Article

A Model for Estimation of Viscosity of Molten Silicate Slag

Isij International - ISIJ INT 01/2005; 45(5):651-656. DOI: 10.2355/isijinternational.45.651
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    ABSTRACT: Integrated gasification combined cycle (IGCC) is a potentially promising clean technology with an inherent advantage of low emissions, since the process removes contaminants before combustion instead of from flue gas after combustion, as in a conventional coal steam plant. In addition, IGCC has potential for cost-effective carbon dioxide capture. Availability and high capital costs are the main challenges to making IGCC technology more competitive and fully commercial. Experiences from demonstrated IGCC plants show that, in the gasification system, low availability is largely due to slag buildup in the gasifier and fouling in the syngas cooler downstream of the gasification system. In the entrained flow gasifiers used in IGCC plants, the majority of mineral matter transforms to liquid slag on the wall of the gasifier and flows out the bottom. However, a small fraction of the mineral matter (as fly ash) is entrained with the raw syngas out of the gasifier to downstream processing. This molten/sticky fly ash could cause fouling of the syngas cooler. Therefore, it is preferable to minimize the quantity of fly ash and maximize slag. In addition, the hot raw syngas is cooled to convert any entrained molten fly slag to hardened solid fly ash prior to entering the syngas cooler. To improve gasification availability through better design and operation of the gasification process, better understanding of slag behavior and characteristics of the slagging process are needed. Slagging behavior is affected by char/ash properties, gas compositions in the gasifier, the gasifier wall structure, fluid dynamics, and plant operating conditions (mainly temperature and oxygen/carbon ratio). The viscosity of the slag is used to characterize the behavior of the slag flow and is the dominating factor to determine the probability that ash particles will stick. Slag viscosity strongly depends on the temperature and chemical composition of the slag. Because coal has varying ash content and composition, different operating conditions are required to maintain the slag flow and limit problems downstream. This report briefly introduces the IGCC process, the gasification process, and the main types and operating conditions of entrained flow gasifiers used in IGCC plants. This report also discusses the effects of coal ash and slag properties on slag flow and its qualities required for the entrained flow gasifier. Finally this report will identify the key operating conditions affecting slag flow behaviors, including temperature, oxygen/coal ratio, and flux agents.
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    ABSTRACT: Several models have been proposed to estimate the viscosity of molten slag using optical basicity. In this work, a new method for viscosity estimation of silicate slag using optical basicity has been proposed. The temperature dependence of viscosity was described by the general Arrhenius equation, and the contributions of bridging oxygen (O0), non-bridging oxygen (O–) and free oxygen (O2–) to viscous activation energy in this equation were taken into account. Compositions of three types of oxygen were calculated from optical basicity using the Toop–Samis model combined with the method proposed by Ottonello and Moretti. The present method has been applied to estimate the viscosity of silicate slag in CaO–FeO–MgO–MnO–SiO2 system. The comparison between estimated and experimental values shows good agreements, with a mean deviation of ?20%.
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