Cellulosic Ethanol and the future of Biofuels: From carbohydrates to hydrocarbons by Efthymios Kallos and Theodora Apostolopoulou ENE505: Energy and the Environment University of Southern California 2007
Abstract This report is aimed at providing a summary of the field of biofuels: the production of liquid fuels from plants. Biofuels are not aiming at solving the world energy problem, but rather at providing a viable alternative to the transportation fuels which are presently derived almost in their entirety from imported oil. Rising oil prices, instabilities in the oil-producing regions of the world and greenhouse gas emissions from fossil fuels provide the motivation behind a field in ferment. As opposed to other renewable intermittent energy technologies such as photovoltaic cells and wind farms, which require (currently inefficient) electrical storage mechanisms in order to function reliably over long periods of time, plants absorb solar energy and store it chemically inside their biomass. It is estimated by our report that 1TW of average power is stored into available for biofuel production biomass in the United States only (the global power consumption is during the year 2007 at 15TW). Even if a small fraction of that stored energy can be retrieved from the biomass, a significant portion of motor fuel could be replaced. The first chapter summarizes the present global situation in terms of energy demand, CO 2 emissions and oil consumption. Chapter 2 provides a basic background on biofuels and examines their potential from an energy perspective. Chapter 3 provides an overview of the biofuel landscape in the United States, which is currently relying on ethanol fuel derived from corn kernels to provide 3% of its transportation fuels, although this type of ethanol could not be expanded into large scale. Chapter 4 examines the details of producing ethanol from the cellulose molecules that comprise the plant walls, which, if harnessed properly, can have much higher efficiencies and energy outputs than crop-derived ethanol because it can consume nontraditional biomass which is not used directly for other purposes. Chapter 5 describes briefly other biofuel production techniques, such as Biodiesel (popular in Germany), sugarcane-derived ethanol (successful in Brazil), Biobutanol and algae cultivation. Finally, we summarize the report in chapter 6.
Table of contents 1 Review of the energy demand trends in the world... 6 1.1 World energy demand... 6 1.2 Energy consumption in the United States... 9 1.3 Oil consumption in the World... 11 1.4 CO 2 emissions... 12 2 Basic concepts of biofuels and their energy potential... 15 2.1 Basic concepts and short history of biofuels... 15 2.2 The potential of biofuel production: An energy perspective... 16 3 Corn ethanol... 20 3.1 Present status in the World and the US... 20 3.2 The Corn Ethanol Cycle... 24 3.3 Advantages and Disadvantages of Corn Ethanol... 26 4 Cellulosic Ethanol... 28 4.1 Basic Concepts of Cellulosic Ethanol... 28 4.2 Comparison of cellulosic ethanol to other biofuels... 33 4.3 Cellulose Pretreatment... 35 4.4 The cellulosic ethanol cycle... 37 4.5 Present state and future of cellulosic ethanol... 40 5 Other types of biofuels... 41 5.1 Sugarcane Ethanol... 41 5.2 Biodiesel... 43 5.3 Biodiesel from Algae... 45 5.4 Biobutanol... 47 6 Summary... 48
List of figures Figure 1 : World energy consumption [1]... 6 Figure 2: Global energy sources [1]... 7 Figure 3: Fossil fuels reserves [1]... 7 Figure 4: Location of fossil fuel reserves [1]... 8 Figure 5: Energy consumption in the US... 9 Figure 6: Energy sources in the US [3]... 9 Figure 7: Global oil demand... 11 Figure 8: CO 2 emission and concentration... 12 Figure 9: Origins of CO 2 emissions... 13 Figure 10: Cost of electricity per MW-hr... 13 Figure 11: Sources of carbon... 16 Figure 12: Photosynthesis efficiency... 17 Figure 13: Potentially useful biomass land area in the US... 18 Figure 14: Summary of solar energy storage in plants [8]... 19 Figure 15: Global Ethanol Production... 20 Figure 16: US corn ethanol production... 21 Figure 17: Location of E85 ethanol stations... 21 Figure 18: A corn ethanol refinery [5]... 22 Figure 19: US Corn crop yield... 22 Figure 20: US Corn Crop Yield per year... 23 Figure 21: The corn ethanol cycle... 24 Figure 22: Energy balance for corn ethanol... 27 Figure 23: Cellulose Molecules... 28 Figure 24: Lignin Molecules... 29 Figure 25: Sugar producers in plant walls... 29 Figure 26: Plant Walls Part 1... 30 Figure 27: Real picture of plant walls... 30 Figure 28: Plant Walls - Part 2... 31 Figure 29: Comparison of starch and cellulose molecules... 32 Figure 30: Switchgrass... 32 Figure 31: Net energy gain of various biofuels [5]... 33 Figure 32: CO 2 emissions of various biofuels [5]... 34 Figure 33: Pretreatment of cellulose... 35 Figure 34: Effect of genetically modified lignin concentration in sugar conversion... 36 Figure 35: Cellulosic ethanol cycle... 37 Figure 36: Xylose fermentation by modified E.Coli... 38 Figure 37: Cellulose fermentation tank... 38 Figure 38: Cellulosic ethanol production... 39
Figure 39: The 6 six planned commercial cellulosic ethanol plants [17]... 40 Figure 40: Sugar canes... 41 Figure 41: Sugarcane harvest in Brazil [19]... 42 Figure 42: Ethanol refinery in Brazil [5]... 42 Figure 43: Biofuel production in the European Union... 43 Figure 45: Biodiesel production in North America... 44 Figure 44: Biodiesel production around the world... 44 Figure 46: Algae growing bags [5]... 45 Figure 47: Biofuel production per acre for various plants... 46 Figure 48: Energy density of Biobutanol... 47
1 Review of the energy demand trends in the world 1.1 World energy demand Figure 1 : World energy consumption [1] In order to understand the impact that biofuels may have in the world economy and energy demand, we first need to understand the trends in world energy consumption. Figure 1 shows the global energy consumption per sector in unites of billion of barrels of oil equivalent (bnboe) [1]. It is clear that since 1971 the demand has doubled, and it is expected to nearly double by 2030. Currently 15TW of power is consumed on average around the world [2], from which roughly 18% goes to transportation. The exact values fluctuate from country to country; for example the US transportation energy demand is 28%, which is higher than the average country value (see Figure 5). Figure 2 shows where the world is getting its energy from. Most of the energy comes from the three large fossil fuels resources, namely coal, oil and natural gas. The next largest energy source is biomass, which especially in 3 rd world countries is a large energy source through burning of firewood to supply heat. Nuclear power is on average a very small contribution, although some countries may utilize it more (e.g. France which gets the majority of its electrical
power from nuclear power plants). Renewable sources currently contribute a very small amount, and that is not expected to change dramatically in the next 25 years. Figure 2: Global energy sources [1] Figure 3: Fossil fuels reserves [1]
Figure 3 examines the availability of fossil fuels in the future. For oil, at the current production rate the reserves (about 1 trillion barrels) are expected to last 41 years (from 2001). There is more oil available underground but it is not economical at the present time to extract it. Of course the energy demand is increasing with time so that time span will actually be smaller. Similarly, gas proven supplies are expected to last 67 years. Coal on the other hand has a vast amount of proven reserves, and probably much more than that since there is no immediate urgency to look for more. Figure 4: Location of fossil fuel reserves [1] The location of the fossil fuel supplies is also of interest. It is interesting to notice in Figure 4 that at least for oil and gas, the 3 largest markets in the world (North America, Europe and Asia Pacific) consume ~70% of the energy stored in fossil fuels but only have 10-15% of the actual deposits in their territory. The security of supplies is therefore not granted, which provides extra motivation for the development of renewable, localized sources of energy.
1.2 Energy consumption in the United States Commerce 18% Residential Industrial 32% US Power Consumption (2005) Total = 3 TW 21% Gas + Diesel 97% Transport 28% Figure 5: Energy consumption in the US Ethanol 3% Figure 5 shows the energy consumption trends in the US, averaged over the year 2005. The total power consumption of 3TW (1/5 th of the world s total) is divided into residential consumption, commercial consumption, industrial consumption and finally transportation. Most of the energy in transportation is provided through gas and diesel, and only 3% currently is provided by ethanol, the liquid fuel derived from biomass. Figure 6: Energy sources in the US [3]
Figure 6 shows where does the United States derives its energy from [3]. The majority comes from petroleum products (including gasoline for transportation), and then its natural gas and coal, and then 8% comes from nuclear energy. Only 6% of the power consumed comes right now from renewable sources, from which the majority (roughly half) originates from biomass, another half from hydroelectric power and a tiny percentage that comes from geothermal, wind and solar energy.
1.3 Oil consumption in the World Figure 7: Global oil demand Presently 80 million barrels of oil are consumed every day around the world, and roughly half of that number covers the increasing transportation needs (Figure 7), especially in the Asian markets. The US presently consumes 1/4 th of this amount, from which 65% is imported [4]. Since the US is on the right hand side of Hubbert curve for oil production, which means that the production is decreasing, as a result the amount of imports is increasing every year in order to cover the ever-increasing demand. This obviously presents problems regarding the security of future supplies of oil.
1.4 CO 2 emissions Figure 8: CO 2 emission and concentration Of increasing interest is also the carbon dioxide concentration in the atmosphere, measured in billion tons per year, as a function of time. The concentration (which is the integral of the yearly emission values) seems to correlate with the average temperature of the planet and its increase could have potentially devastating effects. Figure 8 presents the yearly measured (until 2004) and predicted (from 2005-on) emissions and concentration of CO 2 in the atmosphere, along with a number of scenarios for the future trends (in color). The emission predictions assume a 2% yearly rate of increase in the emissions. The fact is that the CO 2 emissions will not decline unless we switch to burning renewable sources, because the CO 2 emitted originates from carbon molecules stored inside the fossil fuels in the ground; by burning those fuels, the carbon is released in the atmosphere. Biofuels do not suffer from this problem to the extent that other fossil fuels are not used during their production, because the CO 2 emitted when burned is CO 2 that was captured from the plants during their growing season and it is simply being re-emitted into the atmosphere. Finally, Figure 9 presents the details of the origins of CO 2 emissions. It is interesting to notice that emissions from land use, such as deforestation, are comparable to other energy-related emissions such as power (mainly from coal) and transportation.
Figure 9: Origins of CO 2 emissions Figure 10: Cost of electricity per MW-hr
Figure 10 shows the cost of electricity from various renewable sources of energy in 2006, as a function of a potential carbon tax, i.e. assuming that there is a certain amount of tax per ton of CO 2 emitted in the atmosphere. Renewable sources have flat curves (their cost does not depend on carbon), while heavy carbon emitters have the steepest curves (such as coal). The graph shows that a tax of at least $20 per ton is required in order to have an appreciable effect on the electricity production trends. Unfortunately, solar is still extremely expensive at $250 per MW-hr, and gasoline is very low at only $0.35 per gallon, which remains low for most reasonable values of carbon tax. This simply means that the amount of energy we put in our gas tanks per refill (gasoline supplies 130MJ of energy per gallon) is large compared to the damage it produces by releasing CO 2.
2 Basic concepts of biofuels and their energy potential 2.1 Basic concepts and short history of biofuels The term Biofuels refers in general to any type of liquid fuel that can be derived from plant material. The basic idea is that plants can store solar energy during their growing season into chemical energy inside their carbohydrate bonds through the process of photosynthesis. This energy can be extracted afterwards and be converted into liquid fuels (mainly of hydrocarbon structure). Photosynthesis has an energy storage efficiency of 0.1%-1% (see section 2.2), which is less than the 20% efficiency achieved with solar cells; however there is no easy and cheap way to store electrical energy from photovoltaic cells. On the other hand plants over millions of years of evolution have developed those automated storage processes and need no further tweaking: in essence, biofuels use plants as natural batteries. Source plant material for biofuel can be any crop, such as wheat, corn, sugarcane, soybeans etc. However crops cannot be ultimately used in the long term for biofuel production because they are also used as food sources, both for humans and for animals. Alternative plant materials (biomass) include switchgrass, poplar, grass, wood chips or even magazines. About one century ago Henry Ford s first car run on ethanol [5], a biofuel made from peanut oil. However soon after he realized that fuel originating from oil was more powerful per unit volume, and ever since oil and its products (gasoline, diesel) have been dominating the fuel market. In the 1970 s with the OPEC embargo in the US the interest on alternative energy sources and biofuels was increased again temporarily. Since the early 2000 s biofuels have gained momentum again because of the increased oil prices and the unstable situations in the Middle East oil providing countries: the need for oil independency is becoming more and more urgent as supplies run out and prices go up. Biofuels are not aimed at solving the energy problem. Rather, they are aimed towards reducing dependency on oil and at the same time provide a renewable alternative source of energy for the transportation fuels, for which implementation of other renewable options such as solar, wind or hydrogen power is still too far out in the future. Hydrocarbons such as gasoline or biofuel-derived ethanol are great for transportation fuels because they have very high energy density and at the same time they can be transported relatively safely and quickly, without excessive costs.
2.2 The potential of biofuel production: An energy perspective In this section we will examine more closely the potential of biofuels for replacing the transportation fuels in the US, based mostly on energy arguments. Figure 11: Sources of carbon From the discussion in the previous section, carbon is at the present time the best option to provide high energy density, easily transportable fuels. Figure 11 shows the major sources of carbon in the world [1]. Annually, the gasoline consumed contains about 1500 million tons of carbon and the diesel 500 million tons. For comparison, the coal that is burned annually amounts to 5300 million tons (since coal is mostly carbon). In order to have a major impact to the transportation sector, an alternative carbon source must supply for example at least 15% of the carbon presently supplied by gasoline (notice the red line on the graph). The figure shows that simply converting individual crops to fuels is does not provide enough carbon: all the corn in the world would have to be used for ethanol production in order to get only 15% of the motor fuels! Also, the crops are necessary for providing food to humans and animals and therefore can never be fully converted to fuel. The only viable alternative seems to be biomass (on the right hand side of the graph). Today this biomass includes paper and firewood materials; however in the future the total potential of biomass to provide carbon for fuels, if tapped properly, is the only reasonable sustainable source of carbon in the world.
Utilizing the full biomass potential requires harvesting the useful chemical energy locked inside plants other than crops. Roughly 100 billion tons of carbon go back and forth between the plants and the atmosphere annually. Only 0.5% of that amount needs to be retrieved from the plants in order to satisfy the energy needs of 15% of the motor fuel requirements. Solar power in kw per acre captured from plants (energy averaged over 1 year) Solar power on surface (100%) 800 Incident on foliage (70%) Proper wavelengths (25%) Stored Chemically (CH2O) (35%) Actual storage Per capita power consumption (US) 8 10 50 140 560 Figure 12: Photosynthesis efficiency Figure 12 examines the potential of plants to store energy through the photosynthetic process [6]. The solar intensity reaching the surface of the earth, averaged over one year, day and night, for median latitudes, is about 200W/m 2. This translates to about 800kW of average power dissipation per acre (=4047 m 2 ). From that amount of power, roughly 70% reaches the plant leaves since the foliage density is always less than one (the rest of the sunlight hits the ground). 25% of the amount of power incident on foliage is at the right wavelength range to be absorbed by plants. Finally, 35% of that power, or about 50kW per acre, is ultimately stored chemically inside the plants. Realistically, the total actual storage is roughly 0.1% for most crops (corn, wheat etc). The most efficient
plant is sugarcane which has a photosynthetic efficiency of about 1%, which according to these calculations corresponds to 8kW of power per acre. It is interesting to notice that the per capita average power consumption in the US is around 10kW [2], and most developed countries around the world consume 3-5kW per capita. This number means that each of us consumes roughly as much power as it is stored in one acre of vegetation. At the same time, it is worth to note that the average power for each person to simply survive biologically is roughly 100W (2000Kcal per one day), or about the power consumed by a regular incandescent light bulb. Therefore we consume 100 times more power than the absolute required minimum. The next step is to figure out how much biomass is available. In the US, the billion-ton study [3] researched that roughly 1.3 billion tons of biomass can be available every year for biofuel production without affecting food supplies. Assuming that 10 tons of biomass can be produced from 1 acre (currently corn produces 5 tons per acre, but this limit can increase up to 20 tons per acre if modern biomass techniques are introduced), 1.3 billion tons of biomass will require 130 million acres of plant area (see Figure 13). For comparison, the total US surface area is 2.2 billion acres and the whole corn crop production covers 90 million acres [7]. Corn 90M US Surface Area Total = 2.2B acres Useful Biomass: 130M x 8kW = 1TW Other Figure 13: Potentially useful biomass land area in the US 130 million acres store (according to the arguments in section 2.2) 130Mx8kW = 1TW of power. This is roughly 1/3 of the average US power consumption, or approximately the power consumed by the whole transportation sector. However, since this potentially stored energy
cannot be extracted with 100% efficiency, the actual power extracted will be less. Realistically, roughly 1/3 of this amount, or 0.3TW, could be converted to biofuels for energy consumption from the vehicles in the US. So, 30% of the motor fuel in the US could be replaced by ethanol fuel is this energy source is tapped. Note however that this requires advances in technology that are not presently available, but they can be achievable in the near future, mainly the extraction and breakdown of cellulose from biomass (see section 4). Figure 14: Summary of solar energy storage in plants [8]
3 Corn ethanol 3.1 Present status in the World and the US Figure 15 shows the global ethanol production in millions of gallons, varied by continent [9]. The two data bars that stand out are North America which mainly refers to corn ethanol production in the US, and South America, which refers mainly to Brazil s sugarcane ethanol production. Figure 15: Global Ethanol Production The United States currently produces about 3% of its motor fuel from corn (see Figure 6), through making ethanol. Figure 16 shows the total ethanol production in the US, which in 2007 reached slightly over 6 billion gallons. The Department of Energy Policy Act of 2005 mandated that by the year 2012 at least 12 billion gallons of motor fuel must be provided by ethanol [10], in an attempt to gradually reduce dependence on foreign oil. Presently there are 138 refineries that produce ethanol across the US, most of them in the Midwest and central US. Around 50 more refineries are under construction. These refineries supply 1200 stations with E85 gasoline (Figure 17): a blend that contains 85% ethanol and 15% gasoline.
Figure 16: US corn ethanol production There is also the E10 blend, which contains 10% ethanol and 90% regular gasoline, and it is found is most regular gasoline stations as virtually all vehicles can burn it without engine modifications; E10 also boosts the octane rating and produces 20% less emissions. On the other hand the E85 blend corrodes the engines, thus it requires specially modified engines, the so called flex engines (and flex cars). About 6 million out of the 240 million cars in the US are flex cars capable of burning the E85 blend [11]. One gallon of E85 ethanol currently costs $2.4 on average [12]; at the same time its energy density is 30% less than the energy density of gasoline. This is the price after a $0.51 per gallon tax credit that ethanol producers enjoy. From those 6 million flex cars available, only 2% of them actually run on E85. Figure 17: Location of E85 ethanol stations
Figure 18: A corn ethanol refinery [5] US Corn Crop Yield Total = 90M acres Food 20% Ethanol 80% Figure 19: US Corn crop yield Those 6 billion gallons of ethanol are produced by using 20% of the 90 million acre corn supply in the US (Figure 19). This production competes with the corn yields as a food crop since it is one of the major food sources in the US, and increasing demand on ethanol also drives the corn prices up. Corn is also used as a feedstock for animals, which eventually provide other types of
goods such as meat and dairy products, and those prices are also eventually affected. This is one of the main reasons why corn alone cannot provide an adequate solution to the transportation fuels problem: as we also saw in Figure 11, even if the whole US corn production was dedicated to ethanol, it would only supply 15% of all motor fuel. See also Figure 20 for some supplemental corn yield information. Figure 20: US Corn Crop Yield per year
3.2 The Corn Ethanol Cycle The following graph (Figure 21) summarizes the carbon cycle through ethanol production, from photosynthesis to combustion. We will examine those steps in more detail. Figure 21: The corn ethanol cycle Photosynthesis. This part of the cycle happens inside the plants. The plant absorbs solar energy, along with water from the ground and carbon dioxide from the atmosphere. Along the lines presented in Figure 12, a fraction of the incident solar energy is stored inside the plant in the form of carbohydrates, mainly starch, which is a long chain of glucose molecules attached together. During this process oxygen is released into the atmosphere. The general equation of the chemical reactions is 6 CO 2(gas) + 12 H 2 O (liquid) + photons C 6 H 12 O 6(aqueous) + 6 O 2(gas) + 6 H 2 O (liquid) Saccharification (Hydrolysis). This step occurs after the plant is grown, harvested, milled and placed into large tanks where heat is applied, along with special enzymes called amylases [13]. Those enzymes break down the starch molecules into sugars, i.e. simple glucose molecules, which can afterwards be fermented to ethanol. Presently these hydrolytic procedures are expensive and time consuming and they also require fossil fuels to provide the heat required for the enzymes to work properly. Fermentation. This step is identical to the fermentation procedures used to produce wine and other alcohols. The sugars are placed in large tanks, and yeast organisms are fed into them, along with supplied heat (provided also by fossil fuels, typically natural gas or worst coal).
The yeast ferments eats up the sugars, releasing CO 2 (as a by-product) and producing ethylalcohol or ethanol. This decomposition follows the chemical reaction: C 6 H 12 O 6 2C 2 H 6 O + 2CO 2 The ethanol produced has to be distilled and placed into tanks for transportation. Unlike gasoline and diesel, ethanol cannot be transported in current pipelines because it is highly corrosive and it doesn t mix with water remnants in the pipes, which may alter its usefulness. Therefore diesel (more fossil fuel) has to be used for transporting ethanol into the stations. Combustion. After the fuel finds its way inside the fuel tanks of cars, it mixes with oxygen and burns (combusts), releasing energy along with CO 2 and water. The chemical reaction that takes place is: C 2 H 5 OH + 3O 2 2CO 2 + 3H 2 O + heat If air is used instead of oxygen for this process (which is usually the case), then a number of byproducts are also released during combustion. This reaction completes the cycle of the ethanol fuel. It is interesting to notice that the solar energy that was initially captured from the plant is finally released as heat (rapid motion of air molecules) and it is then altered into kinetic motion of the car. The chemical reactions of fermentation + combustion, when added up, they produce exactly the inverse reaction of photosynthesis.
3.3 Advantages and Disadvantages of Corn Ethanol The US initiated ethanol production from corn, mainly because it is one of the most widely available cultivated and available plants in the country. It was the easiest choice, but it can only partially fulfill its purpose as a biofuel crop. That is because, as we saw in section 3.1, there is simply not enough corn mass to supply more than 15% of the transportation fuel in the US. Also, being a food crop there is competition for food supplies, which eventually drives the corn cost upwards. In addition, roughly 80% of the energy input required in the production of ethanol originates from fossil fuels. On top of that a lot of fossil-fuel derived fertilizers are used to grow the corn, and this causes severe soil erosion. Because of the extent of fossil fuel used in the production of corn ethanol, the net energy gain is at most +30% over the fossil fuel energy required to grow the crop (see Figure 22), and the CO 2 emissions are comparable (see Figure 32). Ultimately, although corn ethanol may seem like a green solution, it is mainly a product of fossil fuels. Part of the reason is that only the ears of the corn are used for ethanol production, and the remaining of the stalk is wasted; solutions such as the ethanol derived from cellulose (see section 28) bypass those problem and increase the energy efficiencies by utilizing the whole plant. There are also some problems with the use of ethanol in general. First, ethanol mixes with water and so it cannot be transported in existing oil pipelines: it requires trucks that run on diesel for transportation until another infrastructure is built. Furthermore, ethanol is highly corrosive and can only run in high blends (more than 10%) in specially designed flex-engines. Eventually the car engines will need to be converted to flex, although this is not an impossible task as Brazil has taught us (see section 41). Finally, ethanol has 30% less energy density than regular gasoline, and therefore provides 1/3 less miles per gallon of fuel. Again, this may be a small disadvantage but not a serious one as tanks could be built slightly bigger and also the price of one gallon per unit energy provided is still smaller than gasoline s because of the ever-increasing imported oil prices.
Figure 22: Energy balance for corn ethanol
4 Cellulosic Ethanol In this section we will examine the most promising idea on producing ethanol from biomass, which is to harness the energy stores chemically in the cellulose molecules in the plant walls. 4.1 Basic Concepts of Cellulosic Ethanol Cellulose is the most abundant naturally occurring molecule on the planet [14]. It is most commonly found on the plant walls, along with hemicellulose and lignin (see Figure 25). It was developed by the plants when they moved from the sea to the land roughly 400 million years ago in order to allow them to stay upright and protect themselves from ground predators. Cellulose has crystalline form and consists of 6 carbons sugars, such as glucose chains, and hemicellulose is amorphous and consists of 5 carbon sugars, such as xylose. Lignin on the other hand is a very complex chain of hydrocarbons (see Figure 24) that is very tough to penetrate it has a structure similar to that of asphalt, and it is those lignin molecules that hold cellulose and hemicellulose together. Figure 23: Cellulose Molecules
Figure 24: Lignin Molecules Sugar producers in plant walls Cellulose Hemicellulose Lignin Other 5% 25% 45% C 6 H 12 O 6 25% C 5 H 10 O 5 Figure 25: Sugar producers in plant walls
Figure 26: Plant Walls Part 1 Figure 27: Real picture of plant walls Figure 26 and Figure 27 show in detail (simplified) the composition of a plant wall. 45% of the wall consists of cellulose, 25% consists of hemicellulose and another 25% consists of lignin.
Figure 28: Plant Walls - Part 2 The molecular structure of cellulose is shown in Figure 28. The crystalline-microfibril structure of cellulose is seen to consist of glucose molecules chained together, similar to the starch molecules. However, there are some crucial differences: the orientation of glucose molecules in the cellulose molecules is opposite for every other glucose molecule, as opposed to starch where they are oriented always in the same direction (see Figure 29). This makes cellulose more recalcitrant than starch (i.e., tougher to breakdown) and they are therefore tougher to hydrolyze.
Figure 29: Comparison of starch and cellulose molecules Cellulosic ethanol is of great interest because cellulose can be found in a number of plants that are not food crops, are available abundantly and do not require fertilizers or excess water amounts to grow. This biomass includes switchgrass, willow, poplar, jatropha, woodchips, grass and even magazines. Corn and other food crops on the other hand require copious amounts of fertilizers and water in order to grow efficiently. Figure 30: Switchgrass
4.2 Comparison of cellulosic ethanol to other biofuels Other than the abundance of cellulose in the plants, let us examine further some advantages of cellulosic ethanol compared to other types of biofuels. Figure 31 shows the fossil fuel energy ratio, namely the energy delivered to the customer per 1MJ of fossil fuel energy required to develop and produce the fuel. We see that electricity (from coal) and regular gasoline have both gains lower than unity (they are non-renewable sources). Corn ethanol only offers +30% energy gain at the best case scenario, while sugarcane has 8-fold increase in the energy output because sugarcane is the most efficient photosynthetic (solar light capturing) plant (see more in section XXX later in this text). Cellulosic ethanol on the other hand has the greatest potential from all the other biofuels as the whole plant can be used to produce it. MJ of energy delivered per MJ of fossil energy required 10 8 6 4 2 0 0.45 0.8 2.5 1.3 Electricity Gasoline Biodiesel Corn Ethanol 8 Sugar Ethanol 10 Cellulosic Ethanol Figure 31: Net energy gain of various biofuels [5] It is also interesting to look at the carbon emissions that are produced when making biofuels. Typically papers in the literature compare the emissions produced in making and burning one gallon of biofuel or regular fuel; however it is more accurate to calculate the amount of CO 2 emitted per unit energy consumed, because at the end it is the energy consumption that matters, not the volume consumption. Here we present the amount of CO 2 emitted when making and burning 130MJ of energy, which is roughly the energy contained in 1 gallon of regular gasoline. This is shown in Figure 32.
Pounds of CO 2 emissions per 130MJ (= 1 gallon of gasoline) 50 40 30 20 10 0 50 20 11 21 Electricity Gasoline Biodiesel Corn Ethanol 12 Sugar Ethanol 3 Cellulosic Ethanol Figure 32: CO 2 emissions of various biofuels [5] We see that the worse emitted is electricity, because it relies heavily on burning coal, which has a very high concentration of CO 2. Gasoline and corn ethanol have similar amounts of emissions, and this is due to the large fossil fuels energy required in order to produce ethanol. Sugar ethanol performs a little better because of the higher energy efficiency (see Figure 31), but the ultimate and most promising option is cellulosic ethanol, which, if tapped properly, produce only 3 pounds of CO 2 per 130MJ of energy delivered. It is the only option that can have a significant impact in the reduction of CO 2 emissions.
4.3 Cellulose Pretreatment One of the main issues in making cellulosic ethanol is the penetration of the lignin barriers. Presently it is not easy to penetrate lignin, and therefore the enzymes that are responsible for hydrolyzing cellulose molecules (and converting them to simple sugars) cannot physically access them. In order to overcome this difficulty a process called pretreatment need to be applied, where the biomass is heated in the presence of acids or ammonia in order to make the cellulose available to the enzymes by breaking the lignin walls. Figure 33: Pretreatment of cellulose Pretreatment is a very expensive and not green-friendly process, and it is one of the main reasons why production of cellulosic ethanol is not economically viable yet [15]. Part of the ongoing research on reducing cellulosic recalcitrance is to genetically engineer plants that have weakened lignin walls so that they are more readily accessible to enzymes. Figure 34 shows a recent result from the literature [16] where sugar production from enzymes was increased as the lignin concentration was decreased in different genetically modified versions of the plant. This reduction in recalcitrance could eventually lead to much cheaper pretreatment requirements.
Figure 34: Effect of genetically modified lignin concentration in sugar conversion
4.4 The cellulosic ethanol cycle Figure 35: Cellulosic ethanol cycle The cellulosic ethanol cycle (Figure 35) is very similar to the corn ethanol cycle (Figure 21), apart with a few differences in the initial steps that take place before the fermentation. The cellulosic ethanol requires the pretreatment phase, which raises the cost significantly. The hydrolysis phase is also different compared to corn/sugarcane ethanol because it requires different types of enzymes: those enzymes need to break down the pair-structure of the cellulose molecule, as opposed to breaking down the identically structured starch molecules. Moreover, the enzymes presently used originate from microorganisms found in stomachs of animals such as cows and goats, as it is hard to make them from scratch. These enzymes are very inefficient and slow. Furthermore, the yeast that is used to ferment the glucose molecules to ethanol can only ferment the 6-carbon cellulose-derived glucose molecules, which comprise 45% of the plant material. The 25% that 5-carbon hemicellulose-derived xylose molecules occupy are not fermented and are wasted. Efficiency improvements would require making use of this wasted carbon chemical energy, so a lot of research in under way to create organisms that can ferment 5 carbons sugars [17]. Figure 36 shows some recent results of genetically modified E.Coli that can ferment xylose into ethanol. Unmodified E.Coli ferments the sugars to acids, not ethanol. However the problem now is that E.Coli and other microorganisms cannot survive in high-ethanol environments, so they stop producing ethanol when only ~5% of the concentration is ethanol. Again, ongoing research is aimed at producing bacteria that can survive in the poisonous high-ethanol environments of fermentation.
Figure 36: Xylose fermentation by modified E.Coli Figure 37: Cellulose fermentation tank
Figure 38: Cellulosic ethanol production Figure 37 shows an actual fermentation tank, while Figure 38 graphically represents the various steps in the production of cellulosic ethanol [18].
4.5 Present state and future of cellulosic ethanol The cellulosic ethanol production at this point is still in an experimental and demonstration stage, pursued only by private companies. Not a drop of cellulosic ethanol exists in any car today. This is because it is not yet economically feasible to produce cellulosic ethanol in large quantities, due to the reasons explained in section 4.4. Still, in 2005 the Department of Energy awarded a $385M grant to help build in the next few years 6 commercial cellulosic ethanol plants around the country, which aim to produce (when finished) 130 million gallons of cellulosic ethanol per year, which corresponds to roughly 350MW of power. Remember that the corn ethanol production is presently at 6 billion gallons per year. While the cellulosic ethanol production is still in its infancy, as the technology progresses and the financial and scientific barriers are broken, its production is expected to rise sharply over the next 5-10 years. Figure 39 shows the details of the planned commercial cellulosic ethanol plants. Figure 39: The 6 six planned commercial cellulosic ethanol plants [17]
5 Other types of biofuels In this section we will examine briefly some other types of biofuels, such as the successful example of cane ethanol in Brazil and also some aspects of Biodiesel production. 5.1 Sugarcane Ethanol Ethanol made from sugarcane has been powering Brazilian cars since the 1920s. It wasn t after the oil shocks of 1970s though that the Brazilian government initiated the Pró-Álcool program (1975), in hopes of substituting expensive imported oil with domestic sugar-derived ethanol fuel. Most Brazilian Petrol now is Gasohol which is a 24% ethanol blend, but there are also pumps at the gas stations that deliver pure ethanol. The outcome of this nationwide effort is that while imported oil counted for 75% of Brazil s oil needs during the 1970s, it reduced all the way down to 30 % by 2005 when production reached 4 billion gallons. In 2003 Brazil s first total flex vehicle was introduced. Now 85% of all cars sold in Brazil are flex cars, which means that they can run both on gasoline and ethanol in any blend, and 40% of these cars actually run on ethanol (for Figure 40: Sugar canes the US, these numbers are 3% and 2% respectively, see also section 3.1). Ethanol is sold in Brazil for $2.92 per gallon ($3.88 in energy equivalent units), while gasohol is sold for $4.91/ gallon. Brazil grows sugarcane because its tropical climate is ideal for this fast-growing grass. Unlike corn, in which the starch in the kernel has to be broken down into sugars with expensive enzymes before it can be fermented (see section 3.2), the entire sugarcane stalk is already 20% sugar and fermentation is a very easy and straightforward process. Cane yields 600 to 800 gallons of ethanol per acre, more than twice as much as corn. Moreover, sugarcane delivers eight units of ethanol for each unit of fossil fuels we input, while for corn the net gain is close to zero (see Figure 31).
Figure 41: Sugarcane harvest in Brazil [19] Figure 42: Ethanol refinery in Brazil [5]
5.2 Biodiesel Biodiesel is a diesel equivalent fuel consisting of short chain alkyl (methyl or ethyl) esters, produced using vegetable oils, such as canola (Germany), recycled oils (California), animal fats or algae by transesterification, using catalysts. Biodiesel is mainly produced in Europe (85% of total production) Germany is the word leader in biodiesel production with 0.5 million gallons in 2005. There are currently 2M cars in Germany running on biodiesel and are fueled from 2000 stations, 10% of the country s total stations. It is sold at $6.80 per gallon, 65 cents more expensive than regular diesel [5]. In Figure 43 we can see the trends in biodiesel along with ethanol production in Europe since 1993. Figure 43: Biofuel production in the European Union The next graph is the result of an analysis regarding the potential for biodiesel production around the world. The highest potential lies in South America and southeastern Asia [20], where the tropical climates favors the crop growth and thereby increases the energy potential of the biomass. The detailed biodiesel production for North America is shown in Figure 45.
Figure 44: Biodiesel production around the world Figure 45: Biodiesel production in North America
5.3 Biodiesel from Algae Figure 46: Algae growing bags [5] Algae are the most efficient plants in the world in generating oils, because they are the fastest growing plants on the planet. They were initially considered to be placed next to coal burning plants in order to capture the CO 2 emitted. Figure 47 shows the potential volumes of biofuel production for various types of plants used. Algae have by far the greatest potential because they can produce at least 5000 gallons of oil per acre. Under controlled condition in the labs about 1800 gallons per acre have been demonstrated so far [21]. Furthermore, as opposed to corn which is harvested only once every year, algae can be harvested daily. However its production remains on a private experimental stage because it is still very expensive to grow it. Future advances in technology could help put algae in the map [22].
Biofuel production per acre [gallons] 5000 5000 4000 3000 2000 1000 0 300 700 60 1000 Corn Sugar Biodiesel Cellulose Algae Biodiesel Figure 47: Biofuel production per acre for various plants
5.4 Biobutanol Biobutanol is a 4-carbon alcohol that is derived from biomass fermentation. It is more favorable than ethanol because its structure is more similar to gasoline, and as such it has energy density closer to gasoline (see Figure 48). Energy Density [MJ/gallon] 150 100 121 116 50 0 89 gasoline ethanol butanol Figure 48: Energy density of Biobutanol The advantages of biobutanol include the fact that it can be shipped on existing oil pipelines and it does not require any engine conversion to run (it is non-corrosive like gasoline). Basically biobutanol can be used straight away once it s produced. However there are presently difficulties in finding organisms that can ferment into biobutanol directly. The yields are very low, and there is a lot of research effort in order to synthesize genetically microbes that could ferment into butanol. Although genes can be now designed fully at will, it is not known what the genetic code should be that will lead to the desired function. The first commercial biobutanol will be produced by Dupond and BP at the end of 2007 [23].
6 Summary This report attempted to summarize the present situation of biofuel production around the world, as well as point out the future trends and engineering challenges that are required in order for biofuels to be a true alternative renewable energy source. It was seen that, at least for the US, corn ethanol supplies 3% of its gasoline fuel, and the production is expected to rise by 60% until 2012. However the corn derived ethanol will probably saturate soon after since the corn supplies available for fuel production are limited. Cellulosic ethanol seems at the present time the best alternative, as it can potentially replace 30% of all motor fuel consumed in the US annually. This can happen optimistically by 2030, assuming some significant improvements are made in the energy extraction techniques, such as lignin breakdown and fermentation of hemicellulosic 5-carbon molecules in addition to the cellulosic molecules. Other types of biofuels such as Biodiesel, Biobutanol, or algae-derived ethanol are also promising, each one for its own reasons and drawbacks, but large-scale production lies still several years in the future. Increasing prices of imported oil are making biofuels ever more attractive with every passing day; with some political incentives, scientific breakthroughs and public awareness, biofuels could provide a serious alternative to our fossil-fuel hungry energy needs.
References 1. Koonin, S. 2007 [cited; Available from: nrg.caltech.edu. 2. World energy resources and consumption. [cited; Available from: http://en.wikipedia.org/wiki/world_energy_resources_and_consumption/. 3. BIOMASS AS FEEDSTOCK FOR A BIOENERGY AND BIOPRODUCTS INDUSTRY: THE TECHNICAL FEASIBILITY OF A BILLION-TON ANNUAL SUPPLY. 2005. 4. The world factbook. [cited; Available from: https://www.cia.gov/library/publications/the-world-factbook/print/us.html. 5. Joel K. Bourne, J., Green Dreams, in National Geographic. 2007. 6. Whitmarsh and Govindjee. [cited; Available from: http://www.life.uiuc.edu/govindjee/paper/gov.html 7. Chum and Overend, Advanced Solar Energy, 2003. 15(83). 8. [cited; Available from: http://www.msu.edu/course/isb/202/ebertmay/images/hb_energy_flow.jpg 9. US Farms, I. [cited; Available from: http://www.usfarmsinc.com/ethanol/].. 10. 2005, E.P.A.o. [cited; Available from: http://en.wikipedia.org/wiki/energy_policy_act_of_2005. 11. E85 Vehicles. [cited; Available from: http://e85vehicles.com. 12. E85 Prices. [cited; Available from: http://e85prices.com/. 13. The use of enzymes in starch hydrolysis. 14. Ratliff, E., The plant that will save america, in Wired. 2007. 15. Breaking the Biological Barriers to Cellulosic Ethanol, in DOE/SC-0095 (2006). 2007. 16. Chen and Dixon, Nature Biotechnology, 2007. 25(7): p. 759-761. 17. Service, R., Biofuel researchers prepare to reap a new harvest. Science, 2007. 315: p. 1488. 18. How Cellulosic Ethanol is Made. [cited; Available from: http://genomics.energy.gov/gallery/biomass/detail.np/detail-881.html 19. Sugarcane harvest. [cited; Available from: http://en.wikipedia.org/wiki/image:saccharum-officinarum-harvest.jpg. 20. Johnston, M. and T. Holloway, A Global Comparison of National Biodiesel Production Potentials. 2007. 21. Biodiesel from Algae, in NREL/TP-580-24190. 1998. 22. Directory:Biodiesel from Algae Oil. [cited; Available from: http://peswiki.com/index.php/directory:biodiesel_from_algae_oil. 23. BioButanol. [cited; Available from: http://www.butanol.com/].