Study On The Effects Of Red Mud On Nitrogen Transformation In The Pyrolysis Of Sewage Sludge

Nowdays,the pyrolysis of sewage sludge has drawn a lot of attention due to its efficiency in reducing sludge volume.However,the high nitrogen content in the sludge can be converted into nitrogen-containing gases during the pyrolysis process.This would reduce the quality of syngas and cause serious secondary pollution,thus limiting the resources recovery from the sewage sludge.Therefore,it is necessary to seek appropriate approaches to reduce the emission of nitrogen-containing gases.This thesis focused on the effects and the related mechanism of red mud on the transformation pathway of nitrogen-containing compounds among tar,char,and syngas during the pyrolysis process.The main contents are divided into three parts:1.The effects of the red mud on nitrogen transformation during the sludge pyrolysis processRaw sludge and sludge mixed with red mud were experimented at pyrolysis temperatures of 600,700,800,and 900?,respectively.By studying the distribution of N in the biochar,tar and gas phase,it was found that red mud promoted the fixation of sludge N in the biochar,inhibited its conversion to tar-N,and reduced the emissions of ammonia?NH₃?and hydrogen cyanide?HCN?.At temperature of 600?,the red mud showed the most significant influence on the increase of nitrogen contents in the residues.The pyrolysis-solid-N yield rate of sludge mixed with red mud increased 14.79%compared with that of raw sludge during the pyrolysis process.When the pyrolysis temperature was increased up to 900?,the yield rate of NH₃ in the sludge samples added with red mud could be reduced by 15.10%and the yield rate of HCN could be reduced by 24.72%,compared with those of raw sludge during the pyrolysis process.The analytical results of XPS and GC-MS indicated that the red mud showed significant influences on the morphology of the solid-N functional groups in the residues and the type of nitrogenous compounds in the tar.2.The effects of the red mud on the nitrogen transformation during the pyrolysis of soybean proteinA soybean protein?SP?was used as the model compound to represent sewage sludge.Raw SP samples and treated SP samples with different doses of red mud at mass ratios?red mud/soybean protein?of 0.1,0.2,and 0.6?labeled as SP+RM-0.1,SP+RM-0.2 and SP+RM-0.6?were experimented at the pyrolysis temperatures of 400,500,600,700 and 800?.The results indicated that red mud could promote the fixation of N in the biochar of soybean protein and inhibit the conversion of N to N-tar and help the formation of nitrogenous gases?NH₃-N+HCN-N+N₂-N?.Further studies indicated that higher proportion of red mud can result in higher fixation of N in the residue,more significant inhibition of NH₃ and HCN release and more N₂ emission.It is noted that when the pyrolysis temperature is around 800°C,the pyrolysis Char-N yield rate of SP+RM-0.1,SP+RM-0.2,and SP+RM-0.6 increased by respective 4.71%,10.39%and 21.52%while compared with the soybean protein pyrolysis alone;the?NH₃+HCN?-N yield rate decreased by 26.24%,47.24%and 73.70%,while the N₂-N yield rate increased by 7.09%,7.49%and 16.91%,respectively.Based on the results of XPS analysis of the pyrolysis residues and GC-MS analysis of the tar,it was found that the red mud helped to promote the formation of more stable N-functional groups such as nitrile-N and quaternary N in the residue,and meanwhile inhibited the pyrolysis of agglomerated amine-N compounds in tar.3.The mechanism of red mud inhibition on the release of nitrogen-containing gases during the pyrolysis of sewage sludgeA soybean protein?SP?was used as the model compound to represent sewage sludge.Different mineral compounds in red mud,i.e.Fe₂O₃?Al₂O₃ and CaO,were added at a mass ratio(m_mineral:m_{soybean protein})of 0.1,and were pyrolyzed at different temperatures?400,500,600,700 and 800??.The results indicated that all the added mineral compounds showed significant impacts on the nitrogen transformation,except Al₂O₃.To be summarized,the mechanism of the red mud that inhibit the emission of nitrogen-containing gases was:?1?The Fe2O3 and CaO contained in red mud contributed to the formaion of the stable nitrogen functional groups,which facilitated the fix of the nitrogen in pyrolytical char;?2?Fe₂O₃ and CaO could react with Char-N to form the Fe_x N and CaC_xN_y,the formation and decomposition of he Fe_xN and CaC_xN_y facilitated the conversion of NH₃ and HCN to N₂and H₂,which reduced the release of NH₃ and HCN;?3?in the temperature range of 400-500?,the Fe₂O₃ contained in red mud inhibited the secondary cracking of amine-N compounds;while at temperatures above 500 ?,both Fe₂O₃ and CaO inhibited the secondary cracking of amine-N compounds,which reduced the formation of heterocyclic-N and nitrile-N compounds,and reduced the release of NH₃ and HCN.