Role of ash on the NO formation during char oxidation of biomass

We have investigated the role of ash forming elements on the conversion of char-N to NO. The NO release was measured from combustion of single biomass particles, both in their raw state and after demineralization. The investigated biomasses were: dried distiller's grain with soluble (DDGS), sunflower seed, softwood, straw and torrefied wood. For three of the five raw biomasses, the NO release increased as the char oxidation proceeded. This phenomenon could not be observed for any of the demineralized biomasses. The conversion of char-N to NO was as high as 70% for one of the demineralized biomasses, while this ratio was 15-30% for the raw biomasses. The results show that the ash forming elements, but also the nitrogen content of the char influences the char-N conversion to NO. During oxidation of a single char particle, the NO formation/reduction mechanism can be explained as: the char-N reacts to NO, and this initially formed NO is partly reduced to N2 inside the same particle as the NO was formed. The results imply that both catalytic routes involving inorganic elements as well as non-catalytic routes for heterogeneous NO reduction occur in biomass combustion.