Our kiln system is dry with preheater, precalciner, and bypass system. We are facing the problem of high free lime (reached up to 5-6%) although the liter weight is normal and some time higher than normal (between 1200- 1300 gr/lt). Increasing the gas will increase the liter weight and will create grinding problem for the clinker. What do you think the reason of this problem?

Free lime in clinker is generally controlled by maintaining liter weights. But the problem expressed by you for a gas fired plant seems to be due to basic nature of material, mineralogy, possibly higher free silica etc.If u can shed some more light on the material characteristics then further analysis may be done. What is the rawmeal residue levels and also the PSD of clinker from the crusher? If the rawmix has been corrected properly, pl check the combinability temperature. 

The most likely reason for this problem is that the lime saturation is higher than can possibly combined in the CSAF quaternary system. This situation can arise particularly with high K2O content in clinker.