Hydrogen vent stack fires can create NOx. While not typical, certain municipalities will require air permits to address the emissions from hydrogen flare systems, and even from intermittent ignition of vent stacks. This is highly dependent on the location and interpretation of regulations. A hydrogen flare system is much more likely to require an air permit than a standard vent system.

A vent system should still be installed in case the flare system does not remain ignited.  

At NASA Cape Canaveral, they used a natural gas line connected to the vent stack outlet with a thermal sensor to make sure the pilot was lit. They may also have had a sensor to ensure the H2 fire was lit. In cases where substantial quantities of unused hydrogen is vented and the timing and amount of the flow rate is known and controlled, flaring might be useful. NASA guidelines stipulate that flaring is appropriate for hydrogen vent rates surpassing 0.2 kg/s (~0.44 lb/s). Flare systems themselves must incorporate pilot ignition, flameout warning mechanisms, and a means to purge the vent line, ensuring comprehensive safety measures are maintained throughout the process.

Yes, for all stacks. GH2 has a minimum prescriptive height of 10 ft. There is no minimum prescriptive height for LH2. However, 25 ft has been a best practice for the industry for years. Vent stack outlets that orient the release vertically help reduce the radiation exposure at ground level. Care must be taken to consider varying weather conditions, particularly wind, as well as surrounding exposures that might be elevated in the vicinity of the vent stack outlet, such as nearby equipment, buildings, catwalks, and grade elevation changes.