Corn and Lignocellulosic Ethanol Production Processes
Ethanol Production:Corn and Cellulosic Ethanol Lis Nimani March 28th, 2010
Need for Renewable Energy• The End of the Fossil Fuel Age: – Decline in Oil Production – At current consumption rates, coal reserves should last for about 200 years, oil for 40 years and natural gas for 60 years (BP, 2003).
Need for Renewable Energy – Increase in Greenhouse Gasses
Renewable Resources• Renewable energy can be defined as ‘energy obtained from continuous or repetitive currents of energy recurring in the natural environment’ (Twidell and Weir, 1986) – Or as ‘ energy flows which are replenished at the same rate as they are used’ (Sorensen, 2000).• Sources of Renewable Energy – Solar Energy, Tidal Energy, and Geothermal Energy• Principal Source for Renewable Energy is Solar Radiation – Direct vs. Indirect – Indirect: Hydropower, Windpower, Wavepower, and Bioenergy
Bioenergy• Bioenergy is the general term for energy derived from materials such as wood, straw or animal wastes, which were living matter recently. – Difference between fossil fuels.• Such materials can be burned directly to produce heat or power, but can also be converted into biofuels.• Bioenergy has been used as a form of energy since the invention of fire.
Bioenergy• The conversion of biomass into bioenergy:
Fuel• Fuel is any material that stores energy that can later be extracted to perform mechanical work.• Fuel interacts with oxygen and in doing so releases energy and changes into different chemical compounds-the combustion products.• Fossil Fuels, the result of hundred of millions of years of slow geological change acting on plant or animal matter, are examples of hydrocarbons. – Hydrocarbons consist primarily of carbon and hydrogen• Biofuels contain significant amounts of Oxygen as well.
Bioethanol• World ethanol production for transport fuel tripled between 2000 and 2007 from 17 billion to more than 52 billion liters. – United States predominantly uses Corn and Brazil uses Sugarcane.• In 2009 world wide ethanol fuel production reached 73.9 billion liters. – The United States and Brazil are responsible for 86% of the world’s ethanol production. (2009)• Cars can be created to use ethanol instead of gasoline. – Most cars on the road use 10% ethanol in the gasoline. – Ford’s Model T was the first Flexible –fuel car designed to run on ethanol. – Better performance of engine due to higher compression ratio – However ethanol has lower energy per unit volume than gasoline.
U.S. Policy for Bioethanol • In 2007 the United States passed an Energy bill – The bill requires the US to produce an annual 36 billion gallons of biofuels by 2022, with a 15 Billion gallons/year cap on ethanol made from non-cellulosic material in maize • Cap on non-cellulosic material nearly reached. – Food vs. Fuel debate • Increase on Corn prices • OECD published that the average price of Wheat will increase by 5%. – Need for Cellulosic ethanol • Is it achievable? – Approximately 1.4 Billions tons of biomass available in the U.S. – From this biomass, 80-150 Billion gallons of cellulosic ethanol could be produced.
Production of Bioethanol Chemistry: • Photosynthesis: Glucose is created in the plant by photosynthesis: 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2 • Fermentation: During ethanol fermentation, glucose is decomposed into ethanol and carbon dioxide: C6H12O6 → 2 C2H5OH+ 2 CO2 + heat • Combustion: ethanol reacts with oxygen to produce carbon dioxide, water, and heat: C2H5OH + 3 O2 → 2 CO2 + 3 H2O + heat • Net Reaction: Light → Heat
Production of BioethanolIndustrial Operations:• Corn Ethanol – Dry milling process – Wet milling process• Cellulosic Ethanol• Gasification
Corn Ethanol• Ethanol from corn is produced through fermentation, chemical processing and distillation. – Corn is the main feedstock in the United States – Brazil uses sugarcane.• Two types of Corn ethanol Production: – Dry milling Process – Wet milling Process• In Dry milling, the entire corn kernel is ground into flour.• In Wet milling, the corn grain is steeped in a dilute combination of sulfuric acid and water in order to separate the grain. – Corn oil is a by-product of this process.
Corn Ethanol Corn Kernel: • Endosperm: 82% of the dry weight. It’s the source of starch and protein. • Pericap: Protects the kernel and preserves the nutrients. • Germ: Contains the genetic information. • Tip Cap: Where water and nutrients flow
Corn Ethanol1. Dry Milling Process (82 %)2. Wet Milling Process (18%) Pretreatment Hydrolysis Fermentation Distillation Co-Products
Corn Ethanol Video Tour• Video of the Midwest Grain Producers: http://www.youtube.com/watch?v=npJ1N-1K84E
Dry Milling• Diagram of a Dry Milling Process: – The ICM Inc. process for Corn Ethanol • http://www.icminc.com/
Dry Milling• Delivery/Storage: Grain is delivered to the ethanol plant where it is stored in bins.
Dry Milling• Milling: The grain is screened to remove debris and ground.
Dry Milling• Cooking: During the cooking, the starch in the flour is physically and chemically prepared for fermentation. – Hot Slurry, Primary Liquefaction, and Secondary Liquefaction
Dry Milling• Hot Slurry: The mixed grain is mixed with water, where the pH is set at 5.8. Amylase enzyme is introduced to hydrolyze the starch. The mixture is then heated to 190 F. This temperature is maintained for 30 to 45 minutes.
Dry Milling• Primary Liquefaction: The slurry is pumped to a jet cooker at 221 F and held for 5 minutes. Mixture is then cooled. The mixture can either be cooled through atmospheric or vacuum flash condenser.
Dry Milling• Secondary Liquefaction: After cooling, the mixture is held for 1 to 2 hours at 180-190 F. – This is the time when the amylase enzyme breaks down the starch into more simple dextrins. • Alpha-amylase: randomly hydrolyzes the alpha-1,4-linkage. • Dextrin: low molecular weight carbohydrates produced by hydrolysis.
Dry Milling• A secondary enzyme called glucoamylase, is added before the mixture is pumped into the fermentation tank. – Glucoamylase: An enzyme that breaks the bonds near the ends of large carbohydrates releasing maltose and glucose.
Dry Milling• Simultaneous Saccharification Fermentation: The mixture inside the Fermentation tank is referred as mash. The enzymes break the dextrins into glucose. Yeast is added to convert the sugar to ethanol from Carbon dioxide. – Allowed to ferment for 50 to 60 hours. – Will contain 15% ethanol.
Dry Milling• It is also possible to have saccharification and fermentation done separately. Liquefaction Saccharification Fermentation• Liquefaction: starch to dextrin by amylase.• Saccharification: dextrin to glucose by glucoamylase.• Fermentation: glucose to ethanol.
Dry MillingEthanol Fermentation with Yeast:• Common Yeast: Saccharomyces cerevisiae, S. uvarum, Schizosaccharomyces pombe.• Reaction: C6H12O6 → 2 C2H5OH+ 2 CO2 – Under anaerobic conditions, yeast metabolize glucose to ethanol by the Embden –Meyerhof pathway. – The yield obtained does not normally exceed 90%.• Fermenation Conditions: ph 5-7; temperature 30-37 C. – Above 50 C yeast die, and at low temperatures of 0 to 10 C there is no activity.
Dry Milling• Distillation: Mash is pumped into a distillation system. – The columns utilize the differences in the boiling points of ethanol and water to separate the ethanol. – When completed, the stream contains 95% ethanol by volume.• The residue called Stillage, contains non-fermentable solids and water and is pumped out from the bottom of the columns into the centrifuges.
Dry Milling• Ethanol and water is an azeotropic system. – An azeotrope is a liquid mixture which when vaporized, produces the same composition as the liquid. • Ethanol at 95.63% with water will boil at 78.2 C. • The boiling point of ethanol is 78.4 C and of water is 100 C. • Therefore this is a positive azetrope. – Will not get a concentration higher than 95.63% by simple distillation, therefore need azetropic distillation.
Dry Milling• Types of Distillation Methods: – Pressure swing distillation: At high pressure the boiling point will increase, and the azetropic point will shift to lower b.p. with pressure. Must Distill in two columns at different pressures. The azetropic point will be crossed. – Azeotropic Distillation: Introduce an entrainer to form a ternary azetrope. • Entrainer: will produce a new lower b.p. azeotrope that is heterogeneous which will allow for separation. • Common entrainers are: Benzene, toluene or cylcohexane. – Membrane Permeation Distillation: Using membrane to separate. • Membranes can be more permeable to certain solution
Dry Milling• Dehydration: The ethanol after straight distillation contains 5% water. This water must be dehydrated. This is done by passing it though a molecular sieve to physically separate the remaining water from ethanol based on the different sizes of the molecules.• How is it done? 1. The ethanol containing 5% water passes through sieves. 2. Water is absorbed into the sieves. 3. 200 Proof anyhydrous ethanol is produced. 4. Molecular sieves are regenerated. • Done by heating under vacuum. • Typically last 5 years. – Dehydration can be done by azetropic distillation. • But this process is expensive and complicated.
Dry Milling• Molecular Sieves: They contain tiny pores that can absorb gasses and liquids.
Dry Milling• Denature/Storage: Addition of toxic additives to make ethanol undrinkable for humans. – Denature additives include: methanol, denatonium, gasoline, etc. – Storage tanks are generally sized to store ethanol for 7 to 10 days production capacity.
Dry Milling• By-Products: – Carbon Dioxide: The Carbon Dioxide is captured and purified with a scrubber so it can be sold to the food processing industry. • Carbon dioxide produced during fermentation with yeast. • Recovered through Carbon Dioxide recovery system. – Wet Distillers Grains (WDFs) and Distillers Grains (DDGs). • Feed to Cattle as feedstock.
Dry Milling• Distillers Grain: Stillage from the bottom of the distillation tank contains solids from the grain, yeast, and water during the process.• This stillage is separated through centrifuges. – Make-up water sent back to cook/slurry for water conservation. – Other material sent to multiple effect evaporation. • Concentrated into syrup. • This contains most of the nutritional value of original feedstock (WDG). • Sent through drying system to remove moisture and reduce spoilage (DDG).
Wet Milling• Grain is separated into the germ, fiber, and starch in a steeping process – Uses dilute sulfurous acid.• Many Byproducts produced in addition to ethanol. – Poultry feed, corn oil and syrup, and corn starch.
Wet Milling• Steps for Wet Milling: 1. Steeping: soak corn for 24- 48 hours. 2. Coarse milling: separate the germ from the kernel. 3. Fine milling: separate fiber from endosperm 4. Separation: separate starch from gluten. 5. Hydrolysis 6. Fermentation 7. Distillation 8. Dehydration 9. Denature 10. Storage
Corn Ethanol• Advantages of Wet Milling: – Separate the corn kernel. – Increased byproducts. – Simplified ethanol production• Disadvantages of Wet Milling: – Cost Intensive• How to improve corn ethanol production? – Improve enzymes – Convert more materials (i.e.. Fiber) into ethanol. – Improve separation of corn ethanol and increase efficiency of byproduct usage. – Conserve water and energy better. – Create genetically advanced yeast to improve fermentation.
New Generation Biofuels• 2nd Generation Biofuels: Cellulosic Ethanol – 2007 Energy Bill placed cap on Corn Ethanol production in the United States • Close to the cap, need alternatives to meet demands. – Massive amount of Feedstock available • Includes Crop residues, forest residue, and dedicated energy crops. – Fuel vs. Food competition • Cellulosic Ethanol does not affect the food market – Larger Net Energy Gain than corn ethanol • 1.3 for corn ethanol vs. 2 to 36 for cellulosic ethanol – Less Negative Environmental Impact • GHG reduction for Corn Ethanol at 21.8% vs. Cellulosic Ethanol at 90.9% (EPA.gov)
Lignocellulose• Lignocellulose is composed of Cellulose, Hemicelluloses and Lignin. – Cellulose: is a polysaccharide consisting of a linear chain of several hundred to over ten thousand Beta(1,4) linked glucose units. • Compared to starch, cellulose is much more crystalline. – Making it harder for enzymes and chemicals to access cellulose. • Structural component of cell wall. • Most common organic compound on Earth. – Wood is 40 to 50% cellulose. • Humans do not process the enzyme cellulase to hydrolyze Cellulose.
Lignocellulose• Hemicellulose: Is a polysaccharide that is derived from several sugars. It consists of glucose, xylose, mannose, galactose, rhmanose, and arabinose. – Compromises 20% of the biomass in most plants. – Consists of shorter chains. Around 200 sugar units. – It is branched.• Lignin: cross linked macromolecule consisting of phenylpropane units bound by ether and carbon-carbon bonds. – Second most abundant natural polymer – Allows for water and nutrient transport, as well as structural support – Blocks cellulases as well other bacteria, yeast to reach cellulose • Offers protection to plant.
Cellulosic Ethanol• To produce Cellulosic Ethanol, the Lignin and Hemicellulose must be separated from the cellulose, so the cellulase enzyme can hydrolyze the Cellulose creating glucose units.• These Glucose units are then fermented to produce ethanol.• Ways to make Cellulosic Ethanol: 1. Biochemical Process • Ethanol created through a series of steps: pretreatment, enzymatic saccharification, and fermentation. 2. Thermochemical Process • Ethanol is created by gasification, syngas fermentation or Fischer- Tropsch reforming.
Cellulosic EthanolPretreatment:• Step that separates the Cellulose, Hemicellulose and Lignin. – Prepares the Cellulose for hydrolysis – Very important step because it will influence all of the following processes – Cost Intensive• The objective for pretreatment is to remove the recalcitrance of lignocellulose. – This allows for enzymes and chemicals to have access to cellulose.• It removes the recalcitrance by: – Separating the Lignin and Hemicellulose – Pre-hydrolyze some of the Cellulose – Destroy Crystalline structure of Cellulose – Reducing the Size of the feedstock
Cellulosic Ethanol• Methods: There are many different types of pretreatments to prepare for ethanol production. – They can be classified as physical, chemical, biological.• Some of the more well known Pretreatments are: – Dilute Acid – Steam Explosion – SPORL – Organosolv – Ozone Pretreatment – Ammonia fibre explosion (AFEX) – And more…
Cellulosic Ethanol• Dilute-Acid Pretreatment: Use of acid to reduce the recalcitrance of Lignocellulose. – Very common method – Works better with hardwood and herbage• During Dilute-Acid pretreatment the Cellulose is partially degraded. The Lignin is condensed.• The Hemicellulose is dissolved by acidic hydrolysis and forms HMF and furfural. – Fermentation Inhibitors• No size Reduction• Conditions of Pretreatment: • 150 to 240 C • 2 to 30 minutes • 1 to 4% Sulfuric Acid Concentration
Cellulosic Ethanol• Steam Explosion: Feedstock is heated with high pressure steam. • Works well with herbage and hardwood.• It is done by holding the feedstock at a high temperature and pressure for a few minutes, and then discharging the material from vessel.• This mechanism removes hemicellulose and reduces the size of the feedstock. • Furfural and HMF will result from further degradation. – Inhibitors to Fermentation.• There is no significant degradation of Cellulose as well as no significant delignification of Lignin. The lignin will condense at high temperature and pressure.• Conditions of Pretreatment: • 190 to 230 C • 1.25 to 2.8 MPa • 2 to 10 minutes
Cellulosic Ethanol• Organosolv Pretreatment: Uses aqueous ethanol to fractionate lignocellulose to cellulose, lignin, and hemicellulose. – Works well with all feedstock• The Cellulose is partially hydrolyzed, while the Lignin is depolymerized and dissolved – High quality Lignin application for byproducts.• The Hemicellulose is hydrolyzed and partially degraded. The final products are furfural, HMF, levulinic acid and formic acid. – Chemicals from Hemicellulose – Some Inhibitors• Conditions: • 150 to 180 C • 40 to 60 minutes • 1 to 1.5% H2SO4 • 50 to 70% (v/v) ethanol concentration – Solvent Recovery
Cellulosic Ethanol• SPORL (Sulfite Pretreatment to Overcome Recalcitranc of Lignocellulose): To remove the recalcitrance, a sulfite chemical treatment is done initially. Then a mechanical size reduction is done. – Works well with all feedstock.• During this process the Cellulose is partially hydrolyzed while the Lignin is partially sulfonated and dissolved. – Use of Lignosulfonates as binders, dispersants, emulsifiers.• Hemicellulose is hydrolyzed to sugars. – Minimal fermentation inhibitors created.• The most energy efficient – Sugar production/unit energy consumption• Conditions of Pretreatment: • 170 to 190 C • 15 to 30 minutes • 3 to 7 % Sulfite • 1 to 4% Sulfuric Acid
Cellulosic Ethanol• Ammonia Fiber Explosion (AFEX): The feedstock is heated with concentrated ammonia. The pretreatment ends when the pressure is released. – Works well with herbage.• This process is economical because: – The sugars during the pretreatment are not lost. – No fermentation inhibitors are created. – Ammonia is recovered and reused• Conditions for Pretreatment – 80 to 120 C – 1 to 1.2 kg/kg Ammonia loading – 10 to 30 minutes
Cellulosic Ethanol• Once Pretreatment is finished the Cellylotic Process Begins: – In the hydrolysis process, the cellulose molecules are broken down to simple sugar units. • Two Types of Hydrolysis: – Enzymatic Hydrolysis – Chemical Hydrolysis
Cellulosic Ethanol• Enzymatic Hydrolysis: Cellulose chains are broken into glucose molecules by cellulase enzymes. – Majority of commercial cellulases are produced from fungus, Trichoderma reesei. – Cellulases are composed of endoglucanases and cellobyhydrolases• The enzymatic hydrolysis is controlled by both the enzyme and the substrate.• Reactions are slow and process is expensive.
Cellulosic Ethanol• Chemical Hydrolysis: Cellulose chains are broken into glucose molecules by acid. – Dilute acid can be used at high heat and pressure – Concentrated Acid at lower temperatures and atmospheric pressure.
Cellulosic Ethanol• Fermentation: – Consists of two pathways: • Glucose Fermentation • Hemicellulose Fermentation
Cellulosic Ethanol• Glucose Fermentation: – Fermentation of 6 Carbon sugars to ethanol. – Typically Baker’s yeast is used (Saccharomyces cerevisiae). – The temperature is typically around 30 C, while the pH is adjusted to be around 5.
Cellulosic Ethanol• Pentose Fermentation: – Fermentation of 5 Carbon sugars to ethanol. – S. cerevisiae can not ferment xylose (common sugar found in hemicellulose) to ethanol – Need genetically modified S. cerevisiae to ferment xylose. • Could possibly use Zymomonas mobilis, Pichia stipitis – Can ferment both hexoses and pentoses – Genetically modified Bacteria • E. coli
Cellulosic Ethanol• Different Hydrolysis and Fermentation Pathways:
Cellulosic Ethanol• Separated Hydrolysis and Fermentation (SHF): – Hydrolysis and Fermentation performed separately • This is a good method because each process is done at optimum temperature. • Disadvantages are that high enzyme loadings are needed and get inhibition due to glucose accumulation.• Simultaneous Saccharification and Fermentation (SSF): – Hydrolysis and Hexose fermentation done in single step • Enhanced hydrolysis due to glucose removal simultaneously • Disadvantages are that they are not performed at ideal temperatures.• Simultaneous Saccharification and Co-Fermentation (SSCF): – Hydrolysis and fermentation of both hexoses and pentoses • Cellulase production done separately.• Consolidated Bioprocessing (CBP): – Whole process done in one step. – Similar to how rumen of Cows work.
Cellulosic Ethanol• After Ethanol production, need to retrieve the ethanol – Ethanol is distilled and dehydrated, which then can be stored and used as a Biofuel. – Similar to process of Corn ethanol once fermentation is complete. • Lignin can be used for energy • Do not have WDG’s and DDG’s from Cellulosic Ethanol
Cellulosic Ethanol• Issues that need to be addressed with Cellulosic Ethanol Production: – Process is very expensive due to pretreatment necessary for conversion to ethanol. • Need to remove the recalcitrance of cellulose – Enzymes used in process are expensive. • Enzymes cost 30 to 50 cents per gallon compared to 3 cents per gallon for corn. – Need to produce byproducts from Hemicellulose and specifically Lignin that have value.