Influence of hydrothermal carbonized sewage sludge on coal water slurry performance

Abstract

Coal utilization, as a major energy source, raises sustainability concerns and environmental impacts, prompting researchers to explore blending it with other feedstocks. This study discusses hydrochar coal-water slurry (HC-CWS) preparation conditions, emphasizing apparent viscosity and exploring the influence of high ash content on char reactivity. The study highlights that the presence of free water in sludge is moderately influential, while high amounts of free water in raw sewage sludge (SS) and its near absence during hydrothermal carbonization (HTC) of SS are both unfavorable for enhancing the overall performance of coal-water slurry (CWS). HTC reduces the concentration of hydroxyl functional group, enhancing slurry performance and reducing ash content in HC-CWS, indicating that coal complements hydrochar (HC). High-temperature HC preparation is unsuitable for HC-CWS due to increased viscosity and decreased stability. In terms of ash content, the optimal pH and HC ratio for CWS are determined at 30% HC. The gasification reactivity of HC, prepared at 180 °C with a 30% HC ratio in CWS at R_0.5 is 6 × 10⁻³ and at R_0.9 is 9 × 10⁻³. However, increasing HC to 50% diminishes reactivity under CO₂ atmosphere. The inhibitory effect was observed with an increasing percentage of HC in CWS and the synergy factor decreased in the following order: 10% HC > 30% HC > 50% HC, i.e., from 1.04 to 0.35. The possible reason is the presence of high ash content and their similar initial gasification rates during its early stages.

Graphical abstract

Keywords

hydrochar coal water slurry / high ash content / slurry properties / gasification reactivity

1 Introduction

Coal, a major energy source, plays a crucial role in energy production through coal gasification. In this process, carbon-enriched raw material undergoes thermochemical conversion into syngas, which can be utilized for power generation and upgraded for fuel production. The non-sustainable utilization of coal and its escalating environmental impact has prompted researchers to explore alternatives, leading to the blending of coal with other feedstock like biomass, organic waste, and sewage sludge (SS) [1]. Rapid surge in population and the concomitant production of SS is a key concern from social, economic, and ecological perspectives [2]. Its volume is projected to reach around 90 million tons by 2025. It comprises elevated moisture content along with a substantial presence of pathogens, parasite eggs, heavy metals, and other difficult-to-degrade toxins [3]. Additionally, it emits a strong objectionable odor, rendering it economically unfeasible to be utilized as a fuel source [4]. Different thermochemical conversion techniques can be used for better utilization of SS such as gasification, pyrolysis, and torrefaction [5], but pretreatment is needed because of its high moisture content. For conversion of high-moisture-content feedstock, hydrothermal carbonization (HTC) has demonstrated itself as one of the most effective pretreatment techniques. It uses water as solvent in sub-critical conditions at various residence times, temperature, as well as pressure, resulting in high-value product, i.e., hydrochar (HC) [6,7]. HTC reduces colloidal structure, enhances dehydration properties, and reduces the quantity of sludge. Moreover, coal water slurry (CWS) is a complex solid-liquid mixture utilized as a fuel [8]. Unfortunately, because of less exploration and development, slurries are not widely used. Generally, a typical coal sludge slurry is designed by mixing 50–75 wt % of SS, 25%–50% water, and 1% additives. Numerous studies such as [4,9−13] were focused on CWS mixed with raw SS depicting various flow properties based on quality of feedstock, size of particle, temperature, and pH [14]. A few conclusions could be drawn from previous studies：(1) in terms of viscosity, if the concentration of solids in raw material is high, the resulting CWS will demonstrate non-Newtonian behavior having yield stress, and high power is needed for mixing, (2) coal percentage should be greater than 55% as no significant difference was seen between slurries with solid loading of less than 55 wt %, (3) sludge should not be greater than 15% because mixture is not uniform when the maximum solid loading is exceeded, (4) viscosity increases in un-treated feedstock sludge slurry with increasing shear rate, (5) the greater the amount of sludge, the greater the apparent viscosity (AV) will be, and (6) modification of SS with different salts. Previous research was mainly focused on preparation of coal sludge slurry in terms of viscosity, shear rate, solid concentration, particle size, thixotropy loop area and almost all studies were focused on selecting the optimum ratio of sludge in terms of solid concentration. It has been pointed out that conversion method affects AV. Moreover, due to the floccular structure on the surface of sludge, mixing the sludge with other feedstock will reduce its slurry ability and the resulting slurry will become unstable [4]. The water does not act as a lubricant between solid particles [9]. We postulate that, pre-treatment of SS is necessary for coal slurry preparation because (1) it improves the rheological behavior of CSS [15], (2) it increases stability and heating value of the slurry, (3) it possesses high mineral matter content [9] and the resulting slurry rendered unstable, and (4) during HTC of SS, OH and COOH functional groups are removed which increases the stability of slurry. Through HTC of SS, free water is removed, and there is no increase of water proportion in the slurry, therefore satisfying the general requirement of CWS, i.e., preparing highly concentrated slurry. Increasing HTC conditions influences structural properties of HC, i.e., increasing surface area and pore volume [16]. Numerous co-gasification reactivity studies of char and coal have been performed by thermo-gravimetric analyzer (TGA) under CO₂ atmosphere. Co-gasification of coal with biomass depicts key advantages over coal gasification. Carbon conversion and gas yield tend to increase due to inorganic catalysts in biomass content [17,18]. Recent studies were focused on optimizing operating conditions and on increasing H₂ and improving syngas quality [3,19−26]. Yu et al. [27] reported that gasification efficiency was inhibited by ash content of the biosolids in SS. Hence, it is logical to investigate the gasification reactivity and synergistic effects of HC mixed with coal focusing on ash content. In previous studies, the ratio of sludge to coal did not exceed 25% [9,11]. Researchers concluded that the ratio should not go beyond 15% in terms of AV because the acceptable fluidity for CWS at shear rate of 100 s^–1 is 1000 mPa·s [28]. Although it is mentioned that the conversion process affects the AV, the specific mechanisms or quantitative effects of this impact are not fully understood. Furthermore, pre-treatment of SS is necessary for coal slurry preparation to improve rheological behavior and stability. However, the details of how pre-treatment achieves these improvements, as well as any experimental evidence supporting this assertion, are not provided. Therefore, there is a gap in understanding the precise relationship between the conversion process and the AV of the slurry. Addressing these gaps through further research and analysis would contribute to a more comprehensive understanding of the factors influencing the preparation and stability of coal slurry, as well as the optimization of pre-treatment processes for improved slurry properties. Therefore, the specific objectives of this study are: (1) to investigate the efficacy of pre-treatment of SS for coal slurry preparation, focusing on the mechanisms underlying improvements in rheological behavior and stability; (2) to optimize the HC to coal ratio in HC-CWS beyond previously suggested thresholds (25%), assessing its impact on fixed AV and ash content; (3) to evaluate the non-isothermal gasification of mixture of HC and coal using TGA to investigate potential synergistic effects and gasification reactivity.

2 Experimental

2.1 HTC of SS

HTC tests were performed using a high-pressure batch reactor (HT-250 J0, HTLAB, Beijing, China). The HTC reactor is equipped with electrical heating system and a stirrer capable of withstanding peak pressures and temperatures of 30 MPa and 350 °C, respectively. For each run, 500 g of SS were introduced in reactor vessel along with 50 g of deionized water. The mixture was thoroughly blended at four distinct temperatures (180, 200, 220, 240 °C) for three different durations: 30, 60, and 90 min. Following the closure of the reactor, an inert gas, specifically argon (Ar), was used to remove any traces of air from the HTC reactor, creating an atmosphere without oxygen. Furthermore, the pressure within the reactor was maintained autogenically through the vapor pressure generated by water at the designated operating temperature. After the completion of the HTC reaction, the reactor was cooled down, leading to the collection of HC. The separation of the aqueous and solid phases was achieved using vacuum filtration. The HC obtained from HTC was dried in an oven at 105 °C for 24 h. It was then stored in a desiccator for analysis.

2.2 Preparation of HC-CWS

Shenhua (SH) coal was crushed and sieved by meshes of 40 to 100 µm by coal pulverizer and separated by electrical strainer in different particle sizes. HC-CWS was prepared by mixing coal with calculated amount of deionized water and HC at different proportions, i.e., 10%, 30% and 50% with fixed percentage of naphthalene-sulfonic acid polymer with formaldehyde sodium salt (0.5%). The solid concentration was controlled at 60%. The mixture was stirred at 1000 r·min^–1 for 15 min to ensure the homogenization of slurry and finally the AV of slurry was analyzed by Rheometer. The particle size analysis was performed by Camsizer particle analyzer which is shown in Fig. 1. In this study coarse and fine coal particles ranging from (40 to 100 µm) were analyzed and selected as shown in Fig.1. In addition, the same range was selected for HC prepared at 180 °C at three different residence time, i.e., 30, 60 and 90 min as shown in Fig.1. It is important to consider that excessively fine grinding of coal can actually lead to stability issues in the slurry. When particles are too small, they have a higher tendency to settle out of suspension, which could negatively impact the consistency and homogeneity of the CWS. By selecting particles within a similar size range in our study, it allows for better homogenization and increases the overall stability of the HC-CWS mixture. It can be concluded that chosen particle size range is evenly distributed throughout the slurry and shows a balance between achieving finer grinding, maintaining desirable viscosity and stability characteristics in the slurry for improved performance.

Fig.1 Particle size of (a) raw SS and SH coal and (b) HC prepared at 180 °C at 30, 60, and 90 min.

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2.3 Non-isothermal gasification by TGA

For TGA studies, HC-CWS were dried at 105 °C for 24 h and gasification of raw SS, HC and HC-CWS was analyzed using TGA (NETZSCH STA449 F3). During each run, around 10 mg of sample was loaded in alumina crucible heated under CO₂ atmosphere. The gas flow rate was set at 120 mL·min^–1 during non-isothermal conditions, i.e., 1000 °C at heating rate of 20 °C·min^–1.

The gasification carbon conversion X was calculated as follows:

(1)$X=\frac{m_{\mathrm{o}}-m_{\mathrm{t}}}{m_{\mathrm{o}}-m_{\mathrm{f}}},$

where m_o, m_t and m_f are the initial, instantaneous and final mass of the sample, respectively.

To compare the gasification reactivity with different percentages of HC, the gasification reactivity index R_0.5 and R_0.9 are calculated as follows:

(2)$R_{0.5}=\frac{0.5}{t_{0.5}},$

(3)$R_{0,9}=\frac{0.9}{t_{0.9}},$

where t_0.5 and t_0.9 is the time when carbon conversion rate reaches 50% and 90%, respectively.

2.4 Characterization techniques

Basic composition of HC was analyzed by ultimate analyzer, proximate analyzer and X-ray fluorescence as shown in Tab.1 and Tab.2. Numerous characterization techniques such as Fourier transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and slurry properties were measured to analyze the structure and its correlation with HTC operating conditions.

Tab.1 Proximate and ultimate analysis of HCa)

Sample	Ultimate analysis/(wt %, d)						Proximate analysis/(wt %, d)
	N	C	H	S	O		Ashd	VMd	FCd
SH coal	0.93	77.22	4.550	0.590	9.320		6.890	32.91	60.20
HC 180-30	1.58	20.54	2.461	0.353	9.116		65.95	27.24	6.81
HC 180-60	1.57	19.80	2.101	0.384	12.315		63.83	28.98	7.66
HC 180-90	1.56	19.61	2.025	0.424	12.951		63.43	30.80	9.37

a) Oxygen (%) = 100 – (Carbon (%) + Hydrogen (%) + Nitrogen (%) + Sulfur (%) + Ash (%)).

Tab.2 Ash composition of HC

Sample	SiO2	Al2O3	Fe2O3	P2O5	CaO	K2O	MgO	TiO2	SO3	Na2O
HC 180-30	47.59	18.11	13.02	12.90	2.42	1.68	1.69	0.984	0.772	0.580
HC 180-60	47.21	18.19	13.32	12.70	2.42	1.67	1.67	0.984	0.750	0.605
HC 180-90	47.03	18.09	13.33	13.23	2.35	1.65	1.62	0.985	0.648	0.559

2.4.1 Surface properties

The coal, raw SS, and HC was analyzed using SEM (Quanta 250, FEI, USA) to assess their apparent morphologies. The functional groups were identified using an FTIR (Thermo Scientific Nicolet iS10, USA). The materials hydrophobicity or hydrophilicity was assessed using the contact angle method. In this procedure, about 1 g of powder was compressed into a tablet using a pressure of 20 MPa for a duration of 3 min. Subsequently, a solution containing 0.1 weight percent of KY33 was applied onto the tablet. The contact angle was measured utilizing a contact angle measurement equipment (JC2000D1, Shanghai Zhongchen Digital Technology Co., Ltd., China).

2.4.2 Slurry properties

Apparent viscosities of slurry were analyzed at temperature of 25 °C, shear rate of 200, 0.01 to 100, and 100 to 0.01 s^–1 by Rheometer. The AV was determined at a shear rate of 100 s^–1 three times and averaged.

3 Results and discussion

3.1 Basic properties of HC and HC-CWS

3.1.1 SEM

SEM images of raw SS and HC prepared at different HTC operating conditions were analyzed by SEM as shown in Fig.2. It provides particle morphology, surface features, and structural changes induced by HTC. The SS had dense clusters with no apparent pores or channels, resulting in limited water drainage from the sludge matrix as shown in Fig.2. Additionally, the structures of HCs as shown in Fig.2 underwent substantial alterations with increasing temperature (180, 200, 220 and 240 °C) and residence time (30, 60 and 90 min). The prolongation of carbonization time and temperature resulted in an augmentation of both the fragmentation and porosity of hydrochars [29]. It is due to the emission of volatile gases during devolatilization and the cleavage of chemical bonds in the sludge matrix. The porosity can facilitate more rapid desiccation of the generated hydrochars at HTC temperature. The presence of high reaction temperature and extended residence time has a negative impact on the porous structure of hydrochar. These operating conditions of HTC cause the structure to collapse and become blocked, resulting in limited porosity and a consequently small surface area [30]. The floccular and sponge-like structure of raw SS and HC is the main reason affecting slurry’s water retention properties [31]. The floccular structure of SS is ruptured, its particles become loose and the gap is readily filled amongst particles of coal, increasing the stability of HC-CWS [4]. Furthermore, interestingly, at a temperature of 180 °C the structure was slightly disrupted, leading to an acceptable AV range. As far as the free water in floccular structure is concerned, previous studies have concluded that free water in raw SS is suitable for preparing highly concentrated CWS but they modified its structure with different chemicals, alkali salts, and additives [11]. In summary, it can be inferred that the process of HTC led to the formation of a porous surface leading to pronounced permeability at 180 °C compared to higher temperature.

Fig.2 SEM images of HC prepared at different operating conditions: (a) raw SS, (b) 180 °C-30 min, (c) 180 °C-60 min, (d) 180 °C-90 min, (e) 200 °C-30 min, (f) 200 °C-60 min, (g) 200 °C-90 min, (h) 220 °C-30 min, (i) 220 °C-60 min, (j) 220 °C-90 min, (k) 240 °C-30 min, (l) 240 °C-60 min, and (m) 240 °C-90 min.

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3.1.2 Functional group analysis

The FTIR spectra of HC prepared at various HTC conditions are depicted in Fig.3. Here, it should be noted that FTIR of HC prepared at 180 °C at 30 min was done as it shows the optimum amount of HC in terms of ash content.

Fig.3 FTIR spectra of (a) raw SS and SH coal, (b) 180 °C at 30, 60 and 90 min, and (c–e) HC-CWS (30%) against different time at the same temperature.

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The band seen at 3400 cm^–1 was assigned to the stretching vibration of single bonds in OH or carboxyl groups, as depicted in Fig.3. Additionally, the intensity of the coal peak decreased in comparison to that of the raw sludge as seen in Fig.3. This pertains to the process of removing moisture from untreated sludge during HTC. The intensities of bands from 3000 to 2800 cm^–1, linked to stretching vibration of aliphatic carbon –CH_x, did not exhibit significant fluctuations. Previous study conducted by Malhotra et al. [32] particularly revealed the asymmetric and symmetric –CH stretching of methylene groups at 2925 and 2850 cm^–1, respectively. In HTC, hydroxyl group undergoes reduction, as demonstrated by [8], resulting in enhancement of the slurry’s overall performance. The presence of –OH functional groups has a detrimental effect on the slurry ability of CWS [10]. According to [6], the hydrophobic surface of particles can be converted into a hydrophilic surface due to presence of –OH functional groups in the HC. The apparent surface of hydrophobic particles will undergo a transformation into a hydrophilic surface by the presence of organic molecules containing oxygen in HC. This transformation negatively impacts the capacity of CWS to form a slurry. Consequently, it is necessary to eliminate these functional groups, which further hampers the slurry ability of CWS. Therefore, it is recommended to remove them [33]. The peak at 2920 cm^–1 corresponds to the stretching vibration of alkanes and indicates existence of a –CH₂ on the surface. This methylene group is part of hydrophobic (water-repellent) groups, which enable it to interact with the hydrophobic end of an additive in HC-CWS. Furthermore, at the absorption peak of 2920 cm^–1, the hydrophobicity of the resultant HC and HC-CWS is not affected by HTC, as shown in Fig.3(b). A significant reduction in the relative intensity of HCs was found at 1645 and 1540 cm^–1. The presence of a band at 1645 cm^–1 indicates the stretching vibration of the –C=O functional group in ketone and amide groups [29]. The signal at 1540 cm^–1 was assigned to the asymmetric stretching of –C=O in carboxylic groups. This suggests decarboxylation reactions as confirmed by the alteration in carbon seen in elemental analysis, which indicated liberation of carbon in the form of CO₂ [34]. The presence of a peak between 1150 and 900 cm^–1 [29] indicates a decrease in the ash content of HC-CWS compared to HC as indicated in Fig.3(c–e), which demonstrates that coal effectively decreases the ash content in HC. Hence, the ratio of HC in coal is a crucial parameter that requires meticulous adjustment to enhance the stability of the suspension.

3.1.3 BET

BET low-temperature nitrogen adsorption technique is used to characterize the surface properties. In this study, BET analyzer was used to measure raw SS, coal and HC and their corresponding surface areas and pore volumes as shown in Tab.3.

Tab.3 Specific surface area, pore size and pore volume of raw SS, HC and SH coal

Sample	SH coal	Raw SS	HC (180 °C-30 min)	HC (180 °C-60 min)	HC (180 °C-90 min)
Specific surface area/(m2·g–1)	4.5648	2.0645	19.1363	19.1399	22.2268
Adsorption average pore size/nm	7.3623	18.7025	16.5532	15.6029	14.1106
Desorption average pore size/nm	8.6057	24.0405	24.5947	22.5801	20.0721
Pore volume/(cm3·g–1)	0.008402	0.009653	0.079192	0.074660	0.078408

Specific surface area of raw SS was considerably smaller compared to HC prepared at different reaction conditions as shown in Tab.3. The specific surface area gradually increased with increasing time from 2.064 to 19.136 and 22.226 at 30, 60 and 90 min, respectively, and similar results were found in a previous study [35]. The reason is the decomposition of unstable compounds leading to formation of cracks on its surface in the structure of SEM and the same trend was found in a previous study [31]. The surface properties of SH coal are higher than raw SS. When HC and coal are mixed, their specific surface area was enhanced, attracting the free water in HC-CWS, hence increasing the AV.

3.1.4 Contact angle

The hydrophobicity and hydrophilicity of HC and coal was analyzed by contact angle to further confirm the surface properties of HC and coal as shown in Tab.4. Generally, if the contact angle is greater than 90°, the surface is hydrophobic, and the larger the contact angle is, the surface will be more water-repellant and hydrophobic. Previous results suggested that temperature had no significant impact on hydrophobicity [36]. Since OH functional groups are inherently hydrophilic, the reduction of these groups during HTC transforms HC into a nearly hydrophobic material, aligning with findings from previous literature. Furthermore, the HC surface appeared smooth when the droplet was wiped off. Here, it is worth noting that the hydrophobic surface of HC is what makes the solid fuel cleaner [6].

Tab.4 Contact angle measurements of HC and SH coal

Sample	Maximum contact angle/(° )	Minimum contact angle/(° )
HC 180 °C-30 min	89.51	49.07
HC 180 °C-60 min	89.17	56.97
HC 180 °C-90 min	87.11	56.90
SH coal	85.45	33.55

3.2 Influence of operating conditions on HC-CWS as a function of AV

HC-CWS were prepared at different ratios of HC at controlled solid concentration of 60% and their viscosity was analyzed as shown in Fig.4.

Fig.4 Apparent viscosities of HC-CWS (a) at different ratios of 180 °C and 30 min HC, (b) at different ratios of 180 °C and 60 min HC, and (c) at different ratios of 180 °C and 90 min HC.

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Generally, the acceptable fluidity for CWS at shear rate of 100 s^–1 is 1000 mPa·s. Similar trend can be seen in Fig.4. With the increase of HC proportion, the AV is also increased by twofold [15]. Additionally, HC-CWS prepared at temperatures of 200, 220, and 240 °C were not considered as suitable since their AV exceeded 1000 mPa·s. Furthermore, at a temperature of 180 °C, the structure was slightly disrupted as shown in Fig.2(b, c and d), leading to an acceptable AV range. This could explain the AV of HC prepared at temperatures exceeding 180 °C. HC prepared at 180 °C at 30, 60, and 90 min is the only operating parameter in which viscosity is not greater than 1000 mPa·s as shown in Fig.4(a and b). Therefore, the optimal conditions, i.e., ratio of HC, temperature and residence time in terms of fixed AV is addition of 50% of HC in CWS prepared at 180° C and 90 min, respectively as shown in Fig.4. Due to addition of raw SS, more water was retained in the structure, HC-CWS viscosity increased and the pseudo-plasticity was strengthened [14,31]. Based on these results, different conclusions can be drawn: (1) HC prepared at the same temperature with different holding times has different AV concluding that holding time is also one of the factors influencing the viscosity of slurry, (2) HC prepared at high temperature is not suitable for HC-CWS and (3) the AV of HC-CWS prepared at high temperature seems to fluctuate, even when tested 3 times. The possible reason for its fluctuating nature is its complex composition and structure [31]. In recent studies, it has been concluded that if its structure is well understood and comprehended, we may be able to find the logical reasons behind its unpredictable behavior.

3.3 Fixed AV in terms of ash content and pH

Proximate analysis of HC-CWS with different proportions was measured as shown in Tab.5 since one of our objectives of this study was to optimize the ratio of HC in CWS in terms of ash content and pH. Generally, the acceptable ash content in the gasifiers is about 25%, otherwise it will be detrimental to the gasifier and eventually damage the gasifier [27]. Both raw SS and HC exhibit high acidity, but the addition of coal complements both properties by decreasing ash content and maintaining pH. In previous studies, the ratio of sludge to coal had not tested beyond 25% [9,11] and the ratio should not go beyond 15% in terms of AV [28]. However, in current study, maximum amount of HC was selected and tested to optimize its ratio in terms of fixed AV and ash content. With the increase of the ratio of HC in CWS, the ash content also increases due to the inherent presence of high ash content in raw SS, as well as HC. The ash content and volatile matter in HC-CWS were not affected by holding time, as they increased insignificantly with increasing time. However, the fixed carbon is the only parameter which tends to decrease with increasing ratio of HC. Moreover, the pH remained stable, i.e., not being too acidic or alkaline. Therefore, the optimum pH and ratio of HC in CWS in terms of ash content is 30% of HC without being affected by holding times.

Tab.5 Proximate analysis of HC-CWS with different proportions of HC

Samples	HC-CWS
	Ash (air db)/%				VM				FC				pH
HC	10%	30%	50%		10%	30%	50%		10%	30%	50%		10%	30%	50%
180 °C-30 min	12.20	23.61	36.19		27.50	30.61	29.09		57.90	45.59	32.55		6.37	6.12	5.89
180 °C-60 min	11.76	23.91	34.09		29.90	30.80	29.50		56.97	45.48	36.59		6.20	5.96	5.79
180 °C-90 min	12.29	24.13	33.05		30.21	31.36	31.26		58.21	44.51	37.55		6.94	5.96	5.62

3.4 Non-isothermal gasification of HC-CWS

The CO₂ gasification tests were conducted on raw SS and HC samples prepared at a temperature of 180 °C for 30, 60, and 90 min. The experiments were carried out using TGA under non-isothermal, as illustrated in Fig.5(a) and Fig.5(b). TGA quantifies the mass loss of a substance as it is heated, with respect to temperature, and differential thermogravimetry (DTG) evaluates the rate at which this weight loss occurs in a thermo-balance [33]. The thermal process of all samples exhibited three distinct phases of mass reduction: volatilization and decomposition of volatile matter (240–400 °C), combustion of fixed carbon (400–510 °C), and conversion of the remaining solid into gas (700–1000 °C) as shown in Fig.5. When the temperature was raised to 975 °C, the SS had a significant mass loss [37]. The application of HTC pretreatment can break down volatile matter and enhance the amount of non-active ash in the HC, leading to a reduction in overall mass loss. The impact was intensified by the elevated HTC temperature influencing the decomposition temperature as shown in Fig.5. Research has shown that HTC increases the first volatile matter temperature from 333 to 363 °C. This suggests that HTC enhances its stability, requiring more energy for decomposition [38]. A comparable outcome was noted during the FC combustion phase, with a little rise from 456 to 460 °C. During the gasification conversion phase, the application of HTC pretreatment resulted in a reduction of the initial gasification temperature.

Fig.5 (a) TGA and (b) DTG of HC at 180 °C at different holding times.

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Carbon conversion was calculated as depicted in Fig.6. This study demonstrates the impact of time on the conversion process during CO₂ gasification. The conversion curve and conversion rate curve of various samples exhibit a notable resemblance, indicating that the gasification of the HC samples has experienced changing degrees of decline [39]. The decline in gasification capability of HCs is attributed to the dehydration and decarboxylation processes that occurred during HTC. These reactions lead to a reduction in the O/C and H/C atomic ratios of the sample [40]. These reactions effectively concentrate the carbon content in the molecule relative to oxygen and hydrogen thereby reducing the efficiency of the conversion process. This reduced ratio makes the fuel less reactive during gasification, leading to less efficient conversion. The gasification reactivity is reduced due to (1) fewer oxygenated functional groups and H atoms, (2) energy requirement will be increased to break down stable aromatic structures and (3) decreased volatile matter and increased FC content. Furthermore, the temperature curve at the end of each Fig.6(a–d) is attributed to the catalytic impact of ash content on the gasification of the HCs, as well as the existence of intense decarboxylation and demethylation reactions during the high-temperature HTC process. These reactions lead to a decrease in VM content and FC content, while increasing the ash content. It has been observed that ash has a specific catalytic impact on the gasification performance of SS-derived HC and HC-CWS. It also demonstrates that the carbon conversion rate was higher in HC processed at 180 °C for 30 and 60 min, with both durations showing a similar conversion rate as shown in Fig.6(a). However, the conversion rate of the HC generated at a residence period of 90 min is reduced, indicating that time of HTC affects conversion rate. It was due to extended residence time during HTC, which leads to more complete carbonization, forming stable and condensed aromatic structures that are less reactive during gasification. It also results in a significant loss of volatile matter and a decrease in surface area and porosity, making the HC richer in fixed carbon and harder to gasify. Conversely, shorter residence time results in less extensive carbonization, retaining more reactive oxygenated and aliphatic groups, preserving volatile matter, and maintaining higher surface area and porosity. This enhances the reactivity and improves the carbon conversion rate during gasification. In Fig.6(b–d), it was observed that a 50% concentration of HC in CWS required a shorter duration to achieve complete conversion. The reactivity of gasification was reduced when the quantity of HC in CWS increased. Similar findings had been reported in other studies [19,41]. Additionally, as compared to raw SS, coal needed a shorter time to reach complete conversion as shown in Fig.6(e). It can be concluded that the gasification reactivity of a CWS containing 30% HC was higher compared to other compositions. However, when the HC content was increased to 50%, the reactivity reduced in a CO₂ environment. This information is supported by [17,42]. Moreover, the characterization results justify that the ideal ratio of HC in CWS for optimal gasification reactivity is 30%.

Fig.6 Carbon conversion curves against gasification time (min).

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3.5 Synergistic behavior during co-gasification reactivity

Synergistic effect in the co-gasification of coal and biomass is the phenomenon where the combined effect surpasses the sum of the individual effects observed when each material is gasified independently. The gasification reactivity index is affected by various parameters, including the composition of the solid fuel, particle size, moisture content, and gasification conditions, for instance, temperature, pressure, and gasification agent. Varying fuels and their conditions can provide different reactivity indices, with a higher index indicating a fuel that is more reactive and perhaps more readily gasifiable. The impact of synergy behavior on co-gasification reactivity can be assessed by comparing X-t curves obtained from experimental and computed data. The experimental X-t curve exhibited a lesser magnitude than the calculated X-t curves, during co-gasification reactivity [43]. In their study, Ma et al. [44] introduced the concept of the synergy index (A_X,T) to assess the changes in the synergistic index during co-gasification reactivity at various conversion levels. Equation (4) displays the formula for the synergy factor.

(4)$A_{X,T}=\frac{t_{X,T\mathrm{c}\mathrm{a}\mathrm{l}}}{t_{X,T\mathrm{e}\mathrm{x}\mathrm{p}}},$

where t_X,T,cal and t_X,T,exp represent the experimental and calculated gasification time required for carbon conversion of X when samples are subjected to gasification at a temperature of T, respectively.

Gasification reactivity is considered to be larger than 1 when there is a synergistic influence on co-gasification reactivity during the conversion of X. The changes in the gasification reactivity index of HC-CWS at various conversion levels, with variable proportions, are shown in Fig.7. This work calculates the co-gasification reactivity in non-isothermal conditions, considering different ratios of HC. The reaction rate for raw SS and SH coal at R_0.5 is 3.508 × 10^–2 and 1.224 × 10^–2 as shown in Fig.7(a). However, the reactivity of HC prepared at 180 °C for 30, 60 and 90 min at R_0.5 is around 2.8 × 10^–2, 2.9 × 10^–2, 2.9 × 10^–2, respectively, as shown in Fig.7(b). The reaction rate for 10% of HC is 1.233 × 10⁻² min^–1 at R_0.5 and 1.937 × 10⁻² min^–1 with a ratio of R_0.9 as shown in Fig.7(c) and Tab.6. When the HC is increased to 30%, the gasification performance decreases with R_0.5 at 1.257 × 10⁻² and R_0.9 at 1.929 × 10⁻² min^–1. Moreover, the gasification reactivity experiences a significant decrease when the HC is raised to 50%, in contrast to its levels at 10% and 30% HC. At HC of R_0.5, the reactivity is measured to be 6.527 × 10⁻³ min^–1, whereas with an HC of R_0.9, the predicted value is 9.950 × 10⁻³ min^–1. Therefore, the reactivity index might be arranged in the following order: 10% HC > 30% HC > 50% HC. This pattern corresponds to the results documented by previous researchers [45,46]. Additionally, in terms of synergy factor, the value of A_X,T is decreasing with the increase of HC proportion in CWS as shown in Fig.7(d). The study conducted by Ma et al. [44] found that the synergy factor decreased in the following order: 10% HC > 30% HC > 50% HC, i.e., from 1.04 to 0.35 at R_0.9 respectively. This inhibition is likely caused by reduced contact between HC and coal, as well as their comparable initial rates of gasification reactions during the initial stages (0.016 to 0.014) as indicated in Tab.6, supported by Ding et al. [47]. Furthermore, the high ash content of HC and differences in ash behavior between coal and HC have different ash content and behavior during co-gasification. The interaction of ash components leads to ash-related issues such as slagging, fouling, or the formation of undesirable compounds that negatively impact gasification because it controls the rate at which carbon is converted. This inhibitory impact has been observed in studies conducted by Satyam Naidu et al., and Zhang et al. [48,49]. The presence of K₂O, Al₂O₃, Fe₂O₃, Na₂O, and TiO₂ has been found to have inhibitory effects, as demonstrated in Tab.2 [50].

Tab.6 Initial gasification rate and reactivity index at R0.5 and R0.9

Sample	R0.5/min–1	R0.9/min–1	Initial gasification rate
Raw SS	3.508 × 10−2	2.230 × 10−2	0.018
SH coal	1.224 × 10−2	1.910 × 10−2	0.039
180-30	2.890 × 10−2	2.250 × 10−2	0.010
180-30-10%	1.233 × 10−2	1.937 × 10−2	0.016
180-30-30%	1.257 × 10−2	1.929 × 10−2	0.014
180-30-50%	6.527 × 10−3	9.950 × 10−3	0.014

Fig.7 Carbon conversion curves X against gasification reactivity (min–1) of (a) raw SS and SH coal, (b) 180 °C, 30, 60 and 90 min, (c) different ratios of HC-CWS and (d) synergy index A of HC-CWS.

Full size|PPT slide

These studies suggest that synergistic effects lower than one may be possible during co-gasification of coal and HC, as mixing with coal results in low NH₃ and HCl contents [51,52]. Here, it is worthy to note that when raw sludge and coal was gasified separately, raw SS had higher carbon conversion rate than coal which is attributed to high inorganic content present in SS compared to coal. Secondly, the above carbon conversion curves of HC-CWS are not linear. Therefore, it is recommended that pyrolysis can be done for the stabilization of HC which is further gasified for better conversion efficiency. It should be noted that numerous co-gasification studies of coal and biomass, as referred to in the Introduction section, have been conducted. To the authors’ best knowledge, synergistic effects in non-isothermal co-gasification of SS-derived HC and CWS have not been previously investigated. Therefore, there is still a need to explore concrete reasons behind gasification behavior of HC alone and mixed with CWS. For future studies it is recommended that comprehensive isothermal and non-isothermal gasification studies can be done for better understanding.

4 Conclusions

Coal is one of the major sources of energy, but its utilization is not sustainable which motivates researchers to add coal with other feedstock. In this study, SS-derived HC preparation conditions in terms of AV and influence of high percentage in ash content on the char reactivity is discussed. Basic characterization has been performed and SEM showed that floccular structure of raw SS was destroyed; the particles became looser and readily filled the gap among particles of coal. Addition of coal in HC is complementing both properties, i.e., decreasing ash content and maintaining the pH, and optimum pH and ratio of HC in CWS in terms of ash content is 30% without being affected by holding times. However, in terms of AV the optimum percentage of HC was 50%. HC-CWS prepared at temperatures of 200, 220 and 240 °C were not considered as suitable since their AV exceeded beyond 1000 mPa·s. OH in the HC is damaging to the slurry ability of CWS therefore they should be removed. There is a significant reduction in –OH groups in HC compared to coal. However, when 70% coal is mixed with 30% HC, the –OH group remains relatively balanced, neither significantly reduced nor increased. This complementary property of coal contributes to the overall performance of slurry. The reactivity of HC prepared at the temperature of 180 °C and different residence time, the gasification reactivity at 30% HC in CWS is higher and when the HC amount was increased up to 50%, the reactivity was lowered under CO₂ atmosphere. Synergy factor decreased in the following order: 10% HC > 30% HC > 50% HC, i.e., from 1.07 to 0.35 respectively. This inhibition is caused by reduced contact between HC and coal, as well as their comparable initial rates of gasification reactions during the initial stages.

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Competing interests

The authors declare that they have no competing interests.

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