Article pubs.acs.org/EF
Low-NOx Modification of a Heavy Fuel Oil Swirl Burner Based on Semi-Industrial Scale Experimental Tests Hao Zhou,* Tao Ren, Yan Huang, Shantao Hu, and Kefa Cen Zhejiang University, Institute for Thermal Power Engineering, State Key Laboratory of Clean Energy Utilization, Hangzhou, 310027, P. R. China ABSTRACT: A proposed swirl burner was modified from a burner used in a 300 MWe oil-fueled power plant. In particular, burner construction, including expanded nozzles, swirl numbers, and air ratios were changed. Then, experimental tests were carried out in a semi-industrial test rig with a horizontal cylindrical furnace with a single-swirl burner. The thermal input of the furnace was 9 MW. The species concentrations of O2, CO, and NOx in the flue gas were measured. NOx emission with the original burner was 320 ppm (3 vol % O2). However, NOx emission decreased to 195 ppm (3 vol % O2) with the proposed burner during optimal operation. Through a series of adjusting experiments, a 35−39% abatement of the NOx emission from the single-burner test could be maintained when the original burner was replaced by the proposed one. NOx emission measured from a full-scale industrial furnace is approximately 341 ppm (3 vol % O2) under ordinary operating conditions. The predicted NOx concentration from the industrial furnace could achieve 207.8 ppm (3 vol % O2) employing the proposed swirl burner under optimal operating conditions. The flow fields (including the recirculation zone and turbulent intensity) downstream of the two burners were compared on the basis of the numerical results. A CCD image system was applied to capture the flame structure downstream of the burner. A mushroom-shaped flame structure was observed with the proposed burner, whereas a fanshaped diffusion flame was observed with the original burner. A strengthened division in the flame was formed at the burner outlet with the proposed burner. Temperatures ranged from 1184 to 2114 °C downstream of the burner, which was determined through the two-color pyrometry method for each burner. The minimum temperature of the reference section was reduced by 154 °C, and the average temperature was reduced by 76.9 °C when the proposed burner was applied. The proportion of the temperature region higher than 1500 °C in the reference section was calculated. The initial proportion was 94.96%, which was reduced to 80.91% when the proposed burner was applied. This represents a decrease of 14.05% through the modification.
1. INTRODUCTION There has been an increase in the construction of heavy oilfueled power plants in oil-producing countries to meet increasing energy demand. However, control of NOx emission from oil-fueled industrial furnaces is still a critical issue. Considerable attention has been paid to the precombustion process of oil fuel. The injection, dispersion, and combustion of oil fuels have been extensively investigated.1−6 Numerical and experimental investigations on heavy fuel oil (HFO) combustion have been executed in a vertical cylindrical furnace.7 Because of the high temperatures caused by the high heat content of HFO, NOx emission, especially thermal NOx, is always high. Therefore, reduction of NOx emission from HFO flames under high-temperature conditions needs to be studied. Swirl burners with internal air staging are useful for controlling NOx emission both from solid and liquid fuels.8 Low NOx emissions from coal flames have been achieved in a number of industrial applications with swirl burners.9−14 Most of the NOx emission is formed adjacent to the burner in the high-temperature region. The aerodynamics of the near-burner region influence the flame structure and temperature distribution, which are critical for low NOx combustion in pulverized coal-fired furnaces.15 Adjustments of swirl-burner structure and air distribution have been conducted, and a significant reduction of NOx emission has been obtained in industrial furnaces.16−21 A substantial number of investigations of coal flames has also been carried out. The NOx emissions © 2013 American Chemical Society
have been found to decrease with decreasing thermal input when the thermal capacities were lower than 2−3 MW.22 Low NOx emission from gas and HFO flame has also been reached with thermal capacities of approximately 1 MW at Massachusettes Institute of Technology.8 Optimal NOx emissions of 91 ppm (3 vol % O2) without external air staging and 53 ppm (3 vol % O2) with external staging have been realized.8 In gas and liquid diffusion flames, because of the short flame length, the weak cooling ability of furnaces with high thermal capacity leads to high temperatures in the center of the combustion zone, which is apt to increase the emission of thermal NOx.23 During the experiments of coal flames where the thermal capacities are greater than 4 MW, the prediction of NOx emission can be representative of the industrial-scale furnaces.22 Few experiments have been conducted in large-scale furnaces to investigate the performance of the oil-fueled burners with high thermal capacity. Considerable NOx emission could be formed when oxygencontaining combustion products are at the temperatures higher than 1500 °C. The aim of the present research is to reduce the high-temperature region higher than 1500 °C adjacent to the burner, which has a significant effect on thermal NOx formation. The two-color pyrometry method pioneered by Received: February 10, 2013 Revised: July 23, 2013 Published: July 23, 2013 5029
dx.doi.org/10.1021/ef400238u | Energy Fuels 2013, 27, 5029−5035
Energy & Fuels
Article
Hottel and Broughton24 has been widely extended to determine the temperature distributions in the flame of the industrial applications.25−27 A proposed low-NOx burner with a newly designed air ratio and jet angle between the air tubes was modified from a burner used in a 300 MWe oil-fueled power plant. Comparative tests were conducted in a 9 MW semiindustrial test rig. Comparisons of the temperature distributions, flue gas emissions, and flame structures between the original burner and the proposed burner were made.
2. NUMERICAL MODEL AND PREDICTION OF NOX EMISSION 2.1. Numerical Model. The axial velocity distribution and turbulent intensity adjacent to the burner was simulated by a renormalization-group (RNG) k-ε model through the commercial software FLUENT. The combustion of HFO in swirl burners is a typical example of nonpremixed combustion. As the specifications of the Saudi Arabia #380 HFO was obtained in detail, the combustion model chosen in this study was based on the conserved scalar (mixture fraction) and prescribed probability density function (PDF) approach. This approach is elegant because atomic elements are conserved in the chemical reactions. The thermochemistry can be reduced to a single parameter: the mixture fraction. Species and temperature are modeled as being in chemical equilibrium instead of having to incorporate detailed reaction mechanisms. The details of the nonpremixed combustion model are found in Chapter 15 of Fluent 6.2 User’s Guide.28 2.2. Prediction of NOx Emission in a Full-Scale Industrial Furnace. When the thermal capacity of the test rig is greater than 4 MW, scaling can be successfully carried out through the constant velocity principle.22 Thus, the constant velocity scaling criteria was employed in our research. The NOx emission from a full-scale industrial furnace with the original burners was obtained before the single-burner experiment (NOxo,actual). The NOx emission from the test rig with the original burner (NOxo,test) was obtained during the singleburner tests. The proportionality constant K was calculated for the NOx emission from the single-burner test and the full-scale industrial furnace. The NOx emission from the single-burner test (NOxp,test) was obtained through the single-burner experiment. Thus, the predicted value of NOx emission from the full-scale industrial furnace with the proposed burners (NOxp,prediction) was calculated from the following formulas: K=
Figure 1. Arrangement of the burners in the industrial furnace.
Figure 2. Structure comparison of the two swirl burners. burners with an oil-fit design. The proportion of the primary air was adjusted by changing the fluxes in the primary air tube. The swirl of the two burners was generated by adjustable blades installed in the secondary and tertiary air tubes. The swirl intensity of the two burners was varied with the blade’s axial position in the secondary air tube and with the angle of the blade in the tertiary air tube. A detailed comparison of the parameters is shown in Table 1. In order to inhibit the early mixing of the supplementary air and the HFO, a nozzle with a 20° expanding angle was added to the primary air tube. In addition, the expanding angles of the secondary and tertiary nozzles were both increased from 15° to 20°. The flow ratio of the secondary air was decreased, whereas that of tertiary air was increased in order to inhibit the mixing of the fuel and the combustion air to enhance the lean-oxygen area. All these measures were taken to postpone the ignition of fuel in order to keep flame temperature low, thus minimizing the thermal NOx formation. 3.2. Test Rig. Single-burner tests were conducted in a semiindustrial test rig with a horizontal cylindrical furnace at Zhejiang University. From swirl-stabilized flames of pulverized coal, in which the thermal input was greater than 4 MW, the NOx emission was confirmed to be representative of full industrial-scale applications.22 Although it was difficult to maintain the high quality of the HFO atomization with the small output of an oil gun, the output of the HFO gun from the model burner was chosen as 800 kg/h, which is one-quarter of the output in the industrial furnace. The model burner geometry was scaled to half the geometry of the burner in the industrial furnace. In order to maintain the same volume thermal
NOxo,actual NOxo,test
NOxp,prediction = K ·NOxp,test
(1) (2)
3. EXPERIMENTAL SECTION 3.1. Industrial Furnace and Burners. The arrangement of the burners in the industrial furnace, a wall-fired furnace with 24 swirl burners, is illustrated in Figure 1. The electrical output of the power plant was 300 MWe, with a thermal-to-electric conversion efficiency of about 35%. The thermal input of the furnace was approximately 857 MW. NOx concentration in the flue gas at the furnace exit is usually 350−400 ppm (3 vol % O2) under normal operating conditions. Retrofitting of the furnace, especially the modification of the swirl burners, would have to be conducted in order to meet the requirements for NOx and CO emissions. The structures of the original burner and the proposed one are shown in Figure 2. These two burners were both dual-register swirl 5030
dx.doi.org/10.1021/ef400238u | Energy Fuels 2013, 27, 5029−5035
Energy & Fuels
Article
Table 1. Comparative Operation Parameters of the Two Burners excess air ratio primary air tunnel angle of primary air (deg) primary air ratio (%) primary air velocity (m/s) angle of secondary air (deg) secondary air ratio (%) secondary air velocity (m/s) angle of tertiary air (deg) tertiary air ratio (%) tertiary air velocity (m/s) swirl number of secondary air swirl number of tertiary air
original
proposed
1.08 half direct flow half swirl flow 0 13 23 15 72 46 15 15 40 0.85 1.65
1.08 all swirl flow
Table 2. Characteristics of Saudi Arabia HFO 380
20 15 26 20 45 59 20 40 50 1.18 2.04
75.0 −3 353.1
18305 0.10
flash point (°C) pour point (°C) viscosity at 50 °C (cSt) sulfur (wt %) vanadium (mg/kg)
0.10 0.01 0.9708 14.2
nickel (mg/kg) sodium (mg/kg) aluminum (mg/kg) nitrogen (wt %)
14 13
