Alkali-induced agglomeration of aluminosilicate particles during coal combustion and gasification.

Abstract

This study focuses on the effect of alkali adsorption on the agglomeration of particles of bauxite, kaolinite, emathlite, lime, and two types of coal ash. An agglomeration (adhesion) temperature is defined which characterizes the adhesion propensity of particles. Using a small fluidized bed, a unique experimental technique is developed to measure this agglomeration point in-situ. The effects of alkali adsorption on the agglomeration characteristics of the substrates are determined. The agglomeration temperature of all substrates decreases as the alkali content increases. At low alkali loadings, alkali adsorption enhances particle agglomeration by forming new compounds of lower melting points. At high alkali concentrations, adhesion and agglomeration are caused by a layer of molten alkali which covers the exterior of the particles. Alkali surface composition of particles is studied using a Scanning Auger Microprobe (SAM). Results indicate that the alkali surface concentration decreases as agglomeration temperature increases. SAM depth profiling data provides information on the variations of alkali loading across particles. These results show that an alkali surface product layer is formed where most of the alkali adsorbed is concentrated. The use of additives to scavenge alkali vapors is further studied in a pilot scale downflow combustor under more typical combustion conditions. SAM surface analyses of additive particles indicate three mechanisms of alkali capture. Alkali adsorption by reaction, alkali surface condensation, and alkali nucleation and coagulation with additive particles. These mechanisms may occur independently or simultaneously depending primarily on the alkali vapor concentration and the temperature profile along the combustion furnace. A mathematical model is developed to represent the kinetics and mechanisms of the alkali adsorption and agglomeration process. Modeling results indicate that the adsorption-reaction process is influenced by diffusion of alkali through the surface product layer. The model predictions of the alkali adsorbed as a function of minimum agglomeration temperature agree very well with the experimental results. Alkali-additive interactions in a downflow combustor are also modeled to predict the mechanisms of alkali capture and the overall alkali removal efficiency. Model predictions of the alkali capture agree well with the experimental results.