Analysis and optimisation of a novel 'almond-refinery' concept: Simultaneous production of biofuels and value-added chemicals by hydrothermal treatment of almond hulls

For the first time, this work investigates the achievability of developing a biorefinery concept around almond hulls by hydrothermal treatment (HTT), thoroughly scrutinising the influence of the temperature (200-300 degrees C), pressure (100-180 bar), time (20-180min) and solid loading (5-25 wt%). This process allowed the conversion of almond hulls into four main products: gas (2-13%), bio-oil (2-12%), aqueous (4-69%) and hydro-char (17-89%). The gas consisted of a mix of H-2, CO2, CO and CH4 with a LHV fluctuating from 1 to 13 MJ/m(3) STP. The bio-oil comprised a mixture of alkanes, aldehydes, ketones, phenols, furans, benzenes and nitrogen compounds with a HHV between 21 and 31 MJ/kg. The solid product resembled an energetic hydro-char material (HHV 21-31 MJ/kg), while the aqueous effluent comprised a mixture of value-added chemicals, including saccharides and small oxygenated compounds. The production of biofuels can be maximised at 256 degrees C and 100 bar, using a 5 wt% solid loading for 157 min, conditions at which 43% of the original feedstock can be converted into an elevated energy-filled bio-oil (11% yield, 30MJ/kg), along with a high energetic hydro-char (32% yield, 29 MJ/kg). Regarding value-added chemicals, up to 10% of the almond hulls can be converted into a bio-oil with a high proportion (45%) of phenolic species at 250 degrees C and 144 bar with a solid loading of 5 wt% for 167 min. In comparison, a sugar-rich (81 C-wt%) solution can be produced in high yield (54%), by treating a 24 wt% suspension at 252 degrees C and 180 bar for 153 min. Therefore, the versatility, novelty and intrinsic green and holistic nature of this 'almond-refinery' concept exemplify a landmark achievement in future energy and chemicals production from biomass, which might help render the complete bio-refinery for almond hulls more cost-effectively and ecologically feasible. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

This study investigated the feasibility of developing a biorefinery concept around almond hulls through hydrothermal treatment, converting them into gas, bio-oil, aqueous, and hydro-char. Optimal production conditions were found to be at 256 degrees C and 100 bar, with a 5 wt% solid loading for 157 minutes, resulting in 43% conversion of the original feedstock into high-energy bio-oil and high calorific hydro-char.