Ash partitioning in combustion of pelletized residual agricultural biomass

The use of residual agricultural biomass in the domestic sector has faced limitations for years due to the problems caused by the characteristics of its ash (quantity and composition). Quantifying and characterizing the fractions into which ash is divided during combustion process can help overcome this barrier. Based on ash samples from tests carried out in a laboratory fixed-grate reactor, a methodology for the analysis of ash partitioning during combustion has been presented and applied to four agropellets (one woody and three blended with herbaceous component -mixed agropellets-), both at the ash level and of the main chemical elements that constitute it. Ash has been separated in five fractions, analyzing the results considering the fuel composition and the operating conditions. In addition to detecting important differences in the partitioning among tested fuels, especially between mixed agropellets and the woody one, it should be noted that the analysis of elemental partitioning has shown that the order in the retained percentage of each element in the bottom ash fraction is Si > Ca + Mg > P > K + Na for all agropellets. An increment in the excess air has been corroborated to improve the behavior of the tested mixed agropellets both in sintering and in deposition. In case of the woody pellet, as its much lower sintering status supposes a dominance of fractions related with entrainment, it is preferable working with low excess air. The presented methodology allows estimating the importance and composition of each fraction, which affect the operation and condition the boiler design and is of great importance to achieve larger use of this kind of biomass.