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Thermal Analysis of Chemical-Looping Combustion
0263–8762/06/$30.00+0.00 # 2006 Institution of Chemical Engineers Trans IChemE, Part A, September 2006 Chemical Engineering Research and Design, 84(A9): 795– 806
www.icheme.org/cherd doi: 10.1205/cherd05020
THERMAL ANALYSIS OF CHEMICAL-LOOPING COMBUSTION E. JERNDAL1 , T. MATTISSON2 and A. LYNGFELT2 1
Department of Chemical and Biological Engineering, Chalmers University of Technology, Go¨teborg, Sweden 2 Department of Energy and Environment, Chalmers University of Technology, Go¨teborg, Sweden
I
n chemical-looping combustion, a gaseous fuel is burnt with inherent separation of the greenhouse gas CO2. Oxygen is transferred from the combustion air to the fuel by an oxygen carrier, which is usually a metal oxide, and therefore direct contact between the fuel and the combustion air is avoided. Thus, the products of combustion, i.e., CO2 and H2O, are not mixed with the rest of the flue gases and after condensation almost pure CO2 is obtained, without any energy lost for the separation. A thermal analysis of the process using a large number of possible oxygen carriers was performed by simulating reactions using the HSC Chemistry 5.0 software. Three fuels were used in the investigation, CH4, CO and H2. Based on the ability of the oxygen carriers to convert the fuel to the combustion products CO2 and H2O, stability in air and the melting temperatures of the solid material some metal oxides based on Ni, Cu, Fe, Mn, Co, W and sulphates of Ba and Sr showed good thermodynamic properties and could be feasible oxygen carriers. Only a few of these possible oxygen carrier systems, based on Cu, Fe and Mn, showed complete conversion of the fuel gas, but still the other systems had limited equilibrium restrictions, with only small and acceptable amounts of unreacted CO and H2 released from the fuel reactor. The promising systems were investigated further with respect to temperature changes in the fuel reactor as well as possible carbon, sulphide and sulphate formation in the fuel reactor. For some systems the reactions in the fuel reactor were endothermic, resulting in a temperature drop in the fuel reactor. However, this drop can be limited by applying a sufficient circulation of particles from the air reactor to the fuel reactor. When Ni or Co is used as oxygen carrier the fuel may need to be desulphurized prior to combustion to avoid formation of solid or liquid sulphides or sulphates. On the other hand, to prevent decomposition of the sulphates BaSO4 and SrSO4, in the fuel reactor, to sulphur-containing gases and metal oxides, it is necessary that some sulphur is present in the fuel and that high temperatures are avoided. Formation of carbon should not be a problem as long as the process is run under conditions of high fuel conversion. Keywords: chemical-looping combustion; CO2 capture; oxygen carriers; thermodynamics.
INTRODUCTION
the other fuel gases, i.e., N2 and unused O2 (Mattisson et al., 2001).
CO2 is the primary greenhouse gas and it is generally accepted that CO2 formed by combustion of fossil fuels contribute to an increased global average temperature. One way to achieve combustion without CO2 emissions and still use fossil fuels is separation and sequestration of CO2 (Kaarstad, 1992). Separation can be made by a number of different techniques, but most of them have the disadvantage of requiring a large amount of energy (Mattisson et al., 2001). With chemical-looping combustion, no energy is needed for the separation since CO2 is inherently separated from
Chemical-Looping Combustion Chemical-looping combustion is a method where a gaseous fuel, such as natural gas or synthesis gas, is burnt with oxygen transported from the combustion air to the fuel by an oxygen carrier. This gives the advantage of an exiting gas stream containing only CO2 and H2O. After H2O is condensed, almost pure CO2 is obtained for storage. The chemical-looping combustion system consists of two separate reactors, an air reactor and a fuel reactor. An oxygen carrier, which is usually a metal oxide, transports oxygen from the air reactor to the fuel reactor, see Figure 1. The oxygen carrier is circulating between the reactors and is oxidized in the air reactor, according to
Correspondence to: E. Jerndal, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE 41296 Go¨teborg, Sweden. E-mail: [email protected]
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the overall reaction (1), and reduced back to its initial state by the fuel, according to the overall reaction (2). ð2n þ mÞMex Oy1 þ ðn þ ð1=2ÞmÞO2 ! ð2n þ mÞMex Oy ð2n þ mÞMex Oy þ Cn H2m ! (2n þ m)Mex Oy1
(1)
þ mH2 O þ nCO2
(2)
The total amount of heat evolved from reaction (1) and (2) is equal to the heat released from conventional combustion, where the oxygen is in direct contact with the fuel (Lyngfelt et al., 2001). The process has been successfully demonstrated using gaseous fuel in several prototype units, for instance in a 10-kW unit at Chalmers University of Technology, using oxygen carriers based on both Ni and Fe (Lyngfelt and Thunman, 2004). Oxygen Carriers The rate of reaction and the oxygen transfer capacity of the oxygen carrier are important for the amount of bed material in the reactors and the needed recirculation flow of oxygen carriers (Lyngfelt et al., 2001). Thus, it is important to find an oxygen carrier with sufficient reduction and oxidation rates. Also, it has been established that some metal oxide/metal systems are unable to fully convert the fuel to carbon dioxide and water, resulting in formation of the combustible gases hydrogen and carbon monoxide (Cho, 2005; Mattisson et al., 2005). If the concentrations of these combustible gases are high, the metal oxides are not suitable as oxygen carriers, since a large part of the heating value of the fuel would be lost in the exit stream from the fuel reactor. Further, the particle needs to be resistant to attrition and fragmentation as well as to deactivation by carbon and sulphur species in the reactors. It is also an advantage if the oxygen carrier is cheap and environmentally sound (Cho et al., 2002). In addition to the above mentioned criteria, the following aspects need consideration: . melting temperatures of the compounds used; . oxygen ratios, which is the maximum transported mass of oxygen for a given mass flow of metal oxide; . heat balances of the fuel reactions for the cases where reaction (2) is endothermic; . side reactions in the fuel reactor where carbon and different sulphur-containing compounds may form. A number of papers have been presented where some of these aspects are studied for different oxygen carriers. Overviews of the previous work done on different oxides of transition state metals as oxygen carrier particles were made by Cho (2005) and Brandvoll (2005). Most of the work done on oxygen carriers focuses on oxides of iron, nickel and copper. Only a limited amount of work has been performed on the thermodynamic and thermal aspects of the process. Among those thermodynamic studies, Lyngfelt et al. (2001) have studied heat balances and temperature alterations for iron oxides and nickel oxide, Mattisson and Lyngfelt (2001) have studied the feasibility of using several different metal oxides based on Ni, Cu, Co, Fe and Mn, but also different aspects of NiO as oxygen carrier, including formation of sulphur-containing compounds (Mattisson et al., 2005) and
Kronberger et al. (2005) have simulated energy balances for the process with oxides of Fe, Cu, Ni and Mn. In patent applications oxides of silver, tungsten and molybdenum and sulphates of barium and strontium are also suggested as potential oxygen carriers (Cole, 2003). Cerium has not been proposed as an oxygen carrier in chemical-looping combustion but for oxygen storage in catalysts for car exhaust (Holmgren, 1998). The purpose of the present work is to make a comprehensive thermal investigation of different possible oxygen carriers. THEORY Gas Yield In chemical-looping combustion, it is important to be able to convert a high fraction of the incoming fuel to CO2 and H2O. The fuels studied here are CH4, H2 and CO. To gauge the degree of fuel conversion to carbon dioxide and water, the gas yield was defined as the fraction of the fuel which is oxidized to CO2 or H2O, and is given by equations (3)–(5) below for the different fuels investigated. pCO2 ( pCH4 þ pCO2 þ pCO ) pH 2 O For H2 : gH2 ¼ (pH2 þ pH2 O ) pCO2 For CO: gCO ¼ (pCO þ pCO2 ) For CH4 : gCH4 ¼
(3) (4) (5)
Here, pi is the partial pressure of gaseous species in the product gas. Since the gas yield does not account for the formation of H2 when CH4 is used as fuel, a gas yield based on the fuels heating values was also calculated as
gheat ¼
pCH4 ,in HCH4 (pH2 HH2 þ pCO HCO þ pCH4 HCH4 ) pCH4 , in HCH4 (6)
The results presented for CH4 are highly relevant for common gaseous fuels such as natural gas and refinery gas because of their high fraction of CH4 H2 and CO are relevant when synthesis gas is used in the process. Air Ratio The air ratio, l, is given by the volume fraction of oxygen in the air leaving the air reactor, xO2,ex, and is defined as
l¼
0:21(1 xO2 , ex ) 0:21 xO2 , ex
(7)
The conversion of the gas in the air reactor is simply
gox ¼
1 l
(8) Oxygen Ratio, R0
The oxygen ratio, R0, shows the maximum mass flow of oxygen that can be transferred between the air and the fuel reactor for a given mass flow of circulating oxygen carrier
Trans IChemE, Part A, Chemical Engineering Research and Design, 2006, 84(A9): 795– 806
THERMAL ANALYSIS OF CHEMICAL-LOOPING COMBUSTION particles, and is defined as R0 ¼
(mox mred ) mox
(9)
is defined as the actual amount of O, added with the oxide and/or with steam, over the stoichiometric amount needed for full conversion of the fuel:
6¼ Heat Balance Since the overall heat released in the two reactors is equal to that released in conventional combustion, the heat released or consumed in the fuel reactor may be obtained by comparing the heat released in the air reactor to that released in conventional combustion. The temperature difference in the fuel reactor depends on the oxygen carrier mass flow. A high mass flow gives a small mass difference in the mass conversion between the two reactors and results in a small temperature difference between the reactors. Here, the mass conversion, v, of the oxygen carrier was defined as
v¼
m mox
(10)
The temperature change in the fuel reactor can be calculated from a heat and mass balance over the fuel reactor, i.e., the enthalpy of the incoming particles and fuel to the reactor is equal to the enthalpy of the exiting particles and gas, which can be expressed as X X ni hi ¼ Hprod ¼ yi hi (11) Hreac ¼ where ni is the moles of reactants, yi the moles of products and hi is the enthalpy of component i calculated from ðT hi ¼ h0i þ
cpi (T)dt
(12)
298
where h0i is the heat of formation of substance i at 298 K. Thus, from a molar balance of the incoming and exiting gas and solids to the reactor the temperature difference can be calculated for different changes in solid conversion. A mathematical correlation was adopted for fitting experimental heat capacities. The Kelley equation was used throughout the calculations in the following form cpi ¼ A þ B 103 T þ C 105 T 2 þ D 106 T 2 (13) Carbon Deposition In the fuel reactor, solid carbon may form on the particles under certain conditions through methane decomposition: CH4 ! C þ 2H2 ;
DH ¼ 90:29 kJ at 10008C
(14)
or through the Boudouard reaction: 2CO ! C þ CO2 ;
DH ¼ 167:74 kJ at 10008C (15)
Carbon formation in the fuel reactor depends on temperature, pressure and the amount of oxygen added to the reactor with the metal oxide. The oxygen added ratio, 6,
797
nO, added nO, stoich
(16)
Although steam will likely not be added to the fuel reactor of a real system, the addition of steam is often performed in laboratory experiments in order to suppress carbon formation. The reason for this is that laboratory experiments are usually performed with incomplete conversion of the fuel in order to study the reaction rates, especially at the end of the cycle when the reactivity is low. Thus, laboratory tests are often associated with thermodynamic conditions that are significantly more favourable for carbon formation, as compared to industrial processes, where full conversion of the fuel is desired. Formation of Metal Sulphides and Sulphates The fuel could contain compounds including sulphur, such as H2S or COS. The fate of these species in the fuel reactor may be different and depend upon the oxygen carrier used. They may be oxidized in the fuel reactor forming SO2 or SO3, or they may react with the metal/metal oxide to form sulphides or sulphates. Formation of solid sulphur compounds depends on sulphur compound concentrations as well as temperature and pressure. METHOD To simulate chemical reactions and perform equilibrium calculations, the HSC Chemistry 5.0 software was used. The equilibrium composition is calculated using the Gibbs energy minimization method and a routine described by White et al. (1958). The program finds the most stable phase combination and seeks the phase composition where the Gibbs energy of the system reaches its minimum at a fixed mass balance, constant pressure and temperature (HSC Chemistryw, 2002). One assumption made is that all substances in the solid phase are in pure form and not mixtures. Further, it is assumed that CH4 can be converted to the gaseous components CO2, CO, H2O and H2 in the fuel reactor. H2 can be converted to H2O and CO to CO2. O2 is also included in the equilibrium calculations. Most of the systems investigated were based on metal oxides of the metals Ni, Cu, Fe, Cd, Mn, Co, Zn, Ce, W, Mo and Ag. Furthermore, two systems based on the transition between metal sulphate to metal sulphide were investigated, i.e., Ba and Sr. RESULTS Reduction Table 1 shows the gas yields for all the investigated metal oxide/reduced metal oxide systems at 10008C and 8008C at atmospheric pressure. Of the 27 systems studied, only eight have complete fuel conversion, although roughly half of the systems have conversion above 0.98. Investigations showed that all oxides of silver will decompose
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JERNDAL et al. Table 1. Gas yields for all the investigated oxide systems. Systems with yields below 0.92 are indicated with .
gheat
NiO/Ni CuO/Cu Cu2O/Cu Fe2O3/Fe3O4 Fe3O4/Fe0.945O Fe0.945O/Fe CdO/Cd Mn2O3/Mn3O4 Mn3O4/MnO MnO/Mn Co3O4/CoO CoO/Co ZnO/Zn CeO2/CeO1.83 CeO1.83/CeO1.72 CeO1.72/Ce2O3 Ce2O3/Ce WO3/WO2.96 WO2.96/WO2.722 WO2.722/WO2 WO2/W MoO3/MoO2.889 MoO2.889/MoO2.75 MoO2.75/MoO2 MoO2/MoO BaSO4/BaS SrSO4/SrS
gCH4
gCO
gH2
8008C
10008C
8008C
10008C
8008C
10008C
8008C
10008C
0.9949 1.0000 1.0000 1.0000 0.5529 0.3884 0.9881 1.0000 1.0000 0.0020 1.0000 0.9695 0.0306 0.4595 0.0556 0.0131 0.0000 0.9998 0.9957 0.5760 0.3075 1.0000 1.0000 1.0000 0.3077 0.9824 0.9877
0.9917 1.0000 0.9999 1.0000 0.7579 0.3936 0.9877 1.0000 0.9999 0.0206 1.0000 0.9496 0.0624 0.7658 0.1145 0.0507 0.0000 0.9989 0.9899 0.5821 0.4143 0.9747 1.0000 0.9999 0.4107 0.9749 0.9814
0.9949 1.0000 1.0000 1.0000 0.5406 0.3676 0.9880 1.0000 1.0000 0.0000 1.0000 0.9691 0.0022 0.4426 0.0167 0.0001 0.0000 0.9998 0.9957 0.5647 0.2819 1.0000 1.0000 1.0000 0.2821 0.9822 0.9875
0.9883 1.0000 0.9999 1.0000 0.6820 0.2898 0.9827 1.0000 0.9999 0.0000 1.0000 0.9299 0.0124 0.6917 0.0516 0.0040 0.0000 0.9984 0.9858 0.4804 0.3095 0.9646 1.0000 0.9999 0.3060 0.9648 0.9738
0.9949 1.0000 1.0000 1.0000 0.5408 0.3681 0.9880 1.0000 1.0000 0.0000 1.0000 0.9691 0.0035 0.4429 0.0190 0.0004 0.0000 0.9998 0.9957 0.5649 0.2827 1.0000 1.0000 1.0000 0.2829 0.9822 0.9875
0.9883 1.0000 0.9999 1.0000 0.6820 0.2898 0.9827 1.0000 0.9999 0.0000 1.0000 0.9299 0.0124 0.6917 0.0517 0.0040 0.0000 0.9984 0.9858 0.4804 0.3095 0.9646 1.0000 0.9999 0.3060 0.9648 0.9738
0.9946 1.0000 1.0000 1.0000 0.5264 0.3548 0.9873 1.0000 1.0000 0.0000 1.0000 0.9674 0.0033 0.4288 0.0179 0.0003 0.0000 0.9998 0.9954 0.5506 0.2711 1.0000 1.0000 1.0000 0.2713 0.9812 0.9868
0.9931 1.0000 0.9999 1.0000 0.7841 0.4086 0.9897 1.0000 0.9999 0.0000 1.0000 0.9574 0.0209 0.7916 0.0844 0.0068 0.0000 0.9991 0.9916 0.6102 0.4316 0.9788 1.0000 1.0000 0.4275 0.9789 0.9844
to silver at all temperatures and pressures studied and therefore silver oxides can not be used in the process. Depending upon the oxygen carrier system, the yield can both increase and decrease as the temperature increases. For the systems with high fuel conversions, i.e., higher than 0.92, the higher temperature gave a lower gas yield, with the cadmium system using H2 as the only exception. For systems with low fuel conversion the higher temperature increased the yield, since there is unreacted fuel gas leaving these systems. CH4 and CO reacting with Fe0.945O and WO2.722 were the only exceptions. The effect of an increased pressure was also investigated. As there is no gas expansion during the reaction of the metal oxide with CO and H2, the total pressure will not have any effect on gas yield. However, for methane there is a gas expansion, see reaction (2), and thus systems which do not have complete conversion of the fuel will be affected adversely by the total pressure. For the oxygen carriers which have gas yield above 0.92 at atmospheric pressure, the methane conversion was complete but there was incomplete conversion of H2 and CO. However, for the systems where the gas yield for methane is below 0.92, there was methane released from the outlet and a clear negative effect of pressure was seen. All oxide systems with so poor conversion will later be excluded from further consideration, see oxygen ratio section below.
MoO3, BaSO4 and SrSO4, respectively, in the air reactor at all temperatures and oxygen partial pressures realistic for the process. However, as shown in Figure 2, CuO, Mn2O3 and Co3O4 decompose to Cu2O, Mn3O4 and CoO respectively, already at relatively low temperatures. The temperature of decomposition is dependent on the partial pressure of oxygen. According to Figure 2, the more oxidized form in the system is favoured at low temperatures and high oxygen partial pressures. Thus, at higher temperatures a higher partial pressure of oxygen would be needed in the air reactor to prevent decomposition. In the air reactor, at atmospheric pressure, the oxygen partial pressure in incoming combustion air is 0.21 and this is then lowered as oxygen is consumed. It is essential that the oxygen carrier is able to reduce this pressure and a limit has been set at 0.04, which corresponds to an air ratio of 1.19 and a gas conversion of 0.84 for an atmospheric process.
Oxidation Calculations showed that several of the reduced metal oxides or metals could not be oxidized back to their original form. Ni, Fe, Cd, Zn, Ce, W, Mo, BaS and SrS will be oxidized to NiO, Fe2O3, CdO, ZnO, CeO2, WO3,
Figure 1. Chemical-looping combustion.
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THERMAL ANALYSIS OF CHEMICAL-LOOPING COMBUSTION
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Table 2. Melting temperatures for the metals and metal oxides. Melting point (8C)
Figure 2. The partial pressure of oxygen at which the metal oxides decompose to their more reduced form as a function of temperature.
If pO2 is 0.04, the temperature where Cu2O is formed is 9448C. A similar behaviour can be seen for the systems based on manganese and cobalt. If pO2 is 0.04, the temperature where Mn3O4 is formed is 7388C. Formation temperature of CoO is 8458C at the oxygen partial pressure 0.04. Thus, systems including Mn2O3 or Co3O4 are probably not realistic at atmospheric conditions. Under pressurized conditions the partial pressure of O2 is higher and thus the decomposition temperature would be higher. It should be noted though that a high turbine inlet temperature is desired if the process is to be used in a pressurized combined gas and steam turbine cycle. Melting Points The most suitable design for a chemical-looping combustion process is interconnected fluidized beds (Lyngfelt et al., 2001). Here, it is important to avoid melting and agglomeration of the circulating particles. It is well known that materials become soft at temperatures approaching their melting points. Thus, it is advantageous to operate the process at temperatures far from this temperature. Table 2 show the melting temperatures of the investigated metals and metal oxides. Since the process needs a temperature between 6008C and 12008C, some metals and metal oxides suggested are unsuitable in chemical-looping combustion. Cd, Zn, Ce and MoO3 have melting temperatures that are too low for being used as oxygen carriers. Cu has a relatively low melting temperature of 10858C and therefore a chemical-looping combustion process using copper may need to be conducted at temperatures below 9008C. Cho et al. (2004) noticed agglomeration of freeze granulated Cu-based oxygen carriers, but Garcı´aLabiano et al. (2004a) suggested that agglomeration problems could be avoided by using Cu-based particles prepared by impregnation. Oxygen Ratio, R0 A high oxygen ratio is an advantage for the process since more oxygen can be transported per mass unit of added material. The pairs that are not fully reduced to their metallic form in the fuel reactor like Fe2O3/Fe3O4, Mn3O4/ MnO and WO3/WO2.722 have the disadvantage of showing low oxygen ratios. Table 3 shows the oxygen ratio of the investigated oxygen carriers. It should be noted that the
Ni NiO Cu Cu2O CuO Fe Fe0.945O Fe3O4 Fe2O3 Cd CdO Mn MnO Mn3O4 Mn2O3 Co CoO Co3O4 Zn ZnO Ce Ce2O3 CeO1.72 CeO1.83 CeO2 W WO2 WO2.722 WO2.96 WO3 Mo MoO2 MoO2.75 MoO2.889 MoO3 BaS BaSO4 SrS SrSO4
1455 1955 1085 1235 1446 1538 — 1597 1565 321 — 1246 1842 1562 1347 1495 1830 — 420 1975 798 2230 — — 2400 3407 1724 — — 1472 2623 1927 — — 802 2230 1580 2227 1607
oxygen ratio will decrease when an inert material is used together with the active oxygen carriers. In the analysis of the heat balance, carbon deposition and fate of sulphur species given below, only systems showing sufficient gas yields, having oxides forming in the air reactor and having sufficiently high melting temperatures will be investigated. Thus, the oxide systems indicated with in Table 1 are excluded because of their low ability to oxidize the fuel and Cd is excluded because of its low melting Table 3. Oxygen ratio, R0, for the different pairs of metals/metal oxides. R0 NiO/Ni CuO/Cu Cu2O/Cu Fe2O3/Fe3O4 Mn2O3/MnO Mn3O4/MnO Co3O4/Co CoO/Co WO3/WO2.722 BaSO4/BaS SrSO4/SrS
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0.214 0.201 0.112 0.033 0.101 0.070 0.266 0.214 0.019 0.274 0.348
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JERNDAL et al. Table 4. Reaction enthalpies at 10008C. (DH(kJ mol21O2)
O2 þ 1/2CH4 ! 1/2CO2 þ H2O O2 þ 2H2 ! 2H2O O2 þ 2CO ! 2CO2 O2 þ 2Ni ! 2NiO O2 þ 2Cu ! 2CuO O2 þ 4Cu ! 2Cu2O O2 þ 4Fe3O4 ! 6Fe2O3 O2 þ 4MnO ! 2Mn2O3 O2 þ 6MnO ! 2Mn3O4 O2 þ 3/2Co ! 1/2Co3O4 O2 þ 2Co ! 2CoO O2 þ 0.278/2WO2.722 ! 0.278/2WO3 O2 þ 1/2BaS ! 1/2BaSO4 O2 þ 1/2SrS ! 1/2SrSO4
2401.7 2498.5 2562.8 2468.5 2295.9 2331.7 2478.8 2359.1 2449.4 2446.8 2466.9 2419.6 2481.7 2475.7
temperature. Mo is excluded because MoO3 has too low melting temperature. Ag is excluded because it can not be oxidized in the air reactor.
DH/DHdir
comb CH4
DH/DHdir
comb H2
DH/DHdir
comb CO
1.00 1.00 1.17 0.74 0.83 1.19 0.89 1.12 1.11 1.16 1.04 1.20 1.18
0.94 0.59 0.67 0.96 0.72 0.90 0.90 0.94 0.84 0.97 0.95
1.00 0.83 0.53 0.59 0.85 0.64 0.80 0.79 0.83 0.75 0.86 0.85
The reaction enthalpies for the reaction between the various metals or reduced metal oxides and oxygen at 10008C can be found in Table 4. The ratio of the reaction enthalpy of the oxidation to that of conventional combustion for the three fuels is also shown. Because the overall heat released from chemical-looping combustion is the same as that from normal combustion, it is possible to determine whether the reactions in the fuel reactor are endothermic or exothermic from this ratio. Thus, a ratio above 1 means that the reaction in the fuel reactor is endothermic and a ratio below 1 indicate an exothermic fuel reactor reaction. Thus, it is seen in Table 4 that the reaction in the fuel reactor is always exothermic when H2 or CO is used. As seen from Table 4, some of the reactions are endothermic when methane is used. This means that there will be a temperature decrease in the fuel reactor. It is important that the temperature drop is not too large, since this will mean that the reaction between the fuel and the oxygen carrier will be slower or even stop. The temperature drop was calculated as a function of v for the endothermic reactions, and the results are presented in Figures 3 –5. If
the reaction is exothermic, heat will need to be removed from the reactors to avoid excess temperatures. A high reactor temperature should be avoided because it might lead to sintering or deactivation of the oxygen carrier. The exothermic nature of the reaction in the air reactor may result in temperature gradients within the particles. This temperature increase depends on several factors such as size, porosity, composition, reactivity with the reacting gases and diffusion factors. However, Garcı´a-Labiano et al. (2004b) showed that under conditions present in a chemical-looping combustion system and with particle sizes normally used, the temperature increase was only about 158C, at most, in the particles. Their conclusion was that when using small particles, these can be considered to be isothermal for most reactions. In the calculations of the temperature decrease in Figures 3–5, the following assumptions were made. Temperature in the air reactor has been set to 10008C and the incoming methane is assumed to be preheated to 4008C. Further, the process is thought to be fully adiabatic, meaning that no heat is gained to or lost from the fuel reactor. Because the inlet temperature of methane is 4008C, some of the exothermic reactions may also give a fuel reactor temperature slightly below 10008C after reaction. This fuel reactor temperature decrease is small and should have a negligible effect on the process. The temperature in the fuel reactor would, in some cases, be very low if the metal oxide/sulphate is
Figure 3. Temperature change in the fuel reactor for cobalt and nickel oxides.
Figure 4. Temperature change in the fuel reactor for manganese, iron and tungsten oxides.
Heat Balance
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THERMAL ANALYSIS OF CHEMICAL-LOOPING COMBUSTION
Figure 5. Temperature change in the fuel reactor for strontium- and barium sulphide/sulphate.
entirely reduced. This is of course not viable, since these non catalytic gas solid reactions require a relatively high temperature to proceed. Therefore, temperature decreases to below 6008C are not shown. The implication of this is that the change in solid conversion, Dv, between the fuel and air reactor should not be too high. This is accomplished in practice by maintaining a sufficient circulation of solids. Carbon Deposition Under certain conditions solid carbon deposition on the oxygen carrier particles may occur if a carbon-containing fuel is used. This could have an adverse effect on the process. Formed carbon can be transported back to the air reactor causing CO2 formation, resulting in lower separation efficiency of CO2 (Cho et al., 2005) Parameters influencing carbon formation are temperature, pressure and amount of added oxygen. Normally, low temperatures and small amounts of added oxygen benefit the formation as seen in Figure 6. Here, the oxygen added ratio, equation (16), below which carbon formation is thermodynamically feasible, is shown as a function of the temperature. It is assumed that the fuel is CH4 and that all of the oxygen is added to the fuel reactor with the metal oxide. At low temperatures an increased pressure will favour carbon formation while at high temperatures an increased pressure counteracts carbon formation. This is due to the fact that carbon
Figure 6. The oxygen added ratio, 6, needed to avoid carbon formation when CH4 is used as fuel.
801
is formed by two reactions, the Boudouard reaction dominating at low temperatures and the methane decomposition reaction dominating at higher temperatures. In methane decomposition, less carbon is formed at higher pressures while in the Boudouard reaction more carbon is formed at higher pressures. At temperatures above approximately 9508C, no carbon formation should be expected as long as more than one fourth of the oxygen needed for complete fuel conversion is supplied. When CO is used as fuel, the Boudouard reaction is the only reaction accounting for carbon formation. Therefore, formation is favoured at low temperatures, small amounts of added oxygen and high pressures. Figure 7 shows that at low temperatures more oxygen has to be added to avoid carbon formation while at high temperatures less oxygen has to be added when comparing with CH4 as fuel. Providing oxygen to the fuel by water addition could be interesting when examining oxygen carriers experimentally. Results of water addition could also be of interest if the process is to be used in chemical-looping reforming, where synthesis gas is produced from light hydrocarbons (Ryde´n and Lyngfelt, 2004). As seen in Figure 8, the same amount of oxygen has to be added to avoid carbon formation if it comes from H2O as if it comes from the metal oxide at temperatures above approximately 9508C. Since 6 here is about 0.25, at least 1 mole of H2O has to be added to avoid carbon formation to every mole of CH4. At lower temperatures, less oxygen has to be added to avoid carbon formation when it comes from H2O compared to if the oxygen is added by a metal oxide. With H2O as oxygen source an increased pressure will counteract carbon formation indicating that methane decomposition is the dominating reaction. Fate of Sulphur Species Gas phase reactions Refinery gas and natural gas may contain small amounts of sulphur-containing species like H2S and COS. The most likely of these to occur in significant amounts is H2S and thus its reactions were investigated. H2S in the fuel will be partially oxidized to SO2 by oxidants such as H2O, CO2 or the metal oxide when the fuel is burnt. Formation of COS, SO3 and S2 is generally insignificant. For CuO/Cu, Cu2O/Cu, Fe2O3/Fe3O4, Mn2O3/MnO and Mn3O4/MnO, H2S in the fuel is converted to SO2 to
Figure 7. The oxygen added ratio, 6, needed to avoid carbon formation with CO as fuel.
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JERNDAL et al.
Figure 8. The oxygen added ratio, 6, needed to avoid carbon formation, when H2O is added as oxygen source to CH4.
somewhere between 99.3 and 100%, for every temperature studied between 6008C and 12008C. Calculations including formation of COS, SO3 and S2 showed that these concentrations were very low for all these metal/metal oxide pairs, with concentrations of SO2 þ H2S being at least 151 times larger than COS þ SO3 þ S2 for all systems independent of pressure and temperature. For the NiO/Ni and WO3/WO2.722 systems the conversion of H2S was somewhat lower, as seen in Figures 9 and 10. For NiO/Ni calculations show that the concentration of SO2 þ H2S is at least 52 times larger than the concentration of COS þ SO3 þ S2 and for WO3/WO2.722 the concentration of SO2 þ H2S is at least 77 times larger than the concentration of COS þ SO3 þ S2. As seen from Figures 9– 11, the oxidation of H2S is enhanced at high temperatures and low pressures. For the two systems including cobalt oxides, i.e., Co3O4/ Co and CoO/Co, H2S is partially oxidized to SO2 according to Figure 11. The figure shows that the Co3O4/Co system converts H2S to a much greater extent than the CoO/Co system. For Co3O4/Co, the conversion is almost complete at all pressures and temperatures above 8008C. In contrast, CoO/Co never gives a full conversion of H2S. The concentrations of COS, SO3 and S2 are much lower in the system with Co3O4/Co than in that with CoO/Co. Here, the concentration of SO2 þ H2S is at least 191 times larger than the concentration of COS þ SO3 þ S2, while it is at least 23 times larger for CoO/Co.
Figure 9. The degree of conversion from H2S in the fuel to the oxidized form, SO2, for NiO/Ni.
Figure 10. The degree of conversion from H2S in the fuel to SO2 for WO3/WO2.722.
Figure 11. The degree of conversion from H2S in the fuel to SO2 for Co3O4/Co and CoO/Co.
For systems containing BaSO4/BaS and SrSO4/SrS, SO2 may form by decomposition of the sulphate, predominantly at high temperatures and low total pressures. Figures 12 and 13 show the partial pressure of sulphur-containing gases as a function of the temperature at which the sulphates start to decompose. Decomposition results in formation of sulphurcontaining gases and the metal oxides BaO and SrO, respectively. Thus, to avoid loss of oxygen carrier because of decomposition, the sulphur content of the added fuel must not be too low and the temperature should not be too high.
Figure 12. Decomposition temperature of BaSO4 at different concentrations of sulphur-containing gases.
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Figure 13. Decomposition temperature of SrSO4 at different concentrations of sulphur-containing gases.
Figure 15. Concentration of sulphur-containing gases needed in the gas phase for formation of Cu2S in the Cu2O/Cu system.
Solid phase reactions H2S and SO2 could react with the oxygen carrier, forming sulphides and sulphates. The formation of metal sulphides or sulphates on the oxygen carrier particles could result in deactivation of the particles. Thus, calculations were performed to see if this was possible and, if so, at what partial pressure of sulphur-containing gases. In summary, of all of the investigated systems, only Ni and Co formed sulphides at SO2 and H2S partial pressures and temperatures which may be encountered in a chemicallooping combustion fuel reactor. In the system with Fe2O3/Fe3O4, there is no risk of sulphide or sulphate formation at any concentration of sulphur-containing gases at any temperature. Below, a more detailed analysis of the Mn, Ni, Cu, Co and W based systems is given. As seen in Figures 14 –17, formation might take place at high sulphur-containing gas concentrations. The sulphur concentration needed for formation of these compounds increases with temperature and decreases with total pressure. The fact that formation is enhanced at higher pressures is explained by the higher partial pressure of H2S and SO2 when the total pressure is increased. During the oxidation of CH4, there is a volume expansion by approximately a factor 3 which reduces the concentration of sulphur species. This means that the H2S concentration in the fuel can be approximately three times that shown in the figures below before sulphide or sulphate formation is expected.
With nickel oxide as oxygen carrier, there is a significant risk of sulphide formation at low temperatures as seen in Figure 14. There are several different nickel sulphides, and calculations showed that Ni3S2 is the phase which is most likely to form. In the system containing CuO/Cu, the concentration of sulphur-containing gases, i.e., H2S, SO2, SO3, COS and S2, needed for formation of Cu2SO4 is at least 2.2 vol% at 15 bar and 33 vol% at 1 bar. Fractions that high are not likely to be found in the fuel and therefore the risk of formation is low. Figure 15 shows that when Cu2O is used as oxygen carrier, the sulphide first formed is Cu2S. Formation needs lower concentration of sulphur-containing gases than formation of Cu2SO4 when CuO is used as oxygen carrier, but is still quite unlikely. At 1 bar, Cu2S forms at a sulphur concentration of at least 10 vol% which is just above the range shown in Figure 15. For Mn3O4/MnO, there is no formation of sulphides. For formation of MnSO4, a sulphur concentration of at least 3.1 vol% at 15 bar and 46 vol% at 1 bar is needed at every temperature investigated. Concentrations that high are unlikely to occur and thus sulphate formation should not be a problem. Figure 16 shows that the risk of MnSO4 formation is somewhat greater for the Mn2O3/ MnO system. With respect to cobalt oxide, CoS0.89 may form in the fuel reactor. As shown in Figure 17 there is a significant
Figure 14. Concentration of sulphur-containing gases, i.e., H2S, SO2, SO3, COS and S2, needed in the gas phase for formation of Ni3S2 in the NiO/Ni system.
Figure 16. Concentration of sulphur-containing gases needed in the gas phase for formation of MnSO4 in the Mn2O3/MnO system.
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Figure 17. Concentration of sulphur-containing gases needed in the gas phase for formation of CoS0.89 in the Co3O4/Co and CoO/Co systems.
risk of formation for both systems, especially at low temperatures. For WO3/WO2.722, a sulphur concentration of at least 3.6 vol% at 15 bar and 7.4 vol% at 1 bar is needed for formation of WS2 and therefore sulphide formation should not be expected.
DISCUSSION It is important that the oxygen carrier used in the chemical-looping combustion process can convert the gas almost completely to CO2 and H2O. Combustible gases, such as CH4, CO and H2, leaving the fuel reactor, would mean a loss in combustion efficiency. An oxygen carrier for which minor amounts of fuel gas is unconverted may be acceptable, since only a small amount of oxygen would be needed downstream the fuel reactor to oxidize these species. Based on the results from the reduction and oxidation calculations, it was found that some oxides of copper, iron and manganese are suitable as oxygen carriers since they can fully convert the fuel. Nickel oxide, tungsten oxide, barium- and strontium sulphate also show high gas yield, although they are not able to fully convert the fuel. Cobalt has a somewhat lower gas yield, which is a drawback for this oxide system. More specifically, all iron oxides except Fe2O3/Fe3O4 are thermodynamically unsuitable oxygen carriers. Further, MnO/Mn, ZnO/Zn and all oxides of cerium can be excluded. Low melting temperatures indicate that zinc, cerium, cadmium and molybdenum can not be used in the process. Molybdenum is excluded because its stable oxide MoO3 melts at a low temperature. For copper, a low temperature may be required because of its relatively low melting temperature. Some oxides do not form in the air reactor at conditions feasible for the process. Silver could not be used because it does not form any oxides at the conditions investigated. Mn2O3, CuO and Co3O4 decompose at high temperatures and including these oxides in the process would require a low temperature. If a higher temperature is preferred, the more reduced forms, Mn3O4, Cu2O and CoO should be used. However, it should be noted that by running the process under an increased total pressure, the more oxidized form could be used at a higher temperature without decomposing.
All the systems investigated, except those with copper oxides and the Mn2O3/MnO system, gave a temperature drop in the fuel reactor when methane was burnt. This temperature decrease was most striking for systems with a high oxygen ratio and this could be a problem since the reactions might slow down or even stop at low temperatures. To prevent this temperature decrease from being too large, a high oxygen carrier mass flow is needed. This would result in a low degree of conversion difference between the reactors, and thus a smaller temperature difference. For systems giving an increased fuel reactor temperature, e.g., oxidation of methane with copper oxides, the reactor may be cooled to achieve desired reaction temperatures. This is also the case for CO and H2 combustion, where the reaction in the fuel reactor always is exothermic. Since the chemical-looping combustion process always should be run with full or high fuel conversion, the amount of added oxygen is always well over the amount where carbon formation is possible. Therefore, problems with carbon formation are not expected in a wellmixed fluidized fuel reactor. However, carbon formation may have to be considered for applications where the local or total oxygen supply could be low, e.g., chemical-looping reforming. Further, some studies have found minor carbon formation of NiO based carriers, although more than the thermodynamic amount of oxygen was added. The authors suggested that carbon may be an intermediary product in the reduction process (Cho et al., 2005). H2S in the fuel will be oxidized to SO2 when the fuel is burnt. For most of the systems studied, this oxidation is almost complete at all conditions. Exceptions are systems based on nickel, cobalt and tungsten, where the oxidation to SO2 is low at low temperatures and increased pressures. If there are low concentrations of sulphur-containing gases and a high temperature in the fuel reactor, BaSO4 and SrSO4 will decompose. This decomposition of the sulphates leads to a loss of the active oxygen carrier and thus BaSO4 and SrSO4 cannot be used at these conditions. The three sulphides and sulphates that are most likely to be formed are Ni3S2, MnSO4 and CoS0.89. All these compounds have low melting points, 7898C, 7008C and 8348C, respectively, and therefore it is imperative to avoid formation of these in a fluidized bed fuel reactor. The results are summed up in Table 5 below, where the different oxide systems feasible for the process can be compared. Since this is a theoretical thermodynamic analysis of the process, it should be noted that all the results refer to equilibrium. However, in a real application the rates of reaction, i.e., in the fuel and air reactor, are also very important and have implications with respect to the size of a CLC system. There have been quite a number of investigations in the last few years of oxygen carrier reactivities, especially carriers based on Cu, Mn, Fe and Ni, see for instance Johansson (2005) for a review. The reaction rates vary considerably and are dependant not only on the type of oxygen carrier, but also preparation method and type of inert material used. Abad et al. (2006) point out that there are several resistances that can affect the reaction rates in the fluidized bed reactors. An investigation of the kinetics of the reduction and oxidation reactions with three oxygen carriers based on Cu, Fe, and Ni reduced by the fuel gases CH4, CO and H2 was performed at atmospheric pressure and showed that
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Table 5. Comparison between the different systems.
Fuel conversion NiO/Ni CuO/Cu Cu2O/Cu Fe2O3/Fe3O4 Mn2O3/MnO Mn3O4/MnO Co3O4/Co CoO/Co WO3/WO2.722 BaSO4/BaS SrSO4/SrS
Decomposition temperature for pO2 ¼ 0.04
Melting temperature
I 9448C
Conversion of H2S to SO2
Sulphide/sulphate formed
I
Ni3S2
10858C 10858C MnSO4
7388C L L I I I
8458C
I L I
CoS0.89 CoS0.89
Only at lower temperatures. I ¼ incomplete; L ¼ low.
the kinetic parameters of the reactions were controlled by chemical reactions.
present in the fuel and that excessively high temperatures are avoided. NOMENCLATURE
CONCLUSIONS A comprehensive thermal investigation of oxygen carriers for chemical looping combustion has been performed. Some metal oxides and reduced metal oxide/metal systems based on Cu, Mn and Fe show excellent characteristics and can be used as oxygen carriers in chemicallooping combustion. Oxides of Ni and W as well as BaS and SrS are also highly promising, but have somewhat lower gas yields in comparison to Cu, Mn and Fe. The metal Co and its corresponding oxide, CoO, give a gas yield between 0.93 and 0.97 and this may be too low for practical use. Oxidation investigations indicate that for oxides of copper, manganese and cobalt, it is the more reduced oxide form Cu2O, Mn3O4 and CoO, respectively that should be used, unless the process is performed at low temperatures or higher total pressures. The use of copper is also limited to lower temperatures due to its low melting point. Fe2O3 has the advantages as oxygen carrier of being cheap and easily available (Mattisson et al., 2001). Disadvantages with the iron oxides, the systems with manganese oxides and the tungsten oxides are their relatively low oxygen ratios. Although there was a temperature drop in the fuel reactor for many systems using methane as fuel, the drop can be limited by applying a sufficient rate of circulation of solids between the air and fuel reactor. The possible formation of solid carbon, sulphides and sulphates was investigated. Formation of carbon should not be a problem as long as the process is run under conditions of high fuel conversion, i.e., with a sufficient amount of oxygen continuously added with the particles. With respect to sulphur contamination, H2S in the fuel is converted partially to SO2 as the fuel is burnt. When nickel, cobalt or manganese is used as oxygen carrier the fuel may need to be desulphurized prior to combustion to avoid formation of solid or liquid sulphides or sulphates. To prevent decomposition of the oxygen carriers BaSO4 and SrSO4 to sulphur-containing gases and metal oxides, in the fuel reactor, it is necessary that some sulphur is
cpi Hi Hprod Hreac h0i hi m mox mred ni nO,added nO,stoich pCH4,in pi R0 T xO2,ex yi
specific heat capacity at constant pressure for component i, J mol21 K21 lower heating value of component i, J enthalpy of the exiting particles and gas, J enthalpy of the incoming particles and fuel, J heat of formation of component i at 298 K, (J mol21) enthalpy of component i, J mol21 actual mass of oxygen carriers, kg mass of oxygen carriers in oxidized form, kg mass of oxygen carriers in reduced form, kg amount of reactants, mol actual amount of O added with the oxygen carrier and/or steam, mol stoichiometric amount of oxygen needed for full conversion of the fuel, mol partial pressure of methane in the reactant gas, Pa partial pressure of component i in the product gas, Pa oxygen ratio temperature, K volume fraction of oxygen in the air leaving the air reactor amount of products, mol
Greek symbols gheat gas yield for methane based on the fuels heating values gi gas yield of component i gox conversion of gas in the air reactor DH standard heat of reaction, J l air ratio 6 oxygen added ratio v mass conversion of oxygen carrier
REFERENCES Abad, A., Ada´nez, J., Garcı´a-Labiano, F., de Diego, L.F., Gaya´n, P. and Celaya, J., 2006, Mapping of the range of operational conditions for Cu-, Fe-, and Ni-based oxygen carriers in chemical-looping combustion, Accepted for publication in Chem Eng Sci. Brandvoll, Ø., 2005, Chemical looping combustion-fuel conversion with inherent CO2 capture, Doctoral thesis for the degree of Doktor Ingeniør, Norwegian University of Science and Technology. Cho, P., 2005, Development and characterisation of oxygen-carrier materials for chemical-looping combustion, Thesis for the Degree of Doctor of Philosophy, Chalmers University of Technology. Cho, P., Mattisson, T. and Lyngfelt, A., 2002, Reactivity of iron oxide with methane in a laboratory fluidized bed-application of chemical-looping combustion, Proceedings of the 7th International Conference on Circulating Fluidized Beds, Niagara Falls, Ontario, Canada, 599 –606.
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Cho, P., Mattisson, T. and Lyngfelt, A., 2004, Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion, Fuel, 83: 1215–1225. Cho, P., Mattisson, T. and Lyngfelt, A., 2005, Carbon formation on nickel and iron oxide-containing oxygen carriers for chemical-looping combustion, Ind Eng Chem Res, 44: 668–676. Cole, J.A., 2003, Process for separating synthesis gas into fuel cell quality hydrogen and sequestration ready carbon dioxide, Patent Application Publication, Pub. No.: US 2003/0035770 A1. Garcı´a-Labiano, F., de Diego, L.F., Ada´nez, J., Abad, A. and Gaya´n, P., 2004a, Reduction and oxidation kinetics of a copper-based oxygen carrier prepared by impregnation for chemical-looping combustion, Industrial and Engineering Chemistry Research, 43: 8168–8177. Garcı´a-Labiano, F., de Diego, L.F., Ada´nez, J., Abad, A. and Gaya´n, P., 2004b, Temperature variations in the oxygen carrier particles during their reduction and oxidation in a chemical-looping combustion system, Chem Eng Sci, 60: 851–862. Holmgren, A., 1998, Catalysts for car exhaust: oxygen storage in platinum/ceria and mass transfer in monoliths, Thesis for the degree of Doctor of Philosophy, Chalmers University of Technology. HSC Chemistryw 5.0 for Windows, 2002, Chemical Reaction and Equilibrium Software with Extensive Thermochemical Database, Outokumpu Reaearch Oy. Johansson, M., 2005, Selection of oxygen carriers for chemical-looping combustion using methane as fuel, Thesis for the degree of Licentiate of Engineering, Chalmers University of Technology. Kaarstad, O., 1992, Emission-free fossil energy from norway, Energy Conversion and Management, 33(5–8), 781–786.
Kronberger, B., Lo¨ffler, G. and Hofbauer, H., 2005, Simulation of mass and energy balances of a chemical-looping combustion system, Clean Air: International Journal on Energy for a Clean Environment, 6: 1 –14. Lyngfelt, A., Leckner, B. and Mattisson, T., 2001, A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion, Chemical Eng Sci, 56: 3101–3113. Lyngfelt, A., and Thunman, H., 2004, Construction and 100 h of operational experience of a 10-kW chemical looping combustor, in Thomas, D. (ed.). The CO2 Capture and Storage Project (CCP) for Climate Change Mitigation, Vol. 1—Capture and Separation of Carbon Dioxide From Combustion Sources (Elsevier Science, London, UK). Mattisson, T. and Lyngfelt, A., 2001, Capture of CO2 using chemicallooping combustion, First Biennial Meeting of the ScandinavianNordic Section of the Combustion Institute, April 18 –20, Go¨teborg, Sweden. Mattisson, T., Lyngfelt, A. and Cho, P., 2001, The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2, Fuel, 80: 1953–1962. Mattisson, T., Johansson, M. and Lyngfelt, A., 2006, The use of NiO as an oxygen carrier in chemical-looping combustion, Fuel, 85: 736– 747. Ryde´n, M. and Lyngfelt, A., 2004, Hydrogen and power production with integrated carbon dioxide capture by chemical-looping reforming, 7th International Conference on Greenhouse Gas Control Technologies, Vancouver, Canada. White, W.B., Johnson S.M. and Dantzig, G.B., 1958, Chemical equilibrium in complex mixtures. Journal of Chemical Physics, 28: 751– 755. The manuscript was received 30 September 2005 and accepted for publication after revision 21 April 2006.
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www.icheme.org/cherd doi: 10.1205/cherd05020
THERMAL ANALYSIS OF CHEMICAL-LOOPING COMBUSTION E. JERNDAL1 , T. MATTISSON2 and A. LYNGFELT2 1
Department of Chemical and Biological Engineering, Chalmers University of Technology, Go¨teborg, Sweden 2 Department of Energy and Environment, Chalmers University of Technology, Go¨teborg, Sweden
I
n chemical-looping combustion, a gaseous fuel is burnt with inherent separation of the greenhouse gas CO2. Oxygen is transferred from the combustion air to the fuel by an oxygen carrier, which is usually a metal oxide, and therefore direct contact between the fuel and the combustion air is avoided. Thus, the products of combustion, i.e., CO2 and H2O, are not mixed with the rest of the flue gases and after condensation almost pure CO2 is obtained, without any energy lost for the separation. A thermal analysis of the process using a large number of possible oxygen carriers was performed by simulating reactions using the HSC Chemistry 5.0 software. Three fuels were used in the investigation, CH4, CO and H2. Based on the ability of the oxygen carriers to convert the fuel to the combustion products CO2 and H2O, stability in air and the melting temperatures of the solid material some metal oxides based on Ni, Cu, Fe, Mn, Co, W and sulphates of Ba and Sr showed good thermodynamic properties and could be feasible oxygen carriers. Only a few of these possible oxygen carrier systems, based on Cu, Fe and Mn, showed complete conversion of the fuel gas, but still the other systems had limited equilibrium restrictions, with only small and acceptable amounts of unreacted CO and H2 released from the fuel reactor. The promising systems were investigated further with respect to temperature changes in the fuel reactor as well as possible carbon, sulphide and sulphate formation in the fuel reactor. For some systems the reactions in the fuel reactor were endothermic, resulting in a temperature drop in the fuel reactor. However, this drop can be limited by applying a sufficient circulation of particles from the air reactor to the fuel reactor. When Ni or Co is used as oxygen carrier the fuel may need to be desulphurized prior to combustion to avoid formation of solid or liquid sulphides or sulphates. On the other hand, to prevent decomposition of the sulphates BaSO4 and SrSO4, in the fuel reactor, to sulphur-containing gases and metal oxides, it is necessary that some sulphur is present in the fuel and that high temperatures are avoided. Formation of carbon should not be a problem as long as the process is run under conditions of high fuel conversion. Keywords: chemical-looping combustion; CO2 capture; oxygen carriers; thermodynamics.
INTRODUCTION
the other fuel gases, i.e., N2 and unused O2 (Mattisson et al., 2001).
CO2 is the primary greenhouse gas and it is generally accepted that CO2 formed by combustion of fossil fuels contribute to an increased global average temperature. One way to achieve combustion without CO2 emissions and still use fossil fuels is separation and sequestration of CO2 (Kaarstad, 1992). Separation can be made by a number of different techniques, but most of them have the disadvantage of requiring a large amount of energy (Mattisson et al., 2001). With chemical-looping combustion, no energy is needed for the separation since CO2 is inherently separated from
Chemical-Looping Combustion Chemical-looping combustion is a method where a gaseous fuel, such as natural gas or synthesis gas, is burnt with oxygen transported from the combustion air to the fuel by an oxygen carrier. This gives the advantage of an exiting gas stream containing only CO2 and H2O. After H2O is condensed, almost pure CO2 is obtained for storage. The chemical-looping combustion system consists of two separate reactors, an air reactor and a fuel reactor. An oxygen carrier, which is usually a metal oxide, transports oxygen from the air reactor to the fuel reactor, see Figure 1. The oxygen carrier is circulating between the reactors and is oxidized in the air reactor, according to
Correspondence to: E. Jerndal, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE 41296 Go¨teborg, Sweden. E-mail: [email protected]
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the overall reaction (1), and reduced back to its initial state by the fuel, according to the overall reaction (2). ð2n þ mÞMex Oy1 þ ðn þ ð1=2ÞmÞO2 ! ð2n þ mÞMex Oy ð2n þ mÞMex Oy þ Cn H2m ! (2n þ m)Mex Oy1
(1)
þ mH2 O þ nCO2
(2)
The total amount of heat evolved from reaction (1) and (2) is equal to the heat released from conventional combustion, where the oxygen is in direct contact with the fuel (Lyngfelt et al., 2001). The process has been successfully demonstrated using gaseous fuel in several prototype units, for instance in a 10-kW unit at Chalmers University of Technology, using oxygen carriers based on both Ni and Fe (Lyngfelt and Thunman, 2004). Oxygen Carriers The rate of reaction and the oxygen transfer capacity of the oxygen carrier are important for the amount of bed material in the reactors and the needed recirculation flow of oxygen carriers (Lyngfelt et al., 2001). Thus, it is important to find an oxygen carrier with sufficient reduction and oxidation rates. Also, it has been established that some metal oxide/metal systems are unable to fully convert the fuel to carbon dioxide and water, resulting in formation of the combustible gases hydrogen and carbon monoxide (Cho, 2005; Mattisson et al., 2005). If the concentrations of these combustible gases are high, the metal oxides are not suitable as oxygen carriers, since a large part of the heating value of the fuel would be lost in the exit stream from the fuel reactor. Further, the particle needs to be resistant to attrition and fragmentation as well as to deactivation by carbon and sulphur species in the reactors. It is also an advantage if the oxygen carrier is cheap and environmentally sound (Cho et al., 2002). In addition to the above mentioned criteria, the following aspects need consideration: . melting temperatures of the compounds used; . oxygen ratios, which is the maximum transported mass of oxygen for a given mass flow of metal oxide; . heat balances of the fuel reactions for the cases where reaction (2) is endothermic; . side reactions in the fuel reactor where carbon and different sulphur-containing compounds may form. A number of papers have been presented where some of these aspects are studied for different oxygen carriers. Overviews of the previous work done on different oxides of transition state metals as oxygen carrier particles were made by Cho (2005) and Brandvoll (2005). Most of the work done on oxygen carriers focuses on oxides of iron, nickel and copper. Only a limited amount of work has been performed on the thermodynamic and thermal aspects of the process. Among those thermodynamic studies, Lyngfelt et al. (2001) have studied heat balances and temperature alterations for iron oxides and nickel oxide, Mattisson and Lyngfelt (2001) have studied the feasibility of using several different metal oxides based on Ni, Cu, Co, Fe and Mn, but also different aspects of NiO as oxygen carrier, including formation of sulphur-containing compounds (Mattisson et al., 2005) and
Kronberger et al. (2005) have simulated energy balances for the process with oxides of Fe, Cu, Ni and Mn. In patent applications oxides of silver, tungsten and molybdenum and sulphates of barium and strontium are also suggested as potential oxygen carriers (Cole, 2003). Cerium has not been proposed as an oxygen carrier in chemical-looping combustion but for oxygen storage in catalysts for car exhaust (Holmgren, 1998). The purpose of the present work is to make a comprehensive thermal investigation of different possible oxygen carriers. THEORY Gas Yield In chemical-looping combustion, it is important to be able to convert a high fraction of the incoming fuel to CO2 and H2O. The fuels studied here are CH4, H2 and CO. To gauge the degree of fuel conversion to carbon dioxide and water, the gas yield was defined as the fraction of the fuel which is oxidized to CO2 or H2O, and is given by equations (3)–(5) below for the different fuels investigated. pCO2 ( pCH4 þ pCO2 þ pCO ) pH 2 O For H2 : gH2 ¼ (pH2 þ pH2 O ) pCO2 For CO: gCO ¼ (pCO þ pCO2 ) For CH4 : gCH4 ¼
(3) (4) (5)
Here, pi is the partial pressure of gaseous species in the product gas. Since the gas yield does not account for the formation of H2 when CH4 is used as fuel, a gas yield based on the fuels heating values was also calculated as
gheat ¼
pCH4 ,in HCH4 (pH2 HH2 þ pCO HCO þ pCH4 HCH4 ) pCH4 , in HCH4 (6)
The results presented for CH4 are highly relevant for common gaseous fuels such as natural gas and refinery gas because of their high fraction of CH4 H2 and CO are relevant when synthesis gas is used in the process. Air Ratio The air ratio, l, is given by the volume fraction of oxygen in the air leaving the air reactor, xO2,ex, and is defined as
l¼
0:21(1 xO2 , ex ) 0:21 xO2 , ex
(7)
The conversion of the gas in the air reactor is simply
gox ¼
1 l
(8) Oxygen Ratio, R0
The oxygen ratio, R0, shows the maximum mass flow of oxygen that can be transferred between the air and the fuel reactor for a given mass flow of circulating oxygen carrier
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(mox mred ) mox
(9)
is defined as the actual amount of O, added with the oxide and/or with steam, over the stoichiometric amount needed for full conversion of the fuel:
6¼ Heat Balance Since the overall heat released in the two reactors is equal to that released in conventional combustion, the heat released or consumed in the fuel reactor may be obtained by comparing the heat released in the air reactor to that released in conventional combustion. The temperature difference in the fuel reactor depends on the oxygen carrier mass flow. A high mass flow gives a small mass difference in the mass conversion between the two reactors and results in a small temperature difference between the reactors. Here, the mass conversion, v, of the oxygen carrier was defined as
v¼
m mox
(10)
The temperature change in the fuel reactor can be calculated from a heat and mass balance over the fuel reactor, i.e., the enthalpy of the incoming particles and fuel to the reactor is equal to the enthalpy of the exiting particles and gas, which can be expressed as X X ni hi ¼ Hprod ¼ yi hi (11) Hreac ¼ where ni is the moles of reactants, yi the moles of products and hi is the enthalpy of component i calculated from ðT hi ¼ h0i þ
cpi (T)dt
(12)
298
where h0i is the heat of formation of substance i at 298 K. Thus, from a molar balance of the incoming and exiting gas and solids to the reactor the temperature difference can be calculated for different changes in solid conversion. A mathematical correlation was adopted for fitting experimental heat capacities. The Kelley equation was used throughout the calculations in the following form cpi ¼ A þ B 103 T þ C 105 T 2 þ D 106 T 2 (13) Carbon Deposition In the fuel reactor, solid carbon may form on the particles under certain conditions through methane decomposition: CH4 ! C þ 2H2 ;
DH ¼ 90:29 kJ at 10008C
(14)
or through the Boudouard reaction: 2CO ! C þ CO2 ;
DH ¼ 167:74 kJ at 10008C (15)
Carbon formation in the fuel reactor depends on temperature, pressure and the amount of oxygen added to the reactor with the metal oxide. The oxygen added ratio, 6,
797
nO, added nO, stoich
(16)
Although steam will likely not be added to the fuel reactor of a real system, the addition of steam is often performed in laboratory experiments in order to suppress carbon formation. The reason for this is that laboratory experiments are usually performed with incomplete conversion of the fuel in order to study the reaction rates, especially at the end of the cycle when the reactivity is low. Thus, laboratory tests are often associated with thermodynamic conditions that are significantly more favourable for carbon formation, as compared to industrial processes, where full conversion of the fuel is desired. Formation of Metal Sulphides and Sulphates The fuel could contain compounds including sulphur, such as H2S or COS. The fate of these species in the fuel reactor may be different and depend upon the oxygen carrier used. They may be oxidized in the fuel reactor forming SO2 or SO3, or they may react with the metal/metal oxide to form sulphides or sulphates. Formation of solid sulphur compounds depends on sulphur compound concentrations as well as temperature and pressure. METHOD To simulate chemical reactions and perform equilibrium calculations, the HSC Chemistry 5.0 software was used. The equilibrium composition is calculated using the Gibbs energy minimization method and a routine described by White et al. (1958). The program finds the most stable phase combination and seeks the phase composition where the Gibbs energy of the system reaches its minimum at a fixed mass balance, constant pressure and temperature (HSC Chemistryw, 2002). One assumption made is that all substances in the solid phase are in pure form and not mixtures. Further, it is assumed that CH4 can be converted to the gaseous components CO2, CO, H2O and H2 in the fuel reactor. H2 can be converted to H2O and CO to CO2. O2 is also included in the equilibrium calculations. Most of the systems investigated were based on metal oxides of the metals Ni, Cu, Fe, Cd, Mn, Co, Zn, Ce, W, Mo and Ag. Furthermore, two systems based on the transition between metal sulphate to metal sulphide were investigated, i.e., Ba and Sr. RESULTS Reduction Table 1 shows the gas yields for all the investigated metal oxide/reduced metal oxide systems at 10008C and 8008C at atmospheric pressure. Of the 27 systems studied, only eight have complete fuel conversion, although roughly half of the systems have conversion above 0.98. Investigations showed that all oxides of silver will decompose
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JERNDAL et al. Table 1. Gas yields for all the investigated oxide systems. Systems with yields below 0.92 are indicated with .
gheat
NiO/Ni CuO/Cu Cu2O/Cu Fe2O3/Fe3O4 Fe3O4/Fe0.945O Fe0.945O/Fe CdO/Cd Mn2O3/Mn3O4 Mn3O4/MnO MnO/Mn Co3O4/CoO CoO/Co ZnO/Zn CeO2/CeO1.83 CeO1.83/CeO1.72 CeO1.72/Ce2O3 Ce2O3/Ce WO3/WO2.96 WO2.96/WO2.722 WO2.722/WO2 WO2/W MoO3/MoO2.889 MoO2.889/MoO2.75 MoO2.75/MoO2 MoO2/MoO BaSO4/BaS SrSO4/SrS
gCH4
gCO
gH2
8008C
10008C
8008C
10008C
8008C
10008C
8008C
10008C
0.9949 1.0000 1.0000 1.0000 0.5529 0.3884 0.9881 1.0000 1.0000 0.0020 1.0000 0.9695 0.0306 0.4595 0.0556 0.0131 0.0000 0.9998 0.9957 0.5760 0.3075 1.0000 1.0000 1.0000 0.3077 0.9824 0.9877
0.9917 1.0000 0.9999 1.0000 0.7579 0.3936 0.9877 1.0000 0.9999 0.0206 1.0000 0.9496 0.0624 0.7658 0.1145 0.0507 0.0000 0.9989 0.9899 0.5821 0.4143 0.9747 1.0000 0.9999 0.4107 0.9749 0.9814
0.9949 1.0000 1.0000 1.0000 0.5406 0.3676 0.9880 1.0000 1.0000 0.0000 1.0000 0.9691 0.0022 0.4426 0.0167 0.0001 0.0000 0.9998 0.9957 0.5647 0.2819 1.0000 1.0000 1.0000 0.2821 0.9822 0.9875
0.9883 1.0000 0.9999 1.0000 0.6820 0.2898 0.9827 1.0000 0.9999 0.0000 1.0000 0.9299 0.0124 0.6917 0.0516 0.0040 0.0000 0.9984 0.9858 0.4804 0.3095 0.9646 1.0000 0.9999 0.3060 0.9648 0.9738
0.9949 1.0000 1.0000 1.0000 0.5408 0.3681 0.9880 1.0000 1.0000 0.0000 1.0000 0.9691 0.0035 0.4429 0.0190 0.0004 0.0000 0.9998 0.9957 0.5649 0.2827 1.0000 1.0000 1.0000 0.2829 0.9822 0.9875
0.9883 1.0000 0.9999 1.0000 0.6820 0.2898 0.9827 1.0000 0.9999 0.0000 1.0000 0.9299 0.0124 0.6917 0.0517 0.0040 0.0000 0.9984 0.9858 0.4804 0.3095 0.9646 1.0000 0.9999 0.3060 0.9648 0.9738
0.9946 1.0000 1.0000 1.0000 0.5264 0.3548 0.9873 1.0000 1.0000 0.0000 1.0000 0.9674 0.0033 0.4288 0.0179 0.0003 0.0000 0.9998 0.9954 0.5506 0.2711 1.0000 1.0000 1.0000 0.2713 0.9812 0.9868
0.9931 1.0000 0.9999 1.0000 0.7841 0.4086 0.9897 1.0000 0.9999 0.0000 1.0000 0.9574 0.0209 0.7916 0.0844 0.0068 0.0000 0.9991 0.9916 0.6102 0.4316 0.9788 1.0000 1.0000 0.4275 0.9789 0.9844
to silver at all temperatures and pressures studied and therefore silver oxides can not be used in the process. Depending upon the oxygen carrier system, the yield can both increase and decrease as the temperature increases. For the systems with high fuel conversions, i.e., higher than 0.92, the higher temperature gave a lower gas yield, with the cadmium system using H2 as the only exception. For systems with low fuel conversion the higher temperature increased the yield, since there is unreacted fuel gas leaving these systems. CH4 and CO reacting with Fe0.945O and WO2.722 were the only exceptions. The effect of an increased pressure was also investigated. As there is no gas expansion during the reaction of the metal oxide with CO and H2, the total pressure will not have any effect on gas yield. However, for methane there is a gas expansion, see reaction (2), and thus systems which do not have complete conversion of the fuel will be affected adversely by the total pressure. For the oxygen carriers which have gas yield above 0.92 at atmospheric pressure, the methane conversion was complete but there was incomplete conversion of H2 and CO. However, for the systems where the gas yield for methane is below 0.92, there was methane released from the outlet and a clear negative effect of pressure was seen. All oxide systems with so poor conversion will later be excluded from further consideration, see oxygen ratio section below.
MoO3, BaSO4 and SrSO4, respectively, in the air reactor at all temperatures and oxygen partial pressures realistic for the process. However, as shown in Figure 2, CuO, Mn2O3 and Co3O4 decompose to Cu2O, Mn3O4 and CoO respectively, already at relatively low temperatures. The temperature of decomposition is dependent on the partial pressure of oxygen. According to Figure 2, the more oxidized form in the system is favoured at low temperatures and high oxygen partial pressures. Thus, at higher temperatures a higher partial pressure of oxygen would be needed in the air reactor to prevent decomposition. In the air reactor, at atmospheric pressure, the oxygen partial pressure in incoming combustion air is 0.21 and this is then lowered as oxygen is consumed. It is essential that the oxygen carrier is able to reduce this pressure and a limit has been set at 0.04, which corresponds to an air ratio of 1.19 and a gas conversion of 0.84 for an atmospheric process.
Oxidation Calculations showed that several of the reduced metal oxides or metals could not be oxidized back to their original form. Ni, Fe, Cd, Zn, Ce, W, Mo, BaS and SrS will be oxidized to NiO, Fe2O3, CdO, ZnO, CeO2, WO3,
Figure 1. Chemical-looping combustion.
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Table 2. Melting temperatures for the metals and metal oxides. Melting point (8C)
Figure 2. The partial pressure of oxygen at which the metal oxides decompose to their more reduced form as a function of temperature.
If pO2 is 0.04, the temperature where Cu2O is formed is 9448C. A similar behaviour can be seen for the systems based on manganese and cobalt. If pO2 is 0.04, the temperature where Mn3O4 is formed is 7388C. Formation temperature of CoO is 8458C at the oxygen partial pressure 0.04. Thus, systems including Mn2O3 or Co3O4 are probably not realistic at atmospheric conditions. Under pressurized conditions the partial pressure of O2 is higher and thus the decomposition temperature would be higher. It should be noted though that a high turbine inlet temperature is desired if the process is to be used in a pressurized combined gas and steam turbine cycle. Melting Points The most suitable design for a chemical-looping combustion process is interconnected fluidized beds (Lyngfelt et al., 2001). Here, it is important to avoid melting and agglomeration of the circulating particles. It is well known that materials become soft at temperatures approaching their melting points. Thus, it is advantageous to operate the process at temperatures far from this temperature. Table 2 show the melting temperatures of the investigated metals and metal oxides. Since the process needs a temperature between 6008C and 12008C, some metals and metal oxides suggested are unsuitable in chemical-looping combustion. Cd, Zn, Ce and MoO3 have melting temperatures that are too low for being used as oxygen carriers. Cu has a relatively low melting temperature of 10858C and therefore a chemical-looping combustion process using copper may need to be conducted at temperatures below 9008C. Cho et al. (2004) noticed agglomeration of freeze granulated Cu-based oxygen carriers, but Garcı´aLabiano et al. (2004a) suggested that agglomeration problems could be avoided by using Cu-based particles prepared by impregnation. Oxygen Ratio, R0 A high oxygen ratio is an advantage for the process since more oxygen can be transported per mass unit of added material. The pairs that are not fully reduced to their metallic form in the fuel reactor like Fe2O3/Fe3O4, Mn3O4/ MnO and WO3/WO2.722 have the disadvantage of showing low oxygen ratios. Table 3 shows the oxygen ratio of the investigated oxygen carriers. It should be noted that the
Ni NiO Cu Cu2O CuO Fe Fe0.945O Fe3O4 Fe2O3 Cd CdO Mn MnO Mn3O4 Mn2O3 Co CoO Co3O4 Zn ZnO Ce Ce2O3 CeO1.72 CeO1.83 CeO2 W WO2 WO2.722 WO2.96 WO3 Mo MoO2 MoO2.75 MoO2.889 MoO3 BaS BaSO4 SrS SrSO4
1455 1955 1085 1235 1446 1538 — 1597 1565 321 — 1246 1842 1562 1347 1495 1830 — 420 1975 798 2230 — — 2400 3407 1724 — — 1472 2623 1927 — — 802 2230 1580 2227 1607
oxygen ratio will decrease when an inert material is used together with the active oxygen carriers. In the analysis of the heat balance, carbon deposition and fate of sulphur species given below, only systems showing sufficient gas yields, having oxides forming in the air reactor and having sufficiently high melting temperatures will be investigated. Thus, the oxide systems indicated with in Table 1 are excluded because of their low ability to oxidize the fuel and Cd is excluded because of its low melting Table 3. Oxygen ratio, R0, for the different pairs of metals/metal oxides. R0 NiO/Ni CuO/Cu Cu2O/Cu Fe2O3/Fe3O4 Mn2O3/MnO Mn3O4/MnO Co3O4/Co CoO/Co WO3/WO2.722 BaSO4/BaS SrSO4/SrS
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0.214 0.201 0.112 0.033 0.101 0.070 0.266 0.214 0.019 0.274 0.348
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JERNDAL et al. Table 4. Reaction enthalpies at 10008C. (DH(kJ mol21O2)
O2 þ 1/2CH4 ! 1/2CO2 þ H2O O2 þ 2H2 ! 2H2O O2 þ 2CO ! 2CO2 O2 þ 2Ni ! 2NiO O2 þ 2Cu ! 2CuO O2 þ 4Cu ! 2Cu2O O2 þ 4Fe3O4 ! 6Fe2O3 O2 þ 4MnO ! 2Mn2O3 O2 þ 6MnO ! 2Mn3O4 O2 þ 3/2Co ! 1/2Co3O4 O2 þ 2Co ! 2CoO O2 þ 0.278/2WO2.722 ! 0.278/2WO3 O2 þ 1/2BaS ! 1/2BaSO4 O2 þ 1/2SrS ! 1/2SrSO4
2401.7 2498.5 2562.8 2468.5 2295.9 2331.7 2478.8 2359.1 2449.4 2446.8 2466.9 2419.6 2481.7 2475.7
temperature. Mo is excluded because MoO3 has too low melting temperature. Ag is excluded because it can not be oxidized in the air reactor.
DH/DHdir
comb CH4
DH/DHdir
comb H2
DH/DHdir
comb CO
1.00 1.00 1.17 0.74 0.83 1.19 0.89 1.12 1.11 1.16 1.04 1.20 1.18
0.94 0.59 0.67 0.96 0.72 0.90 0.90 0.94 0.84 0.97 0.95
1.00 0.83 0.53 0.59 0.85 0.64 0.80 0.79 0.83 0.75 0.86 0.85
The reaction enthalpies for the reaction between the various metals or reduced metal oxides and oxygen at 10008C can be found in Table 4. The ratio of the reaction enthalpy of the oxidation to that of conventional combustion for the three fuels is also shown. Because the overall heat released from chemical-looping combustion is the same as that from normal combustion, it is possible to determine whether the reactions in the fuel reactor are endothermic or exothermic from this ratio. Thus, a ratio above 1 means that the reaction in the fuel reactor is endothermic and a ratio below 1 indicate an exothermic fuel reactor reaction. Thus, it is seen in Table 4 that the reaction in the fuel reactor is always exothermic when H2 or CO is used. As seen from Table 4, some of the reactions are endothermic when methane is used. This means that there will be a temperature decrease in the fuel reactor. It is important that the temperature drop is not too large, since this will mean that the reaction between the fuel and the oxygen carrier will be slower or even stop. The temperature drop was calculated as a function of v for the endothermic reactions, and the results are presented in Figures 3 –5. If
the reaction is exothermic, heat will need to be removed from the reactors to avoid excess temperatures. A high reactor temperature should be avoided because it might lead to sintering or deactivation of the oxygen carrier. The exothermic nature of the reaction in the air reactor may result in temperature gradients within the particles. This temperature increase depends on several factors such as size, porosity, composition, reactivity with the reacting gases and diffusion factors. However, Garcı´a-Labiano et al. (2004b) showed that under conditions present in a chemical-looping combustion system and with particle sizes normally used, the temperature increase was only about 158C, at most, in the particles. Their conclusion was that when using small particles, these can be considered to be isothermal for most reactions. In the calculations of the temperature decrease in Figures 3–5, the following assumptions were made. Temperature in the air reactor has been set to 10008C and the incoming methane is assumed to be preheated to 4008C. Further, the process is thought to be fully adiabatic, meaning that no heat is gained to or lost from the fuel reactor. Because the inlet temperature of methane is 4008C, some of the exothermic reactions may also give a fuel reactor temperature slightly below 10008C after reaction. This fuel reactor temperature decrease is small and should have a negligible effect on the process. The temperature in the fuel reactor would, in some cases, be very low if the metal oxide/sulphate is
Figure 3. Temperature change in the fuel reactor for cobalt and nickel oxides.
Figure 4. Temperature change in the fuel reactor for manganese, iron and tungsten oxides.
Heat Balance
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Figure 5. Temperature change in the fuel reactor for strontium- and barium sulphide/sulphate.
entirely reduced. This is of course not viable, since these non catalytic gas solid reactions require a relatively high temperature to proceed. Therefore, temperature decreases to below 6008C are not shown. The implication of this is that the change in solid conversion, Dv, between the fuel and air reactor should not be too high. This is accomplished in practice by maintaining a sufficient circulation of solids. Carbon Deposition Under certain conditions solid carbon deposition on the oxygen carrier particles may occur if a carbon-containing fuel is used. This could have an adverse effect on the process. Formed carbon can be transported back to the air reactor causing CO2 formation, resulting in lower separation efficiency of CO2 (Cho et al., 2005) Parameters influencing carbon formation are temperature, pressure and amount of added oxygen. Normally, low temperatures and small amounts of added oxygen benefit the formation as seen in Figure 6. Here, the oxygen added ratio, equation (16), below which carbon formation is thermodynamically feasible, is shown as a function of the temperature. It is assumed that the fuel is CH4 and that all of the oxygen is added to the fuel reactor with the metal oxide. At low temperatures an increased pressure will favour carbon formation while at high temperatures an increased pressure counteracts carbon formation. This is due to the fact that carbon
Figure 6. The oxygen added ratio, 6, needed to avoid carbon formation when CH4 is used as fuel.
801
is formed by two reactions, the Boudouard reaction dominating at low temperatures and the methane decomposition reaction dominating at higher temperatures. In methane decomposition, less carbon is formed at higher pressures while in the Boudouard reaction more carbon is formed at higher pressures. At temperatures above approximately 9508C, no carbon formation should be expected as long as more than one fourth of the oxygen needed for complete fuel conversion is supplied. When CO is used as fuel, the Boudouard reaction is the only reaction accounting for carbon formation. Therefore, formation is favoured at low temperatures, small amounts of added oxygen and high pressures. Figure 7 shows that at low temperatures more oxygen has to be added to avoid carbon formation while at high temperatures less oxygen has to be added when comparing with CH4 as fuel. Providing oxygen to the fuel by water addition could be interesting when examining oxygen carriers experimentally. Results of water addition could also be of interest if the process is to be used in chemical-looping reforming, where synthesis gas is produced from light hydrocarbons (Ryde´n and Lyngfelt, 2004). As seen in Figure 8, the same amount of oxygen has to be added to avoid carbon formation if it comes from H2O as if it comes from the metal oxide at temperatures above approximately 9508C. Since 6 here is about 0.25, at least 1 mole of H2O has to be added to avoid carbon formation to every mole of CH4. At lower temperatures, less oxygen has to be added to avoid carbon formation when it comes from H2O compared to if the oxygen is added by a metal oxide. With H2O as oxygen source an increased pressure will counteract carbon formation indicating that methane decomposition is the dominating reaction. Fate of Sulphur Species Gas phase reactions Refinery gas and natural gas may contain small amounts of sulphur-containing species like H2S and COS. The most likely of these to occur in significant amounts is H2S and thus its reactions were investigated. H2S in the fuel will be partially oxidized to SO2 by oxidants such as H2O, CO2 or the metal oxide when the fuel is burnt. Formation of COS, SO3 and S2 is generally insignificant. For CuO/Cu, Cu2O/Cu, Fe2O3/Fe3O4, Mn2O3/MnO and Mn3O4/MnO, H2S in the fuel is converted to SO2 to
Figure 7. The oxygen added ratio, 6, needed to avoid carbon formation with CO as fuel.
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Figure 8. The oxygen added ratio, 6, needed to avoid carbon formation, when H2O is added as oxygen source to CH4.
somewhere between 99.3 and 100%, for every temperature studied between 6008C and 12008C. Calculations including formation of COS, SO3 and S2 showed that these concentrations were very low for all these metal/metal oxide pairs, with concentrations of SO2 þ H2S being at least 151 times larger than COS þ SO3 þ S2 for all systems independent of pressure and temperature. For the NiO/Ni and WO3/WO2.722 systems the conversion of H2S was somewhat lower, as seen in Figures 9 and 10. For NiO/Ni calculations show that the concentration of SO2 þ H2S is at least 52 times larger than the concentration of COS þ SO3 þ S2 and for WO3/WO2.722 the concentration of SO2 þ H2S is at least 77 times larger than the concentration of COS þ SO3 þ S2. As seen from Figures 9– 11, the oxidation of H2S is enhanced at high temperatures and low pressures. For the two systems including cobalt oxides, i.e., Co3O4/ Co and CoO/Co, H2S is partially oxidized to SO2 according to Figure 11. The figure shows that the Co3O4/Co system converts H2S to a much greater extent than the CoO/Co system. For Co3O4/Co, the conversion is almost complete at all pressures and temperatures above 8008C. In contrast, CoO/Co never gives a full conversion of H2S. The concentrations of COS, SO3 and S2 are much lower in the system with Co3O4/Co than in that with CoO/Co. Here, the concentration of SO2 þ H2S is at least 191 times larger than the concentration of COS þ SO3 þ S2, while it is at least 23 times larger for CoO/Co.
Figure 9. The degree of conversion from H2S in the fuel to the oxidized form, SO2, for NiO/Ni.
Figure 10. The degree of conversion from H2S in the fuel to SO2 for WO3/WO2.722.
Figure 11. The degree of conversion from H2S in the fuel to SO2 for Co3O4/Co and CoO/Co.
For systems containing BaSO4/BaS and SrSO4/SrS, SO2 may form by decomposition of the sulphate, predominantly at high temperatures and low total pressures. Figures 12 and 13 show the partial pressure of sulphur-containing gases as a function of the temperature at which the sulphates start to decompose. Decomposition results in formation of sulphurcontaining gases and the metal oxides BaO and SrO, respectively. Thus, to avoid loss of oxygen carrier because of decomposition, the sulphur content of the added fuel must not be too low and the temperature should not be too high.
Figure 12. Decomposition temperature of BaSO4 at different concentrations of sulphur-containing gases.
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Figure 13. Decomposition temperature of SrSO4 at different concentrations of sulphur-containing gases.
Figure 15. Concentration of sulphur-containing gases needed in the gas phase for formation of Cu2S in the Cu2O/Cu system.
Solid phase reactions H2S and SO2 could react with the oxygen carrier, forming sulphides and sulphates. The formation of metal sulphides or sulphates on the oxygen carrier particles could result in deactivation of the particles. Thus, calculations were performed to see if this was possible and, if so, at what partial pressure of sulphur-containing gases. In summary, of all of the investigated systems, only Ni and Co formed sulphides at SO2 and H2S partial pressures and temperatures which may be encountered in a chemicallooping combustion fuel reactor. In the system with Fe2O3/Fe3O4, there is no risk of sulphide or sulphate formation at any concentration of sulphur-containing gases at any temperature. Below, a more detailed analysis of the Mn, Ni, Cu, Co and W based systems is given. As seen in Figures 14 –17, formation might take place at high sulphur-containing gas concentrations. The sulphur concentration needed for formation of these compounds increases with temperature and decreases with total pressure. The fact that formation is enhanced at higher pressures is explained by the higher partial pressure of H2S and SO2 when the total pressure is increased. During the oxidation of CH4, there is a volume expansion by approximately a factor 3 which reduces the concentration of sulphur species. This means that the H2S concentration in the fuel can be approximately three times that shown in the figures below before sulphide or sulphate formation is expected.
With nickel oxide as oxygen carrier, there is a significant risk of sulphide formation at low temperatures as seen in Figure 14. There are several different nickel sulphides, and calculations showed that Ni3S2 is the phase which is most likely to form. In the system containing CuO/Cu, the concentration of sulphur-containing gases, i.e., H2S, SO2, SO3, COS and S2, needed for formation of Cu2SO4 is at least 2.2 vol% at 15 bar and 33 vol% at 1 bar. Fractions that high are not likely to be found in the fuel and therefore the risk of formation is low. Figure 15 shows that when Cu2O is used as oxygen carrier, the sulphide first formed is Cu2S. Formation needs lower concentration of sulphur-containing gases than formation of Cu2SO4 when CuO is used as oxygen carrier, but is still quite unlikely. At 1 bar, Cu2S forms at a sulphur concentration of at least 10 vol% which is just above the range shown in Figure 15. For Mn3O4/MnO, there is no formation of sulphides. For formation of MnSO4, a sulphur concentration of at least 3.1 vol% at 15 bar and 46 vol% at 1 bar is needed at every temperature investigated. Concentrations that high are unlikely to occur and thus sulphate formation should not be a problem. Figure 16 shows that the risk of MnSO4 formation is somewhat greater for the Mn2O3/ MnO system. With respect to cobalt oxide, CoS0.89 may form in the fuel reactor. As shown in Figure 17 there is a significant
Figure 14. Concentration of sulphur-containing gases, i.e., H2S, SO2, SO3, COS and S2, needed in the gas phase for formation of Ni3S2 in the NiO/Ni system.
Figure 16. Concentration of sulphur-containing gases needed in the gas phase for formation of MnSO4 in the Mn2O3/MnO system.
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Figure 17. Concentration of sulphur-containing gases needed in the gas phase for formation of CoS0.89 in the Co3O4/Co and CoO/Co systems.
risk of formation for both systems, especially at low temperatures. For WO3/WO2.722, a sulphur concentration of at least 3.6 vol% at 15 bar and 7.4 vol% at 1 bar is needed for formation of WS2 and therefore sulphide formation should not be expected.
DISCUSSION It is important that the oxygen carrier used in the chemical-looping combustion process can convert the gas almost completely to CO2 and H2O. Combustible gases, such as CH4, CO and H2, leaving the fuel reactor, would mean a loss in combustion efficiency. An oxygen carrier for which minor amounts of fuel gas is unconverted may be acceptable, since only a small amount of oxygen would be needed downstream the fuel reactor to oxidize these species. Based on the results from the reduction and oxidation calculations, it was found that some oxides of copper, iron and manganese are suitable as oxygen carriers since they can fully convert the fuel. Nickel oxide, tungsten oxide, barium- and strontium sulphate also show high gas yield, although they are not able to fully convert the fuel. Cobalt has a somewhat lower gas yield, which is a drawback for this oxide system. More specifically, all iron oxides except Fe2O3/Fe3O4 are thermodynamically unsuitable oxygen carriers. Further, MnO/Mn, ZnO/Zn and all oxides of cerium can be excluded. Low melting temperatures indicate that zinc, cerium, cadmium and molybdenum can not be used in the process. Molybdenum is excluded because its stable oxide MoO3 melts at a low temperature. For copper, a low temperature may be required because of its relatively low melting temperature. Some oxides do not form in the air reactor at conditions feasible for the process. Silver could not be used because it does not form any oxides at the conditions investigated. Mn2O3, CuO and Co3O4 decompose at high temperatures and including these oxides in the process would require a low temperature. If a higher temperature is preferred, the more reduced forms, Mn3O4, Cu2O and CoO should be used. However, it should be noted that by running the process under an increased total pressure, the more oxidized form could be used at a higher temperature without decomposing.
All the systems investigated, except those with copper oxides and the Mn2O3/MnO system, gave a temperature drop in the fuel reactor when methane was burnt. This temperature decrease was most striking for systems with a high oxygen ratio and this could be a problem since the reactions might slow down or even stop at low temperatures. To prevent this temperature decrease from being too large, a high oxygen carrier mass flow is needed. This would result in a low degree of conversion difference between the reactors, and thus a smaller temperature difference. For systems giving an increased fuel reactor temperature, e.g., oxidation of methane with copper oxides, the reactor may be cooled to achieve desired reaction temperatures. This is also the case for CO and H2 combustion, where the reaction in the fuel reactor always is exothermic. Since the chemical-looping combustion process always should be run with full or high fuel conversion, the amount of added oxygen is always well over the amount where carbon formation is possible. Therefore, problems with carbon formation are not expected in a wellmixed fluidized fuel reactor. However, carbon formation may have to be considered for applications where the local or total oxygen supply could be low, e.g., chemical-looping reforming. Further, some studies have found minor carbon formation of NiO based carriers, although more than the thermodynamic amount of oxygen was added. The authors suggested that carbon may be an intermediary product in the reduction process (Cho et al., 2005). H2S in the fuel will be oxidized to SO2 when the fuel is burnt. For most of the systems studied, this oxidation is almost complete at all conditions. Exceptions are systems based on nickel, cobalt and tungsten, where the oxidation to SO2 is low at low temperatures and increased pressures. If there are low concentrations of sulphur-containing gases and a high temperature in the fuel reactor, BaSO4 and SrSO4 will decompose. This decomposition of the sulphates leads to a loss of the active oxygen carrier and thus BaSO4 and SrSO4 cannot be used at these conditions. The three sulphides and sulphates that are most likely to be formed are Ni3S2, MnSO4 and CoS0.89. All these compounds have low melting points, 7898C, 7008C and 8348C, respectively, and therefore it is imperative to avoid formation of these in a fluidized bed fuel reactor. The results are summed up in Table 5 below, where the different oxide systems feasible for the process can be compared. Since this is a theoretical thermodynamic analysis of the process, it should be noted that all the results refer to equilibrium. However, in a real application the rates of reaction, i.e., in the fuel and air reactor, are also very important and have implications with respect to the size of a CLC system. There have been quite a number of investigations in the last few years of oxygen carrier reactivities, especially carriers based on Cu, Mn, Fe and Ni, see for instance Johansson (2005) for a review. The reaction rates vary considerably and are dependant not only on the type of oxygen carrier, but also preparation method and type of inert material used. Abad et al. (2006) point out that there are several resistances that can affect the reaction rates in the fluidized bed reactors. An investigation of the kinetics of the reduction and oxidation reactions with three oxygen carriers based on Cu, Fe, and Ni reduced by the fuel gases CH4, CO and H2 was performed at atmospheric pressure and showed that
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Table 5. Comparison between the different systems.
Fuel conversion NiO/Ni CuO/Cu Cu2O/Cu Fe2O3/Fe3O4 Mn2O3/MnO Mn3O4/MnO Co3O4/Co CoO/Co WO3/WO2.722 BaSO4/BaS SrSO4/SrS
Decomposition temperature for pO2 ¼ 0.04
Melting temperature
I 9448C
Conversion of H2S to SO2
Sulphide/sulphate formed
I
Ni3S2
10858C 10858C MnSO4
7388C L L I I I
8458C
I L I
CoS0.89 CoS0.89
Only at lower temperatures. I ¼ incomplete; L ¼ low.
the kinetic parameters of the reactions were controlled by chemical reactions.
present in the fuel and that excessively high temperatures are avoided. NOMENCLATURE
CONCLUSIONS A comprehensive thermal investigation of oxygen carriers for chemical looping combustion has been performed. Some metal oxides and reduced metal oxide/metal systems based on Cu, Mn and Fe show excellent characteristics and can be used as oxygen carriers in chemicallooping combustion. Oxides of Ni and W as well as BaS and SrS are also highly promising, but have somewhat lower gas yields in comparison to Cu, Mn and Fe. The metal Co and its corresponding oxide, CoO, give a gas yield between 0.93 and 0.97 and this may be too low for practical use. Oxidation investigations indicate that for oxides of copper, manganese and cobalt, it is the more reduced oxide form Cu2O, Mn3O4 and CoO, respectively that should be used, unless the process is performed at low temperatures or higher total pressures. The use of copper is also limited to lower temperatures due to its low melting point. Fe2O3 has the advantages as oxygen carrier of being cheap and easily available (Mattisson et al., 2001). Disadvantages with the iron oxides, the systems with manganese oxides and the tungsten oxides are their relatively low oxygen ratios. Although there was a temperature drop in the fuel reactor for many systems using methane as fuel, the drop can be limited by applying a sufficient rate of circulation of solids between the air and fuel reactor. The possible formation of solid carbon, sulphides and sulphates was investigated. Formation of carbon should not be a problem as long as the process is run under conditions of high fuel conversion, i.e., with a sufficient amount of oxygen continuously added with the particles. With respect to sulphur contamination, H2S in the fuel is converted partially to SO2 as the fuel is burnt. When nickel, cobalt or manganese is used as oxygen carrier the fuel may need to be desulphurized prior to combustion to avoid formation of solid or liquid sulphides or sulphates. To prevent decomposition of the oxygen carriers BaSO4 and SrSO4 to sulphur-containing gases and metal oxides, in the fuel reactor, it is necessary that some sulphur is
cpi Hi Hprod Hreac h0i hi m mox mred ni nO,added nO,stoich pCH4,in pi R0 T xO2,ex yi
specific heat capacity at constant pressure for component i, J mol21 K21 lower heating value of component i, J enthalpy of the exiting particles and gas, J enthalpy of the incoming particles and fuel, J heat of formation of component i at 298 K, (J mol21) enthalpy of component i, J mol21 actual mass of oxygen carriers, kg mass of oxygen carriers in oxidized form, kg mass of oxygen carriers in reduced form, kg amount of reactants, mol actual amount of O added with the oxygen carrier and/or steam, mol stoichiometric amount of oxygen needed for full conversion of the fuel, mol partial pressure of methane in the reactant gas, Pa partial pressure of component i in the product gas, Pa oxygen ratio temperature, K volume fraction of oxygen in the air leaving the air reactor amount of products, mol
Greek symbols gheat gas yield for methane based on the fuels heating values gi gas yield of component i gox conversion of gas in the air reactor DH standard heat of reaction, J l air ratio 6 oxygen added ratio v mass conversion of oxygen carrier
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