Biohydrogen production from biowaste: Assessment of the flammability of bioreactors gaseous mixtures

The European Union's 2030 strategy targets sustainable growth through a comprehensive circular economy framework, which includes the management of food waste. Within this context, the organic fraction of municipal solid waste (OFMSW) represents a promising substrate for fermentative biofuel production. Dark fermentation stands out among biological hydrogen production methods for its practical advantages, including efficient organic waste degradation and high hydrogen generation rates. Unlike the established process of anaerobic digestion for biogas, Dark fermentation enables the co-production of biohydrogen (bio-H2) and volatile fatty acids with a smaller volumetric footprint. A significant safety concern in bio-H2 systems is the potential formation of explosive atmospheres (ATEX zones) due to accidental leaks from process components. This study investigates a pilot-scale bioreactor producing bio-H2 from organic waste, specifically evaluating the flammability characteristics-such as the lower flammability limit and laminar burning velocity-of the resulting gaseous mixtures (containing H2, CH4, and CO2) under varying operational conditions. The findings indicate that supplementing sewage sludge with food waste elevates both the hydrogen concentration and the flammability of the gas mixture. Furthermore, CO2 dilution raises the lower flammability limit and reduces the laminar burning velocity, primarily by lowering the adiabatic flame temperature. This inhibitory effect of CO2 is linked to a reduction in the mole fraction of key free radicals (H, O, and OH), thereby slowing the chain-branching reactions that are critical for flame propagation.