The energy penalty of producing hydrogen-containing fuels from hydrogen

Phasing out fossil carbon-based fuels as part of international climate change mitigation policies has put hydrogen on the agenda as an alternative transport fuel, but also as a chemical feedstock. Production, handling and transport of hydrogen are, however, energy intensive and complicated and therefore several approaches have been suggested for converting hydrogen into hydrogen-based energy carriers. Examples are so-called solar fuels or electro-fuels, producing methane or methanol, or ammonia. For the production of hydrogen the
current routes using natural gas or other fossil carbon sources are being replaced by zero-carbon technologies based on renewable electricity. This paper will address the energy penalty resulting from the “degrading” of hydrogen to lower energy content fuels that are easier to handle. Exergy analysis is used, limited to the chemical exergies of the species considered. H2 can be produced via bio-methane steam reforming or water electrolysis. It is here reported that production of ammonia from H2 will give an exergy loss of 18-51%, production of synthetic methane 32-51% and production of methanol 18-40%, respectively. Producing ammonia from hydrogen obtained via steam reforming of methane gives a larger energy penalty than the widely used Haber-Bosch process. For methane and methanol the removal of CO2 from flue gas for reaction with electrolysis H2 gives a 4-7% lower exergy loss than when using CO2 removed from air. Methanol production gives 11-14% smaller exergy losses than producing methane. The smallest exergy losses (~ 18%) may be obtained with methanol production via electrolysis of water and reaction with CO2 from flue gas, or ammonia produced via electrolysis of water and a reversed fuel cell for ammonia production with nitrogen from an air separation process. When use as fuel is aimed at it can be argued that the chemicals produced cannot compete with the electricity spent (and battery technology).