Environmental Engineering Reference
In-Depth Information
Box 9.3
Limits on Ethanol Production from Sugarcane
Sugarcane (
Saccharum ofi cinalis
) is a perennial tropical grass whose average global
yield is now about 65 t/ha (FAO 2011d). In the wet tropics it does not need any irriga-
tion; the best Brazilian cultivars do not need any nitrogen fertilizer (or only minimal
supplementation), thanks to endophytic N-i xing bacteria in stems and leaves; and the
ethanol production does not require any external fuel as it can be entirely fueled by
the combustion of bagasse, the i brous residue that remains after expressing the juice
from cane stalks. Sucrose is about 12% of the harvested crop (or 7 t/ha), and even if
it were converted to ethanol as efi ciently as it is now in Brazil—producing 82 L/t
(Bressan and Contini 2007)—1 ha of cane would yield less than 5,500 L of ethanol.
Some 23 Mha were planted to sugarcane in all tropical and subtropical countries
in 2010, and even if all that land produced cane for ethanol, the annual fuel yield
would be about 2.6 EJ, equal to about 7% of the world's 2005 gasoline consumption
of roughly 37 EJ. In order to supply the global gasoline demand, the entire demand
cane would have to be planted on some 320 Mha, or 20% of the world's arable land.
But high-yielding sugar cane can be grown only in the tropics, and this means that
about 60% of all the farmland that is now under cultivation in that region would have
to be devoted to cane for ethanol, clearly an impossibly high claim.
arable land shortage makes impossible (or quite irrational) in China, India, or Indo-
nesia. There is no need for academic debates regarding that choice: as Jean Ziegler,
the UN Special Rapporteur on the Right to Food, put it, “It is a crime against
humanity to convert agriculturally productive soil into soil which produces food-
stuffs that will be burned as biofuel” (Ferrett 2007). Other crop-based options,
ranging from biodiesel made from rapeseed to the exploitation of tropical species,
including oil palm and jatropha, offer no overall advantages compared to corn or
sugarcane, and that is why the future of liquid biofuels is now seen to depend on
the success of cellulosic ethanol, on enzymatic conversion of cellulose in crop resi-
dues and wood.
Availability, performance, and cost of the requisite enzymes and problems with
scaling up bench experiments to large continuous or batch operation producing
billions of liters a year are the most obvious challenges. But is should be also remem-
bered that feedstock supplies are neither as abundant nor as easily harvested as is
often assumed. Corn stover, America's most abundant crop residue, illustrates some
of these challenges. Different assumptions regarding stover:grain ratios, high average
moisture content, and recycling requirements have resulted in estimates of annual
U.S. stover harvests as low as 64 Mt and as high as 153 Mt of dry matter (Kadam
and McMillan 2003).
