Overview of ash-related matters during pressurised entrained-flow gasification

Observations from recent campaigns of the pilot-scale pressurised entrained-flow biomass gasifier (PEBG) firing ash-rich fuels revealed the apparent need for engineering and operational modifications to the gasifier to accommodate for ash behaviours that occur during the process. As part of a number of activities intended to identify the main modifications that are needed, the purpose of this literature review was to learn from the reported experiences of coal gasifiers and the knowledge surrounding ash transformations during PEFG. In addition, identification of activities that would further progress the development of pressurised entrained-flow gasification (PEFG) of biomass was also an objective. It was found that although PEFG of coal has been commercialised for over six decades, there remains significant technical ash-related aspects that are currently been studied. These include: (1) the actual fractionation of ash-forming matter during the process, (2) the corrosion and spallation of refractory linings by ash slags, and (3) the detailed modelling of the ash behaviour with consideration for the form of the inorganics within the parent fuel and the physical and chemical transformations that they undergo. The commonly reported measures to manage the behaviour of ash slag include fluxing to modify the viscosity—either by blending with different coals or additives—as well as the use of oxygen lances to significantly increase temperature and slag fluidity to avert imminent slag blockages. Staged gasification, where different ratios and amounts of fuel and oxidant ratios are fed through multiple feeders, is another method to control ash behaviour and to attain higher temperatures near slag outlets. Gasifiers utilising cooling screens/membrane walls also appear to be more common among the latest offerings from commercial vendors. The PEFG of biomass fuels entails different ash behaviours than those experienced from PEFG of coal. The comparatively high levels of Ca and K in woody biomass means that deposits are likely to consist of large amounts of crystalline material with sparse melt. If the PEBG is to fire untreated pulverised fuels, it is apparent that a fluxing additive or fuel blending will be needed. Furthermore, careful control of the the temperatures inside the reactor is needed to maintain proper ash flow. With respect to this, there exists designs of gasifiers with configurations that allow higher temperatures to be attained closer to the outlet of the gasifier to reduce chances of slag blockages. However, material considerations remain of concern. Hence, ash-refractory interactions should be studied prior to deciding what refractory to install during modifications/rebuilding of the PEBG. It is important to consider not only the chemical properties, but also the thermo-mechanical limits of the materials. The fragmentation and release of gaseous species from biomass fuel particles during PEFG should be studied in detail. The physical characterisation of possible biomass slags should also be investigated. With such knowledge, realisation of mathematical models for the ash conversion and behaviour would allow them to be implemented within CFD packages as a submodel to predict ash behaviours under different operation scenarios.Problem-Oriented Project (POP): Improved slagging reactor outlet for biomass gasifier