## Yeast Fermentation

Yeasts are capable of converting sugar into alcohol by a biochemical pro­cess called fermentation. The yeasts of primary interest to industrial fer­mentation of ethanol include Saccharomyces cerevisiae, Saccharomyces uvarum, Schizosaccharomyces pombe, and Kluyveromyces spp. Under anaerobic con­ditions, yeasts metabolize glucose to ethanol primarily via the Embden — Meyerhof pathway. The Embden-Meyerhof pathway of glucose metabolism is the series of enzymatic reactions in the anaerobic conversion of glucose to lactic acid (or ethanol in this case), resulting in energy in the form of adenosine triphosphate (ATP) [17]. The overall net reaction represented by a stoichiometric equation involves the production of two moles of ethanol from each mole of glucose as shown below. However, the yield attained in practical fermentation attempts does not usually exceed 90-95% of the theo­retical value. In this case, the theoretical value (i. e., 100% of yield) means that exactly two moles of ethanol are produced from each mole of glucose input to the fermenter. Therefore, this 100% yield is equivalent to the mass conver­sion efficiency of 51%, which is defined later in this section. The following stoichiometric equation shows the basic biochemical reaction in the conver­sion by fermentation of glucose to ethanol, carbon dioxide, and endothermic heat.

C6H12O6 = 2 C2H5OH + 2 CO2
ДИ°298 = 92.3 kJ/mol

Theoretically, the maximum conversion efficiency of glucose to ethanol is 51% on a weight basis, which comes from a stoichiometric calculation of:

2 * (Molecular wt of Ethanol)/(Molecular wt of Glucose) = (2 * 46)/(180)

= 0.51

However, some glucose is inevitably used by the yeast for production of cell mass and for metabolic products other than ethanol, thus reducing the conversion efficiency from its theoretical maximum of 51%. In practice, 40 to 48% of glucose, on a weight basis, is actually converted to ethanol. With a 46% fermentation efficiency, 1,000 kilograms of fermentable sugar would produce about 583 liters of pure ethanol, after taking into account the density of ethanol (specific gravity at 20°C = 0.789), or

(1,000 kg sugar) * (0.46 kg ethanol/kg sugar)/

(0.789 kg ethanol/L) = 583 L ethanol

Conversely, about 1,716 kilograms of fermentable sugar are required to produce 1,000 liters of ethanol, when a 46% mass conversion efficiency is assumed. Mash typically contains between 50 and 100 grams of ethanol per liter (about 5 to 10% by weight) when the fermentation step is complete. This is called distilled mash or stillage, which still contains a large amount of non­fermentable portions of fibers or proteins.

## Combined RASH and Organosolv Pretreatment

Attempts have been made to improve overall process efficiency by combining the two individual pretreatments of rapid steam hydrolysis and organosolv. Rughani and McGinnis [53] have studied the effect of a combined RASH — organosolv process upon the rate of enzymatic hydrolysis and the yield of solubilized lignin and hemicellulose. A schematic diagram of the process is shown in Figure 4.10. For the organosolv pretreatment, the steam generator is disconnected and the condensate valve closed. The rest of the reactor setup is similar to the typical RASH procedure.

The organosolv processes at low temperature are generally ineffective in removing lignin, as explained earlier; however, combining the two processes leads to increased solubilization of lignin and hemicellulose. RASH temper­ature is the major factor in maximizing the percentage of cellulose in the

 FIGURE 4.10 A combined RASH and organosolv pretreatment scheme. (Modified from Rughani and McGinnis, 1989. Combined rapid steam hydrolysis and organosolv pretreatment of mixed southern hardwoods, Biotechnol. Bioeng, 33: 681-686,)

final product. The maximum yield of solubilized lignin was obtained at a temperature of 240°C for RASH and 160°C for the organosolv process.

## Utilization of Biomass Synthesis Gas

Synthesis gas obtained by biomass gasification can be utilized in a variety of ways and end-uses. Although all conventional syngas utilization methods

 Distillate+ FIGURE 5.16 Transformation of syngas into liquid transportation fuels.

are conceivable and applicable, the process economics, based on the current fuel market and the current level of available technology, may not be favor­able for the manufacture of bulk petrochemicals that have traditionally relied on the syngas derived from either natural gas (NG) or coal. However, in cer­tain niche markets, biomass synthesis gas can favorably compete against the NG-based syngas or coal-syngas. Furthermore, biomass syngas is CO2 neutral and renewable as well as possibly being better suited for small — to medium-scale operations.

Syngas obtained from biomass gasification can be used for indirect lique­faction processes by which syngas is converted to liquid transportation fuels such as methanol, dimethylether (DME), ethanol, higher alcohols, gasoline, diesel, and jet fuels, as shown in Figure 5.16. The following will have to be accomplished in order to convert the biomass syngas into clean liquid fuels that can be used in the conventional energy/fuel infrastructure:

(a) Innovative process integration schemes need to be devised.

(b) Highly efficient energy integration between the intraprocess and interprocess steps needs to be devised and achieved.

(c) Effects of trace minerals in the biomass syngas on the process cata­lysts need to be fully understood and managed.

(d) Robust and highly effective catalyst systems need to be developed and demonstrated on a long-term basis.

(e) Efficient conversion technology of CO2-rich syngas needs to be devel­oped and refined.

(f) Biomass pretreatment technology needs to be enhanced.

(g) Conversion technologies using mixed feedstock need to be developed and refined.

(h) Gas clean-up technology needs to be sufficiently enhanced in terms of the efficacy and cost.

(i) Gas separation and purification technology needs to be enhanced.

## Fluidized Bed Gasifier

Fluidized bed gasifiers have a lower gas flowrate, higher residence time, uniform temperature, and good mixing which avoids clinker formation and possible defluidization. They are well suited for active reactants such as bio­mass and low-rank coal. If char particles are entrained, they can be recycled back by a cyclone. Ash particles that are removed at the bottom can exchange heat with incoming recycled gas and steam. Fluidized bed reactors require moderate oxygen and steam requirements and extensive char recycling. They can accept a wide variety of feedstock (e. g., solid waste, wood, and high ash coals) with a larger particle size then what is normally required for the entrained flow gasifiers. Commercial applications of fluidized bed reactors are high-temperature Winkler (HTW) and KRW designs, the latter gasifier is a part of the Pinon Pine Coal gasification plant [6].

Different fluidized bed gasifiers can vary in ash conditions (dry ash or agglomerated) and design configurations to improve char use. Both dry ash and agglomerating fluidized beds are operated with crushed feed (about M in. size). The acceptability of fines is good for the dry ash bed and better for the agglomerating bed. The reactor is generally operated at 925-1025°C. The caking coal may be processed in a dry ash fluidized bed, but it can cer­tainly be processed in an agglomerating bed. The dry ash bed is preferred for low-rank coal and agglomerating bed can process any rank of coal. The main technical issue of the fluidized bed is the carbon conversion per pass. For this reason, for a less reactive coal mixture, a circulating fluidized bed is preferred.

Mixed feedstock has been tested in fluidized bed reactors at the Royal Institute of Technology at Stockholm, Sweden. They found that for a coal- biomass mixture, the char from woody biomass is very sensitive to the ther­mal annealing effect which occurred at low (650°C) temperature and short soak time (less than 8 min). A mixture of birch and coal gasification [106] showed synergies by an enhanced gasification rate in the presence of oxy­gen and reduction of char formation. Also both tar and ammonia formations were lowered in the mixture gasification. Currently, there are very few large fluidized bed gasifiers in operation.

## Sustainability

A principal reason for the use of biofuels and bioenergy is its renewable nature. Utilizing biomass as a biofuel feedstock means that carbon dioxide in the atmosphere that was absorbed and transformed via a plant’s photosyn­thesis is released back into the atmosphere upon combustion of the biofuel. By considering the starting feedstock of the biomass and the end product of biofuels only, without considering any ancillary carbon energy input during the conversion process, the system can be said to be carbon neutral. Carbon neutrality means achieving net zero carbon emission, thus leaving a net zero carbon footprint. Furthermore, by maintaining a balance between plant growth and biomass use, the energy system is both renewable and sustain­able. Sustainability of a feedstock is defined by availability of the feedstock, very positive and beneficial impact on GHG emissions, and no negative impact on biodiversity and land use [9].

There is a growing consensus that carbon dioxide emission and its accu­mulation in the atmosphere is a major culprit of global climate change via human interference with natural cycles of greenhouse gases. Two of the more direct causes for carbon dioxide accumulation in the modern era have been recognized as combustion of fossil fuels and land-use change, in par­ticular, deforestation. The use of biofuels undoubtedly reduces the use of fossil fuels and helps restore the needed balance between the carbon dioxide uptake and release.

Agriculture is also expected to continue to evolve and adapt to new tech­nologies and changing circumstances. Biotechnology is advancing agricul­ture by making available genetically altered varieties of corn and soybeans as well. According to the National Corn Growers Association, biotech hybrids accounted for 40% of the total planted acreage of the United States in 2004 [10]. Crop yields are very important because they directly affect the amount of residue generated as well as the amount of land needed to meet food, feed, and other demands. A joint study sponsored by the U. S. Department of Energy and the U. S. Department of Agriculture offers three scenarios in its report (2005) [10]:

• Scenario 1: Current sustainable availability of biomass feedstocks from agricultural lands

• Scenario 2: Biomass availability through a combination of technol­ogy changes focused on conventional crops only

• Scenario 3: Biomass availability through technology changes in both conventional crops and new perennial crops together with signifi­cant land usage change

Current availability is the baseline case that summarizes sustainable bio­mass resources under current crop yields, tillage practices (20-40% no-till for major crops), residue collection technology (~40% recovery potential), grains to bioethanol and biodiesel production, and use of secondary and tertiary residues [10]. Summing up, the total amount of biomass currently available in the United States as of 2005 for bioenergy and bioproducts was about 194 million dry tons annually. This was about 16% of the 1.2 billion dry tons of plant materials produced on agricultural land of the United States. The single largest source of this biomass potential in 2005 was corn residues or corn stover totaling close to 75 million dry tons [10,11]. Considering that the U. S. corn production was 282 million metric tons in 2005 and 333 million metric tons in 2010, the biomass potential from corn residues alone would have totaled more than 85 million dry tons in 2010. On the other hand, the total biomass derived from forestlands in the United States was estimated to be about 142 million dry tons in 2005 [10]. Therefore, from the standpoints of resource availability and sustainability, intensive R&D efforts focused on efficient conversion of corn residues into biofuels as well as cost-effective conversion of cellulose into ethanol are rationally grounded and well justified.

According to a more recent study conducted and reported in the U. S. Billion-Ton Update [11], for the baseline scenario, projected consumption of currently used resources, the forest residues and wastes, the agricultural res­idues and waste, and energy crops show a total of 1,094 million dry tons in 2030. Under the baseline assumptions, up to 22 million acres of cropland and 41 million acres of pastureland shift into energy crops by 2030 at a simulated farmgate price of \$60 per dry ton [11]. This study also shows that the total biomass potential in the United States for the currently used and potential forest and agricultural biomass at \$60 per dry ton or less, under the baseline scenario, sharply increases from 473 million in 2012, to 676 million in 2017, to 914 million in 2022, and to 1094 million dry tons in 2030.

## 2.2.4.Є Enzymatic Extraction

The enzymatic process uses select enzymes to degrade algae cell walls and in the process system water acts as a solvent medium for enzyme action. No additional solvent is involved, therefore this process facilitates an easier downstream fractionation. The advantages of the enzymatic extraction pro­cess include:

(a) The process does not require dry cakes for oil extraction.

(b) No caustic chemicals are required or involved.

(c) Mild process conditions are used.

(d) The process can be synergistically integrated with other processes such as ultrasonification.

(e) Environmental impact is minimal.

The process is also in a beginning stage and finding efficient and robust enzymes for the process is a challenge. Unless a cost-effective enzyme is developed and proven for the process, this process would be more expen­sive costwise than hexane solvent extraction. However, there are ample opportunities with this approach, inasmuch as the process can be utilized in conjunction with many other mechanical extraction technologies. In addi­tion, a drastically different product portfolio may be developed for highly value-added coproducts and by-products.

## Ethanol as Oxygenated and Renewable Fuel

Oxygenated fuel is conventional gasoline that has been blended with an oxygenated hydrocarbon to achieve a certain desired concentration level of oxygen in the blended fuel. Oxygenated fuel is required by the Clean Air Act Amendments of 1990 for areas that do not meet federal air quality standards, especially those for carbon monoxide. The oxygen present in the blended fuel helps the engine to burn the fuel more completely, thus emitting less carbon monoxide. Extra oxygen already present in situ in the oxygenated fuel formulation helps efficient conversion into carbon dioxide rather than carbon monoxide. Gasoline blends of at least 85% ethanol are considered alternative fuels under the Energy Policy Act of 1992 (EPAct). E85 is used in flexible fuel vehicles (FFVs) that are currently offered by most major automobile manufacturers. FFVs can run on 100% gasoline, E85, or any combination of the two and qualify as alternative fuel vehicles under EPAct regulations.

Reformulated gasoline is a formulation of gasoline that has lower controlled amounts of certain chemical compounds that are known to contribute to the formation of ozone (O3) and toxic air pollutants. It is less evaporative than conventional gasoline during the summer months, thus reducing evaporative fuel emission and leading to reduced volatile organic compound emission. It also contains oxygenates, which increase the combustion efficiency of the fuel and reduce carbon monoxide emission. The Clean Air Act Amendments of 1990 require RFG to contain oxygenates and have a minimum oxygen con­tent of 2.0% oxygen by weight. RFG is required in the most severe ozone nonattainment areas of the United States. Other areas with ozone problems have voluntarily opted into the program. The U. S. EPA has implemented the RFG program in two phases: Phase I for 1995 to 1999 and Phase II having begun in 2000.

To be more specific, the Clean Air Act Amendments mandated the sale of reformulated gasoline in the nine worst ozone nonattainment areas beginning January 1, 1995. Initially, the U. S. EPA determined the nine regulated areas to be the metropolitan areas of Baltimore, Chicago, Hartford, Houston, Los Angeles, Milwaukee, New York City, Philadelphia, and San Diego. The impor­tant parameters for RFG by the Clean Air Act Amendments of 1990 were

1. At least 2% oxygen by weight

2. A maximum benzene content of 1% by volume

3. A maximum of 25% by volume of aromatic hydrocarbons

As of 2011 in the United States, RFG is required in cities with high smog levels and is optional elsewhere. RFG is currently used in 17 states and the District of Columbia. About 30% of gasoline sold in the United States as of 2011 is reformulated.

Methyl tertiary-butyl ether was one of the most commonly used oxygen­ated blend fuels until recent claims arose of health and environmental prob­lems associated with MTBE used as a blending fuel. Tertiary-amyl methyl ether (TAME), ethyl tertiary-butyl ether, and ethanol have also been used in oxygenated and reformulated fuels. Responding to the rapid phase-out of MTBE in the United States, ethanol has gained the most popularity as a blending fuel, based on its clean burning, relatively low Reid vapor pres­sure, the renewable nature of the fuel, minimal or no health concerns, and relatively low cost.

The RFG should have no adverse effects on vehicle performance or the durability of engine and fuel system components. However, there may be a slight decrease in fuel mileage (1 to 3% or 0.2-0.5 mile/gallon) in the case with well-tuned automobiles due to the higher concentrations of oxygenates that inherently have lower heating values. However, RFG burns more com­pletely, thereby reducing formation of engine deposits and often boosting the actual gas mileage, particularly for older engines.

The Reid vapor pressure is crucially important information for blended gasoline from practical and regulatory standpoints. Evaporated gasoline compounds combine with other pollutants on hot summer days to form ground-level ozone, commonly referred to as smog. Ozone pollution is of particular concern because of its harmful effects on lung tissue and breath­ing passages. Therefore, the government, both federal and state, imposes an upper limit as a requirement, which limits the maximum level reformu­lated gasoline can have as its Reid vapor pressure. By such regulations, the government not only controls the carbon monoxide emission level, but also limits the evaporative emission of the fuel. Due to this limit, certain oxygen­ates may not qualify as a gasoline blending fuel even if they may possess excellent combustion efficiency and high octane rating. One such example is methanol. Furthermore, the legal limits for the Reid vapor pressure depend
upon many factors including current environmental conditions, geographi­cal regions, climates, time of the year (such as summer months vs. winter months), and the like. It should also be noted that ground-level ozone is harmful to humans, whereas stratospheric ozone is essential and beneficial for global environmental safety.

The oxygenated fuel program (OFP) is a winter-time program for areas with problems of carbon monoxide air pollution. The oxygenated winter fuel program uses normal gasoline with oxygenates added. On the other hand, the reformulated gasoline program is for year-round use to help reduce ozone, CO, and air toxins. Although both programs use oxygenates to reduce CO, RFG builds on the benefit of oxygenated fuel and uses improvements in the actual formulation of gasoline to reduce pollutants including volatile organic compounds [35].

Although methyl-tertiary-butyl ether was once credited with significantly improving the nation’s air quality, it has been found to be a major contributor to groundwater pollution. Publicity about the leaking of MTBE from gasoline storage tanks into aquifers as well as its adverse health effects has prompted legislators from the midwestern United States to push for a federal endorse­ment of corn-derived ethanol as a substitute oxygenate. Many U. S. states including California and New York mandated their own schedules of MTBE phase-outs and bans. This MTBE phase-out has served as an incentive for corn ethanol industries for marketing their products as being environmen­tally more acceptable than other alternatives and at the same time renewable.

The Energy Policy Act of 2005 (EPAct 2005, P. L. 110-58), established the first-ever renewable fuels standard (RFS) in federal law, requiring increas­ing volumes of ethanol and biodiesel to be blended with the U. S. fuel sup­ply between 2006 and 2012. The Energy Independence and Security Act of 2007 (P. L. 110-140, H. R. 6) amended and increased the RFS, requiring 9 billion

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gallons of renewable fuel use in 2008, stepping up to 36 billion gallons by 2022, as shown in Figure 3.8. A major portion of the increase is expected to come from cellulosic ethanol.

Considering the annual gasoline consumption in the United States to be approximately 140 billion gallons and also assuming that all gasoline sold in the United States is blended with ethanol up to 10% (i. e., E10), the total annual demand for ethanol by E10 in the United States would be about 15.5 million gallons. One can readily notice that this estimated saturation point for etha­nol demand in the United States for E10 blend is not far from the 2010 total U. S. ethanol production from corn, which was 13.2 million gallons. Thus, it is evident that the RFS numbers for future years are based on (a) expanded use of nonethanol renewable fuels such as biodiesel, (b) increased availability of cellulosic ethanol, (c) expanded adoption of alternative fuel vehicles (AFVs) and flexible fuel vehicles (FFVs), and more.

## Coproducts of Cellulosic Ethanol Technology

In order to reduce the ethanol production cost from lignocellulosic materials, it is imperative to expand or develop the market for the process coproducts, by-products, or derivatives. Unlike the mature corn ethanol industry, the by­product (or coproduct) industry for the lignocellulosic ethanol industry is not very well defined or established yet. Potential coproducts include hemicel — lulose hydrolyzate (xylose), cellulose hydrolyzate (glucose of mixed sugars), cell mass, enzymes, soluble and insoluble lignins, lignin-derived chemicals and fuels, solid residues, and so on. Other valuable products include xylitol, which is a sugar alcohol sweetener and is produced by hydrogenation of xylose (an aldehyde) into a primary alcohol.

## Plasma Pyrolysis

Of the three categories mentioned above, the most extensive scientific stud­ies are performed on plasma pyrolysis [24]. Different types of organic waste such as plastic, used tires, agricultural residue, and medical waste have been studied both at the laboratory and pilot-scale level [60]. Plasma pyrolysis generally produces two products, a combustible gas and a carbonaceous resi­due (char), both of which can recover useful materials. It can recover valu­able chemicals (e. g., ethylene and propylene) and carbon black from tires. Although plasma pyrolysis of solid waste still needs some technical develop­ment, plasma pyrolysis of hazardous gases and liquids is a proven commer­cial technology such as the PLASCON process (developed by CSIRO and SRL Plasma Ltd. in Australia which is now owned by Dolomatrix International Ltd.). For MSW and RDF, plasma pyrolysis is combined with plasma gasifi­cation to produce useful synthesis gas. Also, for these types of solid waste, plasma gasification and vitrification are preferred over plasma pyrolysis. Small-scale plasma pyrolysis is practiced to treat polymers, medical waste, and low-level radioactive wastes [7].

## Production and Use of Vegetable Oils

Vegetable oils have long been used by humans throughout the entire world for a variety of applications, including traditional uses such as cooking, food ingredients, medicinal ingredients, lubrication, and heating and lighting as well as more recent uses such as motor oil, lubricants, drying oils, alternative diesel fuel, raw materials for biodiesel, and so on.

In order to distinguish vegetable oil for fuel applications from biodiesel, some people refer to it as waste vegetable oil (WVO) if it is originated from discarded sources such as used restaurant grease, and as straight vegetable oil (SVO) or pure plant oil (PPO) if it has not undergone any chemical treatment or reaction such as transesterification, just as the name implies.

According to the USDA [4, 5], the total world production of major vegetable oils in 2008-2009 includes: 43.19 million metric tons of palm oil, 36.26 of soybean oil, 20.22 of rapeseed oil, 11.46 of sunflower seed oil, 5.15 of peanut oil, 5.10 of palm kernel oil, and 4.72 of cottonseed oil. The world consumption of vegetable oils has been steadily increasing at a rate of 4.5% annual growth and their trade volumes of import and export have also been steadily increasing. See Table 2.1.