Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Optimization of bio-oil production from solid digestate by microwave- assisted liquefaction M. Barbanera a,b, ⁎ , C. Pelosi c , A.R. Taddei d , F. Cotana b a Department of Economics, Engineering, Society and Business Organization, University of Tuscia, 01100 Viterbo, Italy b CIRIAF – Biomass Research Centre – Department of Engineering, Via G. Duranti 67, 06125 Perugia, Italy c Department of Economics, Engineering, Society and Business Organization, Laboratory of Diagnostics and Materials Science, University of Tuscia, 01100 Viterbo, Italy d Interdepartmental Centre of Electron Microscopy (C.I.M.E.), and Department of Environmental Sciences, University of Tuscia, 01100 Viterbo, Italy ARTICLE INFO Keywords: Microwave Liquefaction Solid digestate Bio-oil Response Surface Methodology ABSTRACT Microwave-assisted direct liquefaction of solid digestate was carried out in polyethylene glycol and glycerol employing sulfuric acid as catalyst, in order to convert it into a biofuel. Response Surface Methodology (RSM) coupled with Box-Behnken Design (BBD) with a total of 15 individual experiments was used to optimize the conditions of three independent variables (temperature, reaction time and solvent-to-biomass ratio) related to the bio-oil yield, the higher heating value (HHV) of bio-oil and the energy use of microwave treatment. Desirability function was employed to determine the optimal reaction conditions of the liquefaction process. The results showed that at the optimal conditions the bio-oil yield, the HHV of bio-oil and energy use were re- spectively 59.38%, 28.48 MJ/kg, and 115.93 Wh. The predicted responses showed a good compliance to the obtained experimental data. The optimized bio-oil was further characterized using FTIR analysis while the properties of the solid digestate and the liquefaction residue were analyzed by means of SEM analysis. 1. Introduction Nowadays, global challenges related to resources depletion, air pollution, and cost of oil require significant improvements in the search of alternative renewable energy resources. Furthermore, since it is ex- pected that the world energy demand will double by 2050 [1], the research on biomass-based fuels is one of the trending strategies to fulfill energy requirements. Currently, the significant increase of ex- ploitation of anaerobic digestion plants for biogas or biomethane pro- duction is leading to the development of a significant environmental issue related to the digestate disposal [2]. Up to now, the solid fraction of digestate has been mostly used as a biofertilizer without any further processing because its contains high content of nutrients (N, P, K) and organic matter [3]. However, its overuse can cause significant en- vironmental threats to the soil matrix and to human beings [4] due to ammonia emissions and nitrate and heavy metals leaching as well as the presence of pathogens and land usage [5] Therefore, in order to overcome these drawbacks it is necessary to develop new strategies for sustainable recovery of solid digestate. Whereas the energy efficiency of the anaerobic digestion process is rather low (33–50%) [5], digestate could be conveniently employed for energy purposes. In most biogas plants, digestate is separated into two fractions, liquid (rich in nitrogen) and solid (rich in lignin), being the latter suitable for thermo-chemical processes, such as pyrolysis, gasification and direct liquefaction, in order to convert it into biofuels (e.g. bio-oil) or gas fuels (e.g. H 2 ) [6]. However, pyrolysis process shows certain disadvantages, due to the high operating temperature which could lead to cross-linking reactions between hydrocarbons and aromatics, decreasing the bio-oil yield [7]. Besides the digestate remarkable ash content in digestate can cause significant slagging and fouling problems during the gasification reac- tions [8]. On the other hand, direct liquefaction of biomass has at- tracted wide research interests since biomass can be successfully con- verted into multifunctional bio-oil at milder temperatures and in presence of solvents, such as phenol, monohydric alcohols and poly- hydric alcohols, and acid or basic catalysts [9,10]. In the liquefaction process, cellulose, hemicellulose and lignin are reduced into fragments of smaller molecules but the effective reaction pathways are scarcely detectable; it is assumed that polysaccharides are firstly degraded into glycosides and then hydrolysed to form levulinic glycosides. Instead lignin affects the recondensation reactions after the liquefaction step leading to the formation of insoluble precipitates [11]. Microwave (MW) assisted liquefaction has been proven to be an interesting alternative to conventional heating, because it allows to obtain a uniform fast internal heating, to accelerate the kinetic reaction https://doi.org/10.1016/j.enconman.2018.06.066 Received 11 April 2018; Received in revised form 28 May 2018; Accepted 18 June 2018 ⁎ Corresponding author at: Department of Economics, Engineering, Society and Business Organization, University of Tuscia, 01100 Viterbo, Italy. E-mail address: m.barbanera@unitus.it (M. Barbanera). Energy Conversion and Management 171 (2018) 1263–1272 0196-8904/ © 2018 Elsevier Ltd. All rights reserved. T