Abstract

Ash deposits in commercial coal-fired boilers frequently pose serious maintenance challenges and decrease thermal efficiency. A better understanding of fundamental thermal transport properties in ash deposits can help mitigate their negative effects. In order to characterize the thermal properties of boiler-side deposits, this work presents a thermal transport model and in-situ measurements of effective thermal conductivity in coal ash deposits. A simple model of the thermal transport through an ash deposit, with and with out slagging, was developed. The model approximates the deposit by dividing it into four regimes: particulate, sintered, solidified slag, and molten slag. The development of this model was auxiliary to the primary focus of this study: the in-situ measurement of effective thermal conductivity of ash deposits. Deposits of loosely-bound particulate ash were obtained experimentally using a down-fired drop tube reactor. Pulverized coal was fired and deposits were collected on an instrumented deposition probe. An approach is presented for making in-situ measurements of the temperature difference across the ash deposits, the thickness of the deposits, and the total heat transfer rate through the ash deposits. Using this approach, the effective thermal conductivity was determined for coal ash deposits formed under oxidizing and reducing conditions. Three coals were tested under oxidizing conditions: IL #6 Crown III coal, IL #6 Patiki coal and WY Corederro coal. The WY coal exhibited the lowest range of effective thermal conductivities (ke =0.05 to 0.175 W/mּK) while the IL #6 coals showed higher effective thermal conductivities (ke =0.2 to 0.5 W/mּK). The IL #6 Crown III coal and the WY Corederro coal were also tested under reducing conditions. A comparison of the ash deposits from these two coals, formed under oxidizing or reducing conditions, showed larger effective thermal conductivities in deposits formed under reducing conditions. The IL #6 Crown III coal exhibited the greatest increase (as high as 50%) in ke, under reducing conditions, over that measured in oxidizing conditions. For all of the experiments conducted, an increase in effective thermal conductivity with deposit thickness was observed, with sintering likely causing the increase in ke.

Degree

MS

College and Department

Ira A. Fulton College of Engineering and Technology; Mechanical Engineering

Rights

http://lib.byu.edu/about/copyright/

Date Submitted

2008-12-17

Document Type

Thesis

Handle

http://hdl.lib.byu.edu/1877/etd2764

Keywords

coal, ash, deposit, thermal conductivity, in-situ, thermal transport, slag

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