Urea decomposes at 133°C, so you shouldn't even get close to that. This may leave residuals in your sample, but if you need urea you will have to deal with this problem.
I think 1st treat/heat the graphene material in vacuum oven and the degas within 60-80C for the BET analysis. You have to use lower temperature at the time of degas as the graphene material may burned out.