By Dan Burden, content specialist, AgMRC, Iowa State University, djburden@iastate.edu.
Profile revised January 2008.
Overview
One of the attractions of biofuels is they can be used in most internal combustion engines with little or no modification. Ethanol and biodiesel are the two most immediate candidates for adoption into the existing petroleum fuel infrastructure. For decades, ethanol has been used alone and with petroleum-based gasoline in internal combustion engines. Now, the renewable nature of its feedstocks has focused national attention and government assistance on ethanol as a renewable fuel. Present U.S. ethanol production is primarily by the bioconversion of the grain from corn and wheat, and in some instances, sugar cane.
Current grain-based ethanol production systems are an obvious first step in developing an agricultural-based industrial sector that addresses part of our national biofuel need. The industry is based on existing and proven crop production and transportation infrastructure models as well as a proven, workable fermentation production technology. However, grain-based production is limited by available grain feedstocks and their prices as valuable multi-use commodities. Any way to increase the efficiency of the grain-based processing systems increases their profitability for their owners and investors.
For this reason, research engineers and industrialists have sought to make conventional grain-based ethanol systems more efficient by not just fermenting (fermentation processing) the grain, but by bioprocessing as much of the plant (cellulose and hemicellulose) as possible. This technology looks to digest (enzymatic digestion) much of the plant into usable subunits that can then be efficiently converted (bio-catalysis) to sugars for fermentation processing. The less usable co-products (lignin, ash and hard-to-process proteins) can be combusted to provide power and heat for the ethanol production facility and the residual non-combustible ash and gypsum can become a marketed co-product for field fertilizer. This is the basic system envisioned for cellulosic ethanol production. Cellulosic (plant fiber) “conversion,” along with hydrogen, is viewed by many environmental and social policy organizations as being the transportation fuel future of the United States, if not the world.
Conventional Ethanol Versus Cellulosic Ethanol
Cellulosic ethanol exhibits a net energy content three times higher than conventional ethanol from corn kernel grain, and some of the cellulosic production systems emit far lower net levels of greenhouse gases. Conventional, grain-based ethanol uses fossil fuel to produce heat for fermentation and other aspects of processing, and produces greenhouse gas emissions. Cellulosic ethanol production uses part of the input-biomass feedstock (lignin, hard-to-process proteins) instead of fossil fuel. This very positively changes the "Well to Wheel" life-cycle analysis model used to calculate overall greenhouse gas emissions from fuels and internal combustion engines. Life-cycle analyses look at the environmental impact of a product from its inception to the end of its useful life.
Cellulosic ethanol also may provide additional positive environmental benefits in the form of reductions in greenhouse gas emissions and air pollution. Some researchers calculate that since lignin is a renewable fuel with no net greenhouse gas emissions, the greenhouse gases produced by the combustion of biomass are essentially offset by the CO2 absorbed by the plant material (biomass crop) because it sequesters carbon during its growth.
Conventional ethanol and cellulosic ethanol result in the same product, ethanol, but are produced using different feedstocks and processes. Conventional ethanol is predominantly derived from corn grain. Corn is converted to ethanol in either a dry- or wet-mill process. In dry-milling 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, germ (oil) and protein from the starch before it is fermented into ethanol.
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 bio-fuel. 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 our dependence on imported oil, increase our energy security and reduce our trade deficit. Rural economies will benefit from new job creation and in the form of increased farm incomes.
Producing Cellulosic Ethanol
In general, the bioconversion of cellulose to ethanol requires three major processing steps: pretreatment, saccharification and fermentation. 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
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), autohydrolysis, 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, it is generally carried out at an elevated temperature (to 240 100 centigrade degrees) for various times. At higher acid concentrations, it can be carried out at temperatures as low as 30 degrees Centigrade. 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 non-fibrous 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.
Saccharification
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 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.
Feedstocks
Cellulosic ethanol can be produced from a wide variety of cellulosic biomass feedstocks. These include agricultural plant wastes (corn stover, cereal straws, sugar cane 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 there is little to no production and collection infrastructure in place 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 their 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 services providers with another lucrative crop-based profit center.
The popular press has promoted cellulosic processing as a system whereby any cellulose-containing waste (farm field, industrial and municipal) 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 re-starts.
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 “bio-remediation” 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.
Conclusion
Research continues to push forward. Attempts are being made to address production system insufficiencies and the feedstock homogeneity challenges. The high cost of cellulose enzymes is a key barrier to economical cellulosic ethanol production. Various initiatives and national research programs are attempting to remedy this situation. Biofuel development is now a national priority. It is clear that technology and economic drivers will eventually take cellulosic conversion from its production system infancy into an adopted fuel production technology. Which feedstocks will go into it and how widespread the application and adoption of the technology are anyone’s guess.