Comparative studies on catalytic hydrothermal liquefaction of mixed household waste into bio crude

The hydrothermal liquefaction (HTL) of mixed household waste (MHHW) was carried out at varying temperatures from 260 to 360 °C, residence times of 30–90 min, and catalyst loads of 2.5–12.5 wt. %, in the presence of four heterogeneous catalysts, namely, bentonite clay, diatomaceous earth (DE), nanoporous ZnO (nano ZnO), and zeolite ZSM-5. Maximum bio crude yield of 48.10% was obtained at 360 °C and 75 min, in the presence of 5 wt. % nano ZnO. Among the naturally occurring catalysts, DE yielded a bio crude percentage of 48.52% at 12.5 wt. %, 360 °C, and 75 min. The addition of catalysts increased the bio crude yield and inhibited the formation of bio char and gaseous products. Also, the bio char and bio crude obtained from catalytic HTL (cat-HTL) presented with higher carbon and hydrogen content, thus possessing a significantly higher H/C ratio and HHV. Among cat-HTL, DE and nano ZnO yielded higher carbon and energy recovery percentage for bio char and bio crude, respectively. Also qualitatively, cat-HTL in the presence of DE yielded a higher fraction of hydrocarbons, when compared with non-cat HTL and cat-HTL in the presence of nano ZnO. N-containing compounds, phenols, furfurals, etc., were found to be present in the range of 16.2 to 18.6% for all bio crudes.