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NEW TRENDS

The carbon cycle usage of carbon negative fuels is crucial to not only
transition away from fossil fuels, but also prevent
of the ad end products further global warming from harmful GHGs.
and the negative GHG
End Product 2 - Digestate

End Product 1 - Fuel The other end-product of the AD process is an
organic digestate, created from broken-down
Carbon neutral fuels, by definition, do not add net material from the anaerobic digestion of biode-
carbon dioxide emissions to the atmosphere. gradable feedstock.
RNG is carbon neutral because, when burned as All of the nutrients contained with the feedstock
a fuel, releases carbon dioxide that was not trap- of AD, including nitrogen, phosphorus, and po-
ped underground in fossil fuel reserves, but rather tassium (collectively known as NPK), are embo-
was originally assimilated by plants from the atmo- died within digestate, so it can be used as a soil
sphere. Thus, burning RNG simply releases the amendment in agriculture, landscaping, storm
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carbon dioxide that plants naturally release when water management and consumer-based
they respirate, die, and decay, and no new carbon horticulture.
is added into the atmosphere. By using digestate, we can effectively reduce our
The organic feedstocks used in the AD process, dependence on synthetic fertilizers, which relies
would naturally release carbon dioxide as they de- on the Haber-Bosch process to fix nitrogen. The
cay. Burning fossil fuels rapidly releases carbon Haber-Bosch process fixes most of the world’s ni-
dioxide that would have otherwise been trapped trogen and accounts for 1-2% of the world’s total
underground, while burning RNG releases carbon energy consumption and 3-5% of the world’s total
dioxide recently taken up by plants that would na- natural gas consumption. According to the Food
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turally be released from organic matter. The bur- and Agricultural Organization (FAO) of the United
ning of fossil fuels thereby disturbs the slow carbon Nations, emissions from synthetic fertilizers grew
cycle and rapidly increases atmospheric carbon at an average rate of 3.9% annually from 1961 to
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dioxide concentrations. Timing is key here: Since 2010, with absolute values increasing more than
the burning of fossil fuels releases carbon dioxide nine-fold, from 0.07 to 0.68 Gt CO equivalents an-
2
at a much more rapid pace than the natural carbon nually. By reducing our usage of these synthetic
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cycle, these carbon emissions cannot be absorbed fertilizers, and instead using digestate, we are not
by carbon sinks (e.g., ocean, soil, plants) at an only curtailing GHG emissions, but also reducing
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equivalent rate. And thus, the rate of carbon input our dependence on conventional natural gas.
is faster than the rate of absorption, and the bur- Moreover, introducing digestate into the soil is an
ning of fossil fuels results in a net increase of atmo- important part of the carbon sequestration mecha-
spheric greenhouse gas emissions. nism as the carbon within the feedstock of AD is
Additionally, when the source of biogas is organic embodied in the digestate. Thus, adding digestate
waste and diverted from landfills, the biogas is de- to soil effectively puts the carbon within plants back
emed carbon negative. The rationale behind this is into the soil, which closes the carbon cycle.
predicated on the Global Warming Potential (GWP)
of each greenhouse gas. 13
The impact on global warming of each GHG de- From waste to clean energy
pends on its intrinsic ability to absorb heat. In this
regard, methane emissions have a cumulative ca- Food Waste
pacity for heat absorption 28 times greater (over The Food waste has steadily increased in the
100 years) than that of CO . Due to the different many counties with the majority of all food wa-
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2
GWP potentials of the greenhouse gases, their ste going into landfills.
emissions are reported in CO -equivalents (e.g.,
2
1kg of methane is equivalent to 28 kg CO -eq AD is not only producing renewable fuel, but it
2
emissions). is also diverting emissions from food waste that
When organic waste is disposed of in landfills, it would have been sent to landfills. The Food and
degrades and emits methane; whereas, when the- Agricultural Organization (FAO) of the United Na-
se same organics are naturally converted into bio- tions quantified that global food waste produces
gas as a fuel and combusted, it emits CO . When around 4.4 GtCO annually, or around 8% of total
2
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organic waste is diverted from landfills and used anthropogenic greenhouse gas emissions. If food
to produce biogas, landfill methane emissions are waste were its own country, it would thereby be
prevented, and CO is emitted instead of methane. the third largest emitting country in the world, after
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Since methane emissions are prevented and sin- China and the United States (see Figure 3).
ce CO has a 28-times lower GWP than metha-
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ne, biogas reduces atmospheric greenhouse gas Removing food waste from landfills we remove the
emissions and is, thereby, considered a carbon methane emissions. Considering that methane,
negative fuel. compared to CO , has a 28x higher global warming
2
Biogas is one of the few existing energy sources potential, reducing methane from landfills can gre-
that can claim the title of carbon-negativity. The atly abate the adverse effects of climate change.
94 94 Impiantistica Italiana - Maggio-Giugno 2021