Carbon particle type characterization of the carbon behaviour impacting on a commercial-scale Sasol-Lurgi FBDB gasifier

Char-form analysis, whilst not yet an ISO standard, is a relatively common characterization method applied to pulverized coal samples used by power utilities globally. Fixed-bed gasification coal feeds differ from pulverized fuel combustion feeds by nature of the initial particle size (+6 mm, -75 mm). Hence it is unlikely that combustion char morphological characterization schemes can be directly applied to fixed-bed gasifier chars. In this study, a unique carbon particle type analysis was developed to characterize the physical (and inferred chemical) changes occurring in the particles during gasification based on coal petrography and combustion char morphology. A range of samples sequentially sampled from a quenched commercial-scale Sasol-Lurgi fixed-bed dry-bottom (FBDB) Gasifier were thus analysed. It was determined that maceral type (specifically vitrinite and inertinite) plays a pivotal role in the changes experienced by carbon particles when exposed to increasing temperature within the gasifier. Whole vitrinite particles and vitrinite bands within particles devolatilized first, followed at higher temperatures by reactive inertinite types. By the end of the pyrolysis zone, all the coal particles were converted to char, becoming consumed in the oxidation/combustion zone as the charge further descended within the gasifier. The carbon particle type results showed that both the porous and carbominerite char types follow similar burn-out profiles. These char types formed in the slower pyrolysis region within the pyrolysis zone, increasing to around 10% by volume within the reduction zone, where 53% carbon conversion occurred. Both of these char forms were consumed by the time the charge reached the ash-grate at the base of the reactor, and therefore did not contribute to the carbon loss in the ash discharge. It would appear as if the dense char and intermediate char types are responsible for the few percent carbon loss that is consistently obtained at the gasification operations. The carbon particle type analysis developed for coarse coal to the gasification process was shown to provide a significant insight into the behaviour of the carbon particles during gasification, both as a stand alone analysis and in conjunction with the other chemical and physical analyses performed on the fixed-bed gasifier samples.