Hydrothermal carbonization reaction severity as an indicator of human-excreta-derived hydrochar properties and it's combustion

Reut Yahav Spitzer; Yonas Zeslase Belete; Hunter A Johnson; Sofiya Kolusheva; Vivian Mau; Amit Gross

doi:10.1016/j.scitotenv.2023.162176

Hydrothermal carbonization reaction severity as an indicator of human-excreta-derived hydrochar properties and it's combustion

Sci Total Environ. 2023 May 10:872:162176. doi: 10.1016/j.scitotenv.2023.162176. Epub 2023 Feb 11.

Authors

Reut Yahav Spitzer¹, Yonas Zeslase Belete¹, Hunter A Johnson¹, Sofiya Kolusheva², Vivian Mau¹, Amit Gross³

Affiliations

¹ Zuckerberg Institute for Water Research, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sde Boker campus, 84990, Israel.
² Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, 8410501 Be'er Sheva, Israel.
³ Zuckerberg Institute for Water Research, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sde Boker campus, 84990, Israel. Electronic address: amgross@bgu.ac.il.

PMID: 36775163
DOI: 10.1016/j.scitotenv.2023.162176

Abstract

Hydrothermal carbonization (HTC) is an emerging technology that may potentially address sanitation problems and energy scarcity. However, the significance of the parameters that govern HTC (e.g., temperature and time) is not fully understood, in particular for human excreta. A simplified coalification model was used to describe the 'strength' of thermal reactions by combining temperature and time into a single parameter, the severity factor. This study is the first to assess the extent to which a severity coalification model can predict the properties of human-excreta-derived hydrochar for a given severity with different combinations of reaction time and temperature. HTC experiments with raw human excreta were undertaken with 50 mL batch reactors at five different severities. Severity was established with different combinations of temperature (180 °C, 210 °C, and 240 °C) and reaction time based on the severity-factor equation. The resulting hydrochars were tested for combustion properties, and the respective gas emission as well as, physicochemical and surface area parameters. Significant correlations were found between severity and yield (R² = 0.88), carbon content (R² = 0.85), and calorific value (R² = 0.90), with the properties being similar for a given severity but varying with different severities. Hydrochar's contact angle increased from 53.1° to 81.3° with increasing SF, while surface area remained low, ranging from <1 to 5.1 m²g^-1, with no definite correlation to SF. Combustion profiles for a given severity were generally similar, but the ignition, peak, and burnout temperatures differed between severities. Gram-Schmidt curves indicated that gas emission profiles are similar for a given severity but vary with different severities. The main gases emitted in combustion were virtually identical in all treatments, and included CO₂, alkenes (C9, C10), CH₄, and H₂O. It is concluded that many properties of hydrochar can be inferred from the severity factor.

Keywords: Combustion; Human excreta; Hydrochar properties; Hydrothermal carbonization; Severity factor.