Category Archives: Biofuels and Bioenergy

Ethanol from Corn

3.1 Fuel Ethanol from Corn

Ethanol is one of the simplest alcohols that have long been consumed in human history. Ethanol can be readily produced by fermentation of simple sugars that are obtained from sugar crops or converted from starch crops. This has long been practiced throughout the world. Feedstock for such fer­mentation ethanol includes corn, barley, rice, and wheat. This type of etha­nol may be called grain ethanol, whereas ethanol produced from cellulosic biomass such as trees and grasses is called cellulosic ethanol or biomass ethanol. Both grain ethanol and cellulosic ethanol are produced via biochemical pro­cesses, whereas chemical ethanol is synthesized by chemical synthesis routes that do not involve any fermentation step.

Ethanol, ethyl alcohol [C2H5OH], is a clear and colorless liquid. Ethanol has a substituted structure of ethane, with one hydrogen atom replaced by a hydroxyl group, — OH. Ethanol is a clean-burning fuel thanks to its oxygen content and has a high octane rating by itself (Research Octane Number (RON) of 108.6). Therefore, ethanol is most commonly used as an oxygen­ated blend fuel to increase the octane rating of blend gasoline as well as to improve the emission quality of gasoline engines. Due to the presence of the oxygen atom in its molecular structure, ethanol is classified as an oxygenated fuel. In many regions of the United States, ethanol is blended up to 10% with conventional gasoline. The blend between 10% ethanol and 90% conventional gasoline is called "E10 blend" or simply "E10". Ethanol is quite effective as an oxygenated blending fuel, because its Reid vapor pressure is marginally low; that is, it does not increase the volatility of the blend gasoline significantly, unlike methanol. Reid vapor pressure (RVP) is defined as the absolute vapor pressure exerted by a liquid at 100°F (37.8°C) as determined by the standard testing method following ASTM-D-323. The test method also applies to volatile crude oil and volatile nonviscous petro­leum liquids, except liquefied petroleum gases (LPG). Even though the Reid vapor pressure of ethanol is lower than that of methanol, fuel experts still consider it higher than they desire.

Ethanol can be produced from any biological materials that contain appre­ciable amounts of sugars or feedstock that can be converted into sugars. The former include sugarbeets and sugarcane, whereas the latter include starch and cellulose. For example, corn contains starch that can be easily converted into sugar and is, therefore, an excellent feedstock for ethanol fermentation. Because corn can be grown and harvested repeatedly, this feedstock emi­nently qualifies as a renewable feedstock, that is, this feedstock is not going to be simply depleted by exhaustive consumption.

Fermentation of sugars produces ethanol and this process technology has been practiced for well over 2,000 years in practically all regions of the world. Sugars can also be derived from a variety of sources. In Brazil, as an example, sugar from sugarcane is the primary feedstock for the country’s ethanol industry which has been very successful and active. In North America, the sugar for ethanol production is usually obtained via enzymatic hydrolysis of starch-containing crops such as corn or wheat. The enzymatic hydrolysis of starch is a simple, relatively inexpensive, and effective process, and is also a mature commercial technology. Therefore, this process is used as a baseline or a benchmark against which other hydrolysis processes can be compared. The principal merit of ethanol pro­duction by fermentation of sugar/starch is in its technological simplicity and efficiency, however, its demerit is that the feedstock tends to be expen­sive and also competitively used for other principal applications such as food. Therefore, "food versus fuel" or "food versus oil" is an unavoidable critical issue addressing the risk of diverting farmland or crops for produc­tion of biofuels including corn ethanol to the detriment of the food supply on a global or regional scale. It has also contributed to the increase in food price, which in turn raises the cost of feedstock and hurts the profitability of the ethanol industry.

Technoeconomically speaking, this high cost of feedstock can be favorably offset to a certain extent by the sale of by-products or coproducts such as dried distillers grains (DDGs), provided that the high oil price is sustained in the market. Many corn refineries produce both ethanol and other corn by­products such as cornstarches, sweeteners, and DDGs so that the capital and manufacturing costs can be kept as low as possible by maximizing the over­all process revenue. While they are manufacturing ethanol, corn refiners also produce valuable co-products such as corn oil and corn gluten feed. The North American ethanol industry is, therefore, investing significant efforts in developing new value-added by-products (and coproducts) that are higher in value and minimizing the process wastes, thus constantly making the grain ethanol industry more cost-competitive.

Corn refining in the United States has a relatively long history going back to the time of the Civil War with the development of the cornstarch hydro­lysis process. Before this event, the main sources for starch had been com­ing from wheat and potatoes. In 1844, the Wm. Colgate & Company’s wheat starch plant in Jersey City, New Jersey, unofficially became the first dedicated cornstarch plant in the world. By 1857, the cornstarch industry accounted for a significant portion of the U. S. starch industry. However, for this early era of corn processing, cornstarch was the only principal product of the corn refin­ing industry and its largest customer was the laundry business. Cornstarch has also been used as a thickening agent in liquid-based foods such as soups, sauces, and gravies.

The industrial production of dextrose from cornstarch started in 1866. This industrial application and subsequent scientific developments in the chemistry of sugars served as a major breakthrough in starch technology and its processing. Other product developments in corn sweeteners fol­lowed and took place with the first manufacture of refined corn sugar, or anhydrous sugar, in 1882. In the 1920s, corn syrup technology advanced significantly with the introduction of enzyme-hydrolyzed products. Corn syrups contain varying amounts of maltose (a disaccharide formed by a condensation reaction of two glucose molecules joined with an a (1^4) bond) and higher oligosaccharides. Even though the production of etha­nol by corn refiners had begun as early as after World War II, major quan­tities of ethanol via this process route were not produced until the 1970s, when several corn refiners began fermenting dextrose to make beverage and industrial alcohol. As such, the corn refiners’ entry into the fermen­tation business has become a significant milestone for major changes and transformation of the industry, especially in the fuel ethanol indus­try. The corn refining industry seriously began to develop an expertise in industrial microbiology, fermentation technology, separation process technology, energy integration and process design, and by-product and waste utilization.

As of today, starch and glucose (or dextrose) are still important products of the corn wet milling industry. However, the products of microbiology and biochemical engineering including ethanol, fructose, food additives, and target chemicals have gradually outpaced them. New research and devel­opments have significantly expanded the industry’s product/by-product/ coproduct portfolio, thus making the industry more profitable, flexible to market demands, competitive, and technologically advanced.

Lignocellulosic materials such as agricultural, hardwood, and softwood residues are also potential sources of sugars for ethanol production. The cel­lulose and hemicellulose components of these materials are essentially long and high molecular weight chains of sugars. They are protected by lignin, which functions more like "glue" that holds all of these materials together in the structure. Therefore, the liberation of simple sugars from lignocellulosic materials is not as simple and straightforward as that from sugar crops or starch crops. However, the biggest undeniable advantage of cellulosic etha­nol is its use of nonfood feedstock and no detrimental use of arable land for fuel production. Details of cellulosic ethanol technology are covered in Chapter 4 and, therefore, not repeated here.

Ethanol plays three principal roles in today’s economy and environment and they are

1. Ethanol in the United States replaces a significant amount of imported oil with a renewable domestic fuel.

2. Ethanol is an important oxygenated component of gasoline reformu­lation to reduce air pollution in many U. S. metropolitan areas, which are not achieving air quality standards mandated by the Clean Air Act Amendments (CAAA) of 1990. Ethanol is a cleaner-burning fuel due to its oxygen-containing molecular structure and also a superior gasoline blend fuel due to its renewability as a fuel and relatively low Reid vapor pressure of the blended fuel.

3. Ethanol provides a major income boost to farmers and agricultural communities where most ethanol feedstock is produced. Global corn prices have escalated more sharply than other crops due to the increased demand and higher corn prices have in turn motivated farmers to increase corn acreage at the expense of other crops, such as soybeans and cotton, raising their prices as well.

Ethanol, blended with gasoline at a 10% level (E10) or in the form of ethyl tertiary-butyl ether (ETBE) synthesized from ethanol, is effective in reducing carbon monoxide (CO) emission levels, ozone pollution, and NOx emissions from automobile exhaust. Two of the major barriers to the wide acceptance of ethanol as a gasoline blend fuel are: (1) its Reid vapor pressure being not low enough, and (2) its high moisture-absorbing (hygroscopic) characteristics. As mentioned earlier, the Reid vapor pressure of ethanol is lower than that for methanol; however, it is still marginally high.

In its early years the U. S. fuel ethanol industry was expanding to meet the increased demand for oxygenated fuel that resulted from a withdrawal of methyl-tertiary-butyl ether (MTBE) from the domestic gasoline marketplace. In response to sharply rising national concern about the presence of MTBE in groundwater as well as potential risk to public health and the environment, the U. S. Environmental Protection Agency (EPA) convened a Blue Ribbon Panel to assess policy options regarding MTBE. The Blue Ribbon Panel rec­ommended that the use of MTBE be dramatically reduced or eliminated. The EPA has subsequently stated that MTBE should be removed from all gasoline. Many U. S. states including California and New York mandated their own schedules of MTBE phase-outs and bans. As of September 2005, 25 states had signed legislation banning MTBE. According to a survey conducted in 2003, 42 states reported that they had action levels, cleanup levels, or drinking water standards for MTBE [1]. It is a remarkable turnaround in the chemical and petrochemical marketplace considering that MTBE used to be the fastest growing chemical in the United States in the 1990s. Recovering or retrofit­ting the MTBE plant investments would become an issue for this industry for years to come. Even with a rapid decline and disappearance of MTBE in the U. S. market, global production of MTBE has remained relatively constant at about 18 million tons/year, as of 2005, mainly due to the growth in Asian markets, where the use of ethanol or other oxygenated replacements is not established and ethanol subsidies are not provided.

United States fuel ethanol production has been increasing very rapidly for the first decade of the twenty-first century. According to the Renewable Fuels Association (RFA) [2], the U. S. ethanol production in 2002, 2003, and 2004 was 2.13, 2.80, and 3.40 billion U. S. gallons, respectively. Considering the pro­duction level of 2000 being 1.63 billion gallons, this is more than a twofold increase over five years. The U. S. production of ethanol in 2006, 2007, 2008, 2009, and 2010 was 4.9, 6.5, 8.9, 10.75, and 13.2 billion U. S. gallons, respectively. Comparing between the 2007 and 2010 statistics, it took only four years to double U. S. production. The trend in ethanol production in the United States is presented in Figure 3.1, which shows an exponential growth in ethanol production in the United States for the first decade of the twenty-first cen­tury. Due to the high cost of petroleum crude in recent years, the role of etha­nol has realistically expanded far beyond the oxygenated fuel additive into that of a true alternative renewable transportation fuel. The increased use of ethanol in the United States has significantly contributed to the alleviation of dependence on imported petroleum.

Corn refining has also become America’s premier by-products industry, and its success has set a desirable business model for future biofuel indus­tries. Increased production of amino acids, proteins, antibiotics, and biode­gradable plastics has added further value to the U. S. corn crop. In addition to cornstarches, sweeteners, and grain ethanol, corn refiners also produce corn

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FIGURE 3.1

Ethanol production in the United States.

oils as well as a variety of important feed products. Corn products in the modern world are found in a large variety of areas and applications: (a) live­stock feed grains; (b) food ingredients including sweeteners, starches, and polyols; (c) oil products including corn oil, acid oil, middlings, and corn wax oil; (d) cornstarches for papermaking and corrugated products; (e) personal care products utilizing natural polymers; (f) health and nutrition including sugar-free and low-sugar foods; (g) animal feeds including corn gluten meal, corn germ meal, and steepwater grain solubles; (h) pharmaceutical products including anhydrous dextrose; (i) manufacture of biodegradable polymer, poly(lactic acid; PLA), using cornstarch; and more.

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Corn is the most traded crop product in the world with the United States being the leading exporter and Japan being the largest importer. The U. S. annual export of corn was about 50 million metric tons in the fiscal year 2010. Since 1980, the annual amount of U. S. export of corn has been fluctuating between 35 and 60 million metric tons. Even though the United States dominates the global trading market of corn, it accounts for about 15.2% of the total U. S. corn production. As such, corn prices are largely determined by supply-and-demand relationships in the U. S. mar­ket. The U. S. corn crop was valued at $66.7 billion in the fiscal year 2010 and production for the year was 331 million metric tons, which is equiva­lent to 12.1 billion bushels. The U. S. corn growth/production accounted for about 39% of the world production [3]. About 80 million acres were planted to grow corn and most corn production was in the heartland of the United States. Of the total corn produced in the United States in 2010, about 34.9% or 116 million metric tons were used for corn ethanol produc­tion. Figure 3.2 shows the breakdown of 2010 end-uses of corn by end-use categories and sectors in the United States.

The production of ethanol from starch and sugar-based resources in the United States reached 13.2 billion U. S. gallons in 2010. The amount of gaso­line used by the United States for transportation was approximately 140 bil­lion U. S. gallons per year in 2011, with ethanol used as a blend stock of up to 10% in marketed gasoline (E10) and also with a smaller E85 (85% ethanol) market. As ethanol production increases, the demand for ethanol in the fuel supply chain has nearly reached the 10% "blend wall" of 14 billion gallons. The Energy Independence and Security Act (EISA) of 2007 requires a manda­tory Renewable Fuel Standard (RFS) requiring transportation fuels sold in the United States to contain a minimum of 36 billion gallons of renewable biofuels by 2022, including advanced and cellulosic biofuels and biomass — based diesel. The EISA further specifies that 21 billion U. S. gallons of the 2022 total biofuel blends in gasoline must be derived from noncornstarch products. Certainly, biomass-derived methanol, biomass-derived diesel, cel — lulosic ethanol, algae biodiesel, vegetable oil biodiesel, and others qualify as noncornstarch based biofuels.

Historical Perspective of Alcohol Fermentation Technology

Although less heralded, cellulosic ethanol has a fairly rich history. One of the first recorded attempts at commercializing a cellulosic ethanol process was made in Germany as early as in 1898. The process was based on the use of dilute acid to hydrolyze the cellulose to glucose and the subsequent fermentation of glucose to ethanol. The reported productivity was 7.6 liters of ethanol per 100 kg of wood waste, equivalent to 18 gallons per short ton. As an early process, the conversion of wood waste into ethanol was quite remarkable; the process was further enhanced in Germany to yield about 50 gallons of ethanol per short ton of biomass. In the United States this pro­cess was further enhanced during World War I by adopting a single-stage dilute sulfuric acid hydrolysis process, by which the overall ethanol yield per input biomass was about 50% lower than the original German version, but the throughput of the process was much higher. This American process was short-lived, due to a significant decrease in lumber production in the post­war era. However, this process was brought to commercial operation again during World War II for production of butadiene by ethanol conversion to ultimately produce synthetic rubber. Even though the process achieved an ethanol yield of 50 gallons/dry ton of wood cellulose, this level of productiv­ity was far from profitable and the process was halted after the war. Even though commercial production had been stopped, active research on cellu — losic ethanol continued throughout the world, intensifying even more as a result of several rounds of petroleum crises, booming bioethanol consump­tion, and rapid advances in biotechnology.

In 1978, Gulf Oil researchers [33] designed a commercial-scale plant pro­ducing 95 x 106 liters per year of ethanol by simultaneous enzymatic hydro­lysis of cellulose and fermentation of resulting glucose as it is formed, thereby overcoming the problem of product inhibition. The process con­sisted of a unique pretreatment which involved the grinding and heating of the feedstock followed by hydrolysis with a mutant bacterium, also specially developed for this purpose. Mutated strains of the common soil mold tricho- derma viride were able to process 15 times more glucose than natural strains. Simultaneous hydrolysis and fermentation reduced the time requirement for the separate hydrolysis step, thus reducing the cost and increasing the yield. Also, the process did not use acids which would increase the equipment costs. The sugar yields from the cellulose were about 80% of what was theo­retically achievable, but the small amount of hemicellulose in the sawdust was not converted. This fact demonstrated a need for an effective pretreat­ment to cause hemicellulose separation.

As advances in enzyme technology have been realized, the acid hydrolysis process has been gradually replaced by a more efficient enzymatic hydro­lysis process. In order to achieve efficient enzymatic hydrolysis, chemical or biological pretreatment of the cellulosic feedstock has become necessary to prehydrolyze hemicelluloses in order to separate them from the lignin. The researchers of the Forest Products Laboratory of the U. S. Forest Service (USFS) and the University of Wisconsin-Madison developed the sulfite pre­treatment to overcome recalcitrance of lignocellulose (SPORL) for robust enzymatic saccharification of lignocellulose [34].

Cellulase is a class of enzyme that catalyze cellulolysis, which breaks cellu­lose chains into glucose molecules. In recent years, various enzyme compa­nies and biotechnology industries have contributed significant technological breakthroughs in cellulosic ethanol technology through the development of highly potent cellulase enzymes as well as the mass production of these enzymes for enzymatic hydrolysis with economic advantages. Research, development, and demonstration (RD&D) efforts in cellulase enzyme by many international companies such as Novozymes, Genencor, Iogen, SunOpta, Verenium, Dyadic International, and national laboratories such as National Renewable Energy Laboratory (NREL), are quite significant.

Cellulosic ethanol garnered strong endorsements and received significant support from the U. S. President George W. Bush in his State of the Union address, delivered on January 31, 2006, that proposed to expand the use of cellulosic ethanol. In this address, President Bush outlined the Advanced Energy Initiative (AEI 2006) to help overcome America’s dependence on for­eign energy sources and the American Competitiveness Initiative (ACI 2006) to increase R&D investment and strengthen education. The Renewable Fuel Standard (RFS) program was originally enacted under the Energy Policy Act of 2005 (EPAct 2005) and established the first renewable fuel volume mandate in the United States. The original RFS is referred to as RFS1. RFS1 required 7.5 billion gallons of renewable fuel to be blended into gasoline by 2012. The origi­nal timeline and renewable fuel volume mandate were revised and expanded.

The Energy Independence and Security Act (EISA) of 2007 established long­term renewable-fuels production targets through the second Renewable Fuel Standard (RFS2). The RFS2 expanded upon the initial corn-ethanol produc­tion volumes and timeline of the original RFS, under which the U. S. EPA is responsible for implementing regulations to ensure that increasing volumes of biofuels for the transportation sector are produced. The U. S. EPA released its final rule for the expanded RFS2 in February 2010, through which its statutory requirements established specific annual volumes, for the total renewable fuel volume, from all renewable fuel sources [35]. As a mandate potentially affecting the long-term future of corn ethanol, the RFS2 man­dates that the country as a whole is required to blend 36 billion gallons of renewable fuels into the transportation fuel sector by 2022, of which 16 bil­lion gallons is expected to come from non-corn based ethanol. The U. S. EPA implementation of the RFS2 would position the United States for making significant improvements in the greenhouse gas footprint due to the trans­portation sector. In February 2010, the White House under the leadership of President Barack Obama released "Growing America’s Fuel," which is a comprehensive roadmap to advanced fuels deployment [36].

In 2004, the researchers at the National Renewable Energy Laboratory, in collaboration with two major industrial enzyme manufacturers (Genencor International and Novozymes Biotech), achieved a dramatic reduction in cellulase enzyme costs, which was one of the major stumbling blocks in the commercialization of cellulosic ethanol. Cellulases belong to a group of enzymes known as glycosyl hydrolases, which cleave (hydrolyze) chemical bonds linking a carbohydrate to another molecule. The novel technology involves a cocktail of three types of cellulases: endoglucanases, exogluca — nases, and beta-glucosidases. These enzymes synergistically work together to attack cellulose chains, pulling them away from the crystalline structure, breaking off cellobiose molecules (two linked glucose residues), splitting them into individual glucose molecules, and making them available for further enzymatic processing. This breakthrough work is claimed to have resulted in twenty — to thirtyfold cost reduction and earned NREL and col­laborators an R&D 100 Award [37].

This is certainly a milestone accomplishment in cellulosic ethanol tech­nology development; however, further cost reductions are required in cel- lulase enzyme manufacture, new routes need to be developed to enhance enzymatic efficiencies, the development of enzymes with higher heat toler­ance and improved specific activities is highly desired, better matching of enzymes with plant cell-wall polymers needs to be achieved, a high-solid enzymatic hydrolysis process with enhanced efficiency needs to be devel­oped, and more. The U. S. Department of Energy Workshop Report summa­rizes, in scientific detail, the identified research needs in the area (Biofuels Joint Roadmap [26]).

In recent years, major advances have also been made utilizing genetic engineering and advanced microbiology in the development of robust microbe systems that are capable of efficiently co-fermenting both C5 and C6 sugars and that are resistant to inhibitors and tolerant against process variability.

Another effort for production of cellulosic ethanol is via catalytic conver­sion of gaseous intermediates produced by thermochemical conversion of cellulosic materials without the use of enzymes. Certainly, there is a trade­off between the purely chemical route and the enzymatic route in various aspects, including conversion efficiency, product selectivity, reaction speed, capital cost, overall energy efficiency, raw material flexibility, and more. A large commercialization effort launched by Range Fuels in 2007, based on catalytic conversion of thermochemical intermediates derived from biomass, was shut down in 2011 without meeting its original goals.

Pyrolysis or Thermal Decomposition

Pyrolysis or thermal decomposition is the molecular breakdown of organic materials such as hydrocarbons via cleavages of chemical bonds at elevated temperatures without the involvement of oxygen or air. Typical chemical bond cleavages during pyrolysis involve the C-C and C-H bonds at most operating temperatures, whereas double bonds of C=C, C=O are substan­tially more difficult to break at most practical conditions. As can be imagined by the chemical bonds that are typically broken during pyrolysis, the pyroly­sis reaction starts at a temperature as low as 150-200°C, where the intrinsic reaction rate is very slow and the extent of reaction is far from completion in any reasonable time. It should be clearly noted that this low temperature is not a typical operating temperature for pyrolysis, considering that the pyro­lytic decomposition reactions are still present at this low temperature, even though not active at all. Most practical pyrolysis of hydrocarbons without using any catalyst is pursued at a temperature higher than 450°C. Pyrolysis involves the concurrent change of chemical compositions and physical phases, and the process is irreversible.

If high molecular weight hydrocarbons are pyrolyzed in an oxygen — deprived environment at a temperature of 450-650°C, lighter hydrocarbons (reduced carbon numbers and lower molecular weights), hydrogen, and solid char would be typically formed. Lighter hydrocarbons nearly always involve methane (CH4), ethylene (C2H4), ethane (C2H,), and other fragmented hydro­carbons, of which methane is most dominant. Depending upon the pyroly­sis conditions, liquid range hydrocarbons, C4 — C15, are also obtained. Char formation is believed to be via a route similar to the formation of polycy­clic aromatic hydrocarbons (PAHs) which involves polymerization of highly reactive free radicals of fragmented hydrocarbons and unsaturated hydro­carbon species. Hydrogen formation during pyrolysis is via cleavage reac­tions of C-H chemical bonds of the original and intermediate hydrocarbon molecules and some of the hydrogen molecules formed in such a manner are recombined with methyl radicals and ethylene thus producing methane and ethane. As illustrated, pyrolysis of hydrocarbons can yield materials of all three different phases (i. e., solid, liquid, and gas) as its end products depend­ing upon the treatment conditions. Furthermore, the actual number of chem­ical reactions involved and the number of final and intermediate chemical species are countless. Therefore, pyrolysis collectively represents a class of chemical reactions taking place as thermochemical decomposition. A more generalized chemical reaction equation for hydrocarbon pyrolysis may be written as follows:

CHb ^ c ■ CH4 + CdHe + CfHg + h ■ H2
a = c + d + f
b = 4c + e + g + 2h

CuHvOw ^ CkH°wl + CmHn°w2 + p ‘ H2O + q ■ H2 + r ‘ CO2 + s ‘ CO
u = k + m + r + s
v = l + n + 2p + 2q
w = wl + w2 + p + 2r + s

In the first reaction expression, methane is explicitly written in the product side, because methane is always a dominant hydrocarbon product species of hydrocarbon pyrolysis.

Biomass chemical compounds are far more oxygenated than straight hydrocarbons and biomass also contains a high level of moisture. Therefore, thermal decomposition or pyrolysis of biomass also generates carbon oxides in addition to the aforementioned pyrolysis products of condensable hydro­carbons, methane, and hydrogen. If biomass is microbially degraded in anaerobic conditions, it generates a product gas rich in methane and carbon dioxide. This product gas is called biogas, or landfill gas. A process system developed for exploiting this biogas is called an anaerobic digester, which can produce methane rich gas from waste materials on a small-scale unit.

The scientific definition of pyrolysis presented in this section precludes oxygen involvement in its mechanistic reaction steps. This was necessary in defining pyrolysis as a thermochemical reaction by itself. However, it should be clearly noted that the actual pyrolysis reaction can also occur as a component reaction of many reactions simultaneously taking place in many different chemical process environments, including both oxidative as well as reducing environments. In such environments, pyrolytic decomposition reactions compete with other chemical reactions also occurring in the sys­tem and as such the reaction environment becomes that much more complex in terms of both the nature and the total number of simultaneous reactions taking place in the system.

Of course, it is also true that pyrolysis alone in the absence of oxygen can be targeted in certain process environments, as is the case with fast pyrolysis of biomass. Typical fast pyrolysis processes are operated at a temperature that is substantially lower than typical gasification temperatures of steam gasification, Boudouard reaction, hydrogasification, and partial oxidation. Hence, fast pyrolysis as a transformation process is more or less strictly a combination of devolatilization and pyrolytic decomposition reactions in an oxygen-deprived environment. Furthermore, it must be clearly understood that the principal targeted product of fast pyrolysis of biomass is liquid- phase bio-oil, not gaseous synthesis gas.

Biomass can be gasified without any gasifying agent additionally intro­duced into the reactor and this type of gasification is called pyrogasification.

Pyrolytic gasification or pyrogasification of biomass takes advantage of both pyrolysis and gasification and it can be carried out both catalytically [31, 32] and noncatalytically. In pyrogasification, no separate gasifying medium or oxygen (or air) is introduced, it is expected that a gasifying reactant such as steam has to be in situ provided from biomass pyrolysis. In pyrogasification, biomass pyrolysis also produces biochar and this biochar reacts with steam via steam gasification to generate product gas.

Pyrolytic gasification of wood using a stoichiometric nickel aluminate catalyst was carried out by Arauzo and coworkers [32] in a fluidized bed reactor and near-equilibrium yields of products were obtained above 650°C. Although they obtained 85-90% gas yields, tar production was not detected. They further tested the process using a modified nickel-magne­sium aluminate stoichiometric catalyst and also an addition of potassium as a promoter. They found that magnesium addition to the catalyst crystal­line lattice enhanced attrition resistance of the catalyst with a minor loss of gasification activity and an increased production of coke. However, they found little effect from an addition of potassium component. Catalyst foul­ing by carbon deposition, that is, surface coverage by coke, was shown and regeneration of magnesium-containing catalyst by carbon burn-off was demonstrated.

Asadullah et al. [31] comparatively evaluated the catalytic performances of Rh/CeO2/SiO2, steam reforming catalyst G-91, and dolomite for a number of different biomass gasification modes, including pyrolytic, CO2, O2, and steam. With respect to the biomass conversion to product gas and selectivity of useful gaseous species, Rh/CeO2/SiO2 has shown superior results in all gasification modes. In the pyrogasification case, about 79% of the carbon in biomass was converted to the product gas at 650°C. There was no tar detected in the effluent gas stream. The gasifier used for their experiment was a lab — scale continuously fed fluidized bed reactor.

Economic and Environmental Issues Related to Waste Conversion

Waste is a relatively inexpensive material, but there are some economic and environmental issues associated with its treatment that can make it unattractive for commercial use. For example, landfills are useful sources for methane, however, a continuous collection of methane can be an issue. Depending on the size of the landfill, single or multiple sources of collection points of methane need to be equipped and this will require building a care­fully designed methane collection infrastructure. Also, if a power plant is to be built on or near landfill, its size will depend on the amount of the con­tinuous methane supply from the landfill. Generally the size of the power plant will vary from 1 to 100 MW, a size that is not always most economical. Getting a local permit for such an operation can also be an issue. Landfill gas generally contains about 50 to 55% methane, remaining carbon dioxide, and some other trace materials including hydrogen, nitrogen, and the like. This gas cannot be transported by a natural gas pipeline, which generally contains about 95% methane, without a further purification and upgrading process. Such a process can be expensive.

Numerous localities (particularly on the East Coast) have used MSW to support local power needs. Like landfill gas, the size of such a power plant will depend on the amount and continuous supply of MSW. For these rea­sons, these plants are now largely located in densely populated areas where a significant amount of MSW can be easily transported to a common collec­tion point. An installation of a power plant using MSW as raw material in remote areas can be economically unattractive. The same principle applies to other kinds of waste such as rubber tires, agricultural and forestry resi­dues, and so on, where transportation to a central location can be very expensive. This problem is further compounded because of low mass and energy densities of cellulosic waste. Sizing and separation of waste mate­rials for a particular treatment technology can also become a significant economic issue.

The treatment of waste can also lead to significant environmental issues. Depending on the composition of waste, thermochemical treatment can lead to significant amounts of impurities in the exit gas or remaining solids. Both of these impurities need to be eliminated either by pretreating waste or puri­fying the gaseous product and solid streams. These processes can be expen­sive depending upon the nature and amount of impurities. The problem can be further compounded for a mixed waste.

Algae Harvesting by Flocculation

Algae flocculation is a method of separating algae from its medium by using chemicals to force the algae to form lumps or aggregates. The main disad­vantage of this separation method is that the additional chemicals are diffi­cult to remove or recover from the separated algae, thus making it inefficient and uneconomical for commercial use. The cost to remove or recover these chemicals may be too expensive to be commercially viable, unless a techno­logical breakthrough is achieved.

Flocculating agents, or flocculants, are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc. In general, there are two types of flocculants commonly used: inorganic flocculants and organic polymer/polymer electrolyte flocculants. Alum (hydrated potassium aluminum sulfate (KAl(SO4)212H2O)) and ferric chloride (FeCl3) are the chemical flocculants most frequently used to har­vest algae [20]. A commercial product called "Chitosan," popularly used for water purification, can also be used as a flocculant but is far more expen­sive than other flocculants. The shells of crustaceans, such as shrimp, lobster, crabs, and crayfish, are ground into powder and processed to obtain chitin, a polysaccharide found in the shells, from which chitosan is derived via de­acetylation. Water that is more brackish or saline requires additional chemi­cal flocculant to induce flocculation [20]. High molecular weight organic polymers are considered good flocculants, because several segments of a polymer can attach themselves to the surface of a colloidal particle and the remainder of the segments are extended into the solution [25].

Harvesting via chemical flocculation alone, by current technoeconomic standards, is a method that may be too expensive for large operations. In addi­tion to the chemical flocculation discussed above, there are different methods of flocculation of algae, including autoflocculation [26], bioflocculation [27], and electroflocculation. Bioflocculation has been studied and practiced in wastewater and sewage treatment. The efficacy of algae flocculation depends upon a large number of factors that are usually poorly understood or ignored, including cell size, cell shape, cell wall thickness, cell surface and interfacial properties, and so on. By considering these properties as well as appropri­ate combinations of flocculation with other concentrating techniques, there is room for substantial enhancement in algae flocculation practice.

Ethanol Purification and Product Separation

Ethanol is subsequently separated from the mash by distillation, in which the components of a solution (in this case, water and ethanol) are separated by differences in boiling point (or individual vapor pressure). Separation is technically limited by the fact that ethanol and water form an azeotrope, or a constant boiling solution, of 95.63 wt% alcohol and 4.37 wt% water. This azeotrope is a minimum boiling mixture (or a positive azeotrope), for which the boiling temperature of the azeotrope is lower than that of the individual pure components, that is, water and ethanol. The minimum boiling tempera­ture at the azeotropic concentration is 78.2°C, whereas the normal boiling points of ethanol and water are 78.4°C and 100°C, respectively.

The 5%, more precisely 4.37 wt%, water cannot be separated by conven­tional distillation, because the minimum boiling temperature is attain­able at the azeotropic concentration, not at the pure ethanol concentration. Therefore, production of pure, water-free (anhydrous) ethanol requires an additional unit operation step following distillation. Dehydration, a relatively complex step in ethanol fuel production, is accomplished in one of two methods. The first method uses a third liquid, most commonly benzene, which is added to the ethanol/water mixture. This third com­ponent changes the boiling characteristics of the solution (now a ternary system instead of a binary system), allowing separation of anhydrous ethanol. In other words, this third component is used to break the azeo­trope, thereby enabling conventional distillation to achieve the desired goal of separation. This type of distillation is also called azeotropic dis­tillation, because the operation separates mixtures that form azeotropes. The second method employs molecular sieves that selectively absorb water based on the molecular size difference between water and ethanol. Molecular sieves are crystalline metal aluminosilicates having a three­dimensional interconnecting network of silica and alumina tetrahedra. Molecular sieves have long been known for their drying capacity (even to 90°C). There are different forms of molecular sieves that are based on the dimension of effective pore opening, and they include 3A, 4A, 5A, and 13X. Commercial molecular sieves are typically available in powder, bead, granule, or extrudate forms.

Ionic Liquid Pretreatment

Ionic liquids are a relatively new class of solvents that have recently gained popularity as environmentally friendly alternatives to organic solvents. An ionic liquid is a salt composed of anions and cations that are poorly co-ordi­nated and which has a melting point typically below 100°C. Ionic liquids are also referred to as liquid electrolytes, ionic melts, liquid salts, ionic glasses, and the like. There are thousands of substances that fall into this category. Ionic liquids have been demonstrated as very efficient solvents in the fields of hydrogenation, esterification, nanomaterial synthesis, biocatalysis, and selective extraction of aromatics [55, 56].

The development of a novel biomass pretreatment technology using ionic liquids has only recently been initiated. The first demonstration of an ionic liquid as a cellulose solvent under relatively mild processing conditions was reported in 2002 by Swatloski [56]. In experiments using a range of anions and 1-butyl-3-methylimidazolium cations, some ionic liquids were able to completely dissolve microcrystalline cellulose, and the cellulose was recovered through the addition of an antisolvent such as water or eth­anol. Moreover, the recovered product could be regenerated into a wide range of shapes and morphologies. The most effective cellulose solvents were the ionic liquids that contain chloride anions. An important finding associated with this novel pretreatment method is that enzymes can more efficiently hydrolyze into glucose the amorphous cellulose produced by ionic liquids than the microcrystalline cellulose found in lignocellulose naturally [55, 57].

Ionic liquids are an exciting area of new scientific discovery and inher­ently possess many processing merits in lignocellulose pretreatment. More in-depth R&D work needs to be conducted, however, before a commercially viable process can be fully developed and exploited.

Conversion of Waste to Biofuels, Bioproducts, and Bioenergy

6.1 Introduction

As the world population grows and its natural resources diminish, the concept of waste is changing. Generally waste is considered as those chem­icals and materials that are either used and discarded or those that are perceived to have little direct use potential for human or animal needs. Currently, the preferred mode is to recycle and reuse waste or discard it in landfills. Not all materials are recyclable or can be reused without further treatment or conversion. The new concepts of Enhanced Waste Management (EWM) and Enhanced Landfill Mining (ELFM) put landfill­ing of waste in a sustainable context [1, 2]. In these new concepts, a landfill is no longer considered as a final solution but a temporary storage place before the stored waste is reused through an appropriate conversion pro­cess [3]. Thus, ELFM offers an opportunity to select an appropriate path for the conversion of waste into either materials (waste to product, WtP) or energy (waste to energy, WtE) and thereby reuse all waste to the extent new technologies and environmental regulations allow. The success of these new concepts depends not only on the technological improvements and breakthroughs but also on a multitude of socioeconomic barriers such as regulations, social acceptance, economic uncertainty, and feasibility of a particular technology in the given environment which prevents the emis­sions of CO2 and pollutants [2].

An integrated solid waste management is typically governed by the pro­cess (often known as the "Ladder of Lansink") which specifies a generally accepted hierarchy of preferred methods of dealing with different types of waste. Direct recycling and reuse of waste is preferred, however, this is not always possible. Numerous technologies are now available to convert each type of waste either to energy or to a reusable product. This chapter exam­ines these technologies and associated processes to obtain the desirable out­comes. The chapter also briefly examines the environmental and economic issues associated with various conversion processes.

Moving Bed Gasifier

The British Gas Lurgi gasifier (BGL), which is a moving bed design type, is currently the only large-scale (>50 MWe) gasification technology with sig­nificant operational experience in the processing of mixed feedstock of coal and waste materials. BGL has the most operational experience in process­ing fuels of widely differing fuel properties. This technology is now owned by Advantica, a company leader in the large-scale gasification of variable property feedstock. Global Energy (part of ConocoPhillips since 2003) is the U. S. leader in mixed feedstock gasification. The BGL technology creating new plants in the United States and Europe for moving bed gasification of the materials which include coal, municipal refuse, and sewage sludge feed­stock. Global Energy also operates the SVZ plant in Germany and a plant in Scotland alongside its natural gas-fired power plant for co-gasification using the BGL gasifier [6].

In a moving bed gasifier (often called fixed bed gasification), large par­ticles (about 2 in.) of feedstock move down while gases move upward (or sometimes downward). The acceptability of fines is limited for a dry ash moving bed, although it is better for a slagging moving bed reactor. At the top of the bed, solids get heated by hot gases which get cooled before leav­ing the reactor. This is considered a drying zone. The solids then go through carbonization, gasification, and combustion zones as they move downward [6]. At the bottom of the reactor, oxygen reacts with the remaining char. In a dry-ash version (i. e., Lurgi dry ash gasifier) the temperature is moderated to below ash slagging temperature by reaction of char with steam. The ash below the combustion zone is cooled by the entering steam and oxidant. In a slagging version, less steam is used which maintains the temperature above the ash slagging temperature. The temperature in the reactor may vary from 300-650°C depending on the nature of the feedstock. The dry ash moving bed reactor prefers low-rank coal and slagging moving bed reactor prefers high-rank coal [6]. Both the dry ash (with some modifications) and slagging moving bed accept caking coals. The moving bed reactor technology has (a) a low oxidant requirement which produces hydrocarbon liquids such as oils and tars, (b) high "cold gas" thermal efficiency, when the heating values of the hydrocarbon liquids are included, and (c) a high steam requirement for the dry ash moving bed and low steam requirement for the slagging moving bed [6]. The key technical issue with moving bed technology is the utiliza­tion of fines and hydrocarbon liquids coming from the product gas. Two existing commercial processes using a moving bed reactor are the Lurgi dry- ash gasifier, which is used for town gas production and chemicals from coal in South Africa, and the BG Lurgi (BGL) slagging gasifier, which is currently used to process solid waste and a mixture of coal and waste.

In summary, although updraft and downdraft gasifiers have been used for small-scale applications, fluid bed, circulating fluidized bed, and entrained bed gasifiers are extensively used for large-scale operations. Updraft gener­ates a high amount of tar, whereas the fluid bed and CFB generate a medium amount of tar and downdraft and the entrained bed generates a low amount of tar. Syngas can be produced by fluid bed, CFB, and entrained bed opera­tions. Fluid bed and CFB have higher particle loading and can use larger particle sizes whereas the entrained bed requires a large amount of carrier gas and it has a particle size limit [6].

Biomass Feedstocks

Even though biomass includes all plants and plant-derived materials, all plant matter is not equal as biomass feedstock for bioenergy and biofuels production. The success of the biofuels and bioenergy industry depends on the quality and quantity of biomass available as well as the ability to utilize it cost-efficiently to produce biofuels and bioenergy [12]. Various factors are taken into consideration for suitability determination and choice of biomass for the biofuels program and they include:

• Sustainable feedstock production

• Arable land requirement

• Feedstock logistics

• Regional strength

• Food crops or not

• Grains or nongrains

• Feedstock properties and compositions

• Pretreatment cost

• Availability of efficient conversion/transformation technology

• Feedstock cost

• Capital investment and operating cost involved

• Environmental benefits

• Desirable biofuel products and their values

A variety of biomass feedstock can be converted into alternative trans­portation fuels. Currently, a dominant majority of biofuels produced in the United States are corn ethanol. Although corn is an excellent source of starch which can be easily converted into fuel ethanol and is also the most abun­dantly produced U. S. crop, the corn feedstock also has concerns of being a food crop and demanding high feedstock cost. More R&D is being focused on the development of cellulosic feedstock such as corn stover, switchgrass, and woody cellulose. Figure 1.2 shows resource-based biorefinery pathways, as presented in the Biomass Program’s website, U. S. DOE, Office of Energy Efficiency and Renewable Energy [12]. The Biomass Feedstock Platform has three phases of program foci in its R&D Platform and they are

• In the immediate and near term, it will focus on the sustainable pro­duction, collection, and use of readily available low-cost agricultural residues and industrial wastes.

• In the near to mid-term, it will address additional agricultural and for­estry residues and a potentially few dedicated energy crops.

• In the longer term, it will involve the development and use of both herbaceous and woody dedicated energy crops.

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Подпись: Distribution and End Uses Biofuels: • Bulk Distribution Infrastructure • Retail Marketing Network • Vehicles Biopower: • Grid Connectivity • Co-firing Bioproducts: • Bulk Distribution Infrastructure • Chemical Intermediate Market • Consumer Market
Подпись: ◄

FIGURE 1.2

Resource-based biorefinery pathways by U. S. DOE, EERE. (From U. S. Department of Energy, 2011. Energy Efficiency & Renewable Energy, Biomass Program. Biofuels, Biopower, and Bioproducts: Integrated Biorefineries. http://www1.eere. energ y. gov/biomass/in dex. html.)

Biofuels are classified into two broad categories, namely first-generation and second-generation biofuels, based on the kinds of feedstock as well as the types of process technologies applied or applicable. First-generation (1G) biofuels refer to fuels that have been derived from biological sources such as starch, sugar, animal fats, and vegetable oil. These conventional biofuels are produced from oil crops, sugar crops, and cereal crops, using estab­lished technology. Some of the most typical types of first-generation biofu­els include vegetable oils, conventional biodiesel, bioalcohols, biogas, and biosyngas. On the other hand, second-generation (2G) biofuels are derived from cellulosic materials (lignocellulosic feedstock). These raw materials for 2G biofuels may result in more biofuel per unit area of agricultural land and also require less chemical and energy input for production and har­vesting, which in turn results in a higher net energy yield [9]. As such, raw materials for 2G biofuels may be considered more sustainable than those for 1G biofuels and do not create the issues of "food versus fuel," either. Many second-generation biofuels are under active research and develop­ment and include cellulosic ethanol, algae fuel, biohydrogen, biomethanol, bio-dimethylether (bio-DME), Fischer-Tropsch diesel, 2,5-dimethyl furan (DMF), mixed alcohols, and wood diesel. The second-generation biofuels are also referred to as advanced sustainable biofuels.