基于铁基载氧体的污泥化学结构热解分子动力学模拟

Molecular dynamics simulation of chemical structure pyrolysis of sludge based on iron-based oxygen carrier

市政污泥的低碳资源化利用,是“无废城市”建设、应对全球气候变化的重要保障。化学链燃烧/气化技术是通过载氧体在污泥燃料和空气之间的循环反应实现污泥热解继而燃烧和碳捕集。采用廉价易得的铁基载氧体开展市政污泥化学链燃烧研究,为探究市政污泥基于铁基载氧体的化学链燃烧初始阶段的热解机理,采用X射线光电子能谱、^(13)C固体核磁等表征手段结合工业分析、元素分析,确定了典型市政污泥所含元素种类、化学价态以及成键方式,从而确定其分子团化学结构;以AlFeO_(3)的载氧体形式,构建了污泥独立热解和载氧体表面热解2种结构模型。通过反应力场分子动力学模拟(ReaxFF MD),主要针对升温速率和热解温度2个影响因素进行模拟。结果表明:不同升温速率下,污泥独立热解的产物主要为有机气体,升温速率过高不利于污泥分子团热解,选取16 K/ps较为合适;热解温度的升高和载氧体的存在均促进了污泥化学结构的热解,减少了焦油生成;无载氧体作用时,N元素在不同升温速率下,主要迁移至重质焦油中参与后续的燃烧反应;热解温度对含氮产物的生成影响不大。而载氧体的存在促使含氮活性基团的生成,进而产生NO并部分还原为N_(2);S元素主要迁移至小分子量碎片中,或以H_(2) S形式析出。因此,铁基载氧体提升了固体燃料的热解速率,可减少焦油产生,降低了NO_(x)生成,但对SO_(2)排放影响不大。

The low carbon emission and utilization of municipal sludge is an important guarantee for the construction of"waste free city"and coping with global climate change.Chemical looping combustion/gasification technology can realize the sludge pyrolysis,combustion and carbon capture by looping of oxygen carrier between sludge fuel and air reactors.The cheap and readily available iron-based oxy⁃gen carrier was used to carry out the chemical looping combustion of municipal sludge.In order to explore the pyrolysis mechanism of mu⁃nicipal sludge at the initial stage of chemical looping combustion based on the iron-based oxygen carrier,the X-ray optoelectronic energy spectroscopy,^(13)C solid nuclear magnet and the proximate analysis,elemental analysis were used here to determine the type,the chemical valence state and bonding mode of the typical municipal sludge,so as to determine the chemical structure of molecular groups.Two struc⁃tural models of sludge independent pyrolysis and oxygen carrier surface pyrolysis with AlFeO_(3) as oxygen carrier were constructed in this pa⁃per.The molecular dynamics simulations by the reaction force field(ReaxFF MD)is mainly simulated under the two influence factors of heating rate and pyrolysis temperature.The simulation results show that,under different heating rates,the products of sludge independent pyrolysis are mainly organic gas,and the too higher heating rate is not conducive to the pyrolysis of sludge molecular groups,16 K/ps is suitable.The increase of the pyrolysis temperature and the presence of the oxygen carrier both promote the pyrolysis of the sludge structure and reduce the generation of the tar oil.Without the oxygen carrier,the N mainly migrates into the heavy tar at different heating rates to participate in the subsequent combustion reaction.The pyrolysis temperature has little effect on the generation of the nitrogen-containing products.But the presence of Fe-based oxygen carrier promptes the generation of nitrogen-containing active groups,and then produces NO,while part of NO will be reduced into N_(2).The S elemental mainly migrates into the small molecular fragments or precipitates in the form of H_(2)S.Therefore,iron based oxygen carriers improve the pyrolysis rate of solid fuels,and have a tendency to reduce the production of tar,reduce the generation of nitrogen oxides,but have little impact on sulfur dioxide emissions.