Date of Award

5-2006

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

Adriaan R.P. van Heiningen

Second Committee Member

Doug Bousfield

Third Committee Member

Michael Boyle

Abstract

Kraft black liquor, the bi-product of the kraft pulping process, is concentrated in evaporators and burned at high solids content in recovery boilers. The purpose of burning black liquor is two fold: 1) energy recovery through combustion of the organic content and, 2) recovery of the inorganic pulping chemicals as smelt. In the lower portion of a recovery boiler a char bed forms. In the active layer of the char bed finally separation of unburned char carbon and inorganics is affected primarily by gasification of organic carbon. Pyrolysis and gasification of char carbon in black liquor and carbon structures impregnated with catalytic alkali carbonates has been extensively studied. Gasification rates of kraft black liquor are significantly higher than most carbon alkali carbonate mixtures because of the intimate contact created by the pulping process. Significant efforts have been undertaken to measure kraft black liquor char gasification in experimental systems free of mass transfer limitations. There is however a lack of reliable experimental data collected under mass transfer limiting conditions similar to a recovery boiler char bed. The purpose of this study was to generate data to model the mass transfer limiting conditions of a char bed. In the current study a series of pyrolysis and C02 gasification experiments were conducted on a bed of kraft black char under a perpendicular impinging jet. It was demonstrated through C02 gasification impingement experiments and a series of gas samples collected from a recovery boiler char bed, that char carbon gasification rates are externally mass transfer limited. A net molar out-flow caused by the conversion of one mole of reactant C02 to two moles of product CO via gasification, is the primary driving force for the external mass transfer limitation. In addition to complete consumption of the char carbon, the recoveries of the inorganics (Na, K, S, C03-2 & NOx,) in the char bed are critical in the chemical recovery function of the char bed. Pyrolysis and gasification impingement conditions were varied in our experiments to provide insight into conditions favorable for each analyte. Inorganic analyte recoveries were consistent with the external mass transfer limitations observed in the gas phase results. Our studies are the first to describe the external mass transfer limitations that govern char carbon consumption and inorganic recoveries in a kraft black liquor recovery boiler char bed. A modified shrinking core model, that includes the application of a unique out-flow term can be applied to gas phase data to determine the external mass transfer resistance and char carbon gasification rates in a char bed.

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