Research on Homogeneous Oxidation of NO and SO2 in Flue Gas by

Energy & Fuels 2003, 17, 1549-1553

1549

Research on Homogeneous Oxidation of NO and SO2 in Flue Gas by Chain Reactions Qiang Song* Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

Yasumori Shibamori, Masayoshi Sadakata, and Mitsuo Koshi Department of Chemical System Engineering, The University of Tokyo, Tokyo 113-8656, Japan Received June 10, 2003. Revised Manuscript Received August 28, 2003

A new process to oxidize NO and SO2 in flue gas simultaneously was presented, on the basis of atmospheric chemistry research. The mechanism of this process was analyzed. Methods to initiate the chain reactions were investigated, and the thermal decomposition of additives in flue gas at medium temperature was suggested to be promising, because of its low cost and high efficiency. With nitric acid as the additive, numerical simulations and experiments were developed, to investigate the possibility of applying this new process in flue gas. Numerical and experimental results showed good coherence. Both results showed that the simultaneous oxidation of NO and SO2 can be realized under the proper conditions. There was a temperature window for the oxidation of NO and SO2, and 750 K was determined to be an optimum temperature. The oxidation ratio of SO2 increased fast initially and then slowly as the HNO3 concentration increased, whereas the oxidation ratio of NO decreased slowly as the HNO3 concentration increased. The oxidation ratio of NO decreased as the NO concentration increased. The oxidation ratio of SO2 increased initially and then decreased as the NO concentration increased, which showed a competition between the chain reactions and the termination reactions. Surface reactions are one consumption source of key radicals, and minimizing the wall surface effect by coating inert material enhanced the simultaneous oxidation of NO and SO2.

Introduction SOx and NOx emitted from coal-fired power stations can react with water vapor and other chemicals in the air and lead to acid deposition. They have significant effects on environment and human health. Strict regulations have been enacted in many countries to control such pollution, and low-cost, high-efficiency techniques are in urgent demand. The wet desulfurization process has been developed commercially. However, its system is complicated and costly, and it requires a considerable amount of water consumption and water retreatment. Therefore, this technique is inapplicable for places where the water reserves are rather limited. The dry desulfurization process is recommended for such cases, because its water demands are less. However, the SO2 removal rate of the dry process is generally low and the byproduct is mainly useless sulfite, which requires a large amount of space for storage. It is important to find a novel method to make the dry desulfurization process more efficient and the byproduct more useful. SO3 is much more active than SO2. If SO2 in flue gas could be oxidized to SO3, then the desulfurization efficiency could be greatly enhanced and the byproduct (sulfate) can be utilized in many fields. * Author to whom correspondence should be addressed. E-mail: [email protected].

Some papers have shown that NOx in flue gas can promote the desulfurization reaction and modify the desulfuriztion byproduct to useful sulfate under certain conditions (especially in the presence of NO2).1-3 As we know, NO2 is an active acid gas, so it can also be absorbed by alkaline sorbents used in desulfurization. If NO in flue gas could be oxidized to NO2, not only could the desulfurization process be improved, but the combined removal of SOx and NOx also might be realized. Therefore, the oxidation of NO and SO2 is of great importance for flue gas pollution control. Some work has already been developed in regard to the oxidation of NO;4-6 however, in these cases, no consideration was (1) Li, Y.; Loh, B. C.; Matsushima, N.; Nishioka, M.; Sadakata, M. Chain Reaction Mechanism by NOx in SO2 Removal Process. Energy Fuels 2002, 16, (1), 155-160. (2) Tsuchiai, H.; Ishizuka, T.; Nakamura, H.; Ueno, T.; Hattori, H. Removal of Sulfur Dioxide from Flue Gas by the Absorbent Prepared from Coal Fly Ash: Effects of Nitrogen Oxide and Water Vapor. Ind. Eng. Chem. Res. 1996, 35, 851-855. (3) Ishizuka, T.; Kabashima, H.; Yamaguchi, T.; Tanabe, K.; Hattori, H. Initial Step of Flue Gas DesulfurizationsAn IR Study of the Reaction of SO2 with NOx on CaO. Environ. Sci. Technol. 2000, 34, 2799-2803. (4) Zamansky, V. M.; Ho, L.; Maly, P. M.; Seeker, W. R. Oxidation of NO to NO2 by Hydrogen Peroxide and Its Mixtures with Methanol in Natural Gas and Coal Combustion Gases. Combust. Sci. Technol. 1996, 120, 255-272. (5) Hori, M.; Matsunaga, N.; Malte, P. C.; Marinov, N. M. The Effect of Low-Concentration Fuels on the Conversion of Nitric Oxide to Nitrogen Dioxide. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Sydney, Australia, 1992; pp 909916.

10.1021/ef0340209 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/16/2003

1550 Energy & Fuels, Vol. 17, No. 6, 2003

Song et al.

given in regard to the oxidation of SO2. On the basis of the atmospheric chemistry,7 NO and SO2 can be oxidized through a chain-reaction route. If such chain reactions were initiated in flue gas, the simultaneous oxidation of NO and SO2 could be obtained with low cost. Little research has been done on this process so far. In this work, numerical simulations and experiments were obtained to investigate the possibility of applying this process in flue gas and the influences of the parameters on the oxidation efficiencies. Mechanism Analysis Research on acid deposition showed that the gasphase oxidation of SO2 to SO3 and NO to NO2 proceeds in such a way, under such atmospheres, that the reactions and their relative atmospheric rates are given as8

OH + SO2 + M f HOSO2 + M HOSO2 + O2 f SO3 + HO2 HO2 + NO f OH + NO2

(slow) (fast) (fast)

HNO3 + M f OH + NO2 + M

(5)

OH radicals can be produced via reaction 5. Combined with reactions 1-3, the simultaneous oxidation of NO and SO2 may occur. However, the aforementioned initiation reaction and chain reactions are just one reaction route in practice. Some other adverse reactions also exist that might terminate the chain reaction by consuming the key radicals. For example,

OH + NO f HNO2

(6)

OH + HO2 f H2O + O2

(7)

(1)

OH + HNO3 f H2O + NO3

(8)

(2)

OH + NO3 f NO2 + HO2

(9)

(3)

These three reactions can form a chain reaction, and the net reaction is

SO2 + NO + O2 f SO3 + NO2

possibility of initiating and sustaining the chain reactions in flue gas. At a certain temperature, nitric acid can homogeneously decompose as follows:

(4)

Flue gas includes NO, SO2, and O2. If some OH or HO2 radicals are present, the chain reactions (reactions 1-3) might be initiated and the simultaneous oxidation of NO and SO2 might occur. There are two types of methods to generate radicals. One is to impose an external energy on the flue gas directly, such as via discharge or electron beam. The flow rate of flue gas in practice is large; therefore, these types of methods will consume a large amount of energy and are costly. Another way is to use additives. With the assistance of external energy (e.g., discharge) or hot flue gas, additives can decompose and generate OH or HO2 radicals. The simplest and cheapest way to introduce OH or HO2 radicals into flue gas is via thermal decomposition of the additives in hot flue gas. According to the literature, appropriate additives might be hydrogen peroxide,9 hydrocarbons,10,11 nitric acid,12,13 etc. As a reliable OH radical source, nitric acid was used as the additive in this work, to investigate the (6) Lyon, R. K.; Cole, J. A.; Kramlich, J. C.; Chen, S. L. The Selective Reduction of SO3 to SO2 and the Oxidation of NO to NO2 by Methanol. Combust. Flame 1990, 81, 30-39. (7) Barbara, J. F. P.; James, N. P., Jr. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments and Applications; Academic Press: New York, 2000. (8) Gleason, J. F.; Howard, C. J. Temperature Dependence of the Gas Phase Reaction HOSO2 + O2 f HO2 + SO3. J. Phys. Chem. 1988, 92, 3414-3417. (9) Bohn, B.; Zetzsch, C. Rate Constants of HO2 + NO Covering Atmospheric Conditions. 1. HO2 Formed by OH and H2O2. J. Phys. Chem. A 1997, 101, 1488-1493. (10) Pilling, M. J., Ed. Low-Temperature Combustion and Autoignition; Comprehensive Chemical Kinetics, Vol. 35; Elsevier Science B.V.: Amsterdam, The Netherlands, 1997. (11) Kan, C. S.; Calvert, J. G.; Shaw, J. H. Oxidation of Sulfur Dioxide by Methylperoxy Radicals. J. Phys. Chem. 1981, 85, 11261132. (12) Ballod, A. P.; Titarchuk, T. A.; Tiker, G. S.; Rozovskii, A. Ya. Heterogeneous/Homogeneous Decomposition of Nitric Acid. Kinet. Catal. 1990, 30, 677-681.

Surface reactions also consume some key radicals, such as OH and HOSO2.14 Further research is required to investigate the result of competitions between these reactions in flue gas. Numerical Simulation To give good instruction for the experimental research, numerical simulation was developed first to find the optimum reaction conditions. Modeling Method and Conditions. The Senkin subprogram (for plug flow with constant pressure and temperature) of the Chemkin program (Version 3.6) was used to develop the simulation. The kinetic parameters of radical reactions were provided by Prof. Koshi’s laboratory, which were accumulated from broad sources (e.g., National Institute of Standards and Technology, Gas Research Institute, published papers) and updated occasionally over a long period of time. This database was tested, in part, experimentally and exhibited good coherence with the experimental data. During the simulation, the pressure was set constant as atmospheric pressure and the concentrations of SO2 and O2 were set as 2000 ppmv and 5%, respectively, similar to those in typical flue gas. Other parameters (reaction temperature, residence time, additive dosage, and NO concentration) were changed individually, to investigate their influence and to determine the optimum oxidation conditions. Numerical Result and Discussion. The influence of reaction temperature was investigated for four residence times, as shown in Figure 1. In the figure, T is reaction temperature, XNO the oxidation ratio of NO, and XSO2 the oxidation ratio of SO2. Figure 1 shows that a temperature window exists for the oxidation of NO and SO2. As the residence time increased, this temperature window became wider and shifted to a lower (13) Margitan, J. J. Mechanism of the Atmopheric Oxidation of Sulfur Dioxide. Catalysis by Hydroxyl Radicals. J. Phys. Chem. 1984, 88, 3314-3318. (14) Martin, D.; Jourdain, J. L.; Le Bras, G. Discharge Flow Measurements of the Rate Constants for the Reaction OH + SO2 + He and HOSO2 + O2 in Relation with the Atmospheric Oxidation of Sulfur Dioxide. J. Phys. Chem. 1986, 90, 4143-4147.

Homogeneous Oxidation of NO and SO2 in Flue Gas

Energy & Fuels, Vol. 17, No. 6, 2003 1551

Figure 2. Influence of HNO3 dosage on oxidation ((9) NO oxidation and (2) SO2 oxidation). Conditions are as follows: optimum temperature was 750 K; gas composition was 2000 ppmv SO2, 300 ppmv NO, and 5% O2.

Figure 1. Influence of reaction temperature and residence time on the oxidation of (a) NO and (b) SO2. Gas composition was 2000 ppmv SO2, 300 ppmv NO, 150 ppmv HNO3, and 5% O2. Residence times were (9) 0.1 s, (b) 0.5 s, (2) 1.0 s, and (1) 3.0 s.

temperature range and the corresponding maximum oxidation ratios of NO and SO2 increased. Only a small difference was observed between residence times of 1 and 3 s, indicating that the process with HNO3 was fast and the major portion of the process can be realized within 1 s. The residence time is very important for industrial design. For a flue gas, it is better to limit the residence time within 1 s. For a residence time of 1 s, the optimum reaction temperature for the oxidation of NO and SO2 is 750 K. The influence of the additive dosage was investigated at typical flue gas compositions and the optimum reaction temperature with a residence time of 1 s. The result was given in Figure 2, in which CHNO3 is the HNO3 concentration in the reactant gas. The result shows that the oxidation ratios of both NO and SO2 initially increased quickly and then slowly as the HNO3 concentration increased. Therefore, a further increase in the HNO3 concentration is not very effective in improving the oxidation of NO and SO2, because of the restriction of other fixed parameters. As shown in the mechanism analysis, NO is key in the propagation of the chain reactions. The net reaction for the chain reactions is SO2 + NO + O2 f SO3 + NO2, which indicates that the NO concentration limits the oxidation of SO2, because the concentration of NO is much lower than that of SO2 in the flue gas. Moreover, NO competes with SO2 in reactions with OH radicals. The influence of NO was investigated at a typical flue gas composition and the optimum reaction temperature with a residence time of 1 s. The result was given in Figure 3, in which CNO represents the NO concentration.

Figure 3. Influence of NO on oxidation ((9) NO oxidation and (2) SO2 oxidation). Conditions were as follows: optimal temperature was 750 K; gas composition was 2000 ppmv SO2, 150 ppmv HNO3, and 5% O2.

The oxidation ratio of NO decreased as the NO concentration increased, because of the fixed additive amount. The oxidation ratio of SO2 initially increased and then decreased as the NO concentration increased, which showed a competition between the chain reactions and the adverse reaction of NO with the OH radical. A NO concentration of 300 ppmv seemed to be an optimum value. Numerical results gave good instruction for the experimental design. In the next section, the experimental work was developed. Because of the limitations of the measurement equipment, the composition of simulated flue gas in the experiments was different from that previously discussed. Numerical simulation under the experimental conditions was also developed, and the result was compared with the experimental results presented later in this paper in Figures 5-7. The experimental and numerical results showed good coherence, which indicated good accuracy of the present numerical simulation. Experimental Section Experimental System. A schematic of the experimental setup is shown in Figure 4. The setup included three components: a gas-mixing area, the reactor, and the measurement equipment. HNO3 was used as an additive, and the simulated flue gas consisted of SO2, NO, and O2, with the balance being N2. Each type of reactant gas came from a standard gas cylinder and

1552 Energy & Fuels, Vol. 17, No. 6, 2003

Song et al.

Figure 4. Schematic of experimental setup. its flow rate was controlled by the mass flow controller. HNO3 was carried by N2 through a bubbler filled with 61 wt % HNO3. All the gases mixed in a gas-mixing chamber and then flowed into the tube reactor through a quartz-glass nozzle at a constant total flow rate (2 SLM). Experiments were conducted in a quartz-glass tube reactor (inner diameter of 31 mm and length of 1 m) embedded in an electric furnace (inner diameter of 50 mm and length of 600 mm), whose temperature was monitored to be stable (within (3 K). The reactor has a 400mm-long uniform temperature distribution in the heating region. A gas analyzer (Horiba model PG250) and Fourier transform infrared (FTIR) equipment (Horiba model FT-730G) were used to monitor the compositions of the inlet and outlet gases of the reactor. The oxidation product of SO2 should be SO3. However, SO3 is very active and can react quickly with water vapor in the reactant gas to form sulfuric acid (H2SO4) at temperatures below ∼600 K. The H2SO4 will condense on the cooler pipe wall. Thus, SO3 cannot be detected in this case and only the change in SO2 concentration can be studied. During the experiments, some colorless oily droplets formed on the cooler wall near the end of the reactor. High-pressure liquid chromatography (HPLC) was used to analyze the contents of the droplets, by dissolving them in solution. Experimental Result. HPLC analysis of the reaction product that had condensed on the cool reactor wall showed that its main content was H2SO4. Because the reaction products either existed as gases or condensed on the walls and no SO3 was found in the gas product and little H2SO3 was found in the condensed droplets, it can be determined that the change in the SO2 concentration during the process was caused by the oxidation of SO2. The oxidation ratio of SO2 was defined as the difference in the SO2 concentration between the inlet and outlet gases of the reactor divided by the SO2 concentration of the inlet gas. During this process, NO can also be oxidized to NO2. The gas analyzer and FTIR equipment can measure the concentrations of NO and NO2. Additional NO and NO2 were generated, because of the decomposition of HNO3. Here, the oxidation ratio of NO was defined as the difference in the NO concentration between the inlet and outlet gases of the reactor divided by the NO concentration of the inlet gas. The influence of reaction temperature is given in Figure 5. The result showed that the oxidation ratios of NO and SO2 initially increased and then decreased as the reaction temperature increased. A temperature window existed, similar to that observed with the numerical result. The optimum temperature was 750 K, which is the same temperature as that determined from the numerical results. The experimental maximum oxidation ratios were 80.5% for NO and 6.4% for SO2. The numerical maximum oxidation ratios were 100% for NO and 9.5% for SO2. The influence of the HNO3 concentration is given in Figure 6. The oxidation ratio of NO initially increased quickly and then decreased slowly, because the decomposition of HNO3 generated additional NO. The oxidation ratio of SO2 initially increased quickly and then slowly as the HNO3 concentration increased. The turning points were located at HNO3 concentrations of 85 ppmv in the experiments and 45 ppmv in the simulations. At the turning point, the experimental oxidation

Figure 5. Comparison of (9) experimental results and (2) numerical results, in regard to the influence of reaction temperature. Gas composition was 1000 ppmv SO2, 100 ppmv NO, 57 ppmv HNO3, and 5% O2.

Figure 6. Comparison of (9) experimental results and (2) numerical results, in regard to the influence of HNO3 concentration. Conditions were as follows: optimal temperature was 750 K; gas composition was 1000 ppmv SO2, 100 ppmv NO, and 5% O2. ratios were 88.4% for NO and 7.4% for SO2; the numerical oxidation ratios were 96.3% for NO and 10.2% for SO2. The influence of the NO concentration is given in Figure 7. The oxidation ratio of NO decreased as the NO concentration increased. The oxidation ratio of SO2 increased initially and then decreased as the NO concentration increased. The turning point occurred at a NO concentration of ∼120 ppmv. At the

Homogeneous Oxidation of NO and SO2 in Flue Gas

Energy & Fuels, Vol. 17, No. 6, 2003 1553

Table 1. Experimental Results with Uncoated and Coated Reactors

oxidation ratio of NO (%) oxidation ratio of SO2 (%)

uncoated reactor

twice-coated reactor

four-times-coated reactor

numerical simulation

88.4 7.4

92.9 9.9

98.9 12

96.3 10.2

the previous experiments. One tube reactor was coated with boron oxide two times and another was coated four times, before either reactor was used in the experiments. Because a HNO3 concentration of 85 ppmv is the turning point at the gas composition of 1000 ppmv SO2, 100 ppmv NO, and 5% O2, the experiments with coated reactors were developed under the following conditions: the reaction temperature was 750 K, the total gas flow rate was same as that previously mentioned, and the simulated flue gas consisted of 1000 ppmv SO2, 100 ppmv NO, 85 ppmv HNO3, and 5% O2. The experimental results with coated and uncoated reactors are given in Table 1. For comparison, the numerical results under the same conditions are also given in Table 1. Consistent with the previous analysis, the oxidation ratios of NO and SO2 increased as the wall effect was weakened by coating. The more times the reactor was coated, the weaker the wall effect became. Interestingly, the oxidation ratios of NO and SO2 in the experiment with the four-times-coated reactor was greater than the numerical results. This indicated that some inaccuracy existed in the simulation, which led to lower calculated oxidation ratios. Better results can be expected in practice.

Conclusions

Figure 7. Comparison of (9) experimental results and (2) numerical results, in regard to the influence of NO concentration. Conditions were as follows: optimal temperature was 750 K; gas composition was 1000 ppmv SO2, 57 ppmv HNO3, and 5% O2. turning point, the experimental oxidation ratios were 67.2% for NO and 6.7% for SO2; the numerical oxidation ratios were 96.8% for NO and 10.6% for SO2. The results indicated both the occurrence of the chain reactions and a competition between the favorable chain reactions and the adverse reaction of NO with OH. Discussion and Further Research. The experimental results show that the new process to oxidize NO and SO2 simultaneously, based on the chain reactions, can be realized in flue gas. The experimental results show the same tendencies under the influence of parameters (reaction temperature, HNO3 and NO concentrations) as numerical results. However, some deviations were observed. The experimental values were much lower than the calculated values. These deviations might originate from assumptions made in the simulation or from the improper setting of the dynamic parameters. Surface reactions were omitted in the simulation, on the assumption that the process occurs in a homogeneous gas phase. As previously mentioned, surface reactions are a consumption source of key radicals, such as OH and HOSO2. In the experiments, the tube reactor has a low volume:surface ratio. The reactor wall effect should be weakened in favor of the propagation of the chain reactions. Coating the reactor wall with inert materials is a good choice. In our case, boron oxide was used as the inert material, and the coating procedure was taken as depicted in the literature.15 To compare the coating effect, two new tube reactors were prepared, which were totally the same as the reactor used in (15) Krasnoperov, L. N.; Niiranen, J. T.; Gutman, D.; Melius, C. F.; Allendorf, M. D. Kinetics and Thermochemistry of Si(CH3)3 + NO Reaction: Direct Determination of a Si-N Bond Energy. J. Phys. Chem. 1995, 99, 14347-14358.

A new process was presented to oxide NO and SO2 in flue gas simultaneously via chain reactions. On the basis of the mechanism analysis, a numerical simulation was developed and the result showed that this new process was possible for application in flue gas. There was a temperature window for the oxidation of NO and SO2, and the optimum temperature was 750 K. The oxidation ratio of SO2 initially increased quickly and then slowly as the HNO3 concentration increased, whereas the oxidation ratio of NO decreased slowly as the HNO3 concentration increased. The oxidation ratio of NO decreased as the NO concentration increased. The oxidation ratio of SO2 increased initially and then decreased as the NO concentration increased, which showed a competition between the chain reactions and the termination reactions. Experimental research was developed on a flow tube reactor. Simultaneous oxidation caused by chain reactions was realized experimentally. The experimental results showed the same tendency as numerical results in regard to the influence of parameters on the simultaneous oxidation. On the basis of the analysis of the deviations between experimental results and numerical results, further experiments were developed, by coating the reactor with inert material to weaken the wall effect. Comparison of the results with coated and uncoated reactors showed that weakening the wall effect can largely enhance the simultaneous oxidation of NO and SO2. Acknowledgment. This work was supported by Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Corporation. The authors appreciate their grant in aid for this research. EF0340209