Multi-product biorefineries from lignocelluloses: a pathway to revitalisation of the sugar industry?

Abstract

Background: Driven by a range of sustainability challenges, e.g. climate change, resource depletion and expanding populations, a circular bioeconomy is emerging and expected to evolve progressively in the coming decades. South Africa along with other BRICS countries (Brazil, Russia, India and China) represents the emerging bioeconomy and contributes significantly to global sugar market. In our research, South Africa is used as a case study to demonstrate the sustainable design for the future biorefineries annexed to existing sugar industry. Detailed techno-economic evaluation and Life Cycle Assessment (LCA) were applied to model alternative routes for converting sugarcane residues (bagasse and trash) to selected biofuel and/or biochemicals (ethanol, ethanol and lactic acid, ethanol and furfural, butanol, methanol and Fischer-Tropsch synthesis, with co-production of surplus electricity) in an energy self-sufficient biorefinery system.

Results: Economic assessment indicated that methanol synthesis with an internal rate of return (IRR) of 16.7% and ethanol-lactic acid co-production (20.5%) met the minimum investment criteria of 15%, while the latter had the lowest sensitivity to market price amongst all the scenarios. LCA results demonstrated that sugarcane cultivation was the most significant contributor to environmental impacts in all of the scenarios, other than the furfural production scenario in which a key step, a biphasic process with tetrahydrofuran solvent, had the most significant contribution.

Conclusion: Overall, the thermochemical routes presented environmental advantages over biochemical pathways on most of the impact categories, except for acidification and eutrophication. Of the investigated scenarios, furfural production delivered the inferior environmental performance, while methanol production performed best due to its low reagent consumption. The combined techno-economic and environmental assessments identified the performance-limiting steps in the 2G biorefinery design for sugarcane industry and highlighted the technology development opportunities under circular bioeconomy context.

Keywords: Biochemical; Biofuel; Biorefinery; Life cycle assessment (LCA); Multi-products; Sugarcane residues; Techno-economic evaluation.

Fig. 1

Block flow diagram of the biochemical scenarios

Fig. 2

Block flow diagram of the thermochemical scenarios

Fig. 3

System boundary for investigated biorefinery scenarios

Fig. 4

The specific energy consumption (kW per tonne feedstock to biorefinery) of the studied biorefinery scenarios—sugar mill demand is excluded—(scenario 1 ethanol; scenario 2 ethanol, LA; scenario 3 ethanol, furfural; scenario 4 butanol; scenario 5 methanol; scenario 6 FTS; Detail data are presented in Table S3 of Additional file 1)

Fig. 5

Detailed installed cost of the biochemical biorefinery scenarios (CHP is excluded which costs $61–63.1 million) (a biochemical scenarios, b thermochemical scenarios)

Fig. 6

Detailed sales of the studied scenarios based on the products (relevant data are given in Table S4 of Additional file 1)

Fig. 7

Results of economic sensitivity analysis of the studied biorefineries

Fig. 8

Characterised LCIA profiles of biorefineries; (unit: 1 tonne of product as defined in the caption; Method CML-IA 2 baseline). a Scenario 1 ethanol; b scenario 2 lactic acid; c scenario 3 furfural; d scenario 4 butanol; e scenario 5 methanol; f scenario 6 FT syncrude

Fig. 9

Characterised LCIA profiles for comparison of biorefinery scenarios (including biogenic C, method: CML-IA baseline 2)

Fig. 10

Water consumption of investigated scenarios per hour

Fig. 11

Comparison of different scenarios for production of 1 tonne bioethanol (method: CML-IA baseline)

Fig. 12

Characterised LCIA profiles of investigated biorefineries (method: EI 99H)

Fig. 13

Sustainability analyses of the investigated scenarios