## 3 Results and Discussion
Pressure fluctuations in the system depend mainly on the reactor volume, solid particle size distribution, as well as the kinematic viscosity and flow rate of the reactor slurry. Therefore, pressure drops can be significantly used to indicate the degree of reactor clogging. An overall high ash content increases the probability of flow clogging in a tubular reactor (>24.1%). Additionally, the possibility of clogging increases with the increasing kinematic viscosity (>8.6 mm\({}^{2}\)/s) and solid particles' diameter. Therefore, the ash content has a significant impact on the possibility of reactor clogging. Potentially, this problem can be solved by increasing the heating efficiency of the reactor, allowing rapid liquefaction of the feedstock, reducing the ash content of the  
Figure 4: Effects of temperature for (**a**) 600 K and (**b**) 400 K on biocrude oil (BC) and aqueous phase (AQ) gas, and Vt, volatile yield (wt.%). Figure 5: Effect of the slurry flow rate in the axial direction on the reactor temperature (**a**). Effect of the slurry flow rate in the axial direction on the convective heat transfer coefficient and the outlet temperature in the tubular reactor (**b**). feedback, or increasing the tube diameter. Figure 6b shows the temperature distribution of the reactor, where it can be seen that this temperature gradient was mainly due to heat transfer from the reactor wall to the slurry mixture, which had a temperature of about 300 K at the reactor inlet. For the most part, the sludge suspension behaved according to the thermodynamic properties of subcritical water. In Figure 6c, using contours plot of the gas mass fraction in the external view of the reactor and its cross-section, it can be seen that the fluid velocity was lower at the reactor wall and the reactor wall was warmer, which accumulated more gas. In Figure 6c, the biocrude oil concentration is presented, which was irregularly distributed in the middle of the tubular reactor due to hydrodynamic conditions related to mass and heat exchange in the mass flow. Trajectories of the solid particles were significantly affected by the ash flow and the solid particle size, and thus the trajectories of particles were streamlined. Figure 6: _Cont_. Figure 7 shows the effects of time and temperature (a) and of time and the sludge flow rate (b) on biocrude yield. As the time of the conducted reaction and the temperature increased, the biocrude oil yield increased, with the increase in temperature playing a much greater role than the time of the HTL process. The hydrolysis and dissolution stage of the solids proceeded rapidly, reaching completion after 7 min at 330 \({}^{\circ}\)C, 43.2% of the solids were dissolved. At temperatures above 500 K over 3 min, 58.3% solids' conversion was achieved. At a predicted temperature of 300 \({}^{\circ}\)C, the gas yield increased from 6.6 wt.% to 20.2 wt.% as the reaction time increased from 1 to 60 min. Temperature played a more significant role in obtaining a higher gas yield during the process time. For a given reaction time of 1 min, the gas yield increased from 6.6 wt.% to 65.0 wt.% when the temperature increased from 300 \({}^{\circ}\)C to 600 \({}^{\circ}\)C. Therefore, the highest temperature increase (600 \({}^{\circ}\)C and 60 min) and highest gas yield (66.9 wt.%) were obtained. Figure 7b shows the effects of flow rate and temperature on the biocrude oil yield produced. As the flow rate increased, the yield of biocrude oil decreased because the residence time of the slurry in the reactor decreased. There are two opposite inhibiting processes at play here: temperature and flow rate, with flow rate having a much greater role in the decreased yield of biocrude oil produced. Figure 6: Contours plot of slurry velocity in the external view of the reactor and in the cross-section, m/s (a), slurry temperature in the external view of the reactor and in the cross-section, K (b), biocrude mass fraction in the external view of the reactor and in the cross-section (c).