Integration of Calcium and Chemical Looping Combustion using

ARTICLE pubs.acs.org/est

Integration of Calcium and Chemical Looping Combustion using Composite CaO/CuO-Based Materials Vasilije Manovic and Edward J. Anthony* CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1 ABSTRACT: Calcium looping cycles (CaL) and chemical looping combustion (CLC) are two new, developing technologies for reduction of CO2 emissions from plants using fossil fuels for energy production, which are being intensively examined. Calcium looping is a two-stage process, which includes oxy-fuel combustion for sorbent regeneration, i.e., generation of a concentrated CO2 stream. This paper discuss the development of composite materials which can use copper(II)-oxide (CuO) as an oxygen carrier to provide oxygen for the sorbent regeneration stage of calcium looping. In other words, the work presented here involves integration of calcium looping and chemical looping into a new class of postcombustion CO2 capture processes designated as integrated CaL and CLC (CaL
CLC or Ca
Cu looping cycles) using composite pellets containing lime (CaO) and CuO together with the addition of calcium aluminate cement as a binder. Their activity was tested in a thermogravimetric analyzer (TGA) during calcination/reduction/oxidation/carbonation cycles. The calcination/reduction typically was performed in methane (CH4), and the oxidation/carbonation stage was carried out using a gas mixture containing both CO2 and O2. It was confirmed that the material synthesized is suitable for the proposed cycles; with the very favorable finding that reduction/oxidation of the oxygen carrier is complete. Various schemes for the Ca
Cu looping process have been explored here that would be compatible with these new composite materials, along with some different possibilities for flow directions among carbonator, calciner, and air reactor.

1. INTRODUCTION Carbon dioxide is a major greenhouse gas responsible for climate changes.1 The negative environmental effects of such emissions represent a growing problem as the utilization of fossil fuels such as coal is increasing and can be expected to do so for the near- to medium-term future.2,3 Therefore, technologies associated with CO2 capture and storage (CCS) are increasingly considered to be likely contributors to reduce these emissions.1,4 Three main scenarios for CO2 separation and capture are postcombustion processes for traditional coal-fired power plants, precombustion processes for gasification or reforming, and oxyfuel combustion.5
7 Some proposed technologies incorporate processes from different scenarios, and one example of this is the calcium looping cycle, which is in general regarded as a postcombustion CO2 capture technology. However, about 30% of the heat generated in a power plant integrated with a calcium looping cycle CO2 capture system is generated in the sorbent regenerator (calciner) employing oxy-fuel combustion.7 Calcium looping cycles (CaL) represent a new important class of technology which is based on the reversible chemical reaction between lime (CaO) and CO2. CaOðsÞ þ CO2ðgÞ ¼ CaCO3ðsÞ

ΔHr0

¼
179 kJ=mol ð1Þ

The forward reaction, namely carbonation, is an exothermic reaction, while the reverse reaction, calcination, represents sorbent regeneration which is an endothermic process requiring heat. Published 2011 by the American Chemical Society

That heat can be supplied to the reactor (calciner) directly or indirectly from a combustor, or generated “in situ”, usually by combustion of a gas (CH4) or biomass.8 To produce a highlyconcentrated CO2 stream, nearly pure oxygen is used (oxy-fuel combustion), which is an expensive step because air separation is highly energy intensive. Another class of solid looping cycles is chemical looping combustion (CLC), a combustion technology with inherent CO2 separation.9 In this technology an oxygen carrier, typically a metal oxide, transfers the oxygen from the air to the fuel such that the combustion air and the fuel are never mixed, and the obtained flue gas is a concentrated CO2 stream. Gaseous fuels such as syngas from coal gasification or natural gas are preferred fuels, but combustion of solids is also feasible.10,11 Metal oxides in the oxidized form (MexOy) are the source of oxygen for the fuel oxidation, which occurs in a fuel reactor. ð2n þ mÞMex Oy þ Cn H2m f ð2n þ mÞMey Oy
1 þ
nCO2 þ mH2 O

ð2Þ

O2 carriers are usually oxides of Fe, Ni, Cu, or Mn supported by inert materials such as Al2O3, ZrO2, TiO2, or MgO. After reaction with the fuel, the reduced form of the O2 carrier Received: July 4, 2011 Accepted: October 24, 2011 Revised: September 15, 2011 Published: October 24, 2011 10750

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(MeyOy
1) is transported to the air reactor, and oxidized by air. Mey Oy
1 þ 0:5O2 f Mex Oy

ð3Þ

The oxidized form of the O2 carrier is returned to the fuel reactor for a new cycle. In summary, sorbent regeneration in CaL occurs through combustion of a suitable fuel using pure oxygen, which has been demonstrated at a pilot-scale facility.12 However, the expense involved in providing pure oxygen diminishes the economic advantages of the technology.12,13 Moreover, a high temperature (>900 °C) is required for CaCO3 decomposition to produce a highly concentrated CO2 stream which unfavorably affects sorbent activity over multiple cycles.14 An interesting possibility is to provide the calciner with the necessary heat generated in a CLC process and this is a study dealing with that concept for CaL. Practically, this means supplying heat for an endothermic reaction by a CLC process, which is an idea presented by Lyon and Cole,15 and more recently considered by Abanades et al. 16,17 for steam reforming in particular, and their work adds the critical refinement that the heat from the copper oxide reduction drives the calcination step, thus ensuring that a pure CO2 stream can be produced in the calciner without the use of oxygen. It should also be noted that Abanades et al. have also proposed that this approach can be used for CCS, based on process simulations and have considered a large number of alternatives to the copper system in their patent application.17 What has not been done so far is to demonstrate this approach experimentally. Recently we have studied the preparation of CaO-based pellets for CO2 capture,18,19 and found that aluminate cements are suitable binders. These pellets show a high long-term CO2 capture activity and strength,20 which we attributed to the favorable influence of alumina compounds in the pellet structure. Moreover, the pellet preparation procedure, using a mixture of hydrated lime and aluminate cement,19 allows doping of pellets during preparation by other suitable compounds which can act as catalysts and/or oxygen carriers. Therefore, those pellets are an excellent candidate for integration of CaL with CLC. Oxidation of the O2 carrier in a CLC is exothermic, while the reaction in the fuel reactor, i.e., reduction, can be either endothermic or exothermic, depending on the type of O2 carrier and fuel. In practice, CuO serves as a suitable oxygen carrier for integration with CaL because both its oxidation and reduction reactions by methane (CH4) are exothermic. 2CuðsÞ þ O2ðgÞ ¼ 2CuOðsÞ

ΔHr0 ¼
156 kJ=mol

ð4Þ

4CuOðsÞ þ CH4ðgÞ ¼ 4CuðsÞ þ CO2ðgÞ þ 2H2 OðgÞ ΔHr0 ¼
178 kJ=mol

ð5Þ

The exothermic reduction of CuO can provide heat for the endothermic calcination. The other advantages of this well investigated oxygen carrier21,22 are a high oxygen transport capacity, favorable kinetics and thermodynamics which enable complete conversion of CH4 into CO2 and H2O, and finally its cost (it is one of the cheaper metal oxygen carriers). It has been shown that CuO-based oxygen carriers need a support, and the most promising performance has been demonstrated using alumina (Al2O3).21,22 Moreover, if such a system were to be developed for fluidized bed application, the support ought to provide resistance to attrition to minimize the production of fine particulates, since copper is potentially somewhat

toxic. Therefore, aluminate pellets, for which we have already developed an inexpensive preparation procedure and which we have extensively investigated,18
20 are a good candidate to simultaneously provide support for the CuO carrier and sorbent for CO2 capture. The main objective of this study was to synthesize a new class of pellets, based on CaO and CuO with calcium aluminate cements as a binder/support. These pellets were explored as a means of integration of CaL and CLC in order to prove the proposed concept of Ca
Cu looping cycles.

2. MATERIALS AND METHODS Cadomin (CD) limestone from Canada (see elsewhere19 for its elemental analysis), with a particle size 0.25
1.4 mm, was used as a natural CaO-containing material. The high content of SiO2 (5.47 wt %) and Al2O3 content (1.54 wt %) in CD limestone indicate the presence of silicate and aluminosilicate impurities. A powder of CuO, 98% of particles 80% of the particles 800 °C. However, in this experiment carbonation took place only when the temperature dropped below 700 °C, which is, on first consideration, an unexpected result. However, it should be noted here that oxidation of Cu is a rapid and highly exothermic reaction, which rapidly releases heat into the pellet particles causing their heating above the programmed temperature, i.e., the temperature measured by thermocouple. That is, the thermocouple measures the temperature in the gas around the pellets, but the reaction occurs through the volume of “superheated” pellet particles. It appears that the temperature inside the pellet particles is more than 100 °C higher than that in the surrounding gas, which is important to consider in the design of any process. Namely, this phenomenon can cause undesired sintering in pellet particles (or even agglomeration), and its effects should be mitigated in practice to avoid “superheating”. On the other hand, this effect also offers the scenario in which the pellets are oxidized by flue gas (smaller O2 concentration), which means slower conversion rate and heat release (when compared with that during oxidation by air), and as noted earlier helps purify the CO2 stream produced. Furthermore, the direction of solid material transport from calciner to air reactor becomes more desirable than that from carbonator to air reactor (this scheme and further discussion is presented below). The TGA tests presented in Figure 1 show that the carbonation of pellets previously reduced can provide some benefits. Therefore, further runs aimed at testing pellet activity during a multicycle operation were done under conditions used in the run

Figure 4. SEM images of interior of CaO/CuO-based pellet: (a) original, (b) calcined
reduced, and (c) after 10 calcination
reduction
oxidation
carbonation cycles.

for Figure 1b. Furthermore, there is evidence that H2O(g) enhances carbonation,24 and its presence is expected during a real Ca
Cu looping cycle process in both the carbonator (flue gas usually contains steam) and calciner (reduction of CuO by CH4 generates steam). Therefore, steam effects were also tested, i.e., the cycles were performed with steam and without steam present. The conversion profiles obtained during three-cycle runs are presented in Figure 2. As is expected, steam enhances carbonation and it is more pronounced with cycle number. Oxidation and reduction are not noticeably affected by steam, and complete oxidation conversions are reached in all three cycles, which is important for practical application. The same result was obtained in a 10-cycle run (not presented here), which is in agreement with results presented in the literature.23 As has been seen for other CaO-based sorbents,14,18
20 carbonation conversions decreased with the cycle number, but were still high after the third cycle (∼50% with steam present). It can be seen that both O2 and CO2 carrying capacity (mass of O2 and CO2 per 10753

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Table 1. EDX Analyses of Calcined
Reduced CaO/ CuO-Based Pellet (For Which SEM Image Is Presented in Figure 4b) site 1

site 2

site 3

site 4

site 5

Al2O3 [wt%]

5.73

3.66

2.15

11.56

8.78

CaO [wt%]

17.03

10.38

6.96

76.20

83.63

CuO [wt%]

77.24

85.96

90.89

12.25

5.59

Figure 6. Schematic representation of CaL integrated with CLC (CaL
CLC).

Figure 5. XRD spectra of CaO/CuO-based pellet samples: original, calcined at 800 °C in N2, reduced at 800 °C in CH4, and after 10 calcination/reduction/oxidation/carbonation cycles.

100 g of sorbent) are comparable with those obtained with materials used solely for transport of O2 in CLC or for transport of CO2 in CaL processes. Moreover, the synthesis procedure of CaO/CuO-based pellets allows managing the CaO/CuO ratio, i.e., varying O2 carrying capacity depending on a demand for O2 in a particular process. Pore surface area and pore volume distribution of the tested pellets were determined by nitrogen physiosorption and results are presented in Figure 3. The BET pore surface area of the calcined sample is 1.69 m2/g, which is lower than is expected for this kind of material. The possible reasons for this are a high content of CuO in the sample (45 wt %), and a preparation procedure which is a type of wet mechanical mixing. Namely, a powder of CuO, which is a lightly porous material (see SEMEDX analyses below) was used for the synthesis. Somewhat higher pore surface areas, 6.53 and 10.31 m2/g, have been reported for two oxygen carriers (82.5 wt % CuO supported by Al2O3) prepared by coprecipitation.25 The pore size distribution has a peak at ∼3 nm, which corresponds to the first peak of the BJH pore volume distribution. Another very pronounced peak of pore volume distribution is placed at ∼100 nm. Figure 4 presents SEM analyses of CaO/CuO-based pellets taken from the interior of the particles. The original sample has low porosity; however, after calcination
reduction it becomes a more porous material. Two types of morphology are noticeable (they were analyzed by EDX), and results are presented in Table 1. It can be seen that sites which contain larger grains

(1, 2, and 3) are rich in Cu, and sites with smaller grains are mainly CaO. This confirms that the high content of CuO in the pellets is a cause for their low pore surface area (Figure 3). However, the lower porosity of CuO sites is not critical because TGA experiments (Figures 1 and 2) confirmed high reactivity of the oxygen carrier. A more developed porous pattern can be seen for CaO sites and their morphology is similar to that seen for calcium aluminate pellets.19 It should be noted that the smaller contact area between CuO and surrounding CaO/Al2O3 material may reduce formation of copper aluminate (CuAl2O4) during cycles. Finally, as is usual with other CaO-based sorbents for CO2 capture, cyclic calcination/carbonation causes sintering; therefore, larger grains and pores are present in the residue after 10 cycles (Figure 4c). XRD analysis of CaO/CuO-based pellets confirms the results of TGA tests. The XRD diffractograms obtained, with the most intense peaks designated, are presented in Figure 5. As expected, the main compounds present in the crystal phase of the original samples are CaCO3 (calcite and aragonite), Ca(OH)2 (portlandite), CuO (copper oxide), and CaAl4O7 (grossite). During calcination, the calcium compounds decomposed forming CaO (lime). It is interesting to note that copper aluminates are not formed after calcination, and moreover, their presence is not identified either in the reduced or in the cycled sample. Despite the fact that copper aluminates are fully reducible,25 their absence in this case is desirable because Al2O3 remains available to form Ca12Al14O33 (mayenite) which improves the performances of the CaO sorbent.19 The main change after reduction is the formation of elemental copper (Cu). Moreover, it should be noted here that the presence of Cu2O (cuprite) is also seen in the sample after these cycles. This is an indicator that these pellets were “preheated” during reduction as well during cycles. Most likely their temperature was higher due to larger mass of “XRD sample” (100 mg) than was the case in other TGA tests (∼3 mg). Namely, the formation of Cu2O is characteristic for both reduction of CuO and oxidation of Cu at temperatures ∼800 °C or higher.23 This implies that operation at lower temperatures and its control during the process is important to maintain desired sorbent performances. Finally, it should be mentioned that some Cu2O detected by XRD may be formed due to partial oxidation of Cu by air during handling with the samples. The research presented here has demonstrated that integration of CaL with CLC is a real possibility with practical application. The composite materials containing CaO and CuO are good candidates for this process. A block diagram of the proposed process that is compatible with the composite materials we have developed is presented in Figure 6. As can be seen, this system for 10754

dx.doi.org/10.1021/es202292c |Environ. Sci. Technol. 2011, 45, 10750–10756

Environmental Science & Technology postcombustion CO2 capture has three reactors: carbonator, calciner, and air reactor. Three main possibilities for the direction of solids circulation are worthy of attention: a Carbonator f calciner f air reactor f carbonator b Carbonator f calciner
carbonator c Carbonator f air reactor f calciner f carbonator The first route (a) is represented by solid arrows, and it describes a basic cycle which starts with CO2 capture in the carbonator. Carbonated
oxidized sorbent (CaCO3/CuO) circulates from the carbonator to the calciner where calcination takes place due to heat of exothermic reduction of CuO (reaction 5). Calcined
reduced solid material is then transferred to the air reactor to oxidize Cu (reaction 4). These regenerated pellets are now ready for a new CO2 capture step in the carbonator. The second route (b) is interesting because only the carbonator and calciner are required for the process: oxygen is always present in the flue gas. This means that regenerated solid carrier is directly returned from the calciner to the carbonator where, apart from CO2 capture, oxidation of Cu by oxygen from flue gas takes place. This route provides an interesting opportunity for flue gases with a high O2/CO2 ratio. It is also possible to employ a high air/fuel ratio in the combustor, which can enhance combustion efficiency and supply excess oxygen to the carbonator for oxidation of Cu. In this case one should be aware that CO2 capture efficiency can be diminished (more N2 in the carbonator coming with the excess air in the combustor). However, this type of cycle does not require an air reactor—since the carbonator becomes at the same time an air reactor. A combination of routes (a) and (b) is also very interesting and should be considered as a valuable possibility. Namely, the flow of regenerated pellets from the calciner can be divided into two streams: one entering the air reactor and the other entering the carbonator. The ratio of flow rates of these two streams can be regulated according to the amount of oxygen available in the carbonator and/or according to the desired distribution of heat between the air reactor and carbonator. The third route (c) is the reverse of the first scenario (a), which means the transfer of carbonated sorbent to the air reactor. After oxidation of Cu, the pellets transfer to the calciner, and after regeneration/reduction they continue to the carbonator. This flow direction requires strict control of the temperature in the air reactor; otherwise a significant amount of CaCO3 can be calcined at an elevated temperature diminishing CO2 capture efficiency (CO2 leaves air reactor with the N2 stream). This route is also discussed in detail by Abanades et al.16,17 and it should be noted that the direction which includes oxidation of CaCO3/ Cu appears to be the only possible one in the case of sorptionenhanced reforming (SER) because copper is always in reduced form in the reformer and must be first oxidized before CaCO3 calcination. A techno
economic study can explore which route is most promising. However, it is shown here that a new class of looping cycles for postcombustion CO2 capture is feasible and worthy of consideration, and it is experimentally feasible to employ CaL and CLC instead of the integration of CaL with oxy-fuel combustion. The Cu-based oxygen carrier is a promising candidate for CLC due to its exothermic reduction, and even more so, in that CaO and CuO can be used in a mixed pellet, having advantages such as heat release/consumption in the same pellet particle. Moreover, CaO/Al2O3 acts as a solid porous support for CuO. That also has the benefit that another support for CuO is not required, reducing total solids flow. Fluidized bed (FBC)

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reactors are most likely to be employed in the proposed process, but other options, such as fixed beds, are also open to consideration. Finally, the possibility that this type of system may also offer a method of final purification of the CO2 from oxygen is a valuable consideration which has not been explored here for reasons of space.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: (613) 996-2868; fax: (613) 992-9335.

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