Advances in Applied Microbiology (eBook)

Joan W. Bennett, Geoffrey M. Gadd, Allen I. Laskin (Herausgeber)

eBook Download: EPUB

2002 | 1. Auflage
268 Seiten
Elsevier Science (Verlag)
978-0-08-056907-9 (ISBN)

Infos

Advances in Applied Microbiology offers intensive reviews of the latest techniques and discoveries in this rapidly moving field. The editors are recognized experts and the format is comprehensive and instructive.

Key Features
* Covers topics of historical interest
* Includes a discussion on foodborne pathogens
* Entire sections devoted to various topics such as genomics, and microbial genetics

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Advances in Applied Microbiology offers intensive reviews of the latest techniques and discoveries in this rapidly moving field. The editors are recognized experts and the format is comprehensive and instructive. - Covers topics of historical interest- Includes a discussion on foodborne pathogens- Entire sections devoted to various topics such as genomics, and microbial genetics

Textauszug

Bioethanol Technology: Developments and Perspectives

Owen P. Ward; Ajay Singh    Department of Biology, University of Waterloo, Waterloo, Ontario, N2L 3G1Canada

I Introduction

Accumulation of CO2 in the atmosphere is long recognized as a major contributor to global warming and climate change (Revelle and Suess, 1957). Bioethanol used as a replacement for gasoline reduces vehicle CO2 emissions by 90% (Tyson et al., 1993). With respect to global warming, ethanol from biomass reduces net CO2 emissions since fermentation CO2, produced during ethanol production, is part of the global carbon cycle (Wyman, 1994). Ethanol has an important impact on automobile tailpipe emissions, producing a significant demand for use of ethanol as an oxygenate (Putsche and Sandor, 1996). With the phase out of the oxygenate methyl tert-butyl ether (MTBE), which reduces CO emissions by improving oxidative combustion, ethanol can replace MTBE as an oxygenate (Blackburn et al., 1999; Unnasch et al., 2001). A disadvantage of ethanol is that it has only 65–69% of the energy density of hydrocarbon fuels (Lynd, 1996).

Brazil produces 12.5 billion liters of ethanol from cane sugar, which is used as a 22% blend with gasoline or as neat ethanol fuel in their Otto cycle engine (Rosillo-Calle and Cortez, 1998). The United States produces 5 billion liters mainly from corn, used mainly as 10% in gasoline, but some as 85% ethanol that can be used in flexible fuel vehicles produced by Ford and Chrysler at no extra cost (Sheehan, 2001).

Because of the problems associated with conversion of lignocellulose to fermentable sugars, ethanol plants have relied on sugar- and starch-based substrates, and have been slow to take on the risks of lignocellulose-based fermentation (Claassen et al., 1999). Nevertheless, several bioethanol production plants, having capacities in the range 1–20 million gal/year, are under construction or are being commissioned. These plants use microbial processes to produce ethanol from lignocellulose, sugar cane waste, and municipal solid waste. It has been estimated that the United States potentially could convert 2.45 billion metric tons of biomass to 270 billion gallons of ethanol each year, which is approximately twice the annual gasoline consumption in the United States (Gong et al., 1999). Shell predicts that fuel from biomass will overtake oil by 2060 (Lynd et al., 1999). The National Science and Technology Council predicts that 50% of organic chemicals will be produced from plant material by 2020 with biobased processes playing a central role (Lynd et al., 1999).

The value of ethanol as an oxygenate and octane booster is $0.80–90 s/gal (Sheehan and Himmel, 1999). The U.S. highway bill includes an extension to the ethanol tax incentive program to 2007, which adds about $0.50/gal to the value of ethanol for the fuel market, allowing ethanol to sell for $1.20–1.40/gal (Sheehan and Himmel, 1999). Ultimately, technological developments must be such as to eliminate the need for the tax incentive.

II Feedstock Supply

The potential to use lignocellulose biomass for energy production derives from its position as the most abundant and renewable organic material in the biosphere, accounting for 50% of world biomass (Goldstein, 1981; Lutzen, 1983). Potential feedstocks include food crops, crop residues, and woody biomass. The chemical composition of various biomass feedstocks is presented in Table I. The low cost and chemical composition of crop residues make them attractive as feedstocks (Kaylen et al., 2000). Oak Ridge National Laboratory’s feedstock supply cost curves show costs ranging from $15–44/dry ton (dt) (Walsh, 1997 and a typical cost assumption is $25/dt (Wooley et al., 1999). A scale that benefits from most of the economies of scale is 2000 dt/day feedstock with a feedstock collection radius of up to 40 miles. Current consumption of biomass in the corn-refining industry and the pulp and paper industry are 52 and 100 million tons/year, respectively (Lynd et al., 1999). Annual available collectable waste cellulosic biomass at  < $45/ton is 140 million tons/year (Lynd, 1996). If available agricultural residues in the United States were converted to ethanol, production could expand to 38–53 billion liters per year, which would achieve a 10% blend of all gasoline used in the United States (Sheehan, 2001). By way of comparison, U.S. production of primary building blocks for organic chemicals (ethylene, propylene, benzene, toluene, xylene, butadiene) is 64 million tons/year 1997 (Lynd et al., 1999).

Table I

Proximate Composition Of Biomass Feedstocka

Hexosans

30–42

40–55

40–45

3–8

Pentosans

10–40

8–30

30–35

6–8

Lignin

10–30

15–30

5–12

< 1%

Ash

2–20

0.2–1.0

4–6

1–2

Starch

< 1%

< 1%

< 1%

10–70

Protein

< 1%

< 1%

4–13

10–60

Oil

< 1%

< 1%

< 1%

10–25

Extraneous materials

7–10

0.5–8.0

< 1%

< 1%

Because type and availability varies with geographic region, climate, and environmental conditions, agriculture, and technology (Kuhad and Singh, 1993), generic processes for conversion to energy must be versatile and robust as well as cost effective. The more customized they have to be made to deal with variations in feedstock the less advanced each process is likely to be. For example, a major problem in lignocellulose pretreatment and hydrolysis is the variation in lignin and hemicellulose composition with plant species, cultivation method, and harvest time. Because of these problems, ethanol plants have relied on sugar- and starch-based substrates, and have been slow to take on the risks of lignocellulose-based fermentation (Claassen et al., 1999).

III Alternative Technologies for Biomass Conversion

Major options for production of energy and/or fuel from biomass are fermentation-based bioconversion processes and physicochemical technologies, and competition will occur between these approaches (Singh and Mishra, 1995). While the major target product in fermentation is ethanol, processes for methane production from waste are widely practiced. An acetone–butanol process was operated commercially in the past and may be revived (Spivey, 1978; Lovitt et al., 1988). Microbial production of hydrogen is at a less advanced state and commercial viability remains in question.

Major physicochemical processes are combustion, gasification, pyrolysis, and hydrothermal upgrading. Fermentation and physicochemical processes are differentiated with respect to manner of substrate utilization: thermal technologies can use total crops, producing heat and/or synthesis gas and/or pyrolysis oil, whereas fermentation processes produce specific energy/fuel products plus substantial valuable or waste by-products. Since the volume of by-products can be high (similar in magnitude to the volume of the main product), optimizing the beneficial use and economic value of the by-products is a significant challenge. Gasification-based power generation from lignin-rich residues is considered to be an attractive way to realize value from the residues of processes featuring enzyme hydrolysis (Lynd et al., 1999).

IV Biomass Conversion to Usable Fermentation Feedstock

There are five basic steps involved in bioethanol production: biomass production, conversion to usable fermentation feedstock, microbial fermentation of feedstock to ethanol, beneficial use of residual unfermented material, and environmental management of the process. These processes are currently implemented commercially using cane sugar and starch-based substrates. The intractable nature of cellulose, present in the native state as lignocellulose, is the key barrier to commercially viable use of cellulose as a fermentation feedstock, and development of effective methods for its conversion to fermentable feedstock is the top priority (Ghosh and Singh, 1993). Gasification, acid hydrolysis, and pretreatment/enzyme hydrolysis are options for overcoming recalcitrance. Various physicochemical pretreatment methods used in lignocellulose substrate preparation for bioethanol production are shown in Table II.

Table II

Various Pretreatment Methods Used In Lignocellulose Substrate Preparation

Pretreatment type

Specific method

Mechanical

Weathering and milling—ball, fitz, hammer,...

Produkt-Details

• Erscheint lt. Verlag: 11.9.2002
• Sprache: englisch
• Themenwelt: Sachbuch/Ratgeber
• Themenwelt: Naturwissenschaften ► Biologie ► Mikrobiologie / Immunologie
• Themenwelt: Technik
• ISBN-10: 0-08-056907-2 / 0080569072
• ISBN-13: 978-0-08-056907-9 / 9780080569079