Browsing by Author "Takuwa, Tsuyoshi"

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    Formation Characteristics of Fine Particulates with Na Compounds during Coal Combustion
    (2003-10) Takuwa, Tsuyoshi; Mkilaha, Iddi; Naruse, Ichiro
    Some metal compounds in coal vaporize and form fumes during the combustion. The fumes are usually exhausted through the flue gas. For coal-fired combined power generation systems such as pressurized fluidized bed combustion (PFBC), hot metallic vapors may contact with the surfaces of gas-turbine blades. Since this contact of the hot vapors with the surface has a corrosive effect, it is necessary to control the formation of those fumes, which contain alkali metal compounds. In this paper, the evolution behavior of alkali metal compounds, especially sodium compounds, has been studied, using an electrically heated drop tube furnace with a low-pressure impactor. The main objective in this study is to elucidate the conditions and the possible mechanisms to form alkali metal compounds in particulate matter during combustion. Two types of coal with different sodium contents were tested, where the coal conversion characteristics were established. Furthermore, the evolution and inclusion of sodium compounds into the sub-micron particles were studied in relation to the particle size distribution and sodium fraction distribution in the collected particulates. The study proved that the evolution and inclusion of sodium on sub-micron particles depended on the functions of the coal type. The reaction-controlled mechanism and heterogeneous condensation via chemical reactions during the combustion much more influenced the inclusion of sodium in sub-micron particles. At the coarse particles of above about 0.5 μm, the reaction that formed the particles was mainly via gas film diffusion surrounding the particle.
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    Mechanisms of fine particulates formation with alkali metal compounds during coal combustion
    (2006-02) Takuwa, Tsuyoshi; Mkilaha, Iddi; Naruse, Ichiro
    Two types of coals with different sodium (Na) and potassium (K) concentrations were burned in an electrically heated drop tube furnace, to study the formation of particulates in association to alkali metal compounds from the coals. The particulates formed from these coals during combustion were separated by a low pressure impactor (LPI). The particulates collected in each stage of the LPI were analyzed, using an atomic adsorption spectrometer (AAS). The results obtained show that ash particles have bimodal particle size distribution for both coal types. This tendency could be due to the difference of the inherent minerals in raw coals. It was further observed that Na and K were enriched in the fine particulates for both types of coal. There was a dependence of the Na enrichment on the fine particles on concentration of excluded mineral in the raw coals. This work, therefore, showed that the appearance of alkali metal in the ash particles related to the nature of Na and K concentrations in raw coals.