Hydrothermal carbonization of sludge: Effect of steam release on products properties and wall sticking phenomenon

In order to understand hydrothermal carbonization (HTC) process more effectively, the effect of steam-release process at different temperatures (160, 180, 200, and 220 degrees C) on product properties and wall sticking phe-nomenon were studied. Experimental results indicated that the approximate equilibrium moisture content (AEMC) of hydrochar obtained with (SRHC) and without (HC) steam-release process were both negatively correlated with zeta potential (R = -0.96326 for HC and R = -0.95825 for SRHC). More importantly, the absolute value of the zeta potential of SRHC was lower than HC indicating steam-release process significantly improve dewatering performance of sludge after HTC treatment. The results of FE-SEM and N2 adsorp-tion-desorption experiments indicated steam-release process was favorable for the formation of porous hydro-char. The variation of surface functional groups showed that the intensity of all peaks in SRHC was significantly lower than HC, indicating steam-release process which significantly affects surface chemical properties of samples. As for liquid, TOC, TN and COD value of the condensate were lower than filtrate, indicating steam-release process which produces "cleaner" condensed liquid with less organic matter content. The wall sticking phenomenon in the steam-release process became more severe with increasing temperature and the amount of sticky wall sample (SWS) increased from 0.392 g at 160 degrees C to 2.131 g at 220 degrees C, which indicated that tem-perature was key factor affecting the sticking phenomenon on the inside wall of the reactor. The contact angle of SWS and SRHC decreased from 60.083 degrees and 77.283 degrees at 160 degrees C to 51.317 degrees and 67.967 degrees at 220 degrees C, and the polar free energy of SWS (12.52-14.00 mN/m) was significantly higher than that of SRHC (1.85-4.53 mN/m), indi-cating increasing hydrophobicity and reducing polar free energy of sample were key factors in improving sticking phenomenon.