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Ind. Eng. Chem. Res. 2005, 44, 668-676
Carbon Formation on Nickel and Iron Oxide-Containing Oxygen Carriers for Chemical-Looping Combustion Paul Cho,*,† Tobias Mattisson,‡ and Anders Lyngfelt‡ Departments of Environmental Inorganic Chemistry and Energy Conversion, Chalmers University of Technology, S-412 96 Go¨ teborg, Sweden
For combustion with CO2 capture, chemical-looping combustion with inherent separation of CO2 is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g., metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. Carbon deposition on oxygen-carrier particles was investigated to assess whether it could have adverse effects on the process. The oxygen-carrier particles used were based on oxides of nickel and iron and produced by freeze granulation. They were sintered at 1300 °C for 4 h and sieved to a size range of 125-180 µm. The study of carbon deposition was performed in a laboratory fluidized-bed reactor, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions. The particles with nickel oxide were tested at 750, 850, and 950 °C, and the particles with iron oxide at 950 °C. On the oxygen carrier with nickel oxide, only minor amounts of carbon formed during most of the reduction. However, when more than 80% of the oxygen available was consumed, significant carbon formation started. The formation of carbon was also clearly correlated to low conversion of the fuel. No carbon was formed on the oxygen carrier based on iron oxide. The interpretation for the actual application of this process is that carbon formation should not be a problem, because the process should be run under conditions of high conversions of the fuel. 1. Introduction The concentration of CO2 in the atmosphere has risen to about 370 ppm today from the preindustrial value of 280 ppm, and it is generally accepted that a reduction in emissions of greenhouse gases is necessary. This could be achieved by increasing the efficiency of energy conversion and use of energy, as well as increasing the use of renewable energy sources, such as biofuel and wind power. However, in the short term, fossil fuels are still the dominant energy source worldwide. An additional option for achieving reduction would be separation and sequestration of CO2 before or after combustion of fossil fuel. A number of known techniques can be used to carry out this separation, but a significant disadvantage with most of these techniques is the large amount of energy that is required to obtain the CO2 in pure form, which means that the efficiency of power plants decreases by 7-12 percentage units.1 It is thus important to find a method whereby CO2 can be separated from the flue gases with a small loss in efficiency. In chemical-looping combustion, a metal oxide is used as an oxygen carrier that transfers oxygen from the combustion air to the fuel.2 The main advantage with chemical-looping combustion compared to normal combustion is that CO2 is inherently separated from the other flue gas components, i.e., N2 and unused O2, and thus, costly equipment and energy consumption for separation of CO2 are avoided.3,4 * To whom correspondence should be addressed. Tel.: +46(0)31-7722869. Fax: +46(0)31-7722853. E-mail: pcho@ chem.chalmers.se. † Department of Environmental Inorganic Chemistry. ‡ Department of Energy Conversion.
1.1. Chemical-Looping Combustion System. The chemical-looping combustion system is composed of an air reactor and a fuel reactor (see Figure 1). Lyngfelt et al. proposed a design in which these reactors were interconnected fluidized beds.5 The gaseous fuel is introduced to the fuel reactor which contains an oxygen carrier that is reduced according to the reaction
(2n + m)MexOy + CnH2m f (2n + m)MexOy-1 + nCO2 + mH2O (1) where MexOy is the fully oxidized oxygen carrier and MexOy-1 is the reduced oxygen carrier. The flue gas stream leaving the fuel reactor contains only CO2 and H2O if full oxidation of the fuel has been achieved, which means that pure CO2 can be obtained by cooling the gas and removing the water condensate. The reduced oxygen carrier, MexOy-1, is then transported to the air reactor and oxidized by oxygen in the air according to the reaction
2MexOy-1 + O2 f 2MexOy
(2)
The flue gas stream leaving the air reactor contains N2 and some remaining unused O2. The oxidized oxygen carrier, MexOy, is then transported back to the fuel reactor for another cycle. As a result, CO2 can be inherently separated in this combustion process, thus potentially avoiding costs and energy consumption associated with a gas separation process. The total amount of heat released in reactions 1 and 2 is equal to the heat released from normal combustion. The process has been successfully demonstrated in a 10 kW unit.6
10.1021/ie049420d CCC: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 669 Table 1. Oxygen Carriers oxygen carrier N6AN F6A
active metal oxidea NiOc Fe2O3c
supportb NiAl2O4 Al2O3d
a Mass fraction, 60%. b Mass fraction, 40%. c Johnson Matthey Gmbh. d Alcoa.
Figure 1. Chemical-looping combustion.
1.2. Carbon Formation. The carbon formation mechanisms have been studied in connection with synthesis gas production by partial oxidation of methane with a suitable catalyst. There are two possible ways of carbon formation: through methane decomposition
CH4 f C + 2H2
(3)
or through the Boudouard reaction
2CO f C + CO2
(4)
Claridge and co-workers reported that a significant amount of carbon can be deposited on palladium and nickel catalysts.7 A mixture of methane and carbon monoxide, pure methane, or pure carbon monoxide was used as the gas feedstock. Kinetically, both the Boudouard reaction (eq 4) and methane decomposition (eq 3) are known to be slow in the absence of a catalyst, but both reactions can be readily catalyzed by many transition metals such as nickel and iron. They found that at higher temperature (above 617 °C), the amount of carbon obtained from pure carbon monoxide via the Boudouard reaction is very low compared to the amount deposited from methane decomposition and at lower temperature, i.e., 397 °C, the Boudouard reaction (eq 4) is favored. In chemical-looping combustion, i.e., in the reduction of metal oxide with methane in the fuel reactor, side reactions might occur that result in the formation of solid carbon on the particles. The conditions for which carbon formation is thermodynamically possible depend on the amount of oxygen added with the nickel oxide as well as the temperature and pressure. Ishida and co-workers investigated carbon deposition on oxygen-carrier particles based on nickel and iron oxides mixed with either yttria-stabilized zirconia (YSZ), Al2O3, or TiO2 at 600 °C using thermogravimetrical analysis (TGA).8 The particles were prereduced by a mixture of hydrogen and nitrogen gas, and the carbon deposition was studied with a mixture of carbon monoxide and nitrogen gas. The carbon deposition was indicated by the increased weight of the particles in TGA. They suggested that the carbon formation was caused by the Boudouard reaction (eq 4), and found that the carbon deposition rates for iron-oxide-based particles were lower than those for nickel-oxide-based particles except for Fe2O3/YSZ particles. For nickel oxide, the binders affect the carbon deposition rate in the order Al2O3 > YSZ > TiO2. They also studied effect of temperature in the interval of 550-900 °C for the NiO/YSZ
and found that, at 900 °C, there was no carbon deposition. They found that the carbon deposits decreased with increased reaction temperature and H2O/CO ratio. Jin and co-workers investigated carbon deposition on oxygen-carrier particles based on nickel oxide mixed with either YSZ or NiAl2O3 at 600 °C also using TGA, with either methane or humidified methane as the fuel.9 They found that the carbon deposition is mainly caused by methane decomposition (eq 3) and that, by adding water vapor at the ratio of H2O/CH4 ) 2.0, carbon deposition could be completely avoided in NiO/NiAl2O3. Carbon deposition on the oxygen-carrier particles CoONiO/YSZ at temperatures of 600, 700, and 800 °C was also studied.10 Carbon deposition on the particles was observed at 700 and 800 °C if no water vapor was added. However, when water vapor was added, at H2O/CH4 ) 2.0, carbon formed only at 800 °C. Ryu and co-workers investigated the effects of different reaction temperatures on carbon deposition with an oxygen carrier of NiO/bentonite using TGA.11 A mixture of methane and nitrogen was used as the fuel, and the investigated temperatures were in the range of 6501000 °C. They showed that carbon formation on the particles decreased with increasing reaction temperature and, at temperatures above 900 °C, there was no carbon deposition. They found that, at 900° C, carbon formation was seen when 90% or more of the available oxygen in the metal oxide was consumed. 1.3. Objective. In chemical-looping combustion, it could be important to avoid carbon formation during the oxidation of the fuel. The carbon could be transferred to the air reactor and oxidized to carbon dioxide, resulting in lower separation efficiency of carbon dioxide. It has also been suggested that carbon deposition could have adverse effects on the particles.8 Previous work has shown carbon formation on a number of ironand nickel-based oxygen carriers . This work, however, was done under highly reducing conditions, and its interpretation for an actual application of chemicallooping combustion is not clear. The aim of the present work is to use another experimental approach to assess whether carbon formation is a potential difficulty for the industrial application of chemical-looping combustion. Particles based on nickel oxide and iron oxide were selected for this study. The support material used, Al2O3, was also the material that had the highest carbon deposition for NiO in the previously mentioned study by Ishida et al.8 2. Experimental Section 2.1. Oxygen-Carrier Preparation. Oxygen-carrier particles based on nickel oxide and iron oxide were prepared using freeze-granulation (see Table 1). In this process, a commercial metal oxide powder and an aluminum oxide powder were mixed with distilled water and a small amount of dispersion agent (Duramax D-3021). The mixture was then ground in a ball mill for 17 h, and the resulting slurry was made into frozen spherical particles by being sprayed through a nozzle
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Figure 2. Experimental setup.
into liquid nitrogen. The water in the frozen particles was then removed by freeze-drying. The particles were then pyrolyzed to remove organic material and sintered at 1300 °C for 4 h. The heating rate was 2 °C/min to 200 °C, 1 °C/min to 450 °C, and 10 °C/min to 1300 °C. The oxygen carriers were then sieved to a size range of 125-180 µm. The mass ratio of Fe2O3 to Al2O3 was 6/4. Metal oxides with oxidation number 2+ for the metal, such as NiO, CuO, and MnO, form metal aluminum spinel compounds (metal aluminates, MeAl2O4, where Me ) Ni, Cu, Mn) by solid-state reaction with Al2O3. Because these compounds are unreactive with methane or oxygen, the active metal oxide was added in excess to obtain a NiO/NiAl2O4 mass ratio of 6/4. 2.2. Experimental Setup and Procedure. The experimental setup is shown in Figure 2. The quartz reactor in the electric heater had a length of 820 mm, with a porous quartz plate of 30-mm diameter placed 400 mm from the bottom of the electric heater. The inner diameter of the bottom section was 19 mm, and that of the top section was 30 mm. The temperature of gas entering the bed was measured 5 mm below the porous quartz plate using a 10% Pt/Rh thermocouple enclosed in a quartz shell. The temperature of the fluidized bed was measured 10 mm above the porous quartz plate with the same type of thermocouple. The amount of oxygen carrier used in experiments was 15 g, and the size of the oxygen-carrier particles was 0.125-0.180 mm. In this work, the carbon deposition on the oxygen carriers was measured at 750, 850, and 950 °C for particles with nickel and at 950 °C for particles with iron oxide. The particles were preheated in an inert atmosphere (N2, 1000 mL/min) to the desired experimental temperature and then exposed to 5% O2 in N2 for 1000 s to ensure that the particles were fully oxidized before the reduction period. After the oxidation and reduction periods, inert gas (N2) was introduced for 200 s to avoid air and methane mixing during the shift between the reduction and oxidation periods. The particles were then exposed to either 100% CH4 or a mixture of 50% CH4 and 50% H2O. The flow of the CH4 was 450 mL/min in both cases. The minimum fluidizing velocities, umf; fluidizing velocities, u; and ratios u/umf for reducing, inert, and oxidizing gases are reported in Table 2. The minimum fluidizing gas velocity was
calculated by using equations described by Wen and Yu.12 The properties of the reducing gas vary with the degree of gas conversion. The lower value of umf is calculated assuming full conversion to carbon dioxide and water and the higher value assuming no conversion, i.e., methane. Similarly, the higher value of u was calculated assuming full methane conversion to carbon dioxide and water and the lower value assuming no conversion. The H2O was introduced to study the suppressing effects of steam on carbon formation during the reduction period. The reduction period varied between 30 and 300 s. After a subsequent period of inert gas flow, a new cycle was started by exposing the oxygen carriers to 5% O2 in N2. The typical number of the cycles varied from 6 to 14. The gas flows were led through mass flow controllers (Brooks 5850E) to preprogrammed magnetic three-way valves that supplied one of the gases CH4/H2O, N2, or O2/N2 to the reactor while leading the remaining two gases to a fumehood. The flue gas from the reactor was led to an electric cooler (M&C Products ECP 1000), where the condensed water was removed, and then led to a gas analyzer (Rosemount NGA-2000), where the concentrations of CO2, CO, CH4, and O2 were measured. The high-purity gases were delivered by AGA Gas. The pressure difference between the inlet and outlet gases was measured. 2.3. Data Evaluation. The gas yield, γ, of methane to carbon dioxide was defined as
γ)
pCO2,out
(5)
pCH4,out + pCO2,out + pCO,out
The conversion of oxygen carriers, or the degree of oxidation, was defined as
X)
m - mred mox - mred
(6)
The reduced form could be either the metal or a metal oxide with lower oxidation number than the oxidation number of fully oxidized metal oxide. Thus, the difference between mox and mred in eq 6 is the maximum theoretical amount of oxygen in the oxygen carrier that can be removed through reaction with methane. The conversion of oxygen carrier, X, as a function of time during the reduction period can be calculated from the outlet gas concentrations by
X)1-
n˘
(4p + 3pCO,out - pH ,out) dt ∫tt nOout ptot CO ,out 1
2
0
2
(7)
The partial pressure of H2 was estimated assuming equilibrium of H2, H2O, CO, and CO2 in the shift reaction (eq 8)
CO + H2O f CO2 + H2
(8)
by using relation 9
pH2,out )
2KpCO,out(pCO2,out + pCO,out) pCO2,out + KpCO,out
(9)
where K, the equilibrium constant for reaction 8, is 0.67 at 950 °C, 0.89 at 850 °C, and 0.92 at 750 °C. In a
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 671 Table 2. Minimum Fluidizing Velocities, umf; Fluidizing Velocities, u; and Ratios u/umf for the Reduction and Oxidation Periods reduction umfa (cm/s)
inert
ub (cm/s)
umf (cm/s)
u/umf
oxidation
u (cm/s)
u/umf
umf (cm/s)
u (cm/s)
u/umf
9.5
7.1
1.3
10.6
8.2
without steam with 50% steam
1.3-1.9 1.3-1.5
4.8-14.3 9.5-19.0
F6A at 950 °C 2.5-11.3 1.3 6.4-15.2
without steam with 50% steam
0.7-1.1 0.7-0.8
4.8-14.3 9.5-19.0
N6AN at 950 °C 4.4-20.1 0.8 11.3-27.0
9.5
12.6
0.8
10.6
13.8
without steam with 50% steam
0.8-1.1 0.8-0.9
4.4-13.1 8.7-17.5
N6AN at 850 °C 3.8-16.9 0.8 9.5-22.6
8.7
11.0
0.8
9.7
12.0
without steam with 50% steam
0.9-1.2 0.9-1.0
4.0-8.0 8.0-15.9
N6AN at 750 °C 3.3-9.3 0.8 7.9-18.5
8.0
9.5
0.9
8.8
10.3
a The lower value of u mf was calculated assuming full methane conversion to carbon dioxide and water and the higher value assuming no conversion. b The higher value of u was calculated assuming full methane conversion to carbon dioxide and water and the lower value assuming no conversion.
Table 3. Oxygen Ratio, RO, for Iron and Nickel Oxide Systems and for the Investigated Oxygen Carriers MexOy/MexOy-1 NiO/Ni Fe2O3/Fe3O4a
oxygen carrier 0.21 0.03
N6AN F6A
0.16 0.02
a
RO value for Fe2O3/Fe is 0.30, and the corresponding value for the iron-oxide-based oxygen carrier is 0.18.
previous paper,13 the estimated partial pressure of H2 was found to be well correlated to the measured concentration at 950 °C. The conversion of oxygen carrier, X, as a function of time during the oxidation period was defined as
X ) X0 +
∫tt nO2ptot(n˘ inpO ,in - n˘ outpO ,out) dt 4
2
2
2
(10)
where X0 is the conversion at the start of the oxidation period, t2 is the time when the oxidation started, and t4 is the time when the oxidation period ended. Because different oxygen carriers can transfer different amounts of oxygen per unit mass, it is an advantage to be able to compare the carriers using a mass-based conversion. In this work, the mass conversion was defined as
ω)
of carbon introduced during the reduction period and is defined as
m ) 1 - RO(1 - X) mox
(11)
where RO is the oxygen ratio, defined as
RO )
(mox - mred) mox
(12)
The oxygen ratio shows the maximum mass flow of oxygen that can be transferred between the air and fuel reactors in chemical-looping combustion for a given mass flow of circulating oxygen-carrier particles. The oxygen ratios, RO, for both pure iron and nickel oxide systems and for the oxygen carriers used in this work are reported in Table 3. The carbon formation during the reduction period was calculated by integrating the total amounts of carbon dioxide and carbon monoxide produced during the subsequent inert and oxidation periods. The carbon formation ratio, C/Ctot, is the amount of carbon formed during the reduction period divided by the total amount
∫tt n˘ out(pCO ,out + pCO,out) dt 3
C ) Ctot
2
1
∫t
t3
0
n˘ out(pCH4,out + pCO2,out + pCO,out) dt
(13)
where t0 is the time when the reduction started, t1 is the time when the inert period started, and t3 is the point of time during the oxidation period when no more carbon dioxide or carbon monoxide was detected. The oxygen-added ratio, σ, is defined as the actual amount of O added to the gas, nO,added, to the stoichiometric amount needed for full conversion of CH4 to CO2 and H2O, nO,stoich
σ)
nO,added nO,stoich
(14)
3. Results In Figure 3, the outlet concentrations of CO2, CO, CH4, and O2 are shown as functions of time for the oxygen carrier N6AN without steam addition. The length of the reduction period in Figure 3 are (a) 80, (b) 110, and (c) 180 s. In Figure 3a-c, because of the residence time of the gases between the three-way valve and the gas analyzer, the outlet gas concentration signal is delayed by 15-20 s depending on the flow rate of the gas. During the first 100 seconds of Figure 3a-c, almost all of the reacted CH4 is converted to CO2 and H2O. For Figure 3b and c, as the reactions proceed, the oxygen in the particles is depleted, and the outlet concentration of CO2 decreases, whereas the concentrations of CO and CH4 increase. The subsequent slow decrease of the CO concentration in the inert period and peaks of CO and CO2 in the beginning of the oxidation period indicate carbon formation during the previous reduction period. In contrast, in Figure 3a, almost no carbon was formed during the reduction period, as seen from the low CO and CO2 concentrations throughout the inert and oxidation periods. Nevertheless, the low and decreasing CO concentration in the inert period indicates that a small amount of carbon has been formed during the reduction period. The C/Ctot ratio obtained for the case shown in Figure 3a was 0.5%, whereas C/Ctot was 2.4% and 18.7% for the cases shown in Figure 3b and 3c. The three cases
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Figure 4. Carbon formation ratio, C/Ctot, as a function of length of reduction period, (]) without steam and (+) with 50% steam for N6A particles: (a) 950, (b) 850, and (c) 750 °C.
Figure 3. Concentrations of CO2 (s), CO (- - -), CH4 (- - -), and O2 (- - - -) as a function of time for N6AN particles during the reduction period (without steam), the inert period, and the beginning of the oxidation period: (a) 80 s of reduction time at 950 °C, (b) 110 s of reduction time at 850 °C, and (c) 180 s of reduction time at 950 °C.
shown in Figure 3a-c were chosen to illustrate (a) negligible carbon formation, (c) extensive carbon formation, and (b) the conditions where the formation of carbon has just started. Note, in Figure 3c, that, in the latter part of the reduction period, the sum of the CO, CO2, and CH4 concentrations is much less than 100%. There is good reason to believe that the balance is H2 formed from reaction 3. In all cases, the outlet concentration of O2 was zero during most of the oxidation period except for a rapid increase to the inlet concentration at the end of the oxidation period (not shown). This indicates that all of the O2 going into the reactor was rapidly consumed and that the rate of oxidation was limited by the supply of O2 to the reactor. The carbon formation ratio, C/Ctot, is shown in Figure 4 as a function of reduction time for N6AN at 950, 850, and 750 °C. The carbon formation ratios were low for N6AN at all investigated temperatures up to 100 s (see Figure 4a-c). It is clearly seen that the carbon formation ratios are lower with 50% steam present, but the difference is rather small. After 100 s, the carbon formation ratios increased with the length of the reduction period. Less carbon was produced at higher temperature. It should be mentioned that all available oxygen left in the N6AN is consumed when the reduction period exceeds 120 s, if full conversion of the methane is assumed. For the iron-based oxygen carrier F6A, there was no carbon formation, except for a very small amount formed after 300 s of reduction period (see Figure 5). It should take only 15 s of reduction before all available oxygen is consumed if full conversion is assumed, i.e., from Fe2O3 to Fe3O4. However, because of the low reactivity, the conversion of the gas is low, typically 30%, and approximately 50 s is needed to reach full conversion. Figure 5 also shows the mass conversion versus time. Notice that all of the mass conversion values shown have decreased below 98%, which means that the particles have lost more than 2% of their mass, i.e., oxygen. According to the definition of conversion, X, for the iron oxide system, the conversion reaches 0%
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 673
Figure 5. Carbon formation ratio, C/Ctot, and mass conversion, ω, for F6A, as a function of length of reduction period, (]) without steam and (+) with 50% steam.
Figure 7. Carbon formation ratio, C/Ctot, as a function of the gas yield at breakpoint, γ, i.e., at the end of the reduction period, (]) without steam and (+) with 50% steam: (a) N6AN, 950 °C; (b) N6AN, 850 °C; and (c) N6AN, 750 °C.
Figure 6. Carbon formation ratio, C/Ctot, as a function of the gas yield, γtot, of methane to carbon dioxide during the reduction period (]) without steam and (+) with 50% steam: (a) N6AN, 950 °C; (b) N6AN, 850 °C; (c) N6AN, 750 °C; and (d) F6A, 950 °C.
when the mass conversion reaches 98%. This means that all of the conversion values in Figure 5 are below 0%, and this is explained by the further reduction of Fe3O4 to FeO. This is possible only because of the low conversion of the fuel because the further reduction to FeO cannot, thermodynamically, oxidize the fuel fully to CO2 and H2O. Figure 6 shows C/Ctot as a function of the total gas yield over the reduction period for N6AN at 950, 850, and 750 °C and for F6A at 950 °C. For N6A, it is clear that less carbon was formed when a higher degree of methane conversion to carbon dioxide was achieved. As the investigated temperature of the reduction period decreased from 950 to 850 °C, the slope of the C/Ctot increased. However, no significant difference in slope
was noted when the temperature was decreased to 750 °C (see Figure 6a-c). For F6A, there was no carbon formation despite the much lower values of γtot, except for a small amount, 0.6%, at the lowest γtot value of 12.6% (see Figure 6d). The difference illustrates the catalytic effect of the Ni. The gas yield at breakpoint is the instantaneous value of the gas yield at the moment that the reduction period ends and the inert period starts. Because of the time delay caused by gas residence time, the gas yield at breakpoint is defined at the point when the sum of the CO2, CO, and CH4 concentrations is decreased by 50%, i.e., in the middle of the transient. C/Ctot as a function of the gas yield at breakpoint for oxygen carrier N6AN at 950, 850, and 750 °C is shown in Figure 7a-c. For all investigated temperatures, significant amounts of carbon formed only for low values of gas yield at breakpoint. This indicates that significant carbon formation starts only after the particles have lost so much oxygen that they are not able to convert much of the fuel. The mass conversion, ω, describes mass changes in the oxygen carrier. The dashed vertical line at 84% in Figure 8a-c marks the point of mass conversion where the theoretical amount of oxygen in oxygen carrier is consumed by methane, i.e., 1 - RO. For N6AN, C/Ctot increases rapidly when the mass conversion values are below 87%, i.e., when 80% of oxygen has been consumed (see Figure 8a-c). As shown before, there was no carbon formation on F6A for all but one case studied, and in all tests, the mass conversion passed the theoretical limit of reduction to Fe3O4 (see Figure 5). It should also be pointed out that all experiments with F6A resulted in agglomeration if the reduction period lasted longer than 70 s. This was seen both in the absence and in the presence of steam. However, the particles were still fluidized, i.e., there was no agglomeration, after 20 oxidation/reduction cycles when the length of the reduction period was kept at 50 s. This was seen both with and without addition of
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Figure 10. (s) Thermodynamic conditions for carbon formation from CH4. For values of the oxygen-added ratio, σ, above the lines, carbon is not formed. Pressures of (+) 1, (0)15 and (]) 30 bar.
Figure 8. Carbon formation ratio, C/Ctot, as a function of the mass conversion, ω, after the reduction period (]) without steam and (+) with 50% steam: (a) N6AN, 950 °C; (b) N6AN, 850 °C; and (c) N6AN, 750 °C. The dashed vertical line shows when the theoretical amount of oxygen in the oxygen carriers is consumed.
Figure 9. Pressure drop over the reactor in oxidation periods with F6A. The upper curve shows when the bed is fluidized and the lower curve when it is defluidized because of agglomeration.
steam. Whether the particle bed was fluidized was clearly indicated by the pressure fluctuations, as can be seen in Figure 9, where the pressure drops over the reactor for a fluidized bed and a defluidized, i.e., agglomerated, bed are shown for F6A. 4. Discussion Most studies concerning carbon deposition on oxygencarrier particles have been done using TGA and high concentrations of reducing gas. In this study, carbon formation was investigated as function of the conversion of the fuel to assess whether it could be a problem in an actual process where high conversion of fuel is needed. From the outlet gas concentrations of carbon dioxide and carbon monoxide during the inert and oxidation periods, the carbon formation on oxygen-
carrier particles during the reduction period was obtained. With this approach, the carbon formed during high methane conversion can also be detected. Because there was no other source of oxygen during the inert period following the reduction period, the carbon formed on the particles was oxidized by oxygen from the oxygen carrier to carbon monoxide; cf. Figure 3b and c. For the oxygen carrier based on nickel oxide, N6AN, the carbon formation was more dependent on the length of the reduction period, i.e., mass conversion of the oxygen carrier, than on the steam input and temperature. The amount of carbon deposition increased rapidly after 80% of the available oxygen was consumed (see Figure 8). This rapid increase was clearly correlated with low conversion of the fuel (see Figure 7a-c). Figure 10 shows when carbon is formed assuming thermodynamic equilibrium. At the highest temperature, i.e., at 950 °C, no carbon formed when the oxygenadded ratio, cf. eq 14, was above 25%. This also happens to be the amount of oxygen needed to oxidize methane to CO and H2, which is consistent with reaction 3 representing the most important mechanism for carbon formation at these temperatures. The gas yield, eq 5, gives some indication of how much oxygen has transferred to the gas. In fact, if no CO or steam were added and if no H2 formation occurred, then the gas yield, γ, would be equal to the oxygen-added ratio, σ. Now, if CO is formed, the actual gas yield is lower than the actual oxygen-added ratio. Thus, from Figure 10, we should expect that carbon formation should not take place when σ is higher than 25%, and consequently, carbon formation would not be expected at γ > 25%. As can be seen in Figure 7, significant carbon formation does not take place for γ > 25%. However, some small amount of carbon was formed during the period when fuel conversion was high, mostly below 1% (C/Ctot) with a highest value of 1.8%. This suggests that carbon is a possible reaction intermediate and that methane conversion proceeds via carbon and hydrogen oxidation on the surface of the oxygen carrier. Thus, a possible route for methane oxidation is reaction 3 followed by reactions 15-18
C + H2O f CO + H2
(15)
C + CO2 f 2CO
(16)
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 675
including subsequent oxidation of CO and H2
H2 + NiO f H2O + Ni
(17)
CO + NiO f CO2 + Ni
(18)
Thus, catalytically enhanced carbon formation could also be an advantage and contribute to the reactivity of the oxide. Lyngfelt and co-workers reported that there was no detectable carbon dioxide in the outlet gas from the air reactor in 10-kW chemical-looping combustion,6 also using an oxygen carrier based on nickel oxide, which suggests that the lifetime of any carbon formed in the fuel reactor is short. Another possible reaction route could be via reforming, reaction 19, which is known to be catalyzed by the presence of nickel
CH4 + H2O f CO + 3H2
(19)
followed by reactions 17 and 18. No carbon formation occurs on the oxygen carrier based on iron oxide, F6A; however, the particles tend to agglomerate when the time of reduction is higher than 60 s. This corresponds to a value of ∆ω somewhat larger than 2%, which is the point where Fe3O4 starts to reduce to FeO (wustite). This finding suggests a connection between agglomeration and wustite formation during the reduction period. Furthermore, this result implies that the tendency toward agglomeration of particles with iron oxide should be tested under conditions where wustite is not formed, as wustite formation is not expected under the conditions of an actual chemical-looping process. Considering carbon formation, the available interval of mass conversion, ∆ω, is about 10% for the N6A (see Figure 8a-c). As shown in a previous paper,13 the oxygen-carrier circulation mass flow is related to the interval of mass conversion, ∆ω, i.e., less solids needs to circulate for a large ∆ω. However, because of the endothermic reaction in the fuel reactor, heat must be transferred by oxygen-carrier particles from the air reactor where heat is released. Thus, the temperature difference between the air and fuel reactors depends on the solids circulation mass flow. For example, a ∆ω value of 10% will give approximately a 300 °C difference, and a ∆ω value of 2% will give a 60 °C difference. To have a high enough temperature in the fuel reactor, the temperature difference should be low, and consequently, ∆ω must be low, i.e., a few percent. This can be compared to the high ∆ω value needed before the particles lose reactivity and carbon starts to form, which is well above 10% (see Figure 8a-c). 5. Conclusions The carbon formation conditions on oxygen carriers based on nickel oxide and iron oxide have been investigated. For the oxygen carrier based on nickel oxide, carbon formation was strongly dependent on the availability of oxygen. When sufficient oxygen in the oxide was still available, there was limited formation of carbon independent of either addition of steam or temperature in the investigated range of 750-950 °C, but when more than 80% of the available oxygen was consumed, rapid formation of carbon started. The onset of carbon formation was related to a dramatic fall in conversion of the fuel. The interpretation for the actual application of this process is that carbon formation should not be a
problem, because the process should be run under conditions of high conversions of the fuel. On the other hand, the formation of small amounts of carbon at high fuel conversion suggests that carbon could be a reaction intermediate in the oxidation of methane. For the particles with iron oxide, no or very little carbon was formed, even when the conversion of the fuel was very low. There seems to be a connection between agglomeration of particles with iron oxide and wustite formation. Acknowledgment The authors express thanks to the Swedish National Energy Administration for financial support. Notation C ) amount of carbon formed during the reduction period (mol) Ctot ) amount of carbon introduced during the reduction period (mol) m ) actual mass of oxygen carrier sample in the reactor (g) mox ) mass of the sample when fully oxidized (g) mred ) mass of the sample in the reduced form (g) nO ) amount of oxygen in the oxygen carrier that can be removed from fully oxidized oxygen carrier (mol) nO,added ) actual amount of O added to the gas with the oxygen carrier and steam (mol) nO,stoich ) stoichiometric amount of oxygen needed in the gas for full conversion of CH4 to CO2 and H2O (mol) n˘ in ) molar flow of the gas entering the reactor (mol/s) n˘ out ) molar flow of the gas exiting the reactor after the water has been removed (mol/s) pi,in ) partial pressure of gas i entering the reactor (Pa) pi,out ) partial pressure of gas i exiting the reactor after the water has been removed (Pa) ptot ) total pressure (Pa) RO ) oxygen ratio (eq 12) t ) time (s) X ) conversion of oxygen carrier, or the degree of oxidation (eq 6) Greek Letters ∆p ) pressure difference (Pa) ∆ω ) change of mass conversion during reduction or oxidation γ ) gas yield of methane to carbon dioxide (eq 5) σ ) oxygen-added ratio (eq 14) ω ) mass conversion of oxygen carrier (eq 11)
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Received for review July 2, 2004 Revised manuscript received December 1, 2004 Accepted December 1, 2004 IE049420D
