# Influence of Hydrothermal Carbonization on Catalytic Fast Pyrolysis of Agricultural Biomass
## 3 Results and Discussion
It is clear, when looking at the trend shown in Figure 3, that the HTC process starts at temperatures lower than 180 \({}^{\circ}\)C, since the trend line reaches \(Y_{m}=1\) for the temperatures slightly lower than 170 \({}^{\circ}\)C. This is in good qualitative agreement with the work of [PERSON] and [PERSON], reporting carboxyl and carbonyl groups in biomass degrading above 150 \({}^{\circ}\)C [31], and small deviation could be explained by the specifics of the feedstock in this particular study, as well as by the fact that 3.5 \({}^{\circ}\)C tolerance for the temperature, controlled manually, to some extent influenced the results, thus indirectly influencing the slope of the trend line in Figure 3.  
Energy yields, shown in Figure 4, are higher than respective mass yields, which is caused by the fact that \(Y_{e}\) is the result of the multiplication of \(Y_{m}\) by respective energy densification ratio (ED) [85; 86]. Interestingly, the slope for \(Y_{m}\) is much steeper than the slope for \(Y_{e}\), implying a more extensive difference between \(Y_{m}\) and \(Y_{e}\) for higher HTC temperatures, thus implying ED increases with higher temperatures of the HTC process. This has been confirmed by many studies [87; 88; 89]. For both \(Y_{m}\) (Figure 3) and \(Y_{e}\) (Figure 4), the best fit was achieved by a linear function. On the other hand, the best fit for ash yield (Figure 5) was achieved using a 2 nd-order polynomial. Moreover, the polynomial reaches values close to 1.0 at temperatures below 100 \({}^{\circ}\)C. This seems sensible since \(Y_{a}<1\) indicates that a part of the inorganic content of raw biomass was washed out during the treatment, and it has been reported in the literature that washing biomass with hot water can remove a part of its ash [90; 91]. Overall, HTC treatment led to a decrease in volatile matter content and a simultaneous increase in fixed carbon content (Figure 6), which is typical for HTC of many different types of biomass, as reported in the literature [92]. Moreover, according to expectations, HTC treatment led to an increase in higher heating value (HHV), as shown in Figure 7, which has also been reported in the literature [93; 94]. Figure 4: Energy yield for the performed HTC experiments. Figure 3: Mass yield for the performed HTC experiments. HTC of miscanthus, in line with the expectations, led to an increase in carbon content and a simultaneous decrease in the oxygen content of the valorized samples on a dry ash-free basis (Figure 8), when compared to the raw biomass. Since HHV and C content are not affected by the presence of a dry ash-free basis, the HVC and HVC are not affected by the presence of dry ash-free basis. Figure 5: Ash yield for the performed HTC experiments. Figure 6: Proximate analysis of the raw sample, before HTC, and hydrochars obtained at respective temperatures. Figure 7: Higher heating values of the raw sample, before HTC, and hydrochars obtained at respective temperatures. are strongly correlated [95,96], this is a good explanation of the observed HHV increase (Figure 7). A decrease in the oxygen content seems to be beneficial in terms of subsequent pyrolysis of such materials since it has been reported in the literature that high oxygen content of biomass has a detrimental influence on the quality of pyrolysis oil, including parameters such as pH and viscosity [23,74], thus influencing combustion as well as injection and atomization properties of the pyrolysis oil [97,98]. [PERSON]'s diagram (Figure 9) shows clearly that the HTC process resembles the natural process of coalification, judging by the trend and respective location of raw miscanthus and hydrochars obtained at different temperatures, with respect to peat, lignite, and hard coal. The hydrochars were still in the typical area of biomass [88,99, 100, 101, 102]. However, what needs to be taken into account is the initial location of the raw miscanthus.