Hydrogen Generation from Wood Chip and Biochar by Combined Continuous Pyrolysis and Hydrothermal Gasiﬁcation

energies

Article Hydrogen Generation from Wood Chip and Biochar by Combined Continuous Pyrolysis and Hydrothermal Gasiﬁcation

Bingyao Zeng 1 and Naoto Shimizu 2,*

1 Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Hokkaido, Japan; [email protected] 2 Field Science Center for Northern Biosphere, Research Faculty of Agriculture, Hokkaido University, Sapporo 060-8589, Hokkaido, Japan * Correspondence: [email protected]; Tel./Fax: +81-11-706-3848

Abstract: Hydrothermal gasiﬁcation (HTG) experiments were carried out to extract hydrogen from biomass. Although extensive research has been conducted on hydrogen production with HTG, limited research exists on the use of biochar as a raw material. In this study, woodland residues (wood chip) and biochar from wood-chip pyrolysis were used in HTG treatment to generate hydrogen. This research investigated the effect of temperature (300–425 ◦C) and biomass/water (0.5–10) ratio on gas composition. A higher temperature promoted hydrogen production because the water–gas shift reaction and steam-reforming reaction were promoted with an increase in temperature. The methane concentration was related positively to temperature because of the methanation and hydrogenation reactions. A lower biomass/water ratio promoted hydrogen production but suppressed carbon-

monoxide production. Most reactions that produce hydrogen consume water, but water also affects the water–gas shift reaction balance, which decreases the carbon-monoxide concentration. By focus-

Citation: Zeng, B.; Shimizu, N. ing on the practical application of HTG, we attempted biochar treatment by pyrolysis (temperature ◦ ◦ Hydrogen Generation from Wood of heating part: 700 C), and syngas was obtained from hydrothermal treatment above 425 C. Chip and Biochar by Combined Continuous Pyrolysis and Keywords: hydrothermal gasiﬁcation; pyrolysis; hydrogen production; bioenergy Hydrothermal Gasiﬁcation. Energies 2021, 14, 3793. https://doi.org/ 10.3390/en14133793 1. Introduction Academic Editor: Maurizio Volpe Renewable and environmentally-friendly energy sources have attracted attention because of global warming and the gradual depletion of fossil resources. Biomass from Received: 5 June 2021 plants can be used as a material or energy source [1], which is representative of a renewable Accepted: 21 June 2021 Published: 24 June 2021 energy source, and includes woodland residues, municipal waste, energy crops, aquatic plants, agricultural crops and animal waste. Plants accumulate energy and absorb carbon

Publisher’s Note: MDPI stays neutral dioxide, and release energy and carbon when burnt. Therefore, bioenergy is a carbon- with regard to jurisdictional claims in neutral energy source [2]. In Japan, eight million tons of forest residue are generated published maps and institutional affil- annually, but the utilization rate is only 9% [3]; most forest residue is landfilled in situ. The iations. purpose of this study was to extract biofuel from conifer wood chips. Coniferous wood chips contain 31–49% cellulose, 9–23% hemicellulose and 20–35% lignin [4]. Because lignin hinders cellulose and hemicellulose decomposition, to convert wood chips to bioenergy, lignin needs to be decomposed first. Common methods, such as burning, hydrothermal treatment and pyrolysis have been studied to decompose the lignin [5]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Pyrolysis can be traced back to the ancient Egyptian period and has been used ex- This article is an open access article tensively in coke and charcoal production in modern times [5]. Biomass pyrolysis is a distributed under the terms and process of thermal degradation of biomass raw materials without air/oxygen and is used conditions of the Creative Commons to produce solid (biochar), liquid (tar and other organic matter) and gas products [6]. In Attribution (CC BY) license (https:// general, biochar is burned and used for power generation. Biochar can be used to generate creativecommons.org/licenses/by/ hydrogen or syngas that contains carbon monoxide and hydrogen by hydrothermal gasifi- 4.0/). cation (HTG) [7]. Hydrogen is considered a clean energy carrier. When burned, hydrogen

Energies 2021, 14, 3793. https://doi.org/10.3390/en14133793 https://www.mdpi.com/journal/energies Energies 2021, 14, 3793 2 of 11

combines with oxygen to produce only water. Therefore, infrastructure that is based on hydrogen energy has been regarded as an ideal solution to environmental problems [8–10]. Syngas could convert to C8–C16 by the Fischer–Tropsch process reaction, to form the main content of jet fuel [11–13]. Hydrothermal treatment is a chemical reaction produced by high temperature and pressure with water. In this critical situation, water acts as a solvent and has a catalytic effect. For hydrothermal gasification, the effect of temperature and biomass/water (B/W) ratio is important. Temperature affects the decomposition rate of cellulose, hemicellulose, and lignin, and has a significant impact on the conversion of low-molecular-mass compounds. In HTG, almost half of the hydrogen is produced from water, which influences the B/W ratio’s effect on gas yield and composition [7]. Although the use of HTG and pyrolysis technology to treat biomass has been studied extensively, limited research exists that combines pyrolysis and HTG. We have found some documents that use hydrothermal gasification to treat char, but almost no biochar comes from pyrolysis. In fact, the author did not find any literature using hydrothermal gasification to treat char from pyrolysis [7]. Compared with conventional thermal power generation, HTG removes impurities, such as ash, from char, so the amounts of SOx and NOx that are emitted from syngas use are small [14]. The HTG of pyrolysis chars is environmentally friendly and leads to cost reductions because it can operate without desulfurization equipment. This research confirmed the optimal condition to produce syngas by conducting batch HTG experiments. The biochar from pyrolysis is treated by HTG under optimal conditions to produce syngas.

2. Materials and Methods Wood chips from north Hokkaido conifers were used in the HTG experiment, and Larix kaempferi wood chips from Sunagawa were used in the pyrolysis experiments. All standard samples were used as received. Sulfuric acid was filtered with 0.45 µm filter paper. Conifer wood chips were used in the HTG experiment to determine the optimal conditions for hydrogen production (experimental group 1). An autoclave200 (Model 122841, SUS 316 Tsukuba, Japan) was used for the HTG experiments. A diagram that shows the structure of the reactor is shown in Figure1. The experimental conditions of Group 1 are outlined in Table1 and the experimental process is shown in Figure2. Firstly, we fixed the reactor body by a heater and unscrewed the screw to open the reactor. Wood chips (1 g) and pure water were placed in the reactor (the quality of pure water was determined from the B/W ratio, which is the mass ratio of biomass and pure water). Then, we tightened the screws to close the reactor. Nitrogen purging was conducted three times to remove air from the reactor. In each purge, the internal pressure of the reactor reached 4.0 MPa. To obtain the most accurate gas component measurement, all nitrogen gas was discharged and the initial pressure of group 1 was 0.1 MPa. Heating started after nitrogen purging. The heating rate was adjusted by the proportional–integral–derivative control system, and the holding time after exceeding the target temperature was defined as the reaction time. After each reaction, the reactor was cooled to room temperature overnight. The gas and solid residues were collected, separately, from the reactor. Gas composition analysis was performed by gas chromatography (GC). An electric furnace (FO300) (Yamato Scientific Co., Ltd., Tokyo, Japan) was used to dry the solid phase at 105 ◦C for 24 h in air, and the mass balance was calculated. The calculation of mass balance will be introduced below. The HTG experiment was carried out from 300 ◦C to 425 ◦C, with a B/W ratio of between 0.5 to 10. Energies 2021, 14, x FOR PEER REVIEW 3 of 12

treated to produce hydrogen under the optimal condition, as determined from group 1. The specific condition is shown in Table 2. Figure 3 shows the structure of the pyrolysis reactor. The experimental conditions of group 2 are outlined in Table 3, and the experimental flow is shown in Figure 4. After improving the equipment in the reference, the continuous pyrolysis experiment was undertaken at Mitsui Chemical Factory (Hokkaido, Japan). The wood chip was fed into the bottom of the pyrolysis reactor and risen by rotation of the screw. The heating part released heat and raised the temperature of the heat

Energies 2021, 14, x FOR PEER REVIEWconduction part to reach the reaction temperature. At a high temperature, wood chip3 ofun- 12 dergoes a pyrolysis reaction in the reactor; the oil is removed and collected through various outlets, and the char is discharged from the top of the reactor. The wood chips were heated for 8 h at 700 °C in air to recover the char and oil. The oil phase was filtered using 0.45treated µm tofilter produce paper hydrogen (mixed cellulose under the esters optima membrane).l condition, For as biochar, determined elemental from analysisgroup 1. wasThe performedspecific condition to calculate is shown the inhigh Table heatin 2. gFigure value 3(HHV) shows usingthe structure the DuLong of the pyrolysisformula. Energies 2021, 14, 3793 Charreactor. (1 g)The and experimental pure water cond(2 mL)itions were of usedgroup in 2 each are outlinedHTG experiment, in Table 3, with and a the flow experi- 3that of 11 wasmental the sameflow isas shown group 1.in GasFigure from 4. theAfter HTG im wasproving measured the equipment by GC. in the reference, the continuous pyrolysis experiment was undertaken at Mitsui Chemical Factory (Hokkaido, Japan). The wood chip was fed into the bottom of the pyrolysis reactor and risen by rotation of the screw. The heating part released heat and raised the temperature of the heat conduction part to reach the reaction temperature. At a high temperature, wood chip undergoes a pyrolysis reaction in the reactor; the oil is removed and collected through various outlets, and the char is discharged from the top of the reactor. The wood chips were heated for 8 h at 700 °C in air to recover the char and oil. The oil phase was filtered using 0.45 µm filter paper (mixed cellulose esters membrane). For biochar, elemental analysis was performed to calculate the high heating value (HHV) using the DuLong formula. Char (1 g) and pure water (2 mL) were used in each HTG experiment, with a flow that was the same as group 1. Gas from the HTG was measured by GC.

Figure 1. Schematic diagram of a hydrothermal reactor. 1 = pressure gauge; 2 = reactor body; Figure 1. Schematic diagram of a hydrothermal reactor. 1 = pressure gauge; 2 = reactor body; 3 = 3 = heater; 4 = thermocouple; 5 = control unit. heater; 4 = thermocouple; 5 = control unit.

Table 1. Experimental conditions for HTG (group 1).

Temperature (◦C) Time (min) Atmosphere Initial Pressure (MPa) B/W Ratio 300 5 380 4 0.5 425 0 300 5 380 4 nitrogen 0.1 1 425 0 300Figure 51. Schematic diagram of a hydrothermal reactor. 1 = pressure gauge; 2 = reactor body; 3 = 380heater; 44 = thermocouple; 5 = control unit. 10 425 0

Figure 2. Experimental flow of HTG.

FigureFigure 2. 2. ExperimentalExperimental flow ﬂow of of HTG. HTG.

To study the practicality of HTG, a combined pyrolysis/HTG experiment was carried out (experiment group 2). In this experiment, biochar from pyrolysis with HTG was treated to produce hydrogen under the optimal condition, as determined from group 1. The speciﬁc condition is shown in Table2. Figure3 shows the structure of the pyrolysis reactor. The experimental conditions of group 2 are outlined in Table3, and the experimental ﬂow is shown in Figure4. After improving the equipment in the reference, the continuous pyrolysis experiment was undertaken at Mitsui Chemical Factory (Hokkaido, Japan). The

wood chip was fed into the bottom of the pyrolysis reactor and risen by rotation of the screw. The heating part released heat and raised the temperature of the heat conduction part to reach the reaction temperature. At a high temperature, wood chip undergoes a Energies 2021, 14, 3793 4 of 11

pyrolysis reaction in the reactor; the oil is removed and collected through various outlets, and the char is discharged from the top of the reactor. The wood chips were heated for 8 h at 700 ◦C in air to recover the char and oil. The oil phase was filtered using 0.45 µm filter paper (mixed cellulose esters membrane). For biochar, elemental analysis was performed to calculate the high heating value (HHV) using the DuLong formula. Char (1 g) and pure water (2 mL) were used in each HTG experiment, with a flow that was the same as group 1. Gas from the HTG was measured by GC.

Table 2. Experimental conditions for HTG (group 2).

Temperature (◦C) Time (min) Atmosphere Initial Pressure (MPa) B/W Ratio 300 5 Energies 2021, 14, x FOR PEER REVIEW nitrogen 0.1 0.5 4 of 12 380 4 425 0

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FigureFigure 3. 3.Continuous Continuous pyrolysis pyrolysis reactor. reactor. 1: 1: heating heating part; part; 2: 2: heat heat conduction conduction part; part; 3: 3: screw;screw; 4: 4: outlet.outlet.

Table 3. Experimental conditions for pyrolysis (group 2).

Temperature of Heating Part (◦C) Initial Pressure (MPa) Time (h) Atmosphere Figure 3. Continuous pyrolysis reactor. 1: heating part; 2: heat conduction part; 3: screw; 4: outlet. 700 0.1 8 air

Figure 4. Experimental flow of combined experiment.

Table 1. Experimental conditions for HTG (group 1).

Temperature (°C) Time (min) Atmosphere Initial Pressure (MPa) B/W Ratio 300 FigureFigure 4. 4. Experimental5Experimental flow ﬂow of of combined combined experiment. experiment. 380 4 0.5 Table 1. Experimental conditions for HTG (group 1). 425 0 Temperature300 (°C) Time (min)5 Atmosphere Initial Pressure (MPa) B/W Ratio 300380 5 4 nitrogen 0.1 1 380425 4 0 0.5 425300 0 5 300380 5 4 10 380425 4 0 nitrogen 0.1 1 425 0 300 5Table 2. Experimental conditions for HTG (group 2). Temperature380 (°C) Time4 (min) Atmosphere Initial Pressure (MPa) B/W10 Ratio 425300 05 380 4 nitrogen 0.1 0.5 Table 2. Experimental conditions for HTG (group 2). 425 0 Temperature (°C) Time (min) Atmosphere Initial Pressure (MPa) B/W Ratio 300 5 380 4 nitrogen 0.1 0.5 425 0

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The mass balance calculation is expressed by Equations (1)–(3) [15]. After filtering and drying the char, the dry char mass was calculated as the total mass after drying minus the dry filter paper mass. The total liquid mass (on the filter paper and char) was calculated as the char and filter mass before drying minus their masses after drying. The total liquid mass was determined by adding the mass of the separated liquid portion. According to the principle of conservation of mass, the mass of generated gas was obtained by subtracting the char and liquid mass from the sum of the total mass of pure water and biomass before treatment.

Msolid = solid mass after drying (1)

Mliquid = measured liquid mass + solid mass before drying − Msolid (2)

Mgas = total material mass input − Msolid − Mliquid (3) The concentrations of C, H and N were measured at the Institute of Creation Research (Hokkaido University, Sapporo, Japan). The oxygen concentration was calculated using Equation (4). The elemental analysis and the DuLong formula (Equation (5)) [16] were used to calculate the HHV. The HHV is an important property to deﬁne the energy content of the fuel [7].

Xωt.%O = 100 − (ωt.%C + ωt.%H + ωt.%N + ωt.%Ash) (4)

MJ 33.5 Xωt.%C 142.3 Xωt.%H 15.4 Xωt.%O HHV = + − (5) kg 100 100 100 The gas concentration was measured by a GC4000 (GL Sciences Inc., Tokyo, Japan). The concentrations of CO, CO2 and CH4 were measured by an FID detector, using H2 as a carrier gas. H2 was measured by a thermal conductivity detector. All experiments were repeated once, and the measurement and calculation results were averaged.

3. Results and Discussion 3.1. Group 1 3.1.1. Mass Balance Group 1 was used to determine the optimal condition for hydrogen production. The mass balance results are provided in Figure5. As the treatment temperature increased from 300 ◦C to 380 ◦C, the yield of liquid decreased significantly and the gas yield increased. A high temperature is required to promote gas production because extensive biomass depolymerization occurs when the temperature is sufficient to destroy the bonds and the concentration of free radicals increases [2]. As the free radical reaction was promoted, the gas yield increased and the oil yield decreased [17–19]. The char showed a more stable yield with a change in temperature. As the temperature increased, the solid-phase yield decreased. The solid phase included ash, unreacted biomass and biochar that was generated by the hydrothermal reaction [7,20]. As the temperature increased, biomass decomposition was promoted, which converted biomass to gas, oil and biochar, and resulted in a decrease in total solid yield. In addition, for the experiment with a B/W ratio of 10, an interesting result could be observed. The yield of the liquid first decreased as the temperature rose, and after a reaction temperature over 380 ◦C, it increased again. As shown in Figure6, most of the liquid products came from ionic reactions. In the experiment with a B/W ratio of 0.1, the ion product in the water increased due to the increasing temperature, which promoted the ionic reaction and increased the generation rate of the liquid phase. When exceeding 380 ◦C, the generation rate of the liquid phase exceeded the consumption rate, resulting in an increase in the yield of the liquid phase. In the experiments of B/W ratios of 1 and 0.1, the specific gravity of water was relatively low, which led the rate increase of the ionic reaction to be insignificant, meaning the liquid yield did not increase even after the temperature rose above 380 ◦C. Energies 2021, 14, x FOR PEER REVIEW 6 of 12

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product in the water increased due to the increasing temperature, which promoted the productionic reaction in the and water increased increased the due generation to the increasing rate of the temperature,liquid phase. whichWhen promotedexceeding the380 ionic°C, the reaction generation and increasedrate of the the liquid generation phase ex raceededte of the the liquid consumption phase. When rate, resultingexceeding in 380 an °C,increase the generation in the yield rate of ofthe the liquid liquid phase. phase In ex theceeded experiments the consumption of B/W ratios rate, of resulting 1 and 0.1, in thean increasespecific gravityin the yield of water of the was liquid relatively phase. low,In the which experiments led the rateof B/W increase ratios of of the 1 and ionic 0.1, reac- the Energies 2021, 14, 3793 6 of 11 specifiction to be gravity insignificant, of water meaning was relatively the liquid low, yield which did led not the increase rate increase even after of the the ionic tempera- reac- tionture torose be aboveinsignificant, 380 °C. meaning the liquid yield did not increase even after the temperature rose above 380 °C.

FigureFigure 5. 5.Mass Mass balance balance of of group group 1. 1. Figure 5. Mass balance of group 1.

Figure 6. Gas component analysis result of group 1. FigureFigure 6.6.Gas Gas componentcomponent analysisanalysis resultresult of of group group 1. 1. 3.1.2. Gas Component Analysis 3.1.2. Gas Component Analysis 3.1.2. TheGas gasComponent components Analysis were analyzed by gas chromatography. Figure 6 shows that a higherTheThe temperature gasgas componentscomponents could wereyieldwere analyzedaanalyzed higher gas byby co gasgasncentration. chromatography.chromatography. Except for FigureFigure hydrogen,6 6shows shows methane, that that a a higher temperature could yield a higher gas concentration. Except for hydrogen, methane, highercarbon temperature monoxide and could carbon yield dioxide, a higher the gas remaining concentration. gases Except were almostfor hydrogen, all nitrogen. methane, The carbon monoxide and carbon dioxide, the remaining gases were almost all nitrogen. The carbonprinciple monoxide of HTG forand lignocellulosic carbon dioxide, biomass the remaining is outlined gases in Figurewere almost 7 [21–24]. all nitrogen. Temperature The principle of HTG for lignocellulosic biomass is outlined in Figure7[ 21–24]. Temperature is principleis the most of HTGimportant for lignocellulosic parameter in biomass the hydr isothermal outlined reaction.in Figure Temperature 7 [21–24]. Temperature affects the the most important parameter in the hydrothermal reaction. Temperature affects the rate israte the of most radical important reaction, parameter and indirectly, in the it hydr affectsothermal the rate reaction. of ionic Temperaturereaction by affecting affects thethe of radical reaction, and indirectly, it affects the rate of ionic reaction by affecting the ionic rateionic of product radical andreaction, permittivity and indirectly, of the solvent it affects [25–28]. the rate The of ion ionic product reaction is greatest by affecting at 300 the °C product and permittivity of the solvent [25–28]. The ion product is greatest at 300 ◦C[29], ionic[29], whichproduct means and permittivity that the ionic of reactionthe solvent is promoted; [25–28]. The cellulose, ion product hemicellulose is greatest and at 300 lignin °C which means that the ionic reaction is promoted; cellulose, hemicellulose and lignin are [29],are decomposed which means to that produce the ionic char reaction and tar. is Wh promoted;en the HTG cellulose, temperature hemicellulose exceeds 300and °C,lignin the decomposed to produce char and tar. When the HTG temperature exceeds 300 ◦C, the areion decomposedproduct decreases to produce with anchar increase and tar. in Wh temperature,en the HTG which temperature means thatexceeds the free-radical300 °C, the ion product decreases with an increase in temperature, which means that the free-radical ionreaction product becomes decreases dominant. with an The increase biomass in decomptemperature,oses into which low-molecular-mass means that the free-radical materials, reaction becomes dominant. The biomass decomposes into low-molecular-mass materials, reactionsuch as aldehydesbecomes dominant. and ketones, The biomassand produces decomp gas.oses Because into low-molecular-mass this research was carried materials, out such as aldehydes and ketones, and produces gas. Because this research was carried such as aldehydes and ketones, and produces gas. Because this research was carried out out above 300 ◦C, the ion product decreased with an increase in temperature and the free-radical reaction was promoted, which resulted in increased gas production.

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above 300 °C, the ion product decreased with an increase in temperature and the free- radical reaction was promoted, which resulted in increased gas production. The hydrogen concentration increased with an increase in temperature because of the promoting effect on the water–gas shift reaction and the steam-reforming reaction (Equations (6) and (7)) [8]. An increase in temperature also promoted methane production. The reactions that produced methane were mainly methanation (Equations (8) and (9)) and hydrogenation (Equation (10)) reactions. Although the reaction was exothermic, methane was produced even above 425 °C [21]. A larger water input resulted in a higher hydrogen concentration. Approximately half of the hydrogen was produced by the water–gas shift reaction [28,29]. Most water- producing reactions consume water. Therefore, the increase in water input promoted hydrogen production. The concentration of carbon monoxide increased as the B/W ratio increased. The water–gas shift reaction had a large effect on HTG. A larger water input resulted in more carbon monoxide conversion to hydrogen. Among the given conditions, the most suitable condition for producing hydrogen was 425 °C and a B/W ratio of 0.5.

CO + HO↔CO +H ∆H = 41 MJ/kmol (6)

CHO +6HO↔6CO + 12H ∆H = +206 MJ/kmol (7)

CO + 3H ↔CH +HO ∆H = 206.2 MJ/kmol (8)

CO +4H ↔CH +2HO ∆H = 206.2 MJ/kmol (9) Energies 2021, 14, 3793 7 of 11 CO + 2H ↔CH +0.5O ∆H = 131 MJ/kmol (10)

Figure 7.7. HTG mechanism of lignocellulosic biomass.biomass.

3.2. GroupThe hydrogen 2 concentration increased with an increase in temperature because of the promoting① Pyrolysis effect on the water–gas shift reaction and the steam-reforming reaction (Equations (6) and (7)) [8]. An increase in temperature also promoted methane production. The3.2.1. reactions Mass Balance that produced methane were mainly methanation (Equations (8) and (9)) and hydrogenation (Equation (10)) reactions. Although the reaction was exothermic, methane The continuous pyrolysis test treated 488 g wood chips in 700 °C over 8 h. The mass was produced even above 425 ◦C[21]. balance is shown in Figure 8. Approximately half of the wood chips were converted to A larger water input resulted in a higher hydrogen concentration. Approximately biochar. half of the hydrogen was produced by the water–gas shift reaction [28,29]. Most water- producing reactions consume water. Therefore, the increase in water input promoted hydrogen production. The concentration of carbon monoxide increased as the B/W ratio increased. The water–gas shift reaction had a large effect on HTG. A larger water input resulted in more carbon monoxide conversion to hydrogen. Among the given conditions, the most suitable condition for producing hydrogen was 425 ◦C and a B/W ratio of 0.5.

CO + H2O ↔ CO2 + H2 ∆H = −41 MJ/kmol (6)

C6H12O6 + 6H2O ↔ 6CO2 + 12H2 ∆H = +206 MJ/kmol (7)

CO + 3H2 ↔ CH4 + H2O ∆H = −206.2 MJ/kmol (8)

CO2 + 4H2 ↔ CH4 + 2H2O ∆H = −206.2 MJ/kmol (9)

CO + 2H2 ↔ CH4 + 0.5O2 ∆H = −131 MJ/kmol (10)

3.2. Group 2 1 Pyrolysis

3.2.1. Mass Balance The continuous pyrolysis test treated 488 g wood chips in 700 ◦C over 8 h. The mass balance is shown in Figure8. Approximately half of the wood chips were converted to biochar.

3.2.2. Elemental Analysis and HHV Calculation Table4 shows the elemental analysis of the biochar from pyrolysis. The HHV was 26.26 MJ/kg, which is higher than lignite coal (15–20 MJ/kg) [30].

Table 4. Elemental analysis of biochar.

C (wt %) H (wt %) N (wt %) O (wt %) Ash (wt %) 70.41 4.46 0.30 23.88 0.95 Energies 2021, 14, x FOR PEER REVIEW 8 of 12

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Figure 8. Mass balance of pyrolysis.

3.2.2. Elemental Analysis and HHV Calculation Table 4 shows the elemental analysis of the biochar from pyrolysis. The HHV was 26.26 MJ/kg, which is higher than lignite coal (15–20 MJ/kg) [30].

Table 4. Elemental analysis of biochar.

C (wt %) H (wt %) N (wt %) O (wt %) Ash (wt %) 70.41 4.46 0.30 23.88 0.95 FigureFigure 8. 8. MassMass balance balance of of pyrolysis. pyrolysis. ② HTG 2 HTG 3.2.2. Elemental Analysis and HHV Calculation 3.2.3. Mass Balance 3.2.3.Table Mass 4 Balanceshows the elemental analysis of the biochar from pyrolysis. The HHV was The temperature and pressure change curves of HTG using biochar as a raw material 26.26 MJ/kg,The temperature which is higher and pressure than lignite change coal curves (15–20 of MJ/kg) HTG using [30]. biochar as a raw material are shown in Figure 9. From 15 min, the pure water temperature exceeded the boiling are shown in Figure9. From 15 min, the pure water temperature exceeded the boiling Tablepoint, 4. Elementalsteam was analysis generated of biochar. and the pressure began to increase. Considering the HTG point, steam was generated and the pressure began to increase. Considering the HTG mass balance, as shown in Figure 10, temperature appeared to have little effect on the massC (wt balance, %) as shown H (wt in %) Figure 10, N temperature (wt %) appeared O (wt to%) have little Ash effect (wt %) on the mass balance. A comparison of the HTG mass balances of groups 1 and 2 (Figure 5) shows mass70.41 balance. A comparison4.46 of the HTG 0.30 mass balances23.88 of groups 1 and0.95 2 (Figure 5) showsthat the that liquid the liquidyield of yield theof biochar-based the biochar-based experiment experiment increased, increased, and andthe gas the gasyield yield decreased. Biochar contains char, unreacted biomass and ash. Because char is difficult to decreased.② HTG Biochar contains char, unreacted biomass and ash. Because char is difﬁcult to decompose, thethe generationgeneration speed speed of of low-molecular-mass low-molecular-mass compounds, compounds, which which is required is required for for gas production, is low. Low-molecular-mass substances react with water to generate 3.2.3.gas production,Mass Balance is low. Low-molecular-mass substances react with water to generate gas. Therefore,gas. Therefore, a lower a lower concentration concentration of low-molecular-mass of low-molecular-mass substances substances leads toleads reduced to reduced water The temperature and pressure change curves of HTG using biochar as a raw material consumption.water consumption. Compared Compared with the with experiment the experiment using wood,using thewood, HTG the of HTG group of 2 group yielded 2 arelessyielded shown gas (300less in gas◦FigureC: (300 –27%, 9.°C: 380From –27%,◦C: 15 –27%,380 min, °C: 425the –27%,◦ pureC: –24%)425 water °C: and –24%) temperature more and liquid more exceeded (300 liquid◦C: (300 +45%, the °C: boiling 380 +45%,◦C: point,+89%,380 °C: steam 425 +89%,◦C: was +89%).425 generated °C: +89%). and the pressure began to increase. Considering the HTG mass balance, as shown in Figure 10, temperature appeared to have little effect on the mass balance. A comparison of the HTG mass balances of groups 1 and 2 (Figure 5) shows that the liquid yield of the biochar-based experiment increased, and the gas yield decreased. Biochar contains char, unreacted biomass and ash. Because char is difficult to decompose, the generation speed of low-molecular-mass compounds, which is required for gas production, is low. Low-molecular-mass substances react with water to generate gas. Therefore, a lower concentration of low-molecular-mass substances leads to reduced water consumption. Compared with the experiment using wood, the HTG of group 2 yielded less gas (300 °C: –27%, 380 °C: –27%, 425 °C: –24%) and more liquid (300 °C: +45%, 380 °C: +89%, 425 °C: +89%).

Figure 9. Temperature–pressure changes of group 2 HTG.

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Figure 9. Temperature–pressure changes of group 2 HTG.

Figure 10. Mass balance of HTG (group 2). Figure 10. Mass balance of HTG (group 2). 3.2.4. Gas Component Analysis 3.2.4. Gas Component Analysis The gas components are shown in Figure 11. Carbon dioxide was detected in gases fromThe HTG gas at 300components and 380 ◦ C.are An shown analysis in Figure of gas 11 from. Carbon 425 ◦C dioxide HTG also was indicated detected the in gases pres- encefrom of HTG carbon at 300 dioxide, and 380 carbon °C. An monoxide, analysis of methane gas from and 425 hydrogen. °C HTG also The indicated same as Figurethe pres-6 , exceptence of for carbon hydrogen, dioxide, methane, carbon carbon monoxide, monoxide methane and and carbon hydrogen. dioxide, The the same remaining as Figure gases 6, wereexcept almost for hydrogen, all nitrogen. methane, Because carbon char monoxide is difﬁcult and to decompose, carbon dioxide, the rate the of remaining production gases of low-molecular-masswere almost all nitrogen. compounds Because required char is diffic for gasult production to decompose, was slow.the rate Therefore, of production HTG atof 300low-molecular-mass and 380 ◦C produced compounds almost no required gas. Above for gas 425 production◦C, the char was decomposed slow. Therefore, to produce HTG low-molecular-massat 300 and 380 °C produced compounds, almost such no gas. as aceticAbove acid 425 and°C, the HMF, char which decomposed were converted to produce to gas.low-molecular-mass The reactions that compounds, produce methane such asare acetic mainly acid methanationand HMF, which (Equations were converted (8) and (9) to) andgas. hydrogenationThe reactions that (Equation produce (10)) methane reactions. are Hydrogenmainly methanation is produced (Equations mainly by (8) the and water– (9)) gasand shift hydrogenation reaction and (Equation the steam-reforming (10)) reactions. reaction Hydrogen (Equations is produced (6) and mainly (7)). by The the reaction water– thatgas producesshift reaction carbon and monoxide the steam-reforming is the water–gas reaction shift (Equations reaction (Equation (6) and (7)). (6)). The Although reaction it Energies 2021, 14, x FOR PEER REVIEW 10 of 12 isthat possible produces to generate carbon monoxide hydrogen fromis the pyrolysis water–gas char shift by reaction HTG, a temperature(Equation (6)). above Although 425 ◦C it isis required.possible to generate hydrogen from pyrolysis char by HTG, a temperature above 425 °C is required. The char obtained by pyrolysis has a higher HHV, which leads to a higher yield of hydrogen, which can be obtained by treating these chars by hydrothermal gasification. Because pyrolysis requires a lot of energy, we do not recommend only adding pyrolysis as a pre-treatment for producing hydrogen; also, phenolic components and tar become causes of trouble and blockage in the single continuous pyrolysis system. However, since the purpose of pyrolysis is mostly bio-oil production, the treatment of the remaining char will be very important. We believe hydrothermal gasification is a premium method of treating char from the pyrolysis of biomass.

Figure 11. Gas component analysis result of group 2. Figure 11. Gas component analysis result of group 2. The char obtained by pyrolysis has a higher HHV, which leads to a higher yield of hydrogen,4. Conclusions which can be obtained by treating these chars by hydrothermal gasiﬁcation. BecauseThis pyrolysis study compared requires athe lot use of energy, of wood we chip do notor pyrolytic recommend char only as the adding feedstock pyrolysis to ex- as aamine pre-treatment optimal HTG for producing conditions hydrogen; for hydrogen also, phenolicgeneration. components The temperature and tar becomeof HTG causesaffects ofthe trouble concentration and blockage of each in gas. the singleA higher continuous temperature pyrolysis promotes system. hydrogen However, and sincemethane the purposeproduction of pyrolysisas it favors is mostlythe water–gas bio-oil production,shift, methanation the treatment and hydrogenation of the remaining reactions. char will Be- cause of the effect of the B/W ratio, a larger water input yielded a higher hydrogen concentration. Of the given conditions, the most appropriate conditions to produce hydrogen are 425 °C and a B/W value of 0.5. It is possible to treat biochar with HTG to produce carbon monoxide and hydrogen. A temperature greater than 425 °C is found to be required; biochar is a stable product. The present research proposes that hydrogen is generated from biochar by combining continuous pyrolysis and HTG. From hydrothermal gasification at B/W ratio 0.5 and 425 °C, the gas obtained has almost the same composition as that from biomass (H2 17.85%, CO2 22.96%, CH4 10.61%, CO 6.42%) and biochar (H2 18.73%, CO2 17.12%, CH4 7.53%, CO 5.97%). Rather than using biochar, we recommend using biomass as the raw material for HTG to produce hydrogen. From the perspective of energy production, although the same concentration of hydrogen can be extracted from the unit weight of biochar and biomass, the gas yield of HTG using biomass as a raw material (2 g) is higher than that of biochar (1.2 g). In addition, in the pyrolysis experiment of this study, only about 3.5 g of biochar could be extracted from 5 g of biomass. This means that in order to produce the same amount of hydrogen, pyrolysis-HTG treatment would consume more biomass than the HTG treatment. Even though pyrolysis also produces some by-products with higher HHV, we believe that the pyrolysis-HTG reaction is disadvantageous compared with the HTG reaction. However, the research provides a new idea for the treatment of biochar in thermal decomposition, which is the utilization of HTG to treat biochar to produce hydrogen. This method can not only obtain a higher concentration of hydrogen but also reduce the emission of harmful gases such as SO2, which is environmentally friendly.

Author Contributions: Conceived and performed experiments, analyzed data, prepared the manuscript draft, writing, B.Z.; Conceptualization, methodology, review, editing, supervision, N.S. Both authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data on the compounds are available from the authors.

Energies 2021, 14, 3793 10 of 11

be very important. We believe hydrothermal gasiﬁcation is a premium method of treating char from the pyrolysis of biomass.

4. Conclusions This study compared the use of wood chip or pyrolytic char as the feedstock to examine optimal HTG conditions for hydrogen generation. The temperature of HTG affects the concentration of each gas. A higher temperature promotes hydrogen and methane production as it favors the water–gas shift, methanation and hydrogenation reactions. Because of the effect of the B/W ratio, a larger water input yielded a higher hydrogen concentration. Of the given conditions, the most appropriate conditions to produce hydrogen are 425 ◦C and a B/W value of 0.5. It is possible to treat biochar with HTG to produce carbon monoxide and hydrogen. A temperature greater than 425 ◦C is found to be required; biochar is a stable product. The present research proposes that hydrogen is generated from biochar by combining continuous pyrolysis and HTG. From hydrothermal gasiﬁcation at B/W ratio 0.5 and 425 ◦C, the gas obtained has almost the same composition as that from biomass (H2 17.85%, CO2 22.96%, CH4 10.61%, CO 6.42%) and biochar (H2 18.73%, CO2 17.12%, CH4 7.53%, CO 5.97%). Rather than using biochar, we recommend using biomass as the raw material for HTG to produce hydrogen. From the perspective of energy production, although the same concentration of hydrogen can be extracted from the unit weight of biochar and biomass, the gas yield of HTG using biomass as a raw material (2 g) is higher than that of biochar (1.2 g). In addition, in the pyrolysis experiment of this study, only about 3.5 g of biochar could be extracted from 5 g of biomass. This means that in order to produce the same amount of hydrogen, pyrolysis-HTG treatment would consume more biomass than the HTG treatment. Even though pyrolysis also produces some by-products with higher HHV, we believe that the pyrolysis-HTG reaction is disadvantageous compared with the HTG reaction. However, the research provides a new idea for the treatment of biochar in thermal decomposition, which is the utilization of HTG to treat biochar to produce hydrogen. This method can not only obtain a higher concentration of hydrogen but also reduce the emission of harmful gases such as SO2, which is environmentally friendly.

Author Contributions: Conceived and performed experiments, analyzed data, prepared the manuscript draft, writing, B.Z.; Conceptualization, methodology, review, editing, supervision, N.S. Both authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data on the compounds are available from the authors. Acknowledgments: The continuous pyrolysis reactor was provided by Mitsui Chemical, Hokkaido. We thank Laura Kuhar, from Edanz (https://www.edanz.com/ac; accessed date 4 June 2021), for editing a draft of this manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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