Article pubs.acs.org/IECR
Sintering of Limestone in Calcination/Carbonation Cycles Chunbo Wang,† Xing Zhou,† Lufei Jia,*,‡ and Yewen Tan‡ †
School of Energy and Power Engineering, North China Electric Power University, Baoding City, Hebei Province, 071000, People’s Republic of China ‡ CanmetENERGY, 1 Haanel Drive, Ottawa, Ontario K1A 1M1, Canada ABSTRACT: A custom-built experimental system that can measure sample weight changes at high temperatures is used to study the effect of operating parameters (sintering time, atmosphere, temperature, particle size, etc.) on the sintering of CaO under Calooping process conditions. The pore structures of the sintered CaO samples are measured by N2 absorption/desorption method. The results show that both the rate of carbonation reaction and the final carbonation ratio of the CaO decrease as the sintering duration is increased, especially during the chemical reaction controlled stage. The carbonation of nascent CaO is degraded if steam was added in the calcination process. The sintering of CaO is accelerated by higher temperatures and that will lead to the lower carbonation conversion ratios for the CaO. The highest carbonation ratio is not obtained for the smallest particles (75−97 μm) but for the middle sized particles (150−250 μm) for all the CaO samples for any sintering durations. The biggest CaO particles (355−450 μm) have the lowest carbonation conversion ratio. The surface area and the pore volume of the CaO consistently decrease by the prolonged sintering time. It is interesting to note that for the nascent CaO steam presence will cause the surface area and the pore volume to decrease. But the reverse is true for CaO samples sintered for 6 and 12 min postcalcination. For all CaO samples with different sintering times, steam will change the pore size distribution for pores larger than 200 Å. Finally, similar results are obtained for another two different limestones.
1. INTRODUCTION It is generally accepted that a reduction of the greenhouse gas (GHG) emissions that promote climate change is necessary. Coal-fired power plants are one of the major sources of CO2 emissions, which account for about 33−40% of the total anthropogenic emissions of carbon worldwide.1 Among the CO2 capture methods, Ca-looping is considered one of the most promising technologies2,3 because the sorbent used, limestone, is widely available and cheap. The advantages of the Ca-looping process compared to other technologies include lower energy penalty, use of mature fluidized bed technology, an inexpensive and nontoxic sorbent, and potential synergy by using the spent sorbent in the cement industry. One issue with the Ca-looping technology that needs to be addressed is the decay of reactivity of the Ca-based sorbents derived from the naturally occurring limestone after multiple cycles.4 It has been known that sintering of sorbents is one of the main causes for the reactivity decay. Borgwardt5 fitted a sintering relationship to German and Munir’s6 experimental data, which describes the relationship between BET surface area S (as measured by nitrogen adsorption) and sintering time: ⎛ S0 − ⎜ ⎝ S0
S⎞ ⎟ = Kst ⎠
It should be noted that Borgwardt’s study is suitable for air combustion of the circulating fluidized bed boiler (CFBB); the concentration of CO2 and steam are 12.2% and 7.3%,5 respectively. These conditions are different when compared with those found in the calciner of the Ca-looping technology which is about 80% CO2 and up to 20% steam. Silcox’s formula does not consider the influence of steam. It has been known that the CO2 and steam can accelerate the sintering of CaO.5,7−10 In Ca-looping technology the calcination of limestone occurs at ∼80% CO2 and ∼20% steam. Therefore, the sintering of the CaO might be more serious during the calcination than that in air combustion mode. However, one interesting phenomenon reported in the literature is that many studies have shown that steam could improve the carbonation of CaO in the Ca-looping process.11−16 Wang et al.11 found that steam (8% and 15% v/v) could improve the carbonation of fly ash in oxy-fuel CFB conditions, and suggested that carbonation of CaO may also include a twostep route when water vapor is present: CaO···Ca(OH)2··· CaCO3. Yang and Xiao12 studied the effects of steam on CaO carbonation performance in a pressurized thermogravimetric analyzer (TGA). They reported that steam increased CaO carbonation performance significantly and suggested this effect should be attributed to steam’s catalytic effect on the reaction of CaO and CO2. Manovic and Anthony13 suggest that steam enhances solid-state diffusion in the diffusion controlled reaction phase only. Arias et al.14 tested the effect of steam in the initial carbonation period where the reaction is in the fast
γ
(1)
where Ks is the rate constant for the temperature (min−1) and t is time (min). The exponent γ is found to lie in the vicinity of 2.7, which is consistent with the mechanism of lattice diffusion. An alternative sintering expression adopted by Silcox et al.7 is dS = −Ks(S − Sas)2 m2g −1s−1 dt © 2014 American Chemical Society
Received: Revised: Accepted: Published:
(2) 16235
May 20, 2014 August 26, 2014 September 29, 2014 September 29, 2014 dx.doi.org/10.1021/ie502069d | Ind. Eng. Chem. Res. 2014, 53, 16235−16244
Industrial & Engineering Chemistry Research
Article
Table 1. Composition of the Limestones (wt %) BD (wt %) SD (wt %) MCH(wt %) a
SiO2
Al2O3
Fe2O3
TiO2
P2O5
CaO
MgO
SO3
Na2O
K2O
LOIa
1.07 2.15 1.55
1.08 1.12 0.96
