Ind. Eng. Chem. Res. 1996, 35, 2469-2472
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RESEARCH NOTES A Novel Chemical-Looping Combustor without NOx Formation Masaru Ishida* and Hongguang Jin Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226, Japan
A novel combustor without flame is proposed by employing chemical-looping reactions. Here we report the promising results on the novel combustor by kinetic and crystallographic study. We found that the NiO particles mixed with YSZ (yttria-stabilized zirconia) have very good properties for the oxygen carrier in the loop with respect to the reaction rate, conversion, and physical strength; especially the rate of oxidation of Ni is increased significantly. Furthermore, we observed that NOx was not generated in this combustor. Another significant feature, concerning the greenhouse impact, is that CO2 can easily be recovered. Introduction The development of new combustion technology for saving energy and for suppressing environmental impact is of decisive importance, since the conversion of fuel-based chemical energy into thermal energy in the traditional combustor results in (i) the largest irreversible loss related to the second law of thermodynamics in power station and combustion engine due to the serious imbalance between the energy levels of the reactants and of the products (Gaggioli, 1983; Jin and Ishida, 1993) and (ii) a great amount of NOx formation as mainly classified into the thermal and fuel NOx. However, great innovation in fuel conversion in a combustor has been scarce and in preliminary stages in nature. More recently, we can find that there is a tendency to develop a new combustion approach to effectively utilize the fuel energy. For example, several investigators have examined such issues theoretically from the thermodynamics viewpoint (Richter and Knoche, 1983; Ishida et al., 1987). Although the technology of solid-gas reactions regarding chemical-looping reactions has been applied in several processes, especially in chemical industry such as catalytic cracking, hydrogen generation, and direct removal of hydrogen sulfide (H2S) from coal gasification, it was clarified that there still remains several key obstacles to overcome, such as a very low rate of oxidation and shrinkage of particles at high temperature, etc. (Efthimiadis et al., 1993; Lew et al., 1992). With an increase of concern with environmental problems such as suppression of NOx and recovery of CO2, we focus on developing the chemical-looping combustor by conducting experiments (Ishida and Jin, 1994a,b). Of particular relevance to this paper are the proposal and validation of a novel combustor without NOx formation on the basis of the chemical-looping reactions in which the fuel does not contact with air, somewhat like the fuel cell that has broken through the traditional concept of combustion. A Novel Chemical-Looping Combustor The proposed novel combustor on the basis of chemical-looping reactions is composed of two reactors, a reduction reactor and an oxidation reactor. A repre* To whom correspondence should be addressed.
S0888-5885(95)00680-4 CCC: $12.00
sentative example of the chemical-looping combustor can be seen in the flow of Ni and NiO between the following two reactors:
Reduction reactor: CH4 (H2, CO) + NiO f H2O + CO2 + Ni (1) Oxidation reactor:
Ni + 1/2O2 f NiO
(2)
where fuel, CH4 or H2 (CO), reacts with NiO in the reduction reactor (1), producing water and/or carbon dioxide from its top and Ni from its bottom. The solid product, Ni, is transported to the oxidation reactor and reacts with oxygen in air in the oxidation reactor (2), producing high-temperature flue gas and NiO. Here, solid particles, in the forms of NiO and Ni, are recycled. The sum of two reactions cancels the terms related to NiO and Ni and yields combustion of the fuel. The gases from these two reactors are at high temperature since the oxidation of nickel is highly exothermic and the high temperature of nickel oxide particles raises the temperature of gas from the reduction reactor. Hence, this combustor can be applied to generate electricity or to supply heat for the process. When reduction is endothermic, heat of the amount Qab at middle temperature (600-900 K) is absorbed. Hence, the heat Qre released from the oxidation reactor at high temperature (1200-1550 K) is equal to the sum of the combustion heat Qc and the heat Qab absorbed from the reduction reactor. It means that a greater amount of high-level heat can be obtained by utilizing the middle-temperature heat absorbed from reduction. This allows a higher thermal efficiency of the combustor or power plant to be obtained. Experimental Section A significant technical issue which might arise from introducing the looping material in this combustor is the compatibility of the looping material with this novel combustor; that is, we have had to find a suitable looping material to overcome the barriers of the low oxidation rate and shrinkage of particles. Multiple experiments by Ishida and Jin (1994a) established a choice of suitable material, i.e., the NiO mixed with YSZ (NiO/YSZ). Here, nickel oxide produced by Kanto © 1996 American Chemical Society
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Figure 1. Effect of two kinds of particles on reduction and oxidation.
Chemical Co., Inc., and yttria-stabilized zirconia (YSZ, stabilized by addition of 8% Y2O3) obtained from Tosoh Corp. were used. The nickel oxide powder was first mixed with YSZ powder and ground, and particles of about 2 mm diameter were prepared. They were then dried at 90 °C for 30 min in a constant-temperature oven, then heated by the temperature rise of 200 °C/h, and sintered at 1300 °C for 6 h. We first examined the basic reaction kinetics for reduction and oxidation. The reactivity evolution (conversion vs time) experiments were carried out by using hydrogen or natural gas as the fuel in a thermogravimetric analyzer (TGA). The main part of this TGA is a Cahn 2000 electrobalance housed in a two-port jar with one port connected to the reactor tube. It can handle weights up to 3.5 g and is sensitive to weight changes as small as 0.1 µg. The gas flow rate of fuel and air was fixed at 350 mL/s. To emphasize an outstanding finding, we compared only two kinds of particles in this paper: a particle of the mixture of NiO and YSZ (weight ratio of NiO to YSZ is 3:2) and a pure NiO particle. Particle form, instead of powder form, was adopted to meet the needs of process requirements and of cyclical use of the material. The pore structure inside the particle before and after a cycle of reaction is examined by scanning electron microscopy (SEM). The specimens for SEM were prepared by mounting each particle in an epoxy resin. In order to validate whether this combustor forms NOx, we also measured the quantity of NOx from the oxidation reactor by a NOx meter (Yanaco ECL-77A) with full scale 25 ppm or 500 ppm NOx. The air flow rate was controlled and fixed at the flow rate of 500 cm3/ min to meet the needs of requirement of the NOx meter. The outlet gas of oxidation was directly connected to the inlet of the NOx meter. About 170 particles (2 g) of NiO/YSZ (3:2) are set in a quartz tube reactor. Reduction by H2 was first conducted at 873 K, and then oxidation by air took place at 1473 K (200 K higher than that in the TGA experiment in Figure 1). Results and Discussion Figure 1 shows the reaction results of both a NiO/ YSZ particle and a pure NiO particle. The fractional oxidation X is plotted against the reaction time, where X is defined as X ) (Wi - W)/(Wi - Wf), where W, Wi, and Wf respectively are the weight, initial weight, and final weight at complete reduction.
For reduction of NiO, the result indicates that both materials have a high reduction rate and the time required for 80% conversion is less than 10 min for the above two samples. For oxidation at 1000 °C, however, we observed a much different behavior. The oxidation rate of pure NiO was extremely low, especially after the reaction at the outer surface layer was finished. On the contrary, the NiO/YSZ particle permits extremely fast oxidation compared to the pure NiO sample. The fact that the addition of YSZ to NiO gives a high solid diffusivity for the oxide ion especially at high temperature and high porosity turns out to be a crucial feature that YSZ plays an important role in increasing the oxidation rate. It means that oxidation of Ni/YSZ particles is fast enough to meet the need of a real reactor. Furthermore, an experiment on regeneration, ten cycles of reduction and oxidation, indicates that the reaction kinetics is not changed because of the high physical strength of particle mixed with YSZ, which is favorable for cyclic use of particles. As a result, YSZ plays dual roles: the role as an oxygen-permeable material and the role as a material to enhance the mechanical strength of the particle for cyclic use and against abrasion. The SEM photos for the inner part of the particle are shown in Figure 2. It is found that the size of grains for both samples is as fine as 0.3-0.5 µm and that there are many pores, but it can be noted from comparison of A0 and A1 photos that there is little difference between the two cross-sectional structures before and after a cycle of reactions. For the pure NiO particle, after a cycle of reaction up to 800 °C, shrinkage of the particle is observed from the change of the surface structure (from B0 to B1). The change in the particle volume from B0 to B1 resulted in the increase of the particle density by 64%. X-ray defraction tests indicated that the specimens are composed of NiO, ZrO2, and a compound between ZrO2 and Y2O3. Hence, on the basis of the kinetic aspects by TGA and the internal inspection of particles by SEM, we can conclude that the looping material of NiO/YSZ particles plays a crucial role in enhancing the rate of oxidation and avoiding shrinkage of particles and is a suitable material for this novel combustor. Particularly, we also identified the feature of no NOx formation. It is well-known that NOx is one of the most important subjects in recent environmental problems. Many attempts to suppress NOx may mainly be classified as two groups: (1) removing it from the exhaust gas of a combustor and (2) decreasing its formation at the initial stage of combustion. Although recent technologies can suppress NOx formation to some extent, they cannot thoroughly eradicate the generation of NOx. Figure 3 depicts the result of NOx measurement in the oxidation reactor. We observed that NOx was not generated in the oxidation reactor. Curve A in Figure 3 represents the change of the oxygen concentration. At the beginning of oxidation, the N2 stream was switched to an air stream. After that, a lower oxygen concentration implies that oxidation of Ni with air proceeded, but NOx concentration was kept zero; that is, no NOx was generated. This finding is an important breakthrough in the reduction of NOx formation. Since the fuel and air go through different reactors in this chemical-looping combustor, oxidation of Ni/YSZ particles at high temperature is independent of fuel and hence no fuel NOx exists in this oxidation reactor. On
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Figure 2. Cross section of particles by SEM. A0 is fresh sample of NiO/YSZ (3:2) and A1 that after a cycle of reaction (reduction at 873 K and oxidation at 1273 K); B0 is a fresh sample of pure NiO and B1 that after a cycle of reaction.
the other hand, formation of thermal NOx usually increases exponentially with reaction temperatures and the local frame temperature may become higher than 2000 °C in ordinary combustors. In this combustor, however, oxidation of Ni/YSZ takes place at the temperature of 1200 °C, much lower than that in the existing combustor, and a high heat capacity of solid particles brings about a lower local temperature of oxidation. Hence, NOx is not generated in the chemi-
cal-looping combustor, and this combustor makes the most of an opportunity to thoroughly eradicate the generation of NOx. Another significant feature of this novel combustor is that CO2 can readily be recovered when hydrocarbon or synthetic gas containing carbon monoxide and hydrogen is used. In a conventional combustor, the produced CO2 is diluted by a great amount of nitrogen in air. Recovery of such CO2 needs a high-energy
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with oxygen-permeable ceramics (YSZ) is a quite suitable material for this novel combustor. Acknowledgment This work was supported by the Ministry of Education and Welfare (Monbusho) of Japan and, in part, by the Tokyo Electric Power Co. (TEPCO) of Japan. We are grateful to T. Okamoto for many helpful experiments. Literature Cited
Figure 3. NOx measurement.
penalty. However, the exhaust gas produced in this chemical-looping combustor consists of only CO2 and water vapor and is not diluted by nitrogen; i.e., CO2 in the exhaust gas is highly concentrated compared to the conventional power plant. Hence, the CO2 may easily be recovered by a simple physical manner: we only need to cool the exhaust gas to condense the water vapor in it to liquid water. No extra energy expenditure is required for recovering CO2 from the exhaust gas (Ishida and Jin, 1994b). This feature will contribute to suppressing greenhouse impact. Conclusions The promising results of no NOx formation and effective recovery of CO2 together with lower irreversible loss indicated that this novel chemical-looping combustor may lead to a new technology for suppressing the environmental impact and to a new step for saving energy. At the same time, the results of an experimental study indicated that the material by mixing NiO
Efthimiadis, E. A.; Sotirchos, S. V. Effects of Pore Structure on the Performance of Coal Gas Desulfurization Sorbents. Chem. Eng. Sci. 1993, 48, 1971. Gaggioli, R. Second Law Analysis for Process and Energy Engineering. In Efficiency and Costing; Second Law Analysis of Process; Gaggioli, R. A., Ed.; ACS Symposium Series 235; American Chemical Society: Washington, DC, 1983; p 2. Ishida, M.; Jin, H. A Novel Combustor Based on Chemical-looping Reactions and its Reaction Kinetics. Chem. Eng. Jpn. 1994a, 27, 296. Ishida, M.; Jin, H. A New Advanced Power-Generation System Using Chemical-looping Combustion. EnergysInt. J. 1994b, 19, 415. Ishida, M.; Zheng, D.; Akehata, T. Evaluation of a Chemicallooping Combustion Power-Generation System by Graphic Energy Analysis. EnergysInt. J. 1987, 12, 147. Jin, H.; Ishida, M. Graphical Exergy Analysis of Complex Cycles. EnergysInt. J. 1993, 18, 615. Lew, S.; Sarofim, A.; Flytzani-Stephanopoulos, M. Sulfidation of Zinc Titanate and Zinc Oxide Solids. Ind. Eng. Chem. Res. 1992, 31, 1890. Richter, H. J.; Knoche, K. F. Reversibility of Combustion Processes. In Efficiency and Costing; Second Law Analysis of Process; Gaggioli, R. A., Ed.; ACS Symposium Series 235; American Chemical Society: Washington, DC, 1983; p 71.
Received for review November 10, 1995 Accepted April 23, 1996X IE950680S X Abstract published in Advance ACS Abstracts, June 1, 1996.
