Peter L. Rozelle

Prediction of Sorbent Performance in a Circulating Fluidized Bed Boiler Based on Petrographic Properties

A method for prediction of sorbent consumption is presented here and has been developed based on plant operating data for a boiler in which several limestone and dolostone products were tested under similar firing conditions. The method considers the characteristic partitioning of calcium and sulfur between the flyash and bottom ash stream for the boiler, the feed particle size distribution of the sorbent, and petrographic properties of the sorbents. The predictions of sorbent usage were compared to plant operating data for five sorbents, of two distinct petrographic types. The plant operating data used featured full load operation. The five sorbents tested were all from Pennsylvania, and each contained greater than 40 wt. % CaO. In four of the five cases, the predicted sorbent usage was within 10 wt. % of the average full load sorbent usage by the boiler.

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.

Infiltration velocity and thickness of flowing slag film on porous refractory of slagging gasifiers

Two analytical formulations that describe the fluid interactions of slag with the porous refractory linings of gasification reactors have been derived. The first formulation considers the infiltration velocity of molten slag into the porous microstructure of the refractory material that possesses an inherent temperature gradient in the direction of infiltration. Capillary pressures are assumed to be the primary driving force for the infiltration. Considering that the geometry of the pores provides a substantially shorter length scale in the radial direction as compared with the penetration direction, a lubrication approximation was employed to simplify the equation of motion. The assumption of a fully developed flow in the pores is justified based on the extremely small Reynolds numbers of the infiltration slag flow. The second formulation describes the thickness of the slag film that flows down the perimeter of the refractory lining. The thickness of the film was approximated by equating the volumetric slag production rate of the gasification reactor to the integration of the velocity profile with respect to the lateral flow cross-sectional area of the film. These two models demonstrate that both the infiltration velocity into the refractory and the thickness of the film that forms at the refractory surface were sensitive to the viscosity of the fluid slag. The slag thickness model has been applied to predict film thicknesses in a generic slagging gasifier with assumed axial temperature distributions, using slag viscosity from the literature, both for the case of a constant slag volumetric flow rate down the gasifier wall, and for the case of a constant flyash flux distributed uniformly over the entire gasifier wall.