Electronic Thesis and Dissertation Repository

Degree

Master of Science

Program

Chemical and Biochemical Engineering

Supervisor

Prof. Charles Xu, Prof Honghi Tran

Abstract

Agglomeration is a major operational problem during the combustion of biomass containing a high amount of alkali compounds. The agglomerate formation is mainly due to the presence of alkali elements in biomass ashes that form low-melting compounds in the combustion.

In the first part of this project, the critical amount of liquid that would lead to a severe agglomeration/de-fluidization was studied and determined in a lab scale bubbling fluidized bed (BFB) heated at elevated temperatures. To simulate the biomass ashes, various percentages of KCl and KCl-K2SO4 compounds at eutectic composition were mixed with silica sand as the bed material. The results indicated that the formation of channeling/bed-material agglomeration was severe in the presence of KCl or mixture of KCl-K2SO4 with an amount of 0.4-0.6 wt.% with respect to the weight of bed material, being in a good agreement with that was reported by the author’s group in a previous study carried out in a cold BFB test rig using glycerol-water mixture to simulated melted biomass ash. Kaolin and aluminum sulfates were investigated as additives in the BFB and proved to be effective for preventing the bed material agglomeration.

In the second part of this research, we examined the de-fluidization time for BFB combustion of corn stalk (with a high K, Mg, Ca-containing ash) with different bed materials operating at different superficial air velocities. Corn stalk has a high tendency for the unwanted bed agglomeration problems during combustion due to its high contents of K, Ca and Mg in the ash. In combustion of Corn stalk, there was a lower deposition tendency of K compounds onto the olivine bed material than that onto the silica sand material, which might account for the longer de-fluidization time (>12h) for the olivine bed material than that for the silica bed material (8h) during corn stalk combustion. With increasing the superficial gas velocity, the deposited amount of the alkali/alkaline earth elements (K, Ca, Mg) in the bed material (either silica or olivine sand) reduced substantially, which would contribute to reduced tendency of bed agglomeration and de-fluidization.


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