Of all the trace elements emitted during coal combustion, mercury is most problematic. Mercury from the atmosphere enters into oceanic and terrestrial waters. Part of the inorganic Hg in water is converted into organic Hg (CH3Hg), which is toxic and bioaccumulates in human and animal tissue. The largest source of human-caused mercury air emissions in the U.S is from combustion coal, a dominant fuel used for power generation. The Hg emitted from plants primarily occurs in two forms: elemental Hg and oxidized Hg (Hg2+). The coal chlorine content and ash composition, gas temperature, residence time and presence of different gases will decide the speciation of Hg into Hg0 and Hg2+. For Wyoming coal the concentrations of mercury and chlorine in coal are 120ppb and 140ppb. In order to understand the basic process of formulation of HgCl2 and Hg0 a numerical model is developed in the current work to simulate in the detail i) heating ii) transient pyrolysis of coal and evolution of mercury and chlorine, iii) gas phase oxidation iv) reaction chemistry of Hg and v) heterogeneous oxidation of carbon during isolated coal particle combustion. The model assumes that mercury and chlorine are released as a part of volatiles in the form of elemental mercury and HCl. Homogenous reaction are implemented for the oxidation of mercury. Heterogeneous Hg reactions are ignored. The model investigates the effect of different parameters on the extent of mercury oxidation; particle size, ambient temperature, volatile matter, blending coal with high chlorine coal and feedlot biomass etc,. Mercury oxidation is increased when the coal is blended with feedlot biomass and high chlorine coal and Hg % conversion to HgCl2 increased from 10% to 90% when 20% FB is blended with coal. The ambient temperature has a negative effect on mercury oxidation, an increase in ambient temperature resulted in a decrease in the mercury oxidation. The percentage of oxidized mercury increases from 9% to 50% when the chlorine concentration is increased from 100ppm to 1000ppm. When the temperature is decreased from 1950 K to 950 K, the percentage of mercury oxidized increased from 3% to 27%.

Orientador: Leonardo Goldstein Junior / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-09T04:22:28Z (GMT). No. of bitstreams: 1 RamirezBehainne_JhonJairo_D.pdf: 3210351 bytes, checksum: f10fc08456d97b1335e17aab68707b93 (MD5) Previous issue date: 2007 / Resumo: Este trabalho teve o propósito de avaliar as emissões totais de mercúrio produzidas pela combustão de misturas de carvão mineral brasileiro CE-4500 com dolomita em regime de fluidização rápida. Amostras do gás de combustão foram coletadas na chaminé do sistema após o filtro de mangas do combustor, através de uma sonda isocinética de laboratório operada com base nos métodos U.S.E.P.A. n. 29 e ASTM-D-6784 (Ontario-Hydro). Resíduos sólidos gerados pelo processo também foram coletados, e junto com as amostras obtidas na chaminé, foram analisados através da técnica de espectrofotometria de fluorescência atômica de vapor frio. Em onze testes experimentais, obteve-se informação relacionada com os perfis axiais de pressão estática e temperatura, a concentração de O2, CO e NOx nos gases de combustão, o rendimento de captura do separador de partículas do sistema e a eficiência da conversão de carbono no processo. Os resultados do estudo revelaram que, em média, 6% do mercúrio total liberado nos efluentes do processo saiu pela chaminé com o gás de combustão e com as partículas não coletadas pelo filtro de mangas. Adicionalmente foi demonstrado que o valor da relação molar cálcio-enxofre não influencia significativamente nas emissões totais do poluente no gás de combustão. O balanço de mercúrio no processo mostrou um desvio médio de 40%, valor considerado satisfatório tendo em conta as dificuldades para a determinação de um poluente presente em baixas concentrações. / Abstract: In this work, total mercury emissions from fast fluidized bed combustion of CE-4500 Brazilian coal and dolomite mixtures were quantified. Stack gas samples were collected downstream of a baghouse by using an isokinetic probe and U.S. E.P.A. n. 29 / ASTM-D-6784 (Ontario-Hydro) method procedures. The ash samples trapped in the particulate control device and those accumulated in the bottom of the reaction chamber were also analyzed. A cold-vapor atomic fluorescence spectrophotometer was employed to determine the mercury concentration in the stack gas and ash samples. Additional experimental information about the combustion process was obtained, such as, pressure and temperature axial profiles, O2, CO and NOx concentrations in the exhaust gas, baghouse performance and carbon conversion efficiency. Results showed that approximately 6% of the total mercury released by the process in gaseous and particulate form was emitted from the stack, and this level was not affected by the amount of dolomite being used. A deviation close to 40% in the mercury mass balance for the process was verified. Results were satisfactory taking into consideration the inherent difficulties to quantify small concentrations of the pollutant in the process. / Doutorado / Termica e Fluidos / Doutor em Engenharia Mecânica

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