First generation

Alcohol is obtained by the process of fermentation of the sugars present in the different raw materials, which can be metabolized by microorganisms of the genera Saccharomyces, Zymomonas, Kluyveromyces, and Zygosaccharomyces (Table 2).

Raw materials with available fermentable sugars

These sugars are present in sugarcane, sugar beets and sweet sorghum. Use of these raw materials requires only milling, fermentation, distillation and denaturalization (so that it is not apt for human consumption). Moreover, for its use in mixtures with gasoline, it must be dehydrated (^{Solomon et al., 2007}), which is necessary to displace its azeotropic point. Methods of dehydration include adsorption with molecular meshes, pervaporation, vacuum distillation, extractive distillation with solvents or with salts or with both at the same time. Ethanol is also dehydrated with hybrid processes that combine two or more of the above processes (^{Uyazán et al., 2004}).

In Brazil, they are researching how to reduce ethanol production costs through integration in the use of energy and systems of co-generation in production plants (^{Cortes-Rodriguez et al., 2018}) and decreasing expenditures of vapor and water (^{Pina et al., 2017}), or the use of sugarcane bagasse in the integration of first generation (1G) and second generation (2G) ethanol production processes; 2G processes involve the use of lignocellulosic substrates (as will be discussed below) and the use of sweet sorghum as complementary raw material, grown in areas neighboring the sugarcane and harvested between cycles of the main crop (^{Jonker et al., 2015}). Also studied is the sustainable development and updating of the technologies of biorefineries for producing bioelectricity (^{Khatiwada et al., 2016}). After more than 50 years of experience with 1G ethanol, progress was oriented more toward optimization of the integral use of raw materials and the byproducts of the processes, integration of first generation processes with second generation processes, analysis of the effects of the varieties of the same raw material, crop conditions, harvest seasons and methods, and storage time of the raw material on ethanol yields (^{Gumienna et al., 2016}; ^{Dos Passos Bernardes et al., 2016}). One important aspect is mechanized harvest of sugarcane, which increases the presence of inorganic compounds such as potassium, calcium, silica, iron and copper, relative to the traditional process of burning and manual cutting. ^{Thai et al. (2012)} report increases of 13 % calcium, 32 % magnesium and 7.6 % silica in the sugarcane juice after mechanical harvesting, relative to traditional burning and cutting. These minerals affect fermentation because magnesium increases ethanol yield, but an excess of copper reduces yields to 0.35 g g^-1 (^{Costa et al., 2015}).

Interest in sweet sorghum and other sorghum types for ethanol production has increased. This crop can be grown with little water (compared with other cereals) and is highly efficient photosynthetically for conversion of CO₂ to biomass. Moreover, total sugar content of the juice of the sweet sorghum stem is comparable with that of sugarcane juice (^{Chuck-Hernandez et al., 2012}). Sugarcane processing machinery has been adapted to harvest and mill sweet sorghum. ^{Peralta-Contreras et al. (2013)} proposed designing machinery specifically for sorghum stems to improve juice extraction indexes. Sorghum has many qualities, but its high moisture content (70%) and the low bulk density limit the distances within which transporting it after harvest is profitable (^{Chuck-Hernandez et al., 2012}; ^{Zegada-Lizarazu and Monti 2012}). Also, conservation of the sugars in the stems is difficult because the native bacteria of the crop, such as Leuconostoc, degrade a large proportion of sugars during transport and storage. Inhibitors of Leuconostoc growth during transport, such as SO₂ gas, have been tested. However, storing the gas in hermetic tanks at the sorghum collection sites, as well as its elimination when it arrives at the processing plant, make it an expensive system (^{Lingle et al., 2012}).

Extracted sweet juices lose 20 % of their fermentable sugar content in three days at ambient temperature and 40 % to 50 % in seven days (^{De Oliveira Filho et al., 2016}). Other affected aspects are decreased pH, increase in total acidity and quantity of reducing sugars. Furthermore, the presence of ethyl acetate, acetaldehyde and ethyl carbamate in sweet juices have a negative effect on fermentation, causing low yields (^{De Oliveira Filho et al., 2016}). Juices from milled material must be filtered, clarified and pasteurized. Filtering removes lignocellulosic fractions, leaves, dirt and other contaminants from the collection stage. Clarifying is done to eliminate inorganic compounds, and pasteurization eliminates the microorganisms present. For clarification, the processes of sedimentation, sulfation, and carbonatation generate byproducts known as sludge. To generate less contaminating sludge from clarification, natural products are used, such as extracts from leaves and seeds of Moringa oleifera Lam., which remove more iron and calcium than synthetic polymers (^{Costa et al., 2015}). High-pressure treatment (600 MPa 6 min^-1) is an alternative to high temperatures for elimination of microorganisms and inactivating enzymes from sweet juices (^{Huang et al., 2015}). Also, ohmic heating for 1 min and 32 V cm^-1 has been tested to inactivate polyphenol oxidase and reduce microorganisms (^{Saxena et al., 2016}). High-pressure methods and ohmic heating maintain juice quality stable and inhibit enzymatic activity, thus increasing its useful life in storage. Therefore, the search for methods of conserving raw materials and sweet juices for long periods without loss of fermentable sugars requires further study.

First generation technology has been applied industrially for 40 years. For this reason, besides the studies mentioned, there are many studies related to analysis of environmental problems this technology generates. Among these problems is the biomass produced, separated in the process of distillation and known as vinasse, which is produced at a ratio of 12 L vinasse L^-1 ethanol (^{Silva et al. 2007}). Vinasse is a liquid with suspended particles, acid pH and high chemical oxygen demand (COD) because of the organic matter present. Forty years ago, vinasse was a highly contaminating liquid residue that caused serious environmental problems in the aquifers where it was discharged. Its final disposal was a problem because there was little technology to treat it and costs were high. This situation changed because of new technologies with better results that decreased operation costs. To treat residues of alcoholic fermentation, physicochemical, chemical and biological methods were tested. The biological methods are the most appropriate due to the large quantity of organic biodegradable compounds present in their composition (^{Sheehan and Greenfield, 1980}). Several alternatives were proposed for the treatment or reuse of vinasses, among which is feeding chickens (^{Hidalgo et al. 2009}), production of biogas (^{Cruz-Salomón et al., 2017}), and fertigation (^{Mijangos et al., 2014}). In addition, standardized methods were created to analyze life cycles to determine entrance and exits of energy, evaluate generation of byproducts and residues in the entire supply chain, and minimize impacts from crop establishment to consumption of fuel ethanol (^{Cavalett et al., 2012}; ^{Gallejones et al., 2015}; ^{Miret et al., 2016}).

From the above, it can be inferred that the main interests concerning raw materials with high fermentable sugar content are increasing yields, improving energy balances, diversifying raw materials, and diminishing environmental impact.

Raw materials with high starch content

Other first-generation sources used to obtain ethanol are raw materials that have a high starch content, such as cereals, tubers and rhizomes. For these raw materials, the following steps are used for ethanol production: milling, liquification, saccharification, fermentation, distillation and dehydration. The country with more published studies on ethanol from starch between 2012 and 2015 was China, the US and India with 187, 181 and 74 published documents, respectively. The main research topics center in three fundamental areas: understanding agronomic aspects and their effect on ethanol yields, new operations in industrial processes, and new laboratory techniques to predict starch contents and ethanol yields.

In agronomy, the study of maize and sorghum with several focuses is highlighted, especially on final ethanol yield, for example effects of deficient irrigation (^{Liu et al., 2013}), content of moisture at harvest (^{Huang et al., 2012}), fungus infections and harvest maturity (^{Dien et al., 2012}). In other cereals, transgenic rice with high carbohydrate content was analyzed to evaluate its ethanol yields (^{Kim et al., 2015}).

In industry, the method most used to fragment the cereals is dry milling (^{Orts and McMahan 2016}), in which whole cereal grains are converted into flour, which is processed without separating the grain components. The flour is mixed with water to form a paste and α-amylase, pullulanase and amyl glycosidase are added to convert the starch into glucose. Ammonia is added to the process as a nitrogen source for the yeast and to control system pH. This high-glucose mixture is converted to ethanol by action of the yeast, while the solid residue, which has high fiber, lipid and protein contents, is toasted and converted into a byproduct known as distillers degraded grain soluble (DDGS). DDGS are mixed with syrup from the process and sold as a supplement for livestock. Wet milling is less used because of its high cost and water consumption, but starch from the grains is isolated and ethanol yield is higher (^{Kandil et al., 2011}). Dry milling begins the process with starch, which is mixed with lipids and proteins, and in the processes of liquefication and saccharification these macromolecules decrease enzyme activity because they act as inhibitors of the process (^{Srichuwong et al., 2010}). For this reason, we recommend investigating the interactions of the non-starch components (lipids and proteins) with starch and their effects during hydrolysis. To understand whether pigments are also inhibitors of the process ^{Wang et al. (2016)} validated the use of pigmented sorghum and determined that the high content of anthocyanins had no effect on ethanol yield.

In the industrial stage there are innovative methods for extracting fermentable sugars and non-structured carbohydrates with a diffusion process that uses biomass from chopped sweet sorghum stems and grains (^{Appiah-Nkansah et al., 2016}). The process of milling and elimination of physical barriers that limit interaction of enzymes with the starch was studied and, according to ^{Chuck et al. (2012)}, ethanol yield improves when the sorghum grains are decorticated. In the studies mentioned, cereal treatment technologies and ethanol yield are compared; energy expenditures decrease and ethanol yields increase. However, quantitative analysis of expenditures and comparison of different process proposals should be studied. It is also necessary to evaluate water balances and economic analyses of the process before initiating industrial scaling.

In the laboratory, development of a practical method of detecting approximate starch and ethanol contents using spectroscopy FT-NIR is highlighted (^{Li et al., 2015}). This could speed up industrial scale analysis to control the ethanol production process systematically.

Fruits, rhizomes, roots and tubers are non-conventional sources of starch used particularly in tropical countries. ^{Graefe et al. (2011)} analyzed ethanol production from bananas in Costa Rica and Ecuador. Bananas have an advantage over tubers because of their ripening and, because they are climacteric, they naturally hydrolyze starch and thus no enzyme treatment is required to reach the fermentable sugars (^{Asiedu, 1987}; ^{Bugaud et al., 2009}). Production of ethanol from bananas at different stages of ripeness showed that the highest ethanol yield was from green bananas, immediately after harvesting. Ethanol yield from overripe bananas was 23 % less than with immature bananas. This result is attributed to a decrease in dry-base matter because of the metabolic activity during ripening (^{Hammond et al., 1996}). Due the higher yields, it is recommended to used green bananas to produce ethanol. However, technical problems exist, such as peeling the fruit because the shape of the bananas makes automatizing the process difficult, and the peel is strongly attached to the pulp at that physiological stage of the fruit.

One problem of biotechnological processes is inhibition by the end-product. In the measure that alcoholic fermentation occurs, ethanol is produced, which inhibits the metabolic pathway and the process of bioconversion is detained until it causes cell death. A method used to decrease this inhibition is to ferment the sugars immediately after their release. This is achieved by simultaneous saccharification and fermentation (SSF). Fermentable sugars are produced by starch hydrolysis, and fermentation is carried out in the same reactor. Several raw materials were processed in this way, and the results are better than separate saccharification and fermentation. With green bananas, theoretic ethanol efficiencies above 95 % were obtained in an enzyme SSF system and the use of glucoamylases, pectinolytic enzymes and yeast (^{Bello et al., 2012}). Feasibility of a non-domesticated variety of mandioca (Manihot glaziovii), as a non-food raw material with a high starch content, was evaluated for production of ethanol by SSF. Ethanol concentrations of 190 g L^-1 were obtained. Another technological development in inhibitors is Zimomonas mobilis bacteria isolated from the African palm and Mexican pulque. They are osmotolerant and resistant to high concentrations of ethanol and permit fermentation at high concentrations, or very high gravity (VHG). The advantage is that at high sugar concentrations (up to 360 g L^-1) there is no contamination by other bacteria. In these conditions, ethanol concentration from sweet sorghum was 17.6% v/v (^{Deesuth et al., 2016}). Process integration can produce better yields than individual processes. ^{Chu-ky et al. (2016)} tested simultaneous very high gravity liquefication, saccharification and fermentation (SLSF-VHG) in rice by-products (broken rice grain) and obtained a theoretical 83.2% ethanol.

In Brazil and Colombia new sources and methods for ethanol production are being sought. ^{Soccol (1997)} achieved degradation of raw mandioca starch granules during fermentation in solid state with amyloglucosidase from Rhizopus oryzae.^{Cinelli et al. (2015)} described the method of cold hydrolysis of starches in which the process’ energy demand decreases because of starch hydrolysis at temperatures below its gelling temperature, that is, with starch in its granular form. This technology generates another perspective of the process as an alternative method that can be carried out without the liquefication stage, which is achieved through amylolytic enzyme complexes composed of endo-amylases, exo-amylases and debranching enzymes. In Indonesia, ^{Jusuf and Ginting (2014)} and ^{Kusmiyati (2015)} studied sweet potato (Ipomoea batatas L.) and the tuber Iles Iles (Amorphophalus campanulatus), respectively, as sources for ethanol production. These tubers have high potential for transformation because of their high yield per unit of cultivated area. Sweet potatoes yield more than 30 t ha^-1 of tubers, which are 22.5 % starch, wet base, while Iles Iles has the advantage of being cheap and not used for human food. As an alternative source, ethanol yield of starch extracted from Taro corm (Colocasia esculenta), a perennial plant of the Araceae family was evaluated. Taro is cultivated in the tropics and subtropics of South America, Asia, Oceania and Africa (^{Wu et al., 2016}). With this raw material, theoretic yields of 94.2% ethanol were achieved, an attractive yield for ethanol production.

Second generation

In the last decade, most of the research in liquid fuels evaluated the commercialization of ethanol produced with farm residues (cereal straw, leaves and dry branches of forest crops) or industrial residues (sugarcane bagasse and DDGS), composed mainly of cellulose, hemicellulose and lignin; these biofuels are 2G.

The amount of residue generated by an industry and that can be used to produce 2G ethanol is illustrated by Brazilian ethanol production units, which processed more than 602 x10⁶ t of sugarcane and produced 24 x 10⁹ L of ethanol from 2013 to 2014. Each ton of sugarcane processed generates 270 to 280 kg of bagasse (^{Canilha et al., 2012}), demonstrating the high potential of residual biomass for 2G ethanol production with these lignocellulosic residues.

These residues are the source for cogeneration of heat and electricity in distilleries through thermochemical processes, such as direct combustion, pyrolysis or gasification (^{Henrique et al., 2014}), but their biochemical transformation has certain technical limitations due to the chemical nature of the lignocellulosic residues, which is heterogeneous and different pretreatments, or modifications to the raw material, must be applied. It is necessary to conduct laboratory studies that show the technical feasibility of bioconversion of lignocellulosic residues to ethanol.

Enzyme hydrolysis of cellulose is a limiting step in 2G ethanol production because it needs complex enzymatic cocktails to de-polymerize the residues, in addition to the complexity of the structural arrangement of the components of the lignocellulosic residues that make hydrolysis difficult. Lignin is a main component of these residues, forming a structural arrangement together with the cellulose and the hemicellulose that makes it difficult for cellulases to access the cellulose hydrolyze it to glucose. Moreover, some cellulolytic enzymes are often adsorbed by the hydrophobic surface of lignin that, therefore, prevents them from acting on cellulose (^{Huron et al., 2016}). Phenolic compounds (tannins) of lignin, when de-polymerized, also become inhibitors of fermentation, so that the effects and interactions of the inhibitors of the processes of fermentation should be analyzed and understood to enable improvement of production yields.

As mentioned in the section about the first generation, green or immature banana pulp can be used to produce ethanol, but one of the limiting steps is the peeling process. Eliminating the banana peel can be avoided because both the banana peel and the pulp have high carbohydrate content (^{Gebregergs et al., 2016}), and both parts have been studied for ethanol production (^{Parthiban et al., 2011}). Therefore, ethanol can be obtained from unpeeled green bananas. However, it should be taken into account that some carbohydrates in the pulp and peel of green bananas are complex polysaccharides and cannot be metabolized by yeasts. For this reason, it is necessary to hydrolyze the lignin, the cellulose and pectin to convert them into simple sugars. This is an example of a raw material for ethanol production found in 1G and 2G groups.

Milling bananas with their peel has technical problems derived from the high viscosity and presence of fibers during and after milling (^{Afanador, 2005}). ^{Oberoi et al. (2011)} report the use of dryers such as electric ovens to obtain a dry brittle material that can be ground in laboratory mills, but in industrial processing this would increase energy costs. Here, there is a knowledge gap regarding decreasing viscosity in this step of the process without drying and milling or the use of alternative energy (solar energy, for example) for the drying process.

Second generation technology has given way to the concept of biorefineries because through 2G processes ethanol is obtained as well as methanol, synthesis gas, 2.5-dimethylfurane and tannins, a determining factor for obtaining diverse products from lignocellulosic raw materials. In this sense, one of the goals is to integrate 1G and 2G technologies. ^{Flório and Junior (2013)} performed a heat analysis of production of electricity and 2G ethanol from sugarcane bagasse and obtained better heat effectivity since, besides fermenting sugarcane sucrose and the glucoses that constitute cellulose, the process was optimized, and they sought to ferment pentoses (mainly xylose) that form a structural part of the hemicellulose. The process of bioethanol production was simulated with the mixture glucose/xylose, and the effect of temperature, pH and sugar concentrations was evaluated. The variables of the process were reproduced using different conditions of operation and it was demonstrated that simulation is a useful tool for generating an optimal profile of alcoholic fermentation with the glucose/xylose mixture (^{Reyes et al., 2016}).

With the concept of biorefinery, ^{Xu et al. (2018)} propose obtaining diverse products from lignocellulosic raw material using a modified method of simultaneous saccharification and fermentation (MSSF), which involves ecological technologies like hydrothermal pre-treatments and organic solvents for production of a high cellulose content solid to be transformed into ethanol. In addition, solvents are recycled to produce adhesives based on plant proteins, and the xylose present in the aqueous phase is used to obtain furfural. This type of integrated proposals may be attractive for promoting commercial ethanol production from lignocellulosic biomass.

Saccharomyces cereviciae and Zymomonas mobilis, used to ferment hexoses, cannot use pentoses as a substrate because the metabolic route for pentose conversion is unlike that of hexoses. For this reason, other yeasts and bacteria are being studied. Candida shehatae (^{Guan et al., 2013}) and Pichia stipitis (^{Travaini et al., 2016}) have a high potential for fermenting pentoses; their disadvantage is their low tolerance to ethanol, and in the processes with these microorganisms inhibition by the end-product occurs. As a solution to this technical problem, Kluveromyces marxianus yeast has been proposed; it tolerates ethanol and can ferment hexoses and pentoses (^{Lin et al., 2013}). To increase use of pentoses, process integration through use of fungi that metabolize wood without pre-treating the lignin is being investigated. ^{Mattila et al. (2017)} used a single step and a single microorganism for production of bioethanol with phlebioid fungus species, achieving an ethanol yield of 5.9 g L^-1.

Third generation

The search for raw materials for ethanol production has increased interest in rapid generation of biomass with high energy density (energy crops). Among these crops are found perennial grasses, micro- and macro-algae, and cyanobacteria, all grouped in the denomination of third generation (3G) biofuels. Algae are not seen as raw material for ethanol production, but as producers of hydrogen as the substrate for thermochemical conversion and as lipids for biodiesel (^{Brennan and Owende 2010}). Starches can be obtained by converting biomass from macroalgae. The starches are then hydrolyzed and fermented to produce bioethanol (^{Adams et al., 2009}; ^{Khambhaty et al., 2013}; ^{Scholz et al., 2013}; ^{Sudhakar et al., 2016}) and biobutanol, which has a higher energy density and greater compatibility with gasoline than bioethanol (^{Dürre, 2008}). One study on the economic feasibility of microalgae as raw material for biorefineries was reported by ^{Konda et al. (2015)}. They highlight the potential advantages of producing a broad portfolio of chemical products from microalgae to give economic viability to industrial clusters denominated biorefineries. In the laboratory, isolated alga strains are handled, but in natural conditions water sources have a problem called eutrophication, which is the proliferation of algae because of excess nutrients in the water. To solve this problem, using these algae was proposed for ethanol production and water quality improvement. ^{Chen et al. (2017)} patented a pre-treatment technique using electro-coagulation and acid saccharification of algae in lakes. They obtained up to 156 mg glucose g^-1 of algae, which is available for transformation to ethanol. The type of macroalgae most used industrially are seaweeds, which are pluricellular organisms that efficiently convert nutrients in the water and CO₂ into biomass. There are around 9,200 species of seaweed, but only 221 are economically important (^{Mohammed, 2013}). Macroalgae are raw material for bioethanol production because they do not have a high content of lipids, as do microalgae, but they do have a high content of sugars and other carbohydrates that can be fermented. The alga growth variables studied are those that affect production of carbohydrates, such as light, temperature, nutrients, salinity and pH. With respect to the effect of light, according to ^{George et al. (2014)}, a photosynthetic light intensity of 60 μmol m^-1s^-1 and 12:12 h (light:dark) cycles achieved a biomass production of 7.9 mg L^-1 d^-1 with Ankistrodesmus falcatus. It is important to consider that the pretreatments, liquefication and saccharification of the macroalgae have difficulties similar to those of the second-generation raw materials since they also contained polysaccharides that must be hydrolyzed to fermentable sugars, an aspect that requires more research. Microalgae have a high content of lipids, but also of carbohydrates. For this reason, Scenedesmus sp. has been proposed as raw material for integrated production of ethanol and biodiesel (^{Sivaramakrishnan and Incharoensakdi, 2018}).

Another high-density crop in lakes and lagoons is water hyacinth (Eichhornia crassipe), which is an environmental problem because of its high growth rate; it prevents light and oxygen from penetrating these bodies of water and cause the death of other species in these ecosystems. Water hyacinth is composed mainly of cellulose, which can be converted to glucose, the fermentable substrate for ethanol production (^{Zhang et al., 2018}). Different pre-treatments were tested for water hyacinth, and the most effective was the combination of diluted acid and microorganisms (Phanerochaete chrysosporium), which resulted in highly purified cellulose (39.4 %) and high content of reducing sugars (430.66 mg g⁻¹).

Fourth generation

Advances in bioengineering have led to the concept of fourth generation biofuels (4G), which use genetically modified organisms (GMO) that capture more CO₂, such as genetically modified sugarcane that has high content of lipids for simultaneous production of ethanol and biodiesel (^{Huang et al., 2016}). Also, fermentation with genetically modified E. coli produces triglycerides from sugarcane sweet juices that are later transformed to biodiesel. Amyris (USA), LS9 (USA) Sapphire Energy (USA), Solazyme (USA) and Terrebonne (Canada) study the way to scale the process to industrial levels (^{Steen et al., 2010}; ^{Westfall and Gardner, 2011}). The use of GMO has had positive results in yield and efficiency. According to ^{Tanimura et al. (2015)}, the genetically modified yeast Scheffersomyces shehatae JCM 18690, can simultaneously hydrolyze and ferment starch with a productivity of up to 0.92 g L^-1 d^-1 after ten days, comparable to the highest yield reported in ethanol production from maize with S. cerevisiae. ^{Huerta et al. (2005)} increased the theoretical yield of converting glucose or xylose into ethanol up to 27 % by genetically improving ethanologenic strains of E. coli KO11. One of the main objectives was to optimize the expression of the gene Pdc_Zm, which codes for pyruvate decarboxylase enzymes, and to find or generate more active versions of the same gene.

Future trends

The largest proportion of the costs of producing biofuels is associated with raw materials (^{Neto et al., 2016}). Thus, proposals for the industrial sector and systems of technological innovation in biofuels is focused on reducing production costs and on finding new raw materials that meet economic and environmental requirements and do not affect human food security.

One viable alternative for bioethanol production is process integration, which helps to improve productivity and use of energy resources and has an impact in reducing operation costs. One example is the production of 1G and 2G ethanol from sugarcane and its bagasse, which can reduce the final cost of ethanol enough to make it competitive with fossil fuels (^{Neto et al., 2016}). Technical analyses and economic studies should be conducted on integrated processes. From sugarcane, a raw material of high productivity, applicable results are obtained from integrated processes at an industrial scale, but other less productive raw materials, such as those with high starch content, should be analyzed in technological integration processes. Moreover, using carbon dioxide from the ethanol production plant (fermentation stage) in algae production offers a new perspective for integration of technologies 1G/3G.

The concept of biorefineries is a way of integrating processes; it broadens the spectrum of products of a production plant and, therefore, market possibilities. But its level of complexity is advanced, posing challenges in the integral use of raw materials and calling for further study because of the number variables involved.

A recent proposal concerns synthetic ethanol production from organic wastes such as paper, wood and cow manure. The residues are dried in the same plant where ethanol is produced. They are stored for a short period and introduced into the gasifier together with carbon to produce carbon monoxide and hydrogen (“syngas”) as well as pure carbon. “Syngas” is refined and subjected to a catalytic process before being compressed for transfer to the reactor for ethanol synthesis. In this process, collection of the raw material has a major impact on the cost of the end product, and the carbon used to improve economic efficiency increases costs. The cost of ethanol, with an internal return rate of 10 %, was 0.433 USD L^-1 with paper wastes, 0.51 USD L^-1 with wood waste and 1.45 USD L^-1 with cow manure. Economic feasibility of producing ethanol was greater with the mixture of wood waste and carbon. Using carbon in the mixture may be an option for obtaining energy with household generated organic waste (^{Gwank et al., 2018}).

Petrochemical installations for producing and refining oil contribute CO₂ emissions as does combustion of the energy compounds (gasoline, diesel, butane and natural gas). Combustion of oil derivatives produces only emissions and no fixation which occurs with other gases (for example, nitrogen), and there is no stage at which these emissions can be decreased; thus, it causes environmental pollution. In contrast, CO₂ generated by ethanol combustion is less than that generated by fossil fuels and can be fixed through photosynthesis and thus its release into the atmosphere is lower, contributing less to the greenhouse effect (^{Quintella et al., 2011}).

Life cycle analysis (LCA) of biofuels helps to understand the impact of the generated CO₂ on the environment. An evaluation conducted by the Centro de Tecnología Copersucar (CTC) showed that for each ton of sugarcane, the net effect is fixation of 694.7 kg CO₂, considering the entire cycle, from sugarcane cultivation up to its final use as ethanol. These results showed that emission of 206.8 kg CO₂ per t of sugarcane is avoided when ethanol is used instead of gasoline (^{Paula et al., 2010}).

Diverse factors should be considered during the life cycle analysis of ethanol. Sugarcane under given conditions can generate negative CO₂ balances, but because of the variety of raw materials with which ethanol can be obtained, each of them should be analyzed specifically. For example, application of chemical fertilizers and irrigation water for the crop that can be used to produce ethanol inevitably involves CO₂ emissions in the processes of production (^{Gelfand et al., 2011}). Clearing and converting land to crop fields damage the soil carbon store; this is denominated carbon debt (the amount of CO₂ released by land use change). Production of precise reliable tools for determining and quantifying each relevant contribution, anthropogenic or not, in the life cycle of ethanol is a challenge.