عنوان مقاله [English]
The concern about limited fossil resources has led researchers to use renewable and new energies. Bioethanol is one of the most important and widely used biofuels that can replace fossil fuels. Bioethanol can be produced from various agricultural products and waste, such as grains, molasses, fruit, lignocellulosic materials, and algae. Molasses, an amorphous sugar extracted from sugar cane and sugar beet, is considered one of the most essential raw materials of sugar base. Sugars are usually produced with Saccharomyces cerevisiae yeast and bio-ethanol. Starch-based materials, such as wheat and corn, must first be enzymatically hydrolyzed, and then bioethanol is obtained with the help of the fermentation process. Agricultural wastes such as cereal straw, sawdust, and black liquor are considered lignocellulosic raw materials. Lignocellulosic raw materials are first de-ligninized. Then, the enzymatic hydrolysis process is performed, and finally, with the help of fermentation, bioethanol is obtained. This review article first deals with the production status of this biofuel in the world's leading countries. Then, it briefly mentions the bioethanol production process from sugar, starch, and lignocellulosic raw materials and the challenges of each method. It also examines the types of yeasts and compares them and the effective parameters in the fermentation, pre-treatment, hydrolysis, and distillation processes.
Today, the bioethanol production industry is one of the most essential green business activities. The sources of natural fuels such as fossil fuels and coal are rapidly running out; Therefore, fuels such as ethanol, methane, and hydrogen have been considered by researchers and industry owners. Bioethanol can be produced from various agricultural products and waste, such as grains, molasses, fruit, whey, lignocellulosic materials, and algae. Bioethanol with the chemical formula (CH3-CH2-OH) is produced through a biological process. In this process, biomass is converted into ethanol through a biochemical process. Bioethanol has many advantages over traditional fuel. Premature combustion, prevention of cylinder knocking due to high octane number and high heat of vaporization, ability to mix with gasoline, and reduction of hydrocarbon and carbon monoxide emissions due to the presence of a high percentage of oxygen in the structure of bioethanol are its prominent features. Statistics show that 4.2 million people die yearly due to the inhalation of gases resulting from the combustion of fossil fuels. Therefore, the idea of finding an accessible and environmentally friendly source of energy production, which is generally produced from agricultural waste, is attractive. Because despite the production of a valuable product, it creates an economic benefit from a raw material that has no use. Also, the disposal of agricultural waste is a fundamental environmental problem that can be solved mainly by starting the bioethanol production cycle. Currently, the United States and Brazil are the two countries that account for 70-80% of the world's bioethanol production.
To review various articles about the production process of bioethanol from sugar-based and lignocellulosic materials, Google Scholar, ScienceDirect, ResearchGate, Scopus, ACS, SID, Civilica, Ganj Irandak databases with keywords Bioethanol, Lignocellulosic materials, sugar-based materials, biofuels, searched. The texts used in this article include various articles, web pages, theses, and organizational reports. Most of the articles published in the mentioned scientific databases have been used in this study.
Bioethanol can be a good substitute for gasoline for the following reasons: 1) It can be made from renewable raw materials. 2) It is less toxic than other fuels. 3) It can be used directly or mixed with gasoline in different fuel ratios. 4) Emits fewer pollutants than fossil fuels Because the oxygen in ethanol improves combustion. However, according to the statistics of the REN21 international network in 2016, the share of biofuels is only 3% of the consumed fuels in the world. Several factors can be the reason for the lack of acceptance of biofuels. Easy access to fossil fuels, lack of seriousness of policymakers, and concern about basic raw materials such as sugar and starch, which are people's food, can be the most important reasons. The cost of bioethanol production is an essential function of the raw material. Currently, about 98% of the ethanol produced in the world is obtained through the fermentation of sugars. For this reason, success in the economic production of bioethanol and its competition with gasoline is formidable. For bioethanol production to be economical, the cost of its raw materials should be low, and easy access to raw materials should be possible. Corn is the main ingredient for bioethanol production in America, as the first leading country in the bioethanol industry. Maize is mainly used as animal feed. In the United States, about 38% of corn is used for animal feed, 29% for bioethanol production, and the rest for industrial use and export. As the second producer of bioethanol, sugarcane is the primary raw material in bioethanol production in Brazil. Sugarcane is mainly grown to meet the global demand for sugar, and about 80% of the world's sugar production comes from sugarcane and the remaining 20% from sugar beet. According to the above content, the direction of research and the bio-ethanol production industry is expected to go towards agricultural waste and cheap and abundant raw materials. In other words, the first generation of bio-ethanol production, which uses sugar and starch-based materials, should be replaced, and the second generation of bio-ethanol production, which uses lignocellulosic materials, should be returned to be able to compete economically with fossil fuels. Lignocellulosic materials can be used for biofuel production due to their abundance and availability. Lignocellulosic compounds include different types of plants, agricultural wastes, and urban solid wastes, which consist of cellulose, hemicellulose, and lignin. Lignocellulosic materials account for more than 90% of the dry weight of plant cells. The method of producing bioethanol from lignocellulosic materials may not be as developed as that of producing bioethanol from sugar-based materials. However, due to the availability of lignocellulosic raw materials, bioethanol production has a massive potential. Lignocellulosic raw materials do not have the high cost of sugar and starch and do not threaten society's food security. The biochemical process is a standard method for bioethanol production due to selectivity and high efficiency of biomass conversion. The biochemical method includes pre-processing, enzymatic hydrolysis, fermentation, and distillation. During the biochemical process, lignocellulosic biomass must undergo chemical or physical pre-processing. In addition, there is a need for enzymes to hydrolyze polysaccharides, as well as a requirement for yeasts to ferment sugars into bioethanol. The primary purpose of pre-processing is to prepare the biomass for the next steps and make the process more efficient Because the lignocellulosic substrate has low porosity due to having three main parts: cellulose, hemicellulose, and lignin and shows resistance to enzymatic hydrolysis. This process can happen by reducing the size of lignocellulosic biomass to better access enzymes to sugars. Carrying out high-quality pre-processing increases efficiency and the availability of fermentable sugars in the following steps. If pre-processing is not done on lignocellulosic compounds, hydrolysis efficiency will reach less than 20%. In general, about 33% of the cost of the bioethanol production process from lignocellulosic compounds is related to the pre-processing stage, which shows the importance of this stage in the efficiency of bioethanol production. In the process of producing bioethanol from lignocellulosic biomass, the hydrolysis stage is a challenging stage from the technical and economic perspective, requiring more studies and research. The hydrolysis process breaks the long chain of carbohydrates by enzyme or acid. This stage is essential in bioethanol production because it affects the quality of the fermentation process. The hydrolysis process is necessary because microorganisms are only able to digest simple sugars. Due to the high cost of enzymes, enzymatic hydrolysis is economically challenging for commercial purposes. Fermentation is one of the vital processes in bioethanol production, where ethanol is produced directly from the metabolic activity of yeast. For example, Zymomonas mobilis is a yeast that consumes glucose as "food" and excretes ethanol as a product. Several yeast species are used to convert sugars into bioethanol, But on an industrial scale, the yeast Saccharomyces cerevisiae is the most common microorganism used for bioethanol production; among the many factors affecting bioethanol production temperature, sugar concentration, pH, fermentation time, stirrer speed, and inoculation rate. As a summary of the article, bioethanol production methods are mainly of plant and food origin, which has caused severe challenges for bioethanol production. Therefore, the production of bioethanol from lignocellulosic materials has attracted the attention of researchers. However, the production process of bioethanol from lignocellulosic materials is more complicated and involves more problems than sugar and starch-based materials. Therefore, the development of agricultural solid waste and lignocellulosic materials for bioethanol production, in addition to reducing the volume of junk and its management, also creates added value. Therefore, it is necessary to carry out extensive research to commercialize the process.