Over the last decades, several indices based on ash chemistry and ash fusibility have been used to predict the ash behaviourduringcoalcombustion, namely, itstendencyforslaggingandfouling. However, due to the physical chemical differences between coals andbiomass,inthis work only the applicabilityofan ash fusibility index (AFI) to the combustion andco-combustionofthree typesofbiomass(straw pellets, olive cake and wood pellets)withcoals was evaluated. The AFI values were comparedwiththe behaviourofashduringcombustioninapilotfluidizedbedandaclose agreement was observed between them. Forabetter understandingofthe mechanisms associatedwithbedash sintering, they were evaluated by SEWEDS and the elements present on the melted ash were identified. Evidencesofdifferent sintering mechanisms were found out for the fruitbiomassand herbaceousbiomasstested, depending on the relative proportionsofproblematic elements. The particles deposited onafoulingprobe insertedinthe FBC were analyzed by XRD and the differences between the compounds identified allowed concluding that the studied biomasses present different tendencies forfouling. IdentificationofKCl and K2SO4inthe deposits confirmed the highertendencyforfoulingoffruitbiomasstested rather than wood pellets.