# Influences of the Pretreatments of Residual Biomass on
## 1 Introduction  
The EU's Renewable Energy Directive (RED II) has set the goal of achieving a 14% renewable energy share in the transport sector by 2030 [1], and residual biomasses and agro-industrial waste can be exploited as sources to produce sustainable second generation biofuels [2], which are expected to significantly reduce greenhouse gas emissions [2, 3]. The gasification of residual biomass, to produce advanced biofuels, is a promising technology to achieve the goals of RED II. Gasification is a mature thermochemical conversion process suitable for biomasses, with syngas (mixture of H\({}_{2}\), CO, CO\({}_{2}\), CH\({}_{4}\), possibly diluted by steam and/or N\({}_{2}\)[4]) as the main product; syngas is primarily used to generate heat and electricity, and is potentially exploitable to synthesize advanced biofuels (the latter option has not yet been fully implemented at the industrial scale) [5]. Gasification consists of partial oxidation of the carbon contained in the biomass (or in other carbonaceous fuels) at high temperature (750-1150 \({}^{\circ}\)C [6]), using a controlled amount of an oxidant agent (air, pure oxygen, steam, or mixtures of them) [6]. Pure oxygen ensures the production of a high heating value and nitrogen-free syngas, the latter feature being advantageous for the synthesis of biofuels; however, the provision of pure oxygen requires an air separationunit (ASU), which is usually associated with high capital and operational costs [5]. The chemical looping gasification (CLG) process is a new gasification concept, which avoids nitrogen dilution without requiring an ASU and allows for decent fuel conversions [5; 7; 8]. The difference between CLG and conventional gasification methods is represented by the oxygen source: the gaseous oxidant agents are replaced by the lattice oxygen provided by metal oxides (Me\({}_{\text{X}}\)O\({}_{\text{y}}\)) [9]. These metal oxides for CLG are called oxygen carriers (OC). A suitable reactor configuration for CLG is the dual fluidized bed reactor: one fluidized bed works as a gasifier (steam and/or CO\({}_{2}\) as fluidizing agents, while the OC particles provide oxygen); the other bed works as a burner (air as the oxidizing and fluidizing agent, the OC recovers oxygen); the OC particles continuously circulate from one bed to the other [10]. The ongoing European research project CLARA (chemical looping gasification for sustainable production of biofuels, G.A. 817841 [11]) aims to develop an efficient technology to produce liquid fuels by the Fischer-Tropsch synthesis [12], which converts the syngas obtained by the CLG of selected biogenic residues [13]. Concretely, CLARA's final objective is to prove the feasibility of a complete residual biomass-to-fuel chain up to the 1 MW\({}_{\text{th}}\) scale in an industrially relevant environment, with a target cold gas efficiency of 82%, carbon conversion of 98%, and the level of tar in outlet syngas lower than 1 mg Sm\({}^{-3}\)[14; 15]. Even though CLG is rather flexible concerning the nature of raw solid fuel, one of the main focuses and novelty points of CLARA deals with the study of biomass pretreatments, carried out in order to improve fuel performances during their thermochemical conversion, in terms of fuel energy density and avoidance of sintering/agglomeration phenomena of fluidized bed particles with fuel ashes [15; 16]. CLARA has selected wheat straw as a residual biomass of interest for CLG, investigating the effects from several pretreatments, such as torrefaction, washing, addition of minerals; those pretreatments, described in detail elsewhere [15; 16; 17], were mainly focused on the issue of agglomeration avoidance during the reduction-oxidation cycles of CLG, involving both OC particles and biomass ashes [15]. This work focuses on torrefaction and washing influences on the pyrolytic behavior of pretreated biomass. Torrefaction is a mild form of pyrolysis at temperatures ranging between about 200 and 300 \({}^{\circ}\)C, in an inert environment [18]. [PERSON] et al.