1450
Energy & Fuels 2009, 23, 1450–1455
Characterization of Chemical Looping Pilot Plant Performance via Experimental Determination of Solids Conversion Philipp Kolbitsch,* Tobias Pro¨ll, Johannes Bolhar-Nordenkampf, and Hermann Hofbauer Institute of Chemical Engineering, Vienna UniVersity of Technology, Getreidemarkt 9/166, Vienna 1060, Austria ReceiVed September 26, 2008. ReVised Manuscript ReceiVed December 11, 2008
Chemical looping combustion (CLC) is a novel combustion process with inherent CO2 separation. A CLC reactor system consists of an air reactor (AR) and a fuel reactor (FR). An oxygen carrier (OC) that circulates between the two reactors transports the necessary O2 for combustion. Because the solids conversion of the OC has an impact on total solids inventory, it is an important parameter for the performance of a CLC combustor. In this study, the solids conversion of a Ni-based OC in a dual circulating fluidized bed (DCFB) reactor system is determined. The pilot rig is fueled with H2 or CH4 at a fuel power of 65-140 kW. FR temperature, air/fuel ratio, and solids inventory are also varied. From the results obtained, the global solids circulation rate (GS) is calculated. GS shows a linear trend with the gas velocity in the AR and is nearly independent of the total solids inventory in the reactor system. It is shown that, in all cases, the solids conversion in the FR (XS,FR) is very low; in most cases, XS,FR is even 99.9 vol %) is used for this purpose because it does not interfere with gas analysis and is commercially available. The OC transports the necessary oxygen from the AR to the FR. In this study, a Ni-based OC with NiAl2O4 support is used (see Table 1). To characterize OCs, different numbers have been introduced. The oxygen ratio R0 R0 )
MOC,ox - MOC,red MOC,ox
(5)
with the molar masses of the fully oxidized and reduced OC MOC,ox and MOC,red, respectively, specifies the possible transportation of oxygen in the carrier in kg of O2/kg of OC. The higher the R0, the less solids circulation is necessary to operate a chemical looping combustor. The conversion XS of the OC is defined via the actual molar mass MOC (6)
Assuming stationary conditions, the difference in conversion between the inlet and outlet of a reactor is given by ∆XS ) XS,in - XS,out ) XS,AR - XS,FR
(7)
XS,AR and XS,FR describe the mean solids conversion of the particles present in the AR and FR, assuming perfect mixing of solids in the fluidized beds. The impact of a particle residence time distribution is neglected. The mass flow of circulating solids and the solids circulation rate can be directly calculated from the solids conversion. The required mass flow of the OC in a chemical looping reactor system is calculated via Ominm ˙ fuel R0∆XS
(8)
Table 1. OC Characterization parameter active material support material NiO content R0 MOC,ox MOC,red dp
with the combustion efficiency ηcomb and the oxygen demand Omin. From this relation, the solids circulation rate GS in the air reactor is calculated with the air reactor cross-section AAR as GS )
MOC - MOC,red XS ) MOC,ox - MOC,red
m ˙ OC ) ηcomb
Figure 2. Principle setup of the DCFB reactor system.14
unit
value
wt % kg/kg g/mol g/mol µm
Ni/NiO NiAl2O4 40.0 0.08568 114.26 104.47 90-210
m ˙ OC AAR
(9)
The higher the difference of conversion between the two reactors, the lower the required solids circulation rate, but owing to the smaller amount of oxidized OC particles in the fuel reactor, the required solids inventory increases.13 Experimental Section DCFB Reactor System. In this study, solids samples from a dual circulating fluidized bed (DCFB) reactor system are analyzed and used to determine the performance of the reactor system. Figure 2 shows the setup of the DCFB reactor system described by Kolbitsch et al.14 The AR is operated in the fast fluidization regime (7) Ishida, M.; Jin, H. Energy 1994, 19, 415–422. (8) Lyngfelt, A.; Leckner, B.; Mattisson, T. Chem. Eng. Sci. 2001, 56, 3101–3113. (9) Ryu, H. J.; Bae, D. H.; Jin, G. T. The World Congress of Korean and Korean Ethnic Scientists and Engineers, Seoul, Korea, 2002; pp 738743. (10) Abad, A.; Garcia-Labiano, F.; Adanez, J.; de Diego, L.; Gayan, P.; Celaya, J. 8th International Conference on Greenhouse Gas Control Technologies, Trondheim, Norway, 2006. (11) Hofmann, R.; Binder, F.; Werner, A. Bulk Solids Handl. 2005, 25, 378–385. (12) Binder, F.; Werner, A. Bulk Solids Handl. 2006, 26, 476–483. (13) Abad, A.; Adanez, J.; Garcia-Labiano, F.; de Diego, L. F.; Gayan, P.; Celaya, J. Chem. Eng. Sci. 2007, 62, 533–549. (14) Kolbitsch, P.; Bolha`r-Nordenkampf, J.; Pro¨ll, T.; Hofbauer, H. 9th International Conference on Circulating Fluidized Beds, Hamburg, Germany, 2008; pp 795-800.
1452 Energy & Fuels, Vol. 23, 2009
Kolbitsch et al.
Figure 3. Experimental setup for the solids sampling from the DCFB.
and transports the OC particle to the turbulent FR. Between AR and FR, two loop seals are placed to avoid leakage of CO2 (CO2 slip to AR) and dilution of the CO2 stream (air slip to FR). A third loop seal in the FR return leg allows the solids backflow to the FR. All loop seals are fluidized with steam. Solids sampling points are arranged in the upper loop seal (SSP1) and in the lower loop seal (SSP2). Solids Sampling Setup. Figure 3 shows the basic setup for the solids sampling from the reactor system. A heat-resistant pipe is introduced directly into the fluidized bed in the downward direction (A). In this way, solids are prevented from advancing toward the instrumentation during times when no solids are extracted. The pipe is then bent into a vertical direction. A ball valve (B), which does not distract the solids path in the fully open position, seals the connection to the fluidized bed at this point. From there, the solids pipe is connected to the solids deposit (D) with another ball valve (C) in between. The solids deposit outlet is equipped with a filtering device (E) and another valve (F). Between the two solids valves (B) and (C), a connection for inert gases with another valve (G) is attached. In the case of hot sampling, the solids container (D) is cooled with cold water to avoid overheating of the instrumentation. The applied solids container is shown in Figure 4. The gas-solids mixture enters the container through a downcomer pipe. At its end, the solids will form a pile, which will eventually advance into the downcomer pipe. This should seal the gas path toward the ambiance and therefore stop the solids from being withdrawn from the fluidized bed. The downcomer pipe in the container also supports the separation of gas and solids, owing to the strong decrease of velocity in the turn section. Solids Sampling Procedure. To collect solids from the fluidized bed, the solids container is first flushed with argon. This prevents the entering solids from reacting with the ambient oxygen in the air. For this purpose, valves C, F, and G are opened until the solids container has been fully flushed. Now, valve F is closed, and valve B is opened. The inert gas flows toward the reactor and blows all solids from previous experiments into the reactor. To start the solids flow to the container, valve G is closed and valve F is simultaneously opened. As soon as enough solids are collected, valve F is closed again. The opening of valve G means that the leftover solids in the downcomer pipe are pushed back into the reactor. Closure of valve B disconnects the fluidized bed from the sampling equipment. Finally, valves G and F are opened again to flush all reactive gases (steam from loop seals) from the reactor out of the solids container.
Figure 4. Solids container for collecting the solids from the fluidized bed. It mainly consists of a container with a removable cap, a downcomer pipe, and a gas exit opening.
Dependent upon the amount of collected solids, the apparatus experiences a temperature increase. Therefore, the solids container is placed in cold water for cooling during the whole procedure. In this way, the sampling equipment is protected from overheating, the entrained solids are instantly cooled, and further OC reactions are suppressed. Evaluation of the Samples. Once the sampled solids have reached ambient temperatures, the solids can be evaluated. For this purpose, two samples of approximately 15 g are placed in a heatresistant ceramic shell and its mass m0 is determined. The samples are then heated to 1000 °C and kept at this temperature for 3 h in oxidizing conditions in an oven. After cooling to ambient temperature, the mass of the fully oxidized sample mox is determined. From these values, the solids conversion can be calculated via
XS ) 1 -
mox - m0 moxR0
(10)
Chemical Looping Pilot Plant Performance
Energy & Fuels, Vol. 23, 2009 1453
Table 2. Error Assessment for the Solids Sampling and the Evaluation of the Solids Conversion XSa ∆XS error
worst case
typical case
O2 in solids content steam in solids content scale accuracy inert impurities
+7.52 × 10-1 +1.62 × 10-2 (5.71 × 10-3 +1.10 × 10-2
+2.87 × 10-2 +6.47 × 10-4 (7.28 × 10-4 +8.10 × 10-3
a
The sign of the number indicates in which direction XS is affected.
Whenever more than one sample is evaluated for one position, the mean solids conversion is calculated from all solids samples. Error Assessment. The sampling and evaluation of the solids are accompanied by different inaccuracies. These may occur during the sampling itself but also during weighing. Therefore, an error assessment is indispensable. In the following, the effect of five possible errors on the results is investigated: (1) oxidation of the OC by air in the solids container, (2) oxidation of the OC by steam in the solids container, (3) accuracy of the scale used, (4) inert impurities in the solids samples, and (5) insufficient time for full oxidation in the oven. The error assessment is performed for two different cases: a worst case and a typical case. In the worst case, the total sampled mass and the evaluated mass are lower than the smallest evaluated sample (mS ) m0 ) 2.0 g) and the solids conversion is set to XS ) 0.75 and 0.0, respectively, depending upon which value leads to higher errors. The typical case represents typical results and sample masses (mS ) 50.0 g, m0 ) 20.0 g, and XS ) 0.25). One possible error in the evaluation occurs when the solids container is not sufficiently flushed with inert gas prior to the sampling procedure. This leads to oxidation of the OC by the ambient air, and the results pretend a higher value for the solids conversion. The same effect occurs when the solids container is not properly flushed after the sampling procedure. The steam from the loop seal could oxidize the OC. The equilibrium concentration of H2 in this case is very small though ( 1.0, XS,FR is lower than in the case of lower solids inventory (30.0 kg). Because an increase of solids inventory should lead to higher gas conversion in the AR and FR, respectively, and the AR is operated at O2 excess, this result is very surprising. One possible reason is the temperature difference between the two experiments. At λ > 1.0, the average temperature in AR and FR is approximately 25 K higher in Figure 6 than in Figure 7. That could be a possible explanation, but as shown later, the effect of temperature on XS,FR is rather low. In a second series of experiments, the reactor system is fueled with natural gas at a fuel power of approximately 140 kW. Figure 8 shows the effect of the air/fuel ratio variation on solids conversion, CH4 conversion, and CO2 yield. The OC is again very much reduced in the FR. Even at λ > 1.0, XS,FR does not exceed 0.20. ∆XS is almost constant in this variation, and therefore, the courses of XS,AR and XS,FR are almost parallel. The CO2 yield is strongly increased when λ is increased, but the course is flattened when λ > 1.0. Another very interesting aspect is the CH4 conversion. XCH4 decreases with an increase
(14)
and therefore promotes the CH4 decay. A temperature variation for the same setup at a global air/ fuel ratio of 1.1 is shown in Figure 9. Again, the solids conversion in the FR is very low. Up to 900 °C, XS,FR is below 0.15 and nearly constant. At 950 °C, an increase of XS,FR is observed. ∆XS is almost constant in these experiments. The CO2 yield shows a steady increase with the temperature and reaches a value of 0.891 at 950 °C. The CH4 conversion shows a very interesting trend, which is influenced by two phenomena. A temperature increase promotes the decay of CH4 and reaches a maximum of XCH4 ) 0.954 at ≈900 °C. At this temperature, XCH4 starts to decline, as XS,FR increases. This may occur because of the decrease of free Ni surface in the FR. Effect of Increased Solids Conversion in the FR. During all experiments, very low solids conversion in the FR has been measured (see Figures 5-9). This indicates that, in this CLC pilot rig, the AR is the limiting reactor for oxygen transport. The solids conversion could be increased by an increase in the AR size and, thus, its solids inventory and solids residence time. This does not necessarily mean, however, that this measure will improve the CH4 conversion and CO2 yield. The experiments for H2 combustion have shown very high gas conversion. It is unlikely that an increase of the solids conversion will further improve these results. These experiments, however, have been performed at low fuel power. In the case of CH4 combustion, there is still potential for further improvement. Much improvement is expected, however, for an increase in reactor height. Unfortunately, the height of the air and fuel reactor is limited to 4.1 and 3.0 m, respectively, owing to the laboratory height. It has to be mentioned that the reactor height of a CFB is not scalable and, thus, should be much larger in the pilot rig. Nevertheless, the impact of increased solids conversion is investigated in a dynamic experiment. For this purpose, the OC is fully oxidized by replacement of the fuel with air. After full oxidation of the OC, the fuel is changed to CH4 again and CH4 conversion and CO2 yield are measured. Contacting the fuel with highly oxidized solids results in very low H2 and CO emissions but an increased CH4 content in the offgas. In total, the CO2 yield is decreased. The maximum CO2 yield is reached when stable operation is reached at very low solids conversion. This result further supports the hypothesis that steam reforming of CH4, which is supported by high Ni content (and thus low solids conversion), is a major reaction path in the overall oxidation reaction. Carbon Formation. Because carbon formation in the FR would lead the CO2 emissions from the AR and thus results in
Chemical Looping Pilot Plant Performance
Energy & Fuels, Vol. 23, 2009 1455
Figure 10. Solids circulation rate GS versus the ratio of superficial to terminal velocity in the AR. Table 3. XS,FR and Minimum Active Content of NiO (wNiO,active,min) for Different Highly Reducing Operation Cases (λ < 0.90) and Fuels λ
Pth (kW)
mOC,tot (kg)
fuel
XS,FR
wNiO,active,min
0.796 0.829 0.836 0.851 0.886 0.889 0.895
84.4 63.6 140 62.9 140 63.3 63.0
55 65 65 75 65 75 65
H2 H2 CH4 H2 CH4 H2 H2
0.117 0.123 0.105 0.136 0.111 0.133 0.132
0.353 0.351 0.358 0.346 0.356 0.347 0.347
a decreased CO2 capture efficiency, it has been discussed as a problem in CLC. The amount of carbon produced in the FR can be derived from CO2 measurements in the AR exhaust gas. In industrial steam-reforming applications, a significant amount of steam is added to the hydrocarbon feedstock. In all experiments performed in the CLC pilot rig, no additional steam is added to the fuel. Some steam, however, is available from the fluidization of the loop seals. This amount would correspond to a H2O/CH4 ratio of approximately 0.3:1. Despite this very low H2O/CH4 ratio and the very low solids conversion of the particles, no carbon formation has been measured in any operation case. Active NiO Content in OC. The NiO content of the fully oxidized particle is 40 wt % (see Table 1). It has to be assumed, however, that some of these molecules are trapped in the NiAl2O4 structure and cannot participate in oxygen transfer. In a highly reducing atmosphere, the maximum amount of O2 should be removed from the OC. From the solids conversion of the particle in these conditions, the active NiO content can be calculated. Table 3 shows some of the results of highly reducing conditions in the FR (λ < 0.90). From these results, the minimum content of active NiO wNiO,active,min (on the basis of the fully oxidized particle) is calculated. This value is found to be at least in the range of 35-36 wt %. The remaining 4-5 wt % of NiO is either too deep in the particle structure to participate in oxygen transfer or even higher reducing conditions are necessary for its use. Solids Circulation Rate. From the derived values for solids conversion, the global solids circulation rate GS,AR in the reactor system is calculated via eqs 8 and 9. In Figure 10, GS,AR is plotted against the ratio of superficial velocity to terminal velocity U/Ut. For the case mOC,tot ) 65 kg, a linear fit is calculated for a wide range of U/Ut. The total solids inventory in the reactor system has only a minor influence. Conclusions The solids conversion of the OC in CLC is an important parameter to be determined. In this investigation, the solids
conversion is measured at two different positions in a CLC pilot rig. The pilot rig is designed as a DCFB reactor system and fueled with CH4 and H2, respectively. The CLC reactor system is operated at 65-140 kW fuel power with a Ni-based OC. The solids are sampled at the solids inlet and outlet of the FR. The results clearly show that, independent of the fuel, the solids conversion XS is very low in the FR (≈0.20). Higher values are observed in only a few operating conditions. The H2 conversion is very close to the thermodynamic maximum but is limited when very low solid inventories are applied. Experiments with CH4 as fuel show very high CH4 conversion and high CO2 yield. The CH4 conversion shows only a slight dependence upon the fuel reactor temperature and air/ fuel ratio. Maximum CH4 conversion is observed at an air/fuel ratio below 0.90 and high fuel reactor temperatures (>900 °C). Further, high CO2 yields (up to 0.90) are observed, which increase with air/fuel ratio and fuel reactor temperature. Carbon formation has not been observed in any operating conditions. With the results obtained, the solids circulation rate is calculated for all operating cases. The solids circulation rate follows a linear trend with U/Ut. The total solids inventory in the reactor system has only a minor influence on the solids circulation rate. The NiO content in the original particle is 40 wt %. The active NiO content, however, is slightly less and found to be at least in the range of 35-36 wt %. This is determined via the solids conversion of the particle in highly reducing conditions in the FR (λ < 0.90). Acknowledgment. This work was part of the EU financed project CLC GAS POWER (FP6 Contract Number 019800), coordinated by Chalmers University of Technology. The project is also part of phase II of CCP (CO2 Capture Project) via Shell.
Nomenclature AAR ) air reactor cross-section, m2 dp ) particle diameter, µm GS ) solids circulation rate, kg m-2 s-1 m ˙ i ) mass flow of substance i, kg s-1 mOC ) OC mass, kg mOC,tot ) total mass of the OC in the reactor, kg MOC ) molar mass of the OC, g mol-1 MOC,ox ) molar mass of the fully oxidized OC, g mol-1 MOC,red ) molar mass of the fully reduced OC, g mol-1 n˙i ) molar flow of substance i, mol s-1 m0 ) mass of raw sample, kg mOC ) mass of OC in the reaction zones, kg mox ) mass of oxidized sample, kg mS ) total sample mass, kg Omin ) oxygen demand for full oxidation, kg/kg Pth ) thermal power, kW R0 ) oxygen transport capacity, kg/kg T ) temperature, K U ) superficial velocity, m/s Ut ) terminal velocity, m/s xi ) volume fraction of substance i XCH4 ) CH4 conversion XH2 ) H2 conversion XS ) solids conversion γCO2 ) CO2 yield ηcomb ) combustion efficiency λ ) global air/fuel ratio EF8008184
