Biomass to Biofuels: Strategies for Global Industries

By Hideaki Yukawa and Hans P. Blaschek

Focusing on the key challenges that still impede the realization of the billion-ton renewable fuels vision, this book integrates technological development and business development rationales to highlight the key technological.developments that are necessary to industrialize biofuels on a global scale. Technological issues addressed in this work include fermentation and downstream processing technologies, as compared to current industrial practice and process economics. Business issues that provide the lens through which the technological review is performed span the entire biofuel value chain, from financial mechanisms to fund biotechnology start-ups in the biofuel arena up to large green field manufacturing projects, to raw material farming, collection and transport to the bioconversion plant, manufacturing, product recovery, storage, and transport to the point of sale. Emphasis has been placed throughout the book on providing a global view that takes into account the intrinsic characteristics of various biofuels markets from Brazil, the EU, the US, or Japan, to emerging economies as agricultural development and biofuel development appear undissociably linked.

• Language: English
• Publisher: Wiley
• Release date: Aug 24, 2011
• ISBN: 9781119965497

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Part I

Structure of the Bioenergy Business

1

Characteristics of Biofuels and Renewable Fuel Standards

Alan C. Hansen, Dimitrios C. Kyritsis and Chiafon F. Lee

1.1 Introduction

Liquid biofuels currently in commercial use comprise primarily ethanol-derived fuels, mainly from grain, sugarcane or sugar beet, and biodiesel produced from a variety of vegetable oils and animal fats. It is expected that, in the future, a greater diversity of primary raw materials for manufacturing renewable transportation fuels will be used, including an array of recycled materials. For example, ethanol production from cellulosic material is likely, as well as butanol production from grain and possibly also from cellulose. Furthermore, the use of hydrogenation-derived renewable diesel and gasoline from fats, waste oils, or virgin oils processed either pure or blended with crude oil using petroleum refinery or similar operations, is being explored as an alternative [1]. In addition, the conversion of biomass to liquid fuel via pyrolysis is receiving attention, as well as the production of alkanes from the hydrogenation of carbohydrates, lignin, or triglycerides. Although methane production from waste materials is already well established, its use as a biofuel for transportation remains marginal to this date. In the long term, hydrogen derived from biomass is considered as the ideal fuel, because its combustion yields zero carbon dioxide. However, there are several technical hurdles that will need to be circumvented before this vision becomes reality, including not only the production of hydrogen from renewable materials but also safe methods for the storage and transport of hydrogen fuels [2].

In this chapter, the characteristics of biofuels will be focused primarily on ethanol and biodiesel, although other biofuels will also be mentioned when comparing the key properties of these materials.

1.2 Molecular Structure

Although in general, petroleum-based fuels are a blend of a very large number of different hydrocarbons, biofuels may consist of pure single-component substances such as hydrogen, methane or ethanol; alternatively, as in the case of biodiesel, they may be a mixture of typically five to eight esters of fatty acids, the relative composition of which is dependent on the raw material source. This relatively finite number of fatty acid esters in biodiesel contrasts with the much broader and more complex range of hydrocarbons that exists in petroleum. In addition, these biofuels are typically blended with petroleum-based fuels. A primary factor that distinguishes fuel alcohols and biodiesel from petroleum-based fuels is the presence of oxygen bound in the molecular structure. Alcohols are defined by the presence of a hydroxyl group (−OH) attached to one of the carbon atoms. For example, the molecular structure of ethanol is C2H5OH, and that of butanol is C4H9OH. Butanol is a more complex alcohol than ethanol as the carbon atoms can form either a straight-chain or a branched structure, thus resulting in different properties. Butanol production from biomass tends to yield mainly straight-chain molecules.

While straight vegetable oils have been used to power diesel engines, their viscosity is much greater than that of conventional diesel fuel. This is an important difference, as conventional engines have not been designed to be operated with relatively viscous fluids, and hence problems may be encountered when fuel vegetable oil is injected into the engine. In order to reduce the viscosity, one widely used method is to transesterify the vegetable oil or animal fat via a chemical reaction between the oil or fat and a mixture of an alcohol and a catalyst, as shown in Figure 1.1. The alcohol typically used in the reaction is methanol, thus creating methyl esters. It is worth noting that although ethanol (creating ethyl esters) and

Figure 1.1 Transesterification reaction to produce biodiesel (R represents a primary alkyl radical). Typical alkali catalysts are sodium hydroxide and potassium hydroxide, with sodium hydroxide being more commonly used because of its availability as a drain-cleaning chemical. Acid catalysts may also be used, the choice being dependent on the amount of free fatty acids (FFA) in the raw oil [3]. Apart from corrosion problems, the use of these homogeneous catalysts involves process steps for the removal of FFAs and water from the feedstock and catalyst from the products. An alternative being explored is that of heterogeneous nanocatalysts, such as zeolites [4], that eliminate steps in conventional biodiesel production other alcohols can also be used, the process requirements are different when using these latter feedstocks.

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Figure 1.2 Fatty acid profiles of five biodiesel source materials. C18:1 denotes that there are 18 carbon atoms in the chain, and one double bond [6]

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Vegetable oils and animal fats consist of a mixture of fatty acids with carbon chains of different lengths as well as different degrees of unsaturation (i.e., the number of double bonds that may exist in the fuel molecule). Figure 1.2 shows the fatty acid ester composition of selected source materials for producing biodiesel. Both the chain length and the degree of unsaturation have a major effect on the fuel properties of the esters in the biodiesel [5]. For example, an increase in chain length leads to a higher propensity for the fuel to self- ignite under conditions of high heat and pressure (this property is measured by the cetane number), while increasing the degree of unsaturation causes the cetane number rapidly to drop. However, the value of a higher cetane number with a longer carbon chain length is offset by increasing cloud and pour points; this results in the fuel gelling or solidifying at higher temperatures than would diesel fuel. These factors are discussed later in the chapter.

1.3 Physical Properties

To a significant extent, engine manufacturers design their products based on expectations regarding the properties of the fuel to be used by the consumer. The introduction of biofuels such as ethanol and biodiesel has naturally led not only to a closer scrutiny of the standards of these new fuels, but also to attempts at characterizing their properties in detail. In turn, this information is used to modify engine control parameters in order to account for changes in fuel properties that could affect fuel atomization and combustion should the engine remained unchanged. This need for retrofit explains in part the resistance initially displayed by various automobile manufacturers to implement the use of ethanol. A comparison of the key properties of biofuels and petroleum-derived fuels is provided in Table 1.1. The physical properties of the fuel influence how well the fuel mixes with air and, as a result, its ability to combust. The mixing process relies on sufficient atomization of the fuel and its dispersion to occur either at the level of the intake of the engine or in the combustion chamber.

For most spark ignition (SI) engines, the fuel is injected into the inlet port immediately before the intake valve during the intake stroke. Mixing of the fuel and air is achieved as these two components enter the cylinder. Direct injection into the combustion chamber has also been introduced as a means to permit a much leaner overall combustion process. Notably, both processes require the fuel to be sufficiently volatile to facilitate mixing. The volatility of a fuel is determined by the Reid Vapor Pressure (RVP), a single digit measure of a fuel's propensity to evaporate. In the United States and other countries, the RVP is regulated for both conventional gasoline and ethanol blends to reduce evaporative emissions, as well as to prevent the occurrence of vapor locks in the fuel systems. Although ethanol has a much lower RVP than gasoline, it is well known that the addition of ethanol to gasoline raises the RVP, with a maximum being reached at about 10% ethanol.

The ability of the SI engine fuel to vaporize at engine start-up when cold is another very important characteristic, as it determines the ease with which the engine will start, as well as how it performs initially during vehicle idling, acceleration, or cruising. Ethanol has a much higher latent heat of vaporization than gasoline fuel (see Table 1.1), which means that more heat is required to cause fuel ethanol to vaporize as compared to conventional gasoline. This is the primary reason why a blending limit of 75–85% ethanol (E85) is specified by regulators, in order to ensure that there is a sufficient amount of an adequately volatile gasoline to vaporize, mix with the air, ignite, and thus allow the engine to start when cold. In this context, there have been recent indications that electrostatically assisted atomization may become a useful technology because of the relatively high electric conductivity of ethanol, which is five orders of magnitude higher than the one of hydrocarbons [20–23]. For example, engines have been developed and used in Brazil and Europe that run on E100; however, these engines either rely on gasoline being provided for cold starting, before switching to ethanol once the engine has warmed up, or they may use a specifically designed process for heating the fuel and the air in the manifold before the engine is started. One major advantage of combustion engines that run on E100 is their ability to tolerate the presence of up to 5% water in the ethanol fuel. This is important, as the cost of fuel ethanol production – particularly at the downstream purification step – is dramatically reduced when this specification is allowed. Notably, such relatively large amounts of water could not be accommodated in typical ethanol–gasoline blends as the ethanol would separate from the gasoline.

For diesel engines, the fuel atomization process is critical as there is very little time for the fuel to be injected into the combustion chamber, vaporized, mixed with air, and then chemically reacted and burnt. The physical characteristics that affect the atomization and fuel–air mixing process include: (i) fuel density; (ii) viscosity; and (iii) surface tension. These properties are strongly influenced by the fatty acid profiles of the biodiesel fuels, which in turn vary with the biomass raw material from which these fuels derive (see Figure 1.1) [15]. As compared to conventional diesel, biodiesel typically exhibits higher values for all of the characteristics listed above. For example, the density of biodiesel is approximately 7% higher than that of diesel fuel; this difference results in the biodies droplets penetrating deeper into the combustion chamber because of their higher momentum when injected.

Table 1.1Biofuel properties compared to petroleum-based fuels

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(A/F)s: Stoichiometric air/fuel ratio; BD: Biodiesel; LHV: Lower heating value; RVP: Reid vapor pressure; LPG: Liquefied petroleum gas; Cetane Rating is an estimation of ignition quality; Octane rating is also known as antiknock index or octane number. The values of these properties were assembled from data in Refs [7–19]

Furthermore, compared to that of diesel fuel, the vapor pressure of biodiesel is much lower, which would be expected to have a significant influence on the spray evaporation process. Likewise, the heat of vaporization of biodiesel is lower at low temperatures, but it becomes higher at high temperatures. As the spray droplets are heated quickly during the vaporization process in the engine, it is expected that the evaporation of the biodiesel would be less efficient compared to petroleum-derived diesel fuel at those high temperatures

Surface tension is one of the most important properties in spray breakup and collision/ coalescence models. Empirical correlations have suggested that surface tension is a linear function of temperature, and therefore that it is relatively independent of the specific methyl ester mix that composes different varieties of biodiesel [6]. Consequently, at room temperature the surface tension of biodiesel is about the same as that of diesel. However, relative to conventional diesel the surface tension decreases more slowly with increasing temperatures, and thus becomes significantly higher than that of diesel at higher temperatures.

Compared to diesel, the heat capacity of biodiesel per unit mole is almost 50% higher at temperatures near these fuels' boiling points. However, when it is compared per unit mass – as used in the energy equation – the heat capacity of biodiesel is lower than that of diesel. This suggests that biodiesel droplets are heated faster than diesel droplets.

Liquid viscosity is an important parameter with regard to droplet atomization, drop internal flow, and wall film motion. Notably, the liquid viscosity of biodiesel is higher than that of diesel, especially at low temperatures where most of the atomization processes take place during the injection process in the engine. Therefore, it is to be expected that the atomization process will be affected by the viscosity difference. Figure 1.3 illustrates the differences in viscosity of biodiesel fuel made from the five raw materials shown in Figure 1.2, as compared to diesel fuel. Remarkably, all of these biodiesel fuels have higher viscosities; however, among the biodiesel fuels there is substantial variation caused by differences in fatty acid content.

Figure 1.3 Variation with temperature of measured kinematic viscosity of biodiesel made from five source materials, compared to no. 2 diesel fuel and to the ASTM standard D975 for diesel fuel [6]

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Liquid thermal conductivity affects the heat transfer between the drop interior and the surface. Notably, the thermal conductivity of the biodiesel is slightly lower than that of the diesel [6].

The vapor heat capacity of the fuel is an important parameter that influences the thermal energy balance and temperature distribution of gas mixtures surrounding the spray drops, which in turn affects the transient heat transfer from the surrounding gas mixture to the drop surface. This is particularly important when the fuel drops rapidly vaporize so that the fuel–air mixture become richer. The vapor heat capacity of biodiesel is slightly lower than that of the diesel [6].

The transport properties of the vapor phase – that is, diffusivity, viscosity, and thermal conductivity – can all be estimated for biodiesel mixtures. Typically, the diffusivity for biodiesel vapor is much lower than that for diesel by as much as a factor of 20. The viscosity of biodiesel vapor is about 60% higher than that of diesel, while its thermal conductivity is about 30% lower than that of diesel [24].

Recent studies using multidimensional spray and combustion modeling have been conducted to investigate the effects of varying the fuel's physical properties on the spray and combustion characteristics of diesel-engines when these are operated using various biodiesel fuels [17, 18, 25–28]. The properties of typical biodiesel fuels that have been used in these studies, and the simulation results obtained, are compared with those of conventional diesel fuels. The sensitivity of the computational results to individual physical properties is also investigated by changing one property at a time. Exploitation of these results provide a guideline on the desirable characteristics of blended fuels. The properties investigated in Refs [17,18] included: (i) liquid density; (ii) vapor pressure; (iii) surface tension; (iv) liquid viscosity; (v) liquid thermal conductivity; (vi) liquid specific heat; (vii) latent heat; (viii) vapor specific heat; (ix) vapor diffusion coefficient; (x) vapor viscosity; and (xi) vapor thermal conductivity. The results suggest that the intrinsic physical properties of each of these fuels significantly impact spray structure, ignition delay and burning rates in a wide range of engine operating conditions. Moreover, these observations support the view that there is no single physical property that dominates the differences of ignition delay between diesel and biodiesel fuels. However, the most impactful of these characteristics seem to be liquid fuel density, vapor pressure, and surface tension. This latter observation can perhaps be ascribed to the importance of these parameters on the atomization, spray, and mixture preparation processes.

The spray atomization model thus developed, and which is used to model the breakup of fuels in diesel engines, relies heavily on the physical properties of the fuels being analyzed. As described earlier, significant differences exist in density, viscosity, surface tension and thermal conductivity between diesel and biodiesel fuels. Using this model and the fatty acid profiles of the source oils for biodiesel (as shown in Figure 1.2), the physical properties and critical temperature of soybean, coconut, palm, and lard biodiesels have been predicted. It is particularly noteworthy that these properties differ considerably between each of the biodiesel fuels analyzed. Moreover, a recent study has shown the effect that these differences have on fuel vaporization [6]. Due to its lower boiling point and critical temperature, coconut biodiesel shows a tendency to vaporize faster than any of the other pure biodiesel fuels when injected under engine-like conditions. The biodiesel fuels that behave most like pure conventional diesel include palm and lard biodiesel. Computed spray structures also demonstrate a relationship between atomized droplet diameter and fuel vaporization. Significant differences in the spray and vaporization between diesel–biodiesel blends of B2 (2% biodiesel: 98% diesel), B5 and B20 have also been demonstrated. These blends were modeled using the spray code including multicomponent fuel effects [17, 29–43]. At low blend percentages, such as B2 and B5, simulations for the biodiesel blends predict vaporization similar to that of diesel fuel. However, as the blend percentage increases to more than 5%, the fuel vapor mass is shown to decrease. The vapor mass composition is also affected by the blend percentage and lower volatility of biodiesel. The diesel fuel blends have a lower overall spray tip penetration than pure biodiesel; this characteristic is partially due to differences in molecular weights and densities of the fuels.

Another important characteristic of diesel fuels is their ability to retain liquid properties under cold weather conditions. On the other hand, when ambient temperatures are low enough, some hydrocarbons in diesel fuel begin to solidify, thereby inhibiting the flow of fuel from the storage tank to the fuel injection pump via the filter system. Such a property is represented by the cloud point and cold filter plugging point (CFPP), for which biodiesel fuels generally have higher temperatures; this makes them susceptible to clogging of the fuel system and preventing the engine being started when cold. This property is strongly affected by the fatty acid profile of the biodiesel, and is influenced by the relative proportion of saturated and unsaturated fatty acids. The higher the proportion of the saturated component, the higher the cloud point and CFPP temperature. Nonetheless, additives such as malan-styrene esters and polymethacrylate have been used successfully to address this limitation [19, 44–47].

Energy density is a measure of how much energy a fuel contains; this characteristic has a direct impact on how much power an engine produces by combusting this particular fuel. The lower heating values (LHV) reported in Table 1.1 illustrate the variation in energy content for a range of fuels. Ethanol has about a 30% lower energy content than gasoline on a per unit volume basis; this translates into a lower distance traveled per tank of fuel compared to gasoline. Likewise, biodiesel exhibits an energy content that is approximately 9% lower than that of conventional diesel. Here, the difference on a volume basis is lower because the higher density of biodiesel compared to standard diesel fuel helps to offset the difference in energy content. Note also that the energy content of butanol is much greater than that of ethanol, making it an attractive alternative fuel for SI engines.

1.4 Chemical Properties

The chemical properties of biofuels are strongly affected by their different molecular structures, as described earlier in the chapter. The presence of oxygen in the molecule (see Table 1.1) naturally causes a leaner combustion process in existing engines because this oxygen also participates in the combustion process. In the case of gasoline engines – which must run with an air–fuel mixture that is consistently close to the chemically correct or stoichiometric ratio to achieve complete combustion – the addition of ethanol to gasoline results in there being extra oxygen in the combustion chamber contributed from the ethanol, thus making the mixture leaner. The addition of 10–15% ethanol to gasoline is widely practiced, as gasoline engines are able to tolerate such volumes of ethanol in gasoline with only small changes to the air–fuel ratio. In addition, in modern engines a sensor in the exhaust detects changes in the mixture and provides feedback to the system that controls the mixture ratio. However, higher blends require engine modifications and, as a result, manufacturers have introduced so-called 'flexible-fuel vehicles' that are able to run on fuels ranging from pure gasoline to E85, a blend discussed earlier. These flex-fuel vehicles are particularly common in Brazil for example, where the consumers have a large choice (63 different models in 2007) and the market penetration of this type of automobiles is high, as demonstrated by a total of 85.6% of the new automobiles sold in 2007 in that country [48].

One very attractive chemical property of ethanol is its resistance to self-ignition, as reflected by its high octane number (see Table 1.1). The octane number is an important parameter that establishes whether or not a fuel will knock in a given SI engine under given operating conditions. The higher the octane number of a fuel, the higher resistance it has to knock; consequently, ethanol can be used as an octane enhancer for gasoline. One of the reasons for the higher octane rating exhibited by ethanol is the relatively high heat of vaporization that characterizes this compound (Table 1.1). A higher heat of vaporization results in cooler fuel–air mixtures, which in turn slows down the combustion and provides a higher resistance to knock. Moreover, a higher octane number also means that an engine can burn ethanol at a higher compression ratio. Consequently, engines designed to run on E100 use a comparatively higher compression ratio; this in turn increases the engine overall efficiency and offsets to some extent the lower energy content of ethanol.

In a similar way, the cetane number (or cetane rating) provides a measure of compression ignition. The higher the cetane number is, the greater the ignition quality of a fuel and the shorter the ignition delay. This is an important characteristic, since long ignition delays result in most of the fuel being injected before ignition occurs. In turn, this results in very fast burning rates and very high rates of pressure rise once ignition starts such that, in some cases, diesel knock can occur. Most of the biodiesel fuels listed in Table 1.1 have higher cetane ratings than those of the diesel fuels available in the US (about 43), but they have ratings comparable to the ratings of the diesels available in Europe (about 50).

1.4.1 Oxidation and Combustion Chemistry

Biological processes can yield light fuel molecules such as bio-hydrogen or bio-methane, the combustion chemistry of which is well established. The oxidation of these fuels proceeds along the well-established mechanisms summarized in classical texts [49, 50]. Here, we focus instead on the chemical issues associated with the combustion of heavier biofuels, with a particular emphasis on the combustion of alcohols and fatty acid esters.

1.4.1.1 Alcohols: Ethanol

The combustion chemistry of light alcohols has been studied extensively. The main variations that occur with respect to the combustion of hydrocarbons are due to the presence of a hydroxy group, and have been identified in the early studies conducted by Norton and Dryer [51], summarized in the classical text authored by Glassman [49], and reviewed more recently in a more brief form by Law [50]. Especially for ethanol, it is safe to say that the combustion chemistry is well understood [51–54]. Figure 1.4 (adapted from Ref. [54]) shows, for example, a very convincing agreement between species and droplet evaporation rates for ethanol droplets in microgravity for computations performed with mechanisms proposed by two different research groups.

Figure 1.4 Vaporization rate and species computations for ethanol droplet combustion based on two different ethanol mechanisms. The vaporization rate is presented in the top figure in terms of the rate coefficient Kb, along with the mass fraction of water in the liquid phase. The bottom figure shows the distribution of several species mass fraction and temperature as a function of the nondimensional distance from the droplet center. The solid and dashed lines indicate computations from two different groups. Reprinted from Combustion and Flame, A. Kazakov,J. Conley, andF. L. Dryer, Detailed modeling of an isolated ethanol droplet combustion under microgravity conditions, 134, 301–314. Copyright 2003, with permission from Elsevier

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There are two fundamental initial decomposition mechanisms of the alcohol molecules that occur during the combustion process. In a first pathway, the alcohol molecule is attacked by radicals at a location different than the one of the –OH bond, and an oxygenated radical is formed that ultimately leads to the formation of an aldehyde. This points to the need for a highly efficient oxidation process during power generation, because inefficiencies there can lead to incomplete oxidation and subsequent release into the atmosphere of particularly dangerous pollutants, such as formaldehyde and acetaldehyde. In the alternative pathway, the hydroxy group is displaced from the alcohol molecule and an alkyl radical forms that ultimately can lead to the formation of an olefin. A crucial transport phenomenon that couples with combustion chemistry is the strong capability – especially of light alcohols – to absorb water vapor. This characteristic of alcohol fuels to 're-absorb' in the liquid phase a product that was released during combustion is unique. This phenomenon is of course included in the modeling of practical combustion calculations.

Focusing on the particular case of ethanol, the two general pathways described above lead to the formation of acetaldehyde and ethylene as major intermediate species. The kinetic path that leads to acetaldehyde formation is as follows:

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where X and M denote combustion intermediates that can act as collision partners. Alternatively, the ethanol molecule can be attacked by atomic hydrogen and yield an ethyl radical that ultimately produces ethylene:

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The relative importance of these two routes apparently depends on the overall stoichiometry and, perhaps intuitively, the ethylene pathway is preferred over the acetaldehyde pathway in mixtures that are richer in fuel.

An interesting characteristic of ethanol is that, compared to gasoline, ethanol flames produce significantly increased amounts of soot at slightly elevated pressures (starting from as low as 2 atm.). This is rather counterintuitive, as ethanol is an oxygenated fuel and thus one would expect a soot-less combustion.

The combustion kinetics of heavier alcohols has, on the other hand, received significantly less attention. Nevertheless, it is expected that mechanisms as reliable as those already described for ethanol will be rapidly established for other alcohols if, for example, the interest in butanol use as a transportation fuel is sustained [55]. To this end, the recent detailed measurements of an impressive variety of intermediate species for butanol oxygen flames by Yang et al. [56] will be instrumental in guiding the modeling of the oxidation process of this alcohol. However, a complicating issue is the existence of alcohol isomers. The isomers of butanol are shown in Figure 1.5. It is noted that bio-butanol (i.e., butanol produced from the ABE fermentation processes) contains only three of the isomers and does not include tert-butanol, which is a product of the biodegradation of methyl tert-butyl ether (MTBE). The oxidation mechanisms of these chemicals can differ substantially. Notably, the chemistry of a complex fuel mixture is not simply the sum of the chemistries of each of the constituents, as intermediates of the oxidation of each individual component can affect the oxidation of the others. As a result, the determination of detailed kinetics for blends as opposed to pure chemicals constitutes a crucial step for the detailed study of biofuel chemistry.

1.4.1.2 Biodiesel: Esters

As explained in detail in Section 1.2, most currently available biodiesels consist of fatty acid methyl esters (FAME) with alkyl chains that are typically 16–20 carbon molecules long. Detailed kinetic modeling of such molecules is a task beyond current computational capabilities. In view of this, methyl-butanoate [n-C3H7C(=O)OCH3] has been used as a surrogate for the purposes of detailed modeling. The justification provided for this approach in the first study reporting a detailed mechanism for the oxidation of this chemical was that "...although methyl butanoate does not have the high molecular weight of a biodieselfuel, it has the essential chemical structural features namely the RC(=O)OCH3 structure." [57]. This generates the question of whether the modeling of methyl butanoate kinetics that has since followed is to be envisioned as targeting the qualitative characteristics of long-chain methylester combustion, or whether it is rather a prerequisite for the development of the tools that will ultimately address this issue in the future.

Figure 1.5 The molecular structures of butanol isomers

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Recent results [58–60] have indicated that methyl butanoate is of limited use for the quantitative simulation of biofuel combustion. Specifically, the motored engine experiments with the long-chain ester (decanoate) reported by Szybist et al. [60] indicated a negative temperature coefficient behavior (essentially the chemical process that determines ignition delay), which the advanced modeling of butanoate in Refs [58,59] did not show. If such an important characteristic of combustion chemistry cannot be effectively captured by the kinetics of the surrogate, then perhaps the results of methyl butanoate studies are more important in terms of development of methodologies rather than in terms of, for example, emission or ignition delay modeling for biodiesel.

In this context, a recent study by Huynh and Violi [61] is very interesting because it points to the possibility of ab initio calculations of biofuel combustion chemistry, starting from no less than quantum chemistry of the fuel molecule. The establishment of this methodology as a foundation of multiscale computations that would ultimately lead to the description of much-needed mechanisms to be used as input for engine-combustion codes such as KIVA would be a major contribution. The implied undertaking is clearly nontrivial: the most comprehensive mechanisms that have been described to date pertain to methyl butanoate, a molecule that has a carbon chain almost four times shorter than the esters typically encountered in biodiesel. Remarkably, the model consists of a little less than 300 species and 1500 chemical reactions; moreover, it has only been tested for pressures that are significantly lower than those under which diesel combustion occurs in a typical engine! An alternative approach to ab initio calculations can be the one recently proposed by Brakora et al. [62], which combines a methyl butanoate mechanism with a skeletal kinetic mechanism for hydrocarbon combustion, and is thus able to show negative temperature coefficient behavior. Clearly, such an approach would be computationally less intensive, but the establishment of ab initio calculations would offer the exciting possibility of chemically tailoring the fuel for optimum emission characteristics.

With these methodological issues in mind, the conclusions based on methyl butanoate studies that can be reasonably expected to hold true even for heavier esters is the presence of several light oxygenates in the combustion intermediates. These oxygenates may constitute novel pollutants when one compares these to the ones emitted from the combustion of nonoxygenated fuels. There is evidence that all these fuels generate several pollutants, including formaldehyde, methanol, and acetaldehyde. It is only reasonable to expect that heavier and potentially more toxic oxygenates may also appear during the combustion of heavier fuels.

1.4.2 Oxidative Stability

An additional issue that relates to biodiesel oxidation is that of its oxidative stability. At the heart of the matter lie the unsaturated components (i.e., the components that contain double bonds), which are substantially present in soybean and canola/rapeseed oil, although it has been pointed out that even small amounts of unsaturated components also can cause serious problems. These components are unstable with regard to oxidation for long-term storage, and they undergo both auto-oxidation and photo-oxidation. The underlying fundamental processes involved have been reviewed in detail by Knothe [63], and a detailed report on the issue is provided in Ref. [64]. A major conclusion is that oxidation can be catalyzed or inhibited by minor components present in fuel mixtures. This observation suggests that it could be possible to delay, but not avoid, oxidation with appropriate additives that could compensate for the lack of naturally occurring antioxidants. Naturally occurring antioxidants are substances that have vitamin E activity, but other antioxidants have also been proposed that are specific for biodiesel storage.

The primary products of this type of oxidation are allylic (i.e., containing the group CH2=CHCH2-) hyperperoxides (i.e., organics of the form R–O–O–H). These are unstable substances that can form a variety of secondary products of either smaller (acids, aldehydes) or larger molecular weights (dimers). The oxidation is facilitated by the presence of: (i) metal impurities; (ii) an elevated temperature; (iii) exposure to air and, to a very significant extent; by (iv) exposure to light, which can facilitate oxidation by as much as 30 000 times, thus leading to photo-oxidation. Proneness to both auto-and photo-oxidation varies with biodiesel FAME types, with linolenate being more vulnerable to auto-oxidation than linoleate and oleate, although oleate exhibits a very intense light-induced acceleration of oxidation. Macroscopically, these chemical processes manifest themselves through an increase in viscosity and the corrosion of engine components.

Although oxidation is a widely recognized problem, to this date there are no widely accepted quantitative measurement methods to control the quality of biodiesels with regards to the extent of oxidation that they have undergone. The development of appropriate quantitative analysis methods is difficult given, on the one hand, the involvement of multiple chemical reaction mechanisms, and on the other hand the parallel involvement of physical chemical phenomena. A discussion concerning the determination of the oxidative stability of biodiesel fuels is included in Section 1.5.

1.4.3 Emissions

The presence of oxygen in biofuels is beneficial as it reduces harmful exhaust emissions to a significant degree. Like biodiesel, oxygenated diesel fuels have been found to effectively reduce soot emission as compared to conventional diesel combustion. Recent chemical kinetic modeling with reactions describing soot formation have provided a more detailed description of soot formation processes from oxygenated fuels [65–67]. In particular, numerical modeling has shown that oxygenated diesel fuel reduces the production of soot precursors–and therefore also soot and particulate matter (PM)–through several key mechanisms. The first of these mechanisms proceeds via a natural shift in pyrolysis and decomposition products, while the second proceeds via the presence of high concentrations of radicals, such as O, OH, and HCO, which result from the addition of oxygenate compounds. In turn, these radicals promote carbon oxidation to CO and CO2, thereby limiting the availability of carbon for soot precursor formation. Finally, high radical concentrations (primarily OH) serve to limit aromatic ring growth and soot particle inception. All of these factors contribute to a reduction of soot due to the presence of oxygen in biodiesels.

Another important issue relates to the production of NOx upon biodiesel combustion. The initial intuitive expectation is that oxygenated esters are characterized by increased NOx emissions when compared to nonoxygenated, petrol-derived diesel. However, this hypothesis is invalidated by experimentation. Here, the underlying issue is that, in general, diesel and biodiesel do not have the same energy content and physical properties. As a result, one has to use caution regarding the basis chosen for the comparisons that are performed (i.e., what parameter is kept constant when comparing various fuels; the total mass of fuel injected; the total energy content of the mixture; the total engine load, etc.). Furthermore, the injection strategy can also be important [68–71]. For example, when using the classical scheme of a late single injection, there is a monotonic increase of NOx emissions with increasing biodiesel content. However, when an early injection scheme is used, the effect of a longer ignition delay (due to the fact that biofuels have a higher boiling point than petroleum-derived fuels) competes with the effect of a higher oxygen content and, as a result, the NOx emissions decrease when the biofuel content increases (up to a local minimum at 50% biodiesel per volume). No blanket statement can be issued for these phenomena; rather, appropriate engine and fuel designs must be employed in order to minimize biodiesel emissions.

More than any other oxygenated fuels, the high oxygen content of methanol and ethanol (see Table 1.1) help to reduce carbon monoxide emission levels, by 25–30% according to the US Environment Protection Agency. Methanol or ethanol gasoline blends also dramatically reduce emissions as compared to conventional fuels. Another issue is that of carcinogenic oxygenated emissions in the form of light ketones and aldehydes. Importantly, this particular problem has perhaps not yet received the attention it deserves. The situation is not worrisome, as long as ethanol remains an additive at relatively low levels and in the order of 10%, although it may have to be revisited if E-85 or E-100 technologies are to be generalized. In this context, the experience from neat-ethanol vehicles in Brazil cannot be transferred to the US because the emissions regulations there are less stringent [72]. An additional important issue that relates to ethanol emissions concerns ethanol substituting MTBE. The latter's use as an octane enhancer was initiated during the late 1970s with the purpose of gradually substituting lead. Utilization of the chemical was boosted in the US by the requirements on oxygenated components for gasoline that were mandated by the Clean Air Act Amendments of 1990. However, MTBE was quickly shown to be a carcinogenic groundwater pollutant. In fact, the Energy Policy Act of 2005 both abolished the gasoline marketers' obligation to use MTBE and provided no MTBE liability protection. Ethanol is currently used as an oxygenate additive to gasoline, although its performance is inferior to MTBE with regards to combustion chemistry. This issue may have to be revisited if heavier alcohols (e.g., butanol) emerge as widely used biofuels and oxygenated additives because of the toxicity of their aqueous solutions.

1.5 Biofuel Standards

Fuel quality standards are vital in order to ensure engine–fuel compatibility and reliability. Engine manufacturers depend on fuels meeting these standards in order to be able to address warranty issues pertaining to the fuel, as well as ensuring that their engines are optimized with regards to performance, efficiency, durability, and meeting emissions regulations. Standards have been developed in a number of countries for ethanol and biodiesel, the two primary biofuels that have been commercialized to date.

Common blends of ethanol with gasoline in the US are E10 (10% ethanol and 90% unleaded gasoline) and E85 (85% ethanol). The ASTM D4806 standard specification covers anhydrous denatured fuel ethanol intended to be blended with unleaded or leaded gasoline at 1 to 10 volumetric percentage for use as a SI automotive engine fuel. This standard has been in place since 1999, and is used as a basis for standards in a number of other countries such as Canada, Australia, and China. A European Standard (EN 15376) forundenatured ethanol as a blending component for gasoline up to 5% was finalized in 2008. Likewise, Brazil, a leading ethanol producer and user, has published specifications for quality that apply up to 25% anhydrous ethanol content in gasoline. The ASTM D5798 standard was first published in 1999 to address the quality specification for E85. This standard covers a fuel blend, nominally 75 to 85 volumetric percentage denatured fuel ethanol and 25 to 15 additional volumetric percentage hydrocarbons for use in ground vehicles with automotive SI engines. A summary of the key properties specified in the standards that address the characteristics of ethanol and its production is provided in Table 1.2.

Standards for biodiesel are well established in many countries. The American ASTM D6751 standard was first published in 2002, and has been revised a number of times to address issues such as oxidation stability. The European Union Standard EN 14214 was approved in 2003. Most countries have tended to develop standards similar to, or that refer to, the American ASTM D6751 and the European Union EN 14214 standards. These standards cover two types of characteristics: (i) properties directly affected by the fatty acid (FA) profile of the biodiesel; and (ii) parameters related to production and storage. The EN14214 and ASTM D6751 specifications have similar requirements in order to regulate levels of contaminants and the effect of different source materials on fuel quality. These specifications are for pure biodiesel (B100) prior to use or blending with diesel fuel. The key properties specified in the standards and that address the characteristics of biodiesel and its production are listed in Table 1.3.

Table 1.2 Key properties of ethanol in quality standards, and their importance

As mentioned above, the relatively limited oxidative stability of biodiesel has been a major issue for engine manufacturers. The European biofuel standard EN 14214 includes a standardized test of oxidative stability (EN 14112) that is a Rancimat apparatus test. In this assay, a sample of the fuel under consideration is exposed to a hot air stream at a temperature of 110 °C for a period of several hours (minimum 6 h). The volatile compounds generated as a result of the oxidation by the hot air stream contain organic acids that are collected in a beaker of deionized water, the conductivity of which is recorded as a function of time. However, it is clear that organic acid content is only one of the consequences of oxidative instability, and there is no universal acceptance on whether a Rancimat apparatus test is the proper way to test stability. A proposal for a US standard that would operate on a similar principle (ASTM D6751) has recently been rejected. There is considerable industrial activity on the matter, and an extensive compendium of experimental data has been assembled by Lapuerta et al. [73]. The quantities that are measured in some of the proposed tests are summarized in Table 1.4.

Table 1.3 Key properties of biodiesel in quality standards, and their importance

aProperty related to production and storage.

bProperty affected by fatty acid profile of source material.

cProperty affected by both fatty acid profile and production.

1.6 Perspective

Biofuel production is a rapidly growing industry in many parts of the world. At present, ethanol and biodiesel are the primary alternatives to gasoline for SI engines or diesel for compression-ignition engines, respectively. Other biofuels such as bio-butanol, biomass- derived hydrocarbon fuels and hydrogen in the longer term are currently under investigation, and may be regarded as next-generation fuels. Because of limited manufacturing capacities, ethanol and biodiesel are blended with petroleum-based fuel mostly in relatively small percentages, although higher percentages of ethanol (typically up to 85%) can be used in flexible-fuel vehicles. In the case of biodiesel, existing compression-ignition engines can run on a 100% blend (B100), but engine manufacturers are reluctant to go beyond 2–5% biodiesel in a blend because of fuel stability and fuel quality issues. While ethanol is a single-component fuel in contrast to gasoline and diesel fuel, biodiesel can be produced from any vegetable oil or animal fat, and comprises a mixture of saturated and unsaturated fatty acid esters that can have a substantial effect on the properties of the fuel,including cetane number, oxidative stability and cold weather properties. In order to ensure compatibility with existing engine technologies, it is important to characterize the properties of these biofuels.

Table 1.4 Standardized tests of biodiesel oxidative stability (adapted from Ref. [63])

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In the transition from burning petroleum-based fuels in engines to accommodating the combustion of biofuels in the same engine, manufacturers face a dilemma: (i) to invest resources into the development of new engines that have the flexibility of running on either biofuels or petroleum-based fuels or a blend of the two; or (ii) to continue the development of technologies such as homogeneous charge compression ignition (HCCI) that may yield a major leap forward in engine emissions reduction and efficiency. Even the application of HCCI to biofuels has shown some interesting results, where the unique properties of biofuels may be leveraged to achieve reduced emissions and more efficient combustion [41, 42, 71, 74–101]. HCCI technology represents to some degree the convergence of SI and compression-ignition engine technologies, which therefore could imply sufficient flexibility in fuel usage that would span both gasoline and diesel, thereby posing the possibility of having an engine that would run on either ethanol or biodiesel. The development of such technologies can be expected to be costly to begin with, until sufficient market penetration is achieved. It can be said that the major advances in electronic technologies have created major opportunities for engine manufacturers to be able to control more precisely the operation of their engines. Linked with these advances is the potential to develop sensors that can monitor fuel characteristics and allow the engine to respond to the combustion of different fuels by altering the engine settings so as to achieve optimal engine performance on whichever fuel is chosen by the consumer. Although existing flexible-fuel vehicles operate to some extent in this way, there is substantial room for improvement in the flexible- fuel vehicle concept.

Quality standards exist for both ethanol and biodiesel that specify fuel characteristics within limits established through numerous measurements and fuel evaluations. Some tests within these standards are still subject to debate and modification, such as the oxidation stability test for biodiesel. Especially in the case of biodiesel, biological sources for oil to be converted into biodiesel can vary considerably even within the same plant species; thus, the regulation of quality is particularly important for ensuring that a relatively consistent fuel product is marketed. In recent years, the biofuel industries in the US and in Europe have made considerable progress towards more effective fuel quality regulation via renewable fuel standards. These standards address the production, distribution, storage and sale of biofuels, so as to provide a reproducible quality fuel for the consumer. Questions arise however, with regards to the blending processes with biofuels and petroleum-based fuels, and even within different batches of biofuels. For example, mixing a palm oil-based biodiesel with a soybean-based biodiesel could alter the resultant properties of the fuel, such as its oxidative stability and its response to cold weather. However, if this blend were to meet the published fuel quality standards, it would be regarded as acceptable. A substantial amount of research effort has been expended into the development of additives that can address the issues of oxidative stability and cold-weather performance. In the case of oxidative stability, even the natural antioxidants in vegetable oils (such as the tocopherols) can have a positive effect [102, 103]. Other synthetic antioxidants such as tert-butyl- hydroquinone have been identified as being very effective [46], although cost is a factor that will need very much to be investigated in the selection of the most suitable antioxidants. The same process applies to cold-weather fuel property improvers.

From an emissions standpoint, biofuels generally have a positive effect in reducing harmful emissions such as particulates or soot, carbon monoxide, and unburned fuel. The oxygen typically bound within the biofuel molecules contributes to cleaner combustion. Yet, some uncertainty remains as to whether NOx emissions are increased with biodiesel, and it appears that the engine technology and operational characteristics as a result of injecting and combusting biodiesel play a role in NOx production. However, strategies for reducing NOx emissions are well established, and should be able to overcome any issues with increased NOx output. Of greater concern from a health standpoint are the emissions of unregulated carcinogenic compounds, even in relatively small quantities, that may have a thitherto unseen consequence on public health and the urban environment. For example, MTBE was phased out because of its toxicity in ground water even in small quantities. Whilst the replacement of MTBE by ethanol was seen as a natural and effective step forward, there is growing concern that ethanol combustion generates levels of aldehydes that can be expected to impact air pollution. A study of air quality effects of using 10% ethanol in gasoline in the US state of New Mexico [104] showed increased levels of peroxyacetyl nitrate (PAN) and aldehydes in winter. PAN has a major effect on ozone formation, and is also a potent eye irritant and phytotoxin [104]. It can be concluded from these studies that further research is needed to establish the long-term effects of the increased consumption of biofuels on atmospheric pollution, particularly in the case of presently unregulated combustion products.

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