Environmental Engineering Reference
In-Depth Information
12.7.2 Biofuels
The ecological impact of biomass fuels is even more controversial than the harmful
emissions for which they are responsible. Tractors and farm machinery emit carbon
dioxide just by being turned on. Added to this is the amount of energy required
to produce fertilizer and pesticides. Nitrogen fertilizers increase nitrous emissions,
which are harmful to the environment. Even the processing of biomass raw materials
to produce biofuels is energy-intensive. Huge quantities of carbon dioxide are
created if the energy needed comes from fossil energy sources.
The real diffi culty in evaluating biomass fuels and their ecological impact is illus-
trated by the example of biomass cultivation in tropical rainforest regions - a topic
that has recently been a target of criticism. When tropical rainforests are cleared to
cultivate biomass, considerable quantities of carbon dioxide are released through
the usual slash and burn methods. The subsequent cultivation of raw materials for
the extraction of biofuels then has a negative environmental impact for many years.
In other words, from the standpoint of the environment, it would have been better
to burn crude oil from the start.
On the other hand, the results of the production of bioethanol on existing farmland
in Brazil are better than in Germany or the USA. Factories in Brazil mostly burn
the sugarless residue of sugar cane and extract the energy from this for ethanol
production. As a result, ethanol production there is largely carbon-neutral. Other
countries use substantial quantities of fossil fuel to do the same thing. This can
totally cancel out the climate benefi ts of bioethanol.
Another critical point with biomass fuels is the limited development potential. If all
the available land in the world were devoted to growing biomass for biofuels, it
would still probably not be enough to allow biofuels to replace total oil require-
ments. The second-generation fuels would not totally invalidate this argument but
would at least defuse it. If one removes one's own energy needs for fuel production,
the net yield per hectare with BtL fuels is around three times higher than with
biodiesel (Figure 12.20). The land utilization of solar systems is clearly more
biogas
gross
net
BtL
bioethanol
biodiesel
vegetable
oil
0
1000
2000
3000
4000
5000
6000
litre diesel equivalent per hectare
Figure 12.20
Fuel yield per hectare for different biofuels. (1 hectare = 2.47 acres).
