Abstract
Anthropogenic greenhouse gas emissions are the main cause for global warming and originate for 80% from the burning of fossil fuels to generate heat and electricity. (Inter)national policies strive to reduce these emissions, especially in the industrial sector. The use of renewable fuels could potentially be the most powerful part
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in such a reduction. However, the switch to alternative sources will take some decennia. In the meantime the industry should search for intermediate solutions.
It is important to develop and apply reliable comparison methods. The thesis describes a method to assess the sustainability of chemical manufacturing using the polyamide-6 industry as a case study. We have established five polyamide-6 process designs starting from fossil- or renewable raw materials, which can be considered to be representative for polyamide-6 manufacturing. Widely different options for improved manufacturing processes are investigated, and mutually compared with respect to the (combined) potential to reduce natural raw material consumption and fossil carbon dioxide emissions.
We have defined ways to characterize the performance: (extended) carbon atom efficiency, balancing equation, exergy analysis, primary energy demand (PED), and carbon dioxide emission.
PED of the benchmark process (starting with benzene/ammonia) is 174.6 MJ/kg PA6 and in accordance with the PlasticsEurope data and with the allocation of ammonium sulphate co-production. The alternative fossil process (starting with 1,3-butadiene/hydrogen cyanide) has a demand of 149.0 MJ. The glucose based processes via L-lysine have PEDs ranging from 203.6 to 228.8 MJ. The glucose based process via 6-aminocaproic acid (6-ACA) reveals 110.5 MJ.
The energy efficiency of the 6-ACA process is significantly higher than for the other processes. However, the fossil based processes, particularly the Butadiene process, have the technological potential to surpass the biobased processes with respect to energy efficiency and primary energy loss.
If fossil based steam and electricity is used in PA-6 production, the cradle-to-grave fossil CO2 emission is comparable for all processes, except for the 6-ACA process (12.6 – 15.1 vs. 7.0 kg CO2/kg PA6 for the 6-ACA process). The same comparative conclusion holds for the cradle-to-plant gate emission (11.2 – 13.8 vs. 7.0 kg CO2/kg PA6 for the 6-ACA process).
If sustainable steam and electricity is used, the cradle-to-grave fossil CO2 emission of the fossil- routes are 9.8 and 11.8 kg CO2/kg PA6), however, higher than in the biobased routes (6.0 – 7.4 kg CO2). However, the cradle-to-plant gate fossil CO2 emission of the alternative fossil route and the biobased routes are comparable (6.0 – 7.5 kg CO2/kg PA6) but lower than for the benchmark route (9.5 kg CO2).
From an energy efficiency perspective alone, it is questionable for the studied processes if the biobased process is preferred. When not accounting for end-of-life emissions, e.g. if they are compensated via carbon capture and storage or direct air CO2 capture, a shift to renewable energy sources in the existing process must be prioritized. However, if the total life-cycle greenhouse gas emissions are considered this result may change and favour the biobased route, though at the expense of increased energy use.
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