# Effective Use of Flax Biomass in Biorefining Processes
## 1 Introduction  
Cultivated flax (_Linum usitatsissimum_ L.) belongs to the agricultural species with a very wide range of applications. It is an annual plant with medicinal and dietary properties, also used in industry. Three types of flax are distinguished in cultivation forms: fiber flax, linseed and dual purpose [1]. The total cultivated area of flax in the world in 2021 was 4,384,000 ha, of which only 241,103 ha was used for fiber flax [2]. Linseed varieties are focused on a high yield of seeds, which are characterized by the highest content of alpha-linoleic acid among plants [3, 4]. Flax seeds contain 33-45% fat, and linseed oil over 50% of highly saturated fatty acids--mainly alpha-linolenic acid (omega 3) and linoleic acid. The main producers of linseed are Russia (1,492,119 ha), Kazakhstan (1,366,068 ha) and Canada (403,500 ha) [2]. Flax seeds are used in the food and baking industry (bread, biscuits and breakfast cereals), the oil industry (edible oil, paints, varnishes, printing inks), the pharmaceutical industry and as dietary supplements. The basic raw material obtained from the cultivation of fibrous forms is straw and fiber. Flax straw is used to obtain long fiber used to produce high-quality yarns and fabrics, and short fiber, which is used to produce carded yarns, insulating mats or paper [5, 6]. Fiber flax is mainly cultivated in France (112,580 ha), Belarus (42,300 ha) and Russia (36,483 ha) [2]. In addition, to increase the profitability of flax cultivation, breeding works are carried out to produce dual-purpose varieties [7, 8, 9]. It is an attempt to combine a high yield of seeds with a high yield of monomorphic fiber, intended for technical purposes. However, the fact that natural best fibers constitute a negligible part of the global textile market makes it difficult to sell basic raw materials obtained from flax, especially fiber. The main reason for this is the replacement of flax fiber with cotton and synthetic fibers, and the high cost of natural fiber products. Moreover, weather conditions have a great influence on the quality of the fiber. All this makes it necessary to develop alternative directions for the use of flax biomass [10]. One of them is the use of flax biomass in biorefining processes. Lignocellulosic biomass is the oldest and most widespread source of renewable energy, the third largest natural energy source in the world. The use of biomass is inextricably linked to the reduction of greenhouse gas emissions, improvement of energy security and socio-economic development. The cultivation and processing of flax plants can be climate-neutral and harmoniously integrated into a circular bioeconomy. As mentioned, thanks to the multi-directional use of flax raw materials, by-products do not have to be waste, but a raw material for further production. In this study, one of the planned biorefining processes of flax biomass is the process of obtaining 2G bioethanol, because according to the assumptions of the RED II Directive, it introduced the obligation to reach 14% share of RES in transport by 2030, including at least 3.5% from advanced biofuels (from non-food plant matter) [11]. The structure of plant biomass is formed by lignocellulosic, in which cellulose and hemicellulose are potential substrates in the process of obtaining biofuels, but lignin consisting of aromatic compounds is a strong obstacle in this process. Therefore, there is a need to carry out pretreatment, the purpose of which is to loosen the compact structure of lignocellulosic and delignify the raw material [12]. The next stages of processing plant biomass into bioethanol are enzymatic hydrolysis and ethanol fermentation. An effective and economical solution seems to be the SSF process combining cellulose hydrolysis with sugar fermentation in one bioreactor [13; 14; 15]. Natural plant raw materials often contain substances that have antibacterial or anti-fungal properties, which reduces the number of microorganisms and helps to keep them at a safe, low level.