Effect of Different Hydrothermal Parameters on Calorific Value and Pyrolysis Characteristics of Hydrochar of Kitchen Waste

Abstract

Kitchen waste (KW) has high water content and organic matter, which has great potential for energy application, but how to treat it effectively has always been a difficult problem. In this study, the feasibility of transforming kitchen waste into hydrochar as a solid fuel to replace the primary energy by hydrothermal carbonization (HTC) is put forward. An experimental study of proportioned KW under HTC conditions (reaction temperature, residence time, liquid–solid ratio) is carried out by elemental analysis, industrial analysis, energy density calculation, and calorific value measurement. The results show that a proper extension of the residence time to 3.5 h at a liquid–solid ratio of 10 and a reaction temperature of 265 °C for the kitchen waste after HTC would result in a maximum calorific value of 30.933 MJ/Kg for the prepared hydrochar. The maximum ash content of hydrochar at a 265 °C reaction temperature is 3.94% < 10%, which is much lower than the standard of extra-low-ash coal, and the study shows that the hydrochar prepared from kitchen waste by HTC for combustion will greatly improve the combustion efficiency. The three types of kitchen waste under HTC conditions are compared with the three typical types of coal in China by the van Krevelen diagram. The results show that the combustion properties of hydrochar from kitchen waste prepared by the HTC method are better than those of lignite close to bituminous coal and can provide a theoretical basis for replacing primary energy in the future. Finally, by combining thermogravimetric analysis with mathematical modeling, it is calculated that the hydrochar of kitchen waste has a lower activation energy than those of other biomass, which can better facilitate the reaction. The experimental results further determine the feasibility of hydrochar as an alternative primary energy source and provide a theoretical basis for the future conversion of kitchen waste into hydrochar as a solid fuel through HTC.

1. Introduction

With the world population’s growth and the development of science and technology, more and more municipal solid wastes are produced. According to data estimates, by 2025, the global annual output of municipal solid domestic waste will reach 2.2 billion tons [1]. Among them, kitchen waste accounts for a large proportion, and about 1.3 billion tons of kitchen waste are produced yearly [2]. Due to the high water content of kitchen waste, high organic matter (containing a large amount of starch, protein, and lipids), if not properly treated and disposed of, will be a tremendous waste of resources. The role of microorganisms in nature is also the ease of decay, breeding a large number of harmful substances, causing air, soil, and water pollution, even though the food chain, among others, channels back to the human body, endangering human health [3,4]. In recent years, people have paid more and more attention to the resource and harmless treatment and disposal of kitchen waste. At present, the main treatment methods of kitchen waste mainly include biochemical treatment methods such as landfill, anaerobic fermentation, and composting, and thermochemical treatment methods such as incineration and pyrolysis. Among them, incineration and landfill are low-cost treatment methods with low energy utilization rates and high environmental pollution. For example, harmful gases and leachate produced by kitchen waste after landfill will seriously pollute the surrounding air, soil, and groundwater [5,6]. Composting and anaerobic fermentation are generally regarded as more environmentally friendly treatment methods, and anaerobic fermentation especially, as a mature process, is regarded as the best choice for producing methane and hydrogen. However, anaerobic fermentation also has some disadvantages, such as complicated reactor start-up, difficult control of complex parameters, unstable process, and long reaction time [7,8]. However, the pyrolysis process for treating high-moisture biomass, such as kitchen waste, has some disadvantages, such as high energy consumption and low yield of pyrolytic carbon, which are often not economically feasible [9,10].

In recent years, hydrothermal carbonization has had the advantages of mild reaction conditions and low energy consumption, which can transform waste biomass into carbon materials with high added value. At present, it is widely used in the fields of energy, environment, materials, soil improvement, and nutrient recovery [11,12]. The hydrothermal reaction is a thermochemical reaction process that takes subcritical water as the reaction solvent and reaction medium and uses high temperature and high pressure to realize biomass depolymerization, dehydration, decarboxylation, reforming, and polymerization [13]. After hydrothermal carbonization, biomass’s hydrogen content and oxygen content will obviously decrease compared with the raw materials due to the removal of hydroxyl and carboxyl groups [14]. Unlike thermochemical conversion methods such as pyrolysis, the hydrothermal carbonization reaction does not need to dry the water-containing polymer raw materials, and there is no restriction on the water content of the raw materials [15]. Therefore, the money, energy, and time required for drying raw materials are saved. Many companies have developed hydrothermal chemical and fuel processes [16]. According to the mechanism of hydrothermal carbonization, the carbohydrates, protein, and lipids contained in kitchen waste will be hydrolyzed to form monosaccharides, amino acids, glycerol, and fatty acids, which will be dissolved in the liquid phase, then undergo dehydration, decarboxylation, and deamination in the liquid phase, and then form furan and benzene derivatives through polymerization and aromatization, which will be polymerized to generate hydrothermal carbon particles after reaching a specific concentration [17,18]. The primary reaction process is shown in Figure 1.

The thermal analysis controls the sample under a specific temperature program to observe the process of mass change with temperature or time, to obtain relevant information such as weight loss ratio, weight loss temperature, and decomposition residue, and to evaluate the biomass energy potential through pyrolysis, which has been widely accepted [19]. In order to further evaluate biomass pyrolysis characteristics and energy analysis, detailed kinetic research and mathematical model establishment are essential [20]. There are many kinetic fitting methods, and the equal conversion method is a widely reported model-free method for estimating kinetic parameters and conversion degrees [21], including the Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods, among which the Coats–Redfern method is a standard and robust simulation method [22].

In this paper, kitchen waste was used as raw material to prepare hydrochar by changing the parameters of hydrothermal carbonization (reaction temperature, residence time, liquid–solid ratio). Through comprehensive analysis of elemental analysis, industrial analysis, energy production rate calculations, and higher heating value (HHV) measurements, the conditions that have the most significant influence on the calorific value of hydrochar prepared by hydrothermal carbonization of kitchen waste were found, and the characteristics of hydrochar were explored through a series of characterization analyses such as infrared and thermogravimetry. In order to further understand the hydrochar pyrolysis mechanism, it is necessary to analyze the pyrolysis kinetics and calculate the kinetic parameters (activation energy, pre-exponential factor), with the aim of better determining the applicability of hydrocarbon alternative energy sources.

2. Materials and Methods

2.1. Materials

Kitchen waste mainly consists of starch, protein, cellulose, and lipids. In this study, rice, cabbage, pork, and orange peel in a ratio of 5:3:1:1 were used as typical models representing kitchen waste (KW).

All kitchen waste was purchased from a market in Changchun. Before the experiment, the kitchen waste was dried at 105 °C for 24 h, and then KW raw materials were crushed and ground into powder by a pulverizer, which was sieved by 120–150 mesh and stored in a cool place for subsequent experiments.

2.2. Hydrothermal Carbonization of Kitchen Waste

In this experiment, a 100 mL micro-mechanical stirred autoclave (K-PSA, China) was used for the hydrothermal carbonization experiment, with the highest heating temperature of 350 °C and the highest pressure of 26 MPa. The experimental device is shown in Figure 2. According to the experiment, a certain amount of pretreated KW raw material was mixed with 60 mL of ultrapure water and then put into the reaction kettle and stirred evenly. Then, the reactor was sealed, the gas valve was opened, and nitrogen was introduced for 3 min to exhaust the air in the reactor. The temperature and residence time of the reactor were set, and the hydrothermal carbonization experiment was carried out. After the reaction, a vacuum filter (FY-1H-N, China) was used for solid–liquid separation. After drying in a solid drying oven at 105 °C for 24 h, the weighing records of hydrochar were obtained, labeled, and sealed for storage. For subsequent experimental analysis, writing labels were considered according to W-X, S-X, G-X, where W-X stands for reaction temperature, °C (185, 205, 225, 245, 265); for example, W-185 represents the hydrochar prepared by the hydrothermal carbonation of kitchen waste at a reaction temperature of 185 °C, a residence time of 2.5 h, and a liquid–solid ratio of 10. S-X stands for residence time, h (1.5, 2.5, 3.5, 4.5, 5.5); for example, S-1.5 represents the hydrochar prepared by the hydrothermal carbonization of kitchen waste at a reaction temperature of 245 °C, residence time of 1.5 h, and liquid–solid ratio of 10. G-X stands for the ratio of ultrapure water to raw material (5, 10, 15, 20); for example, G-5 represents the hydrochar prepared by the hydrothermal carbonation of kitchen waste at a reaction temperature of 245 °C, residence time of 2.5 h, and liquid–solid ratio of 5.

2.3. Characterization of Hydrochar

Elemental analysis and industrial analysis were carried out by an elemental analyzer (EA3000, Italy) and an industrial analyzer (GYFX7700, China), and the higher heating value (HHV) of the sample was measured by a calorimeter (ZR9300, China). C, H, N, V_ad, and A represent the contents of carbon, hydrogen, nitrogen, volatile matter, and ash of the material, respectively, expressed as dry basis mass percentages.

The energy density (ED) of solid hydrochar was calculated by the following equation [23].

(1)$ED=\frac{Q_{\mathrm{k}\mathrm{w}}}{Q_{\mathrm{h}}}\times 100\mathrm{\%}$

where $Q_{\mathrm{k}\mathrm{w}}$ is the calorific value of raw materials, MJ/kg; $Q_{\mathrm{h}}$ is the calorific product value, MJ/kg.

Sample morphological features were observed under low vacuum using a scanning electron microscope (Quanta450FEG, Czech Republic). Scanning 16 times in the range of 4000–400 cm⁻¹ by a Fourier transform infrared spectrometer (NicoletiS20, China), and taking KBr as the background, before the test, the sample and KBr were mixed and ground at the ratio of 1:50 and pressed into thin slices under a pressure of 8 MPa by a tablet press for 30 s.

2.4. Pyrolysis Experiment

The thermogravimetric analyzer (TG209F1, Germany) is shown in Figure 3, in which protective gas and purge gas can be seen, and inert N₂ is usually used as the protective gas, introduced into the furnace body through the weighing chamber and the connection area of the support. The furnace body, a heater, operates under a specific temperature program and can be filled with different dynamic atmospheres. During the test process, the high-precision balance connected to the lower part of the sample support can sense the current weight of the sample at any time and send the data to the computer, which will draw the curve of the sample weight versus temperature/time (TG curve).

A small number of samples were placed in an alumina crucible and weighed in the internal balance of the thermogravimetric analyzer. N₂ was used as a protective gas and purging gas, and the flow rates were 20 mL/min and 50 mL/min, respectively. At the initial temperature of 30 °C, the temperature was raised to 980 °C at 20 K/min.

3. Results and Discussion

3.1. Influence of Hydrothermal Parameters on Characteristics of Hydrochar

In this study, a mixture of rice, cabbage, pork, and orange peel is selected as the typical representation of kitchen waste for the hydrothermal carbonization experiment. The essential characteristic parameters of hydrochar are shown in Table 1.

In elemental analysis, the instrument does not detect the content of S in hydrochar; the absence of sulfur compounds in hydrochar can be associated with the formation of sulfur compounds in gaseous form. As can be seen from Table 1, the change in reaction temperature has the most obvious influence on the element content of hydrochar under the conditions of three hydrothermal parameters. With the increase in reaction temperature, the content of H decreases to a certain extent, reaching the lowest value of 5.291% at 245 °C. The O content decreases significantly from 22.075% to 13.050%. This mainly shows that dehydration and decarboxylation occurs during hydrothermal carbonization. The content of C increases from 67.142% to 73.418%, the calorific value increases from 26.922 MJ/Kg to 30.933 MJ/Kg, and the energy density decreases from 69.664% to 60.631%, which indicates that the energy of hydrochar also increases with the increase in carbonization temperature. The fixed carbon (FC) content of hydrochar continues to increase, and the higher the carbonization degree, the lower the corresponding hydrochar volatile matter (V_ad). When the temperature reaches the highest, 265 °C, the V_ad content reaches the lowest, 45.23%, and the FC content reaches the highest, 50.83%, indicating that the higher the hydrothermal carbonization temperature, the stronger the devolatilization of hydrochar, thus forming more fixed carbon. C is the central combustible part of fuel, indicating that hydrochar with high C content has a high calorific value. According to China’s national standard GB/T15224.12018, the ash content continues increasing, reaching the highest 3.94% < 10% at 265 °C, which is far lower than the standard of ultra-low-ash coal. Using this kind of hydrochar for combustion will significantly improve the combustion efficiency.

When the residence time changes, the C content generally increases with the increase in residence time, but the change degree is not particularly obvious. The C content is 72.847% when the residence time is 1.5 h and 73.058% when the residence time is 5.5 h. However, the O content is generally decreasing. The O content is 15.319% when the residence time is 1.5 h and 13.620% when the residence time is 5.5 h. The corresponding calorific value fluctuates between 29.840 MJ/Kg and 30.998 MJ/Kg, and the energy density reaches the lowest value of 60.504% at the residence time of 5.5 h. When the residence time is in other periods, the C content and calorific value do not change much, similar to that of Gupta et al. [24].

According to the data analysis in Table 1, when the liquid–solid ratio changes, it has no significant effect on the degree of hydrothermal carbonization. The C, H, and N content in hot coke are 73.685–72.602%, 5.580–5.383%, and 4.154–4.523%, respectively, but the content of O increases from 14.331% to 15.922%. O can be combined with H and C in hydrochar to form compounds (such as H₂O, CO₂) that occupy combustible C and H elements so that the combustible elements in hydrochar are relatively reduced, the calorific value of hydrochar is reduced from 30.825 MJ/Kg to 29.834 MJ/Kg, and the energy density is also increased. N is an impurity in hydrochar, and its content is minimal under the change in three parameters, which has little effect on the calorific value of hydrochar. However, it can react with O at high temperatures to form nitrogen oxides, which pollute the atmosphere. The content of N and S elements in hydrochar studied in this paper is very low, so it can be used as a clean fuel to reduce pollution and protect the environment.

The H/C ratio and O/C ratio in hydrochar can usually be used to judge the reaction’s carbonization degree and the product’s aromaticity. In order to better understand the reaction trend and carbonization degree in the hydrothermal carbonization process of kitchen waste, the van Krevelen diagram is used for analysis. According to China’s national standard GB/T5751-2009 coal classification, the atomic ratios of H/C and O/C of three typical coals are plotted on the same diagram for comparison, as shown in Figure 4. It is evident from the figure that the change in reaction temperature has the most significant influence on the hydrothermal carbonization of kitchen waste. With the increase in reaction temperature, the O/C and H/C ratio diagram gradually moves from the upper right corner to the lower left corner, indicating that dehydration and decarboxylation mainly occur during the reaction process. When the reaction temperature increases from 185 °C to 245 °C, the O/C ratio decreases from 0.247 to 0.145, and the H/C ratio decreases from 1.07 to 0.875, which indicates that increasing the reaction temperature is conducive to promoting dehydration and decarboxylation in the process of hydrothermal carbonization and enhancing the aromatization of hydrochar. If the reaction temperature is increased to 265 °C, the promotion effect is not apparent. For the influence of residence time and liquid–solid ratio on the hydrothermal carbonization of kitchen waste, the change in O/C and H/C ratio is not so noticeable, and they are all concentrated in the vicinity of the smoky coal seam. In addition, the flammability can be judged by the ratio of O/C to H/C. Generally, the lower the atomic ratio of H/C to O/C, the more favorable it is for combustion. The less water vapor and smoke that are released during combustion, the higher the combustion utilization rate is. It can be seen from the figure that anthracite and bituminous coal with good combustion properties have extremely low O/C and H/C ratios. Increasing the hydrothermal carbonization temperature makes the hydrochar approach from the lignite layer to the smoky coal layer, which indicates that the hydrochar prepared by hydrothermal carbonization of kitchen waste has the potential as solid fuel.

Compared with three typical coals, the combustion properties of hydrochar prepared by hydrothermal carbonization of kitchen waste are greatly improved, which is superior to lignite to some extent, and it is a kind of carbon material similar to bituminous coal. Jang et al. found that the O/C to H/C ratio of livestock manure after hydrothermal carbonization is closer to that of bituminous coal when the reaction temperature rises, and this treatment process can produce renewable fuel similar to low-grade coal, which is similar to this experiment [25]. For different kinds of raw materials, the degree of hydrothermal carbonization reaction is different, and the atomic ratio of products is also different, which can provide a more valuable scheme for the future development of biomass treated by hydrothermal carbonization as solid fuel.

3.2. Physicochemical Properties of Hydrochar

In order to comprehensively evaluate the influence of hydrothermal carbonization technology on kitchen waste, the hydrochar under hydrothermal reaction temperature is selected as the most influential parameter for physical and chemical analysis. The influence of reaction temperature on the infrared spectrum of hydrochar of kitchen waste is shown in Figure 5. The infrared spectrum is divided into characteristic frequency and fingerprint regions [24]. The range of 4000 cm⁻¹–2500 cm⁻¹ is the X-H stretching vibration region, and X can be atoms such as O, N, C, or S. The vibration around 3700 cm⁻¹–3200 cm⁻¹ is the O-H peak. When the hydrothermal reaction temperature gradually rises to 265 °C, the peak decreases obviously, because the high temperature promotes the dehydration reaction of raw materials [26]. The expansion and contraction of saturated C-H bond occur between 2820 cm⁻¹ and 3000 cm⁻¹, related to the C-H vibration of aliphatic groups in hydrochar. The symmetric expansion and contraction of the R₂CH₂ group (methylene group) are around 2930 cm⁻¹, and the antisymmetric expansion and contraction are around 2850 cm⁻¹, which indicates that aliphatic C-H still exists in hydrochar after the HTC process [27]. The range of 2500 cm⁻¹–1900 cm⁻¹ is the triple bond and accumulated double bond region, and there is no absorption peak in this region in the figure. The range of 900 cm⁻¹–1200 cm⁻¹ is the double bond stretching vibration area, which mainly includes three kinds of stretching vibration. The C=O stretching vibration (ester, aldehyde, ketone, acid) appears between 1760 cm⁻¹ and 1660 cm⁻¹. The peak intensity around 1700 cm⁻¹ belongs to a carboxyl group, a carbonyl group, or an ester group, which appears in hydrochar under all parameters, indicating many oxygen-rich functional groups on the surface of hydrochar [28]. For the vibration of the C=C double bond of the aromatic substance at 1455 cm⁻¹ [27], after hydrothermal carbonization, the C=C in biochar becomes stronger obviously, which indicates that the increase in the carbonization degree of biochar leads to the increase in aromatic structure and increases with the increase in hydrothermal carbonization temperature. The peaks in the range of 1060 cm⁻¹–1270 cm⁻¹ can be attributed to C-O-C stretching in sugars and aliphatic ethers, and the number of peaks in this area decreases with the increase in hydrothermal reaction temperature due to the large number of carbohydrates removed by hydrolysis and dehydration reactions [29]. Because the degradation of hemicellulose in kitchen waste occurs near 1040 cm⁻¹, it is the stretching vibration of the C–O bond in hydroxyl, ester, and ether [30]. The peak of HTC kitchen waste at 890 cm⁻¹ is the aromatic compound C–H vibration. In addition, there is a soft vibration peak near 600 cm⁻¹, which may be caused by the stretching vibration of the C-H bond [25]. To sum up, kitchen waste is characterized by an enhanced aromatic structure, fatty substances, and many oxygen-rich functional groups on its surface after hydrothermal carbonization.

3.3. Morphology

In Figure 6, it can be seen from the Scanning electron microscope (SEM) image under the condition of reaction temperature that the morphology of hydrochar of kitchen waste is more complex than that in the figure, and there are always gaps and holes at different reaction temperatures. The number of holes increases more obviously with the increase in reaction temperature, mainly due to the continuous precipitation of volatile matter in KW, resulting in the increasing number of micropores and holes. HC produced at 185 °C (Figure 6a) has almost no structural damage, coarser texture, and more fiber residue [31]. With the increase in reaction temperature, the fiber network of raw materials is destroyed, and spherical particles begin forming on the biomass’s surface, making the surface morphology rougher. The carbon can retain the skeleton structure when the reaction temperature is 185–225 °C (Figure 6a–c). As the temperature continues to rise to 265 °C (Figure 6e), the carbon skeleton structure is destroyed, resulting in fracture and fragmentation, which may be due to the high temperature promoting the decomposition of cellulose, causing it to lose its skeleton-supporting function, resulting in the plastic deformation and fracture of hydrochar.

3.4. Analysis of Pyrolysis Characteristics of Hydrochar

Effect of Temperature on Pyrolysis Characteristic of Hydrochar

According to the inflection point of the thermogravimetry (TG) curve, as shown in Figure 7, the influence of reaction temperature on the pyrolysis of hydrochar can be divided into three stages. The change in mass in the first stage is relatively small. Because of the low pyrolysis temperature, only the water and volatilization in hydrochar are analyzed, so the dehydration peak appears first. When the reaction temperature is 265 °C and 245 °C, the dehydration peak appears earlier in the range of 155–309 °C, and the dehydration peaks all appear in the range of 190–309 °C at different reaction temperatures. Then, the curve is stable and enters the second stage, where the mass changes significantly, which is the main weightlessness stage. This is because, with the increase in pyrolysis temperature, hemicellulose and cellulose in hydrochar gradually decompose, which is the temperature stage corresponding to the pyrolysis peak. In the third stage, the fluctuation in mass change is minimal and tends to be gentle because all the relatively easily decomposed components in hydrochar have been completely decomposed, and only a small amount of carbonaceous material is decomposed in the remaining part, which is a slow pyrolysis stage. This can be seen from Figure 8 of the differential thermogravimetric (DTG) curve of hydrochar at different reaction temperatures (205–265 °C). There are two prominent weightlessness peaks in the pyrolysis process of five kinds of hydrochar, and the maximum weightlessness range is 250–550 °C. The first weightlessness peak of five kinds of hydrochar appears at 205 °C. Because of the complex composition of kitchen waste, the pyrolysis of sugars is easy to volatilize and produce CO, CO₂, and some hydrocarbons, which can occur at a lower temperature [32]. The appearance of the second weightlessness peak includes the pyrolysis of cellulose, hemicellulose, protein, lipids, and other substances. The weightlessness peaks of W-185, W-205, and W-225 appear at about 375 °C. However, the weightlessness peaks of W-245 and W-265 of hydrochar shift backward at 425 °C and 435 °C, respectively, which may be due to the increase in reaction temperature in hydrothermal carbonization, which affects the pyrolysis temperature of the prominent weightlessness peaks of hydrochar. The higher the reaction temperature, the higher the pyrolysis temperature of the prominent weightlessness peaks.

From Table 2, it can be seen that the main weightlessness temperature intervals and pyrolysis peaks of hydrochar after HTC of kitchen waste all increase with the increase in HTC temperature, and the maximum weightlessness temperatures of W-185, W-205, and W-225 are relatively close to each other. There is a significant change from W-245, reaching a maximum weightlessness temperature range of 381.0–546.9 °C, in W-265, with a maximum weightlessness temperature of 438 °C. The reason is that as the reaction temperature increases in the HTC process, the decomposition process of hemicellulose in kitchen waste is accelerated, so organic matter such as cellulose is exposed to speed up the pyrolysis process. With the increase in HTC temperature, the maximum weightlessness rate decreases, reaching a minimum of 4.32%/min at W-265, which is nearly half of the maximum weightlessness rate of W-185. The residual mass reaches a maximum of 50.41% at W-265, This is due to the fact that the HTC of kitchen waste will pyrolyze some of the cellulose, hemicellulose, protein, and other materials, and the pyrolysis process will slow down as the temperature of HTC increases.

3.5. Pyrolysis Kinetics of Hydrochar

In this paper, the kinetic analysis of the primary pyrolysis zone in the second stage of pyrolysis is carried out, and the kinetic parameters of hydrochar at the reaction temperature are solved by thermogravimetric data. For this research, domestic and foreign scholars have put forward different kinetic research models [22]. The Coats–Redfern method has achieved good results in studying the pyrolysis kinetics of materials. This paper also uses this algorithm to calculate the activation energy (E) and pre-exponential factor (A) to reveal the pyrolysis characteristics of kitchen waste hydrochar under different conditions of hydrothermal carbonization [33,34].

The reaction rate formula of hydrochar is:

(2)$\frac{d\alpha }{dt}=k\left(T\right)f\left(\alpha \right)$

$k\left(T\right)$ follows the Arrhenius equation and, because it is a non-isothermal reaction, Equation (1) can be transformed into:

(3)$\frac{d\alpha }{dT}=\frac{A}{\beta }exp\left(-\frac{E}{RT}\right)f\left(\alpha \right)$

where $f\left(\alpha \right)$ is a function of the reaction mechanism; $A$ is the pre-exponential factor, 1/s; $E$ is the apparent activation energy of the reaction, kJ/mol; $R$ is the ideal gas constant; $T$ is the thermodynamic temperature, k; $\beta$ is the heating rate, K·min⁻¹; $\alpha$ is the conversion rate at time t. It is defined as follows:

(4)$\alpha =\frac{m_0-m_t}{m_0-m_f}$

where $m_t$, $m_0$, and $m_f$ are the t-moment mass, initial mass, and residual mass of hydrochar, respectively.

For the reaction-type determination $f\left(\alpha \right)$ functional form, the general hypothesis $f\left(\alpha \right)$ is independent of temperature T and time t. It is only related to the reaction degree $\alpha$. For the series reaction $f\left(\alpha \right)={\left(1-\alpha \right)}^n$, n is the reaction order and is substituted into Equation (2).

(5)$\frac{d\alpha }{dT}=\frac{A}{\beta }exp\left(-\frac{E}{RT}\right){\left(1-\alpha \right)}^n$

The Coats–Redfern method is used to integrate both sides of the equation.

When n = 1:

(6)$ln\left[\frac{-ln\left(1-\alpha \right)}{T^2}\right]=-\frac{E}{RT}+ln\left[\frac{AR}{\beta E}\left(1-\frac{2RT}{E}\right)\right]$

when n ≠ 1:

(7)$ln\left[\frac{1-{\left(1-\alpha \right)}^{1-n}}{T^2\left(1-\alpha \right)}\right]=-\frac{E}{RT}+ln\left[\frac{AR}{\beta E}\left(1-\frac{2RT}{E}\right)\right]$

Researchers have put forward various analytical models for biomass pyrolysis kinetics, among which the first-order reaction model is widely accepted [35].

However, for most reactions, $\frac{2RT}{E}\ll 1$; therefore, n = 1 can plot $\frac{1}{T}$ with $ln\left[\frac{-ln\left(1-\alpha \right)}{T^2}\right]$ as the Y axis. As shown in Figure 6, its slope is $-\frac{E}{R}$ and its intercept is $ln\left[\frac{AR}{\beta E}\left(1-\frac{2RT}{E}\right)\right]$. The activation energy (E) and the pre-exponential factor (A) can be obtained. Under the condition of changing the hydrothermal carbonization temperature of kitchen waste, the first-order kinetic model is adopted to solve the thermal kinetic parameters of hydrochar in the mid-temperature range of pyrolysis. As shown in Figure 9, an excellent fitting straight line is obtained, with R² above 0.9. Combined with Equation (6), the pyrolysis kinetic parameters of hydrochar at different reaction temperatures can be calculated. Table 3 shows the calculation results of pyrolysis kinetics.

The activation energy (E) indicates the problematic degree of the chemical reaction, and pre-exponential factor (A) indicates the complexity degree of the sample or surface structure during pyrolysis [36]. As shown in Table 3, with the increase in HTC temperature (185–265 °C), the pre-exponential factor of kitchen waste hydrochar reaches the maximum value of 54.95 min⁻¹ at the HTC temperature of 185 °C. The activation energies are all 30 KJ/mol, and the lowest is 28.69 KJ/mol at the HTC temperature of 265 °C. Ming et al. found that the pyrolysis reactions of rice and pork at different conversion rates are more complex at a heating rate of 60 °C/min, and its pre-exponential factor is much larger than the hydrochar maximum in this study, which indicates that rice and pork are more complex than hydrochar pyrolysis. However, the activation energies of rice and pork are both above 100 KJ/mol, which is much higher than the activation energy of hydrochar in this study. Low activation energy will promote the reaction more effectively [37]. As the KW component is mainly composed of carbohydrates and other substances, the KW after HTC will show a lower value compared with the activation energy of other biomass [38].

4. Conclusions

In this paper, the influence of the HTC method on the performance of kitchen waste hydrochar as fuel was explored, the pyrolysis kinetics of hydrochar under HTC conditions with the greatest influence was analyzed, and the following conclusions were drawn:

Under the change in HTC conditions of kitchen waste (reaction temperature, residence time, liquid–solid ratio), the hydrochar was analyzed by elemental analysis, industrial analysis, energy density calculation, and calorific value measurement, and the HTC conditions that had the greatest influence on the calorific value of kitchen waste hydrothermal coke were comprehensively evaluated. The research showed that kitchen waste underwent hydrolysis, dehydration, condensation, and other reactions in HTC, and the influence of reaction temperature on hydrochar > residence time > liquid–solid ratio. With the increase in reaction temperature, the contents of H and O in hydrochar decreased obviously, and the calorific value reached the highest of 30.933 MJ/Kg at 265 °C. At this temperature, hydrochar had a low V_ad content and high FC content, which were 45.23% and 50.83%, respectively. This showed that the higher the reaction temperature, the stronger the devolatilization of hydrochar, thus forming more fixed carbon. The highest ash content of W-265 was 3.94% < 10%, which is far below the standard of ultra-low-ash coal. The results showed that the hydrochar prepared by HTC from kitchen waste will greatly improve the combustion efficiency.

Through the van Krevelen diagram, the kitchen waste under three HTC conditions was compared with three typical coals in China. The results showed that the combustion properties of kitchen waste hydrochar prepared by the HTC method were better than those of lignite and close to bituminous coal, which can provide a theoretical basis for replacing primary energy in the future.

Thermogravimetric analysis showed that the higher the HTC temperature, the better the pyrolysis stability of hydrochar. Combined with pyrolysis kinetics, the Coats–Redfern integral method was used for kinetic analysis and parameter calculation. The lowest activation energy and pre-exponential factor of W-265 were 28.69 KJ/mol and 14.04 °C/min, respectively. The results showed that the lower activation energy will promote the reaction more effectively and improve the applicability of kitchen waste hydrochar as a potential alternative to primary energy.

Author Contributions

Conceptualization, Y.S.; Methodology, Y.S. and C.L.; Software, R.C.; Validation, C.L.; Investigation, J.W. and Y.W.; Writing—original draft, C.L. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Simple reaction process of main components of kitchen waste (carbohydrate, protein, lipid) during hydrothermal carbonization.

Figure 2. Hydrothermal carbonization experimental device diagram.

Figure 3. Thermal analyzer schematic diagram.

Figure 4. Van Krevelen diagram of hydrochar under three different HTC conditions and van Krevelen diagram of three typical coals in China.

Figure 5. Fourier Transform Infrared (FT-IR) spectra of hydrochar at different HTC temperatures.

Figure 6. Scanning electron microscope (SEM) micrographs taken at 50 μm (a–e) and 10 μm (an image labeled in (e)) of the hydrochar produced at increasing HTC temperatures (185–265 °C).

Figure 7. Thermogravimetry (TG) curves of hydrochar produced at increasing HTC temperatures (185–265 °C) when the heating rate is 20 K/min.

Figure 8. Derivative thermogravimetric (DTG) curves of hydrochar produced at increasing HTC temperatures (185–265 °C) when the heating rate is 20 K/min.

Figure 9. Fitting curves for pyrolysis of hydrochar produced at increasing HTC temperatures (185–265 °C) under the main weightlessness interval.

Table 1

Basic characteristic parameters of hydrochar.

Sample	Ultimate Analysis(wt.%)				Proximate Analysis(wt.%)			O/CH/C		HHV(MJ/Kg)	ED(%)
	C	H	N	Oa	FC	Vad	A
W-185	67.142	5.985	3.858	22.075	37.380	61.680	0.940	0.247	1.070	26.922	69.664
W-205	67.799	5.583	3.627	20.641	42.180	55.470	2.350	0.228	0.988	27.197	68.960
W-225	70.128	5.448	3.938	17.636	45.530	51.620	2.850	0.189	0.932	28.517	65.768
W-245	72.599	5.291	4.438	14.052	49.570	46.810	3.620	0.145	0.875	29.671	63.210
W-265	73.418	5.353	4.239	13.050	50.830	45.230	3.940	0.133	0.875	30.933	60.631
S-1.5	72.847	5.652	3.942	15.319	46.620	51.140	2.240	0.158	0.931	29.840	62.852
S-3.5	72.793	5.630	4.195	14.302	46.970	49.950	3.080	0.147	0.928	30.257	61.986
S-4.5	72.868	5.535	4.044	14.623	48.800	48.270	2.930	0.151	0.912	30.193	62.117
S-5.5	73.058	5.656	4.306	13.620	46.600	50.040	3.360	0.140	0.929	30.998	60.504
G-5	73.685	5.580	4.154	14.331	46.660	51.090	2.250	0.146	0.909	30.825	60.843
G-15	71.770	5.537	3.894	15.849	46.300	50.750	2.950	0.166	0.926	30.699	61.093
G-20	72.602	5.383	4.523	15.922	50.630	47.800	1.570	0.164	0.890	29.834	62.865

O^a% = 100 − (C% + H% + N% + S% + Ash%); FC = 100 − V_ad − A.

Table 2

Pyrolysis characteristic parameters of water carbon at different HTC temperatures.

Hydrochar	WeightlessnessTemperatureInterval (°C)	MaximumWeightlessnessTemperature (°C)	Maximum WeightlessnessLoss Rate (%/min)	Residual Mass (%)
W-185	313.4–498.3	369	7.93	37.21
W-205	331.0–516.3	378	6.36	41.47
W-225	341.1–527.2	384	5.54	46.45
W-245	355.0–537.8	429	4.46	47.80
W-265	381.0–546.9	438	4.32	50.41

Table 3

Kinetic parameters of hydrochar pyrolysis at different HTC temperatures.

Hydrochar	Weightlessness TemperatureInterval (°C)	Fitted Curve	E (KJ/mol)	A (min−1)	Average R2
W-185	313.4–498.3	y = −4027x − 7.29	33.48	54.95	0.95350
W-205	331.0–516.3	y = −3678x − 7.95	30.58	50.19	0.96446
W-225	341.1–527.2	y = −3473x − 8.33	28.87	16.75	0.97385
W-245	355.0–537.8	y = −3464x − 8.41	28.80	15.42	0.98261
W-265	381.0–546.9	y = −3451x − 8.50	28.69	14.04	0.97804

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