Nitric Oxide Reduction by Non-hydrocarbon Fuels. Implications for

828

Energy & Fuels 2000, 14, 828-838

Nitric Oxide Reduction by Non-hydrocarbon Fuels. Implications for Reburning with Gasification Gases P. Glarborg,† P. G. Kristensen,‡ and K. Dam-Johansen Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark

M. U. Alzueta,* A. Millera, and R. Bilbao Department of Chemical and Environmental Engineering, Centro Polite´ cnico Superior, University of Zaragoza, Maria de Luna, 3, 50015 Zaragoza, Spain Received September 14, 1999

The ability of non-hydrocarbon fuels such as CO and H2 to reduce nitric oxide under conditions relevant for the reburning process is investigated experimentally and theoretically. Flow reactor experiments on reduction of NO by CO and H2 are conducted under fuel-rich conditions, covering temperatures of 1200-1800 K and a range of stoichiometries and reactant levels. Bench and pilot scale results from literature on reburning with CO, H2, and low calorific value gases are also considered. The experimental data are interpreted in terms of a detailed reaction mechanism, and the reactions responsible for removal of NO are identified. The experimental results indicate that under typical reburn process conditions these non-hydrocarbon fuels may remove 20-30% of the nitric oxide entering the reburn zone. However, results indicate that the process potential increases with temperature and reburn fuel fraction, and at high temperatures and reburn fuel fractions of about 30%, the reduction efficiency approaches that of hydrocarbon gases. If dilution effects and the lowering of the primary zone NO (maintaining the overall load) are accounted for, the reduction potential is further increased. Modeling results indicate that the mixing process may affect the NO reduction in the reducing zone. The modeling predictions are in qualitative agreement with the experimental results but tend to underestimate the reduction of NO. Conversion of NO to N2 in the reburn zone proceeds primarily through the following sequence: H + NO + M h HNO + M, HNO + H h NH + OH, NH + NO f N2 + ... The implications of the results for reburning with fuels with a low hydrocarbon content are discussed, with special emphasis on gasified fuels.

Chemical reactions that convert nitric oxide to molecular nitrogen are of large practical interest. Such reactions are used extensively to control emissions of nitrogen oxides from combustion processes. It is wellknown that nitric oxide can be removed by reaction with hydrocarbon-derived radicals such as HCCO and CHi (i ) 0-3), reactions used for NO abatement in the reburn process.1-3 Also nitrogen-containing species such as ammonia, urea, isocyanic acid, and even hydrogen cyanide are, under proper reaction conditions, efficient in reducing NO.4 These reactions are used actively in selective noncatalytic reduction of nitric oxide, and they

act to limit the NO emission from low-temperature combustion facilities such as fluidized beds. The ability of non-hydrocarbon fuels such as hydrogen and carbon monoxide to reduce NO to N2 is still unresolved. Selected low-pressure flame results for fuelrich H2/O2/NO/Ar 5 and pilot scale reburn experiments using CO, H2, or CO/H2 mixtures as reburn fuel6-8 indicate that hydrogen and carbon monoxide are capable of converting NO to N2 under proper reaction conditions. However, other flame data9 as well as flow reactor results2 show very little reduction of NO. Conversion of NO to N2 in the absence of hydrocarbon radicals and reactive nitrogen species such as amines

* To whom correspondence should be addressed. Fax: +34 976 761879. E-mail: [email protected]. † Fax: +45 45 882258. E-mail: [email protected]. ‡ Present address: Danish Gas Technology Centre, Hørsholm, Denmark. (1) Bowman, C. T. Twenty-fourth Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 859-878. (2) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1997, 109, 25-36. (3) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. (1998a). Combust. Flame 1998, 115, 1-27. (4) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1996, 107, 211-222.

(5) Seery, D. J.; Zabielsky, M. F. Eighteenth Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; pp 397404. (6) Chen, S. L.; McCarthy, J. M.; Clark, W. D.; Heap, M. P.; Seeker, W. R.; Pershing, D. W. Twenty-First Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 1159-1169. (7) Chen, S. L.; Kramlich, J. C.; Seeker, W. R.; Pershing, D. W. JAPCA 1989, 39, 1375-1379. (8) Bortz, S. J.; Offen, G. R. Joint Symposium on Stationary Combustion NOx Control, New Orleans, LA, 1987, paper P-362. (9) Cattolica, R. J.; Cavolowsky, J. A.; Mataga, T. G. Twenty-Second Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998, pp 1165-1173.

Introduction

10.1021/ef990186r CCC: $19.00 © 2000 American Chemical Society Published on Web 06/08/2000

Nitric Oxide Reduction by Non-hydrocarbon Fuels

Energy & Fuels, Vol. 14, No. 4, 2000 829

and cyanides potentially occurs through the reaction sequence (Table 1)

H + NO + M h HNO + M

(27)

HNO f NHi NHi + NO f N2 + ... While the H + NO + M10 and NHi + NO reactions11-14 are fairly well-established, the conversion of the HNO species to amine radicals needs further assessment. The objective of the present study is to evaluate the ability of CO/H2 to reduce nitric oxide under fuel-rich conditions. Flow reactor experiments are conducted in the present work to study reduction of NO by CO and H2. Also pilot scale experiments6-8 from literature are considered. All the experimental data are interpreted in terms of a detailed reaction mechanism, and the reactions responsible for removal of NO are identified. The experimental conditions that are optimal for reducing NO are identified, and the implications of the results for reburning with fuels with a low hydrocarbon content are discussed, with special emphasis on gasification gases based for instance on biomass fuels. Experimental Section Two different flow reactor systems were used in the present work. Both the setups as well as the experimental procedures are described in detail elsewhere, and only a brief description is given here. Common for the two configurations was that they involved a gas feeding system, a gas reaction system, and a gas analysis system. In the “premixed” setup15,16 pure gases from gas cylinders were mixed prior to preheating and entrance into the reaction system. The reaction system included a ceramic tubular reactor heated by an electrical oven that allowed temperatures of up to 1800 K. The reactor was an alumina tube of 20 mm inside diameter and 2500 mm in length, with a central zone of approximately 800 mm where the temperature was constant. In the “fast-mixing” setup4,17,18 the gaseous components were led to the quartz reactor in as many as four separate streams. The main flow contained nitrogen, oxygen, and water, while the other reactants were supplied through three injector tubes. To achieve a well-defined reactor volume, the main flow and the three injector flows were heated separately and mixed in cross-flow at the reactor inlet. The reactor tube used in these experiments, with a radius of 40 mm and a length of 180 mm, was placed in a three zone electrically heated oven, allowing a uniform temperature profile within (5 or (20 K over the (10) Glarborg, P.; Østberg, M.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Twenty-Seventh Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998, pp 219-226. (11) Tsang, W.; Herron, J. T. J. Phys. Chem. Ref. Data 1991, 20, 609-663. (12) Glarborg, P.; Kristensen, P. G.; Dam-Johansen, K.; Miller, J. A. J. Phys. Chem. 1997, 101, 3741-3745. (13) Dean, A. M.; Bozzelli, J. W. 1998. Combustion Chemistry of Nitrogen. In Combustion Chemistry II; Gardiner, W. C., Jr., Ed.; Springer-Verlag: Berlin, in press. (14) Miller, J. A.; Glarborg, P. Int. J. Chem. Kinet. 1999, 31, 757765. (15) Bilbao, R.; Millera, A.; Alzueta, M. U. Ind. Eng. Chem. Res. 1994, 33, 2846-2852. (16) Bilbao, R.; Alzueta, M. U.; Millera, A. Ind. Eng. Chem. Res. 1995, 34, 4531-4539. (17) Johnsson, J. E.; Glarborg, P.; Dam-Johansen, K. Twenty-Fourth Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992, pp 917-923. (18) Hulgaard, T.; Dam-Johansen, K. AIChE J. 1993, 39, 13421354.

reactor. The product gas was quenched by cold gas at the outlet of the reactor tube. Both reactors were designed to obtain a good plug flow approximation. Water was added to the gas by saturating nitrogen in a thermostatically controlled water bath. All tubes containing water vapor were heated above the dew point. The concentrations of CO, CO2, O2, and NO were measured continuously by spectrophotometric and paramagnetic analyzers with an accuracy of (3%, but at least (10 ppm. In addition, in the “premixed heating” reactor experiments, CO and CO2, were also determined by gas chromatography. Reaction Set. The mechanism used in the present work is based on the kinetic model of Glarborg, Alzueta, Dan-Johansen, and Miller,3 which describes hydrocarbon/nitric oxide interactions under stoichiometric to fuel-rich conditions. The H/N/O subset was modified according to the recent work of Miller and Glarborg14 on the thermal DeNOx chemistry subset, and in the present work a few additional changes were made, as discussed below. The CO/H/N/O subset of the complete mechanism is listed in Table 1; the reaction numbering used throughout the paper refers to this listing. The full mechanism can be obtained from the authors. With a few exceptions,3,10 thermodynamic data were taken from the Sandia Thermodynamic Database.19 In the H2/O2/NO system removal of nitric oxide under reducing conditions occurs by different mechanisms, dependent on temperature and stoichiometry. In the absence of oxygen, NO is removed at medium temperatures (900-1500 K) by the sequence20

H2 + NO h HNO + H HNO + NO h N2O + OH

(41) (-130)

If O2 is present, the radical pool builds up to higher levels and the comparatively slow reactions -41 and -130 become less significant. Under these conditions, removal of NO is expected to occur mainly by the sequence

H + NO + M h HNO + M HNO + H h NH + OH

(27)

Hr ) 16.4 kcal/mol (93)

followed by reaction of NH with NO. However, a number of other pathways from HNO into the amine pool may possibly contribute. At high temperatures, nitric oxide may be reduced directly by the endothermic reaction with hydrogen atoms,

H + NO h N + OH

(-103)

The interest in the present study is the medium-temperature range, i.e., 1200-1800 K, which is of interest for reburning. Under these conditions the nitroxyl species (HNO) constitutes a key intermediate in the reduction of NO. Recent experimental and theoretical work has improved our understanding of both the thermochemistry21-23 and reactions10,20,24-28 of this component. The reaction forming nitroxyl under reburn conditions, H + NO + M h HNO + M (27) was recently studied in the 1000-1200 K range with N2 as a collision (19) Kee, R. J.; Rupley, F. M.; Miller, J. A. The Chemkin Thermodynamic Database; Sandia Report SAND87-8215, 1991 update; Sandia National. Laboratories: Livermore, CA, 1991. (20) Diau, E. W. G.; Lin, M. C.; He, Y.; Melius, C. F. Prog. Energy Combust. Sci. 1995, 21, 1-23. (21) Lee, T. J.; Dateo, C. E. J. Chem. Phys. 1995, 103, 9110-9111. (22) Dixon, R. N. J. Chem. Phys. 1996, 104, 6905-6906. (23) Anderson, W. R. Combust. Flame 1999, 117, 394-403. (24) Soto, M. R.; Page, M. J. Chem. Phys. 1992, 97, 7287-7296. (25) He, Y.; Lin, M. C. Int. J. Chem. Kinet. 1992, 24, 743-760. (26) Natarajan, K.; Mick, H. J.; Woiki, D.; Roth, P. Combust. Flame 1994, 99, 610-616. (27) Mebel, A. M.; Diau, E. W. G.; Lin, M. C.; Morokuma, K. J. Phys. Chem. 1996, 100, 7517-7525.

830

Energy & Fuels, Vol. 14, No. 4, 2000

Glarborg et al.

Table 1. H/N/O Reaction Subset of the Full Mechanism, Expressed as k ) ATβ exp(-Ea/RT), the Units Being Calories, cm3, mole, and seconds (28) Allen, M. T.; Dryer, F. L. Combust. Flame 1998, 112, 302-311.

no. 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 61 62

reaction O + OH h H + O2 O + H2 h H + OH OH + H2 h H2O + H OH + OH h H2O + O H + H + M h H2 + M enhanced third-body efficiencies: H + H + H2O h H2 + H2O H + O + M h OH + M enhanced third-body efficiencies: H + OH + M h H2O + M enhanced third-body efficiencies: O + O + M h O2 + M enhanced third-body efficiencies: H + O2 + M h HO2 + M enhanced third-body efficiencies: H + O2 + N2 h HO2 + N2 HO2 + H h H2 + O2 HO2 + H h OH + OH HO2 + H h O + H2O HO2 + O h OH + O2 HO2 + OH h H2O + O2 HO2 + HO2 h H2O2 + O2 HO2 + HO2 h H2O2 + O2 H2O2 + M h OH + OH + M enhanced third-body efficiencies: H2O2 + H h HO2 + H2 H2O2 + H h H2O + OH H2O2 + O h HO2 + OH H2O2 + OH h H2O + HO2 H2O2 + OH h H2O + HO2 duplicate reaction CO + O + M h CO2 + M enhanced third-body efficiencies: CO + OH h CO2 + H CO + O2 h CO2 + O CO + HO2 h CO2 + OH NO + H + M h HNO + M enhanced third-body efficiencies: NO + O + M h NO2 + M enhanced third-body efficiencies: NO + OH + M h HONO + M enhanced third-body efficiencies: NO + HO2 h NO2 + OH NO2 + H h NO + OH NO2 + O h NO + O2 NO2 + O(+M) h NO3(+M) low-pressure limit: enhanced third-body efficiencies: NO2 + NO2 h NO + NO + O2 NO2 + NO2 h NO3 + NO NO3 + H h NO2 + OH NO3 + O h NO2 + O2 NO3 + OH h NO2 + HO2 NO3 + HO2 h NO2 + O2 + OH NO3 + NO2 h NO + NO2 + O2 HNO + H h NO + H2 HNO + O h NO + OH HNO + OH h NO + H2O HNO + O2 h NO + HO2 HNO + NO2 h HONO + NO HNO + HNO h N2O + H2O HNO + NH2 h NH3 + NO H2NO + M h HNO + H + M enhanced third-body efficiencies: H2NO + M h HNOH + M enhanced third-body efficiencies: H2NO + H h HNO + H2 H2NO + H h NH2 + OH H2NO + O h NH2 + O2 H2NO + O h HNO + OH H2NO + OH h HNO + H2O H2NO + HO2 h HNO + H2O2 H2NO + O2 h HNO + HO2 H2NO + NO h HNO + HNO H2NO + NO2 h HONO + HNO HNOH + M h HNO + H + M HNOH + H h NH2 + OH HNOH + H h HNO + H2 HNOH + O h HNO + OH HNOH + O h HNO + OH duplicate reaction

H2O ) 0 H2O ) 5 H2O ) 5 H2O ) 5 N2 ) 0, H2O ) 10

H2O ) 5

H2O ) 5

N2 ) 1, O2 ) 1.5, H2O ) 10, CO2 ) 3 N2 ) 1.7, O2 ) 1.5, H2O ) 10 H2O ) 5

N2 ) 1.5, O2 ) 1.5, H2O ) 18.6

H2O ) 10 H2O ) 10

A

β

Ea

2.0 × 1014 5.0 × 104 2.1 × 108 4.3 × 103 1.0 × 1018

-0.40 2.67 1.52 2.70 -1.00

0 6 290 3 450 -2 486 0

3 3 3 3 3

6.0 × 1019 6.2 × 1016

-1.25 -0.60

0 0

3 3

1.6 × 1022

-2.00

0

3

1.9 × 1013

0.00

-1 788

3

2.1 × 1018

-1.00

0

3

6.7 × 1019 4.3 × 1013 1.7 × 1014 3.0 × 1013 3.3 × 1013 1.9 × 1016 1.3 × 1011 4.2 × 1014 1.3 × 1017

-1.42 0.00 0.00 0.00 0.00 -1.00 0.00 0.00 0.00

0 1 411 875 1 721 0 0 -1 630 11 980 45 500

3 3 3 3 3 3 3 3 3

1.7 × 1012 1.0 × 1013 6.6 × 1011 7.8 × 1012 5.8 × 1014

0.00 0.00 0.00 0.00 0.00

3 755 3 576 3 974 1 330 9 560

3 3 3 3

6.2 × 1014

0.00

3 000

3

1.5 × 2.5 × 1012 5.8 × 1013 4.0 × 1020

1.30 0.00 0.00 -1.75

-760 47 700 22 930 0

3 3 3 10

7.5 × 1019

-1.41

0

3

5.1 × 1023

-2.51

-68

3

2.1 × 1012 8.4 × 1013 3.9 × 1012 1.3 × 1013 1.0 × 1028

0.00 0.00 0.00 0.00 -4.08

-480 0 -238 0 2 470

3 3 3 3

1.6 × 1012 9.6 × 109 6.0 × 1013 1.0 × 1013 1.4 × 1013 1.5 × 1012 5.0 × 1010 4.4 × 1011 1.0 × 1013 3.6 × 1013 2.0 × 1012 6.0 × 1011 9.0 × 108 3.6 × 106 2.8 × 1024

0.00 0.73 0.00 0.00 0.00 0.00 0.00 0.70 0.00 0.00 0.00 0.00 0.00 1.63 -2.83

26 123 20 900 0 0 0 0 2 940 2 650 0 0 25 000 2 000 3 100 -1 250 64 915

3 3 3 3 3 3 3 3 3 3 14 3 3 3 14

1.1 × 1029

-4.00

44 000

14

3.0 × 107 5.0 × 1013 2.0 × 1014 3.0 × 107 2.0 × 107 2.9 × 104 3.0 × 1012 2.0 × 104 6.0 × 1011 2.0 × 1024 4.0 × 1013 4.8 × 108 7.0 × 1013 3.3 × 108

2.00 0.00 0.00 2.00 2.00 2.69 0.00 2.00 0.00 -2.84 0.00 1.50 0.00 1.50

2 000 0 0 2 000 1 000 1 600 25 000 13 000 2 000 58 930 0 378 0 -358

3 3 3 3 3 14 14 3 3 14 14 14 14 14

107

ref



Table 1. (Continued) no.

reaction

A

63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

HNOH + OH h HNO + H2O HNOH + HO2 h HNO + H2O2 HNOH + O2 h HNO + HO2 HNOH + NO2 h HONO + HNO HNOH + NH2 h HNO + NH3 HONO + H h NO2 + H2 HONO + O h NO2 + OH HONO + OH h NO2 + H2O NH3 + M h NH2 + H + M NH3 + H h NH2 + H2 NH3 + O h NH2 + OH NH3 + OH h NH2 + H2O NH3 + HO2 h NH2 + H2O2 NH2 + H h NH + H2 NH2 + O h HNO + H NH2 + O h NH + OH NH2 + OH h NH + H2O NH2 + HO2 h H2NO + OH NH2 + HO2 h NH3 + O2 NH2 + NO h NNH + OH NH2 + NO h N2 + H2O NH2 + NO2 h N2O + H2O NH2 + NO2 h H2NO + NO NH2 + H2NO h HNO + NH3 NH2 + HONO h NH3 + NO2 NH2 + NH2 h N2H2 + H2 NH2 + NH h N2H2 + H NH2 + N h N2 + H + H NH + H h N + H2 NH + O h NO + H NH + OH h HNO + H NH + OH h N + H2O NH + O2 h HNO + O NH + O2 h NO + OH NH + H2O h HNO + H2 NH + NO h N2O + H NH + NO h N2O + H duplicate reaction NH + NO h N2 + OH NH + NO2 h N2O + OH NH + NH h N2 + H + H NH + N h N2 + H N + OH h NO + H N + O2 h NO + O N + NO h N2 + O N2H2 + M h NNH + H + M enhanced third-body efficiencies: N2 ) 2, O2 ) 2, H2O ) 15 N2H2 + H h NNH + H2 N2H2 + O h NH2 + NO N2H2 + O h NNH + OH N2H2 + OH h NNH + H2O N2H2 + NO h N2O + NH2 N2H2 + NH2 h NNH + NH3 N2H2 + NH h NNH + NH2 NNH h N2 + H NNH + H h N2 + H2 NNH + O h N2O + H NNH + O h N2 + OH NNH + O h NH + NO NNH + OH h N2 + H2O NNH + O2 h N2 + HO2 NNH + O2 h N2 + H + O2 NNH + NO h N2 + HNO NNH + NH2 h N2 + NH3 NNH + NH h N2 + NH2 N2O + M h N2 + O + M enhanced third-body efficiencies: N2 ) 1.7, O2 ) 1.4, H2O ) 12 N2O + H h N2 + OH N2O + H h N2 + OH duplicate reaction N2O + O h NO + NO N2O + O h N2 + O2 N2O + OH h N2 + HO2 N2O + OH h HNO + NO N2O + NO h N2 + NO2 CO + NO2 h CO2 + NO CO + N2O h N2 + CO2 CO2 + N h NO + CO CO2 + NH h HNO + CO

2.4 × 2.9 × 106 3.0 × 1012 6.0 × 1011 1.8 × 106 1.2 × 1013 1.2 × 1013 4.0 × 1012 2.2 × 1016 6.4 × 105 9.4 × 106 2.0 × 106 3.0 × 1011 4.0 × 1013 6.6 × 1014 6.8 × 1012 4.0 × 106 5.0 × 1013 1.0 × 1013 2.3 × 1010 2.8 × 1020 1.6 × 1016 6.5 × 1016 3.0 × 1012 7.1 × 10 8.5 × 1011 5.0 × 1013 7.2 × 1013 3.0 × 1013 9.2 × 1013 2.0 × 1013 5.0 × 1011 4.6 × 105 1.3 × 106

99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 a

Estimated upper limit, in agreement with Fernańdez et al.31

β

Ea

ref

-1 192 -1 600 25 000 2 000 -1 152 7 350 5 960 0 93 470 10 171 6 460 566 22 000 3 650 0 0 1 000 0 0 -814 41 258 268 268 1 000 -4 940 0 0 0 0 0 0 2 000 6 500 100

2.9 × 1014 -2.2 × 1013

2.00 2.69 0.00 0.00 1.94 0.00 0.00 0.00 0.00 2.39 1.90 2.04 0.00 0.00 -0.50 0.00 2.00 0.00 0.00 0.425 -2.65 -1.44 -1.44 0.00 3.02 0.00 0.00 0.00 0.00 0.00 0.00 0.50 2.00 1.50 slow -0.40 -0.23

14 14 14 14 14 3 3 3 3 3 3 3 3 3 3 3 3 3 3 14 14 14 14 3 3 3 3 3 3 3 3 3 3 3 see text 3 3

2.2 × 1013 1.0 × 1013 2.5 × 1013 3.0 × 1013 3.8 × 1013 6.4 × 109 3.3 × 1012 5.0 × 1016

-0.23 0.00 0.00 0.00 0.00 1.00 0.30 0.00

0 0 0 0 0 6 280 0 50 000

3 3 3 3 3 3 3 3

5.0 × 1013 1.0 × 1013 2.0 × 1013 1.0 × 1013 3.0 × 1012 1.0 × 1013 1.0 × 1013 6.5 × 107 1.0 × 1014 1.0 × 1014 8.0 × 1013 5.0 × 1013 5.0 × 1013 2.0 × 1014 5.0 × 1013 5.0 × 1013 5.0 × 1013 5.0 × 1013 4.0 × 10

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

1 000 0 1 000 1 000 0 1 000 1 000 0 0 0 0 0 0 0 0 0 0 0 56 100

3 3 3 3 3 3 3 14 3 3 3 3 3 3 3 3 3 3 3

3.3 × 1010 4.4 × 1014

0.00 0.00

4 729 19 254

3 3

6.6 × 1013 1.0 × 1014 1.3 × 10-2 1.2 × 10-4 5.3 × 105 9.0 × 1013 3.2 × 1011 1.0 × 1014

0.00 0.00 4.72 4.33 2.23 0.00 0.00 0.00 slow

26 630 28 000 36 560 25 080 46 280 33 800 20 237 30 000

3 3 3 3 3 3 3 estda see text

106

0 0

832


Glarborg et al.

Figure 1. Reduction of NO by CO in the fast-mixing flow reactor. Comparison between experimental data2 (symbols) and model predictions (lines) for NO, CO, and CO2, as a function of temperature. The inlet conditions are shown as set 1 in Table 2. partner by our group10 and by Allen and Dryer.28 In contrast to previous estimates,11 these studies, which were conducted in flow reactors, indicate a strong fall off of k27,N2 at high temperatures, limiting the importance of HNO for NO conversion under these conditions. The rate of the HNO + H reaction (-93) is determined from the rate constant for the reverse reaction, NH + OH h HNO + H (93). This is a fast radical-radical reaction, for which no measurements are available. The uncertainty in the rate constant for reaction -93 involves uncertainties in the estimate for k93 as well as in the thermodynamic data for HNO and NH. Reaction -93 competes with a number of HNO consumption reactions that recycle HNO back to NO. The most important of these under reducing conditions are

HNO + H h NO + H2

(41)

HNO + OH h NO + H2O

(43)

Both of these reactions are fast. The rate constant for reaction 41 has been measured directly over a range of temperatures and is now well-established,10 while the value of k43 is based on an estimate. Reactions 41 and 43 not only recycle HNO to NO but, together with reaction 27, they also constitute a sequence of reactions that catalyze radical removal and thereby may limit the fuel oxidation rate under reducing conditions.10 In addition to reaction -93, a number of HNO reactions may feed into the amine pool. Reactions between nitroxyl and the radical pool forming NH or NH2, e.g.

HNO + H h NH2 + O

∆Hr ) 27.0 kcal/mol (-77)

HNO + O h NH + O2

∆Hr ) 0.4 kcal/mol (-95)

HNO + OH h NH + HO2

∆Hr ) 52.4 kcal/mol

are too slow to be significant. Reactions between HNO and stable species such as H2 and CO

HNO + H2 h NH + H2O HNO + CO h NH + CO2

∆Hr ) 1.4 kcal/mol (-97) ∆Hr ) 16.4 kcal/mol (-135)

are potentially important, because these steps are not very endothermic and concentrations of H2 and CO may reach significant levels under reducing conditions. These reactions

Figure 2. Reduction of NO by H2 (upper) and CO (lower) in the fast-mixing flow reactor. Comparison between experimental data (symbols) and model predictions (lines) for NO as a function of temperature. The inlet conditions are shown as sets 2-5 (CO) and 6-9 (H2) in Table 2. were recently studied in the reverse direction by Ro¨hrig and Wagner29 in a shock tube. With our current thermodynamic data for HNO,21-23 their values for k97 (NH + H2O) and k135 (NH + CO2) correspond to comparatively fast rates for HNO + H2 and HNO + CO. Modeling of H2 or CO reburning, with the rate coefficients proposed by Ro¨hrig and Wagner for reactions 97 and 135, indicates a considerable conversion of HNO to NH, followed by subsequent reduction of nitric oxide, even at low temperatures. This behavior is not supported by our experimental data (Figures 1 and 2), which show no or negligible reduction of NO below 1300 K. For this reason we have chosen to omit the two reactions from the mechanism until the findings of Ro¨hrig and Wagner29 have been confirmed. Another route that potentially leads from HNO into the NHi radical pool involves formation of H2NO or HNOH,

HNO + H + M h H2NO + M

(-48)

HNO + H + M h HNOH + M

(-59)

H2NO + M h HNOH + M

(49)

followed by rapid conversion of H2NO/HNOH to NH2

H2NO + H h NH2 + OH

(51)

HNOH + H h NH2 + OH

(60)

Reactions 51 and 60 both proceed close to collision frequency and the HNO f H2NO/HNOH f NH2 route is limited by the rate of the recombination reactions -48 and -59. Our earlier estimate30 for the H2NO dissociation reaction 48 results in significant NO removal by this pathway at lower tempera(29) Ro¨hrig, M.; Wagner, H. G. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 1073-1076. (30) Glarborg, P.; Dam-Johansen, K.; Miller, J. A.; Kee, R. J.; Coltrin, M. E. Int. J. Chem. Kinet. 1994, 26, 421-436.



tures. The present values for k48 and k59, adopted from Dean and Bozzelli,13 are considerably lower, making the H + HNO + M route to H2NO/HNOH insignificant. This is in agreement with the current experimental data as well as those of Kristensen et al.4 for reactive nitrogen conversion under reducing conditions. In the presence of CO, the reaction

CO + NO h CO2 + N

(-134)

might conceivably contribute to removal of NO. Indeed, with the rate constant we have used previously3 for the reverse reaction, CO2 + N h CO + NO (134), a significant fraction of NO was removed by reaction with CO at higher temperatures. However, recent work by Fernańdez et al.31 in the temperature range 285-1140 K has shown reaction 134 to be very slow, indicating that this step is insignificant under combustion conditions. In the present work we have used a rate constant for (134) that is in agreement with the upper limit by Fernańdez et al.31 and also with the shock tube measurements of Lindackers et al.32 However, following the discussion by Fernańdez et al.,31 we estimate that our value is a conservative upper limit for this spin-forbidden reaction. Carbon monoxide may at high temperatures and very reducing conditions be partly converted to hydrocarbons. The full mechanism used in the modeling includes C1/C2 hydrocarbon oxidation3 and thereby also the reverse reactions through CO to hydrocarbon chemistry. However, except when methane is added to the reburn gas, formation of hydrocarbon radicals is not significant under the conditions of the present study and does not contribute to NO removal.

Results and Discussion To evaluate the potential of CO and H2 for reducing nitric oxide under reburn conditions, a number of flow reactor experiments were conducted and interpreted in terms of the detailed chemical kinetic model described above. The calculations were performed using Senkin,33 a plug-flow code that runs in conjunction with the Chemkin library.34 In addition to the flow reactor results, bench and pilot scale results from literature on reburning with CO, H2, and low calorific gases were analyzed. To simulate the mixing process, which may have a significant impact on the reburn chemistry, we apply our adaption3,35,36 of the approach of Zwietering.37 In this approach a secondary stream (the bulk flow) entrains into the primary stream (the reburn jet) with an exponential rate. The reburn fuel jet is assumed to be heated rapidly by the penetrating hot bulk gas. The mixing time has been taken as the time for which 90% of the cross-flow is mixed with the reburn fuel jet. In the fast-mixing flow reactor this time is 5 ms,4 while in (31) Fernańdez, A.; Goumri, A.; Fontijn, A. J. Phys. Chem. 1998, 102, 168-172. (32) Lindackers, D.; Burmeister, M.; Roth, P. Combust. Flame 1990, 81, 251-259. (33) Lutz, A. E.; Kee, R. J.; Miller, J. A. Senkin: A Fortran Program for Predicting Homogeneous Gas-Phase Chemical Kinetics with Sensitivity Analysis; Sandia Report SAND87-8248; Sandia National Laboratories: Livermore, CA, 1987. (34) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics; Sandia Report SAND89-8009, Sandia National Laboratories, Livermore, CA, 1989. (35) Alzueta, M. U.; Bilbao, R.; Millera, A.; Glarborg, P.; Østberg, M.; Dam-Johansen, K. Energy Fuels 1998, 12, 329-338. (36) Østberg, M.; Glarborg, P.; Jensen, A.; Johnsson, J. E.; Pedersen, L. S.; Dam-Johansen, K. Twenty-Seventh Symposium (Int.) on Combustion, The Combustion Institute: Pittsburgh, PA, 1998; pp 3027-3035. (37) Zwietering, T. N. Chem. Eng. Sci. 1959, 11, 1-15.

Table 2. Experimental Conditionsa set CO (%) H2 (%) NO (ppm) O2 (%) H2O (%) res time (s) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.50 1.96 1.92 1.86 2.02 1.99 1.93 1.89 1.82 0.50 1.00 5.00 10.0 20.0 5.00 5.00 20.0 20.0

927 297 292 281 307 300 291 285 275 900 900 900 900 900 900 900 900 900

0.20 0.16 0.25 0.42 0 0 0.16 0.25 0.42 2.00 2.00 2.00 2.00 2.00 1.00 5.00 1.00 5.00

1.85 4.70 4.00 4.50 4.90 4.90 4.70 4.60 4.50 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00

145/T(K) 114/T(K) 112/T(K) 108/T(K) 118/T(K) 118/T(K) 114/T(K) 112/T(K) 108/T(K) 280/T(K) 280/T(K) 280/T(K) 280/T(K) 280/T(K) 280/T(K) 280/T(K) 280/T(K) 280/T(K)

a The experiments are conducted at constant mass flow, and thereby the residence time is dependent on the reaction temperature. Experiments 1-9 are performed in the fast-mixing flow reactor, while experiments 10-18 were conducted in the premixed flow reactor.

bench and pilot scale experiments we expect it to be considerably longer. Previous modeling35,36,38 indicates that this and similar adaptations of the Zwietering approach are adequate for describing the chemistry/ mixing interaction in reburn modeling. However, a validation of the approach is difficult due to a lack of suitable experimental data. Flow Reactor Results. A number of flow reactor experiments on reburning with either CO or H2 were performed in the two flow reactor setups. A listing of the experimental conditions is found in Table 2. The experiments were conducted by maintaining a constant mass flow rate and varying the reactor temperature. This way the gas residence time in the reactor depended on the temperature, as shown in Table 2. Figures 1 and 2 show comparison between flow reactor data and model predictions for the CO/O2/H2O/ NO and H2/O2/H2O/NO systems, diluted in N2. These experiments were conducted in the fast-mixing setup, which is characterized by a mixing time of about 5 ms.3,4 Under the conditions of Figure 1, mixing is not important for the results, but, at the highest temperatures in Figure 2, the chemistry and mixing times are comparable. The results of Figure 1 are obtained under very diluted, reducing conditions and temperatures of 9001400 K, with an accuracy of (5 K. It is noteworthy that no reduction in NO is observed below 1400 K. Nitric oxide is reduced neither by CO, H2 (formed in the oxidation), or radicals in the O/H radical pool. This observation puts strict upper limits to the rate-limiting reactions in the NO reduction pathways proceeding through the sequence NO f HNO f NHi f N2. In particular, the results in combination with those of Figure 2 strongly suggest that reactions of HNO with CO (-135) and H2 (-97) feeding into the amine pool are very slow and that a NO reduction pathway through H2NO/HNOH is insignificant. Figure 2 shows results for reduction of NO by H2 and CO, respectively, as a function of stoichiometry and (38) Cha, C. M.; Kramlich, J. C.; Kosaly, G. Twenty-Seventh Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; pp 1427-1434.

834


Figure 3. Reduction of NO by CO in the premixed flow reactor. Comparison between experimental data (symbols) and model predictions (lines) for NO, and CO, as a function of inlet CO concentration. The inlet conditions are shown as sets 1014 in Table 2.

temperature. Compared to the data of Figure 1, the fuel/ oxygen ratios are higher and the data extend to 1550 K (but with a larger uncertainty in temperature, (20 K). Consistent with the data of Figure 1 and data reported previously,10 little NO reduction is observed in the lower part of the temperature regime. However, at higher temperatures some reduction of the inlet NO is obtained, increasing from about 10% at 1400 K to 2030% at 1550 K. The observed level of reduction is higher for CO than for H2 and appears to be independent of the oxygen concentration in the range investigated. The modeling predictions show very little reduction of NO, and effects of reburn fuel type, stoichiometry, or temperature are not discernible. The underestimation of the NO removal is largest for the CO experiments. The higher efficiency of CO compared to H2 in reducing NO cannot be explained in terms of the modeling. We attribute the higher efficiency of CO partly to heterogeneous effects, in that reduction of NO by CO to some extent may be promoted catalytically by the walls of the quartz reactor. However, underprediction of the reburn efficiency of CO/H2 appears to be a general feature of the mechanism, as seen in the following. Figures 3-5 show results of CO reburn in the premixed flow reactor. In these experiments, reactant levels representative of practical applications of reburning were used and temperatures ranged as high as 1800 K. Figure 3 shows the effect of inlet CO level (0.5-20%) and temperature (1473-1773 K) on NO and CO at conditions with 2% O2. Consistent with the results of Figure 2 the reduction of NO by CO is seen to increase with temperature, again ranging roughly from 0 to 30%. The modeling predictions are seen to be in qualitative

Glarborg et al.

Figure 4. Reduction of NO by CO in the premixed flow reactor. Comparison between experimental data (symbols) and model predictions (lines) for NO and CO, as a function temperature. The inlet conditions are shown as sets 15 and 17 in Table 2.

Figure 5. Reduction of NO by CO in the premixed flow reactor. Comparison between experimental data (symbols) and model predictions (lines) for NO and CO, as a function of temperature. The inlet conditions are shown as sets 16 and 18 in Table 2.

agreement with the flow reactor results, but underestimate the removal of NO at higher temperatures.



Figures 4 and 5 compare experimental data and modeling predictions for two levels of CO (5 and 20%) and two levels of O2 (1%, Figure 4; and 5%, Figure 5) in the same temperature range. Reduction levels for NO range up to about 25%, again increasing slightly with fuel/oxygen level and with temperature. Modeling predictions generally underpredict the reduction of NO observed experimentally, but correctly reflect the dependence of stoichiometry and temperature. In summary, the flow reactor results indicate a NO reduction potential of hydrogen and carbon monoxide of up to 30%, increasing slightly with fuel/air ratio and with temperature. The model correctly identifies the effect of reaction conditions on the NO reduction, but generally underpredicts the level of reduction achieved, in particular at higher temperatures. The reduction of NO proceeds largely through the sequence

H + NO + M h HNO + M HNO + H h NH + OH

(27) (-93)

followed by reaction of NH with NO (reactions 98 and 99). Even though the rate of reaction -93, as discussed above, is subject to uncertainty, it is not possible by increasing this value to improve modeling predictions at high temperatures without hampering the agreement at lower temperatures. Simulation of Bench and Pilot Scale Results. While there is a significant amount of pilot scale data reported on reburning with hydrocarbon fuels, mainly natural gas, data on reburning with CO or H2 are scarce. Chen and co-workers6,7 have performed bench scale experiments in a 25 kW furnace, using propane or natural gas as the primary fuel. They have reported reburn results for H2 over a range of stoichiometries6 and data for CO reburn for one single condition.7 Bortz and Offen8 performed pilot scale experiments on reburning in a 2 MW test furnace to assess the potential of coal gasification gases with low amounts of hydrocarbons as reburning fuels. Figure 6 shows comparison between the bench scale results of Chen and co-workers6,7 for reburning with CO and H2 and our modeling predictions. The experimental data, carried out at a reburn fuel injection temperature of 1673 K, indicate that both hydrogen and carbon monoxide have a considerable potential for reducing NO. The data for H2 show that the NO removal increases monotonically with reburn fuel fraction under the conditions investigated. This is in contrast to observations for hydrocarbon reburn fuels,6 but in agreement with our model predictions. The data show that the reburn potential of H2 exceeds 50% at this high temperature, but it requires a high reburn fuel fraction. Our modeling predictions indicate that the results of the bench scale experiments are quite sensitive to mixing of the reburn jet with the bulk flow. Calculations were performed with assumed mixing times of 10 ms (dashed lines, Figure 6) and 100 ms (solid lines, Figure 6). Predictions with the longer mixing time show a significantly enhanced NO reduction compared to the fast mixing calculations. Unfortunately, no experimental information is available on mixing times in the bench scale facility. Mixing of the reburn zone effluents and the burn out air was not found to be important, contrary

Figure 6. Bench scale results6,7 for reburning with H2 (upper) and CO (lower) as function of stoichiometry (excess air ratio). Model predictions (present work) are performed with mixing times of 10 (dashed lines) and 100 ms (solid lines). Upper figure: Primary fuel, C3H8; reburn fuel, H2; primary zone stoichiometry, SR1) 1.1; reburn zone stoichiometry, SR2 variable; burn out zone stoichiometry, SR3) 1.1; reburn fuel injection temperature, T2i ) 1673 K; reburn zone residence time, τ2) 0.4 s; inlet NO level, NOprim ) 630 ppm. Lower figure: primary fuel, natural gas; reburn fuel, CO; SR1) 1.1; SR2 variable; SR3 ) 1.1; T2i ) 1673 K; τ2 ) 0.4 s; NOprim ) 240 ppm.

to reburning with hydrocarbons.35 If the flue gas at the rich-lean transition contains significant amounts of cyanides and amines, the burn out air mixing may affect the selectivity of these species for forming NO or N2 in the oxidation.35,36 However, when the reburn fuel does not contain hydrocarbons, as here, the amounts of HCN and NH3 entering the burn out zone are negligible and the mixing process is less important. Since the primary NO levels were different in the two studies (630 and 240 ppm, respectively), it is not possible directly to compare the efficiency of CO and H2, but our modeling predictions indicate that they are similar. Figure 7 shows comparison between the pilot scale results of Bortz and Offen8 on reburning with a low calorific value gas and natural gas, respectively, and our calculations. A mixing time of 100 ms was assumed in the calculations. The low calorific gas, consisting of CO/ H2 with some CO2 and N2, is seen to be less efficient than natural gas as the reburn fuel, with an NO reduction using a reburn fuel fraction of 20 of about 45%, compared to more than 70% for natural gas. Some of the reduction observed is due to dilution; the NO reduction caused by chemical reaction of the low calorific gas is about 20-30%. The modeling predictions are in good agreement with the pilot scale results for natural gas reburning, but underestimate the reburn efficiency of the low calorific

836


Figure 7. Pilot scale results8 for reburning with a low calorific gas (upper) and natural gas (lower) as a function of stoichiometry (excess air ratio). Model predictions (present work) are performed with an assumed mixing time of 100 ms for 90% mixing: primary fuel, Hallmark coal (C, 73.9%; H, 5.2%; O, 8.2%; S, 1.7%; N, 3.5%; ash, 7.5% (all wt %)); reburn fuel, synthetic low calorific gas (CO, 16%; H2, 25%; CO2, 14%; N2, 45%; (all vol %)) or natural gas (here assumed to be CH4); primary zone stoichiometry, SR1 ) 1.1; reburn zone stoichiometry, SR2 variable; burn out zone stoichiometry SR3 ) 1.2; reburn fuel injection temperature, 1590 K; temperature gradient, -278 K/s; reburn zone residence time, 0.5 s; inlet NO level (corrected to 3% O2), 750 ppm.

value gas. This difference is similar to that seen for the laboratory experiments and may be attributed to inadequacies in the reaction mechanism. However, since mixing affects the predicted NO removal, it may also be influenced by underestimation of the mixing time or deficiencies of the mixing model. Implications for Reburning with Low Hydrocarbon Content Fuels. The results presented in this work show that both CO and H2 have a certain potential for NO reduction under reburn conditions. This is interesting in relation to the use of reburn fuels with a low hydrocarbon content, as is the case of gasification gases, e.g. from biomass fuels. Biomass is becoming an increasingly important fuel in the European Union and other countries, because of its low emission generation and in particular because it is considered CO2 neutral. Biomass can be burned directly or can be processed in a number of ways, for instance by pyrolysis or gasification. Biomass pyrolysis results in a comparatively low amount of gases, accompanied by a significant fraction of tars, while gasification processes yield a high amount of gases, primarily CO and H2, with minor amounts of hydrocarbons. The exact composition of the gasification gas is very dependent on the operating conditions used, i.e., type of biomass, type of gasifier, gasifying agent, etc.

Glarborg et al.

Figure 8. Modeling results of the effect of hydrocarbon content in the reburn gas for an excess air ratio of 0.7.

In this work, we have simulated the use of an idealized biomass gasification gas as reburn fuel, choosing a basis gas with 50% CO and 50% H2. To this gas we have then added various amounts of hydrocarbons in the form of methane. The presence of other components, such as tar, fuel nitrogen, or alkali metals, was not considered even though these may conceivably affect the reburn chemistry. Gasification gases typically contain small amounts of hydrocarbons; a gas with 3% methane was chosen as representative. Since the NO removal potential of gasification gases may be insufficient due to their low hydrocarbon content, the possibility of combining this gas with for instance natural gas is interesting. To simulate such blends, we have made additional predictions for methane contents in the reburn fuel of 10 and 25%, respectively. Calculations were conducted following the procedure of Alzueta et al.35 who evaluated the potential of lowtemperature natural gas reburning. Results were obtained for both the reburn and burn out zones, with temperatures in the reburn zone between 1300 and 1800 K, and 100 K lower in the burn out zone. A primary stoichiometry of SR1 ) 1.1 and a primary NO concentration of 500 ppm were chosen, together with stoichiometries in the reburn zone of 0.7 and 0.9 and a value of 1.2 for the burn out zone stoichiometry. Residence times in both the reburn and burn out zone were 500 ms. Mixing in the reburn zone was described by the Zwietering37 approach, as outlined above, while mixing of the burn out air with the bulk gas coming from the reburn zone was assumed to occur during 20 ms, by adding equal amounts of burnout air each 2 ms, i.e., 10 times. Figures 8 and 9 show the modeling predictions for NO versus reburn temperature at the outlet of both the reburn and burn out zones for two different stoichiom-


Figure 9. Modeling results of the effect of hydrocarbon content in the reburn gas for an excess air ratio of 0.9.

etries. The concentrations of NO at the outlet of the reburn zone were corrected for the effect of dilution due to burn out air, to show the calculated results corresponding only to chemical reaction. Some interesting observations can be made from the results. The process potential seems to depend significantly on the hydrocarbon content of the reburn gas, but the impact of the hydrocarbons varies with reaction conditions. Under typical reburn conditions with an excess air ratio in the reburn zone of 0.9 (Figure 9), the NO removal generally increases with the hydrocarbon concentration. The exception is at lower temperatures, where the calculations indicate the existence of an optimum level of hydrocarbons in the reburn gas. At the more fuel-rich conditions of Figure 8, a low hydrocarbon content enhances the NO reduction. This is consistent with the general observation that hydrocarbon based reburn fuels yield an optimum NO reduction at a stoichiometry of about 0.9, while the efficiency of non-hydrocarbon fuels increases with the fuel/air ratio. The predicted effect of temperature on the overall NO reduction efficiency is fairly small for the low HC gases, but the model most likely underestimates this effect. For the reburn fuels with a higher hydrocarbon content, the effect of temperature is quite significant, in agreement with results for natural gas reburning.35 The reburn zone stoichiometry SR2 and the reburn fuel composition affects significantly the product distribution at the rich-lean transition, changing the yields of NO, HCN, NH3, and N2. The results show that at the outlet of the reburn zone, NO conversions ranging from 30 to almost 100% can be attained, depending on the reburn fuel composition and the reaction temperature. In general, at the outlet of the reburn zone, the NO concentration decreases as the amount of methane in the reburn gas is increased. However, the influence


of the CH4 level is different for the two stoichiometries. For the leaner conditions, i.e., SR2 ) 0.9, NO decreases monotonically with the level of methane, while at the richer stoichiometry little difference is seen for levels of CH4 of 10 and 25%. In both cases, as the methane level is increased, NO is mainly converted to HCN and NH3. For simplicity those products have not been shown in the figures. It is notable that the minimum NO concentration at the outlet of the reburn zone is seen to occur at lower temperatures, approximately at 1400 K, for the leanest stoichiometry, while it occurs at temperatures of 1600 K and higher for a reburn stoichiometry of 0.7. This is closely related to the oxidation regime of methane, which is shifted to lower temperatures as the oxygen availability for reaction becomes higher. The importance of the addition of burn out air increases with the hydrocarbon content of the reburn fuel and with the fuel/air ratio. For high levels of CH4 in the reburn gas, considerable amounts of both HCN and NH3 are formed, a high fraction of which is recycled to NO at the rich-lean transition. For this reason, no net reduction of NO is observed and only at very high temperatures is the reduction process effective. For the leaner conditions in the reburn zone, the impact of burn out air addition is comparatively lower, but the NO reduction process is overall more efficient. Very similar NO profiles are seen at the outlet for the reburn gas composition with no methane and with a percentage of 3% CH4. Since in those cases there is little or no nitrogen intermediate species, the NO remains basically at the same level after the addition of air. However, as the presence of methane is increased, the amount of HCN and NH3 formed increases as well, and the fate of these components after the addition of burnout air becomes important. It is interesting to note that the minimum NO concentration is obtained at different temperatures for the two levels of methane in the reburn gas (10 and 25%). The increased effectiveness of the high CH4 content reburn fuel with temperature is attributed to the fact that some unreacted methane from the reburn zone is still present and active when the burnout air is added. Conclusions The ability of non-hydrocarbon fuels such as CO and H2 to reduce nitric oxide under conditions relevant for the reburning process has been investigated experimentally and theoretically. Flow reactor experiments on reduction of NO by CO and H2 were conducted under fuel-rich conditions, covering temperatures of 12001800 K and a range of stoichiometries and reactant levels. Bench and pilot scale results from literature on reburning with CO, H2, and low calorific value gases were also considered. The experimental data have been interpreted in terms of a detailed reaction mechanism, and the reactions responsible for removal of NO were identified. The results show that these non-hydrocarbon fuels may remove 20-30% of the NO entering the reburn zone. The process potential increases with temperature and reburn fuel fraction, and at high temperatures and high reburn fuel fractions, the reduction efficiency approaches that of hydrocarbon gases. Dilution effects

838


and the lowering of the primary zone NO (maintaining the overall load) further reduce the NO emission. The modeling predictions are in qualitative agreement with the experimental results, but tend to underestimate the reduction of NO. Conversion of NO to N2 proceeds primarily through the sequence H + NO h HNO h NH f N2. The calculations indicate that the mixing process may affect the NO reduction in the reducing zone. A parametric study of the effect of hydrocarbon content of the reburn gas indicates that the optimum reburn fuel composition depends on reaction conditions. The results have implications for reburning with fuels based on gasification.

Glarborg et al.

Acknowledgment. The work is part of the research programs of CHEC (Combustion and Harmful Emissions Control), which is co-funded by the Danish Technical Research Council, Elsam (the Jutland-Funen Electricity Consortium), Elkraft (the Zealand Electricity Consortium) and the Danish Ministry of Energy; and of the Department of Chemical and Environmental Engineering of the University of Zaragoza (financial support from CICYT, Project QUI97-1112, is acknowledged). EF990186R

Nitric Oxide Reduction by Non-hydrocarbon Fuels. Implications for

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