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Recovering energy from plastics

  1. Oct 21, 2015 #1


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    Is there an energy efficient way of recovering energy from waste plastics, i know they can be turned back to oil, but is this economical? I am thinking of things like plastic toys and pats from domestic appliances.
    Last edited: Oct 21, 2015
  2. jcsd
  3. Oct 21, 2015 #2
    Most plastics will burn, so they are a potential source of heat which can then be used in many ways, for example to heat water.
    However they don't burn efficiently in normal open air, and if simply set fire to they will produce a lot of sooty carbon compounds, (+CO, CO2 +water)
    Burning efficiency could be very much improved by powderising the plastic first then burning it in a environment with added oxygen.
    A jet engine uses a similar idea to burn kerosine (aka paraffin) very efficiently, by first turning it into a vapour and then adding highly compressed air before igniting.
    Liquid kerosine/paraffin by itself doesn't burn well at all if just ignited in normal atmosphere.
    Last edited: Oct 21, 2015
  4. Oct 22, 2015 #3
    There are commercial firms that buy and degrade old plastic into small pieces to power high-temperature incinerators. Here's an interesting link from an industry trade association showing the energy ratings of different fuels and plastics are at the very top of the list. (https://www.plasticsindustry.org/AboutPlastics/content.cfm?ItemNumber=793&navItemNumber=1124 [Broken])
    Last edited by a moderator: May 7, 2017
  5. Oct 26, 2015 #4


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    As a notachemist, I can only gather data, and propose that it is theoretically possible.

    Focusing on HDPE, as "waste plastic" is somewhat of a nightmare, I've come up with the following:

    HDPE [High Density Polyethylene, C2H4)n]
    Specific heat: 1900 J/(K * kg) [ref]
    Change in temperature required for pyrolysis HDPE to fuel: 410 K (430°C Th & 20°C Tc) [ref]
    Energy required to change temperature: 780,000 joules [ref: maths]
    Energy released by burning 1 kg: 46,300,000 joules [ref, page 8]
    Ratio: 59:1 [ref: maths]​

    From this, it is obvious that it is theoretically possible to convert HDPE plastic into petrol, economically, from an energy standpoint.

    As to whether or not a device can do this, "financially" economically, is an engineering problem.
  6. Oct 26, 2015 #5


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    Actually polyethylenes burn quite clean.
  7. Oct 26, 2015 #6
    Check out "Thermal depolymerization" and "Molten salt oxidation"
  8. Oct 26, 2015 #7


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    I think Wolram's question stemmed from a debate as to whether or not it was better to convert plastics to oil, or just burn them.
    According to my latest calculation, there are 660,000,000 tons of plastic in landfills in the United States.
    I'm guessing there is at least that much elsewhere.
  9. Nov 2, 2015 #8
    If you could find a way to efficiently convert plastics to liquid fuels (and took the time to talk to a patent attorney), you'd get very rich very quickly. The problem is that even the simplest of plastics (poyethylene) is made of very large polymer molecules, which have to be broken down into much shorter chains (think ball of yarn vs. 1-cm snippets.) That takes energy and, usually, metallic "cracking" catalysts. You can do it, but you'd be competing with the very same process that starts with crude oil - and the latter has a few advantages, in scale and in the cost of feedstock. There might be a way to add shredded plastic to the crude, but even then the cost of transporting and shredding would have to be pretty low, to make the overall effort profitable.
    Probably best to recover what energy you can by burning plastics directly - that's what's going to happen to any liquid fuel you might be able to make.
  10. Nov 24, 2015 #9
    Most of the plastics can be pyrolized to oil, which can be used in medium speed reciprocating engine after removing salts and ashes, which are well presented in the fuel.
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