Hey;
All of us know that there is a single preheater kiln and double preheater kiln, If by the new technology we can increase the capacity of single preheater kiln to 5,000 or 6,000 tpd and maybe more (I don't know). What is the need of double preheater kiln with 5,000 or 6,000 tpd?
Double preheater = single preheater x 2
Double preheater = I.D fan x 2
Douple preheater = Refractory lining x 2
and etc.
Regard
You are wrong to assume that everything will be doubled. For same capacity, if you use two strings instead of one, all the equipment (cyclones) will be smaller. So steel and refractory quantities will be less than double.
As the cyclones will be smaller, the height of the tower will also be shorter. This can have a huge impact on civil engineering cost.
It is also worng to consider that you would need 2 ID fans. I know many plants working with 2 identical strings and only one ID fan.
The final choice should be driven by an economical study taking into account the points above.
Best regards