Subscriber access provided by - Access paid by the | UCSF Library
Article
Numerical and experimental studies of NO formation mechanisms under methane MILD combustion without preheated air Shiying Cao, Chun Zou, Qingsong Han, Yang Liu, Di Wu, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501943v • Publication Date (Web): 04 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Numerical and experimental studies of NO formation mechanisms under methane MILD combustion without heated air Shiying Cao, Chun Zou*, Qingsong Han, Yang Liu, Di Wu, and Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China
KEYWORDS: NO formation mechanism; MILD combustion; role of NO2; non-preheated air ABSTRACT: MILD (moderate or intense low oxygen dilution) combustion a newly developing and innovative combustion technology. A MILD combustion without preheated air was achieved with inner flow recirculation and simulated by a CFD (computational fluid dynamics) model with GRI-3.0. Comparisons between the experimental and predicted results, including temperature and O2, CO, and NO concentration fields, demonstrate that the model closely predicted MILD combustion without preheated air and is suitable for studying the NO formation mechanism under these conditions. According to the distribution of NO and NO2, there are three zones in the furnace: a central zone (Ⅰ), a transition zone (Ⅱ) and a recirculation zone (Ⅲ). The NO concentration is lowest in the central zone, highest in the recirculation zone and intermediate in the transition zone. The NO2 concentration is high in regions where the NO concentration is low and is low in regions where the NO concentration is high. The NO formation mechanism is that NH3, N and HCN are mainly produced in the recirculation zone, and the majority of NH3, N and HCN are recycled into the central and 1
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 30
transition zone. In the central zone, the paths (NH3→NH2→HNO→NO), (N→NO) and (NNH→NH→HNO→NO) are main paths for NO formation, and the majority of NO is oxidized to NO2 by R186. The majority of NO2 in the central zone is transported to the transition zone via diffusion and convection. In the transition zone, the majority of NO2 is reduced to NO, and the main resource of NO formation. Unreacted NO2 in the transition zone is transported to the recirculation zone, and reduced to NO. Moreover, NNH is the main Nitrogen-containing
radical
produced
from
N2
by
the
reverse
direction
R209
(NNH+HH2+N2) in the recirculation zone. NNH formed in the recirculation zone is recycled into the central and transition zone, and reduced to N2 by R204 (NNHN2+H).
1. Introduction The reduction of pollutant emissions from combustion processes is subjected to extensive research due to consistently rigorous regulations. One of the primary pollutants from the combustion process is NOx, and considerable progress has been made to reduce NOx emissions from combustion systems by utilizing flame cooling, air staging, exhausted gas recirculation, lean premixed combustion, re-burning and low-NOx burners. In general, these methods aim to reduce the residence time or avoid high O2 concentrations in high temperature regions to suppress thermal NO formation. MILD (moderate or intense low oxygen dilution) combustion is an attractive method due to its high efficiency and low pollutant emissions1-4. MILD combustion is a newly developing and innovative combustion technology5.6. In the MILD combustion regime, the temperature of the reactant mixture should be above that of auto-ignition, and the oxygen concentration in the reactant mixture should be diluted with combustion products to very low levels, typically no more than 3 to 5%7. These conditions 2
ACS Paragon Plus Environment
Page 3 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
result in pyrolysis depression and flame thickening, which produce an invisible flame and a distributed reaction in the MILD combustion regime8.9, as well as a uniform temperature distribution10.11. As a consequence, the pollutant emissions, particularly NOx12.13, are much lower than those from conventional flames. The NOx formation mechanism under MILD combustion has been investigated experimentally and numerically by many researchers. Wünning14 illustrated that MILD combustion lowers NOx emissions via the suppression of thermal NOx formation with experimental and numerical techniques. Dally15 used a jet hot co-flow (JHC) burner to simulate the heat and exhaust gas recirculation and found that a different NO formation mechanism may be active under MILD combustion; however, the detailed mechanism of NO formation was not provided. G.G. Szegö16 investigated the effects of heat extraction, air preheating, excess air, fuel type and dilution on NOx emission experimentally under MILD combustion with a parallel jet burner system. This work indicated that the prompt-NO and/or N2O-intermediate pathways are of comparable significance to the thermal-NO pathway. Mancini17 assessed the potential of several different NOx models to predict NO emissions in MILD combustion. These models suggested that the prompt NO and re-burning mechanisms are negligible and that NOx is primarily formed by the thermal path. Hamdi18 studied the relationships between the preheat temperature and NO formation and claimed that the thermal NO mechanism is dominant at higher preheat temperatures, while the prompt route is dominant at lower preheat temperatures. A.V. Sepman19 investigated NO formation with different degrees of preheating using experiments and simulations. Rate-of-production analysis of the results showed that NO formation is predominantly caused by the Fenimore mechanism. Many researchers have studied NO formation mechanisms by numerical simulation. Abbas Khoshhal20 numerically investigated the influence of fuel temperature on NOx 3
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
emission in MILD combustion. This work demonstrated that the modified N2O-intermediate NOx mechanism is suitable for correctly predicting NOx levels in MILD combustion. Yang21 combined the N2O route with thermal, prompt and NO re-burning models to predict NO emission. This researcher reported that NO formation by N2O intermediate mechanisms is of outstanding importance in MILD combustion. D. Tabacco22 used Chemkin code to investigate the NOx formation mechanism. He found that the N2O-intermediate mechanism is important in MILD combustion, and the thermal NO route makes significant contributions to the emission of NO, while prompt NO was negligible. Mardani23 studied the NO formation mechanism for H2-CH4 combustion under MILD combustion using CFD and WSR analysis with the JHC burner. The effects of O2 concentration, fuel hydrogen content, and Reynolds number on NO formation were studied. The results demonstrated that the NNH and N2O routes are the most important pathways in NO formation under MILD combustion. Galletti24 investigated NO formation mechanisms numerically and experimentally and reported that the NNH and N2O routes are the dominant formation pathways under the MILD combustion regime. Xuan Gao25 explored the influence of air temperature and hydrogen concentration on NO formation mechanisms and claimed that the NNH and prompt routes play a significant role in NO formation under MILD combustion. Thus, the NO formation mechanism may vary in MILD combustion according to the different methods that are used to achieve MILD combustion. Recently, researchers achieved MILD combustion by inner flow recirculation without preheated air26-28. However, few studies have focused on the NO formation mechanism under MILD combustion without preheated air, which is the primary objective of this work. Firstly, MILD combustion was achieved by an inner flow recirculation without preheated air, and the temperature field and main species concentrations were measured in the furnace. Secondly, the numerical results were compared with the experimental results to validate the correctness 4
ACS Paragon Plus Environment
Page 4 of 30
Page 5 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
of the combustion model and the chemical mechanism, GRI 3.0, used in the study. Finally, the NO formation mechanisms were studied by numerical simulation.
2． ．Experimental Setup and Operating Conditions A laboratory scale MILD furnace was used in this work, as shown in Figure 1. This furnace was designed and modified according to the MILD combustion furnace (MCF) that has been described in detail elsewhere16,26 from the University of Adelaide. The main difference between the original and modified furnace is the removal of the U-Shaped cooling tubes in the present MILD burner to make inside flow fluent. Another difference is the size of the modified combustion chamber, with dimensions of 250 mm×250 mm×550 mm. It is insulated with 75 mm-thick extreme temperature refractory and three layers of 50 mm-thick high-temperature ceramic fiberboards. The furnace has a maximum thermal power of 20 kW. The burner is composed of a single fuel tube through a bluff-body, an air annular channel around the bluff-body in the center of the furnace, and four exhaust ports symmetrically arranged in a ring with 43 mm between the center of the fuel tube and the centers of the exhaust ports. The diameters of the fuel tube, bluff-body, and air channel are 6 mm, 22 mm, and 26 mm, respectively. The time-averaged furnace temperature was measured with a bare, fine-wire type R (Pt-Pt-13% Rh) thermocouple. A stainless steel water-cooled probe was used to sample the gas to measure local mean O2, CO, CO2, and NOx concentrations. KANE 9106 was used to measure concentrations of major species including flue-gas date. The accuracies of analyzer measurements were estimated to be: O2 (±0.1%: 0-25%), CO2 (±0.3%: 0-99.9%), CO (±20 ppm: 0-400 ppm, ±5%: 401-2000 ppm, ±10%: 2000-10000 ppm, ±0.1%: 0-10%), NO (±5 ppm: 0-100 ppm, ±5%: 100-5000 ppm), SO2 (±5 ppm: 0-100 ppm, ±5%: 100-5000 ppm). CH4 was used as fuel in this study. It took approximately 6 hours to warm up from the 5
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
cold-state to stable MILD combustion. The thermal power is 9.5 kW, the excess air ratio is 1.25, and ambient air temperature is 306 K. For this study, the inlet air and fuel velocities are U air =
22.38 m/s and U fuel =9.32 m/s. There are 3（row）×6 （column） ports equally spaced
in the back of furnace, as shown in Fig. 1. The distances from each column to the bottom are 135, 225, 315, 405, 495 and 585 mm, respectively. The thermocouple and species profiles in the combustion chamber were measured by traversing the probe through each port in the X-direction 25 mm from the furnace center to the wall.
3. Numerical Methods Steady Favre-averaged Navier-Stokes equation are solved by finite volume code FLUENT in ANSYS 15.0 package. The comparison between the results of 3-D grid and 2-D grid found that the differences are less than 10%, and can be acceptable. Consequently, in order to save computational time consumption, 2-D grid was used for the simulation according to the suggestion of Parente29. The computational domain covers the combustion chamber so it is 550 mm long with a 125 mm width. A grid independence study was conducted and a refined structured of 29800 cells was selected. The turbulence flow is simulated using modified k-ε equations. According to Christo and Dally30, the Ce1 coefficient is changed from 1.44 to 1.6 because both fuel jet and air jet are round jet. The interaction between turbulence and chemistry was modeled by the eddy dissipation concept (EDC) model31, as recommended by other researchers29-33. GRI 3.034, which includes 53 species and 325 chemistry mechanisms, was applied to the simulations in this work because it has been successfully utilized in modeling the MILD combustion regime in previous studies19,35. Because the time scale of NO formation is probably close to that of fuel oxidation in MILD combustion, NO formation was simulated by GRI 3.0 and EDC 6
ACS Paragon Plus Environment
Page 6 of 30
Page 7 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
model instead of the post-processing method in order to ensure the precision of simulation. To reduce the computational cost of integration, the in-situ adaptive tabulation (ISAT) method of Pope36 with error tolerance (etol) for a value of etol≤1×10-7 was applied, because the results are found to be independent of the error tolerance. The discrete ordinates (DO) model, in which absorption coefficients are computed using the weighted sum of the gray gases model (WSGGM)37, was adopted to account for radiation. Second-order upwind scheme was applied to solve all transport equations and the SIMPLE algorithm was employed to handle pressure-velocity coupling. The residuals were kept lower than 10-6 for energy and DO intensity and 10-5 for all other variables as a convergence criterion.
4. Results and Discussions 4.1 Achievement of MILD combustion and validation of model In this study, we judged MILD combustion according to the three conditions below: In this study, we judged MILD combustion according to the three conditions below: 1) No visible flame in the furnace. 2) Nearly uniform temperature field. According to Kumar’s definition8, the normalized spatial temperature variation, β , is defined as β = T ' Tmean , in which T mean= ∫ TdV / ∫ dV and T ′2 = ∫ (T ′ − Tmean ) 2 dV / ∫ dV . The variation’s value should be less than 15% for a nearly uniform
temperature field. 3) Low pollutant emissions. The emissions of NOx and CO should be below 100 ppm in the flue gas. Six hours after ignition, as shown in Fig. 2(a), there were no visible flames in the furnace, as shown in Fig. 2(b). Based values of 108 measurement points in the furnace, the maximum and minimum temperatures were 1634 K and 635 K, respectively. The normalized 7
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
spatial temperature variation was 8.4%. Therefore, the temperature field was uniform in this case. The concentrations of NOx and CO in the flue gas were 8 ppm and 17 ppm, respectively. Thus, it was clear that MILD combustion was achieved. Temperature and O2 concentration are, respectively, the main parameter and component in MILD combustion. Thus, prediction of these parameters is the foundation for simulation of MILD combustion. CO and NO concentrations are minor components in MILD combustion, and the prediction of these parameters provides a representation of the level of MILD combustion simulation. Therefore, the predicted temperature, O2, CO and NO concentration fields were compared with those that were measured in the furnace. The center of the fuel jet at the exit of the burner is defined as the origin. The vertical axis along the fuel jet corresponds to the z-axis, and the x-axis is the horizontal axis crossing the center of the burner, as shown in Fig. 1. The results on the central plane (plane XOZ) are shown in Fig. 3 a-d. It can be seen in Fig. 3 that the models used in the study predict MILD combustion accurately. Therefore, the present model is suitable for the study of the NO formation mechanism in MILD combustion without preheated air.
4.2 NOx formation mechanism discussion 4.2.1 NOx distribution in the furnace Fig. 4 (a)-(c) shows the contour map of predicted NO, NO2, and N2O concentration fields in the x-z plane, respectively. The colored arrows represent the air and fuel injection locations and the outgoing exhausted streams (E, A and F represent exhaust, air, and fuel, respectively). The three zones in the furnace are the central zone (Ⅰ), the transition zone (Ⅱ) and the recirculation zone (Ⅲ), as shown in the Fig. 4(a) and (b). It can be observed that the NO concentration is the lowest in the central zone，the highest in the recirculation zone and intermediate in the transition zone. The NO2 distribution is inversely related to the NO 8
ACS Paragon Plus Environment
Page 8 of 30
Page 9 of 30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
distribution, as shown in Fig. 4(b); the NO2 concentration is high in the region where the NO concentration is low, and it is low in the region where the NO concentration is high. This result implies that NO2 is closely associated with NO in MILD combustion. As seen from Fig. 4(c), the N2O distribution is quite different from the NO and NO2 distributions. High N2O concentrations occur in the narrow area between the fuel jet and the back-flow above the middle of furnace. The N2O concentration is one order of magnitude smaller than that of NO. According to the simulation results, the average concentrations of NO, NO2, and N2O in the furnace are 6.05, 0.17 and 0.14 ppm, respectively. The ratios of NO, NO2, and N2O to the total NOx are 95.13%, 2.67% and 2.2%, respectively. The NO, NO2 and N2O emissions in the flue gases are 7.05, 0.02, and 0.05 ppm, respectively. The percentages of NO, NO2 and N2O in the NOx emissions in the flue gas are 99%, 0.3% and 0.7%, respectively. These results demonstrate that the emission of NO is still dominant in the total NOx emission in MILD combustion, whereas NO2 and N2O are the intermediate products in the formation of NO in MILD combustion. To investigate and validate the NO formation mechanism, the representative lines of X=0 mm, Y=0 mm or 10 mm, and Z=100 mm were selected.
4.2.2 NO formation mechanism analysis at the center line (X=0 mm, Y=0 mm) The profile of NO, NO2, and N2O concentrations at the center line (X=0 mm, Y=0 mm) is displayed in Fig. 5(a). The NO2 concentration increases slowly, reaches its maximum at Z=400 mm, and drops off quickly beyond that until it reaches 0 ppm at Z>500 mm. The NO concentration was very low until Z=360 mm and increased drastically beyond that to its maximum at Z=470 mm. The N2O concentration increases slowly in the region of Z>480 mm. Corresponding to Fig. 4(a), the central zone is located at 0
