Figure 2 showed the surface morphologies of the hydrochar at different magnifications. In the traditional HTC process, the structure of hydrochar is related to the water-solubility of feedstock. For water-soluble feedstock, hydrochar was tend to be spherical particles, such as cellulose [11] [12] . As shown in Figure 2(a), the hydrochar appeared as blocks with agglomerated particles attached. The formation of blocks may due to the lack of available space for particle growth at the high concentration of NPEOs. With the concentration decreasing, small particles formed. As shown in Figure 2(b), some particles were hollow hemispherical with small blocks attached. This may suggest that the spherical particles was hollow and some of them was broken. As shown in Figure 2(c), smaller agglomerated particles were also spherical. As shown in Figure 2(d), small spherical particles and blocks were attached to the big spherical particles. The formation of small blocks may result from the big blocks and particles which broke in HTC process.

Various functional groups of the hydrochar were characterized by FTIR. Figure 3 shows the FTIR spectra of the hydrochar. The band at 3438 cm⁻¹ was attributed to -OH stretching vibration in hydroxyl groups. Asymmetric and symmetric -C-H stretching of methylene groups were identified at 2922 cm⁻¹ and 2864 cm⁻¹ [13] , respectively.

The band at 1698 cm⁻¹ implied the stretching vibration of -C=O in ketone and amide groups [14] . The peak at 1450 cm⁻¹ was assigned to the -C=O stretching in aromatic ring carbons. In addition, the peak at 910 cm⁻¹ was attributed to the out-of-plane bending vibration in aromatic nucleus -CH structure, which suggested the occurrence of aromatization during HTC [15] . The peaks at 1159 cm⁻¹ and 1123 cm⁻¹ were attributed to symmetric and asymmetric stretching of sulfonated groups, respectively. S-O stretching at 1370 cm⁻¹, asymmetric C-S stretching at 910 cm⁻¹ and S-O bending vibration at 660 cm⁻¹ related to sulfonic acid group [16] . These suggested the introduction of sulfonic acid group in the HTC process.

The ultimate analysis of the raw waste liquid and hydrochar are listed in Table 2. By comparing the raw waste liquid, C content of the hydrochar increased from 42.17 to 69.48 wt%, showing that C was enriched in the HTC process. H and O content of the raw waste liquid decreased from 10.04 and 47.43 to 5.32 and 20.10 wt%, respectively, suggesting the dehydration in the HTC process. S content of the hydrochar increased from 0.00 to 4.80 wt%, proving that sulfonic acid group was formed in the HTC process.

Figure 4 indicates the classification of the hydrochar as a solid fuel based upon Van Krevelen diagram [17] . For comparison, H/C vs. O/C atomic ratios of Anthracite, bituminous, lignite, and peat were also plotted in Van Krevelen diagram. The hydrochar was in the region of lignite, implying that its element composition was similar to lignite.

Table 3 presents the proximate analyses and the high heat values (HHV) of the hydrochar. The fuel ratio (FC/ VM) ranks carbonaceous product as an alternative fuel. Channiwala and Parikh [18] have developed a unified correlation to predict HHV of solid fuel expressed by elemental composition:

where C, H, S, O, N and A represent the mass percentages on dry basis of carbon, hydrogen, sulfur, oxygen, nitrogen and ash contents of fuels, respectively.

Moisture of the hydrochar was low, because they were obtained after dried. FC/VM was 1.10 and HHV was 28.38 MJ/kg, showing that it could be use as an alternative fuel.

Figure 5 illustrates the combustion profiles of the hydrochar. For a better evaluation of the different combustion behaviors, the entire combustion profile was divided into several stages according to the rate of weight loss in the DTG curve [19] . Table 4 lists the combustion stages, characteristic temperatures and residue of the hydrochar.

As shown in Figure 5, the overall decomposition of the hydrochar was in the temperature range of 85˚C - 657˚C. Stage D exhibited a peak due to moisture content. Stage E were in the temperature range of 85˚C - 342˚C for the devolatilization and combustion. The stage F took place in the temperature range of 342˚C - 647˚C for the further release of VM and char combustion, respectively [20] [21] .

T_i refers to the temperature at which fuels begin to burn. T_i is a critical parameter to minimize fire and explosion as solid fuel [20] . T_m corresponds to DTG peak temperature. T_b refers to the temperature at which the rate of weight loss diminishes to 1 wt%/min at the end of the combustion profile.

T_i of the hydrochar was 185˚C, which implied the higher difficulty of ignition. This can be explained by the low VM [21] . T_m of the hydrochar was 384˚C, so it needed a long reaction time to achieve the maximum decomposition rate. Especially, T_b of the hydrochar was 657˚C, which implies that it had a high HHV and it needed a long reaction time to achieve the complete combustion.

Notes, Stage D―Dehydration process, Stage E―Devolatilization and combustion phase, Stage F―Char combustion phase, Last stage―Burnout phase, T_i―Ignition temperature, T_m―Maximum combustion rate temperature, T_b―Burn out temperature (T_b).