Investigation on the Physical and Chemical Properties of Hydrochar

Jun 23, 2015 - The results indicated that the properties of both hydrochar and HDPC .... to a weight loss of 50% by oxidation and volatilization of th...
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Investigation on the Physical and Chemical Properties of Hydrochar and Its Derived Pyrolysis Char for Their Potential Application: Influence of Hydrothermal Carbonization Conditions Xiangdong Zhu, Yuchen Liu, Feng Qian, Shicheng Zhang,* and Jianmin Chen Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, People’s Republic of China S Supporting Information *

ABSTRACT: Hydrothermal carbonization (HTC) is an aqueous-phase procedure to prepare charred material using biomass. To obtain a charred material with high porosity, ash content, and thermal recalcitrance, it is necessary to investigate the influence of HTC conditions (peak temperature, retention time, and feedstock type) on the properties of hydrochar and its derived pyrolysis char (HDPC). Additionally, the relative importance of these conditions for the selected properties was also investigated by heterogeneity index. The results indicated that the properties of both hydrochar and HDPC samples were greatly influenced by the HTC process. The ash content and major metal elements (Na, Mg, K, and Ca) of hydrochar and HDPC samples were strongly influenced by the feedstock type; other properties, such as surface area, carbon sequestration potential, total carbon, total nitrogen, and dissolved organic carbon were moderately influenced by the feedstock type. Overall, this study provided new insights into the relative importance of different HTC conditions in the properties of hydrochar and HDPC samples, which was an important process toward obtaining a “required” charred material for environmental remediation.

1. INTRODUCTION In the past few years, much attention has been focused on charred materials as an additive to improve soil properties and reduce global warming.1−5 Several methods are widely used to produce charred material, including slow pyrolysis,1,6−8 gasification,9 and hydrothermal carbonization (HTC).4 Most research has focused on pyrochar, which is produced via slow pyrolysis under a nitrogen (N2) or oxygen-limited atmosphere.10−12 In contrast, little attention has been paid to the application of charred materials derived from the HTC process.13,14 HTC occurred at relatively low temperatures (180−300 °C) under autogenous pressures, simultaneously producing gases, organic substances, bio-oil, and carbon-rich solid residue termed hydrochar.4,15 Hydrochar materials are composed of spherical microparticles with abundant carbon content. However, because of its low process temperature, hydrochar has some limiting factors, such as low ash content, low porosity, low aromatic structure, and low recalcitrance. The hydrochar surface is also gathered by acidic composition, such as phenolic and organic acid compounds. These shortcomings hinder its effective and straightforward use in environmental and agricultural applications.16−20 The ash content and surface area of biochar are key determinants of its ability to improve soil fertility and adsorption of pollutants. Furthermore, an obvious inhibiting effect of hydrochar on the germination of plant is observed, resulting from the dissolution of organic carbon;18 however, pyrochar has no effect on germination. Hydrochar has reduced stability and, therefore, exhibits substantially different characteristics from those of biochar derived from a slow-pyrolysis process.21,22 Hence, to adjust the properties of hydrochar, a post-thermal treatment stage under an inert atmosphere (slow pyrolysis) is required, resulting in a © XXXX American Chemical Society

charred material with higher environmental performance, termed hydrochar-derived pyrolysis char (HDPC). In comparison to compost,23 slow pyrolysis is a more useful activation method for the charred material, with regard to the enhancement of porosity, ash content, and thermal recalcitrance. It has been well-documented in the literature that the properties of hydrochar are greatly influenced by carbonization conditions (such as peak temperature and retention time) and feedstock type.24 For example, an increasing peak temperature is associated with higher ratios of carbon to oxygen (C/O) and carbon to hydrogen (C/H) and a decrease in volatile matter content.25 It also has been shown that the availability of nutrient elements (such as N, P, and K) within the hydrochar varied as a result of different HTC peak temperatures and retention times.26 Additionally, considerable progresses have been made toward understanding the properties of hydrochar through the study of the material derived from the HTC process of homogeneous substrates (such as cellulose, lignin, xylose, etc.);25,27 however, comparatively little is known about the properties of hydrochar derived from heterogeneous substrates. Moreover, to the best our knowledge, little information is available on the effects of HTC conditions on HDPC materials. It is important to develop an understanding of the relationships between hydrochar and HDPC samples, because HTC conditions may be important in determining the properties and specific functionality of the resulting HDPC material. Received: March 10, 2015 Revised: June 23, 2015

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DOI: 10.1021/acs.energyfuels.5b00512 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Compositions, Physicochemical Properties, and Structural Characteristics of Hydrochar Samples feedstock SW SW SW SW SW HT SW SW SW SW HR RS BB SS PP PN EP HF

HTCa condition 180-1 210-1 240-1 270-1 300-1 240-0 240-0.5 240-2 240-4 300-1 300-1 300-1 300-1 300-1 300-1

yield (%)

ash (%)

Na (g/kg)

Mg (g/kg)

K (g/kg)

Ca (g/kg)

pH

TCb (%)

DOCc (mg/g)

TNd (%)

SAe (m2/g)

V tf (cm3/g)

R50g

CSh (%)

64.7 58.5 49.0 39.8 38.6 0.23 66.9 58.0 47.8 46.1 0.16 36.0 33.6 34.3 25.7 37.3 25.7 0.16

8.6 8.2 8.3 8.2 7.9 0.03 6.4 7.1 7.4 6.1 0.12 26.3 0.6 1.5 1.9 3.0 36.9 1.30

0.04 0.02 0.02 0.04 0.02 0.39 0.03 BDL 0.01 BDL 0.50 0.01 BDL BDL BDL 0.04 0.24 1.41

0.16 0.27 0.25 0.33 0.64 0.56 0.30 0.29 0.33 0.32 0.10 3.52 0.03 0.32 0.30 0.86 12.7 1.75

0.12 0.13 0.12 0.12 0.15 0.10 0.29 0.14 0.14 0.21 0.39 0.42 0.01 0.43 0.09 0.29 1.89 1.38

11.7 11.8 13.2 10.9 7.38 0.20 9.81 10.5 10.5 8.61 0.16 5.63 BDL 3.21 2.97 7.05 47.1 1.41

5.0 5.0 4.8 4.7 4.9 0.03 5.0 4.9 4.8 4.7 0.02 5.3 4.3 4.7 4.7 5.2 6.1 0.12

51.8 53.9 57.6 64.2 69.5 0.12 53.8 57.4 62.6 65.8 0.08 55.9 75.0 73.7 75.8 72.5 48.2 0.16

5.58 5.49 6.41 5.52 4.72 0.11 11.48 8.53 7.68 8.15 0.22 6.31 4.20 6.94 3.56 9.14 3.38 0.39

0.49 0.61 0.60 0.72 0.87 0.22 0.42 0.49 0.66 0.68 0.19 1.43 0.66 0.73 1.35 8.79 3.13 0.60

6.9 9.7 4.2 5.6 4.5 0.36 3.8 5.1 6.6 4.6 0.22 6.0 3.3 2.9 3.4 7.9 4.2 0.39

0.023 0.054 0.038 0.015 0.009 0.66 0.022 0.032 0.026 0.025 0.22 0.035 0.008 0.021 0.009 0.020 0.016 0.57

0.38 0.41 0.44 0.47 0.48 0.10 0.40 0.41 0.44 0.45 0.05 0.49 0.51 0.50 0.50 0.46 0.48 0.03

26.6 27.1 26.2 25.3 27.2 0.03 30.6 29.0 27.8 28.7 0.06 24.7 27.8 26.9 23.1 25.6 17.1 0.15

a

HTC condition refers to the temperature-retention time. bTC is the total carbon. cDOC is the dissolved organic carbon. dTN is the total nitrogen. SA is the BET surface area. fVt is the total pore volume. gR50 is the novel recalcitrance index. hCS is the potential for carbon sequestration.

e

°C/min, followed by an isothermal period of 60 min. The produced pyrolysis chars are designated PX-T-R, where X, T, and R are as previously defined. 2.2. Characterization of Samples. All of the hydrochar and HDPC samples underwent the following analyses: yield, elemental analysis, dissolved organic carbon, ash content, pH, major metal composition (Na, Mg, Ca, and K), surface area and pore volume, and thermogravimetry (TG) analysis. A CHN element analyzer (Vario EL III, Germany) was used to measure the content of C, N, and H. The ash content was measured by heating the samples at 600 °C for 2 h in an air atmosphere. The pH of the samples was determined in a suspension of 1:10 sample/deionized water using a combination electrode. The suspension was shaken for 1 h before pH measurement. For the analysis of dissolved organic carbon (DOC), 0.08 g of sample was suspended in 20 mL of 0.05 M K2SO4 with 1 h.29 Supernatant was filtered and analyzed using a total organic carbon (TOC) analyzer (Analytikjena, multi N/C 3100). The concentrations of major metals in the samples were measured in the digestion solution using inductively coupled plasma (ICP, P4010, Hitachi). N2 adsorption at −196 °C was performed with a Quantasorb SI instrument (Quantachrone, Boynton Beach, FL) to determine Brunauer−Emmett−Teller (BET) surface area and pore volume. The TG analyses of samples were performed by a TG analyzer (PerkinElmer, Inc.), heated from 30 to 1000 °C in an air atmosphere at a rate of 20 °C/min. To discern the effect of feedstock on the functional groups and phase structures of the samples, the selected hydrochar and HDPC samples also underwent Fourier transform infrared (FTIR, Nexus 470), nuclear magnetic resonance (NMR, Bruker DSX 300), and X-ray diffraction (XRD) analyses. 2.3. Calculations. A novel recalcitrance index, R50, was used to evaluate the quality of samples for carbon sequestration, as calculated by eq 1

The objectives of this study are (1) evaluation of how the three main HTC parameters (peak temperature, retention time, and feedstock type) affect the selected properties of hydrochar and HDPC material and (2) investigation of the relative importance of these factors for the selected properties, in relation to soil amendment and carbon sequestration performance on the basis of the heterogeneity index.28 In the present study, seven common biomass feedstock materials are used to produce hydrochar and HDPC. This study is the first attempt to characterize HDPC samples prepared from various hydrochar materials. A better understanding in the properties of hydrochar and HDPC will provide a basis for expanding the potential applications of the asprepared charred material.

2. EXPERIMENTAL SECTION 2.1. Hydrochar and Pyrolysis Char Production. Seven common types of biomass were used as feedstocks: Salix psammophila wood (SW), rice straw (RS), bamboo (BB), soybean straw (SS), pomelo peel (PP), pine needle (PN), and Enteromorpha prolifera (EP). As shown in Table S1 of the Supporting Information, the biomass exhibited various characteristics. The HTC process can be summarized as follows: A mixture of feedstock (15 g) and deionized water (150 mL) were placed into an autoclave. The reactor was programmed to heat until reaching a predefined peak temperature and then hold for the specified retention time. After the reaction, the hydrochar products were washed several times with deionized water. Then, these samples were separated from water using filtration and dried at 80 °C for further analysis. To examine the effect of the HTC temperature, the SW feedstock was carbonized at five different peak temperatures (180, 210, 240, 270, and 300 °C) with 1 h for the different reaction extent of biomass composition. The SW feedstock was carbonized for five different retention times (0, 0.5, 1, 2, and 4 h) at a peak temperature of 240 °C to evaluate the effect of the retention time. To investigate the effect of feedstock, all seven dried and ground feedstocks were carbonized at 300 °C for 1 h. The hydrochar materials thus synthesized were denoted as HX-T-R, where X is the feedstock designation defined above, T is HTC peak temperature, and R is HTC retention time. To activate the hydrochar, the dried samples were thermally treated at 700 °C under a constant flow of N2 (1 L/min) at a heating rate of 4

R 50 =

T50,sample T50,graphite

(1)

where T50,sample and T50,graphite are the temperature values corresponding to a weight loss of 50% by oxidation and volatilization of the prepared sample and graphite, respectively. Graphite (purity of 99.9995%, 100 mesh, Alfa Aesar) was the reference substance. The values of T50,sample and T50,graphite were collected directly from TG thermograms that had been previously corrected for the removal of water and ash content, using eq 230 B

DOI: 10.1021/acs.energyfuels.5b00512 Energy Fuels XXXX, XXX, XXX−XXX

a

180-1 210-1 240-1 270-1 300-1

SW SW SW SW SW HT SW SW SW SW HR RS BB SS PP PN EP HF

20.8 22.9 21.1 21.5 21.9 0.04 23.9 22.9 23.3 24.4 0.05 24.4 20.5 19.9 15.5 20.0 17.2 0.15

yielda (%) 19.9 23.7 18.5 13.1 15.6 0.22 14.7 15.6 13.8 10.8 0.19 37.9 0.44 2.08 4.08 4.45 47.0 1.19

ash (%) 0.06 0.06 0.08 0.05 0.02 0.41 0.11 0.09 0.06 0.07 0.23 0.07 0.01 0.03 BDL 0.10 0.33 1.29

Na (g/kg) 0.85 0.92 0.86 0.55 1.21 0.27 0.98 0.75 0.71 0.65 0.17 5.05 0.10 0.57 1.02 1.53 15.9 1.56

Mg (g/kg) 0.55 0.43 0.34 0.24 0.35 0.30 0.70 0.35 0.33 0.40 0.37 0.57 0.01 0.30 0.14 0.43 3.15 1.55

K (g/kg) 38.0 32.7 30.8 20.0 11.9 0.40 29.1 26.8 21.6 17.0 0.23 7.94 0.24 5.88 6.94 13.2 60.5 1.34

Ca (g/kg) 11.6 11.5 11.6 9.2 8.8 0.13 11.6 11.7 11.2 9.2 0.10 8.7 7.2 9.1 8.9 8.7 11.2 0.13

pH 73.7 70.9 72.4 79.0 81.5 0.06 76.5 77.2 79.1 81.2 0.04 53.9 90.1 90.1 87.6 84.5 44.4 0.25

TC (%)

The yield of pyrolysis char was compared to the original mass of the feedstock. bVmic is the micropore volume.

300-1 300-1 300-1 300-1 300-1 300-1

240-0 240-0.5 240-2 240-4

HTC condition

feedstock 0.36 0.41 0.26 0.12 0.34 0.38 0.30 0.18 0.12 0.13 0.42 0.35 0.17 0.10 0.15 0.14 0.29 0.47

DOC (mg/g)

Table 2. Compositions, Physicochemical Properties, and Structural Characteristics of HDPC Samples 0.63 0.66 0.71 0.99 0.94 0.21 0.72 0.75 0.98 0.92 0.15 1.32 1.00 1.00 1.74 2.38 2.33 0.41

TN (%) 354 324 350 366 315 0.06 369 331 323 406 0.09 207 351 342 333 333 116 0.31

SA (m2/g) 0.24 0.23 0.23 0.37 0.17 0.30 0.24 0.21 0.20 0.27 0.12 0.15 0.19 0.19 0.19 0.19 0.08 0.25

Vt (cm3/g) 0.15 0.15 0.16 0.09 0.15 0.20 0.17 0.15 0.15 0.17 0.06 0.09 0.16 0.16 0.15 0.15 0.05 0.33

Vmicb (cm3/g) R50 0.57 0.59 0.59 0.60 0.60 0.02 0.59 0.60 0.60 0.60 0.01 0.59 0.63 0.63 0.57 0.59 0.52 0.06

18.4 20.4 19.0 21.7 22.7 0.09 22.8 22.6 23.5 25.0 0.10 19.3 25.5 24.1 18.3 20.0 11.6 0.22

CS (%)

Energy & Fuels Article

C

DOI: 10.1021/acs.energyfuels.5b00512 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

in the utilization of HDPC samples, because the natural alkalinity of char can resist soil acidity and improve crop growth and yield.11 In addition, the pH of hydrochar and HDPC samples was independent of the HTC process (HT, R, and F < 0.15). As shown in Figure S1a of the Supporting Information, the ash content of hydrochar samples exhibited a low RF (especially for the BB, SS, and PP samples), indicating separation of the soluble mineral constituent from the feedstock to the liquid phase. After thermal treatment, HDPC products exhibited similar RF values to those of the hydrochar samples, because of the retention of the inorganic fraction and loss of organic fractions during the pyrolysis process. In addition, the ash content of the hydrochar and HDPC samples showed significant variation among the different feedstock types, because of the different mineral constituents in the raw feedstock, as indicated by its high HF (1.30 and 1.19). More importantly, the ash content of HDPC samples was greatly enhanced (expect for the BB sample), which may be desirable for soils that would benefit from mineral inputs and an associated increased pH. 3.2. Major Metal Elements. As with the ash content, there were no significant differences in the concentrations of major metal elements (Na, Mg, K, and Ca) between the hydrochar samples produced at different HTC peak temperatures and retention times (HT = 0.10−0.56 and HR = 0.10−0.50). However, the examined metal elements were greatly influenced by the feedstock type, as indicated by its higher value (HF = 1.38−1.75). After pyrolysis, the major metal elements of the HDPC samples also showed similar trends, with the individual metal elements of the HDPC greatly dependent upon the feedstock type (HF = 1.29−1.56). Thus, to improve soil fertility through the addition of hydrochar and HDPC samples as soil amendment, the selection of feedstock type should be paid greater attention than other reaction conditions (such as HTC peak temperature and retention time). Currently, there is very little information available on the retention ability of metal elements in the HTC process. Hence, the effect of feedstock type on RF was also calculated to investigate the retention ability of metal elements in the hydrochar and HDPC samples, as shown in panels b and c of Figure S1 of the Supporting Information, respectively. It is obvious that the majority of both Na and K (except for the EP sample) were lost within the hydrochar samples, because of the solubilization of the minerals (see Figure S1b of the Supporting Information). However, Mg and Ca exhibited lower solubility than Na and K, especially in the RS and EP samples, respectively. These results also indicated that the retention of metal elements was strongly influenced by the type of metal element and the metallic phases. In addition, the RF values of the HDPC samples were similar to those of the hydrochar samples, indicating lower volatility and loss during the process of thermal activation. Nevertheless, the retention of major metal elements is favorable for increasing the soil pH and cation-exchange capacity (CEC) and, thus, improving soil fertility.12 3.3. Recalcitrance and Stability. The recalcitrance index, R50, was used to evaluate the quality of char for carbon sequestration. Harvey et al. presented the division of R50 into three classes: class A (R50 ≥ 0.70), class B (0.50 ≤ R50 < 0.70), and class C (R50 < 0.50), which have carbon sequestration potentials comparable to graphite or soot, intermediate carbon sequestration potential, and uncharred plant biomass, respec-

Wi ,cor = 100 + [100(Wi ,uncor − W200,uncor) /(W200,uncor − Wcutoff,uncor)]

(2)

where Wi,cor and Wi,uncor are the corrected and uncorrected weights, respectively, W200,uncor is the weight at 200 °C, and Wcutoff,uncor is the weight at the temperature when no further oxidation is apparent. The heterogeneity index, which was obtained from the coefficient of variation (CV), was used to examine the extent to which particular reaction conditions (i.e., HTC peak temperature, HTC retention time, and feedstock type) affected specific properties of the samples more, as calculated by eq 3 H(T,R,or F) =

standard deviation mean value

(3)

where HT is the HTC-peak-temperature-dependent heterogeneity, HR is the HTC-retention-time-dependent heterogeneity, and HF is the feedstock-dependent heterogeneity. Generally, a larger value of H associated with a certain reaction condition suggests that it has a greater influence on the specific property of the sample.28 If the values HT, HR, and HF for a certain property are all less than 0.15, then this evaluated property is independent of the HTC processes. The potential for carbon sequestration (CS) was used to evaluate the retention capability of the final carbon in soil,28 as calculated by eq 4

CS =

yield(%)hydrochar/HDPC C(%)hydrochar/HDPC R 50 C(%)feedstock

(4)

where C(%)feedstock is the percentage of the total carbon contained in the feedstock and C(%)hydrochar/HDPC is the percentage of the total carbon contained in the hydrochar or HDPC sample. The relative enrichment factor (RF) was used to evaluate the degree of enrichment with respect to a specified composition in the resultant products, as calculated by eq 5 RF =

Yhydrochar/HDPC C hydrochar/HDPC 100

Cfeedstock

(5)

where Yhydorchar/HDPC is the yield of the hydrochar or HDPC sample (%), Chydorchar/HDPC is the content of the examined composition in the hydrochar or HDPC sample (g/kg for metal element and % for ash content), Cfeedstock is the content of the examined composition in the original feedstock (g/kg for metal element and % for ash content). Generally, the RF value of the examined composition in a product is less than 1, indicating that the composition exhibits solubilization or volatilization during the preparation process.31

3. RESULTS AND DISCUSSION 3.1. Proximate Analysis. The bulk physicochemical properties of the hydrochar and HDPC samples are shown in Tables 1 and 2, respectively. The yields of the hydrochar samples decreased with increasing peak temperatures and retention times, because of the continuous decomposition of the feedstock component. As suggested by the higher HT value (0.23) than HR and HF (value of 0.16), the yield was more dependent upon te HTC peak temperature. Interestingly, the HTC peak temperature and retention time showed little effect on the yields of the HDPC samples, which were more sensitive to feedstock type, as indicated by the higher HF value (0.15), than both the HT and HR values (0.04 and 0.05, respectively). Acidic pH values were observed for the hydrochar samples, because of the solubilization of the inorganic fraction and acidic products (such as organic acid and phenolic compounds) migrating from the liquid to the solid residue. As expected, after thermal treatment, the HDPC samples exhibited alkaline pH, because of decomposition of the acidic products and separation of alkali salts from the organic composition.32 This is important D

DOI: 10.1021/acs.energyfuels.5b00512 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. (a) Correlation between carbon content (ash free) and recalcitrance index (R50) of hydrochar and HDPC samples. (b) Typical van Krevelen plot of hydrochar and HDPC samples.

tively.30 As shown in panels a and c of Figure S2 of the Supporting Information, increased HTC peak temperatures and retention times led to gradual increases in the T50 of hydrochar samples. Accordingly, the R50 of hydrochar samples increased and the hydrochar became more recalcitrant as the peak temperatures and retention times rose. In addition, hydrochar samples produced with different feedstock types had similar R50 values, and the R50 of hydrochar samples were independent of the HTC process (HT, R, and F < 0.15). As shown in Figure 1a, a positive correlation was obtained between the carbon content (ash free) and the R50 value, indicating that thermal recalcitrance of samples was mainly influenced by its degree of carbonization. Obviously, the majority of hydrochar samples belonged to class C (Table 1); therefore, post-activation for hydrochar is required to enhance its capability for resisting abiotic and biotic degradation. After thermal treatment, the R50 values of the HDPC samples were significantly increased, as indicated in panels b and d of Figure S2 of the Supporting Information. In addition, the R50 values of HDPC samples were independent of the HTC process (HT, R, and F < 0.15). The variation of thermal recalcitrance between hydrochar and HDPC samples was also confirmed by the derivative thermogravimetric (DTG) thermograms. As shown in Figure S3 of the Supporting Information, the DTG curves contained two general regions of weight loss. The first region of weight loss occurred within the temperature range of 300−350 °C, which resulted from the thermal oxidation of residual cellulosic and low-boiling products derived from the HTC process.33 The second region of weight loss was observed above 400 °C and was due to thermal oxidation of the more recalcitrant organic substances with the structure of lignin or thermally produced carbonized/aromatic compounds.30 Obviously, in hydrochar samples derived from higher HTC peak temperature and retention time, the weight loss was focused within the region of much higher temperatures. Such differences indicated that the hydrochar samples produced at higher peak temperatures and retention times were more recalcitrant, which was consistent with the qualitative assessment of the R50 values for those samples. For hydrochar samples produced from the various feedstocks, the weight loss was all focused in the second region (above 400 °C). Only the PN hydrochar sample exhibited a low-temperature peak at around 280 °C; thus, PN hydrochar was less recalcitrant, as confirmed by its R50 value. As shown in Figure S4 of the Supporting Information, the region of weight loss of the HDPC samples was within a higher

temperature range, because of the increased aromatic structure, further confirming changes in the R50. In addition, the HDPC samples produced at differing HTC peak temperatures and retention times exhibited similar regions of weight loss, further indicating that the recalcitrance of HDPC samples was less affected by the HTC process. 3.4. Surface Area and Porosity. The N2 adsorption− desorption isotherms and pore size distribution of selected hydrochar samples and HDPC samples are shown in Figure S5 of the Supporting Information. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, all of the N2 isotherms for hydrochar samples were of type III, suggesting low porosity. After heating, the N2 isotherms for HDPC samples were changed to types I and IV, reflecting the evolution of micro- and mesoporous structures. As shown in Figure S5 of the Supporting Information, it was evident that mesopores (diameter > 2 nm) within the HDPC sample greatly increased and the main mesopores had a diameter of less than 10 nm. As shown in Table 1, the surface area and pore volume of the hydrochar samples were extremely low (