锐钛型TiO_2对尿素热解过程的影响
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摘要
通过实验的方法收集了不同温度下纯尿素和尿素/TiO_2混合物热解后的固体残留物,使用红外光谱(IR)及气相色谱质谱联机(GC-MS)方法对这些热解残留物进行成分分析;使用热重-红外联机(TG-FTIR)技术研究尿素及三聚氰酸在有无催化剂TiO_2的情况下的热解特性及气体产物的生成规律;根据Coats-Renfern方法对尿素热解第一阶段的非等温热失重率曲线的数据进行动力学研究,建立动力学方程。结果表明,100～250℃的尿素热解残留物中主要为尿素和缩二脲,300～400℃的尿素热解残留物中主要为三聚氰酸等含氮杂环有机化合物;锐钛型TiO_2能促进尿素和三聚氰酸的热解反应,缩短其反应进程,HNCO与水蒸气在TiO_2表面易发生反应;尿素第一阶段热解的反应级数为2,单独热解时活化能为113.25kJ/mol,指前因子A为2.01×1011min-1,在催化剂TiO_2的作用下,活化能E为77.42kJ/mol,指前因子A为4.82×107min-1。
The solid residues after pyrolysis of pure urea and urea/TiO_2 at different temperatures were collected and analyzed by using infrared spectroscopy(IR) and gas chromatography mass spectrum(GC-MS). Thermogravimetric-infrared(TG-FTIR) technique was used to analyze the pyrolysis characteristicsof urea and cyanuric acid and the transformation mechanism of gaseous compounds in the pyrolysis process with and without TiO_2. The kinetic equations and parameters were obtained from the for non-isothermal weight loss curves according to the Coats-Redfern method. The results showed that urea andbiuret were the main pyrolytic residues of urea at 100—250℃, while the nitrogenous heterocyclic organiccompounds such as cyanuric acid were the main pyrolytic residues of urea at 300—400℃. Anatase TiO_2 can promote the pyrolysis reaction of urea and cyanuric acid and thus shorten the reaction process, since HNCO and water vapor can react easily on the surface of TiO_2. The reaction order of urea for the pyrolysisalone in the first stage of pyrolysis is 2, the activation energy is 113.25kJ/mol, and the pre-exponentialfactor A is 2.01×1011min-1. Under the presence of TiO_2, the activation energy E is 77.42kJ/mol, and the pre-exponential factor A becomes 4.82×107min-1.
The solid residues after pyrolysis of pure urea and urea/TiO_2 at different temperatures were collected and analyzed by using infrared spectroscopy(IR) and gas chromatography mass spectrum(GC-MS). Thermogravimetric-infrared(TG-FTIR) technique was used to analyze the pyrolysis characteristicsof urea and cyanuric acid and the transformation mechanism of gaseous compounds in the pyrolysis process with and without TiO_2. The kinetic equations and parameters were obtained from the for non-isothermal weight loss curves according to the Coats-Redfern method. The results showed that urea andbiuret were the main pyrolytic residues of urea at 100—250℃, while the nitrogenous heterocyclic organiccompounds such as cyanuric acid were the main pyrolytic residues of urea at 300—400℃. Anatase TiO_2 can promote the pyrolysis reaction of urea and cyanuric acid and thus shorten the reaction process, since HNCO and water vapor can react easily on the surface of TiO_2. The reaction order of urea for the pyrolysisalone in the first stage of pyrolysis is 2, the activation energy is 113.25kJ/mol, and the pre-exponentialfactor A is 2.01×1011min-1. Under the presence of TiO_2, the activation energy E is 77.42kJ/mol, and the pre-exponential factor A becomes 4.82×107min-1.
引文
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[19] ALSHEHRI S M, AL-FAWAZ A, AHAMAD T. Thermal kineticparameters and evolved gas analysis(TG-FTIR-MS)for thiourea-formaldehyde based polymer metal complexes[J]. Journal of Analytical&Applied Pyrolysis, 2013, 101(5):215-221.
[2] OGIDIAMA O V, SHAMIM T. Performance analysis of industrialselective catalytic reduction(SCR)systems[J]. Energy Procedia, 2014,61:2154-2157.
[3] SCHABER P M, COLSON J, HIGGINS S, et al. Thermaldecomposition(pyrolysis)of urea in an open reaction vessel[J].Thermochimica Acta, 2004, 424(1):131-142.
[4]刘成武,刘政修,王力彪,等.脱硝尿素热解系统沉积物形成原因分析及对策[J].热力发电, 2013, 42(3):53-57.LIU C W, LIU Z X, WANG L B, et al. Deposit formation in denitrationurea pyrolysis system:cause analysis and countermeasures[J]. ThermalPower Generation, 2013, 42(3):53-57.
[5] KLEEMANN M, ELSENERl M M,KOEBEL A, et al. Hydrolysis ofisocyanic acid on SCR catalysts[J]. Industrial&Engineering ChemistryResearch, 2000, 39(11):4120-4126.
[6] KR?CHER O, ELSENER M. Materials for thermohydrolysis of urea ina fluidized bed[J]. Chemical Engineering Journal, 2009, 152(1):167-176.
[7] BERNHARD A M, PEITZ D, ELSENER M, et al. Catalytic ureahydrolysis in the selective catalytic reduction of NOx:catalystscreening and kinetics on anatase TiO2and ZrO2[J]. Catalysis Science&Technology, 2013, 3(4):942-951.
[8] BERNHARD A M, PEITZ D, ELSENERl M, et al. Hydrolysis andthermolysis of urea and its decomposition byproducts biuret, cyanuricacid and melamine over anatase TiO2[J]. Applied Catalysis B:Environmental, 2012, s115/116(15):129-137.
[9] BERNHARD A M, CZEKAI I, ELSENER M, et al. Adsorption andcatalytic thermolysis of gaseous urea on anatase TiO2, studied byHPLC analysis, DRIFT spectroscopy and DFT calculations[J]. AppliedCatalysis B:Environmental, 2013, s134/135:316-323.
[10] FANG H L, DACOSTA H F M. Urea thermolysis and NOx, reductionwith and without SCR catalysts[J]. Applied Catalysis B:Environmental, 2003, 46(1):17-34.
[11] BRACK W, HEINE B, BIRKHOLD F, et al. Kinetic modeling of ureadecomposition based on systematic thermogravimetric analyses of ureaand its most important by-products[J]. Chemical Engineering Science,2014, 106(6):1-8.
[12] EICHELBAUM M, FARRAUTO R J, CASTALDI M J. The impact ofurea on the performance of metal exchanged zeolites for the selectivecatalytic reduction of NOx:Part I. Pyrolysis and hydrolysis of urea overzeolite catalysts[J]. Applied Catalysis B:Environmental, 2010, 97(1):90-97.
[13]高俊华,邝坚,宋崇林,等.国Ⅳ柴油机SCR后处理系统结晶体成分分析[J].燃烧科学与技术, 2010, 16(6):547-552.GAO J H, KUANG J, SONG C L, et al. Components of crystal fromSCR system of a diesel engine compliant with china stage IV emissionstandard[J]. Journal of Combustion Science&Technology, 2010, 16(6):547-552.
[14] RANGEL L S, ROSA J R D L, ORTIZ C J L, et al. Pyrolysis of ureaand guanidinium salts to be used as ammonia precursors for selectivecatalytic reduction of NOx[J]. Journal of Analytical&AppliedPyrolysis, 2015, 113:564-574.
[15] DéSILETS S, BROUSSEAU P, CHAMBERLAND D, et al. Analyses ofthe thermal decomposition of urea nitrate at high temperature[J].Thermochimica Acta, 2011, 521(1):59-65.
[16] BRILL T B, ARISAWA H, BRUSH P J, et al. Surface chemistry ofburning explosives and propellants[J]. Journal of Physical Chemistry,1995, 99(5):1384-1392.
[17] LUNDSTR?M A, SNELLING T, MORSING P, et al. Ureadecomposition and HNCO hydrolysis studied over titanium dioxide,Fe-beta andγ-alumina[J]. Applied Catalysis B:Environmental, 2011,106(3/4):273-279.
[18] CZEKAJ I, KR?CHER O. Decomposition of urea in the SCR process:combination of DFT calculations and experimental results on thecatalytic hydrolysis of isocyanic acid on TiO2, and Al2O3[J]. Topics inCatalysis, 2009, 52(13-20):1740-1745.
[19] ALSHEHRI S M, AL-FAWAZ A, AHAMAD T. Thermal kineticparameters and evolved gas analysis(TG-FTIR-MS)for thiourea-formaldehyde based polymer metal complexes[J]. Journal of Analytical&Applied Pyrolysis, 2013, 101(5):215-221.
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