Influence of Wet Torrefaction Pretreatment on Gasification of Larch

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Cite This: Energy Fuels XXXX, XXX, XXX-XXX

Influence of Wet Torrefaction Pretreatment on Gasification of Larch Wood and Corn Stalk Shumin Fan,*,†,‡ Li-Hua Xu,‡ Hueon Namkung,§ Guangri Xu,† and Hyung-Taek Kim*,‡ †

School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang, Henan 453003, People’s Republic of China ‡ Department of Energy Systems Research, Ajou University, Woncheon-dong, Youngtong-gu, Suwon 16499, Republic of Korea § Clean Fuel Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea S Supporting Information *

ABSTRACT: Biomass was wet-torrefied to improve its properties prior to gasification. Two kinds of biomass were employed as feedstock, including the larch wood and corn stalk. For both larch wood and corn stalk, the oxygen content of torrefied samples reduced greatly, while the carbon content increased. All of these features had a positive effect on the gasification process, which was discussed in the research. During steam gasification, all of the wet-torrified biomass samples yielded higher amounts of H2 and CO compared to the raw biomass samples, showing the superiority of the wet torrefaction (WT). During CO2 gasification, the CO production was also significantly improved by WT. The kinetic characteristic parameters for the gasification of raw and torrefied biomass samples were determined using the random pore model. It was found that the activation energy of gasification could be reduced by the WT process. Fisher et al.16 conducted torrefaction prior to gasification of biomass samples. The results showed that the reactivity of torrefied samples was relatively lower compared to that of the raw sample. Chen et al.17 also observed the effects of torrefaction on CO2 gasification of rice straw. The reactivity of char from torrefied rice straw was always lower than that of raw rice straw during gasification. In the studies of Fisher et al.,18 McNamee et al.,19 and Wang et al.,20 the same result was obtained. There are also different findings proving the positive effect of torrefaction. In the study by Xue et al.,21 the reactivity of gasification was increased by torrefaction at high conversion rates. Karlström et al.22 studied CO2 gasification of torrefied straw. The torrefaction showed a positive effect on the reactivity of straw. Thus, it is believed that the torrefaction has no systematic influence on reactivity of gasification. Almost all of the researchers are investigating the gasification after pyrolysis of torrefied biomass to study the effect of torrefaction on gasification, which is really complex. In our research, the biomass samples were wet-torrefied first and then gasified directly to study the influence of torrefaction on syngas production and its kinetics during gasification. The products during gasification were well-recovered for quantitative analyses. The random pore model (RPM) was used to study the kinetics during gasification of biomass samples. This research focused on the effect of WT on gasification and provided a comprehensive evaluation, which is meaningful for the utilization of biomass.

1. INTRODUCTION Among the renewable energy sources, biomass is currently the major renewable energy source in use and accounts for approximately 10% of global annual energy consumption.1 The energy in biomass could be used to produce power, heat, and chemicals. The gasification technology is considered to be the most effective method for using biomass. However, the thermochemical conversion process of biomass to obtain heat and power is difficult to handle because of the inherent properties of biomass. The lower heating value, energy density, hydrophobic character, and grindability all make biomass less attractive compared to coal. The whole efficiency is also dramatically reduced during application of biomass.2 As a result, the pretreatment process prior to biomass application is definitely essential to avoid the aforementioned problems and upgrade the fuel properties of biomass.3,4 Biomass could be upgraded to be comparable to coal by torrefaction technology. Wet torrefaction (WT) occurs in hydrothermal media under high pressure at 180−260 °C. In WT, the biomass is immersed in the liquid phase to prevent energy cost on the latent vaporization heat. Thus, the pressure of WT is usually set to be higher than the saturated vapor pressure of the water at a certain WT temperature. There are also three kinds of products after WT: the solid product, gas product, and aqueous liquid.5−11 The characteristics of the gasification of the torrefied biomass have been investigated, and different results have been reached. In the research by Raut et al.,12 it was found that the gas yield with the torrefied wood was lower than that of raw wood steam gasification. Many other studies have proven that the torrefaction could promote the gasification performance. Chen et al.13 found that H2 with torrefied bamboo was significantly higher compared to that of raw feedstock. Deng et al.14 and Couhert et al.15 also obtained the same conclusion. © XXXX American Chemical Society

Received: June 24, 2017 Revised: November 5, 2017 Published: November 8, 2017 A

DOI: 10.1021/acs.energyfuels.7b01785 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Proximate and Ultimate Analyses of Raw and Wet-Torrefied Samples proximate analysis (wt %, dry basis)

ultimate analysis (wt %, dry and ash-free basis)

sample

A

VM

FC

C

H

N

S

O

RLW RCS WTLW WTCS

0.18 1.85 0.10 0.92

87.10 77.16 78.77 69.30

12.72 20.99 21.13 29.78

47.80 44.92 50.9 50.89

7.38 6.54 6.65 5.05

0.20 1.28 0.21 1.28

0.12 0.34 0.15 0.36

44.50 46.92 42.09 42.42

Figure 1. Schematic diagram of the high-pressurized batch reactor for WT: (1) carrying gas (N2/CO2), (2) gas inlet, (3) thermocouple, (4) pressure gauge, (5) cooling system, (6) three curved blade type stirrer, (7) electric furnace, (8) stainless vessel, (9) chiller, (10) wire mesh, (11) gas outlet, and (12) system controller. captured, as shown in part 11 of Figure 2. The liquid product adhering to the wall and pipe was also recovered. 2.3. Other Analysis Methods. Surface structure parameters of samples were analyzed with Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods using ASAP 2020 (Micromeritics). The analysis was performed by N2 adsorption/desorption at the temperature of liquefied nitrogen (−196 °C), and the relative pressure of N2 varied from 0.01 to 1 (the ratio of low-temperature adsorption equilibrium pressure to saturation pressure). The condensed liquid product was analyzed by gas chromatography− mass spectrometry (GC−MS, Agilent GC−MS 7890A/5975C). The column was HP-5MS, 30 m × 0.25 mm × 0.25 mm. The chromatographic peaks were determined by the database of the National Institute of Standards and Technology (NIST) MS library. 2.4. Equations. Carbon conversion (Xc) is calculated by the following equation (eq 1):

2. EXPERIMENTAL SECTION 2.1. Characteristics of Biomass Samples. Korean larch wood and corn stalk were selected as the objects of the research, which were denoted as RLW and RCS, respectively. Ultimate and proximate analyses were conducted on a CHN-2000 elemental analyzer (LECO Co., St. Joseph, MI, U.S.A.) and a TGA-701 thermogravimeter (LECO Co., St. Joseph, MI, U.S.A.). The results were shown in Table 1. Moreover, the higher heating value (HHV) was calculated on the basis of the ultimate analysis results.23 The HHVs of larch wood and corn stalk are calculated as 19.20 and 17.97 MJ/kg, respectively. All of the samples were ground and meshed to 0.355−0.5 mm. 2.2. Experimental Setup and Procedure. In this research, WT experiments were carried out in a 5 L high-pressurized batch reactor (87 mm inner diameter and 125 mm height, Figure 1). All experiments with the same condition were duplicated 3 times. The values were collected and averaged for data processing. The weight ratio of the dry feedstock over distilled water was 1:10. Then, the reactor was sealed and purged with the carrier gas (N2) for 10 min. The timing started right when the temperature reached 200 °C, and the pressure during WT was 20 bar. The holding time was 30 min. The collected solid products were dried in the furnace at 105 °C for 48 h and then balanced. Finally, the dried biomass was kept in a desiccator for further use. The wet-torrefied larch wood and corn stalk are denoted as WTLW and WTCS, respectively. The gasification experiments were conducted in the fixed-bed reactor (Figure 2) at 700−900 °C. The reactor was heated to the preset temperature under a N2 atmosphere (flow rate of 1 L/min). A total of 0.500 g of biomass sample was used for gasification. The gasification started by adding a steam or CO2 for 30 min (100 mL/ min). The syngas was analyzed by a non-dispersive infrared sensor (NDIR, A&D 9000 series). The condensed liquid product was

Xc =

12(mol CO + mol CO2 + mol CH4) mass biomasspct carbon

(1)

where molCO, molCO2, and molCH4 are the moles of CO, CO2, and CH4, respectively, massbiomass is the mass of the biomass sample, and pctcarbon is the mass percentage of carbon in the ultimate analyses. Gasification is a heterogeneous reaction between gas and solid. If the partial pressure in the gas phase is kept constant during the reaction, the reaction rate constant could be expressed using the Arrhenius equation (eq 2)

⎛ E ⎞ k = A exp⎜ − a ⎟ ⎝ RT ⎠ B

(2) DOI: 10.1021/acs.energyfuels.7b01785 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Porous Properties of Biomass Samples sample

specific surface area (m2/g)

pore volume (cm3/g)

average pore size (nm)

RLW WTLW RCS WTCS

0.5 1.1 2.7

0.0021 0.0018 0.0064

61.0 67.2 17.8

Figure 2. Schematic diagram of the fixed-bed reactor. where Ea is the activation energy, A is the pre-exponential factor, T is the reaction temperature, and R is the ideal gas constant. The RPM considers the pore structure, its evolution, the overlapping of pore surfaces during gasification, and the area available for reaction.24,25 The reaction rate is described using the following equation (eq 3):

dXc = (1 − Xc) 1 − Ψ ln(1 − Xc) dτ

(3)

where ψ is a structural parameter, defined in the following equation (eq 4):26,27 ψ=

Figure 3. Effect of the temperature on the syngas yield during steam gasification of larch wood and corn stalk.

4πL0(1 − ε0) s0 2

(4)

The properties of WTLW and WTCS were listed in Table 1. The volatile matter of RLW was 87.1% and decreased to 78.77% after WT at 200 °C. In comparison to the RLW, the fixed carbon content after WT at 200 °C increased from 12.72 to 21.12%. It was remarkable that the fixed carbon content was enhanced with the increase of the WT temperature for both WTLW and WTCS. The devolatilization and carbonization of hemicellulose occurred during WT, decreasing the volatile matter and moisture. Table 1 also indicates that the ash contents were decreased by WT for both biomass samples. The lower ash content after WT is the result of the leaching effect in wet chemistry. In fact, there is competition between these opposite effects. If the leaching effect is dominant, the ash content of torrefied biomass may decrease and vice versa. Thus, there is no consistent trend for the changes of the ash content after WT.28 For corn stalk, the same conclusion has been found. Ultimate analyses were also shown in Table 1. It was notable that the carbon content increased, while the oxygen and

where ε0, L0, and S0 are the initial porosity, pore length, and specific surface area, respectively. The cold gas efficiency (ηCGE) is an important parameter to evaluate the economic performance of gasification. It is calculated according to the following equation (eq 5):

ηCGE =

(vol H2HHVH2) + (vol COHHVCO) + (vol CH4HHVCH4) mass biomassHHVbiomass × 100

(5)

where volH2, volCO, and volCH4 are the volumes of H2, CO, and CH4, respectively, HHVCH4 is 39.8 MJ Nm−3, HHVCO is 12.6 MJ Nm−3, HHVH2 is 12.8 MJ Nm−3, and massbiomass and HHVbiomass are the mass and HHV of the biomass for gasification.

3. RESULTS AND DISCUSSION 3.1. Results of WT. The proximate and ultimate analyses are important factors to evaluate fuel properties of biomass. C

DOI: 10.1021/acs.energyfuels.7b01785 Energy Fuels XXXX, XXX, XXX−XXX

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hydrogen decreased as the WT temperature increased. It could be deduced that the volatile released during WT is mostly a combination of oxygen and hydrogen. Thus, the volatiles are affected by WT, leading to an increase of the carbon content. In fact, the release of hydroxyl groups in the form of water during WT could cause a lower oxygen content and a higher carbon content, effectively upgrading the biomass. The changes of the contents of carbon, hydrogen, and oxygen accordingly lead to lower H/C and O/C ratios, which could efficiently reduce smoke and energy loss during a further application process.29 After WT, very few changes were made to nitrogen and sulfur contents. In all, the fuel properties of the biomass samples were effectively improved by WT. The physical properties of all biomass samples under the optimal conditions were shown in Table 2. For RLW, the specific surface area was too small that it could not be measured. After WT, the specific surface area was increased to 0.48 m2/g. For corn stalk, the pore volume and specific surface area both increased, while the average pore size decreased. During the WT process, there were gaseous products released from the decomposition of hemicellulose, which made the pores enlarge to generate more open pores. Meanwhile, there was volatile tar produced in the form of a semi-precipitated state, plugging some pores and forming new pores. This impact made the pore structure complicated and resulted in an increased specific surface area and a decreased average pore size. In conclusion, the specific surface area was increased after WT for both larch wood and corn stalk. The morphology and structure change after WT were also studied, which were shown in the Supporting Information. 3.2. Syngas Production during Steam and CO 2 Gasification. The larch wood and corn stalk samples were subjected to WT and then steam gasification from 700 to 900 °C. The composition of the syngas generated during gasification is not only one of the indicators of the overall performance of the process but also affects the potential uses of the syngas. The carbon balance during gasification was analyzed to check the feasibility of the measurement, which was shown in the Supporting Information. Figure 3 showed the CO, CO2, H2, and CH4 molar yields (normalized per mole of carbon in the larch wood) with temperatures during steam gasification. As shown, the production of H2 and CO was improved by WT, showing the superiority of wet-torrefied samples. At 700 °C, gasification of WTLW produced 0.21 mol of H2/mol of C, a 75% increase compared to that of RLW (0.12 mol of H2/mol of C). At 900 °C, the gasification of WTLW produced the highest amount of hydrogen, 0.76 mol of H2/mol of C, a 4% increase compared to that of RLW (0.73 mol of H2/mol of C). The rate of hydrogen production increased from 700 to 900 °C as a result of the higher rate of conversion. The difference in hydrogen production between WTLW and RLW became smaller with the temperature. The molar yields of CO increased from 700 to 900 °C. The effect of WT was apparent at all temperatures for higher yields of CO. The use of WTLW produced 0.39 mol of CO/mol of C at 900 °C, leading to a 30% increase compared to that of RLW. The CH4 production was very low, and its change with the temperature was not obvious. H2 was produced from the water-gas shift reaction, which is exothermal. The production rate of H2 is constrained at a higher temperature. The variation in CO production is caused by the comprehensive results of the temperature dependence of the Boudouard reaction and the water-gas shift reaction, which are

Figure 4. Effect of the temperature on the syngas yield during steam gasification of larch wood and corn stalk.

Table 3. Cold Gas Efficiency during Gasification ηCGE (%) during steam gasification ηCGE (%) during CO2 gasification

T (°C)

RLW

WTLW

RCS

WTCS

700 800 900 700 800 900

12.91 35.08 67.46 18.07 23.10 40.02

21.36 55.83 72.81 18.55 23.72 46.11

23.25 45.69 60.65 16.34 26.62 53.40

37.90 59.05 68.72 18.63 33.01 60.94

Table 4. Reaction Rate Constant Calculations during Steam Gasification Using the RPM RPM sample RLW

WTLW

RCS

WTCS

T (°C)

k (×10−4, s−1)

ψ

R2

700 800 900 700 800 900 700 800 900 700 800 900

0.35 4.12 9.12 0.48 4.49 10.18 0.42 4.92 9.95 0.66 5.87 11.71

1.94 1.52 1.27 1.66 1.41 0.56 2.18 9.87 4.17 2.56 2.78 3.81

0.987 0.999 0.999 0.998 0.999 0.999 0.999 0.998 0.996 0.999 0.995 0.993 D

DOI: 10.1021/acs.energyfuels.7b01785 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 5. Gasification rate versus carbon conversion at different temperatures.

different from that of larch wood, with a lower amount of H2 and a higher amount of CO production. Many studies have proven that the gasification performance varied with the biomass feedstock.22,30,31 The H2/CO mole ratios were closely related to the application of gasification, which were studied and shown in the Supporting Information. The larch wood and corn stalk samples were also gasified under CO2 atmospheres. Figure 4 showed the syngas production during CO2 gasification of larch wood and corn stalk. During CO2 gasification, CO is the main product. As the temperature increased, the CO production increased because of the endothermic characteristics of the Boudouard reaction. The CO production of WTLW was higher than that of RLW. The H2 production during CO2 gasification was almost zero at 700 and 800 °C. At 900 °C, there was a little amount of H2 during gasification of WTLW. The CH4 production during CO2 gasification was also very low. The CO production of WTCS was also higher than that of RCS at all three temperatures. Cold gas efficiency (ηCGE) is an important parameter to evaluate the economic performance of gasification, which was shown Table 3. It was indicated that ηCGE increased with the gasification temperature. At 900 °C, ηCGE of all samples reached the maximum value. It could be seen that the cold gas efficiency could be improved by WT pretreatment for both larch wood and corn stalk. At 900 °C, ηCGE of WTLW and WTCS during steam gasification was 72.81 and 68.72, respectively. It was higher than ηCGE obtained during CO2 gasification, which was 46.11 and 60.94 for WTLW and WTCS, respectively.

Table 5. Physical Properties during the Gasification Process sample WTLW

WTCS

time (min)

specific surface area (m2/g)

pore volume (cm3/g)

average pore size (nm)

0 5 10 15 20 30 0 5 10 15 20 30

0.5 371.1 352.6 330.3 310.1 301.3 2.7 200.0 230.8 317.5 428.9 337.9

0.0021 15.5 13.9 12.8 5.7 0.01 0.0064 0.0059 0.0101 0.0227 0.0394 0.0125

61.0 2.6 2.5 2.5 2.3 2.2 17.8 5.1 6.5 27.5 3.8 3.3

endothermic and exothermic, respectively. Thus, a higher temperature promotes a higher CO production. The effect of the temperature on gasification was obvious. Thus, the effect of WT became more apparent for CO production and less obvious for H2 production with the increase of the temperature. The difference of the syngas production between raw and wettorrefied larch wood samples became smaller at a higher temperature, which was shown in the Supporting Information. The effect of WT on corn stalk is similar to that on larch wood. During corn stalk gasification, all of the WTCS gasification yielded higher amounts of H2 and CO compared to that of RCS. The gasification performance of corn stalk was E

DOI: 10.1021/acs.energyfuels.7b01785 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 6. Arrhenius plots during gasification.

Table 6. Activation Energies and Pre-exponential Factors of Steam Gasification Using RPM sample RLW WTLW RCS WTCS

Ea (kJ/mol)

A (s−1)

156.4 146.6 150.3 140.3

× × × ×

1.05 4.06 5.65 2.59

Table 7. Reaction Rate Constant Calculations during CO2 Gasification

R2 4

10 103 103 103

RPM

0.952 0.966 0.945 0.949

sample RLW

WTLW

3.3. Kinetic Study of Gasification. According to the steam gasification performance of WTLW and WTCS, the reaction rate constants were calculated using the RPM, as presented in Table 4. It was clear that the rate constants of wettorrefied samples were higher than that of raw biomass samples at all temperatures. For example, at 900 °C, the rate constant for the WTCS was 11.71 × 10−4 s−1, 1.2 times as much as that of RCS (9.95 × 10−4 s−1). Meanwhile, the rate constant for the WTLW was 10.18 × 10−4 s−1 using RPM at 900 °C, 1.1 times as much as that of RLW (9.12 × 10−4 s−1). RPM assumes that the internal surfaces of the pore structure also serve as reaction interfaces. The cylindrical pores having uneven diameters enlarge as the internal surfaces erode during the progress of the reaction and eventually merge together.32 On the basis of eq 4, ψ is a dimensionless structural parameter that provides information for the structure of the initial sample

RCS

WTCS

T (°C)

k (×10−4, s−1)

ψ

R2

700 800 900 700 800 900 700 800 900 700 800 900

0.23 1.21 7.31 0.35 2.75 8.94 0.24 1.51 7.03 0.38 2.75 8.42

9.45 2.82 1.27 9.07 3.44 2.02 8.90 3.05 1.47 6.51 2.72 1.01

0.998 0.999 0.998 0.998 0.999 0.999 0.998 0.998 0.998 0.999 0.994 0.998

and influences subsequent gasification rates. The RPM tries to find a maximum surface over the range of 2 ≤ ψ ≤ ∞. The gasification rate increases slightly in the early stages of conversion and then showed a moderate peak. After that, the reaction rate slows until reaching zero. The maximum surface happened from two opposite effects: the growth of reaction F

DOI: 10.1021/acs.energyfuels.7b01785 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

samples at all temperatures during CO2 gasification. In addition, the activation energy was also calculated, as shown in Figure 6. The fitting results for A and Ea were listed in Table 8. The activation energies with WTLW and WTCS were 150.9 and 146.8 kJ/mol, both lower than those of the corresponding raw biomass samples. The WT destroyed the surface of biomass and created numerous pores and cracks on the biomass surface. In addition, the improvement of fuel properties, including the increased carbon content and surface area, may all have positive effects on gasification, thus enhancing the reactivity during gasification and reducing the activation energy. A wide range of activation energies of CO2 gasification of biomass samples has been reported, from 80 to 400 kJ/mol.20 The results in our research were consistent with other studies. Furthermore, the activation energy obtained from CO2 gasification was found to be higher than that from steam gasification.

Table 8. Activation Energies and Pre-exponential Factors of CO2 Gasification Using RPM sample RLW WTLW RCS WTCS

Ea (kJ/mol)

A (s−1)

163.2 150.9 157.3 146.8

× × × ×

1.22 4.87 6.77 3.17

R2 5

10 103 103 103

0.994 0.992 0.998 0.990

surfaces associated with the pore and the loss of reaction surfaces as they progressively collapse by intersection. The initial increase in rate is attributed to the growth of reaction surfaces from the initial pore. This effect is overshadowed in a later process by the intersection of the growing surfaces, which decrease the reaction rate. At low values of ψ, the gasification rate decreases almost monotonically without a maximal rate peak. Figure 5 showed the steam gasification rate curve versus carbon conversion. It was observed that the gasification rates increased with the temperature for all samples. After WT, the reaction rate of larch wood and corn stalk displayed a higher reaction rate than their raw samples. During RLW and WTLW gasification, the reaction rate decreased almost monotonically without a maximal rate peak, which was consistent with the ψ values (ψ < 2) shown in Table 4. On the contrary, during RCS and WTCS gasification, the maximal reaction rate existed, which was also consistent with the ψ values calculated (ψ > 2). In conclusion, the conformity and validity of RPM were demonstrated. To see the real pore structure evolution during gasification to verify the prediction of RPM, the physical properties were analyzed. The samples during gasification were taken out for pore structure analysis during the gasification process. The results were shown in Table 5. With the structure parameter for WTLW, the maximum surface area was expected to appear in the initial stages of gasification. The results from Table 5 showed that the specific surface area at the beginning of the conversion reached the maximum value and then decreased all of the time, which was consistent with the prediction. The results indicated that the fine pore condition was obtained in the initial stage; thus, only coalescence occurred in the reaction. Everson et al. also found the decreasing reaction rate during the gasification process and successfully explained the conversion using the RPM. For WTCS, the surface area was predicted to grow first and then decrease because of the coalescence. The wide change of the average pore size also proved the growth and coalescence during the conversion.33 Lu et al.,34 Liu et al.,35 and Kajitani et al.36 also analyzed the gasification using RPM and determined the structure parameters with the value greater than 2. In addition, the activation energy was calculated. Using the data from the RPM shown in Table 4, the activation energy, Ea, could be obtained using the Arrhenius law, as shown in Figure 6. The fitting results for A and Ea were listed in Table 6. The results showed that the activation energy of gasification after WT was lower than that of raw biomass. Thus, the activation energies obtained from steam gasification of RLW and RCS were determined to be 156.4 and 150.3 kJ/mol, which were reduced to 146.6 and 140.3 kJ/mol for the WTLW and WTCS. On the basis of the gasification of larch wood and corn stalk under a CO2 atmosphere, the reaction rate constants at the three temperatures were calculated using the RPM, as presented in Table 7. It was also found that the rate constants of wet-torrefied samples were higher than those of raw biomass

4. CONCLUSION In this research, larch wood and corn stalk samples were studied to produce the syngas by gasification, which was clean and promising. The biomass samples were wet-torrefied prior to gasification to improve the properties of biomass. The following conclusions can be reached from this research: After WT, the volatile matter in biomass decreased, while the fixed carbon content increased. The changes of the contents of carbon, hydrogen, and oxygen accordingly lead to lower H/C and O/C ratios. In addition, the specific surface area was increased, which could efficiently improve the fuel properties of the biomass samples. During steam gasification, all of the wet-torrified biomass samples yielded higher amounts of H2 and CO compared to the raw biomass samples, showing the superiority of the WT. During CO2 gasification, the CO production was also significantly improved by WT. The structure change during the gasification process was explained successfully by the RPM. The activation energies with the wet-torrefied samples were lower than those of the corresponding raw biomass samples. The WT technology is not only a pretreatment method to improve the fuel properties of biomass but also improves the reactivity during gasification, which is meaningful for the utilization of biomass.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01785. Carbon balance analysis (S1), morphology and structure of fuel particles (S2), H2/CO mole ratios during gasification (S3), and syngas production at 1000 °C (S4) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shumin Fan: 0000-0003-3591-1456 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.energyfuels.7b01785 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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ACKNOWLEDGMENTS This work was supported by the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP). Financial support was granted by the Ministry of Trade, Industry and Energy, Republic of Korea (Project 2015 4010 200820).

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DOI: 10.1021/acs.energyfuels.7b01785 Energy Fuels XXXX, XXX, XXX−XXX