Kinetics of solid-solid reaction between cotton char and Ni/olivine in chemical looping gasification
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摘要: 本实验利用微型流化床反应分析仪(MFBRA)研究了棉秆半焦(CSC)与载镍橄榄石(Ni/olivine)的固-固化学链反应特性,利用模型拟合法在等温条件下对29种模型函数进行拟合计算,从中选取了最优的三种模型,计算出棉秆半焦和载氧体的固-固反应动力学。结果表明,CO和CO2是CSC与Ni/olivine反应的主要气体产物,固-固反应过程中,先析出CO后再析出CO2,CSC并不会完全转换成CO2,产气中CO的浓度比CO2大;随着反应温度的升高,产气中CO和CO2的浓度和产率增加。CO、CO2和CSC利用三种不同模型函数计算出来的活化能平均值分别为27.5、46.4和69.8 kJ/mol。利用热重研究了CSC和Ni/olivine非等温反应特性及动力学,结果表明,CSC和Ni/olivine的反应从750 ℃开始,在890 ℃时反应速率达到了峰值,非等温反应活化能为72.05 kJ/mol,这与MFBRA等温动力学活化能基本相似,说明生物质化学链气化过程中,半焦和镍基载氧体的固-固反应较容易发生。Abstract: Direct solid-solid reaction characteristics of cotton stalk char (CSC) and nickel loaded olivine (Ni/olivine) were studied using micro-fluidized bed reaction analyzer (MFBRA). Model fitting method was used to fit 29 model functions under isothermal conditions. Three optimal models were selected to calculate kinetics of the solid-solid reaction of cotton char and oxygen carries. The results show that CO and CO2 are main gas products of CSC and Ni/olivine. CO is first and then CO2 is evolved during the reaction. CSC does not completely convert into CO2, and concentration of CO is higher than that of CO2. With increasing reaction temperature, concentration and yield of CO and CO2 in the product gas increase obviously. The average activation energies for CO, CO2 and CSC are 27.5, 46.4 and 69.8 kJ/mol, respectively. The non-isothermal reaction characteristics and kinetics of CSC and Ni/olivine were studied by thermogravimetric analyzer (TGA). The results show that reaction of CSC and Ni/olivine starts at 750 ℃, and reached the peak at 890 ℃. The activation energy of non-isothermal reaction is 72.05 kJ/mol, which is in accordance with the result of MFBRA. It indicates that the solid-solid reaction between char and nickel-based oxygen carrier easily occurs in during chemical looping gasification of biomass.
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表 1 29种固-固反应模型函数
Table 1 29 model functions of solid -solid reactions
Number Function name Mechanism G(x) 1 Maple power order (Exponential nucleation) Phase boundary reaction (One-dimensional), R1, n = 1 $ x $ 2 Maple power order (Exponential nucleation) n = $ \dfrac{1}{2} $ $ {x}^{\frac{1}{2}} $ 3 Maple power order (Exponential nucleation) n = $ \dfrac{1}{3} $ $ {x}^{\frac{1}{3}} $ 4 Maple power order (Exponential nucleation) n = $ \dfrac{1}{4} $ $ {x}^{\frac{1}{4}} $ 5 Maple power order (Exponential nucleation) n = $ \dfrac{2}{3} $ $ {x}^{\frac{2}{3}} $ 6 Parabola order One-dimensional diffusion, 1D, D1 Deceleration curve of α-t $ {x}^{2} $ 7 2 order Chemical reaction, F2, Deceleration curve of α-t $ {\left(1-x\right)}^{-1} $ 8 2/3 order Chemical reaction $ {{\left(1-x\right)}}^{-\frac{1}{2}} $ 9 Reaction order Chemical reaction $ {\left(1-x\right)}^{-1}-1 $ 10 Shrink cylinder (area) Phase boundary reaction, Cylindrical Symmetry, R2, Deceleration curve of α-t, n = $ \dfrac{1}{2} $ $ 1-{\left(1-x\right)}^{\frac{1}{2}} $ 11 Shrink ball (volume) Phase boundary reaction, Spherical Symmetry, R3, Deceleration curve of α-t, n = $ \dfrac{1}{3} $ $ 1-{\left(1-x\right)}^{\frac{1}{3}} $ 12 Reaction order n = $ \dfrac{1}{4} $ $ 1-{\left(1-x\right)}^{\frac{1}{4}} $ 13 Reaction order n = 2 $ 1-{\left(1-x\right)}^{2} $ 14 Reaction order n = 3 $ 1-{\left(1-x\right)}^{3} $ 15 Reaction order n = 4 $ 1-{\left(1-x\right)}^{4} $ 16 Maple single law, First order Random nucleation and subsequent growth, There is only one core on each particle, A1, F1, Sigmoid curve of α-t, n = 1, m = 1 $ -{\rm{ln}}\left(1-x\right) $ 17 Avrami-erofeev equation Random nucleation and subsequent growth, A2, Sigmoid curve of α-t, n = $ \dfrac{1}{2} $, m = 2 $ {\left[-{\rm{ln}}\left(1-x\right)\right]}^{\frac{1}{2}} $ 18 Avrami-erofeev equation Random nucleation and subsequent growth, A3, Sigmoid curve of α-t, n = $ \dfrac{1}{3} $, m = 3 $ {\left[-{\rm{ln}}\left(1-x\right)\right]}^{\frac{1}{3}} $ 19 Avrami-erofeev equation Random nucleation and subsequent growth, A4, Sigmoid curve of α-t, n = $ \dfrac{1}{4} $, m = 4 $ {\left[-{\rm{ln}}\left(1-x\right)\right]}^{\frac{1}{4}} $ 20 Avrami-erofeev equation Random nucleation and subsequent growth, A1.5, n = $ \dfrac{2}{3} $ $ {\left[-{\rm{ln}}\left(1-x\right)\right]}^{\frac{2}{3}} $ 21 Avrami-erofeev equation Random nucleation and subsequent growth, n = 2 (Code: AE2) $ {\left[-{\rm{ln}}\left(1-x\right)\right]}^{2} $ 22 Avrami-erofeev equation Random nucleation and subsequent growth, n = 3 (Code: AE3) $ {\left[-{\rm{ln}}\left(1-x\right)\right]}^{3} $ 23 Avrami-erofeev equation Random nucleation and subsequent growth, n = 4 (Code: AE4) $ {\left[-{\rm{ln}}\left(1-x\right)\right]}^{4} $ 24 Jander equation Spherical Symmetry, 3D, D3, Deceleration curve of α-t, n = 2 $ {\left[1-{\left(1-x\right)}^{\frac{1}{3}}\right]}^{2} $ 25 Jander equation Three-dimensional diffusion, 3D, n = $ \dfrac{1}{2} $ $ {\left[1-{\left(1-x\right)}^{\frac{1}{3}}\right]}^{\frac{1}{2}} $ 26 Jander equation Three-dimensional diffusion, 2D, n = $ \dfrac{1}{2} $ $ {\left[1-{\left(1-x\right)}^{\frac{1}{2}}\right]}^{\frac{1}{2}} $ 27 Z-L-T equation Three-dimensional diffusion, 3D $ {\left[{{\left(1-x\right)}^{-}}^{\frac{1}{3}}-1\right]}^{2} $ 28 Valensi equation Two-dimensional diffusion, Cylindrical Symmetry, 2D, D2, Deceleration curve of α-t $ x + \left(1-x\right){\rm{ln}}\left(1-x\right) $ 29 Ginstling-brounshtein equation Spherical Symmetry, 3D, D4, Deceleration curve of α-t $ 1-\dfrac{2}{3}x-{\left(1-x\right)}^{\frac{2}{3}} $ 表 2 基于MFBRA等温反应动力学参数
Table 2 Kinetic parameters of isothermal reaction by MFBRA
Sample Model function E/(kJ·mol−1) A R2 CO G(16) 27.54 1.53 0.995 G(20) 28.20 0.74 0.996 G(24) 26.71 0.16 0.994 CO2 G(11) 47.23 0.11 0.977 G(28) 45.22 0.12 0.989 G(29) 46.62 0.04 0.989 CSC G(11) 69.41 1.35 0.995 G(12) 70.51 1.51 0.999 G(24) 69.36 1.70 0.991 表 3 基于热重非等温反应动力学参数
Table 3 Kinetic parameters of non-isothermal reaction by TGA
Sample E/(kJ·mol−1) A R2 CSC 72.05 1.33 0.953 -
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