超细煤粉化学氧化改性对NO_(x)生成的影响

Effect of chemical oxidation modification of superfine pulverized coal on NO_(x)formation

有机含氧官能团对煤的热解和燃烧过程具有重要影响。目前关于引入的外来官能团对煤热转化过程中生成NO_(x)的影响的研究较为欠缺。本研究使用过氧化氢、过氧乙酸、硝酸和过硫酸铵4种典型的化学试剂对2种煤阶的煤粉(无烟煤煤粉和烟煤煤粉)进行了化学氧化改性,利用^(13)C-NMR和XPS分别获得了改性前后煤粉含氧官能团和含氮官能团的变化规律,并对原煤和改性煤进行了热解和燃烧实验。^(13)C-NMR结果表明:过氧乙酸溶液和过氧化氢溶液均可通过氧化使煤中烷基芳香碳和氧基取代芳碳断键。5%(质量分数)过氧乙酸溶液可在煤中引入醇羟基、羧基等官能团,1%和5%(质量分数)的过氧化氢溶液与煤粉反应无法引入含氧官能团。XPS结果表明:5%过氧乙酸溶液可将煤粉表面的吡咯型氮(N-5)、吡啶型氮(N-6)氧化为氧化物氮(N-X)结构。此外,过氧乙酸溶液和过二硫酸铵溶液可将煤中芳香胺结构氧化为—NO_(2)基团,此结构在180℃开始受热分解。在热解过程中,过氧乙酸溶液改性后的煤粉在更低的温度下释放出了更多的NO_(x),N_(2)O释放温度降低至200℃,NO_(x)生成途径发生了改变。过氧乙酸溶液对两种煤的改性效果一致,且对于无烟煤来说,粒径越小,过氧乙酸溶液改性效果越明显。过氧乙酸溶液改性后,煤粉中燃料氮向气相氮的转换得到调控与强化,为后续煤燃烧烟气中NO_(x)的脱除创造了更多的时间与空间,丰富了NO_(x)生成理论。基于实验结果,本研究提出了过氧乙酸改性与空气分级技术耦合的NO_(x)减排新设想。

The presence of organic O-containing functional groups in coal can significantly affect the pyrolysis and combustion processes.However,there is a scarcity of studies investigating the impact of O-containing structures introduced into coal macromolecules on NO_(x)formation during coal combustion.Representative chemical reagents,namely hydrogen peroxide,peracetic acid,nitric acid and ammonium persulfate were selected to modify the superfine powder coal of 2 ranks(bituminous coal and anthracite).^(13)C-NMR and XPS were applied to obtain the changes of O-containing and N-containing functional groups,and the pyrolysis and combustion experiments of raw and modified coal were conducted successively.The ^(13)C-NMR results reveal that both peracetic acid solution and hydrogen peroxide solution can break alkyl aromatic and O-substituted aryl carbon by oxidation in coal.A 5%(mass fraction)peracetic acid solution is able to introduce functional groups such as alcohol hydroxyl group and carboxyl group into coal,whereas 1%and 5%(mass fraction)hydrogen peroxide solutions do not exhibit such performance.XPS results indicate that a 5%peracetic acid solution can oxidize pyrrole nitrogen(N-5)and pyridine nitrogen(N-6)on the surface of pulverized coal into oxide nitrogen(N-X)structure.Additionally,the aromatic amine structure in the coal can be oxidized to form—NO_(2) group by peracetic acid solution and ammonium persulfate solution.This structure begins to decompose upon heating at 180℃.During the pyrolysis process,the samples modified by peracetic acid solution release more NO_(x)at a lower temperature,consequently lowering the release temperature of N_(2)O to 200℃,thereby altering the NO_(x)generation pathway.The two kinds of coal have the same modification effect.For anthracite,with the particle sizes decreasing,the modification effect becomes more obvious.After peracetic acid modification,the conversion of fuel nitrogen to gas phase is strengthened,which enriches NO_(x)formation theory and creates more time for the subsequent removal of NO_(x).Based on the experimental results,a novel approach to reduce NO_(x)emission is proposed,which involves coupling peracetic acid modification with air classification technology.