Grasses are very abundant forms of biomass (U. S. Dept, of Agriculture, 1948). About 400 genera and 6000 species are distributed all over the world and grow in all land habitats capable of supporting higher forms of plant life. Grasslands cover over one-half the continental United States, and about two — thirds of this land is privately owned. Grass, as a family (Gramineae), includes the great fruit crops, wheat, rice, corn, sugarcane, sorghum, millet, barley, and oats. Grass also includes the many species of sod crops that provide forage or pasturage for all types of farm animals. In the concept of grassland agriculture, grass also includes grass-related species such as the legumes family—the clo­vers, alfalfas, and many others. Grasses are grown as farm crops, for decorative purposes, for preserving the balance of productive capacity of lands by crop rotation, for controlling erosion on sloping lands, for the protection of water sheds, and for the stabilization of arid areas. Many advances in grassland agriculture have been made since the 1940s through breeding and the use of improved species of grass, alone or in seeding mixtures; cultural practices, including amending the soil to promote herbage growth best suited for specific purposes; and the adoption of better harvesting and storage techniques. Until the mid-1980s, very little of this effort had been directed to energy applications. A few examples of energy applications of grasses can be found such as the combustion of bagasse for steam and electric power, but many other opportuni­ties exist that have not been developed.

Perennial grasses have been suggested as candidate feedstocks for conversion to synfuels. Most perennial grasses can be grown vegetatively, and they reestab-

lish themselves rapidly after harvesting. Also, more than one harvest can usually be obtained per year. The warm-season grasses are preferred over the cool-season grasses because their growth increases rather than declines as the temperature rises to its maximum in the summer months. In certain areas, rainfall is adequate to permit harvesting every 3 to 4 weeks from late February into November, and yields between about 18 and 24 t/ha-year of dry grasses may be obtainable in managed grasslands. Some tropical and semitropical grasses are very productive and can yield as high as 50 to 60 t/ha-year on good sites (Westlake, 1963). The tropical fodder grass Digitaria decumbens has been grown at yields of organic matter as high as 85 t/ha-year (Westlake, 1963). Table 4.11 lists some promising grasses that have been proposed as energy resources in the United States (Cushman and Turhollow, 1991).

An example of a tropical grass that has been grown commercially as a combination foodstuff and fuel crop for many years is sugarcane (Saccharum

“Cushman and Turhollow (1991). bAveraged by site; data are for range of sites.

Thick-stem grass.

Thin-stem grass.

“Productivity rates after 1- to 2-year establishment period. Productivity rates after 1- to 2-year extablishment period.

spp.), but rising production costs, alternative sweeteners, and the nebulous mixture of changing social, political, and agricultural policy issues have not been kind to insular sugar planters (Alexander, 1993). A great deal of informa­tion has been accumulated about sugarcane, and it might well be used as a model tropical herbaceous crop for other biomass energy systems. It grows rapidly and produces high yields; the fibrous bagasse is used as boiler fuel for the generation of electric power; and sugar-derived ethanol is used as a motor fuel in gasoline blends (gasohol). Sugarcane plantations and the associated sugar processing and ethanol plants are in reality biomass fuel farms. About one-half of the organic material in sugarcane is sugar and the other half is fiber. Total cane biomass yields have been reported to range as high as 80 to 85 dry t/ha-year. Normal cultivation provides yields of about 50 to 59 dry t/ha-yr (Westlake, 1963). Studies on sugarcane managed specifically as an energy crop have been underway in Puerto Rico for several years. “First- generation energy cane” consisting of conventional varieties managed for opti­mal growth with irrigation averaged 186 green t/ha-year of whole cane includ­ing detached trash, whereas second generation yields exceeded 269 green t/ha-year (Alexander, 1983). At an average of 40 wt % dry matter, these yields range from 74 to 108 dry t/ha-year. Presuming the energy content of energy cane is about 18.5 GJ/dry t, the energy yields correspond to 1369 to 1998 GJ/ ha-year, or the equivalent of 232 to 338 BOE/ha-year, a very high yield.

In moderate climates, switchgrass (Panicum virgatum) has been recom­mended as one of the model biomass energy crops for North America because of its high yield potential, adaptation to marginal sites, and tolerance to water and nutrient limitations (Sanderson et ah, 1995). It is a warm-season grass native to much of North America and is a major species in tall prairie grasses. Average yields are reported to range from 5.5 to 11.3 dry t/ha-year in the midwestern and eastern United States (Wright, 1994). In the southwestern United States, evaluation of eight switchgrass cultivars showed that for six locations in Texas, single harvests of fertilized plots of the Alamo cultivar afforded the highest aver­age yields, 10.7 to 15.7 dry t/ha-year (Sanderson et ah, 1995).

Other productive grasses that have been given serious consideration as raw materials for the production of energy and synfuels include the perennials Reed canary grass, tall fescue, crested wheatgrass, weeping lovegrass, and Bermuda grass, the annual sorghum and its hybrids, and others. It is apparent that there are many grasses and related biomass species that can be considered for energy applications. They have many of the desirable characteristics needed for terrestrial biomass energy systems.