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Bioconversion and Biorefineries of the Future Linda L. Lasure, Pacific Northwest National Laboratory Min Zhang, National Renewable Energy Laboratory Contents I. Introduction a. Sources and nature of biomass b. Bioconversion c. Definition of a biorefinery II. Bioconversion, microbial biodiversity and transformation of lignocellulosic feedstocks III. Bioconversion and century-scale impacts on petroleum use and carbon sequestration IV. Local Biorefineries V. Large consolidat
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  Bioconversion and Biorefineries of the Future Linda L. Lasure, Pacific Northwest National Laboratory Min Zhang, National Renewable Energy Laboratory Contents I.   Introduction a.   Sources and nature of biomass  b.   Bioconversion c.   Definition of a biorefinery II.   Bioconversion, microbial biodiversity and transformation of lignocellulosic feedstocks III.   Bioconversion and century-scale impacts on petroleum use and carbon sequestration IV.   Local Biorefineries V.   Large consolidated Biorefineries and century-scale impacts on energy and GHG mitigation a.   Example I: Ethanol from corn stover in the USA i.   Energy from corn stover ii.   Impact on GHG production iii.   Life cycle analysis of impacts on energy and GHG mitigation for corn stover derived ethanol iv.   Century-scale impacts on GHG mitigation  b.   Example II: Polylactide from corn VI.   Summary VII.   References I. Introduction Although the environmental, economic and social impacts are still being debated, it is clear that the atmospheric concentrations of greenhouse gases (GHG) are increasing. The atmosphere in 2000 held about 774 Pg (774 billion metric tonnes) of carbon (C) as carbon dioxide (CO 2 ), corresponding to an average concentration of 369 parts per million  by volume (vppm) (Marland & Boden, 2001). At the present rate of emission, the  projected total will double by the end of this century. To achieve the goal of stabilizing the total carbon in the atmosphere at about 550 vppm, it will be necessary to reduce GHG emissions. The USA alone is expected to release 1.8 Pg of carbon in 2010 and 2.1 Pg of carbon in 2020, about 25% of the world total (EIA 2000). To maintain the target carbon dioxide concentration through the end of this century, current anticipated emissions must  be reduced by 1500 Pg C (Edmonds. this volume). Most of current carbon emission to the atmosphere is a direct result of the use of fossil fuels. The enormity of the problem suggests that many different and coordinated actions must be taken. One approach to reduction of carbon emissions is to substitute renewable  biological sources for fossil sources of combustion fuel and other products. The US 1  Government has enunciated the goal of displacing 10% of the petroleum used in the USA with biomass derived fuel and products by 2020 (DOE Vision for Bioenergy and Biobased Products in the United States, October 2002). The vision is the development of the technical, commercial and political infrastructure analogous to the current oil refinery concept. In the “biorefinery” renewable biomass is “cracked” to useful components using “bioconversion” technology. The resulting components are separated into useful streams for production of fuels, power and products. Here we explore the current  biorefinery concept with respect to its potential contribution to reduction in greenhouse gas emissions. Sources and nature of biomass The biomass of the world is synthesized via the photosynthetic process that converts atmospheric carbon dioxide to sugar. Plants use the sugar to synthesize the complex materials that are biomass. Biorefineries require a large and constant supply of biomass. Biomass for use in the biorefinery could include grains such as corn, wheat and barley, oils, agricultural residues, waste wood and forest trimmings and dedicated energy crops such as switchgrass (Panicum virgatum ) or hybrid poplar (  Populus ). Use of grains and oils for energy reduces their availability for use as food or feed. The part of the plant that remains following harvesting of the grains and oils-- the stover and straw == are also sources of biomass but their use does not reduce the supply of food. Corn stover is the leading candidate as a biomass source to support a lignocellulosic Biorefinery because of large quantitieis available. It has been estimated that in the USA there is a potential supply of between 60 to 100 million tons per year (Elander, 2002; Kadam & McMillan, 2003). Municipal solid waste and waste from wood processing and from forest thinning operations are additional sources of biomass for use in producing fuel, power and  products in biorefineries. All forms of biomass have the same major components--cellulose, hemicellulose, and lignin. Cellulose is the largest fraction (40 to 50%), hemicelluse is next (20 to 30%) and lignin is usually 15 to 20% of biomass. The structures of these substances are shown in Figures 1,2 and 3. Figure 1 Cellulose structure 2  Because of its potential importance as a biomass source we use corn stover to exemplify the biorefinery concept. The cellulose, hemicellulose and lignin components of corn stover fall well within the typical composition of biomass: about 40% cellulose, 25% hemicellulose and 18% lignin. The cellulose is composed of linear polymers of the six-carbon sugar glucose linked by 1,4 glycosidic bonds. Hemicellulose is a complex of  primarily five carbon sugars, the majority of which are xylose and arabinose. X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4  X 1 -> 4 X 1 > 4 X !   !   !   !   !   !   !  Glc A 1 Glc 1 A 2  FeA 5 A 1 Glc !  X 1 X 1   X 1 -> 4  X 1 -> 4  X 1 -> 4  = xylan backbone X = xylopyranose A = arabinofuranose Glc = galactopyranose FeA = ferulic acid Figure 2 Hemicellulose Structure Lignin is a complex polymeric heterogeneous material composed of variously substituted benzene rings. Figure 3 Lignin Monomeric structures 3  Bioconversion The term bioconversion is used in several ways. One definition limits the term to the conversion of organic wastes to methane via a biological process involving living organisms (also known as anaerobic digestion). This process does not lend itself very well to the biorefinery concept, as the biomass is converted to the simple one carbon compound, methane. Further refining of methane may yield hydrogen, but few other  products are easily or economically produced. This is but one example of bioconversion. Here we consider bioconversion to be the use of biological processes to transform  biomass materials from one form to another. Such conversions involve the use of enzymes, microbes or other biological agents, alone or in combination. It is important to note that in the ‘biorefinery’ the bioconversion processes involve the use of both physical and chemical methods. For example, the current process for “cracking” corn involves, steeping the corn kernels (a biological process), followed by grinding and separation, followed by conversion of the corn starch by thermostable alpha amylase in a jet cooking step at 100 C (a combination of biological and thermal processes), followed by treatment with a second enzyme to produce a 95% glucose syrup. In another example, the hemicellulose fraction of a lignocellulosic material such as corn stover (wheat and rice straw, as well) can be separated by a thermo-chemical treatment prior to use of enzymes to convert the cellulose fraction to glucose. It is interesting to compare the process of anaerobic digestion that yields methane to the “biorefinery” process that yields ethanol. Anaerobic digestion employs, for the most part, complex biomass that is converted directly by microorganisms. In what may be called the current “biorefinery concept”, the  biomass is “pretreated” to produce simple sugars. This is so because the organism used for centuries to make ethanol, the yeast, Saccharomyces cerevisiae , does not secrete the enzymes required to convert complex biomass to sugars. The yeast must have glucose in order to make ethanol. The current lignocellulose-based biorefinery concept is based on the corn-milling model in which the biomass is converted to glucose Definition of Biorefinery The term “biorefinery” has been used to describe the, as yet unrealized, manufacturing  paradigm for converting “lignocellulosic biomass and” to valuable products. The  biorefinery is analogous to the petroleum refinery in that in it the biomass is “cracked” into separated components and each is converted to a separately marketed product. A  biorefinery, then, is as a processing unit that refines biomass. This definition includes existing processing plants wherein grains (corn, wheat, barley and sorghum) and sugar cane are converted to starch, sugars, ethanol, organic acids and polymers. It is important to note that this definition of a biorefinery does not limit the method of conversion of crops to “bioconversion” alone. The biorefinery of the future is likely to integrate both  bioconversion and chemical “cracking” technologies. In addition to ethanol, it is envisioned that in the future many valuable bio-based products will be produced from low value biomass. The concept is illustrated in Figure 4. 4
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