The detective said the valve cover on the stand-pipe was rusted shut, yet when the cover was removed, shiny new brass threads are seen inside the pipe, indicating it is nearly new.
Liquid Nitrogen, which transports at -40°, flowing from a poorly-secured hose into an un-mated stand pipe would've instantly frozen the jacket wrapped around the joint and iced the hose from the joint backward to the truck.
Lincoln's plan to replace the oxygen in the boiler room with Nitrogen failed to take into account the -40° temperature of the gas, which would have manifested almost instantly, without the wispy, cough-inducing vapors seen during the officers' approach to the kidnap victim.
The placards on the tanker they commandeer are not the green, Class 2 1977 Compressed Gas placards as they should be, rather they are Class 3 1993 Flammable Liquid placards, indicating the tanker actually carried Diesel fuel.
Liquid Nitrogen isn't transported in simple tanker trucks like the one at the gas station. It would require a heavily-insulated tanker.
Rhyme knows that nitrogen is hypoxic, but no-one knows the size of the building. In addition to the temperature flaws with liquid nitrogen that others have highlighted, trying to find a balance point that would affect the kidnappers but not kill everyone in the building would be impossible. If it had any effect at all, there is a massive likelihood that they all die, Police included.