A new dissection methodology and investigation into coal property transformational behaviour impacting on a commercial-scale Sasol-Lurgi MK IV fixed-bed gasifier

Abstract

Gasification behaviour is particle dependent, whilst gasifier (reactor) behaviour is an averaging process of individual responses of each particle [Glover, 1991]. It was hypothesized in the case of the present study that if it were possible to extract and analyse particles from different reaction zones within a gasifier, it may be likely to enhance the understanding as to the contribution that these particles make towards gasification. This better understanding of the particle-type compositional responses could act as an enabler to further influence gasifier performance. The primary focus of this study was to develop a "new-to-Sasol" sequential (axial) sampling methodology of the entire GG41 commercial-scale Sasol-Lurgi FBDB MK IV gasifier (isolated after running on medium load), having a Bosman skirt feeder configuration and standard ash grate. Detailed characterisation profiles of the chemical, physical, and petrographical properties in the sample increments were expected to deliver distinct profiles of the drying, pyrolysis, reduction, and combustion (ash-bed) zones. The input value of this experimental data, in the form of a "pancake" model to the existing one-dimensional kinetic models at Sasol Technology R+D were expected to advance the prediction capability significantly. The Sasol Gasification Operations could then possibly benefit in terms of an improved control philosophy of their process, yielding better gasification stability, improved efficiency and availability. The kinetic model development was not considered to be part of this particular study. The "controlled" GG41 gasifier turn-out and sampling methodology employed, provided excellent residual internal profile results, clearly showing the complexity and heterogeneity of the coal properties studied with respect to transformational behaviour during gasification. By keeping the turn-out sample increment size constant (3m3), whilst maintaining a constant ash-grate discharge speed (3rph) to assimilate plug-flow conditions; 32 fractional "slices" of the entire gasifier contents (100t) were successfully extracted. Truck "sub-sampling" of the increments taken, was also found to be successful. The entire sampling methodology is considered to be plausible when observing the "pan-cake" model derived for the GG41 gasifier, in that minimal scatter in the data trends are evident. The GG41 drying zone was found to be prevalent in the Bosman skirt feeder region in the uppermost part of the fixed bed gasifier. Volatile matter recondensation and some evidence of particle size segregation and primary fragmentation was observed as the charge descended within this reaction zone. The pyrolysis zone was found to be the largest reaction zone, followed by the reduction zone and the combustion (ash-bed) zones. Whilst the boundary of the pyrolysis zone was very clearly defined by the residual volatile matter distribution profile, distinctive regional overlap characterised by a "fast devolatilisation" region and a "slow pyrolysis with gasification" region, observed in the bottom half of the pyrolysis zone, were evident. A gasification region was observed to occur at the end of the pyrolysis zone, where the coal char C02 reactivity was found to reach a maximum, midway into the reduction zone, characterising the onset of an oxidation front occurring above the combustion zone position, where 65 % carbon conversion had occurred and the char was subsequently rapidly consumed. Melting of the ash-forming minerals tended to act as a "glue", agglomerating particles together and thereby increasing the amount of the coarse +25mm fraction in the bottom half of the gasifier. Rapid carbon consumption occurred in the low ash-bed, but 8% fixed carbon was still evident in the ash at the grate position at the base of the reactor. The char types responsible for the carbon losses have however been quantified in this study. From the unique petrographic char morphology results obtained, it was found that the maceral type appears to play a pivotal role in the changes experienced by carbon particles when exposed to increasing temperature. Whole vitrinite particles and vitrinite bands within particles devolatilised first, followed at higher temperatures by reactive inertinite types. The porous chars remained predominantly isotropic, while the mixed and dense chars showed varying degrees of anisotropy, which is related to the aromaticity of the carbon. At the end of pyrolysis, all the coal was found to be essentially converted to char, but chars continued to increase as the charge further descended within the gasifier, becoming consumed in the oxidation / combustion zone. Comparison of the GG41 gasifier (4 m diameter) internal profile, on a factorised basis with a quarter-scale Lurgi gasifier (1.13 m diameter) operating on Indian coal, provided an excellent basis for comparison. The good agreement obtained, substantiates credibility of the current findings with respect to repeatability, albeit that different coal types and reactor geometries were being compared. This also has implications regarding scale-ability to smaller gasifier sizes, should the need arise to conduct small scale gasification experiments, which can be scaled up to a commercial sized 4m diameter reactor. The positive "bench-mark" findings indicate that considerable benefit could be obtained by carrying out more extensive and detailed studies on other Sasol-Lurgi MK IV gasifier operating systems, i.e. low load and high load conditions; as well as internal mechanical influences on performance, such as the rubble finger positioned on the ash grate, and "Unifeeder" compared with the Bosman skirt feeder configuration. The findings of this study also clearly indicate where optimisation opportunities exist for the MK IV gasifier, i.e. the char type responsible for carbon-loss, the oxygen scavenging effect of minerals, volatile matter recondensation in the cooler zone of the gasifier, localised oxygen sparging that causes hot-spots just above the ash grate and reduction of the top size of the feed coal to a "thermally stable" size, which could result in a more stable gasifier operation with possible process efficiency benefits. It is likely that this thesis will ultimately have a major impact on the profitability of many existing and new Sasol-Lurgi gasification plants. It also represents a contribution towards a better understanding of the key phenomena involved in the fixed-bed gasification process.