This study has focused on identifying the cause of agglomeration that occurred in a domestic commercial-scale circulating fluidized bed boiler. Solid refuse fuel (SRF) was fed into the target facility to produce electricity. Agglomeration occurred in the combustor and cyclone during commercial operation. The bed material, clinkers produced in the combustor and cyclone, and boiler ash were collected, and components that are known to cause agglomeration were analyzed. Additionally, the possibility of slagging and fouling formation was predicted using components obtained by XRF analysis. The melting temperature of the bed material was decreased by complex reactions of low-boiling-point metal, alkaline metal and sulfur, and chlorine components. Then, agglomeration was generated because the bed material and ash were melted and combined. Basicity (B/A), which can lead to slagging, was estimated to be above 1.0 (reference 0.5 < B/A < 1.0). The boiler ash had a basicity of 1.83. The slag viscosity index (SVI) was estimated to be between 18.83 and 49.78 (reference 65 < SVI < 72). The boiler ash and combustor clinker had 3.30 and 4.40 total alkali (TA) values, respectively (reference 0.3 < TA < 0.4). This condition determined that slagging and fouling formation easily progressed. This result is expected to be utilized as data for preventing agglomeration formation and clinker generation.