|
|
| Fig. 1: Ethanol Share of U.S. Gasoline Consumption, 2009-12. [1] (Courtesy of the U.S. Department of Energy.) |
Ethanol is an alcohol that can be created from a wide variety of plant materials and feedstocks and is used in liquid from to fuel in motor vehicles. Today, corn starch and sugarcane are the two main feedstocks used, respectively producing starch- and sugar-based ethanol. Cellulose can also be used to produce ethanol, however, doing so requires additional processes using enzymes to break down the cellulosic materials into sugars. Cellulosic materials, which provide structure to plants, comprise the stems, stalks, and leaves of plants as well as trunks of trees. Cellulose and hemicellulose, which collectively are referred to as cellulosic materials, can be broken down into sugars, which can then be fermented into ethanol. Cellulosic materials being studied for the production of biofuels includes those found in switchgrass, prairie grasses, cornhusks, wood chips, forestry materials and residues as well as other inedible agricultural plant waste. The ethanol produced from these cellulosic materials is referred to as cellulosic ethanol.
The abundance of cellulosic materials, which boils down to about 60 to 90 percent of earths biomass measured by weight, along with the fact that they are not used for food and feed (unlike corn and sugarcane), are significant reasons why cellulosic ethanol and other cellulose-based biofuels are so appealing to scientific and political scholars.
Today, corn-based ethanol comprises nearly 10 percent of U.S. motor fuel. This number has steadily increased over the past several years, jumping about 3% since 2009. (See Fig. 1) However, Congress is worried about driving up the price of corn because it is used as feed for livestock and poultry, rising corn costs would subsequently raise the prices in supermarkets. Lawmakers have resorted to capping the total production of corn-based ethanol and have called for a steady increase in the use of advanced biofuels. Congress concerns are grounded in the world energy consumption which has doubled in the past 30 years, and it will almost double again in the next 30 years. By the year 2030, we will need 30 TW of average power, from which 15% will be transportation energy, mostly oil.
Currently, transportation energy is 35 million barrels of oil per day. From all the oil consumed in the world, 50% goes directly to transport. 1/4 of all oil in the world is consumed in the US, from which 65% is imported and the demand is going to rise.
Cellulosic ethanol also has the hypothetical ability to provide substantial lifecycle GHG reductions compared to petroleum-based gasoline.
Using biomass for transportation fuels raises questions concerning the logistics of feedstock production such as land use and land use change, fertilizer and pesticide use, water consumption, and energy used for production and cultivation. However, grasses and trees typically require minimal labor and generally have lower fertilizer and pesticide needs and resources as opposed to other row crops such as corn. Grasses such as switchgrass require a low level of attention and are perennial crops that do not need to be re-planted for about 20 years and provide as easily accessible feedstock to produce cellulosic ethanol.
The main idea and potential benefits associated with biofuels is to extract the energy that is stored in plants, turn it into cellulosic ethanol, and replace a significant amount of the oil demand, mainly for transportation purposes. Biofuels like cellulosic ethanol are self-sustaining, reliable energy sources which, in principle, have smaller net CO2 emissions than fossil fuels and bio fuels. Renewable Fuel Standard (RFS) goals for biofuels penetration are based on specific GHG reductions from the fossil fuel it replaces. Cellulosic Biofuels would result in a 60% reduction.
The chemical make-up of ethanol is uniform across the board whether it is produced from corn, sugarcane, or cellulose; however, the differences lie in the production processes and the necessary technologies in different stages of development. While corn- and sugar-based ethanol production technologies have been produced at a commercial scale for decades, some of the technologies needed to manufacture cellulosic ethanol, an advanced biofuel, are relatively new. Today, there still no fully operational commercial-size cellulosic ethanol facilities in the United States.
 |
| Fig. 2: Cellulosic Ethanol Production Cycle. |
The process of producing ethanol from cellulosic materials is also far more complicated than the processes employed for starch- or sugar-based ethanol. Enzymes must break up the complex cellulose-hemicellulose-lignin structure in which cellulosic materials are found before the fermentation process can begin. The cellulosic ethanol conversion process consists of two basic steps: pretreatment and fermentation. This two-step process is what is responsible for the increase in time, expense, and complexity of converting the cellulosic biomass into ethanol, relative to the procedures used to convert corn or sugarcane into ethanol.
Pretreatment is necessary to prepare cellulosic materials for hydrolysis, which converts the hemicellulose and cellulose into glucose. Standard pretreatment includes a chemical pretreatment step involving acid and a physical pretreatment step such as grinding. These steps make the cellulose more accessible to the cellulases, which are the enzymes that digest cellulose and turn it into glucose. These enzymes catalyze its conversion to sugars in the successive steps and begin the breakdown of hemicellulose into glucose. Following pretreatment, the conversion of cellulose to glucose is completed using a chemical reaction called hydrolysis, normally employing enzymes secreted by certain organisms (typically fungi or bacteria) to catalyze the reaction. The pretreatment and hydrolysis process usually results in one co-product, lignin, which can be burned to generate heat or electricity. Using lignin instead of a fossil-based energy source to power the conversion process reduces cellulosic ethanol's life-cycle greenhouse gas (GHG) emissions, compared to corn-based ethanol.
Once the sugars have been derived from the cellulosic materials, they are fermented using yeast or bacteria in processes similar to those used for the corn-based ethanol production. The liquid resulting from the fermentation process contains ethanol and water; the water is removed through distillation, again similar to the corn-based ethanol process. Finding the most effective and low-cost enzymes for the pretreatment process and organisms for the fermentation process has been one of the main areas of research in the development of cellulosic ethanol. The type of feedstock and method of pretreatment both influence the amount of ethanol produced. Currently, one dry short ton of cellulosic feedstock yields about 60 gallons of ethanol. Projected yields with anticipated technological advances are as high as 100 gallons of ethanol per dry short ton of feedstock.
There are four primary factors that determine the cost of the finished product: the feedstock, chemical processing and pretreatment, refining and finishing the crude product to a usable state, and the transportation and distribution of finished fuel. However, the most significant and alarming cost can be found in the pretreatment phase. Pre-treatment is considered one of the most expensive processing steps in the bioconversion of lignocellulosic biomass, which accounts for up to 40% of the total processing cost. In addition, this trickles down and greatly affects the cost of operations such as enzymatic hydrolysis and fermentation [2]. The catch with cellulosic ethanol is the energy required to make the enzyme. The enzyme in question is called "cellulase," which is the top-selling industrial enzyme on the market. Cellulase is typically used in a process called biostoning, which is employed to make pre-washed jeans, and also is a component in laundry detergent which is the agent responsible for removing fuzz from the cotton fibers and ultimately enhancing the brightness of the fabric. However, cellulase is not economically ideal to be used in the pretreatment process of biomass. The fungus that must be cultured and fed which makes these enzymes is costly in terms of energy. The amount of energy it takes to make the enzyme exceeds the energy produced by hydrolyzed glucose before the enzyme breaks. Thus creating the crux of the issue. There is a negative net gain of energy. The cellulose enzymes are simply too costly to use in the cellulosic ethanol process.
So, in order for this particular avenue to work economically, massive government subsidies must be granted. Additional costs resulting from inefficient pre- treatment include detoxification, limited enzymatic hydrolysis rate, high enzyme loading, low product concentration, and complicated product purification. Therefore, pre-treatment can be seen as a key step in limiting the realistic possibility of bioconversion in terms of cost effectiveness. The long-term potential of advanced biohydrocarbons is linked to the ability of producers to create liquid fuels using cost-effective catalysts. However, looking at existing catalytic processes, the DOE has a projected cost of cellulase enzymes for the production of ethanol between $0.30-0.50 per gallon of ethanol. In contrast, the chemical catalysts in the petroleum industry are estimated to cost about $0.01 per gallon of gasoline [2].
Currently, there are no low-cost technologies to convert the large fraction of energy in biomass into liquid fuels efficiently and in a cost effective manner. Production costs must be reduced greatly, and the volume of cellulosic ethanol necessary for widespread use still needs to be developed. The lower limit benchmark for commercial scale processing of biomass is about 150,000 metric tons per year. The optimization of advanced biohydrocarbon production processes is an essential step to allow biorefineries to produce up to commercial volumes and Congress' expectations and standards.
Looking forward, there are still important challenges that remain for commercial use and production of cellulosic biofuel. Total production costs for many of these revolutionary projects remain higher than the cost of petroleum- based fuels on both a volumetric and energy-content basis. Cellulosic ethanol also faces the same market and regulatory challenges to overtake a share of the fuel market that is faced by other types of ethanol. Ultimately, cellulosic ethanol is currently an emerging technology and will require continued technological advancements and reduced costs to become commercially viable.
© Bobby Zarubin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Biofuels Issues and Trends," U.S. Energy Information Administration, October 2012.
[2] V. Bekmuradov, G. Luk, and R. Luong, "Improved Cellulose and Organic-Solvents Based Lignocellulosic Fractionation Pre-treatment of Organic Waste for Bioethanol Production," Am. J. Eng. Res. 3, No. 6, 177 (2014).