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Novel Pathways for the Production of Ethanol, Biogas and Biodiesel
By Alwin Hoogendoorn, Han van Kasteren
The Royal Society of ChemistryCopyright © 2011 Alwin Hoogendoorn and Han van Kasteren
All rights reserved.
The current world fossil oil production is struggling to meet demand and may even show a decline in the coming years. Currently, one can observe a period of economical stress in the European transportation biofuels industry. There are huge differences between EU installed production capacity and EU actual production in both the fuel ethanol and the biodiesel industry (see also Sections 2.1 and 4.1). However, a positive side effect of this current financial situation is that industrial process innovation is stimulated and transitions towards a second generation of biofuels production, providing higher efficiencies and better economics, may succeed more easily.
According to the authors, both the prospect of (future) declining oil production and unattractive profit margins for current production processes, result in the urgent need for new and more energy efficient production pathways for transportation biofuels. These new and more efficiently produced biofuels are sometimes referred to as second-generation biofuels.
With this book we intent to provide insight into 3 new promising and innovative pathways for the biological production of the main transportation biofuels: biodiesel, ethanol and methane. The pathways we intend to describe are nonconventional and should provide higher product yields, less stringent feedstock specifications, lower chemical additive demand, lower waste production and much better energy balances when compared to the more traditional production methods for, in particular, biodiesel and ethanol.
The two pathways described in Chapters 2 and 3 are both based on the biological conversion of syngas into either ethanol or methane. These two pathways are intended to have amongst other features higher product yields, high energy-production efficiencies, less complicated syngas cleaning, and can be produced from various kinds of lignocellulosic biomass. For these two pathways, a lot of attention is given towards technological and engineering aspects like gasifier selection, syngas cleaning, process design, reactor configuration and product upgrading (be it either ethanol distilling and dehydration or biogas purification).
Enzymes can be used instead of NaOH or KOH in order to produce biodiesel that consists of 100 wt.% methyl- or ethylesters. The pathway described in Chapter 4 involves the enzymatic production of a new kind of biodiesel where, amongst others, much less glycerol waste is produced and up to 10% higher product yield is obtained. This new kind of biodiesel consists of 75–85 wt.% methylesters (and the remainder monoglycerides and diglycerides) while still maintaining a good viscosity between 5–6 mm2/s.
The contents of this book will not only reflect extended desk-top research but will also show practical experimental results and an engineering perspective. For each of the 3 pathways there is a comparison made to competing production methods (research, patents), bacteria/enzymes (types, metabolism, reactions, inhibition, cultivation), process designs, pilot or laboratory experimental results (when available), discussion of fuel specifications and demands and a balanced discussion of the general economic and technical feasibility.
Therefore, it is intended to make this book attractive for both education purposes (scientists and students) and professionals within the current biofuels industry with a special interest in these novel pathways.CHAPTER 2
Biological Conversion of Syngas into Ethanol
2.1 First Generation of Ethanol Production
Alcoholic beverages often containing 5–10 vol.% of ethanol have been produced by mankind for thousands of years. Alcohol distillation in order to obtain a higher alcohol concentration is also known to have existed since ancient times. The world ethanol production amounts to 66 million m3 per year (2008 data;) with both Brazil (approximately 38% production share) and the USA (approximately 50% production share) as the main producing countries. The overwhelming majority of this 66 million m3 of ethanol production is used for transportation purposes. New fuel-ethanol production plants are being built every year. The yearly production capacities of these new plants are often in the range of 100–400 million litres ethanol in order to obtain the benefit of relatively lower operational and investment costs at larger plant sizes.
The main EU feed stocks for alcohol production are molasses, starch, beets, C-sugar, wheat and corn. France, Germany, UK and Spain are considered to be the EU largest producers. The figure below shows a Swedish ethanol production plant that uses corn as the main feedstock.
The current EU application of ethanol for transport fuel applications is around 60% of the total ethanol production, and equals about 2.2 million m3 per year (2008 data;). The total installed EU fuel ethanol production capacity amounts to 6.3 million m3, which means that many EU ethanol production plants are either temporarily out of operation or not reaching full capacity. A similar situation of economical stress is also observed within the first-generation biodiesel industry (see also Chapter 4). However, a positive side effect of this current financial situation is that industrial process innovation is stimulated and transitions towards a second generation of biofuels production, provided higher efficiencies and better economics, may succeed more easily.
Starch containing feedstock will need to be treated in liquefaction and hydrolysis steps in order to produce sugars prior to fermentation. The residence time in the fermentation tanks where the ethanol is produced is often around 3 days (batch processes). An ethanol concent around 65–80 g/l is commonly produced. From this, an average ethanol production of 20–27 g/l fermentor volume/day can be derived (Figure 2.1).
2.2 Introduction of Biological Conversion of Syngas into Ethanol
This chapter describes a joint e?ort by Ingenia, Telos and Wageningen University that has been sponsored by the Dutch Senternovem NEO EOS pro- gramme. The project started in January 2007 and finished in December 2008 and was a follow up for an earlier desk-top feasibility study that was executed in 2006.
The aim of this project is to develop a process for ethanol production by fermentation of syngas obtained by gasification of plant (residues) and waste materials. Much R&D effort is currently focused on alcohol production by fermentation of sugars obtained by enzymatic hydrolysis of (hemi)cellulose. A drawback of this enzymatic pathway is still the enzyme costs. The main advantage of the syngas route compared to the enzymatic route is that it also allows for conversion of nonbiodegradable feedstock components (e.g. lignin) to ethanol.
We calculated mass and enthalpy flows for ethanol production from lignocellulose via gasification and syngas fermentation and hydrolysis and yeast fermentation (Figure 2.2). The gasification and syngas fermentation route consists of gasification (57% of C to CO, 43% to CO2, 100% of H to H2), cooling, fermentation (100% conversion of incoming CO and H2, ethanol as only product) and distillation steps. The hydrolysis and yeast fermentation route consists of hydrolysis (100% conversion of C6 and C5 to monomers), solids separation, alcohol fermentation (100% conversion of C6, 33% conversion of C5), methane fermentation (rest of C5), furnace (100% combustion of lignin) and distillation steps. In addition we assumed 1 J thermal energy gives 0.37 J electrical energy. Enthalpy calculations were based on standard enthalpies of formation from the elements and specific heat capacities from various sources. For the distillation, the top and bottom product composition, reflux ratio and 100% heat recovery are assumed. The water content of the incoming lignocellulose in the gasifier is 13% (w/w). The calculations showed that 70% of the energy available in lignocellulosic feedstock can be recovered as ethanol via the syngas route if the syngas is completely converted to ethanol, compared to 45% via the enzymatic hydrolysis route with genetically modified yeast. The enzymatic hydrolysis route gives methane (15% of energy available in feed- stock) and electricity (14% of energy available in feedstock) as secondary products. Applying the syngas route; heat from the gasifier is used partially in distillation (reboiler) and the rest is used to generate electricity (4% of energy available in feedstock) (Figure 2.3).
In addition, the syngas route can deal with feedstock mixtures (even including plastics or rubbers) and with feedstock variability, which the enzymatic hydrolysis is not able to do. The third advantage of the syngas route is that it requires fewer process steps and avoids the costly enzymatic hydrolysis. The syngas route shares some of its process layout with the Fischer–Tropsch route, but has the advantage that it can be applied on smaller scale: 30 kton y-1 is an economically feasible scale of operation for the syngas route, but the Fischer–Tropsch route requires far bigger capacities to be cost effective. The advantages of small-scale operation with respect to feedstock logistics and transport costs are self-evident. A similar outcome was published recently in a study on conversion technologies for ethanol production from lignocellulosic biomass. The overall process yield applying gasification and fermentation and hydrolysis and fermentation were, respectively, 418 and 270 L of ethanol per ton of dry wood. Furthermore, anaerobic bacteria used for syngas fermentation are sulfur tolerant and not sensitive to the CO/H2 ratio. Therefore, fermentation route does not require an expensive sulfur-gas cleaning process and strict control of the CO/H2 ratio in comparison with chemical catalysis of syngas. The syngas fermentation is not operated at high temperature and pressure, unlike a chemically catalytic process. Last, but not least, product specificity of syngas fermentation is higher than that of chemical catalysts.
In order to obtain the estimated 70% (on energy basis) ethanol/feedstock yield, several hurdles have to be overcome. The most important hurdle is the slow absorption of syngas in water that makes it very difficult to reach the high ethanol yield at acceptable conversion rate (i.e. with acceptable equipment size) and power requirement. A second hurdle is the lack of understanding of the physiology and kinetics of the bacteria that convert the syngas.
2.3 Clostridium ljungdahlii and other Strains
Several species from different bacterial strains (Acetobacteira, Butyribacteria, Clostridia, Eubacteria, Moorella, Oxobacteria and Peptostreptococci) are capable of conversion of CO and H2 to alcohol. Different mesophilic bacterial strains were compared in a literature study on availability, ethanol concentration and available data based on the work that has been done in the past with these strains (Table 2.1). Most of the work published on gasification and ethanol production by bacterial fermentation has been done with Clostridium ljungdahlii. The highest ethanol concentration published (48 g L-1) was also achieved with C. ljungdahlii.
Bacterial fermentation of CO, CO2 and H2 to ethanol using Clostridium ljungdahlii gives the following equations:
6CO + 3H2O) [??] C2H5OH + 4CO2 ΔG = -216 kJ/mol (2.1)
6H2 + 2CO2 [??] C2H5OH + 3H2O ΔG = -97 kJ/mol (2.2)
In an experimental study it is said that, while C. Ljungdahlii strains will simultaneously convert all three components CO, CO2 and H2, CO is considered to be preferred substrate for the two strains with ATCC numbers 55988 and 55989. During experiments with these 2 strains, CO syngas conversions around 90% and H2 syngas conversions around 70% were achieved.
The distinctive feature of the followed pathway of these micro-organisms seems to involve the reduction of carbon dioxide to a methyl group and then its combination with a molecule of carbon monoxide and CoA to form acetyl-CoA. This combination of reactions has been designated as the acetyl-CoA pathway. The C. Ljundahlii bacterium was first isolated by researchers at the University of Arkansas from animal waste in 1987, with the wild strain producing predominantly acetate in favour of ethanol. In fact, early studies showed an ethanol-to-acetate product ratio of 0.05 mol mol-1 and an ethanol concentration of lower than 0.1 g L-1. Great scientific effort was undertaken over almost twenty years to improve the product ratio and increase the ethanol concentration. Cultivation at decreased pH (4.5), medium adaptation (yeast extract removal from medium, Ca-d-pantothenate and cobalt limitation) and oversupply of syngas are mentioned in the literature as successful approaches to increase the ethanol concentration and improve the product ratio.
The aim of this part of the research was to obtain a stock culture of viable cells capable of converting syngas to alcohol. C. ljungdahlii (DSMZ 13528) was obtained from Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (DSMZ, DE). (Figure 2.4)
The vacuum dried culture of C. ljungdahlii was cultivated under strictly anaerobic conditions in reinforced Clostridial medium (RCM) (Difco) (containing only 38 g L-1 of Difco). Butyl-rubber-stopped bottles of 120 ml contained 50 ml medium. The medium was reduced with cysteine (0.5 g L-1) prior to sterilization. Bottles were autoclaved for 25 min at 121 °C. The gas phase was pressurised to 170 kPa and was composed of 100% N2. Cultures were incubated at 37 °C (without shaking). All the cultivation conditions mentioned here were held the same in all cultivation experiments, which will be explained below.
After a week, the cell material cultivated in RCM is prepared for preservation at -80 °C. A quantity of 5 ml of 20% glycerol solution was added in 15 ml serum bottles and butyl rubber stopped and autoclaved for 25 min at 120 1C. The gas phase was here also pressurised to 170 kPa and was composed of 100% N2. A quantity of 5 ml of the cell material grown in RCM for a week was added to these serum bottles. The bottles were placed at -80 °C.
184.108.40.206 Analytical Methods
Gas samples were analysed by gas chromatography on a GC-14B gas chromatograph (Shimadzu, JP) fitted with a TCD detector. The injector and detector temperatures were 90 and 150 °C, respectively. H2 and CO were analysed using a molsieve packed column (13 × 60/80 mesh, 2 m length, 2.4 mm internal diameter; Varian, NL). Argon was the carrier gas at a flow rate of 30 mL min-1 and the oven temperature was 100 °C. Organic acids and alcohols were analysed by gas chromatography using a Chrompack CP9001 gas chromatograph (Varian, NL) equipped with a FID detector. The injector and detector temperatures were, respectively, 250 and 300 °C. Organic acids were analysed using a chromosorb 101 column (80/100 mesh, 180 cm length, 2 mm internal diameter, Varian, NL). Alcohols were analysed on a sil 5 CB column (2.5 m length, 0.32 mm internal diameter, df: 1.20, Varian, NL). The oven temperature for organic acids and alcohols were, respectively, 170 and 50 °C. The carrier gas was nitrogen and ?ow pressure for organic acids and alcohols were, respectively, 300 and 50 kPa. Glucose was analysed by HPLC using a Polyspher OA HY column (300–6.5 mm, Merck, DE) and a RI SE-61 refractive index detector (Shodex, JP). The mobile phase was 2 N H2SO4 at a flow rate of 10 mL min-1. The column temperature was 60 °C.
Excerpted from Transportation Biofuels by Alwin Hoogendoorn, Han van Kasteren. Copyright © 2011 Alwin Hoogendoorn and Han van Kasteren. Excerpted by permission of The Royal Society of Chemistry.
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