Ind. Eng. Chem. Res. 2007, 46, 8597-8601
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Spinel-Supported Oxygen Carriers for Inherent CO2 Separation during Power Generation Peter Erri and Arvind Varma* School of Chemical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907
Chemical looping combustion potentially offers inherent CO2 capture during power generation. Implementation of the process, however, is hindered by the lack of suitable materials to function as oxygen carriers. In this context, spinel-supported nickel oxides were investigated, using modified TGA to study the redox characteristics of NiO/NiAl2O4 and (NiO)1-y(MgO)y/Ni(1-x)MgxAl2O4, prepared by solution combustion synthesis using varying precursor ratios. The long-term performance of the promising stable oxide (NiO)0.79(MgO)0.21/Ni0.62Mg0.38Al2O4 was tested by cycling the oxidant/reductant gases 100 times at 1000 °C, where it exhibited excellent reactivity and stability. Finally, attrition resistance of the oxygen carriers was determined via a modified ASTM test, which indicated mechanical strength superior to that of commercially available iron oxide. 1. Introduction Numerous studies suggest that global climate change is linked to increasing levels of CO2 in the atmosphere, caused by society’s reliance on fossil fuels for energy.1 To limit this climate change, decreased CO2 emissions have been agreed upon, with the United States as a signatory, under the 1992 United Nations Framework Convention on Global Climate Change. In this context, carbon sequestration for power plants is an attractive option as power generation accounts for 32% of the annual CO2 emissions in the U.S.2 By separating and storing the greenhouse gas emissions, the currently available fossil fuels could be utilized without harming the global environment. A prohibitive factor, however, is cost as the U.S. Department of Energy estimates that electricity prices would increase $0.020.04/kWh (i.e., 25-50%) if the currently proposed technologies, such as amine absorption, were implemented. Considering that ∼90% of the cost associated with sequestration stems from the separation of the gas itself ($100-200/ton of C),3 it is important to consider alternative approaches to combustion. One such option is chemical looping combustion (see Figure 1), which offers inherent CO2 capture while using air as an oxidant. In this case, the fuel (coal gas or natural gas) is not burned directly with air (hence eliminating the need to separate N2 from CO2), but is instead used to reduce a metal oxide, acting as an oxygen carrier. This reduced carrier is then looped to an air reactor where it is reoxidized, releasing thermal energy for power generation. By separating the combustion in two steps, a highpurity CO2 stream can be obtained from the fuel reactor by simple condensation of the water present. Despite the potential of the method, a major obstacle to commercial utilization remains the lack of an oxygen carrier which can withstand the severe operating conditions such as cyclic oxidation/reduction at elevated temperatures. Various transition metals have been proposed as oxygen carriers, such as Fe, Cu, Mn, and Ni, with the latter exhibiting excellent performance when supported on aluminate spinel structures to provide mechanical strength and sintering resistance.4-6 This was also observed in our recent work where NiO was supported with two spinel structures: NiAl2O4 and Ni1-xMgxAl2O4. Here, the addition of Mg to the oxygen carrier * To whom correspondence should be addressed. Tel.: (765) 4944075. Fax: (765) 494-0805. E-mail:
[email protected].
Figure 1. Schematic diagram of chemical looping combustion.
was found to enhance stability during cycling under alternating dilute methane and oxygen streams.7 In the present work, NiO/MgO/spinel mixed oxides are further investigated, varying the synthesis method, the active oxide (NiO) to support ratio, and the Mg-Ni mole ratio. These complex oxides were characterized by phase structure and attrition rate studies, and their redox performance was evaluated thermogravimetrically, under alternating coal gas (H2-COCO2 mixture) and oxygen streams simulating chemical looping combustion. The use of coal gas as reductant was prompted by the continued importance of coal for power generation. 2. Determination of Required Oxygen Capacity To allow for direct comparison of the synthesized oxygen carriers, the required particle circulation rate was mapped with respect to oxygen capacity and oxide conversion in the fuel reactor. This was done for the probable configuration of the reactors, where they are positioned in a manner allowing the particles to move between them by gravity.8 In this case, the air reactor may be approximated as a pneumatic transport reactor, where air lifts the particles and the superficial gas velocity controls the overall particle flow rate. This limits how fast the oxygen carriers can be circulated through the system, as their maximum velocity for ideal pneumatic transport is given by the following equation, derived from a momentum balance:9
Up ) Ug - Ut
(1)
where Ug and Ut are the superficial gas and particle terminal velocities, respectively. Since the air velocity is determined by the desired output temperature (higher flow results in lower temperature), it cannot be increased arbitrarily. Based on these considerations and the
10.1021/ie070068o CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007
Figure 2. Calculated oxygen carrier particle circulation rate map. Table 1. Assumed Process Conditions for Circulation Rate Calculations Texit P fuel flowa dp Fs a
1200 °C 13 atm 1 kmol/s 0.125 mm 2 g/cm3 0.01
35% H2, 18% CO, 47% CO2.
process parameters given in Table 1, mass and energy balances over the system yielded the flow rate map shown in Figure 2. Here, the oxygen carrier particle flow rate is simply the stoichiometric oxygen needed for complete fuel combustion divided by the carrier oxygen capacity and the oxide conversion in the fuel reactor. The maximum particle flow rate is established by eq 1, where Ut is determined by particle characteristics and Ug by the requirement that all energy needed to raise the air reactor effluent to 1200 °C is supplied by Ni oxidation. It may be seen from Figure 2 that, at low oxygen capacity, the required particle flow rate exceeds the upper bound. The importance of oxide conversion in the fuel reactor is also seen, since for a given particle flow rate the required oxygen capacity increases significantly with decreasing conversion. 3. Experimental Section 3.1. Oxygen Carrier Preparation. The nickel oxides were synthesized by the solution combustion technique, described in detail elsewhere.10 Briefly, with the desired ratio of cations, the metal precursors in the form of nitrates were mixed in water along with glycine. This solution was then heated on a hot plate, boiling off water and eventually igniting the mixture. The combustion wave then progressed through the vessel, yielding a fine mixture of Ni and metal oxides. For the synthesis of NiO as an example, the overall reaction between the metal nitrates and the glycine fuel is represented by
Ni(NO3)2 + 1.11φH2N(CH2)CO2H + 2.4975(φ - 1)O2 f NiO+ φ[2.22CO2 + 2.78H2O] + [1 + 0.555φ]N2 (2) An important parameter associated with this reaction is the fuel to oxidizer ratio (φ), where a value of unity indicates that all oxygen needed for combustion is supplied by the nitrates, while φ > 1 (
