Yi Niu

Comparative evaluation of thermal decomposition behavior and thermal stability of powdered ammonium nitrate under different atmosphere conditions

In order to analyze the thermal decomposition characteristics of ammonium nitrate (AN), its thermal behavior and stability under different conditions are studied, including different atmospheres, heating rates and gas flow rates. The evolved decomposition gases of AN in air and nitrogen are analyzed with a quadrupole mass spectrometer. Thermal stability of AN at different heating rates and gas flow rates are studied by differential scanning calorimetry, thermogravimetric analysis, paired comparison method and safety parameter evaluation. Experimental results show that the major evolved decomposition gases in air are H2O, NH3, N2O, NO, NO2 and HNO3, while in nitrogen, H2O, NH3, NO and HNO3 are major components. Compared with nitrogen atmosphere, lower initial and end temperatures, higher heat flux and broader reaction temperature range are obtained in air. Meanwhile, higher air gas flow rate tends to achieve lower reaction temperature and to reduce thermal stability of AN. Self-accelerating decomposition temperature of AN in air is much lower than that in nitrogen. It is considered that thermostability of AN is influenced by atmosphere, heating rate and gas flow rate, thus changes of boundary conditions will influence its thermostability, which is helpful to its safe production, storage, transportation and utilization.

Effects of mechanical activation methods on thermo-oxidation behaviors of pyrite

To explore the effects of mechanical activation methods (ball mill, planetary mill and rod mill) on the oxidation and the spontaneous combustion of pyrite, the kinetic curves of non-activated pyrite and mechanically activated pyrite were created by simultaneous thermal analysis. The structural characteristics and changes of mechanically activated pyrite were investigated by X-ray diffraction and SEM, and the relationship between the mean diameter and the grinding time was obtained by using a laser particle size analyzer. The kinetic model of pyrite and the kinetic parameters were deduced using Bagchi method. The relationship between the kinetic parameters indicates that, pyrite activated by ball milling shows the best thermal stability at the same diameter. By comparing and analyzing the X-ray diffraction patterns, results show that different mechanical activation ways played different roles in structural changes of pyrite.

Application of the global soot model based on smoke point in simulation of multiple laminar diffusion flames

ABSTRACT In this study, a new conservation scalar, sooting tendency parameter, which has the same control differential equation with mixture fraction except the boundary values at fuel inlets, is introduced to evaluate the average laminar smoke point (LSP) height of multiple fuels mixed in combustion region. According to theoretical analysis, the average LSP height of the mixed fuels in combustion region will be equivalent to the ratio of mixture fraction and sooting tendency parameter. The soot oxidation model is also modified in present work to allow it to be used with mixture fraction combustion model. Then this improved global soot model is implemented in Fire Dynamics Simulator and two different laminar flames are simulated in one case to validate the predictability. Good agreements between the predicted and measured soot volume fraction demonstrate the applicability of accounting for the sooting propensity of different fuels by the introduced scalar, and the reasonability of the modification of the soot oxidation model. The difference of stoichiometric mixture fraction between the fuel used in combustion model and the realistic fuels will lead to some error in soot prediction, yet which can be reduced by multiplying the boundary value of sooting tendency parameter by a safety factor (SF) greater than 1. The introduction of the SF is also very essential in fire protection design, because it will lead to the increase of predicted soot volume fraction, making the fire risk assessment more conservative.