Numerical Simulation Of Single-partical Char-nitrogen Conversion

As important primary energy of China, combustion of coal Brings serious environmental problems. NOx causes environmental problems, such as acid rain and photochemical smoke and so on, as one of environmental pollutant. at the same time,NOx is Harmful to human health. So the problem of NOx caused the wide attention around the world. heat-engine plant is a primary source of NOx. Meanwhile, Coal-fired power plants has a major share in it. Severe environmental problems and Stringent emission limits of new national standard put forward new requirements to emission reduction of NOx. In order to further reduce the emissions of NOx, clearer understanding of the conversion of coal nitrogen to NOx is needed. the conversion of coal nitrogen to NOx divides into the conversion of volatile nitrogen to NOx and the conversion of char nitrogen to NOx. However, char nitrogen conversion mechanism is still not clear currently. It is a effective way to realise the char nitrogen conversion process by employing single partical model for calculation of char nitrogen conversion.Based on energy conservation equation that describing the change of temperature in the process of combustion and component mass conservation equation that describing the change of gas component in the process of combustion, this artical employed implicit finite volume method to discrete equation. Reaction source term included in equation is determined by corresponding reaction mechanism, including the mechanism of char burnout and char nitrogen conversion. The mechanism of char burnout includes the four-step two-phase surface model and the heterogeneous volume reaction betweeen CO and O2 and heterogeneous surface reaction between CO2 and char. The mechanism of char nitrogen conversion includes the production mechanism of NO and the reduction mechanism of NO. A direct oxidation of char nitrogen to NOx is selected as the production mechanism of NO, taking no account of intermediates for NO production during char combustion and N2 O which has a remarkable effects in the temperature range in fluidized beds. the reduction mechanism of NO takes into account surface reduction between NO and char and volume reduction between CO and NO.The calculation of the distribution of gases inside the pattical is conducted for the importance of diffusion in the process of combustion. Mainly includes the distribution of the volume concentration and the influence of temperature, partical size and gas concentration. The calculation result indicates that temperature and partical size increment is negative for gas diffusion,meanwhile,gas concentration promotes gas diffusion to some extent.In the calculation of char nitrogen conversion mainly calculate the conversion ofchar nitrogen to NO and the influence of temperature, partical size and the concentration of O2 and NO.The conversion of char nitrogen to NO increases with the growth of temperature under the pulverized coal combustion condition. The environmental oxygen concentration has a complex impact on the conversion of char nitrogen to NO. The conversion of char nitrogen to NO almost constant with the growth of the environmental oxygen concentration in the temperature range under 1200℃. While the conversion of char nitrogen to NO increases with the growth of the environmental oxygen concentration, and the conversion almost constant when the environmental oxygen concentration higher than 10%. The conversion of char nitrogen to NO decreases with the growth of partical size. As the initial environmental NO concentration increases, the conversion of char nitrogen to NO decreases. The impact of the initial environmental NO concentration on the conversion of char nitrogen to NO in hign temperature is smaller than the impact in low temperature.