The principle aim of this paper is to understand the crystallization of coal ash slags and the effects on the viscosity by means of high temperature viscosity measurements, in combination with FactSage modeling, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Four coal ashes with the fusion temperatures between 1130 and 1470 °C applied in entrained flow gasifiers in China were prepared for this study. The thermodynamic modeling was carried out using FactSage 6.2 software to predict the composition of homogeneous liquid slag systems as well as heterogeneous slag systems. It can be concluded that the viscosity of coal ash samples is closely related to the phases at high temperatures. The viscosity increases significantly until the mass percentage of the solid phases reaches a certain value (15.15−33.82%). For the coal ash samples enriched in Al 2 O 3 and SiO 2 with high AFT, mullite is the first solid phase forming in the liquid slag above 1600 °C, followed by quartz, anorthite, and ferro-cordierite, until the molten slag finally transforms to a 100% solid state. For the coal ash samples enriched in CaO and Fe 2 O 3 with relatively lower AFT, anorthite is the first solid phase forming and separating from the liquid slag, followed by the ferrous aluminosilicate. The crystallization temperature of solid phases, as well as the crystallization rate, is determined by the chemical composition of coal ash samples. The XRD findings were further supported with FactSage thermochemical modeling. The Krieger−Dougherty equation combining with the Watt and Fereday model was used to simulate the viscosity results, which provided a good fit for coal ash samples with a low proportion of crystalline phases.

The gasifier represents an attractive alternative to the well-established thermal treatment system for municipal sewage sludge, but special attention should be paid to the high phosphorus content in sewage sludge. Phosphorus volatilization during the co-gasification process of municipal sewage sludge and coal mixtures was investigated using a laboratory-scale highfrequency furnace. The transformation of phosphorus-containing compounds in slag at different temperatures was also studied.The results indicate that the volatilization ratio of phosphorus monotonically increases with the increasing gasification temperature and the volatilization of phosphorus mainly takes place during the pyrolysis process. The organophosphorus compound makes up the main part of volatized phosphorus when the pyrolysis temperature is not higher than 1100°C. Inorganic phosphorus does not volatilize obviously until 1200°C. After gasification, most phosphorus in mixtures deposits in slag in the form of phosphorus-containing glass.

A slag flow submodel has been developed to simulate the slag flow and phase transformation behaviors in coal gasifiers. The volume of the fluid (VOF) model is used to capture the free surface of the slag flow, and the continuum surface force (CSF) model is employed to calculate the surface tension between the gas phase and the liquid slag phase. The slag is treated as a Newtonian fluid when the slag temperature is above the critical viscosity temperature (T cv), and plastic fluid is treated when the slag temperature is between the flow temperature (T f) and the T cv. The ash particle deposition, viscosity−temperature dependence, and different thermal conductivity for different slag phase are all included in the present simulation. For membrane wall coal gasification, the liquid slag and solid slag layer increases along the flow and total slag thickness increases as the operating temperature decreases. The velocity profiles and viscosity profiles at different operating temperatures are performed. The liquid slag flow will produce fluctuations when the slag temperature decreases to the lowest at the bottom of the gasifier. In addition, the temperature difference (T o − T f) between 150 and 200 °C is suitable for a membrane wall coal entrained-flow gasifier. For refractory wall coal gasification, the thicker refractory bricks can effectively prevent the heat lost from the gasifier wall, so the slag flow is steady when the operating temperature is higher than the critical operating temperature. An expression of solid slag layer formation criterion has been deduced from heat-transfer balance. The critical operating temperature of the different slag mass flow rate is studied by heat-transfer balance. In addition, the solid slag layer will rapidly increase as the operating temperature decreases to critical operating temperature.

Gasification slags are byproducts of the coal gasification process, including fine slags (slags from filter) and coarse slags (slags from lock hopper). The characteristics of gasification slags with different particle sizes were investigated by the losson-ignition (LOI) method, scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), pore structure analysis, X-ray diffractometry (XRD), and X-ray fluorescence spectrometry (XRF). The relationships between the particle size and the characteristics of slags, which include residual carbon content, surface characteristics, pore structure, crystal mineral content, and ash composition, were analyzed and obtained. For fine slags, carbon content, specific surface area, porosity, and crystal content increase with particle size. For coarse slags, medium sized (105−280 μm) particles contain the most residual carbons and crystal minerals, followed by the smaller (0−105 μm) particles. Si/Al/Ca/Mg/K tends to concentrate in larger coarse slags, while Fe/S/P tends to concentrate in smaller particles. Different characteristics of different sized slags are mainly due to their different processes experienced in the gasifier. It is considered that particle fragmentation, ash agglomeration, and slag deposition should be taken into account.

A comprehensive model has been developed to analyze the multiphase flow and heat transfer in the radiant syngas cooler (RSC) of an industrial-scale entrained-flow coal gasification. The three-dimensional multiphase flow field and temperature field were reconstructed. The realizable k−ϵ turbulence model is applied to calculate the gas flow field, while the discrete random walk model is applied to trace the particles, and the interaction between the gas and the particle is considered using a two-way coupling model. The radiative properties of syngas mixture are calculated by weighted-sum-of-gray-gases model (WSGGM). The Ranz−Marshall correlation for the Nusselt number is used to account for convection heat transfer between the gas phase and the particles. The discrete ordinate model is applied to model the radiative heat transfer, and the effect of ash/slag particles on radiative heat transfer is considered. The model was successfully validated by comparison with the industrial plant measurement data, which demonstrated the ability of the model to optimize the design. The results show that a torch shape inlet jet was formed in the RSC, and its length increased with the diameter of the central channel. The recirculation zones appeared around the inlet jet, top, and bottom of the RSC. The overall temperature decreased with the heat-transfer surface area of the fins. The concentration distribution, velocity distribution, residence time distribution, and temperature distribution of particles with different diameters have been discussed. Finally, the slag/ash particles size distribution and temperature profile at the bottom of the RSC have been presented.