Numerical simulation of bituminous coal combustion in a full-scale tiny-oil ignition burner: influence of excess air ratio

Abstract

The progression of ignition was numerically simulated with the aim of realizing a full-scale tiny-oil ignition burner that is identical to the burner used in an 800 MWe utility boiler. The numerical simulations were conducted for four excess air ratios, 0.56, 0.75, 0.98 and 1.14 (corresponding to primary air velocities of 17, 23, 30 and 35 m/s, respectively), which were chosen because they had been used previously in practical experiments. The numerical simulations agreed well with the experimental results, which demonstrate the suitability of the model used in the calculations. The gas temperatures were high along the center line of the burner for the four excess air ratios. The flame spread to the burner wall and the high-temperature region was enlarged in the radial direction along the primary air flow direction. The O₂ concentrations for the four excess air ratios were 0.5%, 1.1%, 0.9% and 3.0% at the exit of the second combustion chamber. The CO peak concentration was very high with values of 7.9%, 9.9%, 11.3% and 10.6% for the four excess air ratios at the exit of the second combustion chamber.

Keywords

numerical simulation / tiny-oil ignition burner / pulverized coal / temperature field

Introduction

The traditional firing up of a boiler requires that oil guns be arranged at the secondary air nozzles to pre-heat the combustion chamber of a furnace to its operating temperature. Pulverized coal is then put into the burner and the fuel-rich primary air-coal mixture is ignited by a high-temperature oil flame. According to statistics, the firing up of a 300 MWe unit requires approximately 100 tonnes of oil. However, oil resources are limited. To save energy, a tiny-oil ignition centrally fuel-rich swirl burner was proposed and experiments were conducted to determine their ignition characteristics [1-4]. In tiny-oil ignition technology, oil guns are arranged in the primary air duct so that the heat of oil combustion may be used to ignite the pulverized coal directly. Oil energy is used more efficiently by employing the tiny-oil technology. Therefore, oil guns with a smaller delivery capacity can meet the firing up needs and the partial-load operation of boilers. As a result, oil can be saved. In a tiny-oil burner, the atomized oil from the main oil-gun ignites and combusts in an adiabatic chamber. Subsequently, an oil flame ignites the atomized oil from the auxiliary oil-gun. Cone separators are typically installed in the primary air-coal mixture duct to concentrate pulverized coal into the central zone of the burner. The fuel-rich primary air-coal mixture passes into the first combustion chamber where the fuel-rich primary air-coal mixture is ignited by a high-temperature oil flame formed by the main and auxiliary oil-guns. The burning pulverized coal and the oil flame from the first combustion chamber are then directed to the second combustion chamber where the coal is ignited. In bituminous coal ignition experiments that were conducted with tiny-oil ignition burners, the pulverized coal was ignited using excess air ratios of 0.56 to 1.14 and a primary air temperature of 15°C. The coal feed rate was 4 t/h and the total oil flow rate was 100 kg/h. The gas temperature increased gradually along the center line of the burner in the direction of the primary air flow. The wall temperatures of the first and second combustion chambers were low and, therefore, the burner wall was safe. The O₂ concentrations at the exit of the burner ranged from 0.01% to 3.70%. The CO concentrations were more than 0.01 [3]. More details can be found in Ref. [3].

Because of the extremely high costs of performing experiments, the use of numerical models to simulate and predict problems such as combustion phenomena is the best choice. CFD has the capacity of providing detailed information about coal combustion [5-10]. Presently, research on tiny-oil burners is mainly done in China. Li et al. [12] studied the effects of operational parameters of less-oil ignited horizontal bias pulverized-coal burner on ignition operation states. Li et al. [12] simulated the ignition process of pulverized coal with less oil in the abundant oxygen ignition burner. Fu et al. [13] found that the increase of the primary air in the combustion chamber was negative to the ignition of the flow of pulverized coal using numerical software. In this paper, the details of bituminous coal combustion were investigated in a tiny-oil ignition centrally fuel-rich swirl burner. The fuel was composed of pulverized coal and oil. The temperature field, O₂ concentration and CO concentration distribution were obtained from the numerical simulation which will be of benefit to future laboratory experiments and engineering applications.

Numerical simulations of tiny-oil burners

Simulation objects

Figure 1 shows the tiny-oil ignition apparatus. The pulverized coal was carried from the feeder to the tiny-oil ignition burner by the primary air from one blower. Oil was drawn from the oil tank and sent to the main and auxiliary guns. The oil guns employed mechanical and air atomization. Compressed air was administered as atomized air and entered the oil guns which provided O₂ for the initial combustion of the oil. The main body of the air consumed during oil combustion was supplied by another blower. The gas temperature was measured using thermocouples. The end of a bare thermocouple was exposed in the burner and, therefore, the temperature measured was higher than that of the local gas because of the high amount of flame radiation. However, the temperature measured should be lower than that of the local gas because of the radiation between the bare thermocouple and the burner wall, which are in close proximity. The measurement error caused by these two effects was approximately 8%. In Fig. 1, x is the distance from the temperature measurement point to the central pipe exit along the axial line; r is the distance from the temperature measurement point to the centerline of the burner along the radial direction; r is positive on the side of the main oil gun and negative on the side of the auxiliary oil gun; r₁ is the distance from the temperature measurement point to the centerline of the burner along the radial line at the exit of the first combustion chamber and r₂ is the distance from the temperature measurement point to the centerline of the burner along the radial line at the exit of the second combustion chamber .

Computational domain and mesh generation

Figure 2 illustrates the three-dimensional mesh that was generated for the whole burner based on the structural design of the burner.

Selection of the model

The commercially available FLUENT 6.3.26 software was used to calculate the co-combustion of oil and coal in a start-up ignition burner by employing a range of widely used numerical models. Gas turbulence was specifically taken into account by the realizable k-ϵ model [14]. The Lagrangian stochastic tracking model was used to analyze the gas/particle flow field [15]. Radiation was described using the P-1 model [16] and the devolatilization process was modeled using the two-competing-rate Kobayashi model [17]. For gaseous phase combustion, a two-fraction approach was used to calculate the mixture fractions and their variances using transport equations [18]. The individual-species molar fraction, temperature and density were derived from the predicted mixture fraction distribution. It was assumed that the volatile reaction rate was rapid enough to ensure chemical equilibrium and this assumption was made with a PDF approach. In the injection plane of the mesh, two different types of particles were introduced. The coal was set as a primary stream and the oil as a secondary stream. This method allows each fuel in the binary blend to be described separately with different properties and initial conditions. Char combustion was described by a diffusion/kinetics model [19].

Operating parameters

Table 1 lists the parameter settings which are consistent with the parameters of the operation of a power plant. Table 2 presents the ultimate analysis results and other characteristics of the 0# light diesel oil used in the experiments. Table 3 gives the ultimate analysis of the bituminous coal used in this study.

Results and discussion

Distribution of gas temperature in the burner

Figure 3 depicts the distribution of the gas temperature along the burner center line from the numerical simulation using tiny-oil ignition and excess air ratios of 0.56, 0.75, 0.98 and 1.14 (the primary air velocities were 17, 23, 30 and 35 m/s). The gas temperature distributions were similar for the different excess air ratios along the burner center line for the simulations and the experiment. In accord with the overall trend in the numerical simulation results, the gas temperature along the burner center line should decrease as the coal precipitates volatile, absorbing heat from the high-temperature oil flame. The gas temperature increased as the pulverized coal was gradually ignited along the direction of the primary air flow. For all four excess air ratios the gas temperature tended to be steady from x = 1 m to the nozzle of the burner and, therefore, this mean combustion was also steady. The gas temperature along the center line remained high from x = 1 m to the nozzle of the burner at a temperature higher than 1100°C, which is advantageous for pulverized coal ignition. With an increase in the excess air ratio, the corresponding primary air velocity increased and subsequently the amount of oxygen increased. The gas temperature along the center line increased at the same physical positions after the combustion became steady. At excess air ratios of 0.56, 0.75, 0.98 and 1.14 the gas temperatures at the center exit of the burner were 1182°C, 1360°C, 1437°C and 1615°C, respectively. Figure 4 demonstrates the maximum temperatures at different sections along the flow direction of the primary air and the radial positions of the maximum temperatures at different excess air ratios. The distribution of the maximum gas temperature was similar to the distribution of gas temperature along the center line in the simulations. At first, the maximum temperature is located on the side of the main oil gun, however, it gradually moves to the side of the auxiliary oil gun along the direction of primary air flow. Figure 5 displays the gas temperature field of the inner burner at an excess air ratio of 0.75 (corresponding to the primary air velocity of 23 m/s). The primary air velocity is designed to be 23 m/s in practical operation and, therefore, the experiment with an excess air ratio of 0.75 can be treated as standard for the ignition process. From the gas temperature distribution, a high-temperature region exists separately at the center lines of the main oil gun and the auxiliary oil gun, indicating a shift of the maximum temperature from the side of the main oil gun to the side of the auxiliary oil gun. The flame spread to the burner wall along the primary air flow direction and the high-temperature region in radial direction expanded. This phenomenon can be explained by Figs. 4 and 5. Two oil guns were arranged on either side of the burner center line. During the process of ignition, a high-energy igniter was used to ignite the atomized oil from the main oil gun first. The initial ignition heat was from the oil flame on the side of the main oil gun. Therefore, the maximum gas temperature was initially on the side of the main oil gun. With the primary air flowing to the nozzle of the burner, the oil from the auxiliary oil gun was then ignited. Because the oil flow rate of the auxiliary oil gun was 65 kg/h which is more than that of the main oil gun (35 kg/h), more heat was released and more pulverized coal was ignited by the auxiliary oil gun. The maximum gas temperature gradually moved from the side of the main oil gun to the side of the auxiliary oil gun. The high-temperature region expanded as the flame spread along the primary air flow direction.

Figure 6(a) shows the calculated temperature field and the measured temperature field at the exits of the first and second combustion chambers at different excess air ratios. There was little difference between the numerical simulation results and the experimental results for excess air ratios of 0.56, 0.75, 0.98 and 1.14. The numerical simulation results indicated a gas temperature distribution that was similar to the one at the exits of the first and second combustion chambers at different excess air ratios. The gas temperature was the highest along the center of the burner and gradually decreased toward the burner wall. This temperature distribution resulted from the main and auxiliary oil guns both at the centre of the primary air-coal mixture duct where the high-temperature oil flame was formed. Cone separators were installed in the primary air-coal mixture duct to concentrate the pulverized coal into the central zone of the burner. Pulverized coal was then heated by the oil flame and was ignited. Therefore, pulverized coal combustion was intense in the central zone of the burner where the gas temperature was the highest. Because the flame gradually spread to the burner wall along the primary air flow direction, the diameter of the burner duct was large and the unburned primary air-coal mixture kept on entering the flame boundary while an air membrane formed near the wall. The air membrane had a cooling effect on the burner wall. Therefore, the gas temperature decreased gradually from the center to the wall. At low temperatures, the burner wall was safe for operation. With pulverized coal combusting along the primary air flow direction, the gas temperature at the exit of the second combustion chamber was higher than that at the exit of the first combustion chamber at equivalent points. When the excess air ratio increased, the high-temperature zone narrowed. This occurred because the corresponding velocity of the primary air increased as the excess air ratio increased. Additionally, the momentum of the primary air-coal mixture increased while the coal diffusion rate decreased, making it more difficult for the flame to spread in the radial direction. The residence time of the pulverized coal in the burner shortened. Moreover, the primary air was entering the boundary of the flame. All this resulted in the narrowing of the high-temperature zone.

Distribution of O₂ and CO concentrations at the exits of combustion chambers

Figure 7 exhibits the distribution of calculated O₂ concentration along the radial direction at the exits of the first and second combustion chambers and measured O₂ concentration at the exits of the second combustion chambers for different excess air ratios. The oil mainly concentrated and combusted at the center of the burner. The high pressure blower could only provide some of the air that was required for the combustion of oil. The remaining air requirement was provided by the primary air that carried the pulverized coal. The oil flame ignited the pulverized coal in the center of the burner first. The pulverized coal combustion time in the center of the burner was the longest. Therefore, O₂ consumption in the centre of the burner was high and as a result these two effects resulted in a low O₂ concentration in the centre of the burner. The pulverized coal was only in contact with the center of the oil and coal combustion flame of the burner for a short time and, therefore, the coal combustion time was short while the O₂ concentration outside the center of the burner was high. Near the burner wall, the O₂ concentration was approximately 21%. Therefore, the pulverized coal did not burn near the wall. The O₂ concentration distribution was related to the gas temperature distribution at the exits of the first and second combustion chambers. More O₂ was consumed at the equivalent points at higher gas temperatures. With an increase in the excess air ratio, the primary air quantity increased which means more O₂ was supplied to the burner. At excess air ratios of 0.56, 0.75, 0.98 and 1.14, the lowest O₂ concentrations at the exit of the first combustion chamber were 4.8%, 8.9%, 10.2% and 11.3% and decreased to 0.5%, 1.1%, 0.9% and 3.0%, respectively, at the exit of the second combustion chamber. The O₂ concentrations at the exit of the second combustion chamber were 0.01%, 0.01%, 0.21% and 3.70% in the experiments. There exist little differences between the simulation results and the experimental results. The O₂ concentration was quite low at the center of the exit of the second combustion chamber at different excess air ratios. In a traditional burner, the primary air only provides O₂ for volatile combustion. By comparison with a traditional burner, the primary air in a tiny-oil ignition burner provides O₂ both for the volatile combustion and for the partial char combustion. Therefore, the pulverized coal combusted under oxygen-poor conditions. Because O₂ was limited and pulverized coal combustion was intense in the centre of the burner, the O₂ was consumed completely here. The numerical simulation results agree well with the experimental results, suggesting that the combustion in the centre of the burner was very good.

Figure 8 shows the distribution of the CO concentration along the radial direction at the exit of the second combustion chamber under different excess air ratios. The CO concentration was high because of the lack of O₂ in the center of the burner. The CO concentration was zero near the burner wall. In accord with the CO and O₂ concentration distributions, O₂ spread to the burner center with difficulty because of the large momentum of the primary air-coal mixture and the limited length of the burner. Similarly, it was difficult for CO to spread to the burner wall. With a lack of O₂ in the center of the burner the pulverized coal burned incompletely, which produced much CO. As the excess air ratio increased, the quantity of O₂ in the center of the burner also increased while the pulverized coal burned more completely and the CO concentration peak decreased. The CO concentration peaks were 12.6%, 11.3%, 10.8% and 9.2% at excess air ratios of 0.56, 0.75, 0.98 and 1.14 at the exit of the second combustion chamber. The maximum CO concentration is 0.01 in the Testo 350M instrument. The CO concentration measured in the experiments was out of the range of the instrument. When the excess air ratio was increased, CO formation decreased at equivalent points. There were two reasons for this. On the one hand, the increased amount of air resulted in an increase in the amount of O₂ in the centre of the burner and pulverized coal burned more completely. Additionally, the CO concentration decreased in the center line. One the other, an increase in the excess air ratio leads to the momentum for the primary air-coal mixture. Hence, the residence time of the pulverized coal in the burner is shorter. The amount of pulverized coal that comes into contact with the high temperature oil flame decreases and, therefore, the range for CO formation is smaller. A lower excess air ratio is beneficial for the ignition of pulverized coal in a tiny-oil burner.

Conclusions

A tiny-oil ignition burner was numerically simulated using excess air ratios of 0.56, 0.75, 0.98 and 1.14 at a coal feed rate of 4 t/h. The numerical simulations agree well with the experimental results, demonstrating the suitability of the model used in the calculation. It was found that for the four excess air ratios investigated, the gas temperature was higher in the center of the burner. The high-temperature region was enlarged along the primary air flow direction. These developments ensured the successful ignition of the pulverized coal. The gas temperature was high in the central zone of and at the exit of the burner. The pulverized coal combusted well in the center of the burner, and the O₂ concentrations were low at the exit of the second combustion chamber. The CO concentrations were high at the exit of the second combustion chamber and pulverized coal combusted under oxygen-poor conditions in the centre of the burner. However, the pulverized coal near the burner wall did not ignite as it did not come into contact with the high temperature flame. As the excess air ratio increased from 0.56 to 1.14, the high-temperature region in the radial direction decreased. Therefore, a lower excess air ratio is beneficial for the ignition of pulverized coal in a tiny-oil burner.

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