During thermochemical conversion, alkali metals (Na, K, Mg, P, Ca) present in the ash react with silica — originating both from the biomass itself and from soil introduced dur­ing harvesting — to produce a sticky, mobile liquid phase that can contribute to slag­ging, deposition, and corrosion of process equipment. As noted above, water leaching and fuel additives can be used to reduce the damaging effects of ash components, including alkali metals.

1.2.2 Carbohydrate/Lignin Ratio

In biological processing, carbohydrate present in cellulose (and potentially hemicellulose) is converted to fuels and/or chemicals, while the lignin fraction remains unaffected. Fur­thermore, the recalcitrance of cellulosic biomass to bioconversion typically increases with increasing lignin content, requiring more severe pretreatment, which decreases process efficiency. Bioconversion processes, therefore, favor feedstocks with high carbohydrate to lignin ratios. Representative cellulose, hemicellulose, and lignin values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.

1.3 Desirable Traits and Potential Supply of Cellulosic Energy Crops

Given the world’s finite land resource, the most important trait for cellulosic energy crops is productivity — the annual dry matter produced per unit land area. As listed in Table 1.1, productivity of the crops considered in this book ranges from 0.1 to 1.75 Mg/ha/yr (dry basis) for wheat straw, to as high as 44 Mg/ha/yr (dry basis) for miscanthus. The best energy crops will also have few inputs and low production costs. Easily established, robust perennial crops having long life spans (e. g. >10 years) are favored over annual crops, as are those having low fertilizer, pesticide, and insecticide requirements. Native, non-invasive species that provide good habitats for wildlife are preferred.

Feedstocks used in thermochemical processing should be harvested when moisture con­tent is relatively low to minimize preliminary energy intensive drying. Low moisture is not as critical in bioconversion feedstocks, for which wet storage can sometimes be a viable option. Ideally, ash content should be low (e. g. <1%), ash melting temperatures should be high (e. g. >1500°C), with low levels of particularly damaging elements, including alkali metals, alkaline earth metals, silicon, chlorine, and sulfur.

Conventional plant breeding — which involves manipulating the genes of a species via selection and hybridization so that desired genes are packaged together in the same plant and as many detrimental genes as possible are excluded — has traditionally been used to enhance desired agronomic traits such as productivity, water use efficiency, and crop lifespan. Breeding systems have been developed, and continue to be developed, that can be used to improve virtually all plant species. The productivity of corn, for example, has more than quadrupled since the 1930s largely through conventional breeding [30]. Biomass productivity can potentially be increased even further using more sophisticated biotechnology techniques. Recent molecular and genetic studies have identified a number of regulators of plant biomass production — for example, vegetative meristem activities, cell elongation, photosynthetic efficiency, and secondary wall biosynthesis — that might be manipulated to enhance energy crop yields [31].

The potential to produce viable energy feedstocks is vast. A detailed study led by the Oak Ridge National Laboratory estimates that the United States could produce 602-1009 million dry tons annually by 2022, and 767-1305 million annual dry tons by 2030, at a price of $60 per dry ton [32]. (The low value in the range assumes a 1% annual increase in yield; the high value, a 4% annual increase.) This excludes resources that are currently being used, such as corn grain and forest products industry residues. When currently used resources are included, the total biomass estimate jumps to over one billion dry tons per year for the lower productivity case — enough to displace about half of the country’s current gasoline consumption (134 billion gallons/year) if converted to ethanol at a yield of 100 gallons/dry ton. Estimates for the global annual supply of biomass feedstocks range from 100 to 400 EJ/year — equivalent to 6 to 24 billion dry tons. If converted to ethanol, this represents 120-460% of current global gasoline consumption (338 billion gallons/year).