Journal of The Energy Institute最新文献

The co-combustion of ammonia and pulverized coal can effectively reduce the carbon emission of thermal power generation. However, ammonia, as a carbon-free fuel, is rich in large amounts of nitrogen, which increases the risk of high NOx emissions. Therefore, it is important to clarify the influence mechanism of ammonia on the NO formation of coal volatile-N and coal char-N in the co-combustion, and to reveal the N oxidation pathway in different combustion stages.

In this study, simulations were carried out on the CHEMKIN software to investigate the generation characteristics of NO and the transformation mechanism of fuel-N at different combustion stages of ammonia-coal co-firing. The study showed that ammonia-blending combustion promoted the release of coal volatiles and the oxidation to NO. In the total NO generation during the ammonia-coal co-firing, the proportion of NO produced by ammonia-coupled coal char combustion was very low. Compared with ammonia-coupled coal combustion, the amount of NO produced in ammonia-coupled coal volatile combustion was significantly reduced. Sensitivity analysis and rate of production (ROP) analysis indicated that the increase of H, OH, and O free radicals promoted the NO formation, and that NH_i free radicals played an important role in the NO reduction. By analyzing the elementary path of NO generated from ammonia-coupled pulverized coal, coal volatiles and coal char combustion at 1400 °C and 10 % ammonia ratio, it can be seen that the main path of NO formation during ammonia-coupled coal volatiles combustion is VOL→HCN→NCO→NO, CHAR→NO, NH₂→HNO→NO, compared with ammonia-coupled coal combustion. The proportion of NH₂→NH→NO reaction paths decreased, while the proportion of NH₂→N₂, NCN→NCO→N₂, and NH₂→NNH→N₂ reaction paths increased respectively, indicating that separation combustion promoted the reduction of NO by NH_i free radicals while inhibiting the oxidation of N-containing components.

Application of renewable methanol as an alternative fuel is a promising method for both CO₂ and NO emission reduction in thermal power plants fueled by coal. This work gives the first insight into coal-methanol co-combustion from the perspective of NO emission control with a wide range of methanol blending ratio (0%–100 %) involved. Air-staged strategy commonly applied in thermal power plants fueled by coal was considered, and the effects of some key parameters, including burnout air ratio, burnout air injection position and furnace temperature, were analyzed. Experimental results show a significant potential of NO emission reduction in coal-methanol co-combustion, as NO emission from methanol combustion is less than 30 % of that from coal combustion. The correlation between NO emission and methanol blending ratio is approximately linear. Air-staged strategy is still effective for NO emission reduction in coal-methanol co-combustion, and the effects of the key parameter is similar to that in coal combustion. Increase of burnout air ratio and delay of burnout air injection are beneficial, and NO emission can be reduced by more than 70 % compared with that under unstaged condition. Furnace temperature rise is harmful, whereas the corresponding NO emission increase is lower than 30 ppm (@6 % O₂).

Chemical looping water gas shift (CL-WGS)is prospective to generate high-purity hydrogen with integrated CO₂ capture. However, this technology is impeded by the lack of active oxygen carriers at mid-temperatures. Here, we synthesized several Ni-doped CoFe₂O_4-δ to modulate oxygen vacancies and investigate its effect on promoting hydrogen production reaction via chemical looping water gas shift at 650 °C. The findings delineate that doping Ni considerably lowers the energy barriers associated with the oxygen vacancies formation, thereby augmenting their concentration. The underlying mechanism elucidates that within the CL-WGS process, the transfer of lattice oxygen acts as the rate-limiting step. Ni_xCo_1-xFe₂O₄ lowers the formation energy of oxygen vacancies and facilitates the bulk lattice oxygen diffusion through the bulk. Hence, Ni_0.5Co_0.5Fe₂O₄ demonstrates the most reduction depth and reversibility via redox reactions, resulting in an elevated hydrogen yield (∼15.5 mmol g⁻¹) at 650 °C, which surpasses the yield from undoped CoFe₂O₄ by 1.4 times. This performance remains consistently high with only a minimal decline over 100 cycles. The findings introduce a promising approach to promote the reactivity of oxygen carriers, particularly for mid-temperature applications.

Heavy tar is a crucial intermediate product during coal combustion. To explore the transformation pathway of N atoms in heavy tar under oxy-coal combustion, a comprehensive molecular model of heavy tar with typical N-containing functional groups was constructed. Different temperatures and chemical equivalence ratios were set for the oxy-coal combustion. The ReaxFF was employed to study various products' distribution and molecular numbers. The reaction network among different precursors and NOx was extracted, and the NO to N₂ conversion mechanism was summarized. The results indicated that, like char combustion, the proportion of heavy tar gradually declined, the proportion of light tar and organic gas first rose and then gradually declined, and the proportion of inorganic gas continuously rose during heavy tar combustion. As the temperature increased, the proportion of cyanide precursors decreased, while the proportion of amine precursors and NOx increased. The oxidation of N-containing intermediates became more intense as the O₂ content rose, but this oxidation effect was inhibited, and the NOx generation was reduced as the O₂ content further increased. NO could bond with NH_i, HNO, CN, and activate NO, decomposing to produce N₂O, and N₂O reacted with H radical to produce N₂.