The main objective of this work was to compare the difference of chars produced via LTP and HTC in terms of their product yield and properties. For simplifying, char samples were named as following the format such as “Process Temperature” (e.g., H220 stands for hydrothermal carbonization conducted at 220 °C, whereas L220 stands for low temperature pyrolysis conducted at 220 °C). The proximate analysis, HHV, product yield and fuel rate are summarized in Fig. 1(a,b).
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Although processed under the same reaction temperature and holding time, a higher yield of chars was observsevd with LTP. The downtrend of hydrochar yields lowed down once the reaction temperature of HTC exceeded 260 °C, while this phenomenon occurred near 340 °C in the LTP process. The results indicate that cellulose degraded significantly before 260 °C by HTC and 340 °C by LTP. It is similar to the previous study9. Regardless of the processes, the fixed carbon content increased, and the volatiles content declined with the increments of reaction temperature. For example, H320 has 52.10% fixed carbon and 47.35% volatiles. L440 has 71.25% fixed carbon and 25.00% volatiles. Compared to raw materials (13.56% fixed carbon, 85.47% volatiles), they both changed significantly.
In the LTP process, the ash content increased with increase in temperature as a result of the release of volatiles. Interestingly, the ash content of HTC did not show the same growth trend. Specifically, under the same reaction temperature, the ash content in HTC experiments was significantly lower than that of the LTP and fluctuated between 0.13% and 0.61%. For instance, even maximum ash content (0.61%) of HTC which was observed with H200 was much smaller than that of the L200 (1.05%). It could be explained by the dual effects by elution and accumulation of the ash. As is reported in Fig. 1(a), regarding the ash content, the ash removed by subcritical water is greater than its accumulation in chars.
The change of HHV versus temperature was similar to that of the fixed carbon. It proves that the content of fixed carbon is a major factor determining the HHV. In this study, the maximum HHV of the chars from HTC reached 29.61 MJ/kg (H320), and the maximum HHV of the chars from LTP reached 31.40 MJ/kg (L440). In the temperature range of 180–320 °C, the fuel rate of both HTC and LTP significantly increased from 0.16 to 1.10 and 0.84, respectively. The fuel rate of hydrothermal carbon has been higher at lower temperatures, such as H260 which had a fuel rate of 0.74, while the L260 has fuel rate of 0.24. In comparison, at the higher reaction temperature, chars from LTP had a higher fuel rate. For example, the fuel rate of L440 reached 2.85.
With the results from the proximate analysis, HHV as well as char yield, it was found that regardless the process of HTV or LTP, the rising tendency of fixed carbon content was highly consistent with that of HHV, both the tendencies of volatiles and char yield were similar. In addition, the char yields of H200 and L300 were quite similar (66.50% and 66.74%, respectively). While the chars from L300 has better basic fuel characteristics with 30.59% fixed carbon content (which was 15.71% of H200), 23.43 MJ/kg HHV (which was 20.54 MJ/kg of H200) and 0.45 fuel rate (which was 0.19 of H200). The HHV of H240 was closed to that of the L300 (23.99 MJ/kg and 23.43 MJ/kg), and both were higher than that of the standard lignite (23.00 MJ/kg). As was shown in Table 1, H240 and L300 have extremely similar composition of elements which could explain the similarity of HHV. However, H240 has lower fixed carbon content (26.07% to 30.59% of L300) and higher volatiles content (73.70% to 67.80% of L300). It shows that compared with L300, H240 has stronger ignition performance and the higher heat release in the same heat production condition. The HHV of H300 and L340 were similar (28.52 MJ/kg and 28.73 MJ/kg) and reached the HHV of bituminous which was 27.17 MJ/kg29. In the comparison of elements, H300 has more C content which was 73.03% (65.59% of L340) and less O content which was 20.61% (28.05% of L340). In addition, a group of chars samples which were H220 and L220 under the same reaction temperature were selected for process comparison. It was found that H220 has slightly higher HHV and fuel rate but has lower chars yield. The characteristics compared to the latter are based on these groups.
A close inspection of the raw material and selected samples was performed using scanning electron microscopy (SEM) technique. Images shown in Fig. 2 reveals different transformations in the morphologies of the hydrochars versus the pyrolytic carbon. As the figure shows, the structure of the raw material is less porous than the char samples of HTC and LTP. In the image of H200, H220 and H240, the hydrochars contained aggregates of spherical microparticles. These microparticles are originated from the decomposition of cellulose, and the subsequent precipitation and growth into spheres30. However, in the image of H300, these spherical microparticles almost disappeared completely. This is most likely due to that they have been broken down into small molecules and was dissolved in the liquid phase under this temperature. Comparing the images of L220, L300, and L340, it was found that higher reaction temperatures of LTP contributed to the formation of pores and resulted in increased roughness on the biomass surface. By the result of char yield and SEM image from Fig. 2 (H220 and L220, H300 and L300), it could be concluded that the lignocellulosic structure of the HTC was decomposed much more significantly than LTP under the same reaction temperature.
As shown by the results of the elemental analysis in Table 1, regardless of the process of HTC or LTP, carbonization caused the aggregation of carbon element and the releasing of hydrogen and oxygen. To analysze the variation in elemental composition, the ratios of hydrogen to carbon (H/C) and oxygen to carbon (O/C) of selected samples were ploted in Fig. 3 in the form of “Van Krevelen diagram”31. The diagram confirms that both of the HTC and LTP had a significant effect on the elemental composition of the products. Specifically, in Fig. 3, after the treatment of HTC, when the reaction temperature increased to 300 °C, the atomic H/C and O/C ratios dropped from 1.55 and 0.73 to 0.82 and 0.21, repectively. The atomic H/C and O/C ratios of L340 were 0.77 and 0.22, respectively. Four typical coals such as anthracite, bituminous, sub-bituminous, and lignite were used for comparision with the prepared chars. It was found that the H/C and O/C ratios of H300 and L340 were in the sub-bituminous region. In addition, with the increasing of temperature, the H/C and O/C atomic ratios of the samples decreased and became closed to that of coal.
Comparing the fiber analysis results of H220 and L220 (Table 1), it is obvious that most of the hemicellulose was removed at a lower temperature in the HTC process. However, the hemicellulose content in L220 is very close to that of the feedstock, indicating that most of the hemicellulose in pyrolytic carbon was not degraded in 220 °C via LTP process. This also explains the fact that the yield of hydrothermal carbon is generally higher than that of pyrolytic carbon at the same temperature (Fig. 1b). Therefore, it was considered that the degradation of hemicellose is the critical factor which influenced the results of the proximate analysis, char yield, and HHV in lower temperature region. Regadless of the processes, the content of cellulose was always experienced an initial stage of ascending and a subsequent desending. For example, the content of cellulose (40.92%) in H200 was higher than that of the raw material (34.04%) and H240 (34.21). For the LTP treatment, the content was 43.06% in L300 was higher than that of L220 (36.45%) and L340 (30.28%). Above all, it can be concluded that the increasing of cellolose content in lower temperature region was mainly due to the removal of hemicellulose. The subsequent decrease in the content of cellulose is mainly due to the decomposition of the cellulose.
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The FT-IR spectra of the selected samples are shown in Fig. 4. It could be observed that the raw materials and treated samples showed similar spectral patterns, however, the intensities of infrared absorption peaks are differnt. Besides, it is worth noticing that due to the presence of CO2 during the measurement, a peak around – cm−1 was observed. In region 1, due to the vibration of –OH stretching, the peak between cm−1 and cm−1 can be found. It is obvious that there was a decrease in the intensity of –OH peak with the increase in the reaction temperature regardless of HTC or LTP32. The peaks between and cm−1 represent the aliphatic groups of CH, CH2 and CH3, respectively33. The content of aliphatic compounds was increased by the HTC treatment as slight incerase of the peak intensity was found, while it was not obvious with the LTP process. Various functional groups were identified in region 3. The peak around cm−1 represents the C–H stretching vibration and deforming vibration of cellulose and hemicellulose. With the rising reaction temperature, the dehydration and depolymerization of cellulose and hemicellulose caused the decrease in peak strength. The peaks between cm−1 and cm−1 are the characteristics of C–O–C vibrations in the cellulose and hemicellulose. These peaks shrinked with the increase in temperature by both HTC and LTP processes, which proved the decomposing of cellulose and hemicellulose34. In addition, compared with the raw material, new absorption peaks appeared at – cm−1 prove the presence of C–OH in alcohol groups33. The peak appeared at cm−1 (– cm−1,C–O in methoxy) decreased, because the C–O band was broken due to the decarboxylation reation. It indicated that lignin was degraded during HTC and LTP processes. The peaks appeared at 765 cm−1 could be assigned to the C–H group in substituted aryl, which was observed in the samples treated with higher reaction temperatures such as H300 and L340. It means the aromatization of lignin was more pronounced in H300 and L340. Comparing H300 and L300 which were prepared under the same reaction temperature, it was found that H300 showed weaker absorption peaks of –OH (– cm−1) and C–H which at cm−1, while had stronger absorption peaks of C–H at 765 cm−1. It shows the HTC process enabled stronger carbonization at the same reaction temperature.
The ash content and composition of feedstock are important indexes which need to be assessed for fuel utilization. Most lignocellulosic biomasses are rich in inorganic elements which are the main components of ash in raw materials and chars. These inorganic elements, including magnesium (Mg), sulfur (S), calcium (Ca), manganese (Mn), copper (Cu), zinc (Zn) and iron (Fe), often existed in the biomass in the oxidized forms, MgO, SO3, CaO, Mn3O4, CuO and Fe2O3, respectively. These can cause slagging, fouling and corrosion of boiler. It has shown in Fig. 1(a) that the ash contents in HTC samples were significantly lower than that of the LTP samples prepared under the same reaction temperature and the values are fairly low (0.13–0.61%), which is due to the leaching of inorganic compositions. The different behavior observed for the LTP process is the consequence of the concentration effect. Table 2 shows the effect of HTC and LTP processes on the inorganic composition yields of hydrochar and pyrolytic carbon. The results suggest that HTC and LTP are different in the mechanism of yielding inorganic elements. It is evident that the HTC process removed the considerate amount of the inorganic components (26–90%) from the raw feedstock, especially magnesium, calcium and manganese. In comparison, the contents of inorganic elements in pyrolytic carbon were higher than that of the raw material. With the increase in reaction temperature, the volatiles was separated from the sample and the inorganic elements were hence concentrated in the produced chars. The observations are in coherence with the finding reported in the literature9,24,25.
To evaluate the effects of HTC and LTP processes on the combustion properties, the samples were subjected to thermo-gravimetric (TG) analysis under an oxidizing atmosphere. The TG and DTG curves of raw materials, pyrolytic carbon and hydrochar are shown in Fig. 5(a–h), respectively. For clarity, the regions at temperatures lower than 200 °C (L340 and H300 were lower than 150 °C) and higher than 350 °C (L340 and H300 were higher than 300 °C) are excluded. Table 3 presents the corresponding combustion parametres for selected samples obtained from TG and DTG analysis. Ti (ignition temperature) means the lowest temperature of the combustion reaction. Temperature interval was defined as the range where the sample mass decreasing rates are above 1%/°C. Vmax and Tmax were the maximum value of weight loss rate and its corresponding temperature. ΔT was the maximum value of the temperature difference between actual temperature and set temperature due to the burning of the sample. In the pyrolytic carbons, it was observed that the ignition temperature of the sample decreased from 265 °C to 221 °C when the pyrolysis temperature increased. Hydrothermal carbon also showed a similar trend in the change of ignition points. Generally, in a wider pyrolysis temperature range (<800 °C), the ignition point of the fuel shifted to a higher temperature range with the deepening degree of carbonization and decreasing content of volatiles. However, selected samples did not show the similar trend. It implies that the most important factor affecting the ignition is not the content of volatiles in the lower temperature range.
It could be deduced that smaller particle size and less moisture content could effectively promote the combustion of the fuel and lower the ignition temperature. Figure 6 shows the particle size distributions of the selected samples. The results indicate that the percentage of smaller size particles (<100 μm, 100–250 μm and 250–600 μm) increased in both processes. In the LTP process, larger particle size (>600 μm) was reduced from 34.27% (Raw) to 6.14% (L340). It decreased to 9.95% (H300) by the process of HTC. Interestingly, In the range of 100–250 μm, the yields of chars showed an opposite trend which increased from 61.81% (Raw) to 78.11% (L340) in LTP while decreased to 47.68% in HTC. It is mainly due to the fact that a large number of particles was received in the range of 0–250 μm by HTC. At the same reaction temperature, the HTC samples showed higher pulverization than those obtained via LTP process. For example, L300 produced 12.84% pyrolytic carbons in range of 100–250 μm while the H300 produced 32.94% hydrothermal carbon. In the range of less than 100 μm, L300 received almost no product while H300 had 9.42% hydrothermal carbon. Altogether, through the particle size distribution analysis, it was found that the percentage of smaller size particles increased in both of processes, and it was considered to be one of the main reasons for the decreasing of the ignition temperature.
Moisture resistance is one of the important indicators of biomass fuels. Higher moisture resistance can reduce transportation costs and improve the combustion characteristics of the fuel. Figure 7(a,b) show the moisture uptake profiles of all samples under the storage conditions of 30 °C and 70% relative humidity within 10 h. It is evident that the moisture of the chars reached saturation in 10 h. Compared with the raw material, the treatments of LTP and HTC both improved the moisture resistance of the chars. In Fig. 7(a), it was found that the hydrophobicity of the samples increased with the increase in reaction temperature by LTP before 280 °C. For example, the saturated moisture content of the chars decreased from 8.73 wt.% to 3.88wt.%. Interestingly, this decreasing trend did not continue in the higher temperature region (280 °C–440 °C). Instead, the saturated moisture content was increased to around 5 wt.% (4.90 wt.% for L420 and 5.22 wt.% for L440). In Fig. 7(b), it was evident that saturated moisture content decreased from 8.73 wt.% to around 2 wt.%. In addition, it is worth mentioning that the hydrophobicity of H260, H280, H300, and H320 were quite consistent. It was also found that the change of fixed carbon, volatiles and HHV slowed down. The reason might be that large amount of cellulose and hemicellulose degraded before 260 °C by HTC. Above all, H260 is a better choice as a fuel candidate in terms of energy consumption, char yield, fuel rate, and hydrophobicity. Comparing L200 and H200, it was found that H200 had lower saturated moisture content (4.83%) than that of L200 (7.26%). Comparing L300 and H300, it was also found that H300 had lower saturated moisture content (2.14% for H300, 4.14% for L300). Based on the results, under the same reaction temperature, the chars produced by HTC have stronger hydrophobicity than that of the chars prepared by LTP. Therefore, through both the analyses of particle size distribution and moisture resistance, particle size distribution and moisture resistance have more serious effects on the ignition temperature than the degree of carbonization.
Similar to the change of the ignition point, Tmax also decreased with the increase in reaction temperature and reduced from 268 °C to 224 °C by LTP. While the Vmax had the opposite trend which increased from −1.43E-02%/s to −1.00E-02%/s. With the deepening of reaction degree by LTP process, the temperature interval of the resulting charcoal was increased from 20 °C (261 °C–281 °C for Raw) to 27 °C (250 °C–277 °C for L340 and 221 °C–248 °C for L300). In selected HTC samples, a similar pattern of Tmax, Vmax and temperature interval was not shown. However, ΔT of carbon samples of HTC and LTP showed consistent trends. Temperature differences of pyrolytic carbon samples and hydrothermal carbon samples increased from 45 °C (Raw) to 63 °C (L340) and 62 °C (H300), respectively. It was considered that ΔT has a close relationship with HHV.
Integrated combustion characteristics index (S) was used to further evaluate the combustion performance of HTC and LTP in the same reaction temperature. It could be described by the following formula:
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