Sustainable production of biohydrogen: Feedstock, pretreatment methods, production processes, and environmental impact

A significant increase in the use of hydrogen, expected to reach between 667 and 4000 TWh, is forecasted for the whole EU in 2050. Electrolysis is believed to be a "silver bullet" due to its synergy with the needs of the grid. However, biohydrogen generation could be complimentary to electrolysis since it does not depend on electricity prices. This review presents a comprehensive picture of the landscape in biohydrogen production, showing stateof-the-art research on different biohydrogen production processes and highlighting potential problems and shortcomings for different processes, including microbial-based production and thermal processes. The work shows that "colour coding" used nowadays is insufficient in terms of providing accurate information regarding the sustainability of particular biohydrogen production technologies. Instead, LCA can provide substantial information for each investigated process. However, there is a need for a wider scope of LCA studies since currently published studies present a syndrome of "carbon tunnel vision", often ignoring impacts other than global warming. Moreover, studies often tend to exclude the impact of capital goods production, which might provide an incomplete overview of such technologies. Moreover, it should not be overlooked that biohydrogen is capable of achieving negative values of CO2 emissions if CCS is implemented.