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Pyrolysis product evaluation and prediction of coals of different rank
Pretorius, Gustav Nico
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When coal is heated slowly (<10 °C/min), as in the Lurgi Fixed Bed Dry Bottom (FBDB) gasification process, products formed in the pyrolysis region of the gasifier include gas liquor, condensable tars, oils and non-condensable gases. Knowledge of the temperature profile together with coal behaviour (partitioning into char, tar, gas and liquid) is therefore very important when designing a gasification plant. Extensive research on the effect of coal type on the flash pyrolysis at high heating rates of different coals has been reported, but only limited research on the tar, oil and naphtha composition produced by slow heating rates exists. When screening coals for suitability in Lurgi FBDB gasification, slow heating rate pyrolysis predictions and a total mass balance of elements (carbon, hydrogen, nitrogen, sulphur and oxygen) of the product yields obtained based on only raw coal information (proximate-and ultimate analysis) will be very advantageous and needed. With the support of validated existing pyrolysis models e.g. FLASHCHAIN® and statistical regression the determination of which coals are more amenable to chemicals and gas production can be ascertained in an uncomplicated fashion. The aim of this study was to evaluate/develop models for five coals of different rank in order to predict the char, tar, water and gas yields, as well as the tar composition (naphtha, oils and tars) when heated at slow heating rates (<10°C/min). A modified Fischer Assay setup, developed at the North-West University, was used in order to investigate pyrolysis at temperatures higher than that of the ISO 647 standard, for the pyrolysis experiments, i.e. final temperatures of 520 °C, 720 °C and 920 °C. The five coals studied (A-E) were characterised and classified as follows: Coal A, B and C were ranked as lignite B coals, coal D as rank C bituminous, and coal E as subbituminous. Anthracite was not included in this study, since the selection was based on coals which are conventionally used in Lurgi FBDB gasification. The Fischer Assay pyrolysis experiments showed that temperature has a large effect on the pyrolysis product yield, i.e. the volatile matter released increased with an increase in pyrolysis temperature. Tar yields of all coals increased to a maximum at 720 °C, before decreasing or remaining the same at higher temperature, which means that a maximum tar yield has been reached by 720 °C. Only the char yield was found to be rank dependent with large variations being observed for the derived tar yields. Coal B had the highest tar yield (8.8 wt%.) compared to that of coals A, C, D and E (2.8wt%, 2.9wt%, 3.2wt% and 4.9wt% respectively). An elemental balance of the pyrolysis products derived from coal B, relative to the other coals, showed that more carbon and hydrogen partitioned into the gas phase than into the tar fraction, where the opposite was seen. This partitioning therefore seems to correlate well with the tar yield observed. The elemental carbon and oxygen contents of the tars formed at a final pyrolysis temperature of 920 °C have been found to correlate well with the elemental analysis of the raw coals. Carbon contents in the original coals are ranked as follows: coal A < coal C < coal B < coal D < coal E, whereas the carbon content in derived tars formed at a pyrolysis temperature of 920 °C are ranked as follows : coal A < coal B < coal C < coal D < coal E. Identical trends were found for oxygen. Coals of lower rank therefore form tars that are more oxygen rich than coals of higher rank. This was seen evident in the gas yield results, where lower rank coals formed more oxygen gas species than higher ranked coals. No rank dependence was observed for the results obtained using Simdis and SEC-UV. From the Simdis results, the only temperature dependence found was that the tar heavy residue fraction at 920 °C was higher than observed at 520 °C, which indicates that lighter tars cracked while being heated to 920 °C. From the GC-MS results it appears as if tars derived from higher rank coals form more phenolic compounds, and a similar trend is seen for furans, but no rank dependence is apparent for cresol and paracresol and long chain alcohols. For the gas species it was observed that the CO and CO2 yields were rank dependent and closely related to the oxygen content in the original coal. Lower rank coals had higher oxygen contents and produced more oxygen containing gases (CO and CO2). On the other hand, no rank dependence was found for the H2 yields. FLASHCHAIN® was able to provide accurate predictions of the char yield, and good trends of tar and gas yield for all 5 coals. Elemental carbon in the char was also accurately predicted at 720 °C and 920 °C, and elemental nitrogen in the char was accurately predicted at 520 °C and 720 °C. Elemental carbon content in tar formed at 920 °C was also accurately predicted. Statistical regression was applied in order to determine the relationship between raw coal properties and pyrolysis products and its compositions. A number of coal properties were found to have statistically significant relationships with some of the coal pyrolysis products, but no strong correlation was found for the gas yield. These properties include inherent moisture content, ash content, elemental carbon, oxygen and sulphur and Al2O3 for the char yield. The statistically significant properties that correlated well with the water yield include inherent moisture content, ash content, elemental carbon, nitrogen, oxygen and sulphur and Al2O3. Finally, XRF (x-ray fluorescence) ash constituents (MgO, CaO, TiO2 and Fe2O3) seemed to have statistically significant correlations with the gas species and tar composition, which could indicate catalytic effects of the minerals occurring during heating.
- Engineering