“…Natural gas blending rules also limit the amount of H 2 in pipelines in order to minimize H 2 leaks and metal embrittlement, although compositional tolerances are unclear. Various studies indicate different maximum blending levels (Altfeld and Pinchbeck, 2013;Melaina et al, 2013;Hodges et al, 2015); the overall conclusion is that the safe level for H 2 blended into natural gas pipelines is ≤20%, and must be assessed on a case-by-case basis due to differences in pipeline materials, operating pressures, and states of repair. Other impurities that must be completely removed from natural gas (e.g., H 2 S, Hg, oils) and/or biogas (amine, chlorine and siloxane compounds) (Luostarinen et al, 2011;Götz et al, 2016;Corvini et al, no date) are not produced from CO 2 RR so would not be problematic unless contamination or degradation of materials occurred.…”