Fe3O4 Cycle Complemented with

Jun 25, 2013 - ABSTRACT: A modified FeO/Fe3O4 redox cycle of producing hydrogen from water at a constant temperature of 800 °C is experimentally stud...
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Experimental Studies on the FeO/Fe3O4 Cycle Complemented with Carbon Gasification for Producing Hydrogen Yaping Zhou,† Yan Sun,† Wei Su,‡ and Li Zhou*,‡ High Pressure Adsorption Laboratory, †Department of Chemistry, School of Science, and ‡School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: A modified FeO/Fe3O4 redox cycle of producing hydrogen from water at a constant temperature of 800 °C is experimentally studied. Instead of solar energy, Fe3O4 was reduced to FeO by CO, the product of carbon gasification. The carbon was from coal carbonization. It was shown that the rate of both oxidation (reaction of FeO with H2O) and reduction (reaction of Fe3O4 with CO) was fast; however, the reduction must stop before formation of metal iron. The addition of 5−10 mol % Mo in iron oxide improved the stability and the reactivity of the latter during reaction cycling. Hydrogen was produced continuously in a process consisting of three reactors that couple the FeO/Fe3O4 redox cycle with carbon gasification (at 900 °C). The residual gas in the reactor and pipeline at the end of reduction may contaminate the hydrogen product; however, an evacuation step complemented to the end of reduction guarantees a hydrogen concentration higher than 95%.

1. INTRODUCTION The exponential increase of the world population and the accelerated development of the global economy in the past century have accelerated the speed with which fossil fuels step closer to depletion. In the meantime, emissions of massive greenhouse gases deteriorate the environment and affect humanity’s living. While a strategy of carbon capture and storage (CCS) is proposed, whether the strategy functions or not is doubtful. A more practical and efficient strategy is to regulate the world population1 and rely on green and sustainable energy, among which hydrogen is a known option. Combustion of hydrogen generates water, and decomposition of water generates hydrogen. Therefore, hydrogen realizes clean combustion and guarantees sustainable supply. Apparently, sustainable hydrogen must come from water but not from natural gas or other fossil fuels. The technology to produce sustainable hydrogen is the first and most serious challenge to access the epoch of hydrogen energy. Because fossil fuels are not renewable, a technology relying on the expense of fossil fuels is not justified for the production of sustainable hydrogen, and the technology of water electrolysis is not either because of a high energy penalty. The authors previously defined a theoretical energy gain (TEG) to examine the theoretical energy efficiency of a proposed technology,2 and a process that produces fuel but cannot pass the TEG examination is certainly of no practical value. TEG is the ratio of theoretically releasable energy of products over the theoretically invested energy to acquire the products. The TEG of water electrolysis is 1.0 because the reaction of consuming hydrogen is equal to the reciprocal of producing hydrogen. The real energy gain must be far less than unity because of the low efficiency of real processes; therefore, researchers turn to natural energy for the splitting of water. Natural energy is costless; however, a rather high cost must be paid to the equipment of collecting and/or using the energy. In addition, hydrogen productivity of such a process is too low to be relied upon industrially. Permissible production of hydrogen from water with a reasonable TEG value is possible in terms of chemical cycles © 2013 American Chemical Society

that generate hydrogen. Both valence states of hydrogen and oxygen were changed in complete splitting of water, and the TEG value was thus limited. However, the TEG value must be higher if a water-decomposing technology only changes the valence state of hydrogen, leaving oxygen unchanged, which is realized through chemical reaction cycles.3 Miscellaneous thermochemical cycles were designed to release hydrogen from water, among which the redox cycle involving molten iron or iron oxides received industrial interest.4−6 The FeO/Fe3O4 cycle was initially realized in terms of solar energy:7 Fe3O4 releases oxygen and changes back to FeO at high temperatures (1600−2200 °C) that were reached and maintained by solar energy. However, the big temperature difference between FeO oxidation (at 800 °C when hydrogen is produced) and Fe3O4 reduction makes the cycle slowly and energetically expensive because the heat load is proportional to the product of the mass of reaction materials/ containers and the temperature difference and it takes a long time to reach the target temperature. The authors modified the cycle.8 Instead of relying upon solar energy, carbon monoxide was resumed to reduce Fe3O4 back to FeO. As a result, both oxidation and reduction reactions occur at a constant and relatively low temperature (800 °C) and the FeO/Fe3O4 cycle reduces to the following general reaction: H 2O + C = H 2 + CO

(1)

It was shown that the TEG of the revised cycle is 3.24.2 All drawbacks of the initial FeO/Fe3O4 cycle have thus been removed. Because of the FeO/Fe3O4 cycle, H2 and CO are separately produced as pure gases; therefore, the separation cost is saved. Carbon monoxide used for the reduction of Fe3O4 comes from the following reaction: CO2 + C = 2CO

(2)

Received: March 21, 2013 Revised: June 22, 2013 Published: June 25, 2013 4071

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CO2 was the product of Fe3O4 reduction, and carbon was the product of biomass/coal carbonization. Carbon may be renewable if it is out of biomass, and it is not renewable if it is out of coal. Nonetheless, extraction of carbon out of coal provides a simple and clean way to use coal energy. Coal plays a major role in meeting the world energy requirement presently and for the near future because the reserve of coal may last well over 250 years.9 Direct burning of coal generates serious pollution, and a lot of money is presently spent to alleviate the pollution. It is aimed, therefore, to change coal to clean gaseous fuel and inorganic carbon in the present study. The gaseous fuel may merge to the civil gas system, and the inorganic carbon may switch to the FeO/ Fe3O4 cycle for hydrogen production. All potential polluting elements contained in coal could be separated or destroyed before combustion. Therefore, coal carbonization is incorporated into the study. Our investigation aims to experimentally show that power hydrogen can be produced from water together with the clean release of coal energy.

Figure 1. Experimental setup for the study of the oxidation/reduction reaction and iron oxides: (1) Ar cylinder, (2) CO cylinder, (3−5) mass flow controllers, (6−8) solenoids, (9) mass flow meter, (10 and 11) temperature controllers, (12) reactor, (13) condenser, (14) steam generator, (15) mass spectrograph, and (16) computer.

2. EXPERIMENTAL SECTION 2.1. Materials. Oxidation and reduction reactions are cycling between FeO and Fe3O4; however, iron oxide itself cannot stay stable upon reaction cycling, and other ingredients were added to improve the morphology and reactivity of iron oxides, as was suggested in the literature.10−12 Samples of iron oxide and its mixture with other elements were prepared using the precipitation method. The aqueous solution of iron nitrate (pH 0.23) together with the nitrates of another metal (or ammonium molybdate) was mixed together according to a prescribed ratio. Then, the mixture was dropped in ammonia−water (pH 12.33) under gentle agitation, and ironic hydroxide was precipitated (pH 7.80) together with the other metal element. The sediment was aged for 4 h, filtered off of the liquid, washed with anhydrous alcohol, and dried at 100 °C. The sediment was finally roasted at 600 °C for 3 h. Composition of oxides was analyzed on the basis of X-ray diffraction (XRD) of the powder sample. Experiments of coal carbonization were conducted on samples of lignite, lean coal, and coking coal. The sample sources and the results of proximate analyses are listed in Table 1. To minimize the generation of tar, about 10%

Coal carbonization experiments were carried out on a vertical oven shown schematically in Figure 2. The oven is composed of three zones

Figure 2. Coal carbonization apparatus. that are electrically heated and independently conditioned. The lengths of three zones are 205, 205, and 220 mm from the bottom up. The coal sample of known weight and particle size of 0.4−0.8 mm was put in the bottom zone with a packing height of about 100 mm. The heating rate and the target temperature were conditioned separately for three zones. The target temperature of the low zone is usually 800 °C, and the target temperature of the middle and top zones is 500 and 300−400 °C, respectively. The target temperatures were maintained for 90 min before the end of each run. The effluent gas and condensate were collected separately in a bottle or a water-displacement container, as shown in Figure 2. Composition of the incondensable gas was

Table 1. Major Information of Coal Samples sample

lignite

lean coal

coking coal

sources

Taiyuan, Shanxi

Yangzhuan, Huaibei

Liulin, Shanxi

moisture (%) ash (%) volatile (%) carbon (%)

18.11 17.35 37.55 26.99

6.38 9.63 12.31 71.68

1.25 5.74 25.18 67.83

catalyst that is usually applied for the cracking of coal tar was mixed with the coal sample. The catalyst applied was nickel-based or blended with other minor elements, such as Ce, Zr, La, or Mo.13−16 2.2. Apparatus and Methods. The oxidation/reduction reaction was carried out on a vertical quartz tube reactor with dimensions of 25 mm outer diameter and 3 mm wall thickness. The reactor was surrounded by an electric oven possessing a constant temperature zone for about 120 mm, and the temperature was set at 800 °C. A sample of iron oxides containing 0.05 mol of Fe was put in the constant temperature zone. The experimental apparatus is schematically shown in Figure 1. Argon with a purity of 99.995% was used to carry water vapor passing through the reactor. Argon was replaced by carbon monoxide with a purity of 99.5% at the end of oxidation, and the reduction period began. A QMS series gas analyzer purchased from Stanford Research Institute was used to analyze the composition of the reactor effluent stream, and a calibration curve was prepared in terms of gas chromatography and used to correct the report of the gas analyzer.

Figure 3. Apparatus of the coupling redox cycle with charcoal gasification. 4072

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reactors operating at 800 °C. The reactor was made of quartz with a dimension of ϕ 25 × 4 × 700 mm and was heated by an electric heating jacket. There are four heating zones along the reactor from the top to the bottom: preheating, top-heating, middle-heating, and lowheating zones. The target temperature of each zone was separately conditioned. A total of 10 g of Mo-modified ferric oxide was put in the middle zone that was designed for the occurrence of core reactions. The charcoal gasification proceeds in the right longer reactor running at 900 °C. The reactor was made of stainless steel with a dimension of ϕ 25 × 4 × 900 mm and was also heated and conditioned separately in four zones. A charcoal support was set at the lower position, and 60 g of charcoal was added. A quartz container of charcoal was placed above the reactor, allowing for intermittent carbon supply. The operation system was maintained at a gauge pressure of 0.05 MPa. The gas product was collected by the water-displacement method and analyzed by the above-mentioned QMS series gas analyzer corrected with a calibration curve. Details of the apparatus were previously described.17

determined by gas chromatography. The weight of possible condensate and solid residues was measured by an electronic balance, and the weight of charcoal was determined by subtracting ash content from total residues. The quantity of tar cannot be directly and exactly determined; however, it is right the difference of the sample weight and the total weight of known quantities.

Wtar = Wsample − Wsol − Wliq − Wgas

(3)

Coupling of the redox cycle with charcoal gasification was tested in a process that consists of three reactors shown in Figure 3. The oxidation and reduction reactions cycled upon the left two shorter

3. RESULTS AND DISCUSSION 3.1. Reaction Course of Oxidation/Reduction. The reaction course was recorded when a long enough time was allowed for either oxidation or reduction to occur at appropriate conditions. The reaction condition was determined in reference to the phase diaphragm shown in Figure 4,18 and the recorded composition of the reactor off-gas is shown in Figure 5 for the oxidation and reduction reactions. The

Figure 4. Phase diagram in reducing ferric oxides with CO.

Figure 5. Composition of the reactor effluent stream during (a) oxidation and (b) reduction.

Figure 6. XRD diagrams of samples drawn at stages 2 and 3. 4073

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Figure 8. SEM photos of ferric oxide including 10 mol % Mo (a) before and (b) after redox reactions.

Figure 7. SEM photos of ferric oxide (a) before and (b) after redox reactions.

generation rate of hydrogen was very fast, and most hydrogen was generated in the initial 15 min, as shown in Figure 5a. Besides, the effluent stream did not contain CO or CO2 for the oxidation period. There are three reduction stages at 800 °C, as recognized in the phase diaphragm. The three stages are also reflected in the composition curve of CO, as shown in Figure 5b. The CO content in the gas stream entering the reactor was about 70%. However, the CO content in the effluent stream was increased from zero because of consumption in the reduction of Fe2O3 to Fe3O4, and initiation of the second reduction stage, i.e. Fe3O4 reduced to FeO, caused a sudden decrease of the CO content and formed a little peak on the CO composition curve. The CO content in the effluent stream increases gradually following the increasing conversion; however, the increasing rate of the CO content slowed again, and a shoulder is shown on the composition curve when the third reduction stage, i.e., FeO reduced to Fe, begins. Both content curves of CO and CO2 become leveler following the completion of reduction reactions. Samples of metal oxide were drawn at stages 2 and 3. The first reduction stage was too short to allow for drawing a sample. However, the result of XRD analyses for samples S2 and S3 shown in Figure 6 supported the explanation for reduction stages. As was previously indicated,2 generation of metal iron must be avoided because the reduced tiny iron particles permitted entrainment in the gas phase and, ultimately, eluted from the reactor. In addition, iron is a catalyst for the Boudouard reaction at 800 °C19 and causes carbon deposition or formation of metal carbides. Therefore, the reduction reaction must stop at the end of the second reduction stage. 3.2. Effect of Iron Oxide Additives. The morphology of iron oxide is basically aggregates of small particles with a

Figure 9. TG curves of three coals: (1) lean coal, (2) coking coal, and (3) lignite.

dimension of several hundred nanometers, as shown with the scanning electron microscopy (SEM) image in Figure 7a. However, agglomeration between particles occurred after sintering at 800 °C for a longer time, and the particle size increased to 2−3 μm, as shown in Figure 7b. Although agglomeration is inevitable after sintering at 800 °C, it was effectively retarded because of the addition of Mo in ferric oxide. The particle size is about 1−2 μm after reaction cycling for the Mo-added sample, as shown in Figure 8, and both morphology and specific surface area of oxides were kept stable for a long run. However, too much content of Mo negatively 4074

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catalyzed the reduction reaction, and a better effect was shown for the content of 5−10 mol %. Other elements were also tested for additives, and Ce or Zr showed a satisfactory effect as well. 3.3. Result of Catalyzed Carbonization of Coals. Different carbonization conditions were tested for different types of coals based on the thermogravimetry (TG) curves shown in Figure 9. The Ni/γ-Al2O3 catalyst was mixed with lignite powder according to the mass ratio of 1:10, and the carbonization result of lignite is shown in Figure 10 for different

Figure 11. Variation of off-gas composition with time for the initial period of oxidation.

content was maintained at 95% for repeated cycles, as shown in Figure 12. Except CO and CO2, the impurity includes a minor

Figure 10. Result of catalyzed carbonization of lignite at different temperatures: (1) tar, (2) incombustible gas, (3) char, and (4) combustible gas.

temperatures. Different catalysts were tested for the carbonization of coking coal and lean coal, and the results are shown in Table 2. While less than 10% tar was generated for lignite at Table 2. Result of Catalyzed Carbonization of Coking and Lean Coals catalyst Ni/Ce-ZrO2/γ-Al2O3 9% Ni/10% MoO3/γ-Al2O3 Ni/La2O3−MgO/γ-Al2O3 Ni/γ-Al2O3 Ni/CeO2/γ-Al2O3 Ni/La2O3−MgO/γ-Al2O3

gas (wt %)

combustible fraction of gas (%)

Coking Coal 13.62 13.92 15.12 Lean Coal 11.31 8.32 8.86

tar (wt %)

89.37 91.93 90.38

4.29 4.57 3.59

94.44 96.32 96.58

1.14 2.58 2.68

Figure 12. Results of continuous running on hydrogen production via the FeO/Fe3O4 (containing 10% Mo) cycle.

content of nitrogen that comes in initially with air or enters the system with feeding charcoal.

4. CONCLUSION (1) Pure hydrogen was generated when FeO reacts with H2O at 800 °C. However, hydrogen may be contaminated with CO, CO2, and N2 that left the reactor and pipeline at the end of reduction or entered the system with feeding charcoal during continuous production. (2) The reduction of ferric oxide consists of cascade reactions at 800 °C; however, the reduction must stop at the second step of Fe3O4 to FeO, and the reaction rate is quite fast until that point. (3) The addition of 5−10 mol % Mo or Ce and some other elements into iron oxide considerably improved the stability and reactivity of the latter during cycling on oxidation/reduction reactions. (4) Catalytic carbonization converts coal to flammable gas and charcoal. The former may be merged to the civil gas system, and the latter may be used in the Fe3O4/FeO redox cycle to produce hydrogen. Only a small amount of tar was yielded, and it may disappear in a large-scale carbonization reactor. (5) Hydrogen with a purity higher than 95% was continuously produced when coupling the Fe3O4/FeO redox cycle with charcoal gasification. The feasibility and potential of the technology is thus evidenced for industry.

carbonization temperatures higher than 700 °C, less than 5% tar was generated for the coking and lean coals. Most mass of coal converted to gaseous fuel or charcoal that was applicable for the production of hydrogen. However, no liquid fuels were collected in experiments. In addition to gaseous products, 65−70 wt % lignite converted to inorganic carbon (charcoal) and 75−80 wt % coking coal and 83−86 wt % lean coal converted to inorganic carbon. 3.4. Result of the Coupling Redox Cycle with Charcoal Gasification. Hydrogen was produced in the oxidation period; however, the hydrogen purity varies with time in the oxidation period, as shown in Figure 11. Some CO and CO2 were left in the reactor and pipeline as residual gas at the end of reduction and contaminated hydrogen. Therefore, the reactor and part of the connecting pipeline were isolated and evacuated at the end of the reduction period. As a consequence, the hydrogen 4075

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: 86-22-87891466. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (Grant 21076142) is sincerely appreciated. The authors also appreciate the efforts of graduate students D. S. Huang, Y. Zhang, C. F. Liu, C. X. Jia, X. H. Zhang, N. W. Huang, J. J. Yu, and J. J. Zheng, who took part in the research.

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