VOLUME 23
JUNE 2009 Copyright 2009 by the American Chemical Society
Articles High-Temperature CaO Hydration/Ca(OH)2 Decomposition over a Multitude of Cycles Shiying Lin,*,† Yin Wang,‡ and Yoshiizo Suzuki‡ Japan Coal Energy Center, 3-14-10 Mita, Minato-ku, Tokyo 108-0073, Japan and National Institute of AdVanced Industrial Science and Technology, Japan ReceiVed December 14, 2008. ReVised Manuscript ReceiVed March 2, 2009
CaO hydration and Ca(OH)2 decomposition cycles can be used to supply or transport energy (heat). The reaction rates and properties of these materials are important parameters for the design of a reactor. In this work, the rates of CaO hydration and Ca(OH)2 decomposition were measured continuously over 20 hydration/ decomposition cycles by means of a high-pressure thermogravimetric apparatus. CaO was completely converted to Ca(OH)2 in all cycles, but the CaO hydration rate decreased with increasing number of cycles. Ca(OH)2 was decomposed to CaO completely in all cycles. The extent and rate of conversion for Ca(OH)2 decomposition were not influenced by increasing cycle number. Eutectic melting of CaO and Ca(OH)2 was not observed on the particles after CaO hydration or after Ca(OH)2 decomposition. The compressive strength of CaO particles decreased with increasing cycle number, and the average compressive strength of CaO particles after 20 cycles was about 16.5 kg/cm2.
1. Introduction The world’s energy consumption is increasing continuously, thus raising the cost of oil and polluting the environment. To save energy and protect the environment, energy should be used with higher efficiency. To this end, chemical looping is used to save, store, and transfer energy. By exploiting reversible phase changes of chemical substances, chemical looping allows heat energy to be converted to chemical energy and then released again at a desired temperature and location (raising heat temperature, as heat pump effect), so that energy is used more efficiently. One such reversible phase change is that between CaO and Ca(OH)2, as shown in following equations. Ca(OH)2 f CaO + H2O; endothermic
(1)
CaO + H2O f Ca(OH)2 ; exothermic
(2)
* To whom correspondence should be addressed. Phone: +81-29-8618224. Fax: +81-29-861-8209. E-mail:
[email protected]. † Japan Coal Energy Center. ‡ National Institute of Advanced Industrial Science and Technology.
Erin1 first proposed the use of the CaOSCa(OH)2 phase change to create a high-temperature heat pump. Ca(OH)2 decomposes to CaO at low temperature, and then CaO reacts with steam to release heat at high temperature.2 The CaOSCa(OH)2 phase change is also used in an in situ CO2 removal gasification method known as the HyPr-RING (hydrogen production by reaction-integrated novel gasification) method.3 In this method, CaO hydration occurring in a gasifier is used to restore the reactivity of the CaO sorbent for CO2 absorption while releasing high-temperature heat (approximately 873-973 K) for the hydrocarbon gasification. The mechanism and reactivity of the CaO hydration and Ca(OH)2 decomposition have been studied extensively. Halstead and Moore4 and Samms5 studied CaO hydration equilibrium (1) Erin, G. J. Solid State Chem. 1977, 22, 51. (2) Fujimoto, S.; Bilgen, E.; Ogura, H. Energy ConVers. Manage. 2002, 43, 947–960. (3) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Energy ConVers, Manage. 2004, 46 (6), 869–880. (4) Halstead, P. E.; Moore, A. E. J. Chem. Soc. 1957, 3873–3875. (5) Samms, J. A. C. J. Appl. Chem. 1968, 5, 18.
10.1021/ef801088x CCC: $40.75 2009 American Chemical Society Published on Web 05/06/2009
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Table 1. Limestone Composition and Experimental Conditions limestone composition [wt %] particle size [mm]
CaCO3
MgCO3
others
0.38
98
1.67
0.33
experimental conditions
limestone predecomposition CaO hydration Ca(OH)2 decomposition
temp. [K]
total pressure [MPa]
steam partial pressure [MPa]
1023 923 823
0.1 3.0 0.1
0 2.0 0.06
under various steam pressures. Matsuda and co-workers6,7 studied the CaO hydration rate under steam pressures of 0.015-0.0157 MPa at 773 K, and Chen et al.8 studied the effects of porosity on hydration of CaO materials. We previously studied the CaO hydration rate at temperatures up to 1023 K.9,10 The rate of CaO hydration was measured and examined in terns of parameters of temperature, steam pressure, and particle size. Our results have shown that CaO hydration rate increases with increasing steam pressure up to 1023 K. The CaO hydration rate increases by a power of 2 with respect to the difference between the reactant steam pressure and the equilibrium steam pressure. However, we have not yet intensively studied CaO hydration and Ca(OH)2 decomposition over many cycles, especially at high temperatures. In this study, CaO hydration and Ca(OH)2 decomposition cycles were measured continuously by means of a high-pressure thermogravimetric apparatus. Variations in reaction rates as well as changes in the properties, morphology, and compression strength of solid materials were investigated over multiple CaO hydration and Ca(OH)2 decomposition cycles.
thermocouple was set 5 mm under the sample pan, to monitor the temperature of the sample. High-pressure (∼5 MPa) steam was generated by forcing water through multiple stainless steel coils housed in an electric furnace (1073 K). The steam line was heated to at least 723 K by means of an electric heater until the steam reached the reactor. 2.3. Experimental Procedure. Figure 2 shows the experimental conditions (temperature and pressure) and sample weight fluctuations during an experiment. About 0.35 g of a limestone particle sample was used as an initial reaction material. The limestone particles were first predecomposed (CaCO3 f CaO) in the reactor at 1023 K under an N2 atmospheric pressure of 0.1 MPa. As a result of the predecomposition phase change of CaCO3 to CaO, the weight of the sample decreased. Details regarding this predecomposition process were described in the previous studies.9 Following limestone predecomposition, the reactor temperature was reduced and the pressure was increased (by N2) to attain CaO hydration conditions. This pressure increase caused the weight of the sample to increase (buoyancy up). When the temperature and pressure stabilized under the hydration conditions, steam was injected into the reactor to initiate the CaO hydration. The steam supply was adjusted to about 1 L/min for CaO hydration, while about 0.5 L/min N2 (purge gas) was supplied from the balance chamber. During the hydration, the sample weight was increased owing to the phase change of CaO to Ca(OH)2. After hydration was complete, as evidenced by a stabilized sample weight, the steam supply was stopped, the pressure was reduced to atmospheric pressure, and the temperature was decreased to 823 K to promote Ca(OH)2 decomposition. Both the pressure decrease (buoyancy down) and the phase change of Ca(OH)2 to CaO caused the sample weight to decrease rapidly. When the Ca(OH)2 decomposition was complete as evidenced by a stabilized sample weight, both pressure and temperature were raised to hydration conditions to start the next cycle. The conversions of CaO hydration and Ca(OH)2 decomposition were calculated from eq 3
2. Experimental Section 2.1. Samples. Chichibu limestone (Japan; CaCO3, 98%) was used as the starting material (Table 1). The limestone was crushed and sieved into samples with average particle diameters of 0.38 mm for our experiments. 2.2. Thermogravimetric Apparatus. The high-pressure thermogravimetric apparatus (PGT) used to measure the rates of CaO hydration and Ca(OH)2 decomposition is shown schematically in Figure 1. A sample basket (8 × 8 × 2 mm) made of platinum mesh (200 mesh) allows measurement of large samples of particles (up to 1 g). The apparatus consists of three sections: a balance chamber, a reactor, and a cooling section. The stainless steel balance chamber was designed to withstand pressures of 5.0 MPa and contains a balance bar. Any movement of the balance bar with weight changes in the sample is detected by a laser displacement sensor and recorded on a computer. The reactor is made of stainless pipe with an inner diameter of 20 mm and a length of 500 mm. Between the reactor and the balance chamber is a cooling section constructed of double pipes: the inside pipe is a gas tube (internal diameter, 6 mm) that transfers the purge gas (N2) from the balance chamber to the reactor, and the outside pipe is cooled by N2 gas to intercept the heat conducted from the reactor to the balance chamber. The sample pan hangs on the balance bar by a platinum wire, where it is passed through the inside pipe of the cooling section. A (6) Matsuda, H.; Ishizu, T.; Lee, S. K.; Hasatani, M. Kagaku Kogaku Ronbunshu 1985, 11 (5), 542–548. (7) Matsuda, H.; Ishizu, T.; Lee, S. K.; Hasatani, M. Kagaku Kogaku Ronbunshu 1987, 13 (1), 20–28. (8) Chen, M.; Wang, N.; Yu, J. K.; Yamaguchi, A. J. Eur. Ceram. Soc. 2007, 27, 1953–1959. (9) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Energy Fuels 2006, 20, 903–908. (10) Wang, Y.; Lin, S. Y.; Suzuki, Y. Fuel Process. Technol. 2008, 89, 220–226.
X)
|
MCa,init·WCh WCa,init·(MCa,init - MCa,prod)
|
(3)
where, X is the conversion for CaO hydration or Ca(OH)2 decomposition, WCa,init and MCa,init are the weight [kg] and molecular weight [kg · kmol-1] of the initial calcium reactant (CaO or Ca(OH)2), respectively, Wch is the weight change [kg] during the thermogravimetry experiments, and MCa,prod is the molecular weight of the calcium product [kg · kmol-1]. We found that the CaCO3 in the limestone predecomposed completely to CaO under N2 at 1023 K. Therefore, we assumed that the CaO produced by CaCO3 decomposition was the reactant for the subsequent hydration reaction. For Ca(OH)2 decomposition, since the combining pressure decrease, WCh is the difference between total weight change and the weight change caused by pressure decrease. The rate of CaO hydration was obtained from eq 4, in which t is reaction time.
RX)0.1 or 0.5 )
dx dt(1 - X)
|
(4) X)0.1 or 0.5
2.4. Analysis of Solid Products. Before analysis, solid samples were carefully placed in airtight bottles to protect them from moisture. CaO products formed by limestone predecomposition and by Ca(OH)2 decomposition were confirmed by means of X-ray diffraction (XRD). The specific surface areas of the CaO products of limestone decomposition and Ca(OH)2 decomposition were measured by means of a N2 absorption analyzer (COULTER Omnisorp 100). The morphologies of CaO and Ca(OH)2 products were examined by scanning electron microscopy (SEM). The compression strength of the CaO particles produced was measured by placing a particle under a plate and then loading force on the plate. The compressive strength [kg/cm2] was defined as the loading
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Figure 1. High-pressure thermogravimetric apparatus.
Figure 3. Equilibrium constant of CaO + H2O S Ca(OH)2 for the experimental conditions used here. For comparison, the experimental conditions reported by Matsuda et al. are also plotted.
pressure experimental conditions from other studies are also shown in this figure.
3. Results and Discussion
Figure 2. Variations in temperature and pressure during an experiment, and resulting changes in sample weight.
force per cross-sectional area of the particle at the point when the particle broke. Figure 3 shows the temperature and steam pressure conditions of CaO hydration and Ca(OH)2 decomposition in a CaO-Ca(OH)2 equilibrium phase diagram. For comparison, temperature and steam
3.1. Profiles of CaO Hydration and Ca(OH)2 Decomposition for a Multitude of Cycles. Figure 4 shows the weight change during CaO hydration and Ca(OH)2 decomposition over 20 cycles. Pressure effects have been eliminated from this plot. When steam was injected to initiate hydration, the sample weight increased with time and the final weight increase almost equaled the mass of the phase change for CaO f Ca(OH)2. This result reflects that CaO was completely converted to Ca(OH)2 by means of hydration. The weight increase for all 20 hydration/ decomposition cycles was similar (Figure 4), meaning that the
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Table 2. Hydration Rates Obtained from Experimental Data (Figure 7) and Model Calculations (eqs 7, 10, and 13) model calculation results RX)0.5/ RX)0.1 1 cycle 10 cycles 20 cycles
experimental reaction gas film ash layer data control diffusion control diffusion control (Figure 7) (eq 7) (eq 10) (eq 13) 1.48 1.42 1.40
1.22
1.8
0.25
extent of CaO hydration conversion was not hindered with increasing number of CaO hydration and Ca(OH)2 decomposition cycles. However, the decrease in sample weight during Ca(OH)2 decomposition over 20 cycles was almost equal to the mass of the phase change of Ca(OH)2 to CaO, indicating that complete Ca(OH)2 decomposition also occurred. Figure 4 also shows that Ca(OH)2 decomposition conversion was not influenced by increasing the number of CaO hydration and Ca(OH)2 decomposition cycles. From eq 3, variations in conversion of CaO hydration and Ca(OH)2 decomposition with reaction time were calculated; these results are shown in Figures 5 and 6, respectively. The hydration rate of the first cycle was faster than those of the other cycles, and the times required for complete conversion were 8, 10, 11.5, 12.2, 13, 13.5, and 13.8 min for cycles of 1, 2, 3, 4, 5, 10, and 20, respectively. In other words, the hydration rate decreased with increasing cycle number. However, the decrease in hydration rate also became smaller with each subsequent cycle, especially after 5 cycles had been completed. The plot of Ca(OH)2 decomposition conversion versus time for all cycles (Figure 6) seems to show the same pattern. However, in this case complete conversion was observed after only 3-4 min, meaning that the Ca(OH)2 decomposition rate
Figure 6. Conversions of Ca(OH)2 to CaO with reaction time for various cycles.
was not influenced by an increasing number of CaO hydration and Ca(OH)2 decomposition cycles. Comparison of Figure 6 with Figure 5 reveals that the Ca(OH)2 decomposition rate was faster than the CaO hydration rate. 3.2. Illustration of Hydration Rate Obtained for a Multitude of Cycles. Figure 7 shows CaO hydration rates (RX)0.1 and RX)0.5) obtained from the slopes of the conversion-time curves of Figure 5. RX)0.1 is an initial reaction rate without product layer effect, whereas RX)0.5 shows the reaction rate at 50% conversion. The ratio of RX)0.5/RX)0.1 is typically used to describe the reaction progress. From Figure 7, we can see that the ratios RX)0.5/RX)0.1 were about 1.48, 1.42, and 1.4 for cycles 1, 10, and 20, respectively. In a previous study,9 we compared experimental rates of hydration with a Shrinking core model with constant size particles as follows.
Figure 7. Variation in hydration rate with increasing number of cycles. Figure 4. Variation of sample weight over 20 hydration/decomposition cycles.
Figure 5. Conversions of CaO to Ca(OH)2 with reaction time for various cycles.
Figure 8. Logarithmic plot of hydration rates as a function of cycle number.
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Energy & Fuels, Vol. 23, 2009 2859
Figure 10. Specific surface area variation with increasing number of cycles.
Figure 9. XRD spectra of CaO produced by limestone predecomposition and Ca(OH)2 decompositions after 1, 5, and 20 cycles.
did not influence the hydration rate in the present particle size range, even for a multitude of cycles. However, as seen in Figure 7, both the initial rate (RX)0.1) and the average rate (RX)0.5) were extremely high at the first cycle, fell rapidly at the second cycle, and continued to slowly decrease until the fifth cycle. After the fifth cycle, the rate change for hydration for both the initial and average rates was much smaller. We discuss the causes of these variations in section 3.3. Figure 8 shows a logarithmic plot of hydration rate as a function of the reciprocal of cycle number. The effect of cycle on rate can be numerically expressed by the slope of the line obtained from linear regression of the data in this plot
Reaction control: t ) 1 - (1 - X)1 / 3 τ 1 dt ) dx τ 3(1 - X )2 / 3 3 dX ) dt(1 - X ) τ(1 - X )1 / 3 Gas film diffusion control: t )X τ dt ) dx τ dX 1 ) dt(1 - X ) τ(1 - X ) Product layer diffusion control: t ) 1 - 3(1 - X )2 / 3 + 2(1 - X ) τ 1 dt )2 - 1 dX τ (1 - X )1 / 3
(
)
(5) (6)
Rcyc ) a·exp
( cycb )·R
cyc)1 ;
a ) 0.337; b ) 1.088
(14)
where Rcyc and Rcyc)1 are rate at a number of cycle and at one cycle, respectively. Cyc is the number of cycle. In previous study,9 we concluded that, the initial rate expression of CaO hydration is
(7) R)
-8400 1 ·0.0069 exp ·(PH2O - P *H2O)2 RT d 0.11 p
(
)
(15)
(8)
To account for the effect of increasing cycles in the present experiments, this initial rate expression is rewritten as
(9)
Rcyc ) 0.337 exp
(10)
(11) (12)
1 dX (13) ) 2/3 dt(1 - X ) 2τ[(1 - X ) - (1 - X )] Accordingly, the reaction rates R at X ) 0.1 and 0.5 were calculated from eqs 7, 10, and 13 and are shown in Table 2. The results show that RX)0.5/RX)0.1 for film diffusion and reaction control are 1.8 and 1.22 times, respectively. Because the experimental results shows that RX)0.5/RX)0.1 was 1.48-1.4, the real reaction more closely resembled the reaction control. The ratio of RX)0.5/RX)0.1 for the model with product layer diffusion control was 0.25, much different from the obtained experimental result. This discrepancy indicates that product layer diffusion
1 · ( 1.088 cyc ) d
0.11 p
·0.0069 exp
· ( -8400 RT ) (PH2O - P *H2O)2 (16)
3.3. Properties and Morphologies of Materials as a Function of Increasing Number of Cycles. Crystal, Specific Surface Area, and Pore Structure. Crystals of CaO produced by limestone predecomposition and produced after cycle by Ca(OH)2 decomposition over a multitude of cycles were confirmed by XRD, and the results are shown in Figure 9. The CaO peaks were lowest for limestone predecomposition and grew with increasing number of CaO hydration and Ca(OH)2 decomposition cycles. After five cycles, the growth was slowed. These results indicate that CaO crystallization was smallest for the limestone predecomposition, increased quickly with increasing CaO-Ca(OH)2 cycles until five cycles were achieved, and then increased more slowly after five cycles. Figure 10 shows the specific surface area of CaO produced by limestone predecomposition and Ca(OH)2 decomposition during multiple cycles. The CaO produced by limestone predecomposition had a much larger specific surface area than CaO produced by Ca(OH)2 decomposition, and the specific surface area of CaO
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Figure 11. SEM photographs of CaO and Ca(OH)2 particles after 5 cycles.
Figure 12. SEM photographs of CaO and Ca(OH)2 particles after 10 cycles.
produced by Ca(OH)2 decomposition decreased with increasing number of cycles.
By examining the XRD and specific surface area results, we concluded that the fast hydration rate observed for the first cycle
CaO Hydration/Ca(OH)2 Decomposition
was caused by small crystals (i.e., large surface area) of CaO produced from limestone predecomposition.11 As the number of cycles increased to five, crystal growth and hence surface area reduction occurred rapidly, leading to a rapid decrease in hydration rate. SEM images of CaO and Ca(OH)2 particles after 5 and 10 cycles are shown in Figures 11 and 12, respectively. The phase change of CaO to Ca(OH)2 or Ca(OH)2 to CaO during hydration or decomposition did not cause the particle’s shape to change nor did the particle’s morphology change with increasing cycles. Many cracks were present on the surfaces of both CaO and Ca(OH)2 materials. These cracks allowed reactant gas diffusion through the product layer, thus allowing complete CaO hydration to occur. Comparison of the images between the 5th and 10th cycles revealed that many cracks were still present on the particle over a multitude cycles. This observation indicates that macropores were not significantly reduced by increasing the number of CaO hydration and Ca(OH)2 decomposition cycles and that reactant gas (H2O) was able to diffuse through the product layer sufficiently to permit hydration. Eutectic Melting of CaO and Ca(OH)2. Notably, eutectic melting of CaO and Ca(OH)2 was not observed in the present studies. Curran and Gorin12 reported that the melting temperature was as low as 1050 K when the CaO/Ca(OH)2 ratio was about 3/7 mol/mol. However, the eutectic melting phenomenon was not observed in the SEM images of CaO and Ca(OH)2 particles. The reason for this lack of eutectic melting may be that the heat transfer rate was faster enough for protecting location temperature up to 1050 K. CompressiVe Strength. The compressive strength of material is an important parameter for reactor design. The average (11) Kantiranis, N. Construct. Build. Mater. 2003, 17, 91–96. (12) Curran, G. P.; Fink, C. E.; Corin E. CO2 Acceptor Gasification Process. Proceedings of the 8th Synthetic Pipeline Gas Symposium, Chicago, IL, 1976.
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compressive strengths of the CaO particles clearly decreased with increasing cycle number, such as from about 28 kg/cm2 initially to 16.5 kg/cm2 after 20 cycles. 4. Conclusions The variations of reaction rate for a multitude of CaO hydration and Ca(OH)2 decomposition cycles were investigated by use of a high steam-pressure thermogravimetric apparatus. Properties of the CaO and Ca(OH)2 particles produced during these cycles, such as crystal growth, specific surface area, pore structure, and compressive strength, were also examined. The following results were obtained. (1) CaO was completely converted to Ca(OH)2 regardless of the number of cycles, because the particles remained porous even after 20 cycles. (2) CaO hydration rate decreased with increasing number of cycles, since crystal growth and specific surface area reducing by the progression of cycles as described by the following equation. Rcyc ) 0.377·exp
·R ( 1.088 cyc )
cyc)1
(17)
(3) Ca(OH)2 was completely decomposed to CaO, and the conversion rate was not influenced by the increasing number of cycles. (4) Eutectic melting of CaO and Ca(OH)2 was not observed on the particles following CaO hydration and Ca(OH)2 decomposition for a multitude of cycles. (5) The compressive strength of CaO particles decreased with increasing number of CaO hydration and Ca(OH)2 decomposition cycles. Nevertheless, the particles still had a compressive strength of 16.5 kg/cm2 after 20 cycles. EF801088X
