| Oxy-fuel combustion is one of the several promising new technologies which can realize the integrated control of CO2, SO2, NOx and other pollutants. Most studies have found significantly lower NOx emissions and unstable ignition combustion characters during Oxy-coal combustion.When fuels are burned in the high CO2 concentration environment, its NOx generation and conversion process combustion characteristics can be very different from conventional air-fired combustion. Such changes imply that the high CO2 concentration atmosphere have physically and chemically impacts on the combustion processes. Up to now, detailed and quantitative researches on the high CO2 concentration atmosphere's effect on the NOx and flame characteristics are still limited, while high CO2 concentration's chemical effect on NOx conversion has not been well studied.In this paper, NOx combustion chemistry under O2/CO2 atmosphere as well as air atmosphere was studied using detailed kinetic model. A suitable reaction mechanism was chosen based on the comparison between the calculation result and the experimental data. The influence of various parameters (atmosphere, temperature, pressure, CO2 concentration, O2 concentration) on NOx conversion was also investigated. Compared with O2/CO2 atmosphere, air atmosphere is favored to achieve lower NO emission at fuel-rich condition. At fuel-lean condition, there is not much difference of NO emission between different atmospheres, and O2/CO2 atmosphere helps to control NO emission a little bit. And the trend is reversed in lower temperature window from 1250K to 1450K, only at fuel-rich condition; higher temperatures cause lower NO emissions under both atmospheres. It can be concluded that high CO2 concentration atmosphere does affect the NO pollutant emission behaviour. The chemical effects of high CO2 concentration on NO formation and destruction process was studied. The dominant reaction steps contribution to production and destruction of NO as well as the most important reactions for NO reduction under different atmospheres were discussed. On the basis of investigations through elementary chemical reactions, it can be concluded that high CO2 concentration play a pronounced role on NOx conversion process. The effect of high CO2 concentration atmosphere on laminar burning velocity, ignition delay and adiabatic temperature characters of three liquid hydrocarbons (n-pentane, n-hexane, n-heptane) were also studied in this paper. Detailed chemical kinetics model was employed and evaluated in comparison with the well-defined laboratory datas in the literature. Reasonably good agreement is observed between the model simulations and the experimental results.Based on the verified models, the influences of various parameters (equivalence ratio, O2 concentration, CO2 concentration, and atmosphere) on the combustion characteristics were also investigated. It can be concluded that high CO2 concentration atmosphere has negative effect on the n-pentane, n-hexane and n-heptane flame's ignition delay, flame speed and adiabatic temperature characteristics. On the basis of simulation through elementary chemical reactions, the results confirm that high CO2 concentration atmosphere's chemical effects play a pronounced role on the combustion characteristics.At last, uncertainty analysis was used to investigate the precision property of detailed chemical kinetic models. A algorithm for assessing and evaluating the impact of uncertainties in chemical kinetic models was presented. The method was also validated through analysis on different kinetic mechanisms applied in the process of modeling NOx emission in methane flame. The algorithm, which provided a basis for further studies, was more efficient and general compared with other methods.This work helps to develop a better understanding of the Oxy-combustion flame, and of NOx reduction in Oxy-combustion systems.
|
PDF Full Text Request