Page 43 - Advances in Eco-Fuels for a Sustainable Environment

20 Advances in Eco-Fuels for a Sustainable Environment

Table 2.3 Second-generation biofuels and feedstocks

GHG
Fuel Feedstock Energy density (MJ/kg) CO 2 (kg/kg)
Cellulosic Wood, grass, – –
ethanol inedible parts of
plants
Algae-based Different fuels Can be used to produce any See specific
biofuels a made from algae fuel from Table 2.2, as well as fuels in
jet fuel Table 2.2
Biohydrogen Algae breaking 123 b None
down water
Methanol c Inedible plant 19.7 1.37
matter
Dimethylfuran d Fructose from 33.7 –
fruits and
vegetables
Fischer- Paper & pulp 37.8 2.85
Tropsch manufacturing
biodiesel waste

a
More expensive at present, may yield 10–100 more fuel per unit surface area in future.
b
When compressed to 700atm.
c
More toxic and less energy dense than ethanol.
d
Energy density close to gasoline. Toxic to respiratory tract and nervous system.
rapeseed also showed high yields initially. However the need for cropland instead of mar-
ginal land for their attainment, and substantial rate drops when marginal land was used,
diminished strongly the industrial interest in these species [12].
Waste vegetable oil (WVO), used as fuel for more than a century. Some of the earliest diesel
-
engines ran on vegetable oil (hempseed oil). WVO is considered a second-generation biofuel
with an extremely low environmental impact and is easily converted to low-sulfur biodiesel (it
could be burned as such in some older engines). It is considered one of the best sources of bio-
diesel and could, if proper collection systems were in place, meet much of the European
biodiesel demand.
Municipal solid waste (MSW) including landfill gas, human waste (excess sludge from waste-
-
water processes), and grass and yard clippings, which are, in many cases, simply disposed of
as waste.
These feedstocks’ carbon footprint is much less than traditional fossil fuels, and they
have some considerable advantages: grasses are usually perennial and fast growing on
marginal land with little fertilizer needs, so they can be harvested often with high net
energy yields (up to 540%) although they can only be used for bioethanol production.
Third-generation biofuel has only recently entered mainstream terminology, refer-
ring to algae-derived fuels, which previously were lumped in with the second-
generation. Algae and microalgae, such as land crops, can be used for direct energy
production by lipid extraction, or as feedstock for other biofuel production via fermen-
tation processes. When it became apparent that algae could be capable of much higher
and advantageous yields than other feedstock (they can grow much faster than crops),
it was suggested that they be “promoted” to their own class.