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Syngas Chemical Looping Gasification Process: Oxygen Carrier Particle Selection and Performance Fanxing Li, Hyung Ray Kim, Deepak Sridhar, Fei Wang, Liang Zeng, Joseph Chen, and L.-S. Fan* William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed March 17, 2009. ReVised Manuscript ReceiVed June 7, 2009
The syngas chemical looping (SCL) process coproduces hydrogen and electricity. The process involves reducing metal oxides with syngas followed by regeneration of reduced metal oxides with steam and air in a cyclic manner. Iron oxide is determined to be a desired oxygen carrier for hydrogen production considering overall properties including oxygen carrying capacity, thermodynamic properties, reaction kinetics, physical strength, melting points, and environmental effects. An iron oxide based particle can maintain good reactivity for more than 100 reduction-oxidation (redox) cycles in a thermogravimetric analyzer (TGA). The particle exhibits a good crushing strength (>20 MPa) and low attrition rate. Fixed bed experiments are carried out which reaffirm its reactivity. More than 99.75% of syngas is converted during the reduction stage. During the regeneration stage, hydrogen with an average purity of 99.8% is produced.
Introduction Coal is relatively cheap and abundant compared to natural gas and crude oil. With the increasing demand for clean and affordable energy carriers, efficient and environmentally friendly coal conversion technologies are highly desirable. Conventional coal gasification processes can produce hydrogen, electricity, and liquid fuels. However, it is capital-intensive and requires significant amounts of parasitic energy.1,2 The chemical looping process provides an option to efficiently convert coal-derived syngas into hydrogen and/or electricity with 100% CO2 capture.3-6 Chemical looping processes convert carbonaceous fuels into carbon-free energy carriers, such as H2 and electricity, through the looping of oxygen carriers, typically metal oxides. As shown in Figure 1, the chemical looping process involves two steps, i.e., the reduction step and the oxidation step. In the reduction step, carbonaceous fuels such as methane, syngas, hydrocarbons, and/or coal react with a metal oxide based oxygen carrier particle. As a result, the metal oxide is reduced to a lower oxidation state while the fuels are oxidized to a mixture of steam and CO2. In the oxidation step, steam or air oxidizes the reduced metal oxide particle, producing hydrogen or heat. In the chemical looping process, the flue gas from the reduction step is never mixed with the product from the oxidation step. Therefore, the energy-intensive CO2 separation steps are avoided. The chemical looping concept can be traced back to nearly 100 years ago, when the steam-iron process was used in * To whom correspondence should be addressed. Telephone: +1 (614) 688-3262. Fax: +1 (614) 292-3769. E-mail:
[email protected]. (1) Shoko, E.; McLellan, B.; Dicks, A. L.; da Costa, J. C. D. Int. J. Coal Geol. 2006, 65 (3-4), 213–222. (2) Lewandowski, D.; Gray, D. Presented at the Gasification Technologies Conference, 2001. (3) Gupta, P.; Velazquez-Vargas, L. G.; Fan, L. S. Energy Fuels 2007, 21 (5), 2900–2908. (4) Thomas, T.; Fan, L.-S.; Gupta, P.; Velazquez-Vargas, L. G. U.S. Patent 11010648, 2004. (5) Fan, L. S.; Li, F.; Ramkumar, S. Particuology 2008, 6 (3), 131– 142. (6) Li, F.; Fan, L. S. Energy EnViron. Sci. 2008, 1, 248–267.
Figure 1. Schematic process diagram of chemical looping processes.
commercial plants to generate hydrogen from syngas through the use of iron-based looping particles.7 The steam-iron process only partially converts the reducing gas. Moreover, the ironbased looping medium has poor recyclability, especially with the presence of sulfur.7,8 With the introduction of less costly hydrogen production techniques using oil and natural gas as feedstock in the 1940s, the steam-iron process became less competitive and was then phased out. In the 1950s, the chemical looping scheme was adopted for CO2 generation used for the beverage industry. Oxides of copper or iron were used as the looping particle, and carbonaceous material was used as the feedstock.9 Since the 1980s, the renewed interest in processes with high energy conversion efficiency and a low CO2 capture penalty prompted the revival of chemical looping. Proven to be advantageous via thermodynamic analysis,10-12 the chemical (7) Hurst, S. J. Am. Oil Chem. Soc. 1939, 16 (2), 29–36. (8) Gasior, A.; Forney, J.; Field, J.; Bienstock, D.; Benson, H. U.S. Department of the Interior-Bureau of Mines, 1961. (9) Lewis, W. K.; Gilliland, E. R. U.S. Patent 26665972, 1954. (10) Ishida, M.; Jin, H. G. Energy 1994, 19 (4), 415–422. (11) Ishida, M.; Zheng, D.; Akehata, T. Energy 1987, 12 (2), 147–154.
10.1021/ef900236x CCC: $40.75 2009 American Chemical Society Published on Web 07/09/2009
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Figure 2. Schematic of the syngas chemical looping process.
looping concept has been extensively studied over the last 2 decades. Previous studies concerning coal-based chemical looping processes focused on the development of the cyclic chemical intermediates13,14 and their application in chemical looping combustion (CLC) processes for power generation from gaseous fuels.15,16 Studies on the utilization of chemical looping gasification strategy for hydrogen production, however, remain limited, focusing mostly on theoretical analyses.17,18 This paper discusses the syngas chemical looping (SCL) process for hydrogen and electricity coproduction, with the focus on the oxygen carrier particle selection, optimization, and characterization. In the following sections, the SCL process is briefly discussed. Various factors for oxygen carrier particle selection are elaborated, revealing that an iron oxide based oxygen carrier is suitable for the SCL process. The reactivity, recyclability, physical strength, and attrition rate of an iron oxide based oxygen carrier particle are then presented. Fixed bed experiments that mimic the reducer and oxidizer operations are also reported. Experimental results show that the iron oxide particle can fully oxidize syngas into CO2 and steam. Moreover, the reduced particles can react with steam, generating hydrogen with purity in excess of 99.8%. Syngas Chemical Looping Process Overview. The SCL process converts gaseous fuels such as syngas and gaseous hydrocarbons into hydrogen and/or electricity with integrated CO2 capture.3,5 The simplified process flow diagram of the SCL process is shown in Figure 2. As can be seen, the SCL process converts coal-derived syngas into hydrogen and electricity using three reactors: the reducer, the oxidizer, and the combustor. Such conversion is realized through the assistance of iron-based oxygen carrier particles circulating among the three reactors. In the SCL process, coal is first gasified into raw syngas in a commercial gasifier. The raw syngas, which contains principally CO, H2, CO2, and contaminants such as particulates, sulfur, and halogen compounds, ammonia, and mercury, is cooled down (12) Richter, H. J.; Knoche, K. F. ACS Symp. Ser. 1983, 235, 71–86. (13) Gupta, P. Ph.D Dissertation, The Ohio State University, Columbus, Ohio, 2006. (14) Ryu, H.-J.; Jin, G.-T. Hwahak Konghak 2004, 42 (5), 588. (15) Chuang, S. Y.; Dennis, J. S.; Hayhurst, A. N.; Scott, S. A. Combust. Flame 2008, 154 (1/2), 109–121. (16) Mattisson, T.; Garcia-Labiano, F.; Kronberger, B.; Lyngfelt, A.; Adanez, J.; Hofbauer, H. Int. J. Greenhouse Gas Control 2007, 1 (2), 158– 169. (17) Svoboda, K.; Siewiorek, A.; Baxter, D.; Rogut, J.; Puncochar, M. Chem. Pap. 2007, 61 (2), 110–120. (18) Xiang, W.; Chen, Y. Energy Fuels 2007, 21 (4), 2272.
and introduced to a particulate removal unit followed by a hot/ warm gas cleanup unit for particulate and sulfur removal. The particulate-free syngas is then sent to the bottom of the reducer. In the reducer, the oxygen carrier oxidizes the syngas to a gaseous mixture of CO2 and H2O. Meanwhile, the oxygen carrier is reduced to a mixture of Fe and FeO. The reducer can be slightly endothermic to slightly exothermic depending on the syngas composition and the extent of the solid reduction. The heat required or generated can be provided to or removed from the reducer by the oxygen carrier particles with ease. This eliminates the need for heat exchanging units for the reducer operations. The reduced particles obtained from the reducer are sent to the oxidizer to react with steam via the steam iron reaction. In the oxidation step, the reduced particles are regenerated to Fe3O4, producing a hydrogen-rich gaseous product stream. After condensing out the unconverted steam, the high-purity hydrogen product is obtained. The oxidizer is slightly exothermic; by introducing steam at a temperature lower than its operating temperature, the oxidizer can be adjusted to achieve adiabatic operation. Both the reducer and the oxidizer are countercurrent moving beds with reactant gases injected from the bottom and the oxygen carrier particles introduced at the top. The countercurrent gas-solids contacting pattern enhances the conversions of both reactant gases and the oxygen carrier particles. Discussions on the rationale of using countercurrent moving bed are elaborated in Gupta et al.3 The Fe3O4 particles discharged from the oxidizer are introduced to an entrained bed combustor. The combustor uses air to convey the particles to the reducer inlet. In the combustion step, the air oxidizes Fe3O4 to Fe2O3, releasing heat. The sensible heat carried by the hot exhaust air from the combustor is used for power generation while the regenerated Fe2O3 particles enter the reducer to perform another redox cycle. The fines resulting from the particle attrition are purged out before the reducer, and fresh makeup particles are added to the particle stream. The SCL process can also be configured so that a fraction or all of the particles discharged from reducer is directly introduced to the combustor. By doing so, more electricity will be generated at the expense of the decreased hydrogen production. The step of combusting the reduced particles characterizes CLC, which differs from the chemical looping gasification in which the particle is regenerated using steam instead of oxygen. As can be seen in Table 1, both the chemical looping gasification strategy and CLC strategy are utilized in the SCL process. Thus,
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Table 1. Reactor Type, Main Reactions, and Operating Conditions for SCL Reactors name reducer
reactor type
reactions
temperature °C
pressure MPa
countercurrent
Fe2O3 + CO/H2 f 2FeO + CO2 /H2O(g)
750-900
3.0
500-750
3.0
950-1150
3.2
FeO + CO/H2 f Fe + CO2 /H2O(g)
moving bed oxidizer
countercurrent
3FeO + H2O(g) f Fe3O4 + H2
moving bed combustor
Fe + H2O(g) f FeO + H2
entrained bed
4Fe3O4 + O2 f 6Fe2O3
Table 2. Comparisons of the Key Properties of Different Metal Oxide Candidatesa cost oxygen capacity (wt %)c thermodynamics: syngas conversiond thermodynamics: steam conversione reduction kinetics/reactivityf melting pointsg strength environmental and health impacts
Fe2O3
NiO
CuO
Mn3O4
CoO
+b 30 Fe2O3/Fe3O4/FeO 100/83.2/42.5 Fe/FeO/Fe3O4 55.9/15.8/0.05 ≈ 1275 + ≈
21 Ni/NiO 99.3 -
≈ 20 CuO/Cu2O 100/100 -
20 Mn3O4/MnO 100/0 -
+ 1452 -
+ 1026 ≈
≈ 1260 ≈ -
21 Co/CoO 96.3 Co 3.5 1480 ≈ -
a Refs 3, 21-23. b +, positive; -, negative; ≈, neutral. c Maximum possible oxygen carrying capacity by weight percent, pure basis; achievable using excess fuel (actual). d Maximum theoretical conversion of a syngas (66.6% CO and 33.3% H2) to CO2 and H2O with the presence of the given metal oxide at 850 °C (calculated by Aspen Plus). e Maximum theoretical conversion of steam with the presence of the given metal oxide at different oxidation states at 850 °C (calculated by Aspen Plus). f Reactivity refers to the rates of the reactions between metal oxides and syngas (CO and H2). g Lowest melting points of the metal/metal oxides under various oxidation states (°C); Co O and Co O are not considered in this case since they are 3 4 2 3 difficult to be oxidized.
hydrogen and electricity can be coproduced from the SCL process with 100% CO2 separation at no additional cost. Oxygen Carrier Selection. Table 1 generalizes the key reactions and the operating conditions of the SCL reactors. As seen in Table 1, the oxygen carrier particles play an important role in the SCL process, because they participate in all the key chemical reactions in the process. The functions of the oxygen carrier particles include oxidation of the syngas in the reducer, production of hydrogen in the oxidizer, and generation of heat to produce power in the combustor. Thus, the particle performance is crucial to the process operation. This section discusses the criteria on the oxygen carrier selection. A number of factors need to be considered to obtain the optimum oxygen carrier particle. Such factors include thermodynamic properties, reactivity, recyclability, cost, melting temperatures, physical strength, and health and environmental impacts. Among these factors, thermodynamic properties underlie the functionality of the particle. Specifically, the oxidized particle must fully oxidize syngas and the reduced particle must convert a significant portion of steam into hydrogen. More detailed information on the thermodynamic feasibilities of various particles can be found in the various studies reported in the literature.3,17,19,20 These studies identified the oxides of Fe, Mn, Ni, Cu, and Co as potential candidates for hydrogen generation. This paper approaches the particle selection using a systematic comparison. Table 2 generalizes the various factors that affect the particle performance in the looping process. As can be seen in Table 2, although the iron-based oxygen carrier particle has relatively slow reduction kinetics, it possesses favorable thermodynamic properties and is less costly. Moreover, iron-based particles have good physical strength, high melting temperatures, and fewer environmental concerns. Therefore, iron oxide is a (19) Svoboda, K.; Siewiorek, A.; Baxter, D.; Rogut, J.; Pohorely, M. Energy ConVers. Manage. 2008, 49 (2), 221–231. (20) Svoboda, K.; Slowinski, G.; Rogut, J.; Baxter, D. Energy ConVers. Manage. 2007, 48 (12), 3063–3073.
favorable choice for the SCL process. The performance of the iron-based particles is further discussed in the following sections. Experimental Section Particle and Pellet Preparation. The Fe2O3 composite particles are prepared using a sol-gel method.3,4 First, a solution of FeCl3 · 6H2O in isopropyl alcohol with a concentration of 1 g/mL and a solution of aluminum isopropoxide (Alfa Aesar) in isopropyl alcohol at a 1:1 volume ratio are prepared. The FeCl3 solution is then mixed with the aluminum isopropoxide solution in constant agitation. The mixture is then placed under an oven at 50 °C for curing to form gel. The gel is subsequently dried under vacuum at 50 °C. Once all the alcohol is evaporated, the gel is heated in a muffle furnace at 500-600 °C for 2 h with continuous flow of air. The resulting particles, in powder form, are then processed into pellets using a TDP benchtop single-punch tablet press. The pellets are cylindrical with a 5 mm diameter and 1.5-4.5 mm in height. Prior to experiments, the pellets are sintered at 900 °C for 12 h. The particles/pellets contain Fe2O3 up to 70% by weight. Particle Reactivity and Recyclability. A Perkin-Elmer Pyris 1 thermogravimetric analyzer (TGA) is used to characterize the reactivity and recyclability of different particles. The schematic of the experimental setup is shown in Figure 3. Powder samples are directly used in the TGA, whereas pellet samples are broken in a mortar and then sieved into different size ranges before being loaded into the TGA. Unless otherwise mentioned, broken pellet samples with sizes ranging between 710 µm and 1 mm are used in the TGA. Before each experiment, around 20 mg of particles is loaded into a quartz crucible. Next, the TGA is purged with N2 to introduce an inert atmosphere. The crucible is then heated to the desired reacting temperature, 900 °C. To compare the reactivity of various Fe2O3-based samples, about 160 mL/min of reducing gas, composed of 37.5% H2 balanced with (21) Adanez, J.; de Diego, L. F.; Garcia-Labiano, F.; Gayan, P.; Abad, A.; Palacios, J. M. Energy Fuels 2004, 18 (2), 371–377. (22) Garcia-Labiano, F.; de Diego, L. F.; Adanez, J.; Abad, A.; Gayan, P. Chem. Eng. Sci. 2005, 60 (3), 851–862. (23) Perry, R. H.; Green, D. W. Perry’s Chemical Engineers’ Handbook, 8th ed.; McGraw-Hill: New York, 2008;
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Figure 3. Schematic of the experimental setup for the particle reactivity and recyclability studies.
Figure 5. Schematic of the fixed bed reactor setup. Table 3. Inlet Gas Composition during the Reduction Experiment
Figure 4. Schematic of the entrained bed setup for particle attrition studies.
N2, is introduced to the TGA. The weight change of the sample is recorded as a function of time, and the experiment is stopped after the sample weight stabilizes. This point corresponds to the maximum achievable reduction under the given conditions. Since the rate of weight change (oxygen carrying capacity change) is proportional to the rate of the reduction reaction, the reactivity can be quantified using the TGA curve. In the present study, two values, i.e., the maximum rate of weight change (wt %/min) and the time required to reach 80% conversion, are used to compare the reactivity of the particles during reduction, with a similar analysis used to obtain oxidation reactivity. Following the complete reduction of the previous sample, the TGA is purged with N2. Then, 200 mL/ min of oxidization gas, consisting of 45% air balanced with N2, is fed into the TGA. To simulate redox cycles, the reducing and oxidizing gases are alternately introduced to the TGA with 15 min of N2 flushing in between. The change in reactivity is then monitored across cycles to calculate recyclability. Particle (Pellet) Strength. The crushing strength of the cylindrical-shaped particles/pellets is determined using a modified version of the ASTM D4179 standard using a hydraulic press installed with a digital pressure transducer. During testing preparation, a pellet is loaded into the hydraulic press. Next, the manually operated press compresses the pellet in an axial direction. A computer records the pressure at which the pellet is crushed. In order to obtain a reliable mean crushing strength and distribution of the pellet crushing strength, more than 50 pellets are tested for each sample composition. The attrition rate of the particles/pellets is tested in an entrained flow reactor that simulates the combustor operation, as shown above in Figure 4. The reactor is 2.7 m in height with an outer diameter (o.d.) of 2.54 cm and inner diameter (i.d.) of 1.91 cm. Gas can be introduced to the bottom of the reactor through the distributor.
type of gas
CO2
H2
CO
N2
total
flow rate (mL/min, STP) concentration (%)
13.0 2.6
127.1 25.1
252.2 49.9
113.3 22.4
505.6 100.0
Cylindrical composite pellets weighing 328.5 g that have been reduced and oxidized for two cycles are used as the fresh sample. Before each experiment, the composite pellets are loaded to the bottom of the reactor. The valve is then opened to send air to the reactor at 5.43 L/s. Such an air flow rate corresponds to a superficial gas velocity of ∼18 m/s, which is higher than the terminal velocity of the pellets. As a result, the air pneumatically conveys the pellets to the top of the reactor, through the U bend, and eventually to the funnel-shaped pellet collector. The particles, after being collected, are sieved into four different size ranges, i.e., >2.8 mm, 2.8-1.98 mm, 0.71-1.98 mm, 710 µm) in the system as a function of the conveying cycles given the addition of 0.57% (by weight) fresh makeup pellets each cycle. With 0.57% fresh pellet makeup rate, the total weight of pellets circulating inside the SCL process remained stable. Thus, the attrition test results can be used as an estimate of the pellet attrition/purge rate during commercial SCL operations.
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2CO f C + CO2
Figure 8. Attrition rate of the composite pellets in an entrained flow reactor.
Figure 9. Composition of the exhaust gas stream from the fixed bed reactor during the reduction of the Fe2O3 composite pellets (dry basis). Table 5. Gas and Solids Conversions in the Fixed Bed Reduction Experiment CO conversion H2 conversion carbon content particle before before breakthrough after experiment conversion after breakthrough (%) (%) (%) experiment (%) 99.76%
99.75%
0.02
94.6
Since the purged particle can easily be captured and repelletized for reuse, the fresh particle makeup cost is low at the current attrition rate. Fixed Bed Reactor Experiments. Pellet Reduction Experiment. The reduction experiment validates the reducer concept in the SCL process. The composition of the exhaust gas (dry basis) during the reduction experiment is plotted in Figure 9. The concentration of the N2 carrier gas is not shown. The CO and H2 conversions before breakthrough, the carbon content, and the solids conversion after the experiment are given in Table 5. Before breakthrough, both CO and H2 were almost completely oxidized given the rather short gas residence time in the reactor (∼6 ms). This is mainly due to the presence of Fe2O3, which, as shown in Table 2, is capable of oxidizing nearly 100% of the syngas. As the reaction proceeded, the Fe2O3 phase disappeared due to particle reduction, and the breakthrough subsequently took place. The reduction of Fe2O3 to lower oxidation states such as Fe3O4, FeO, or Fe led to significantly decreased CO and H2 conversion. This phenomenon agrees with predictions based on the thermodynamic properties of the metal oxides in Table 2. The solid analysis showed that the particles were reduced by 94.6% with small amounts of carbon deposition on the surface of particles (0.02 wt %). The carbon deposition resulted from the reverse Boudard reaction:
Methods that minimize carbon deposition have been discussed by Gupta et al.3 The reduction experiment in the fixed bed reactor validates that the presence of Fe2O3 can oxidize syngas into an exhaust stream of CO2 and steam. Therefore, a readyto-sequester CO2 stream can be obtained by condensing out the steam from the reducer effluent gas. Moreover, Fe2O3 is reduced by syngas to its metallic form with insignificant carbon deposition. Pellet Oxidation Experiment. The reduced particles from previous experiment were oxidized using 20% steam balanced with N2 in the same fixed bed reactor. Figure 10 shows the concentration of H2 and CO (dry basis) exiting from the fixed bed reactor. Although water is injected at a constant rate, it evaporates in the capillary tube in a “batch mode”. The instability in steam flow rate leads to the fluctuation in hydrogen concentration at the outlet. Table 6 shows the average steam conversion before breakthrough, the average and the lowest hydrogen purity (normalized to N2- and moisture-free basis), and the compositions of the composite particle after the experiments. As can be seen from Figure 10 and Table 6, before the breakthrough, nearly 80% of the steam is converted due to the presence of metallic iron. After the complete oxidation of the iron phase to higher oxidation states, the breakthrough occurs, characterized by a sharp decrease in steam conversion. The average H2 purity is 99.8% with CO as the only impurity. The CO is formed due to steam gasification of the carbon deposited during the syngas reduction stage, as shown below: C + H2O f CO + H2 Characterization of the solid sample after the steam oxidation experiments showed that the carbon content in the solid sample remained undetectable. This suggests that all the carbon formed during reduction stage was gasified during the steam oxidation stage. Also, the particle was almost completely regenerated to Fe3O4. After the steam oxidation experiment, the pellets in the fixed bed were fully reoxidized with air to Fe2O3, releasing heat. The TGA recyclability tests performed on the reoxidized pellets showed that the pellets were recyclable with reactivity comparable to that of fresh pellets. Concluding Remarks The studies indicate that an iron oxide based oxygen carrier particle is suitable for the SCL gasification process for hydrogen
Figure 10. Composition of the exhaust gas stream from the fixed bed reactor during the oxidation of the reduced Fe2O3 composite pellets using steam (dry basis).
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Table 6. Gas and Solids Conversions in the Fixed Bed Oxidation Experiment average steam conversion before breakthrough (%)
average H2 purity in the experiment (%)
lowest H2 purity in the experiment (%)
Fe3O4 content after the experiment (%)a
79.10
99.80
99.66
99
a
The product from the steam oxidation experiment is a mixture of FeO and Fe3O4. Percentage of Fe3O4 denotes the percentage (by weight) of Fe3O4 in the solid mixture. Estimated based on the average oxidation state of iron in the solid sample taken after the fixed bed experiment.
and electricity production. Adding supports to the iron oxide drastically increases the reactivity and recyclability of the oxygen carrier. The TGA experiments showed that the iron oxide composite particle can maintain recyclability for more than 100 cycles. The pelletized particle showed good crushing strength (>20 MPa) and a low attrition rate (
