Page 54 - Materials Chemistry, Second Edition

40 N. E. Korres

2.1 Ethanol Production from Starch and Sugar Crops

Efforts, worldwide, to replace conventional fuels with biofuels can be seen by the
high growth in ethanol and biogas during the past decade. According to the US
Energy Information Administration (EIA 2013) total world liquid biofuels pro-
duction increased almost sixfold over the period 2000–2010, more specific from
315,000 to 1,856,000 barrels per day. Ethanol (from Zea mays or maize) has been
the leading biofuel in the Unites States and in Brazil (from Saccharum officinarum
or sugarcane), (Moschini et al. 2012) whereas biodiesel (from Brassica napus or
rapeseed) is by far the leading biofuel in EU (Korres et al. 2011).
The superiority of maize as main ethanol feedstock stems mainly from its
advantage over other feedstocks in economic efficiency of conversion into ethanol
(i.e., fuel yield of maize for e.g., is higher than that of barley and sorghum) (Board
2009). Dry milling and wet milling are the two processes for (first-generation)
ethanol production from maize with the former being the most common. The
coproducts from the conversion of maize to ethanol are known as distillers dried
grains (DDGs) and maize oil from dry and wet milling, respectively, that can serve
as a portion of livestock feed rations (Aines et al. 1986—cited in Board 2009).
Grain sorghum (Sorghum bicolor) is also used as feedstock for the production of
bioethanol (NSP 2012) and produces roughly the same amount of ethanol as maize
although the crop’s yield per unit area is lower than that of maize. Sorghum also
produces DDGs and is completely interchangeable with maize in the ethanol
production process (NSP 2008). Barley (Hordeum vulgare) is also being used in
three US ethanol plants (RFA 2008). Research on hulless barley varieties as a
potential feedstock to increase ethanol output in comparison to conventional barley
varieties hence making this feedstock more attractive is under process (Board
2009).
Crops with high sugar content (i.e., sugarcane and Beta vulgaris or sugar beet)
are easier to process into ethanol than starch crops since the sugar required by
fermentation is already present. The fermenting and distilling technology for
ethanol production from these crops is not much different than that used in
breweries (Board 2009). One ton of sugarcane produces about 19.3 gallons of
ethanol, a greater ethanol output per acre compared to maize. Sugar beet shows a
great potential for ethanol production which with current conversion technologies
yields an ethanol output per unit area close to that of sugarcane. Nevertheless,
sugar beet at present is a high-cost input for biofuel production and is not used for
that purpose (Salassi and Fairbanks 2006). Sweet sorghum (Sorghum spp.), which
contains carbohydrates in fractions of both sugar and starch, is another feedstock
candidate (Lau et al. 2006) as well as energy cane, a breed of sugarcane that
produces high amounts of sugar and stalk for ethanol conversion. Nevertheless,
despite the high ethanol yields from first-generation bioethanol (Fig. 4), sustain-
ability criteria, environmental and economic concerns diverse bioenergy market
toward second-generation bioethanol, i.e., bioethanol; production from lignocel-
lulosic materials.