# Paper-Mill Wastes for Bioethanol Production in Relation to Circular Economy Concepts: A Review
## 5 RPS for Energy Applications
### Bioethanol Production from Lignocellulosic Biomass  
Ethanol (or EtOH, C\({}_{2}\)H\({}_{6}\)O) is a colorless liquid with a slight odor, soluble in water, flammable, and volatile. Ethanol, unlike fossil fuels, is a renewable energy source produced through the fermentation of sugars. As a high-octane fuel (98), it has replaced lead as an octane enhancer in petrol. Blending ethanol with gasoline fuel benefits the engine, burns more completely, and reduces polluting emissions [119]. Ethanol can also be used as a fuel for power generation, such as in solid oxide fuel cells, and as an ingredient in the chemicals industry [120; 121]. Bioethanol is produced from natural matter. first-generation bioethanol feedstock is mainly edible food crops such as rice, corn, sugarcane, and vegetable oils like soybean oil and olive oil. Production of first-generation bioethanol has its disadvantages, competing with the food supply and land utilization. However, this form of biofuel is commercially available and known for its yield and production process [14]. Second-generation bioethanol refers to ethanol produced from nonedible feedstocks such as woody biomass, herbaceous biomass, solid waste, and animal fat. Compared to the first generation, the amount of energy that can be produced per land area is much bigger. Some drawbacks, however, are higher capital costs in production due to sophisticated processing equipment and lower energy density vs. first-generation bioethanol [122]. The conversion of lignocellulosic biomass into bioethanol generally starts with feedstock preparation that involves cleaning and size reduction. This is done by milling, grinding, or chopping and is crucial for the removal of impurities and increasing surface area; however, this step is energy-consuming [123]. The process is followed by four steps: pretreatment for the degradation of the lignocellulosic matrix, hydrolysis of cellulose in the lignocellulosic materials to fermentable reducing sugars, fermentation of the sugars to ethanol, and lastly, distillation and recovery of the produced ethanol (Figure 5). Pre-treatment is needed to reduce the lignin and hemicellulose amount, as they hinder hydrolysis by impeding the access of cellulase enzymes to cellulose [124; 125]. The pre-treatment should also reduce cellulose crystallinity and increase its porosity or surface area, which can significantly improve hydrolysis. In addition to being cost-effective, it is required to prevent the loss of carbohydrates and the formation of hydrolysis-inhibiting by-products [126; 127]. This process has been intensively studied as it is a major technical and economical bottleneck in the bioconversion of lignocellulosic to bioethanol, accounting for 40% of total costs [128], hence enabling scaling up production [129; 130]. During pre-treatment, inhibitory compound may be generated. These compounds have a negative effect on enzymes and microorganisms, affecting catalytic processes and reducing ethanol production yield. Table 5 presents current industrial pre-treatment methods. Figure 5: Stages in second-generation bioethanol production. Hydrolysis is usually catalyzed by cellulase enzymes, and fermentation is carried out by yeast or bacteria. Factors that have been identified to affect the hydrolysis of cellulose include the porosity (accessible surface area) of the feedstock, cellulose fiber crystallinity, and lignin and hemicellulose content [10]. Ethanol is produced by fermenting soluble sugars using a variety of microorganisms. The fermentation is conducted in anaerobic conditions, with the maximum theoretical yield being 0.51 kg of ethanol and 0.49 kg of CO\({}_{2}\) per kg of glucose [133]. There are several possibilities for the integration of hydrolysis and fermentation steps. These include separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and consolidated bioprocessing (CBP) [134]. SHF consists of two separate hydrolysis and fermentation steps. In SSF, both steps are conducted simultaneously in the same vessel.