Future Perspectives of  Biofuels and Biotechnology

Long term, regardless of the conversion technology, the supply of renewable feedstock must fit the process for which it is destined. While thermochemical processes, of course, have much less restrictions regarding the biomass sources supplied, the biological conversion systems described here are dramatically impacted by the feedstock characteristics and quality. Corn-based ethanol processes require a high starch content and the seed grain suppliers have improved the corn yields with high starch content kernels. However, the nascent biomass energy industry is being built upon available residues from the food and feed industry and, for maximum cost effectiveness, this must change.
An important additional aspect to consider is that there are numerous types of plant biomass feedstocks available for biofuels production,ranging from herbaceous corn stover and switchgrass to woody sources such as hardwoods and pine forest residues. Their composition varies considerably with differences in the levels of cellulose, hemicellulose and lignin, as shown in Table 1. Among the critical differences from a process perspective are the levels of acetic acid and lignin, as these are important contributors to fermentation inhibition. Process

Table 1. Composition of selected biomass sources

modifications must be considered when feedstocks containing considerable acetylated hemicellulose, such as pine, or high levels of lignin, such as hybrid poplar or sugar cane bagasse, are the available biomass source because of their expected higher levels of inhibitor caused by the feedstock chemical composition. The bold entries in the table are particularly important characteristic for bioconversion, including high glucan as a benefit and high lignin or mineral content as detriments.
Current biomass ethanol processes are using either wheat straw, for example at the Iogen pilot plant or waste sugar cane residues, bagasse, corn plant residue or corn stover. While these materials have a beneficial composition (Table 1), wheat straw resources are limited, sugar cane planting range is limited due to its current climatic range for growth and the US National Corn Growers Association estimates corn ethanol production will peak at approximately 15 billion gallons (www.ncga.com). Additionally, the environmental impacts of corn production and energy balance of corn ethanol have become important considerations before this upper limit is reached. To change this situation, dedicated bioenergy crops must be selected, tested for bioconversion capabilities, domesticated and grown on a large scale. In the mid-1990s, the US DOE Oak Ridge National Laboratory (ORNL) selected hybrid poplar and switchgrass as short rotation woody and herbaceous dedicated bioenergy crops needing development. Both are native North American species that have a wide grow range. In addition, hybrid poplar is a hardwood that grows very rapidly and, in spite of a relatively high lignin content, is readily conversed to ethanol with proper pretreatment and SSF processes. For trees, storage is not an issue since they are a year-round source waiting to be harvested to support continuous biorefinery operation. Switchgrass is a highyielding prairie grass that requires minimal fertilizer inputs, has deep root systems providing carbon sequestration benefits, grows in regions with low rainfall and can be planted and harvested reasonably well with existing planting equipment.140 These two plant species are not the only potential dedicated crop selections as other native grasses, willow species and tropical energy cane have specific benefits for certain agronomic regions.
To maximize productivity of biomass to ethanol and other chemicals selected dedicated energy crop species must be improved by plant genetic engineering and breeding with the aim of building in plant characteristics that are superior to the best available cultivar regarding ease of complete bioconversion to product. Such research has begun with a forage species, alfalfa, aimed at the reduction of lignin content. The Samuel Roberts Noble Foundation has shown genetic alteration of six different enzymatic steps involved in lignin production with reduced lignin content and proportionately improved enzymatic hydrolysis of the plant residues.Similar breeding and genetic research on potential crops is under way aimed at improving not only the yield of the selected crop, but also modification of the plant characteristics by addition of hydrolytic enzymes involved in bioprocessing. Improvement of hybrid poplar took a significant leap forward with the completion of its genome sequence.Work has been under way at ORNL to utilize this genomic sequence for selected genetic modification and gene knockouts to improve this fast-growing tree for bioconversion. Indeed, the simultaneous improvements of switchgrass and hybrid poplar are central themes of the ORNL-centered multi-partner US DOE Bioenergy Centers awarded in the Fall of 2007 (www.bioenergycenter.org). The ideal eventual process will be comprised of dedicated, high-yielding biomass feedstocks specially bred for minimal impact on the environment while having composition and structural characteristics that facilitate rapid carbohydrate depolymerization and conversion to ethanol or other chemicals. This will be accomplished by concerted and linked research on both improvement of the plant species and optimization of the conversion process for this improved feedstock, permitting an economically sustainable biofuels industry to grow and displace significant levels of petroleum.