Article pubs.acs.org/EF
Evaluation of a Spray-Dried CuO/MgAl2O4 Oxygen Carrier for the Chemical Looping with Oxygen Uncoupling Process Iñaki Adánez-Rubio, Pilar Gayán,* Alberto Abad, Luis F. de Diego, Francisco García-Labiano, and Juan Adánez Department of Energy and Environment, Instituto de Carboquímica (ICB), Consejo Superior de Investigaciones Científicas (CSIC), Miguel Luesma Castán, 4, Zaragoza 50018, Spain ABSTRACT: The chemical looping with oxygen uncoupling (CLOU) process is a chemical looping combustion (CLC) technology that allows for the combustion of solid fuels with inherent CO2 separation. As in CLC technology, in the CLOU process, the oxygen necessary for fuel combustion is supplied by a solid oxygen carrier, which is moving between two reactors: the fuel and air reactors. The CLOU technology uses the property of some metal oxides (CuO, Mn2O3, and Co3O4), which can generate gaseous oxygen at high temperatures. The oxygen generated by the oxygen carrier reacts directly with the solid fuel, which is mixed with the oxygen carrier in the fuel reactor. The reduced oxygen carrier is transported to the air reactor, where it is oxidized by air. In this work, a material prepared by spray drying containing 60 wt % CuO and 40 wt % MgAl2O4 as supporting material was evaluated as an oxygen carrier for the CLOU process using different installations. First, the oxygen release rate and the fluidization behavior, with regard to the agglomeration and attrition rate, were analyzed in a thermogravimetric analyzer (TGA) and in a batch fluidized bed, respectively. Then, the effects of the main operating conditions, such as the temperature, solids flow rate, and gas velocity in the fuel reactor, on the oxygen-carrier capability to release gaseous oxygen were analyzed in a continuous CLOU unit using N2 and CO2 as fluidization media. In addition, the effect of the oxygen concentration in the air reactor on the capability of the oxygen carrier to be regenerated was evaluated. The results obtained showed that this oxygen carrier has suitable characteristics for the CLOU process. Nevertheless, after 40 h of continuous operation at high temperatures, the particle integrity decreased significatively, indicating the need to improve the lifetime of this kind of material for use in an industrial CLOU process.
1. INTRODUCTION To stabilize the CO2 concentration in the atmosphere, several measures must be taken. Among them, the carbon capture and storage (CCS) would contribute 15−55% to the cumulative mitigation effort worldwide until 2100.1 The CCS is a process involving the separation of CO2 emitted by industry and energy-related sources and then storage of it for a long period of time. The chemical looping combustion (CLC) process has been suggested among the best alternatives to reduce the economic cost of CO2 capture in power plants2 and to increase the efficiency with respect to other CO2 capture processes.3 In this process, CO2 is inherently separated from other combustion products, N2 and unused O2, through the use of a solid oxygen carrier, and thus, no energy is expended for the separation. The CLC process has been demonstrated for gaseous fuel combustion, such as natural gas and syngas, in 10− 140 kWth units using oxygen-carrier materials based on Ni,4,5 Cu,6,7 or Fe.8,9 A huge number of oxygen carriers have been proposed for CLC with gaseous fuels, which have been reviewed by Adánez et al.10 Because solid fuels are considerably more abundant and less expensive than natural gas, it would be highly advantageous if the CLC process could be adapted for these types of fuels. The first option to use solid fuels in a CLC process was to use syngas in the fuel reactor coming from a previous gasifiying stage. In this technology, experience gained in the development of CLC for syngas can be useful,11,12 but it is necessary to use pure oxygen for the gasification of the solid fuel, causing an important energy penalty because of oxygen © 2012 American Chemical Society
separation from air. The second option of development is the CLC with solids fuels, where the solid fuel is directly introduced to the fuel reactor. Devolatization of coal particles happen in the CLC system, and the remaining char must be gasified. Thus, a gasification agent, e.g., H2O or CO2, must be used as the fluidization gas. The oxygen carrier reacts with volatiles and the gas product of coal gasification. Because of the slow gasification reaction rate, the residence time of char particles must be high in the gasification step to have high CO2 capture efficiencies.13,14 To increase the gasification rate, temperatures higher than 1000 °C have been proposed.14 Because a partial loss of the oxygen carrier is expected in the purge stream of ash, low-cost and environmental friendly materials are preferred in this CLC option, e.g., natural minerals or industrial/waste products.15−19 To overcome the low reactivity of the char gasification stage in the direct solid-fueled CLC, an alternative process was very recently proposed.20 The chemical looping with oxygen uncoupling (CLOU) process is based on the strategy of using oxygen carriers that release gaseous oxygen at high temperatures, thereby allowing the solid fuel to burn with gasphase oxygen. In this way, the slow gasification step of the direct solid-fueled CLC is avoided, giving a much faster solid conversion.20−22 Received: February 8, 2012 Revised: April 2, 2012 Published: April 12, 2012 3069
dx.doi.org/10.1021/ef3002229 | Energy Fuels 2012, 26, 3069−3081
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Article
shown in reaction 6, which can be advantageous for the heat balance of the CLOU system.
Figure 1 shows a schematic diagram of a CLOU system. As proposed by Mattisson et al.,20 the CLOU process is composed
4CuO + C → 2Cu 2O + CO2 ΔHr900 °C = −132.9 kJ/mol of C
(6)
Figure 2 shows the partial pressure of oxygen as a function of the temperature for the CuO/Cu2O system, calculated using
Figure 1. Schematic diagram of the CLOU process for solid fuels.
of two interconnected fluidized-bed reactors: the fuel reactor and the air reactor. In the fuel reactor, CO2 and steam are produced by different reactions. First, the oxygen carrier releases oxygen according to 2MexOy (s) ↔ 2MexOy − 1(s) + O2 (g)
(1)
and the solid fuel begins devolatilization, producing a solid residue (char) and volatile matter as gas products. coal → volatile matter + char
(2) Figure 2. Equilibrium oxygen concentrations over the CuO/Cu2O system as a function of the temperature.
Then, char and volatiles are burnt as in usual combustion according to reactions 3 and 4. char + O2 → CO2
(3)
y⎞ ⎛ volatile matter+⎜x + ⎟O2 → xCO2 + y H 2O ⎝ 2⎠
(4)
HSC software.23 The oxygen concentration at equilibrium conditions greatly depends upon the temperature. It is clear from this figure that the air- and fuel-reactor temperatures in the process must be adjusted to the thermodynamic equilibrium of the system. On the one hand, the metal oxide is stable in the air reactor below 950 °C if the maximum oxygen concentration from the air reactor is 4.5 vol %. A higher temperature must be avoided in the air reactor if high use of oxygen in the air is desirable. On the other hand, it would be desirable to have a low oxygen concentration at the exit of the fuel reactor to obtain a high-purity CO2 stream. However, in a previous work,22 it was found that an oxygen concentration in the fuel-reactor outlet close to the thermodynamic equilibrium was necessary to avoid unburnt compounds. An equilibrium concentration of 1.5 vol % O2 can be reached in the fuel reactor at 900 °C for the CuO/Cu2O system, whereas the equilibrium concentration increases up to 12.4 vol % at 1000 °C. Therefore, it could be advantageous to operate the fuel reactor at lower temperatures than the air-reactor temperatures, if the oxygencarrier reactivity is high enough for full combustion. Mattisson et al.20,21 developed oxygen carriers with 60 and 40 wt % CuO. Cyclic testing with solid fuels verified that oxygen was released close to the equilibrium pressure in the temperature range of 880−985 °C, and the material could also be regenerated close to equilibrium. When solid fuel particles were added to a bed of oxygen-carrier particles, a very rapid release of oxygen and combustion of fuel started. Thus, the conversion rate of the fuel could be increased by almost of 2 orders of magnitude compared to coal gasification in a CLC system with steam.21 Although no permanent agglomeration was detected, some defluidization phenomena were reported. On the basis of previous experience24−26 on the development of Cu-based materials for the CLC process, an analysis about the suitability of several Cu-based materials was carried out at ICB, CSIC.27,28 Particles prepared by several methods with different supports and metal oxide contents were tested in a
In the air reactor, the oxygen carrier is regenerated by oxygen from the air, following the next reaction. 2MexOy − 1(s) + O2 (g) ↔ 2MexOy (s)
(5)
After steam condensation, a pure CO2 stream can be obtained. In the CLOU process, the fluidization gas can be recycled CO2. In this way, the steam duty of the plant for the gasification step and energy penalties related are reduced. The reduced oxygen carrier (MexOy−1) is transported to the air reactor, where the oxygen carrier is regenerated to the initial oxidation stage with oxygen of the air according to reaction 5, and ready for a new cycle. The exit stream of the air reactor contains only N2 and unreacted O2. Therefore, the CLOU process has a low-energy penalty for CO2 separation, and low CO2 capture costs are expected. The heat release over the fuel and air reactors is the same as for conventional combustion. Thus, metal oxides used in oxygen carriers for CLOU must evolve gaseous oxygen at high temperatures. Besides, this O2 release must be reversible to oxidize the oxygen carrier in the air reactor. Thus, a special requirement is needed for the oxygen carrier to be used in the CLOU process in comparison to oxygen carriers for normal CLC, where the fuel must be able to react directly with the oxygen carrier without any release of gas-phase oxygen. Only those metal oxides that have a suitable equilibrium partial pressure of oxygen at temperatures of interest for combustion (800−1200 °C) can be used as CLOU oxygen carriers. The possible metal oxides that have the property of release oxygen are limited: CuO, Mn2O3, and Co3O4.20 Copper has the highest oxygen transport capacity (10 g of O2/100 g of CuO compared to 3 g of O2/100 g of Mn2O3 and 6 g of O2/100 g of Co3O4). Moreover, for the CuO/Cu2O system, the reaction with C is exothermic in the fuel reactor, as 3070
dx.doi.org/10.1021/ef3002229 | Energy Fuels 2012, 26, 3069−3081
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Article
thermogravimetric analyzer (TGA) and batch fluidized bed. Impregnated particles on Al2O3 were suitable for CLC but not for CLOU because they lost the oxygen release property when CuAl2O4 is formed. It was found that particles with 60 wt % CuO and using MgAl2O4 as the supporting material prepared by mechanical mixing were adequate for use as an oxygen carrier for the CLOU process. This material showed high reactivity and oxygen transport capacity and suitable attrition resistance and does not have a tendency to agglomerate during operation in a batch fluidized-bed reactor. After the screening tests, a batch of 60 wt % CuO supported on MgAl2O4 was prepared by spray drying (Cu60MgAl). The proof of the concept of the CLOU process was demonstrated using this material as an oxygen carrier,29 burning coal in a 1.5 kWth continuously operated unit consisting of two interconnected fluidized-bed reactors. The effects of the fuel-reactor temperature, coal feeding rate, and solids circulation flow rate on the combustion and CO2 capture efficiencies were investigated. Fast reaction rates of oxygen generation were observed with the oxygen carrier, and full combustion of coal was attained in the plant using a solids inventory of ≈235 kg/ MWth in the fuel reactor. In addition, values close to 100% in carbon capture efficiency were obtained at 960 °C. An assessment about the suitability of this material during longterm operation in a CLOU system is necessary. The aim of this work was focused on the oxygen-carrier behavior during long-term operation in a continuous CLOU unit. The effect of operating conditions, such as the temperature and gas velocity in the fuel reactor, solids circulation rate, and oxygen concentration in the air reactor, on the oxygen-carrier capability to release gaseous oxygen has been evaluated without fuel feeding. The evolution of the physical and chemical characteristics of the Cu60MgAl oxygen carrier was also analyzed.
coupled to an ultrathin window PGT Prism detector for energydispersive X-ray (EDX) analysis. Table 1 shows the main properties of this oxygen carrier. It has a low porosity and a very low superficial area. The crushing strength of
Table 1. Properties of the Fresh and Used Oxygen-Carrier Particles of Cu60MgAl fresh CuO content (wt %) oxygen transport capacity, ROC (wt %) crushing strength (N) real density (kg/m3) porosity (%) specific surface area, BET (m2/g) XRD main phases a
used batcha
used CLOU unita
60 6
60 6
60 6
2.4 4600 16.1
