Pyrolysis of brewer’s spent grain biomass to form functional adsorbers

Abstract

PYROLYSIS OF BREWER’S SPENT GRAIN BIOMASS TO FORM FUNCTIONAL ADSORBERS

D. BLEUS 1, B. JOOS 2,3, W. MARCHAL 1, D. VANDAMME 1

1 Analytical and Circular Chemistry (ACC), Institute for Materials Research (IMO), Hasselt University, Hasselt, Belgium. 4 Design and Synthesis of Inorganic Nanomaterials (DESINe), Institute for Materials Research (IMO-IMOMEC), Hasselt University, 3590 Diepenbeek, Belgium 3 EnergyVille, Thor Park, 3600 Genk, Belgium

1. Keywords Biomass, valorisation, pyrolysis, adsorbers 2. Highlights - Brewer’s spent grain and malt dust biomass streams were extracted using bio-based solvents to recover (poly-)phenolic compounds. - The extracted biomass was pyrolyzed using a lab-scale pipe furnace setup. - The resulting biochar was then physically activated to obtain activated carbon (AC). - The AC adsorbers will further be employed as adsorber material in solid phase separation/purification of (poly-)phenolic compounds.

3. Purpose The proposed research abstract elaborates a novel, circular valorization methodology for brewer spent grain (BSG) through green solvent-extraction and subsequent pyrolysis and activation of the biomass resource to produce activated carbon (AC) materials. Finally, the obtained AC will be investigated for use as adsorber for separation and purification of extraction mixtures. BSG is a nutrient-rich side product obtained from the beer brewing process. Many types of BSG have proven to be naturally abundant in phenolic compounds, which could find application as anti-oxidants 1 in a variety of food, feed, non-food products and pharmaceuticals. As an industrial side stream that exceeds 3.4 million tons per year in the EU alone 2 , a viable valorization route would create both economic and ecological opportunity. Malt dust, on the other hand, is a lesser-known and underexplored waste stream that encompasses all fine particulate matter separated from the freshly germinated and dried barley. It is captured before the brewing process, and therefore still retains most naturally present nutrients and extractable components. To create a sustainable 3 valorization route for these biomass streams, extractions should be carried out using bio-based or biocompatible solvents, while also minimizing the required energetic budget. Biomass-based adsorbers show well-documented potential in traditional solvent purification and extractive recuperation procedures. 4- 5 Hence, in this project biomass-based adsorber materials are put forward as a promising tool in the efficient recuperation of solvent after the extraction step has been carried out. Extraction or ‘washing’ pretreatment steps have an impact on the obtained pyrolysis products, and can yield increased surface area carbonaceous materials. 6,7 It is therefore postulated that an optimized extraction process can have a synergistic effect in the production of high surface area AC materials. Not only would the successful implementation of these adsorbers boost the applicability of bio-based solvents in the industry, but it would also increase the utilization of biomass side streams that would otherwise be discarded as waste in landfills or incineration plants. 8

4. Materials and methods BSG and malt dust were obtained from a local brewery. The samples were dried at 60°C and stored at -20°C until further use. Different bio-based solvent systems are prepared and evaluated for their efficiency. The mixtures were extracted using maceration at 70°C, 90°C, and 120°C, respectively. The mixture was then centrifuged at 4000 rpm for 30 minutes, before being filtered off under vacuum. The filtered extracts were then analysed on HPLC- MS. The solid biomass precipitate obtained after centrifugation was collected, excess bio-based solvent was removed by vacuum filtration, and finally dried in vacuo at 60°C to remove remaining water. The extracted biomass with residual bio-based solvent was then pyrolyzed in a tube furnace at 700°C, under N 2 atmosphere. The resulting biochar was collected and yield was determined gravimetrically, before performing physical activation on part of the biochar material. Resulting biochar and AC materials were then analysed using BET (Brunauer–Emmett–Teller) specific surface area using gas adsorption measurement, scanning electron microscopy for surface morphology, Ultimate analysis for elemental CHNO-composition, and thermogravimetric analysis for volatiles and ash content determination.

5. Results and discussion Preliminary results confirm the findings of earlier studies regarding the extraction efficiency of bio-based solvents as a valid alternative to optimized extraction methods that utilize classical solvents, such as acetone:H 2 O mixtures. 9 Using an optimized extraction setup at elevated temperatures (120°C), extraction efficiencies were improved over classical maceration methods. Through the combination of centrifugation and vacuum filtration, (poly-)phenolic extracts have been separated from the extracted solid biomass residue, obtaining pure liquid extracts suitable for direct analysis on HPLC-MS. Qualitative identification of various (poly-)phenolic compounds was performed through an optimized separation method on C18 column, with acidified H 2 O:MeOH elution gradient. ‘Wet’ biomass was pyrolyzed at 700°C and steam-activated at 800°C in small scale experiments, obtaining AC materials with promising surface areas, exceeding 500 m 2 /g.

6. Conclusions and perspectives Promising preliminary results confirm the plausibility of the above-mentioned methodology for circular valorisation of BSG and malt dust biomass. On one hand, phenolic anti-oxidant compounds were qualitatively extracted from BSG and malt dust biomass streams, using green solvents. The obtained AC materials will be further analysed for chemical and phyiscal functionality, with additional optimizations of the lab-scale pyrolysis and activation process still to be performed. Future perspective includes the setup of small-scale preparative phenolic isolation, and elution tests, which will be performed with the obtained AC adsorbers.

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