We Have Met the Future and It Is Green - Sustainable Design: The Science of Sustainability and Green Engineering - page 310

Kamaal suggested switchgrass ( Panicum virgatum ) as a biofuel to replace coal.
Switchgrass, a perennial warm-season grass with coarse stems, has a heating value
of 18.3 GJ/ton, with lower percentages of ash and sulfur than those in coal.
Although her analysis suggested that switchgrass contains only about half of the
heating capacity of coal, other economic analyses could prove this comparison to
be incomplete. In fact, switchgrass compares favorably to other biofuel feedstocks
(see Table 8.1). A study of recent facility management reports noted that the price

Table 8.1 Comparison of Physicochemical Properties of Switchgrass ( Panicum virgatum )asa
Biofuel Feedstock Relative to Selected Alternative Fuels a

Alternative Fuel

Switchgrass

Fuel Property Value Value Fuel Type

Energy content (dry) (MBtu Mg − 1 ) 17.4 18.6 Wood

26.0 Coal

Moisture content (harvest) (%) 15 45 Poplar

Energy density (harvest) (MBtu Mg − 1 ) 14.8 10.2 Poplar

Net energy recovery (MBtu Mg − 1 ) 17.0 16.4 Poplar

Storage density [kg m − 3 (dry weight)] 150 Poplar chips

(6 × 5 ft) round bale 133

(4 × 5) ft round bale 105

Hopped 108

Hollocellulose (%) 54-67 49-66 Poplar

Ethanol recovery (L kg − 1 )

280 205 Poplar

Combustion ash (%) 4.5-5.8 1.6 Poplar

Ash fusion temperature ( ◦ C)

1016 1350 Poplar

1287 Coal

Sulfur content (%) 0.12 0.03 Wood

1.8 Coal

Source: S. B. McLaughlin, R. Samson, D. Bransby, and A. Wiselogel “Evaluating physical, chemical, and
energetic properties of perennial grasses as biofuels,” Proceedings of the 7th National Bioenergy Conference:
Partnerships to Develop and Apply Biomass Technologies , Nashville, TN, September 15-20, 1996.
a Energy content of switchgrass was determined from six samples from Iowa. Bale density and chopped density
of switchgrass are from Alabama (Bransby, Auburn). Poplar chip density is from studies of White et al. (M. S.
White, M. C. Vodak, and D. C. Cupp, “Effect of surface compaction on the moisture content of piled green
hardwood chips,” Forest Products Journal , 34, 59-60, 1984). Poplar energy moisture content, combustion ash,
and ash fusion temperatures are from NREL, as are the ash fusion temperatures and sulfur contents of all fuels.
Energy density is the energy per unit of wet harvest weight. Net energy recovery considers energy lost in
drying fuel prior to combustion. The holocellulose content of switchgrass is from seven varieties in Alabama
S. E. Sladden and D. I. Bransby, “Improved conversion of herbaceous biomass to biofuels: potential for
modification of key plant characteristics,” Technical Report ORNL/Sub/88-SC011/1, Oak Ridge National
Laboratory, Oak Ridge, TN, 1989) and from seven hybrid poplar vareties in Pennsylvania T. W. Bowersox,
P. R. Blankenhorn, and W. K. Murphey, “Heat of combustion, ash content, nutrient content, and chemical
content of Populus hybrids,” Wood Science , 11, 257-262, 1979). Ethanol yields are averages of simultaneous
saccharification and fermentation recovery on three analyses per species using a standard recovery procedure
for all feedstocks. Ethanol yields can probably be improved somewhat by tailoring reaction mixtures to each
specific feedstock; thus, those should be considered preliminary measures of potential recovery.