| Toxic heavy metal mercury derived from coal combustion will cause serious damage to human living environment. Biomass is concerned because of its advantage of resource-rich, carbon-cycle, low-pollution. The research of co-combustion biomass with coal has great practical significance. This new combustion method can not only take full adavantage of natural resource, but also control the emission of pollutants.Biomass has substantial content of alkali metals. The effect of alkali metal on mercury oxidation in flue gas during co-combustion biomass with coal was studied based on previous mercury oxidation mechanism in present work. It focused on the dynamics mechanism of homogenous mercury oxidation and the reactions of alkali metals. The thermodynamics model of co-combustion biomass with coal was established with CEA, a thermodynamics software which is developed based on the principle of Gibbs free energy minimization. The transformation of alkali metals in flue gas was studied. The thermodynamics calculation results suggest that alkali metals react with chloride or sulfur oxide in flue gas. At high temperatures, alkali metals exist mainly in alkali chloride (ACl). But below 1100K, the thermodynamics stable substance is A2SO4. The temperature of ACl transition to A2SO4 moves towards lower temperature.On the other hand, Hg/C/H/O/N/S/Cl/K/Na kinetics model was proposed and established based on previous kinetics data. Kinetics calculation and sensitivity analysis, which was focused on the transformation of alkali in flue gas and the effect of concentration of Cl2 and SO2 on mercury oxidation, was performed with CHEMKIN 4.1, chemical kinetics software. According to the kinetics model, it's concluded that as the concentration of Cl2 and SO2 changes, the influence degree of alkali on mercury will change. Mercury oxidation will be greatly inhibited when the concentration of Cl2 and SO2 is low, while it will be little promoted when the concentration of Cl2 and SO2 is higher. The sensitivity analysis shows that the alkali metals influence mercury oxidation mainly by the reaction with Cl2 and SO2, and it will be suppressed when alkali reacts with Cl2 but promoted when alkali reacts with SO2.The effect of biomass co-firing with coal on Hg oxidation was further studied with CHEMKIN 4.1. Chemical and phase equilibrium calculations results show that mercury oxidation will be promoted during co-combustion biomass with coal, compared with during coal combustion. The high content of Cl in the biomass is the most important factor to promote Hg oxidation, and the presence of K and Na will further promote Hg oxidation. And this also further confirms the thermodynamic calculations. Mercury oxidation is also influenced by the concerntration of alkali, chlorine and sulfur in fuel, so right mix ratio should be choosed when biomass fired with coal. In addition, the calculation results indicate that reaction reactivity is controlled by temperature. At different temperatures, the mercury oxidation degree and the influence of alkali metals on mercury oxidation are different. Mercury oxidation can be promoted by alkali metals only at suit temperatures.Based on Hg/C/H/O/N/S/Cl/K/Na kinetics model, the physical model of combustion was established with FLUENT, a computational fluent dynamics (CFD) software. The CHEMKIN kinetics mechanism was imported to the physical model to simulate real combustion. Chemical reactions are combined with physical fields, the influence of flow field on chemical reaction was considered. The physical model calculation results are consistent with experimental datas, and it was verified to be feasible. |
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