Process heater stacks are not lined with ceramic fiber as the flue as velocity in stack is high. they are lined with castable refractory.
The density of the refractory varies from on designer to other. In my experience* it has been 1100 kg/cu.m. But I will suggest please see the drawing to get refractory type used. Once refractory specification is known one can get the density from its datasheet. Refractories lighter than 1100 kg/cu.m density are very weak and susceptible to failure during stack erection.
regards