Ash Behaviour during Combustion of Agropellets Produced by an Agro-Industry—Part 2: Chemical Characterization of Sintering and Deposition

Eight different pellets (one woody and seven blends of woody and herbaceous biomass) produced by an agro-industry were tested in a fixed-bed reactor to characterize their behaviour during their combustion process. The objective was to analyze the possible problems that could arise and, thus, achieve advances in the greater penetration of these agropellets in the energy market. The blends’ design and tests results are presented in the first part of this article. The results of the bottom ash and fly ash samples obtained from the combustion tests using SEM-EDS and P-XRD techniques were analyzed in order to delve into the sintering and deposition phenomena, respectively. Regarding the sintering, a clear relationship has been found between the results of the SEM-EDS analysis and the initial composition of the ash of the fuels. Additionally, the analysis of the results confirms a different ash behaviour regarding the sintering phenomenon depending on the ratio between the amounts of Si, Ca + Mg and K + Na. With respect to deposition, it has been determined that in woody pellets, showing the lower sintering degree, the predominant deposition mechanism is by inertial impact, while in blend pellets it is by condensation, which increases when the percentage of herbaceous in the mixture increases. It has been proved that, in order to compensate (at least partially) for the negative effect of the herbaceous components in the blends, working with a higher λ value is interesting, as it achieves a decrease in the combustion temperature (with λ >1), which implies both a decrease in the sintering degree and in the deposition, improving therefore the ash behaviour in the combustion of agropellets