Figure 6.6 shows the aggregated environmental impact comparison between recycled and virgin PP fibres across all impact categories. Because the fibre-based scenarios all have similar impact magnitudes, we cluster all fibre-based scenarios together in Fig. 6.6. Since there are orders of magnitude differences between use of any fibre and steel, we have selected the best performing fibre to compare to SRM in Fig. 6.7. As can be seen in Fig. 6.6, the production of industrial recycled PP fibre has minimum impacts on the environment under all categories considered. Generally, virgin PP fibre has higher environmental impacts than either of the recycled fibres, especially in terms of resource consumption.
To produce 40 kg of PP fibres, the industrial recycled fibre life cycle produces 81.7 kg of CO2 equivalent. As explained in Fig. 6.5, processing of domestic PP
Fig. 6.6 LCIA results from the three scenarios producing 40 kg of PP fibre
Fig. 6.7 LCIA results of the industrial recycled PP scenario (40 kg of PP fibre) vs. the reinforcing steel scenario (364 kg of SRM)
waste into fibre needs more complex and energy intensive processes, such as PP waste collection and sorting, PP reprocessing, and fibre production. Hence, the production of domestic PP fibre consumes more energy and produces more CO2 equivalent (109 kg). For the virgin PP fibre, 137 kg of CO2 equivalent is predominately associated with the production of PP granulates and PP fibre, and also with propylene production (as Fig. 6.3).
The process used to produce the industrial recycled PP fibre contributes only minor eutrophication impact (0.033 kg of PO4 equivalent). However, for the domestic recycled PP fibre, the waste washing process emits more significant PO4 equivalent (0.069 kg of PO4 equivalent), and leads to higher eutrophication impacts in comparison to industrial recycled PP. It is also interesting to note that despite the comparatively high total volume of water used in producing domestic recycled PP, the eutrophication impact of the waste water is low compared to virgin PP production. For example, the catalytic cracking unit used for virgin propylene production has substantial eutrophication impact in comparison. Overall, the virgin PP fibre production process causes the highest eutrophication impact (0.085 kg of PO4 equivalent).
When it comes to water consumption, manufacturing domestic recycled PP fibre consumes much more water (0.99 m3) than producing either industrial recycled PP fibre or virgin PP fibre, which explains the differences in eutrophication impacts between domestic and industrial recycled PP. High water consumption occurs when reprocessing domestic PP waste, because the shredded PP fragments are washed in large volumes of sodium hydroxide solution. After washing, effluent is neutralised, then discharged to domestic sewage. Substantial environmental benefits, including reduced water use and reduced eutrophication impact, could be made, if the effluent was treated and recycled back to the washing process.
In terms of natural resource consumption (in this case fossil fuel), virgin PP fibre production consumes three times more fossil fuel (91.3 kg) than the production of either recycled PP fibres. As well as oil/coal consumed for electricity production, the propylene monomer is extracted through the catalytic cracking of crude oil.
In Fig. 6.7 we compare the environmental impacts of industrial recycled PP fibre with SRM. Across all categories the environmental impacts of producing the industrial recycled PP fibre is negligible in comparison. As shown in Fig. 6.7, the production of 364 kg of SRM emits 15 times the CO2 equivalent than production of 40 kg of industrial recycled PP fibre. The eutrophication impact of the SRM production is 33 times higher than that of industrial recycled PP fibre. 20.9 m3 of water is needed for the production of 364 kg SRM, which is consistent with American government survey results for water use in steel production (Walling and Otts 1967). The water consumption is 20 times higher than that of even domestic recycled PP fibre. 245 kg of oil equivalent is also needed, which is 11.5 times more than that of industrial recycled PP fibre. Although any PP substitution leads to reduced impacts, comparing the best performing PP fibre process to the SRM, the production of industrial recycled PP fibre can save 93% of CO2 equivalent, 97% of PO4 equivalent, 99% of water and 91% of oil equivalent.
Uncertainty is an inherent feature of LCA, and it can be caused by multiple reasons: missing inventory data or inventory data inaccuracy; model uncertainty; uncertainty due to choices of allocation rules; impact factors and system boundaries; spatial and temporal variabilities; and epistemological uncertainty. In Figs. 6.6 and 6.7 the uncertainty range per impact category is shown, and expresses the 95% confidence interval. Across all categories except fossil fuel use, SRM shows much greater uncertainties than all other product alternatives, primarily as a result of the larger raw material quantities associated with SRM. This is particularly evident in water use, where uncertainty is very large. For global warming potential and water use, the recycled and virgin PP fibres show similar degrees of uncertainty. For eutrophication potential, only industrial recycled PP fibre shows a small level of uncertainty, whilst the other fibres are subject to greater uncertainty. This may be the result of limited data describing the variance in input raw materials and output flows associated with industrial recycled PP processing inventory data. However, the impact of uncertainty on the project conclusions is negligible. For the comparative assessment, the impacts of industrial recycled PP fibre are clearly smaller than all other scenarios considered, particularly in comparison to SRM.
Figure 6.8 shows contribution of major sub-stages to the overall impacts, for each PP production pathway. As can be seen, in the industrial recycled PP fibre production processes, the global warming potential, eutrophication impact, water use and fossil fuel consumption are dominated by the fibre production process. Reprocessing industrial PP waste into recycled PP granulates also gives rise to a
Fig. 6.8 Contribution of major sub-stages for three PP fibre scenarios to the overall impacts within each impact category
substantial proportion of the burdens to environment. For the domestic recycled PP fibre processes, the fibre production process is again the dominant source of impact. However, the domestic waste collection, sorting and reprocessing stages are also significant, and emit considerable CO2 equivalent and PO4 equivalent, as well as consuming substantial fossil fuels. Most notably, large amounts of water are needed to wash and separate plastic wastes. Improvements in processing and water recycling at this stage can make domestic recycling more competitive with industrial PP recycling in terms of environmental impacts. For the virgin PP fibre, the production of virgin PP granulates emits considerable CO2 equivalent and PO4 equivalent, and consumes significant water. Obviously, substantial fossil fuels are also needed in this sub-stage because crude oil is used as raw material for production of virgin PP granulates. As shown in Fig. 6.8d, 83% of crude oil use is for monomer production and the remaining 17% is consumed for energy.
In the production of SRM, the EAF is the main process used to produce iron. In Fig. 6.9, which compares the SRM scenario impacts to the industrial recycled PP scenario, the EAF is energy intensive and emits substantial amounts of CO2 equivalent and PO4 equivalent, and also consumes considerable quantity of fossil fuels. Whether iron is produced by EAF or BOF processes, the iron will be continuously casted and roll milled, which also emits significant CO2 equivalent and
Fig. 6.9 Contribution of major sub-stages for the SRM scenario within each impact category, compared to the total impacts for the industrial recycled PP scenario
PO4 equivalent. 40% of the total CO2 equivalent contribution is from EAF, while 52% of the total is from continuous cast and rolling mill processes. 46% of the total eutrophication impact originates during the EAF process and 49% from the continuous cast and rolling mill. Casting and roll milling also require orders of magnitude larger amounts of water. Regarding the use of the fossil fuels, 9% of total consumption is for producing heat in the BOF process, while 89% of total consumption is within the EAF process.