Browsing by Author "Valentim, B."
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- Characterization of Coal Maceral Groups by Micro-Raman SpectroscopyPublication . Valentim, B.; Guedes, A.; Prieto, A. C.; Lemos de Sousa, M. J.Uma caracterização rápida e não-destrutiva da organização estrutural do material carbonoso pode ser conseguida através da espectroscopia micro-Raman, uma vez que podem ser obtidos espectros de áreas micrométricas. Uma aplicação óbvia desta técnica é a caracterização pontual de macerais do carvão. As análises de espectroscopia micro-Raman efectuadas em diferentes grupos macerais do mesmo carvão revelam, para além da presença dos habituais picos aos 1580 cm-1 e 1350 cm-1 no espectro de primeira ordem, dois picos de fraca intensidade: um próximo dos 1170 cm-1 nos macerais do grupo da inertinite e outro aos 1470 cm-1 nos macerais de vitrinite e liptinite. Os diferentes grupos macerais apresentam várias diferenças no espectro de primeira ordem, um estreitamento do pico 1580 cm-1 e seu deslocamento para números de onda mais elevados da vitrinite para a liptinite e inertinite; um deslocamento do pico 1350 cm-1 para números de onda mais baixos da vitrinite para a liptinite e para a inertinite, conjuntamente com o seu estreitamento na inertinite e um ligeiro aumento da razão entre a intensidade dos picos 1350/1580 da vitrinite para a inertinite e para a liptinite. A fast and non-destructive characterization of the local state of carbonaceous material may be achieved with a micro-Raman spectrometer, since spectra from micrometric areas can be recorded. Therefore, this technique can obviously be applied for characterization of coal macerals. Raman spectral analysis performed on different coal maceral groups of the same coal reveals not only the presence of the usual peaks at 1580 cm-1 and 1350 cm-1on the first-order Raman spectrum, but also the presence of two additional weaker peaks, one around 1170 cm-1 in inertinite and another around 1470 cm-1 appearing in vitrinite and liptinite. Differences of the first-order spectrum of the different macerals include a shift of the 1580 cm-1 peak towards higher wavenumbers accompanied by its narrowing from vitrinite to liptinite and inertinite; a shift of the 1350 cm-1 peak towards lower wavenumber from vitrinite to liptinite and to inertinite together with a small narrowing in the inertinite and a slight increase in the 1350/1580 peak intensity ratio from vitrinite to inertinite and to liptinite.
- Combustion studies in a fluidised bed—The link between temperature, NOx and N2O formation,char morphology and coal typePublication . Valentim, B.; Lemos de Sousa, M. J.; Abelha, P.; Boavida, D.; Gulyurtlu, I.Five commercially available high volatile bituminous coals from different origins were studied with the objective of characterizing their petrographic nature with respect to emissions of NOx and N2O. The chars produced [at temperatures ranging from 700 to 1000 °C] from these coals were also petrographically analyzed to assess the contribution of char to NOx and N2O formation during combustion. Vitrinite-rich coals produced higher porous chars (cenospheres and tenuinetworks) than those that are rich in inertinite. The former coals were, however, found to release lower concentrations of NO. Consistent with previous works, N2O emissions were observed to decrease significantly with temperature, however, on the whole, the N2O emissions from vitrinite-rich high volatile coals were less than those from inertinite-rich coals. Additionally, high porous chars were found to give rise to lower emissions of NO and N2O.
- A study on coal blending for reducing NOx and N2O and levels during fluidized bed combustionPublication . Boavida, D.; Abelha, P.; Gulyurtlu, I.; Valentim, B.; Lemos de Sousa, M. J.The objective of the study was to investigate the effect of blending on the emissions of NOx and N2O during the fluidized bed combustion of five coals with different levels of nitrogen, volatile matter, and fixed carbon contents, and diverse coal and char petrographic composition. The devolatilization chars were produced in a fluidized bed combustor. Using the petrographic analyses of the coals, sixteen coal blends were made to carry out the combustion tests. The combustion of individual coals and the sixteen blends was carried out in an electrically heated atmospheric fluidized bed combustor. The emissions of NOx reached maximum values at a bed temperature around 1173 K and decreased with lower combustion temperatures. The amount of nitrous oxide formed decreased, for all five coals, as the bed temperature increased from 1053 to 1303 K. These trends are consistent with the observed experimental data obtained by other researchers. It was observed that for ratios of 1.3 to 1.6 of fixed carbon to volatile matter, both NOx and N2O emission levels were minimal and, outside this range, both oxides showed a sharp increase in their formation/emissions. The coal blends were also found to show similar trends with increasing the bed temperatures. The results serve to establish the relative importance of volatile and fixed carbon reactivity and coal and char petrography, which is believed to significantly contribute to devolatilization mechanisms and char reactivity.
- The identification of unusual microscopic features in coal and their derived chars: influence on coal fluidized bed combustionPublication . Valentim, B.; Lemos de Sousa, M. J.; Abelha, P.; Boavida, D.; Gulyurtlu, I.During the petrographic study of seven feed coals from different origins, it was found that these coals presented microfeatures such as: material size, shape, weathering, thermally affected particles and contamination. After devolatilization under fluidized bed conditions, some chars presented the consequences of the above mentioned microfeatures, i.e., unreacted coal, unswelled particles, coatings and microstratification. Since the amounts of the microfeatures observed were low (less than 1%), the present study is essentially observational/ descriptional. However, it seems very likely, from the observations that were made, that the occurrence of one or more of these microfeatures in coal, depending on their kind and abundance, may have significant effect on the coal devolatilization.