Co-pyrolysis of microalgae and glucose: nitrogen evolution and incorporation in biochar

Abstract

Introduction: Algal biomass is a promising sustainable feedstock for a wide range of different biobased products, e.g. green chemicals, fuels, … In this regard, algae could be used to produce green fertilizers in the form of biochar. However, one of the main limiting factors to their widespread application is their tendency to release large amounts of hazardous nitrogen containing compounds during their pyrolysis [1]. Therefore, new strategies to limit these emissions are crucial. Hence, the idea to co-pyrolyse microalgal biomass with reducing sugars, aiming on the interaction of their carbonyl groups with the amine compounds which results in the incorporation of nitrogen in the biochars carbon matrix.

Aim: This investigation aims to assess whether the addition of glucose to microalgae slow pyrolysis increases the retention of nitrogen in the solid biochar. Different ratios of glucose were added to assess the interaction effect and to determine the optimal mixing ratios. Furthermore, to elucidate on the possible mechanisms involved in the pyrolysis process several hyphenated analytical techniques (e.g. py-GC-MS and TG-IR) were performed. This way an attempt was made to link micro-scale pyrolysis behaviour with bench-scale experiments.

Methods: Microalgal biomass (Spirulina sp.) was mixed with D-(+)-glucose (>99.8%, Fisher chemicals) using different mixing ratios (25-75 wt%, 50-50 wt%, 75-25 wt%) hereafter samples were pyrolysed in a slow pyrolysis set-up until 700 °C. After this the resulting biochars were removed and yields were determined. The ultimate and proximate composition and surface functional groups of the biomass and biochars. were measured. Lastly, to elucidate on the biomass behaviour during the pyrolysis process, TGA, TG-IR and Py-GC-MS.

Results: After pyrolysis, biochar yields increased when glucose was added, from 19.4 wt% (0% GL) to 24.6% (75% GL). Curiously, blending SP and GL always gave increased biochar yields compared with their respective separate pyrolyses, Figure 1B. This indicates that some form of interaction between both compounds took place during the pyrolysis process. Furthermore, absolute nitrogen retentions increased threefold when GL was added in large quantities, from 10.7% to 34.6%, Figure 1A. To verify which reactions were responsible for this large nitrogen incorporation PY-GC-MS was performed, here several Maillard reaction intermediate tracer compounds were detected (e.g. methylglyoxal, 2,3-butanedione, 2,3-pentanedione, 5-hydroxymethylfurfural, …). Indicating that these are pivotal for an extended incorporation of nitrogen in the aromatic biochar carbon matrices.

Conclusion: Biochar yields increased via the interaction of glucose with microalgae. Furthermore, adding glucose to microalgae has a beneficial effect on the incorporation of nitrogen in the biochar. It was postulated that Maillard type reaction (between a reducing sugar and proteins) were responsible for this increase.