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Article

Analysis of the Impact of Biomass/Water Ratio, Particle Size, Stirring, and Catalysts on the Production of Chemical Platforms and Biochar in the Hydrothermal Valorization of Coffee Cherry Waste

by
Alejandra Sophia Lozano Pérez
*,
Valentina Romero Mahecha
and
Carlos Alberto Guerrero Fajardo
Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Carrera 30 No. 45-02 Ed. Química, Bogotá 111321, Colombia
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(17), 7415; https://doi.org/10.3390/su16177415
Submission received: 25 April 2024 / Revised: 7 June 2024 / Accepted: 22 June 2024 / Published: 28 August 2024
(This article belongs to the Topic New Research on Waste Treatment, Disposal and Valorization)
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Abstract

:
In Colombia alone, 12.6 million bags of green coffee are produced, but at the same time, 784,000 tons of waste biomass are dumped in open fields, of which only 5% is recovered or used, and 10 million tonnes of coffee emit 28.6 million tonnes of CO2 eq annually. This presents a worrying dilemma, and the need to develop a technology to transform the waste into usable products is increasing. As a response to this, the valorization of coffee waste was explored through the production of biochar and platform chemicals by implementing a set of hydrothermal experiments with different biomass/water ratios (1:5, 1:10, 1:20, 1:40), particle sizes (0.5, 1, 2, 5 mm), stirring rates (5000 and 8000 rpm), and catalysts (H2SO4, NaHCO3 and CH3COOH) at 180, 220, and 260 °C in a batch reactor with autogenous pressure. Notably, the smaller B:W ratios of 1:20 and 1:40, as well as smaller particle sizes of 0.5 and 1 mm, yielded higher amounts of platform chemicals, while stirring showed minimal influence. CH3COOH significantly enhanced the process compared to other catalysts. The biochar was characterized as anthracite, and this obtaining of coal-like materials from biomass itself represents a remarkable feat. Said anthracite presented little to no variation in physical parameters, while catalysts induced functionalization. By optimizing factors like B:W ratio, particle size, and catalyst application, valuable insights have been gained into enhancing the yield of platform chemicals and quality of biochar from coffee waste. The findings not only contribute to sustainable waste management practices but also highlight the importance of exploring innovative solutions for utilizing agricultural by-products effectively.

1. Introduction

Coffee, a vital global agricultural commodity, has an annual production exceeding 169.50 million bags (10 million tons), predominantly cultivated on small family-owned farms covering less than 10 hectares [1,2]. The production process, while essential, generates significant solid waste and wastewater with high carbon content, posing environmental challenges [3]. By innovating alternative uses for coffee by-products and enhancing water filtration and recycling systems, the coffee industry can reduce its environmental impact and sustain growth [4,5,6].
Biomass valorization involves converting biomass into valuable products like biofuels and chemicals through various processes. Thermochemical methods such as pyrolysis, gasification, and liquefaction are widely used [7]. Low-temperature hydrothermal carbonization (LHW) and hydrothermal carbonization (HTC) are hydrothermal processes that transform biomass into valuable products. LHW operates at temperatures ranging from 120 to 180 °C, transforming wet biomass into aqueous platform chemicals. HTC, conducted at temperatures between 180 and 260 °C, converts wet biomass into hydrochar, a solid product rich in carbon that is commonly referred to as biochar [8]. Biochar produced through HTC is a stable, carbon-rich material that can be used for a variety of applications, particularly in the context of biomass valorization. This process transforms biomass into a stable, porous, and lightweight material with a high surface area, making it suitable for various applications [9,10,11]. Biochar has been shown to be effective in improving soil fertility, reducing greenhouse gas emissions, and mitigating climate change. It can also be used for waste management, contaminant remediation, and energy production. The properties of biochar, such as its high carbon content, alkaline pH, and large surface area, make it a valuable tool for enhancing soil physical, chemical, and biological conditions [11,12,13].
The optimization of parameters can lead to improved reaction kinetics, increased product yields, and reduced energy consumption, leading to more cost-effective and environmentally friendly processes [14,15]. Temperature is a crucial parameter in hydrothermal reactions, with temperatures ranging from 120 to 500 °C commonly used. Higher temperatures can lead to increased reaction rates and product yields, but they can also result in the decomposition of the desired products [16]. Therefore, it is essential to optimize the temperature for each specific reaction system to maximize product yields while minimizing decomposition [17]. The use of different temperatures can favor the production of aqueous, oil, gas, and solid phases of hydrothermal valorization [18]. Additionally, a study on the hydrothermal valorization of winery and distillery by-products discusses the impact of temperature on the hydrochar production process and demonstrates that the carbonization temperature is a crucial parameter, affecting product yield and quality [19].
The biomass-to-water (B:W) ratio is an important parameter in hydrothermal reactions, with ratios ranging from 10 to 30% commonly used [20]. Higher B:W ratios can lead to increased product yields, but they can also result in higher energy requirements and equipment limitations. In the context of HTC, previous research shows that the B:W ratio has a significant impact on the HTC process, with a lower B:W ratio leading to a higher carbonization degree and a higher yield of solid products [21].
On the other hand, smaller particle sizes lead to increased reaction rates and product yields. Particle sizes ranging from 0.1 to 1 mm are commonly used in hydrothermal reactions, with smaller particle sizes leading to increased surface area and reaction rates [22]. A consistent particle size is essential for ensuring uniform heat and mass transfer during these processes [23]. The size of the particles affects the heating rate of solid fuel, controlling the rates of drying and reactions [24]. Additionally, the surface area of the particles is a key factor influencing reactions. Smaller particle sizes lead to a more intense penetration of activated agents and the development of pores, which can impact the efficiency of reactions and the overall effectiveness of the valorization process [25]. Stirring can significantly impact the morphology and phase composition of the products obtained. The stirring rate can affect the probability of spontaneous nucleation and the supersaturation during crystal growth, leading to changes in the morphology of the products [26]. In the hydrothermal synthesis of hydroxyapatite (HAp), higher stirring rates can lead to the formation of longer whiskers, while lower stirring rates can result in the formation of hexagonal rods and agglomerates [27]. The most homogeneous product is usually obtained at intermediate stirring rates, while lower or higher stirring rates can lead to less homogeneous products [28].
Catalysts are important in hydrothermal reactions, with various catalysts being commonly used to increase reaction rates and product yields [29]. The type and amount of catalyst used can significantly impact the reaction kinetics and product distribution, making it essential to optimize catalyst selection and dosage for each specific reaction system. Homogeneous catalysts promote water–gas shift reactions by favoring C-C bond breakup, thus improving H2 yields [30].
There is a lack of studies of valorization processes implementing coffee cherry waste, which comprises silverskin, husk, parchment, mucilage, and spent coffee grounds (SCG), with most of the research focused on SCG [31,32,33] and husk [34,35] for the production of biochar. This study innovates by focusing not only on the effect of parameters on the yield and quality of biochar, but also the platform chemicals produced during the hydrothermal treatment process by implementing a biomass rarely used. The article also shows the influence of physical parameters in the production of chemicals and biochar and how they can improve/decrease the production, concentration, and yield of the process. This study will also investigate the effect of homogeneous catalysts on the activation energy of the hydrothermal process, promoting the hydrolysis and conversion of the coffee cherry waste into valuable by-products. The results of this study will provide insights into the optimal conditions for the LHW-HTC valorization of coffee cherry waste, maximizing product yields while minimizing decomposition and energy requirements.

2. Materials and Methods

2.1. Biomass Characterization

In order to understand the initial biomass (Figure 1), a previous characterization was performed. A small summary of the techniques and technical reports used are presented:
In the case of proximate assay, the assays consisted of moisture (NREL/TP-510-42621) [36], ash (NREL/TP-510-42622) [37], volatile matter (ASTM E872-82) [38], and fixed carbon for ultimate assay, the Technical Report ASTM D5373-21 [39] was followed, and finally, the chemical composition was measured through the procedure presented in a study entitled “Methods for Dietary Fiber, Neutral Detergent Fiber, and Nonstarch Polysaccharides in Relation to Animal Nutrition” [40], published in the Journal of Dairy Science.

2.2. Hydrothermal Experiments

The experiments were conducted in a 500 mL batch reactor (Figure 2). Initially, tests were performed by varying the biomass-to-water ratio (1:05, 1:10, 1:20, and 1:40), where 20, 10, 5, and 2.5 g of biomass was weighed with 80, 90, 95, and 97.5 g of deionized water (respectively). Subsequently, the mixture was placed in the reactor at different temperatures, namely 180 °C, 220 °C, and 260 °C, in accordance with previous research [41]. Biochar and platform chemicals were produced under autogenous pressure (less than 5 MPa) for 1 h (measured after reaching the desired temperature) with a particle size of 2 mm (obtained after milling in a hammer mill and sieving in a 2 mm stainless steel wire cloth 12 inch-diameter ASTM E11 Test Sieve).
Subsequently, to analyze the impact of biomass particle size during hydrothermal carbonization, a separate set of experiments was carried out using four different particle sizes: 0.5 mm, 1 mm, 2 mm, and 5 mm (all obtained after milling and sieving in different sizes test sieves). These experiments were conducted at the same temperatures (180 °C, 220 °C, and 260 °C) under autogenous pressure for 1 h (measured after reaching the desired temperature) and a biomass-to-water ratio of 1:20 (5 g biomass/95 g of water) was used.
Finally, to inquire into the impact of stirring during the process, a set of experiments were performed with stirring at 5000 and 8000 revolutions per minute at the same temperatures (180 °C, 220 °C, and 260 °C) under autogenous pressure for 1 h (measured after reaching the desired temperature) with a particle size of 0.5 mm and a biomass-to-water ratio of 1:20 (5 g biomass/95 g of water).

2.3. Characterization of Fractions

The solid and liquid fractions were separated using vacuum filtration and measured to verify a mass balance. The solid fraction was recovered, washed with water, ethanol, and acetone, and then dried at 105 °C until reaching a constant weight. The solid was characterized with elemental analysis (Thermo Flash 2000, manufacturer Thermo Fisher Scientific, MA, USA) following the guidelines outlined in ASTM D5373-21 [39]. pH and conductivity were used to follow the reactions taking place. Additionally, HPLC-RI was used to quantify the platform chemicals (PC). The yields were measured using Equation (1):
Y i e l d = ( g r a m s   o f   P C ) ( g r a m s   o f   l i g n o c e l u l l o s i c   s t r u c t u r e   i n   b i o m a s s ) × 100 %

2.4. Analytical Methods

The method implemented in previous articles ([21,41] and their supplementary material [42]) was used for the quantification of PC. Said method was implemented with a Hitachi Elite LaChrom (Tokyo, Japan), Hitachi L-2490 refraction index detector at 40 °C, SHODEX Sugar SH1821 column at 60 °C, mobile phase of 0.005 M H2SO4, and a flow rate of 0.5 mL/min, producing peaks that are resolved (Figure 3), except for glucose, galactose, arabinose, and xylose, which coeluted, and hence were measured as overall sugars. The platform chemicals measured (pure standards) were xylose, glucose, galactose, arabinose, formic acid, levulinic acid, hydroxymethylfurfural (HMF) and furfural, all with corresponding retention times presented in Table 1.

2.5. Homogeneous Catalysts

To analyze the effect of catalysts in the hydrothermal reaction, a set of acid and basic homogeneous catalysts were used to make the reactions more efficient and specific, as well as to functionalize the biochar. H2SO4 [0.1 M], CH3COOH [0.1 M], and NaHCO3 [0.1 M] solutions were prepared to replace the water in the biomass/water 1:20 ratio, 0.5 mm (5 g biomass/95 g catalysts solution). Reactions were performed at 180 °C, 220 °C, and 260 °C for 1 h. The obtained fractions (solid and liquid) were characterized as previously mentioned.

3. Results and Discussion

A preliminary characterization of the coffee waste was performed and is presented in Table 2. This allows for a better understanding of the start conditions and if the biomass is usable for valorization processes.
The chemical composition of coffee cherry, pulp, husk, and spent coffee grounds (SCG) reveals distinct differences between the various waste streams. Cellulose and hemicellulose are present in varying amounts, with coffee husk having the highest cellulose content at 43%, followed by coffee cherry and SCG. Hemicellulose is highest in SCG and coffee cherry, while pulp and husk have lower amounts. Lignin is highest in coffee pulp at 20.07%, followed by SCG and coffee cherry.
The data also highlight differences in the elemental composition of the waste streams. SCG has the highest carbon content at 68.52%, while coffee cherry and husk have lower amounts. Hydrogen is highest in SCG, followed by husk and coffee cherry. Nitrogen is similar in coffee cherry, pulp, and SCG, but lower in husk. Sulfur is present in coffee cherry and husk, but not reported for SCG. Moisture, ash, and volatile matter contents also vary significantly between the waste streams, with SCG having the highest ash content and volatile matter, and husk having the highest fixed carbon content. These differences can inform strategies for processing and utilizing these waste streams.
The biomass used for the essays corresponds to coffee cherry waste where there is a high amount of volatile matter (80.81%) which can be transformed into PC, it has a relatively low amount of ashes which can be toxic for the process and reactor. On the other hand, it has a high amount of cellulose (27.6%) and hemicellulose (12.5%) which can be hydrolyzed into sugars. It also has a high content of lignin (13.7%), which can act as a shield for the transformation of cellulose and hemicellulose.

3.1. Biomass/Water Ratio Influence

From the hydrothermal processes, three phases were obtained. Two of these were measured directly, and the gas fraction was obtained through mass difference. The amounts are presented in Table 3.
The purpose of this set of experiments was to compare the ratios of solid fraction, liquid fraction, and gas fraction under a specific biomass–water ratio at different reaction temperatures. It was observed that when the temperature increased, the solid fraction decreased, which can be attributed to the carbonization of the biomass that occurs during hydrothermal reactions at 260 °C. Additionally, the gas fraction increased at higher temperatures. However, the trend for the liquid fraction was not clear when comparing the same biomass–water ratio at different temperatures.
The changes in pH and conductivity in a liquid fraction at different temperatures can be explained by the concentration of ions and the nature of the solution [43]. When analyzing the pH and conductivity trends, it is observed that the 1:10 ratio has the lowest pH, becoming more basic as the B:W ratio decreases. In terms of conductivity, it consistently drops as the concentration of B:W reduces.
There is a decrease in pH as the B:W ratio becomes smaller, and the solution becomes more basic due to the nature of the ions present. pH is a measurement of the concentration of hydrogen ions in a sample, with lower pH values indicating higher hydrogen ion concentrations, hence acidity [44]. As the B:W ratio decreases, the concentration of basic ions increases relative to acidic ions (due to the increase in water content), leading to a rise in pH towards basic levels. Conductivity, on the other hand, is a measure of the concentration of ions in a sample that can transmit an electric current. The drop in conductivity as the B:W ratio reduces is linked to the relative concentration of ions in the solution [45]. A decrease in the concentration of ions, which occurs as the B:W ratio decreases, results in lower conductivity levels (Figure 4). Therefore, the observed trends in pH and conductivity can be attributed to the changing balance of acidic and basic ions in the solution, impacting both the pH levels and the conductivity as the B:W ratio varies.
Figure 5 shows how when the B:W ratio decreases, the pH decreases as well. The conductivity has the same behavior, decreasing from 12 mS/cm to 3 mS/cm (for 220 °C) and 15 mS/cm to 3 mS/cm (for 260 °C), as the B:W changes from 1:5 to 1:40. This shows a bigger shift in conductivity than when the hydrothermal reaction takes place at 180 °C. The change in the initial conductivity and the pH from the aqueous phase is due to the fact that at higher temperatures, the water’s acidity increases, allowing for the hydrothermal reactions to be more efficient and for the process not only to stop at sugar production but to lead to the production of acids, furfural, and HMF. It can also be seen that there is a different tendency whereby even though the conductivity decreases, the pH becomes more acidic as the B:W ratio decreases.
The observed phenomenon where conductivity decreases while pH becomes more acidic as the B:W ratio decreases can be explained by the production of weak acids that impact pH without significantly affecting conductivity. This suggests that the increase in weak acids leads to a more acidic pH environment while not exerting a substantial influence on the conductivity of the solution. The relationship between weak acids and pH is well established in chemistry. Weak acids partially dissociate in solution, releasing hydrogen ions that lower the pH. This increase in weak acids can cause a more acidic pH environment, as observed in the scenario described. Conductivity, on the other hand, is primarily influenced by the concentration of ions that can carry an electric current [46]. While the weak acids may contribute to the pH change, they might not significantly alter the overall ion concentration responsible for conductivity. Therefore, the conductivity decreases due to other factors, such as changes in the concentration of other ions or the nature of the solution, while the weak acids predominantly impact the pH levels.
After following the reaction, a proper quantification was performed of the different conditions of temperature and the influence of the B:W in the reactions taking place with the initial lignocellulosic structure of the biomass previously characterized (Table 2). The yields are calculated over and above the initial percentage of hemicellulose and cellulose that can be transformed into platform chemicals.
Figure 6 presents the different yields of the platform chemicals and the total yield at 180 °C. The total yield increases as the B:W ratio decreases. The lower amount of biomass particles in water can lead to better extraction and yield and higher surface contact of water molecules with the biomass due to several factors. Reducing the amount of biomass particles in water increases the surface area available for water molecules to interact with the biomass, leading to better extraction and yield. This increased surface area allows for more efficient mass transfer and diffusion of compounds from the biomass to the water, enhancing the extraction process [47]. A lower amount of biomass particles in water improves the solvent-to-solute ratio, allowing for more efficient extraction of bioactive compounds. This improved ratio ensures that the solvent has adequate capacity to dissolve the desired compounds, leading to higher yields [48]. With a lower amount of biomass particles, water molecules can more easily penetrate the biomass, facilitating the extraction of bioactive compounds. This enhanced solvent penetration is particularly important for extracting compounds that are deeply embedded within the biomass structure [49]. Decreasing the amount of biomass particles in water reduces diffusion resistance, allowing for faster and more efficient extraction of bioactive compounds. This reduced resistance enables the compounds to move more freely from the biomass to the water, enhancing the overall extraction process [50]. Finally, by using a lower amount of biomass particles, there is less risk of over-extraction, which can lead to the degradation of the bioactive compounds [51]. This ensures that the extracted compounds maintain their quality and functionality.
Regarding the reactions that take place at 180 °C, Figure 5 shows that both sugars and formic acid are the most produced. The sugars are primarily produced from the hydrolysis of complex lignocellulosic structures present in the biomass. This process involves breaking down the polysaccharides into monosaccharides through hydrolysis, resulting in the production of sugars as a significant product [52]. Formic acid is generated as a byproduct of the instability and degradation of compounds like HMF (5-Hydroxymethylfurfural), furfural, and levulinic acid under the high-temperature conditions of 180 °C. These compounds can undergo further reactions leading to the formation of formic acid as a degradation product [53].
Figure 7 shows the total yields and individual yields of the platform products produced in these HTC conditions at 220 °C. It shows, similarly to the situation that took place at 180 °C, that the best B:W ratio is 1:40, although in Figure 4, the tendency after 1:20 ratio becomes rather constant. Under these conditions, the production of formic acid and levulinic acid is favored over the production of sugars. There is enough energy to transform the sugars into levulinic acid, meaning that the sugar content decreases compared to the one obtained in 180 °C. Both HMF and furfural are obtained in very low quantities or null quantities from coffee waste.
Figure 8 presents the concentrations at the highest temperature from HTC (260 °C). Opposite to the results found in 180 °C and 220 °C, the yield obtained with the biomass/water ratio 1:0 was the highest. It is worth noting that it mainly comprises the production of formic acid due to the degradation of the other platform chemicals and how these conditions tend to favor mostly the production of biochar and not so much the production of platform chemicals. Perhaps, since the energy is higher at 260 °C, and the main product is biochar, the use of a smaller B:W ratio can lead to bigger degradation of the platform chemicals due to high radiation.
Comparing the total yield of 180 °C, 220 °C, and 260 °C, correspondingly 20.42% (sugars/formic acid), 17.29% (formic acid/levulinic acid), and 28.48% (formic acid/levulinic acid), both the amount and content of the platform chemicals produced shifts in the temperature. In all the cases, an improvement of three times the initial yield can be obtained selecting the correct B:W ratio for the hydrothermal valorization conditions. For the specific biomass of coffee cherry waste, the use of lower B:W ratios can improve the variety and obtention of platform chemicals without leading to degradation. According to the results, to continue the process of optimization, the B:W ratio of 1:20 is selected to be maintained as a constant to evaluate the particle size influence. Conductivity and pH are not good parameters to use for the follow-up of reactions that switch the initial concentration due to the fact that the final pH and conductivity are not comparable because the initial amount of H+ and Ions are different.

3.2. Particle Size Influence

After analyzing the impact of the biomass–water ratio on the hydrothermal process, experiments were conducted to investigate the effect of varying particle size on the solid, liquid, and gas fractions. The results of the mass balance for different particle sizes at varying temperatures are presented in Table 4.
The solid fraction decreased at higher temperatures due to biomass carbonization, a trend that was consistent regardless of particle size. This behavior was observed in experiments with different biomass–water ratios. For example, at 260 °C, the weight of the dry solid remained similar across different particle sizes. Nevertheless, no clear trend or significant result was observed for the liquid fraction, highlighting the importance of analyzing the impact of particle size on hydrothermal reaction yields.
A previous follow-up was performed for the aqueous phases of the hydrothermal reactions obtained at 1 h, 1:40 B:W ratio, and different particle sizes (Figure 9). In all the cases (180 °C, 220 °C, and 260 °C), a similar behavior was observed, with a similar behavior overall in all of the particle sizes except for the 2 mm size, this size producing the most basic sample and the highest conductivity. The size of a sample can significantly impact its conductivity and pH levels. When a sample has a larger size, it generally has a higher conductivity due to the increased concentration of ions present in the solution. This is because larger samples contain more particles, which can release more ions into the solution upon dissolution or hydrolysis. A higher concentration of ions in the solution results in a higher conductivity, as ions are responsible for carrying the electric current in the solution [54].
On the other hand, the pH of a sample can also be affected by its size [55]. A larger sample size may produce a more basic pH due to the presence of more basic compounds or ions in the solution. This is because basic compounds or ions can neutralize acidic compounds or ions, leading to an increase in pH. Additionally, the increased concentration of ions in the solution can also contribute to a more basic pH, as some ions can act as buffers and regulate the pH of the solution [56].
Figure 10 shows the products obtained at 180 °C; the total yield of 2 and 5 mm is very similar, in both cases producing a higher amount of sugars and formic acid, with smaller particle sizes (0.5 and 1 mm) having total yield increases of around 5%, which shows the influence that particle size has in the hydrothermal reactions. Smaller particles have a larger surface area per unit mass, which enhances the contact between the biomass and the hydrothermal medium. This increased surface area facilitates the hydrolysis and degradation of the biomass, leading to higher yields of platform chemical [57].
Smaller particles have better heat and mass transfer properties due to their increased surface area. This improved heat and mass transfer leads to more efficient and complete reactions, resulting in higher yields of platform chemicals [58]. Smaller particles have lower diffusion resistance, which allows for faster and more efficient transport of reactants and products to and from the reaction site. This reduced diffusion resistance leads to higher yields of platform chemicals [59]. Smaller particles have a higher specific surface area, which enhances their hydrolysis. This increased digestibility leads to higher yields of platform chemicals during hydrolysis [60]. A smaller particle size improves the pumpability and mixing of the hydrothermal medium, leading to more uniform and efficient reactions. This optimized pumpability and mixing result in higher yields of platform chemicals [61].
The reaction presented at 180 °C favors the production of sugar over the other platform chemicals due to the lack of enough energy to transform said sugars into other platform chemicals. Lower particle sizes also allow the reaction to happen more uniformly, and that can be seen in a higher production of furfural and HMF. Less degradation takes place because of the lower energy compared to reactions at 220 °C and 260 °C.
As for Figure 11, where the temperature analyzed is 220 °C, the particle with the highest yield is 1 mm, being one of the smallest particles used. This shows that a tendency can be observed. The behavior mentioned previously and the effects of smaller particle sizes influence the overall production of platform chemicals in a positive way [62,63]. In this temperature, the sugars continue to have a higher level of production, followed by the formic acid and levulinic acid, with a very small yield of HMF and null furfural, which is due to a higher energy in the system and the degradation of said molecules. In this case, similarly to before, an increase of around 5% can take place with the right conditions and particle size.
Finally, Figure 12 shows the influence of the particle size in a hydrothermal reaction at 260 °C. At this temperature, the particles size of 0.5 mm and 1 mm present the highest yield of sugar, formic acid, levulinic acid, HMF, and furfural. This is also the only condition where furfural is formed. The correct tuning of the particle size at 260 °C can lead to a 9% increase in the total yield of the valorization process. It is also worth highlighting that when comparing Figure 10 to Figure 6, a higher diversity in platform chemicals can be seen, and not only formic acid is produced. This confirms that the B:W ratio and particle size not only influence the yield of the reaction but the selectivity of it, leading to a higher/lower production of certain platform chemicals depending on the conditions chosen.
In conclusion, the optimization of particle size in coffee waste processing has shown that using smaller particle sizes (0.5 and 1 mm) leads to higher yields and efficiency in the production of platform chemicals through hydrothermal processes. The results verify that smaller particle sizes offer increased surface area, improved heat and mass transfer, reduced diffusion resistance, enhanced digestibility, and optimized pumpability and mixing [64,65]. These factors collectively contribute to more efficient and complete reactions, ultimately resulting in higher yields and enhanced process efficiency when utilizing smaller particle sizes in coffee waste conversion processes.

3.3. Stirring Influence

In addition to observing the influence of the particle size and biomass–water ratio, the next variable studied was the stirring speed. Table 5 presents the results of the mass balance of solid, liquid, and gas in the hydrothermal processes, comparing the reaction without stirring, at 5000 rpm, and at 8000 rpm.
When comparing the results with different stirring speeds, it was noticed that the stirring had no significant effect on the solid and gas fractions. However, the liquid fraction increased slightly with higher stirring speeds at 180 °C and 220 °C. At 260 °C, the liquid fraction remained virtually unchanged.
As for the follow-up process performed with pH and conductivity (Figure 13), the changes are small to none in the aqueous phases obtained from the hydrothermal processes. In some cases, it can be seen that the conductivity of the sample without stirring had a slightly higher conductivity than the other with stirring, but, other than that, by analyzing the preliminary data obtained, there is no significant increase/decrease in the charged species or H+ in the media.
When analyzing the quantification results presented in Figure 14, there is almost no change in the results between the two speeds of stirring, and, opposite to expected, there is a higher yield in the experiments performed without agitation, producing higher concentrations of all the platform chemicals. The phenomenon observed in the experiments, where there is almost no change in the results between the two speeds of stirring and a higher yield in experiments performed without agitation, can be explained by the fact that stirring helps reactions by keeping the reacting mixture homogeneous [66]. However, once above a certain speed threshold, increasing the stirring speed may not have any significant additional effect on the reaction rate [67]. Additionally, stirring at high speeds can cause the liquid to be distributed all over the walls, which may not be beneficial for the reaction. Furthermore, the higher yield in experiments performed without agitation could be due to the fact that the reaction is taking place in a 500 mL batch reactor with only 100 mL of mixture. In such a small volume, the reactants may be able to mix efficiently without the need for agitation, leading to a higher yield. Focusing on the specific platform chemicals obtained, there is a high number of sugars, followed by formic acid and levulinic acid. The ratios between the produced platform chemicals in each stirring condition remain equal.
In Figure 15, the previously seen behavior where there is no significant change between the stirring at 5000 and 8000 rpm is maintained, showing that both speeds are above the threshold and there is no longer a significant difference between them. On the other hand, there is a decrease in the total yield without stirring. This can be because of the favoring of the production of organics and a gas phase when applying stirring to the system due to a more efficient heat transfer.
Finally, Figure 16 displays a very similar yield at all of the stirring speeds, though, as seen before, there is a slight increase in the experiments performed without stirring, as well as the obtention of HMF and furfural, two products that are not obtained by implementing stirring. Figure 16 also shows that adding stirring to the system at 260 °C helps with the production of formic acid over sugar. The parameter of stirring may also influence the production of the biochar, a parameter that will be evaluated later in the article.
In conclusion, the experiments conducted in a 500 mL batch reactor with 100 mL of mixture have shown that there is minimal difference in results between the two stirring speeds. Surprisingly, higher yields were obtained in experiments conducted without agitation, leading to higher concentrations of all platform chemicals. This phenomenon may indicate a preference for certain reactions over others in the absence of agitation, highlighting the complex interplay between reaction conditions and outcomes.

3.4. Catalyst’s Influence

After optimizing all the physical properties in order to enhance the production of platform chemicals, the enhancement of chemical properties (through the usage of catalysts) appeared as an interesting pathway. Homogeneous catalysts, such as CH3COOH, H2SO4, and NaHCO3, are used in hydrothermal valorization (HTC) processes for several reasons. Firstly, these catalysts can improve the yield of platform chemicals by enhancing the reaction rate and selectivity. For example, CH3COOH and H2SO4 can act as catalysts in the hydrolysis and dehydration reactions, while NaHCO3 can be used as a catalyst in the methanol synthesis reaction [68].
The active Bronsted acid sites on these catalysts play a crucial role in improving the yield of platform chemicals. Bronsted’s acid sites are electron-deficient centers that can accept electron pairs from other molecules, leading to the formation of a coordinate covalent bond [69]. In the context of hydrothermal valorization, these sites can react with the organic compounds present in the biomass feedstock, promoting the formation of desired products [70]. Furthermore, homogeneous catalysts can help functionalize biochar, which is a residual byproduct from the thermochemical degradation of biomass [71]. By activating and functionalizing the biochar, its physicochemical properties can be improved, making it more suitable for various applications, including catalysis and catalyst supports.
Table 6 shows the results for the different fractions obtained through hydrothermal reactions with coffee waste.
In this set of experiments, the impact of an acid catalyst (CH3COOH) and a basic catalyst (NaHCO3) on the hydrothermal reaction was compared. The results showed that a basic medium at a higher temperature resulted in a higher solid fraction compared to the solid fraction obtained with the acid catalyst and without any catalyst. However, when no catalyst was present or an acid catalyst was used, the liquid and gas fractions were favored. There was no clear trend between the results in an acid medium and without any catalyst regarding the liquid and gas fractions. Therefore, it is important to compare the yields of the reactions in detail to determine which products are favored under these conditions.
In this case, no previous follow-up was performed, because each catalyst has an initial pH/conductivity and there is no way to properly see if the changes in those parameters are due to reactions of the biomass or only the catalysts itself. Some tests were performed with H2SO4, but due to the highly acidic conditions of the sample, it was not possible to inject it into the HPLC-IR, and the results will only be evaluated based on the biochar produced with said catalyst.
Figure 17 shows the individual and total yields without and with catalysts at 180 °C. As for the acid catalysts, it improves the over yield of the reaction, almost doubling to 62.51% the rate of valorization. It leads to the selective production of sugars and formic acid and to decreasing (completely) the production of levulinic acid, HMF, and furfural. On the order hand, the results obtained for the basic catalyst decrease the yield of all of the platform chemicals but produce a harder, darker, rock-like biochar with characteristics not present in either non-catalyzed reactions or acid-catalyzed ones.
As for the reactions at 220 °C (Figure 18), both catalysts improve the yield from the initial non-catalyzed reaction. Reactions with CH3COOH produce the highest yield of HMF and furfural found yet with coffee residues, increase the production of formic acid up to 75% and produce four times more sugar than a reaction without catalyst. It is worth noting that the total yield is higher than 100%, this due to the fact that the yield is calculated from the hemicellulose and cellulose percent found in the biomass, but by finding a yield higher than 100%, it can be said that other components from the biomass aside from hemicellulose and cellulose are being hydrolyzed by the catalyst and producing sugar, formic acid, HMF and furfural, which can be starches, sugars from the waste itself, fibers, oils, and others.
In the case of the basic catalyst, it increases the sugar and formic acid production, and decreases the levulinic acid production. HMF and furfural are not produced. The overall yield increases in a 10% and biochar characteristics are also harder, darker and more coal-like than the other biochars produced.
Figure 19 shows how reactions at 260 °C are also favored by the use of a weak acid catalyst, but do not have such a good yield compared to reactions at 220 °C, this because at 260 °C the production of biochar is favored as well, leading to mostly the degradation of the platform chemicals to formic acid, and the formation a carbon-like solid. Performing the reaction without catalyst leads to a small production of HMF, but, due to the high energy, the addition of a catalyst leads to the drop in HMF and furfural production. On the other hand, the acid catalyst produces mainly formic acid, in a highly selective reaction, while the basic catalyst produces similar parts of sugar, formic acid and a slight increase in the levulinic acid production.
Catalysts play a crucial role in selective reactions with biomass in hydrothermal processes [72], particularly for coffee waste. The use of weak acid catalysts in hydrothermal processes for coffee waste leads to an overall increase in yield and production of formic acid, while basic catalysts result in a slightly higher yield and increase in sugar, levulinic acid, and formic acid yield. The choice of catalyst depends on the desired product and the specific biomass feedstock.

4. Biochar Characterization

To characterize biochar, several techniques can be employed, including elemental analysis, scanning electron microscopy (SEM), Van Krevelen diagrams, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), nuclear magnetic resonance spectroscopy (NMR), Brunauer–Emmett–Teller (BET), Raman, and density analyzer, among others [73,74]. In this manuscript, Elemental analysis and Van Krevelen were used to visualize the distribution of oxygen, carbon, and hydrogen in the biochar.
After obtaining the different biochars, the samples obtained at 260 °C were characterized using elemental analysis, and they were positioned in the Van Krevelen diagram (Figure 20). The initial biomass can be seen in the zone of biomass; an experiment performed at 180 °C is represented with the pink triangle, being the closest to a lignite with a higher hydrogen and oxygen content that the samples obtained at 260 °C. The triangles colored green present the characterization of the biochars obtained using catalysts, shown in the lignite/coal zone, but also present sulfur, sodium and aluminum in their structures, showing signs of functionalization and their potential use as catalysts (Figure 21).
The other samples show a very similar O/C ratio and H/C ratio, and are in the zone of anthracite, showing that even though the parameters of B:W ratio, particle size, and stirring can influence of the yield of platform chemicals, under the condition of a 500 mL batch reactor filled, they have little influence on the characterization of the biochar. On the other hand, parameters such as temperature, time, and catalysts [41] have the highest influence on the production of biochar.

5. Conclusions

In conclusion, this study of coffee waste valorization has been successful in demonstrating the potential of this abundant waste material as a valuable resource. The investigation of various factors, including the influence of stirring, B:W ratio, particle size, and catalysts, has provided valuable insights into the optimization of the conversion process. Specifically, the B:W ratio of 1:20 and 1:40, along with the particle size of 0.5 and 1 mm, were found to produce the highest yield, while stirring did not significantly affect the yield.
Furthermore, the application of catalysts, such as CH3COOH and NaHCO3, resulted in a significant improvement in the yield, with an over 100% yield achieved with CH3COOH. The characterization of the biochar produced from the coffee waste revealed that it was primarily composed of anthracite, with the physicochemical conditions of stirring, B:W ratio, and particle size having little impact on the final product. However, the application of catalysts resulted in functionalization and a change in the hardness of the solid product.
Overall, this study has demonstrated the potential of coffee waste as a valuable resource to produce biochar, and the optimization of the conversion process through the investigation of various factors. The application of catalysts, in particular, has shown great promise in improving the yield and quality of the final product, providing a valuable contribution to the field of waste valorization and sustainable resource management.

Author Contributions

Conceptualization, A.S.L.P. and V.R.M.; methodology, A.S.L.P.; validation, A.S.L.P. and V.R.M.; formal analysis, A.S.L.P. and V.R.M.; investigation, A.S.L.P. and V.R.M.; resources, C.A.G.F.; data curation, A.S.L.P.; writing—original draft preparation A.S.L.P. and V.R.M.; writing—review and editing, C.A.G.F.; visualization, A.S.L.P.; supervision, C.A.G.F.; project administration, C.A.G.F.; funding acquisition, A.S.L.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by MINCIENCIAS, grant number Contrato de financiamiento de recuperación contingente No. 80740-101-2022. Implementación de una biorrefinería hidrotermal para la producción de productos químicos de alto valor agregado, mediante el uso de biomasas residuales de procesos agroindustriales, en alianza intersectorial (academia-industria)”. Código 1101-914-91642.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data are found the present manuscript.

Acknowledgments

We thank the Universidad Nacional de Colombia and the Departamento de Química-Facultad de Ciencias for their support and the possibility of using equipment and techniques that allowed the development of this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Coffee cherry waste used in the present research.
Figure 1. Coffee cherry waste used in the present research.
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Figure 2. The implemented reactor consists of a jacket (A), where the 500 mL stainless steel vessel is placed, eight threaded studs (B) in the upper part that are adjusted; a heating blanket allows for the regulation of the reaction temperature (C and D), and the coil (I) is used for the circulation of water to stabilize the temperature with the operation of the chiller (F) and its respective pump (E), (G) corresponds to the cooling hoses and (H) tabs for recirculating water. The nozzles (K) allow for the control of gas extraction, and if necessary, an inert gas can be added to add pressure to the system (J). Finally, the system pressure is monitored using the manometer located in the reactor (L).
Figure 2. The implemented reactor consists of a jacket (A), where the 500 mL stainless steel vessel is placed, eight threaded studs (B) in the upper part that are adjusted; a heating blanket allows for the regulation of the reaction temperature (C and D), and the coil (I) is used for the circulation of water to stabilize the temperature with the operation of the chiller (F) and its respective pump (E), (G) corresponds to the cooling hoses and (H) tabs for recirculating water. The nozzles (K) allow for the control of gas extraction, and if necessary, an inert gas can be added to add pressure to the system (J). Finally, the system pressure is monitored using the manometer located in the reactor (L).
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Figure 3. Standards used for analytic method development.
Figure 3. Standards used for analytic method development.
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Figure 4. pH and conductivity of liquid fraction at 180 °C with different B:W ratios, 1 h, and 2 mm particle size.
Figure 4. pH and conductivity of liquid fraction at 180 °C with different B:W ratios, 1 h, and 2 mm particle size.
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Figure 5. pH and conductivity of liquid fraction at (a) 220 and (b) 260 °C with different B:W ratios, 1 h, and 2 mm particle size.
Figure 5. pH and conductivity of liquid fraction at (a) 220 and (b) 260 °C with different B:W ratios, 1 h, and 2 mm particle size.
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Figure 6. HPLC-RI quantification of liquid fraction at 180 °C with different B:W ratios, 1 h, and 2 mm particle size.
Figure 6. HPLC-RI quantification of liquid fraction at 180 °C with different B:W ratios, 1 h, and 2 mm particle size.
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Figure 7. HPLC-RI quantification of liquid fraction at 220 °C with different B:W ratios, 1 h, and 2 mm particle size.
Figure 7. HPLC-RI quantification of liquid fraction at 220 °C with different B:W ratios, 1 h, and 2 mm particle size.
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Figure 8. HPLC-RI quantification of liquid fraction at 260 °C with different B:W ratios, 1 h, and 2 mm particle size.
Figure 8. HPLC-RI quantification of liquid fraction at 260 °C with different B:W ratios, 1 h, and 2 mm particle size.
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Figure 9. pH and conductivity quantification of liquid fraction at (a) 180 °C, (b) 220 °C, and (c) 260 °C with different particle sizes, 1 h, and 1:20 B:W ratio.
Figure 9. pH and conductivity quantification of liquid fraction at (a) 180 °C, (b) 220 °C, and (c) 260 °C with different particle sizes, 1 h, and 1:20 B:W ratio.
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Figure 10. HPLC-RI quantification of liquid fraction at 180 °C with different particle sizes, 1 h, and 1:20 B:W ratio.
Figure 10. HPLC-RI quantification of liquid fraction at 180 °C with different particle sizes, 1 h, and 1:20 B:W ratio.
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Figure 11. HPLC-RI quantification of liquid fraction at 220 °C with different particle sizes, 1 h, and 1:20 B:W ratio.
Figure 11. HPLC-RI quantification of liquid fraction at 220 °C with different particle sizes, 1 h, and 1:20 B:W ratio.
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Figure 12. HPLC-RI quantification of liquid fraction at 260 °C with different particle sizes, 1 h, and 1:20 B:W ratio.
Figure 12. HPLC-RI quantification of liquid fraction at 260 °C with different particle sizes, 1 h, and 1:20 B:W ratio.
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Figure 13. pH and conductivity quantification of liquid fraction at (a) 180 °C, (b) 220 °C and (c) 260 °C with different stirring speeds, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
Figure 13. pH and conductivity quantification of liquid fraction at (a) 180 °C, (b) 220 °C and (c) 260 °C with different stirring speeds, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
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Figure 14. HPLC-RI quantification of liquid fraction at 180 °C with different stirring speeds, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
Figure 14. HPLC-RI quantification of liquid fraction at 180 °C with different stirring speeds, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
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Figure 15. HPLC-RI quantification of liquid fraction at 220 °C with different stirring speeds, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
Figure 15. HPLC-RI quantification of liquid fraction at 220 °C with different stirring speeds, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
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Figure 16. HPLC-RI quantification of liquid fraction at 260 °C with different stirring speeds, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
Figure 16. HPLC-RI quantification of liquid fraction at 260 °C with different stirring speeds, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
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Figure 17. HPLC-RI quantification of liquid fraction at 180 °C with no stirring, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
Figure 17. HPLC-RI quantification of liquid fraction at 180 °C with no stirring, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
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Figure 18. HPLC-RI quantification of liquid fraction at 220 °C with no stirring, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
Figure 18. HPLC-RI quantification of liquid fraction at 220 °C with no stirring, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
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Figure 19. HPLC-RI quantification of liquid fraction at 260 °C with no stirring, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
Figure 19. HPLC-RI quantification of liquid fraction at 260 °C with no stirring, 1 h, 0.5 mm particle size, and 1:20 B:W ratio.
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Figure 20. Van Krevelen characterization of biochars obtained from coffee cherry waste.
Figure 20. Van Krevelen characterization of biochars obtained from coffee cherry waste.
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Figure 21. Nomenclature, conditions, and elemental ratios for biochars obtained and presented in Van Krevelen diagram.
Figure 21. Nomenclature, conditions, and elemental ratios for biochars obtained and presented in Van Krevelen diagram.
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Table 1. Retention time for standards used.
Table 1. Retention time for standards used.
Platform ChemicalsRetention Time (minutes)
Glucose16.789
Xylose17.179
Galactose17.255
Arabinose17.916
HMF35.637
Levulinic acid22.127
Formic acid20.415
Furfural51.349
Table 2. Characterization of coffee waste [41].
Table 2. Characterization of coffee waste [41].
EssayCoffee Cherry (%)Pulp (%)Husk (%)SCG (%)
Dry matter (%)95.5
Hemicellulose12.53.60712.1
Lignin13.720.07917.8
Cellulose27.625.884323.6
C45.27 44.9568.52
H4.862 5.3411.04
N1.4711.50.601.40
S0.138 0.03-
Moisture (Initial Biomass)80.7973.858.8855.2
Moisture (BHP) *10.94---
Ashes7.796.290.7915.3
Volatile matter (Initial biomass)10.069.8075.8583.3
Volatile matter (BHP) *80.81---
Fixed carbon1.36-14.481.5
* Biomass for hydrothermal processes.
Table 3. Matter balance of fractions in hydrothermal reactions with biomass waste (2 mm) and for 1 h.
Table 3. Matter balance of fractions in hydrothermal reactions with biomass waste (2 mm) and for 1 h.
T (°C)B:W RatioWLF (g)WWSF (g) WDSF (g)WGF (g)
1801:0537.00057.14211.0085.858
1801:1050.94437.9856.25611.071
1801:2072.25818.2012.0839.542
1801:4083.2677.6871.2359.046
2201:0534.50455.40213.62110.094
2201:1057.54426.6014.26415.854
2201:2078.25214.3051.8197.444
2201:4081.0726.1150.79212.814
2601:0553.47226.2628.93520.267
2601:1069.76214.3933.09715.845
2601:2071.3827.1511.25821.468
2601:4075.5932.6250.52021.782
Weight liquid fraction (WLF), weight dry solid fraction (WDSF), weight wet solid fraction (WWSF), weight gas fraction (WGS).
Table 4. Matter balance of solid, liquid, and gas fractions in hydrothermal reactions with biomass waste at a biomass–water ratio of 1:20 and a reaction time of 1 h.
Table 4. Matter balance of solid, liquid, and gas fractions in hydrothermal reactions with biomass waste at a biomass–water ratio of 1:20 and a reaction time of 1 h.
T (°C)Particle Size (mm)WLF (g)WWSF (g)WDSF (g)WGF (g)
1800.570.8099.8891.62819.303
180182.58711.6771.6715.736
180272.25818.2012.0839.542
180567.07810.6911.65322.232
2200.569.02611.2131.57019.761
220171.15412.7071.54216.139
220278.25214.3051.8197.444
220572.14110.6311.98617.228
2600.583.9207.1591.2968.921
260183.8278.3141.2427.859
260271.3827.1511.25821.468
260573.2517.9451.25118.804
Weight liquid fraction (WLF), weight dry solid fraction (WDSF), weight wet solid fraction (WWSF), weight gas fraction (WGS).
Table 5. Matter balance of solid, liquid, and gas fractions in hydrothermal reactions with biomass waste at a biomass–water ratio of 1:20, a particle size of 0.5 mm, and a reaction time of 1 h.
Table 5. Matter balance of solid, liquid, and gas fractions in hydrothermal reactions with biomass waste at a biomass–water ratio of 1:20, a particle size of 0.5 mm, and a reaction time of 1 h.
T (°C)Stirring (RPM)WLF (g)WWSF (g)WDSF (g)WGF (g)
180W/O70.8099.8891.62819.303
180500073.09911.4261.65715.476
180800075.8189.9891.61014.194
220W/O69.02611.2131.57019.761
220500076.1439.4181.60114.439
220800080.6728.0481.48111.280
260W/O83.9207.1591.2968.921
260500083.6835.8671.20510.451
260800084.0646.2891.2009.647
Weight liquid fraction (WLF), weight dry solid fraction (WDSF), weight wet solid fraction (WWSF), weight gas fraction (WGS).
Table 6. Matter balance of solid, liquid, and gas fractions in catalyzed hydrothermal reactions with biomass waste at a biomass–water ratio of 1:20, a particle size of 0.5 mm, and a reaction time of 1 h.
Table 6. Matter balance of solid, liquid, and gas fractions in catalyzed hydrothermal reactions with biomass waste at a biomass–water ratio of 1:20, a particle size of 0.5 mm, and a reaction time of 1 h.
T (°C)Catalysts (0.1 M)WLF (g)WWSF (g)WDSF (g)WGF (g)
180W/O70.8099.8891.62819.303
180CH3COOH84.0218.9091.6937.070
180NaHCO368.50816.7881.07914.704
220W/O69.02611.2131.57019.761
220CH3COOH82.1929.8571.6247.951
220NaHCO365.05215.5361.39819.413
260W/O83.9207.1591.2968.921
260CH3COOH81.6538.3431.56110.004
260NaHCO376.7017.2851.03716.014
Weight liquid fraction (WLF), weight dry solid fraction (WDSF), weight wet solid fraction (WWSF), weight gas fraction (WGS).
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Lozano Pérez, A.S.; Romero Mahecha, V.; Guerrero Fajardo, C.A. Analysis of the Impact of Biomass/Water Ratio, Particle Size, Stirring, and Catalysts on the Production of Chemical Platforms and Biochar in the Hydrothermal Valorization of Coffee Cherry Waste. Sustainability 2024, 16, 7415. https://doi.org/10.3390/su16177415

AMA Style

Lozano Pérez AS, Romero Mahecha V, Guerrero Fajardo CA. Analysis of the Impact of Biomass/Water Ratio, Particle Size, Stirring, and Catalysts on the Production of Chemical Platforms and Biochar in the Hydrothermal Valorization of Coffee Cherry Waste. Sustainability. 2024; 16(17):7415. https://doi.org/10.3390/su16177415

Chicago/Turabian Style

Lozano Pérez, Alejandra Sophia, Valentina Romero Mahecha, and Carlos Alberto Guerrero Fajardo. 2024. "Analysis of the Impact of Biomass/Water Ratio, Particle Size, Stirring, and Catalysts on the Production of Chemical Platforms and Biochar in the Hydrothermal Valorization of Coffee Cherry Waste" Sustainability 16, no. 17: 7415. https://doi.org/10.3390/su16177415

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