biomethane match quite well with the analogous fossil fuels. One of the conditions
with biofuels is that the use of biomass as source of fuels must not conflict with its
use as food or feed. This point has been debated for long time and, nowadays,
second- and third-generation biomasses are used that do not conflict with food.
Nevertheless, the use of arable land for growing biomass for food/feed or energy is
still a point of argument. Marginal lands are today evaluated for growing non-food
biomass together with the use of non-drinkable water. Such marginal lands can be
either polluted soils not suited for growing crops because biomass can uptake
pollutants from soil and transfer them to the food chain, or poor soils
(low-carbon/nitrogen, arid) which would not guarantee an intensive production of
edible biomass and increase the cost. The efficient use of biomass is a must and the
recently developed biorefinery approach is the most valuable strategy [7].
Biofuels, such as bioethanol, long-chain hydrocarbons, biodiesel (esters of fatty
acids), and biomethane, are today used at different rates in various parts of our
planet. They are derived from a variety of biomasses, which have different production costs and impacts [8]. Biofuels are said “zero-emission” fuels, a belief
partly supported by Life Cycle Assessment (LCA), a useful environmental appraisal
methodology, which, nevertheless, is essentially static and needs to be adapted to
the dynamic nature of biosystems. LCA studies often suffer the definition of system
boundaries, the quality of available data, the completeness of data, the normalization of data, the attempt to use LCA in an absolute, instead of comparative,
mode. All biofuels are “not really zero-emission” because biofuels do not offset
combustion CO 2 emissions.
There are at least three key points that must be taken into due consideration for a
correct assessment of biofuels. A first key point is that biomass uses carbon existing
in land for growing and at the end of the day the change of soil from a baseline
should be taken into due consideration in assessing the carbon balance using bio
versus fossil fuels. In a wild environment, the carbon content of soil is kept almost
constant because dry-fallen-down biomass re-enters soil. In a managed land, harvesting biomass makes the soil depauperated of carbon. Therefore, the amount of
carbon in soil decreases and this amount is usually not considered in assessment
studies. In a sense, biofuels, as fossil fuels, even if at a reduced rate, are moving
carbon from soil to the atmosphere: fossil-C is taken from deep deposits and
biomass is made from surface carbon. A different methodology should be used for a
correct assessment of biofuels and the real contribution they can give to reducing
the amount of carbon transferred from soil to the atmosphere and the overall
increase of GHGs. A methodology that takes into account the amount of carbon in
soil, the so-called Annual Basis Carbon (ABC) [9], has been proposed which,
integrated with LCA, may give a more correct assessment of the potential of
biofuels in mitigating CO 2 emissions. This potential is also limited by the fact that
biomass for producing biofuels demands water: the water footprint of biomass is a
second key factor that is quite important [10]. Water footprint is divided into three
classes: blue water (freshwater), green water that evaporates from soil, and gray
water or processed wastewater [11]. In addition, we must consider that in order to
keep high the productivity of land, N-based fertilizers are used that emit N 2 O. In the
4.5 Quasi-zero-Carbon Emission Sources of Energy: Use of Biomass
53
with biofuels is that the use of biomass as source of fuels must not conflict with its
use as food or feed. This point has been debated for long time and, nowadays,
second- and third-generation biomasses are used that do not conflict with food.
Nevertheless, the use of arable land for growing biomass for food/feed or energy is
still a point of argument. Marginal lands are today evaluated for growing non-food
biomass together with the use of non-drinkable water. Such marginal lands can be
either polluted soils not suited for growing crops because biomass can uptake
pollutants from soil and transfer them to the food chain, or poor soils
(low-carbon/nitrogen, arid) which would not guarantee an intensive production of
edible biomass and increase the cost. The efficient use of biomass is a must and the
recently developed biorefinery approach is the most valuable strategy [7].
Biofuels, such as bioethanol, long-chain hydrocarbons, biodiesel (esters of fatty
acids), and biomethane, are today used at different rates in various parts of our
planet. They are derived from a variety of biomasses, which have different production costs and impacts [8]. Biofuels are said “zero-emission” fuels, a belief
partly supported by Life Cycle Assessment (LCA), a useful environmental appraisal
methodology, which, nevertheless, is essentially static and needs to be adapted to
the dynamic nature of biosystems. LCA studies often suffer the definition of system
boundaries, the quality of available data, the completeness of data, the normalization of data, the attempt to use LCA in an absolute, instead of comparative,
mode. All biofuels are “not really zero-emission” because biofuels do not offset
combustion CO 2 emissions.
There are at least three key points that must be taken into due consideration for a
correct assessment of biofuels. A first key point is that biomass uses carbon existing
in land for growing and at the end of the day the change of soil from a baseline
should be taken into due consideration in assessing the carbon balance using bio
versus fossil fuels. In a wild environment, the carbon content of soil is kept almost
constant because dry-fallen-down biomass re-enters soil. In a managed land, harvesting biomass makes the soil depauperated of carbon. Therefore, the amount of
carbon in soil decreases and this amount is usually not considered in assessment
studies. In a sense, biofuels, as fossil fuels, even if at a reduced rate, are moving
carbon from soil to the atmosphere: fossil-C is taken from deep deposits and
biomass is made from surface carbon. A different methodology should be used for a
correct assessment of biofuels and the real contribution they can give to reducing
the amount of carbon transferred from soil to the atmosphere and the overall
increase of GHGs. A methodology that takes into account the amount of carbon in
soil, the so-called Annual Basis Carbon (ABC) [9], has been proposed which,
integrated with LCA, may give a more correct assessment of the potential of
biofuels in mitigating CO 2 emissions. This potential is also limited by the fact that
biomass for producing biofuels demands water: the water footprint of biomass is a
second key factor that is quite important [10]. Water footprint is divided into three
classes: blue water (freshwater), green water that evaporates from soil, and gray
water or processed wastewater [11]. In addition, we must consider that in order to
keep high the productivity of land, N-based fertilizers are used that emit N 2 O. In the
4.5 Quasi-zero-Carbon Emission Sources of Energy: Use of Biomass
53