(best examples and good practices) on household organic waste ...

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choices for a commercial plant would either be steam or exhaust gases from the
syngas combustor (which may be a gas turbine). Exhaust gases primarily
contain steam <strong>and</strong> carbon dioxide, but also some oxygen. The use of exhaust
gases would alter the process from being a pure pyrolytic process to a partial air
gasification process. Examples of potential commercial processes of pyrolysis
for MSW include Nexus <strong>and</strong> Thide Environmement technologies.
The Nexus process pyrolyses unsorted MSW waste in containers at 500°C. This
should be equivalent to pyrolysis on a packed bed. Heating can take many
hours. The gaseous output is 64%, including steam <strong>and</strong> oil vapor. The
remainder is classified as solids. The solids contain carbon char, metals, glass
<strong>and</strong> ash. The gaseous output is burnt in a boiler without treatment or cooling.
Exhaust gases from the boiler are filtered <strong>and</strong> scrubbed to remove acid gases.
Because of the high oil content, it is probably not practical to cool <strong>and</strong> clean the
syngas for use in a gas turbine. In the Thide Environmement process, pyrolysis
also occurs at 500°C. Pyrolysis takes place in an externally heated rotating
drum. Heat <strong>and</strong> mass balance data for this process have not been published.
Pyrolysis processes are already in commercial use by the metals industry for
treating contaminated non-ferrous scrap. An example of this is the Alcan
process for delacquering aluminum cans. Two options exist therefore for
recovering contaminated metals – separate then pyrolyse, or pyrolyse then
separate. The first is probably cheaper but the second may recover more metal.
Pyrolysis has been extensively researched with respect to the conversion of
polymers back to petrochemical feedstocks. Polyethylene (PE) <strong>and</strong>
polypropylene (PP) decompose rapidly at temperatures between 400 <strong>and</strong> 600°C
to give a complex mixture of olefins <strong>and</strong> alkanes. At 400°C, the yield is mainly
waxes. The gaseous fraction increases with temperature. Polystyrene (PS)
initially decomposes at 290°C to yield styrene, diphenylbutene <strong>and</strong>
triphenylbutene. After prolonged heating, or at higher temperatures, these
components primarily form toluene, ethyl benzene, cumene <strong>and</strong>
triphenylbenzene. PVC begins to degrade rapidly above 250°C, yielding
hydrogen chloride gas. In addition to hydrogen chloride, small quantities of
benzene <strong>and</strong> other hydrocarbons are released. At higher temperatures, the
dehydrochlorinated polyene undergoes further cracking to yield a mixture of
aliphatic <strong>and</strong> aromatic compounds <strong>and</strong> a carbonaceous char. PET degrades at
about 300°C to yield a mixture of terephthalic acid monomer <strong>and</strong> vinyl ester
oligomers. Longer reaction times <strong>and</strong> higher temperatures produce volatiles,
including formic acid, acetaldehyde, carbon oxides, ethylene <strong>and</strong> water.
The pyrolysis of plastics with a high PVC content requires special techniques.
One approach is to add lime. The lime reacts with the PVC to form calcium
chloride. In fluidized bed, the calcium chloride forms undesirable
agglomerates. Hydrogen chloride is released from PVC at temperatures well