I recommend that you watch the "Planet of the Humans" documentary to understand why there is no cheap solar energy, no so-called "green" energies: https://www.youtube.com/watch?v=Zk11vI-7czE
So there will never be any "green" planes, no matter if they use hydrogen or kerosen. Since the minimum required amount of energy to fly from point A to point B with N passengers will always be the same.
There are some truths in the movie (e.g. biomass would not work), but many times he is plain wrong. E.g. 8% solar efficiency, when my neighbours installed 21% one and 15% is common on utility scale. No solar panel on the market has just 10 years+ lifespan, many come with 20+ year warranty and could still produce energy afterwards with tiny decrease in efficiency.
Not necessarily, it depends on the byproducts - hypothetically, suppose when you burned the fuel the emissions were SOMEHOW 100% methane. You're essentially turning CO2 into methane in this farcical hypothetical, which is probably worse than burning normal fossil-kerosene.
Obviously the emissions won't be methane, but you'll need to track the CO2e of every type of emission it produces other than CO2.
Battery planes will eat the market from below, step by step.
The process is straightforward:
1. A lot of cheap solar energy, from the best location.
2. Electrolyse to produce hydrogen.
3. Capture CO2. Either from air or from carbon intensive processes (e.g. producing cement).
4. Use Sabatier reaction to bond CO2 and hydrogen to produce carbohydrate fuel.
5. Since we use same amount of CO2 to produce as there will be burn this carbon neutral fuel.
It's much easier to do it at scale, than redesign, get approved new planes, replace all existing fleets and the infrastructure. This could easily take 60-120 years, when we need to act much sooner to prevent worst consequences of global warming.
Of course, today this would not work as individual steps are too expensive. Though all of the steps could be much cheaper as solar energy panel shows.
It gives possibility for oil rich countries to transform to post carbon economy when they resources will be depleted and consumers would prefer carbon neutral fuels.
I guess all of this sovereign funds would be much better of investing in this tech than pouring money in SoftBank to do WeWork style opportunities.
Google crowd oil or solar kerosene for papers about it.