A System to Convert Waste Plastic into Clean Energy
Design Team
Anna Craver, Katherine Dixon, Chris Flanagan,
Brittne Lynn, Mason Riley
Design Advisor
Prof. Yiannis Levendis
Email:[email protected]
Abstract
The 20th century will forever be known as the Petroleum Age. However, the increasing
need for energy and depleting values of fossil fuels necessitates a jump start to a new era.
This design, planted on the firm foundation that is the research of Professor Yiannis
Levendis, will function as a stepping stone into the much needed “iEnergy” Age. Why
waste plastics? Three reasons: 1) Waste plastics are capable of generating energy
comparable to that of gasoline, 2) Plastics pollution is an ever increasing concern for
marine environments and landfills at the very least, and 3) Plastics degrade over time in
the recycling process (meaning that the grade of plastic decreases each time it is recycled
until it is no longer valuable). This design has been procured to extract the maximum
amount of thermal energy from waste plastics, while not negatively impacting the
environment by releasing toxic chemicals into the air (what commonly happens when
solid plastic is ignited).
Waste plastic
inserted into system
Compression
Chamber
Exhaust gasified plastic
to be harnessed as fuel
Liquefied plastic gasifying in
pyrolysis chamber
Waste plastic
liquefied
Energy added to system (eventually
harnessed from exhaust gasified plastic)
The Need for the Project
Waste plastics are highly
sensible to use as an alternative
form of power generation
because there are limited uses for
recycled plastic and they possess
approximately 44 MJ/kg of
energy, which is comparable to
that of gasoline.
With traditional energy sources depleting rapidly by the minute, it
has become increasingly evident that other, less traditional energy
methods must gain momentum to balance the needs of modern society.
In tandem with this comes the necessity to take preventive and
regenerative actions on the environment itself. Waste plastics is not
only a major solid pollutant to the environment, but also has a large
amount of energy that can be extracted with the right tools. With that
in mind, managing waste plastics can not only help clean up the
environment, but also supplement the increasing demand for energy all
over the globe.
The Design Project Objectives and Requirements
The objective of this design is to
Design Objectives
create a system that will gasify
The primary design objective is to create a continuously fed system
plastic, mix this gas with air and
that will convert waste plastic into energy through a sequential process
finally combust the mixture in
in a clean and efficient manner. This system must gasify the plastic in
order to produce thermal energy.
the absence of oxygen so that the plastic will not combust prematurely.
Then the gasified plastic must be mixed with air, to finally combust the
gasified plastic to produce thermal energy. A flow chart of this process
is shown to the left for reference.
Design Requirements
For the prototype model, the system must efficiently convert at least 1
gram of plastic per minute to thermal energy. Two major challenges
were to control the gasification of the plastic without premature
combustion, and dealing with temperatures from 273K to in excess of
1273K inside the system. A thermal analysis is shown to the left as
reference.
Design Concepts considered
We considered two options for
The project was comprised of two major sub-systems, the feeder, and
both of the subsystems.
the pyrolysis chamber. For the feeder sub-system, the design came in
the form of a chamber with liquefied plastic to maintain a seal. To feed
that chamber, there could be either cartridges that would lock in place,
and dispense additional plastic. Alternatively, to add plastic to the
melting chamber, the concept of using a chamber with sealed doors on
the top and bottom will act as an air-lock, allowing the feeder to be
purged before releasing the plastic into the melting chamber. For the
doors, various mechanisms were considered, specifically, butterfly
valves, trap doors, sliding plates, and rotating chambers. For the
pyrolyzer system, a proposed design was to use a vertically stacked set
up. The chambers were to be placed within and on top of each other.
Essentially, the pyrolysis chamber would be seated within the
combustion chamber. This set up maximizes the amount of heat
transferred between the various levels. On the outer most layer, the
combustion chamber was to be wrapped with external insulation. An
alternative set up was a side-by-side design, using heat exchangers to
transfer the heat between the chambers, again, insulating the external
walls of the chamber.
Recommended Design Concept
Our finalized design concept uses
In the recommended design, the most beneficial set-up for the system
a compartmentalized design, with
is a compartmentalized design with stacked chambers seen in the
stacked chambers to facilitate heat
figure on the left (refer to page 1 for chamber labels). The chambers
and pressure flow through the
are stacked so the natural flow of plastic waste is from top to bottom:
system.
compression chamber to liquefaction chamber to pyrolysis chamber.
The final feeding idea chosen for the preliminary design uses
liquefied plastic in the liquefaction chamber. Not only will the liquid
create a seal between the pyrolysis chamber and the atmosphere air,
but it also allows for continuous flow into the pyrolysis chamber. The
liquid pool of plastic will be pressurized by nitrogen in a capped
chamber that will force the liquid into the pyrolysis chamber, while
the pyrolysis chamber is pressurized to a slightly smaller degree, in an
attempt to control flow of the system. This pressure gradient will
ensure the correct flow of the system as a whole.
The compression chamber is one key component in keeping a
continuous flow. The top of this compression chamber has an air tight
valve that one can open to insert solid plastic that will then enter the
system. The valve is resealed, and the initial chamber is pressurized
to the same pressure as the liquefaction chamber. The chamber is to
be purged then pressurized with nitrogen to rid the chamber of
oxygen.
The recommended pressure gates at the top of the
compression chamber and between the liquefaction chamber and
compression chamber are butterfly valves. The floor of the
compression chamber flips and drops all material into liquefaction
chamber to melt and to help continuously feed the system.
The bottom chamber is the pyrolysis chamber, where the plastic
is gasified. This is final step is the most critical, where the gasified
plastics will leave this chamber ready to combust and generate the
energy desired. The chamber is heated by a source beneath it which is
currently an external source to better control the heat generation rate.
To determine the estimated temperature ranges, a thermal flow
analysis and a mathematical model were generated and compared.
These two methods made it possible to determine the required
dimensions of the product based on the desired power output.
To test the system, three different stages were designed to test
each chamber separately, building from the liquefaction chamber
down. The first stage of testing was used to prove that plastic can be
liquefied and to determine a melting temperature range for the
different plastics in use. Pyrex glasses and a 290 Watt heater were
used to create liquefied plastic. The second stage of the testing used a
steel crucible will small holes (a scaled down version of the
liquefaction chamber), wrapped with a high power heat wrap, and the
compression chamber to determine the mass flow rates of the plastics.
It was found that Polyethylene powder flows through one hole .125”
in diameter at an average rate of 1 g/min. The third stage of the
testing includes testing the pyrolysis chamber (see in the figure
above). This stage of the test includes the crucible set inside the
pyrolysis chamber with the feeder on top.
Financial Issues
The total cost for the prototype
and testing materials was $661.55
dollars.
.
The main financial issues in manufacturing the prototype would be
due to the machining cost of the liquefaction chamber. The prototype
price of 661.55 dollars does not include machining costs since all
parts were made by design group members. If the system were to be
scaled up, safety materials pertaining to the high temperatures of the
combustion chamber would also be of financial concern.
Recommended Improvements
Some recommended
improvements are to better
harness the current exhaust gasses
as fuel for sustaining the system
as well as increasing system
insulation.
The next major step is to bring the exhaust gasses from the pyrolysis
chamber back to the system by finishing the design of the combustion
chamber, so that the system is completely self-sustaining. Another
major improvement would be to create a more robust insulation
system to reduce losses to the outside environment.