Environ. Sci. Technol. 2011, 45, 821–826
Ironmaking with Ammonia at Low Temperature SOU HOSOKAI,† YOSHIAKI KASIWAYA,‡ KOSUKE MATSUI,† NORIYUKI OKINAKA,† A N D T O M O H I R O A K I Y A M A * ,† Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, N13 W8, Kita-ku, Sapporo 060 8628, Japan, and Graduate School of Energy Science, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Received August 23, 2010. Revised manuscript received October 23, 2010. Accepted November 10, 2010.
This paper describes the reduction of hematite with ammonia for ironmaking, in which the effect of temperature on the products was examined. The results showed that the reduction process began at 430 °C during heating, and with an increase in temperature, the reduction mechanism changed apparently from a direct reduction of ammonia (Fe2O3 + 2NH3 f 2Fe + N2 + 3H2O) to an indirect reduction via the thermal decomposition of ammonia (2NH3 f N2 + 3H2, Fe2O3 + 3H2 f 2Fe + 3H2O) at temperatures over 530 °C. The final product obtained at 600 and 700 °C was pure metallic iron, in contrast with that formed at 450 °C, that is, a mixture of metallic iron and iron nitride. The results suggest the possibility of using ammonia as a reducing agent for carbonless ironmaking, which is operated at a much lower temperature than 900 °C in conventional coal-based ironmaking.
1. Introduction Conventional coal-based ironmaking is under increased pressure to drastically reduce the emissions of greenhouse gases (GHG), because ironmaking blast furnaces need approximately 500 kg of coke as fuel, packing materials, and a reducing agent to produce one ton of hot iron. The World Steel Association announced that the gross crude steel production of 66 countries in 2009 was over 1200 million metric tons (mmt), which uses 600 mmt of coke and emits 2200 mmt of carbon dioxide to the air. This value reached as much as 7.6% of the 29 000 mmt gross GHG emissions in 2009. Therefore, replacing carbon with hydrogen in ironmaking would be an effective way of reducing GHG emissions if hydrogen is produced from nonfossil fuels. However, the problem of how to transport hydrogen has been a major bottleneck in the development of the hydrogen power technology, because hydrogen gas is too light and not easily liquefied for transportation. Recently, ammonia (NH3) has drawn worldwide attention as a hydrogen transport medium (1) with a large storage density of up to 17.6 mass% and 120 kg-H2 /m3-liquid NH3. In fact, ammonia is much easier to liquefy than hydrogen and needs only cooling to -33.4 °C at 0.1 MPa or pressurizing up to 0.873 MPa at 0 °C, compared with a hydrogen dew point of -252.9 °C and a volumetric density of 70 kg-H2 /m3* Corresponding author phone: +81 11 706 6842; fax: +81 11 726 0731; e-mail: takiyama@ eng.hokudai.ac.jp. † Hokkaido University. ‡ Kyoto University. 10.1021/es102910q
2011 American Chemical Society
Published on Web 12/02/2010
liquid H2. To confirm these benefits, comparative analyses of implementing ammonia from the production, storage, transportation cost, and energy use aspects have been reported (2, 3). The results revealed that ammonia could be a lower cost and a more efficient fuel than hydrogen. Furthermore, significant ammonia infrastructure in place for fertilizer use will allow for faster development of an ammonia economy, because large quantities of ammonia are already being produced at a rate of 160 Mt/year. Ammonia can be conventionally produced from natural gas or coal using the Haber-Bosch process (4). More than 90% of the world ammonia production currently uses the Haber-Bosch synthesis process, which is based on combining hydrogen and nitrogen over an iron oxide catalyst. Ammonia can also be produced by thermochemical synthesis (Al2O3 + N2 + 3C f 2AlN + 3CO, 2AlN + 3H2O ) Al2O3 + 2NH3) and electrochemical synthesis (5), in which Murakami et al. proposed an electrolysis reaction using H2 (6), H2O (7), CH4 (8), and HCl (9) as H sources and N2 as a source of N at an ambient pressure. This technique is currently being commercialized; however, the cost of producing ammonia is expected to be quite low. For hydrogen production from the decomposition of ammonia, many catalysts have been reported (10-17), of which ruthenium, platinum, and iron worked as effective catalysts for lowering the decomposition temperature and removing a low concentration of ammonia from the fuel gas produced during coal gasification. The ammonia-decomposed hydrogen is supplied to the fuel cell to generate electricity. Thus far, a combination of decomposing ammonia in a fuel cell (18-20) and directly feeding it to a solid oxide fuel cell (SOFC) has been studied (21, 22). The above-mentioned reports have greatly contributed to endorse the idea that ammonia can be a new energy media for developing a hydrogen economy, in which ammonia would be used as not only an energy and chemical source but also a reducing agent. In fact, ammonia can be a reducing agent for NOx reduction (23). In the ironmaking industry, an alternative reducing agent for iron ore is strongly required, and therefore, hydrogen reduction of iron ore has been studied thus far (24-26). The use of ammonia decomposition to produce hydrogen and the successive hydrogen reduction of iron ore seems to be practical as an indirect method for the reduction of ammonia. In contrast, the direct use of ammonia as a reducing agent in the ironmaking process is simpler than indirect reduction. The conventional coal-based ironmaking blast furnace has a thermal reserve zone at 900 °C, in which the CO reduction rate of wustite (FeO + CO f Fe + CO2) is strongly controlled by the rate of the Boudouard reaction (C + CO2 f 2CO). The problems that arise in this technology are the slow kinetics of wustite reduction, the requirement of a large furnace, the preparation of strong coke as the packing material, and GHG emission. If ironmaking with ammonia has large kinetics, it will alleviate the above problems. Surprisingly, however, to the best of our knowledge, there have been no papers published on ironmaking with ammonia. Therefore, the objective of this paper is to experimentally study ammonia reduction of iron oxide and examine the effect of temperature on the reduction products. The results will encourage the use of ammonia in the ironmaking process as a basic strategy to reduce GHG emitted by the ironmaking industry.
2. Thermodynamic Consideration We first consider the changes in the Gibbs free energy of the iron oxide reduction for each reducing agent, H2, and NH3. VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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noted that ammonia can be decomposed at a relatively low temperature. In this case, ammonia-decomposed hydrogen will reduce hematite, too. These results show that the use of ammonia as a reducing agent is promising from the thermodynamic point of view. The phase diagram also shows that reduction at temperatures lower than 600 °C results in a direct change from magnetite to metallic iron, without the generation of wustite (FeO). We will now show these advantages experimentally.
3. Experimental Investigation
FIGURE 1. The change of Gibbs free energy with temperature in Fe2O3 reduction reaction with H2 and NH3, together with ammonia decomposition. This suggests ammonia reduction occurs at much lower temperature than hydrogen one, in contrast it is known that ammonia can thermally decompose at low temperature only under the existence of catalyst. Figure 1 shows the calculations carried out using commercial software (HSC Chemistry v. 5.11, Outotec). It is apparent that the change in the Gibbs free energy for ammonia reduction was lower than that for hydrogen at temperatures higher than 250 °C. This means that thermodynamically, ammonia reduction has an advantage, as ammonia brings a higher equilibrium conversion in the reduction of hematite. The change in the Gibbs free energy for ammonia reduction of hematite decreased with an increase in temperature. This indicates that the equilibrium conversion of ammonia increases with temperature. At the same time, it should be
3.1. Experimental Procedure. Figure 2 shows the schematic diagram of the experimental apparatus, which consists of gas flow controllers, a tubular fixed-bed reactor, and a quadrupole mass spectrometer (QMS) for analyzing gaseous compounds at the reactor downstream. The gas flow controllers were connected to pure argon (99.999%) and 10% ammonia (Argon balance). The fixed bed reactor was made of a transparent quartz tube with an inner diameter of 8 mm. The fixed bed was 0.27 g of reagent Fe2O3 (99.9% purity grade, Kojundo Chemical Laboratory Co., Ltd.) with a particle diameter of c.a. One µm. The fixed bed of hematite particles with a height of 10 mm was supported by quartz wool. The fixed bed temperature was monitored and controlled by a K thermocouple placed underneath the quartz wool. Pure Ar and 10% NH3, at a flow rate of 100 mL-STP/min each, were mixed to make a 5% NH3 gas stream with a flow rate of 200 mL-STP/min. After the confirmation of gas replacement with QMS, the fixed bed was heated at a rate of 10 °C/min. When the fixed bed reached the desired temperature, the temperature was kept constant until the reduction reaction was completed. After the completion of the reduction reaction, the gas was replaced with pure argon at a rate of 200 mL-
FIGURE 2. Schematic diagram of a fixed-bed reactor for ammonia reduction of hematite. A mixed gas of ammonia and argon with NH3 concentration from 0 to 10% was inflowed to fixed beds of hematite powder which was supported by quartz wool, and then outflowed gas was monitored by Quadrupole mass spectrometer (QMS). The temperature of the fixed-bed was monitored and controlled with a thermocouple underneath the quartz wool. Residual ammonia in the exhaust gas was completely absorbed in hydrogen chloride solution. 822
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FIGURE 3. Changes in the QMS relative intensity of the exhaust gas during heating up and holding at 450, 600, and 700 °C. The relative intensity of ammonia, hydrogen, nitrogen, and steam was normalized by argon intensity. Heating rate during heating up was 10 °C/min. STP/min. Then, the fixed bed was cooled down to ambient temperature. The reduction ratio of the sample was determined by the change in weight of the fixed beds before and after the experiments. The phase of the products after the reduction reaction was identified by X-ray diffraction (XRD). 3.2. Results and Discussion. The completion of the reduction reaction can be determined by the termination of steam generation because only steam is produced, without NOx, during the reduction reaction in this temperature range. In fact, we did not observe NOx generation at any temperature. Figure 3 shows the changes in the QMS relative intensity of each gas component during heating and holding at each temperature. The relative intensities are the intensity values normalized by the argon intensity. Therefore, the profiles corresponded to the changes in the molar flux of the gas components. Figure 4 shows the differential of the relative intensity during the heating to 700 °C. The value shows the rate of increase in the molar flux. Thus, a positive value indicates an increase in the molar flux. It is apparent that the ammonia flux started to decrease at around 430 °C. At the same time, the nitrogen and steam fluxes began to increase to compensate for the decrease in the ammonia flux. The increase in steam flux indicates the commencement of the reduction of hematite because steam is the gas component that contains an O atom. Therefore, we concluded that the ammonia reduction of hematite started at a temperature of 430 °C. The steam flux increased with the increase in temperature. When the temperature was fixed at
the holding temperature, the steam flux was almost stable and then decreased, which must correspond to the termination of the reduction reaction. The termination of the reduction of hematite occurred earlier at higher temperatures because of the higher reaction rate. Figure 5 shows the XRD patterns of the fixed-bed particles after ammonia reduction for each temperature. When the holding temperature was higher than 600 °C, hematite was almost completely reduced to metallic iron as expected thermodynamically. The reduction ratio, which is defined as the percentage of the amount of reduced oxygen in hematite, was 99% at 600 °C and 100% ((3% error) at 700 °C on a weight basis. At 450 °C, hematite was reduced to not only metallic iron but also iron nitride, represented by Fe4N. Although Fe4N was produced, hematite was completely reduced by ammonia at temperatures higher than 450 °C. The mechanism for the reduction reaction of hematite with ammonia is discussed here in detail. As shown in Figure 4, the increasing rate of ammonia flux decreased and that of N2 and H2O fluxes increased at 430 °C. In contrast, hydrogen was not generated prior to 500 °C. Therefore, at the temperature range of 430-500 °C, the reduction of hematite with ammonia predominantly occurred. The reduction with hydrogen from ammonia decomposition may occur, but the rate of the reduction is much higher than that of the hydrogen generation. Therefore we cannot observe the hydrogen generation in this temperature range. When the temperature exceeded 500 °C, hydrogen generation began because of the VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Changes in the differential of relative intensity for each gas component during heating up to 700 °C. Positive value means the increase in the flux. decomposition of ammonia. The increasing rate of hydrogen flux reached a peak at around 530 °C and then decreased. At the same temperature range, the H2O flux rate peaked and then started decreasing. In other words, the generation of H2 inhibited the reduction reaction of hematite with ammonia at this temperature range. The inhibition effect must be due to the competition between ammonia and hydrogen for the reduction reaction with hematite in the so-called Langmuir-Hinshellwood mechanism. When the temperature rose above 530 °C, the H2O flux rate increased again. Thus, we conclude that ammonia decomposition to produce H2 and the successive reduction reaction of hematite with hydrogen should occur at this temperature range. The reduction in the increase rate of H2 flux must be due to the hydrogen consumption during the reduction reaction of hematite with hydrogen rather than with ammonia. The above discussion refers only to a low reduction ratio less than 20%, which corresponds to the state of reduction from hematite to magnetite. Even at temperatures below 500 °C, ammonia decomposition occurred as the reduction reaction progressed. As can be seen in Figure 3, in spite of the pseudo steady state of the H2O flux at a fixed temperature, the ammonia flux was still decreasing, and the hydrogen and nitrogen fluxes were increasing. In other words, ammonia decomposition was accelerated as the hematite reduction progressed. Figure 6 shows the changes of the reduction ratio with time, which was estimated from the changes in H2O flux. At each holding temperature, the reduction ratio increased almost linearly according to the pseudo steady state of H2O flux. Especially at the holding temperature of 450 °C, comparing the rate of reduction in Figure 6, we see that the 824
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FIGURE 5. XRD patterns of fixed-bed material during the reduction of hematite with ammonia. slope of the change in the reduction ratio increased at the holding time of around 3000 s. At the same time, the ammonia flux decreased while hydrogen and nitrogen fluxes increased. For that holding time, the reduction ratio was around 11.1%, indicating the completion of the reduction of hematite to magnetite. In conclusion, magnetite has some catalytic effect in the decomposition of ammonia. Because of the acceleration of the ammonia decomposition in the presence of magnetite, hydrogen generation was also accelerated. As a result, the net reduction rate increased because the competitive reaction, Langmuir-Hinshelwood mechanism mentioned above, was reduced. As shown in Figure 5, at the holding temperature of 450 °C, we obtained a mixture of Fe and Fe4N as the product. Here, we discuss the mechanism for Fe4N generation. Figure 5 also shows the XRD patterns for the holding periods of 4000 and 6000 s at the holding temperature of 450 °C. After 4000 s, only a magnetite pattern was obtained. This is in agreement with the reduction ratio as shown in Figure 6. After 6000 s, Fe4N patterns appeared and a small peak of Fe at the diffraction angle of around 44° was obtained. Based on these finds, we can describe the possible reactions for generating Fe4N as follows: 8/11Fe3O4 + 2NH3 f 6/11Fe4N + 1/11H2 + 32/11H2O(g) + 8/11N2 (1) 4Fe + NH3 T Fe4N + 3/2H2
(2)
At the moment, we cannot distinguish between the contributions of the two reactions. However, considering the reactor characteristic of the fixed beds, we speculated some reaction
FIGURE 6. Changes in the reduction ratio estimated from the QMS intensity of H2O in the exhaust gas. Wustite line is not appeared at 450 °C because it does not exist stable at this temperature. mechanisms. The residence time in the fixed-bed was less than 0.95 ms at a temperature of 450 °C. However, the residence time was still too long when compared to the ammonia decomposition rate. Therefore, we obtained more than 80% of the ammonia conversion ratio even at temperatures of 450 °C. In this case, there were longitudinal distributions of the gas concentration along with the gas stream in the fixed bed. In other words, the top of the fixed bed was in the ammonia concentrated atmosphere throughout the experiment, while the bottom of the fixed bed was in the hydrogen concentrated atmosphere. In the top area of the fixed bed, Fe4N would be produced due to reactions 1 and 2. In contrast, Fe4N cannot be produced in the bottom area of the fixed bed because of the low ammonia concentration, while metallic iron will be produced due to the high hydrogen concentration there. As a result, the product was a mixture of Fe and Fe4N. At fixed bed temperatures of 600 and 700 °C, the same reactions may occur but the product was only metallic iron without Fe4N. The same trend was also reported for ammonia decomposition with an iron catalyst (27). At this temperature range, we will need to consider the decomposition rate of Fe4N for a better understanding of the experimental results. Figure 7 shows the phase diagram for the stable products of Fe-O-N as a function of temperature and ammonia fraction. The upper right region, described by the broken line, is the equilibrium line for reaction 1. At the temperature of 700 °C, the ammonia fraction measured in the exhaust gas was still in the Fe4N
FIGURE 7. Phase diagram for the products in the reduction of Fe2O3 with NH3. Closed circle and open one show measured fractional ammonia at the inlet and outlet of fixed bed, respectively, at each temperature. Arrows mean changes in fractional ammonia during the experiments. stable zone. On the other hand, Fe4N is an unstable material, of which the Gibbs free energy for the decomposition reaction is always a negative value even for temperatures above 0 °C. By taking these results into consideration, we can summarize that the rate of Fe4N decomposition is enough slow for VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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obtaining Fe4N at 450 °C, while the rate is too fast to obtain Fe4N at over 600 °C. In fact, Fe4N production will never be a serious problem in the high temperature ironmaking process, because Fe4N is easily decomposed in the absence of NH3. To summarize, the ammonia reduction of hematite in a fixed-bed reactor at different constant temperatures of 450, 600, and 700 °C was studied and the following conclusions were derived: (1) The ammonia reduction of hematite started at only 430 °C, which was much lower in comparison to carbon reduction at 900 °C. The low-temperature ironmaking operation will give the ironmaking industry a significant advantage as a carbonless process if ammonia is used as an alternative energy delivery medium. (2) With increasing temperature, the rate-limiting reaction in the reduction of hematite shifted from direct reduction with ammonia (Fe2O3 + 2NH3 f 2Fe + N2 + 3H2O) to indirect reduction at around 530 °C, which consisted of ammonia decomposition (2NH3 f N2 + 3H2) and hydrogen reduction (Fe2O3 + 3H2 f 2Fe + 3H2O). At this temperature, the reduction ratio was approximately 20%. With an increase in the reduction ratio, ammonia decomposed more quickly. This implies that reduction products such as magnetite and metallic iron served as catalysts for ammonia decomposition. (3) At a reduction temperature of 450 °C, the product was a mixture of metallic iron and iron nitride, Fe4N. This was probably caused by the slow decomposition rate of iron nitride. The conventional ironmaking blast furnace has a thermal reserve zone at 900 °C, in which the CO reduction rate of wustite (FeO + CO f Fe + CO2) is strongly controlled by the small reaction rate of Boudouard reaction (C + CO2 f 2CO). As a result, the conventional process has an essential problem of slow kinetics in the reduction of iron ore. From the above conclusions, we would like to propose a new ironmaking process using ammonia, which operates at much lower temperatures. This will offer many benefits such as accelerating the reduction rate without the generation of wustite, scaling down the furnace size, releasing the restriction of coke strength property, minimizing GHG emission, and maximizing energy saving.
Acknowledgments This research work was partially supported by JFE Steel Corporation. We gratefully appreciate their cooperation.
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