Understanding the enhancement effect of high-temperature steam on the carbonation reaction of CaO with CO 2

Transcription

1 5th IEAGHG Network Meeting and Technical Workshop n High Temperature Solid Looping Cycles Cambridge University 2th-3th of September, 2013 Understanding the enhancement effect of high-temperature steam on the carbonation reaction of Ca with C 2 Zhenshan Li, Yang Liu, Ningsheng Cai Department of Thermal Engineering Tsinghua University 2013/09/02

2 utline Background Interaction of H 2 with oxygen vacancy Solid product nucleation and growth Product layer diffusion Validation Conclusions

3 Background carbonation 600 o C, 20% steam Indirect sulphation. 600 o C, no steam 400 o C, 20% steam 400 o C, no steam Ca + S 2 Ca + C 2 NH = N + 1.5H 2 Steam is always present in flue gases. Steam can enhance the carbonation, indirect and direct sulphation, and can decrease the oxidation of NH 3 on the Ca. Vasilije Manovic. Ind. Eng. Chem. Res., 2010, MC. Stewart et al. Environ. Sci. technol., 2010, MC. Stewart et al. Fuel, 2012,

4 Background The steam-enhanced carbonation reaction of Ca with C 2 is a widely observed phenomenon, but its mechanism is unclear. Manovic et al. and Arias et al. suggested that steam does not affect the initial reaction rate in the kinetically controlled stage but enhances the diffusion in the product layer. Hu et al. concluded that the promoting effect is due to the enhanced ion mobility in the presence of steam. Wang et al. postulated that the existence of Ca(H) 2 contributes to the carbonation process and that the carbonation of Ca(H) 2 is much faster than that of Ca. However, Yang and Xiao found that Ca(H) 2 formation may not be the main reason for the significant carbonation enhancement. This research aimed to (1)apply the theories of defect chemistry and ion diffusion to analyze the steam enhancement, (2)discuss the effects of steam fraction and temperature on carbonation.

5 The interaction of H 2 molecule with oxygen vacancy Dissociative adsorption of H 2 occurs on the Ca and CaC 3 surface. xygen vacancies are shown to dissociate H 2 molecules by transferring one proton to nearby oxygen atoms and forming two hydroxyl groups for every vacancy adsorption dissociative adsorption H (g) + v =2H 2 Schaub R, et al. Phys Rev Lett 2001; 87: (I) xygen vacancy (II) bridging hydroxyl group (III) terminal hydroxyl group (IV) water molecule. Red atoms:

6 The interaction of H 2 molecule with oxygen vacancy The interaction of H 2 molecule with oxygen vacancy H (g) + v =2H 2 Defect equilibrium K exp( S / R)exp( H / RT ) [H ] /([v ][ ] p ) H 2 The electroneutrality condition The lattice structure balance 2[v ] [H ] [A] [ ] [v ] [H ] [] The fraction of proton defect y H [H ] [A] Kp 2 2 2[A] 8[A] [A] 4[A] (1 1 ) Kp H [] [] KpH [] [] ( KpH4) 2 [] H 2 2 [A] y 2 The relationship of proton defect with steam is built. H p H

7 o Solid product (a) 550 o Cnucleation and (b) 650 o Cgrowth 550 o C 650 o C 750 o C 550 o C (b) 650 o C (c) 750 o C Solid product grows with the morphology of the island shape A part of solid reactant surface is covered by the product islands the fraction of reactant surface covered by product: The other part of solid reactant surface remains in contact with gas the fraction of fresh reactant surface: 1

8 (a) 550 o C Solid product nucleation and growth C 2 is in contact with Ca and CaC 3 surface. When C 2 makes contact with Ca surface, the carbonation reaction can directly occur on the Ca surface; while for the CaC 3 surface, the diffusion of ions through the CaC 3 product layer is the controlled step. Ca conversion X ( X X )(1 ) I II (b) 650 o C the fraction of Ca surface exp[ k ( C C ) t] s C C,e 2 2 Ca conversion during the product diffusion controlled stage the critical Ca conversion (critical thickness of product layer)

9 Product layer diffusion CaC 3 C Ca C 2 Without steam present Proposed by Bhatia and Perlmutter and recently validated by Fan et al.: C 3 2- diffuses inward from the CaC 3 /gas interface to the CaC 3 /Ca interface 2- diffuses outward from the CaC 3 /Ca interface to the CaC 3 /gas interface C 2 With steam present 2- H - H + C 2-3 CaC 3 h C 2-3 and H + diffuse inward 2- and H - diffuse outward J J 2J 2J H H C 3 Ca h h p 0 J + - J J J H C 3 H J H

10 Product layer diffusion C 3 2- and H + diffuse inward while 2- and H - diffuse outward C 2 Fick diffusion solid conversion 2- H - H + C 2-3 J J J H C 3 CaC 3 h Ca h h p 0 ' dx D II ' X D y y v H H dt D X X H II [0.5(1 ) ](1 ) m ' v I H diffuses faster than v ' DH is bigger than ' Dv y H Kp 2 2 2[A] 8[A] [A] 4[A] (1 1 ) Kp H [] [] KpH [] [] ( KpH4) 2 [] H 2 2 [A]

11 Validation of H formation Ca+C 2 =CaC 3 CaC 3 Ca C+H 2 =H 2 +C 2 Ca+C 2 =CaC 3 Water gas shift reaction (WGS) 500 o C in bubbling fluidized bed reactor, 50g Ca C Co ad H + C ad CH ad CH ad C 2 + H ad 2H ad H 2 H - is required for WGS because H - reacts with C to form CH. CH finally generates C 2 and H 2. Hydroxyl groups are formed; otherwise, WGS reaction cannot occur.

12 Potons defects fraction H Validation 20vol% H 2 content: 10vol% 2vol% Temperature ( o C) Effect of steam fraction and temperature on the proton defects ' dx D II ' X D y y v H H dt D X X Ca conversion H II [0.5(1 ) ](1 ) m ' v I 15 vol% C 2 at 400 o C (a) experiment, 0vol% steam experiment, 2vol% steam experiment, 20vol% steam simulated result Time (min) Effect of steam fraction on Ca conversion in the product layer diffusion controlled stage proton defects diffuse faster than oxygen vacancies. increasing steam fraction, proton defect increases, whereas oxygen vacancies decreases. further increasing steam has no big effect, because the amount of H 2 molecule dissociation is limited by the density of oxygen vacancies. 2[v ] [H ] [A]

13 Validation Potons defects fraction H. Ca conversion vol% 10vol% 0.2 experiment, 0vol% steam 0.2 experiment, 2vol% steam 0.1 H 2 content: (a) 2vol% experiment, 20vol% steam simulated result Temperature Time (min) ( o C) Effect of steam fraction and temperature on the proton defects Ca conversion (b) 15 vol% C 2 at 500 o C experiment, 0vol% steam experiment, 2vol% steam experiment, 20vol% steam simulated result Time (min) Effect of steam fraction on Ca conversion in the product layer diffusion controlled stage increasing temperature, proton defect decreases, whereas the oxygen vacancy diffusion coefficient and oxygen vacancy fraction increased. Consequently, the weak steam-induced enhancement effect on carbonation ensued

14 Rate Equation Method 600 o C 400 o C Effect of steam fraction on Ca conversion in the fast reaction stage in TGA with 15 vol% C 2 Two stages: fast and slower stages The developed model can reasonably predict Ca conversion in the kinetically controlled stage, diffusion-limited regime, and transition without/with steam

15 1 Validation: indirect sulphation & catalytic NH 3 oxidation G.J. Zijlma et al. Fuel. 2000,79(12): Ca conversion Indirect sulphation. experiment, 0vol% steam experiment, 7.5vol% steam experiment, 15vol% steam simulated result 系列 Time (min) MC. Stewart et al. Environ. Sci. Technol., 2010, 44: The enhancement of steam on the sulphation behavior can also be explained and predicted with the developed methods. Steam fraction increases, H increases, while v decreases. The fraction of NH 3 oxidation decreases while the fraction of NH3 for N reduction increases, therefore, N decreases with steam increasing. N and NH 3 concentrations, vs, H 2,850 o C, 2 vol% 2. NH 3 oxidation on Ca v NH N reduction N + 1.5H 2 NH 3 + H NH 2 + H 2 NH 2 + N N 2 + H 2

16 Conclusions H - formation caused by the dissociation of H 2 molecules may explain enhanced carbonation. The relationship of oxygen vacancies with H - was established and integrated into a new carbonation model. This new model included a simplified rate equation model to describe product island formation and a multi-ion diffusion model to describes product layer diffusion. Carbonation and sorption-enhanced water gas shift reactions in a fluidized bed reactor were conducted to validate the formation of H -. The new developed models were validated by experimental data.

17 This research is supported by the National Natural Science Funds of China ( , ) and by the National Basic Research Program of China (No.2011CB707301)..