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feedstock recycling and energy recovery When we discuss the processes of feedstock recycling and energy recovery first, we realize that they have the potential to save ca. 30 MJ/kg of primary energy in comparison with landfill and thus use most of the energy content of waste plastics. The blast furnace and thermolysis process perform best, closely followed by fluid-bed combustion and hydrogenation. The waste incineration and gasification processes perform significantly worse. in the case of waste incineration this is primarily because only 34% of the generated steam was assumed to be utilized as energy, which corresponds to the present average of German waste incineration plants. The poor performance of the gasification processes is attributable to the choice of the complimentary process — a European mix in which 73% of the synthesis gas is produced from natural gas (far more energy efficient), 22% from vacuum residue oil, and only 5% from lignite if energy recovery could be optimized in waste incineration, for example, by fully utilizing the generated steam4 or if for the gasification processes a complimentary process is chosen that gasifies lignite only,5 all investigated feedstock recycling and energy recovery processes would save 24-30 MJ of primary energy as indicated by the additional bars in Figures 13.22 and 13.23.

The impact on the global warming potential depends upon the use of energy resources of each process. The main greenhouse factors involved are gaseous emissions of CO2 and methane. Fluid-bed and fixed-bed gasification and waste incineration yield increased gas emissions in comparison to landfill because they release them immediately and not in more than 100 years, as assumed for the landfill.

Energy recovery in a cement kiln was not investigated in the main study, whose results are reported here, but in a separate investigation initiated by APME [12]. Here plastic waste substituted coal as fuel for the cement kiln. The investigation showed a significantly better resource saving potential than feedstock recycling (29-33 MJ/kg vs. 26-30 MJ/kg); the saving potential for eutrophication and for acidification was also improved.

MECHANICAL RECYCLING REPLACING VIRGIN PLASTIC MATERIALS We turn now to mechanical recycling and compare this route of filling the basket of goods with the three best feedstock recycling routes. As can be seen mechanical recycling enables in certain cases to save part of the processing energy of plastic waste and is in these cases to be ecologically the preferred process. But, there are also examples of mechanical recycling that save comparable or even less energy than the best universal processes. In these cases feedstock recycling is the ecologically preferred process.

The positioning of this "break-even point" depends on the efficiency by which recyclate is able to replace virgin resin (Fig. 13.24). If 1 kg of recyclate is able to substitute 1 kg of virgin resin and the final products fulfill identical practical

4 Several plants of this type exist in Germany where the steam is used in industrial plants close by.

5 This would be the case for the Schwarze Pumpe plant in Germany.

Figure 13.24. Possible dependence of substitution factor on amount of recyclate in product.

functions, this would mean dealing with a substitution factor equals 1. If, on the other hand, a bag has to be 30% thicker in order to have the same impact strength as a bag made from virgin material, the substitution factor equals only 0.77. This substitution factor certainly varies with the ratio between recyclate and virgin material in the final product. It will mostly be 1 at low percentage of recyclate and drop drastically with increasing percentage of recyclate in the final product. Two limiting cases can be assumed in theory, one where clean scrap substitutes virgin resins up to 100% with only a certain drop in properties, and one where worked up waste substitutes virgin resin only up to a certain content, and this with a much larger drop in properties.

The critical substitution factors, or break-even points, between mechanical recycling and feedstock recycling (i.e., when the ecological effects of mechanical recycling are identical to those of the best feedstock processes) have been calculated for the ecological categories considered for the investigated here routes of mechanical recycling (Fig. 13.25).

The conclusion is that mechanical recycling substituting virgin resins in its application has the potential of saving significantly more primary resources than the other routes of recycling because both the energy content (some = 40 MJ/kg) and the process energy used to produce them (another 40 MJ/kg) are largely conserved. But each mechanical recycling route is different as shown with three examples only. This means a general route of mechanical recycling does not exist. In addition, different polymers such as polypropylene (PP), polystyrene (PS), PET, and polyvinyl chloride (PVC) can be recycled each in a very special process.

Therefore, individual cases have to be considered. A general environmental positioning in comparison to other routes is impossible. The procedure developed in this project enables us to position individual processes if the environmental data are available.

Input

Best feedstock process

Bottle

Film

Cable conduit

Energy total (MJ)

Blast furnace

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