BIOFUELS SUPPLEMENT
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A bi-weekly summary of world developments on biofuels, produced by the Global Knowledge Center on Crop Biotechnology, International Service for the Acquisition of Agri-biotech Applications SEAsiaCenter (ISAAA)
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June 17, 2011
In This Issue:
News and Trends
- Report Lists "Who's Who" in Global Biofuels
- First Ethanol-powered Bus Fleet Makes Debut Run in Brazilian City
- Genetic Manipulation of Lignin Synthesis in Switchgrass Reduces Recalcitrance and Improves Ethanol Production
Energy Crops and Feedstocks for Biofuels Production
- Sugar Release in Populus Tree Variants Affected by Lignin Content
Biofuels Processing
- Alkaline Hydrogen Peroxide (AHP) Pretreatment of Corn Stover
- Targeting Complete Utilization of Sugar Beet Pulp as Feedstock for Ethanol Production
Biofuels Policy and Economics
- Incorporating Biogeophysical Factors in Climate Effect Analysis of Perennial Bioenergy Crops Show Net Cooling Effect
* NEWS AND TRENDS *
Report Lists "Who's Who" in Global Biofuels
http://www.thebioenergysite.com/articles/833/global-bioenergy-whos-leading-whos-following
http://www.thebioenergysite.com/articles/941/global-bioenergy-whos-leading-whos-following-part-2
The Bioenergy Site features a global bioenergy report by Heather Youngs, from the Energy Biosciences Institute at the University of California, Berkeley (United States). The report describes countries who are major players in bioenergy. Some highlights of the report are:
(1) Brazil has changed the status of sugar cane ethanol from an agriculture product to a fuel, so bioenergy crop will be regulated now as a strategic fuel rather than an agricultural product; Brazil tightly controls the fuel market;
(2) Sustainability is an issue which has been factored in many national biofuel programs; some countries are reportedly struggling with water deficits (presumably due to increased bioenergy crop cultivations);
(3) biogas (methane and carbon dioxide from the anaerobic microbial conversion of organic wastes) continues to be one biofuel growth area; China and India have good biogas programs for rural/village-scale applications, while Germany leads in large-scale biogas applications with about 4,000 biogas plants;
(4) the United Kingdom is active in renewable fuels, and are "concerned about limited resources for biomass and how they can they proceed in a way that is socially and environmentally sustainable";
(5) the goals of bioenergy programs are country-dependent; for developing countries, the major concern is the enabling of small farmers to provide a basic means for energy access on a daily basis (cooking, heating electricity); for developed countries, the goal is "to meet a volume or percentage requirement from a renewable source".
Scientists from the Samuel Roberts Noble Foundation, Oak Ridge National Laboratory, and Georgia Institute of Technology (United States) report their attempts to produce low-lignin switchgrass, by genetic manipulation of its lignin biosynthetic pathway. Switchgrass is a high-yielding perennial prairie grass, with low agronomic inputs, and is considered a potential (lignocellulosic) feedstock for bioethanol production in the United States.
Recalcitrance, on the other hand, is the property of (lignocellulosic) biomass to resist pretreatment (usually a severe thermochemical process applied to biomass, to remove lignin, and to modify the structure of cellulose/hemi-cellulose fractions for easier processing to ethanol). The lignin content in the biomass is considered to be a major cause of recalcitrance. Thus, attempts have been made to use molecular biology techniques to produce low-lignin bioenergy crops, with the objective of reducing biomass recalcitrance and to reduce pretreatment cost.
The scientists reduced the lignin content in switchgrass by "down-regulation of the switchgrass caffeic acid O-methyltransferase gene". The genetically modified switchgrass required less severe preatreatment, required a lower enzyme dose for the subsequent saccharification process (by 300 percent to 400 percent), and increased ethanol yields by about 38 percent. They were able to show that the "apparent reduction in the recalcitrance of transgenic switchgrass has the potential to lower processing costs for biomass fermentation-derived fuels significantly" and that "modified transgenic switchgrass lines should yield significantly more fermentation chemicals per hectare". The study is published in the (United States) Proceedings of the National Academy of Sciences (PNAS) (URL above).
Fast-growing trees, such as those from the genus, Populus, are considered as potential (lignocellulosic) feedstock sources for "second generation" biofuels. Thus, research on improving the processability of fast-growing trees for biofuel production is one area of active research. As with the other types of lignocellulosic feedstocks (such as grasses), the reduction of "biomass recalcitrance" is a major challenge for improving the processability for biofuel-ethanol production. The lignin content and the syringyl-to-guaiacyl (S/G) ratio in biomass have been recently used to quantitatively assess biomass recalcitrance. Syringyl (S) and guaiacyl (P) are two of three major structural units of lignin (the third structural unit is p-hydroxyphenyl (P)). The S/G ratio is a quantity has been found to correlate with many variables associated with biomass recalcitrance.
Scientists from the Oak Ridge National Laboratory, and the National Renewable Energy Laboratory (United States) report that the sugar release in Populus tree variants may be affected by both lignin content and the S/G ratio. They found that for pretreated samples with S/G ratios less than 2.0, a strong negative correlation between sugar release and lignin content was found. For pretreated samples with S/G ratios higher than 2.0, "sugar release was generally higher, and the negative influence of lignin was less pronounced". They also found that glucose release was correlated with both lignin content and S/G ratio, but xylose release depended on the S/G ratio alone.
Some samples with average values of lignin content and S/G ratios were observed to exhibit "exceptional sugar release", prompting the researcher to conclude that "factors beyond lignin and S/G ratio influence recalcitrance to sugar release". A critical need for deeper understanding of cell wall structure may be needed "before plants can be rationally engineered for reduced recalcitrance and efficient biofuels production".
Researchers from the University of Michigan (United States) report the use of alkaline hydrogen peroxide (AHP) for the pretreatment of corn stover (leaf- and stalk- residues of the corn plant after harvest). Pretreatment is usually the first step in the processing of lignocellulosic biomass (such as corn stover) into biofuel ethanol. Its objective is to remove the tight lignin wrapping around the biomass, and to expose/transform the carbohydrate fractions (cellulose/hemicellulose) so that these are easier to process into ethanol-fermentable sugars.
Hydrogen peroxide (H2O2) is an oxidizing chemical which is commonly used as a disinfectant. It's oxidizing ability is considered sufficient for lignin removal in oxidative pretreatment processes. A mixture of hydrogen peroxide and sodium hydroxide constitutes AHP. The use of AHP is considered to have the following advantages:
(1) the chemicals (hydrogen peroxide and sodium hydrogen peroxide) are considered "environmentally benign",
(2) operating temperatures are milder (21 oC to 50 oC, in contrast to other treatments with operating temperatures higher than 100 oC),
(3) the chemical cost compared to the use of other effective pretreatment chemicals (such asa ionic liquids and phosphoric acid/ethanol) are lower.
The researchers studied the effects of biomass loading, hydrogen peroxide loading, residence time, and pH control, in combination with subsequent enzymatic digestion (using a commercial enzyme preparation, optimized mixtures of four commercial enzymes, or optimized synthetic mixtures of pure enzymes). They found that a combination of a biomass loading of 10%, a hydrogen peroxide loading of 0.5 g/g biomass, and the use of a commercial enzyme mixture at a protein loading of 15 mg/g glucan gave a monomeric glucose yield of 95%.
They concluded that AHP can be a good pretreatment method for corn stover, but additional improvements in the AHP process, such as peroxide stabilization, peroxide recycling, and improved pH control, could be helpful. The full results of the study is published in the open access journal, Biotechnology for Biofuels (URL above).
Sugar beet pulp is a by-product from the processing of sugar beet, a high-sucrose-containing crop which is cultivated for sugar (sucrose) production. It is largely lignocellulosic in nature. The high fiber (i.e. cellulose) content of sugar beet pulp makes it a good material for fodder. With the present trend toward the use of lignocellulosic biomass for "second generation biofuels", sugar beet pulp is considered a potential biofuel feedstock. The low lignin and high sugar contents of sugar beet pulp particularly good material characteristics. A feedstock with low lignin content translates to lower pretreatment cost, while one with a high sugar content translates to higher ethanol yields.
Pretreatment of lignocellullosic biomass is the process by which the lignin in the biomass is removed, resulting in a cellulose/hemicellulose-rich material which can be more easily processed for ethanol production. Pretreatment is usually followed by enzymatic treatment; the enzymes convert the cellulose/hemicellulose into ethanol-fermentable sugars.
Scientists from the Laboratory of Food Chemistry, Wageningen University (Netherlands) investigated the pretreatment conditions of sugar beet pulp which can be done within "commercially reasonable time spans" and "with economically reasonable enzyme levels". They found that hydrothermal treatment at 140 degrees Celsius allowed lower enzyme levels which could convert 90% of cellulose into ethanol-fermentable sugars in 24 hours. More severe treatments (such as acid treatment) destroyed and solubilized the sugars, with the production of subsequent production of the sugar-degradation products (furfural, hydroxymethylfurfural, acetic acid and formic acid). These products can inhibit ethanol fermenting organisms.
The full paper is published in the open access journal, Biotechnology for Biofuels (URL above).
Scientists from the Arizona State University, Stanford University, Carnegie Institution for Science (all in the United States) report the use of a Weather Research and Forecasting (WRF) Model to evaluate the climate effects of converting agricultural areas in the central United States to perennial crops for bioenergy production. Examples of these bioenergy crops include, perennial grasses (switchgrass, Panicum Virgatum L. or miscanthus, Miscanthus X giganteus).
One difference in their approach is the accounting of biogeophysical impacts on climate, "by considering properties that directly influence the manner in which energy is absorbed at the surface and redistributed to the overlying atmosphere". Their study showed that the biogeophysical effects resulting from the hypothetical conversion of annual-to-perennial bioenergy crops across the central United States impart a significant local-to--regional cooling. This cooling, they say, would have considerable implications for the reservoir of stored soil water. Increases in transpiration and higher albedo (reflectivity of the surface) were considered as factors to the cooling effect. A carbon emissions reduction of 78 tons per hectare can be potentially realized. This figure in emissions reduction is said to be about six times larger than the annual biogeochemical effects that arise from the offsetting of fossil fuel use).
Their study demonstrated that biogeophysical effects are important aspects of climate impacts of biofuels, even at the global scale. The full report of their study is published in the Proceedings of the National Academy of Sciences (PNAS) (URL above).