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Evaluating ash slagging can alleviate the operational problems and improve the efficiency of coal-fired boilers. This paper presents methods to quantify the extent of slagging in a 500 MWe tangential firing boiler by determining the thermal resistance and relating it to ash properties. To determine the thermal resistance by slagging, the operational data of the boiler were extracted for six coal-blend cases and analyzed using a one-dimensional process model. Although the conventional slagging indices based on ash composition have generated conflicting results, new indices based on thermo-mechanical analysis (TMA) of the coal-blend samples provided reasonable agreement with the operational data. In particular, the second peak of the TMA associated with the growth rate was correlated with the thermal resistance of the superheaters, and the third peak representing the slag melting was inversely correlated with that of the furnace wall. It was also found from the numerical modeling of the slag flow on the furnace wall that the critical viscosity temperature at 25 Pa·s can be positively related with the thermal resistance, and consequently, with the slagging propensity of specific coal ashes.

The entrained flow reactor (EFR) is frequently applied to analyze the reactivity of pulverized coal char under controlled atmospheres of temperature and partial pressure. The rate constants for a s...

The viscosity-temperature relation of slag determines its behavior inside an entrained flow coal gasifier. However, existing prediction models give results with large variations among them. We investigated the influence of different viscosity models in the prediction of the steady and transient behaviors of slag flow on the wall of a gasifier undergoing gas temperature changes. Five viscosity models adopted showed differences in the temperature (T250) at 25 Pa∙s as large as 97 K for the selected slag composition, which was used as the interface temperature between the solid and liquid slag. When the predicted viscosity and corresponding T250 increased, the solid slag became thicker and the dynamic response of the slag became slower. In contrast, the differences in the liquid slag thickness were small. The influence of T250 predicted was dominant, compared to that of different viscosity curves of the liquid slag.

In an entrained-flow coal gasifier with water-cooled walls, it is important to maintain an appropriate range of slag thickness on the wall for protecting the refractory and preventing blockage at the slag tap. Owing to the difficulty in measurement within the gasifier, the heat absorption on the wall and downstream syngas composition are monitored as indicators of slag thickness and gas temperature. However, there is a certain delay between these parameters that can be highlighted by the dynamic modeling of slag behaviors on the wall and reactions/flow in the gas regime. In this study, a new dynamic model for the slag layer was developed, which solves for the energy, heat transfer, and flow of the solid and liquid slag layers including the transformation between the two phases. The model was applied to a commercial gasifier varying operating parameters, and the transient behaviors of the slag layers were analyzed focusing on the thickness at the slag tap and the heat absorption on the wall. When the operating conditions changed abruptly, the slag layer required 10 min or longer to reach the new steady state. The thinning of the slag layer was faster than its thickening. In addition, the response of heat absorption was significantly faster than that for the slag thickness at the slag tap. The characteristic times of the slag layer were shorter with changes in gas temperature than with changes in the mass rate and temperature of the slag deposition. During the transient states, the temperature profile within the liquid slag became non-linear, which suggests that existing models assuming a linear temperature profile may not be appropriate for dynamic simulations.

A reduced-order model (ROM) for an entrained flow coal gasifier provides a real-time dynamic prediction of key performance parameters under various operating conditions. However, existing 1D ROMs cannot consider coal burners located on the sidewalls in Shell coal gasifiers frequently applied in commercial plants. In this study, a pseudo-2D ROM was developed to consider the characteristic flow pattern of the gasifier and coupled with a discretized slag-layer model for the wall. Some input parameters specific to the gasifier were determined from a detailed computational fluid dynamics study. Further, the measured differences between input coal ash and bottom slag composition were incorporated in the slag viscosity. The ROM was validated in two ways using the design and operation data of a 300-MW IGCC plant. First, the steady-state prediction showed good agreement with the design data at various loads in terms of syngas composition, heat duty, and exit gas temperature. This also implied that the predicted slag thickness on the wall was reasonable. Second, the ROM was applied to actual operation data over 7 h with two events. For a gradual increase in the target O2/coal ratio and sudden changes in the coal throughput, the dynamic response of the measured heat duty was successfully reproduced by the ROM. The predicted exit gas temperature reflected the instantaneous fluctuations of O2/coal ratio and appeared more reasonable than the correlation-based value used by the operators. The ROM clearly showed differences in the response rate between the exit gas temperature, heat duty, cold gas efficiency, and slag thickness for changes in the operation variables. It can be used as a real-time online simulator for advanced gasifier operation.

To improve availability and lower the electricity production costs in an integrated gasification combined cycle (IGCC) plant, it is essential to minimize operation problems in the coal gasifier. Using computational fluid dynamics, this study evaluated the performance of the coal gasifier at Taean IGCC with 300 MWe capacity both with and without steam in the gasifying agent under loads between 50 and 100%. The swirling flow developed by the horizontally biased coal burners had a huge influence on the particle trajectories, reactions, and temperature distribution. In particular, the gas temperature was higher in the upward swirling flow, whereas the downward flow carrying more particles developed a lower temperature region below the burner level. These phenomena became more prominent at lower operational loads. Despite the design conditions including steam in the gasifying agent, the operation without it was stable at 0.36–1.07% lower cold gas efficiencies with increased heat transfer to the wall. With steam injection, 37–98 °C lower gas temperature and corresponding increase in the slag thickness were expected in the lower part of the gasifier due to an increase of endothermic char reactions by 4.6–7.6%. This could be severe for low-rank coals with higher inherent moisture and lower energy content compared with design coal. Therefore, a low operating load with steam injection requires careful control of slag composition and flowability for prevention of slag discharge problems.

In entrained coal gasification, the properties of slag and its behavior on the wall are crucial in selecting specific coal types and controlling the gasifier operation. The temperature of critical viscosity (T cv ) is the key parameter that characterizes the slag flowability. This study evaluates the theoretical influences of T cv , critical viscosity and flux addition on the slag behavior in a commercial coal gasifier using a numerical model. The results showed that a lower slag T cv value led to an exponential decrease in the solid slag thickness (δ S ) owing to an increased heat flux to the wall and a reduced temperature difference between the slag and the coolant. In contrast, the influence of T cv on the liquid slag thickness (δ L ) was relatively small. When T cv remained constant, a higher critical viscosity value decreased the velocity of the liquid slag close to the solid slag, leading to a gradual increase in both δ S and δ L . Addition of flux was effective in decreasing δ S for coals with high T cv values, while the impact of increased slag flow rates on δ L was minimal and could be disregarded. The relative magnitude of total slag thickness by changes in T cv and critical viscosity was almost the same for different ash:flux ratios, gas temperatures, and wall angles. The influence of T cv was dominant over that of critical viscosity for slags with critical viscosity values above 10 Pa s.

In an entrained coal gasifier, coal ash turns into molten slag which deposits on the walls to form solid and liquid slag layers. It is important to understand the slag behavior to prevent the blockage of slag tap and maintain the desired level of heat absorption on the wall. In this study, we proposed a comprehensive numerical model to predict the flow and heat transfer characteristics of the slag layers. The model can be used as the wall condition for detailed computational fluid dynamic or process simulations of the gasifier. The numerical model was evaluated in comparison with existing analytical models for a commercial coal gasifier under three exemplary operating conditions. The new model adequately predicted the thickness, velocity, and temperature of the slag layers, as well as the heat flux to the wall. Under particular operating conditions that had discrepancies between the model predictions, the reasons were discussed in relation to the respective model formulation and assumptions/simplifications introduced. It was shown that the temperature profile within the liquid slag was close to a linear relationship at sufficiently high gas temperatures, while it became parabolic when the gas temperature fell below T cv . Applying the analytical models requires caution under the conditions in which the assumptions on the temperature profile are not valid. The numerical model can be applied to various operating conditions without such limitations and be extended to incorporate different slag property submodels.

Coal gasification offers a flexible and efficient conversion of the solid fuel into CO- and H2-rich synthetic gas (syngas) for production of various chemicals and energy products. Since the hot syngas leaving a gasifier contains various impurities such as acidic gases and particulates, it needs to be cooled down for cleaning prior to conversion into the final product. A dedicated heat exchanger called a syngas cooler (SGC) is used to lower the gas temperature while recovering the thermal energy. This study investigated the heat transfer characteristics in a commercial-scale SGC consisting of a series of concentric helical coil channels. First, the detailed flow and heat transfer pattern in the unique heat exchanger were analyzed using computational fluid dynamics (CFD) for various operating loads and fouling conditions. The predicted heat transfer rate was used to derive correlations for Nusselt number for the channel sections of the SGC. Second, a one-dimensional model of the equipment was proposed for fast-response process simulations. In terms of heat transfer rate and gas temperature, the process model showed a reasonable accuracy compared to the CFD results for the tested cases.

The gas reversing chamber (GRC) is the top part of a syngas cooler in the Shell coal gasification process, in which the quenched syngas flowing from the gasifier is turned downward into evaporator channels. Using computational fluid dynamics (CFD), this study investigated the detailed gas/particle flow and heat transfer characteristics in the GRC of an integrated gasification combined cycle (IGCC) process with 300 MWe capacity for operational loads between 50 and 100%. The gas flow rapidly changed its direction to downward after impinging onto the wall opposite to the inlet. This led to the formation of a higher velocity region along the opposite side, increasing the gas flow rate in the outer channel of the evaporator. In contrast, a region with low velocity below 2 m/s developed towards the inlet side above the evaporator, which may lead to the significant deposition of fly slag particles onto the structural elements. Larger wall heat flux appeared along the main gas stream with a maximum of 180 kW/m2 at full load. Convection accounted for about 70% of the total heat transfer rate, of which the coefficient was correlated to Nu = 8.778 Re0.435Pr0.3. Many particles impinged onto the wall along the main gas stream, especially at the joint connecting the transfer duct and the GRC. However, erosion by fly slag was not expected to be significant, mainly due to the reduced gas velocity at the high operating pressure (43 bar).

In a coal gasifier for IGCC, hot syngas leaving the gasifier at about 1550oC is rapidly quenched by cold syngas recycled from the gas cleaning process. This study investigated the flow and heat transfer characteristics in the gas quench system of a commercial IGCC process plant under different operating pressures. As the operating pressure increased from 30 bar to 50 bar, the reduced gas velocity shortened the hot syngas core. The hot fly slag particles were retained within the core more effectively, and the heat transfer became more intensive around the hot gas core under higher pressures. Despite the high particle concentrations, the wall erosion by particle impaction was estimated not significant. However, large particles became more stagnant in the transfer duct due to the reduced gas velocity and drag force under higher pressures.

In the Shell coal gasification process, the hot syngas at about 1550 °C leaving the gasifier exit is immediately quenched by recycled cold syngas for protection of the system from corrosion and fly slag deposition. As the gas quench system is composed of membrane walls with evaporating water inside, understanding the heat transfer is important in the design and operation of gasification process. This study investigated the detailed flow and heat transfer characteristics of the gas quench system by computational fluid dynamics (CFD) for operational loads of 50–100% in a 300 MWe integrated coal gasification combined cycle (IGCC) process. Based on the CFD results, one-dimensional process model for heat transfer was proposed. The approach for considering three gas streams (hot, cold and mixed syngas) was effective in predicting the trends of heat transfer and gas temperature in the quench system.

In the unlikely of nuclear reactor meltdown, the leaked core melt or corium must be contained in a device called core-catcher so that the corium can be cooled and stabilized. The ex-vessel behavior of corium involves complex physical and chemical mechanisms of flow propagation, heat transfer, and reactions with sacrificial substrates. In this study, the detailed characteristics of corium flow and heat transfer were investigated by using a commercial CFD code for VULCANO VE-U7 test reported in the literature. The volume-of-fluid (VOF) model was used to predict the interfacial surface formation of corium and the surrounding air, and the discrete ordinate model was adopted to calculate radiation between corium and the surroundings. It was found that cooling via radiation through the top surface of corium had a dominant effect on the temperature and viscosity profiles at the front of the corium flow.

The behaviors of the slag layers formed by the deposition of molten ash onto the wall are important for the operation of entrained coal gasifiers. In this study, the effects of design/operation parameters and slag properties on the slag behaviors were assessed in a commercial coal gasifier using numerical modeling. The parameters influenced the slag behaviors through mechanisms interrelated to the heat transfer, temperature, velocity, and viscosity of the slag layers. The velocity profile of the liquid slag was less sensitive to the variations in the parameters. Therefore, the change in the liquid slag thickness was typically smaller than that of the solid slag. The gas temperature was the most influential factor, because of its dominant effect on the radiative heat transfer to the slag layer. The solid slag thickness exponentially increased with higher gas temperatures. The influence of the ash deposition rate was diminished by the high-velocity region developed near the liquid slag surface. The slag viscosity significantly influenced the solid slag thickness through the corresponding changes in the critical temperature and the temperature gradient (heat flux). For the bottom cone of the gasifier, steeper angles were favorable in reducing the thickness of the slag layers.