Optimizing Synergy between Phosphogypsum Disposal and Cement

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Optimizing Synergy between Phosphogypsum Disposal and Cement Plant CO2 Capture by the Calcium Looping Process Jian Sun, Wenqiang Liu,* Wenyu Wang, Yingchao Hu, Xinwei Yang, Hongqiang Chen, Yang Zhang, Xian Li, and Minghou Xu* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China ABSTRACT: In this work, a novel joint processing route that integrates the disposal of phosphogypsum waste with CO2 emissions reduction in a cement plant was proposed. The route mainly includes three parts: direct aqueous carbonation of phosphogypsum, use of the obtained carbonation product for CO2 capture in the calcium looping process (CLP), and manufacture of cement clinker using the spent CaO-based sorbent. The direct use of the CO2 derived from cement plant flue gas (20 vol % CO2) is able to convert 94.5% of CaSO4 in the phosphogypsum into CaCO3. However, a long time of 90 min is required for the completion of the conversion. Therefore, we proposed to introduce a part of the highly concentrated CO2 gas stream separated from the CLP and, hence, to increase the overall CO2 concentration of the carbonation gas stream. It was found that only 45 min is needed to achieve a comparable carbonation level when the gas stream containing 45 (or 60) vol % CO2 was used. Moreover, the solid carbonation residues derived from phosphogypsum carbonation possess relatively good cyclic CO2 capture performance, which is superior to the CaCO3 reagent. It indicates that the joint processing route proposed here is feasible, which can not only recycle the phosphogypsum waste but also reduce the CO2 emissions in the cement plant. the flue gas and a calciner for the regeneration of CaO-based sorbent.16−19 Meanwhile, in the calciner, the highly concentrated CO2 gas stream is produced in an oxy-fuel combustion process for compression and sequestration.20−22 The application of CaO-based CLP technology can avoid complex process retrofitting for a cement plant because of its technical compatibility with cement production operating conditions.12 Moreover, the spent sorbent removed from the CLP can be used as the raw material for cement clinker production,23,24 greatly reducing the cost of raw materials. Comprehensively considering the disposal of phosphogypsum waste and CO2 emissions reduction of the cement industry, a novel concept of integrating phosphogypsum disposal with cement plant CO2 removal is proposed. As shown in Figure 1, the joint processing route can be divided into three major parts: (1) the direct aqueous carbonation of phosphogypsum waste, (2) the CO2 removal of cement plant flue gas by a CLP using the obtained solid carbonation residues in step 1, and (3) the manufacture of cement clinker using the spent sorbent purged from CLP. Moreover, the joint processing route may be more applicable for the energy-intensive industrial parks containing both phosphate fertilizer plants and cement plants, largely reducing the cost of transportation. The principle of phosphogypsum carbonation is based on the reaction CaSO4·2H2O(s) + 2NH4OH(l) + CO2(g) → CaCO3(s) + (NH4)2SO4(aq), which refers to the previous work where the mineral carbonation of flue gas desulfurization gypsum was investigated.25 Moreover, the way also could be used in the preparation of CaO-based sorbent for CO2 capture

1. INTRODUCTION Phosphogypsum (PG) is the byproduct from the wet-acid process for phosphoric acid production, which is mainly composed of calcium sulfate dihydrate (CaSO4·2H2O) with high contents of impurities such as soluble P2O5, F, and heavy metals.1,2 About 5 t of PG are obtained in the manufacture of 1 t of phosphoric acid.3 In 2013, over 70 million t of phosphogypsum were produced in China, whereas the comprehensive utilization rate was merely 27%. Currently, phosphogypsum is mainly recycled as a cement retarder, loadbearing building material, soil stabilizer, and agriculture fertilizer.4−7 However, the remaining unrecycled waste phosphogypsum is landfilled, which will cause farmland occupation and severe environmental pollution to the areas surrounding landfill sites.8 Therefore, it is urgent to implement efficient disposal of phosphogypsum waste aiming to ensure sustainable development of the phosphate industry. On the other hand, CO2 emission from cement production process accounts for 5−7% of the world’s total CO2 emissions; hence, there is a huge pressure to reduce CO2 emission in the cement industry.9−11 Precombustion, oxy-combustion and postcombustion are the three commonly used strategies for CO2 capture, and postcombustion CO2 capture technologies are considered to be the most suitable for application to the cement industry.12 Particularly, the calcium looping process (CLP) is regarded as the most promising technology for CO2 capture in the cement industry in comparison with other postcombustion CO2 capture technologies, such as membrane separation and MEA absorption.13−15 The principle of CLP for CO2 capture is based on the reversible carbonation/decarbonation reaction of CaO-based sorbent. The CaO-based sorbent is continuously circulated between the two interconnected fluidized-bed reactors, a carbonator for CO2 removal from © XXXX American Chemical Society

Received: November 30, 2015 Revised: January 7, 2016

A

DOI: 10.1021/acs.energyfuels.5b02786 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Specific novel process flow diagram of integrating phosphogypsum disposal with cement plant CO2 capture.

2. EXPERIMENTAL SECTION

and composite catalyst for sorption-enhanced steam−methane reforming.26 It was found that the prepared CaO-based sorbent displays favorable CO2 sorption capacity and the catalyst used in sorption-enhanced steam−methane reforming can help to produce hydrogen with high purity (>90 vol %). In the joint processing route, the desulfurization exhaust of a cement plant is used as the main source for CO2. As mentioned above, the obtained solid carbonation residues are used as the sorbent to remove CO2 from the cement plant flue gas, and the byproduct, (NH4)2SO4, is a valuable fertilizer that provides both nitrogen and sulfur for growing plants.27 Most of the spent sorbent purged from the calciner is used for cement clinker manufacture, and the feasibility of the plan has been confirmed by researchers.28,29 The remaining spent sorbent is used to remove the sulfur compounds in cement flue gas, and the obtained desulfurization gypsum can be reused after carbonation processing. The typical CO2 concentration of cement plant flue gas is around 14−33 vol %.30 However, it is known that a faster conversion rate for phosphogypsum into CaCO3 can be obtained, when a higher CO2 concentration is applied.31 To increase the overall CO2 concentration of the carbonation gas stream, a part of the highly concentrated CO2 stream (red dotted line in Figure 1) separated from the CLP can be introduced. Therefore, the carbonation rate, particle size distribution, and microstructure of the solid carbonation residues derived from phosphogypsum with various CO2 concentrations (20, 45, 60, and 100 vol %) were studied in this work. Moreover, the cyclic CO2 capture performance of the obtained solid residues was also investigated because it is vital to evaluate the feasibility of the joint processing route.

2.1. Raw Materials. The waste residue of phosphogypsum was from a phosphate fertilizer plant located in Tongling of the Anhui province, China. After being dried at 45 °C for 12 h, the powders that passed through the 220 μm sieve were collected. An ammonia reagent containing 25−28 wt % NH3 (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) was used as the alkaline solution. 2.2. Direct Aqueous Carbonation Experiments. As shown in Figure 2, the direct aqueous carbonation experiments were conducted

Figure 2. Reactor for direct aqueous phosphogypsum carbonation. via injecting the CO2/N2 gas mixture into the phosphogypsum slurry (containing 200 g of phosphogypsum). Here, the solid-to-solution (15%) and ammonia content (120%) were chosen according to reported optimal parameters in previous work.31 The focus of the work was to investigate the effect of CO2 concentration on the carbonation of phosphogypsum. Therefore, the constant CO2 flow rate of 2.5 L/ min with four different CO2 concentrations (20, 45, 60, and 100 vol %) was implemented via adjusting the flow rate of N2. The pH of the slurry was monitored through precise pH test paper strips during the carbonation process. Then, about 15 mL of slurry was sampled by syringe after 5, 15, 20, 30, 45, 60, and 90 min and was filtered using a vacuum suction filter. About 200 mL of deionized water was used to B


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Energy & Fuels wash the filter residue, aiming to remove other ionic impurities. Finally, the residues were dried overnight at 105 °C and subjected to further chemical analysis. 2.3. Cyclic CO2 Sorption Testing. The cyclic CO2 sorption performance of the solid carbonation residues from phosphogypsum was tested in a Pyris 1 TGA (PerkinElmer). The reaction temperature, heating rate, residence time, and reaction gas switching were controlled by the Pyris software. In each test, a sample of ∼20 mg was placed in a platinum sample pan hung in a quartz tube. Under a N2 atmosphere of 100 mL/min, the sample was first heated to 850 °C at a rate of 20 °C/min and maintained for 10 min to ensure that the contained CaCO3 was calcined completely. Then, the cyclic carbonation/calcination tests were carried out. The sample was cooled to 650 °C at a rate of −25 °C/min, and the gas was switched to 15 vol % CO2 (maintaining a constant total flow rate of 100 mL/min) immediately when the temperature reached 650 °C. The sample was subjected to carbonation for 30 min at this condition. Next, the sample was reheated to 850 °C under a N2 flow of 100 mL/min and kept there for 2 min to regenerate the CaO. Then, a complete carbonation/ calcination cycle was accomplished, which was repeated 19 times to investigate the CO2 sorption performance of the sample. It is worth recognizing that the 30 min carbonation is unrealistic for the fluidizedbed-based reactors, but it is the most typical carbonation time under laboratory test conditions in order to study comprehensively the CO2 uptake of sorbent, to understand better the two-stage reactions of sorbent and CO2. The CO2 capture capacity (Cn, g of CO2/g of calcined sorbent, g/g) and CaO carbonation conversion (Xn, %) of sorbent are defined by the following formulas (eqs 1 and 2)32 m − mo Cn = n mo (1)

Xn =

(mn − mo)MCaO × 100% moφMCO2

Figure 3. XRD pattern of raw phosphogypsum.

particle diameter [d(0.5)] of phosphogypsum is 77.038 μm, and over 65% of the particles are smaller than 100 μm. 3.2. Effect of CO2 Concentration on Phosphogypsum Carbonation. Investigating the effect of CO2 concentration on the carbonation of phosphogypsum would be helpful for the practical application of the novel joint processing route. Therefore, the direct aqueous carbonation of phosphogypsum was carried out using a gas stream containing different CO2 concentrations. The relative mass fraction ratios of S and Ca (S/Ca) in the final solid carbonation residues are obtained according to the XRF data, as depicted in Figure 4. A sharp reduction of the S/Ca ratio was observed with the increase of the carbonation duration, and the ratio (∼3%) remained constant after a certain time, indicating the termination of carbonation. It was also found that a shorter time is required to complete the carbonation of phosphogypsum under higher CO2 concentration. During the process of the carbonation of phosphogypsum, the decrease of S content in solid carbonation residue indicates that the SO42− is dissolved into the solution.33 Therefore, we can simply speculate the mechanism of direct aqueous carbonation of phosphogypsum, as shown in Figure 5. The Ca2+ diffuses from the surface of the phosphogypsum particle and is dissolved into the solution. Then, the CO2 gas was injected into the phosphogypsum slurry to react with the ammonia solution, forming CO32−.34 The precipitation of CaCO3 occurs via combining Ca2+ with CO32−, and thus more SO42− is dissolved in order to maintain the ionization equilibrium in the solution. The conversion rate of phosphogypsum (Wc, %) is defined as the ratio of the actual purity and the theoretical purity of CaCO3 in the carbonation products, which is an intuitive parameter to evaluate the carbonation degree of phosphogypsum. The specific calculation formula is as follows (eq 3)

(2)

where mn is the mass of the carbonated pellets in the nth cycle; mo is the mass of the initial calcined sorbent; φ is the mass fraction of CaO in the initial calcined sorbent, which is calculated through the mass loss for the decomposition of the CaCO3 contained in the testing sorbent in the first cycle; and MCaO and MCO2 are the molar weight of CaO and CO2. 2.4. Characterization Testing. The compositions of the raw phosphogypsum and solid carbonation residues were analyzed using a focused-beam X-ray fluorescence (XRF) spectrometer (model EAGLE III). An X-ray diffractometer (XRD, model X’Pert PRO) was used to analyze the phase compositions of the samples, and their particle size distributions were measured by a laser particle size analyzer (MALVERN, model Master Min), using water as the dispersant. The microstructures of the fresh and cycled solid carbonation residues were scanned by a field emission scanning electron microscope (FSEM, model Nova NanoSEM 450) and a scanning electron microscope (SEM, model JSM-6510LV). In order to obtain better electronic signals, the samples were coated with a layer of platinum prior to image acquisition. The specific BET surface area of the calcined samples was tested using a surface area analyzer (Micromeritics, model ASAP 2020). The method of N2 adsorption and desorption isotherm was used to analyze the specific surface area and the average pore width of the sorbent.

Wc =

3. RESULTS AND DISCUSSION 3.1. Analysis of Phosphogypsum. As shown in Figure 3, the major constituent of phosphogypsum is calcium sulfate dihydrate (CaSO4·2H2O), and silica (SiO2) is the minor phase. The XRF analysis results of phosphogypsum are shown in Table 1, and the content of combined water in the phosphogypsum was tested according to Chinese standard methods for chemical analysis of gypsum (GB/T5484-2000). Therefore, the purity of CaSO4·2H2O in the phosphogypsum was 85.09%, calculated by the stoichiometric ratio. The median

ΔWMCaCO3 PM t CO2

× 100% (3)

Where Pt (%) is the theoretical purity of CaCO3 in the carbonation product, which is calculated by assuming that all the CaSO4 in the phosphogypsum was converted into CaCO3. The Pt is 76.84% for the raw phosphogypsum containing 85.09% of CaSO4·2H2O.

ΔWMCaCO3 MCO2

is the actual purity of CaCO3

in the carbonation products, ΔW is the weight loss percentage of carbonation product due to the decomposition of CaCO3, C


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Energy & Fuels Table 1. XRF Analysis of Raw Phosphogypsum component wt %

CaO 30.59

SO3 44.74

Al2O3 0.62

SiO2 4.52

Fe2O3 0.32

P2O5 1.14

K2O 0.23

combined water 17.81

Figure 4. Relative mass fraction ratios between S and Ca in the solid carbonation residues with different CO2 concentration.

and MCaCO3 and MCO2 are the molar weights of CaCO3 and CO2. As shown in Figure 6b, it was found that the carbonation reaction rate becomes faster with the increase of CO 2 concentration. For instance, over 94% of the phosphogypsum was converted to carbonate calcium in approximately 30 min when 100 vol % CO2 was used, while 90 min was needed to achieve the comparable level when using 20 vol % CO2. Similar results were found by Lee et al.,31 who investigated the mineral carbonation of flue gas desulfurization gypsum. They believed that the slow reaction rate at the lower CO2 concentration might be attributed to the lower temperature of the slurry. The generated exothermic energy during the carbonation process is eliminated by the N2 escaping from the slurry when low CO2 concentration is used, thus resulting in the low slurry temperature. Additionally, the pH value of the slurry gradually decreases with the prolongation of carbonation time, as shown in Figure 6a. It is found that at the 90 min carbonation, the

Figure 6. Effect of CO 2 concentration on phosphogypsum carbonation: (a) pH of slurry and (b) carbonation rates of product.

conversion rate of phosphogypsum remains almost stable and the pH value of the slurry is maintained between 6.0 and 7.0. Therefore, it is easy to determine the completion node of carbonation via the method of pH monitoring. The feasibility of directly using the desulfurization exhaust from a cement plant (20 vol % CO2) for the carbonation of phosphogypsum (the conversion rate of phosphogypsum is up to over 94.5% after 90 min) has been proved. However, it takes a long time (90 min) to achieve the high conversion rate of phosphogypsum, which is not desirable for practical applica-

Figure 5. Schematic diagram of the mechanism of direct aqueous carbonation of phosphogypsum. D


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Energy & Fuels tion. Therefore, it is necessary to increase the CO 2 concentration, aiming to shorten the carbonation duration. In our novel concept, higher CO2 concentration can be obtained via introducing a part of the highly concentrated CO2 stream separated from the CLP. Further, the cyclic CO2 sorption performance of solid carbonation residues used in the CLP is closely related to their CaCO3 content. Also, the high content of CaSO4 in the solid carbonation residues may deteriorate the property of cement that is produced from the spent sorbent.35 Therefore, to ensure a high content of CaCO3 and a low content of CaSO4 in the solid carbonation residues, the carbonation duration should not be shorter than 60 min for 20 vol % CO2 and 45 min for 45 and 60 vol % CO2, while 30 min is enough for 100 vol % CO2 (as shown in Figure 6b). 3.3. Characterization of Solid Carbonation Residue. The X-ray diffractogram of the solid carbonation residues (20 vol % CO2) with different carbonation durations is shown in Figure 7. It is clear to see that the peaks of gypsum and

Figure 8. XRD patterns of solid carbonation residues with different CO2 concentrations for 30 min carbonation (g, gypsum; b, bassanite; s, silica; c, calcite; v, vaterite).

shape is observed in the higher magnification micrographs (50 000×) of the solid carbonation residues in the case of using 20 vol % CO2. More calcite was observed at a longer carbonation time of 60 min, because of its higher carbonation rate. However, the carbonation product using 100 vol % CO2 for 30 min carbonation shows cluster shapes (Figure 9f,i), being significantly different from the solid carbonation residues using 20 vol % CO2. Additionally, the microstructure of the carbonation product using 100 vol % CO2 is more porous, and smaller grains were observed. The laser particle size analysis results of the raw phosphogypsum and the solid carbonation residues obtained under different carbonation conditions are depicted in Figure 10. Under the constant CO2 concentration of 20 vol %, about 88.7 vol % of the carbonation product particles are less than 20 μm after 90 min carbonation, while only 65.5 vol % of the particles are less than 20 μm after 30 min carbonation (Figure 10a). As shown in Figure 10b, the raw phosphogypsum has a clearly discernible peak near 100 μm, which gradually becomes smaller with the prolongation of carbonation duration. This is in accordance with the FSEM results shown in Figure 9. The CO2 concentration also affects the particle size distribution during the same carbonation duration of 30 min. Moreover, the CO2 concentration is also closely related to the particle size distribution of the obtained carbonation products, as shown in Figure 10c,d. The median particle diameter [d(0.5)] of the solid carbonation residues is reduced from 13.778 to 5.186 μm when the CO2 concentration increases from 20 to 60 vol %. However, it is interesting to find that the median particle diameter increases to 8.529 μm when further increasing the CO2 concentration to 100 vol %. The exorbitant CO2 concentration causes an extraordinarily fast carbonation reaction, which easily leads to the crystal nucleus aggregation growth, accelerating the formation of bulky grains and thus increasing of median particle diameter. 3.4. Cyclic CO2 Sorption Performance of Solid Carbonation Residue. The feasibility of the above joint processing route also relies on the CO2 sorption performance of the products derived from the direct aqueous carbonation of phosphogypsum. Therefore, the cyclic CO2 capture capacities of the solid carbonation residues derived from phosphogypsum carbonation using 20 vol % CO2 for various durations (30, 60, and 90 min) were investigated in a TGA. The solid carbonation

Figure 7. XRD patterns of solid carbonation residues with different carbonation durations using 20 vol % CO2 (g, gypsum; b, bassanite; s, silica; c, calcite; v, vaterite).

bassanite gradually disappear with the increase of carbonation durations. Meanwhile, the amounts of calcite and vaterite (polymorphs of CaCO3) increase, which is consistent with the conversion rate of phosphogypsum shown in Figure 6b. The emergence of vaterite peaks with prolonged carbonation duration (≥45 min) may be ascribed to the higher CO32− concentration in the phosphorus gypsum slurry.36 Moreover, higher CO2 concentration promotes the formation of calcium carbonate within the same carbonation durations of 30 min, as shown in Figure 8. Correspondingly, the obvious peaks of gypsum are observed when using 20 vol % CO2, while the gypsum phase disappears completely in the case of 100 vol % CO2. The morphological changes of the solid carbonation residues with different carbonation durations and CO2 concentrations are shown in Figure 9. Clearly, the direct aqueous carbonation (30 min) causes an obvious size reduction of phosphogypsum particles, as observed by comparing part a with parts b and c of Figure 9. Moreover, the original smooth surface of the phosphogypsum particles, being tabular in shape, becomes coarse and irregular. Calcite with a cubic or rhombohedral E


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Figure 9. FSEM images of raw phosphogypsum and solid carbonation residues under different carbonation conditions.

designated as PG20-T90, and the other samples are also named following the rule. It is found that the solid carbonation residues experiencing longer carbonation durations display better cyclic CO2 capture performance (Figure 11). As discussed, more active CaCO3 can be converted from phosphogypsum with prolonged carbonation durations, and hence higher CO2 capture capacity can be obtained. For instance, PG20-T90 (90 min carbonation) shows the highest carbonation conversion (Xn) of 72.04% in the first cycle, in comparison with 55.84% for PG20-T30 (30 min carbonation).

Figure 10. Laser particle size analysis of raw phosphogypsum and solid carbonation residues under different carbonation conditions.

residues used for CO2 uptake testing are denoted as PG#-Tm, where # refers to the CO2 concentration of the gas stream for phosphogypsum carbonation and m refers to the carbonation duration. For instance, a sample derived from the phosphogypsum carbonation under 20 vol % CO2 for 90 min is

Figure 11. (a) Carbonation conversion of solid carbonation residues derived from phosphogypsum carbonated under 20 vol % CO2 for various durations and (b) 30 min carbonation conversion profiles in the first cycle. Calcination, 2 min at 850 °C in 100 vol % N2; carbonation, 30 min at 650 °C in 15 vol % CO2. F


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reagent, which is particularly apparent in the first few cycles. Particularly, the CO2 sorption capacities of the PG60-T45 and PG100-T30 are also comparable to that of pure CaCO3 reagent in the first 10 cycles (Figure 12b). Although they possess lower CO2 capacities than pure CaCO3 reagent (0.174 g/g) after 19 cycles, the lowest of their capacities is over 0.1 g/g, which is still useful for CO2 capture in the CLP. The analysis results of N2 adsorption−desorption isotherms, pore size distributions, and specific surface area of the three calcined solid carbonation residues are displayed in Figure 13. Three well-distinguished regions of the adsorption−desorption isotherms can be found: a linear increase of the adsorbed volume at a low relative pressure due to monolayer adsorption on the outer surface (A), a horizontal transition stage at a medium relative pressure (B), and a sharp increase in the adsorbed volume at a high relative pressure due to multilayer adsorption (C).47 The results reflect that the calcined solid carbonation residues are nonporous. From Figure 13b,c, it can be seen that the three samples display a similar pore size distribution and specific surface area, which is responsible for their similar carbonation conversions in the first few cycles. Moreover, the average crystallite sizes of CaO were calculated with the Scherrer’s equation using the most intense peak of CaO appearing at 37.4°,48,49 as shown in Figure 14. It is found that the average CaO crystallite size of the above three sorbents has no obvious difference, all being around 85 nm. This is in agreement with their similar carbonation conversions during 19 cycles. 3.5. Discussion. The CO2 concentration plays an important role in the carbonation of phosphogypsum. Although higher CO2 concentration is beneficial to shorten the carbonation duration, we found that a carbonation duration ranging from 30 to 90 min is still needed for the complete carbonation of phosphogypsum under different CO2 concentration (20−100 vol %). Therefore, the production of solid carbonation residues would not be a continuous process, while the production of cement is a continuous flow process. Actually, during a realistic operation, a certain amount of fresh limestone is supplemented in order to ensure the continuous operation of the joint processing route, as shown in Figure 1. Moreover, the major composition of spent sorbent is CaO, which would largely reduce the energy consumption during the precalcination process to decompose the carbonate minerals during cement production. From the above results, we can infer that the joint processing route integrating phosphogypsum disposal with CO2 removal in a cement plant is sustainable and environment-friendly. An economic evaluation of the cost of the raw materials would be helpful for the practical application of the route; thus, a rough cost estimation has been done here. We assumed 60 vol % CO2 for a carbonation duration of 45 min, 15% of solid-to-solution and 120% of ammonia content are used in the phosphogypsum carbonation process, and the final conversion rate of phosphogypsum is 97.65% (Figure 6). In Table 2, the unit prices of raw materials were derived from a chemical materials supply Web site (chem.1688.com) in November 2015. In order to ensure the rationality of the rough cost estimation, a price range of each kind of raw material was found and adopted. It is found that producing 1 t of solid carbonation residues needs 1.54 t of phosphogypsum, 1.38 t of ammonia, 8.90 t of industry water, and 0.33 t of CO2, while 0.98 t of (NH4)2SO4, an agricultural fertilizer, could be obtained as the byproduct. Therefore, the net cost of the raw materials to produce 1 t of

However, the CaSO4 is almost completely converted into CaCO3 when the carbonation duration is over 60 min, and therefore, the enhancement of the cyclic CO2 capture capacity is very limited because of their similar carbonation rate. For instance, PG20-T60 (60 min carbonation) displays a comparable CO2 sorption performance with PG20-T90 over 19 cycles. Similar to traditional calcium sorbent from naturally occurring limestone, the CO2 capture performance of the solid carbonation residues gradually decreases with the increase of cycle number. Sorption capacity loss is a common problem for CaO-based sorbents during cyclic carbonation/calcination cycles, and the sintering of sorbent is responsible for this phenomenon.37−40 Particularly, higher calcination temperature would cause severe decay of cyclic CO2 sorption performance.41−43 In the first cycle, a typical solid/gas reaction is found during the 30 min carbonation process, as shown in Figure 11b. There is a rapid increase in carbonation conversion in the first 2−3 min, which is controlled by the chemical reaction between CaO and CO2. After the fast stage, the increase of conversion becomes slow because of the formation of the CaCO3 product layer blocks the pore of the sorbent.44−46 As can be seen from the above discussions, the solid carbonation residue with 90 min phosphogypsum carbonation time owns the best cyclic performance for capturing CO2, but 90 min may be too long for practical application. Hence, it is necessary to investigate the solid carbonation residue with high CO2 capture performance in shorter carbonation durations, which could be achieved through using higher CO 2 concentration during phosphogypsum carbonation, as shown in Figure 12. It is found that the cyclic CO2 sorption performance of both PG60-T45 and PG100-T30 are comparable to or better than that of PG20-T90 over 19 cycles. Moreover, the carbonation conversions of all the solid carbonation residues are higher than that of pure CaCO3

Figure 12. (a) Carbonation conversion and (b) CO2 capture capacity of solid carbonation residues derived from phosphogypsum under different carbonation conditions. The testing conditions are as same as for Figure 11. G


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Figure 13. (a) N2 adsorption and desorption isotherms, (b) pore size distributions, and (c) specific surface area of PG20-T90, PG60-T45, and PG100-T30.

in the phosphogypsum into CaCO3 when using the higher CO2 concentration, and the final carbonation rate can be nearly up to 95%. Moreover, the solid carbonation residues show a relatively good cyclic CO2 sorption performance, indicating its suitability to be used in the CLP. The joint processing route is very economical for practical application because the used raw materials are phosphogypsum waste and the spent sorbent purged from the CLP can be used for cement clinker production.

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AUTHOR INFORMATION

Corresponding Authors

*W.L.: tel, +86 027 87542417-8301; fax, +86-27-87545526; email, [email protected]. *M.X.: tel, +86 027 87542417-8301; fax, +86-27-87545526; email, [email protected].

Figure 14. XRD patterns of PG20-T90, PG60-T45, and PG100-T30 after initial calcination.

solid carbonation residues for CO2 capture in the CLP is around $89−107. The cost could be cut further when recycling of the excess ammonia could be achieved. Comprehensively considering the environmental cost of phosphogypsum disposal, the reduction of limestone usage, and the energy saving for carbonate decomposition when using the spent sorbent (mainly CaO) as the raw materials for cement clinker production, the superiority of the joint processing route was particularly prominent.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (51306063 and 21306059), the Ph.D. Program Foundation of the Ministry of Education of China (20130142120047), and the Foundation of State Key Laboratory of Coal Combustion (FSKLCCB1602) is appreciated. Also, the authors acknowledge the support of Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education and the Analytical and Testing Center at the Huazhong University of Science and Technology.

4. CONCLUSIONS It is feasible to integrate the direct aqueous carbonation disposal of phosphogypsum waste with the CO2 removal of cement plant flue gas via a calcium looping process. A part of the highly concentrated CO2 stream separated from the CLP was introduced into the desulfurization exhaust from a cement plant (20 vol % CO2) to increase the overall CO2 concentration of the carbonation gas stream. It is found that the shorter carbonation duration is needed to convert the CaSO4 contained

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REFERENCES

(1) Yang, X.; Zhang, Z.; Wang, X.; Yang, L.; Zhong, B.; Liu, J. Thermodynamic study of phosphogypsum decomposition by sulfur. J. Chem. Thermodyn. 2013, 57, 39−45.

Table 2. Raw Material Costs of Production of Solid Carbonation Residue raw materials

byproduct solid carbonation product a

PG ammonia (>25%) industrial water CO2 (NH4)2SO4 PG60-T45

unit pricea ($/t)

amount (t/t PG60-T45)

cost ($/t PG60-T45)

3−4 100−155 0.55 − 60−120 −

1.54 1.38 8.90 0.33 0.98 1

∼5−6 ∼138−214 ∼5 − ∼59−118 ∼89−107

The unit prices of raw materials were derived from a chemical materials supply Web site (chem.1688.com) in November 2015. H


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Optimizing Synergy between Phosphogypsum Disposal and Cement

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