Paper written for U of M ESPM 3605 - Recycling: Extending Raw Materials
Abstract
Pyrolysis of waste plastic is a complex topic that has many
aspects. In this paper we explore the
mechanisms available to implement commercial-scale pyrolysis plants that may be
profitable and effective in reducing the environmental impact of plastic
waste. We start by presenting a flow for
such a plant, and identifying tasks that are problematic in the current state
of the art. Then, using recent research
papers, we identify choices that can be made in the processing to reduce cost
and improve the quality or usefulness of the output.
Introduction
Recently there have been two review articles (5, 6) that
discuss the current state of the art in pyrolysis of plastic. Pyrolysis is one way of handling the massive
problem of waste plastic which is filling our landfills and getting into our
oceans, lakes and rivers (5, 6). Synthesizing those articles and primary current
research, we identify a process flow for a canonical pyrolysis plant and
identify areas of that process that can be modified based on current research.
Basic outline of the plant and processes
1. Sources
of waste plastic.
·
Plastic manufacturing plants
·
Municipal recycling facilities
·
Agricultural farms
·
Wire recycling facilities
These sources vary greatly in the kind of plastic available,
the cleanliness of the plastic, the quantity available and the distance the
material needs to be transported.
2. Cleaning, if needed, to some standard of cleanliness.
3. Separation of the plastic into those acceptable to the
pyrolysis process being used and those not acceptable. It is possible that more than one pyrolysis
process would be used to handle different types of plastic.
4. Grinding (if needed) and sorting to facilitate heating
and eliminate some contaminates.
5. Flow an inert atmosphere into the pyrolysis chamber to
remove oxygen, and potential use of one or more catalysts or other materials to
reduce the needed time for the reactions and to increase the production of
desired outputs. Heat to the desired
temperature(s) at a specific rate and for a defined period. The various research papers identify
different optimal temperatures for different types of plastic and for different
amounts of liquid, gas and solid output and the materials can be added in
batches or on a continuous basis, and the products removed by different
methods.
6. Capture the
gas and condense it partially into liquids, and save it into tanks. Some gas will remain and potentially be fed
through a water and chemical bath to remove contaminates, and then into tanks
or bags for saving. Some solids will
accumulate on the chamber and be scraped off.
7. Gas may
be used (burned) onsite to heat the next batches of plastic, or compressed to
be sold. The liquid is captured into tanks and saved to be sold or also used to
heat the next batches of plastic. The solid char is landfilled or packaged to
be sold.
Sources of income
·
Excess gas output
|
·
Liquid fuel output
|
·
Char output
|
·
Use of some heat for driving turbines or
district heating
|
·
Tipping fees
|
·
Sale of cleaned and separated plastics not
suitable for pyrolysis
|
Costs
·
Plant design (environmental impact studies, architecture
and industrial design)
|
·
Plant site (acquisition or rent, maintenance,
and taxes)
|
·
Plant equipment (acquisition and maintenance)
|
·
Transportation of plastic input (labor, fuel,
equipment, maintenance)
|
·
Labor to separate, clean and load the pyrolysis
chamber
|
·
Landfill costs for the contaminates and
incompatible plastics
|
·
Nitrogen gas and catalysts and their disposal
|
·
Water and sewer for cleaning
|
·
Fuel at
least for startup
|
·
Transportation of plant outputs
|
·
Insurance and ongoing environmental compliance
including escrow for cleanup
|
·
Sales and general administrative costs
|
Materials and Methods
I have used Google Scholar and Web of Science searches to
identify articles that address areas of the process.
Results
Current Commercial Activity in Pyrolysis
Commercial scale pyrolysis is underway in a number of
places, often focused on bio-mass like wood (3). There are also many plants that process
tires, including one that processes 100 tons per day of scrap tires built by
Klean Industries of Vancouver, Canada (4).
There is less literature on pyrolysis of plastic on a commercial scale. There is a guide for Plastics-to-Fuel project
developers (8) that includes a list of suppliers, developers and active
projects.
Research-based process improvements
There are papers with ideas to help drive process
improvements in the areas of sources of waste plastic; the types of plastic and
other matter (like manure or rice stalks) to combine into the mix in the
pyrolysis chamber; recommendations for the chemical and flow rate for the inert
atmosphere; catalysts (if any) that should be used, and recommendations for the
pattern of heating (temperature level and duration) to get the best output
profile.
Sources of waste plastic
Municipal Solid Waste (MSW) is the most common source for
real-world plants (8, 9). Agriculture,
industrial waste and pre-consumer scrap from plastics molding, etc. plants are
also sources of good quality plastic scrap (10). The cleaner the incoming plastic, the less
cost there is in cleaning it. And, if a
large enough supply of a particular polymer can be found, the pyrolysis
parameters can be tuned for that polymer, resulting in more valuable outputs.
Types of plastic and other materials to use in the pyrolysis chamber
Many of the studies do not use PVC due to the high chlorine
content and its propensity to produce HCl in the gas. There are a number of ways to handle this.
The most widely used adsorbents are FeOOH, Fe3O4 and Fe2O3
(5). Polyethylene terephthalate (PET)
from daily usage and polyvinyl chloride (PVC) from industrial usage can be
co-pyrolyzed with good results (1), however the oil produced from PET was ~43%
benzoic acid which was acidic and damaging to the reactor. HDPE has the highest higher heating value
[HHV] (5). Thus, it is potentially best
used in energy recovery facilities instead of pyrolysis. The use of PET or PETE increases the amount
of CO and CO2 formed early in the process (9), probably due to the
oxygen content of the underlying monomers.
Chemical and flow rate for the inert atmosphere
Nitrogen was almost always used as the inert atmosphere,
given the pricing issues with other gasses.
However, hydrogen was used to do liquefaction in (5).
Others have used an argon atmosphere (2). The size of the reactor
chamber and how tightly it is closed affects the needed flow rate of the
gas. Obviously, a lower the flow rate
can lower the cost of the pyrolysis reaction.
Small reactors in the labs used a flowrate of 100 ml min−1 (2) or 30
ml/min during the process, but during the purging before starting heating the flow
rate was 100 ml/min for a period of 15–20 min (9). In volume, nitrogen (N2) costs
$0.027 per cubic foot, while argon is $0.075 and hydrogen is $0.062 in 2013
(14). Thus, we would use 2.5 cubic feet per 12 hour batch in the lab for the
100 ml per minute flow rate. Assuming a 2000
times scale, we are at $135 for nitrogen for a 10 ton batch.
Catalysts
Catalysts are not always used, but there are a wide range of
choices in catalyst. Various forms of zeolite are very common (5). The use of natural zeolite vs. commercial
Y-zeolite had only a slight change in the output, so the less expensive natural
zeolite is preferred. (7) Catalysts
increase the speed of the reaction and can help with removing impurities.
Heating pattern (pressure, level and duration)
There are many available papers on the duration and
temperature pattern controlling the results from pyrolysis (5). The use of increased pressure is also a main
parameter (6). Rohit Kumar Singh and
Biswajit Ruj (9) lay out many of the parameters and results for their selected
mix of plastics from municipal solid wastes.
The heating rate is important, as the remaining solid residue increases
with an increase in heating rate regardless of the type of samples used (2). The peak high temperature is crucial to how
much cracking happens and thus how much gas vs. liquid oil is produced.
There seems to be a potential for a linear program to
calculate the optimal choices each day based on the relative commodity prices
for gas, char and liquid fuel, as well as available stocks of waste
plastic. Based on my personal
experience, this would be similar to the “cheaper wiener” program where the
University of Wisconsin worked with Oscar Mayer in the 1970’s to calculate the
optimal amount of cereal grain, pork and beef to use in the wieners that day;
or on programs run by oil refineries to optimize the output of the cracking
processes to maximize revenue from the output.
Discussion
The economics of these processes have been elusive,
primarily as there are few in operation and the companies involved have many
intellectual property concerns in this developing field.
Costs for starting a plant are significant, with production
plants proposed starting at $16 million (13) to $90 million. (11) The seller of pyrolysis reactors on
Alibaba.com offered a waste tire cost benefit analysis (12): converted from the
Chinese yuan to $ at $.15 per yuan.
Description
|
$/10 Ton Load
|
|
1
|
10 tons of waste tires
|
10*$225=$2225
|
2
|
0.4 tons of coal
|
0.4*$120=$48
|
3
|
Water and Electricity
|
100*$0.15=$15
|
4
|
4 workers for 12 hours
|
4*$15=$60 (In China. In
the US these are probably around $12 per hour, or $576)
|
Total Cost
|
2225+48+15+60=$2348
|
|
Output Description
|
$/10 ton load
|
|
1
|
Crude Oil
|
10T*45%*$675=$3037.50
|
2
|
Carbon Black
|
10T*35%*$78=$273
|
3
|
Steel Wire
|
10T*13%*$330=$429
|
Total Income
|
3037.5+273+429=$3739.50
|
|
Net Income before fixed
costs per 10 ton load
|
3739.5-2348=$1391.5
|
Of course, this is not in any way precise, as the commodity
costs and values vary a lot day- to-day.
For instance, steel is now closer to $90 a ton than the $330 a ton shown
here. And, this does not include the
cost of the nitrogen gas. A single manual-loading
batch reactor unit costs between $40,000 and $90,000 with a 2 year warrantee
and an expected life of 5-8 years.
Automated continuous flow plants would cost much more. Land costs, environmental studies, transportation
of the equipment, etc. are significant, and vary a lot based on the site and
the nature of the business.
The Ocean Recovery Alliance (8) provides a link to an Excel
spreadsheet that would calculate the cost per gallon or barrel for resulting
fuel and an internal rate of return for a plant, if you fill in the numerous
parameters for all the costs and returns.
The production of a business plan for a pyrolysis plant
requires knowledge of the proprietary processes and costs related to them and
the site characteristics, to identify all actions needed to be environmentally
safe, and robustly operate.
Conclusions
The pyrolysis of waste plastic is rapidly becoming a
well-known scientific area that has the potential to be very useful in
environmentally-sound handling of municipal solid waste.
The science is showing that many parameters control the
operation and results of the pyrolysis process.
The recent research is working to allow definition of formulae to
optimize the operation, based on costs of the inputs and values of the outputs
of the pyrolysis processes. This is consistent with existing practices in
petrochemical refineries and other commodity using and producing industries
where linear programming has been used for decades.
The economics of the waste plastics pyrolysis industry is
unclear at the moment, as there are very few active plants. The costs of building a plant aren’t yet well
established, as most are pilots in one form or another. It is clear that pyrolysis is a good
candidate for future handling of plastic waste, and it deserves to be further
studied.
References
1.
O. Cepeliogullar, A.E. Putun (2013) Utilization
of two different types of plastic wastes from daily and industrial life,C.
Ozdemir, S. Sahinkaya, E. Kalipci, M.K. Oden (Eds.), ICOEST Cappadocia 2013, ICOEST Cappadocia, Turkey), pp. 1–13.
2.
B.L.F. Chin, S. Yusup, A. Al Shoaibi, P. Kannan, C. Srinivasakannan, S.A.
Sulaiman (2014) Kinetic studies of co-pyrolysis of rubber seed shell with high
density polyethylene, Energy Convers
Manage, 87:746–753.
3.
Juhani Isaksson (2015) Commercial scale gasification to replace fossil fuel in
power generation – Vaskiluodon Voima140 MW CFB Gasification Project, Valmet
Technologies Oyj, Presentation Slides, October 2015 at IEA Bioenergy 2015
conference.
4.
Klean Industries, Inc. (2015) Klean Industries: Visit a Commercially Operational 100TPD
Scrap Tire Pyrolysis Facility, Press Release, June 10, 2015; site visited April
14, 2016.
5.
Bidhya Kunwar, H.N. Cheng, Sriram R Chandrashekaran, Brajendra K Sharma (2016) Plastics to fuel: a review, Renewable and Sustainable Energy Reviews, 54:421–428.
6.
Shafferina Dayana
Anuar Sharuddin, Faisal Abnisa,
Wan Mohd Ashri Wan
Daud, Mohamed Kheireddine
Aroua (2016) A
review on pyrolysis of plastic wastes, Energy Conversion and
Management,
115:308–326.
7.
Mochamad Syamsiro, Harwin Saptoadi, Tinton Norsujianto, Putri Noviasri, Shuo
Cheng, Zainal Alimuddin, Kunio Yoshikawa (2014) Fuel
Oil Production from Municipal Plastic Wastes in Sequential Pyrolysis and Catalytic
Reforming Reactors, Energy
Procedia, 47, 2014, 180-188.
8.
Ocean Recovery Alliance (2015) 2015
PLASTICS-TO-FUEL PROJECT DEVELOPER’S GUIDE, (http://www.oceanrecov.org/assets/files/Valuing_Plastic/2015-PTF-Project-Developers-Guide.pdf Visited April 15, 2016).
9.
Rohit Kumar Singh, Biswajit Ruj (2016) Time
and temperature depended fuel gas generation from pyrolysis of real world
municipal plastic waste, Fuel,
174:164–171.
10.
N. Miskolczi, A. Angyal, L. Bartha, I. Valkai (2009) Fuels by pyrolysis of waste plastics from
agricultural and packaging sectors in a pilot scale reactor Fuel Process Technol,
90:1032–1040.
11.
Plastics News (2016) RES Polyflow
to build $90 million plastics-to-fuel plant in Indiana (http://www.plasticsnews.com/article/20151222/NEWS/151229948/res-polyflow-to-build-90-million-plastics-to-fuel-plant-in-indiana
Visited April 23, 2016).
12.
Alibaba.com (2016) 20 tons per
day waste tyre pyrolysis plant with 50% oil output, (http://www.alibaba.com/product-detail/20-tons-per-day-waste-tyre_60125526152.html?spm=a2700.7743248.51.41.aHnuxB
visited April 24, 2016)
14.
State of New Jersey (March 1, 2013) Price
List – Praxair (https://wwwnet1.state.nj.us/treasury/dpp/ebid/Buyer/GetDocument.aspx?DocId=19789&DocName=T0081PriceList.pdf&DocLoc=15
Visited April 26, 2016).
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