Prospective life cycle assessment of adipic acid production from forest residue

Abstract

Moving from a fossil-based to a bio-based economy requires the development of novel technologies for the production of bio-based chemicals and materials, and the Swedish forestry sector may play a major role in this. These technologies may become part of novel biorefinery concepts that combine the production of bulk and fine chemicals. This paper presents the life cycle assessment (LCA) of such a technology, in its early stages of development, which targets the biochemical production of adipic acid from Swedish forest residue. Adipic acid is a bulk chemical and its yearly production is approximately 2.3 million tonnes. It is mainly used as a precursor in the production of nylon, and its current fossil-based production process emits significant quantities of nitrous oxide (N2O), a strong greenhouse gas. Preliminary calculations showed that, compared to conventional adipic acid production, eliminating the emission of N2O would lead to a reduction of GWP by 75%, and that switching from a fossil-based to a biomass-based feedstock would reduce GWP by an additional 10%. This LCA focused on the technology for producing adipic acid, but also considered its connection with other technologies in the biorefinery concept. An anaerobic digestion process is used to produce biogas from the waste water. As well, lignin is produced as a by-product. The heat from biogas and lignin incineration can be used to meet the energy demands of the adipic acid production. Lab-scale experimental results were used in the assessment, and scaling up these results to an industrially relevant process capacity was done by using process modelling and simulation. Furthermore, an assessment was done of the impact of extracting forest residue from the Swedish forest. The results of this LCA show that the environmental impacts of producing adipic acid from forest residues, except for acidification potential, are significantly lower than those of its conventional production. The environmental hotspot of the technology is the downstream processing stage due to its need of additional energy which was assumed to be fossil. This additional energy is needed because of the low concentration of adipic acid in the fermentation broth. Further improvements can thus be made by minimizing fossil energy use in this stage, or by aiming for a higher end concentration of adipic acid in the fermentation broth. Lastly, the extraction of forest residue was found to only marginally contribute to global warming.