It should burn pretty cleanly (as it's just carbon), but who knows what it would be mixed with in an actual application. Could be mixed with all kinds of plastics and resins that are nasty when burnt.
Graphite is "just carbon" too, but you cannot simply burn it by putting a lit flame to it -- it's simply not hot enough to combust it. To put it in perspective, graphite is frequently used as a crucible to melt aluminum in. Aluminum won't melt in your home fireplace (it's not hot enough), but graphite crucible will easily hold molten aluminum without combusting itself.
Graphite is also used to make rocket exhaust nozzles; it is really thermally stable stuff. Diamond by contrast is much easier to burn. The good news is that if you want to dispose of graphene, just stick it with a lot of other graphene and make pencils. It’s not as though we worry about disposing of pencil “leads” after all.
It should burn with all the side products of most artificial carbon chains (aromatics and poisonous carbon oxides). Those aren't hard to deal with, but don't expect it to burn cleanly.
It should also be hard to burn. It will probably require some water vapor to burn at all, and do it slowly.
I'm not sure I understand how you arrived at your first point. It seems to me that at a suitable temperature and oxygen content atmosphere, it should burn about as clean as natural gas. Why would you expect otherwise?
I also do not understand your second point. Surely, it is oxygen that is the missing ingredient in the burn rather than water vapor. What does water vapor provide to increase the rate of combustion?
About the first point, you are expecting total combustion of a solid. That is hard to achieve. By your theory coal should also burn cleanly, and it evidently doesn't.
With enough temperature and a high oxygen pressure, graphene will burn cleanly. But also will your furnace, and my bet is that both will do so at roughly the same temperature and pressure.
About the second point, oxygen does not react well with graphite because it's entire surface does not let its electrons go very easily. Graphene has similar properties. Water does catalyze the burning of graphite, it may very well do that to graphene too.
Coal has lots of other stuff mixed in to carbon: notably sulfur, and up to 10% of non-burning minerals (ash), often containing significant amounts of radioactive uranium and thorium.
Pure carbon (as in graphene) should burn much more cleanly, provided abundant oxygen.
Why dispose them? They’re a great carbon sink and as long as we don’t make short graphene chains I think they’re also not toxic for the environment. The process described uses methane (greenhouse gas) and if the process would be done with renewable energy it could even perhaps be carbon negative?
Yes, there are some papers that suggest that adding graphene-like elements to for example concrete will make them stronger. I think that if we can find a carbon neutral process of creating graphene it could be a great additive and a carbon sink (finally making a useful product out of carbon). It can even be used to detect damages in concrete (electrical resistivity value reduces at maximum compressive load). Here is an interesting paper: www.mdpi.com/2071-1050/9/7/1229/pdf but there are more applications once you start looking.
