Effect of competitive char burning patterns on the efficiency of coal blends combustion

摘要

Blends in pf power stations offer several advantages; however, practice shows that burnout may not comply with the expectedrnweighted average from the single coals. Burnout depends upon char reactivity, which is a function of coal rank and maceralrncomposition. This work deals with the effect of single char reactivity on the combustion of their blends. Samples of coals andrnashes were obtained from power stations in Chile. Chars from those coals, obtained in a DTF reactor, were characterized byrnoptical microscopy and combustion profiles and activation energies were obtained from thermal analysis. Char characteristicsrnwere correlated with the Petrographic properties and burnout of the coal, the latter calculated from “loss on ignition” (LOI)rnmeasurements in fly ashes. As expected, char reactivity and burnout diminish with increasing rank. For blends, burnout mayrndeviate from expected averages depending on the differences among char reactivity, hence on differences in rank of the singlerncoals. It was found that in blend combustion, chars of similar reactivity burn under an assumed parallel pathway, with nornsignificant deviations from the expected burnout. As the difference in rank and char reactivity widens, chars compete for oxygenrnin detriment of the less reactive char and a lower burnout than expected is obtained. For larger differences of char reactivity, andrnif one char is highly reactive a parallel-series path way is assumed. The reactive char burns rapidly, while the less reactive char isrnleft to burn in a diluted cloud of particles with better oxygen access. This leads to greater than expected burnout. Char reactivityrnseem to govern these modalities for char combustion, and the reactivity may be related to characterization of the constituent coalsrnof the blend. Thus, in blend combustion, burnout and efficiency may be partially predicted.