Tsuyoshi Takuwa

Effect of Mineral Matters in Coal on Formation Behaviors of Particulate Matter and Alkali Metal Compounds During Coal Combustion

Combustion tests for 2 types of coal (SC and BT coals), which have similar combustion performance and main fraction in the ash compositions, were conducted, using an electrically heated drop tube furnace. The burning particles were sampled by the Low Pressure Impactor (LPI) and analyzed concentration of alkali metal in the collected particles. Furthermore, they were observed by a scanning electron microscope (SEM) to discuss formation behaviors of the fine particles during combustion. In order to elucidate the relationship between formation characteristics of fine particulates during combustion and coal types, analysis of included and excluded mineral particles in the raw coals was conducted by a Computer Controlled SEM (CCSEM). Additionally, the composition analysis of the mineral particles in the raw coal was also carried out, using an energy dispersive X-ray spectrometer (EDS). As a result, the particle size distribution of ash particulates formed was different each other for two types of coal. Especially for the fine particulates with the size of less than 1 μm, the result for SC coal showed much more fine particulates formation than those for BT coal. Sodium compounds were enriched in the fine particulates for both the coals. Shapes of the fine particles with the size less than 1 μm for both the coals were spherical. For the coarse particles, however, spherical particles were observed for only SC coal. This difference was due to difference of the characteristics of mineral particles in the raw coals. The CCSEM analysis indicated that the excluded mineral matters, which tended to fragment during combustion, were contained more in SC coal than those in BT coal. This is the reason why SC coal forms more fine particulates during combustion than BT coal. From the EDS analysis, moreover, content of sodium in the excluded minerals of SC coal was higher than that of BT coal. This result suggests that SC coal tends to form the spherical fine particulates, in which the sodium compounds are enriched, during combustion.© 2005 ASME

Effect of Coal Type on Emission Characteristics of Alkali Metal Compounds in Particulate Matters

Some metal compounds in coal vaporize and form fumes during combustion. The fumes are generally exhausted through the flue gas. For coal-fired combined power generation systems such as pressurized fluidized bed combustion (PFBC), hot vapors may contact with the surfaces of gas-turbine blades. As this contact of the hot vapors with the surface has corrosive effect, it is necessary to control the formation of those fumes, which mainly contain alkali metal compounds. In this paper, the evolution behavior of alkali metal compounds, especially for 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 content 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 formed and sodium fraction distribution in the collected fine particulates. The study proved that the evolution and inclusion of sodium in the sub-micron particles depended on function of type of coal via its composition and the form by which sodium compounds existed in coal. The reaction-controlled mechanism and heterogeneous condensation via chemical reactions during combustion affected the inclusion of sodium in the sub-micron particles. In the coarse particles of above about 0.5 μm, reaction that formed those particles was mainly via gas film diffusion surrounding the particle.Copyright