Energy Fuels 2010, 24, 131–135 Published on Web 08/27/2009
: DOI:10.1021/ef900524b
Computational Fluid Dynamics Modeling of NOx Reduction Mechanism in Oxy-Fuel Combustion† Huali Cao,‡ Shaozeng Sun,*,‡ Yinghui Liu,§ and Terry F Wall§ ‡ Combustion Engineering Research Institute, Harbin Institute of Technology, Harbin, 150001, China, and §Department of Chemical Engineering, University of Newcastle, NSW 2308, Australia. †Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies.
Received May 25, 2009. Revised Manuscript Received July 27, 2009
Oxy-fuel combustion is a promising carbon capture technology in which both the conversion of fuel-N to NO and the reduction of recycled NO contribute to lowering of final NO exhausted from the coal combustion system. Combustion characteristics for both air and oxy-fuel conditions were numerically investigated in a pilot scale test facility for an Australian sub-bituminous coal. On the basis of De Soete’s mechanism, the additional reactions for formation of fuel-NO and reduction of recycled NO were added into computational fluid dynamics (CFD) codes, using user-defined functions (UDFs). The NOx predictions in air and oxy-fuel combustion were compared to experimental data. The NO emission in oxy-fuel condition is predicted to be significantly lower than that in air combustion, even without recycled NO. The effect of the nitrogen partitioning ratio between volatile and char on the NOx emission was also investigated.
performed in the laboratory2-10 and semi-industrial scale test facilities.11-20 Some laboratory tests typically use O2 and CO2 to simulate the oxy-fuel combustion environment in oncethrough systems, in which NO was added to CO2 to simulate recycle of NOx in the furnace.2,5-9 Okazaki and Ando2 investigated reduction of NO under recycling conditions. They concluded that the influence of CO2 on NO emission is negligible despite its high concentration. Furthermore, an effect of the interaction of fuel-N and recycled NO was detected. Moreover, reduction of NO to molecular N2 due to chemical reaction in the combustion zone is taken as the main reason for the overall decrease (50-80%) in NO during recycling. Hu et al.7 evaluated three coals for a wide range of stoichiometries with the recycling ratio ranging from 0 to 0.4
1. Introduction The role of carbon dioxide as a dominant contributor to global warming has resulted in the development of carbon capture and storage technologies. There are three main technological processes for CO2 capture which are currently investigated, namely postcombustion capture, precombustion capture, and oxy-fuel combustion. Oxy-fuel combustion and CO2 capture is a near-zero emission technology that can be adapted to both new and existing pulverized coal-fired power stations. A schematic of oxy-fuel combustion is shown in Figure 1. Oxygen is separated from air and then mixed with a recycle stream of flue gases, leading to an increase of CO2 in the flue gas from approximately 17 to 70% by mass. The CO2 can then be captured by cooling and compression for subsequent transportation and storage. Recently, oxy-fuel combustion has attracted considerable interest as an option for a significant and simultaneous reduction of NO and CO2 emissions from existing fossil fuel power plant. NOx emissions generated per unit energy are reduced as the recycled NO is reduced or reburned as it is recirculated through the flame.1 The replacement of air with O2/CO2 leads to modified distributions of temperature and species, as well as radiation flux, resulting from the property differences between N2 and CO2. This also results in changes in NOx formation and reduction in the flame. Experimental studies on NOx emission characteristics in coal-fired O2/CO2 combustion have been
(7) Hu, Y. Q.; Kobayashi, N.; Hasatani, M. Energy Convers. Manage. 2003, 44, 2331–2340. (8) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 833–840. (9) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 2109–2115. (10) Andersson, K.; Normann, F.; Johnsson, F.; Leckner, B. Ind. Eng. Chem. Res. 2008, 47, 1835–1845. (11) Kimura, N.; Omata, K.; Kiga, T.; Takano, S.; Shikisima, S. Energy Convers. Manage. 1995, 36, 805–808. (12) Takano, S.; Kiga, T.; Endo, Y.; Miyamae, S.; Suzuki, K. IHI Eng. Rev. 1995, 28, 160–164. (13) Kiga, T.; Takano, S.; Kimura, N.; Omata, K.; Okawa, M.; Mori, T.; Kato, M. Energy Convers. Manage. 1997, 38, S129–134. (14) Nozaki, T.; Takano, S.; Kiga, T.; Omata, K.; Kimura, N. Energy 1997, 22, 199–205. (15) Croiset, E.; Thambimuthu, K. V. Greenhouse Gas Control Technol. 1999, 581–586. (16) Chatel-Pelage, F.; Marin, O.; Perrin, N.; Carty, R. A pilot-scale demonstration of oxy-combustion with flue gas recirculation in a pulverized coal-fired boiler. The 28th International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, FL, March 10-13, 2003; pp 1407-1417. (17) Chui, E. H.; Douglas, M. A.; Tan, Y. Fuel 2003, 82, 1201–1210. (18) Chui, E. H.; Majeski, A. J.; Douglas, M. A.; Tan, Y.; Thambimuthu, K. V. Energy 2004, 29, 1285–1296. (19) Tan, Y.; Croiset, E.; Douglas, M. A.; Thambimuthu, K. V. Fuel 2006, 85, 507–512. (20) Stadler, H.; Ristic, D.; Forster, M.; Schuster, A.; Kneer, R.; Scheffknecht, G. Proc. Combust. Inst. 2009, 32 (2), 3131–3138.
*Corresponding author. Tel.: þ86 451 86412238. Fax: þ86 451 86412528. E-mail address:
[email protected]. (1) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283–307. (2) Okazaki, K.; Ando, T. Energy 1997, 22, 207–215. (3) Hu, Y.; Naito, S.; Kobayashi, N.; Hasatani, M. Fuel 2000, 79, 1925–1932. (4) Croiset, E.; Thambimuthu, K. V. Fuel 2001, 80, 2117–2121. (5) Hu, Y. Q.; Kobayashi, N.; Hasatani, M. Fuel 2001, 80, 1851–1855. (6) Liu, H.; Okazaki, K. Fuel 2003, 82, 1427–1436. r 2009 American Chemical Society
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: DOI:10.1021/ef900524b
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Figure 1. Schematic of oxy-fuel combustion. Table 1. Coal Proximate and Ultimate Analysis high heating value (dry) moisture (as received) ash (dry) volatile matter (dry) fixed carbon (dry) C (dry) H (dry) N (dry) S (dry) O (dry)
unit
coal A
MJ/kg % % % % % % % % %
23.73 11.5 19.3 25.7 55.0 63.5 2.83 0.73 0.24 13.4
Figure 2. Fuel-NO reaction path in Fluent.
combustion, in which fuel NO was modeled by the eddydissipation concept. Andersson et al.10,26 investigated high temperature NO gas chemistry in lignite-fired oxy-fuel flames and concluded that the reverse Zeldovich mechanism contributes to NO reduction during oxy-fuel combustion at high temperatures. This paper presents a preliminary numerical study of NOx emission characteristics on a pilot scale facility (IHI). On the basis of the De Soete’s fuel-NO mechanism, a simplified reaction path was incorporated into Fluent using using userdefined functions (UDFs) to improve the capacity of CFD code for NO formation and reduction in oxy-fuel combustion.
and concluded that the reduction ratio (RR) increased with the fuel equivalence ratio and recycling ratio. Recently, Dhungel et al.21 examined NO emission behavior during oxycoal combustion in a 20 kW electrically heated furnace and concluded that the pathways of NO reduction in oxy-fuel were fundamentally similar to those in air combustion. Mackrory et al.22 investigated oxy-fuel combustion in a multifuel flow reactor (MFR) and found that oxy-fuel combustion can result in lower NOx emissions than air combustion independent of the reduction of recycled NOx, which was different from previous studies. The work performed on the semi-industrial scale test facility typically achieves a reduction of the total NOx emission between 50 and 80% under oxy-fuel combustion conditions compared to air-firing. Nozaki et al.14 discussed the differences in NOx emission for air and oxy-fuel conditions on the basis of measurements of HCN and NH3 in the flame and concluded that the recycled NOx is rapidly reduced to HCN or NH3 in the flame. As a powerful tool, computational fluid dynamics (CFD) has been used to provide detailed information in oxy-fuel process or burner design.23 Toporov et al.24 performed numerical simulations in a small scale swirl burner with modified devolatilization and char oxidation, as well as gas phase combustion submodels. Gharebaghi et al.25 carried out numerical modeling of both air and oxy-pulverized fuel (PF) combustion with recycled flue gas (RFG) on a pilot scale test facility, indicating that improved and modified physical submodels are required. Chui et al.17 incorporated both experiments and modeling of the nitrogen chemistry during oxy-fuel
2. Pilot Scale Experiments In this study, the model validation data was obtained from the pilot scale test facility under both air and oxy-fuel retrofit combustion conditions, located in Aioi, Japan.27 The furnace has a nominal thermal load of 1.2 MW, with an IHI-TR pulverized coal burner (a central oil gun, a nonswirling primary flow, and swirling secondary flow). In air combustion, the primary air is used to transport coal with a separate secondary air stream used to burn out coal. In an oxy-fuel retrofit condition, part of the recirculated flue gas known as primary RFG is used to transport coal, and the remaining flue gas known as secondary RFG was introduced directly into the O2 feed. The oxygen concentration in secondary RFG is up to 35% in order to maintain the oxygen concentration at the burner inlet around 26 wt %. An Australian sub-bituminous coal with elemental and proximate analysis as shown in Table 1 was investigated on this pilot scale facility in both air and oxy-fuel combustion conditions. Results from the pilot scale experiments show a 45% increase in NOx concentration (parts per million) in the flue gas compared to air-firing, due to the lower flue gas flow rate at oxy-fuel conditions. The amount of NOx released in terms of milligrams per megajoule is much smaller for oxy-fuel combustion, which is approximately one-third of the total NOx produced by air combustion.
(21) Dhungel, B.; Maier, J.; Scheffknecht, G. Emission behaviour during oxy-coal combustion. AIChE 2007 Annual meeting, Salt Lake City, UT, November 4, 2007. (22) Mackrory, A. J.; Lokare, S.; Baxter, L. L.; Tree, D. R. An investigation of Nitrogen evolution in oxy-fuel combustion. 32rd International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, FL, June 10-15, 2007. (23) Li, Z. Q.; Jing, J. P.; Ge, Z. H.; Liu, G. K.; Chen, Z. C.; Ren, F. Numer. Heat Transfer, Part A: Appl. 2009, 55 (6), 1–20. (24) Toporov, D.; Bocian, P.; Heil, P.; Kellermann, A.; Stadler, H.; Tschunko, S.; Forster, M.; Kneer, R. Combust. Flame 2008, 155, 605–618. (25) Gharebaghi, M.; Goh, B.; Ma, L.; Pourkashanian, M.; Williams, A. Numerical investigation of Oxy-coal combustion in pilot scale and the challenges. 25th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 8-20, 2008.
3. Numerical Models The numerical simulations were performed using the CFD code Fluent 6.2. A two-dimensional furnace geometry was created using Gambit, 2.2 version, consisting of 50 000 cells, which had been optimized, with the grid independent solution. The combustion modeling work of coal A has been (26) Normann, F.; Andersson, K.; Leckner, B.; Johnsson, F. Fuel 2008, 87, 3579–3585. (27) Wall, T. F.; Liu, Y.; Spero, C.; Elliott, L.; Khare, S. Chem. Eng. Res. Des. 2009, 87, 1003–1016.
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: DOI:10.1021/ef900524b
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Figure 3. Simplified reaction scheme for NOx prediction in the coal combustion process. Table 2. Reaction Rate Constant for Fuel NO Mechanism reaction expression 1
volatile-N f HCN
2
HCN f NH3 þ ...
3
NH3 þ O2 f NO þ ...
4
NH3 þ NO f N2 þ ...
5
CHi þ NO f HCN þ ...
6
char þ NO f NO
7
char þ NO f N2 þ ...
8
NO þ N f N2 þ O
rate constant S Y M Svol, HCN ¼ vol NM, wvol, N Vw, HCN d½HCN P ¼ -k2 YHCN YO2 RT , k2 ¼ 1:94 1015 expð -78400=RTÞ dt d½NH3 ¼ -k3 XNH3 XO2 a , k3 ¼ 4:0 106 expð -133900=RTÞ dt d½NH3 ¼ -k3 XNH3 XNO , k4 ¼ 1:8 108 expð -113000=RTÞ dt d½NO P ¼ -k5 YCHi YNO RT , k5 ¼ 2:7 106 expð -18800=RTÞ dt Sc YN, char Mw, NO Schar, NO ¼ Mw, N V d½NO ¼ k7 AE PNO , k7 ¼ 4:18 104 expð -34700=RTÞ dt d½NO ¼ 2k8 ½O½N2 , k8 ¼ 1:8 1011 expð -38370=TÞ dt
28
ref 31 32 29 29 33 34 35 36
Table 3. Volatile-N and Char-N Mass Fraction
undertaken by Khare for coal firing in air and oxy-fuel combustion conditions. The flow field was solved using the standard k-ε turbulence model with the SIMPLE algorithm for velocity-pressure coupling. The volatile release is based on the single kinetic rate model, and the char burnout model is defined by the kinetic-diffusion limited combustion model, in which parameters were derived from drop tube furnace (DTF) experiment results. Further details on combustion models can be found in ref 28. NOx formation and destruction is normally described as postprocessing of the CFD converged cases in which four reaction processes were included, namely, thermal NO, prompt NO, fuel NO, and NO reburning. The fuel-NO mechanism is dominant in the formation of NO during coal combustion. Figure 2 shows the different available options in Fluent, in which the gas reaction rates are derived from the De Soete mechanism,29 with HCN or NH3 as intermediate N species. However, the predictive capability of CFD in NOx emission characteristics is not accurate enough. It might due to (1) the mechanism of NO reduction in the flame is not totally described in CFD software or (2) the partitioning ratio between volatile N and char N which depends on the coal-N content, heating rate, and final temperature is still not well-understood. In this study, the following reaction paths (as shown in Figure 3) were used to describe the fuel-NO formation and recycled-NO reduction in oxy-fuel conditions. It is assumed that volatile N releases as HCN, then HCN decays subsequently and forms NH3, and NH3 undergoes further reaction to produce either NO or N2.29 Recycled NO was expressed as initial NO at the inlet. NO in the furnace react with hydrocarbon radicals then form HCN again. The char nitrogen is released to the gas phase as NO directly. The rate constants of reactions were given in Table 2. The coal bound N partition between volatile and char in air combustion is derived from FG-DVC (functional group-depoly-
coal A fuel N (daf %) volatile-N mass fraction (%) char-N mass fraction (%)
0.90 0.338 0.562
merization vaporisation cross-linking), which has been verified by Backreedy et al.30 The uncertainty of the coal-N partitioning ratio between volatile and char in oxy-fuel combustion is being further studied. In this paper, it is assumed that the partition ratio in oxy-fuel is similar to that for air combustion. The volatile-N and char-N mass fraction were given in Table 3. The volatile N is assumed to be released as HCN; the source of HCN from volatiles is related to the rate of volatile release.31 The subsequent decay of HCN yields NH3, due to the reactions such as: CN þ OH f NH þ CO and O þ HCN f NCO þ H f NH þ CO The conversion of HCN to NH3 is described in the overall reaction rate.32 NO in the flame is recycled back to HCN by reacting with hydrocarbon radicals through: CHi þ NO f HCN þ ::: and the rate parameters for the global reaction were decided following the work of Chen et al.33 (31) Chui, E. H.; Hughes, P. M. J. Combust. Sci. Technol. 1996, 119, 51–75. (32) Mitchell, J. W.; Tarbell, J. M. AIChE J. 1982, 28, 302–311. (33) Chen, W.; Smoot, L. D.; Hill, S. C.; Fletcher, T. H. Energy Fuels 1996, 10, 1046–1052. (34) Lockwood, F. C.; Romo-Millares, C. A. J. Inst. Energy 1992, 65, 144–152. (35) Levy, J. M.; Chan, L. K.; Sarofim, A. F.; Beer, J. M. NO-char reactions at pulverized coal flame conditions. The 18th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1981; pp 111-120. (36) Hanson, R. K.; Salimian, S. Combust. Chem. 1984, 361–421.
(28) Khare, S. P.; Wall, T. F.; Farida, A. Z.; Liu, Y.; Moghtaderi, B.; Gupta, R. P. Fuel 2008, 87, 1042–1049. (29) De Soete, G. G. Overall reaction rates of NO and N2 formation from fuel nitrogen. The 15th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1975; pp 1093-1102. (30) Backreedy, R. I.; Fletcher, L. M.; Ma, L.; Pourkashanian, M.; Williams, A. Combust. Sci. Technol. 2006, 178, 763–787.
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Table 4. Comparison of NO Concentration Measurements with Predictions IHI test results outlet NOx (average) coal type
combustion mode
ppm
mg/MJ
coal A
air mode oxy mode
808 1152
321 107
prediction ppm 638 without NO recycle with 1000 ppm NO recycle
mg/MJ
ηN (%)
260 76 112
34.5 28.3
809 1323
Figure 5. Nitrogen intermediates NH3 and HCN.
Figure 4. Comparison of flame temperature in air and oxy combustion.
The char-bound N is assumed to convert to NO and N2 in oxy-fuel combustion,22 in which the char-N contribution to NO destruction is less significant. Thus, the total NO, HCN, and NH3 source terms can be expressed as SNO ¼ S3 - S4 - S5 - S7 - S8
ð1Þ
SHCN ¼ S1 - S2 þ S5
ð2Þ
SNH3 ¼ S2 - S3 - S4
ð3Þ
which are included in the mass transport equations for NO, HCN, and NH3 species. Fluent solves the mass transport equation for the NO species: Figure 6. NO rate off-axis 0.04 m for air and oxy-fuel combustion.
D ðFYNO Þ þ r 3 ðFvBYNO Þ ¼ r 3 ðFDYNO Þ þ SNO Dt
still need to be improved to provide predictions that are more accurate in oxy-fuel conditions. 2. NOx Emission in Air and Oxy-Fuel Combustion. The modified NOx mechanism as mentioned above was added into Fluent to predict NO emission in oxy-fuel combustion. In oxyfuel combustion, the NO back to furnace was considered by adding initial NO (1000 ppm) in a secondary stream. On the basis of converged combustion cases, NO emission for coal A was predicted in air and oxy-fuel combustion, separately. The comparison between experimental data and the prediction at the exit of the furnace in air and oxy-fuel combustion is shown in Table 4. It shows that the NO emission (milligrams per megajoule) in oxy-fuel is significantly less than in the air condition. The nitrogen conversion ratio (ηN) was calculated assuming that fuel N causes overall NOx. The nitrogen conversion ratio is compared between the cases with recycled NO and without recycled NO. The concentration of HCN and NH3 is higher under oxyfuel conditions (as shown in Figure 5), which indicates that oxy-fuel combustion is more prone to produce reburning
in which the source term SNO is determined by the NO formation and destruction mechanism. The mass transport equations for the intermediate species HCN and NH3 are also solved. The reaction schemes (S2 and S5) were incorporated into Fluent using user-defined functions (UDFs), combined with other Fluent internally calculated NOx reactions. 4. Results and Discussions 1. Combustion Characteristics. The in-furnace gas temperature was measured by an optical pyrometer and was corrected to the actual flame temperature due to emission from the gas and from the wall. The comparison between air and oxy-fuel combustion for given flame temperatures is shown in Figure 4. Both the experimental data and the prediction in the oxy-fuel condition have a delayed ignition, and lower flame temperatures were observed under oxy-fuel combustion. It has been found that the combustion models 134
Energy Fuels 2010, 24, 131–135
: DOI:10.1021/ef900524b
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the coal nitrogen released is split between volatiles and residual char. In order to investigate the influence of high-temperature volatile yield and N partition ratio between volatile and char on NO emission, NOx emission from coal A under air conditions was considered, in which volatile N and char N are assumed to convert to HCN and NO, respectively. Figure 8 shows the NO emission at the outlet of the furnace (dry %) versus N percentage retained in char in air combustion at different volatile yields. It indicates that the NO emission behavior is more sensitive to the nitrogen partitioning ratio between volatile and char than the amount of volatile released at high temperature. In addition, char N contributes less to NO reduction in the flame. However, the coal-N partition between volatile and char in oxy-fuel conditions has not as yet been reported.
Figure 7. NOx predictions with and without recycled NO in oxy-fuel combustion.
5. Conclusions Oxy-fuel combustion as a cost-effective carbon capture and storage (CCS) technology can be used for both new and retrofitted power plants. A pilot scale test facility in air and oxy-fuel retrofitted combustion has been numerically investigated. Compared with experimental data, the numerical results showed the delayed ignition and slightly lower peak flame temperature under the oxy-fuel condition. On the basis of the existing NO formation mechanism (De Soete’s mechanism), the modified pathways of NO formation and reduction were added into Fluent to predict NO emission at both air and oxy-fuel combustion, including decays of HCN to yield NH3 and hydrocarbon radicals reacting with NO to produce HCN. Both experimental results and predictions show that NO emission in oxy-fuel conditions, which comes from fuel NO and recycled NO, can be reduced to a level less than that of air combustion. It was observed that higher nitrogen intermediate species during the combustion zone in oxy-fuel conditions is favorable for NO reduction. And, the results show that the NO formation rate could be suppressed by NOx recycling back to the furnace. A sensitivity study of the N percentage retained in the char and high-temperature volatile yields shows that NO emission is more sensitive to the N partitioning ratio between volatiles and char which required further study.
Figure 8. Influence of N% retained in char on NO emission.
environments than in air combustion. The highest nitrogen intermediates concentration appears prior to the highest temperature in the furnace. Two mechanisms might lead to nitrogen intermediates formation: (1) fuel-N partitioning between volatile and char and volatile-N release as HCN or NH3; (2) NO reduction to HCN by hydrocarbon fragments. The modeling work here assumes that the fuel-N partitioning ratio is same. Therefore, there should be more than 35% difference between air and oxy-fuel conditions due to the latter one. The NO rates along the furnace (off-axis 0.04 m) under air and oxy-fuel combustion (as shown in Figure 6) were compared. The maximum of NO rates in the near-burner zone are comparable under air and oxy-fuel without recycled NO condition, which are much higher than that of oxy-fuel with recycled NO condition. The NO formation occurs delayed under oxy-fuel condition due to ignition delayed. Figure 7 shows the comparison between oxy-fuel conditions with recycled NO (1000 ppm) and without recycled NO. A dotted line representing the difference between two cases has been added. The fact that the differences at the beginning are higher than the subsequent concentration might be due to the fact that the NOx formation is slower or inhibited by recycled NO in the secondary stream, which is consistent with experimental work by Mackrory et al.22 3. Sensitivity Analysis of N Partition Ratio on the Emission of NOx. Modeling fuel-NOx formation requires both the rate of coal nitrogen release and the form of nitrogen released. And,
Acknowledgment. The authors greatly acknowledge the financial support provided by the Natural Science Foundation of China (Project No. 50876025), The Chinese Scholarship Council, and Cooperative Research Centre for Coal in Sustainable Development.
Nomenclature a = order of oxygen reaction A = pre-exponential factor, s-1 AE = BET surface area, m2/kg E = activation energy, J/g mol k = reaction constant rate, s-1 Mw,i = molecular weight of species i P = pressure, pa R = universe gas constant, 8.314 J/mol K S = source term Xi = mole fraction of species i Yi = mass fraction of species i F = density, g/cm3 vB = velocity, m/s ηN = nitrogen conversion efficiency 135
