time of the HTC process increased, the polarity index decreased, causing a better dewaterability performance of the material. In addition, the lowest value for the polarity index, describing the hydrophobicity of hydrochar, was calculated for HTC_2h. This result is in line with the highest value of the contact angle determined for this sample (Table 3).

The HTC process resulted in changes in the ash composition of hydrochars in comparison to sewage sludge (

Table 4. Results of XRF analysis for ash derived from SS and hydrochars, %wt.

Oxide	SS	HTC_0.5h	HTC_1h	HTC_2h
Na2O	0.7367	0.3982	0.4549	0.3546
MgO	4.3358	4.1074	4.1114	4.1368
Al2O3	7.5850	7.8706	7.9795	8.0470
SiO2	26.5249	27.0726	26.9042	26.3887
P2O5	21.8986	21.5918	21.8070	21.5085
SO3	4.0171	4.4008	3.6788	4.6645
Cl	0.0549	0.0340	0.0267	0.0304
K2O	1.9380	1.3713	1.3655	1.2909
CaO	20.8553	21.0838	21.361	21.4059
Fe2O3	9.2019	9.1599	9.3642	9.1782
ZnO	0.6566	0.6768	0.6972	0.6919
PbO	0.0385	0.0353	0.0385	0.0375
Cr2O3	0.0647	0.0525	0.0454	0.0429
NiO	0.0791	0.0798	0.0777	0.0824
CuO	0.1170	0.1144	0.1241	0.1124
TiO2	1.0303	1.1142	1.0801	1.1633
MnO	0.3377	0.3548	0.3565	0.3640
Rb2O	0.0055	0.0066	0.0069	0.0066
SrO	0.1879	0.1884	0.1947	0.1923
ZrO2	0.1879	0.0647	0.0565	0.0631
SnO2	0.0143	0.0132	0.0185	0.0182
BaO	0.2043	0.1951	0.2360	0.2095

). The distribution of the identified oxides in the sewage sludge ashes can be ordered from highest to lowest values: SiO₂ > P₂O₅ > CaO > Fe₂O₃ > Al₂O₃ > MgO > SO₃ > K₂O > TiO₂. After the HTC process, minor changes were found regarding applied residence time. For HTC_0.5h and HTC_2h, the composition is arranged as follows: SiO₂ > P₂O₅ > CaO > Fe₂O₃ > Al₂O₃ > SO₃ > MgO > K₂O > TiO₂, while for HTC_1h, no changes were observed. The HTC process decreased the content of Na₂O, MgO, P₂O₅, Cl, K₂O, Cr₂O₃, and ZrO₂ and increased Al₂O₃, CaO, ZnO, TiO₂, MnO, Rb₂O, and SrO, while the other oxides did not follow any particular trends. Within the increasing time, the content of Al₂O₃, CaO, ZnO, and MnO increased and the content of K₂O and Cr₂O₃ decreased. Correspondingly, this suggests that longer residence times affect the accumulation and leaching of individual metal oxides [54]. It has been observed that the HTC process has a significant effect on reducing Na content. Na and K are mainly present as highly soluble ions such as sodium or potassium nitrate and chloride and are therefore easily removed by HTC [55,56]. P also accumulated in the hydrochars, and no significant change in the content of this element in the ash was observed. This suggests that the temperature applied does not indicate a strong correlation with the content of this element. There is a necessity to recover this element and find a method to release it into the HTC liquid. For example, Song et al. [57] examined the efficiency of phosphorus removal and the properties of the corresponding hydrochars with the addition of various acids.

On the basis of the XRF results, indicators suggesting operational problems during the combustion of sewage sludge and hydrochar were determined (

Table 5. Slagging and fouling indices.

Index	SS		HTC_0.5h		HTC_1h		HTC_2h
	Value	Evaluation	Value	Evaluation	Value	Evaluation	Value	Evaluation
RB	37.07	Low melting point	36.12	Low melting point	36.66	Low melting point	36.37	Low melting point
RB/A	1.05	Medium slagging tendency	1.00	Medium slagging tendency	1.02	Medium slagging tendency	1.02	Medium slagging tendency
SR	43.54	High slagging tendency	44.08	High slagging tendency	43.58	High slagging tendency	43.18	High slagging tendency
RS	1.36	Medium slagging tendency	1.28	Medium slagging tendency	1.31	Medium slagging tendency	1.35	Medium slagging tendency
Fu	2.82	High slagging tendency	1.77	High slagging tendency	1.86	High slagging tendency	1.68	High slagging tendency
LF	2.74	-	2.75	-	2.72	-	2.78	-
Fe2O3/CaO	0.44	Formation of eutectics	0.43	Formation of eutectics	0.44	Formation of eutectics	0.43	Formation of eutectics

). They characterize the behavior of the ash and its tendency of deposition on the heating elements, which can cause slagging, corrosion, and growth and agglomeration of ashes on heating elements. Typical indices include: S_R—viscosity index, R_S—slagging index, F_u—fouling index. In the case of the slagging index, both hydrochars and sewage sludge displayed a medium tendency to slagging (R_S = 0.6–2.0). Concerning the viscosity index, worse results were achieved, with a high tendency towards slagging (S_R < 65) for all samples. The fouling index also showed a high tendency towards slagging and fouling (0.6 < F_u < 40). The ratio of Fe₂O₃/CaO suggested that eutectics may be present, which enhance slag formation (0.3–3.0). As no significant changes in the individual parameters were observed, it was very difficult to assess the effect of hydrothermal carbonization on the ash properties of the treated sewage sludge. This was also influenced by the lack of specific trends for individual parameters.

The process of hydrothermal carbonization also affects the combustion of the material and its characteristic combustion parameters. In general, the combustion process can be divided into two main stages and classified on the basis of mass and heat changes. The first stage is combustion, when oxidation and a release of volatile substances, which is an exothermic process, occurs as well as oxidation of the residue. The second stage, on the other hand, is the combustion of char [46]. Regarding the combustion profiles for hydrochars and sewage sludge, the TG/DTG/DSC curves are depicted in Figure 5. For sewage sludge (Figure 5a), ignition occurred at 205 °C, and the first stage lasted until a temperature of 373 °C was reached. The combustion rate in the first stage was 2.34%/min and was lower than the combustion rate of the second stage, which was 3.86%/min. The second stage ended when the sample burned out and the temperature at that moment was 526 °C. In connection with hydrochars, for each of the test samples, i.e., HTC_0.5h, HTC_1h, and HTC_2h (Figure 5b–d), the results are very similar, proving that HTC unified the fuel properties of the pretreated material. Furthermore, the TG profiles of hydrothermally carbonized samples differed and moved slightly towards higher temperatures in comparison to sewage sludge. In addition, a sharp decrease was observed between 250–350 °C. The first stage lasted until 363 °C, 360 °C, and 359 °C, respectively. The combustion rates of stage I and stage II were also very similar, for stage I: 2.87%/min, 2.91%/min, 2.67%/min and for stage II: 2.03%/min, 2.17%/min, 2.19%/min. Therefore, it can be seen that the process temperature did not have a significant effect on the individual combustion temperatures, whereas the HTC process itself most certainly did. In addition, aafter the hydrothermal treatment, a much higher quantity of solid residue was collected: for HTC_0.5h—45.97%, for HTC_1h—45.21%, and for HTC_2h—48.33% in comparison to sewage sludge which equaled 32.35%, indicating that inorganics which accumulated in the solid HTC products were slightly related to a longer residence time.

The characteristics’ temperatures, times, and combustion indices, summarized in

Table 6. Combustion parameters.

Parameter	SS	HTC_0.5h	HTC_1h	HTC_2h
Ti, °C	205	247	243	247
ti, min	17.97	22.05	21.65	22.02
Tb, °C	526	478	472	475
tb, min	49.83	45.05	44.48	44.80
t0.5, min	19.62	23.87	20.64	23.63
t1, min	25.00	27.77	27.58	27.53
T1, °C	276	306	304	304
DTG1, %/min	2.34	2.87	2.91	2.67
T2, °C	499	401	399	398
DTG2, %/min	3.86	2.03	2.17	2.19
DTGmean, %/min	1.00	0.80	0.81	0.76
Di, %·min−3	0.0052	0.0047	0.0049	0.0044
Db, %·min−4·10−5	9.6	9.6	11.5	9.1
S, %·min−2·°C−3·10−8	10.6	7.9	8.5	7.1
Hf, °C	822.4	1038.9	984.2	1041.5

for the individual samples, also show the effect of the HTC process on the combustion of hydrochars in comparison to sewage sludge. Characteristics’ temperatures included the following: T_i—ignition temperature defining the beginning of the combustion, T₁—the maximum peak temperature and T_b—burnout temperature, indicating ignition at higher temperatures and later ignition times in comparison to sewage sludge. Hydrothermally carbonized samples at 0.5 h, 1 h, and 2 h ignited at 247 °C, 243 °C, and 247 °C, respectively, for periods of 22.05, 21.65, and 22.02 min, whereas sewage sludge was recorded as 205 °C and 17.97 min. Burnout temperatures also reached similar values: 478 °C, 472 °C, and 475 °C, and burnout times equalled 45.05, 44.48, and 44.48 min unlike 526 °C and 49.83 min found for sewage sludge. One of the indicators determined is the combustion index S, which reflects the ignition, combustion and burn-up properties of the fuel. Sewage sludge had the highest S index, which decreased after the HTC process. This suggests that the high volatile content of the sewage sludge caused an increase in this index. Another indicator, H_f, describes the rate and intensity of the combustion process: the lower the indicator, the higher the intensity of the sample. The highest value of this index was observed for sewage sludge, indicating slightly better combustion properties for SS than for hydrochars. The parameter D_i was also determined, which reaches the highest value for SS, indicating that more volatile substances were separated from the fuel and that combustion occurs easily in the initial phase.

4. Conclusions

The residence time of the hydrothermal carbonization process did not significantly affect the physical and chemical properties of hydrochar taking into account the fuel properties of hydrochar. However, when a longer residence time was applied, a more hydrophobic character of the hydrochar was found suggesting more efficient dewaterability. In addition, at a longer residence time, a slightly higher mass yield was achieved. The combustion behavior and key combustion parameters of the hydrochar in comparison to sewage sludge were determined, confirming that hydrochar was combusted at higher temperatures for a shorter time and resulted in a higher ash content after the combustion process. Therefore, the operational problems during the combustion process of hydrochar based on XRF data were identified indicating that a slagging and fouling tendency might occur.