## 1 Introduction  
Over the recent decades, the utilization of empty fruit bunch (EFB) to produce renewable energy source has been studied extensively. This, EFB typically, is a leftover of the fresh fruit bunches after their fruit harvesting. Moreover, it is the major downstream residue from the Southeast Asian palm oil industry. This industry, recently, has produced 70 million metric tons of crude palm oil (CPO) just within one production year 2017-2018 amounting their production even up to 35% of the total production of edible vegetable oils [1]. Theoretically, palm oil plantations can produce more than 100 million metric tons of EFB since 1.42-1.88 tons of EFB are generated for every one ton of CPO produced [2]. However, unfortunately, most of these EFB is dumped and pose serious environmental problems. Therefore, it is highly desirable to utilize these biomasses for sustainable energy production.  
Biochemical conversion of lignocellulosices involving chemical pretreatment followed by enzymatic hydrolysis and finally anaerobic fermentation is a well-established production line for lignocellulosic bioethanol production. In this classical approach, due to an inherited complex cell wall structure, effective fractionation of major lignocellulosic biomass components (cellulose, hemicellulose, and lignin) is crucial for effective subsequent processing. Undoubtedly, there have been several pretreatment studies involving EFB to deconstruct its native web into its constituents. In this regard, steam explosion, which uses high-pressure steam to breakdown biomass structure, has been widely adapted strategy [3]. The effects of several factors like biomass particle size, moisture, pressure, residence time, reaction temperature and catalysts on the steam explosion were consistently investigated for several decades [4]. Previously, [PERSON] et al. used alkaline reagent (3% NaOH at 160 \({}^{\circ}\)C) to catalyze steam explosion pretreatment and dissolved out C5 sugar-based components as well as lignin components from biomass [5]. Their yields, after enzymatic hydrolysis, for glucose and xylose heightened up to 93% and 78% respectively. In another study, the steam explosion of rose gum tree (Eucalyptus grandis) was carried out under the temperature range of 200-210 \({}^{\circ}\)C and residence time of 2-5 min after impregnation with 0.087% and 0.175% (_w/w_) H\({}_{2}\)SO\({}_{4}\)[6]. Their results showed that 0.175% (_w/w_) H\({}_{2}\)SO\({}_{4}\)-impregnated chips at 210 \({}^{\circ}\)C for 2 min was the optimal condition for hemicellulose extraction. Today steam explosion is seen as one of the most widely employed and cost-effective process for lignocellulosic pretreatment, yet the process still needs to be improved to realize industrialization [7]. In addition to this steam explosion strategy, another effective fractionation scheme for lignocellulosicoses is organosolv treatment, so-called organosolvation. This process, in principle, extracts lignin from lignocellulosic with organic solvents, such as alcohols, acetone and organic peracids [8]. Previous studies have shown considerable effects of organosolv treatment (diethylene glycol) of rice straw and EFB as compared to soda pulping by [PERSON] et al. [9] keeping in view the heterogeneous yet complex cell wall structure of EFB, it is highly likely to investigate an appropriate pretreatment strategy for commercial production of biofuels [10]. Therefore, it has to be investigated the proper pretreatment process of EFB. In this study we adapted a two-step pretreatment strategy to fractionate EFB biomass into its sugar and non-sugar components. For this two-step pretreatments, EFB was first subjected to acid-catalyzed steam explosion (ACSE) followed by alkali-catalyzed organosolv treatment (ACO). Pretreatment efficacy was evaluated in terms of yields, crystallinity, and chemical composition of each solid fraction.