Kyei-Sing Kwong

Viscosity Determination of Molten Ash from Low-Grade US Coals

Abstract In entrained slagging gasifiers, the fluidity of the molten ash is a critical factor for process control since it affects slag formation, the capture of inorganic constituents, refractory wear, and slag drainage along the gasification chamber walls. The use of western coal, or mixtures of eastern and western coals as gasifier feedstock, is likely to occur as western coals become available and technological issues that hinder their use are being resolved. In the present work, the viscosity of synthetic slags with ash chemistries simulating the western U.S. coals, was experimentally measured at a Po2 = 10−8 atm in the temperature range of 1773–1573 K (1500–1300 °C) using a rotating-bob viscometer. Alumina spindles and containment crucibles of both alumina and zirconia were used. Crystallization studies of this slag using a confocal scanning laser microscope found that a (Mg,Fe)Al2O4-based spinel precipitated at temperatures below 1723 K (1450 °C), and this agreed with FactSage equilibrium phase prediction. The same spinels were observed in the post-viscometry experiment slags when ZrO2 crucibles were used and assumed to be in equilibrium with the slag at the higher temperatures. Zirconia dissolution resulted in a slight increase in the solid fraction present in slags at lower temperatures, compared to spinel fraction. Crystal precipitation changed the apparent activation energy and required a longer stabilization times for viscosity measurements. The viscosity results were used in predictive equations based on Veytsman and Einsteins models, with critical nucleation temperatures and the solid fraction calculated with FactSage. In the simulated eastern/western coal feedstock blends based on ash compositions, the fractions of the solid precipitates were also calculated using the thermodynamic program FactSage for each blend composition, and the plastic viscosity of each eastern/western coal slag blend was predicted using Veytsmans model and compared to available experimental data.

A Slag Management Toolset for Determining Optimal Coal Gasification Temperatures

Abstract Gasifier operation is an intricate process because of the complex relationship between slag chemistry and temperature, limitations of feedstock materials, and operational preference. High gasification temperatures increase refractory degradation, while low gasification temperatures can lead to slag buildup on the gasifier sidewall or exit, either of which are problematic during operation. Maximizing refractory service life and gasifier performance require finding an optimized operating temperature range which is a function of the coal slag chemistry and viscosity. Gasifier operators typically use a slag’s viscosity-temperature relationship and/or ash-fusion fluid temperature to determine the gasification temperature range. NETL has built a slag management toolset to determine the optimal temperature range for gasification of a carbon feedstock. This toolset is based on a viscosity database containing experimental data, and a number of models used to predict slag viscosity as a function of composition and temperature. Gasifier users typically have no scientific basis for selecting an operational temperature range for gasification, instead using experience to select operational conditions. The use of the toolset presented in this paper provides a basis for estimating or modifying carbon feedstock slags generated from ash impurities in carbon feedstock.

Development of Slag Management System

Degradation of refractory liners is one of the key factors limiting the service life of entrained flow slagging gasifiers, which is caused primarily by refractory/slag interactions. Slag originates from impurities in the carbon feedstock, typically coal and/or petcoke, which melt and coalesce during gasification, flowing over the refractory liner in the gasifier and interacting with it. Slagging gasifier operators attempt to minimize refractory degradation by controlling slag viscosity (and interactions) through the gasification process temperature. A computer model utilizing empirical and neural network calculations was developed to predict T100, T250, fluid temperature, and liquidus temperature of slag compositions based on chemistry and a newly developed “similarity index”. Development of the model and its application in designing slags to minimize refractory degradation will be discussed.

A Thermodynamic Study of Mixed Carbon Feedstock Gasification Slags

Integrated Gasification Combined Cycle used in power and chemical production is considered a clean technology, with the ability to capture almost all CO2, NOx, and SOx emissions. In entrained bed slagging gasifiers, molten slags formed from feedstock’s non-volatile impurities contribute to gasifier liner degradation and can cause gasifier clogging, affecting system efficiency and operation. Increased petcoke use as a key feedstock in addition to or as a replacement for coal has drastically modified slag chemistry, leading to unknown chemical/physical slag properties and behavior in the gasifier. In this work, thermodynamic phase equilibria in synthetic slags (Al2O3-CaO-FeO-SiO2-V2O3) were evaluated under simulated gasifier conditions to establish an understanding of the phase equilibrium in these slag systems. The effects of V2O3 content, slag chemistry, and additives on amorphous and crystalline phases were studied. In this study, increasing calcium oxide and iron oxide additive agents was found to lower the slag melting temperature and caused the karelianite (V2O3) crystal size to increase. Equilibrium phase diagrams showing the additive effect on the mixed coal-petcoke slag systems studied were constructed.