Biofuels in Perspective

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Biological Hydrogen Production by Anaerobic Microorganisms 199 is the case but hydrogen is only formed when it is efficiently taken away by methanogens. Thermodynamically, acetate can only be oxidized to carbon dioxide at a very low hydrogen partial pressure (P(H2)). Thus, hydrogen inhibits its own formation. This is not only the case for acetate, but also for a variety of other organic substrates. 1 Thus far, formation of hydrogen-rich gas in dark fermentation is only feasible from sugars, although it is restricted by thermodynamics. This is also true for thermophilic microorganisms, even though they harbor the biochemical potential to convert complex organic carbon completely to carbon dioxide and hydrogen. Thus, energy input is needed to overcome the thermodynamic barrier. 11.3 Thermodynamics of Hydrogen Formation Carbohydrates are the main substrates for fermentative bacteria that produce hydrogen. 3–5 Cellulose and hemicellulose are the most abundant polysaccharides available in nature, and therefore, glucose and xylose are the predominant monomeric sugars used for hydrogen formation. Starch and sucrose may also be available in large amounts as these are used as storage material in various plants. Fermentation of glucose and xylose results in hydrogen formation, but in addition, depending on the microorganism, also acetate, butyrate, lactate, ethanol and some other organic compounds (e.g. formate, butanediol, succinate) are produced. Under standard conditions complete oxidation of glucose to CO2 and H2 is thermodynamically not possible: glucose + 12 H2O → 6HCO3 − + 6H + + 12 H2 �G 0′ =+3.2kJ/mol The oxidation of glucose proceeds only to acetate, and even in this case other end products are usually produced as well. Intermediates of the metabolism are used as electron acceptors resulting in branched pathways, leading to butyrate, lactate, ethanol, alanine etc. (Figure 11.1). Production of acetate is coupled to the synthesis of ATP by substrate-levelphosphorylation, whereas production of the other products yields less or no ATP. These branched pathways enable the microorganism to adjust the metabolism so that an optimal ATP gain and thermodynamic efficiency of ATP synthesis are accomplished. 6 The oxidation of glucose to 2 acetate and 4 H2 is thermodynamically feasible, but in natural environments this only occurs when the hydrogen concentration is kept low by hydrogen-consuming methanogens. 1 glucose + 2H2O → 2 acetate − + 2HCO3 − + 4H2 �G 0′ =−206.1 kJ/mol The need for a low hydrogen partial pressure can be explained best when we look at the hydrogen producing reactions. During catabolism (e.g. the Embden-Meyerhof pathway) reducing equivalents are produced in the form of NADH (glyceraldehyde-3-P dehydrogenase reaction) and in the form of reduced ferredoxin (pyruvate:ferredoxin oxidoreductase reaction) (Figure 11.1). The midpoint redox potential of the couples NAD + /NADH and oxidized ferredoxin/reduced ferredoxin are −320 mV and −398 mV, respectively. 6 These electron carriers must be recycled continuously for catabolism to proceed. Various fermentation reactions exist that can accomplish this recycling of electron mediators, but the
200 Biofuels lactate 6 glucose 1 GAP 3 pyruvate 2 Acetyl-CoA Acetate NAD(H 2) Fd(H 2) Low p(H2) glucose High p(H2) 4 5 H 2 H 2 lactate GAP pyruvate Acetyl-CoA Acetate NAD(H 2) Fd(H 2) H 2 glucose + 2 H 2O 2 acetate + 2 CO 2 + 4 H 2 glucose + 2 H2 O 1 acetate + 1 CO 2 + 2 H 2 + 1 lactate Figure 11.1 Scheme of a typical carbon (grey)- and electron (black) flow during glucose fermentation. For simplicity pathways to ethanol, butyrate and alanine are not shown. At low P(H2) reducing equivalents are transferred mainly to H2, involving glyceraldehyde-3-P dehydrogenase (1), pyruvate:ferredoxin oxidoreductase (2), NADH:ferredoxin oxidoreductase (3), NADH-dependent hydrogenase (4) and ferredoxin-dependent hydrogenase (5). At high P(H2) reducing equivalents are partly transferred to lactate, involving lactate dehydrogenase (6) conversion that is most important is determined by the Gibbs free energy change at standard conditions (�G ◦′ ) of the individual conversions (Table 11.1). From these reactions it can be seen that the formation of hydrogen by reduction of protons with NADH as electron donor is thermodynamically unfavorable: the midpoint redox potential of the couple H + /H2 being –414 mV. When NADH is reoxidized only via the formation of ethanol, lactate or alanine etc., the amount of hydrogen that is produced would never exceed the amount of CO2. Nevertheless, available fermentation data show that the H2/CO2 ratio can easily reach values higher than 1 and even reach 2, as was reported for Thermotoga maritima (Table 11.2). This indicates that NADH is not only used for the exergonic dehydrogenase catalyzed reactions, but that under certain conditions NADH oxidation also results in proton Table 11.1 Gibbs free energy values for different fermentative reactions. Data were calculated using Thauer et al. 6 , Amend and Shock 7 and Amend and Plyasunov 8 Fermentative reaction �G 0 ‘ kJ/reaction NADH + H + + pyruvate− → NAD + + lactate− −25.0 2NADH + 2H + + acetyl-CoA → 2NAD + + ethanol + CoA −27.5 NADH + H + + pyruvate− + NH4 + → NAD + + alanine + H2O −36.7 NADH + H + → NAD + + H2 +18.1 2 Ferredoxin(red) + 2H + → 2 Ferredoxin(ox) + H2 +3.1
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Biofuels Biofuels. Edited by Wim So
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Biofuels Edited by WIM SOETAERT Ghe
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Contents Series Preface ix Preface
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Contents vii 6.3 Biomass Gasificati
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Series Preface Renewable resources,
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Preface This volume on Biofuels fit
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Editors List of Contributors Wim So
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1 Biofuels in Perspective W. Soetae
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Table 1.1 Approximate average world
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Table 1.3 Energy yields of bio-ener
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Biofuels in Perspective 7 is burnt
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2 Sustainable Production of Cellulo
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Sustainable Production of Cellulosi
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Figure 2.9 2004 US adoption rates o
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3 Bio-Ethanol Development in the US
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Bio-Ethanol Development in the USA
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Biorefineries in Production (115) B
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Cost of Cellulosic Ethanol, $ per g
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4 Bio-Ethanol Development(s) in Bra
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Bio-Ethanol Development(s) in Brazi
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Share of energy consumption 100% 90
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Table 4.2 Main technological improv
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4.7.4 Use of Fertilizers and Pestic
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78 Biofuels Table 5.1 Biodiesel pro
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80 Biofuels CH 2 O CH O COR R1 + 3
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82 Biofuels short reaction times. 9
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84 Biofuels Table 5.4 Overview on h
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86 Biofuels Table 5.5 Critical cond
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88 Biofuels methoxide as catalyst u
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90 Biofuels KOH Methano Oil/Fat Aci
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92 Biofuels 16. J. Graille, P. Loza
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6 Bio-based Fischer-Tropsch Diesel
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6.2.1.2 Catalysts Bio-based Fischer
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Bio-based Fischer-Tropsch Diesel Pr
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7 Plant Oil Biofuel: Rationale, Pro
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Table 7.1 Market milestones for pla
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Water CO 2 Figure 7.1 Closed CO 2 l
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Plant Oil Biofuel: Rationale, Produ
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130 Biofuels Among the attractive f
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132 Biofuels Table 8.1 Enzymatic tr
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134 Biofuels conversion was maintai
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136 Biofuels (diglycerides) decreas
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138 Biofuels ME content (wt.%) Reac
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140 Biofuels Ratio of initial react
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142 Biofuels (a) 34 kDa 31 kDa 34 k
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144 Biofuels preparation of whole-c
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ability to reduce flocculation (% o
Page 163 and 164: 148 Biofuels 5. R. C. Strayer, J. A
Page 165 and 166: 150 Biofuels 46. H. Fukuda, Immobil
Page 168 and 169: 9 Production of Biodiesel from Wast
Page 170 and 171: Production of Biodiesel from Waste
Page 174 and 175: 9.3.2 Processing of Crude and Waste
Page 176 and 177: Heavy layer from alkaline neutralis
Page 182 and 183: CPO I 1 1 Heating 60°C Reaction st
Page 184 and 185: References Production of Biodiesel
Page 186 and 187: 10 Biomass Digestion to Methane in
Page 188 and 189: Cow manure Pig manure Yard manure B
Page 190 and 191: Number of plants 3000 2500 2000 150
Page 192 and 193: Biomass Digestion to Methane in Agr
Page 196 and 197: Table 10.4 Alternative forms of ene
Page 200 and 201: Wet fermentation 3 - 10% TS applica
Page 202 and 203: Rel. Frequency [%] 35 30 25 20 15 1
Page 204 and 205: Steam 2 1 Energy crops (& manure) D
Page 210: Biomass Digestion to Methane in Agr
Page 213: 198 Biofuels 11.1 Introduction Hydr
Page 217 and 218: Table 11.2 Continued Organism Domai
Page 219 and 220: 204 Biofuels NADH is that the react
Page 221 and 222: 206 Biofuels Clostridium thermocell
Page 223 and 224: 208 Biofuels in particular in Cl. p
Page 225 and 226: 210 Biofuels ferredoxin-dependent m
Page 227 and 228: 212 Biofuels Concentration (mM) 45
Page 229 and 230: 214 Biofuels genes of the glycolysi
Page 231 and 232: 216 Biofuels 11. S. Tanisho and Y.
Page 233 and 234: 218 Biofuels 49. M. J. Axley, D. A.
Page 235 and 236: 220 Biofuels 83. P. J. Silva, E. C.
Page 238 and 239: 12 Improving Sustainability of the
Page 240 and 241: Table 12.1 Corn ethanol dry mill: e
Page 242 and 243: Table 12.2 Atmospheric CO2 (equival
Page 244 and 245: Table 12.3 Incremental CO2 equivale
Page 246 and 247: Improving Sustainability of the Cor
Page 248 and 249: Improving Sustainability of the Cor
Page 250 and 251: Index italic entries indicate refer
Page 252 and 253: iorefineries 3, 10, 11, 41 and corn
Page 254 and 255: policy (official) 59-61 production
Page 256 and 257: NADH 200-6, 207, 210 natural gas 1
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Biofuels in Perspective

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