Ethanol fuel from biomass: A review

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Dec 29, 2017 - Another enzyme (zymase), also present in the yeast, then converts the glucose and fructose into ethanol and CO2. Typically, fermentation of ...
EJCEES, 2017, Volume 1, Issue 1, Pages 1-7

European Journal of Chemistry, Environment and Engineering Sciences ,

Journal homepage: www.ejcees.eu

Issue REVIEW 6

Ethanol fuel from biomass: A review

, ARTICLE INFO Page Article history:

Received 30th October 2017 1901

Accepted 26th November 2017 Available online 29th December 2017 -Keywords: Bioethanol 1909 Biomass Feedstock Hydrolysis 017 Volume 8 Fermentation

, Issue 6 , Page 1901 1909

1.

Sofien Chnitia,b* and Mnasser Hassounab* aUniversité

de Rennes 1, ENSCR, CNRS, UMR 6226, 11 allée de Beaulieu, 35700 Rennes, France.

bEcole

Supérieure des Industries Alimentaires de Tunis, Université Carthage et sis Avenue de la République, B.P 77, 1054 Amilcar, Tunisie. *Corresponding Author: [email protected] (S.Chniti)

ABSTRACT Bioethanol is a promising renewable biofuel produced from agricultural crops (sugarcane, sugar beet, corn, wheat) and cellulosic feedstock. Conventional bioethanol (1st generation) production based on edible agricultural products conflicts with food supply and causes food price increase. As an alternative to edible agricultural feedstock, lignocellulosic biomass (2nd generation) has been gaining attention as a sustainable feedstock (pulp, stover, stalk, stems and leaves) for bioethanol production. Ligncellulosic biomass based bioethanol requires a multi-step complex conversion technology due to its rigid structure, comprised of milling (size reduction), pretreatment, hydrolysis and fermentation.

Introduction

Ethanol (C2H5OH) is soluble in water and has a density of 789 g/L at 20°C. Catalytic hydration of petroleum products (ethylene) produces synthetic ethanol. Alcoholic fermentation of the simple sugars present in biomass produces bioethanol (Gnansounou et al., 2005). Absolute and 95% ethanol is a good solvent and is used in many industrial products, such as paints, perfumes and tinctures. Solutions of ethanol (70-85%) are used as disinfectants in medicine. Except for uses in motor fuels, the largest market share of synthetic ethanol is in industrial applications because synthetic ethanol is cheaper than ethanol derived from biomass. Bioethanol, however, captures the alcoholic beverage market and a small share of the vehicle fuels market. Ethanol intended for non-food uses is made unfit

for human consumption by addition of small amounts of toxic or unpleasant substances, such as methanol or gasoline. The main reasons of the glory of bioethanol are the following: (1) use as an octane enhancer in unleaded gasoline in place of methyl tertio-butyl ether; (2) use as an oxygenated compound for cleaning the combustion of gasoline and improving air quality; (3) use as an alternative fuel for reducing CO2 emission and limiting the risk of climate change; and (4) use as renewable energy as a partial substitute for oil, thus increasing security of fuel supply. The objective of this paper is to review the state of the art in industrial bioethanol production from a bioprocess engineering point of view for productivity improvement.

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European Journal of Chemistry, Environment and Engineering Sciences 2.

Fermentation substrates

The majority of the world’s bioethanol production is based on sugar crops, such as sugarcane and sugar beet. Grains, such as corn, are also used in many areas as raw material for microbial ethanol production. Lignocellulosic biomass is envisaged to provide a significant portion of the feedstock for ethanol production in the medium and long term due to its low cost and high availability (Kadar et al., 2007; Nguyen et al., 2017). For a given production line, the comparison of feedstock includes the chemical composition of the biomass; cultivation practices; availability of land and land use; use of resources; energy balance; emission of greenhouse gases, acidifying gases and ozone depletion gases; emission of minerals into water and soil; emission of pesticides; soil erosion; contribution to biodiversity and landscape value losses; farm-gate price of the biomass; logistic cost (transport and storage of the biomass); direct economic value of the feedstock, taking into account the co-products; and creation or maintenance of employment. Different authors have proposed criteria and methodologies for assessing the sustainability of energy crops. Bioethanol-to-ethanol crops comprise (1) multipurpose crops that are also devoted to food markets; and (2) dedicated ethanol crops. Although the latter are cultivated especially for ethanol production on non-agricultural lands (fallow or undeveloped lands), the former provide almost all of the feedstock used to date for ethanol production (sugarcane in Brazil and corn in the United States). In most industrialized countries, the development of biomass-to-ethanol conversion emerged as alternative markets for sugar and grain surpluses, as the feedstock cost often represents more than 75% of the ethanol production in these cases.

3.

Microorganisms

used

for

bioethanol

production Depending on the raw material used, different species of microorganisms can be used for bioethanol

production. Yeasts are the commonly preferred microorganisms for industrial-scale production of bioethanol (Chniti et al., 2014); Saccharomyces cerevisiae is commercialized worldwide for this Fungi such as Trichoderma reesei and Aspergillus niger produce the enzyme cellulose, which can excellently convert the cellulose present in sugarcane bagasse into fermentable sugars for ethanol production (Ben Tahar et al., 2017). In tropical areas, bioethanol has been produced by using Zymomonas mobilis, but its spectrum of carbohydrate fermentation is not very broad, which makes it less suitable. Zymomonas mobilis (Yang et al., 2016; You et al., 2017) and Escherichia coli (Trinh et al., 2008) have been successfully transformed into efficient ethanol producers.

4.

Main steps in biomass-to-ethanol processes

Once the biomass is delivered to the ethanol plant, it is stored in the warehouse and conditioned to prevent early fermentation and bacterial contamination. Through pre-treatment, carbohydrates are extracted or made more accessible for further extraction. During this step, simple sugars may be made available in proportions that depend on the biomass and the pre-treatment process. A large portion of fiber may remain for saccharification through hydrolysis reactions or other techniques to yield simple sugars, which are then fermented. In batch fermentation, the hydrolysate, yeasts, nutrients and other ingredients are added from the beginning of the step. In the fed-batch process, one or more inputs are added as fermentation progresses. The continuous process, in which ingredients are constantly added and products removed from the fermentation vessels, are also used. In efficient processes, the cell densities are increased by recycling or immobilizing the yeasts to improve their activity and increase fermentation productivity. The fermentation reactions occur at 2530°C and last for 6-72 h, depending on the composition of the hydrolysate and the type, density and activity of the yeasts. The broth typically contains 8-14% v/v ethanol. Above this concentration,

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European Journal of Chemistry, Environment and Engineering Sciences inhibitions of yeasts may occur and reduce their activity. Distillation yields an azeotropic mixture (95.5% alcohol, 4.5% water), often termed “hydrous” ethanol (99.6% alcohol, 0.4% water). The remaining flow from the distillation column, known as vinasse or stillage, can be valorized to produce co-products, which may include process. Products include steam and electricity, products for feeding animals, more or less concentrated stillage to be used as fertilizer and other valuable by-products.

5.

Dissaccharides to ethanol process

The most common disaccharide used for bioethanol production is sucrose, which is composed of glucose and fructose. Sucrose is an abundant, readily available and inexpensive substrate for industrial biotechnology processes, and its use has been demonstrated with much success in the production of fuel ethanol (Marques et al., 2015). Fermentation of sucrose is performed using commercial yeast such as Saccharomyces cerevisiea. The chemical reaction is composed of enzymatic hydrolysis of sucrose followed by fermentation of simple sugars. First, invertase (an enzyme present in the yeast) catalyzes the hydrolysis of sucrose to convert it into glucose and fructose. C12H22O11 + H2O Sucrose Water

Invertase

C6H12O6 + C6H12O6 Glucose

Fructose

beet, but sweet sorghum may also be used. Bioethanol (>60%) fuel worldwide comes from sugarcane juice, which is extracted from fiber using either a series of tandem roller mills or diffuser technology. The extracted juice contains water (84%) and sugars (14%). The next steps depend on whether ethanol alone is produced with sugar. After purification and evaporation, juice passes through in the crystallisation step. In addition to crystalline sugar, molasses is obtained as a by-product and may then be fermented, distilled and dehydrated to yield anhydrous ethanol.

6.

Starch to ethanol process

Starch stored in grains as long chains of α-glucose monomers, 1000 or more monomers for one amylase molecule and 1000 to 6000 or more monomers for amylopectin. To convert starch to ethanol, the polymer of α-glucose is broken into glucose through a hydrolysis reaction with gluco-amylase enzyme. (C6H10O5)n + nH2O Starch

Water

Glyco-amylase

nC6H12O6 n Glucose

The resulting sugar is known as dextrose or Dglucose, which is an isomer of glucose. The enzymatic hydrolysis is then followed by fermentation to yield anhydrous ethanol. In the bioethanol fuel industry, grains (corn, wheat or barley) mainly provide starch (Fig. 1).

Another enzyme (zymase), also present in the yeast, then converts the glucose and fructose into ethanol and CO2. C6H12O6 Glucose

2CH3CH2OH + 2CO2 Ethanol

Typically, fermentation of 100 g glucose by Saccharomyces cerevisiae yields 45-49 g ethanol, the theoretical limit being 51.1 g. However, practical efficiency of fermentation is about 92% of this yield. Sucrose comes mainly from sugarcane and sugar

Figure 1: Starch multiscale structure: (a) starch granules from normal maize (30 μm), (b) amorphous

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European Journal of Chemistry, Environment and Engineering Sciences and semicrystalline growth rings (120–500 nm), (c) amorphous and crystalline lamellae (9 nm), with magnified details of the semicrystalline growth ring, (d) blocklets (20–50 nm) constituting a unit of the growth rings, (e) amylopectin double helixes forming the crystalline lamellae of the blocklets, (f) nanocrystals, other representation of the crystalline lamellae called starch nanocrystals when separated by acid hydrolysis, (g) amylopectin’s molecular structure, and (h) amylose’s molecular structure (0.1– 1 nm) (Mohammadinejad et al., 2016). Corn (60-70% starch) is the dominant feedstock in the starch–to-ethanol industry worldwide. In the dry milling process, grain is ground to a powder that is hydrolyzed, and the sugar contained in the hydrolysate is converted to ethanol. The remaining flow containing fiber, oil and protein is dried and converted into a by-product known as distillers dried grains (DDG) or DDGS when it is combined to process syrup. DDGS is a very valuable by-product of dry mills sold as animal feed. Another by-product may be CO2, which can be sold for different applications (e.g., carbonated beverages or dry ice). Dry mills are dominant in grain-to ethanol Industry. However, in a number of large facilities, the mills are kinds of biorefineries in which the grains are wet milled first for separating the different components (i.e starch, protein, fiber and germ) before these intermediates are converted into final co-products.

7. Lignocellulosic to ethanol process Lignocellolisic materials represent an abundant feedstock for ethanol production. Because of their structure, pre-treatment is necessary to make them accessible for enzymatic attack.

7.1.

Structure of lignocellulosic materials

Lignocellulose, which is the principal component of the plant cell walls (Fig. 2), is mainly composed of cellulose, hemicelluloses and lignin, extractives and several inorganic materials. The compositions of each vary depending on the origin of the lignocellulosic material (Single et al., 2012; Saini et al., 2014).

Cellulose molecules consist of long chains of β-Dglucose monomers gathered into micro-fibril bundles. This cellulosic fraction can be converted into glucose by enzymatic hydrolysis or by chemical methods (Mosier et al. 2005).

Figure 2: General structure of lignocellulosic materials. (Montoya et al., 2014). The hemicelluloses can be xyloglucans or xylans, depending on the type of plant. The backbone of the former consists of chains of β-glucose monomers to which chains of xylose (5-C sugar) are attached. The latter are mainly composed of xylose linked to arabinose or/and other compounds that vary from one biomass source to the other. The hemicellulose molecules are linked to the micro-fibrils by hydrogen bonds. Lignins are phenolic compounds that are formed by polymerization of three types of monomers (p-coumaryl, coniferyl and synapyl alcohols), the proportion of which differs significantly depending on whether the plant is a gymnosperm, woody angiosperm or grass. Lignin adds compressive strength and stiffness to the cell wall. Lignocellulose is abundant in nature and does not compete with food. Typical sources of lignocellulosic biomass are agricultural and forestry residues (e.g., bagasse of sugarcane or sweet sorghum, corn stover, grasses, woody biomass), industrial wastes and dedicated woody crops (poplar). Once the

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European Journal of Chemistry, Environment and Engineering Sciences lignocellulosic biomass is pre-treated and hydrolyzed, the released sugars are fermented; the down steam process is similar to that of sweet juice and starch.

7.2.

Pre-treatment

Various pre-treatment methods are now available to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose, and lignin components. Physical (mechanical), physico-chemical, chemical and biological processes have been used for pretreatment of lignocellulosic materials (Mauriya et al., 2015). The performance of a few methods is assessed with regard to the yield of fermentable sugars, inhibitors, the recycling of chemicals, the production of wastes and the investments. This comparison shows that carbonic acid and alkaline extraction have the best performance. However, the most common methods are steam explosion and dilute acid prehydrolysis, which are followed by enzymatic hydrolysis. In the steam explosion method, the lignocellulosic materials are treated with highpressure saturated steam (0.69-4.83 MPa) at a high temperature (160-260°C) for several seconds to a few minutes. The pressure then is suddenly dropped to atmospheric levels, causing the materials explosion. Most of the hemicelluloses solubilized during the process whose efficiency depends on the temperature and residence time. Lower temperature and longer residence time yield higher efficiency. Sulphuric acid or carbon dioxide is often added to reduce the production of inhibitors and improve the solubilization of hemicelluloses. Steam explosion has a few limitations: (1) the lignin-carbohydrate matrix is not completely broken down; (2) degradation products are generated that reduce the efficiency of the hydrolysis and fermentation steps; and (3) a portion of the xylan fraction is destroyed (Fig. 3). The use of dilute acid is preferred by the US National Renewable Energy. In this method, the structure of the lignocellulosic materials is attacked by a solution of sulphuric acid (0.5-1%) at about 160-190°C for approximately 10 min.

Figure 3: Schematic representation of the effect of pre-treatment (Parveen et al., 2009). During this reaction, hemicellulose is largely hydrolyzed, releasing different simple sugars (xylose, arabinose, mannose and galactose) as well as other compounds of the cellulosic matrix, a few of which can inhibit the enzymatic hydrolysis and fermentation. The stream is then cooled. Part of the acetic acid, much of the sulphuric acid and other inhibitors produced during the degradation of the materials are removed. Finally, neutralization is performed, and pH is set to 10 before hydrolysis and fermentation.

7.3. Enzymatic hydrolysis of cellulose and fermentation of simple sugars Enzymatic hydrolysis of cellulose is achieved using cellulases, which are usually a mixture of groups of enzymes such as endoglucanases, exoglucanases and β-glucosidases acting in synergy, for attacking the crystalline structure of the cellulose, removing cellobiose from the free chain-ends and hydrolyzing cellobiose to produce glucose. Cellulases are produced by fungi, mainly Trichoderma reesei, in addition to Aspergillus, Schisophyllum and Penicillum. The efficiency of cellulose enzymatic hydrolysis is affected by the substrate to enzyme ratio, cellulase dosage and the presence of inhibitors. Cellulase loading may vary from 7 to 33 FPU/g (substrate) depending on the substrate structure and concentration. A high concentration of cellobiose and glucose inhibits the activity of cellulase enzymes and reduces the efficiency of the saccharification. One of the methods used to decrease this inhibition is to ferment the reduced sugars along their release. This is achieved by simultaneous saccharification and fermentation (SSF), in which fermentation using yeasts (Saccharomyces cerevisiae) and enzymatic

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European Journal of Chemistry, Environment and Engineering Sciences hydrolysis are achieved simultaneously in the same reactor (Olofsson et al., 2008). The fermentation of xylose released from the pre-hydrolysis process can be carried out in a separate vessel or in an SSF reactor using a genetically modified strain of the bacterium Zymomonas mobilis that can convert both glucose and xylose. The latter method is named simultaneous saccharification and co-fermentation (SSCF). Compared to the sequential saccharification and fermentation process, SSCF exhibits several advantages, such as lower enzyme requirements, shorter processing time and reduced cost due to economy in fermentation reactors (only one reactor compared to three sets). However, a few disadvantages include the difference between optimal temperatures for saccharification (50-60°C) and fermentation (30°C), inhibition of enzymes and yeast to ethanol and the insufficient robustness of the yeast in co-fermenting C5 and C6 sugars. The main co-product of lignocellulose conversion to ethanol is energy. The effluent from the distillation column that contains most of the lignin and other non-fermentable products is sent to a combined heat and power plant to produce process steam and electricity required by the ethanol plant. Depending on the proportion of lignin in the feedstock, excess electricity may be available for export sale.

8. Conclusions The ethanol fermentation industry is using heterogeneous raw materials rather than pure glucose. Optimization of the pre-treatment strategy aimed at reducing the formation of degradation products and optimizing enzyme mixtures for efficient conversion of pretreated biomass, together with improving fermentation efficiency using genetically modified strains, mixed cultures and the application of thermos-anerobes that can ferment hexose and pentose sugars to improve ethanol yield, are key areas of future research.

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