Rate equation theory of gas-solid reaction kinetics for calcium looping and chemical looping

Transcription

1 4th IEAGHG Network Meeting and Technical Workshop On High Temperature Solid Looping Cycles Tsinghua University 20th-21st of August, 2012 Rate equation theory of gas-solid reaction kinetics for calcium looping and chemical looping Zhenshan Li, Hongming Sun, Jinhua Bao, Ningsheng Cai Department of Thermal Engineering Tsinghua University 2012/08/20

2 Outline Kinetics model for gas solid reaction Solid product nucleation and growth Rate equation method Results and discussions Conclusions

3 Kinetics model for gas solid reaction CaCO CaO CaO + CO 2 = CaCO CaO/fly ash, 5 vol% O 2, 2000 ppm SO 2 (1) Two stages: fast and slower stages (2) High T increase the critical conversion () Most G-S reactions behave similarly Why the critical conversion increases with the temperature increasing? CuO ~1000µm, 5% CO, 95% N 2 Vasilije Manovic. Ind. Eng. Chem. Res. Yuran Li et al. Fuel, 2007, 86, Garcia F et al. Ind. Eng. Chem. Res. 2004, 4,

4 Kinetics model for gas solid reaction (1) Gas external mass transfer (2) Pore diffusion () Adsorption (4) Product layer diffusion (5) Reaction and product growth However, the nucleation and growth of solid product was not considered in detailed in these models. pore 孔 product 固体产物 reactant 固体反应物 (a) shrinking (a) 缩核模型 core model (b) (b) grain 晶粒模型 model (c) pore model (d) (d) nucleation 成核与和生长模型 and growth

5 Kinetics model for gas solid reaction Two famous theories for product growth Wagner theory: focuses on the slower product layer diffusion stage; Cabrera-Mott theory: focuses on initial oxidation stage; A critical assumption in the Cabrera-Mott model is that the oxide film grows in a uniform layer-by-layer fashion. However, the nucleation and growth of solid product occur during the initial reaction stage, and the critical assumption of the Cabrera Mott model is not valid, micro-structural information must be considered in detailed oxidation modeling.

6 Solid product nucleation and growth Fe oxidation at 700 o C, 1020ppm O 2 5.min 8.6min 1min Fe oxidation 1020ppm O o C 800 o C 900 o C

7 Solid product nucleation and growth MgO surfaces reacting with 200 ppm of SO 2 and 5 vol% O 2 for 10 min 550 o C 650 o C 750 o C NiO reduction by H 2 at 800 o C, (a) 2s; (b) 5s; (c) 5s. Hidayat T et al. Metall Mater Trans B. 2009, 40B,

8 Solid product nucleation and growth Oxide islands formed on Cu(110) at different oxidation temperatures, the oxygen pressure is 0.1 torr. Zhou GW. PhD thesis. University of Pittsburgh, 200. Cu 2 O islands at constant oxygen partial pressure of 10-4 and temperature of 1000 o C. CaCO sulfation with SO 2 (516 ppm) at 600 o C for 0min, after 15min,10vol% steam was added

9 Solid product nucleation and growth The formation and growth of the solid product is the most critical step CaO CaO CaO (1) layer growth; (2) island growth; () island-layer growth There are three growth modes for solid products Solid product shows three-dimensional island shaped morphology. High density groups of islands with smaller size are formed at lower temperature while low density, larger sizes of islands are formed at high temperatures. How to describe the island growth in gas-solid reaction model is not clear in previous work

10 Rate Equation Method 不稳定核 成核物理吸附 化学吸附 reaction 解离 成键 The nucleation and growth of solid products are controlled by both the chemical reaction rate and surface diffusion. nucleation R 成核 > R 生长 growth R 成核 < R 生长 晶界迁移 Ostwald 烧结 ripening The driven force for Ostwald ripening Grain boundary and lattice diffusion Diffusion coefficient

11 Rate Equation Method σ s. 1 2 σ s ) 气体反应物 CO 2 固体反应物 CaO 固体生成物 CaCO An island can grow due to the capture of molecules from small islands. Or an island can shrink due to the escaping of molecules to larger islands. (1) surface reaction and the formation of solid product: (2)surface diffusion of single molecule: () single molecule captured by islands: (4) single molecule escape from islands: (5) grain boundary and lattice diffusion: D i σ = ( r / r) s F= kc ( C ) N 2 2 exp( Ei = D ) 0i RT s s 1e, s s 1 N 1/ 2.7 η = DN 1e, s e = k = fd + (1 f ) D s GB L CO CO,e molecular N γω 1 exp( ) RT d s

12 Rate Equation Method. 气体反应物 CO 2 固体反应物 CaO 1 固体生成物 CaCO N s : islands composed of s molecules N 1 : islands composed of one molecule Rate equation for N 1 : dn1 = { reaction} { captured by island} + { escape from island} dt Rate equation for N s : d dt N s = { grain boundary and lattice diffusion}+{ molecule captured by island}- { molecule escape from island} (2 s ) Island density changing depends on surface reaction, surface diffusion, nucleation and island growth, grain boundary and lattice diffusion

13 Rate Equation Method. 气体反应物 CO 2 固体反应物 CaO N s : islands composed of s molecules 1 固体生成物 CaCO N 1 : islands composed of one molecule Surface coverage: θ = Ns s s= 2 s Rate equation for N 1 : dn dt 1 = Fθ DN(2 σ N + σ N) + (2 η N + η N) s s s s 2 2 s s s= 2 s= Surface reaction Loss of single molecule due to capture by other islands single molecule escapes from other islands dn dt s Grain boundary & lattice diffusion = Fk ( N kn) + Dσ N N Dσ NN η N + η N (2 s ) s 1 s 1 s s s s 1 s 1 1 s s s 1 s s s+ 1 s+ 1 Island density changing depends on surface reaction, surface diffusion, nucleation and island growth, grain boundary and lattice diffusion

14 Results and Discussions ~20µm CaO carbonation, 14vol% CO o C 0.7 CaO Conversion o C 590 o C 640 o C o C calculated resutls Time (min) The output parameters of rate equation are islands density (N s ), and the macroscopic behavior such as solid conversion can be calculated with the islands density. Rate equation can be validated by both microscopic AFM & SEM and macroscopic TGA experiments. More detail can be found in the paper published by Li ZS et al, Energy Fuels, 2012,

15 Results and Discussions Fe oxidation 1020ppm O 2 Fe oxidation at 900 o C, 5vol% O 2 Submitted to Combustion and Flame

16 Conclusions Achieved: A rate equation theory was developed to provide information on the variations in island size distribution with time evolution. The elemental steps of surface reaction, surface diffusion, and grain boundary and lattice diffusion were included in these rate equations. The macroscopic solid conversion can be calculated by use of the island size distribution information. Next step: Mechanism of the effect of H 2 O on gas solid reaction. Effect of impurity and support on islands nucleation, growth and morphology (surface diffusion, capture number or grain boundary diffusion). Sintering of oxygen carrier or sorbent- (Ostwald ripening & islands diffusion & neck sintering ).

17 This research is supported by the National Natural Science Funds of China ( , ) and by the National Basic Research Program of China (No.2011CB70701)..