Re: your second point, splitting methane into H2 and CO2 is not something that's being investigated at lab scale, it's how 95% of global hydrogen production today is done. We're talking close to a trillion cubic meters per year from steam methane reforming.

It's just that most of the time, resulting CO2 is just released instead of being stored. But that is changing, fast.

I was specifically referring to pyrolysis, where unlike steam reforming no oxygen is introduced and so the carbon comes out as solid graphite. Scaling the process, and running it in a continuous-operation reactor, is still an area of active engineering work (see e.g. https://pdfs.semanticscholar.org/d36a/ff0a2db0f99a92c2debd5b...), mostly focused on designing the physical system so that the solid carbon comes out nicely during the reaction instead of gunking up the works.

What are the benefits of pyrolysis versus steam methane reforming (SMR)? My immediate thought would be that you leave energy on the table, and that storage at scale is harder when the end result is graphite. Moreover, SMR is proven technology at very large scale.

One advantage is that it produces high-purity graphite, which is a moderately expensive commodity, used for electronics manufacturing (especially power components in batteries), structural carbon composites, and machine lubricants.

The percentage of carbon captured is also damn near 100% - quite a bit better than gaseous carbon capture techniques I've heard of.

You do leave quite a lot of energy on the table (losing somewhere between 30% and 50%), so this is a technology that is only economically viable with high emissions pricing. But that seems like the way Europe is going anyway.

And yes, SMR is the proven and clearly superior technology right now; I'd call this TRL 4/5 right now. But with another decade of development?

Aha, so it's targeting "CCU". Then OK, I can maybe see there is a point initially, but not at anything near the scale we need.

Global graphite production from mining is at 1 megatonne/year, while synthetic is a few hundred kilotonnes/year. We need CCS to operate at tens of gigatonnes per year, that's four orders of magnitude beyond all the graphite the world needs.

This is the problem with anything CCU - there just aren't viable options with consumption at the gigatonne scale.

I'd imagine extra graphite is pretty easy to store, though?

At the gigatonne scale, nothing is easy to store. Fill the entire Hoover dam with graphite, and you've stored 6 gigatonnes. We need to store tens of gigatonnes per year for a hundred years.

We could put it down in empty mine shafts... or build entire mountain tops out of it!