No abstract

No abstract

This study shows the utility of optical microscopy to trace the origin of unburned carbon in fly ashes from coal blends. Coal blends currently burned in four power groups in Spain have been investigated. Mill samples were separated into size fractions, and petrographic analysis were performed to assess the extent of blend components segregation as a function of different coal grindability. Strong segregation was observed when coals from a wide rank interval were ground together. In this case, anthracite particles accumulated in the largest size fractions, high and medium volatile bituminous coals accumulated in the smallest ones, and semianthracites accumulated in the intermediate size fractions. No clear segregation trend was observed for anthracites ground together with an inertinite-rich semianthracite. Chars from single coals were prepared in a drop tube furnace operated at 1300 °C under 5% oxygen in a nitrogen atmosphere. Petrographic analysis of chars were based on the optical texture and development of porosity, which are features that are maintained even in extensively burned material. This information was used to identify the coal responsible for the unburned carbon particles in fly ashes. Vitrinite- and inertinite-derived material from anthracite and semianthracite were the major components of unburned carbon; however, inertinite from high volatile bituminous coals and other nonassignable inertinites also were recorded. Results indicate a rather different combustibility for the different coals in the blends. High volatile bituminous coals burned almost to completion, whereas the semianthracite-derivedand, particularly, the anthracite-derivedparticles were responsible for most of the unburned material. This was true even for blends where anthracite was only a minor component of the feed blend.

The current work investigates the performance of petroleum coke (PC) as a blended fuel under pulverized-fuel combustion conditions. Three full-scale combustion experiments were carried out: a pure Carboniferous, high volatile bituminous coal and two blends of this coal with different proportions of PC. The samples studied included feed fuels and blends, fly ashes, chars taken at different positions of the combustion chamber, and chars prepared in a drop tube reactor to test the performance of the individual fuels. The addition of PC led to a substantial increase in the unburned carbon of the fly ashes. The petrographic analysis of the granulometric fractions of the fuels revealed that this increase cannot be attributed to an enrichment in coke of the coarser fractions, as reported in the literature. On the contrary, the finer fraction contained slightly more coke than the raw blend. The petrographic analysis of the chars collected with the suction probe and the fly ashes showed that the two blended fuels were strongly enriched in PC-derived material, indicating a poorer combustibility compared to the high volatile bituminous coal. It is concluded that the reactivity of the blends in the later stages of combustion is related with the contents of PC-derived chars and the burnout itself.