Impacts and Action Mechanism of Coal Ash on CaO-Based Sorbents

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Impacts and Action Mechanism of Coal Ash on CaO-based Sorbents for CO2 Capture under Oxy-Fuel Calcination Environment Donglin He, Ge Pu, Changlei Qin, Ruijie Gong, Lili Tan, and Jingyu Ran Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03777 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Industrial & Engineering Chemistry Research

Impacts and Action Mechanism of Coal Ash on CaO-based Sorbents for CO2 Capture under Oxy-Fuel Calcination Environment Donglin He, Ge Pu, Changlei Qin *, Ruijie Gong, Lili Tan, and Jingyu Ran

Key Laboratory of Low-grade Energy Utilization Technologies and Systems of Ministry of Education, College of Power Engineering, Chongqing University, Chongqing 400044, China

*Corresponding author: Tel.: +86-23-65103101. Email: [email protected]. ACS Paragon Plus Environment

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ABSTRACT The emerging calcium looping (CaL) process, basing on the reversible reactions between CaO and CaCO3, is considered to be a potential mid-term mitigation solution in capturing CO2 from coal-fired power plants. Normally, the efficient regeneration of sorbents is realized through oxygen-enriched combustion of coal above 900 °C. However, the minerals in coal will potentially affect sorbents’ CO2 capturing and the effect could gradually intensify as increasing temperature. Therefore, in this work, effects of ash with a series of variables under a more practical oxy-fuel calcination condition are evaluated, and the action mechanism of ash on the sorption process is specially studied. Using a combination of testing approaches, both physical and chemical contributions are observed and identified for the effect of coal ash on the CO2 capturing of CaO-based sorbents. The physical influence, caused by ash deposition and following grain aggregation, on CaO-based sorbents for CO2 capture is found to be inevitable and predominated. Meanwhile, it is also suggested that solid-solid reactions involving Al and Si from coal ash and Ca from sorbents will occur as the aggregation of coal ash intensifying, which could negatively restrain the CO2 capturing of CaO-based sorbents in the later stage of CaL. Furthermore, both physical and chemical mechanisms are proposed in describing and understanding the detailed interaction process between coal ash and sorbents.

Key words: CaL, CO2 capture, Coal ash, Action mechanism, Oxy-fuel combustion

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1. INTRODUCTION It has been widely acknowledged that the anthropogenic emission of CO2 via the combustion of fossil fuels is the main cause for global warming.1 However, current economic growth is heavily dependent on low-cost and plentiful fossil fuels, and they are predicted to be persistently used well into the 21st Century and, most likely, beyond.2 In this context, the controlling of CO2 emission during the utilization of fossil fuels has been attracting worldwide attention. To help mitigate the issue of carbon emissions effectively, CO2 capture and storage (CCS) comes into being as a potential mid-term mitigation of post-combustion CO2 from coal-fired power plants and other stationary sources. Till now, an extensive effort has been devoted to achieve more advanced and less costly CO2 capture technologies. Among the different techniques under study, calcium looping (CaL) is an emerging technology for CO2 capture via the loop of a calcium-based sorbent, which involves the exothermic carbonation and endothermic calcination of CaCO3. CO2 in flue gas is sequestrated via a gas-solid reaction in a carbonator, and then the recycled sorbents with fixed CO2 is transported into a regenerator in the atmosphere of high concentration of CO2. Hence, sequestration of CO2 from a large flow stream of flue gas could be achieved. Techno-economic analysis on the process of calcium looping (CaL) has also demonstrated its economic viability.3 With a good adaptability in capturing CO2 from coal-fired power plants, calcium looping (CaL) has obtained rapid development.4-13 However, the rapid decay of CO2-capturing capacity with increasing number of CaL cycles could largely pull down the utilization efficiency of materials and the operating efficiency of CaL process, and thus, hinders its large-scale industrial application. So far, a large number of researches have been conducted to solve the well-known problem of loss-in-capacity and various strategies have been developed to enhance the CO2-capturing capacity of sorbents.14-20 Accompanied with a large number of efficient solution ACS Paragon Plus Environment

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proposed, calcium looping (CaL) technology is gradually being operated in a number of pilot plants and fluidized bed systems.21-25 It is well-known that practical CaL process has the following features: (i) short residence times in reactors, (ii) low CO2 concentration (≤15 % in volume) when carbonating at around 650 °C, (iii) fast transitions between the carbonation and calcination stages, (iv) high temperature and high CO2 concentration in the regenerator (around 930 °C and at least 70 % CO2 in volume).26 With CaL technology gradually maturing, more practical factors are being considered in recent researches. To achieve the full regeneration and subsequent sequestration of CO2, oxygen-enriched combustion of coal is utilized in the regenerator.26-28 However, the minerals in coal will potentially affect sorbents’ CO2 capturing and the effect could gradually intensify as increasing temperature. In other words, solid impurities such as coal ash derived from combustion would interact with CaO-based sorbents, which is inevitable and could frequently affect characteristics of CaO-based sorbents for CO2 capture due to the deposition and fusion of coal ash.28-31 Karalambula et al.31 investigated minerals’ attachment to calcium-based sorbent particles and found that iron (Fe) from coal tends to attach itself to calcium (Ca) as a result of solid-solid reactions occurring during coal gasification. Kuramoto et al.32 studied the deactivation of CaO-based sorbents by coal-derived minerals during CO2 sorption reactions at elevated temperature and pressure, and it suggested that the sorbents tended to undergo a solid-solid reaction with coal ash. When CaL process was operated in a pilot-scale dual-fluidized bed for CO2 capture, Lu et al.5 and Hughes et al.27 observed a thin shell on the surface of calcined CaO-based sorbents due to coal ash deposition. Our previous studies demonstrated that physical ash deposition and the subsequent aggregation could be the dominant inducement to the inhibition of sorbents’ CO2 capturing capacity.33 ACS Paragon Plus Environment

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By reviewing the literature, it is known that the existence of coal ash could affect CO2 capture of CaO-based sorbents, however, the mechanism of how coal-derived ash act during the interaction with CaO-based sorbents is still unclear. Moreover, the impacts of ash, especially with the consideration of more practical oxy-fuel calcination, are lack of comprehensive researches. The research status gives rise to the current work. In this paper, besides the evaluation of effects of ash with a series of variables under a more practical oxy-fuel calcination condition, we attached our importance to understand the action mechanism of ash on the sorption process and the proposal of models that could describe the interaction process.

2. EXPERIMENTAL 2.1. Sample Preparation and Analysis One typical limestone from Shandong Province (denoted as “L”) was chosen as the sorbent precursor for CO2 capture in the work. It was firstly milled and sieved by filter screens to get a uniform particle size of two bituminous coal ash, CB and CC, which presented similar characteristic. As the CO2 capture


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characteristics and chemical compositions of LCB and LCC are similar, thus only LCA and LCB were further studies in the following work.

Figure 1. Cyclic carbonation behavior of freshly calcined limestone (L) and its mixtures with coal ash (LCA, LCB and LCC) under oxygen-enriched calcination. (Carbonation at 650 °C in 15 vol. % CO2 with N2 balanced for 15 min; Calcination at 900 °C in 73 vol. % CO2 with O2 balanced for 8 min)

To fully understand the effect of coal ash on lime, carbonation behavior of LCA and LCB were investigated with the variation of ash content and the results are summarized in Figure 2. As shown in Figure 2a, CO2 sorption reactivity of LCA was consistently inhibited with the presence of coal ash regardless of its content. The freshly calcined L demonstrated a superior carbonation conversion of around 52 %, while the initial conversions of LCA10, LCA20, LCA30 and LCA40 were 47 %, 42 %, 35 % and 29 %, respectively. This trend was still pronounced after 20 carbonation/calcination cycles, that carbonation conversions of LCA10, LCA20, LCA30 and LCA40 were 1.9 %, 2.4 %, 3.1 % and 3.7 % lower than the 7.7 % carbonation conversion of natural lime L. In other words, coal ash CA undoubtedly possesses negative influence on the CO2 sorption process of pure lime L. Turning our attention to Figure 2b, it was interesting to find a different impact of coal ash CB on the CO2 capture behavior of natural lime L. Although the initial conversions of LCB10, LCB20, LCB30 and LCB40 were orderly 6.9 %, 11.9 %, 16.2 % and 21.8 % lower than that of pure lime L, carbonation conversion of LCB10 and LCB20 started to exceed that of L at 7th and 10th cycle, respectively, and led to 1.4 % and 0.6 % higher final conversions than the 7.7 % L. These results indicate that the interaction between coal ash CB and lime could largely depend on their proportion. While a small amount of coal ash CB provide some ACS Paragon Plus Environment

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stability for natural lime to resist the loss-in-capacity problem, negative impact will appear when the amount of ash CB is above a certain limit. Based on the representative chemical composition and interesting experimental observation, hence, further study would be focused only on coal ash CB.

Figure 2. Effect of ash content on the cyclic carbonation of LCA and LCB under oxygen-enriched calcination. (Carbonation at 650 °C in 15 vol. % CO2 with N2 balanced for 15 min; Calcination at 900 °C in 73 vol. % CO2 with O2 balanced for 8 min)

Sintering that occurs at high temperature not only determines sorbents’ intrinsic loss on CO2 capture capacity, but also affects complex interaction between coal ash and the sorbents. Therefore, the role of temperature at which coal ash was formed and calcium-based sorbents were calcined should be well studied. In this section, coal ash preparation temperature and CaL calcination temperature were always keep consistent (at 900 °C, 950 °C or 1000 °C) to match the actual working conditions. As the results shown in Figure 3, the initial carbonation conversions of pure lime L at 900 °C, 950 °C and 1000 °C were 52.3 %, 24.1 % and 9.4 %, respectively, comparing to 36 %, 17 % and 8.7 % of sample LCB. As temperature increasing, the degradation of initial carbonation conversions from L to LCB tends to weaken with reduction ratios of 31 %, 29 % and 7 % were observed. However, after experiencing 20 carbonation/calcination cycles, it was interesting to find that the final carbonation conversions at 900 °C, 950 °C and 1000 °C decreased from 7.74 % to 7.44 %, 5.74 % to 4.63 % and 3.02 % to 2.10 % when comparing L with LCB, with reduction ratio of 3.9 %, 19.3 % and 30.5 %, respectively. It is indicates that the difference of coal combustion and CaL calcination temperature probably changes the


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characteristic of coal ash or the pattern of that impacting on sorbents. Based on these interesting results, further studies will be conducted to analyze the phenomenon.

Figure 3. Carbonation conversions of L and LCB under various ash formation and CaL calcination conditions. (The temperature for coal ash formation and CaL calcination was 900 °C, 950 °C and 1000 °C, respectively. Coal ash was prepared in 70 vol. % O2 with N2 balanced while CaL calcination was conducted in 73 vol. % CO2 with O2 balanced)

3.2. Carbonation Behaviors of Large-mass Mixtures in a Dual-temperature Reactor Although coal ash induced loss-in-capacity of CaO-based CO2 sorbents has been observed obviously in TGA, sorbent/ash mixtures with a large mass (4 g, nearly three orders of magnitude) were further tested in a dual-temperature fixed-bed to study the amplification effect. The mixtures containing ash CB ranging from 15 wt. % to 45 wt. % were utilized and the results were presented in Figure 4. Though a larger amount of sample was loaded, it is seen that there is the similar tendency of decreasing carbonation conversion with increasing ash content to the TGA experiment. Pure lime L possessed the highest carbonation conversion of 22.4 % at the end of 14th cycle, which was orderly followed by LCB15 and LCB30, and LCB45 demonstrated the minimum conversion of 16.3 %. Moreover, various temperatures, under which coal ash was prepared and sorbent was calcined for regeneration during CaL cycles was adopted to investigate carbonation behaviors of large-mass mixtures and the results were shown in Figure 5. As expected, the difference on carbonation conversion between L and LCB was remained in the whole CaL process, regardless of calcination temperature. In addition, it was calculated to be with reduction ratios of 16.1 %, 18.8 % and 21.6 %, respectively, at 14th cycle with temperature ACS Paragon Plus Environment

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varying from 900 °C to 1000 °C. These results indicate that the impact of coal ash on natural sorbents for CO2 capture would persistently exist in actual CaL process, and it is not easily to be eliminated.

Figure 4. Carbonation conversions of sorbent mixture with coal ash CB ranging from 15 wt. % to 45 wt. % in a dual-temperature fixed bed under oxygen-enriched calcination. (Carbonation at 650 °C in 15 vol. % CO2 with N2 balanced for 15 min; Calcination at 900 °C in 73 vol. % CO2 with O2 balanced for 8 min) Figure 5. Effect of ash formation and CaL calcination temperature on carbonation conversion of LCB in a dual-temperature fixed bed. (The temperature for coal ash formation and CaL calcination was 900 °C, 950 °C and 1000 °C, respectively. Coal ash was prepared in 70 vol. % O2 with N2 balanced while CaL calcination was conducted in 73 vol. % CO2 with O2 balanced)

3.3. Analysis on the Evaluation of Microstructure Based on the results provided in the above sections, it is roughly concluded that coal ash mainly plays a negative role in CaL process, demonstrating an induced loss-in-capacity of natural CaO-based sorbents for CO2 capture. To further understand its intrinsic mechanism, N2 adsorption/desorption was firstly utilized to measure pore structure parameters of pure lime L, coal ash CB and mixture LCB40. As summarized in Figure 6, BET surface area and BJH pore volume of pure lime changed from 12.64 m2/g and 0.0836 cm3/g to 7.81 m2/g and 0.0454 cm3/g, respectively, as CaL cycle number increasing from 1 to 20. However, when a portion of 40 wt. % coal ash was mixed into the lime, its BET surface area and BJH pore volume decreased sharply to 4.49 m2/g and 0.0072 cm3/g only after initial calcination, and still maintained at relatively low values when experiencing 20 CaL cycles. Therefore, it is apparently ACS Paragon Plus Environment

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revealed that the addition of coal ash changes the pore structure of the lime and leads to the poor CO2 capture capacity of ash-contained mixtures.

Figure 6. Pore structure parameters of three samples after 10 min calcination under oxygen-enriched condition and 20 CaL cycles.

Undoubtedly, the addition of coal ash could weaken CO2 capturing capacity of natural CaO-based sorbents as a result of the decrease of specific surface area and pore volume, a more specific understanding of how coal ash impacting on sorbents is still lack of discussing. Therefore, the morphological structure of mixture LCB40 was captured by SEM and the distribution of elements was investigated through EDS technique. As results depicted in Figure 7, surfaces of the initially calcined LCB40 are quite smooth, seems to show there is no coal ash particles adhering to the surfaces. However, the mixtures’ morphological structures are quite different with increasing CaL cycle number as Figure 8 and 9 presented. It shows that a growing number of larger pores were generated during the 5-20th CaL cycles. Meanwhile, grains are observed to increase gradually on the surfaces of cycled LCB40. Moreover, it is interestingly to find that, with the increasing of CaL cycle number, the aggregation pattern of coal ash changes from a spot cluster mode featured by the merging of a few small grains on sorbents to a surface diffusion mode characterized with the formation of fusional shells around particles. To verifying whether the above variation is caused by coal ash, distribution of key elements was area-scanned and presented in Figure 7-9. For the initial calcined LCB40, apart from Ca, the concentrations of Si, Al, Mg and Fe are negligibly low on the surface, with values of 0.2 wt. %, 0.1 wt. %, 0.3 wt. % and 0.6 wt. %, respectively. However, their distributions are quite even. When ACS Paragon Plus Environment

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experiencing 5 CaL cycles, aggregation of Si appears accompanying with Al-gathering phenomenon in the same location. Furthermore, a certain amount increase of the concentration of Si and Al was observed, from 0.2 wt. % and 0.1 wt. % to 0.8 wt. % and 0.3 wt. %, respectively. The variation of element distribution is even more pronounced when LCB40 experiencing 20 CaL cycles. The concentration of Si and Al surprisingly achieved relatively high values of 4.9 wt. % and 2.7 wt. %, respectively, about 6 and 9 times higher than that after just 5 cycles. Besides, the positions occupied by Si in the Si distribution profile are seen to be still occupied by Al. The co-existence of these minerals indicates that some chemical reactions probably occurred among Si, Al and Ca. To further reveal the phenomenon, X-ray diffraction (XRD) results of standard and contrast samples are presented in Figure 10. Peaks of calcium hydroxide (Ca(OH)2), calcium oxide (CaO), and calcium sulfate (CaSO4) are observed in the initial calcined lime. However, XRD pattern of the 5th-cycle-carbonated products becomes complicated and a complex inorganic resultant (Ca2(Al(AlSi)O7) was detected. It worth noting that the feeble peak of Ca2(Al(AlSi)O7 becomes much stronger as CaL process continuously proceeding.

Figure 7. SEM and EDS images showing morphology and element distribution of calcined mixture

LCB40 under oxygen-enriched condition after 1st cycle. Figure 8. SEM and EDS images showing morphology and element distribution of calcined mixture

LCB40 under oxygen-enriched condition after 5th cycle. Figure 9. SEM and EDS images showing morphology and element distribution of calcined mixture

LCB40 under oxygen-enriched condition after 20th cycle. Figure 10. XRD patterns of standard and contrast samples from the various CaL cycles.


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3.4. Discussion on the Action Mechanism The above results demonstrated that coal ash possesses negative impact on CO2 capture reactivity of natural lime to a great extent. And it suggests that physical interaction between coal ash and CaO-based sorbents plays an important role in the process. Although calcined LCB40 demonstrated the maximum reduction in carbonation conversion at 1st cycle when compared with natural sorbents, coal ash was not observed adhering to the surface or forming complex compounds with sorbents as Figure 7 and 10 shown. In other words, the reduction in carbonation conversion of 1st-cycle calcined LCB40 is merely attributed to the decrease of specific surface area of mixtures. Hence, it indicated that the influence of coal ash on sorbents’ CO2-capture capacity should be attributed to physical impact. Furthermore, complex inorganic resultant (Ca2(Al(AlSi)O7) was hardly observed in the first five cycles as Figure 10 shown. Therefore, it is suggested that the influence of physical aspect on CaO-based sorbents for CO2 capture is inevitable and predominates the interaction between coal ash and CaO-based sorbents. Based on the above analysis, a schematic describing how coal ash impact on CaO-based sorbents for CO2 capture in physical aspect is given in Figure 11. It is believed that the CO2 capture of 1st-cycle calcined sorbents is mainly affected by ash deposition. To be more specific, the presence of coal ash initially decreases the specific surface area of mixtures, i.e. ash deposition occupies part of the efficient space/surface that available for the chemical-controlled reaction without the involvement of physical fusion or sorbent-ash reaction, as shown in Figure 7. With increasing carbonation/calcination cycles, ash deposition gradually intensifies and causes grain aggregation on the surrounding sorbents’ surface due to its adhesion property. It is worth noting that the incipient aggregation of coal ash mainly occurs in some spots, most likely in cracks or protuberances as revealed in Figure 8, showing a spot-cluster mode in this stage. As further accumulation of carbonation/calcination cycles, aggregation of coal ash ACS Paragon Plus Environment

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intensifies and previous spot-type influence diffuses following a surface-diffusion mode. Because of the melting propensity of coal ash, spot-type grains easily bridged together in clumps with evident loss in porosity, hence, the negative effect of coal ash still maintains even after 20 CaL cycles. In a word, physical interaction between sorbents and coal ash dominates the preliminary CaL stage and sustains further with pattern changing from spot-cluster mode to surface-diffusion mode.

Figure 11. A schematic diagram of the physical interaction mechanism between coal ash and CaO-based sorbents for CO2 capture.

Although physical deposition and aggregation dominate in the interaction between coal ash and CaO-based sorbents, the chemical reactions should also be considered based on the following interesting phenomena: (1) complex inorganic resultant (Ca2(Al(AlSi)O7) began appearing when experiencing 5 CaL cycles and was even more apparent at the 20th cycle as Figure 10 shown; (2) with increasing CaL cycles, the aggregation of Si is gradually intensified and it always appears accompanying with massive Al-gathering; (3) even in the severe sintering stage, the CO2 sorption characteristics of sorbents with quite poor morphological parameters in physical structure still sharply deteriorated when coal ash mixed. These results clearly suggest that chemical reactions occur and gradually play an important role, especially in the severe sintering stage. To better understand the chemical interaction between coal ash and sorbents, a schematic mechanism is proposed and presented in Figure 12. For Ca-based sorbents without the presence of ash, the porous surface undoubtedly possesses better CO2 sorption characteristics due to high surface area and pore volume. Thus, a thin CaCO3 layer quickly covers the grains and keeps thickening till reaches a critical ACS Paragon Plus Environment

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thickness, at which point the reaction mode changes from chemical control stage to diffusion control stage. With the sintering of surface intensifying, the critical thickness decreases, meanwhile, a great deal of cracks emerges and keeps extending, which leads to the diffusion of CO2 into inner cores of sorbents and the prolongation of diffusion control stage. On the other hand, for Ca-based sorbents with the presence of coal ash, coal ash rarely affect CO2 sorption characteristics of sorbents in chemical aspect due to the lack of sufficient physical contact in preliminary stage. However, with a large amount of cracks or protuberances generating caused by severe sintering, cracks or protuberances provide vast space for physical contacting, hence, suitable chemical reaction conditions are provided for complex solid-solid reactions. As a result, the formation of complex inorganic (Ca2(Al(AlSi)O7) consumes a portion of CaO-based sorbents involved in carbonation in chemical control stage. Furthermore, the deposition of Ca2(Al(AlSi)O7 and coal ash also hinder the diffusion of CO2 in diffusion control stage. In conclusion, solid-solid reactions, which mainly involve Al and Si from coal ash reaction with Ca from the sorbent, could happen as aggregation of coal ash intensifying and would maintain the negative impact in the later stage of CaL process. These conclusions also explain the result why anthracitic coal ash, with high content of Al and Si, possessed stronger influence on the capacity of natural lime sorbents for CO2 capture than bituminous coal ash in Figure 1.

Figure 12. A schematic diagram demonstrating the chemical impact of coal ash on CaO-based sorbents.

4. CONCLUSIONS In an effort to improve the industrial application of CaL process, we investigated the interaction between three different types of coal ash and a typical limestone under various conditions in the ACS Paragon Plus Environment

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continuous TGA environment and lab-scale dual-temperature reactor environment, respectively. Though the interaction between sorbents and ash may be affected by attritions in practical fluidized systems, it needs to note that severe attrition only occurs in initial cycles of carbonation and calcination. In this case, our work adopting TGA and fixed bed rather than a fluidized bed is able to reveal the following conclusions: (1) coal ash could affect the CO2 capture of CaO-based sorbents in the whole CaL process and the extent of the negative impact mainly depends on the element compositions of coal ash; (2) certain coal ash could provide stability for natural lime to resist the loss-in-capacity problem in the presence of a small amount of it; (3) different coal combustion and CaL calcination temperature could probably change the characteristic of coal ash and the pattern of that impacting on sorbents; (4) physical interaction between coal ash and sorbents predominates the impacts in the preliminary stage and it sustains in the whole CaL process with interaction pattern changing from a spot cluster mode to a surface diffusion mode; (5) solid-solid reactions involving Al and Si in coal ash and Ca in the sorbent could appear as aggregation of coal ash intensifying and would contribute to the negative impact in the later stage of CaL process.

ACKNOWLEDGEMENT The authors are grateful to the financial supports from National Natural Science Foundation of China (No. 51606018), National Key R&D Program of China (No. 2017YFB0603300), Chongqing Basic Science and Advanced Technology Research Program (No. cstc2017jcyjAX0324), and Key Laboratory of Low-grade Energy Utilization Technologies and Systems of Ministry of Education (No. LLEUTS-2016004, LLEUTS-201411).


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TABLES Table 1. Chemical compositions of the calcined limestone (L) and coal ash (CA, CB and CC). component

L (wt %)

CA (wt %)

CB (wt %)

CC (wt %)

Al2O3

1.8

38.0

32.2

28.8

CaO

88.3

3.9

13.5

14.3

CeO2

--

Impacts and Action Mechanism of Coal Ash on CaO-Based Sorbents

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