Direct Combustion

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Direct combustion converts biomass to thermal energy. The removal of access moisture as a liquid (dewatering), the complete drying (dehydration), and the compaction or densification of biomass are critical processes to ensure efficient combustion. The energy expended in these processes must be compensated for by increased efficiency of conversion of biomass to energy, to make direct combustion economical. Combustion is carried out in incinerators, boilers, or furnaces under controlled conditions. The burning of biomass consists of a rapid chemical reaction; biomass components are oxidized, with the release of energy, carbon dioxide, and water. Chemical energy is released in the form of radiant and thermal energy, which can be used directly for drying purposes, or it can be converted into hot air, hot water, or steam, for instance, to generate electricity in turbines. Under perfect conditions, each reactant would be totally combusted, but in practice, this does not occur and considerable amounts of ash result. The removal of ash via grates is an important consideration for the efficient running of combustion systems. Plant biomass can be mixed with a range of other combustible materials in incinerators, including domestic and industrial biowastes. Crop biomass feedstocks are particularly suitable for use in combined heat and power (CHP) generators.

Although Jerusalem artichoke tops need extensive drying, they can be a useful source of biomass for direct combustion. Trials in Lithuania have confirmed Jerusalem artichoke's suitability as an energy crop. The bulk density of tops harvested in autumn was 78 kgm3, compared to 65 kgm3 in the spring, while the average net calorific value of dry biomass harvested in autumn was 18.0

MJm-2, compared to 18.5 MJm-2 in the spring. Harvesting in either spring or late summer provided dry matter with the best properties for combustion (Rutkauskas, 2005).

Jerusalem artichoke is unlikely to become a major biomass source for direct combustion because of the drying required. Zubr (1988) also noted that when the aim of energy generation by direct combustion is also the recycling of materials, then using Jerusalem artichoke might be hard to justify when biowastes are available. The crop is therefore more likely to become of greater significance as a wet feedstock in the production of biofuels.

7.3 BIOLOGICAL CONVERSION 7.3.1 Ethanol

Ethanol (ethyl alcohol) can be produced from a wide variety of feedstocks, including wood, wastepaper, and crop residues. Ethanol produced from plant biomass is also known as bioethanol. The production of bioethanol from plant biomass involves the fermentation of pulped, mashed, or juiced plant material by yeasts and bacteria (Wiselogel et al., 1996).

Bioethanol is a colorless, water-soluble, volatile liquid that can be utilized as a versatile fuel and fuel additive. It was recognized from the early days of the internal combustion engine that alcohol could be used as an alternative to fossil fuels, especially gasoline. During the 1920s in France and Germany, mixtures of ethanol and gasoline (e.g., 25% ethanol to 75% gasoline by volume) were used to extend motor fuel supplies and utilize agricultural surpluses. There was a revival of interest in ethanol as a motor vehicle fuel in the early 1970s, following the steep rise in oil prices imposed by OPEC. Bioethanol promised to reduce dependence on imported fossil fuel and extend gasoline supplies. The potential of bioethanol as a modern fuel for automobiles was first realized in Brazil in 1975, with the establishment of the National Alcohol Program. Initially the scheme used blends of gasoline with 10% and 20% bioethanol obtained from sugarcane. By 1995, 35% of passenger vehicles (4.2 million cars) were fueled by pure (100%) bioethanol (gasohol).

Bioethanol is being increasingly added to gasoline worldwide to improve engine performance through octane boosting. A fuel's octane number is a measure of the delay between fuel injection and self-ignition; the higher the octane number, the shorter the delay, and this reduces engine knocking. Lead was added to gasoline to enhance its octane rating but has been phased out, as it is a serious environmental pollutant. Ethanol provides an environmentally friendly alternative; with ethanol, gasoline blends effectively, raising octane rating. The use of ethanol in blends also helps to reduce exhaust emissions, such as carbon monoxide (CO) and volatile organic compounds (VOCs), thereby improving air quality. Many countries have moved toward ethanol and gasoline mixtures for these reasons, for example, 10% bioethanol to 90% gasoline (E10) (Bailey, 1996). Bioethanol can also be converted into ethyl tert-butyl ether (ETBE), which is used as a gasoline additive to enhance air quality. ETBE lowers the vapor pressure of gasoline, reducing the release of organic compounds that contribute to pollution and smog.

The production costs of liquid (transportable) fuels from biomass are generally higher than those from fossil fuel sources. For example, ethanol net production costs were $0.46 per energy equivalent of gasoline in the U.S. in 2005 (Hill et al., 2006). However, the costs of bioethanol are likely to fall due to biotechnological advances and economies of scale, while fossil fuel costs may rise due to future shortages and unpredictability of supplies. Energy content and energy conversion values for ethanol are also less favorable for biomass than fossil fuel feedstocks, with ethanol giving approximately 34% less energy than the equivalent quantity of gasoline. The lower heating value (LHV) for ethanol is around 21.1 MJ l-1 (75,700 Btugallon-1) compared to 32.0 MJ l-1 (115,000 Btugallon-1), while the higher heating values (HHVs) are 23.4 MJ l-1 (84,000 Btugallon-1) for ethanol and 35.0 MJL-1 (125,000 Btugallon-1) for gasoline (Anon., 2006). Therefore, more ethanol is required to do the same amount of work, which needs to be factored into the relative costs to the consumer.

However, bioethanol offers many environmental advantages over fossil fuels that are desirable for meeting the challenges of future energy production. Bioethanol is produced from renewable and sustainable resources, and theoretically makes no net contribution to greenhouse gas emissions. Its production and combustion reduce greenhouse gas emissions by around 12% relative to the fossil fuels it replaces (Hill et al., 2006). As a transport fuel (pure bioethanol or in mixtures with gasoline), it results in considerable reductions in emissions of pollutants (e.g., VOCs and CO). Bioethanol also contains no sulfur, and therefore does not contribute to acid rain. Therefore, demand for bioethanol is forecasted to rise, to enable environmental targets to be met and to stretch fossil fuel supplies. The development of domestically abundant and inexpensive biomass feedstocks will be required to meet future demand, particularly using wastes and biomass from agriculturally marginal land.

There has been a long history of converting Jerusalem artichoke into ethanol. The chemist Anselme Payen advised the French alcohol industry in the late 1800s, for example, to use Jerusalem artichoke tubers as a carbohydrate source for fermentation with yeasts, to produce a beer that could be distilled into pure ethanol. Fermented and distilled tuber extracts of Jerusalem artichoke have continued to be used in beer, wine, and spirit production. Beer made from Jerusalem artichoke tubers is said to have a sweet, fruity flavor; extracts of tubers or stalks can be added at various stages during the brewing process (Fritsche and Oelschlaeger, 2000; Zelenkov, 2000). In France and Germany, topinambur brandy has been made from tubers fermented by yeasts, especially Saccharomyces cerevisiae and Kluyveromyces marxianus (Benk et al., 1970; Hui, 1991). Vodka and sake can also be produced by the fermentation and distillation of Jerusalem artichoke extracts (e.g., Arbuzov et al., 2004; Ge and Zhang, 2005). The quality of spirits obtained from Jerusalem artichoke extracts is determined by their characteristic composition (e.g., inulin content), with esters produced during fermentation giving these spirits a distinctive aromatic quality (Szambelan et al., 2005). All systems that ferment Jerusalem artichoke stalks and tubers need to take into account the inulins present; the chemistry of inulin fermentation has been described in Chapter 5.

Ethanol from Jerusalem artichoke has been recognized as a promising biofuel since at least the 1920s. However, ambitious schemes in the 1930s and 1980s to promote Jerusalem artichoke as a source of fuel ethanol in the U.S. failed, primarily through lack of markets (Amato, 1993). The market for plant-derived ethanol is now growing, however, and improvements in the efficiency of production of ethanol from Jerusalem artichoke continue to be made (e.g., Baev et al. 2003; Filonova et al., 2001; Krikunova et al., 2001; Kobayashi et al., 1995).

Plant biomass feedstocks undergo a series of pretreatments before fermentation, including milling or grinding, and the separation of juice and pulp. Further treatments remove lignin and digest components such as cellulose and hemicellulose into fermentable compounds by partial or complete hydrolysis. Cellulose can be broken down, for example, using sulfurous acids, exogenous enzymes, or enzymes from cellulolytic strains of fungi or bacteria. The inulin present in Jerusalem artichoke can be converted to fermentable sugars by acidic or enzymatic hydrolysis (Figure 7.1a) prior to fermentation with yeasts or bacteria (van Bekkum and Besemer, 2003). This process is sometimes called separate hydrolysis and fermentation (SHF). Acid hydrolysis was the original method of obtaining fermentable sugars from plant feedstocks, using either high acid concentrations at low temperatures or low acid concentrations at high temperatures. A hot acid hydrolysis of pulped Jerusalem artichoke, for instance, was formerly the method of producing fructose and glucose from inulin, prior to fermentation with S. cerevisiae or other distillery yeasts (e.g., Boinot, 1942; Lampe, 1932; Underkofler et al., 1937). Subsequently, enzymic pretreatment has been used to hydrolyze inulin prior to fermentation (Combelles, 1981; Duvnjak et al., 1982; Sachs et al., 1981; Zubr, 1988). A process to produce ethanol from Jerusalem artichoke tubers was designed by Kosaric et al. (1982), for example, using a two-step tuber maceration to yield juice with a 12 to 15% carbohydrate content. This was heated and hydrolyzed enzymatically. After cooling, the juice was fermented in

Biomass (e.g., Jerusalem artichoke tubers)

Biomass (e.g., Jerusalem artichoke tubers)

Pre-treatment (e.g., storage, washing, grinding)

Pre-treatment (e.g., storage, washing, grinding)

Saccharification (acid or enzymatic hydrolysis)

Simultaneous saccharification and fermentation (with inulinase-producing yeasts)

Fermentation (with yeasts or bacteria)

Distillation

Distillation

Ethanol recovery

Ethanol recovery (a)

FIGURE 7.1 Stages in ethanol production from Jerusalem artichoke using (a) acid or enzymatic hydrolysis followed by fermentation with classical yeasts (e.g., S. cerevisiae) or the bacteria Z. mobilis, and (b) inulinase-producing yeast (e.g., K. marxianus).

batches, with yeast being recycled. An ethanol yield of about 90% of the theoretical maximum value was achieved after 28 h (Kosaric and Vardar-Sukan, 2001). Zubr assessed enzymatic pre-treatments of tuber pulp (cv. 'Urodny') prior to fermentation with S. cerevisiae; the highest ethanol yields were obtained in conjunction with the industrial enzyme 'Novo 230' (Zubr, 1988). Extracellular inulinase from Aspergillus niger can also be used in conjunction with S. cerevisiae to ferment tuber juice or pulp (Ohta et al., 2004).

Traditional fermentation yeasts, such as S. cerevisiae, are not adapted to utilize inulin. However, a number of yeast strains have been discovered with inulinase activity, which can both hydrolyze inulin and ferment the resulting sugars (Echeverrigaray and Tavares, 1985; Guiraud et al., 1981a, 1981b; Padukone, 1996). It is therefore possible to produce ethanol from Jerusalem artichoke juice using these yeasts in a single vessel, without prior hydrolysis or saccharification, in a process called simultaneous saccharification and fermentation (SSF) (Figure 7.1b). In practice, enzymatic hydrolysis may still be conducted under acidic conditions, for instance, to utilize the enzymes present in the plant material and to start saccharification prior to the addition of inulinase-producing yeasts. High rates of ethanol production from tuber extracts have been obtained with inulin-fermenting strains of K. marxianus, K. fragilis, Candida pseudotropicalis, C. kefyr, C. macedoniensis, Saccha-romyces fermentati, S. diasticus, Schwanniomyces castellii, and Torulopsis colliculosa (Duvnjak et al., 1981; Ge and Zhang, 2005; Guiraud et al., 1986; Rosa et al., 1986). Improvements have been made to these yeasts to further increase the efficiency of ethanol production; for example, a strain of K. fragilis has been selected with improved ethanol tolerance (Rosa et al., 1988).

There are, in general, four types of ethanol production systems used by industry: batch, fed-batch, continuous, and semicontinuous processes. The batch process is the classical method of producing alcoholic beverages and is the method by which most ethanol is produced today. Batch processes are easily managed and flexible; their main disadvantage is unproductive downtime, though several bioreactors can be run at staggered intervals. Continuous processes have little downtime; microbial cells are born in the bioreactor, replacing those that are washed out. Continuous systems can produce more ethanol, of a more uniform quality, than batch processes and are better suited to large-scale production, but they are less flexible, have higher investment and operating costs, and have a higher risk of adverse microbial mutation due to longer culturing periods.

Fed-batch processing combines batch and continuous cultures, with feedstocks and microbial cultures added at regular intervals and effluent removed discontinuously. Semicontinuous production is effectively repeated fed-batch processing, with culture withdrawn at intervals, and new microbes and fresh medium added (Kosaric and Vardar-Sukan, 2001). The fermentation of Jerusalem artichoke extracts in experimental trials has been conducted using batch, semicontinuous, and continuous processes (e.g., Bajpai and Bajpai, 1991; GrootWassink and Fleming, 1980). Fermentation by all processes can be conducted with either free (e.g., Margaritis and Bajpai, 1982b) or immobilized (e.g., Margaritis and Bajpai, 1983; Margaritis and Merchant, 1984) yeast cells. Free cells are suspended in the culture medium and may aggregate through natural flocculation, while immobilized cells are held on surfaces or within particles. Immobilized cells can occur at higher cell concentrations, and the need for cell recycling systems to replace washed out cells is avoided, although free cells are less prone to inhibition due to the high levels of substrate or alcohol that can occur around immobilized cells.

Optimal rates of fermentation occur at acidic pH levels. In nonacidified conditions (e.g., over pH 6.4) more contaminants occur, which reduces the efficiency of fermentation. Guiraud et al. (1982) found that fermentation with inulin-fermenting strains of a range of yeast species occurred optimally at pH 3.5, with higher-grade alcohol produced after 7 days in a semicontinuous culture (Guiraud et al., 1982). In batch and continuous cultures, with free and immobilized cells, the optimal pH range is 3.5 to 6.0 (e.g., Bajpai and Margaritis, 1987; Margaritis and Bajpai, 1982a; Margaritis et al., 1983b). Fermentation with inulinase-containing yeasts occurs optimally at high temperatures. For instance, Rosa et al. (1987, 1992) reported that batch fermentation with K. marxianus proceeded efficiently at fermentation temperatures of 28 to 36°C and occurred up to 39 to 40°C, although at higher temperatures premature stoppage of fermentation is more likely due to a reduced ethanol tolerance of the yeast. Temperature affects the rate of ethanol production differently in free and immobilized yeast cell systems. Bajpai and Margaritis (1987) found that fermentation with free cells of K. marxianus occurred optimally at 25 to 35°C, while immobilized cells performed well over a wider temperature range of 25 to 45°C. However, Williams and Munnecke (1981) observed a lower optimum temperature for immobilized S. cerevisiae than for free cells, probably due to diffusional limitations arising from the support matrix, resulting in inhibitory levels of ethanol around the cells at higher temperatures.

In addition to yeasts, strains of the bacteria Zymomonas mobilis also efficiently and rapidly ferment Jerusalem artichoke juice. The bacteria do not contain inulinase, so fermentation must be in conjunction with acidic or enzymatic hydrolysis of inulin (Kim and Rhee, 1990). However, Z. mobilis has several properties that are suited to industrial applications. It has a natural tendency to flocculate, raising fermentation efficiency in batch production, and grows in both high sugar and ethanol concentrations without inhibition, while it has more favorable fermentation kinetics than yeasts such as S. cerevisiae (Ingram et al., 1989; Rogers et al., 1979; Toran-Diaz et al., 1983). Moreover, Szambelan et al. (2005) found that concentrations of components other than ethanol, which are potential contaminants, were fewer using Z. mobilis than when using strains of yeast (S. cerevisiae and K. fragilis). The optimal conditions for fermentation with Z. mobilis are typically 30 to 40°C and pH 4.0 to 5.0 (Kosaric and Vardar-Sukan, 2001). Fermentation of Jerusalem artichoke ('Albik' and 'Rubik') tubers by Z. mobilis gave theoretical yields of ethanol, after acid and enzymic inulin hydrolysis, of 86 and 90%, respectively (Szambelan and Chrapkowska, 2003). In a number of other studies with Z. mobilis, ethanol yields of 90 to 94% of the theoretical maximum have been obtained (Allias et al., 1987; Favela-Torres et al., 1986; Szambelan et al., 2005; Toran-Diaz et al., 1985). In continuous production systems, Z. mobilis consistently outperforms yeast strains in terms of ethanol yield (Kosaric and Vardar-Sukan, 2001). However, there is a need to sterilize the culture medium when using Z. mobilis, while yeast strains that produce inulinase may be preferable in economic terms when fermenting Jerusalem artichoke extracts. Z. mobilis can be used in mixtures with yeasts (e.g., S. cerevisiae and K. fragilis), and this can result in higher theoretical yields than obtained from single organisms alone (Szambelan et al., 2004b).

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Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

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