Page 96 - Inpiantistica industriale

NEW TRENDS

Figure 1 - General process
of AD 2

BTS digesters can handle a wide range of • Truly Renewable Natural Gas
feedstocks and manage the co-digestion of The RNG from AD facilities is pipeline-quality and
multiple feedstocks. Nearly any biodegradable fully interchangeable with conventional natural gas
waste can be digested like packaged food wa- - it is the only renewable energy source that can
ste, food processing waste, and wastewater be used with existing natural gas-pipeline infra-
sludges that otherwise pose serious societal, structure. This is a crucial distinction of RNG, as it
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environmental, or financial challenges. enables both the political and economic sphere to
T he Digestion Process transition to a sustainable fuel base by leveraging
existing pipeline infrastructure. Like conventional
Organic feedstocks are fed into the AD faci- natural gas, RNG can be used as a transportation
lity, where they are broken down by various fuel in the form of compressed natural gas (CNG) or
microorganisms in a four-stage process. The liquefi ed natural gas (LNG). 5
purpose of this four-stage process (which in- The RNG present in the Biogas is extracted and
cludes hydrolysis, acidogenesis, acetogenesis, concentrated via a biogas upgrader process sepa-
and methanogenesis) is to convert the proteins, rating
carbohydrates, and lipids within the feedstock Carbon dioxide, water, Hydrogen Sulfi de and Parti-
into carbon dioxide, methane, and digestate. culate CO and Water can be re-cycled
2
The End Product This form of renewable natural gas RNG is ideal for
The resulting products of digesting feedstocks heavy-duty vehicles (HDV) due to its high energy
in an AD facility is biogas and digestate. Metha- density. HDVs require about fi ve-times more power
ne is the primary constituents of biogas, which than passenger vehicles to achieve viable opera-
can be refined into renewable natural gas (RNG) ting mileage.
by removing its non-methane components. 3 To supply this power, an HDV can either rely on
Digestate is heat treated to kill any dangerous fuels or batteries. Unfortunately, even the most
pathogens and can be used as an organic soil advanced batteries cannot compete as they have
amendment without further processing, or the a 30-to-50-times smaller energy density than RNG.
digestate’s value can be increased by drying For an average HDV, this equates to a battery size
it, removing certain nutrients, and pelletizing. of around 800-1000kWh to deliver 800km (500 mi-
les) of range. Even at battery prices approaching
$100/kWh , this represents a massive cost for each
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HDV. While the solar/battery industry is working on
these challenges, equally vital are weight and volu-
me considerations, and, at current energy densities
of state-of-the-art batteries , the necessary battery
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weight would be around 5,000-6,000kg, which de-
creases payload capacity. In fact, for equal ranges,
battery-powered trucks can carry ~40% less pay-
load than trucks powered by fuels . Thus, given the
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energy density weight, and payload requirements
of HDVs, energy-dense fuels (such as truly renew-
able natural gas) are currently a more optimal path
towards carbon neutrality than batteries

• Digestate

The second end-product of AD is digestate,
which is an organic soil amendment that con-
sists of broken-down organic matter. Digestate

92 92 Impiantistica Italiana - Maggio-Giugno 2021