# Production of Cellulosic Ethanol from Enzymatically Hydrolysed Wheat Straws
## 1 Introduction  
In the past decade, due to climate changes there has been an increasing attention on reducing greenhouse gas (GHG) emissions. In accordance with the Paris Agreement (Council Decision (EU) 2016/1841 of the 5 October 2016) from 2023, every 5 years a comprehensive assessment of the progress of the parties will be made on the basis of scientific data and the situation regarding the reduction of emissions, the adjustments made and the support provided will be analysed. Compared to 1990, the mandatory target set for 2030 is at least a 40% domestic reduction economy-wide greenhouse gas (GHG) emissions [1]. Agricultural biomass is considered one of the most important renewable energy resources and contributes to the development of bioenergy generation. It consists of annual and perennial energy crops (green biomass for animals feed), residues from agricultural production (straw, corn stalks, corn cobs, sugar cane, etc.) and the food industry (residues from dairy industry, sugar industry, etc.). In recent years, there has been an increasing trend in obtaining bioethanol from renewable resources such as straws resulted from cereals harvesting. However, huge quantities of wheat straws (WS) are generated annually which could be used for the production of cellulosic bioethanol [2, 3]. Liquid biofuels can partially or completely replace conventional fuels and can be an alternative source in the transport sector (aviation, shipping and heavy freight trucks). Therefore, liquid biofuels are an important solution because they do not require major changes in distribution infrastructure or the transport fleet. International Renewable Energy Agency (IRENA) argues that a reduction in carbon emissions by 2050 is only possible if there is a five-fold increase in biofuel consumption, from 130 billion litres in 2016 to almost 650 billion litres in 2050 [4]. Wheat (Triticum sp.) is the third most cultivated cereal in the world. Based on the data provided by Food and Agriculture Organization of the United Nations (FAOSTAT) in 2020, the worldwide cereal production is 2789.8 million tons. The worldwide cereal production in 2020 (2789.8 million tons) is higher by 3% compared to 2019 (2708.5 million tons) [5]. In 2019/2020 the estimated global wheat production is 762.2-764.1 million tons in comparison with the production of 2018/2019 which was 732.1 million tons (Figure 1) [6; 7]. Therefore, if we take into account a coefficient of 1.3 [8; 9], in 2019/2020 an amount of 990.86-993.33 million tons of wheat residues were produced. The wheat straw is consisted of internodes (57 \(\pm\) 10%), nodes (10 \(\pm\) 2%), leaves (18 \(\pm\) 3%), straw (9 \(\pm\) 4%) and the central axis (6 \(\pm\) 2 %). The chemical composition of the wheat straw contains cellulose (34-40%), hemicellulose (20-25%) and lignin (20%) [10; 11; 12; 13]. Cellulose is a linear polymer composed of \(\beta\)-\(D\)-glucopyranosyl units linked at 1,4 positions [14]. The utilisation of cellulose and hemicellulose sugars present in the hydrolysate of lignocellulosic biomass is essential for the production of bioethanol [15]. Hemicellulose is a complex polysaccharide, found in  
Figure 1: SEM images for untreated WS ((**A**)—E detector-exterior view; and for pretreated WS ((**B**) —SE detector-1% H\({}_{2}\)SO\({}_{4}\) (_v/v_); (**C**)—SE detector-2% H\({}_{2}\)SO\({}_{4}\) (_v/v_); (**D**)—SE detector-3%; H\({}_{2}\)SO\({}_{4}\) (_v/v_)). the cell walls of plants, composed of neutral monosaccharide units: \(D\)-xylose, \(D\)-manose, \(D\)-galactose, \(D\)-glucose, \(L\)-arabinose, \(D\)-glucuronic acid etc. Hemicellulose fractions consist of \(\beta\)-\(D\)-xylopyranose backbone residues linked at 1,4 positions to the units of \(L\)-arabinose, \(D\)-galactose and/or \(D\)-glucuronic acid [16]. The mechanical pretreatment of lignocellulosic materials is an important stage in the process of obtaining bioethanol which reduces the particle size and crystallinity of cellulose, increases the contact surface, dissociates tissues and disintegration of the cell wall [17; 18; 19].