Oxidation kinetics of adsorbent-decorated Fe-based oxygen carrier for chemical-looping combustion
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摘要: 研究了吸附剂修饰合成Fe2O3/Al2O3的氧化动力学。其中，吸附剂（K2O、Na2O、CaO）用于控制化学链燃烧过程中有毒氯化物、硫化物以及重金属的排放。首先在热重分析仪（TGA）上利用合成气作为还原气氛使氧载体呈还原态（FeO/Al2O3），在空气气氛下进行了原FeO/Al2O3以及三种吸附剂修饰FeO/Al2O3的氧化实验，实验温度分别为850、875、900和925℃。通过八种等温动力学模型对900℃下原FeO/Al2O3的氧化过程进行了分析。结果表明，phase boundary-controlled（contracting cylinder）模型能够很好地描述其氧化过程（FeO向Fe2O3转化过程）。利用该模型分别计算了原FeO/Al2O3、K2O修饰FeO/Al2O3、Na2O修饰FeO/Al2O3和CaO修饰FeO/Al2O3的氧化动力学参数，其表观活化能分别为13.71、20.21、21.62和24.20 kJ/mol。通过进行比较依据动力学参数计算得到的转化率随时间的函数以及实验获得的转化率随时间的函数，进一步证实了phase boundary-controlled（contracting cylinder）模型的可靠性以及相应动力学参数的准确性。
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关键词:
- 化学链燃烧 /
- 氧化动力学 /
- 吸附剂修饰铁基氧载体 /
- TGA
Abstract: Adsorbent-decorated Fe2O3/Al2O3 as oxygen carrier (OC) was proposed for restraining the emission of chloride, sulfide and heavy metals during the chemical-looping combustion process of gaseous or solid fuels. Three adsorbents (K2O, Na2O and CaO) were selected for decorating these OC particles. First, the raw Fe2O3/Al2O3 and three adsorbent-decorated Fe2O3/Al2O3 were reduced by synthesis gas and then the oxidation kinetics of four reduced OCs (raw FeO/Al2O3 and three adsorbent-decorated FeO/Al2O3) were investigated by using thermogravimetric analysis (TGA) technique in air atmosphere at four temperatures (850, 875, 900 and 925℃). It was found that the translation of FeO to Fe2O3 can be described by the phase boundary-controlled (contracting cylinder) model, and the apparent activation energy (E) was calculated to be 13.71, 20.21, 21.62 and 24.20 kJ/mol for raw FeO/Al2O3, K2O-decorated FeO/Al2O3, Na2O-decorated FeO/Al2O3 and CaO-decorated FeO/Al2O3, respectively. Last, the reaction mechanism was evaluated through comparing the calculated data from the obtained kinetic parameters and the experimental results, which demonstrated the reliability of the phase boundary-controlled (contracting cylinder) model.-
Key words:
- chemical looping combustion /
- oxidation kinetics /
- adsorbent-decorated Fe2O3/Al2O3 /
- TGA
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图 3 900℃下Fe2O3/Al2O3为氧载体时 TG 随时间的变化
Figure 3 TG as a function of time for reduction (a) and oxidation (b) of raw OC at 900℃
图 4 空气条件下不同氧载体颗粒的转化特性
Figure 4 Conversion characteristics obtained from oxidation of different OC particles in air atmosphere
图 5 八种反应机理模型拟合900℃下原FeO/Al2O3颗粒在空气气氛下的氧化过程
Figure 5 Integral form expressions of eight reaction mechanisms for raw FeO/Al2O3 reacting with air at 900℃
图 6 四种氧载体被空气氧化过程中lnk与1000/T的线性拟合
Figure 6 Plots of lnk vs 1000/T for the oxidation of four OCs with air
图 7 900℃下四种氧载体的实验线
Figure 7 Experimental values of four OC s oxidation at 900℃ vs the calculated model
(a): raw FeO/Al2O3; (b): K2O decorated FeO/Al2O3; (c): Na2O decorated FeO/Al2O3; (d): CaO decorated FeO/Al2O3 ： computed result; ： experimental result
表 1 不同反应动力学机理的动力学方程
Table 1 Kinetic equations for different reaction mechanisms
Reaction mechanism Integral form G(x)=kt 1-D diffusion x2 2-D diffusion x+(1-x)ln(1-x) 3-D diffusion-Jander [1-(1-x)1/3]2 Phase boundary-controlled
(contracting cylinder)1-(1-x)1/2 Phase boundary-controlled
(contracting sphere)1-(1-x)1/3 First order reaction -ln(1-x) 2-D growth of nuclei [-ln(1-x)]1/2 3-D growth of nuclei [-ln(1-x)]1/3 
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表 2 八种反应机理模型对900℃下原FeO/Al2O3氧化过程的线性拟合参数
Table 2 Linear fitting parameters of eight reaction mechanisms for the oxidation process of raw FeO/Al2O3 by air at 900℃
Reaction mechanism Fitting Line R2 1-D diffusion y=-0.17816+0.02369x 0.98406 2-D diffusion y=-0.16251+0.01842x 0.96435 3-D diffusion-Jander y=-0.07135+0.00703x 0.92741 Phase boundary-controlled
(contracting cylinder)y=-0.04508+0.01769x 0.99487 Phase boundary-controlled
(contracting sphere)y=-0.04989+0.01371x 0.98946 First order reaction y=-0.31875+0.05665x 0.97216 2-D growth of nuclei y=-0.21116+0.03167x 0.9973 3-D growth of nuclei y=-0.43192+0.02236x 0.99892
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表 3 四种氧载体与空气反应的线性拟合参数
Table 3 Linear fitting parameters of the oxidation process for four OCs reacting with air
Sample Temperature t/℃ Fitting line R2 Raw FeO/Al2O3 850 y=-0.04555+0.01645x 0.99395 875 y=-0.04345+0.01719x 0.995 900 y=-0.04508+0.01769x 0.99487 925 y=-0.04721+0.01804x 0.99427 K2O-decorated FeO/Al2O3 850 y=-0.04905+0.0147x 0.99359 875 y=-0.03443+0.01564x 0.99516 900 y=-0.05703+0.01592x 0.99403 925 y=-0.0431+0.01699x 0.99627 Na2O-decorated FeO/Al2O3 850 y=-0.04227+0.01465x 0.99528 875 y=-0.04824+0.01509x 0.99521 900 y=-0.04827+0.01588x 0.99723 925 y=-0.04272+0.01693x 0.99339 CaO-decorated FeO/Al2O3 850 y=-0.03476+0.0128x 0.99805 875 y=-0.04066+0.01375x 0.99526 900 y=-0.04435+0.0144x 0.99499 925 y=-0.05129+0.01509x 0.99489
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表 4 不同氧载体与空气在850-925℃条件下的氧化动力学参数
Table 4 Parameters of oxidation kinetics for four OCs reacting with air between 850-925℃
Sample E/(kJ·mol-1) A Raw FeO/Al2O3 13.71 0.07183 K2O-decorated FeO/Al2O3 20.21 0.12848 Na2O-decorated FeO/Al2O3 21.62 0.14702 CaO-decorated FeO/Al2O3 24.20 0.17198
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