Depolymerization to plastic

Depolymerization using either pyrolysis or gasification breaks plastic down to simpler hydrocarbon compounds. In the low-oxygen environment polymers break down into small hydrocarbon molecules, which can be condensed from the hot gas. However, the molecule bond cleavage is more random, generating light hydrocarbon fractions and waxy fractions, which when condensed and combined, form an oily substance that can be used as fuel. In the case of carefully controlled conditions and feedstock both PMMA and PS monomers can be created from pyrolysis allowing for direct conversion to polymers. However, in general terms the output is a range of hydrocarbon fractions which can be used as feedstock for chemical manufacture, polymer production, or fuel, but must be subject to similar refining and production process as other petrochemical feedstock before it can be converted to final products.

While there are many proposals and trials to use pyrolysis and gasification to depolymerize plastics, the focus and promotion is on their ability to produce monomers for re-use with little consideration of associated health and environmental impacts. There is little to no information on the outputs of the pyrolysis or gasification process of plastics, such as toxic emissions, global warming potential, residues, and gas quality. However, certain inferences can be made about these factors based on the experience with pyrolysis and gasification of MSW.

Using mixed inputs of plastic waste has been demonstrated to generate toxic substances in char and emissions such as polyaromatic hydrocarbons (PAHs) and dioxins. Researchers were able to “quantify that the toxicity rating of PCDD/ DF products from pyrolysis was three times the input at full operational

THE KEY FACTOR IN COMMERCIAL FAILURE IS AN INABILITY TO GENERATE SURPLUS ENERGY DUE TO HIGH EXTERNAL ENERGY INPUT REQUIRED TO HEAT THE WASTE TO PYROLYTIC TEMPERATURES. WHEN ENERGY CONSUMPTION IS CALCULATED FOR THE PRE-SORTING AND DRYING OF WASTE, THE ENERGY MASS BALANCE OF PYROLYSIS IS VERY POOR.

performance and eleven times the input at pilot scale, and that these toxins were also present in both gas and oil”. Dioxin was found to be particularly problematic when chlorinated plastics such as PVC were included in the feedstock.

The implications are that entrained dioxin contamination (other POPs may also be present) in the output hydrocarbons will be carried through as contaminants into the final polymer products or fuel. They may also be released in emissions from the process, representing a health risk to workers and the community. The fate of the char material also becomes important, as char, being a carbonaceous material, is an ideal adsorption matrix for dioxins and other unintentionally created POPs. Indeed, activated carbon is injected into waste incinerators for the specific purpose of adsorbing dioxins from the flue gas .

Energy use of pyrolysis and gasification is very high. Pyrolysis operates at temperatures of around 300o – 600 o C, and gasification in the range of 1,200 °C – 1,500 °C. To heat the system to this level requires external sources of energy – usually sourced from fossil fuels. Some waste processing pyrolysis and gasification plants claim to be self-sustaining based on using the hydrocarbons (and char) they generate to power parasitic loads. However, as Rollinson and Oladejo note,

“Modest positive energy balances have been reported but only under the impractical and unsustainable conditions of:

1. When the drying energy has been set outside the system boundary.

2. Without considering fundamental (second law of thermodynamic) heat losses.

3. Discounting essential auxiliary energy to manage the plant such as, but not exclusively, gas cleaning, pre-processing, and supplementary fuels to the reactor.

” In other words, it is not plausible for these systems to be energy selfsufficient and generate energy or fuel as surplus, so their energy balance is therefore essentially negative.