This year EBIO PhD candidate Elisabeth Oehl presented the project at RRB 2023, held in Riga, Latvia, 31 May – 2 June. The presentation was based on the abstract provided below, and the presentation slides can be downloaded here.
Abstract: Electrocatalytic kraft lignin conversion dissolved in industrial black liquor
Elisabeth Oehl1, Roman Tschentscher2, Audrey Minnard2, Francisco Pereira2, Niclas Schupp1, Carl-Johan Hjerpe3, Jonas Kihlman3, Bernd Wittgens2, Siegfried Waldvogel1
1 Department of Chemistry, Johannes Gutenberg-University, Mainz, Germany
2 Process Technology, SINTEF Industry, Oslo, Norway
3 ÅF-Industry AB, Karlstad, Sweden
Today’s Kraft pulp mills have developed from energy consumers to modern biorefineries supplying pulp, green energy and valuable side products. Continuous optimisations enable to valorise lignin fractions from black liquor for chemicals and fuels production. This will not only generate additional income but also increase the pulp production rate, as the load of the recovery boiler is reduced.
In this work, performed within the framework of the EBIO H20202 project, we present the electrochemical conversion of kraft lignin into phenolic compounds for further production of basic chemicals and biofuels.
In the Waldvogel group an anodic degradation of lignin with an activated nickel foam electrode in an aqueous caustic soda solution with successful extractions of up to 1.8 wt% vanillin was published. The experiment was performed using a divided cell with a current density of 38 mA/cm2 at 80 °C to avoid a pressurized system. [1]
It was possible to avoid the issue of pressurization above 100 °C by designing an electrochemical setup which reaches pressures up to 8 bar, enabling the use of high temperatures (160 °C). The vanillin yield from Indulin AT Kraft lignin was 4.2 wt% which is 60% of the possible amount relative to the nitrobenzene oxidation standard. Acetovanillone was also obtain as a by-product in 0.8 wt%.[2] In the further studies with a high-performance oxidiser (peroxodicarbonate) it was possible to facilitate 90% of the possible amount of vanillin. The two-steps ex-cell approach is highly selective. [3]
Weak and intermediate black liquor was supplied from Stora Enso. Without modifications it represents an excellent electrolyte due to its high inorganic content. As electrode materials low-cost metals including nickel and copper were studied in batch using stirred tank batch reactors and flow through cells in multipath operation. Applying chrono-amperometry the optimal temperature window has been identified between 80 and 120 °C, resulting in current densities up to 500 A/m2 without deposit formation on the electrode surfaces, when working in the voltage range up to 2.2 V. Several process parameters, such as the black liquor flow rate and viscosity have been identified as crucial for stable operation both in chronoamperometric runs and in constant voltage runs of 12h and will be discussed in detail. Applying online-GC the formation rate and faradaic efficiencies have been calculated. It was found that the faradaic efficiencies towards lignin oxidation is close to 90%, while the efficiency towards hydrogenation is below 50%. Periodic sampling and analysis using GPC, C13-NMR and GCxGX-FID/MS enabled us to calculate apparent depolymerisation rates towards monomeric methoxy phenols, aldehydes and acids, such as vanillin, and guaiacol. The extent of undesired repolymerisation could be quantified and correlated both to the black liquor composition and the electrode materials. Implementation with further electrocatalytic reduction of produced monomers will be discussed. Further, the current progress towards scale- up of electrodes and cells, options for process intensification and integration into kraft pulp mills will be presented. Additional challenges, such as initial foaming of black liquor during electrochemical conversion and viscosity increase due to water consumption will be addressed.
[1] D. Schmitt, N. Beiser, C. Regenbrecht, M. Zirbes, S. R. Waldvogel, Adsorption and Separation of Black liquor-derived Phenol Derivatives Using Anion Exchange Resins, Sep. Purif. Technol. 2017, 181, 8–17.
[DOI: 10.1016/j.seppur.2017.03.004]
[2] M. Zirbes, S. R. Waldvogel, Electro-conversion as Sustainable Method for the Fine Chemical Production from the Biopolymer Lignin, Curr. Opin. Green Sustainable Chem. 2018, 14, 19–25.
[DOI: 10.1016/j.cogsc.2018.05.001]
[3] WO 2020099350 A1 20200522, 2020.