The active cooling technology based on endothermic hydrocarbon fuels can cause serious coking problems. Ammonia addition inhibits the coke formation at the mild fuel cracking. However, the mechanism of coke deposition under higher fuel conversion conditions remains unclear. Herein, the deep pyrolysis experiments of n-decane with ammonia addition was conducted using the electric heating method. The anti-coking performances of additives were obtained from the coking characteristics at different pyrolysis stages. The results indicate that under deep cracking conditions, the additive reacts with unsaturated free radicals produced by fuel cracking to produce aromatic heterocyclic compounds, inhibiting the formation and growth of polycyclic aromatic hydrocarbons (PAHs), thereby reducing the content of coking precursors. More importantly, the C-N compounds interact with the metals to form the spherical carbon with a predominant diamond structure, depositing as a thin film with passivation effect. As the reaction time proceeds, the film gradually smooths and maintains a thickness of about 4 μ m . In addition, passivation coating of TiN is used to further evaluate the effect of additives on the formation of surface coke. These results reveal that ammonia can ensure effective coking inhibition under long-term deep cracking conditions by affecting the growth of PAHs and coke surface.
Ji Liu,Yuan-gu Xia,Meng-fan Liu,Bin Hu,Hong-jin Zhang,Ji-hong Li,Zhen-xi Zhang,Qiang Lu
Nitrogen oxides are the major pollutants in solid fuel/waste combustion processes, which can be reduced by unburned or additive chars. The surface char–NO reaction is significantly influenced by CO 2 , whereas the impact mechanism is far from conclusive. Herein, fixed-bed experiments and computational methods were adopted to elucidate the role of CO 2 on char–NO reaction. Experimentally, CO 2 exhibited an inhibitive impact on NO reduction at 873 K but a promotive influence at 1023 K and 1173 K. According to the detection of outlet CO, characterization of char, and density function theory (DFT) calculations, CO and gasified char derived from char–CO 2 gasification are both responsible for the promotive effect at 1173 K, which offers a clear image of the gasification mechanism conjectured in previous work. More importantly, a CO 2 -mediated oxygen rearrangement (CO 2 -MOR) mechanism has been found and confirmed for the first time, which can well explain the experimental results observed at 873 K and 1023 K. The adsorbed CO 2 can intervene in the oxygen rearrangement on the char surface to alter the NO reduction behaviors. The present study offers a comprehensive insight into the roles of CO 2 in the heterogeneous reduction of NO on char surface.
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