- Email: [email protected]
petcoke fired circulating fluidized bed boiler with limestone injection using its fly and bottom ash
Journal of Environmental Chemical Engineering 5 (2017) 667–678
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
An investigation into partial capture of CO2 released from a large coal/ petcoke fired circulating fluidized bed boiler with limestone injection using its fly and bottom ash Anantkumar Patela , Prabir Basua,* , Bishnu Acharyab a b
Deaprtment of Mechanical Engineering, Dalhousie University, Halifax, NS, B3H 4R2, Canada School of Sustainable Design Engineering, University of Prince Edward Island, Charlottetown, PEI, C1A 4P3, Canada
A R T I C L E I N F O
Article history: Received 4 October 2016 Received in revised form 25 December 2016 Accepted 28 December 2016 Available online 29 December 2016 Keywords: CFB fly ash CO2 capture Carbonation Ash utilization
A B S T R A C T
Circulating Fluidized Bed (CFB) power plants have been gaining popularity globally due to its ability to utilize less expensive solid fuels, in-situ capture of SOx, low NOx emission and flexible operating characteristics. Higher CO2 emission from sulfur capturing CFB plants is, however, a major shortcoming of this technology. Additionally, fly ash generated from such CFB power plants is not favorable for many commercial applications. This work examined if the fly ash produced in a CFB power plants with limestone feed can capture a part of the CO2 released from it. It studied factors affecting capture of CO2 utilizing by its ash. Two sets of experiments were carried out for dry and hydrated fly ash at 500–750 C and 30–80 C respectively. Tests showed that CaO conversion increased with temperature up to 700 C for the dry case and 50 C for hydrated case thereafter it started to reduce. Effect of partial pressure of CO2 (17 kPa–31 kPa) on the total carbonation reaction was minor, but, the duration of initial rapid CO2 capture stage was inversely proportional to the CO2 partial pressure. For benchmarking the capture characteristics of lime were compared with that of CFB ash at the same conditions. In most cases carbonation reaction in lime followed a similar pattern as in fly ash. The capture characteristics were also similar for bottom ash (particle size, 116 mm–275 mm). However, the amount of CO2 captured by the coarser bottom ash was significantly lower than that by dry fly ash at the same condition. This study, thus showed the potential of using fly ash from sulfur capturing CFB power plants in reducing the CO2 emission from the very plant and thereby achieve better disposal of the fly ash. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Use of coal as a fuel for steam generation began in 1850s, and it played a major role in the Industrial Revolution of the 19th century [1]. Coal is abundant and has the highest reserve-to-production ratio of all fossil fuels [2]. It is estimated to contribute around 40% of the world’s electricity generation [3], but coal-burning also accounts for about 42% of the global CO2 emission [4]. Additionally, coal plants produce several harmful gases and solid products, such as fly ash, bottom ash, boiler slag, and flue gas desulphurization residues, most of which are, to some extent, hazardous for the environment. Increasing global energy demand could see by 2040 about 78% of the global energy come from fossil fuels [5], which
* Corresponding author. E-mail address: [email protected] (P. Basu). http://dx.doi.org/10.1016/j.jece.2016.12.047 2213-3437/© 2016 Elsevier Ltd. All rights reserved.
could further complicate the issue of climate change and regional pollution worldwide. Various carbon-capture and storage (CCS) technologies focus on a reduction of CO2 emission from fossil fuel power plants. There are three major CO2 capture methods from fossil fuel power plants, pre-combustion, post combustion, and oxy-fuel combustion [6,7]. Capture, compression, transportation and storage of carbon dioxide (CCS) are accepted means of large scale reduction in GHG emission, but these are energy intensive and expensive [6]. For example, the increase in energy requirement for a plant with CO2 storage as mineral carbonates could be as high as 60–180% [8]. Circulating fluidized bed (CFB) combustion is a relatively new technology for control of acid rain gases. It has gained much popularity due to its ability to utilize low-grade fuel or mixtures of fuels (such as coal and petroleum coke), fuel flexibility, in-situ capture of SOx and low NOx emission. However, the emission intensity (for GHG gases CO2 and N2O) of a CFB power plant is higher than a that of a pulverised-fuel (PF) fired power plant
668
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
especially when limestone is added in such boilers to capture SO2 released during combustion. Additional CO2 is released here due to calcination of limestone in its furnace [9]. A major challenge in post-combustion CO2 capture from a typical coal or petcoke fired power plant is the low concentration of CO2 in the flue gas from a CFB boiler, which makes its direct compression, transportation and storage difficult. Thus, it is necessary to explore new cost effective and less energy-intensive practical methods for carbon capture. Interestingly, the ash produced from a CFB power plant firing high sulfur fuels do not find much use in building industries due to its large volumetric expansion when cured at normal temperature [10]. Fly ash and bottom ash from typical CFB power plants contain significant amounts of free CaO, as only 35–40% of the lime feed is utilized in SO2 capture [18]. So, disposal of fly ash from such CFB plants is difficult. This work therefore examines if the unutilized CaO in CFB ash can be utilized through carbonation reaction for partial capture CO2 from flue gas for CFB boiler. Carbonation of alkaline solid wastes such as coal ash containing lime can proceed through two routes: dry (gas-solid) carbonation and wet (aqueous) carbonation [11]. In gas-solid carbonation, carbon dioxide reacts with available CaO in the ash to produce CaCO3 as below: CaO + CO2 = CaCO3
(1)
In aqueous carbonation, CO2 gas reacts with water and produces carbonic acid according to the following gas phase reactions [12,11,13,14]: CO2(g) + H2O
(moisture) ! H2CO3(carbonicacid,aq)
H2CO3(carbonicacid,aq) ! H+ + HCO3 CO32(aq)
+ (bicarbonate) ! 2H (aq) +
(2)
With increase in CO2 partial pressure, its dissolution in water increases [11], but the dissolution is also inversely proportional to temperature [15]. The solid phase reaction is: CaO
+ (s) + H2O (l) ! Ca(OH)2(aq) ! Ca2 (aq) + 2OH
CO32
(3)
+
ions reacts with Ca2 to produce CaCO3(nuclei) which converts into CaCO3(s) Ca2+
2 (aq) + CO3 (aq) ! CaCO3(nuclei) ! CaCO3(s)
(4)
The overall reaction can therefore be written as: Ca(OH)2 + CO2 + H2O ! CaCO3 + H2O
(5)
In addition, there is also the formation of calcium bicarbonate, which gives enhanced CO2 diffusion due to its higher solubility [14]. CaCO3 + CO2 + H2O ! Ca(HCO3)2
(6)
Calcium bicarbonate reacts with calcium hydroxide and forms calcium carbonate ass below: Ca(HCO3)2 + Ca(OH)2 ! 2CaCO3 + 2H2O
(7)
A number of studies were conducted for CO2 capture and CaO utilization in carbonation reaction of fly ash or limestone derived materials [16,17,12,18,19,20,21,22,23]. Reddy et al. [12] conducted
carbonation reaction in fly ash fluidized by flue gas in a pilot scale plant at 30–45 C achieving a reduction in CO2 concentration from 13% to 9% within few minutes. They referred the cost estimation of mineralization of one tonne of CO2 at $11 at mineralization capacity of 207 kg CO2/tonne of fly ash for a 532 MW plant. Two step reactions, first, hydration of fly ash to convert CaO to Ca(OH)2 followed by carbonation reaction were employed by Montes-Hernandez et al. [24] for aqueous carbonation reaction and found that one tonne fly ash (4.1% CaO) captured 26 kg of CO2 at 60–90 C. González et al. [25] observed that aqueous carbonation reaction at a particular temperature is dependent on the amount of water present in the system. Studies on the effect of temperature on wet carbonation revealed that carbonation reaction increased up to 60 C with increase in temperature, followed by reduction. They also mentioned that aqueous carbonation required less energy than dry gas-solid carbonation. There has been some argument about natural carbonation of lime through weathering. Comparison of naturally weathered carbonation and accelerated carbonation (at 4 MPa, 90 C) of fly ash showed that the required time duration for about the same level of carbonation was 20 years for the former compare to 2 h for the latter [26]. Investigation of the effect of relative humidity (RH) on the carbonation of Ca(OH)2 shows that the reaction could not take place below a critical RH (8%) and the influence of RH was higher than that of temperature [27]. Wang et al. [18] and Whittington [28] observed that carbonation of Ca(OH)2 is much faster than carbonation of CaO. The conversion of CaO to Ca(OH)2 depends on factors such as hydration temperature, hydration method, reaction time, and particle size. Only a limited number of studies are available on the carbonation reaction of CFB fly ash and presently there is no information on the carbonation reaction of CFB bottom ash of different particle sizes. Wang et al. [18] investigated carbonation reactions in CFB fly ash in oxy-fuel condition (CO2 concentration 80%), and observed that CaO utilization increased with increase in temperature and below a critical temperature (less than 400 C) there was not any significant capture in dry carbonation experiments. Whereas, addition of steam enhanced the carbonation reaction even at low temperature (250 C). Although Wang et al. [18] studied CO2 capture by dry CFB fly ash in oxy-fired condition above 400 C and in presence of water vapour above 250 C, the reaction at low CO2 partial pressure as expected in air fired CFB boiler, and, at low temperature for hydrated fly ash could differ considerably. Comparison of carbonation reaction characteristics of pure lime and CFB fly ash for similar operating condition could provide more insight into the effectiveness of CO2 capture by CFB fly ash. As untreated CFB fly ash contains items like SO3, CaO, LOI, Al2O3, SiO2, a difference in the reaction characteristics of CFB fly ash and pure lime could exist. González et al. [25] investigated carbonation in aqueous condition at 100% CO2 concentration, however, in typical air fired CFB boiler CO2 accounts for less than 20% [18]. Thus it is necessary to evaluate the effect of partial pressure of carbon dioxide on the carbonation reaction of CFB fly ash. In this, paper we investigated carbonation reaction at CO2 concentration between 16% and 36%. The novelty of this work is, firstly, investigation of CO2 capture by un-treated CFB fly ash at low CO2 partial pressure and at different temperature in dry and hydrated conditions. Secondly,
Table 1 Fly ash composition analysis (Unmeasured carbon and nitrogen accounts for the balance). Element
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
SO3
SiO2
V2O5
P2O5
LOI
wt (%)
1.70
37.25
0.87
0.32
0.45
0.11
15.88
0.22
0.22
0.05
11.08
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
669
Fig. 1. Fly ash particle size distribution.
comparison of CO2 capture with pure lime for the same experiments. Thirdly, study of carbonation reaction in CFB bottom ash of different particle size. 2. Methodology Ash for the experiments was collected from the 197 MWe Circulating Fluidized Bed (CFB) power plant in Point Aconi, Nova Scotia, Canada. The CFB power plant uses a mixture of petroleum coke and coal as fuel in a ratio of 80:20. Limestone is added for insitu capture of SO2. Fly ash contains 37.25% unutilized CaO from the sulfation reaction in the furnace. Table 1 gives full composition of the fly ash. The size distribution of the fly ash (Fig. 1) shows that 95% of fly ash particle size falls under 120 mm. The experimental setup comprised a metal cylindrical reactor of diameter 25 mm and height 300 mm closed at both
ends, with a provision to accommodate a gas tube and a thermocouple was made for all the tests performed in this study. A schematic of the experimental setup is shown in Fig. 2. The reactor was heated in a bubbling fluidized bed heater. During the experiment, mixtures of nitrogen (N2) and carbon dioxide (CO2) gases were supplied from the bottom end of the reactor. The exit for outlet gas passes through the cap on top of the reactor that also holds a thermocouple to continuously monitor interior temperature of the reactor during experiments. A Testo 350-XL gas analyser continuously analyzed gases coming out of the reactor. A photograph of the experimental setup is shown in Fig. 3. The aim of this study is to investigate the carbonation reaction in fly ash at different conditions and compare that from similar experiments with high calcium quicklime. The high calcium quicklime (termed lime) contained more than 90% CaO, and was
Fig. 2. Schematic diagram of the experimental setup.
670
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678 Table 3 Experimental test conditions for different samples. Temperature ( C)
30 40 50 60 80 500 600 700 750 a
Fig. 3. Photograph of experimental setup.
collected from Graymont Limited, Havelock, New Brunswick, Canada. 2.1. Test conditions Two sets of experiments were conducted in different conditions. First set was at dry conditions for both dry fly ash and dry lime. Other experiments were conducted at hydrated condition for hydrated fly ash and hydrated lime. As the reactor has a fixed volume and the dry and hydrated samples have different densities it was not possible to accommodate the same mass of hydrated lime sample in the reactor. Hydrated lime sample of smaller mass (i.e., 30 g) was selected. Table 2 gives sample preparation information, such as weight of ash/lime and amount of water added. Dry and hydrated test were performed at high temperature (above 500 C) and low temperature (below 80 C), respectively. Table 3 shows the test matrix prepared for this study. For all the experiments, nitrogen flow rate was maintained constant at 0.46 l/ min and the amount of water for hydration was kept constant at liquid to lime ratio of 0.65. 2.1.1. Effect of temperature in dry and hydrates samples Previous study noted that gas solid carbonation reaction in CFB fly ash below 500 C is insignificant even at CO2 concentration as high as 80% [18]. Also, they noted that CO2 capture increased with temperature. So, all experiments to investigate the effect of temperature for dry conditions were conducted at temperature range from 500 to 750 C, at significantly lower CO2 concentration (16–36%) than that investigated by Wang et al. [18]. Fifty grams of un-treated dry fly ash or quicklime sample were spread on quartz wool and placed in the reactor. Quartz wool served as a highly porous support for the sample, with free access to gas from all directions. Nitrogen (N2) was continuously supplied during the experiment, and, on reaching the desired temperature, it was switched to the mixture of CO2 and N2 gases at 0.20 and 0.46 l/min (as volume: 30% and 70%) respectively. Reacted gas leaving the reactor column was continuously cooled in a Graham (coiled type) condenser and continuously analyzed by a gas analyzer. Carbonation of hydrated fly ash is conducted typically below water evaporation temperature to prevent steam from escaping, Table 2 Mass of sample and water mixed for batch experiments. Dry Fly Ash Hydrated Fly Ash Dry Lime Hydrated Lime Sample Weight (g) 50 Water Added (g) 0
50 12
50 0
30 19.5
CO2 = 0.20 l/min (vol: 30%), N2 = 0.46 l/min (vol: 70%) Hydrated fly ash p p p a , p p N/A
Hydrated lime p p p a , p p
Dry fly ash
Dry lime
N/A
p p p , p
a
p p p , p
a
test conducted for CO2 volume concentration: 16%, 22%, 30%, 36%.
and thus avoiding an expensive pressurized carbonation vessel. This less energy intensive choice simplifies industrial processes and makes it cost-effective. So, the carbonation reaction in hydrated fly ash and hydrated lime was conducted at low temperature range (30–80 C). 2.1.2. Experiments at different CO2 partial pressure To investigate the effect of CO2 partial pressure on carbonation reaction, tests were conducted at four CO2 partial pressures at temperature 50 C for hydrated and 700 C for dry case and by keeping all other parameter constant. The CO2 partial pressures were 17, 22, 31 and 37 kPa (Volume: 16%, 22%, 30%, 36%) keeping N2 flow at 0.46 l/min. 2.1.3. Experiments on effect of particle size To investigate the effect of particle sizes on the CO2 capture, bottom ash of average particle sizes 116 mm, 196 mm and 275 mm were selected and the capture results were compared with the same particle sizes of dry lime experiments carried out at similar operating conditions. Operating condition for all the experiments was fixed at temperature 700 C and CO2 partial pressure at 31 kPa. 3. Result and discussion All experiments in this study have shown an initial stage of rapid CO2 conversion into CaCO3 followed by a rather slow reaction stage, when the CO2 capture level of all dry ash samples dropped to 50%, and, after that, it reduced asymptotically. The similar observation has been seen in studies by Bhatia and Perlmutter [29], Dedman and Owen [30], Silaban [31], Stanmore and Gilot [32] and Mess et al. [33]. The initial rapid reaction is essentially a surface reaction of CO2 with CaO, which was kinetically controlled due to the high gas-exterior surface mass transfer. The carbonation reaction produced a product layer of CaCO3 on the exterior surface of available CaO in ash, which restricts further direct contact of CO2 with available CaO beneath it [34–36]. This mass transfer restriction slows down the reaction, and thus, the reaction was controlled by diffusion of CO2 gas into pores to react with CaO. The CO2 diffusion was very slow in the second stage phase of carbonation of lime. The total CO2 capture was found nearly independent of CO2 partial pressures, as also noted in other studies [33,20,31]. 3.1. Effect of temperature 3.1.1. Dry test Fig. 4 shows the percentage capture of the CO2 entering the reactor in the temperature range 500 C to 750 C by the as received dry fly ash from Point Aconi power plant. It is apparent from the figure that initial level of CO2 capture increased with the increase in flue gas temperature up to 700 C. However, after the initial
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
671
100%
CO2 capture (%)
90% 500 °C
80%
600 °C 70%
700 °C 750 °C
60%
50% 40%
0
5
10
15
20
25
30
time (min) Fig. 4. CO2 captured by dry fly ash.
remaining constant at 0.1 kg CO2 at both 600 C and 700 C, then it dropped to the same level as dry fly ash at 750 C. Interestingly this translate into lower (12.8%) conversion of the CaO in quicklime. One notes here lower level of CaO utilization in case of lime. The reduction in capture at 750 C could be because of the reaction approached to the calcination temperature. Li et al. explained that at 10 kPa CO2 partial pressure, change in Gibb’s free energy becomes zero at 720 C and there could not be any carbonation reaction. It could be stated that at 750 C and at 31 kPa CO2 partial pressure change in Gibb’s energy approached to zero and the reaction approached to calcination reaction. Thus, at 750 C very little carbonation reaction took place in the kinetically controlled regime. Interestingly, both test samples showed similar amount of CO2 capture in fast reaction regime at 750 C.
stage of about 10 min, the capture level reduced slowly below 50% capture for all samples. At 750 C there was a reduction in CO2 capture in the rapid reaction regime because above 700 C, the change in Gibbs free energy would be negligible at the modest partial pressure of CO2 of the present experiments, and, hence, the carbonation reaction could not take place [37]. Fig. 5 shows similar experiments with high-calcium quicklime (lime) instead of fly ash. The dry fly ash samples here typically contained 37.25% CaO in comparison with lime sample which had more than 90% CaO. Except for tests at 500 C the capture level at 10th minute for all tests varied between 100 and 76%. At 750 C the drop to 50% capture level, occurred well after 160 min. Similar to the case of dry fly ash, initial amount of CO2 capture by lime increased rapidly with increasing temperature but up to 700 C and thereafter gradually reduced to its steady-state slow reaction stage. The initial kinetically controlled rapid surface reaction stage captured a significant amount of CO2 and thus temperature had important effect (Figs. 4 and 5), but in the subsequent slow diffusion controlled there was not a noticeable effect on temperature on the amount of CO2 capture. Fig. 6 shows the amount of CO2 captured per unit mass of the sample (including CaO and other components of the ash) in the kinetically controlled regime at different temperatures. For dry fly ash, the highest CO2 captured was 0.04 kg per kg sample at 700 C, and the minimum was 0.01 kg per unit sample at 750 C. The highest value translates into 14% conversion of the CaO in ash. The CO2 captured per unit mass of sorbent was higher for dry lime
3.1.2. Hydrated tests Dry carbonation reaction at lower temperature (30–80 C) being negligible, effort was made to conduct tests through hydration as a part of the available CaO in fly ash and lime could be converted to Ca(OH)2 through hydration. However, it was not possible to verify the extent of Ca(OH)2 conversion by measurement of mass change as significant amount of water was taken up in the reaction to form phases such as hydrated aluminosilicates [38]. Hence, for hydrated fly ash experiments, both free CaO or Ca (OH)2 may be expressed as CaO. A reaction mechanism for Ca(OH)2 and CO2 in presence of water vapour [14] showed that CO2 and Ca(OH)2 dissolve in the nano-
100%
CO2 capture (%)
90% 80%
500 °C 600 °C
70%
700 °C 60%
750 °C
50% 40% 0
20
40
60
80
time (min)
100
120
Fig. 5. CO2 captured by 50 g of high calcium quicklime (lime).
140
160
672
CO2 capture/sample (kg/kg)
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
0.120 0.100 0.080 dry fly ash
0.060
quicklime 0.040 0.020 0.000 450
500
550
600
650
700
750
800
temperature (°C) Fig. 6. CO2 captured per unit sample during rapid reaction stage.
Fig. 7. Ca(OH)2 particle carbonation reaction.
750 C) temperature range. It is apparent that CO2 capture level increased with increase in temperature but only up to 50 C. Above 50 C, the capture started to reduce again because the carbonation reaction of Ca(OH)2 depends on the amount of water on the sorbent surface, relative humidity (RH) and temperature. The solubility of CO2 in water reduces (carbonic acid) with rising temperature, but calcium leaching increases in the water layer (equation (3)) [25,40]. So, the CO2 dissolution reduced above 50 C hindering the further carbonation reaction at 0.65 liquid to lime ratio in this study. It is supported by the observation of Ukwattage et al. [40]. The reaction of CO2 and Ca(OH)2 is dependent on the amount of moisture present in the reaction environment. There could be no
droplets of water formed on the surface of sorbent (Fig. 7), precipitating CaCO3 as product layer through carbonic reactions (Equations (2)–(7)). The initial rapid reaction stage could be considered gas-liquid-solid process [39,13]. At the start of the reaction, CO2 reacted with sorbent in presence of water droplets and formed a product layer of CaCO3. This product layer on the surface of sorbent hindered further carbonation reaction resulting in a transition to a slow second stage reaction that was controlled by diffusion of CO2 in the pores. The change in CO2 conversion or capture by hydrated fly ash with time in the lower temperature range of 30 C–80 C is shown in Figs. 8 and 9. The pattern is similar to that for dry fly ash at higher (600–
100%
CO2 capture (%)
90% 80%
30 °C 40 °C
70%
50 °C 60 °C
60%
80 °C 50% 40% 0
10
20
30
time (min)
40
Fig. 8. CO2 captured by 50 g of hydrated fly ash.
50
60
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
673
100%
CO2 capture (%)
90% 30 °C
80%
40 °C 70%
50 °C 60 °C
60%
80 °C 50% 40% 0
30
15
45
60
75
90
time (min) Fig. 9. CO2 captured by 30 g of hydrated lime.
reaction if CO2 and water were not present simultaneously above a particular relative humidity [27,22,39,13]. Shih et al. [27] noted that RH has more dominant effect on carbonation reaction than that by temperature. At the start all samples for hydrated tests had similar moisture/water content (liquid to lime ratio: 0.65); but increase in temperature reduced the RH in the reaction environment. Although increase in temperature had a positive effect on CO2 capture, the reduction in RH depressed the amount of capture. For this reason, one notes (Fig. 10) RH showed dominant effect causing the capture start to reduce. This observation is in agreement with those made by Shih et al. [27] and Girard et al. [41]. The nano-droplets from the sorbent surface evaporated rapidly with temperatures above 50 C, which reduced, the carbonation reaction significantly. Interestingly, transformation of Ca(OH)2 to CaCO3 by carbonation reactions produced additional water layer that catalyzed the ongoing carbonic reaction [22]. Comparing results from dry experiments with those from hydrated cases, one notes that surface carbonation reaction of Ca (OH)2 on hydrated particles helped capture significantly more amount of CO2 than that by CaO in dry fly ash or lime. The addition of water or hydration of the sample extended the first stage reaction, even at low temperature. This observation had also been noted by Manovic and Anthony [42]. They stated that the steam enhanced carbonation via solid state diffusion in the product layer. In case of this study, where temperature is considerably lower than that of Manovic and Anthony [42], water enhanced the first stage reaction and catalyzed the carbonation reaction even at low temperature [13]. This could be also explained
as during carbonation reaction, release of H2O due to decomposition of Ca(OH)2 increased humidity and surface area eventually favoring the capture [43,27]. In addition to this, Ca(OH)2 has higher specific surface area than CaO which enhanced surface carbonation reaction [44]. Fig. 10 shows the amount of CO2 captured by unit mass of hydrated samples at different temperatures during the initial rapid reaction stage. It can be seen that with increase in temperature, CO2 capture increased but declined above 50 C as the RH in reaction environment decreased with the thickness of water droplets. It can be observed for most experiments that transition from an initial rapid kinetically controlled stage to a slower diffusion stage took place rapidly, and the start of this quick transition was considered at when CO2 capture reached 95%. A comparison of CO2 capture in the kinetically controlled regime for dry and hydrated fly ash at different temperature is shown in Fig. 11. Except for one case, the amount of CO2 capture was higher for hydrated samples at similar conditions. 3.2. Effect of partial pressure 3.2.1. Dry test The decline in the amount of CO2 capture after the initial period of rapid kinetic controlled reaction to asymptotically slower CO2 diffusion regime occurred early for higher CO2 partial pressure. Fig. 12 shows that the time required to reach that stage controlled by diffusion of gas into the pores, was around 30 min at the lowest
CO2 capture/hyd. sample (kg/kg)
0.20 0.18 0.16 0.14 0.12 0.10
hydrated fly ash
0.08
hydrated lime
0.06 0.04 0.02 0.00
20
30
40
50
60
70
80
temperature ( °C) Fig. 10. Mass of CO2 captured per unit hyd. sample during rapid reaction stage.
90
674
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
Fig. 11. CO2 captured by of dry and hydrated fly ash during initial rapid carbonation stage (CO2 partial pressure 31 kPa).
100%
CO2 capture (%)
90% 80%
17 kPa
70%
22 kPa
60%
31 kPa 37 kPa
50% 40% 0
10
20
30
40
50
60
time (min) Fig. 12. CO2 captured by dry fly ash at 700 C.
[29,34]. However, CO2 captured in diffusion-controlled regime (slow reaction stage) was independent of CO2 partial pressure (Fig. 13).
partial pressure (17 kPa) while at the highest partial pressure (37 kPa) it was 8 min. This is because the rate of development of CaCO3 layer on particle surface increased with the increase in CO2 partial pressure during the kinetically controlled stage. However, the latter stage, being diffusion controlled, the effect of partial pressure of CO2 on capture was negligible [29,34]. At a given initial time period, a higher amount of CO2 capture was noted for lime (Fig. 13) compared to that for fly ash (Fig. 12). Except for this, the general characteristics of capture in both samples at different partial pressure were similar. The amount of CO2 captured per unit mass of fly ash or lime sample increased with CO2 partial pressure during the first reaction stage (Fig. 14) because the speed of surface carbonation (kinetically controlled) was higher for high CO2 partial pressure
3.2.2. Hydrated test The effect of CO2 partial pressures on CO2 capture by hydrated fly ash and hydrated lime at 50 C is shown in Figs. 15 and 16, respectively. This characteristic was similar to that noted in dry test cases (Fig. 12 and 13). Also, in the diffusion-controlled regime, no effect of CO2 partial pressure was observed. The reasons for higher capture by hydrated samples in comparison with dry samples have been discussed earlier. A comparison of the CO2 captured per unit mass of hydrated sample for different CO2 partial pressure during the initial fast
100%
CO2 capture (%)
90% 80%
17 kPa
70%
22 kPa 31 kPa
60%
37 kPa 50% 40% 0
20
40
60
80
time (min)
100
Fig. 13. CO2 captured by dry lime at 700 C.
120
140
160
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
675
0.16
CO2 capture/sample (kg/kg)
0.14 0.12 0.10 0.08 dry fly ash at 700 °C
0.06 0.04
quicklime at 700 °C
0.02 0.00 15
20
25
30
35
40
CO2 partial pressure (kPa) Fig. 14. CO2 captured by dry fly ash and lime during rapid reaction stage at 700 C.
100%
CO2 capture (%)
90% 80% 70%
17 kPa 22 kPa 31 kPa 37 kPa
60%
50% 40% 0
10
20
30
40
50
time (min)
60
70
80
90
100
Fig. 15. CO2 captured by hydrated fly ash at 50 C.
reaction stage is shown in Fig. 17. Hydrated lime showed a large increase in CO2 capture when its partial pressure increased from 17 kPa to 22 kPa (Fig. 17), but above 30 kPa there was reduction. This could be because of the carbonation of hydrated lime, being an exothermic reaction, released heat that increased reaction temperature and above a certain temperature, the capture started to reduce. This is evident from Fig. 9, where increase in temperature above 60 C, at 30% CO2 concentration, showed significant reduction in capture in the rapid reaction stage. The change in the temperature inside the reactor due to exothermic carbonation reaction, at 50 C in hydrated lime, at different CO2 partial pressure is shown in Fig. 18. It shows that at CO2 partial pressures 17 kPa and 37 kPa, the increase in temperature was around 5 C and 15 C respectively. The reduction in
amount of CO2 captured in rapid reaction stage above 30 kPa partial pressure (Fig. 17) is because of this sudden increase in localised temperature at the reaction environment. 3.3. Effect of particle size Circulating fluidized bed boilers produce two types of ash; fly ash and bed ash. The ratio of production rates of fly ash and bottom ash depends on a number of factors, and this ratio could be as high as 50:50 for high ash coal. Bottom ash is coarse and typically in 200–1000 mm range while the fly ash is much finer in the range of 20–200 mm. In the Point Aconi plant both sizes were relatively small. The average size of fly ash is the range of 20–50 mm (Fig. 1) while that of bed ash was about 300 mm. So, it
100%
CO2 capture (%)
90% 80%
17 kPa
70%
22 kPa 31 kPa
60%
37 kPa
50%
40% 0
20
40
60 time (min) 80
100
Fig. 16. CO2 captured by hydrated lime at 50 C.
120
140
676
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
CO2 capture/sample (kg/kg)
0.25 0.20 0.15 hydrated fly ash at 50 °C
0.10
hydrated lime at 50 °C
0.05 0.00 15
20
25
30
35
40
CO2 partial pressure (kPa) Fig. 17. CO2 captured by hyd. fly ash and hyd. lime during rapid reaction stage.
80
temperature (°C)
70 60
17 kPa 22 kPa
50
31 kPa 40
37 kPa
30 20 0
10
20
30
40
time (min)
50
60
70
80
Fig. 18. Change in reaction temperature during carbonation reaction of lime at 50 C.
is necessary to examine if the coarser bottom ash from a CFB boiler with limestone addition could make any contribution to the CO2 capture. The effect of particle size on the carbonation reaction of bottom ash plotted on Fig. 19 show no notable difference in CO2 capture by bottom ash of any particle size. A relatively small amount of capture in the initial stage was observed for bed ash all sizes of particles, which carbonated under diffusion controlled regime soon after the start of the experiment. After 5 min, CO2 captured by bottom ash reduced to around 50%
(Fig. 19), whereas dry fly ash captured nearly 98% of the CO2 supplied for similar experiments (Fig. 4). Fig. 20 for lime shows that at 19th minute (in the fast reaction stage), CO2 captured by 116 mm, 196 mm and 275 mm lime particles was 100%, 98% and 95% respectively apparently due to smaller particles’ higher pore surface area than larger particles. Similar results have been observed by Samari [14], and, for the same reason, the transition from rapid kinetically controlled regime to slow diffusion controlled regime occurred early for larger particles than smaller particles.
100%
CO2 capture (%)
90% 80% 116 µm 70%
196 µm
60%
275 µm
50% 40% 0
5
10
15
20
25
30
35
40
time (min) Fig. 19. CO2 captured by bottom ash at 700 C and 31 kPa CO2 partial pressure.
45
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
677
CO2 capture (%)
100% 90% 80%
116 um
70%
196 um
60%
275 um
50% 40% 0
10
20
30
40
50
60
70
80
time (min) Fig. 20. CO2 captured by dry lime of different sizes at 700 C.
4. Conclusion This work investigated into the carbonation of untreated CFB ash in dry and hydrated conditions to capture CO2 from CFB power plants with limestone feed. Experiments identified the factors affecting the carbonation reaction in CFB fly ash, and the results were compared with the tests conducted with high calcium quicklime at same operating conditions. Results showed that carbonation reaction in CFB ash and lime follows the same pattern: initial rapid conversion of the sorbent, followed by a slow reaction. Effect of temperature showed that carbon dioxide capture in the fast-reaction regime increased with temperature, but above a certain temperature (700 C for dry cases and 50 C for hydrated cases) the capture reduced. Increase in temperature from 600 C to 700 C in dry fly ash, and, 40 C to 50 C in hydrated fly ash, extended rapid reaction stage by one minute in both the cases. Hydrated fly ash showed similar or even higher capture level than dry fly ash at significantly lower temperature (30–80 C). The rapid initial CO2 capture by fly ash was kinetically controlled and its duration varied inversely with partial pressure of CO2. At the highest partial pressure of 37 kPa, the time required to reach the slow diffusion stage of capture by hydrated fly ash at 50 C was 30 min, whereas it was 90 min at the lowest partial pressure of 17 kPa. At partial pressures, 37 kPa and 17 kPa, the time required by dry fly ash was 30 min and 8 min respectively at 700 C. There was no significant effect of particle size for bottom ash in the size range of 116 mm to 196 mm. The capture level dropped below 60% in 1 min of the start, while for dry fly ash it took 10 min to drop to this level. Thus, this work concludes that a partial CO2 sequestration by untreated hydrated fly ash could be an effective option for capture and sequestration CO2 from CFB power plant with sorbent injection. References [1] U.S. Department of Energy, A Brief History of Coal Use, February, (2013) . [2] BP, BP Statistical Review of World Energy 2016, British Petroleum, 2016. [3] WCA, Coal Matters, Coal and Electricity Generation, World Coal Association, 2009. [4] IEA, CO2 Emissions from Fuel Combustion, International Energy Agency, 2015. [5] EIA, International Energy Outlook 2016, EIA, 2016 Retrieved from: http:// www.eia.gov/forecasts/ieo/pdf/0484(2016).pdf.. [6] D.M. D'Alessandro, B. Smit, J.R. Long, Carbon dioxide capture: prospects for new materials, Angew. Chem. Int. Ed. 49 (2010) 6058–6082. [7] A. Samanta, A. Zhao, G.K. Shimizu, P. Sarkar, R. Gupta, Post-combustion CO2 capture using solid sorbents: a review, Ind. Eng. Chem. Res. (2012) 1438–1463. [8] IPCC, IPCC Special Report on Carbon Dioxide Capture and Storage, Cambridge University Press, 2005. [9] P. Basu, Circulating Fluidized Bed Boilers: Design, Operation and Maintenance, vol. 2015, Springer International Publishing, Halifax, Canada, 2015. [10] Z. Zhang, J. Qian, C. You, C. Hu, Use of circulating fluidized bed combustion fly ash and slag in autoclaved brick, Constr. Build. Mater. (2012) 109–116.
[11] S.-Y. Pan, E. Chang, P.-C. Chiang, CO2 capture by accelerated carbonation of alkaline wastes: a review on its principles and applications, Aerosol Air Qual. Res. (2012) 770–791. [12] K.J. Reddy, S. Johna, H. Weber, M.D. Argyle, P. Bhattacharyya, Simultaneous capture and mineralization of coal combustion flue gas carbon dioxide (CO2), Energy Procedia (2011) 1574–1583. [13] D.T. Beruto, R. Botter, Liquid-like H2O adsorption layers to catalyze the Ca(OH) 2/CO2 solid gas reaction and to form a non-protective solid product layer at 20C, J. Eur. Ceram. Soc. (2000) 497–503. [14] M. Samari, CO2 Capture from Dilute Sources via Lime-Based Sorbents, Department of Chemical and Biological Engineering; University of Ottawa, Ottawa, 2014. [15] C.Y. Tai, W.-R. Chen, S.-M. Shih, Factors affecting wollastonite carbonation under CO2 supercritical conditions, React. Kinet. Catal. (2005) 292–299. [16] J.C. Abanades, D. Alvarez, Conversion limits in the reaction of CO2 with lime, Energy Fuels (2013) 308–315. [17] K.V. Balen, Carbonation reaction of lime, kinetics at ambient temperature, Cem. Concr. Res. (2005) 647–657. [18] C. Wang, L. Jia, Y. Tan, E. Anthony, Carbonation of fly ash in oxy-fuel CFB combustion, Fuel (2008) 1108–1114. [19] S. Jung, G. Dodbiba, T. Fujita, Mineral carbonation by blowing incineration gas containing CO2 into the solution of fly ash and ammonia for ex situ carbon capture and storage, Geosyst. Eng. (2014) 125–135. [20] G.S. Grasa, J.C. Abanades, CO2 capture capacity of CaO in long series of carbonation/calcination cycles, Ind. Eng. Chem. Res. (2006) 8846–8851. [21] R.R. Tamilselvi Dananjayan, P. Kandasamy, R. Andimuthu, Direct mineral carbonation of coal fly ash for CO2 sequestration, J. Clean. Prod. (2016) 4173– 4182. [22] F. Pontiga, J. Valverde, H. Moreno, F. Duran-Olivencia, Dry gas–solid carbonation in fluidized beds of Ca(OH)2, Chem. Eng. J. 2013 (2013). [23] A. Mazzella, M. Errico, D. Spiga, CO2 uptake capacity of coal fly ash: influence of pressure and temperature on direct gas-solid caronation, J. Environ. Chem. Eng. (2016) 4120–4128. [24] G. Montes-Hernandez, R. Perez-Lopez, F. Renard, J. Nieto, L. Charlet, Mineral sequestration of CO2 by aqueous carbonation of coal combustion fly-ash, J. Hazard. Mater. (2009) 1347–1354. [25] A. González, N. Moreno, R. Navia, CO2 carbonation under aqueous conditions using petroleum coke combustion fly ash, Chemosphere (2014) 139–143. [26] G.N. Muriithi, L.F. Petrik, O. Fatoba, W.M. Gitari, F.J. Doucet, J. Nel, P.E. Chuks, Comparison of CO2 capture by ex-situ accelerated carbonation and in in-situ naturally weathered coal fly ash, J. Environ. Manag. 127 (2013) 212–220. [27] S.-M. Shih, C.-S. Ho, Y.-S. Song, J.-P. Lin, Kinetics of the reaction of Ca(OH)2 with CO2 at low temperature, Ind. Eng. Chem. Res. 38 (1999) 1316–1322. [28] B. Whittington, The chemistry of CaO and Ca(OH) 2 relating to the Bayer process, Hydrometallurgy (1996) 13–35. [29] S.K. Bhatia, D.D. Perlmutter, Effect of the Product Layer on the Kinetics of the Co2-lime Reaction, The American Institute of Chemical Engineers, 1983, pp. 79–86. [30] A. Dedman, A.J. Owen, Calcium cyanamide synthesis, Trans. Faraday Soc. (1962) 2027–2035. [31] A. Silaban, High-temperature High-pressure CO2 Removal from Coal Gas, The Louisiana State University and Agricultural and Mechanical Col., 1993. [32] B. Stanmore, P. Gilot, Review—calcination and carbonation of limestone during thermal cycling for CO2 sequestration, Fuel Process. Technol. (2005) 1707– 1743. [33] D. Mess, A.F. Sarofim, J.P. Longwell, Product layer diffusion during the reaction of calcium, Energy Fuels (1990) 999–1005. [34] P. Sun, J.R. Grace, C.J. Lim, E.J. Anthony, Removal of CO2 by calcium-based sorbents in the presence of SO2, Energy Fuels (2007) 163–170. [35] Z.-S. Li, F. Fang, X.-Y. Tang, N.-S. Cai, Effect of temperature on the carbonation reaction of CaO with CO2, Energy Fuels (2012) 2473–2482. [36] P. Agrinier, A. Deutsch, U. Scharer, I. Martinez, Fast back-reactions of shockreleased CO2 from carbonates: an experimental approach, Geochim. Cosmochim. Acta (2001) 2615–2632.
678
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
[37] Y. Li, S. Buchi, J.R. Grace, C.J. Lim, SO2 removal and CO2 capture by limestone resulting from calcination/sulfation/carbonation cycles, Energy Fuels (2005) 1927–1934. [38] D. Gora, E. Anthony, E. Bulewicz, L. Jia, Steam reactivation of 16 bed and fly ashes from industrial-scale coal-fired fluidized bed combustors, Fuel (2006) 94–106. [39] T. Yang, B. Keller, E. Magyari, Direct observation of the carbonation process on the surface of calcium hydroxide crystals in hardened cement paste using an Atomic Force Microscope, J. Mater. Sci. (2003) 1909–1916. [40] N. Ukwattage, P. Ranjith, M. Yellishetty, H. Bui, T. Xu, A laboratory-scale study of the aqueous mineral carbonation of coal fly ash for CO2 sequestration, J. Clean. Prod. (2015) 665–674.
[41] F. Girard, M. Antoni, S. Faure, A. Steinchen, Influence of heating temperature and relative humidity in the evaporation of pinned droplets, Colloids Surf. A: Physicochem. Eng. Asp. (2008) 36–49. [42] V. Manovic, E.J. Anthony, Carbonation of CaO-based sorbents enhanced by steam addition, Ind. Eng. Chem. Res. (2010) 9105–9110. [43] S.F. Wu, T.H. Beum, J.I. Yang, J.N. Kim, Properties of Ca-base CO2 sorbent using Ca(OH)2 as precursor, Ind. Eng. Chem. Res. (2007) 7896–7899. [44] A.M. Kalinkin, E.V. Kalinkina, O.A. Zalkind, T.I. Makarova, Chemical interaction of calcium oxide and calcium hydroxide with CO2 during mechanical activation, Inorg. Mater. 41 (10) (2005) 1073–1079.
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
An investigation into partial capture of CO2 released from a large coal/ petcoke fired circulating fluidized bed boiler with limestone injection using its fly and bottom ash Anantkumar Patela , Prabir Basua,* , Bishnu Acharyab a b
Deaprtment of Mechanical Engineering, Dalhousie University, Halifax, NS, B3H 4R2, Canada School of Sustainable Design Engineering, University of Prince Edward Island, Charlottetown, PEI, C1A 4P3, Canada
A R T I C L E I N F O
Article history: Received 4 October 2016 Received in revised form 25 December 2016 Accepted 28 December 2016 Available online 29 December 2016 Keywords: CFB fly ash CO2 capture Carbonation Ash utilization
A B S T R A C T
Circulating Fluidized Bed (CFB) power plants have been gaining popularity globally due to its ability to utilize less expensive solid fuels, in-situ capture of SOx, low NOx emission and flexible operating characteristics. Higher CO2 emission from sulfur capturing CFB plants is, however, a major shortcoming of this technology. Additionally, fly ash generated from such CFB power plants is not favorable for many commercial applications. This work examined if the fly ash produced in a CFB power plants with limestone feed can capture a part of the CO2 released from it. It studied factors affecting capture of CO2 utilizing by its ash. Two sets of experiments were carried out for dry and hydrated fly ash at 500–750 C and 30–80 C respectively. Tests showed that CaO conversion increased with temperature up to 700 C for the dry case and 50 C for hydrated case thereafter it started to reduce. Effect of partial pressure of CO2 (17 kPa–31 kPa) on the total carbonation reaction was minor, but, the duration of initial rapid CO2 capture stage was inversely proportional to the CO2 partial pressure. For benchmarking the capture characteristics of lime were compared with that of CFB ash at the same conditions. In most cases carbonation reaction in lime followed a similar pattern as in fly ash. The capture characteristics were also similar for bottom ash (particle size, 116 mm–275 mm). However, the amount of CO2 captured by the coarser bottom ash was significantly lower than that by dry fly ash at the same condition. This study, thus showed the potential of using fly ash from sulfur capturing CFB power plants in reducing the CO2 emission from the very plant and thereby achieve better disposal of the fly ash. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Use of coal as a fuel for steam generation began in 1850s, and it played a major role in the Industrial Revolution of the 19th century [1]. Coal is abundant and has the highest reserve-to-production ratio of all fossil fuels [2]. It is estimated to contribute around 40% of the world’s electricity generation [3], but coal-burning also accounts for about 42% of the global CO2 emission [4]. Additionally, coal plants produce several harmful gases and solid products, such as fly ash, bottom ash, boiler slag, and flue gas desulphurization residues, most of which are, to some extent, hazardous for the environment. Increasing global energy demand could see by 2040 about 78% of the global energy come from fossil fuels [5], which
* Corresponding author. E-mail address: [email protected] (P. Basu). http://dx.doi.org/10.1016/j.jece.2016.12.047 2213-3437/© 2016 Elsevier Ltd. All rights reserved.
could further complicate the issue of climate change and regional pollution worldwide. Various carbon-capture and storage (CCS) technologies focus on a reduction of CO2 emission from fossil fuel power plants. There are three major CO2 capture methods from fossil fuel power plants, pre-combustion, post combustion, and oxy-fuel combustion [6,7]. Capture, compression, transportation and storage of carbon dioxide (CCS) are accepted means of large scale reduction in GHG emission, but these are energy intensive and expensive [6]. For example, the increase in energy requirement for a plant with CO2 storage as mineral carbonates could be as high as 60–180% [8]. Circulating fluidized bed (CFB) combustion is a relatively new technology for control of acid rain gases. It has gained much popularity due to its ability to utilize low-grade fuel or mixtures of fuels (such as coal and petroleum coke), fuel flexibility, in-situ capture of SOx and low NOx emission. However, the emission intensity (for GHG gases CO2 and N2O) of a CFB power plant is higher than a that of a pulverised-fuel (PF) fired power plant
668
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
especially when limestone is added in such boilers to capture SO2 released during combustion. Additional CO2 is released here due to calcination of limestone in its furnace [9]. A major challenge in post-combustion CO2 capture from a typical coal or petcoke fired power plant is the low concentration of CO2 in the flue gas from a CFB boiler, which makes its direct compression, transportation and storage difficult. Thus, it is necessary to explore new cost effective and less energy-intensive practical methods for carbon capture. Interestingly, the ash produced from a CFB power plant firing high sulfur fuels do not find much use in building industries due to its large volumetric expansion when cured at normal temperature [10]. Fly ash and bottom ash from typical CFB power plants contain significant amounts of free CaO, as only 35–40% of the lime feed is utilized in SO2 capture [18]. So, disposal of fly ash from such CFB plants is difficult. This work therefore examines if the unutilized CaO in CFB ash can be utilized through carbonation reaction for partial capture CO2 from flue gas for CFB boiler. Carbonation of alkaline solid wastes such as coal ash containing lime can proceed through two routes: dry (gas-solid) carbonation and wet (aqueous) carbonation [11]. In gas-solid carbonation, carbon dioxide reacts with available CaO in the ash to produce CaCO3 as below: CaO + CO2 = CaCO3
(1)
In aqueous carbonation, CO2 gas reacts with water and produces carbonic acid according to the following gas phase reactions [12,11,13,14]: CO2(g) + H2O
(moisture) ! H2CO3(carbonicacid,aq)
H2CO3(carbonicacid,aq) ! H+ + HCO3 CO32(aq)
+ (bicarbonate) ! 2H (aq) +
(2)
With increase in CO2 partial pressure, its dissolution in water increases [11], but the dissolution is also inversely proportional to temperature [15]. The solid phase reaction is: CaO
+ (s) + H2O (l) ! Ca(OH)2(aq) ! Ca2 (aq) + 2OH
CO32
(3)
+
ions reacts with Ca2 to produce CaCO3(nuclei) which converts into CaCO3(s) Ca2+
2 (aq) + CO3 (aq) ! CaCO3(nuclei) ! CaCO3(s)
(4)
The overall reaction can therefore be written as: Ca(OH)2 + CO2 + H2O ! CaCO3 + H2O
(5)
In addition, there is also the formation of calcium bicarbonate, which gives enhanced CO2 diffusion due to its higher solubility [14]. CaCO3 + CO2 + H2O ! Ca(HCO3)2
(6)
Calcium bicarbonate reacts with calcium hydroxide and forms calcium carbonate ass below: Ca(HCO3)2 + Ca(OH)2 ! 2CaCO3 + 2H2O
(7)
A number of studies were conducted for CO2 capture and CaO utilization in carbonation reaction of fly ash or limestone derived materials [16,17,12,18,19,20,21,22,23]. Reddy et al. [12] conducted
carbonation reaction in fly ash fluidized by flue gas in a pilot scale plant at 30–45 C achieving a reduction in CO2 concentration from 13% to 9% within few minutes. They referred the cost estimation of mineralization of one tonne of CO2 at $11 at mineralization capacity of 207 kg CO2/tonne of fly ash for a 532 MW plant. Two step reactions, first, hydration of fly ash to convert CaO to Ca(OH)2 followed by carbonation reaction were employed by Montes-Hernandez et al. [24] for aqueous carbonation reaction and found that one tonne fly ash (4.1% CaO) captured 26 kg of CO2 at 60–90 C. González et al. [25] observed that aqueous carbonation reaction at a particular temperature is dependent on the amount of water present in the system. Studies on the effect of temperature on wet carbonation revealed that carbonation reaction increased up to 60 C with increase in temperature, followed by reduction. They also mentioned that aqueous carbonation required less energy than dry gas-solid carbonation. There has been some argument about natural carbonation of lime through weathering. Comparison of naturally weathered carbonation and accelerated carbonation (at 4 MPa, 90 C) of fly ash showed that the required time duration for about the same level of carbonation was 20 years for the former compare to 2 h for the latter [26]. Investigation of the effect of relative humidity (RH) on the carbonation of Ca(OH)2 shows that the reaction could not take place below a critical RH (8%) and the influence of RH was higher than that of temperature [27]. Wang et al. [18] and Whittington [28] observed that carbonation of Ca(OH)2 is much faster than carbonation of CaO. The conversion of CaO to Ca(OH)2 depends on factors such as hydration temperature, hydration method, reaction time, and particle size. Only a limited number of studies are available on the carbonation reaction of CFB fly ash and presently there is no information on the carbonation reaction of CFB bottom ash of different particle sizes. Wang et al. [18] investigated carbonation reactions in CFB fly ash in oxy-fuel condition (CO2 concentration 80%), and observed that CaO utilization increased with increase in temperature and below a critical temperature (less than 400 C) there was not any significant capture in dry carbonation experiments. Whereas, addition of steam enhanced the carbonation reaction even at low temperature (250 C). Although Wang et al. [18] studied CO2 capture by dry CFB fly ash in oxy-fired condition above 400 C and in presence of water vapour above 250 C, the reaction at low CO2 partial pressure as expected in air fired CFB boiler, and, at low temperature for hydrated fly ash could differ considerably. Comparison of carbonation reaction characteristics of pure lime and CFB fly ash for similar operating condition could provide more insight into the effectiveness of CO2 capture by CFB fly ash. As untreated CFB fly ash contains items like SO3, CaO, LOI, Al2O3, SiO2, a difference in the reaction characteristics of CFB fly ash and pure lime could exist. González et al. [25] investigated carbonation in aqueous condition at 100% CO2 concentration, however, in typical air fired CFB boiler CO2 accounts for less than 20% [18]. Thus it is necessary to evaluate the effect of partial pressure of carbon dioxide on the carbonation reaction of CFB fly ash. In this, paper we investigated carbonation reaction at CO2 concentration between 16% and 36%. The novelty of this work is, firstly, investigation of CO2 capture by un-treated CFB fly ash at low CO2 partial pressure and at different temperature in dry and hydrated conditions. Secondly,
Table 1 Fly ash composition analysis (Unmeasured carbon and nitrogen accounts for the balance). Element
Al2O3
CaO
Fe2O3
K2O
MgO
Na2O
SO3
SiO2
V2O5
P2O5
LOI
wt (%)
1.70
37.25
0.87
0.32
0.45
0.11
15.88
0.22
0.22
0.05
11.08
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
669
Fig. 1. Fly ash particle size distribution.
comparison of CO2 capture with pure lime for the same experiments. Thirdly, study of carbonation reaction in CFB bottom ash of different particle size. 2. Methodology Ash for the experiments was collected from the 197 MWe Circulating Fluidized Bed (CFB) power plant in Point Aconi, Nova Scotia, Canada. The CFB power plant uses a mixture of petroleum coke and coal as fuel in a ratio of 80:20. Limestone is added for insitu capture of SO2. Fly ash contains 37.25% unutilized CaO from the sulfation reaction in the furnace. Table 1 gives full composition of the fly ash. The size distribution of the fly ash (Fig. 1) shows that 95% of fly ash particle size falls under 120 mm. The experimental setup comprised a metal cylindrical reactor of diameter 25 mm and height 300 mm closed at both
ends, with a provision to accommodate a gas tube and a thermocouple was made for all the tests performed in this study. A schematic of the experimental setup is shown in Fig. 2. The reactor was heated in a bubbling fluidized bed heater. During the experiment, mixtures of nitrogen (N2) and carbon dioxide (CO2) gases were supplied from the bottom end of the reactor. The exit for outlet gas passes through the cap on top of the reactor that also holds a thermocouple to continuously monitor interior temperature of the reactor during experiments. A Testo 350-XL gas analyser continuously analyzed gases coming out of the reactor. A photograph of the experimental setup is shown in Fig. 3. The aim of this study is to investigate the carbonation reaction in fly ash at different conditions and compare that from similar experiments with high calcium quicklime. The high calcium quicklime (termed lime) contained more than 90% CaO, and was
Fig. 2. Schematic diagram of the experimental setup.
670
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678 Table 3 Experimental test conditions for different samples. Temperature ( C)
30 40 50 60 80 500 600 700 750 a
Fig. 3. Photograph of experimental setup.
collected from Graymont Limited, Havelock, New Brunswick, Canada. 2.1. Test conditions Two sets of experiments were conducted in different conditions. First set was at dry conditions for both dry fly ash and dry lime. Other experiments were conducted at hydrated condition for hydrated fly ash and hydrated lime. As the reactor has a fixed volume and the dry and hydrated samples have different densities it was not possible to accommodate the same mass of hydrated lime sample in the reactor. Hydrated lime sample of smaller mass (i.e., 30 g) was selected. Table 2 gives sample preparation information, such as weight of ash/lime and amount of water added. Dry and hydrated test were performed at high temperature (above 500 C) and low temperature (below 80 C), respectively. Table 3 shows the test matrix prepared for this study. For all the experiments, nitrogen flow rate was maintained constant at 0.46 l/ min and the amount of water for hydration was kept constant at liquid to lime ratio of 0.65. 2.1.1. Effect of temperature in dry and hydrates samples Previous study noted that gas solid carbonation reaction in CFB fly ash below 500 C is insignificant even at CO2 concentration as high as 80% [18]. Also, they noted that CO2 capture increased with temperature. So, all experiments to investigate the effect of temperature for dry conditions were conducted at temperature range from 500 to 750 C, at significantly lower CO2 concentration (16–36%) than that investigated by Wang et al. [18]. Fifty grams of un-treated dry fly ash or quicklime sample were spread on quartz wool and placed in the reactor. Quartz wool served as a highly porous support for the sample, with free access to gas from all directions. Nitrogen (N2) was continuously supplied during the experiment, and, on reaching the desired temperature, it was switched to the mixture of CO2 and N2 gases at 0.20 and 0.46 l/min (as volume: 30% and 70%) respectively. Reacted gas leaving the reactor column was continuously cooled in a Graham (coiled type) condenser and continuously analyzed by a gas analyzer. Carbonation of hydrated fly ash is conducted typically below water evaporation temperature to prevent steam from escaping, Table 2 Mass of sample and water mixed for batch experiments. Dry Fly Ash Hydrated Fly Ash Dry Lime Hydrated Lime Sample Weight (g) 50 Water Added (g) 0
50 12
50 0
30 19.5
CO2 = 0.20 l/min (vol: 30%), N2 = 0.46 l/min (vol: 70%) Hydrated fly ash p p p a , p p N/A
Hydrated lime p p p a , p p
Dry fly ash
Dry lime
N/A
p p p , p
a
p p p , p
a
test conducted for CO2 volume concentration: 16%, 22%, 30%, 36%.
and thus avoiding an expensive pressurized carbonation vessel. This less energy intensive choice simplifies industrial processes and makes it cost-effective. So, the carbonation reaction in hydrated fly ash and hydrated lime was conducted at low temperature range (30–80 C). 2.1.2. Experiments at different CO2 partial pressure To investigate the effect of CO2 partial pressure on carbonation reaction, tests were conducted at four CO2 partial pressures at temperature 50 C for hydrated and 700 C for dry case and by keeping all other parameter constant. The CO2 partial pressures were 17, 22, 31 and 37 kPa (Volume: 16%, 22%, 30%, 36%) keeping N2 flow at 0.46 l/min. 2.1.3. Experiments on effect of particle size To investigate the effect of particle sizes on the CO2 capture, bottom ash of average particle sizes 116 mm, 196 mm and 275 mm were selected and the capture results were compared with the same particle sizes of dry lime experiments carried out at similar operating conditions. Operating condition for all the experiments was fixed at temperature 700 C and CO2 partial pressure at 31 kPa. 3. Result and discussion All experiments in this study have shown an initial stage of rapid CO2 conversion into CaCO3 followed by a rather slow reaction stage, when the CO2 capture level of all dry ash samples dropped to 50%, and, after that, it reduced asymptotically. The similar observation has been seen in studies by Bhatia and Perlmutter [29], Dedman and Owen [30], Silaban [31], Stanmore and Gilot [32] and Mess et al. [33]. The initial rapid reaction is essentially a surface reaction of CO2 with CaO, which was kinetically controlled due to the high gas-exterior surface mass transfer. The carbonation reaction produced a product layer of CaCO3 on the exterior surface of available CaO in ash, which restricts further direct contact of CO2 with available CaO beneath it [34–36]. This mass transfer restriction slows down the reaction, and thus, the reaction was controlled by diffusion of CO2 gas into pores to react with CaO. The CO2 diffusion was very slow in the second stage phase of carbonation of lime. The total CO2 capture was found nearly independent of CO2 partial pressures, as also noted in other studies [33,20,31]. 3.1. Effect of temperature 3.1.1. Dry test Fig. 4 shows the percentage capture of the CO2 entering the reactor in the temperature range 500 C to 750 C by the as received dry fly ash from Point Aconi power plant. It is apparent from the figure that initial level of CO2 capture increased with the increase in flue gas temperature up to 700 C. However, after the initial
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
671
100%
CO2 capture (%)
90% 500 °C
80%
600 °C 70%
700 °C 750 °C
60%
50% 40%
0
5
10
15
20
25
30
time (min) Fig. 4. CO2 captured by dry fly ash.
remaining constant at 0.1 kg CO2 at both 600 C and 700 C, then it dropped to the same level as dry fly ash at 750 C. Interestingly this translate into lower (12.8%) conversion of the CaO in quicklime. One notes here lower level of CaO utilization in case of lime. The reduction in capture at 750 C could be because of the reaction approached to the calcination temperature. Li et al. explained that at 10 kPa CO2 partial pressure, change in Gibb’s free energy becomes zero at 720 C and there could not be any carbonation reaction. It could be stated that at 750 C and at 31 kPa CO2 partial pressure change in Gibb’s energy approached to zero and the reaction approached to calcination reaction. Thus, at 750 C very little carbonation reaction took place in the kinetically controlled regime. Interestingly, both test samples showed similar amount of CO2 capture in fast reaction regime at 750 C.
stage of about 10 min, the capture level reduced slowly below 50% capture for all samples. At 750 C there was a reduction in CO2 capture in the rapid reaction regime because above 700 C, the change in Gibbs free energy would be negligible at the modest partial pressure of CO2 of the present experiments, and, hence, the carbonation reaction could not take place [37]. Fig. 5 shows similar experiments with high-calcium quicklime (lime) instead of fly ash. The dry fly ash samples here typically contained 37.25% CaO in comparison with lime sample which had more than 90% CaO. Except for tests at 500 C the capture level at 10th minute for all tests varied between 100 and 76%. At 750 C the drop to 50% capture level, occurred well after 160 min. Similar to the case of dry fly ash, initial amount of CO2 capture by lime increased rapidly with increasing temperature but up to 700 C and thereafter gradually reduced to its steady-state slow reaction stage. The initial kinetically controlled rapid surface reaction stage captured a significant amount of CO2 and thus temperature had important effect (Figs. 4 and 5), but in the subsequent slow diffusion controlled there was not a noticeable effect on temperature on the amount of CO2 capture. Fig. 6 shows the amount of CO2 captured per unit mass of the sample (including CaO and other components of the ash) in the kinetically controlled regime at different temperatures. For dry fly ash, the highest CO2 captured was 0.04 kg per kg sample at 700 C, and the minimum was 0.01 kg per unit sample at 750 C. The highest value translates into 14% conversion of the CaO in ash. The CO2 captured per unit mass of sorbent was higher for dry lime
3.1.2. Hydrated tests Dry carbonation reaction at lower temperature (30–80 C) being negligible, effort was made to conduct tests through hydration as a part of the available CaO in fly ash and lime could be converted to Ca(OH)2 through hydration. However, it was not possible to verify the extent of Ca(OH)2 conversion by measurement of mass change as significant amount of water was taken up in the reaction to form phases such as hydrated aluminosilicates [38]. Hence, for hydrated fly ash experiments, both free CaO or Ca (OH)2 may be expressed as CaO. A reaction mechanism for Ca(OH)2 and CO2 in presence of water vapour [14] showed that CO2 and Ca(OH)2 dissolve in the nano-
100%
CO2 capture (%)
90% 80%
500 °C 600 °C
70%
700 °C 60%
750 °C
50% 40% 0
20
40
60
80
time (min)
100
120
Fig. 5. CO2 captured by 50 g of high calcium quicklime (lime).
140
160
672
CO2 capture/sample (kg/kg)
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
0.120 0.100 0.080 dry fly ash
0.060
quicklime 0.040 0.020 0.000 450
500
550
600
650
700
750
800
temperature (°C) Fig. 6. CO2 captured per unit sample during rapid reaction stage.
Fig. 7. Ca(OH)2 particle carbonation reaction.
750 C) temperature range. It is apparent that CO2 capture level increased with increase in temperature but only up to 50 C. Above 50 C, the capture started to reduce again because the carbonation reaction of Ca(OH)2 depends on the amount of water on the sorbent surface, relative humidity (RH) and temperature. The solubility of CO2 in water reduces (carbonic acid) with rising temperature, but calcium leaching increases in the water layer (equation (3)) [25,40]. So, the CO2 dissolution reduced above 50 C hindering the further carbonation reaction at 0.65 liquid to lime ratio in this study. It is supported by the observation of Ukwattage et al. [40]. The reaction of CO2 and Ca(OH)2 is dependent on the amount of moisture present in the reaction environment. There could be no
droplets of water formed on the surface of sorbent (Fig. 7), precipitating CaCO3 as product layer through carbonic reactions (Equations (2)–(7)). The initial rapid reaction stage could be considered gas-liquid-solid process [39,13]. At the start of the reaction, CO2 reacted with sorbent in presence of water droplets and formed a product layer of CaCO3. This product layer on the surface of sorbent hindered further carbonation reaction resulting in a transition to a slow second stage reaction that was controlled by diffusion of CO2 in the pores. The change in CO2 conversion or capture by hydrated fly ash with time in the lower temperature range of 30 C–80 C is shown in Figs. 8 and 9. The pattern is similar to that for dry fly ash at higher (600–
100%
CO2 capture (%)
90% 80%
30 °C 40 °C
70%
50 °C 60 °C
60%
80 °C 50% 40% 0
10
20
30
time (min)
40
Fig. 8. CO2 captured by 50 g of hydrated fly ash.
50
60
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
673
100%
CO2 capture (%)
90% 30 °C
80%
40 °C 70%
50 °C 60 °C
60%
80 °C 50% 40% 0
30
15
45
60
75
90
time (min) Fig. 9. CO2 captured by 30 g of hydrated lime.
reaction if CO2 and water were not present simultaneously above a particular relative humidity [27,22,39,13]. Shih et al. [27] noted that RH has more dominant effect on carbonation reaction than that by temperature. At the start all samples for hydrated tests had similar moisture/water content (liquid to lime ratio: 0.65); but increase in temperature reduced the RH in the reaction environment. Although increase in temperature had a positive effect on CO2 capture, the reduction in RH depressed the amount of capture. For this reason, one notes (Fig. 10) RH showed dominant effect causing the capture start to reduce. This observation is in agreement with those made by Shih et al. [27] and Girard et al. [41]. The nano-droplets from the sorbent surface evaporated rapidly with temperatures above 50 C, which reduced, the carbonation reaction significantly. Interestingly, transformation of Ca(OH)2 to CaCO3 by carbonation reactions produced additional water layer that catalyzed the ongoing carbonic reaction [22]. Comparing results from dry experiments with those from hydrated cases, one notes that surface carbonation reaction of Ca (OH)2 on hydrated particles helped capture significantly more amount of CO2 than that by CaO in dry fly ash or lime. The addition of water or hydration of the sample extended the first stage reaction, even at low temperature. This observation had also been noted by Manovic and Anthony [42]. They stated that the steam enhanced carbonation via solid state diffusion in the product layer. In case of this study, where temperature is considerably lower than that of Manovic and Anthony [42], water enhanced the first stage reaction and catalyzed the carbonation reaction even at low temperature [13]. This could be also explained
as during carbonation reaction, release of H2O due to decomposition of Ca(OH)2 increased humidity and surface area eventually favoring the capture [43,27]. In addition to this, Ca(OH)2 has higher specific surface area than CaO which enhanced surface carbonation reaction [44]. Fig. 10 shows the amount of CO2 captured by unit mass of hydrated samples at different temperatures during the initial rapid reaction stage. It can be seen that with increase in temperature, CO2 capture increased but declined above 50 C as the RH in reaction environment decreased with the thickness of water droplets. It can be observed for most experiments that transition from an initial rapid kinetically controlled stage to a slower diffusion stage took place rapidly, and the start of this quick transition was considered at when CO2 capture reached 95%. A comparison of CO2 capture in the kinetically controlled regime for dry and hydrated fly ash at different temperature is shown in Fig. 11. Except for one case, the amount of CO2 capture was higher for hydrated samples at similar conditions. 3.2. Effect of partial pressure 3.2.1. Dry test The decline in the amount of CO2 capture after the initial period of rapid kinetic controlled reaction to asymptotically slower CO2 diffusion regime occurred early for higher CO2 partial pressure. Fig. 12 shows that the time required to reach that stage controlled by diffusion of gas into the pores, was around 30 min at the lowest
CO2 capture/hyd. sample (kg/kg)
0.20 0.18 0.16 0.14 0.12 0.10
hydrated fly ash
0.08
hydrated lime
0.06 0.04 0.02 0.00
20
30
40
50
60
70
80
temperature ( °C) Fig. 10. Mass of CO2 captured per unit hyd. sample during rapid reaction stage.
90
674
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
Fig. 11. CO2 captured by of dry and hydrated fly ash during initial rapid carbonation stage (CO2 partial pressure 31 kPa).
100%
CO2 capture (%)
90% 80%
17 kPa
70%
22 kPa
60%
31 kPa 37 kPa
50% 40% 0
10
20
30
40
50
60
time (min) Fig. 12. CO2 captured by dry fly ash at 700 C.
[29,34]. However, CO2 captured in diffusion-controlled regime (slow reaction stage) was independent of CO2 partial pressure (Fig. 13).
partial pressure (17 kPa) while at the highest partial pressure (37 kPa) it was 8 min. This is because the rate of development of CaCO3 layer on particle surface increased with the increase in CO2 partial pressure during the kinetically controlled stage. However, the latter stage, being diffusion controlled, the effect of partial pressure of CO2 on capture was negligible [29,34]. At a given initial time period, a higher amount of CO2 capture was noted for lime (Fig. 13) compared to that for fly ash (Fig. 12). Except for this, the general characteristics of capture in both samples at different partial pressure were similar. The amount of CO2 captured per unit mass of fly ash or lime sample increased with CO2 partial pressure during the first reaction stage (Fig. 14) because the speed of surface carbonation (kinetically controlled) was higher for high CO2 partial pressure
3.2.2. Hydrated test The effect of CO2 partial pressures on CO2 capture by hydrated fly ash and hydrated lime at 50 C is shown in Figs. 15 and 16, respectively. This characteristic was similar to that noted in dry test cases (Fig. 12 and 13). Also, in the diffusion-controlled regime, no effect of CO2 partial pressure was observed. The reasons for higher capture by hydrated samples in comparison with dry samples have been discussed earlier. A comparison of the CO2 captured per unit mass of hydrated sample for different CO2 partial pressure during the initial fast
100%
CO2 capture (%)
90% 80%
17 kPa
70%
22 kPa 31 kPa
60%
37 kPa 50% 40% 0
20
40
60
80
time (min)
100
Fig. 13. CO2 captured by dry lime at 700 C.
120
140
160
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
675
0.16
CO2 capture/sample (kg/kg)
0.14 0.12 0.10 0.08 dry fly ash at 700 °C
0.06 0.04
quicklime at 700 °C
0.02 0.00 15
20
25
30
35
40
CO2 partial pressure (kPa) Fig. 14. CO2 captured by dry fly ash and lime during rapid reaction stage at 700 C.
100%
CO2 capture (%)
90% 80% 70%
17 kPa 22 kPa 31 kPa 37 kPa
60%
50% 40% 0
10
20
30
40
50
time (min)
60
70
80
90
100
Fig. 15. CO2 captured by hydrated fly ash at 50 C.
reaction stage is shown in Fig. 17. Hydrated lime showed a large increase in CO2 capture when its partial pressure increased from 17 kPa to 22 kPa (Fig. 17), but above 30 kPa there was reduction. This could be because of the carbonation of hydrated lime, being an exothermic reaction, released heat that increased reaction temperature and above a certain temperature, the capture started to reduce. This is evident from Fig. 9, where increase in temperature above 60 C, at 30% CO2 concentration, showed significant reduction in capture in the rapid reaction stage. The change in the temperature inside the reactor due to exothermic carbonation reaction, at 50 C in hydrated lime, at different CO2 partial pressure is shown in Fig. 18. It shows that at CO2 partial pressures 17 kPa and 37 kPa, the increase in temperature was around 5 C and 15 C respectively. The reduction in
amount of CO2 captured in rapid reaction stage above 30 kPa partial pressure (Fig. 17) is because of this sudden increase in localised temperature at the reaction environment. 3.3. Effect of particle size Circulating fluidized bed boilers produce two types of ash; fly ash and bed ash. The ratio of production rates of fly ash and bottom ash depends on a number of factors, and this ratio could be as high as 50:50 for high ash coal. Bottom ash is coarse and typically in 200–1000 mm range while the fly ash is much finer in the range of 20–200 mm. In the Point Aconi plant both sizes were relatively small. The average size of fly ash is the range of 20–50 mm (Fig. 1) while that of bed ash was about 300 mm. So, it
100%
CO2 capture (%)
90% 80%
17 kPa
70%
22 kPa 31 kPa
60%
37 kPa
50%
40% 0
20
40
60 time (min) 80
100
Fig. 16. CO2 captured by hydrated lime at 50 C.
120
140
676
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
CO2 capture/sample (kg/kg)
0.25 0.20 0.15 hydrated fly ash at 50 °C
0.10
hydrated lime at 50 °C
0.05 0.00 15
20
25
30
35
40
CO2 partial pressure (kPa) Fig. 17. CO2 captured by hyd. fly ash and hyd. lime during rapid reaction stage.
80
temperature (°C)
70 60
17 kPa 22 kPa
50
31 kPa 40
37 kPa
30 20 0
10
20
30
40
time (min)
50
60
70
80
Fig. 18. Change in reaction temperature during carbonation reaction of lime at 50 C.
is necessary to examine if the coarser bottom ash from a CFB boiler with limestone addition could make any contribution to the CO2 capture. The effect of particle size on the carbonation reaction of bottom ash plotted on Fig. 19 show no notable difference in CO2 capture by bottom ash of any particle size. A relatively small amount of capture in the initial stage was observed for bed ash all sizes of particles, which carbonated under diffusion controlled regime soon after the start of the experiment. After 5 min, CO2 captured by bottom ash reduced to around 50%
(Fig. 19), whereas dry fly ash captured nearly 98% of the CO2 supplied for similar experiments (Fig. 4). Fig. 20 for lime shows that at 19th minute (in the fast reaction stage), CO2 captured by 116 mm, 196 mm and 275 mm lime particles was 100%, 98% and 95% respectively apparently due to smaller particles’ higher pore surface area than larger particles. Similar results have been observed by Samari [14], and, for the same reason, the transition from rapid kinetically controlled regime to slow diffusion controlled regime occurred early for larger particles than smaller particles.
100%
CO2 capture (%)
90% 80% 116 µm 70%
196 µm
60%
275 µm
50% 40% 0
5
10
15
20
25
30
35
40
time (min) Fig. 19. CO2 captured by bottom ash at 700 C and 31 kPa CO2 partial pressure.
45
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
677
CO2 capture (%)
100% 90% 80%
116 um
70%
196 um
60%
275 um
50% 40% 0
10
20
30
40
50
60
70
80
time (min) Fig. 20. CO2 captured by dry lime of different sizes at 700 C.
4. Conclusion This work investigated into the carbonation of untreated CFB ash in dry and hydrated conditions to capture CO2 from CFB power plants with limestone feed. Experiments identified the factors affecting the carbonation reaction in CFB fly ash, and the results were compared with the tests conducted with high calcium quicklime at same operating conditions. Results showed that carbonation reaction in CFB ash and lime follows the same pattern: initial rapid conversion of the sorbent, followed by a slow reaction. Effect of temperature showed that carbon dioxide capture in the fast-reaction regime increased with temperature, but above a certain temperature (700 C for dry cases and 50 C for hydrated cases) the capture reduced. Increase in temperature from 600 C to 700 C in dry fly ash, and, 40 C to 50 C in hydrated fly ash, extended rapid reaction stage by one minute in both the cases. Hydrated fly ash showed similar or even higher capture level than dry fly ash at significantly lower temperature (30–80 C). The rapid initial CO2 capture by fly ash was kinetically controlled and its duration varied inversely with partial pressure of CO2. At the highest partial pressure of 37 kPa, the time required to reach the slow diffusion stage of capture by hydrated fly ash at 50 C was 30 min, whereas it was 90 min at the lowest partial pressure of 17 kPa. At partial pressures, 37 kPa and 17 kPa, the time required by dry fly ash was 30 min and 8 min respectively at 700 C. There was no significant effect of particle size for bottom ash in the size range of 116 mm to 196 mm. The capture level dropped below 60% in 1 min of the start, while for dry fly ash it took 10 min to drop to this level. Thus, this work concludes that a partial CO2 sequestration by untreated hydrated fly ash could be an effective option for capture and sequestration CO2 from CFB power plant with sorbent injection. References [1] U.S. Department of Energy, A Brief History of Coal Use, February, (2013) . [2] BP, BP Statistical Review of World Energy 2016, British Petroleum, 2016. [3] WCA, Coal Matters, Coal and Electricity Generation, World Coal Association, 2009. [4] IEA, CO2 Emissions from Fuel Combustion, International Energy Agency, 2015. [5] EIA, International Energy Outlook 2016, EIA, 2016 Retrieved from: http:// www.eia.gov/forecasts/ieo/pdf/0484(2016).pdf.. [6] D.M. D'Alessandro, B. Smit, J.R. Long, Carbon dioxide capture: prospects for new materials, Angew. Chem. Int. Ed. 49 (2010) 6058–6082. [7] A. Samanta, A. Zhao, G.K. Shimizu, P. Sarkar, R. Gupta, Post-combustion CO2 capture using solid sorbents: a review, Ind. Eng. Chem. Res. (2012) 1438–1463. [8] IPCC, IPCC Special Report on Carbon Dioxide Capture and Storage, Cambridge University Press, 2005. [9] P. Basu, Circulating Fluidized Bed Boilers: Design, Operation and Maintenance, vol. 2015, Springer International Publishing, Halifax, Canada, 2015. [10] Z. Zhang, J. Qian, C. You, C. Hu, Use of circulating fluidized bed combustion fly ash and slag in autoclaved brick, Constr. Build. Mater. (2012) 109–116.
[11] S.-Y. Pan, E. Chang, P.-C. Chiang, CO2 capture by accelerated carbonation of alkaline wastes: a review on its principles and applications, Aerosol Air Qual. Res. (2012) 770–791. [12] K.J. Reddy, S. Johna, H. Weber, M.D. Argyle, P. Bhattacharyya, Simultaneous capture and mineralization of coal combustion flue gas carbon dioxide (CO2), Energy Procedia (2011) 1574–1583. [13] D.T. Beruto, R. Botter, Liquid-like H2O adsorption layers to catalyze the Ca(OH) 2/CO2 solid gas reaction and to form a non-protective solid product layer at 20C, J. Eur. Ceram. Soc. (2000) 497–503. [14] M. Samari, CO2 Capture from Dilute Sources via Lime-Based Sorbents, Department of Chemical and Biological Engineering; University of Ottawa, Ottawa, 2014. [15] C.Y. Tai, W.-R. Chen, S.-M. Shih, Factors affecting wollastonite carbonation under CO2 supercritical conditions, React. Kinet. Catal. (2005) 292–299. [16] J.C. Abanades, D. Alvarez, Conversion limits in the reaction of CO2 with lime, Energy Fuels (2013) 308–315. [17] K.V. Balen, Carbonation reaction of lime, kinetics at ambient temperature, Cem. Concr. Res. (2005) 647–657. [18] C. Wang, L. Jia, Y. Tan, E. Anthony, Carbonation of fly ash in oxy-fuel CFB combustion, Fuel (2008) 1108–1114. [19] S. Jung, G. Dodbiba, T. Fujita, Mineral carbonation by blowing incineration gas containing CO2 into the solution of fly ash and ammonia for ex situ carbon capture and storage, Geosyst. Eng. (2014) 125–135. [20] G.S. Grasa, J.C. Abanades, CO2 capture capacity of CaO in long series of carbonation/calcination cycles, Ind. Eng. Chem. Res. (2006) 8846–8851. [21] R.R. Tamilselvi Dananjayan, P. Kandasamy, R. Andimuthu, Direct mineral carbonation of coal fly ash for CO2 sequestration, J. Clean. Prod. (2016) 4173– 4182. [22] F. Pontiga, J. Valverde, H. Moreno, F. Duran-Olivencia, Dry gas–solid carbonation in fluidized beds of Ca(OH)2, Chem. Eng. J. 2013 (2013). [23] A. Mazzella, M. Errico, D. Spiga, CO2 uptake capacity of coal fly ash: influence of pressure and temperature on direct gas-solid caronation, J. Environ. Chem. Eng. (2016) 4120–4128. [24] G. Montes-Hernandez, R. Perez-Lopez, F. Renard, J. Nieto, L. Charlet, Mineral sequestration of CO2 by aqueous carbonation of coal combustion fly-ash, J. Hazard. Mater. (2009) 1347–1354. [25] A. González, N. Moreno, R. Navia, CO2 carbonation under aqueous conditions using petroleum coke combustion fly ash, Chemosphere (2014) 139–143. [26] G.N. Muriithi, L.F. Petrik, O. Fatoba, W.M. Gitari, F.J. Doucet, J. Nel, P.E. Chuks, Comparison of CO2 capture by ex-situ accelerated carbonation and in in-situ naturally weathered coal fly ash, J. Environ. Manag. 127 (2013) 212–220. [27] S.-M. Shih, C.-S. Ho, Y.-S. Song, J.-P. Lin, Kinetics of the reaction of Ca(OH)2 with CO2 at low temperature, Ind. Eng. Chem. Res. 38 (1999) 1316–1322. [28] B. Whittington, The chemistry of CaO and Ca(OH) 2 relating to the Bayer process, Hydrometallurgy (1996) 13–35. [29] S.K. Bhatia, D.D. Perlmutter, Effect of the Product Layer on the Kinetics of the Co2-lime Reaction, The American Institute of Chemical Engineers, 1983, pp. 79–86. [30] A. Dedman, A.J. Owen, Calcium cyanamide synthesis, Trans. Faraday Soc. (1962) 2027–2035. [31] A. Silaban, High-temperature High-pressure CO2 Removal from Coal Gas, The Louisiana State University and Agricultural and Mechanical Col., 1993. [32] B. Stanmore, P. Gilot, Review—calcination and carbonation of limestone during thermal cycling for CO2 sequestration, Fuel Process. Technol. (2005) 1707– 1743. [33] D. Mess, A.F. Sarofim, J.P. Longwell, Product layer diffusion during the reaction of calcium, Energy Fuels (1990) 999–1005. [34] P. Sun, J.R. Grace, C.J. Lim, E.J. Anthony, Removal of CO2 by calcium-based sorbents in the presence of SO2, Energy Fuels (2007) 163–170. [35] Z.-S. Li, F. Fang, X.-Y. Tang, N.-S. Cai, Effect of temperature on the carbonation reaction of CaO with CO2, Energy Fuels (2012) 2473–2482. [36] P. Agrinier, A. Deutsch, U. Scharer, I. Martinez, Fast back-reactions of shockreleased CO2 from carbonates: an experimental approach, Geochim. Cosmochim. Acta (2001) 2615–2632.
678
A. Patel et al. / Journal of Environmental Chemical Engineering 5 (2017) 667–678
[37] Y. Li, S. Buchi, J.R. Grace, C.J. Lim, SO2 removal and CO2 capture by limestone resulting from calcination/sulfation/carbonation cycles, Energy Fuels (2005) 1927–1934. [38] D. Gora, E. Anthony, E. Bulewicz, L. Jia, Steam reactivation of 16 bed and fly ashes from industrial-scale coal-fired fluidized bed combustors, Fuel (2006) 94–106. [39] T. Yang, B. Keller, E. Magyari, Direct observation of the carbonation process on the surface of calcium hydroxide crystals in hardened cement paste using an Atomic Force Microscope, J. Mater. Sci. (2003) 1909–1916. [40] N. Ukwattage, P. Ranjith, M. Yellishetty, H. Bui, T. Xu, A laboratory-scale study of the aqueous mineral carbonation of coal fly ash for CO2 sequestration, J. Clean. Prod. (2015) 665–674.
[41] F. Girard, M. Antoni, S. Faure, A. Steinchen, Influence of heating temperature and relative humidity in the evaporation of pinned droplets, Colloids Surf. A: Physicochem. Eng. Asp. (2008) 36–49. [42] V. Manovic, E.J. Anthony, Carbonation of CaO-based sorbents enhanced by steam addition, Ind. Eng. Chem. Res. (2010) 9105–9110. [43] S.F. Wu, T.H. Beum, J.I. Yang, J.N. Kim, Properties of Ca-base CO2 sorbent using Ca(OH)2 as precursor, Ind. Eng. Chem. Res. (2007) 7896–7899. [44] A.M. Kalinkin, E.V. Kalinkina, O.A. Zalkind, T.I. Makarova, Chemical interaction of calcium oxide and calcium hydroxide with CO2 during mechanical activation, Inorg. Mater. 41 (10) (2005) 1073–1079.