A policy or infrastructure decision that switches from natural gas to grey hydrogen combustion does not reduce emissions. The study compares five scenarios at a consistent 1 MW net electrical output boundary: natural gas combustion as the reference, grey hydrogen combustion, grey hydrogen in a proton exchange membrane fuel cell, green hydrogen combustion, and green hydrogen in a PEM fuel cell. The gap closes faster as carbon pricing rises and green hydrogen production costs fall, which is the trajectory that both the IEA’s Hydrogen Review and the European Commission’s updated hydrogen targets assume. The Grey Hydrogen TrapThe most policy-relevant finding in the scenario comparison is not the superiority of green hydrogen combined with fuel cells. The more troubling finding is how poorly grey hydrogen combustion performs relative to both the natural gas baseline and the grey hydrogen fuel cell alternative.