Hydrogen production by methane-pyrolysis processes: an evaluation of CO2 emissions and carbon footprint based on material and energy balances

Hydrogen (H2) production processing schemes based on methane (CH4) pyrolysis are developed to reduce and possibly delete the amount of fossil carbon dioxide (CO2) emissions, by exploiting the sequestration of the solid carbon product. Material and energy balances are accounted for by a process simulation analysis using Aspen Plus™. A few aspects of process and plant feasibility at the industrial scale are discussed. The process performances are compared to those of a classical CH4 Steam Reforming plus Water Gas Shift process (SRWGS). All the scenarios investigated have much lower CO2 emissions when compared to SRWGS, with a 49 % to 98 % reduction per unit of H2 produced (i.e., 0.164 – 4.69 kgCO2/kgH2 versus 9.24 kgCO2/kgH2 for SRWGS). This holds if the carbon product of the pyrolysis is stored underground. The yield of H2 is lowered (i.e. 0.156 – 0.247 kgH2 produced per kgCH4 consumed, versus 0.310 for SRWGS). Energy duties are discussed as well. It is suggested that, if a mixture of CH4 and biomethane (bioCH4) obtained from food industry wastes is used as the feedstock, it is possible to delete the overall process carbon footprint with a bioCH4 fraction in the feed between 22 % and 52 %. Above these threshold values of bioCH4 fraction, H2 production could be achieved with a negative carbon footprint. If a 100 % bioCH4 feed were used, the CO2 emission balance would be consistently negative (between –2.6 and –5.0 kgCO2 per kgH2 produced). It is concluded that making H2 by a CH4 pyrolysis process with a suitable bioCH4 fraction in the feed is potentially not only a sustainable route to green H2 but also an interesting alternative technology for the net sequestration of CO2 contained in the atmosphere.