O_(2)/H_(2)O预氧化对高阶煤中N元素迁移路径的影响

Effect of O_(2)/H_(2)O pre-oxidation on the migration pathway of nitrogen element in high rank coal

通过贫氧预氧化可以有效改善高阶煤燃烧过程中着火困难、燃尽率低、NOx排放量高等问题。基于O_(2)和H_(2)O对煤基燃料表面结构的协同活化特性,研究高阶煤O_(2)/H_(2)O预氧化过程中N元素的演化机制。应用等温固定床试验台作为反应器,将高阶煤样品承装于微型流化装置。当反应器温度稳定在1173 K时,在O_(2)/H_(2)O气氛下进行高阶煤活化改性试验。通过控制微型流化装置在反应区的停留时间,调节煤粉样品的预氧化程度。反应尾气由高精度红外烟气分析仪进行在线测量,半焦样品冷却后进行元素分析、工业分析和XPS分析,确定预氧化过程中N元素和C元素的迁移路径和析出规律。试验结果表明:在高阶煤贫氧预氧化过程中,H_(2)O的加入将导致半焦中残留N元素的含量明显降低,气态产物中N_(2),HCN和NH_(3)的占比明显升高,燃料氮向NO的转化率明显降低;预氧化过程中O_(2)和H_(2)O可以协同促进稠环芳香结构转化为小芳香环结构,导致结构内部的N元素逐渐迁移至结构边缘,促进燃料中热稳定性较强的吡啶(N-6)和季氮(N-Q)转化为HCN的前驱体吡咯(N-5);在反应过程中,O_(2)体积分数的提高和H_(2)O的加入可以定向促进结构氮的快速释放,降低N元素剥离过程中固定碳的损耗。因此,O_(2)/H_(2)O预氧化可以有效优化高阶煤在预氧化过程中N元素的析出路径,调控半焦中残留N元素的赋存形态,在一定程度上优化高阶煤的燃料品质。

The oxygen deficient pre-oxidation technology can effectively solve the problems of different ignition,low burnout rate and massive NOx emission in high rank coal based fuel combustion.Based on the co-activation behavior of O_(2)and H_(2)O on the surface of coal based fuel,the evolution mechanism of nitrogen element in the process of O_(2)/H_(2)O pre-oxidation of high rank coal was studied.The isothermal fixed-bed reactor was employed as the experimental system,and the samples were loaded in a micro fluidization device.The tests were performed at 1173 K under O_(2)/H_(2)O atmosphere.The composition of downstream gas was measured on-line by a high resolution infrared flue gas analyzer.Subsequently,the proximate analysis,ultimate analysis and XPS analysis were carried out for determining the migration pathway and precipitation mechanism of nitrogen element and carbon element in the reaction process.The results illustrated that the content of residual nitrogen in semi-char particles decreased obviously,and the proportion of N_(2),HCN and NH_(3) in the gaseous products increased apparently,and the conversion ratio of fuel-N to NO decreased significantly.Furthermore,O_(2)and H_(2)O molecules could accelerate the conversion of condensed aromatic ring structure into small ones and the inside nitrogen migrated to the edge of the structures,promoting the conversion of thermal stable pyridine(N-6)and quaternary nitrogen(N-Q)into pyrrole(N-5)which was the precursor of HCN.In the reaction process,the enhancement of O_(2)and the addition of H_(2)O could promote the rapid release of structural nitrogen and reduce the loss of fixed carbon in the process of nitrogen stripping.Therefore,the O_(2)/H_(2)O pre-oxidation could effectively optimize the release pathway of nitrogen in the process of pre-oxidation,regulate the occurrence of residual nitrogen in semi-char,and gradually optimize the fuel quality of high rank coal.