Page 65 - Advances in Eco-Fuels for a Sustainable Environment

42 Advances in Eco-Fuels for a Sustainable Environment

but are not available on a large-enough scale to meet current fuel demands. Issues that
ought to be taken into consideration in this respect are:
l Water use: the less a crop uses, the better because water is a very limited resource. This is
particularly important in arid places; water use could be mitigated by using crops that do not
require irrigation, or by growing feedstock, where possible, with untreated wastewater (that
will also provide nutrients to the crops).
l Invasiveness: a crop that is difficult to control and may kill native plants is not a good choice.
Biodiversity and the surrounding ecosystem may be severely damaged.
l Fertilizer needs: nutrients are needed for plants to grow. Producing fertilizers from nutrient
elements (from the atmosphere or phosphate rock) is energetically expensive. Some plants
are more frugal with scarce resources than others.
l Limitations: some areas just are not suitable for growing biofuel crops. Alaska and the
Sahara, for instance, are not suited to the rapid, continuous growth required from crops
for fuel supplies, for different reasons. Energy independence will not be possible everywhere
and transportation will be necessary, increasing the biofuel footprint.
It is often claimed that biofuels burn cleaner than fossil ones, resulting in lower GHGs,
particulate matter, and substances causing acid rain emissions. Because alcohols con-
tain very little sulfur, there is practically no SO 2 produced when they burn, and there-
fore almost sulfuric acid. This may not be completely true for some biofuels. When
ethanol is burned, the ideal reaction looks like:

C 2 H 5 OH + 3O 2 ! 2CO 2 +3H 2 O (2.6)
While with butanol the reaction is written as:

2CH 4 H 9 OH + 5O 2 ! 8CO 2 + 10H 2 O (2.7)

From these, it would seem that only CO 2 and water are produced from combustion.
However, alcohols are not exactly “clean-burning” fuels: like any fuel derived from
recently living matter, they also contain nitrogen, producing nitric oxide and other
nitrogen compounds when burned. This, in some cases, not only could offset savings
from not producing sulfur compounds, but could actually worsen long-term trends for
acid rain. Inefficient combustion of alcohol may produce carbon monoxide, formal-
dehyde, ammonia, benzene, and other toxic chemicals. In practice, burning ethanol
produces less CO as the fuel itself supplies some extra oxygen, but the benefits in
terms of CO are lower with butanol because of its lower ratio of oxygen to carbon:
in ethanol, the ratio is one oxygen molecule to two carbon molecules, in butanol, 1:4.
The combustion of a kilogram of gasoline yields approximately 33MJ of energy
and 3.09kg of CO 2 while combustion of one kilogram of ethanol yields 20MJ and
1.91kg CO 2 . By equalizing the ratios, it can be seen that 1.64kg of ethanol must
be burned to get an equivalent amount of energy as gasoline, which brings us up to
3.14kg CO 2 for the equivalent emission. Butanol, on the other hand, produces about
2.37kg CO 2 /kg; as it yields about 29MJ/kg, only about 1.14kg are needed to get the
same energy content of gasoline. Thus, at equal energy content, ethanol combustion
causes more carbon dioxide than gasoline while butanol produces only 2.7kg CO 2 ,
which constitutes an environmental advantage over both gasoline and ethanol.