Hydrothermal Carbonization of Corn Stover: Structural Evolution of Hydro-Char and Degradation Kinetics

Abstract

Hydrothermal processing of biomass may be able to overcome a series of problems associated with the thermochemical conversion of lignocellulosic material into energy and fuels. Investigating the process parameters and an adequate process description is one of the first steps to being able to design and optimize a certain treatment concept. In the present article, we studied process evolution with respect to reaction time in order to evaluate structure changes and kinetics of corn stover decomposition in a hydrothermal reactor. The effect of the biomass-to-H₂O ratio was also investigated. A pilot-scale reactor of 18.75 L was used to conduct hydrothermal processing runs at 250 °C at different reaction times (60, 120 and 240 min) and biomass-to-H₂O ratios (1:10, 1:15 and 1:20). Solid phase products were characterized by thermogravimetry (TG), scanning electron microscopy (SEM), elemental composition (EDX), crystalline phases by X-ray diffraction (XRD) and surface area (BET). For the experiments with a constant reaction time, the yields of hydro-char, aqueous and gaseous phases varied between 31.08 and 35.82% (wt.), 54.59 and 60.83% (wt.) and 8.08 and 9.58% (wt.), respectively. The yields of hydro-char and gases tend to increase with higher biomass-to-H₂O ratios, while aqueous phase yields are lower when using lower ratios. As expected, the yields of liquid and gases are higher when using higher reaction times, but there is a reduction in hydro-char yields. TG showed that 60 min was not enough to completely degrade the corn stover, while 120 and 240 min presented similar results, indicating an optimized time of reaction between 120 and 240 min. SEM images, elemental composition and XRD of hydro-char showed that higher biomass-to-H₂O ratios increase the carbonization of corn stover. The surface area analysis of hydro-char obtained at 250 °C, 2.0 °C/min, a biomass-to-H₂O ratio of 1:10 and 240 min showed a surface area of 4.35 m²/g, a pore volume of 18.6 mm³/g and an average pore width of 17.08 μm. The kinetic of corn stover degradation or bio-char formation was correlated with a pseudo-first-order exponential model, exhibiting a root-mean-square error (r²) of 1.000, demonstrating that degradation kinetics of corn stover with hot-compressed H₂O, expressed as hydro-char formation, is well described by an exponential decay kinetics.

1. Introduction

Corn stover is considered agricultural waste, usually incinerated or used as bedding in poultry farms in Brazilian rural properties. It is an abundant lignocellulosic-base material but is little used to generate income [1]. It is the residue left after harvesting corn grains, consisting approximately of 25% (wt.) of a leafy fraction (leaf + husk + sheaths) and between 70 and 75% (wt.) of fibrous and hard material (stalk + cobs) [2]. Brazil stands out as the third largest producer (115.00 million tons), behind China (272.55 million) and the USA. (383.94 million) [3]. Since millions of tons of corn are produced [4], a large quantity of corn stover waste is generated and discarded, a waste that, if submitted to hydrothermal processing, can be turned into a carbon-based adsorbent, trapping carbon on a solid material (avoiding it to be sent into the atmosphere as CO₂ if burned) and at the same time being of great utility [5,6,7].

Hydrothermal processing of lignocellulosic materials is where water and steam at elevated temperatures and pressures thermochemically convert the base material into higher-value products such as hydro-char [6,7,8,9,10,11,12,13,14,15,16,17]. Academic literature presents some studies on the subject of hydrothermal processing of corn residues [5,6,11,14,16,17,18,19,20,21,22,23,24,25,26,27] and chemically modified ones [26,27].

The influence of process parameters on hydrothermal carbonization, including temperature, reaction time and biomass-to-H₂O ratio, as well as raw material characteristics on the yield of reaction products (solid, liquid and gas) and chemical composition, was systematically investigated by applying statistical methods to a huge collection of experimental data by Li et al. [28]. They reported that hydro-char yield is the main way to analyze the hydrothermal carbonization process, and other variables are less studied, such as the composition of the aqueous phase [6,9,12,14,15,24,29,30,31,32,33,34,35], kinetics [20,24,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60] and morphology [43,44,61,62,63,64,65,66]. Analysis of reaction mechanisms [8], design and application of hydrothermal processing [67,68], hydro-char characterization and its chemistry [10,69], show that the process parameters are of great influence for hydro-char yield, the kinetics of conversion and structural evolution [10,11,67,68,69]. The novelty of this work consists of the simultaneous investigation of the structural evolution of hydro-char together with the kinetics of hydro-char conversion, fronted by two parameters, biomass-to-H₂O ratio and reaction time.

Hydro-char conversion and carbon content can be improved by increasing reaction times, which can be easily carried out for batch systems. In recent years, a large number of authors commented on the variation in crystalline and morphological properties when considering different reaction times [20,24,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60]. As for the biomass-to-water ratio in the hydrothermal process [20,21,34,36,40,44,45,46,48,49,50,52,56,57,61,62], only a few considered the influence of this parameter on the structural evolution and characteristics of hydro-char [61,65]. In this work, the investigation of the structural evolution of hydro-char consisted of different analyses, such as elemental composition, thermogravimetry and X-ray diffraction to check crystalline phases and scanning electron microscopy (SEM) images of the hydro-char produced considering different reaction times and biomass-to-water ratio values. Hydro-char conversion kinetics was modeled as a function of those two process variables.

2. Materials and Methods

2.1. Methodology

The process flow diagram (Figure 1) presents, in a relatively simple way, the whole methodology of this study. Initially, the corn stover residues were collected. Afterwards, corn stover was subjected to drying, milling and sieving pretreatments. The experiments were carried out in pilot scales to investigate the effect of process time and biomass-to-H₂O ratio on the structural evolution of bio-char, yields of reaction products (hydro-char, H₂O and gas) and the kinetics of corn stover degradation. The bio-char submitted to thermogravimetric (TG/DTG), morphological (SEM/EDS), crystalline (XRD), elemental (C, H, N, S), textural (BET) and physicochemical characterization (DM, TOC, Ash).

2.2. Materials, Pre-Treatment and Characterization of Corn Stover

The corn stover residues used were provided by Leibniz-Institüt für Agrartechnik und Bioökonomie e.V (ATB), Potsdam-Bornin [5,6]. The corn stover residues were submitted to the pre-treatments of drying at 105 °C for 24 h using a digitally controlled oven, grinding using a laboratory cutting mill and, finally, sieving to produce particle sizes averaging 2.0 mm of square geometry, as described in the literature [5,6]. The hydro-chars were physicochemically characterized for dry matter, total organic content, ash content, elemental analysis (C, H, N and S) and oxygen content, as described elsewhere [5,6].

2.3. Experimental Apparatus and Procedures

The pilot-scale reactor of cylindrical geometry (V_Reactor = 18.75 L), as described in detail elsewhere [5,6,15], is constructed by stainless steel (Parr, Moline, IL, USA, Model: 4555) and includes a mechanical stirring system with 02 impellers, each containing 3 blades, and 6.78 N.m Torque. The reactor has a ceramic-3 zone heater of 4500 W, a control unit (Parr, USA, Model: 4848) and a degassing system, as shown in Figure 2.

The temperature was measured with aid 02 thermocouples placed inside a thermos well. The reactor operates at a maximum pressure of 131 bar and a maximum temperature of 350 °C. The hydrothermal processing of dried corn stover was carried out with hot-compressed water at 250 °C (saturated pressure of 39.76 bar) for 240 min with biomass-to-water ratios of 1:10, 1:15 and 1:20 at 250 °C with biomass-to-water ratios of 1:10 for 60, 120 and 240 min, as described in detail in the literature [5,6,15]. For the experiments with varying biomass-to-H₂O ratios, 600 g of corn stover residues were placed inside the reactor. Afterwards, 6000, 9000 and/or 12,000 g H₂O, depending on the biomass-to-H₂O ratio, was inserted into the reactor and soaked manually until a homogeneous cake formed and reactor-sealed. The operating temperature and the heating rate were set at 250 °C and 2.0 °C/min, respectively. The reaction times were counted when the reactor reached the set point temperature (τ₀). Once the reaction time was reached for each batch (60, 120 and/or 240 min), the reactor was cooled down to ambient temperature. The gravitational dewatering makes it possible to separate the liquid and solid phases. The moist solid phase inside the reactor was removed and placed inside a mechanical press to remove the residual water. Afterwards, the solid and aqueous phases were determined gravimetrically. The moist solid phase was dried at 105 °C for 48 h to determine the moist content. Samples of moist dewatered solids, liquid phase and dried solid phase stored for physicochemical analysis and morphological characterization are shown in Figure 3.

The investigated process parameters (reaction time, biomass-to-H₂O ratio, temperature, mass of corn stover and mass of H₂O) are described in

Table 1. Process parameters by hydrothermal carbonization of corn stover at 250 °C with a biomass/H₂O ratio of 1:10 after 60, 120 and 240 min and at 250 °C, 240 min, biomass/H₂O ratios of 1:10, 1:15 and 1:20, using a reactor of 18.75 L.

Process Parameters	250 °C
	τ (min)			Biomass/H2O (-)
	60	120	240	1:10	1:15	1:20
Mass of Corn Stover (g)	600.66	600.31	600.10	600.10	600.11	600.28
Mass of H2O (g)	6000.20	6002.70	6000.70	6000.70	9003.10	12,007.00

.

2.4. Characterization of Hydro-Char

2.4.1. Proximate, Ultimate and Elemental Analysis of Hydro-Char

The hydro-char was characterized for dry matter, ash content and elemental analysis (C, H, N and S) [5,6,15].

2.4.2. Thermogravimetric, Morphological and Crystalline Analysis of Hydro-Char

The morphological, crystalline and thermogravimetric characterization was performed by thermogravimetric analysis (TG/DTG), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and BET, as described in detail elsewhere [5,7].

2.5. Material Balances and Yields of Reaction Products

The law of conservation of mass was applied within the reactor, operating as a closed thermodynamic system, batch mode, yielding the following equations:

$$M_{Reactor}=M_{Feed}$$

(1)

$$M_{Reactor}=M_{Solid}+M_{LP}+M_{Gas}$$

(2)

where $M_{Reactor}$ is the mass of reactor, $M_{Feed}=M_{Corn Stover}$ is the mass of corn stover filling the reactor, $M_{Solid}$ is the mass of hydro-char, $M_{LP}$ is the mass of liquid phase products and $M_{Gas}$ is the mass of gas. The process performance evaluated by computing the yields of liquid and liquid reaction products using Equations (3) and (4), and the yield of gas by difference is calculated using Equation (5).

$$Y_{LP}\left[\%\right]=\frac{M_{LP}}{M_{Corn Stover}}\times 100$$

(3)

$$Y_{Solid}\left[\%\right]=\frac{M_{Solid}}{M_{Corn Stover\left(0\right)}}\times 100$$

(4)

$$Y_{Gas}\left[\%\right]=100-\left(Y_{LP}+Y_{Solid}\right)$$

(5)

2.6. Degradation Kinetics of Corn Stover

The degradation kinetics of corn stover by hydrothermal processing at 250 °C with a biomass-to-H₂O ratio of 1:10 for 60, 120 and 240 min, expressed as hydro-char formations, was correlated with a pseudo-first-order exponential model [41].

The thermal degradation process of biomass, in this case, corn stover, follows the idea and/or concept of Yin et al. [41].

By performing a mass balance on the dry biomass, in the beginning, the mass of biomass is equal to the mass of solids

$$M_{Biomass}\left(0\right)=M_{Corn Stover}\left(0\right)=M_{Solids}\left(0\right)$$

(6)

$$M_{Biomass}\left(0\right)=M_{Gas}\left(\tau \right)+M_{Liquids}\left(\tau \right)+M_{Solids}\left(\tau \right)$$

(7)

By considering that the mass of biomass degraded by thermochemical reactions during the hydrothermal processing, here named $M_{Dissolution}\left(\tau \right),$ is the sum of the mass dissolved within the aqueous phase as hydrolysates and the mass of gaseous phase $M_{Gas}\left(\tau \right),$ the $M_{Dissolution}\left(\tau \right)$ is given by Equation (8).

$$M_{Dissolution}\left(\tau \right)=M_{Gas}\left(\tau \right)+M_{Liquids}\left(\tau \right)$$

(8)

The initial mass of biomass,$M_{Biomass}\left(0\right)$, is equal to the sum of the mass of solids and the mass of dissolution at time τ, $M_{Dissolution}\left(\tau \right)+M_{Solids}\left(\tau \right)$.

$$M_{Biomass}\left(0\right)=M_{Dissolution}\left(\tau \right)+M_{Solids}\left(\tau \right)$$

(9)

The time differential of Equation (9) is given as follows:

$$\frac{d{M}_{Solids}\left(\tau \right)}{d\tau }=-\frac{d{M}_{Dissolution}\left(\tau \right)}{d\tau }$$

(10)

Assuming the mass of solid phase products is equal to the mass of hydro-char yields Equation (11).

$$M_{Solids}\left(\tau \right)=M_{Hydrochar}\left(\tau \right)$$

(11)

The time differential of Equation (11) is given as follows:

$$\frac{d{M}_{Solids}\left(\tau \right)}{d\tau }=\frac{d{M}_{Hydrochar}\left(\tau \right)}{d\tau }$$

(12)

The biomass dissolution was described by modified first-order kinetics given by Equation (13).

$$\frac{d{M}_{Solids}\left(\tau \right)}{d\tau }=-K\ast \left[M_{Solids}\left(\tau \right)-a\right]$$

(13)

Dividing Equation (13) by Equation (6) yields Equation (14).

$$\frac{d[M_{Solids}\left(\tau \right)/M_{Solids}\left(0\right)]}{d\tau }=-K\ast \left[Y_{Solids}\left(\tau \right)-A\right]$$

(14)

$$\frac{d{Y}_{Solids}\left(\tau \right)}{d\tau }=-K\ast \left[Y_{Solids}\left(\tau \right)-A\right]$$

(15)

The constant A, defined as $a/M_{Solids}\left(0\right)$, is included as a correction term. By solving the first-order homogeneous and linear differential Equation (15), with initial boundary condition, $\tau =0, Y_{Solids}\left(\tau \right)=Y_{Solids}\left(0\right)$, Equation (16) is yielded.

$$Y_{Solids}\left(\tau \right)=A+\left[Y_{Solids}\left(0\right)-A\right]\ast \exp \left(-K\ast \tau \right)$$

(16)

3. Results

3.1. Thermogravimetric, Morphological and Mineralogical Characterization of Hydro-Char

3.1.1. Thermogravimetric Analysis (TG/DTG)

Influence of Reaction Time

The thermal degradation of corn stover was analyzed via thermogravimetric analysis (TG/DTG) by Sittisun et al. [70] between 25 and 900 °C with heating rates of 10, 20 and 50 °C/min, as well as by Mohammed et al. [21] between 25 and 600 °C with heating rates of 10 °C/min. For heating rates of 10 °C/min, Sittisun et al. [70] reported a mass loss of 92% (wt.) within the temperature interval of 25 and 510 °C, while Mohammed et al. [21] reported a mass loss of ≈ 70.0% (wt.) between 25 and 600 °C. Sittisun et al. [70] reported the occurrence of three degradation steps: the first between 25 and 167 °C due to the removal of moisture; a second between 167 and 368 °C due to the removal of volatile compounds by the degradation of hemicellulose and cellulose; and a third between 368 and 514 °C due to thermal degradation and/or combustion of lignin [70]. Mohammed et al. [21] also reported the occurrence of three disintegration steps: the first between 100 and 250 °C due to the removal of moisture and some volatiles; a second between 250 and 400 °C due to the degradation of hemicellulose and cellulose; and a third above 450 °C due to the thermal degradation of lignin [21]. The thermal degradation of bio-char obtained by hydrothermal processing of corn stover at 250 °C for 240 min with a 1:10 biomass-to-H₂O ratio using a reactor of 18.75 L was analyzed via thermogravimetric analysis (TG/DTG) by Costa et al. [7] between 25 and 800 °C, at 10 °C/min, under a N₂ atmosphere, as well as by Mohammed et al. [21] between 25 and 600 °C, at 10 °C/min, under a N₂ atmosphere.

Figure 4 illustrates the TG–DTG curves of hydro-chars obtained. One observes the presence of three reaction steps, similar to the results reported in the literature [7,21]. The TG–DTG curves for the hydro-chars obtained at 240 and 120 min are identical, showing mass losses between 50.63 and 49.15% (wt.) for the temperature interval of 25–800 °C. On the other hand, the TG–DTG curves for the hydro-chars obtained at 60 min have a mass loss of 69.40% (wt.) for the temperature interval 25–800 °C, showing that a reaction time of 60 min was not enough to carbonize corn stover.

From the TG–DTG curves for the hydro-chars obtained at 60 and 120 min, a small loss of mass around 100 °C associated with the presence of water in hydro-chars can be seen at the beginning. A mass loss of approximately 37.5% (wt.) was observed for the hydro-char obtained at 120 min between 204 and 530 °C, while for the hydro-char obtained at 60 min, the mass loss was around 57.6% (wt.) between 159 and 506 °C. These mass losses are possibly associated with the presence of volatile material produced by the thermal degradation of lignocellulosic material (hemicellulose or cellulose). In addition, the temperatures of peaks in the DTG curve increase as the reaction time increases, showing that an increase in reaction time contributes to the formation of more stable compounds.

Influence of Biomass-to-H₂O Ratio

Hydro-char TG/DTG results obtained using different biomass-to-water ratios are shown in Figure 5 considering a reaction time of 240 min.

The TG–DTG curves for biomass-to-H₂O ratios of 1:10 and 1:15 are identical, showing mass losses between 50.63 and 53.68% (wt.), respectively. However, if one decreases the biomass-to-H₂O ratio of 1:15 to 1:20, the mass loss increases from 53.68 to 92.48% (wt.), increasing the carbonization grade of hydro-char. According to the literature, increasing the quantity of water favors the hydrolysis of the starting material, especially hemicelluloses and celluloses [15,71,72]. Cellulose hydrolyzes into glucose and further transforms into fructose [73]. The hydrothermal decomposition of fructose produces low carbon chain carboxylic acids, dissociating within the aqueous phase and producing H₃O⁺. By increasing the H₂O-to-biomass ratio, a decrease in the concentration of volatile carboxylic acids, particularly acetic acid, within the aqueous phase is expected, causing a diminution in the aqueous phase ionic product and worsening the degradation process of biomass [73]. Studying the hydrothermal processing of Açaí seeds with hot-compressed H₂O at 250 °C with a heating rate of 2 °C/min for 240 min, da Silva et al. [15] stated that hydrolysis is probably the dominant reaction mechanism—but not the only one—as the H₂O-to-biomass ratio increases from 1:10 to 1:20. For hydro-char obtained at 250 °C, after 60 min, with a biomass-to-H₂O ratio of 1:20, a mass loss of 92.48% (wt.) can be seen in Figure 5, proving that most of hemicellulose, cellulose and lignin were not thermally degraded. This is corroborated by the peaks in the DTG curve at 390 °C and 525 °C, respectively, which are characteristics of hemicellulose/cellulose and lignin degradation, showing that an increase in the H₂O-to-biomass ratio worsens the carbonization of corn stover by hydrothermal processing, as most hemicellulose and cellulose still retain their structure.

3.1.2. Scanning Electron Microscopy

SEM analysis was performed to investigate the effect of reaction time and biomass–H₂O ratio on the evolution and/or changes in the microstructure of corn stover after hydrothermal processing with hot-compressed H₂O, as illustrated in detail in Supplementary Figures S1–S13.

Influence of Reaction Time

The microscopies of bio-char obtained are shown in Figure 6. By the SEM images illustrated in Figure 6a–c, at different magnifications (MAG: 200× (a); MAG: 1000× (b); MAG: 5000× (c)), one observes the rupture of polymer bundles (fibers) and opening of pores in the cellular tissue, showing that fragmentation of hemicellulose and cellulose begins [74]. However, the main morphological structure of the corn stover remains practically unchanged since the plant microstructure retains its characteristics for the 60 min reaction time. The results are according to similar studies on the influence of reaction time over the morphology of hydro-chars reported in the literature [21,24,54,62,65].

Figure 7 shows the SEM microscopies of bio-char obtained by the hydrothermal processing of corn stover at 250°C, a biomass/H₂O ratio of 1:10 and 120 min using a reactor of 18.75 L.

The scanning electron microscopies at different magnifications illustrated in Figure 7 (MAG: 200× (a); MAG: 1000× (b); MAG: 5000× (c)) show that the morphological microstructure of corn stover has been drastically changed, as the fibers of plant tissue were destroyed by the fragmentation of hemicellulose and cellulose [73], and the pores on the vegetal tissue are open. In addition, Figure 7c shows the appearance of microspheres, which is in accordance with the results reported by Xing et al. [74]. The plant microstructure no longer retains its original morphology, demonstrating that a reaction time of 120 was able to carbonize corn stover, as described in the Influence of Reaction Time section.

The microscopies of bio-char obtained for 240 min of reaction are shown in Figure 8. The SEM images illustrated in Figure 8a–c, at different magnifications (MAG: 200× (a); MAG: 1000× (b); MAG: 20,000× (c)), show that the microstructure of corn stover has been destroyed due to the fragmentation of hemicellulose and cellulose [7,73]. The cellular tissue was replaced by an aggregate amorphous solid consisting of a layer of microspheres [7]. The results are in accordance with similar studies on the influence of reaction time on the morphology of hydro-chars reported in the literature [21,24,54,62,65].

Influence of Biomass-to-H₂O Ratio

The microscopies of bio-char obtained by the hydrothermal processing of corn stover at 250 °C with a biomass/H₂O ratio of 1:15 after 240 min using a reactor of 18.75 L are shown in Figure 9. da Silva et al. [15] investigated the effect of biomass-to-H₂O ratios on the hydrothermal processing of Açaí seeds, stating that an increase in the H₂O-to-biomass ratio enhances the hydrolysis of cellulose.

The SEM images at different magnifications (MAG: 200× (a); MAG: 1000× (b); MAG: 5000× (c)) show that the microstructure of corn stover has been changed since the pores on the vegetal tissue are open, probably due to the hydrolysis reaction of cellulose and hemicellulose, as reported by da Silva et al. [15]. However, by increasing the magnification 5000×, as shown in Figure 9c, one observes a rupture of polymer bundles (fibers) and the opening of pores in the cellular tissue was not very effective at changing the microstructure of corn stover, and a lower biomass-to-water ratio achieves less carbonization.

The microscopies at different magnifications (MAG: 200× (a); MAG: 1000× (b); MAG: 5000× (c)) of bio-char obtained by the hydrothermal processing of corn stover at 250 °C with a biomass/H₂O ratio of 1:20 after 240 min using a reactor of 18.75 L are shown in Figure 10. The SEM image at a magnification of 5000×, as seen in Figure 10c, shows that the microstructure of corn stover retains its original characteristics; that is, the morphology of corn stover remains practically unchanged, demonstrating that a higher H₂O-to-biomass ratio worsens the carbonization of corn stover as hydrolysis reactions of cellulose and hemicellulose are the major degradation mechanism [15].

3.1.3. Energy Dispersive X-ray Spectroscopy

Table 2. Percentages in mass and atomic mass of hydro-char obtained by the hydrothermal carbonization of corn stover at 250 °C after 60, 120 and 240 min with a biomass-to-H₂O ratio of 1:10 and at 250 °C after 240 min with biomass-to-H₂O ratios of 1:10, 1:15 and 1:20 using a pilot-scale reactor of a reactor of 18.75 L.

Hydro-Char
250 °C, Biomass-to-H2O Ratio of 1:10									250 °C, 240 min
60 (min)			120 (min)			240 (min)			1:15 (gbiomass/gH₂O)			1:20 (gbiomass/gH₂O)
CE	Mass (wt.%)	Atomic Mass (wt.%)	SD	Mass (wt.%)	Atomic Mass (wt.%)	SD	Mass (wt.%)	Atomic Mass (wt.%)	SD	Mass (wt.%)	Atomic Mass (wt.%)	SD	Mass (wt.%)	Atomic Mass (wt.%)	SD
C	43.76	74.14	3.14	50.15	84.94	3.59	58.65	85.11	4.26	76.04	81.05	0.631	6.04	28.01	0.70
O	16.29	20.72	1.33	6.91	8.79	0.68	11.03	12.02	1.01	23.59	18.87	0.632	5.58	19.41	0.63
Si	0.75	0.54	0.05	0.50	0.32	0.04	-	-	-	-	-	-	5.09	10.09	0.18
K	0.17	0.09	0.03	1.02	0.53	0.05	-	-	-	-	-	-	-	-	-
Ca	1.06	0.54	0.05	2.73	1.39	0.08	0.48	0.21	0.04	-	-	-	2.60	3.61	0.10
Mg	-	-	-	-	-	-	-	-	-	-	-	-	0.33	0.77	0.04
Al	-	-	-	-	-	-	-	-	-	-	-	-	2.48	5.12	0.11
S	-	-	-	-	-	-	-	-	-	-	-	-	0.87	1.51	0.05
Fe	-	-	-	-	-	-	-	-	-	-	-	-	13.35	13.31	0.56
Zn	-	-	-	-	-	-	-	-	-	0.366	0.072	0.065	-	-	-
Pt	37.97	3.96	0.87	38.69	4.03	0.90	29.84	2.67	0.76	-	-	-	63.66	18.17	1.72

SD = Standard Deviation; CE = Chemical Elements.

shows the EDX analysis of hydro-chars obtained by the hydrothermal processing of corn stover with hot-compressed H₂O at 250 °C with biomass/H₂O ratio of 1:10 after 60, 120 and 240 min and at 250 °C after 240 min with biomass-to-H₂O ratios of 1:10, 1:15 and 1:20 using a reactor of 18.75 L. The results show that carbon content increases when the reaction time increases between 60 and 120 min, as shown in the results presented in Figure 4, Figure 6c, Figure 7c and Figure 8c, while between 120 and 240 min, the carbon content is almost identical. By decreasing the biomass-to-H₂O ratio to between 1:10 and 1:15, little variation in the carbon content of hydro-chars has been observed, while a drastic decrease in the carbon content occurred between 1:15 and 1:20, demonstrating that a higher H₂O-to-biomass ratio worsens the carbonization of corn stover. The energy dispersive X-ray spectroscopy (EDX) also identified the presence of Si, K, Ca, Mg, Al, S and Fe. The micronutrients (Fe, Si), macronutrients (C, O, Ca, K, Mg, S), as well as heavy metals (Al) identified by EDX in hydro-chars are according to inorganics compounds identified in corn stover after drying at 105 °C [75].

3.1.4. X-ray Diffraction

The X-ray diffraction of hydro-chars obtained by the hydrothermal processing of corn stover with hot-compressed H₂O at 250 °C with a biomass/H₂O ratio of 1:10 after 60, 120 and 240 min and at 250 °C after 240 min with biomass-to-H₂O ratios of 1:10, 1:15 and 1:20 using a reactor of 18.75 L is shown in Figure 11 and Figure 12, respectively.

The diffractogram of hydro-chars illustrated in Figure 11 identified the presence of two crystalline phases-graphite (C) and crystalline cellulose. At 60 min, a peak of graphite of medium intensity (55.7%) was observed at position 2θ: 26.47, and a peak of high intensity (100.0%), characteristic of crystalline cellulose [20,76], was observed at position 2θ: 22.40. At 120 min, a peak of graphite of high intensity (100.0%) was observed at the position 2θ: 26.54, and a peak of high intensity (99.1%), characteristic of crystalline cellulose [20,76], was observed at position 2θ: 22.73. At 240 min, a peak of graphite of high intensity (100.0%) was observed at position 2θ: 26.47, and a peak of low intensity (12.95%), characteristic of crystalline cellulose [20,76], was observed at position 2θ: 22.20. The results show the occurrence of peaks of higher intensity of graphite (C) as the reaction time succeeds.

Figure 12 illustrates the XDR of hydro-chars obtained at 250 °C after 240 min with biomass-to-H₂O ratios of 1:10, 1:15 and 1:20 using a reactor of 18.75 L. The diffractograms identified the presence of two crystalline phases: graphite (C) and crystalline cellulose. For a biomass-to-H₂O ratio of 1:10, a peak of graphite of high intensity (100.0%) was observed at position 2θ: 26.47, and a peak characteristic of crystalline cellulose [20,76] of low intensity (12.95%) was observed at position 2θ: 22.20. For a biomass-to-H₂O ratio of 1:15, a peak of graphite of high intensity (100.0%) was observed on the position 2θ: 26.51 and a peak of low intensity, characteristic of crystalline cellulose [20,76], at position 2θ: 22.20. For a biomass-to-H₂O ratio of 1:20, a peak of graphite of medium intensity (59.38%) was observed at position 2θ: 26.47, and peaks characteristic of crystalline cellulose [20,76] of high intensity (79.0%) and (90.0%) were observed at positions 2θ: 15.53 and 2θ: 21.96, respectively. The results identified the occurrence of peaks of lower intensity of crystalline cellulose as the biomass-to-H₂O ratio decreases, which is in agreement with the results illustrated in Table 2.

3.1.5. Surface Area Analysis

The surface area analysis of hydro-char obtained at 250 °C with a biomass-to-H₂O ratio of 1:10 after 240 min is shown in Figure 13. The hydro-char capacity (X_ADS) increases with relative pressure (P/P₀). The sample density was 2.10 g/cm³, and the BET analysis showed a surface area of 4.35 m²/g, a pore volume of 18.6 mm³/g and an average pore width of 17.08 μm [7].

3.1.6. Proximate, Ultimate and Elemental Analysis of Hydro-Char

Table 3. Elemental characterization of hydro-char for reaction times of 60, 120 and 240 min.

Proximate, Ultimate and Elemental Analysis	Hydro-Char
	250 °C
	τ (min)
	60	120	240
TS 60–105 °C (%MM)	98.65	97.26	97.75
OTS (%TS)	89.17	92.61	88.49*
Ash (%TS)	9.48	4.65	9.26
N (%TS)	0.5629	0.9846	0.8611
C (%TS)	50.57	55.74	59.17
S (%TS)	0.1885	0.2224	0.2353
H (%TS)	6.571	6.97	5.719
O (%TS)	32.63	31.40	24.75

shows the proximate, ultimate and elemental analysis of hydro-char by the hydrothermal carbonization of corn stover at different reaction times. The elemental analysis illustrates that, for a constant biomass-to-H₂O ratio, carbon content increases with reaction time, while that of oxygen decreases, demonstrating that higher reaction times enhance the carbonization of corn stover, as corroborated by the TG/DTG, MEV/EDX and XRD analysis described in Section 3.1.

3.2. Process Parameters, Mass Balances and Yields of Reaction Products

3.2.1. Influence of Reaction Time

Table 4. Process parameters, mass balances and yields of reaction products by the hydrothermal carbonization of corn stover at 250 °C with a biomass/H₂O proportion of 1:10 after 60, 120 and 240 min using a reactor of 18.75 L.

Process Parameters	250 °C
	τ (min)
	60	120	240
Mass of Corn Stover (g)	600.66	600.31	600.10
Mass of H2O (g)	6000.20	6002.70	6000.70
Mechanical Stirrer Speed (rpm)	90	90	90
Initial Temperature (°C)	30	30	30
Heating Rate (°C/min)	2	2	2
Mass of Slurry (g)	6482.40	6402.20	6425.10
Volume of Gas (mL), T = 25 °C, P = 1 atm	8405	12,910	35,225
Mass of Gas (g)	13.195	20.656	57.495
Process Loss (I) (g)	105.265	200.81	118.205
Input Mass of Slurry (Pressing) (g)	6474.70	6335.10	6417.80
Process Loss (II) (g)	7.70	7.10	7.30
Mas of Liquid Phase (g)	5034.80	5321	5288.90
Mass of Moist Biochar (g)	1262.18	898.42	976.64
Process Loss (III) (g)	177.72	115.68	152.26
Mass of Dried Biochar (g)	318.19	263.88	214.99
Mass of H2O(V) (g)	943.99	634.54	761.65
(Mas of Liquid Phase + Mass of H2O(V)) (g)	5978.79	5955.54	6050.55
Process Loss (I + II + III) (g)	290.685	323.59	277.76
Mass of LiquidReaction (g)	269.275	315.774	327.61
Yield of Hydro-char (wt.%)	52.97	43.96	35.82
Yield of Gas (wt.%)	2.19	3.44	9.58
Yield of Liquid Phase (wt.%)	44.84	52.60	54.59

summarizes the mass balances, process parameters and yields of reaction products (hydro-char, aqueous and gaseous phases) of the hydrothermal processing of corn stover at 250 °C and different reaction times of 60, 120 and 240 min.

Figure 14 shows the effect of the reaction time on the yields of products (hydro-char, aqueous phase and gas) by the hydrothermal processing of corn stover at 250 °C with a biomass/H₂O ratio of 1:10 after 60, 120 and 240 min using a reactor of 18.75 L. The yields of reaction products (hydro-char, aqueous phase and gas) were regressed using exponential functions. The results show that applied exponential functions correlated well with the experimental data for solid, aqueous and gaseous phases, with R² (R-Squared) between 0.994 and 0.999. The yield of hydro-char shows a smooth first-order exponential decay behavior, while that of the liquid and gaseous phases shows smooth first-order exponential growth. The yield of hydro-char is according to similar data for the degradation of Brewer’s Spent Grains [34] and maize silage [33].

3.2.2. Influence of Biomass-to-H₂O Ratio

Table 5. Process parameters and material balances by the hydrothermal carbonization of corn stover at 250 °C after 240 min with biomass-to-H₂O ratios of 1:10, 1:15 and 1:20 using a reactor of 18.75 L.

Process Parameters	250 °C
	Biomass/H2O (-)
	1:10	1:15	1:20
Mass of corn stover (g)	600.10	600.11	600.28
Mass of H2O (g)	6000.70	9003.10	12,007.00
Mechanical Stirrer Speed (rpm)	90	90	90
Initial Temperature (°C)	30	30	30
Heating Rate (°C/min)	2	2	2
Process Time (min)	240	240	240
Mass of Slurry (g)	6425.10	9488.90	11,893.90
Volume of Gas (mL), T = 25 °C, P = 1 atm	35,225	32,536	30,518
Mass of Gas (g)	57.495	52.546	48.534
Process Loss (I) (g)	118.205	61.764	664.846
Input Mass of Slurry (Pressing) (g)	6417.80	9476.40	11,890.20
Process Loss (II) (g)	7.30	12.50	3.70
Mas of Liquid Phase (g)	5288.90	8544.30	10,996.00
Mass of Moist Biochar (g)	976.64	851.06	782.45
Process Loss (III) (g)	152.26	81.04	111.75
Mass of Dried Biochar (g)	214.99	205.61	186.57
Mass of H2O(V) (g)	761.65	645.45	595.88
(Mas of Liquid Phase + Mass of H2O(V)) (g)	6050.55	9189.75	11,591.88
Process Loss (I + II + III) (g)	277.76	155.30	780.29
Mass of LiquidReaction (g)	327.61	341.95	365.17
Yield of Solids (wt.%)	35.82	34.26	31.08
Yield of Gas (wt.%)	9.58	8.75	8.08
Yield of Liquid Phase (wt.%)	54.59	56.98	60.83

summarizes the influence of the biomass-to-H₂O ratio on the mass balances, process parameters and the yields of reaction products (hydro-char, aqueous and gaseous phases) by the hydrothermal carbonization of corn stover at 250 °C after 240 min with biomass-to-H₂O ratios of 1:10, 1:15 and 1:20 using a reactor of 18.75 L.

Figure 15 illustrates the influence of the biomass-to-H₂O ratio on the yield of reaction products by the hydrothermal carbonization of corn stover at 250 °C after 240 min with biomass-to-H₂O ratios of 1:10, 1:15 and 1:20 using a pilot-scale stirred tank reactor of 18.75 L. By analyzing the experimental data for the yield of reaction products (hydro-char, aqueous phase and gas), as depicted in Figure 15, one observes a linear behavior for the whole data set, showing a slight decrease in the yields of hydro-char and gas when the H₂O-to-biomass ratio increases, while that of aqueous phase increases. With a higher water quantity, hydrolysis is a favored mechanism of reaction, producing sugars and further decomposing to carboxylic acids, such as formic and acetic acids. The formation of carboxylic acids dissociates in the hot-compressed water, producing acid media for further decomposition of hemicelluloses and celluloses, improving the degradation of biomass. This is according to the reaction mechanism proposed by Wang et al. [65], as an increase in the H₂O-to-biomass ratio produced a solid phase with low carbon content; thus, a biomass-to-H₂O ratio of 1:20 was not enough to carbonize corn stover, as shown in Figure 5.

A smooth decrease in the yield of hydro-char when the H₂O-to-biomass ratio increases was also observed by Kang et al. [20], Arauzo et al. [34], Putra et al. [36], Putra et al. [40], Rather et al. [44], Rather et al. [46], Heilmann et al. [48], Putra et al. [52], Kambo et al. [57], Sliz et al. [61] and Sabio et al. [62], showing the effect of H₂O-to-biomass ratio on the yield of hydro-char is according to similar studies reported in the literature [20,34,36,40,44,46,48,52,57,61,62].

3.2.3. Degradation Kinetics of Corn Stover

Figure 16 summarizes the experimental data for the degradation kinetics of corn stover by hydrothermal processing at 250 °C with a biomass-to-H₂O ratio of 1:10 after 60, 120 and 240 min, expressed as hydro-char formation. The kinetic data were correlated with a pseudo-first-order exponential model described in Section 2.6, and the experimental data were compared to similar studies reported in the literature [33,34,36,40,44,60,74,77].

The constant A, defined as $\left[a/M_{Solids}\left(\tau \right)\right]$, is a correction term, exhibiting a root-mean-square error (r²) between 0.996 and 1.000, as shown in

Table 6. Regression of experimental kinetic data for the yield of hydro-char obtained by hydrothermal processing of corn stover with hot-compressed H₂O at 250 °C with a biomass-to-H₂O ratio of 1:10 after 240 min using Equation (16) compared with similar kinetic data reported in the literature [33,34,36,40,44,60,74,77].

Kinetic Data [33,34,36,40,44,60,74,77]	Regression of Experimental Kinetic Data
	Process Parameters			Kinetic Parameters
	T (°C)	Biomass/H2O (-)	τ (min)	A	K (min−1)	R2
Xing et al. (2016) [74]	260	1:8	0, 15, 30	50.33	0.19231	1.000
Xing et al. (2016) [74]	230	1:8	0, 15, 30	63.77	0.11880	1.000
Putra et al. (2021) [40]	210	1:10	0, 30, 60	59.54	0.14536	1.000
Reza et al. (2014) [33]	200	1:12	0, 20, 60, 180, 360	47.31	0.12266	1.000
Reza et al. (2014) [33]	250	1:12	0, 20, 60, 180, 360	34.44	0.11501	1.000
Arauzo et al. (2018) [34]	220	1:4	0, 120, 240	54.77	0.02199	1.000
Putra et al. (2020) [36,77]	230	1:10	0, 30, 60	57.84	0.14070	1.000
Pala et al. (2014) [60]	225	1:4	0, 10, 30, 60	58.50	0.33176	1.000
Rather et al. (2017) [44]	300	1:8	0, 10, 20, 30, 40	38.26 ± 0.71	0.2273–0.2835	0.999
Teribele	250	1:10	0, 60, 120, 240	36.79	0.02148	0.996

. By analyzing the experimental kinetic data illustrated in Figure 16, one observes that all the decomposition correlates with exponential decay functions [37,38,39,40,41].

4. Conclusions

From the perspective of yields and material balance, experiments on the hydrothermal carbonization of corn stover produced hydro-char yields between 31 and 35 wt.%, aqueous phase between 54 and 61 wt.% and 8 to 9.5 wt.% for gas phases. Yields are affected by reaction time, where hydro-char yields are lower, while aqueous and gas phases increased for longer reaction times. For different biomass-to-water ratios, there is a decrease in solid and gas phases when the water quantity increases. TG analysis suggests that 60 min of reaction is not enough to carbonize lignocellulosic material, and the results for 120 and 240 min are very similar, suggesting that 120 min is an optimized time for carbonization. The carbon content of hydro-char is affected by reaction time but decreases when the water quantity increases (lower biomass-to-water ratios). XRD analysis showed graphite peaks increase with higher reaction times and decreases in crystalline cellulose content. The surface area analysis of solid phase product obtained at 250 °C, 2.0 °C/min, biomass-to-H₂O ratio of 1:10, and 240 min revealed that the most carbonized hydro-char containing peaks of higher intensity of graphite (C) showed a surface area of 4.35 m²/g, pore volume of 18.6 mm³/g and average pore width of 17.08 μm.

The kinetic of corn stover degradation or bio-char formation was correlated with a pseudo-first-order exponential model, exhibiting a root-mean-square error (r²) of 1.000, demonstrating that the degradation kinetics of corn stover with hot-compressed H₂O, expressed as hydro-char formation, is well described by an exponential decay kinetics.