Issue 06/2021

Clean-up ships fuelled
by garbage
Millions of tonnes of synthetic plastics are released to
the environment each year. Of this, a fraction ends
up in one of several oceanic gyres, natural locations
where the currents tend to accumulate floating debris –
including plastics. The largest and best known of these is
the Great Pacific Garbage Patch (GPGP), which is estimated
to cover an area roughly the size of the state of Texas (or
France), and which seems to be increasing in size over time.
Removing this plastic from the oceanic gyres has promise
to return the ocean to a more pristine state and alleviate the
associated burden on wildlife and the food chain. Current
methods to remove this plastic use a boom system to
concentrate the plastic and a ship to harvest it and return to
port to unload the plastic cargo and refuel [1].
Plastic is a natural energy carrier, which suggests the
question: is there enough energy embodied in the plastic to
power the ship and eliminate or reduce the need to return to
port? If so, then can a process be devised to convert plastic
into a form of fuel appropriate for modern diesel engines
that are used to power ships?
Thermodynamic analysis of the energy available in plastics
answered the first question –
yes, there is enough energy in
the ocean plastics, provided that
they are first concentrated using A
booms and that the ship is small
and efficient enough to minimize
its fuel consumption.
The next question was
answered by designing a
process to convert plastics into
a liquid fuel precursor. The most
important step of the process is a
high-temperature reaction called
hydrothermal liquefaction or
HTL. HTL depolymerizes plastics
at high temperature (300–550 °C)
and high pressure (250–300
bar), thereby converting it into a
liquid form. Oil yields from HTL
are typically >90 % even in the
absence of catalysts and, unlike
pyrolysis, yields of solid byproducts
– which would need to
be stored or burned in a special
combustor – are less than 5, thus
conferring certain comparative
advantages to HTL.
Current data on the GPGP
indicates that it contains mainly
polyethylene and polypropylene,
a mixture that is especially
Great Pacific
Garbage Patch
C
~ 1900 km
San Francisco Port
Current with Plastic
Boom 600 m
appropriate for HTL. By-products include a gas that might
be used as a cooking fuel; a solid that could be burned on
board or stored; and process water that is cleaned prior to
release. Further analysis indicated that the use of plasticderived
fuels could reduce fuel consumption, and effectively
eliminate fossil fuel use. The HTL derived fuel could be
termed blue diesel, to reference its marine origin and in
contrast with both traditional marine diesel and green
diesel, derived from land-based renewable resources. The
full feasibility study is available for free online (link below
[1]). Future work will construct the process and test it at
pilot-scale for realistic feeds, to ultimately transition to
shipboard use. AT
[1] Belden, E.R.; Kazantzis, N.K.; Reddy,C.M.; Kite-Powell, H; Timko,M.T.;
Italiani, E.; Herschbach D.R.: Thermodynamic feasibility of shipboard
conversion of marine plastics to blue diesel for self-powered ocean
cleanup; https://doi.org/10.1073/pnas.2107250118
www.wpi.edu
California
0.5 knots
B
D
Current 14 cm s -1
Boom Array System
Reactor
15 knots
Overview of the process for plastic removal out of the GPGP showing (A) the total system overview,
(B) part of the system of collection booms, (C) a single collection boom, and (D) the HTL reactor.
Science and Research
San Francisco Port
Current bioplastics 14 cm s MAGAZINE [06/21] Vol. 16 15
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