Assessing an electrochemical process for the treatment of tar-containing wastewater with hydrogen recovery

Biomass gasification generates syngas with low carbon emissions but produces tar-contaminated wastewater that challenges environmental sustainability. This study explored using an alkaline electrolysis process with a DSA anode to remove organic pollutants from the effluent of a wet scrubber in a biomass gasification plant's syngas clean-up system. The effect of current density (16, 80, 160 mA cm−2), area-to-volume ratio (0.03 and 0.15 cm−1), and organic load on the process performances was assessed by monitoring total organic carbon, chemical oxygen demand and UV–Vis analyses; the phenolic fraction was also analysed with 4-aminoantipyrine colorimetric method. A kinetic analysis with zero-order and pseudo-first-order kinetic models was done for the relevant parameters, and the kinetic constants (kapp) were evaluated under all the explored conditions. Results showed lower organic loads (higher dilution), as well as increasing current densities, have a beneficial effect on the phenol removal and mineralisation, achieving up to 50 % mineralisation in 8 h with 1:100 wastewater at 160 mA cm−2 and 0.15 cm−1. Also, reducing the area-to-volume ratio leads to slower mineralisation processes. On the other hand, adopting low current densities with high organic loads lead to 100 % instantaneous current efficiency, in which the oxygen evolution reaction is fully suppressed, and pure hydrogen is produced. Insights into the economic aspects of the process in terms of energy per order and energy per mass were given: the energy consumption for the most favourable condition is 15.95 kWh kg−1, corresponding to 960 kWh m−3 and a total cost of 72 € m−3.