Coal char temperature profile estimation using optical reflectance for a commercial-scale Sasol-Lurgi FBDB gasifier

In the Sasol-Lurgi fixed-bed dry-bottom (FBDB) gasifier the temperature in the combustion zone should not exceed the melting point of the ash-forming minerals, causing them to melt/flow and agglomerate. Sintering of ash particles is considered desirable in Sasol-Lurgi FBDB gasification, since it promotes easy gas flow, whereas clinkering creates channeling and localized “hot spots”, leading to unstable gasifier operation. Due to the counter-current mode of operation, hot ash exchanges heat with the cold incoming agent (steam and oxygen), while at the same time hot raw gas exchanges heat with cold incoming coal. This results in the ash and raw gas leaving the gasifier at relatively low temperatures compared to other types of gasifiers, which improves the thermal efficiency and lowers the steam consumption.Vitrinite reflectance analyses were performed on a range of Sasol-Lurgi MK IV commercial-scale gasifier turn-out samples, applying ISO standards 7404-5. Average temperature profile measurements of the solid particles, successfully revealed the temperature range occurring within the various zones of the gasifier. The average (mean) temperature ranged from ca. 400 °C up to 850 °C within the pyrolysis region. In this region of the gasifier, the particle surface temperature and peak temperature showed visible evidence of heat transfer limitations occurring through lump coal when compared to the mean particle temperature. This provides some evidence of the complex radial and localized behaviour occurring within the averaged axial sample slices. In the oxidizing and combustion regimes, exothermic conditions prevail and heat transfer differences across the particles are minimized. A characteristic spike, indicative of an increase in temperature, was found in the sample taken directly above the ash-grate, seeming to indicate that agent distribution through the nozzles positioned just above the grate is not uniform, resulting in localized oxygen concentration increases with subsequent “hot-spots” and channel-burning occurring. Homogenization of the ash bed could help to optimize the agent distribution within the reactor.The surface temperature profile of the gasifier solids was thus found to be in reasonable agreement with literature, albeit that different coal types and temperature profile estimation methods were utilized.