Fuel bed evaluations and coal properties transformation in a Sasol–Lurgi fixed bed dry bottom gasifier operating on North Dakota lignite
Abstract
The growth in coal consumption worldwide as well as the high oil prices in the recent past has led to the current increased interest in the application of coal gasification technologies. The Sasoi-Lurgi Fixed Bed Dry Bottom (S-L FBDB) gasification technology is one such technology that has the biggest market share in the world and maintains its competitive edge particularly with regard
to gasification of low grade and low rank coal. To ensure sustained competitive advantage through technology development, it is important to understand the fundamentals of the process as well as the behaviour of coals of different rank in the reactor. The main objective of this study was to investigate the fuel bed behaviour as
well as coal properties transformational behaviour in a S-L FBDB gasifier that gasifies North Dakota (NO) lignite. It was hypothesised in this study that using the FBDB gasifier sampling methodologies available in the literature, with some modifications to suit the context of this study, can help to explain the fuel bed behaviour as well as the coal properties transformational behaviour during gasification of lignite in the S-L FBDB process. To test the hypothesis and to achieve the objectives of the study, two MK IV S-L FBDB gasifiers (i.e. "Albert" and "Bernice") operating at the Great Plains Synfuels Plant of the Dakota Gasification Company (DGC) in the United States of America (USA) were sampled using the Turn-Out method developed by Bunt (2006) and modified in this study to suit lignite. The samples were characterised for their chemical, physical, petrographic and mineralogical properties which were then interpreted in terms of their transformation in the various reaction zones of the gasifiers. The different reaction zones In the "Bernice" and "Albert" gasifiers were successfully identified using chemical analyses (i.e. proximate and ultimate analyses as well as Fischer tar yields). Identification of reaction zones in the S-L FBDB gasifiers operating on lignite is a first in the history of the process. In comparing Secunda GG41 gasifier operating on bituminous coal with the DGC "Bernice" and "Albert" gasifiers operating on lignite, the reaction zones were found to be very different due to, amongst other things, the different operating philosophy, stability and coal rank. About two thirds of the reactor volume, in the case of DGC "Bernice" and "Albert" gasifiers, was found to be drying and devolatilizing the coal, leaving only about a third of the reactor volume for gasification and combustion. Nonetheless, due to the high reactivity of the lignite, more than 98% of the char/fixed carbon was consumed within a third of the remaining gasifier volume and this is a significant new finding. The fact that the entire reactor volume was utilized for drying, devolatilization, gasification and combustion with carbon conversion of >98%, makes the S-L FBDB gasifier very suitable for lignite gasification. In line with the Secunda GG41 gasifier, clear overlaps between the reaction zones were observed in the "Bernice" and "Albert" gasifiers. This therefore confirms the gradual transition from one reaction zone to another as reported in the literature. The volatile matter in the ash from both the "Bernice" and "Albert" gasifiers was about 10% (dry basis). This volatile matter is most probably inorganic in nature given the presence, in the samples obtained from the ash bed, of calcite (CaCO3), gypsum (CaSO4.2H2O) and melanterite (FeSO4.7H2O) which are expected to decompose during volatile matter determination at 900 °C. Using only •the volatile matter, as determined by proximate analyses, to determine the pyrolysis zone position in the reactor will in the case of DGC gasifiers therefore be delusive. As expected, most of the H, N and S were released in the pyrolysis zone of both the "Albert" and "Bernice" gasifiers. A significant increase in the reactivity of the chars from both the "Bernice" and "Albert" gasifiers was observed in the gasification zones. It is due to this increased reactivity that the char/carbon in these gasifiers were consumed ,within only a third of the gasifier volume. The increased reactivity is most probably due to the catalytic reactions effected by the organically bound alkali and alkaline earth metals, particularly calcium as the coal was found to be rich in this element. Thermal fragmentation was found to be severe with the NO lignite tested. The feed coal was found to decrease in size from 3% in the feed to 90% of <6.3 mm fine particles in the drying and pyrolysis zones of both the "Albert" and "Bernice" gasifiers. This is also a new significant finding in the history of the SL FBDB gasification process which is traditionally known to operate on coarse coal. Mineral matter in the feed coal to the "Bernice" gasifier was mainly dominated by the organically bound calcium. The crystalline phases in the gasification and combustion zones were dominated by gehlenite and bredigite which may have formed from the transformation, at higher temperatures, of the organically bound Ca and Mg to GaO and MgO and subsequent interaction with the reactive silica and transformation products of the clays. In the "Bernice" gasifier, a significant amount of calcite was found to be forming in the beginning of the gasification zone, towards the end of pyrolysis, and decomposing slightly in the hotter combustion zone. It is suggested that the calcite was formed from the reaction of GaO (formed from the transformation of the organically bound Ca) with the CO2 from the raw gas in the gasifier. As expected, the glass phase was found to be the major part of the ash minerals in the gasification and combustion zones of the "Bernice" gasifier. This phase was composed mainly of the Ca, Mg, Na aluminosilicates with some Fe. This composition is common to the slag formed from the Fort Union
lignite. There was therefore a significant amount of melting in the hotter reaction zones (i.e. gasification and combustion zones) of these gasifiers. The organically bound Ca, Mg and Na seemed to have played a significant role in the formation of this glass phase in the gasifiers. Oxygen scavenging by the ash minerals in the combustion and gasification zones of both "Albert" and "Bernice" gasifiers was observed. In the "Bernice" gasifier, it was estimated at about 16% of the oxygen fed as agent to the gasifier. From an economic viewpoint this is significant given the high cost of producing the 99% pure oxygen for gasification. The AFT in the feed coal to both "Albert" and "Bernice" gasifiers was found to be higher as compared to the ash samples from the ash bed. This may have
implications on the design and operating philosophy since the gasifiers are normally designed to operate between the initial deformation and flow temperatures of the ASTM ash, which is not the same as the ash formed in the gasifier. This is also another new significant finding. The high concentration of the fluxing elements (i.e. Ca, Mg, Na and Fe) in the dominating glass phase determined in the gasification and combustion zones of these gasifiers was most probably the reason behind this phenomenon. The char particles formed in both "Bernice" and "Albert" gasifiers were, as determined petrographically, mainly dominated by the dense chars which were highly reactive. An induced "coalification" process was observed in both "Bernice" and "Albert" gasifiers with the macerals/char particles being transformed from lignite to bituminous and anthracitic coal particles. In the "Bernice" gasifier, the average temperature of solids in the combustion zone was found to be about 700 °C, peaking at 11 00 °C in the combustion zone. The average temperature is in line with the predicted figures (i.e. 741 °C for the predicted temperature at which the water gas shift reaction was forced into equilibrium). Overall, there was an excellent match in the trends of the chemical, physical, petrographic and mineralogical properties of the samples obtained at different levels of the "Albert" and "Bernice" gasifiers. This may therefore confirm plug flow during the Turn-Out sampling methodology and hence supports the hypothesis of this study.
It is hoped that the results obtained in this study will not only benefit Sasol or Sasoi-Lurgi Technology Company with regard. to the understanding of the reactors and improvement in modelling and design, but will also assist DGC in further optimising their lignite gasification process.
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