Waste Plastic as Energy Material: End-Of-Life Polymer Design-Juniper Publishers
JUNIPER
PUBLISHERS- ACADEMIC JOURNAL OF POLYMER SCIENCE
Abstract
Waste plastic poses a dire threat to life on earth
and addressing this crisis must be considered to be a priority in the
field of polymer science. In this piece, several approaches to waste
plastic handling are discussed, with their costs, benefits and potential
for sustainability briefly outlined.
Keywords: Waste plastic; Plastic pollution; Polymer; Environmental impact
Introduction
Virtually every eco-system on Earth is threatened by
plastic pollution, directly or indirectly [1-6]. The persistent toxic
nature of plastic once it has entered the environment means that plastic
pollution is, arguably, a more severe long-term environmental threat
than climate change. Polymer research must therefore consider
environmental impact as an urgent priority for ensuring the survival of
the polymer industry and, indeed, the human species.
It follows that polymer design philosophy must needs
be more focused on end-of-life usage. This question represents a
convergence of multiple fields of research, depending on which
end-of-life usage case is considered most prominent. Many of the
possible chemical process pathways for waste plastic are
under-researched and poorly understood, leaving a significant knowledge
gap in this area.
Discussion
The approaches to dealing with waste plastic can be
grouped into three main categories; accumulation, complete chemical
conversion, and circular economy. It is useful to classify processes
into these categories based on the eventual end-location of carbon
atoms. In complete chemical conversion, the carbon atoms contained
within plastic are converted into CO2, which is released into the
atmosphere. In waste accumulation processes, carbon atoms are kept out
of the atmosphere and biosphere by way of being retained in one form or
another of permanent storage. In the circular economy, waste plastic is
converted back into virgin polymer, generally via pyrolysis or
gasification processes.
Waste accumulation is perhaps best exemplified by the
use of landfills to store plastic waste, ostensibly indefinitely. This
is demonstrably a non-viable approach for two reasons; the first is that
no practical storage method is stable on anything like the
timeframe for bio-degradation of waste plastic and therefore storage
merely delays plastic’s entry into the biosphere. The second flaw is
that no economic value is generated by plastic in storage and there is
therefore no financial incentive involved aside from the simple need for
disposal. The two approaches that can be considered genuine solutions
to the problem of plastic pollution are complete chemical conversion,
and the circular economy. The circular economy has received a great deal
of attention as it theoretically meets the requirements for genuine
sustainability. It also offers a financial incentive for the collection
of plastic waste, proportional to the price of virgin polymer, making it
economically attractive once the capital cost for the associated
infrastructure has been met. It must be noted, however, that process
inefficiencies as well as fundamental thermodynamics that energy and
work must be added to this cycle in order to fully close the loop. It
follows that until and unless all that energy is obtained renewably, the
circular economy will not be entirely sustainable.
Complete chemical conversion is less attractive in a
more obvious way; it produces waste carbon, by definition. Moreover,
the most common chemical pathway for complete conversion is simple
incineration, which does not generate significant revenue as the
monetary value of low-grade heat is fairly low even in favorable
contexts. Higher-value process pathways do exist, however. In much the
same way that waste plastic can be converted into the raw materials for
polymerization, it can be utilized in a wide range of chemical
processes, all of which merit consideration in the drive to combat
plastic waste pollution. In the current context where the majority of
energy is produced from non-renewable sources, cyclic plastic processing
has to be considered to have a net positive carbon footprint when one
considers the energy input required for it and, similarly, one must also
take into account the carbon emissions that are offset when plastic
displaces another fossil fuel as an energy material.
Most forms of plastic possess higher energy content, on a
per-mass basis, than common fossil fuels such as coal. Moreover,
plastics possess lower fractional carbon content than coal.
Consequently, the carbon emissions of a chemical process utilizing
waste plastic will tend to be significantly lower than those of the
same process utilizing coal. When emissions off-set are included
in the determination of overall carbon footprint, complete
chemical conversion can be a net carbon-negative process
provided that it provides a suitable replacement for existing fossil
fuels. Meanwhile, if the carbon footprint of energy consumption
is factored in, circular cycling of plastic has a net positive carbon
footprint.
In the long term, a circular economy for plastic appears to
be the only genuinely sustainable way to continue to produce
and consume plastic products. In the short- to medium-term,
however, complete chemical conversion can be argued to offer
larger benefits in terms of net CO2 emissions and the associated
climate damage. With new high-value process pathways being
developed, such as the reduction of iron ore [7-9], complete
chemical conversion is also becoming increasingly economically
attractive.
It may well turn out that these two different end-of-life
scenarios have disparate implications for polymer design; a
polymer that is optimized for use as a reducing agent in a blast
furnace at end-of-life may be quite different from one that is
optimized for re-conversion into virgin polymer. It follows that
determining a pathway for the future of the polymer industry
is a significant multi-disciplinary task bringing together several
branches of chemical and metallurgical engineering as well as
environmental science.
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