­­Optimisation of sugar and solid biofuel co-production from almond tree pruning by acid pretreatment and enzymatic hydrolysis

Abstract

Abstract Almond pruning is an important agricultural residue that has been scarcely studied for the production of sugars and solid biofuels. In this work, the production of monosaccharides from almond tree pruning has been optimised by a two-step process scheme: pretreatment with dilute sulphuric acid (0.025 M, at 185.9-214.1 ºC for 0.8–9.2 min) followed by enzyme saccharification of the pretreated cellulose. The application of a response surface methodology allowed mathematical modelling of the hydrolysis, setting pretreatment conditions that maximise both the amount of sugar in the acid prehydrolysate (22.8 kg/100 kg raw material, at 194 ºC for 2.08 min) and the enzymatic digestibility of the pretreated cellulose (45.4%, at 210 ºC for 8 min). The highest overall sugar yields (36.8 kg/100 kg raw material, equivalent to 64.3% of all sugars in the feedstock) were obtained with pretreatments carried out at 197 ºC for 4.0 min, which implies lower energy consumption than previously reported for the same biomass. Under these conditions, moreover, the final solids showed much better properties for thermochemical utilization (22.0 MJ/kg heating value, 0.87% ash content, and 72.1 mg/g moisture adsorption capacity) compared to those of the original prunings.