Hydrothermal carbonization for energy-efficient processing of sewage sludge: A review

Renewable and Sustainable Energy Reviews 108 (2019) 423–440

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Hydrothermal carbonization for energy-efficient processing of sewage sludge: A review

T

Liping Wanga,∗, Yuzhi Changb, Aimin Lic a

School of Ecology and Environment, Inner Mongolia University, Hohhot, 010021, Inner Mongolia, China Environmental Monitoring Center, Jining Environmental Protection Bureau, Ulanqab, 012000, Inner Mongolia, China c School of Environmental Science & Technology, Dalian University of Technology, Industrial Ecology and Environmental Engineering Key Laboratory of Ministry of Education, Dalian, 116024, Liaoning, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Sewage sludge Hydrothermal carbonization Thermochemical conversion Clean biofuel production Resource utilization

Hydrothermal carbonization is an important thermochemical conversion process that can be used as an energyefficient alternative to enhance the dewaterability of sewage sludge and meanwhile to convert sewage sludge into high value-added products, such as clean biofuel, organic fertilizer and precursors of functional materials. This paper presents an overview of the latest development of hydrothermal carbonization in the field of sewage sludge treatment, with a particular focus on critical hydrothermal parameters, physicochemical characteristics of products streams, current understanding on hydrochar formation mechanisms, sewage sludge dewaterability improvement and techno-economic advantages. Recent advances have shown that hydrothermal carbonization of sewage sludge is an exothermal process, which is governed by temperature to a large extent. Both polymerizations of highly reactive intermediates derived from degradation of biopolymers in sewage sludge and solid-solid conversion of their undissolved fractions are regarded as the major mechanisms of hydrochar formation. The high ash content of hydrochar is probably the limiting factor for its potential applications in energy and functional materials. The chemistry in hydrothermal carbonization of sewage sludge, closely related to the process parameters and the chemical composition of sewage sludge, offers huge potential to influence the products distribution and characteristics and the process energetics as desired, which provides a promising opportunity to construct a high-efficiency industrial chain for energy and resources recovery from sewage sludge by a controlled hydrothermal process. This review identifies the current challenges and knowledge gaps, and provides new perspectives for future research efforts targeting at sustainable treatment of sewage sludge by hydrothermal carbonization.

1. Introduction It has been estimated that billions of tons of wastewater, including domestic and industrial wastewater, are generated in the world each year and show an increase trend in the future due to the growth in population and the improvement in living standards [1]. Wastewater treatment plants have proven to be an effective way to eliminate the pollutants such as NH4eN, PO4eP and COD in wastewater, which are becoming more significant to ecological improvement and environmental protection. The biological wastewater treatment, especially activated sludge process, is the most widely used technique for municipal sewage treatment plants in the world. A problem that needs to be considered carefully is the efficient and environmentally friendly disposal of sewage sludge generated as a by-product by these plants during wastewater treatment. ∗

Sewage sludge is a generic term including primary sludge, secondary sludge, digested sludge and various industrial sludges according to the wastewater source and the treatment process. The increasing awareness of the problems associated with sewage sludge are the continuous increase in production, the pollution risks on environment and human health, the high costs of conventional treatments and the low level of resource cycling. It is estimated currently that at least 50 million tons of sewage sludge with moisture content of 80% will be produced within the European Union annually [2]. The annual sewage sludge production in both the United States and China is similar, which are all estimated to be approximately 40 million tons on the 80% moisture content basis [3,4]. Due to high organic substances content and biological activity, sewage sludge normally has less favorable dewatering properties, resulting in the dewatered sludge by mechanical means still with moisture content higher than 65% and with limited

Corresponding author. E-mail address: [email protected] (L. Wang).

https://doi.org/10.1016/j.rser.2019.04.011 Received 2 June 2018; Received in revised form 31 March 2019; Accepted 3 April 2019 1364-0321/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Phase diagram (a) and properties of water at 25 MPa (b) as a function of temperature [22,141]. SCW: supercritical water.

implementation has only been developed with a comparably low effort. There are several literature surveys that cover hydrothermal carbonization, liquefaction and gasification of lignocellulosic biomass [12–14] and hydrothermal liquefaction and gasification of sewage sludge [15,16], but to the best of our knowledge few reviews are specifically focused on hydrothermal carbonization of sewage sludge. To support the further development of hydrothermal carbonization as a possible application for sewage sludge treatment and to help highlight the areas that require more research, a summary of published literature is provided. In particular the critical process parameters, physicochemical characteristics of products streams, hydrochar formation mechanisms, dewaterability improvement and techno-economic advantages will be discussed in detail.

effective disposal routes. As a result, further moisture removal has been one main obstacle to sewage sludge treatment and disposal. Currently sewage sludge management accounts for approximately 50–60% of the total operating costs of wastewater treatment plants [5]. More importantly, because valuable compounds (organic carbon, phosphorous and nitrogenous) and hazardous substances (heavy metals, pathogens, and persistent organic pollutants) are present in one mixture, the traditional application of sewage sludge as a fertilizer in agriculture has become increasingly under pressure. Landfill of sewage sludge is also facing the decline in available land space, together with the potential pollution risk to soil and groundwater. For conventional thermal treatments, such as incineration, pyrolysis and gasification, a predrying procedure is inevitably required, which leads to the most of energy invested and released during these thermal treatments being consumed to remove the moisture retained in sewage sludge. Therefore, exploring innovative and sustainable technologies for sewage sludge treatment are urgently required. Hydrothermal carbonization, known as an exothermally thermochemical process, is considered to convert sewage sludge into carbonaceous products called hydrochar at the typical temperatures around 180–250 °C under the autogenously saturated pressures for several hours, together with the byproducts including a large amount of liquid phase (process water) and a little gas (mainly CO2). It was first introduced by Bergius, the Nobel Prize winner in 1931 for invention and development of chemical high-pressure methods, with the aim of understanding the mechanism of natural coalification by hydrothermal transformation of cellulose into coal-like materials in laboratory as early as 1913 [6]. From then on, extensive studies have been conducted by many scholars to explore the involved mechanisms by model compounds of biomass such as cellulose, lignin, glucose, sucrose, starch, xylose etc. [7–9]. It has been proved that water medium is very important to hydrothermal process and the mechanism is governed by several chains of chemical reactions of hydrolysis, dehydration, decarboxylation, polymerization and coalification, which are only valid up to a certain degree of reaction severity. Technical applications for hydrothermal carbonization have also been a long history and can be traced back to 1850s, which always use the carbonization effect to realize the dewatering and upgrading of low-rank coals [10,11]. The conversion of sewage sludge into high value-added products or biofuel by hydrothermal carbonization has attracted considerable attention recently. This not only solves the problems associated with sewage sludge disposal from the view of resources recycling but also contributes to the greenhouse gas segregation for climate change mitigation, as well as some additional socio-economic benefits. Compared to hydrothermal liquefaction and gasification, however, until recently hydrothermal carbonization attracts great attention as a promising technology for sewage sludge treatment, and the technical

2. The role of process parameters 2.1. Moisture content It has been observed that the hydrous condition is very important to the reaction mechanism of hydrothermal carbonization and the carbonization process is accelerated by water [17]. Water can be considered as a good heat transfer and storage medium, which not only enhances the efficiency of heat transfer in preheating process but also avoids the local overheating due to the exothermal reactions during hydrothermal carbonization. In order to suppress the occurrence of liquefaction and gasification but promote carbonization process, the operation of hydrothermal carbonization is comparatively mild and should be limited to subcritical conditions of water (Fig. 1a). In subcritical conditions, temperature and pressure have a great effect on the properties of water (Fig. 1b). With increasing temperature, a decrease of the density can be observed for high-temperature water, along with a decline in the dielectric constant, while the ionic product increases dramatically [18]. The changes in the extent of dielectric constant is closely related to the corresponding changes of hydrogen bonding structure of water, of which the decrease makes the behavior of water is more like a polar organic solvent. For example, the solvent properties of water at 300 °C are roughly equivalent to that of acetone at 25 °C [19]. As a result, some organic molecules that are previously considered unreactive undergo chemical reactions due to increased solubility in water. Chemical reactions occurring in high-temperature water with high density are generally dominated by ionic pathways, while lowdensity water favors free radical reactions [20]. So water under hydrothermal carbonization conditions provides a favorable medium for ionic reactions of non-polar organic compounds [21]. This contributes primarily to the bond cleavage of hydrogen bonds, especially hydrolysis. The increase in ion product (dissociation constant, Kw) leads to the fact that the concentrations of H+ and OH− ions are higher than 424

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literature.

that in ambient liquid water, favoring the high-temperature water as an effective medium for both acid- and base-catalyzed reactions [22]. On the other hand, water molecules may participate in carbonization reaction steps as reactants or products, facilitating hydrolysis, hydrogen exchange, condensation and cleavage, although the amount of water that is indispensable to keep up these influences is very small. Moisture content represents a significant factor which is found to affect the hydrothermal carbonization process, including products distribution and reaction network. In general, sewage sludge with higher initial moisture content to start hydrothermal carbonization will result in lower hydrochar yield, and also the hydrochar with lower calorific value. This is easily explained by the faster conversion of sewage sludge at lower solid content, which facilitate the organics to be nearly completely dissolved, leaving little residue mainly dominated by mineral matters. However, a high solid load shows a positive influence on the overall residence time, likely by the fast increasing concentration of monomers in the liquid phase, which promotes the polymerization to start earlier, leading to a larger fraction of solid precipitation [23]. It has been reported that the carbonization reaction significantly occurred in subcritical water when the moisture content is decreased, leading to the increased formation of hydrochar from sewage sludge [24]. In order to maximize hydrochar production, the solid load should be as high as possible but the transport of fragments first produced by hydrolysis out of the sludge matrix is probably limited, which might become the rate determining step [25]. Hence typically, hydrothermal carbonization of sewage sludge has been reported at the moisture content of 75–99%. From the point of whole engineering practice, both chemical and economic reasons are decisively under consideration. Sewage sludge after mechanical dewatering, generally with moisture content of around 80–85%, therefore appears to be beneficial for the hydrothermal processes, by reason of high availability of mechanical means for sewage sludge dewatering and high potential of reducing energy input for hydrothermal carbonization system. Until now, however, no publication has been found to systematically investigate the influence of moisture content which needs further investigation.

⎛− 3500 ⎞ TK ⎠

f = 50⋅ts0.2⋅e⎝

=

Ofeed − Ot Ofeed − 6

(1)

The hydrothermal carbonization of sewage sludge has been extensively investigated by a number of workers, mostly at the temperature range of 120–300 °C, aside from the study of He et al. [29], who explored the fuel characteristics of hydrochar obtained at higher temperature of 220–380 °C and a highest fuel ratio of 3.66 was found at 320 °C. Indeed, elevating of reaction temperature adequately has some substantial benefits to carbonization process and products characteristics, for example, increasing reaction rate, promoting conversion level, enhancing carbon and energy content in hydrochar, and improving dewaterability [30]. However, on the premise of satisfying the requirements of sewage sludge disposal routes, the temperature should be low enough to prevent the significant release of organics into process water, avoiding the notable reduction of hydrochar yield and the increase in processing difficulty of the liquids. The breakdown of main macromolecular components of sewage sludge is determined to be temperature dependent. As single substances, polysaccharides, in general, are much easier to be hydrolyzed than proteins, followed by lipids [31]. Nevertheless, when hydrothermal carbonization of sewage sludge is conducted at above 150 °C, the decomposition rate of proteins seems to be higher than that of polysaccharides, suggesting proteins in sewage sludge are more susceptive to temperature [32]. This is probably because sewage sludge is a complex mixture, which leads to the reaction rates of each individual component in sewage sludge are affected by each other and further the kinetics of the overall process. The rapid rise in soluble biopolymer concentrations is found at around 130–150 °C and strongly affected by hydrothermal reactions at above 150 °C [32,33]. When temperature is higher than 180 °C, the soluble polysaccharides and proteins can be hydrothermally decomposed [34]. As a result, some unexpected products with dark brown color are produced following the Maillard reactions (Fig. 2). These formation of polycyclic nitrogenous compounds, due to the condensation reactions between reducing sugars from carbohydrates and N-terminal amines from proteins, have been implicated in poor biodegradability of the process water [35]. In addition, lipid oxidation products may also react with amino acids to form Maillard reaction products as the similar role of reducing sugars in an oxygenlimited environment [36]. As a pretreatment of anaerobic digestion of sewage sludge, reported optimal temperature of hydrothermal carbonization is between 160 and 180 °C [37]. An immiscible liquid phase has been observed at temperature higher than 250 °C, which indicates that hydrothermal liquefaction occurs and lower temperature is desired [38]. Therefore, a moderately feasible temperature range is around 160–250 °C for hydrothermal carbonization of sewage sludge, which is beneficial to produce coal-like products and/or to obtain bioenergy from liquid by-products by subsequent anaerobic digestion in practical implementation.

2.2. Temperature There is no doubt that the reaction network of hydrothermal carbonization and further the products distributions and characteristics are all governed by temperature to a large extent. The key role of temperature is to offer sufficient heat to disintegrate the organic macromolecules for fragmentation and recombination of chemical bonds with high activity. Hydrothermal reactions thus become increasingly important at higher temperatures which significantly accelerate the rates of degradation and polymerization of sewage sludge compounds. There is a competition among reactions of depolymerization and polymerization with the increase in temperature and residence time. During initial stages of carbonization at lower temperature, depolymerization of sewage sludge is always the dominant reactions to form fragments, while polymerization of the extensive fragmented species becomes active at the later stages of higher temperature, which leads to the formation of hydrochar [26]. The formation and enhancement of aromatic structures in hydrochar has also been identified at higher temperatures, which results in the ordering and rearrangement of hydrochar structure [27]. Ruyter [28] developed a model of hydrothermal carbonization (Eq. (1)) by defining a conversion factor (f) according to the combination of residence time (ts, s) and temperature (TK, K), based on the changes of oxygen content (dry ash-free basis) and the assumption of complete conversion with the level of subbituminous coal (6%). Ofeed and Ot are the oxygen percentage of feed and hydrochar, respectively. This semiempirical model indicates the time-temperature equivalence and there appears to be possibility of reaching the similar carbonization level by adjusting residence time at a valid temperature range, whereas the verifiability about that is absent for sewage sludge within published

2.3. Residence time It is not surprising that residence time is an important factor during hydrothermal carbonization of sewage sludge, which shows some influence not only on the energy balance and operating costs of hydrothermal system but also on the products distribution, as well as their chemical composition and characteristics. A longer residence time generally increases carbonization severity. This effect, however, seems to be limited in the relatively short residence time scale used in current studies, because hydrothermal carbonization of sewage sludge is reported to be a comparably slow process where the overall rate of reactions is likely controlled by the diffusion mechanisms of organics decomposition and polymerization [14]. Reported residence time for hydrothermal carbonization of sewage sludge varies from several 425

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Fig. 2. Maillard reaction network based on interactions of glucose and glycine as starting reactants [71].

Fig. 3. Correlation between higher heating values (HHV) and carbon content of sludge and hydrochar [29,32,38–40,81,91,119,129,138].

reactive, which finally leads to a significant increase in hydrochar yield by polymerization for a longer residence time [14]. There is lack of a systematic study to date on the influence of residence time on hydrochar yield over a long enough time scale including an identification of turning point. Within a limited residence time in laboratory, hydrothermal carbonization of sewage sludge with short reaction time ranging from 30 min up to an hour could already lead to the produced hydrochar with a remarkably higher heating value than the original due to the removal of oxygen-rich compounds [40]. Results from some

minutes (15 min) to some hours (24 h), which are too short compared to the nature coalification. As a result, there appears to be little possibility to reach similar result which is obtained at higher temperature only by adjusting the residence time. It has been reported that a higher quantity of hydrochar is obtained from sewage sludge at shorter reaction time [39]. This could be explained by the depolymerization of biomacromolecules in sewage sludge and the further degradation of the resultant intermediates via cleavage, dehydration, decarboxylation and deamination for a long residence time. However, some of the fragments formed from the abovementioned decomposition may be highly 426

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temperature of lower than 200 °C, which have a significant influence on the morphology of the carbon nanomaterials [51]. Given the substantial amounts of heavy metals and salts contained in sewage sludge, it is meaningful to understand the particular mechanism of the potential catalytic effects of them in hydrothermal carbonization, but only limited research exists on this topic.

experiments with increase in residence time seem to show a slow increase in higher heating value [30,39]. While a slight decrease in higher heating value is also observed in other studies even when reaction time is as long as 8 h at 200 °C and 260 °C [38,40], which may be closely related to the increase of ash content in hydrochar with a longer carbonization time resulting from dissolution of biopolymers. It is unclear whether such a contradiction in calorific value is due to the different carbonization effects of sewage sludge, such as the extraction of oxygen-rich substances and/or the hydrolysis of compounds with low higher heating values, or whether the separation operations of hydrochar and liquid, for example, the choice of filtration membrane and washing the solid product or not, lead to the change of chemical composition of hydrochar. However, it is worth noting that the higher heating value (HHV) of hydrochar shows a positive correlation with its carbon content (Fig. 3). Based on the experimental data from published papers, a linear regression relationship was also obtained, which seems to provide a possibility for predicting the HHVs of hydrochar by carbon content with < 10% error. As well as higher temperature, longer reaction time is unfavorable for porous structure of hydrochar, limiting the potential application in adsorption [41]. Nevertheless the increasing of reaction time is significantly beneficial to the improvement of rheological property of sewage sludge, permitting more convenient transportation of products and more compact reactor design [42]. From an industrial point of view, hydrothermal carbonization must be performed in a short residence time to increase efficiency and save cost. Some published results have indicated that the residence time of 30 min that contributes to a significant solubilization and decomposition of the biopolymers and a substantial conversion in sludge properties is usually employed [43,44]. Here it should be noted that this duration is just the reaction time at the target temperature, excluding preheating time. In fact, the heating and cooling rate and stirring characteristics certainly have considerable influence, resulting in varying heat and mass transfer conditions.

2.5. Heating rate In principle, heating rate is one of the most crucial parameters for hydrothermal carbonization, which has a considerable influence on the intermediates formation and products distribution. According to the results obtained by Brand et al. [52] that slow heating rate (2 °C min−1) favored the solid residue with an increased higher heating value and an lower O/C and H/C ratio, compared to fast heating rate (20 °C min−1). A slow heating rate can provide enough reaction time to promote decomposition of biomolecules and recombination of intermediates. Obviously, the same effect also exists for the cooling rate, which is generally out of control when the water/air cooling is employed. It should be noted that the heating rate impact is strongly dependent upon the final temperature. The heating method mostly adopted in hydrothermal carbonization is autoclave reactor with an external electric heater [53], oil bath [30] or superheated steam [54]. In thus hydrothermal carbonization processes, however, the role of heating rate and its influence on reaction mechanisms have not been discussed in detail so far.

3. Fundamental mechanisms Considering the significant effort in trying to understand the hydrothermal reaction process of biomass, the practical application of such technology using hydrothermal carbonization for reforming sewage sludge should be possible in the near future. To date, many chemical reactions that have been qualitatively identified in hydrothermal carbonization are mentioned throughout the literature, which include hydrolysis, dehydration, decarboxylation, polymerization and aromatization. A separate discussion of these general reaction mechanisms have been reviewed by Funke and Ziegler [10]. Nevertheless, the detailed reaction networks including thermodynamics and dynamics are only partly understood due to the high complexity of sewage sludge components and multiple possible reaction pathways. The following sections will be specifically focused on the current understanding of the chemistry taking place within the hydrothermal carbonization of sewage sludge, in order to provide useful information about the possibilities of manipulating the process as we desired.

2.4. Catalyst Compared to direct hydrothermal carbonization, hydrothermal carbonization in the presence of catalyst can enhance hydrochar yield and quality, promote deoxygenation and denitrogenation, and produce functional carbonaceous materials. Different acids, alkalis and metal salt catalysts have been utilized in many hydrothermal carbonization processes [45,46]. Escala et al. [47] have reported that the use of citric acid as a catalyst in hydrothermal carbonization of stabilized sludge accelerated the carbonization reactions, leading to the hydrochar with a higher heating content. Arrhenius acid like H2SO4 is generally used to accelerate the dehydration reactions by catalytic effect [13]. Carbon dioxide produced from decarboxylation is believed to promote the hydrolysis reactions of biopolymers as an acid catalyst due to the formation of carbonic acid in hydrothermal carbonization, but which diminishes with increasing temperature (higher than 260 °C) [48]. Acetic acid, which as primary acid product contributes to a decrease in activation energy of biopolymers decomposition, has been suggested to perform a catalytic role in hydrothermal carbonization [49]. It is very essential and urgent to gain an understanding of the catalysis mechanism of organic acids formed in hydrothermal carbonization process because these acids may play an important role in the reaction network. Nonetheless, the detailed knowledges about the autocatalytic effects of such organic acids on hydrothermal carbonization are limited. Much attention has been focused on the catalyzed hydrothermal carbonization of carbohydrates in the presence of metal ions, which can effectively accelerate the carbonization process and also induce the formation of carbon-metal composite materials with nanoarchitectures, such as (Ag, Cu, Pd or Te)@carbon composite nanocables and microcables [50]. Iron ions and iron oxide nanoparticles have been shown to catalyze the hydrothermal carbonization of starch and rice grains at

3.1. Hydrothermal conversion of organic components It is well known that sewage sludge contains a significant amount of organic matters which are predominantly proteins, polysaccharides, lipids, humic substances and nucleic acids (Table 1), of which the relative amount could vary greatly according to the sewage sources and technological processes. Both proteins and polysaccharides generally account for about 90% of the volatile suspended solids after sewage sludge is concentrated by settling [55]. These organic ingredients that constitute microorganisms and extracellular polymeric substances are first depolymerized into their corresponding monomers during hydrothermal carbonization, followed by the decomposition of the resultant monomers via hydrolysis, dehydration, decarboxylation and deamination, as well as the recombination of produced reactive fragments through condensation polymerization. It is understood that these various reactions occur simultaneously in hydrothermal carbonization of sewage sludge to form a parallel and complex reaction network. Some detailed information about the basic reaction mechanisms can be provided by individual degradation of biopolymers. 427

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Table 1 Chemical composition of sewage sludge. Sewage sludge

Volatile solids (% TS)

Proteins (% TS)

Polysaccharides (% TS)

lipids (% TS)

Humic substances (% TS)

Primary sludge Digested sludge Activated sludge

60–80 30–67 50–88

14–30 15–20 19–41

8–15 8–15 5–10

13–65 5–20 5–12

8–14 11–19 6–20

TS: total solids. Data from Refs. [135–137].

hydrothermal carbonization, from further decomposition of which the intermediate products are mainly fructose, furan derivatives and aldehydes by isomerization, dehydration and CeC bond cleavage [62]. The 5-HMF derived from glucose dehydration are likely the main reactive species in the heterogeneous medium of hydrothermal carbonization, which are highly favorable to the formation of hydrochar by reactions of condensation and polymerization, specifically reverse aldol condensation and intermolecular dehydration [63]. The formation mechanism (Fig. 5) of solid carbonaceous spheres derived from carbohydrates under hydrothermal conditions has been illustrated in detail in the study of Zhang et al. [64]. This hydrochar microspheres generally present a core-shell chemical structure consisting of a highly hydrophobic aromatic nucleus and a hydrophilic shell with a high level of reactive oxygen functional groups [8]. Hemicellulose is a heteropolysaccharide composed mostly of pentose, hexose and uronic acid units and xylan is generally used as the model substance. Compared to cellulose, hemicellulose hydrolyzes more rapidly due to the random and amorphous structure with little strength. As a result, hemicellulose begins to decompose at around 180 °C, while hydrothermal degradation of cellulose requires higher temperatures of about 200 °C [65]. Hemicellulose depolymerization (Fig. 6) takes place first through hydrolysis and cleavage of glycosidic bonds during hydrothermal carbonization, and then the produced xylose is dehydrated and undergoes retro-aldol condensation, which leads to main production of furfural (2-furaldehyde) and small parts of other intermediates such as glycolaldehyde, glyceraldehyde and dihydroxyacetone [66]. It is surprising that the hydrothermal decomposition products of hemicellulose, unlike cellulose, seem to be negative to repolymerization to form solid product, but show a significant gasification activity with increase in temperature and reaction time [67]. Starch is a polysaccharide consisting of a large number of glucose monomers joined by α-1,4 and α-1,6 glycosidic bonds, which is used by bacteria as the energy storage material, specifically glycogen, a more branched amylopectin. The hydrolysis of starches is much easier and faster than cellulose decomposition under hydrothermal conditions [48]. It has been reported that a complete solubilization of starch was observed at 180 °C for 10 min and the highest production of glucose was found at 200 °C for 30 min or 220 °C for 10 min [68]. Higher temperature and longer reaction time result in a significant degradation of glucose which is primarily converted into 5-HMF, while it is significantly more effective to increase the glucose yield and to suppress the production of 5-HMF by precisely controlling the residence time [69]. The addition of CO2 as acid catalyst in hydrothermal system has been found to effectively promote the glucose production but also to enhance the formation of 5-HMF [70]. There is a negligible production of dextrose and maltose from starch hydrolysis at temperatures below 220 °C, whereas a significant caramelization of starch occurs at 220 °C, resulting in the liquid phase with a tea-color [33]. On the basis of published results, it becomes evident that there are three main reaction pathways for polysaccharides in hydrothermal carbonization. Polysaccharides are first converted into different pentose and hexose species according to hydrothermal carbonization severity, and then monosaccharides are transformed into furfural (2-furaldehyde or 5-HMF) intermediates by intramolecular dehydration, followed by the formation of hydrochar from furans by intermolecular dehydration

3.1.1. Proteins Proteins formed by the peptide bonds linking amino acids account for the largest fraction of organic materials in sewage sludge and as such, its breakdown in hydrothermal carbonization is intimately linked to the decrease in volatile solids, which means that hydrothermal carbonization of sewage sludge is a process of devolatilization [32]. Understanding the hydrothermal conversion of proteins is thus important for predicting the changes of sludge characteristics and constructing the reaction mechanisms of hydrothermal carbonization of sewage sludge. Generally speaking, the peptide bonds in proteins have a much higher stability compared to the glycosidic linkages in polysaccharides, leading to the formation of amino acids by hydrolysis with a lower rate constant. The hydrolysis reaction of proteins under hydrothermal conditions begin first by an attachment of proton to the nitrogen atom of the peptide bond, resulting in a splitting of the peptide bond and the forming of carbocation and amino group [56]. After that, hydroxide ion from a water molecule attaches to the carbocation to form carboxy group [57]. Proteins are therefore converted during hydrothermal carbonization to smaller molecular weight peptides, of which the dominant size fraction is progressively decreased with the temperatures increased, but exceedingly small polypeptides (< 1 kDa) are not generated significantly even at 220 °C [33]. It has been reported that the solubilization and decomposition of proteins in sewage sludge are strongly affected by temperature, which realizes a maximum in aqueous phase at around 150 °C [32] and nearly 30% of the initial proteins is decomposed at temperature of 220 °C [33]. Based on the hydrothermal reforming results of protein model compounds such as glycine and alanine (Fig. 4), the primary mechanisms of amino acids decomposition are identified as deamination and decarboxylation to form volatile fatty acids, hydrocarbons, aldehydes, ammonia and amines [58]. Which of the deamination and decarboxylation is the predominant reaction depends upon the type of amino acids. Reported amino acids yields from proteins are generally below 10% at 250 °C, which are significantly lower than that in conventional acid hydrolysis since the produced amino acids subsequently degrade rapidly by hydrothermal hydrolysis [13]. Under hydrothermal carbonization conditions, the decomposition of produced amino acids seems to be easier (with an activation energy of around 154 kJ mol−1) compared to other biomass monomers and can be described to be the first order mechanism although various amino acids have different chemical structures [59].

3.1.2. Polysaccharides Polysaccharides are polymers of monosaccharide like glucose, mannose and fructose, which are present in sewage sludge primarily in the form of cellulose, hemicellulose and starch [60]. Cellulose is a polysaccharide consisting of a long linear chain of glucose monomers that are connected by β-1, 4 glycosidic bonds. Although there is a high degree of crystallinity due to the formation of strong intra- and intermolecule hydrogen bonds, cellulose can be rapidly solubilized and hydrolyzed under hydrothermal carbonization conditions, resulting in production of oligomers and glucose monomers. It appears that cellulose thermohydrolysis rate is lower than glucose decomposition rate at temperature below the critical point of water [61]. While glucose is still proved to be the primary hydrolysis product of cellulose during 428

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Fig. 4. Reaction network of hydrothermal decomposition of alanine (a) and glycine (b) [58].

Fig. 5. Illustration of the chemical formation of carbonaceous spheres [64].

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Fig. 6. Hydrothermal decomposition pathways of xylan [67].

divided into three main fractions: humic acids, fulvic acids and humins. Fulvic acids are soluble in water under all pH conditions, while humic acids and humins are not water-soluble under acidic conditions and in full range of pH, respectively [76]. It is believed that humic substances are very resistant to further biodegradation during wastewater treatment process and thereby accumulated in sewage sludge, in which humic acids comprise over 80% of the humic substances [77]. However, to the best of our knowledge, little attention has been paid to the fate of humic substances during hydrothermal carbonization of sewage sludge. Fekete et al. [78] investigated the subcritical hydrolysis of humic and fulvic acids and found that the reaction temperature plays an important role in their degradation, leading to the generation of alkylbenzenes, polycyclic aromatic hydrocarbons and heteroaromatics as temperature is higher than 220 °C. It is noteworthy, however, that the concentrations of abovementioned compounds are very low with a magnitude of μg L−1. A longer residence time is required for humic substances decomposition, although the occurrence of reactions like aromatization, polycondensation and cleavage of CeC and C-heteroatom bonds is more dependent on temperature than time. As a model compound of sewage sludge, the supercritical water gasification of humic acid was carried out at temperature up to 600 °C with or without catalyst by some researchers, who suggested that humic substances is difficult to gasify due to the recalcitrant condensed structures, resulting in lower carbon conversions [77,79]. Therefore, there are reasons to believe that during the course of ongoing polymerization of highly reactive intermediates from hydrothermal degradation of sewage sludge, humic substances mainly precipitate together with the formed insoluble solids to form hydrochar.

[63]. Condensation and cyclization of intermediates from polysaccharides degradation may play a major role in the increase in aromatics, which has a high stability under hydrothermal conditions and therefore is considered as a basic building block of the resulting hydrochar [71]. As a result, the hydrochar derived from polysaccharides possesses a polyaromatic networks and a polyfuranic structure. 3.1.3. Lipid Lipids in sewage sludge, which is typically in the form of triglycerides consisting of three fatty acids bound to a glycerol backbone, is a composite that originates from the direct adsorption from wastewater, the phospholipids from cell membranes and the by-products of microbial metabolites and cell lysis. Fatty acids present in sewage sludge are predominantly in the range of C16 to C18, presenting in the chemical form of palmitic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), oleic acid (C18:1) and linoleic acid (C18:2) [72]. Under hydrothermal conditions, the volatile fatty acids production from lipid decomposition is believed to be dependent on both the reaction temperature and the degree of saturation of the fatty acid chain. The increase in temperature causes lipids to become increasingly soluble in subcritical water, showing an exponential increase with temperature [73]. The literature has indicated that a significant production of volatile fatty acids was observed at 170 °C and above, which was largely attributed to the breakdown of unsaturated lipids, almost eight times more than saturated lipids [33]. Hydrolysis reactions of lipids occur primarily at the lipids-water interface in the beginning, which gradually becomes homogenous with increase in the level of free fatty acids from lipids decomposition due to their auto-catalytic effect and thereby promotes further the increase of lipids hydrolysis rate [74]. A first-order kinetics for degradation of lipids and long-chain fatty acids have been confirmed [75]. The lipids hydrolysis probably contributes to the main production of volatile fatty acids for hydrothermal carbonization of sewage sludge at temperature below 220 °C [33].

3.2. Hydrothermal carbonization of sewage sludge The real challenge of hydrothermal carbonization for sewage sludge reforming is never in the conversion of sludge into products but rather in understanding the chemistry taking place during the process to establish the routes of sewage sludge resource treatments in a highly efficient and economical way. Quantitative reaction models based on predominating reaction pathways are indispensable for the reactor

3.1.4. Humic substances Humic substances that are formed due to the microbial degradation of organic matters is a heterogeneous mixture of compounds based on the aromatic nuclei with phenolic and carboxylic groups, which can be 430

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Fig. 7. Change of carbon content (a) and carbon efficiency (b) in hydrothermal carbonization of sewage sludge with increasing reaction severity f (Eq. (1)) [30,32,39,47,81,91,119,129].

natural coal range. So if sludge hydrochar is used for energy-related application, there seems to be an order as follows: activated sludge > anaerobic digested sludge > paper mill sludge > primary sludge. Chemical dehydration generally means elimination of hydroxyl groups to form water, while decarboxylation is partially explained by the removal of carboxyl groups as formation of carbon dioxide. The changes of these chemical function groups can be confirmed by the qualitative and semi-quantitative analysis of FTIR and NMR (Fig. SIeS1). There is a steady decline in the intensity of OeH (32003500 cm−1) and C]O (1540 cm−1) for hydrochar with the increasing reaction severity compared to sewage sludge, together with more alkyl (0–48 ppm) and aromatic carbon (112–165 ppm) groups but lower Oalkyl (44–112), carboxyl (160–190 ppm) and carbonyl carbon (190–220 ppm) groups [41,81]. Hydrothermal carbonization below 180 °C appears mainly to be governed by dehydration reaction whereas significant decarboxylation reaction is connected to more severe conditions and begins to become obvious with temperature up to 210 °C [32]. Although the underlying chemistry of dehydration and decarboxylation is not very clear so far, the rate of dehydration seems to be much higher than that of decarboxylation during the whole process of hydrothermal carbonization. From the point of view of an efficient carbon sequestration, it is meaningful to keep decarboxylation as low as possible in order to allow for a high carbon conversion from sewage sludge to hydrochar, although it is favorable for oxygen removal from sewage sludge. An additional benefit of hydrothermal carbonization is the removal of nitrogen and sulfur, as well as the change of fuel-N release pathways during hydrochar combustion where the release of volatile nitrogen as NH3 is improved and the reaction of NH3 with NO is further enhanced similar to selective non-catalytic reduction process, which eliminate the potential risk of forming gaseous pollutants like NOx and SOx if sludge hydrochar is used as solid fuel to instead of fossil fuel [33,82]. Of particular note is that over 90% of the total phosphorus present in sewage sludge is contained in hydrochar as precipitated phosphate salts after hydrothermal carbonization [83]. After incineration of sludge hydrochar or hybrid hydrochar generated from sewage sludge and other phosphorus-containing waste streams, a phosphate-rich ash is obtained to generate phosphorus products by separating heavy metals with chemical treatments or by thermochemical treatment to obtain new mineral phosphates with an increased bioavailability, such as chlorapatite, farringtonite and stanfieldite [84–86]. This multi-step process suggests a sustainable means to achieve municipal wastes management and meanwhile to recover energy and phosphorus from waste streams. Compared to initial sewage sludge, hydrochar generally has lower volatile matter content and higher ash content, along with a slight increase in fixed carbon content. As shown in Table 2, the level of

design and process optimization. The chemistry behind reactions of individual biopolymers in sewage sludge under hydrothermal carbonization is to some extent well studied, but the detailed chemical pathways, kinetics, and interactions between components of sewage sludge are largely unknown. The main focus of this section is on the underlying chemical reactions of sewage sludge in hydrothermal carbonization on the basis of characteristics of products streams given by the published literature. 3.2.1. Hydrochar characteristics The elemental composition of sewage sludge is significantly changed after hydrothermal carbonization, which results in not only the increase in carbon content but also the decrease in oxygen, nitrogen and hydrogen content. It has been observed that the carbon content in hydrochar is higher than that of initial sewage sludge after hydrothermal carbonization and tends to increase as the severity of carbonization is increased (Fig. 7a). The extent of carbon content enhancement is strongly dependent on sewage sludge compositions and hydrothermal conditions. The hydrochar generated from sewage sludge, however, still has a comparatively lower carbon content which is usually lower than 50% (dry basis). Because chemical composition of sewage sludge varies largely depending on the sources and treatment processes of sewage, there is a remarkable difference in carbon content between hydrochar samples produced from different kinds of sewage sludge [47]. The carbon efficiency, defined as the quotient of carbon in hydrochar and that in initial sewage sludge, is bound to decrease with the increasing reaction severity of hydrothermal carbonization (Fig. 7b). The moderate temperature and short reaction time are required in order to sequester the maximum carbon in hydrochar. After hydrothermal carbonization under normal conditions, most publications agree that the majority of carbon (higher than 60%) is still retained in hydrochar as expected, along with about less than 40% being transferred into the liquid-phase and about 2–5% being lost in the gas-phase [39,47]. In 1960, Van Krevelen [80] proposed a diagram of H/C versus O/C to analyze the chemical transformation of cellulose and glucose during hydrothermal carbonization process and founded that the solid products from these substances show the same composition. For various types of sewage sludge, the carbonization causes an obvious decrease in both H/C and O/C ratios due to dehydration and decarboxylation, which are dependent on the reaction severity (Fig. 8). Of special importance is that the decrease of O/C ratio is much higher than that of H/ C ratio, indicating that hydrothermal carbonization is almost a process of oxygen elimination. The carbonization effects on activated sludge is similar to that on anaerobic digested sludge but shows a much higher level, yet both of which are superior to primary sludge and paper mill sludge. It is obvious that the H/C and O/C ratios of hydrochar derived from activated sludge are approaching the region of lignite and even flame coal, while that from primary sludge are far away from the 431

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Fig. 8. Van Krevelen diagram of hydrochar produced from various sewage sludge samples [30,38,129,138].

volatiles decreases significantly as a result of carbonization, approximately 30% reductions under common conditions. It is obvious that the hydrothermal carbonization of sewage sludge is a devolatilization process, largely due to the destruction of oxygen-containing functional groups of biopolymers, for example, the elimination of hydroxyl and carboxyl groups [32]. This reduction of volatiles is bound to promote the upgrading of hydrochar as solid fuel, confirming that carbonization takes place. Sludge hydrochar has already proved to have a good combustion performance that the ignition and burning process is much easier and more stable than sewage sludge [87]. Unfortunately, the ash content of hydrochar, which is regarded as an unwanted fraction, increases accordingly owing to excess loss of volatile matters and retention of minerals. The increasing extent (19–87%) is linked closely to the initial ash content of sewage sludge and hydrothermal conditions. The addition of wood biomass in hydrothermal carbonization of sewage sludge have shown a positive effect not only on reduction of ash content but also on enhancement of hydrochar yield [88]. From the Table 2, it can be seen that the rise in fixed carbon content is significant at high reaction severity, even reaching up to 131–269% for activated sewage sludge at temperatures over 240 °C, which leads to the properties of hydrochar to be more like coal. It is important to note that the relative content of fixed carbon in hydrochar is usually less than 15%. It has clearly been observed from SEM (Fig. 9) analysis that hydrothermal carbonization greatly altered the surface morphology of sewage sludge, including porous structure and surface area. The morphology of hydrochar derived from sewage sludge is generally tedious, which is conspicuously different from hydrothermal carbonization of

Fig. 9. SEM images of sewage sludge (A) and corresponding hydrochar (B) at 180 °C for 60 min as well as hydrochar from fructose (C) at 180 °C for 6 h and from glucose (D) at 180 °C for 24 h. Images (A) and (B) from Ref. [41]; (C) from Ref. [64]; (D) from Ref. [63].

Table 2 Proximate analysis of sewage sludge and corresponding hydrochar. Sewage sludge

Temperature (°C)

Residence time (h)

Hydrochar yield (db, %)

Volatiles (db, %)

Ash (db, %)

Fixed Carbon (db, %)

Reference

Anaerobic digested sludge Hydrochar Anaerobic digested sludge Hydrochar Activated sludge Hydrochar Activated sludge Hydrochar Primary sludge Hydrochar Paper mill sludge Hydrochar

– 200 – 280 – 260 – 240 – 200

– 6.0 – 0.5 – 8.0 – 0.75 – 4.0

– 73.60 – 80.40 – 66.19 – 78.14 – 60.54

0.5

76.00

40.00 48.40 26.06 40.02 49.85 59.53 18.51 34.68 27.24 38.94 47.44 57.86

8.60 14.20 7.07 12.7 2.66 6.14 2.01 7.42 3.90 5.73 5.18 6.86

[39]

210

51.50 37.20 66.87 47.28 47.49 34.33 78.49 57.42 68.56 55.33 47.38 35.28

432

[81] [40] [129] [30] [138]

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Fig. 10. Ammonia (a) and SCOD (b) production in hydrothermal carbonization of sewage sludge with increasing reaction severity f (Eq. (1)) [32,44,98,130].

from lipids is considerably higher than from proteins, and significantly more volatile fatty acids (4–7 times) are produced from primary sewage sludge than from activated sewage sludge because of the higher unsaturated lipid fractions in the former [33]. Melanoidins, the refractory polymers formed through Maillard reactions, are also identified in the liquids as temperature is over 180 °C, such as aldehydes, pyrroles, pyrazines and pyridines, which is responsible for the liquids with brown in color and bad biodegradability [30,99]. It has been reported that 40–70% of nitrogen, 50–70% of potassium and 10–15% of phosphorus in sewage sludge could be transferred into the liquids and their solubilization is highly influenced by temperature than residence time, particularly at temperature higher than 200 °C [100]. Due to the significant fraction of macronutrients and some little micronutrients within the liquid phase where the concentration of N, K and P is generally around 2000–5000 ppm, 100–600 ppm and 10–200 ppm respectively, there is a potential application to make use of the liquids as organic fertilizer. Hydrothermal carbonization is generally more favorable at low pH level while hydrothermal liquefaction is preferred at alkaline condition from point of product yield and C/H/O composition [50]. Due to the presence of organic acids resulting from decomposition of biopolymers, the liquid phase is generally acidic for activated sludge with a pH of around 5.0, whereas it is remarkably more basic for digested sludge probably due to its high buffering capacity [47]. With increasing reaction temperature, the pH of liquid phase decreases first and then increases at a turning point of around 210–220 °C, owing to high production of alkaline groups like ammonia at higher reaction temperatures [100]. The concentration of NH4eN has been found to show an increasing trend with increasing reaction severity since proteins in sewage sludge are prone to ammonification in particular at high temperatures (Fig. 10a). If temperature is elevated up to the liquefaction range (250–370 °C), the pH value of liquid phase from any sewage sludge probably reaches higher than 8.0, showing alkalinity [101]. Besides, the presence of ionic species and soluble salts in the liquids causes a high value of conductivity ranged from 7.5 to 14.3 mS cm−1 [47]. The first solubilization of biopolymers likely plays a pivotal role in the carbonization rate of sewage sludge. As a primary indicator of biopolymers solubilization, the soluble COD (SCOD) correlates closely with the reaction severity (R = 0.90, P < 0.01), showing a stable increase with increasing both temperature and residence time (Fig. 10b). There has been a broad consensus in recent publications that reaction temperature has a greater effect on COD solubilization than residence time [44,102]. Compared to temperature, it has been observed that there is almost no significant increase in release of SCOD after carbonization at 120–180 °C for 10 min and the reaction time of 30 min appears sufficient for the release of SCOD [32,98]. SCOD levels of the liquids obtained from different sewage sludge are also diverse, which

carbohydrates where the formation of microspheres is observed on the hydrochar surface [89]. High reaction temperature and long residence time are unfavorable for the porous structure of hydrochar due to collapse and blocking, leading to a limited porosity and therefore a small surface area [39]. The specific surface area of sludge hydrochar, which is slightly smaller than that of cellulose hydrochar (around 30 m2 g−1) but much lower than that of biochar (even more than 1000 m2 g−1) from pyrolysis [90], is commonly lower than 20 m2 g−1 [91]. This inferior morphology property can be counterbalanced by hydrochar with abundant oxygen-containing functional groups (Fig. SIeS2) on their surface, since surface functional groups seem to play a more important role in the adsorption process than structural morphology [92], indicating the potential in heavy metals and xenobiotic organics retention from soils and water. In contrast to organic substances, heavy metals in sewage sludge cannot be degraded during hydrothermal carbonization, most of which are accumulated in hydrochar as relatively more stable forms. It has been reported that increasing reaction temperature and alkaline reaction condition show positive effects on accumulation of heavy metals in sludge hydrochar and immobilization of them, resulting in decrement of ecological risk [93,94]. The addition of rice husk in hydrothermal carbonization of sewage sludge presents a synergistic effect on the immobilization of heavy metals, which result in all heavy metals in sludge hydrochar with no eco-toxicity and no leaching toxicity at the addition ratio of 0.57 [95]. However, as a result of the less aromatic structure and higher percentage of labile carbon species in hydrochar, the capability of sludge hydrochar for long-term carbon sequestration may be limited in soil applications. The reported mean residence time for hydrochar application to soils ranges from 4 to 29 years under laboratory conditions [96], which is much lower than that for biochar with approximately 2000 years [97].

3.2.2. Liquid product characteristics Liquid is the major by-product that can be easily separated by filtration or centrifugation from hydrochar slurry. A number of organic compounds are detected in the liquids from hydrothermal carbonization of sewage sludge. Some short-chain organic acids like acetic acid, benzene acetic acid, propionic acid and butanoic acid, as well as several other organics such as furanic, phenolic, aromatic, alkene and aldehyde compounds, are all present in the liquids [30], suggesting that the carbonization pathways of sewage sludge follow the reported mechanisms, including hydrolysis, dehydration, decarboxylation, condensation, polymerization and aromatization. For the formation of volatile fatty acids which may come from degradation of both lipids and proteins, the absolute increase in acetic acid concentration is mostly linked to the temperature and residence time, while harsher reaction severities result in a clear decline in the concentration of propionic acid [32,98]. It has been demonstrated that the yields of volatile fatty acids 433

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1402 cm−1 and C]N at 1622-1658 cm−1), polysaccharides (10001100 cm−1) and lipids (2850-2920 cm−1) are identified after hydrothermal carbonization, providing the evidence of decomposition of organic components in sewage sludge [105]. The first attempt to describe the reaction pathways was made by Takamatsu et al. [106], who suggested the hydrothermal decomposition scheme of sewage sludge in terms of the solubilization of solid matters into soluble evaporative and non-evaporative matters as well as the interconversion of them. The differentiation of evaporative and non-evaporative matters was performed according to the drying test at 120 °C. In another study, Imbierowicz and Chacuk [107] proposed a simple scheme that apart from the direct dissolution of sewage sludge particles, the insoluble highmolecular organic compounds from hydrothermal destruction of sewage sludge particles dissolve into liquid phase and degrade into simple gaseous products, respectively. More recently, a simplified framework incorporating exact biochemical intermediates has been applied to hydrothermal decomposition of sewage sludge by Yin et al. [108], which involves two major steps that sewage sludge first dissolves into macromolecular products (like polysaccharides and proteins) and then the conversion of soluble organic matters occurs in liquid phase by hydrolysis and oxidation, leading to the production of final products like ammonia, acetic acid, H2O, CO2 etc. It is clear that these reaction networks pay more attention to the solubilization and decomposition of sewage sludge and do not incorporate polymerization which is considered as an unwanted side-reaction. For hydrochar formation from hydrothermal carbonization of sewage sludge, the simplified reaction pathways are proposed, as shown in Fig. 11. Hydrolysis is deemed to be the first step of hydrothermal carbonization due to lower activation energy than most of hydrothermal reactions including dehydration, decarboxylation, aromatization and condensation [109], where water reacts with extractives or biopolymers and breaks their chemical bonds with lower activation energy, resulting in various products including soluble oligomers, monomers and other intermediates [10]. Dehydration and decarboxylation of the hydrolyzed products take place immediately after hydrolysis [110]. Most of the furans compounds (mainly 2-Furfural and 5-HMF) from dehydration of monosaccharides undergo condensation, polymerization and aromatization in the liquid phase, along with the N-containing ring compounds from Maillard reactions and the phenolic derivatives from hydrolysis of lignin, as well as the highly reactive intermediates like various organic acids, alditols and aldehydes, which are subsequently converted into solid products with or without auto-nucleation [15,111]. This solid product leads to the formation of desired hydrochar by aggregation and carbonization through further intermolecular dehydration, together with the products from liquid-solid and solid-solid reactions by undergoing devolatilization, condensation, dehydration and decarboxylation [38,112]. Condensation polymerization is the most important reaction processes for the formation of hydrochar, which is most likely governed by step-growth polymerization and can be enhanced by higher temperature and longer reaction time [113]. It is noteworthy that the direct conversion of solid sewage sludge to hydrochar is still lack of strong experimental evidences, except for the proof of hydrochar with porous structure which is probably caused by devolatilization mainly at higher temperatures [114]. Besides, nitrogen-doped aromatic structure is probably assigned to the main fraction of the sludge hydrochar matrix. 2. Kinetics of sewage sludge in hydrothermal carbonization. Except for the hydrothermal carbonization mechanisms, there have also been efforts to describe the kinetics of sewage sludge conversion. The developed kinetic models primarily apply the kinetic approach of Arrhenius and the fitting of experimental data to establish apparent kinetics according to first-order reactions. Yin et al. [108] have confirmed that macromolecular products such as proteins and saccharides are the primary intermediates during initial period of hydrothermal carbonization of sewage sludge, with an activation energy of 51 kJ mol−1 for both of them in the temperature range of 180–300 °C.

varied from thousands to tens of thousands milligram per liter. It becomes evident that the changes in SCOD are the result of solubilization and further decomposition of macromolecules during hydrothermal carbonization. Donoso-Bravo et al. [44] reported that the vast majority of SCOD is attributed to proteins which contribute around 55–60%, followed by lipids (15–25%) and polysaccharides (13–15%). The concentrations of soluble proteins and soluble polysaccharides are all strongly affected by carbonization temperature at 150 °C and above [32,33]. The reported values are generally in the range of 200–33170 mg L−1 for soluble proteins and in the range of 230–13760 mg L−1 for soluble polysaccharides, which are dependent not only on hydrothermal conditions but also on initial content of biopolymers in sewage sludge. Compared to hydrothermal conditions, the initial content of biopolymers probably plays a more important role in determining their solubilization [47]. There has been a considerable controversy over whether the concentrations of soluble proteins and polysaccharides follow a progressive increase or present a turning point with the increasing severity of hydrothermal carbonization. Within this literature survey, except for the studies of Xue et al. [98] and Wilson and Novak [33], most studies seem to support the latter, especially for soluble polysaccharides [32,44,102]. It is reasonable that there is a particular inflection point during hydrothermal carbonization of sewage sludge where the decomposition rate of biopolymers began to exceed their solubilization rate due to occurrence of carbonization reactions, further facilitating the conversion of intermediates to hydrochar. If the liquids are the sole substrate for anaerobic digestion, methanogenesis seems to be the most probable speed-limiting step because there are almost no particulate matters that need to be hydrolyzed prior to methanogenesis. It has been reported that the methane yields using the liquids in an anaerobic filter system is up to 0.18 L CH4 g−1 COD and the methane content in biogas are stable in the range of 69–77%, accordingly resulting in the COD removal of 68–75% even at only 5 days of hydraulic retention time [103]. The activity of methanogens, however, could be inhibited at the beginning of anaerobic digestion because the liquids usually contain high concentrations of volatile fatty acids, ammonia, phenolic compounds and furfural derivatives. The upflow anaerobic sludge blanket reactor seems to be superior to sequencing batch reactor and continuous-flow stirred tank reactor, which shows a high organic loading rate of up to 18 g COD L−1 d−1 and a stable COD removal of approximately 70%, along with a mean biogas production of 0.35 L g−1 COD and around 80% of the methane content in biogas [104]. Indeed, the combination of hydrothermal carbonization of sewage sludge and anaerobic digestion of the liquid by-products shows numerous advantages, not only realizing the treatment of the liquids to diminish its environmental toxicity, but also increasing the economic feasibility of hydrothermal carbonization process to support a positive energy balance. 3.2.3. Reaction pathways and kinetics 1. Reaction pathway of sewage sludge in hydrothermal carbonization. From the whole research history of hydrothermal carbonization for sewage sludge, previous investigations have mainly focused on examining how reaction conditions, especially temperature and residence time, affect the yields, compositions and properties of the product fractions. Only a few studies have, to the best of our knowledge, reported the reaction pathways according to a macro approach, but the detailed reaction mechanisms are still unclear due to the complexity of hydrothermal reaction network of sewage sludge. Determination of detailed reaction pathways is necessary to describe the mechanism and kinetics of various products formation in hydrothermal carbonization of sewage sludge. In principle, the basic reaction mechanism of hydrothermal carbonization process relates to the depolymerization of sewage sludge, the decomposition of biopolymers and the recombination of reactive intermediates. The changes of FTIR signals from proteins (NeO at 434

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Fig. 11. Simplified reaction pathways for hydrothermal carbonization of sewage sludge [12,15,111,112].

mechanical technologies after hydrothermal carbonization, the extent of which is associated with the sludge source, hydrothermal severity and dewatering conditions. The specific resistance to filtration of sludge has been found to decrease from 5.43 × 1012 to 2.05 × 1010 m kg−1 at 200 °C for 240 min [117] and the Normalized CST also show a decrease from 16 to 1.2 s L/g TSS at 200 °C for 6 h [118]. This improved mechanical dewaterability positively relates to the increase of both reaction temperature and residence time. Reaction temperature, more importantly than residence time, seems to govern the extent to which the dewatering process occurred. It has been reported that there exists a threshold temperature of around 130 °C for dewaterability improvement by hydrothermal carbonization for sewage sludge [118]. Residence time of 30–60 min is enough to promote substantial enhancement of sewage sludge dewaterability at temperature of 180–210 °C and the final moisture content of hydrochar can be reduced down to be lower than 30% by further mechanical dewatering at increased temperature [43]. Hydrothermal carbonization has been currently regarded as one of the most promising technologies to solve the dilemma of established technologies for sewage sludge dewatering, either still with high moisture content after dewatering run or with high energy consumption. When hydrothermal carbonization is applied to sewage sludge dewatering, the resulting excellent mechanical dewatering performance is linked closely to the thermochemistry occurred in the process, which leads to a destruction of floc structures, less hydrophilic functional groups, small negative surface charge, lower viscosity of water and gas formation, finally resulting in the water free (Fig. SIeS3). The conversion of bound water to free water is significantly facilitated by hydrothermal effects and free water becomes the main form for moisture distribution in wet hydrochar as temperature is higher than 180 °C [41]. Although hydrothermal carbonization causes a significant reduction of sewage sludge particle size which to some extent probably deteriorates the dewaterability, the viscosity reduction of water with increasing of temperature gives rise to a chance to counteract this deterioration of dewaterability by mechanical dewatering at increased temperature

The further degradation of these dissolved biochemical compounds in liquid phase to gaseous products appears to be insignificant at temperature range 150–250 °C, the limited formation of which may mainly derived from direct thermal decomposition of organic compounds on the surface of solid particles, with an activation energy of 63 kJ mol−1 [107]. Nevertheless, at higher temperatures, the decomposition of aqueous phase products is likely one of the major contributors of gas formation [115]. For hydrochar production, Danso-Boateng et al. [116] reported that the global kinetics of primary sewage sludge at 140–200 °C is fitted first-order with an activation energy of 70 kJ mol−1 and higher temperature results in a higher conversion of sewage sludge to hydrochar. To date, detailed kinetic models that consider exact initial compositions, operating conditions and intermediates to further explain the carbonization process of sewage sludge have been seldom reported, except the model (Eq. (1)) proposed by Ruyter [28], which allows the prediction of carbonization progress on the basis of oxygen content and conditions. An ideal approach for modeling the process chemistry of hydrothermal carbonization is to generate molecularly explicit models on the premise of knowledge on molecular compositions of sewage sludge and products. However, the variety of carbonization products is infinite and some individual mechanisms are far from being understood at present, suggesting a molecular-level model for hydrothermal carbonization of sewage sludge still remains challenging. 4. Improvement of dewaterability It is well known that high moisture content has been one major obstacle to sewage sludge treatment and utilization and to some extent determines energetic and economic efficiencies. Water removal is therefore a core aspect for safe disposal of sewage sludge or even energy and resource recovery from sewage sludge. Hydrothermal carbonization has proved to be an effective method to considerably enhance the dewaterability under low energy consumption [47]. As shown in Table 3, the water in sewage sludge is removed easily by the established 435

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Table 3 Dewatering performance of sewage sludge after hydrothermal treatment. Sewage sludge

Temperature

Residence time

Moisture content

(°C)

(h)

Initial (%)

Final (%)

Activated sludge Activated sludge Digested sludge

180 190 205

1.0 0.5 7.0

86 86 80

28 54 48

Activated sludge

205

7.0

90

30

Paper sludge Digested sludge Municipal sludge

220 220 240

0.5 0.5 1.0

76 96 84

60 81 47

Dewatering technology

Reference

Mechanical pressing/6 MPa for 20 min at180 °C Centrifugation/19000 rpm at room temperature Centrifugation/4400 rpm for 10 min and then mechanical pressing/4Mpa at room temperature Centrifugation/4400 rpm for 10 min and then mechanical pressing/4Mpa at room temperature Mechanical pressing/0.6 MPa for 15 min at room temperature Centrifugation/4000 rpm for 10 min at room temperature Mechanical pressing/0.4 MPa for 1000 s at room temperature

[43] [139] [47] [47] [119] [81] [140]

temperature and residence time but also on sewage sludge properties, showing a similarity with lignocellulose and microalgae biomass in the energy enhancement factor [125]. Since higher energy densification leads to a lower solid yield, the energy yield that is the energy ratio of hydrochar and parent sewage sludge is generally considered to represent the carbonization performance from the energy point of view. A decrease in energy yield has been confirmed as reaction temperature and time increased, which implies that some of energy is released as heat during exothermic process of carbonization or is chemically bonded in organics dissolved in liquid phase, but at least over 60% of the gross energy in sewage sludge are available in hydrochar [126]. As a result, hydrothermal carbonization of sewage sludge is conducive to subsequent storage, transport and application, especially if hydrochar is to be used as a solid fuel [116]. For determining energy requirements of hydrothermal carbonization, it should include not only the energy consumption of hydrothermal process but also the additional energetic aspects related to pretreatment of sewage sludge and separation of hydrochar and liquid. Both the results from laboratory and industrial scales indicate that the total energy input for the dewatering process of sewage sludge including hydrothermal carbonization can reduce 61–62% of the heat demand and 65–69% of the electricity demand if established thermal drying is replaced [47,127]. If the excess exothermic heat of reactions and the heat from hydrochar slurry cooling process are recoverable, the overall energy balance will be identified negative, which means that the potential recoverable energy could exceed energy invested [43]. It has been reported that an energy gain is theoretically possible even if the solid content of sewage sludge is as low as 10% [47]. The exothermic heat plays an important role in this favorable energy balance since reactions become more exothermal for both higher reaction temperature and longer residence time [128]. Besides, the energy benefits of hydrothermal carbonization should consider the energy generated from hydrochar combustion and liquids anaerobic digestion. As shown in Fig. 12, besides meeting the energy requirements of running the hydrothermal carbonization, mechanical dewatering and thermal drying process (A), approximately 397 MJ and 271 MJ could still be recovered from hydrochar combustion and from liquids anaerobic digestion respectively for 1000 kg sludge, while additional 351 MJ is required for the direct thermal drying (B) even after combusting the dried sludge. Zhao et al. [129] also reported that about 50% of the energy generated from hydrochar combustion could be recovered for other purposes after meeting the energy needs for hydrothermal carbonization. The biogas energy from liquids anaerobic digestion was identified as 1.3 times of the heat that is required for hydrothermal carbonization [130]. In conclusion, the current theoretical investigations have offered an insight to some extent to what can be expected, although the research on technical development of hydrothermal carbonization systems for sewage sludge is still in an embryonic stage.

[32,42]. Indeed, mechanical dewatering of the resulting suspension after cooling will weaken the hydrothermal effect, leading to hydrochar with moisture content higher than 50% and subsequent thermal dewatering being unavoidable [119]. However, effective moisture diffusivity of sludge hydrochar, which is highly controlled by reaction temperature, is increased during the subsequent drying process due to the increase of drying flux and the decrease of internal heat and mass transfer limitations and sample shrinkage [120]. Moreover, some chemicals like acids, alkalis, hydrogen peroxide and calcium chloride have been introduced to cooperate with hydrothermal carbonization to get a better dewaterability [45,121–123], since solubilization and disintegration of sludge flocs likely play a crucial role in dewaterability improvement. It must be admitted that the promising dewatering result is just an additional benefit of hydrothermal carbonization, and the ultimate goal should be high quality hydrochar production. Therefore, systematic investigation of the synergistic relationship between hydrothermal dewatering and hydrothermal carbonization is required for sewage sludge treatment but still obscure to date. 5. Techno-economic and policy analysis 5.1. Process energetics The investigation and discussion of a developing sludge conversion technology always have to consider its energetic efficiency that is a crucial factor to determine the economic feasibility of the process. Admittedly, owing to the fact that heating energy requirement for water evaporation is avoided, the major energy input for hydrothermal carbonization is only to heat sewage sludge to the target temperatures for activating the carbonization reactions. More importantly, hydrothermal carbonization is described in theory as the most exothermic process compared with fermentation and anaerobic digestion (Fig. SIeS4), which can release almost a third of the combustion energy stored in the carbohydrates throughout dehydration reaction [124]. The similar observation of negative reaction enthalpy has been reported for hydrothermal carbonization of municipal wastes, which was determined by constructing simplified process reactions based on the elemental composition of feedstock and corresponding products (hydrochar, dissolved organics and CO2) [53]. This released energy will increase or maintain the reaction temperature in the reactor if thermal losses are minimized, probably resulting in a significant reduction of energy input after initial heating phase. It has been observed in previous experiments that hydrothermal carbonization of sewage sludge results in an occurrence of energy densification, which is commonly defined as a ratio of higher heating value between hydrochar and sewage sludge [81]. This could be explained by the significant oxygen removal due to dehydration and decarboxylation reactions. The more severe the hydrothermal conditions in particular the reaction temperature, the higher the energy densification. Reported energy density of sewage sludge can be increased up to 3%–36%, which is dependent not only on reaction 436

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Fig. 12. Typical energy balance for hdydrothermal carbonization of sewage sludge including hydrochar and liquid as biofuels [43,47].

uncertainty in the technical and economic aspects of the industrial-scale hydrothermal carbonization process. The future efforts are required to develop new commercial engineering concepts to construct a high-efficiency industrial chain for hydrothermal carbonization of sewage sludge, in order to reduce the uncertainty for future decisions on the suitability of hydrothermal carbonization for the management of sewage sludge.

5.2. Techno-economic analysis Techno-economic considerations are important to identify the profitability of hydrothermal carbonization for sewage sludge treatment and bring the technology to market. To our knowledge, the techno-economic assessments for hydrothermal carbonation of sewage sludge are absent to date from the scientific literature. However, some information could still be obtained from the results of life-cycle costing for solid biofuel production by hydrothermal carbonization of other biomass wastes. Zeymer et al. [131] found that hydrothermal carbonization process becomes profitable, if the levelized production costs of hydrochar from hydrothermal carbonization of municipal green waste are reduced to below 150 €/ton by using the non-pelletizing hydrochar for heating in medium-sized units (500 kW). Lucian and Fiori [132] reported that the production cost of pelletized hydrochar from hydrothermal carbonization of grape marc is 157 €/ton, and the break-even value is determined to be 200 €/ton (corresponding to 8.3 €/GJHHV) with an assumption of the repayment period of 10 years. It is evident that the current hydrochars as biofuel cannot directly compete with fossil fuel, such as bituminous coal with a price about 2.6 €/GJHHV [133]. Nevertheless, considering the deduction of greenhouse gas mitigation costs of hydrochar as a substitute for fossil fuel and the benefit of methane production by anaerobic digestion of process water, the economic performance of hydrothermal carbonization for treatment of biomass wastes like sewage sludge may become more acceptable, but which requires the support of public policies, such as carbon certificates and encouraging the use of biofuels. It has been reported that the costs of conventional sewage sludge disposal on a dry basis vary between 160 €/ton for agricultural use and 330 €/ton for combustion [134]. According to the hydrochar yields in above studies [131,132], the treatment cost of biomass wastes (dry basis) by hydrothermal carbonization can be obtained within the range of 89–114 €/ton. This indicated that hydrothermal carbonization of sewage sludge is a cost-effective process to solve the disposal problem associated with sewage sludge and meanwhile the fossil resources are saved. Moreover, learning effects of hydrothermal carbonization process probably lead to a 50% reduction of total investment costs after the installation of 10 plant units due to the novel components such as the reactor, heat-exchanger and solid-liquid separation system [134]. An optimized calibration of the process design and parameters are also important to improve the economic efficiency. It should be noted here that the current techno-economic analyses of hydrothermal carbonization process are mainly based on the data of hydrochar production from laboratory-scale batch experiments, thus there remains significant

5.3. Future outlook Previous studies have demonstrated the potentials of hydrothermal carbonization in facilitating sewage sludge conversion into high valueadded products in a more energy-saving way. Further studies to fill the current research gaps are needed to technically and economically improve the application of hydrothermal carbonization for sewage sludge treatment. The particular mechanism of the potential catalytic effects of heavy metals in sewage sludge and organic acids formed in the process may play an important role in reaction pathways and products distribution, but only limited research exists on this topic. The development of an overall reaction model associated with process parameters, initial sewage sludge composition and final product yield and characteristics is essential to account for the underlying reactions and to guide the practical application. Regarding hydrochar formation mechanisms of hydrothermal carbonization of sewage sludge, further research efforts are needed to quantitatively identify the contributions of the direct solid-solid conversion and polymerization of dissolved intermediates derived from original biopolymers. It is necessary to determine the physicochemical properties of hydrochar and liquid phase and develop the quality standards regarding their relevance to various applications such as biofuel production, functional carbon materials, soil amendments and liquid fertilizer. Information on ash behavior during hydrothermal carbonization of sewage sludge is crucial to the fuel properties of hydrochar as solid biofuel and fertilizer properties of process water as liquid fertilizer. It is noted that the current researches mainly focus on the investigation based on the batch method, but the design of a reactor of hydrothermal carbonization which can operate in a continuous process for conversion of sewage sludge is important for the large scale application. Construction of a high-efficiency industrial chain (Fig. SIeS5) for hydrothermal carbonization of sewage sludge is meaningful in the future research, based on the detailed investigation of chemical structure of main products and their relevance to potential applications. Further development must be accompanied by the holistic life cycle 437

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assessment to evaluate the whole process energetics from energy quantity and quality aspects, the fates of products and by-products, and the global environmental issues of hydrothermal carbonization system.

2012;32:1186–95. [3] Venkatesan AK, Done HY, Halden RU. United States national sewage sludge repository at Arizona State University-a new resource and research tool for environmental scientists, engineers, and epidemiologists. Environ Sci Pollut Res 2015;22:1577–86. [4] Bureau CS. China statistical yearbook. Chinese Statistical Bureau; 2017http:// www.stats.gov.cn/tjsj/. [5] Murray A, Horvath A, Nelson KL. Hybrid life-cycle environmental and cost inventory of sewage sludge treatment and end-use scenarios: a case study from China. Environ Sci Technol 2008;42:3163–9. [6] Bergius F. Die Anwendung hoher Drücke bei chemischen Vorgängen und eine Nachbildung des Entstehungsprozesses der Steinkohle. Germany: Halle an der Saale; 1913. [7] Falco C, Baccile N, Titirici MM. Morphological and structural differences between glucose, cellulose and lignocellulosic biomass derived hydrothermal carbons. Green Chem 2011;13:3273–81. [8] Sevilla M, Fuertes AB. Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides. Chem Eur J 2009;15:4195–203. [9] Li R, Shahbazi A. A review of hydrothermal carbonization of carbohydrates for carbon spheres preparation. Trends Renew Energy 2015;1:43–56. [10] Funke A, Ziegler F. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuel Bioprod Biorg 2010;4:160–77. [11] Racovalis L, Hobday MD, Hodges S. Effect of processing conduction on organics in wastewater from hydrothermal dewatering of low-rank coal. Fuel 2002;81:1369–78. [12] Toor SS, Rosendahl L, Rudolf A. Hydrothermal liquefaction of biomass: a review of subcritical water technologies. Energy 2011;36:2328–42. [13] Peterson AA, Voge F, Lachance RP, Fröling M, Antal Jr. MJ, Tester JW. Thermochemical biofuel production in hydrothermal media: a review of sub-and supercritical water technologies. Energy Environ Sci 2008;1:32–65. [14] Nizamuddin S, Baloch HA, Griffin GJ, Mubarak NM, Bhutto AW, Abro R, et al. An overview of effect of process parameters on hydrothermal carbonization of biomass. Renew Sustain Energy Rev 2017;73:1289–99. [15] He C, Chen CL, Giannis A, Yang Y, Wang JY. Hydrothermal gasification of sewage sludge and model compounds for renewable hydrogen production: a review. Renew Sustain Energy Rev 2014;39:1127–42. [16] Qian L, Wang S, Xu D, Guo Y, Tang X, Wang L. Treatment of municipal sewage sludge in supercritical water: a review. Water Res 2016;89:118–31. [17] Mok WSL, Antal Jr.. MJ, Szabo P, Varhegyi G, Zelei B. Formation of charcoal from biomass in a sealed reactor. Ind Eng Chem Res 1992;31:1162–6. [18] Watanabe M, Sato T, Inomata H, Smith Jr. RL, Kruse Jr. KA, et al. Chemical reactions of C1 compounds in near-critical and supercritical water. Chem Rev 2004;104:5803–22. [19] Siskin M, Katritzky AR. Reactivity of organic compounds in superheated water: general background. Chem Rev 2001;101:825–36. [20] Kritzer P. Corrosion in high-temperature and supercritical water and aqueous solutions: a review. J Supercrit Fluids 2004;29:1–29. [21] Siskin M, Katritzky AR. Overview of the reactivity if organics in superheated water: geochemical and technology implications. Preprints symposium-American chemical society, division of fuel chemistry, vol. 44. 1999. p. 324–30. [22] Akiya N, Savage PE. Roles of water for chemical reactions in high-temperature water. Chem Rev 2002;102:2725–50. [23] Robbiani Z. Hydrothermal carbonization of biowaste/fecal sludge. Conception and construction of a HTC prototype research unit for developing countries. Switzerland: Eidgenössische Technische Hochschule Zürich; 2013. [24] Gong M, Zhu W, Xu ZR, Zhang HW, Yang HP. Influence of sludge properties on the direct gasification of dewatered sewage sludge in supercritical water. Renew Energy 2014;66:605–11. [25] Hashaikeh R, Fang Z, Butler IS, Hawari J, Kozinski JA. Hydrothermal dissolution of willow in hot compressed water as a model for biomass conversion. Fuel 2007;86:1614–22. [26] Akhtar J, Amin NAS. A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass. Renew Sustain Energy Rev 2011;15:1615–24. [27] Wang L, Li A, Chang Y. Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: heavy metals, volatile organic compounds and combustion characteristics of hydrochar. Chem Eng J 2016;297:1–10. [28] Ruyter HP. A coalification model. Fuel 1982;61:1182–7. [29] He C, Wang K, Yang Y, Wang JY. Utilization of sewage-sludge-derived hydrochars toward efficient cocombustion with different-rank coals: effects of subcritical water conversion and blending scenarios. Energy Fuel 2014;28:6140–50. [30] Danso-Boateng E, Shama G, Wheatley AD, Martin SJ, Holdich RG. Hydrothermal carbonisation of sewage sludge: effect of process conditions on product characteristics and methane production. Bioresour Technol 2015;177:318–27. [31] Hii K, Baroutian S, Parthasarathy R, Gapes DJ, Eshtiaghi N. A review of wet air oxidation and thermal hydrolysis technologies in sludge treatment. Bioresour Technol 2014;155:289–99. [32] Wang L, Li A. Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: the dewatering performance and the characteristics of products. Water Res 2015;68:291–303. [33] Wilson CA, Novak JT. Hydrolysis of macromolecular components of primary and secondary wastewater sludge by thermal hydrolytic pretreatment. Water Res 2009;43:4489–98.

6. Conclusions Hydrothermal carbonization has great potential to be an energyefficient and environmentally friendly reforming process for energy and resources recovery from sewage sludge. The coal-like hydrochar, liquid product with a vast amount of organic compounds and gas by-product consisting mainly of CO2 are produced under mild conditions. The critical hydrothermal conditions and their influence on the reaction mechanisms, products characteristics, dewaterability improvement and process energetics, have been considered in this review paper, along with highlighting the areas that require more research, such as autocatalytic mechanism of hydrothermal carbonization of sewage sludge, overall kinetics model associated with process parameters and initial sludge composition, and techno-economic and life cycle assessment of the process. The reaction temperature is the most crucial condition to reaction pathways and products characteristics compared to other parameters. An increase in reaction severity of hydrothermal carbonization leads to hydrochar with a higher calorific value but a lower mass yield. Hydrothermal carbonization of sewage sludge is found to be a devolatilization process, which results in the immobilization of heavy metals and phosphorus in hydrochar in more stable forms. The furans compounds (mainly 2-Furfural and 5-HMF) from dehydration of monosaccharides and the N-containing ring compounds from Maillard reactions are responsible for hydrochar formation through condensation, polymerization and aromatization in the liquid phase, while the direct reactions of solid sewage sludge components to form hydrochar is still lack of strong evidences. These detailed reaction pathways are still largely unclear to date, although some mechanisms have been deduced from analysis of the products, such as hydrolysis, dehydration, decarboxylation, aromatization and polymerization. Better identification and quantification of the compositions of products is an important step in understanding the suitability of hydrothermal carbonization for the recycling and reuse of sewage sludge as an efficient way. Although hydrochar and liquid products have already shown broad potential applications in many areas, such as material, energy, agriculture and environment, the current results of the limited investigations still do not give a clear picture. The high ash content of hydrochar is probably the limited factor for the potential applications in energy and functional materials. This review paper provides a chance to systematically understand the current research status of hydrothermal carbonization in the field of sewage sludge treatment to promote its further development for sustainable management of sewage sludge. Acknowledgments This work is supported by the National Science and Technology Ministry of China (No. 2012BAC05B04), the Natural Science Foundation of Inner Mongolia (No. 2016BS0202) and the Higher-level Talents of Inner Mongolia University (No. 5165117). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rser.2019.04.011. References [1] Mateo-Sagasta J, Raschid-Sally L, Thebo A. Global wastewater and sludge production, treatment and use. Dordrecht: Springer; 2015. [2] Kelessidis A, Stasinakis AS. Comparative study of the methods used for treatment and final disposal of sewage sludge in European countries. Waste Manag

438

Renewable and Sustainable Energy Reviews 108 (2019) 423–440

L. Wang, et al.

2004;91:93–100. [67] Pińkowska H, Wolak P, Złocińska A. Hydrothermal decomposition of xylan as a model substance for plant biomass waste-hydrothermolysis in subcritical water. Biomass Bioenergy 2011;35:3902–12. [68] Nagamori M, Funazukuri T. Glucose production by hydrolysis of starch under hydrothermal conditions. J Chem Technol Biotechnol 2004;79:229–33. [69] Miyazawa T, Ohtsu S, Nakagawa Y, Funazukuri T. Solvothermal treatment of starch for the production of glucose and maltooligosaccharides. J Mater Sci 2006;41:1489–94. [70] Orozco RL, Redwood MD, Leeke GA, Bahari A, Santos RCD, Macaskie LE. Hydrothermal hydrolysis of starch with CO2 and detoxification of the hydrolysates with activated carbon for bio-hydrogen fermentation. Int J Hydrogen Energy 2012;37:6545–53. [71] Kumar M, Oyedun AO, Kumar A. A review on the current status of various hydrothermal technologies on biomass feedstock. Renew Sustain Energy Rev 2017;81:1742–70. [72] Dufreche S, Hernandez R, French T, Sparks D, Zappi M, Alley E. Extraction of lipids from municipal wastewater plant microorganisms for production of biodiesel. J Am Oil Chem 2007;84:181–7. [73] Khuwijitjaru P, Adachi S, Matsuno R. Solubility of saturated fatty acids in water at elevated temperatures. Biosci Biotechnol Biochem 2002;66:1723–6. [74] Baig MN, Santos RCD, King J, Pioch D, Bowra S. Evaluation and modelling of continuous flow sub-critical water hydrolysis of biomass derived components; lipids and carbohydrates. Chem Eng Res Des 2013;91:2663–70. [75] Fujii T, Khuwijitjaru P, Kimura Y, Adachi S. Decomposition kinetics of monoacyl glycerol and fatty acid in subcritical water under temperature-programmed heating conditions. Food Chem 2006;94:341–7. [76] MacCarthy P. The principles of humic substances. Soil Sci 2001;166:738–51. [77] Gong M, Nanda S, Romero MJ, Zhu W, Kozinski JA. Subcritical and supercritical water gasification of humic acid as a model compound of humic substances in sewage sludge. J Supercrit Fluids 2017;119:130–8. [78] Fekete J, Sajgó C, Kramarics Á, Eke Z, Kovács K, Kárpáti Z. Aquathermolysis of humic and fulvic acids: simulation of organic matter maturation in hot thermal waters. Org Geochem 2012;53:109–18. [79] Azadi P, Afif E, Foroughi H, Dai T, Azadi F, Farnood R. Catalytic reforming of activated sludge model compounds in supercritical water using nickel and ruthenium catalysts. Appl Catal B Environ 2013;134:265–73. [80] Krevelen DWV. Graphical-statistical method for the study of structure and reaction processes of coal. Fuel 1950;29:269–84. [81] Kim D, Lee K, Park KY. Hydrothermal carbonization of anaerobically digested sludge for solid fuel production and energy recovery. Fuel 2014;130:120–5. [82] Zhao P, Chen H, Ge S, Yoshikawa K. Effect of the hydrothermal pretreatment for the reduction of NO emission from sewage sludge combustion. Appl Energy 2013;111:199–205. [83] Heilmann SM, Molde JS, Timler JG, Wood BM, Mikula AL, Vozhdayev GV, et al. Phosphorus reclamation through hydrothermal carbonization of animal manures. Environ Sci Technol 2014;48:10323–9. [84] Hultman B, Levlin E, Mossakowska A, Stark K. Effects of wastewater treatment technology on phosphorus recovery from sludge and ash. Noordwijkerhout NL: Second international conference on recovery of phosphates from sewage and animal wastes. 2001. p. 12–3. [85] Petzet S, Peplinski B, Cornel P. On wet chemical phosphorus recovery from sewage sludge ash by acidic or alkaline leaching and an optimized combination of both. Water Res 2012;46:3769–80. [86] Adam C, Peplinski B, Michaelis M, Kley G, Simon FG. Thermochemical treatment of sewage sludge ashes for phosphorus recovery. Waste Manag 2009;29:1122–8. [87] Zhao P, Shen Y, Ge S, Chen Z, Yoshikawa K. Clean solid biofuel production from high moisture content waste biomass employing hydrothermal treatment. Appl Energy 2014;131:345–67. [88] Zhang X, Zhang L, Li A. Hydrothermal co-carbonization of sewage sludge and pinewood sawdust for nutrient-rich hydrochar production: synergistic effects and products characterization. J Environ Manag 2017;201:52–62. [89] Ryu J, Suh YW, Suh DJ, Ahn DJ. Hydrothermal preparation of carbon microspheres from mono-saccharides and phenolic compounds. Carbon 2010;48:1990–8. [90] Mumme J, Eckervogt L, Pielert J, Diakité M, Rupp F, Kern J. Hydrothermal carbonization of anaerobically digested maize silage. Bioresour Technol 2011;102:9255–60. [91] Zhang JH, Lin QM, Zhao XR. The hydrochar characters of municipal sewage sludge under different hydrothermal temperatures and durations. J Integr Agric 2014;13:471–82. [92] Repo E, Warchol JK, Kurniawan TA, Sillanpää ME. Adsorption of Co(II) and Ni(II) by EDTA- and/or DTPA-modified chitosan: kinetic and equilibrium modeling. Chem Eng J 2010;161:73–82. [93] Huang HJ, Yuan XZ. The migration and transformation behaviors of heavy metals during the hydrothermal treatment of sewage sludge. Bioresour Technol 2016;200:991–8. [94] Zhai Y, Liu X, Zhu Y, Peng C, Wang T, Zhu L, et al. Hydrothermal carbonization of sewage sludge: the effect of feed-water pH on fate and risk of heavy metals in hydrochars. Bioresour Technol 2016;218:183–8. [95] Shi W, Liu C, Shu Y, Feng C, Lei Z, Zhang Z. Synergistic effect of rice husk addition on hydrothermal treatment of sewage sludge: fate and environmental risk of heavy metals. Bioresour Technol 2013;149:496–502. [96] Steinbeiss S, Gleixner G, Antonietti M. Effect of biochar amendment on soil carbon balance and soil microbial activity. Soil Biol Biochem 2009;41:1301–10. [97] Kuzyakov Y, Subbotina I, Chen H, Bogomolova I, Xu X. Black carbon

[34] Inoue S, Sawayama S, Dote Y, Ogi T. Behaviour of nitrogen during liquefaction of dewatered sewage sludge. Biomass Bioenergy 1997;12:473–5. [35] Dwyer J, Starrenburg D, Tait S, Barr K, Batstone DJ, Lant P. Decreasing activated sludge thermal hydrolysis temperature reduce s product colour, without decreasing degradability. Water Res 2008;42:4699–709. [36] Yilmaz Y, Toledo R. Antioxidant activity of water-soluble Maillard reaction products. Food Chem 2005;93:273–8. [37] Zhang J, Wang S, Lang S, Xian P, Xie T. Kinetics of combined thermal pretreatment and anaerobic digestion of waste activated sludge from sugar and pulp industry. Chem Eng J 2016;295:131–8. [38] He C, Giannis A, Wang JY. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: hydrochar fuel characteristics and combustion behavior. Appl Energy 2013;111:257–66. [39] Saetea P, Tippayawong N. Recovery of value-added products from hydrothermal carbonization of sewage sludge. ISRN Chem Eng 2013;2013:1–6. [40] Peng C, Zhai Y, Zhu Y, Xu B, Wang T, Li C, et al. Production of char from sewage sludge employing hydrothermal carbonization: char properties, combustion behavior and thermal characteristics. Fuel 2016;176:110–8. [41] Wang L, Li A, Chang Y. Relationship between enhanced dewaterability and structural properties of hydrothermal sludge after hydrothermal treatment of excess sludge. Water Res 2017;112:72–82. [42] Yanagida T, Fujimoto S, Minowa T. Application of the severity parameter for predicting viscosity during hydrothermal processing of dewatered sewage sludge for a commercial PFBC plant. Bioresour Technol 2010;101:2043–5. [43] Wang L, Zhang L, Li A. Hydrothermal treatment coupled with mechanical expression at increased temperature for excess sludge dewatering: influence of operating conditions and the process energetics. Water Res 2014;65:85–97. [44] Donoso-Bravo A, Pérez-Elvira S, Aymerich E, Fdz-Polanco F. Assessment of the influence of thermal pre-treatment time on the macromolecular composition and anaerobic biodegradability of sewage sludge. Bioresour Technol 2011;102:660–6. [45] Neyens E, Baeyens J, Creemers C. Alkaline thermal sludge hydrolysis. J Hazard Mater 2003;97:295–314. [46] Lynam JG, Coronella CJ, Yan W, Reza MT, Vasquez VR. Acetic acid and lithium chloride effects on hydrothermal carbonization of lignocellulosic biomass. Bioresour Technol 2011;102:6192–9. [47] Escala M, Zumbuhl T, Koller C, Junge R, Krebs R. Hydrothermal carbonization as an energy-efficient alternative to established drying technologies for sewage sludge: a feasibility study on a laboratory scale. Energy Fuel 2012;27:454–60. [48] Rogalinski T, Liu K, Albrecht T, Brunner G. Hydrolysis kinetics of biopolymers in subcritical water. J Supercrit Fluids 2008;46:335–41. [49] Li Y, Lu X, Yuan L, Liu X. Fructose decomposition kinetics in organic acids-enriched high temperature liquid water. Biomass Bioenergy 2009;33:1182–7. [50] Hu B, Yu SH, Wang K, Liu L, Xu XW. Functional carbonaceous materials from hydrothermal carbonization of biomass: an effective chemical process. Dalton Trans 2008:5414–23. [51] Cui X, Antoniett M, Yu SH. Structural effects of iron oxide nanoparticles and iron ions on the hydrothermal carbonization of starch and rice carbohydrates. Small 2006;2:756–9. [52] Brand S, Hardi F, Kim J, Suh DJ. Effect of heating rate on biomass liquefaction: difference s between subcritical water and supercritical ethanol. Energy 2014;68:420–7. [53] Berge ND, Ro KS, Mao J, Flora JR, Chappell MA, Bae S. Hydrothermal carbonization of municipal waste streams. Environ Sci Technol 2011;45:5696–703. [54] Yoshikawa K, Prawisudha P. Hydrothermal treatment of municipal solid waste for producing solid fuel. Berlin: Springer; 2014. [55] Yuan H, Chen Y, Zhang H, Jiang S, Zhou Q, Gu G. Improved bioproduction of short-chain fatty acids (SCFAs) from excess sludge under alkaline conditions. Environ Sci Technol 2006;40:2025–9. [56] Brunner G. Near critical and supercritical water. Part I. Hydrolytic and hydrothermal processes. J Supercrit Fluids 2009;47:373–81. [57] Zhu G, Zhu X, Fan Q, Wan X. Recovery of biomass wastes by hydrolysis in subcritical water. Resour Conserv Recycl 2011;55:409–16. [58] Klingler D, Berg J, Vogel H. Hydrothermal reactions of alanine and glycine in suband supercritical water. J Supercrit Fluids 2007;43:112–9. [59] Abdelmoez W, Nakahasi T, Yoshida H. Amino acid transformation and decomposition in saturated subcritical water condition. Ind Eng Chem Res 2007;46:5286–94. [60] Bazaka K, Crawford R J, Nazarenko EL, Ivanova EP. Bacterial extracellular polysaccharides. Dordrecht: Springer; 2011. [61] Sasaki M, Fang Z, Fukushima Y, Adschiri T, Arai K. Dissolution and hydrolysis of cellulose in subcritical and supercritical water. Ind Eng Chem Res 2000;39:2883–90. [62] Kruse A, Maniam P, Spieler F. Influence of proteins on the hydrothermal gasification and liquefaction of biomass. 2. Model compounds. Ind Eng Chem Res 2007;46:87–96. [63] Titirici MM, Antonietti M, Baccile N. Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/ hexoses. Green Chem 2008;10:1204–12. [64] Zhang M, Yang H, Liu Y, Sun X, Zhang D, Xue D. Hydrophobic precipitation of carbonaceous spheres from fructose by a hydrothermal process. Carbon 2012;50:2155–61. [65] Reza MT, Yan W, Uddin MH, Lynam JG, Hoekman SK, Coronella CJ, et al. Reaction kinetics of hydrothermal carbonization of loblolly pine. Bioresour Technol 2013;139:161–9. [66] Carvalheiro F, Esteves MP, Parajó JC, Pereira H, Gırio FM. Production of oligosaccharides by autohydrolysis of brewery's spent grain. Bioresour Technol

439

Renewable and Sustainable Energy Reviews 108 (2019) 423–440

L. Wang, et al.

[98]

[99] [100] [101]

[102]

[103]

[104]

[105]

[106] [107] [108] [109]

[110]

[111]

[112] [113]

[114]

[115] [116]

[117]

[118] [119]

[120] Mäkelä M, Fraikin L, Léonard A, Benavente V, Fullana A. Predicting the drying properties of sludge based on hydrothermal treatment under subcritical cond itions. Water Res 2016;91:11–8. [121] Abelleira J, Pérez-Elvira SI, Sánchez-Oneto J, Portela JR, Nebot E. Advanced thermal hydrolysis of secondary sewage sludge: a novel process combining thermal hydrolysis and hydrogen peroxide addition. Resour Conserv Recycl 2012;59:52–7. [122] Neyens E, Baeyens J, Weemaes M. Hot acid hydrolysis as a potential treatment of thickened sewage sludge. J Hazard Mater 2003;98:275–93. [123] Guan B, Yu J, Fu H, Guo M, Xu X. Improvement of activated sludge dewaterability by mild thermal treatment in CaCl2 solution. Water Res 2012;46:425–32. [124] Titirici MM, Thomas A, Antonietti M. Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO2 problem? New J Chem 2007;31:787–9. [125] Chen WH, Huang MY, Chang JS, Chen CY. Torrefaction operation and optimization of microalga residue for energy densification and utilization. Appl Energy 2015;154:622–30. [126] Ramke HG, Blöhse D, Lehmann HJ, Fettig J. Hydrothermal carbonization of organic waste. Sardinia: Twelfth International Waste Management and Landfill Symposium; 2009. [127] Stucki M, Eymann L, Gerner G, Hartmann F, Wanner R, Krebs R. Hydrothermal carbonization of sewage sludge on industrial scale: energy efficiency, environmental effects and combustion. J Energy Challenges Mech 2015;2:38–44. [128] Funke A, Ziegler F. Heat of reaction measurements for hydrothermal carbonization of biomass. Bioresour Technol 2011;102:7595–8. [129] Zhao P, Shen Y, Ge S, Yoshikawa K. Energy recycling from sewage sludge by producing solid biofuel with hydrothermal carbonization. Energy Convers Manag 2014;78:815–21. [130] Qiao W, Peng C, Wang W, Zhang Z. Biogas production from supernatant of hydrothermally treated municipal sludge by upflow anaerobic sludge blanket reactor. Bioresour Technol 2011;102:9904–11. [131] Zeymer M, Meisel K, Clemens A, Klemm M. Technical, economic, and environmental assessment of the hydrothermal carbonization of green waste. Chem Eng Technol 2017;40:260–9. [132] Lucian M, Fiori L. Hydrothermal carbonization of waste biomass: process design, modeling, energy efficiency and cost analysis. Energies 2017;10:211. [133] Stemann J, Erlach B, Ziegler F. Hydrothermal carbonisation of empty palm oil fruit bunches: laboratory trials, plant simulation, carbon avoidance, and economic feasibility. Waste Biomass Valoriz 2013;4:441–54. [134] Gasafi E, Reinecke MY, Kruse A, Schebek L. Economic analysis of sewage sludge gasification in supercritical water for hydrogen production. Biomass Bioenergy 2000;32:1085–96. [135] Fytili D, Zabaniotou A. Utilization of sewage sludge in EU application of old and new methods-a review. Renew Sustain Energy Rev 2008;12:116–40. [136] Jin B, Wilén BM, Lant P. A comprehensive insight into floc characteristics and their impact on compressibility and settleability of activated sludge. Chem Eng J 2003;95:221–34. [137] Mikkelsen LH, Keiding K. Physico-chemical characteristics of full scale sewage sludges with implications to dewatering. Water Res 2002;36:2451–62. [138] Lin Y, Ma X, Peng X, Hu S, Yu Z, Fang S. Effect of hydrothermal carbonization temperature on combustion behavior of hydrochar fuel from paper sludge. Appl Therm Eng 2015;91:574–82. [139] Meng D, Jiang Z, Kunio Y, Mu H. The effect of operation parameters on the hydrothermal drying treatment. Renew Energy 2012;42:90–4. [140] Saveyn H, Curvers D, Schoutteten M, Krott E, Van Der Meeren P. Improved dewatering by hydrothermal conversion of sludge. J Residuals Sci Technol 2009;6:51–6. [141] Möller M, Nilges P, Harnisch F, Schröder U. Subcritical water as reaction environment: fundamentals of hydrothermal biomass transformation. ChemSusChem 2011;4:566–79.

decomposition and incorporation into soil microbial biomass estimated by C-14 labeling. Soil Biol Biochem 2009;41:210–9. Xue Y, Liu H, Chen S, Dichtl N, Dai X, Li N. Effects of thermal hydrolysis on organic matter solubilization and anaerobic digestion of high solid sludge. Chem Eng J 2015;264:174–80. Rizzi GP. Chemical structure of colored Maillard reaction products. Food Rev Int 1997;13:1–28. Sun XH, Sumida H, Yoshikawa K. Effects of hydrothermal process on the nutrient release of sewage sludge. Int J Waste Resour 2013;3:1–8. Elliott DC, Biller P, Ross AB, Schmidt AJ, Jones SB. Hydrothermal liquefaction of biomass: developments from batch to continuous process. Bioresour Technol 2015;178:147–56. Bougrier C, Delgenès JP, Carrère H. Effects of thermal treatments on five different waste activated sludge samples solubilisation, physical properties and anaerobic digestion. Chem Eng J 2008;139:236–44. Wirth B, Reza T, Mumme J. Influence of digestion temperature and organic loading rate on the continuous anaerobic treatment of process liquor from hydrothermal carbonization of sewage sludge. Bioresour Technol 2015;198:215–22. Yoneyama Y, Nishii A, Nishimoto M, Yamada N, Suzuki T. Upflow anaerobic sludge blanket (UASB) treatment of supernatant of cow manure by thermal pretreatment. Water Sci Technol 2006;54:221–7. Afolabi OOD, Sohail M, Thomas CLP. Characterization of solid fuel chars recovered from microwave hydrothermal carbonization of human biowaste. Energy 2017;134:74–89. Takamatsu T, Hashimoto I, Sioya S. Model identification of wet-air oxidation process thermal decomposition. Water Res 1970;4:33–59. Imbierowicz M, Chacuk A. Kinetic model of excess activated sludge thermohydrolysis. Water Res 2012;46:5747–55. Yin F, Chen H, Xu G, Wang G, Xu Y. A detailed kinetic model for the hydrothermal decomposition process of sewage sludge. Bioresour Technol 2015;198:351–7. Libra JA, Ro KS, Kammann C, Funke A, Berge ND, Neubauer Y, et al. Hydrothermal carbonization of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis. Biofuels 2011;2:71–106. Reza MT, Uddin MH, Lynam JG, Hoekman SK, Coronella CJ. Hydrothermal carbonization of loblolly pine: reaction chemistry and water balance. Conver Bioref 2014;4:311–21. Reza MT, Andert J, Wirth B, Busch D, Pielert J, Lynam JG, et al. Hydrothermal carbonization of biomass for energy and crop production. Appl Bioenergy 2014;1:11–29. Kruse A, Funke A, Titirici MM. Hydrothermal conversion of biomass to fuels and energetic materials. Curr Opin Chem Biol 2013;17:515–21. Yu Y, Lou X, Wu H. Some recent advances in hydrolysis of biomass in hot-compressed water and its comparisons with other hydrolysis methods. Energy Fuel 2007;22:46–60. Karayıldırım T, Sınağ A, Kruse A. Char and coke formation as unwanted side reaction of the hydrothermal biomass gasification. Chem Eng Technol 2008;31:1561–8. Valdez PJ, Savage PE. A reaction network for the hydrothermal liquefaction of Nannochloropsis sp. Algal Res 2013;2:416–25. Danso-Boateng E, Holdich RG, Shama G, Wheatley AD, Sohail M, Martin SJ. Kinetics of faecal biomass hydrothermal carbonisation for hydrochar production. Appl Energy 2013;111:351–7. Danso-Boateng E, Holdich RG, Wheatley AD, Martin SJ, Shama G. Hydrothermal carbonization of primary sewage sludge and synthetic faeces: effect of reaction temperature and time on filterability. Environ Prog Sustain 2015;35:1279–90. Yu J, Guo M, Xu X, Guan B. The role of temperature and CaCl2 in activated sludge dewatering under hydrothermal treatment. Water Res 2014;50:10–7. Areeprasert C, Zhao P, Ma D, Shen Y, Yoshikawa K. Alternative solid fuel production from paper sludge employing hydrothermal treatment. Energy Fuel 2014;28:1198–206.

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