By Dan Burden, Program Coordinator, AgMRC, Iowa State University, email@example.com.
Revised December 2021
Cellulosic ethanol is a developing industry within the larger biofuels production industry in the US. Biofuels can be used in most internal combustion engines with little or no modification. Ethanol and biodiesel have become common additives or substitutes within the existing petroleum fuel infrastructure. For decades, ethanol has been used alone and with petroleum-based gasoline in internal combustion engines. The renewable nature of ethanol feedstock’s, primarily corn and sugarcane, has focused national and international attention on the development of a new production industry to create fuel-grade ethanol. The addition of ethanol to gasoline can boost octane rating and reduce greenhouse gas (GHG) emissions, providing environmental benefits beyond the “renewable” aspect of the fuel source.
Technology development initially favored using corn and sugarcane as feedstocks because it is relatively simple to convert the starches and sugars naturally present in these feedstocks to glucose, the necessary component to make ethanol. Conventional and cellulosic ethanol manufactures result in the same product, but are produced using different feedstocks and processes. Conventional ethanol is predominantly derived from corn grain, by far the largest grain crop grown in the United States. As corn yields increase, there continues to be ample corn grain for livestock feed, human food, and export to users in other countries, even considering that nearly 5 billion bushels used annually to produce fuel ethanol. Development of cellulosic ethanol processes has been driven by the need to augment corn-based ethanol, thereby allowing corn to be directed into animal diets and human food primarily. However, this strategy has not been necessary so far. Of more importance to the industry is the ability to further ferment the fibrous portion of the corn kernel to derive additional ethanol from each batch. Moreover, the ability to produce ethanol from “woody” wastes, such as paper from municipal waste, and the stalks and stover from field crops that is commonly left in the field after harvest.
To understand cellulosic ethanol production, it is important to know how ethanol has commonly been produced from corn. Corn grain is converted to ethanol in either a “dry-grind” or “wet-mill” process. In dry-grind operations, liquefied corn starch is produced by heating corn meal with water and enzymes. A second enzyme converts the liquefied starch to sugars, which are fermented by yeast into ethanol and carbon dioxide. Wet milling operations separate the fiber and germ from the endosperm, which contains most of the starch and approximately seventy percent of the corn kernel. The starch portion is fermented into ethanol, while the fiber, protein, and oil residues become a nutrient-rich animal feed called “distillers grains”.
With respect to conventional ethanol, technological research primarily is concentrated in two areas: ethanol yield improvement and overall production efficiency. Both seek to reduce production cost and create a viable technology to replace imported oil, as well as make crop, crop residue and alternative feedstock-based ethanol a sustainable, domestic biofuel. Much of this research is applicable to converting or transitioning existing conventional grain-based production facilities to cellulosic systems. Cellulosic ethanol is a potential replacement for gasoline and grain-based ethanol in cars and trucks. Additionally, cellulosic ethanol is promoted, as are other biofuels, to reduce the nation's dependence on imported oil, increase energy security and reduce the trade deficit. Rural economies have benefitted from new job creation and increased farm income.
The Environmental Protection Agency has established production goals under the Renewable Fuels Standard to encourage development of new production systems from other renewable resources, the largest of which is cellulosic plant material. Under the current Administration, federal research dollars have been directed toward improvements in fermentation technology to allow cellulosic ethanol production to be competitive with other biofuels, including grain-based ethanol. Technologically, the primary roadblock has been the inability to break fibrous plant material down to form glucose, which can then be fermented into ethanol. New enzymatic systems are helping to overcome this limitation and, simultaneously, the Department of Energy offered grants to four ethanol producers to build production-scale facilities in Florida, Mississippi, Iowa, and Kansas for cellulosic ethanol production. Other cellulosic production is in various stages of construction in 20 other states. Meanwhile, innovative processes within current corn-based ethanol production systems are being developed to increase efficiency and profitability by using both the corn starch for primary ethanol production and the residual corn fiber for cellulosic ethanol production.
The other significant challenge to cellulosic ethanol production is gathering and storing the cellulosic plant material until it can be used. Unlike corn grain, cellulosic material is bulky and has a relatively high moisture content, making it cumbersome to move and difficult to store for several months, as is necessary for year-round ethanol production. Innovators in the industry, such as POET and DuPont, have developed strategies to accomplish this along with incentives for farmers to gather corn stalks/cobs and to grow specific cellulosic crops, such as switchgrass and miscanthus, for ethanol production.
Currently, there are 213 ethanol plants operating in the United States with total production capacity of 15,069 million gallons per year. Of that, plants producing ethanol from cellulose have a combined production capacity of about 100 million gallons per year, although this is expected to increase as systems improve and cellulosic ethanol production becomes more efficient.
Currently, cellulosic ethanol is marketed with other fuel-grade ethanol in the same distribution system. The product, ethanol, does not differ from conventionally produced ethanol so there is no current need to market them differently. Ethanol is transported by truck and railcar to distribution sites in various regions of the US. It is then blended with gasoline in 90% gasoline: 10% ethanol ratio for motor fuel. “Blender pumps” are located at a limited number of service stations allowing the customer to select the proportion of ethanol that is added to the gasoline, so people could individually choose to use higher blends of ethanol. While blender pumps would be a logical way to encourage increased use of ethanol in motor fuel, the cost of adding blender pumps has kept many service stations from installing them. Unless higher proportions (above 10 percent) of ethanol are mandated to be blended into motor fuel, it appears that marketers are less likely to allow increased ethanol proportions to be used. This dynamic affects sales of conventional ethanol and cellulosic ethanol equally.
Ethanol price closely follows the price of crude oil and, more directly, the price of gasoline. Ethanol producers are constantly challenged to increase production efficiencies in order to maintain favorable profit margins. Where ethanol producers have less control over the market price of ethanol, they also need to deal with fluctuating corn prices which, in recent years, increased dramatically to the point where some ethanol producers had to curtail operations until corn prices went down. The market dynamic for cellulosic feedstock has yet to evolve because there are so few producers at this time. POET, DuPont, and others that are producing or will be producing cellulosic ethanol in the near future have designed contracts with farmers to supply corn stover, grasses, etc. in a form that can be transported by the processor to the processing plant.
Producing Cellulosic Ethanol
In general, the bioconversion of cellulose to ethanol requires three major processing steps: pretreatment, saccharification and fermentation. Plant structure depends on lignin, a sturdy carbohydrate that holds plants upright. In order to use woody wastes, it is first necessary to break down the "woody compounds" into fermentable sugars. This has taken the form of a number of different pretreatment (pre-fermentation) strategies. Pretreatment requirements vary with the feedstock and are often substantially less in the case of various paper and hydrolysis waste streams.
Pretreatment is an essential step for bioconversion of most lignocellulosic materials. Roughly two-thirds of the lignocellulosic materials is present as cellulose and hemicellulose (the two main components of plants that give them structure), and lignin makes up the bulk of the remaining dry mass. To efficiently and economically produce cellulosic ethanol, the complex polymeric structures must be separated into fermentable sugars. The sugars in cellulose and hemicellulose are locked in complex carbohydrates called polysaccharides (long chains of monosaccharides, or simple sugars). Pretreatment breaks apart the structure of biomass to allow for the efficient, effective hydrolysis of cellulosic sugars, but this involves extremely complex chemical engineering. Usually the systems use processes to disrupt the hemicellulose/lignin sheath that surrounds the cellulose in plant material. Pretreatments maximize subsequent bioconversion yields and minimize the formation of inhibitory compounds. They include acid hydrolysis (a controlled breakdown using dilute acid), alkali treatment, ammonia fiber explosion (liquid ammonia under moderate heat and pressure to separate the biomass components), auto hydrolysis, chemical pulping, heat, mechanical size reduction, solvent extraction, steam, steam explosion, weak acid hydrolysis and various combinations of these separate processes. These technologies have different strategies for accessing the cellulose and hemicellulose and then dealing with the lignin, smaller amounts of other proteins, lipids (fats, waxes and oils) and ash. Acid hydrolysis is often used as a pretreatment because it can be adapted to a wide variety of feedstocks. Except for strong hydrochloric acid hydrolysis, acid hydrolysis is generally carried out at higher temperatures; yet higher acid concentrations produce the same results at lower temperatures. Generally an inexpensive process, acid hydrolysis may also produce large quantities of degradation byproducts and undesirable compounds that inhibit other areas of cellulosic processing. Autohydrolysis is the process of converting lignocellulose into fermentable sugars by exposure to high temperature steam. Many lignocellulosic materials contain significant quantities of acetylated hemicellulose. Steam releases these in the form of acetic acid, which subsequently carries out a partial hydrolysis of the hemicellulosic and cellulosic sugars. The principal disadvantage of this approach is that sugar yields are generally very low. Usually performed in a tumbling reactor, steam treatment is a convenient way to facilitate separation of plastics and fibers while increasing digestibility of municipal solid waste. In this experimental process, garbage is introduced into a large, cylindrical, horizontal autoclave that is slowly rotated on its side while steam treatment takes place. The plastic materials collapse and the fibrous materials form a pulp. Metals and other nonfibrous materials (for example, old shoes, composite material containers, plastic objects) are readily separated on screens after treatment. The process known as weak acid hydrolysis consists of sulfur dioxide combined with steam. It is particularly effective as a pretreatment for enzymatic cellulose saccharification. Sulfur dioxide is often used in combination with autohydrolysis because it gives better sugar yields and helps to modify lignin for subsequent extraction or recovery.
Cellulose saccharification is the process of turning polymeric lignocellulosic materials into fermentable sugars and can be accomplished by a number of processes including acid and enzymatic hydrolysis. Acid hydrolysis and enzymatic hydrolysis are currently the main two processes used to create fermentable sugars from cellulosic biomass. Acid hydrolysis processing breaks down the complex carbohydrates into simple sugars. Enzymatic hydrolysis processing uses a complex pretreatment processing stage to reduce the size of the material, making it more efficient than acid hydrolysis. In both processes, enzymes are used to convert the cellulosic biomass into fermentable sugars and then microbial fermentation (as in current corn-based systems) is used to produce ethanol. As with current corn-based systems, carbon dioxide is produced as a co-product in this final stage of production. Dilute acid hydrolysis with 1 percent to 5 percent sulfuric acid is generally considered the most cost-effective means of hydrolysing wood and agricultural residues. Yields of hemicellulosic sugars can be 80 percent to 95 percent of theoretical. Yields of glucose from cellulose are generally less than 50 percent but can approach 55 percent at elevated temperatures. Strong acid hydrolysis, often using a concentrated form of sulfuric acid, usually separates and recycles the acid catalyst, limiting the total acid losses to approximately 3 percent, or the same as the dilute process. Use of the concentrated acid, however, allows lower temperature and pressure hydrolysis with fewer byproducts produced. Concentrated hydrochloric acid at a concentration of about 47 percent is sometimes used for strong acid hydrolysis because it is relatively easy to recover. Hydrolysis with concentrated hydrochloric acid gives one of the highest sugar yields of any acid hydrolysis process. It is carried out at room temperature. The chief drawback is that it is highly corrosive, volatile, expensive and almost complete recovery is essential to make the process economical. Ultimately, the goal is to get higher sugar yields as efficiently as possible without degrading the feedstock materials. This is easier said than done. Each technology has advantages and disadvantages in terms of costs, yields, material degradation, downstream processing and generation of process wastes.
Cellulosic ethanol can be produced from a wide variety of cellulosic biomass feedstocks. These include agricultural plant wastes (corn stover, cereal straws, sugarcane bagasse), plant wastes from industrial processes (sawdust, paper pulp, distiller’s grains) and energy crops grown specifically for fuel production, such as switchgrass. Growing energy crops and harvesting agricultural residuals are projected to increase the value of farm crops, potentially eliminating the need for some agricultural subsidies. Perennial grasses, such as switchgrass and miscanthus, have been discussed as promising feedstocks for cellulosic ethanol production. They use water efficiently and do not need a lot of fertilizers or pesticides. However, their production economics are questionable, and the infrastructure is developing now to drive their acceptance as feedstocks. With
heterogeneous waste streams such as municipal solid waste, it may be necessary to include costly, complex sorting and filtering steps to separate usable material from those that may inhibit the process before carrying out pretreatment. In the case of agricultural residues, particle size reduction can often be done simply with grinding. For dry corn or soybean residues, density is often lower than what is desirable, and particle size reduction is not an issue. Municipal solid wastes present a particular problem because of their extremely heterogeneous nature. Large quantities of plastics, wood, metals and other materials are often present. If certain heavy metals or toxic compounds are present in the mixture when acid is added, they can create severe downstream problems in fermentation and product formation. Many cellulosic ethanol advocates see cellulosic conversion as a one-stop, one-size-fits-all fuel production system. This is partly due to their lack of understanding of the complexity of the chemical engineering and biological components of the processing system and partly due to wishful thinking regarding exploitation of agricultural wastes. Agricultural wastes, agricultural residue material that is plowed into the soil for fertilizer, composted, burned or disposed of in landfills, are more abundant and contain greater potential energy than simple starches and sugars. Additionally, the collection, transportation and perhaps processing of these residues would present farmers and agricultural service providers with another lucrative crop-based profit center. The popular press has promoted cellulosic processing as a system in which any cellulose-containing waste (farm field, industrial and municipal) and waste (newsprint, homeowner leaf and yard waste, sawdust, wild native grasses) can be processed by the same system within the same facility. A range of material this varied would have vast differences in chemical makeup, as well as the contaminant load that each would carry into the system. The biological components of any processing system seldom are amenable to introduced molds, bacteria and other contaminants. In current ethanol production technologies, food production systems and so on, these biological contaminants tend to wreak havoc with production efficiencies and can even curtail operation of a processing facility and force shutdowns, requiring costly restarts.
The problem is that the complex chemical and biological engineering needed to optimally convert (in a manner suitable for profitable, large-scale fuel production) a single homogeneous feedstock, for example, wheat straw, may be very different from another homogeneous feedstock, for example, corn stalks. Add to this the harvest and storage technologies that may not exist at this time (yet have been evolving for over a century for grain-based production systems). However, cellulosic conversion “dialed-in” to a specific feedstock could be a great solution for taking some waste materials from a landfill and converting them to fuel and usable co products. This “bioremediation” of an industrial waste stream has the potential to eliminate a costly expense for a company and turns it into a new profit center. One example is paper sludge, a waste material that goes into landfills at a cost of $60 to $100 a dry ton. Others are various wood wastes and field stovers. Perhaps the most promising application is using cellulosic conversion to “re-use” or “further process” the dried distillers grain component produced as a co-product from conventional grain ethanol production. This system is using an extremely homogeneous pre-processed feedstock, and there is no need for complex new handling and storage systems.
Industry Overview: The Players
The Advanced Ethanol Council (AEC) represents worldwide leaders in the effort to develop and commercialize the next generation of ethanol fuels, ranging from cellulosic ethanol made from dedicated energy crops, forest residues and agricultural waste to advanced ethanol made from municipal solid waste, algae and other feedstocks. The Cellulosic Biofuels – Industry Progress Report 2014 presents an extensive list of the players in this industry along with information about the projects and plants under development and construction worldwide.
- Biofuels for Sustainable Transportation, National Renewable Energy Laboratory.
- Cellulosic Biofuels: Analysis of Policy Issues for Congress, Congressional Research Service, 2010.
- Cellulosic Energy Fact Sheet, National Commission on Energy Policy Forum, The Future of Biomass and Transportation Fuels; Dartmouth College, Carnegie Mellon University and Natural Resources Defense Council; National Commission on Energy Policy; 2003.
- Cellulosic Ethanol, National Renewable Energy Laboratory (NREL), 2007 - Overview of national research into cellulosic ethanol.
- Cellulosic Ethanol, Renewable Fuels Association - Cellulosic ethanol review.
- Ethanol Production and Distribution, U.S. Departmentt of Energy.
- Ethanol, Biomass Program, U.S. Department of Energy.
- Next-Generation Biofuels: Near-Term Challenges and Implications for Agriculture, Economic Research Service, USDA, 2010 - This report assesses the short-term outlook for production of next-generation biofuels and the near-term challenges facing the sector.
- Principles for Bioenergy Development, Union of Concerned Scientists, 2007.
- U.S. Baseline Briefing Book, Food and Agricultural Policy Research Institute (FAPRI), University of Missouri and Iowa State University, 2010 - Assuming some reductions in production, collection and processing costs, FAPRI expects cellulosic ethanol production to expand rapidly after 2015.
- Will Cellulosic Ethanol Take Off?, Technology Review Inc., Massachusetts Institute of Technology, 2007 - With government assistance, fuel from grass and wood chips could be realized in ten years.