Effect of Preoxidation O2

Article pubs.acs.org/IECR

Effect of Preoxidation O2 Concentration on the Reduction Reaction of NO by Char at High Temperature Weidong Fan,* Yu Li, and Meng Xiao School of Mechanical and Power Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ABSTRACT: Preoxidation experiments were carried out to determine the effect of temperature, ranging from 1000 to 1600 °C, on the O2 enhancement of NO reduction by a bituminous coal char. The results showed that there is a critical O2 concentration that gives a maximum amount of NO reduced by char. The effect of O2 on the reduction of NO indicates a balance between the oxidation reaction and the formation of surface complexes on the char. A large number of C(O) surface complexes can form during the gasification process of char but cannot stably exist at high temperature. The presence of active vacant sites in the complexes is an important factor affecting the reduction of NO and char. This analysis provides a theoretical foundation for further reducing NO emissions during coal combustion. In recent work, we found that a four-level air-staged combustion technology realized ultralow NOx emissions of ∼100 mg/Nm3 at 6% O2, which is much lower than that obtained by the current single-level air-staged technology with NOx emissions of about 200−300 mg/Nm3 at 6% O2. The difference is because, in the four-level air-staged case, the O2 concentration along the reducing zone is maintained at a uniform low value that is beneficial to the enhancement of the reaction between NO and char.

1. INTRODUCTION Air-staged combustion is the most sophisticated low-NOx combustion technology for reducing NOx emissions, and it is thus the most widely employed technique in coal-fired power plants. The latest development in air-staged technology is the vertical staging of the air inlets into the furnace,1 also termed overall air-staged technology. In China’s newly built 600 and 1000 MW boilers employing overall air-staged technology, the NOx emissions are no more than 250 mg/m3 for high-volatilecontent coals. This value is far lower than the 800 mg/m3 observed before this technology was implemented. Many old power plants have also achieved low NOx emissions after being retrofitted with this technology. With increasing public demand for reductions in the emissions of gas pollutants, many countries have lowered their NOx emissions limits. For example, as of 2012, the allowed concentration for all new power plants in China is 100 mg of NO2/Nm3 at 6% O2. After 2014, the limit allowed for all old power plants will be 100 mg of NO2/Nm3 at 6% O2.2 Such limits urgently require much more stringent NOx emissions control techniques.2,3 For the further development of overall air-staged technology, it is necessary to explain the principles on which it is based. The combustion air in the furnace is divided into stage-one air and stage-two air (burnout air). Along the axis of the furnace, three zones are formed: the primary combustion zone, the reducing zone, and the burnout zone.1 Maximum burnout air rate can reach 40% of the total air rate. The experimental results of Coda et al.4 clearly showed the mechanism of reducing NOx emissions in overall air-staged technology. In a one-dimensional drop-tube furnace, a bituminous coal was used to carry out a comparison between unstaged combustion and deep air-staged combustion. The results indicated that the peak value of NO in the air-staged combustion was lower than that in unstaged combustion because the former restricted the generation of NO during the early period of coal burning. The NO generated in the primary combustion zone was reduced in the reducing zone © 2013 American Chemical Society

before the injection of burnout air, and the NOx concentration in the reducing zone dropped off rapidly. At the same time, the O2 concentration in the reducing zone was much lower than that found during unstaged combustion. An O2 concentration of nearly zero was reached at the late stage of the reducing zone, just before the injection of the burnout air.1 The presence of reducing species (HCN, NH3, NCO, etc.) in the combustion zone is conducive to NOx destruction.5,6 In addition, the NO reduction zone is actually the region where the heterogeneous reduction reaction between NO and coal char occurs, which is another important mechanism for efficient NOx reduction. Regarding NO reduction by char, Tomita7 pointed out that, although part of the NO generated in the process of coal burning is quickly reduced by the HCN and NHi produced in the pyrolysis of coal, these gaseous products are depleted quickly, and then char becomes the main reducer of NO. Goel et al.8 also pointed out that the heterogeneous reaction between NO and coal char is the main NO-reducing mechanism. Therefore, coal char is very important to the heterogeneous reduction reaction of NO. Some researchers have reported that NO conversion increases upon addition of small amounts of O2 but decreases when excess O2 is added.9−11 Regarding the mechanism by which O2 enhances the rate of reduction of NO by coal char, many studies have demonstrated that great positive effects on this reduction occur as a result of O2 surface complexes on the surface of coal char. Pevida et al. reported that O2 surface complexes play a determining role in NO heterogeneous reduction, influencing the chemisorption step at lower temperatures and, consequently, the NO−C gasification Received: Revised: Accepted: Published: 6101

January 12, 2013 March 18, 2013 April 5, 2013 April 5, 2013 dx.doi.org/10.1021/ie400131y | Ind. Eng. Chem. Res. 2013, 52, 6101−6111

Industrial & Engineering Chemistry Research

Article

Figure 1. Fixed-bed reaction platform.

step, at a fixed high temperature, O2/He with a fixed O2 concentration was sent to a reactor to maintain coal-char oxidation for a period of time. In the second step, the O2/He atmosphere was switched to NO/He with a constant NO concentration, and then the reduction reaction between NO and coal char was performed. If large quantities of complexes containing O2 were produced on the surface of the coal char during the first stage, these complexes would play an obvious role in the enhancement of the reduction of NO by coal char in the second stage. Obviously, by means of such experiments, the impact of O2 on the mechanism of NO reduction by coal char at high temperature could be determined. This research provides a theoretical foundation for developing the technique of reducing NO emissions in coal combustion engineering.

reaction at higher temperatures.12 Suzuki et al.11 concluded that the main reason for the O2 enhancement of the reaction rate of NO and char is the increase of the number of O2-containing surface complexes produced in the course of carbon gasification, which generate free active sites upon decomposition. Yang et al.13 also found that the C(O) complexes produced during oxidative modification play an important role in the NO−C reaction. However, most experimental research on the mechanisms of the heterogeneous reaction between NO and char has been carried out at relatively low temperatures (usually below 1200 °C). Actually, the temperature of the combustion environment in utility boilers is much higher. The average combustion temperature in a furnace is usually above 1400 °C. At such high temperatures, the trends of NO reduction by char would probably show many differences. As the combustion-environment temperature increases, the degree of homogeneous reduction of NO by pyrolysis gas from coal would be enhanced, and the degree of heterogeneous reduction of NO by char would also be enhanced. Particularly, at such high temperatures, it is not clear whether the enhancement effect on the reaction rate of NO by O2 surface complexes on the char still exists. Consequently, some past works on the heterogeneous reaction mechanism between NO and char are not representative of the combustion of pulverized coal in utility boilers. It is highly important to study the effects of the actual combustion-environment temperature in boilers on the O2 enhancement of the reaction rate of NO and char, but little research in this area has been done. Therefore, in this work, a typical high-volatile-content bituminous coal was employed to determine the effect on the O2 enhacement mechanism of the combustion-environment temperature in the range of 1000− 1600 °C. Because complexes containing O2 on the surface of coal char are difficult to measure directly at high temperature, a special type of preoxidation experiment was employed to confirm the influence of O2 on NO reduction by coal char indirectly. These experiments involved two steps. In the first

2. EXPERIMENTAL SECTION 2.1. Instruments. The experiments were carried out in a tube reactor. A diagram of the system is shown in Figure 1. The experimental apparatus consisted of a gas intake system, a gas mixing section, a pusher system, a high-temperature tube furnace, a sampling unit, and a control cabinet. The reactor tube was made of the refractory ceramic alundum, which can withstand temperatures up to 1650 °C. It was 600 mm long and had an inner diameter of 23 mm. A corundum crucible used as a carrier of the sample was placed in the middle of the alundum tube. The corundum crucible was 25 mm long, and its outside diameter was slightly smaller than the inner diameter of the reactor tube. Ten straight holes were evenly distributed on the bottom of the corundum crucible, and the open end of the crucible was connected to a refractory alundum tube that extended outside the furnace. The outside diameter of the refractory alundum tube was smaller than the inner diameter of the reactor tube. The end that extended outside the furnace was connected to a soft pipe used to feed reaction gas. Moreover, it was also fixed by a tube clip driven by a pusher system. This refractory alundum tube was not only a pusher rod but also a gas inlet pipeline. The pusher system consisted of a variablefrequency motor, a ball-bearing lead screw, a linear bearing, and 6102

dx.doi.org/10.1021/ie400131y | Ind. Eng. Chem. Res. 2013, 52, 6101−6111


Article

alundum tube. With the protection of high-purity He gas lasting for a period of time, all surface complexes on the surface of the coal char and other impurities were removed. In the second step, the He gas was switched to O2/He at a specified O2 concentration. Five different O2 concentrations were tested: 0.1%, 0.5%, 1.0%, 1.5%, and 2%. (In some cases, a sixth concentration, namely, 0.25%, was added.) Each O2 concentration corresponded to one test case. This means that, during each experiment, the O2 concentration remained unchanged for 60 s. In the third step, O2/He was switched to NO/He at a fixed NO concentration of 570 ppm. The NO concentration of the exhaust was continuously detected until the coal char was exhausted and NO concentration returned to the initial value, at which point the run was considered to be over.

a guide rail. Therefore, the pushing times under various conditions were the same. The heating device was a rotary tube-type electric heating furnace with a power capability of 3 kW. The heating element consisted of 10 silicon molybdenum rods. Temperature programming was performed using an intelligent temperature controller, and the highest achievable temperature was 1700 °C. The gas-tightness of the experimental system was of critical importance in this study and was thus ensured. The different gases from different gas cylinders entered the gas distribution box through a reducing valve, and each gas stream was controlled by a mass flow meter. Next, the mixed gas with a fixed flow rate entered the reaction tube. After the furnace temperature had risen to a certain value, the corundum crucible with the sample was pushed into the uniform-temperature zone of the reaction tube by the pusher system. The outlet gas from the reaction tube was continuously analyzed using a gas analyzer that was calibrated using standard gases with constant concentrations before each test. 2.2. Coal and Sample Preparation. The coal used in the experiments was Shenhua bituminous coal widely used in tangential-firing furnaces for power generation in China. Engineering practices confirmed that this coal could easily achieve low NOx emissions in air-staged combustion, so it was an ideal representive coal type for this study. Results of its ultimate and proximate analyses are listed in Table 1.

3. RESULTS AND DISCUSSION 3.1. Influence of the O2 Concentration and Analysis. According to past research, there are two explanations for the mechanism by which O2 enhances the rate of reduction of NO by coal char. First, the O2 atmosphere can cause a large number of CO molecules to be produced to accelerate the decomposition of NO on the carbon surface under hightemperature conditions.14 Chan et al.15 concluded that the effect of O2 is to produce CO which codiffuses with the NO and accelerates the rate of NO reduction. However, studies by other researchers have concluded that the enhancement of the NO conversion by the addition of O2 is not due to the presence of CO. Thus, it can be expected that the catalytic reduction of NO by CO is not a major process under the present reaction conditions (high temperatures and CO/NO ratio of 800 °C) was also emphasized by Aarna and Suuberg.22 However, these sites were not formed from the decomposition of C(O) complexes at 950 °C.13 For temperatures above 850 °C, the activity difference of carbon atoms located at different surface sites might not play an important role in the enhancement of the reaction rate.13 It is thus clear that the preceding viewpoints almost agree that temperatures that are too high could cause the decomposition of O2 surface complexes. Hence, the key issue of this research was to determine whether O2 surface complexes

Table 1. Characteristics of Coal ultimate analysis

a

component

contenta (wt %)

C H S N O volatile matter

63.13 3.62 0.41 0.70 9.94 24.22

proximate analysis component

contenta (wt %)

ash moisture fixed carbon

10.7 11.5 53.58

LCVb (kJ/kg)

24136

As air-dried. bLower calorific value.

Some active components in the ash can play a catalytic effect on interactions between various forms of nitrides. Therefore, to exclude the impact of the ash on the C−NO reaction, all samples were de-ashed using an effective method of hydrochloric acid/hydrofluoric acid extracting minerals for ash removal. Next, the ash-free coal samples were converted to char using a tube furnace. The whole char-making proccess required He gas protection. Finally, the char samples were stored in a container filled with He. 2.3. Methods. For each run, a coal-char sample of 0.004 g was deposited in the corundum crucible placed in s container filled with He gas, and the gas was fixed at a constant flow rate of 1 L/min. Then, the corundum crucible was rapidly loaded into the alundum pusher rod filled with He in advance. This procedure was followed to minimize the contact time of the char sample with the outside environment to avoid the adsorption of O2 from the air onto the surface of char sample. Both ends of the sample were sandwiched by two asbestos pads. Thickness around the circumference of the sample was ensured to be the same. At the three temperatures of 1000, 1300, and 1600 °C, isothermal experiments were performed in three steps. In the first step, the corundum crucible filled with char sample immersed in a pure He atmosphere was pushed into the constant-temperature zone located in the middle of the 6103



Article

Figure 2. Effect of the preoxidation O2 concentration on the reduction of NO on char for 100 μm char.

Figure 3. Effect of the preoxidation O2 concentration on the reduction of NO on char for 30 μm char. 6104



Article

on the surface of coal particles are still generated at high temperature (>1000 °C). This research also attempted to demonstrate that, in the absence of O2, these surface complexes can exist for a period of time and have a significant effect on NO reduction. It can be seen from the experimental methods in section 2.3, that the reduction of NO by coal char in the third stage would inevitablely be impacted if O2 surface complexes were produced on the coal-char surface in the second preoxidation stage. On the contrary, if the third-stage process were not affected by the second-stage process, this would indicate that O2 surface complexes and other types of material did not form during the second stage. Therefore, we can indirectly and strictly judge whether an O2 atmosphere can influence the presence of complexes on the coal-char surface by analyzing the experimental results. To confirm that the role of O2 surface complexes in the reduction of NO is more important than the roles of the specific surface area and structural characteristics of the coal-char particles at high temperature and to further show that there is little relationship between the amount of O2 surface complexes and the specific surface area of the coal-char particles, we used coal chars with two different sizes of 30 and 100 μm. Their total specific surface areas were about 10.6 and 3.7 m2/g, respectively. The effects of varying the inlet O2 concentration are presented in Figures 2 and 3. The overall trends demostrated in these figures are nearly the same. At the three temperatures of 1000, 1300, and 1600 °C, the concentration represented by each curve was lower than the inlet concentration of NO. These curves do not show a monotonic relationship between the extent of NO reduction by coal char and the O 2 concentration in the process of preoxidation. However, it is evident from the figures that a low concentration of O2 (0.1%) also led to a measurable reduction in the amount of NO. As the O2 concentration increased, so did the degree of NO reduction. Under the particular set of operating conditions used in these experiments, it was observed that the greatest extent of NO reduction occurred at 1000 or 1300 °C for an O2 concentration of 0.5% and at 1600 °C for an O2 concentration of 0.25%. As the inlet O2 concentration further increased, the overall rate of the carbon−O2 reaction increased, and a lower fraction of the active sites was available for NO reduction. The data in Figures 2 and 3 show that the char was consumed very rapidly at a 2% preoxidation O2 concentration, leading to a comparatively lower NO reduction over the course of the experiment. The use of a higher temperature would lead to complete reduction of NO, and at lower temperatures, NO would not be reduced to a sufficient degree, causing inconsistent analysis results through the amplification of errors that are concomitant with low degrees of NO reduction. The amount of NO reduced by char as a function of the preoxidation O2 concentration is shown in Figure 4. This figure provides a comparison of the influences of different preoxidation O2 concentrations on char with various particle sizes at different temperatures. Each curve in the figure shows a trend in which the reduction amount first increases and then decreased with increasing preoxidation O2 concentration. It is clear that there exists an optimum O2 concentration that maximizes the extent of NO reduction. This optimum O2 concentration at 1000 °C is about 0.5%. With increasing temperature, the optimum O2 concentration decreases. For example, at 1600 °C, the optimum O2 concentration drops to about 0.3%. The influence of the preoxidation O2 concentration actually results from a competition between the influence of O2

Figure 4. Effect of the preoxidation O2 concentration on the reaction between bituminous coal char and NO.

concentration on the amount of complexes and the influence of O2 on the consumption of char in the process of preoxidation. Below the optimal O2 concentration, as the preoxidation O2 concentration increases, the effect of the increase in complex production on the extent of NO reduction exceeds that of the char consumption, leading to a result in favor of NO reduction. However, above the optimal O2 concentration, the char consumption greatly increases, so that its impact exceeds that caused by the increase in complex formation. Perhaps a great consumption of char would even cause its overall surface area to decrease, so that the amount of complexes formed could not increase but significantly decreased instead. Ultimately, this results in a decrease in the degree of reduction of NO. As can be clearly seen in Figure 4, when the preoxidation O2 concentration was above 1%, the amount of NO reduction dropped off rapidly, confirming this viewpoint. Meanwhile, it can be seen that the influence of temperature obviously supports this viewpoint as well. On one hand, the total amount of NO reduction represented by each curve shows a downward trend as the temperature rises. For example, for two particle sizes, the amounts reduced at 1000 °C, 0.7−2.5 mg of NO/g of char, are much higher than the amounts reduced at 1600 °C, 0−0.7 mg of NO/g of char. At the same preoxidation O2 concentration, the higher the temperature, the more char consumed. On the other hand, the optimal O2 concentration decreases as the temperature rises. This illustrates that the increase in char consumption caused by higher temperature plays a more dominant role than the increase of in the amount of complexes formed. The differences in impact for particles of different sizes shown in Figure 4 actually confirm the differences in their specific surface areas. As reported in the literature,11 the impact of differences in particle size at the same temperature is not large. However, this clearly indicates a relationship between the amount of complexes generated and the consumption of char, both of which are individually affected by the preoxidation O2 concentration. At 1000 or 1300 °C, the reduction amount for 30 μm is greater than that for 100 μm. However, the results for the two particle sizes at 1600 °C are reversed. Actually, at the lower temperatures, more surface complexes are produced on the surface of 30 μm particle with a greater specific surface area at the preoxidation stage, which is more conducive to increasing the amount of NO reduction. However, the increase in temperature will cause more consumption of char through burning. Particularly, at 1600 °C, the consumption of the 30 μm char particle is significantly greater than that of the 100 μm particle. 6105



Article

Through the preceding analysis, it can be concluded that there is a critical O2 concentration for the preoxidation stage. If the preoxidation O2 concentration is below or above this value, the ability of preoxidized char to reduce NO will be lower. The capacity for reduction corresponding to the critical value is the greatest. This shows that the preoxidation process can indeed make more O2 surface complexes on the char surface. However, the formation of surface complexes and the oxidation of char from a low-O2 combustion reaction are carried out simultaneously at this stage. Yang et al.13 believed that, with increasing modification time, more C(O) complexes would form, which, in turn, would decompose to give a higher number of free active carbon sites. However, at high temperatures, preoxidation at low O2 concentration still brings an intense gasification reaction for the char. Our past tests showed that a longer preoxidation time result in a significant fall in NO reduction. This indicates that a large amount of char was consumed by its gasification during the preoxidation stage. If an appropriate preoxidation time is used, as shown in Figures 2 and 3, when the preoxidation concentration is less than the critical value, fewer surface complexes are formed, so that it is also not conducive to the total reduction of NO, although the weak oxidation and lower consumption of char are maintained. When the preoxidation concentration is higher than the critical value, a stronger oxidation and a greater consumption of char are also not conducive to the reduction of NO, even though more surface complexes are formed. In particular, the reduction ability of preoxidated coal char declines greatly even though the O2 concentration is only slightly higher than the critical value. At increased reduction temperature, the critical value of the preoxidation O2 concentration is still evident. It was confirmed that an O2 atmosphere can greatly strengthen the ability of char to reduce NO at a high temperature (1600 °C) similar to that used in industrial combustion and that surface complexes can exist at high temperature. Figure 4 shows the trends in the effect of the critical preoxidation O2 concentration on the ability of char to reduce NO under isothermal conditions at the three temperatures of 1000, 1300, and 1600 °C. The critical O2 concentration is in the range of 0.35−0.5%. At 1600 °C, there is a trend of the critical value to move toward lower O2 concentration. 3.2. Discussion of the Mechanism. As mentioned previously, the impact of the preoxidation O2 concentration actually indicates a competition between the effects of the O2 concentration level on the amount of complexes formed and the consumption of char in the process of preoxidation. However, at such high temperatures, can some stable surface C(O) complexes really form on the char surface during the preoxidation stage and remain during the third stage? According to the findings from most studies reported in the literature, there is almost a consistent conclusion that a temperature that is too high would decompose the O2 surface complexes so that C(O) could not exist. At high temperatures, a priority for the direct gasification of char is obtained even with a rather low O2 concentration. This char gasification by O2 is thought to be a key step in this reaction system, because it determines the concentration of active sites and the nature of surface complexes on the surface. Figure 5 shows the trends in the residual mass of char for all runs after the preoxidation stage. Each residual mass value was derived from the difference in mass balance between all of the reactants and products at the preoxidation stage and reduction stage. As the initial mass of the char sample for each case was 4

Figure 5. Effect of the preoxidation O2 concentration on consumption of char during the preoxidation stage.

mg and each residual mass value shown in the figure is below this initial value, char gasification obviously occurred during the preoxidation stage. Even with a minimum O2 concentration, the degree of gasification reaction was also high. For example, at 1000 °C and a preoxidation O2 concentration of 0.5%, the mass loss of 100 μm char was minimal, but it reached as high as about 50% of the initial mass. At 1600 °C and a preoxidation concentration of 1%, the mass loss of 30 μm char reached a maximum value and nearly realized a complete loss. A lower O2 concentration and a lower temperature would lead to less char loss, whereas a higher O2 concentration and a higher temperature would lead to more char loss. The infuence of the preoxidation O2 concentration on the mass of residual char showed a monotonic decreasing trend. This agrees with the viewpoint put forward in the literature.10 Because the infuence of the preoxidation O2 concentration on the mass of residual char showed a monotonic decreasing trend during the preoxidation stage, why is there not a monotonic relationship between the amount of NO reduced and mass of residual char in the reduction reaction? This illustrates that the gasification during the preoxidation stage is not the simple combustion reaction of the char, but rather causes some important changes on the char surface. As mentioned previously, it is possible that a large number of C(O) surface complexes can form during the gasification process but cannot exist stably at high temperatures. Therefore, it is more likely that, by the formation of C(O) surface complexes, the gasification reaction is just a transitional process. C(O) complexes would continue to generate, but would also continue to decompose. After the stage of preoxidation, C(O) complexes cannot exist, but would decompose and generate active vacant sites, denoted as Cf. The existence of active vacant Cf sites is just an important reason to have an influence on the next reduction stage. During the preoxidation stage, the higher the preoxidation O2 concentration, the more active vacant sites Cf formed on the surface. The number of active vacant sites Cf on the surface per unit mass of char and the mass of residual char would produce a combined effect on the reduction reaction in the next step. Figure 6 shows the change in the CO formation concentration as a function of reaction time during the stage of NO reduction by 30 μm char in various preoxidation O2 concentrations at three temperatures. At the same temperature, the lower the preoxidation O2 concentration, the higher the peak value of the CO curve. Therefore, it is qualitatively judged that the integral area of CO concentration on reaction time is greater. Figure 7 shows the curves for the ratio of CO to CO2 yields versus the 6106



Article

Figure 6. Effect of the preoxidation O2 concentration on CO formation during the reduction stage (size of char, 30 μm).

Figure 7. Effect of the preoxidation O2 concentration on the ratio of CO and CO2 yields during the reduction stage.

combination of active vacant sites Cf and NO is easier for CO2 generation. Therefore, the more active vacant sites per unit mass of char formed on the surface, the easier for CO2 to be generated. For the char preoxidized at 0.1% O2 concentration, because of the lower amount of active vacant sites Cf formed per unit mass of coal char during the preoxidation stage and in addition to the interaction of NO and active vacant sites Cf, NO probably reacts on the surface of coal char without any active vacant sites and directly produces CO at high temperature:

preoxidation O2 concentration at the reduction stage for the two sizes of chars at three temperatures. Obviously, the higher the preoxidation O2 concentration, the lower the ratio of CO to CO2. In contrast, at a 0.1% preoxidation O2 concentration, this ratio is much higher than those at other concentrations. The reduction reaction of NO by char can produce CO or CO2 as well. This indicates that the proportion of CO2 yield in the overall reduction products will substantially increase when the preoxidation O2 concentration increases. Obviously, under the conditions of generating more CO2, the char consumption per unit mass of NO reduction would obviously be less. This can explain why there is a critical relationship between the amount of NO reduced and the preoxidation O2 concentration. The increase in the CO2 formation ratio is precisely determined by the number of active vacant sites formed on the surface per unit mass of char. At high temperatures, the

2C + 2NO → 2CO + N2

(1)

The preoxidation O2 concentration of 0.5% might be a balanced O2 concentration. This enables the amount of active vacant sites Cf formed per unit mass of char much higher than that at a 0.1% preoxidation O2 concentration; meanwhile, the 6107



Article

loss of char gasification at the preoxidation stage is relatively much less. Thus, by means of a large number of active vacant sites Cf, a large amount of NO can be reduced through a way of producing CO2. A small amount of NO can be reduced through direct gasification with char to form CO. Although the amount of active vacant sites Cf formed per unit mass of char might be higher at preoxidation O2 concentrations above 0.5%, a substantial decrease in the mass of residual char would make the total amount of NO reduction significantly smaller. The part of Cf would play a catalytic effect on NO decomposed by CO on the char surface. Therefore, the proportion of CO2 generation would increase, whereas the final proportion of CO generation would further decreases. Of course, a secondary reaction between the justproduced CO2 and the unreacted char might occur, so some CO will be generated:

C + CO2 → 2CO

Figure 8. Effect of the inlet O2 concentration on the reduction of NO on char (reaction temperature, 1300 °C; size of char, 30 μm).

carbon−O2 reaction would increase and a lower fraction of active sites would be available for NO reduction. The data in Figure 8 show that the char is consumed very rapidly at an inlet O2 concentration of 0.5%, leading to comparatively lower NO reduction over the course of the experiment. However, in an actual combustion process, the residence time of coal char in the furnace, only a few seconds, is much shorter than the combustion duration under fixed-bed conditions. Therefore, the influence of a low O2 atmosphere on coal-char gasification consumption might be weakened, so the reduction of NO would not be greatly weakened. Under the conditions of an actual furnace, there might be a much higher critical value of O2 that can lead to a maximum NO reduction rate, so a higher O2 concentration can generate more surface complexes on the coal-char surface to enhance the reduction of NO. To obtain a further demonstration under conditions closer to those used for actual combustion, a drop-tube furnace1 was arranged to carry out experiments on the reduction of NO by coal char in the presence of O2. During the experiments, the simulated gas mixture was injected into the furnace at the top, and Shenhua coal char was brought into the reactor tube by the gas using a microscrew trace feeder. Three cases were chosen for the simulated gas mixture, with initial NO concentrations of 300, 500, and 700 ppm, and in each case, the effects of five different O2 concentrations, namely, 0.5%, 1%, 2%, 3%, and 4%, on NO reduction were investigated. The results obtained in these experiments can indirectly and more reliably reveal the effect of NO reduction by coal char in overall air-staged coal combustion. The NO reduction rate does not change monotonically with the O2 concentration for each NO concentration; however, there does exist a critical O 2 concentration of ∼2−3% for the maximum reduction of NO. This again demonstrates that the presence of O2 in the gas could significantly enhance the reduction of NO. By the preceding experiments, it was confirmed that, under the conditions of industrial pulverized-coal combustion, the ability of char to reduce NO would indeed be strengthened at high temperature and with a low-O2-concentration atmosphere. It was introduced that the overall air-staged technology employed widely by large-scale coal-fired boilers in recent years divides the furnace into three independent functional zones, including the primary combustion zone, the reducing zone, and the burnout zone. The main role of the reducing zone is to offer a special space where NO generated in the primary combustion zone can be siginificantly reduced by reducing species (HCN, NH3, NCO, coal char, etc.). However, an O2 concentration of nearly zero was reached at the late stage

(2)

However, not all of the CO2 would have the opportunity to participate in such a secondary reaction. In particular, under conditions of higher preoxidation O2 concentrations, the mass of residual char would be less, so it would be more difficult for CO2 from the reduction reaction to undergo a secondary reaction with the char. This is why the ratio of CO formation from the reduction reaction is lower under conditions with a higher preoxidation O2 concentration. On the contrary, at the lowest preoxidation concentration of 0.1%, part of the CO2 formed from the reduction reaction probably undergoes a secondary reaction with char, and then a much higher proportion of CO is generated under these conditions. 3.3. Discussion of Applications. In the preoxidation experiments described in the preceding sections, no O2 atmosphere was present during the reduction of NO by preoxidized char. However, the independent existence of the preoxidation stage is almost impossible for the reduction stage of an actual air-staged combustion process. Thus, the reduction stage might always be accompanied by an O2 atmosphere. At this stage, through the gasification of char and O2, would a lowO2 atmosphere exist that can create a type of char with the greatest reduction ability? To uncover this issue, we carried out a series of reduction experiments in which different O2 concentrations were maintained during the whole reduction stage. To make the experiments closer to actual combustion conditions, Shenhua coal-char particles were still chosen, but they were not de-ashed and their size was 30 μm. A temperature of 1300 °C was used to capture the outlet NO trends. An O2 concentration range of 0−0.5% was chosen for this set of experiments to mimic the concentration of O2 existing in combustion flue gas at high temperature. This range of O2 concentrations is much lower than that used in the preoxidation experiments, but it just indicates that the consumption of char gasified by O2 during the reduction stage would greatly increase. The effects of varying the inlet O2 concentration on NO reduction are presented in Figure 8. In the absence of O2, it was observed that the NO concentration level declined obviously compared to its inlet level and reached a relatively sufficient degree of reduction. Under the particular set of operating conditions used in this experimental set, it was observed that the maximum degree of NO reduction occurred at an O2 concentration of 0.25%. The NO peak concentration of the reduction curve reached a minimum. As the inlet O2 concentration further increased, the degree of NO reduction began to drop off. This illustrates that the overall rate of the 6108



Article

Figure 9. Influence of multilevel air-staged combustion on NOx emission trends.

of the reducing zone.4 Our previous studies found that a higher reducing zone is not good for increasing NO reduction when the overfire air or burnout air (OFA) ratio is fixed. When the height of the reducing zone is low, the emissions of NO would decrease as the reducing zone moves upward properly. However, when the height of the reducing zone increases to a certain value, the emissions of NO will change slightly. This shows a saturation characteristic of NO reduction. In addition, a greater burnout air rate indicates that a smaller Ms value should be used, and a smaller burnout air rate indicates that a greater Ms value is optimal.1 Here, Ms is the ratio representing the position of the burnout air along the furnace height. These conclusions precisely verified that, regardless of whether the height is too long or the OFA ratio is too large, no O2 atmosphere in the late stage of the reducing zone would form, and then this case would lead to the saturation of NO reduction and make the reduction efficiency drop off significantly. This confirms the conclusion extracted from the experiments reported herein that a certain low O2 concentration can promote NO reduction by char. To maintain a certain low O2 atmosphere along the entire height of the redcuing zone, a feasible approach is to set up a series of independent OFA groups at different positions along the furnace height of the reducing zone, so that one reducing zone can be divided into a number of zones. Thus, the decrease of the height of each reducing subzone and the prompt injection of OFA after each reducing subzone can make the next reducing zone obtain a timely supplement of O2, and then the low-O2 atmosphere along the reducing zone is formed. In this way, the reducing zone is maintained at the optimal low O2 concentration that is beneficial for strengthening the reaction between NO and char. This is just the orientation of further development of overall air-staged technology for realizing ultralow NOx emissions: multilevel air-staged combustion technology. In the case of multilevel air-staged combustion, the O2 concentration in the entire reducing zone can be guaranteed at a certain low value, which is not close to zero. As a result, a greater degree of NO reduction can be achieved, and then the final NO emissions at the outlet of the burnout zone are much lower than those obtained under single-level air-staged combustion conditions.23 In our previous works, we conducted two-level air-staged combustion experiments with a one-dimensional furnace system to confirm this idea.1 Furthermore, we also carried out multilevel air-staged combustion experiments in the latest

work23 and again confirmed the same results. The curves shown in Figure 9 indicate the trends in NOx emissions under the conditions where multistage combustion was applied. A onedimensional furnace system was still employed. The test details can be found in the literature.23 The values of Ms in the experiments were designed with four values at fixed increments. Here, R90 is the particle-size distribution coefficient, defined as the ratio of the weight of pulverized coal with particle diameters greater than 90 μm to the total weight of the sample. In deep air-staged combustion with a ratio of the primary combustion air rate to the burnout air rate of 0.6:0.4, NOx emissions in multistaged combustion under C1, C2, and C3 conditions shown in the figure are much lower than those obtained in single-level air-staged combustion under C7 conditions with the maximum Ms. In multistaged combustion, R is defined as the staged-air volume flow proportions corresponding to each position of all active burnout air. The reason we chose the single-level air-staged combustion with the maximum Ms here as contrast is that NOx emissions can reach the lowest value with the maximum Ms compared to the other three Ms cases. In deep staged combustion, NOx emissions were lower than 200 mg/Nm3 under C1, C2, and C3 conditions, especially C1 conditions with NOx emissions of ∼100 mg/Nm3. However, NOx emissions under C4 and C5 conditions were higher than those in single-level air-staged combustion with the maximum Ms. These phenomena can be explained as follows: Low O2 concentrations of nearly zero cannot cause further reduction enhancement of NO in the late reduction zone. The existence of appropriate O2 can be conducive to NOx destruction in the reduction zone, in which case the overall NOx would be reduced. C1 represents the case with the arrangement of four independent OFA groups; C2 represents the case with the arrangement of three independent OFA groups; and C3−C6 all represent cases with the arrangement of two independent OFA groups. They are called four-level, three-level, and two-level airstaged combustion technologies, respectively. Obviously, higher numbers of burnout air are favorable distributing O2 uniformly in the reducing zone. The O2 concentration in the reduction zone is beneficial to NOx reduction under C1−C3 conditions, and the optimal O2 concentration can be obtained under C1 conditions. However, the O2 concentration in the reduction zone is higher than the critical or appropriate concentration under C4−C6 conditions. Therefore, NOx emissions under 6109


Industrial & Engineering Chemistry Research these conditions are higher than those in single-level air-staged combustion with the maximum Ms. In the literature,23 we also compared multistaged combustion to single-level air-staged combustion with the maximum Ms under shallow-staged combustion, in which the ratio of the primary combustion air rate to burnout air rate is 0.8:0.2. The NOx emissions in multistaged combustion are still higher than that those in single-level air-staged combustion with the maximum Ms. The reason is that in shallow air-staged combustion the O2 in reduction zones have already exceeded its optimal O2 concentration. This is detrimental to the formation of reduction species (HCN, NH3, NCO, etc). Thus, the application of multistaged combustion using shallow staged combustion technology impairs the effect of the reduction zone. Recently, a 600 MWe tangential firing boiler in China was reconstructed using the above technical principle for ultralow NOx emissions. Through performance test after retrofitting, it was confirmed that the reconstruction achieved an actual ultralow NOx emissions level. For example, for bituminous coal with a volatile matter weight percentage (as air-dried) exceeding 30%, the emissions were 500−600 mg/Nm3 at 6% O2 before retrofitting, but afterward, the guaranteed emissions ceiling reached ∼120 mg/Nm3 at 6% O2, and the actual minimum value was 100 mg/Nm3 at 6% O2. The NOx emissions reduction delivered using this latest air-staged technology with the stated technical principles is of tremendous interest for the realization of ultralow NOx emissions.

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported by the National Natural Science Foundation of China (Grant 50876061).

(1) Fan, W. D.; Lin, Z. C.; Kuang, J. G.; Li, Y. Y. Impact of air staging along furnace height on NOx emissions from pulverized coal combustion. Fuel Process. Technol. 2010, 91, 625. (2) Bris, T. L.; Cadavid, F.; Caillat, S.; Pietrzyk, S.; Blondin, J.; Baudoin, B. Coal combustion modeling of large power plant, for NOx abatement. Fuel 2007, 86, 2213. (3) Li, Z. Q.; Chen, Z. C.; Sun, R.; Wu, S. H. New low NOx, low grade coal fired swirl stabilized technology. J. Energy Inst. 2007, 80, 123. (4) Coda, B.; Kluger, F.; Förtsch, D.; Spliethoff, H.; Hein, K. R. G. Coal-nitrogen release and NOx evolution in air-staged combustion. Energy Fuels 1998, 12, 1322. (5) Spliethoff, H.; Greul, U.; Rüdiger, H.; Hein, K. R. G. Basic effects on NOx emissions in air staging and reburning at a bench-scale test facility. Fuel 1996, 75, 560. (6) Li, S.; Xu, T. M.; Hui, S. E.; Zhou, Q. L.; Tan, H. Z. Optimization of air staging in a 1 MW tangentially fired pulverized coal furnace. Fuel Process. Technol. 2009, 90, 99. (7) Tomita, A. Suppression of nitrogen oxides emission by carbonaceous reductants. Fuel Process. Technol. 2001, 71, 53. (8) Goel, S. K.; Morihara, A.; Tullin, C. J.; Sarofim, A. F. Effect of NO and O2 concentration on N2O formation during coal combustion in a fluidized bed combustor. In 25th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; p 1051. (9) Zhang, G.; Yamaguchi, T.; Kawasaki, H.; Suzuki, T. Selective reduction of nitric oxide over platinum catalysts in the presence of sulfur dioxide and excess oxygen. Appl. Catal. B: Environ. 1992, 1, 15. (10) Gupta, H.; Fan, L. S. Reduction of nitric oxide from combustion flue gas by bituminous coal char in the presence of oxygen. Ind. Eng. Chem. Res. 2003, 42, 2536. (11) Suzuki, T.; Kyotani, T.; Tomita, A. Study on the carbon−nitric oxide reaction in the presence of oxygen. Ind. Eng. Chem. Res. 1994, 33, 2840. (12) Pevida, C.; Arenillas, A.; Rubiera, F.; Pis, J. J. Heterogeneous reduction of nitric oxide on synthetic coal chars. Fuel 2005, 84, 2275. (13) Yang, J.; Mestl, G.; Herein, D.; Schlogl, R.; Find, J. Reaction of NO with carbonaceous materials: 2. Effect of oxygen on the reaction of NO with ashless carbon black. Carbon 2000, 38, 729. (14) Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. NO and N2O decomposition over coal char at fluidizedbed combustion conditions. Combust. Flame 1994, 99, 499. (15) Chan, L. K.; Sarofim, A. F.; Beér, J. M. Kinetics of the NO− carbon reaction at fluidized bed combustor conditions. Combust. Flame 1983, 52, 37. (16) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Nitric oxide reduction by char and carbon monoxide: Fundamental kinetics of nitric oxide reduction in fluidizedbed combustion of coal. Fuel 1985, 64, 1306. (17) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Influence of char surface chemistry on the reduction of nitric oxide with chars. Energy Fuels 1993, 7, 85. (18) Rosas, J. M.; Mirasol, J. R.; Cordero, T. NO reduction on carbon-supported chromium catalysts. Energy Fuels 2010, 24, 3321. (19) Guerrero, M.; Millera, A.; Alzueta, M. U.; Bilbao, R. Experimental and kinetic study at high temperatures of the NO reduction over eucalyptus char produced at different heating rates. Energy Fuels 2011, 25, 1024. (20) Sorensen, C. O.; Johnsson, J. E.; Jensen, A. D. Reduction of NO over wheat straw char. Energy Fuels 2001, 15, 1359. (21) Dong, L.; Gao, S.; Song, W.; Xu, G. Experimental study of NO reduction over biomass char. Fuel Process. Technol. 2007, 88, 707. (22) Aarna, I.; Suuberg, E. M. A review of the kinetics of the nitric oxide−carbon reaction. Fuel 1997, 76, 475.

4. CONCLUSIONS There is a critical O2 concentration during the preoxidation stage that can realize a maximum amount of NO reduction by char. Under isothermal conditions at three temperatures of 1000, 1300, and 1600 °C, the critical O2 concentration is in the range of 0.35−0.5%. At 1600 °C, there is a trend of the critical value to move toward a lower O2 concentration. Through the analysis of the trends in residual char mass and curves of the ratio of CO to CO2 yields, it can be concluded that gasification during the preoxidation stage is not a simple combustion reaction of the char, and a large number of surface complexes C(O) can form during the gasification process, but cannot exist stably at high temperatures. The decomposition of C(O) surface complexes would generate active vacant sites, denoted as Cf. The existence of active vacant sites Cf is an important reason that yield an influence on the reduction of NO by char. A series of multilevel air-staged combustion experiments were carried out in our previous works. The case with four-level air-staged combustion realized the lowest NOx emissions of ∼100 mg/Nm3 at 6% O2, which is much lower than the emissions of current widely used single-level air-staged technology with lowest emissions of about 200−300 mg/ Nm3 at 6% O2. This result is because, for the four-level airstaged case, the O2 atmosphere in the entire reducing zone is maintained at a uniform low concentration that is beneficial in strengthening the reaction between NO and char.

■

■

Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-34206049. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6110



Article

(23) Wang, J. C.; Fan, W. D.; Li, Y.; Xiao, M.; Wang, K.; Ren, P. The effect of air staged combustion on NOx emissions in dried lignite combustion. Energy 2012, 37, 725.

6111


Effect of Preoxidation O2

Recommend Documents