Chemical study of bottom ash sintering in combustion of pelletized residual agricultural biomass

Abstract

Although residual agricultural biomass is a high-potential energy source, its use is curtailed by the characteristics of its ash (quantity and composition). This ash can cause problems during combustion, including bottom ash sintering, which is the main focus of this study. Results of bottom ash proportion and sintering degree yielded by combustion tests carried out with four different agropellets (one was exclusively woody, and the others were blended with various herbaceous components –mixed agropellets) under different operating conditions in a laboratory fixed-grate reactor were presented in a previous paper. The inlet air temperature and the excess air were the two controlled operating parameters in the tests. To bring the analysis further, samples of bottom ash collected during these tests have been characterized using SEM-EDS and P-XRD techniques. The results confirmed that, given its high content of alkali metals and its high ratio (K+Na)/(Si+P), the differences in sintering behavior presented by the four agropellets are related to the formation of alkali metal silicates and phosphates; Si+P is the limiting and determining content that condition the sintering severity. Regarding the influence of operating conditions, only small variations in the composition of both the sintered and not sintered fractions of the bottom ash have been detected when varying inlet air temperature and excess air in the analyzed range. Nevertheless, these SEM-EDS results have led to the detection of two opposing phenomena related to increased excess air: higher ash entrainment and lower vaporization, which are associated with the increase of air velocity and the reduction of combustion temperature, respectively. Whereas in the mixed agropellets and in the range of excess air tested these two effects offset each other, woody agropellet is significantly affected by entrainment due to its lack of sintered matter. These results provide useful information towards a solution of the problem posed by sintering in the combustion of residual agricultural biomass.