Biomass is a very important energy source in the sugar mill industry as the main fuel in the power plant boilers. The efficiency of boilers for most sugar mills in Indonesia is quite low and the availability of bagasse is lacking so that in its operations, co-firing has to be applied with an alternative biomass fuels such as rice husk, cane trash, wood and saw dust. Based on the analysis, rice husk has a lower calorific (3248 cal/gr) value than that of bagasse (4076 cal/gr), but it has a higher ash content (20.67%) than that of bagasse (3.66%), so adjustments in boiler operation is required when using rice husk as co-firing. The higher ash content in biomass can affect the performance of the furnace and superheater in the boiler and increase the potential for deposit formation. The composition of rice husk ash was dominated by silica (SiO2) which reached 95.52%. It is higher than the silica content of bagasse which reached 74.99%. The high silica content in rice husks has the potential to conduce melting ash in boiler grate. The alkali content of rice husk which was around 3.64% is less than that found in bagasse which reached 17.33%. Even though, the ash composition analysis shows quite small potential for slagging, fouling, melting, sintering, and agglomeration, but the large amount of ash content in rice husks can increase the potential for the occurrence of the risks.