# _Arthrospira platensis_ as a Feasible Feedstock for Bioethanol Production
## 4 Techno-Economical Considerations  
Production of ethanol through biological fermentation of sugars extracted from sugarcane [78] or corn and wheat [79; 80] (Ist generation biofuel) is technically matured and commercially available. For instance, global leaders in fuel ethanol production such as the United States and Brazil produced about ~57.7 billion L and 27.6 billion L of bioethanol in 2019 [80]. Liquefaction of the starch fraction is accomplished by adding hydrolytic enzymes (\(\alpha\)-amylases) at temperatures of around 90 \({}^{\circ}\)C. After the liquefaction step, the starch molecules are further hydrolyzed by the addition of glucoamylases. This produces sugars, which are readily fermented by yeast (e.g., _Saccharomyces cerevisiae_) to ethanol. The main co-product is usually animal feed, consisting of the remaining fraction of the raw material, mainly proteins and fibers [81]. However, this feedstock is competing with food, making it controversial. On the other hand, lignocellulosic material such as sugarcane bagasse (SCB), oil palm empty fruit bunches (OPEFB), wheat straw, rice straw, corn straw, and soybean bagasse have been proposed as a viable alternative to produce liquid biofuels (2 nd generation), but the technology is still under development as it is still difficult to reach high ethanol concentrations during fermentation. Lignocellulosic materials contain about 50-60% of carbohydrates in the form of cellulose (made up of glucose) and hemicellulose (consisting of various pentose and hexose sugars), which may be fermented to ethanol, and 20-35% lignin. The separation of these compounds requires strong pretreatments, which can be chemical, thermochemical, or biological with the use of cellulases. Even if the efficiency of some methods have been demonstrated at the lab-scale, the main limitation in the industrial development of such processes remains the production costs linked to their complexity. Indeed, the main locks are (1) the complex structure of lignocellulosic materials, making them recalcitrant to hydrolysis, requiring strong pre-treatment and the use of expensive cellulases; (2) the presence of various hexose and pentose sugars in hemicellulose, making fermentation more difficult than from glucose; and (3) the presence of various compounds that inhibit the fermenting organism. Even if some techno-economic evaluations are available in the literature, it is still difficult to evaluate the production costs of such a bioethanol, as large variations in the process design (different pre-treatments and conversion methods, various types of raw materials...) and in the assumptions underlying the techno-economic evaluations have been observed. As the glycogen is a polymer of glucose, just like starch, the conversion of glycogen into bioethanol would thus be closed to a 1 st generation one from a process point of view, and high cost pre-treatments will not be required. A comparison of simplified process diagrams is proposed in Figure 1. However, biomass production costs have to be taken into account. In a recent review, [82] estimated the production costs of _Spirulina_ dry biomass at 5.1 USD kg\({}^{-1}\) assuming a plant with a productivity of 750,000 kg year\({}^{-1}\).