Manipulation of gasification coal feed in order to increase the ash fusion temperature of the coal to operate the gasifiers at higher temperatures
Van Dyk, Johannes Chrisstoffel
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Coal is a crucial feedstock for South Africa's unique synfuels and petrochemicals industry and used by Sasol as a feedstock to produce synthesis gas via the Sasol-Lurgi Fixed Bed Dry Bottom (FBDB) gasification process. The ash fusion temperature (AFT) gives detail information on the suitability of a coal source for gasification purposes, and specifically to the extent ash agglomeration or clinkering is likely to occur within the gasifier. Ash clinkering inside the gasifier can cause channel burning and unstable operation. Sasol-Lurgi FBDB gasifiers are currently operated with the philosophy of adding an excess of steam to the process to control the H2/CO ratio of the syngas produced, but indirectly also to control the maximum gasifier temperature below the AFT of the coal. An opportunity exists to increase the AFT of the coal fed to the gasifiers by adding AFT increasing minerals to the coal blend before it is fed into the gasification process. For the aim of this study a typical Highveld Nr. 4 coal seam was investigated, as being used by the gasification operations in Secunda. In the drying and devolatilization zone no slag formation in the coal was observed. Based on HT-XRD analysis the predominant phases in the untreated coal sample were quartz, muscovite, calcite, dolomite. hematite. anhydrite, rutile and kaolinite. Kaolinite started to decompose to metakaolinite at ±450°C with the formation of amongst others mullite at a temperature of 850°C to 1000°C. Mullite formation can also take place if free Al2O3 is present in the coal that can react with free SiO2. However, free Al2O3 is normally not present in coal and the presence of the aluminasilicate (Al2SiO5) is formed as an intermediate phase due to the decomposition of kaolinite. From 500°C to 900°C, the carbonates, calcite and dolomite, started to decompose with the formation of lime and periclase. The feldspar (CaAl2Si2O8) observed, formed as a reaction product between the SiO2, Al2O3 and Ca-containing species present in the coal. In the gasification zone slag-liquid formed at a temperature from 1000°C. The formation of anhydrite (CaSO4) took place after the formation of calcite. At 1000°C anorthite, initially present as feldspar (CaAl2Si2O8) and gehlenite (Ca2Al2SiO7) became stable, due to partial melting of the low AFT mineral phases. Anorthite and gehlenite were formed as products from anhydrite, alumina and silica at temperatures around 900°C to 1100°C. Mullite decomposed at temperatures >1100°C, while quartz and anorthite were observed up to 1350°C. Above 1350°C the whole mineral phase assemblage in the coal sample was molten. When comparing the base case sample with the Al2O3-manipulated sample, it was clear that the mullite is one mineral that showed a significant difference in formation and mechanistic behaviours. In the combustion zone the decrease in the slag-liquid content confirmed the cooling and actual mineral formation and crystallization within the gasifier combustion zone. The representative coal ash, as it was produced after gasification, showed evidence of crystallization from the melt phase and formed due to the interaction of specific mineral species to produce a molten phase that had the correct chemistry to crystallize again. Mullite formation can also take place when free Al2O3 in the coal is available that can react with free SiO2, also present in the coal. With the addition of y-Al2O3 the free SiO2 in the coal can react with the y-Al2O3 to form mullite (Al6O5(SiO4)2) directly. The Al2O3 in the reactive form acts as a network former where SiO2 can be reacted on, to form mullite. The main conclusion of the addition of y-Al2O3 to the blend is that the slag-liquid content decreased with addition, only when the temperature was greater than 1000oC, which is of importance in Me operating region where the proposed higher gasifier temperature of more than 1250°C, is aimed for. Another observation from the AFT results was that the AFT was definitely non-additive (not a linear weighted calculated average) and not the weighted average AFT as was expected for the other coal properties such as the ash content, for example. The ash slagging behaviour is a non-additive property of individual coal sources in the blend and therefore difficult to predict. Viscosity modelling can be another tool for predicting slag mineral behaviour and used as a predicting tool, as has been done in this study. A higher viscosity for all relative density fractions were observed for all temperature ranges in comparison with the results obtained from the AFT analysis. In general it can be concluded that the unique opportunity that exists to increase the AFT, was tested, proven and mechanistically outlined in this study on the coal source fed to the Sasol-Lurgi FBDB gasifiers. The AFT can be increased to > 1350°C by adding AFT increasing minerals or species, for example Al2O3 or other mineral species, to the coal blend before it is fed into the gasification process. By increasing the AFT, the direct effect will be that steam consumption can be decreased, which in tum will improve carbon utilization.
- Engineering