Utilisation and valorisation of distillery whisky waste streams via biomass electrolysis: electrosynthesis of hydrogen

Abstract

Fuel-flexible hydrogen generation methods, such as electrochemical conversion of biomass, offer a route for sustainable production of hydrogen whilst valorising feedstocks that are often overlooked as waste products. This work explores the potential of a novel, two-stage electrolysis process to convert biomass-containing solid (draff/spent barley) and liquid (pot ale and spent lees) whisky co-products, from the Isle of Raasay Distillery, into hydrogen, using a phosphomolybdic acid (H₃[PMo₁₂O₄₀] or PMA) catalyst. Characterisation results for whisky distillery co-products will be presented, including thermogravimetric, differential scanning calorimetric, CHN elemental, total organic carbon and chemical oxygen demand analysis data. The results indicated that the characteristics of these co-products align well with those reported across the Scotch whisky distillation sector. Subsequently, the concept of thermal digestion of each co-product type, using the Keggin-type polyoxometalate PMA catalyst to abstract protons and electrons from biomass, will be outlined. UV-visible spectrophotometry was employed to assess the extent of reduction of the catalyst, after digestion of each co-product, and indicated that draff and pot ale offer the largest scope for hydrogen production, whilst digestion and electrolysis of spent lees is not viable due to the low biomass content of this distillation co-product. Finally, details of electrolysis of the PMA–biomass solutions using a proton-exchange membrane electrolysis cell (PEMEC) will be provided, including electrochemical data that help to elucidate the performance-limiting processes of the PEMEC operating on digested biomass–PMA anolytes.