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Article

Hydrothermal Carbonization of Spent Coffee Grounds for Producing Solid Fuel

by
Yulin Hu
1,*,
Rhea Gallant
1,
Shakirudeen Salaudeen
1,
Aitazaz A. Farooque
1,2 and
Sophia He
3
1
Faculty of Sustainable Design Engineering, University of Prince Edward Island, Charlottetown, PE C1A 4P3, Canada
2
School of Climate and Adaption, University of Prince Edward Island Charlottetown, Charlottetown, PE C1A 4P3, Canada
3
Department of Engineering, Dalhousie University, Truro, NS B2N 5E3, Canada
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(14), 8818; https://doi.org/10.3390/su14148818
Submission received: 23 June 2022 / Revised: 11 July 2022 / Accepted: 15 July 2022 / Published: 19 July 2022
(This article belongs to the Special Issue Frontiers in Bio-Energy Production and Applications)
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Abstract

:
Spent coffee grounds (SCG) are industrial biowaste resulting from the coffee-brewing process, and they are often underutilized and end up in landfills, thereby leading to the emission of toxic gases and environmental damage. Hydrothermal carbonization (HTC) is an attractive approach to valorize wet biomass such as SCG to valuable bioproducts (i.e., hydrochar). Thus, in this work, the HTC of SCG was carried out in a 500 L stainless steel vessel at 150, 170, 190, 210, and 230 °C for 30 min, 60 min, 90 min, and 120 min and a feedstock to water weight ratio of 1:5, 1:10, and 1:15, and the use of the resulting hydrochar as a solid fuel was evaluated. The results showed that a high energy recovery (83.93%) and HHV (23.54 MJ/kg) of hydrochar was obtained at moderate conditions (150 °C, 30 min, and feedstock to water weight ratio of 1:5) when compared with conventional approaches such as torrefaction. Following this, the surface morphology, functionality, and combustion behavior of this hydrochar were characterized by SEM, FTIR, and TGA, respectively. In short, it can be concluded that HTC is an effective approach for producing solid fuel from SCG and the resulting hydrochar has the potential to be applied either in domestic heating or large-scale co-firing plants.

1. Introduction

Undoubtedly, it is significant to search for renewable and sustainable resources to produce fuels and materials with the aim of lowering the use of fossil fuels and reducing CO2 emissions. To date, a range of biomass and organic waste including first-, second-, and third-generation biomass have been extensively investigated as the raw materials for the generation of biofuels and biomaterials [1,2,3,4]. Biofuel production obtained from first-generation biomass such as corn is a well-established process; however, it would lead to fuel vs. food competition. Second-generation biomass such as agricultural waste could be an alternative resource to first-generation biomass for producing biofuels. Microalgae belongs to third-generation biomass, and it is often utilized as the feedstocks for biodiesel production owing to its high concentration of lipids [5,6].
The consumption of spent coffee grounds (SCG) as a result of the waste originally comes from the coffee-brewing process has gradually increased. In 2020–2021, the global coffee consumption is expected to grow to 167.1 million bags of 60 kg, which leads to the generation of around 6 million tons of SCG per year [7]. This massive amount of SCG poses a serious environmental concern owing to the current waste disposal practices, i.e., landfills [8]. Alternatively, SCG is a carbonaceous material and can be used as the feedstock to produce useful bioproducts. In particular, converting SCG to solid fuel by improving its fuel properties has been investigated by torrefaction of dried SCG at 250–350 °C for 0–30 min by Nepal et al. [9]. Barbanera and Muguerza [10] performed torrefaction of SCG for the production of solid biofuels at 210–260 °C for 90 min. However, an energy-intensive pre-drying step is needed prior to solid fuel preparation via torrefaction, and thus it is not a suitable conversion process for wet biomass such as SCG.
Hydrothermal carbonization (HTC) is an alternative approach for preparing solid fuel, especially for high water containing biowaste. This is due to the fact that water can be used as a reaction medium in the HTC process, and thus the energy intensive pre-drying stage can be eliminated and reduces the overall energy consumption. At subcritical conditions, the physical properties of water are dramatically different from those at ambient conditions. For instance, the dielectric constant of water reduced from 78 F/m at 25 °C and 0.1 MPa to 27.1 F/m at 250 °C and 5 MPa, along with an increase in the ionic product (pKw) from 14 at 25 °C and 0.1 MPa to 11.2 at 250 °C and 5 MPa [11]. Thus, HTC is selected as the conversion technology in this study to convert wet SCG into solid fuel. Based on recent studies, temperature, residence time, feedstock to water weight ratio, and catalyst type/dosage are the key parameters affecting the product yield and properties [3,12].
As earlier described, HTC could be an energy-saving technology for converting high water containing feedstocks such as SCG when compared with conventional approaches such as torrefaction and pyrolysis. Additionally, in consideration of the composition of SCG, it belongs to lignocellulosic biomass primarily containing cellulose, hemicellulose, lignin, fatty acids, and other polysaccharides, thereby ensuring SCGs excellent raw materials for producing fuels and chemicals [9]. Among them, the preparation of solid fuel could be a potential application of SCGs. Till now, most recent studies relating to SCG valorization have primarily been limited to soil conditioner [13], energy storage [14], adsorbent [15], biodiesel [16], and bio-crude oil [17]. Aside from these applications, SCG could be an excellent source for producing solid fuel. In addition, although few studies have investigated the use of SCG for preparing solid fuel, most studies have used traditional torrefaction as the conversion approach, and there is a lack of investigations on the alternative method—HTC [9,10]. As a result, in this study, the non-catalytic HTC of SCG was carried out at temperatures of 150–210 °C, a residence time of 30–120 min, and feedstock to water weight ratio of 1:5–1:15. A series of analytical instruments including CHNS elemental analyzer, bomb calorimeter, scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) were applied to comprehensively characterize SCG-derived hydrochar in terms of elemental composition, higher heating value (HHV), surface morphology, functionality, and combustion behavior, respectively. Overall, the present study can offer a theoretical basis for expanding the application fields of SCG and improving the utilization efficiency of SCG through the use of alternative conversion technology.

2. Materials and Methods

2.1. Feedstock Preparation

In this study, SCG was used as the raw material, which was supplied by local Starbucks in Charlottetown (PE, Canada). The raw materials were oven dried at 105 °C overnight before conducting experiments. The dried SCG was then kept in sealed bags and stored in the fridge at 4 °C. The chemicals used in this study were purchased from VWR International, Part of Avantor (Mount Royal, QC, Canada) and used as received. The moisture and ash content of feedstock was determined to be 62.91 ± 0.87 wt.% and 1.57 ± 0.16 wt.%, respectively.

2.2. HTC Experiments

The experimental procedure and the selection of the reaction conditions were based on our preliminary experiments and previous studies on HTC [18,19], and a simple process flow diagram (PFD) is displayed in Figure 1. HTC experiments of SCG were performed using Parr HP/HT 4580 series reactor (Parr Instrument Company, Moline, IL, USA). The equipment has a volume of 1000 mL, and the maximum heating temperature and pressure is 500 °C and 34.5 MPa, respectively (Figure 2).
For each run, dried SCG and distilled water were loaded to the reactor at a weight ratio of 1:5, 1:10, and 1:15. Following this, the reactor was tightly sealed and pure N2 gas was purged into the reactor to remove oxygen inside the reactor. Then, 20 bar of N2 was purged to the reactor again to create an initial pressure to avoid the boiling of water during the heating process. The reactor was then heated to the desired reaction temperature of 150–210 °C with an interval of 20 °C, and residence time was 30 min, 60 min, 90 min, and 120 min. When the reaction was finished, the reactor was cooled to room temperature, and the gaseous products were released through a release value and the remaining solid–liquid mixture was transferred to a beaker. The solid product was separated from this mixture by vacuum separation, and the solid product retained in the coffee filter was collected and then placed in an oven at 105 °C overnight. The dried solid product from HTC was denoted as the hydrochar. All experiments were conducted at least two times to ensure the reproducibility and reliability of the results, and Excel software was used for data processing. The hydrochar yield can be calculated using Equation (1), and the others yield (combination of gas and liquid) can then be simply determined as:
Hydrochar yield (wt.%) = (Mass of dried hydrochar/Mass of dried feedstock) × 100%
Others yield (wt.%) = (1 − hydrochar yield) × 100%
The energy densification and energy recovery (%) can be determined using Equations (3) and (4), respectively, based on the previous study reported by Shrestha et al. [19] and Hu et al. [20], respectively.
Energy densification = (HHV of hydrochar/HHV of raw feedstock) × 100%
Energy recovery = (HHV of hydrochar × weight of hydrochar)/(HHV of dried feedstock × weight of dried feedstock)

2.3. Characterizations of Feedstock and Hydrochar

The moisture content of feedstock was determined by drying the samples in an oven at 105 °C for 24 h. The ash content of feedstock was measured by combusting the dried samples in a muffle furnace at 575 °C for 3 h. The methods adopted in this study for measuring moisture and ash content followed our previously published papers [20,21]. The elemental composition of feedstock and hydrochar was determined using the Organic Elemental Analysis Equipment (Flash 2000 CHNS-O, Thermo Fisher Scientific, Waltham, MA, USA). The tolerance for this elemental analyzer is total carbon and nitrogen ranges between 100 ppm to 100% level, and 2,5-bis(5-tert-2-benzo-oxazol-2-yl) thiophene (BBOT) is used as the standard to build up the calibration curve. The functionality of feedstock and hydrochar was determined using FT-IR analysis (PerkinElmer, Waltham, MA, USA) in the range of 400–4000 cm−1 with a resolution of 4 cm−1. The morphology of feedstock and hydrochar was characterized using SEM (TM3000, Hitachi, Ibaraki, Japan), and the sample was coated with platinum before the analysis. The imaging was conducted at an acceleration voltage of 15 kV. HHV of feedstock and hydrochar was determined using a bomb calorimeter (Parr 6100 Calorimeter, Parr Instrument Company, Moline, MA, USA). The precision classification of this Parr 6100 Calorimeter given by the manufacture is 0.1–0.2% class. The combustion performance of hydrochar was determined using a TGA analyzer (TGA Q500), and the parameters used in the equipment set-up and analysis were followed by He et al. [22].

3. Results and Discussion

3.1. Effects of Temperature on Products Yield and Properties

Temperature is one of the major reaction parameters affecting the project distribution and properties. In this study, the effect of temperature was investigated by performing HTC experiments at 150–210 °C for 30 min and feedstock to water weight ratio of 1:5, and the results are shown in Figure 3.
As indicated in Figure 3, the yield of hydrochar increased decreased gradually from 81.39 wt.% at 150 °C to 63.78 wt.% at 190 °C and then remained constant with an increase in temperature from 190 °C to 210 °C. An opposite trend was found between HTC temperature and others yield. The similar results were previously reported by Liang et al. [18] where HTC of forest waste was conducted at 200–280 °C for 1 h and feedstock to water weight ratio of 1:5 and Wang et al. [23] where co-HTC of food waste and woody biomass at 180–260 °C for 1 h. This decrease in the hydrochar yield with increasing temperature could be due to the hydrolysis of hemicellulose often occurs at lower temperature, and thereafter the hydrolysis of cellulose takes place when further increasing temperature. At higher temperatures, the decomposition of organics present in the feedstock into liquid phase could be promoted and thus leads to an increase in the others yield [19]. In addition, it should be noted that due to the low temperatures (i.e., 180–210 °C) adopted in HTC runs, the gas yield is negligible [24].
The results of elemental composition (C, H, O, N, and S) of feedstock and hydrochar obtained at 150–210 °C for 30 min and 1:5 feedstock to water weight ratio are presented in Table 1, and a Van Krevelen diagram of feedstock and hydrochar samples is depicted in Figure 4. In addition, the energy densification and energy recovery of hydrochar obtained from 150–210 °C for 30 min and feedstock to water weight ratio of 1:5 were calculated and summarized in Table 1.
As shown in Table 1, the HHV of SCG is 22.83 MJ/kg, while the heating value of all hydrochar samples was found to be higher than that of raw material with energy densification of 1.03–1.21. The highest HHV of hydrochar (27.65 MJ/kg) was obtained at 210 °C, and HHV of all hydrochar samples was higher than that of wood pellet from different species, i.e., wood pellet from softwood: 19.66–20.36 MJ/kg and from hardwood: 17.63–20.81 MJ/kg [25]. This increase in HHV with increasing temperature could be related to an increase in the C content of hydrochar [26], as evidenced by the elemental composition of hydrochar and indicated in Table 1. Similar increase in the HHV with increase in HTC temperature was observed in hydrochar obtained from hemp digestate [27]. Table 1 indicates that increasing temperature led to a decrease in the energy recovery, which is due to higher biomass decomposition and lower hydrochar yield at higher temperatures. Based on this result, temperature of 150 °C was selected for the following investigations on the effects of residence time and feedstock to water weight ratio on the product yield and properties.
As depicted in Figure 4, both H/C and O/C atomic ratios of hydrochar samples were lower than those of feedstock and they decreased with increasing temperature, which is consistent with the results reported by Ipiales et al. [28]. This observation could be due to the fact that the dehydration and decarboxylation reactions are promoted when increasing temperature, and thus the O content continuously reduces at higher temperature. Typically, the lower atomic ratio renders a fuel with higher heating value [19]. Two of the hydrochar samples fell in the category of peat and could be used as feed for heat generation.

3.2. Effects of Residence Time on Products Yield and Properties

As earlier discussed in Section 3.1, a temperature of 150 °C was selected in the studies on the effects of residence time on the project yield and properties. The influence of residence time on products yield was performed at a temperature of 150 °C and feedstock to water weight ratio of 1:5, and the residence time ranged from 30 min to 120 min; the results are shown in Figure 5.
As illustrated in Figure 5, the yield of hydrochar continuously dropped when extending residence time from 30 min to 120 min, which is in a good agreement with the results reported by Cheng et al. [29] where HTC of rape straw was carried out at 180 °C for 15–120 min. This trend could be caused by the stimulated decomposition of large biomolecules from biomass and then partition into liquid phase. In addition, the effect of residence time on the products yield was less significant that the effect of temperature. This is consistent with the results reported by Wang et al. [30], and the results showed that the influence of HTC temperature was stronger than residence time not only in the hydrochar yield but also in the elemental composition, combustion performance, and nutrient composition of hydrochar.
The results of energy densification and energy recovery of hydrochar obtained from 150 °C for 30–120 min and feedstock to water weight ratio of 1:5 are summarized in Table 2. In contrast to the hydrochar yield trend with residence time, HHV of hydrochar continuously increased from 23.54 MJ/kg at 30 min to 24.62 MJ/kg at 120 min, corresponding to an increase in the energy densification from 1.03 to 1.08. These results are similar to those observed for hydrochar produced from SCG by HTC at 180–220 °C for 1–5 h and support evidence that HTC promotes energy densification in hydrochar at severe conditions such as higher temperatures and prolonged residence times [31]. Table 2 also implies that the highest energy recovery of 83.93% was obtained at 30 min due to the compensation of lower hydrochar yield at extended residence times. Thus, the residence time of 30 min was chosen as the optimal residence time for the following investigation on the effects of feedstock to water weight ratio on the products yield and properties.
The results of elemental composition (C, H, O, N, and S) and atomic ratio of O/C and H/C of hydrochar obtained at 150 °C for 30–120 min and 1:5 feedstock to water weight ratio are summarized in Table 3.
The results indicated that atomic O/C ratio decreased with the increase in the residence time, which could be related to the promoted dehydration and decarboxylation reactions at longer residence time. This similar trend was also found by Cheng et al. [32] where hydrochar was obtained from HTC of coconut shell. Table 3 shows that the content of C increased from 52.77% in SCG to 57.32% in hydrochar obtained at 120 min. As for O content, due to the biomass decomposition and intermediate cracking, it reduced from 38.14% in feedstock to 33.66% in hydrochar obtained at 120 min. A similar trend was also found in the content of N and residence time, which could be owing to the promoted denitrogenation at extended residence time.

3.3. Effects of Feedstock to Water Weight Ratio on Products Yield and Properties

As earlier discussed in Section 3.2, the residence time of 30 min was selected in the investigations on the effects of feedstock to water weight ratio on the products yield and properties. In this study, as depicted in Figure 6, the influences of feedstock to water weight ratio on products yield and properties are evaluated at 150 °C for 30 min and feedstock to water weight ratio ranged between 1:5 and 1:15.
Figure 6 indicates that the yield of hydrochar continuously decreased when adding more water into the HTC reactor and thus promotes biomass hydrolysis to small molecules. On the other hand, insufficient water content could cause localized overheating and uneven temperature profile in HTC reactor [33]. Similarly, Gong et al. [34] reported that the formation of hydrochar was favorable when adding a lower amount of water to the reactor. Based on the results and previous studies, we can conclude that the amount of feedstock adding to the reactor should be as high as possible with the consideration of the capacity and technical condition of the reactor.
The results of energy densification and energy recovery of hydrochar obtained from 150 °C for 30 min and feedstock to water weight ratio of 1:5–1:15 are summarized in Table 4. In general, the effect of feedstock to water weight ratio on HHV of hydrochar was minor, and it is undoubtedly that an increase in the HHV from 22.83 MJ/kg in feedstock to 23.54–23.98 MJ/kg was observed after HTC treatment. In addition, the highest energy recovery of 83.93% was obtained at the feedstock to water weight ratio of 1:5.
The results of elemental composition (C, H, O, N, and S) and atomic ratios of H/C and O/C of hydrochar obtained at 150 °C for 30 min and feedstock to water weight ratio from 1:5 to 1:15 are indicated in Table 5. As indicated in Table 5, the influence of feedstock to water weight ratio on the elemental composition of hydrochar was insignificant. This trend is consistent with the results previously observed by HTC of tomato peel waste [35].
Overall, the hydrochar product obtained at temperature of 150 °C, residence time of 30 min, and feedstock to water weight ratio of 1:5 contained a HHV value of 23.54 MJ/kg, together with the highest yield of 81.39 wt.% and the highest energy recovery of 83.93%. As suggested by the ash content of SCG, the total content of volatile matter and fixed carbon (dried basis) is ~98%. When considering the chemical composition of SCG, previous study has reported that cellulose (~12 wt.%), hemicellulose (~39 wt.%), and lignin (~24 wt.%) were main composition of SCG and lipid (~2 wt.%) and protein (~17 wt.%) were also observed in the SCG [36]. Based on the physiochemical analysis, it can be speculated that some fraction of volatile matter was decomposed during HTC reaction, which implies by a dark color of the water phase. As confirmed by TGA analysis, some small molecular weight chemicals could be released from the biomass, especially the protein and hemicellulose due to their relatively lower degradation temperature compared to other chemical composition such as lignin. The high calorific value is larger than that of low-rank fuels such as peat (17.1 MJ/kg) and lignite (17.6–21.9 MJ/kg). With further optimization on the reaction conditions, the HHV of SCG-derived hydrochar can be further improved and become comparable to that of bituminous coal (30.2–31 MJ/kg) and anthracite coal (31.8–34.5 MJ/kg) [31]. Aside from elemental composition and HHV, other physicochemical properties of hydrochar such as functionality, surface morphology, and combustion behavior need to be investigated, as discussed below.

3.4. FTIR Analysis

The functionality of feedstock and hydrochar obtained at 150 °C for 30 min and feedstock to water weight ratio of 1:5 was characterized by FTIR analysis, and the FTIR spectra are depicted in. The interpretation of wavenumber and functional groups is based on previous literature [22,37].
As indicated in Figure 7, the band from 3700–3100 cm−1 could be attributed to the -OH stretching vibration. Clear, the intensity of this band of hydrochar was lower than that of feedstock, which could be due to the dehydration reaction occurred in the HTC process. The peaks at 2924 and 2984 cm−1 can be assigned to -CH bonds of methyl (-CH3) group in the caffeine molecule. Similar, the intensity of these peaks of feedstock were stronger than those of hydrochar, implying that the decomposition of big molecules during HTC [9]. Two peaks at 1739 and 1644 cm−1 can be related to the C=O stretching vibration in carboxyl, and their intensities reduced after HTC treatment caused by the decarboxylation reaction. In addition, the peak at 1458 cm−1 could be correlated to the C=C stretching vibration in the benzene ring skeleton. Although the fingerprint region (region between 1500 and 400 cm−1) is different to interpret, several peaks in both feedstock and hydrochar were still observed such as 1371, 1160, and 1029 cm−1. These peaks could be assigned to the C-O bond.

3.5. SEM Analysis

SEM analysis is used to evaluate the change in the surface morphology of spent coffee ground after HTC process. The SEM images of raw material and the hydrochar sample with the highest energy recovery obtained at 150 °C for 30 min and feedstock to water weight ratio of 1:5, as indicated in Figure 8.
Compared to the original structure of the spent coffee ground, Figure 8 shows that no big difference was observed in the morphology of hydrochar after HTC treatment. Both raw material and hydrochar had a porous structure. However, the results are different from previous literature. For instance, Li and Cai [38] found that the porous structure was only observed for the hydrochar obtained at 240 °C for 60 min rather than in the original material. The different results could be related to the fact that spent coffee ground is the residual after brewing coffee and the brewing process usually consists of roasting, grinding, and mixing with either hot or cold water, which could result in the formation of a porous spent coffee ground. This implies that this raw material could be a promising resource to prepare adsorbent after certain modification or functionalization. Future investigations on the pore size and pore volume using the BET method are required since the pores of the hydrochar samples are expected to gradually expand at higher temperatures and then intensify and lead to pore clogging [39].

3.6. TGA Analysis

TGA is an excellent technique to determine the combustion behavior of fuel materials, and thus TGA analysis of feedstock and hydrochar was performed in this study and the TG-DTG profiles of feedstock and hydrochar obtained at 150 °C for 30 min and feedstock to water weight ratio of 1:5 are depicted in Figure 9. Based on the thermogravimetric (TG) curves, derivative thermogravimetric (DTG) curves were depicted and could be helpful for understanding the rate of biomass decomposition with respect to temperature, the major biomass degradation region, and the degradation temperature. It was found that no bid difference can be observed between SCG and hydrochar in terms of thermal degradation properties, which could be caused by the lower temperature (i.e., 150 °C) used in the HTC process. In general, a slight decrease in the weight was found around 150 °C and this could be due to the removal of moisture and the release of low molecular weight volatile components. This observance is consistent with the previous study regarding combustion profile of SCG and its prepared solid fuel published by Nepal et al. [9]. In addition, a big weight loss was observed in the temperature range of 220–360 °C, which could be related to the decomposition of hemicellulose and cellulose [40]. Another big weight loss was found in the temperature between 450 °C and 520 °C. This weight loss might be caused by the degradation of lignin. Ash content is a very important property that determines the combustion behavior of the fuel pellet, and usually a high ash content of the fuel pellet is not recommended due to the ash deposition and fouling to the boiler. As clearly shown in Figure 9, the ash content in the hydrochar was found to be significantly low.

4. Conclusions

The inappropriate disposal of SCG in the coffee brewing industry by landfill leads to serious environmental problems. SCG is an excellent source for producing value-added bioproducts such as solid fuel; however, the traditional conversion process (i.e., torrefaction) is an ideal approach for high water-containing feedstocks such as SCG. Thus, in this study, SCG was converted to a solid fuel called hydrochar via HTC at temperatures of 150–210 °C, residence times of 30–120 min, and a feedstock to water weight ratio of 1:5–1:15. HTC is an alternative and promising technology to convert wet biomass into hydrochar, and one of its applications is to be used as a solid fuel for domestic heating or co-firing power plant. After HTC experiments, the elemental composition, HHV, functionality, surface morphology, and combustion behavior of hydrochar was determined using CHNS elemental analyzer, bomb calorimeter, FTIR, SEM, and TGA, respectively. The results showed that hydrochar obtained at 150 °C for 30 min and with a feedstock to water weight ratio of 1:5 had the highest energy recovery of 84%, along with a yield of 81.93 ± 2.04 wt.% and HHV of 23.54 MJ/kg. No significant differences were observed in the functionality, surface morphology, and combustion performance of feedstock and hydrochar. In conclusion, SCG might be a promising source to prepare solid fuel via HTC in terms of its relatively high calorific value and excellent combustion performance. Further research is needed to explore the other fuel properties such as combustion emission characteristics.

Author Contributions

Conceptualization, Y.H., R.G. and A.A.F.; Data curation, Y.H.; Formal analysis, S.H.; Funding acquisition, Y.H.; Investigation, R.G.; Project administration, S.H.; Resources, S.S. and A.A.F.; Software, S.S.; Writing—original draft, Y.H.; Writing—review & editing, Y.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by NSERC Discovery Grant (RGPIN-2022-03203) awarded to Dr. Yulin Hu and Undergraduate Student Research Awards awarded to Miss Rhea Gallant.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Acknowledgments

The authors would like to acknowledge the funding from NSERC through the Undergraduate Student Research Awards awarded to Rhea Gallant and the Discovery grant (RGPIN-2022-03203) awarded to Hu. The authors are also grateful to Chris Lacroix (UPEI), Rabin Bissessur (UPEI), Jeffrey Benoit (UPEI), Michael Kozma (UPEI), and Haoyu Wang (Chemical Engineering, UWO) for their technical assistance in this work.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

DTGDerivative thermogravimetric
FTIRFourier-transform infrared spectroscopy
HTCHydrothermal carbonization
HHVHigher heating value
PFDProcess flow diagram
SEMScanning electron microscope
SCGSpent coffee ground
TGAThermogravimetric analysis
TGThermogravimetric

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Sustainability 14 08818 g001 550
Figure 1. The process flow diagram of HTC process to produce hydrochar from SCG.
Figure 1. The process flow diagram of HTC process to produce hydrochar from SCG.
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Figure 2. HTC reactor used in this study.
Figure 2. HTC reactor used in this study.
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Figure 3. Effects of temperature on the product yield.
Figure 3. Effects of temperature on the product yield.
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Figure 4. The Van Krevelen diagram of hydrochar obtained at different temperatures.
Figure 4. The Van Krevelen diagram of hydrochar obtained at different temperatures.
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Figure 5. Effects of residence time on products yield obtained at 150 °C for 30–120 min and 1:5 weight ratio of feedstock to water.
Figure 5. Effects of residence time on products yield obtained at 150 °C for 30–120 min and 1:5 weight ratio of feedstock to water.
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Figure 6. Effects of feedstock to water weight ratio on products yield obtained at 150 °C for 30 min and feedstock to water weight ratio of 1:5 to 1:15.
Figure 6. Effects of feedstock to water weight ratio on products yield obtained at 150 °C for 30 min and feedstock to water weight ratio of 1:5 to 1:15.
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Figure 7. The FTIR spectra of feedstock and hydrochar.
Figure 7. The FTIR spectra of feedstock and hydrochar.
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Figure 8. SEM images of feedstock (A,B) and hydrochar (C,D).
Figure 8. SEM images of feedstock (A,B) and hydrochar (C,D).
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Figure 9. TG (black) and DTG (red) curves of feedstock (a) and hydrochar (b).
Figure 9. TG (black) and DTG (red) curves of feedstock (a) and hydrochar (b).
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Table
Table 1. Elemental composition, HHV, energy densification, and energy recovery of feedstock and hydrochar obtained from 150–210 °C for 30 min and 1:5 feedstock to water weight ratio (* n.d.—not detected).
Table 1. Elemental composition, HHV, energy densification, and energy recovery of feedstock and hydrochar obtained from 150–210 °C for 30 min and 1:5 feedstock to water weight ratio (* n.d.—not detected).
C (%)H (%)O (%)N (%)S (%)HHV (MJ/kg)Energy DensificationEnergy Recovery (%)
Feedstock52.77 ± 1.257.08 ± 0.1238.14 ± 1.372.01 ± 0.01n.d. *22.83 ± 0//
Hydrochar
150 °C55.12 ± 0.017.23 ± 0.1135.87 ± 0.421.78 ± 0.32n.d. *23.54 ± 0.081.0383.93
170 °C58.50 ± 3.937.01 ± 0.3032.44 ± 4.012.05 ± 0.33n.d. *26.55 ± 2.461.1679.95
190 °C62.05 ± 0.337.29 ± 0.0328.40 ± 0.182.26 ± 0.12n.d. *26.44 ± 0.401.1673.88
210 °C65.59 ± 0.907.15 ± 0.0325.08 ± 0.792.18 ± 0.09n.d. *27.65 ± 0.111.2177.05
Table
Table 2. The HHV, energy densification, and energy recovery of hydrochar obtained at 150 °C for 30–120 min and feedstock to water weight ratio of 1:5.
Table 2. The HHV, energy densification, and energy recovery of hydrochar obtained at 150 °C for 30–120 min and feedstock to water weight ratio of 1:5.
HHV (MJ/kg)Energy DensificationEnergy Recovery (%)
Feedstock22.83 ± 0//
Hydrochar
30 min23.54 ± 0.081.0383.93
60 min23.44 ± 0.10 1.0383.37
90 min24.38 ± 0.141.0782.36
120 min24.62 ± 0.241.0881.21
Table
Table 3. The elemental composition and atomic ratio of O/C and H/C of hydrochar obtained at 150 °C for 30–120 min and feedstock to water ratio of 1:5 (* n.d.—not detected).
Table 3. The elemental composition and atomic ratio of O/C and H/C of hydrochar obtained at 150 °C for 30–120 min and feedstock to water ratio of 1:5 (* n.d.—not detected).
Residence Time (min)C (%)H (%)O (%)N (%)S (%)Atomic O/C RatioAtomic H/C Ratio
Feedstock52.77 ± 1.257.08 ± 0.1238.14 ± 1.372.01 ± 0.01n.d. *0.541.61
Hydrochar
3055.12 ± 0.017.23 ± 0.1135.87 ± 0.421.78 ± 0.32n.d. *0.491.57
60 53.39 ± 2.856.90 ± 0.2238.04 ± 3.141.67 ± 0.06n.d. *0.531.55
90 56.98 ± 0.697.41 ± 0.2534.05 ± 0.931.56 ± 0.26n.d. *0.451.56
120 57.32 ± 0.077.52 ± 0.0833.66 ± 0.161.50 ± 0.17n.d. *0.441.57
Table
Table 4. The HHV, energy densification, and energy recovery of hydrochar obtained at 150 °C for 30 min and feedstock to water weight ratio of 1:5–1:15.
Table 4. The HHV, energy densification, and energy recovery of hydrochar obtained at 150 °C for 30 min and feedstock to water weight ratio of 1:5–1:15.
Feedstock to Water Weight RatioHHV (MJ/kg)Energy DensificationEnergy Recovery (%)
Feedstock22.83 ± 0//
Hydrochar
1:523.54 ± 0.081.0383.93
1:1023.85 ± 01.0475.59
1:1523.98 ± 0.061.0572.39
Table
Table 5. The elemental composition and atomic ratio of O/C and H/C of hydrochar obtained at 150 °C for 30–120 min and feedstock to water ratio from 1:5 to 1:15 (* n.d.—not detected).
Table 5. The elemental composition and atomic ratio of O/C and H/C of hydrochar obtained at 150 °C for 30–120 min and feedstock to water ratio from 1:5 to 1:15 (* n.d.—not detected).
Feedstock to Water Weight RatioC (%)H (%)O (%)N (%)S (%)Atomic O/C RatioAtomic H/C Ratio
Feedstock52.77 ± 1.257.08 ± 0.1238.14 ± 1.372.01 ± 0.01n.d.*0.541.61
Hydrochar
1:555.12 ± 0.017.23 ± 0.1135.87 ± 0.421.78 ± 0.32n.d.*0.491.57
1:1055.99 ± 0.817.30 ± 0.0834.88 ± 1.011.83 ± 0.11n.d.*0.471.57
1:1555.62 ± 0.407.39 ± 0.2135.05 ± 0.411.93 ± 0.22n.d.*0.471.60
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Hu, Y.; Gallant, R.; Salaudeen, S.; Farooque, A.A.; He, S. Hydrothermal Carbonization of Spent Coffee Grounds for Producing Solid Fuel. Sustainability 2022, 14, 8818. https://doi.org/10.3390/su14148818

AMA Style

Hu Y, Gallant R, Salaudeen S, Farooque AA, He S. Hydrothermal Carbonization of Spent Coffee Grounds for Producing Solid Fuel. Sustainability. 2022; 14(14):8818. https://doi.org/10.3390/su14148818

Chicago/Turabian Style

Hu, Yulin, Rhea Gallant, Shakirudeen Salaudeen, Aitazaz A. Farooque, and Sophia He. 2022. "Hydrothermal Carbonization of Spent Coffee Grounds for Producing Solid Fuel" Sustainability 14, no. 14: 8818. https://doi.org/10.3390/su14148818

APA Style

Hu, Y., Gallant, R., Salaudeen, S., Farooque, A. A., & He, S. (2022). Hydrothermal Carbonization of Spent Coffee Grounds for Producing Solid Fuel. Sustainability, 14(14), 8818. https://doi.org/10.3390/su14148818

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