Conversion of Fuel-Nitrogen in the Primary Zones of Pulverized Coal

Energy & Fuels 1996, 10, 463-473

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Conversion of Fuel-Nitrogen in the Primary Zones of Pulverized Coal Flames Stephen Niksa*,† and Sunghwan Cho Molecular Physics Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, California 94025 Received June 19, 1995. Revised Manuscript Received November 10, 1995X

Complete distributions of all major products and nitrogen species are reported for the oxidative pyrolysis and combustion of premixed suspensions of subbituminous, Pittsburgh No. 8 hvA bituminous, and a low-volatile bituminous coal after 150 ms. As inlet O2 levels were progressively increased from 0 to 15% in successive tests, the process chemistry moved through oxidative volatiles pyrolysis, volatiles combustion, soot combustion, and char oxidation. However, the different fuel components were consumed sequentially only with the low-volatility coal. Char, soot, and noncondensible fuels burned simultaneously with the subbituminous coal and, to a lesser degree, with the Pittsburgh No. 8. Consequently, hydrocarbon gases from these coals, particularly CH4 and C2H2, were present while most of the char and its residual fuel-N was converted into gases. With all coals, the persistence of gaseous hydrocarbons profoundly affects the conversion of all fuel-N species in the gas phase. As long as detectable amounts of CH4 and C2H2 are present, NO is absent with all coal types and total fixed nitrogen (TFN ) HCN + NH3 + NO) is composed of only HCN with both bituminous coals, plus appreciable amounts of NH3 during intermediate stages with the low-rank coal. But after the hydrocarbons are consumed, HCN and NH3 vanish and most of the remaining char-N and soot-N are converted into NO, so TFN is composed of only NO. The extent of carbon conversion up to the NO inception point is the primary coal rank index for early NO production in coal flames, falling from 38% with the subbituminous to 12% with the low-volatility coal.

Introduction Pulverized coal flames in utility boilers generate NOx primarily by converting the nitrogen in coals’ organic components because boiler operators effectively inhibit thermal NOx production by regulating flame temperatures. In the primary zones of coal flames, fuel nitrogen is first liberated during primary devolatilization as an element in heavy, aromatic compounds collectively called tar.1 Additional fuel nitrogen is expelled from char as HCN (and occasionally NH3) on time scales that are considerably longer than those for tar evolution. At the same time, the volatiles undergo secondary and oxidative pyrolysis in hot, fuel-rich gases that convert most of the nitrogen in tar into HCN.2,3 The remainder is incorporated into soot along with the aromatic components of tar molecules,4,5 counteracting the devolatilization of fuel nitrogen in the primary devolatilization stage. Eventually oxygen contacts the char and soot, liberating additional nitrogen either by direct chemical conversion to NO or by thermal dissociations induced by the higher particle temperatures associated with char combustion. †

FAX: (415) 859-6196. E-mail: [email protected]. Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Freihaut, J. D.; Zabielski, M. F.; Seery, D. J. Symp. (Int.) Combust., [Proc.] 23 1990, 1265. (2) Bruinsma, O. S. L.; Geertsma, R. S.; Bank, P.; Moulijn, J. A. Fuel 1988, 67, 334. (3) Nelson, P. F.; Buckley, A. N.; Kelley, M. D. Symp. (Int.) Combust., [Proc.] 23 1990, 1265. (4) Chen, J. C.; Niksa, S. Symp. (Int.) Combust., [Proc.] 23 1990, 1265. (5) Chen, J. C.; Castagnoli, C.; Niksa, S. Energy Fuels 1992, 6, 264. X

0887-0624/96/2510-0463$12.00/0

NOx forms in two channels, through conversion of HCN via gas phase chemistry and through conversion of the nitrogen in char and soot via heterogeneous chemistry. Interactions among the species in these channels are extremely important because high concentrations of hydrocarbon radical species and relatively long reaction times at modest temperatures bias the gas phase chemical mechanisms toward the production of N2 from both HCN and NO.6 Even though the nitrogen in char and soot may be expelled as NO, the NO can be subsequently reduced to N2 if hydrocarbon radicals are present, as in the rich primary flame zones from low NOx burners or in reburning schemes. Consequently, NOx levels in the exhaust represent the impact of chemistry in primary flame zones in conjunction with the much slower reduction of NO by amines in postflame zones and, perhaps, additional oxidation of fuel-N species by overfire air in staged systems. The latter two processes are discussed, for example, by Wendt et al.7 and Chen et al.8 but are not relevant here. Whereas all three chemical stages require reaction times from 1 to 2 s, we are focusing on chemistry in the first stage that is finished in 50-200 ms. The experiments described in this paper characterize the most important connections among the first conversion channels for fuel combustion and fuel-N conversion, ultimately to identify the most effective NOx reduction agents in primary zones for different coal types. Insofar (6) Wendt, J. O. L. Prog. Energy Combust. Sci. 1980, 6, 201. (7) Bose, A. C.; Dannecker, K. M.; Wendt, J. O. L. Energy Fuels 1988, 2, 301. (8) Chen, S. L.; Cole, J. A.; Heap, M. P.; Kramlich, J. C.; McCarthy, J. M.; Pershing, D. W. Symp. (Int.) Combust., [Proc.] 22 1988, 1135.

© 1996 American Chemical Society

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as the same laboratory system used previously to track the conversion of primary devolatilization products into secondary pyrolysis products5 is used again, we are also in a position to determine if oxygen alters secondary volatiles pyrolysis and if, indeed, the fuel components that burn in coal flames are secondary pyrolysis products, rather than primary devolatilization products. Only a handful of reported studies resolve the progress of flame zone chemistry in flow fields that are simple enough for definitive interpretations.9-18 Yet even within this compact database, there are glaring inconsistencies in the time evolution of the fuel-nitrogen species (whether NO appears before, with, or after HCN for example), and little information about its relation to the hydrocarbon conversion channels. Only four groups acquired the information that relates NO production to the release of both nitrogen and carbon and/ or mass from the fuel suspension, two with relatively dense coal suspensions9-11,14 and two with dilutions to single-particle levels.16,17 These results are not directly comparable because conversion efficiencies of fuel-N into NO with individual particles can achieve 100%, which does not represent pulverized coal suspensions.13,16 Furthermore, the chemical transformations in the primary zones of coal flames are extremely difficult to monitor. Individual coal particles of the mean size in the pulverized fuel size grade ignite, burn out their volatiles, and achieve quasi-steady char oxidation in less than 10 ms. These reaction times to quasi-steady char oxidation are proportional to the square of the particle size,19 so they must be broadly distributed for any pulverized coal suspension. Consequently, burners that operate with realistic coal loadings in one-dimensional flow fields preserve the nominal relationship between distance from the injection point and reaction time, but only within the uncertainties from the inevitable size distribution and the d2 scaling. They must also contend with narrow stability domains that confine their operating ranges of O2 levels to high values. Flat flame burners circumvent the radial gradients and/or entrainment issues in round coal jets, but not these inherent limitations on time-resolved species monitoring. Our strategy to circumvent this limitation is to stabilize a coal burner with an intense external radiant field so that it operates with any inlet O2 level, including none at all in cases that determine the distributions of secondary pyrolysis products. At any particular operating condition, O2 depletion eventually “quenches” the chemistry at an intermediate stage determined by the proportions of coal and O2 at the inlet, especially since (9) Altenkirch, R. A.; Peck, R. E.; Chen, S. L. Combust. Sci. Technol. 1979, 20, 49. (10) Peck, R. E.; Midkiff, K. C.; Altenkirch, R. A. Symp. (Int.) Combust., [Proc.] 20 1984, 1373. (11) Midkiff, K. C.; Altenkirch, R. A. Symp. (Int.) Combust., [Proc.] 21 1986, 1289. (12) Beck, N. C.; Hayhurst, A. N. Combust. Flame 1991, 87, 306. (13) Kramlich, J. C.; Seeker, W. R.; Samuelsen, G. S. Fuel 1988, 67, 1182. (14) Phong-Anant, D.; Wibberley, L. J.; Wall, T. F. Combust. Flame 1985, 62, 21. (15) Okazaki, K.; Shishido, H.; Nishikawa, T.; Ohtake, K. Symp. (Int.) Combust., [Proc.] 20 1984, 1381. (16) Haussmann, G. J.; Kruger, C. H. Symp. (Int.) Combust., [Proc.] 23 1990, 1265. (17) Pohl, J. H.; Sarofim, A. F. Symp. (Int.) Combust., [Proc.] 16 1976, 491. (18) Ghani, M. U.; Wendt, J. O. L. Symp. (Int.) Combust., [Proc.] 23 1990, 1281. (19) Lau, C.-W.; Niksa, S. Combust. Flame 1993, 95, 1.

Niksa and Cho

NO production on short time scales requires an abundance of O or OH radicals.20 Inlet O2 levels are progressively increased in successive cases to move the process chemistry through oxidative volatiles pyrolysis, volatiles combustion, soot combustion, and char oxidation. The order of fuel consumption for a given coal will be apparent in the distributions of all major products, including condensed phase species, and its relation to NO production will be evident in the N-species distributions. Following the description of the test facility and procedures in the next section, detailed product distributions of all fuel and N-species are reported for subbituminous, hv bituminous, and lv bituminous coals. These data are combined with all comparable cases in the literature to illustrate the main impact of coal rank on the initial stages of NO production in coal flames: Coals of progressively lower rank achieve much higher extents of carbon burnout before any NO appears, even though the ultimate NO conversion efficiencies after all carbon is consumed may be similar with all coal types. Experimental Section This experiment is intended to simulate the thermal and chemical environments in the primary zone of a pulverized coal flame without the complications of two-phase turbulent mixing. The coal burner is a one-dimensional flow reactor that imposes uniform radial heat fluxes comparable to those in utility burners. As suspensions move along the flow tube, they rapidly heat to the onset temperature for primary devolatilization and/or particle ignition, release their volatiles, and burn. Residence times and suspension loadings were the same in all cases and the extents of combustion were regulated by adding progressively more O2 to the entrainment stream. Residence times in the furnace were only about 150 ms so particles probably never achieved a quasi-steady temperature. Figure 1 shows the schematic diagram of the laboratory facility. As described by Chen and Niksa,21 the radiant coal flow experiment entrains coal particles through a quartz tube surrounded by an inductively heated graphite cylinder. Heat fluxes into the suspension are comparable to estimates for pulverized coal burners, and nominal particle heating rates exceed 104 K/s for the 90 µm particles used in these experiments. This furnace can be operated to keep temperatures of the entrainment gas cooler than the threshold for secondary hydrocarbon cracking, so that the primary products can be quenched as soon as they are expelled from particles.21,22 However, in all cases reported here, the entrainment streams were definitely hot enough to promote secondary pyrolysis, oxidative pyrolysis, and combustion of volatiles in the furnace. At the top of the system, a coal feeder generates a 1 cm particle suspension in 99.999% Ar on the axis of a 2 cm quartz tube on the centerline of the graphite furnace. An annular coflow stabilizes the suspension and eliminates deposition on the walls. The suspension loading in the core flow at the inlet was 500 particles/cm3 in all cases, corresponding to a nominal mass loading of 0.15 g of coal/g of gas (or 270 mg/L). The furnace hot zone was 12.5 cm in length and operated at 17001720 K for all cases. Wall temperatures were monitored with a disappearing filament pyrometer to an uncertainty of 10 K. The presence of O2 in the flow system raises the stream temperature as it sustains the heat release due to combustion and also accelerates the rates of volatiles pyrolysis. Preliminary runs established that a furnace temperature of 1700 K (20) Peck, R. E.; Glarborg, P.; Johnsson, J. E. Combust. Sci. Technol. 1991, 76, 81. (21) Chen, J. C.; Niksa, S. Rev. Sci. Instrum. 1992, 63 (3), 2073. (22) Chen, J. C.; Niksa, S. Energy Fuels 1992, 6, 254.

Conversion of Fuel-Nitrogen in Pulverized Coal Flames

Energy & Fuels, Vol. 10, No. 2, 1996 465

Figure 1. Radiant coal flow experiment. Table 1. Proximate and Ultimate Analyses coal type

ash (dry wt %)

C (wt % daf)

H (wt % daf)

N (wt % daf)

S (wt % daf)

Oa (wt % daf)

DECS 9 (Dietz, subbituminous B) DECS 12 (Pittsburgh No. 8 hvA bituminous) PSOC-1516 (Lower Kittanin lv bituminous)

6.4 10.3 10.3

75.5 83.3 88.8

5.2 5.4 4.7

0.9 1.4 1.6

0.4 1.3 1.6

17.9 8.4 3.3

a

By difference.

was sufficient to ignite the suspension or, at least, to sustain oxidation at low O2 levels in the available residence time. All runs were made with an entrainment velocity of 18 cm/s at the inlet, for which the measured residence time is 160 ms when no O2 is present. However, cases with O2 operate at hotter temperatures, so residence times decrease slightly with increasing O2 levels due to the diminishing gas densities. Oxygen (99.999%) was added to both the coal particle entrainment stream and to its surrounding sheath flow at the same concentration to create a premixed combustible suspension. A quench nozzle at the furnace outlet blasts argon into the process stream, rapidly quenching all chemistry and nucleating tar into an aerosol. Nominal quenching rates range from 5000 to 10 000 K/s. From the quench nozzle into the product recovery station, the flow tube wall is transpired to inhibit deposition of tar and soot aerosol. Even with transpiration at twice the levels that were effective in earlier primary and secondary pyrolysis experiments, solids deposition was a problem during runs with low O2 levels, as evident from the breached elemental balances reported below. Products are segregated into bulk char samples, tar plus soot aerosol, and noncondensible gases with virtual impaction in an aerodynamic classifier.21 This device creates a virtual impaction surface between two nozzles at the inlet by diverting 95% of the flow radially off the axis. Since the aerosol particles are only a few micrometers in size, they are convected into the annulus and ultimately collected on four stages of glass filters. By virtue of their inertia, char particles penetrate the impaction surface and fall into a wire mesh basket. Pure tar samples for subsequent chemical analyses are prepared by extracting the glass filters with tetrahydrofuran (THF) in an ultrasonic bath, followed by filtration through a 0.2 µm Teflon membrane. The amount of soot is assigned as the membrane residue. The tar solution is concentrated in a Kuderna-Danish concentrator before the remaining solvent is evaporated. Carbon, hydrogen, and nitrogen contents of condensed products are determined with an elemental analyzer calibrated with acetanilide. At the very high combustion temperatures in this furnace with high O2 levels, substantial portions of the ash would be released from the char particles. In lieu of direct determinations of ash loss from individual char samples, we adjusted all mass loss data to a basis of 50% ash depletion, which is surely too high for low O2 levels but is realistic for the combustion cases.

Noncondensible gases are sampled through sidearms on the aerodynamic classifier. Concentrations of CO, CO2, H2O, NH3, NO, and NO2 were monitored on-line with FTIR spectroscopy through a 6.5 m multipass gas cell at 335 K. A side arm in the annulus of the classifier drew gases and aerosol into a tar/ soot filter and then into a heated line that fills the multipath gas sample cell in the FTIR. Preliminary runs established that water condensation on the transfer lines and all species concentrations stabilize within 1 min of sampling. Hence, FTIR scans were acquired after 65 s of operation. Preliminary calibration runs with synthetic gas mixtures established that a correction factor of 38% should be applied to the measured H2O concentrations to account for water condensation. Gas samples were also extracted into multiport sampling valves at 335 K through a port in the classifier for subsequent chromatography. The concentrations of H2 and O2 were determined with gas chromatography on HayeSep into a thermal conductivity cell. The concentrations of HCN and all other hydrocarbons (CH4, C2H2, C2H4, C2H6, C3H6, C3H8, C4’s) were determined with gas chromatography on HayeSep into a flame ionization detector (FID). The yields of oils are based on the FID signal from all hydrocarbons minus the amounts of C1-C3 hydrocarbons. Due to the severe furnace conditions, C2H2, CH4, and oils were the only hydrocarbons present in substantial amounts even when no O2 was present. Three coals were tested. As seen in the coal properties collected in Table 1, the subbituminous, Pittsburgh No. 8 hvA bituminous, and lv bituminous coals in these tests represent major segments of the coal rank spectrum. Coals were prepared by grinding under a dry N2 atmosphere and then sieved with a RoTap into the 75-106 µm range. Size distributions were then improved by wet sedimentation. Before each test the coals were dried overnight under 15 kPa of N2. The overall performance of the product recovery and analysis train is evident in the elemental closures in Table 2. These C/H/N balances are based on the measured yields in individual runs of char, soot, tar, CO2, CO, O2, H2, H2O, NO, HCN, NH3, CH4, C2H2, and oils and the C/H/N contents of char and the aerosol product. Carbon balances are closed to within 5% in 9 of the 14 cases. Soot deposition is probably responsible for breaches in both tests without O2, and an erroneous CO2 measurement is probably responsible for the discrepancy at the highest O2 levels with the subbituminous coal. H balances close to within 5% in only 6 of 14 cases. Our syringe injection

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Niksa and Cho

Table 2. Elemental Closures for Individual Experiments coal

% O2

∑fiC

∑fiH

∑fiN

Dietz subbit

0.0 2.0 4.8 9.9 14.6 2.9 5.6 9.1 14.6 0.0 1.9 4.2 9.1 14.6

89.3 105.0 103.8 101.2 86.2 99.2 100.2 87.5 92.9 91.4 96.4 96.5 96.2 98.7

89.5 99.5 100.8 109.0 94.2 95.1 106.4 96.3 96.7 99.2 92.9 102.6 109.2 107.2

95.3 104.3 89.6 46.8 35.6 116.3 90.9 52.4 46.6 82.5 110.8 76.4 65.4 70.9

Pittsburgh No. 8

Lower Kittaning lvb

Table 3. O2 Concentrations and Carbon Conversions in These Experiments for Combustion of Noncondensible Gasesa and Tar-Derived Productsb O2 requirement, %

carbon conversion, %

coal

gas only

gas + tar-derived products

gas only

gas + tar-derived products

Dietz subb Dietz subb IIc Pittsburgh No. 8c Pittsburgh No. 8 Lower Kittaning

4.0 4.9 4.9 4.5 3.7

7.9 9.0 12.5 9.4 8.8

14.6 20.9 12.6 12.2 5.1

47.1 46.0 52.6 44.0 31.8

a Noncondensibles comprise CH , C H , oils, C H , CO, and H 4 2 2 2 4 2 for nearly complete secondary volatiles pyrolysis. b Tar-derived products comprise soot, tar, and deficits in the C and H balances. c Based on product distribution reported previously by Chen, Casiagnoli, and Niksa.5

procedure in the chromatography of H2 is probably responsible for the breaches at low O2 levels, but only uncertainties in the H2O determination can be responsible for the discrepancies at the highest O2 levels. Closures in the N balances are satisfactory for the cases with little or no O2 but not during combustion because the substantial N2 yields for these cases could not be detected.

Results The O2 requirements and carbon conversions in Table 3 are reference points to apply to the observed product distributions to see if, in actuality, the diverse fuel components in burning premixed coal suspensions are consumed sequentially, beginning with noncondensible fuel compounds, then soot, then char. Two O2 requirements are tabulated, one based on conversion of only the noncondensible fuel compounds into CO2 and H2O and the other on all volatile-derived fuel species. These values are for conversion of secondary pyrolysis products at the specific loading and entrainment flow rate in these experiments. Due to the severe thermal conditions in this furnace, secondary volatiles pyrolysis of all three coals is nearly complete even when no O2 is present. Under these circumstances, noncondensible fuels comprise only CH4, C2H2, oils, CO, and H2. Volatile-derived fuels comprise these noncondensibles plus soot and small amounts of residual tar. These same species are accounted for in the respective carbon conversions in Table 3 along with the diluent species, CO2. Two cases in Table 3 are based on the observed product distributions in this test series with no O2, one each for the subbituminous and low-volatility coal. In these cases, the O2 requirements for volatile-derived fuel

also account for the breaches in the C and H balances in Table 2, assuming that some aerosol volatile matter, not char, was not collected. The cases for the other subbituminous coal and the Pittsburgh No. 8 in Table 3 are based on earlier determinations of the products of primary and secondary pyrolysis for similar coals.5,22 These product distributions have better C/H/N balance closures (3 wt %), and also include the yields and elemental compositions of tar, soot, and char. The subbituminous coal used previously contains more oxygen (24.1%) and less carbon (69.5%) than the subbituminous coal in Table 1, and the tar yield from the Pittsburgh No. 8 used previously is one-third larger than with the coal in Table 1, so samples of the same nominal rank are not directly comparable. Generally speaking, the O2 requirements for noncondensible fuels in Table 3 show the expected rank dependence, passing through a broad maximum for the high-volatile bituminous rank and diminishing by 2050% for lignites and brown coals and by 25% for lowvolatility coals.19 This tendency is due to two compensating factors, diminishing gas yields versus higher fuel quality from coals of progressively higher rank. Total gas yields diminish monotonically for coals of higher rank, falling from 35 wt % for lignites to less than 10 wt % for low-volatility coals.23 But most of this decrease is due to the diminishing yields of the diluents, CO2 and H2O. Also, the contributions from CO fall in tandem with increasing contributions from CH4 and H2 for coals of higher rank, which compensates somewhat for the diminishing gas yields. A more pronounced rank dependence is apparent in the volatiles-derived O2 requirements, which exhibit the tendency in tar yields to pass through a broad maximum for hv bituminous coals, then fall off sharply for low-volatility coals. Unlike the O2 requirements, the carbon conversions for noncondensibles are affected by the abundance of CO2 from low-rank coals. That is why the value for noncondensibles from the subbituminous coal is 3-4 times greater than for the low-volatility coal. Volatilesderived carbon conversions, however, are dominated by contributions for soot/tar, so they also exhibit the broad maximum evident in the analogous O2 requirements. The stoichiometries and conversion indices in Table 3 should be compared to the observed carbon conversions in Figure 2. Here filled data points express the carbon conversion to volatile matter and combustion products based on the observed levels of soot plus tar, CH4, C2H2, oils, CO, and CO2. The smooth curves for the subbituminous and Pittsburgh No. 8 coals also account for the deficits in the C balances, assuming that they are due to unrecovered volatile matter. The O2 conversions in Figure 2 are utilization factors for the O2 fed into the furnace with the coal suspensions, not accounting for any coal-O. Volatiles combustion does not increase the carbon conversion, so char combustion must be responsible for observed carbon conversions greater than the thresholds in Table 3. For the subbituminous coal, devolatilization expels about 40-45 % C. This level is surpassed with 5.5-6.5% O2, which is well below the 8-9% O2 needed to completely burn the volatile matter. Volatiles combustion and char oxidation occur simultaneously with the subbituminous coal. For the Pittsburgh No. 8, the (23) Niksa, S. Energy Fuels 1994, 8, 659.


Figure 2. Extents of carbon conversion (filled symbols) and utilization factors of the O2 in the entrainment gas (open symbols) for three coals. For the carbon conversions for Pittsburgh No. 8 and the subbituminous coal, the lines also account for deficits in the carbon balances, assuming that incomplete recovery of volatile matter, not char, is responsible.

threshold carbon conversion of 45-55% is surpassed with 11-14.5% O2, which brackets the 12.5 O2 requirement for volatiles combustion. These carbon conversion data indicate that char oxidation follows volatiles combustion (which the forthcoming product distributions contradict). For the low-volatility coal, the observed carbon conversions never exceed the threshold value of 32%, so the impact of char oxidation on product distributions from this coal is expected to be minimal. The relative rates of carbon conversion for these three coals are consistent with their relative rates of char oxidation, which diminish for coals of progressively higher rank as their chars become more aromatic.24 The O2 utilization factors for the Pittsburgh No. 8 and lv bituminous in Figure 2 are also consistent with this tendency, but the relatively low values for the subbituminous coal at low O2 levels may also reflect the impact of significant dilution of the volatiles by CO2 and H2O. With all three coals, only 85% of the entrainment O2 is converted in these tests, regardless of O2 level, probably because the O2 fed into the furnace in the annular coflow around the coal suspension was not completely converted. Detailed product distributions will be reported next for each coal, beginning with Pittsburgh No. 8’s. In Figure 3, the distributions are broken down into condensed phase products, major combustion gases, hydrocarbons, and nitrogen species expressed as fractions of the coal-N. Changes in yields of the condensed phase products clearly resolve the sequence of fuel combustion in this multicomponent system. As the O2 level is increased from 3 to 14.6% with Pittsburgh No. 8, the weight loss increases from 53 to 72 wt %, due entirely to char combustion. The aerosol yields fall from 21 to 4 wt % over the same range, indicating that soot oxidation is incomplete. Whereas the carbon conversion data in Figure 2 suggested that the Pittsburgh No. 8 chars were unignited with O2 levels below 14.5%, the weight loss actually increases from 53 to 70% over this range. Clearly, the conversion time scales for char and soot are comparable, and the hypothesis that gas com(24) Hurt, R. H.; Mitchell, R. E. Symp. (Int.) Combust., [Proc.] 24 1992, 1243.


Figure 3. In clockwise order, distributions of the major solid products, hydrocarbons, nitrogen species, and the major gaseous products from the oxidative pyrolysis and combustion of Pittsburgh No. 8 coal in a 1700 K furnace at the indicated inlet O2 levels. The sum of the soot and tar yields (b) as well as resolved yields of soot (3) and tar (1) appear with the major products. Nitrogen levels are expressed as fractions of the coalN. Nominal residence times were fixed in all cases at roughly 150 ms.

bustion precedes soot/tar combustion which precedes char combustion does not apply under these conditions. The breakdown of the condensed products in Figure 3 also indicates that tar does not survive even modest oxidative pyrolysis conditions, although the soot and tar yields were quantitatively resolved only for 6% O2. Even without any O2, the tar yield would have been minimal because this furnace temperature is hot enough to bring secondary pyrolysis nearly to completion, as demonstrated elsewhere with a similar coal.5 For the same reason, no oils were detected with this coal. The distributions of the major combustion products and hydrocarbons in Figure 3 indicate that combustion times for the gaseous volatiles are faster than those for the solid products, although even H2 and the hydrocarbons persist while substantial amounts of soot and char are burning away. Carbon dioxide yields increase monotonically with increasing O2 levels. For the first incremental change in O2 level, from 3 to 6%, almost 70% of the CO2 comes from soot combustion, with additional contributions of 15% each from char and C2H2 oxidation, and 2% from CH4 conversion. So char combustion is important even before the hydrocarbons are burned out. The yields of H2 and CO fall off sharply for O2 levels between 6 and 10%, levels that are much greater than the O2 requirement for combustion of the noncondensible fuels only. Whereas H2 is eliminated under the most oxidizing condition, CO persists at appreciable levels. The H2O yields seem to approach an asymptote because once the noncondensible fuels are consumed only the very small amounts of hydrogen in soot and char are available to produce H2O. Also, the tendency to maintain water gas shift equilibrium as the temperature increases can be satisfied by the compensating changes in the H2 and CO2 levels with little change in the H2O and CO levels. Hydrocarbons persist at much greater O2 levels than the O2 requirement for noncondensible fuels, even though additional hydrocarbons are not expelled from char or soot under these operating conditions. Levels

468


of most of the C2 and all C3 and C4 alkanes and olefins are negligible even for the case with 3% O2 as a consequence of the high extent of sooting for this furnace temperature. As seen in Figure 3, oils are absent at all O2 levels but CH4 and C2H2 are present in substantial amounts provided that the O2 level is less than 10%, which is roughly twice the nominal O2 requirement for noncondensibles combustion. The persistence of the hydrocarbons exerts a very significant impact on the conversion of the nitrogen species, presented in Figure 3 as a percentage distribution based on the nitrogen content of the whole coal. As the O2 level increases from 2 to 14.6%, char-N falls, reflecting the consumption of char by combustion. It may seem surprising that the reduction in the char-N fraction as O2 level is increased from 6 to 10% (from 45.1 to 36.1%) is virtually identical to the change in mass loss fraction (from 48.2 to 38.6 %) because we know that char-N is both burned away and released by thermal dissociations as HCN. The nitrogen content of the aerosol products, tar + soot, also falls continuously, starting from a relatively depleted mass loading due to the release of tar-N as HCN during sooting.4,5 For the aerosol also, the changes in mass loss and N-content are very similar. Evidently, the time scales for direct HCN production are too long to come into play in this firing condition, consistent with a recent model for fuel-N evolution from coal.25 HCN levels fall monotonically until this species is entirely consumed with 11% O2. Ammonia is never appreciable, even under the most fuel-rich operating conditions. Although the N balance is not closed (at 116% in Table 2) by the monitored N-species at 3% O2, the deficits in the N balances for the other operating conditions probably give a fairly accurate estimate of the N fraction as N2. These estimates rise from 9% at 6% O2 to 53% at 14.6%. The most interesting data in Figure 3 are the NO yields. Even after substantial portions of char-N have been converted in 10% O2, virtually no NO is formed. As O2 levels are increased from 3 to 10%, the coal-N fractions diminish by 8.1% each in the char-N and soot-N fractions, and by 50.2% in the HCN fraction. Yet the NO fraction increases by only 2.5% over this range. However, the NO conversion efficiency jumps dramatically for progressively higher O2 levels. As O2 levels are raised from 10 to 15%, there is nearly complete conversion of HCN, char-N, and soot-N to NO: The N-fraction of these three species falls by 17.8% while the N fraction as NO increases by 12%. Only 14% of the coal-N has been converted to NO with 14.6% O2 while two-thirds of the coal-N has been converted into gases. These product distributions show that NO conversion efficiencies remain very low as long as CH4 and C2H2 are present. But in more than 10% O2, no hydrocarbons are available to reduce the NO so all the available N-precursors are converted and persist as NO. It is also interesting to note that the H2 and CO that remain under even the most oxidizing conditions are not effective NO reductants on the short time scales imposed in these experiments (although they are effective in postflame gases on longer time scales.8) (25) Niksa, S. Energy Fuels 1995, 9, 467.

Niksa and Cho

Figure 4. In clockwise order, distributions of the major solid products, hydrocarbons, nitrogen species, and the major gaseous products from the oxidative pyrolysis and combustion of Dietz subbituminous coal at the conditions of Figure 3.

The distributions of hydrocarbons and nitrogen species from the Dietz subbituminous coal and the Lower Kittaning lv bituminous coal reaffirm the crucial interplay among hydrocarbons and NO production attenuated by much higher extents of char burnout with the subbituminous and much lower extents with the lowvolatility coal. Product distributions from the subbituminous coal appear in Figure 4, including the one for secondary volatiles pyrolysis (no O2). The observed weight loss for secondary pyrolysis is 60 daf wt %, which is probably too high by 2-4 wt % because of soot deposition. The aerosol yield is 16%, which is too low by the same amount. Tar yields are minimal. Oils, C2H2, and CH4 are the only appreciable hydrocarbon products, consistent with our previous data for secondary pyrolysis with a similar coal.5 Levels of most of the C2 and all C3 and C4 alkanes and olefins are negligible even for this case with no O2; the C2H4 yield for secondary pyrolysis of 0.2 wt % is the highest among these trace hydrocarbon species. Considering that the levels of all products decay smoothly at progressively higher O2 levels, except C2H2’s which is an intermediate in partial oxidations, our product distributions give no indications that O2 disrupts the course of secondary volatiles pyrolysis in any appreciable way. And the fuel components that sustain combustion are secondary products, not primary devolatilization products. Tar combustion appears to be inconsequential. As the O2 level is increased from 2 to 14.6%, the weight loss increases as the char burns from 56 to 85 wt % and soot yields are halved by combustion from 16 to 8 wt %. Tar yields are below 4 wt % at all O2 levels. They show no dependence on O2 level, except that they are completely eliminated in the most strongly oxidizing case. Even without any O2, the tar yields is minimal because this furnace temperature is hot enough to sustain nearly complete secondary pyrolysis. The combustion product distributions in Figure 4 show virtually the same levels of CO2, H2O, and H2 as the Pittsburgh No. 8’s at all conditions. However, nearly all the increase in CO2 as O2 levels are raised from 2 to 5% can be attributed to char oxidation, in contrast to the much smaller contribution from Pittsburgh No. 8 chars in this operating regime. Moreover,


Figure 5. In clockwise order, distributions of the major solid products, hydrocarbons, nitrogen species, and the major gaseous products from the oxidative pyrolysis and combustion of Lower Kittaning lv bituminous coal at the conditions in Figure 3.

the CO levels are at least twice as large with the subbituminous coal, quadrupling as O2 levels are increased from 0 to 5%. Only a small portion of this enhancement can be attributed to the relatively high CO yields from primary and secondary pyrolysis of this lower rank coal. Rather, CO levels surge during partial oxidation of the noncondensible fuel compounds and/or the carbonaceous fuel species. Although the oils are rapidly eliminated by addition of low levels of O2, CH4 and C2H2 persist with O2 levels that are double the nominal O2 requirement for noncondensibles combustion and at least as large as the requirement for complete volatiles combustion. The C2H2 yield passes through a maximum at about 2% O2 because it is an intermediate in the partial oxidation. As with the Pittsburgh No. 8, the persistence of the hydrocarbons exerts a very significant impact on the conversion of the nitrogen species. As the O2 level increases from 5 to 10 %, the coal-N fractions in char, soot, and HCN fall 50% while the N fraction in NO grows to only 9%. In the incremental change from 10 to 14.6% O2, however, the enhanced N fraction in NO accounts for half the reductions among the other species because no hydrocarbons are present to reduce NO. The NH3 level is never substantial in terms of coal-N fraction, but NH3 is a noticeable intermediate N species in the gas phase at low O2 levels for this coal only (as seen in Figure 6, below). On the basis of the product distributions of the Pittsburgh No. 8 and subbituminous coal, one might suppose that NO conversion efficiencies are simply proportional to the extent of char burnout, at least after the hydrocarbons are consumed. But any relationship of this sort is contradicted by the results for the lowvolatility coal in Figure 5. As the O2 level is increased from 0% to 14.6% with the low-volatility coal, the weight loss increases by only 2 wt % while the N fraction in NO reaches the same level as the subbituminous. Over the same range of O2 levels, the burnout of the subbituminous and Pittsburgh No. 8 coals were 30 and 26 wt %, respectively. Clearly the lv bituminous coal char is very much more resistant to oxidation than all other ranks, as expected. Over the same range of O2 levels, the soot yields fall from 19 to 4 wt %, although the


extent of soot oxidation is negligible as long as hydrocarbons are present. In contrast to both other coals, this one exhibits sequential combustion of its major fuel components. The hydrocarbons, CO, and H2 are burned out first, so that the theoretical O2 requirement in Table 3 is directly applicable. With 4.4% O2, only 1 wt % C2H2 plus CH4 are present and more than half the H2 is gone. But CO reaches its maximum yield at this condition, again reflecting the partial combustion of the noncondensible fuel compounds. With 10% O2, all noncondensible fuels are consumed and soot burns, but not char. With 14.6 % O2, the char is ignited but burning slowly while the soot continues to burn away. The nitrogen species distribution in Figure 5 shows that as the O2 level increases from 2 to 14.6%, char-N is constant, reflecting this char’s slow burning rate. HCN levels pass through a maximum, in contrast to the monotonic decays seen with the other coals. We suspect that the very rapid surge in HCN promoted by the addition of a very small amount of O2 is due to the rapid conversion of cyanogen (C2N2) into HCN via oxidative pyrolysis. We did not monitor C2N2 but its presence seems to be the most likely explanation for the deficit in the N balances for complete secondary pyrolysis with this coal and several other low-volatility samples.4 The HCN is almost entirely consumed in 10% O2. The nitrogen content of the soot also diminishes with O2 levels above 5%, but not completely. NO production is again suppressed by hydrocarbons, but not for O2 levels as large as the thresholds for the other coals. And as O2 levels are increased from 5 to 14.6%, the enlarged N fraction of NO accounts for almost 80% of the N release from HCN, soot, and char. Consequently, the estimated N2 level of 29% for this coal with 14.6% O2 is less than half the estimates for both other coals at this operating condition. To summarize the impact of hydrocarbons on NO production in more conventional terms, the reactor effluent concentrations of HCN, NH3, and NO appear in Figure 6 on the dry basis, adjusted to 0% O2. Total fixed nitrogen (TFN) is the sum of these three concentrations. No NO is present at low O2 levels, so TFN is determined by the HCN concentrations with both bituminous coals, although NH3 is a significant intermediate with the subbituminous. TFN decays for progressively higher O2 levels, reflecting N2 production, and then reaches a rank-dependent minimum. The minimum TFN is only 100 ppm with the Pittsburgh No. 8, occurring with just slightly more O2 than needed to eliminate all hydrocarbon gases. The positions of the minimum TFNs for both other coals are the same, but their magnitudes are 3 times greater. Note that these magnitudes are inconsistent with the coals’ N contents, which increase with rank for these three coals from 0.9 to 1.4 to 1.6%. For O2 levels beyond the point of minimum TFN, NO makes up all the TFN, increasing while most of the nitrogen released from char and soot is converted. None of these chars are nitrogen-free after combustion at the highest O2 level. The extents of conversion of fuel-N into gaseous species falls from 84% with the subbituminous to 68% with the Pittsburgh No. 8 to 48% with the lv bituminous. So the apparent increase in the exhaust NO levels for coals of higher

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Figure 7. Coal-N fractions released from char versus carbon conversion for low-volatility coals4 (3), Pittsburgh No. 8,26 (9) and a subbituminous26 (b).

Figure 6. Concentrations of HCN (b), NH3 (O), NO (9), and their sum (0, TFN) on the dry, O2-free basis for, in descending order, the subbituminous, Pittsburgh No. 8, and low-volatility coal.

rank in Figure 6 would surely be accentuated by additional char oxidation. Discussion This dataset has important implications about the ways that O2 is apportioned among the various fuel components expelled into coal flames, and how representative fuel equivalence ratios can be assigned for flames burning different coal types. These issues are discussed elsewhere, while the impact of coal rank on NO production in primary flame zones is emphasized here. Our key observation is that NO production shuts down if C2H2 and CH4 are present but becomes the main fuel-N conversion channel in their absence. The inception point for NO production is determined by the consumption of O2 among all the fuel components, including soot and char. Hydrocarbon gases survive higher O2 levels only if the char oxidation rate is comparable to the burning rates of soot and noncondensibles, as observed with the subbituminous coal and, to a lesser degree, with the Pittsburgh No. 8. This tendency is the primary coal rank effect on early NO production. It can be evaluated directly on plots of the coal-N fraction converted to NO, fN NO, versus carbon conversion, XC, for two reasons. First, carbon conversion tracks the combustion of all fuel components, regardless of their phase or relative reactivity. Second, the release of fuel-N increases monotonically with XC, as seen in Figure 7. Devolatilization releases carbon faster than nitrogen from all but low-volatility coals,4,5,26 but char oxidation preferentially releases nitrogen from all coal types.26 Consequently, with any coal type, plots of the volatile-N fraction versus XC are nonlinear (26) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Energy Fuels, in press.

Figure 8. (a, top) NO conversion fractions versus XC for the subbituminous, 75.5% C (9), Pittsburgh No. 8, 83.3% (b), and low-volatility coal, 88.8% (3) in this study. (b, middle) Our N versus XC data with additional data from the radiant coal fNO flow experiment for 4 low-volatility coals4 (87.5% C, ]; 88.7%, 3; 89.6%, 4; 89.9%, b) and from an entrained reactor14 with 90 µm particles of a Milmerran lignite, 69.2% C (0) and a Liddel hv bituminous, 80.9% (O) at 100 mg/L. (c, bottom) N versus XC data for a one-dimensional coal flame burning fNO 28 µm particles of subbituminous (3) and hv bituminous (O) coals at 285 mg/L (open symbols) and 470 mg/L (closed symbols).9-11

through XC < 0.3 and then relax to a line that approaches the point of total C and N burnout from slightly above the parity line. Hence, the XC scale is a useful surrogate for fractional nitrogen release throughout all stages of coal combustion, and a linear one during the char oxidation stage. The plot of fN NO vs XC based on our dataset in Figure 8a shows clearly that coal rank determines the value of XC at which NO is first produced. These values increase from 12% burnout for the lv bituminous to 18% for the Pittsburgh No. 8 to 38% for the subbituminous. Beyond the inception point, slopes of the NO conversion


levels are the same for the subbituminous and Pittsburgh No. 8 but twice as high with the lv bituminous. The database that illustrates this rank dependence is expanded considerably in Figure 8b where data from this paper have been supplemented with additional data from combustion of four low-volatility coals in the radiant coal flow reactor,4 and two Australian coals in a drop tube furnace.14 Carbon contents of these additional coal samples are reported in the figure caption to establish that the onset of early NO production in premixed coal suspensions does indeed shift to higher values of XC for progressively lower coal ranks. The database does not determine if NO conversion efficiencies beyond the inception point are also rank dependent, although independent studies at lower temperatures on much longer time scales suggest that NO production efficiencies from char-N are rank-dependent, but with considerable complexity.27 The database in Figure 8b contains all the reported laboratory results that can be presented on these coordinates, except for two cases. One16 is at coal loadings low enough to be in the individual particle regime, where NO conversion efficiencies are too high to compare with those for coal suspensions. The other dataset9-11 is plotted separately in Figure 8c. These results are for a flat flame coal burner operating with 28 µm particles of subbituminous and hvA bituminous coals at two loadings, the least of which is the same as in our experiments. Product distributions were monitored from just above the burner surface, where ignition occurred, through 60 ms when all stable species concentrations had relaxed to asymptotic values. In contrast to the distinct rank dependence in the inception points for NO production in Figure 8b, these data show NO production from the point of ignition with conversion efficiencies that are above and beyond the carbon conversion rates in three of the four cases. The data for XC < 10% are impossible to reconcile against the well-established tendency (in Figure 7) for preferential release of carbon over nitrogen during the devolatilization of coals of these ranks. And even if carbon and nitrogen were released at comparable rates, NO conversion efficiencies are not 100% during this stage of coal combustion. These defects notwithstanding, these data raise two important considerations. First, fractional NO conversions reach asymptotic values that diminish for higher suspension loadings. This tendency is apparent in Figure 8c and is corroborated by Kramlich et al.13 Second, the fine size distribution fed into this burner includes substantial mass in sizes that are smaller than the critical radii for heterogeneous combustion, where envelope flames around volatile clouds around the particles give way to simultaneous oxidation of volatile matter and fixed carbon on the particle surface. Since they have no reducing zones around them, the smallest particles have been purported to produce NO earlier in their combustion histories with higher conversion efficiencies than larger particles.11,13,15,20,29 The suspension loading effect is only a factor on the ultimate fuel-N conversion efficiency. But size disparities may be (27) De Soote, G. G. Symp. (Int.) Combust., [Proc.] 23 1990, 1257. (28) Cho, S.; Marlow, D.; Niksa, S. Combust. Flame, in press. (29) Midkiff, K. C.; Altenkirch, R. A.; Peck, R. E. Combust. Flame 1986, 64, 253.


responsible for the main discrepancy between Figure 8, b and c, that the operating regime in which no NO is formed with larger particles vanishes with much smaller particles of similar coals. Having been taken with only relatively large particles, our product distributions do not directly determine whether attached or envelope flames govern NO conversion efficiencies. But it is difficult to comprehend how the clear evidence for simultaneous oxidation of char, soot, and hydrocarbons can be reconciled against the conceptual model of envelope flames. Burning rates of the noncondensible fuel mixtures from the later stages of secondary pyrolysis are fast enough to sustain envelope diffusion flames around individual particles, approaching those of H2/air.28 Such a flame buffers the char surface with a reducing zone that could, in principle, explain the connection between low NO production efficiencies and high levels of hydrocarbon gases. However, simulations with detailed reaction mechanisms indicate that NO reduction by hydrocarbons in a volatiles cloud is not a plausible hypothesis,13 and fastburning envelope flames cannot pass enough O2 to sustain vigorous char oxidation. Moreover, slip velocities for 90 µm particles are about 10% of their absolute velocities, which were roughly 25 cm/s in our tests. During the roughly 50-100 ms of available residence time in our furnace after ignition, a volatile cloud moving at the slip velocity would travel almost 15-30 particle diameters away from its parent particle. So envelope flames cannot impose the local environment for NO production in our tests. It is more likely that the gas phase is fairly well-mixed because the particles were fully dispersed, although microscale homogeneity is another matter entirely. With these challenges to the envelope flame hypothesis in mind, we propose an “intrinsic reburning” mechanism among the gas phase N species and hydrocarbons to rationalize the inception point for NO production in Figure 8b and, perhaps, its absence in the flame data in Figure 8c. According to this hypothesis, HCN is the only volatile N species in the gas phase at the onset of combustion, because NO and N2 are neither primary devolatilization products nor secondary pyrolysis products and tar-N is either released as HCN or incorporated into soot.4,5 During the initial stages of combustion at low O2 levels, HCN is converted into N2 via an NO intermediate, according to +O

+H

+H,OH

+OH,O2

+N

HCN 98 NCO 98 NH 98 N 98 NO 98 N2 At progressively higher O2 levels, a point would eventually be reached where an abundance of OH and O2 shuts down NO conversion into N2 by depleting the supply of N. During the initial stages of coal combustion, however, “intrinsic reburning” promotes NO reduction into HCN, according to +O

+H

C2H2 98 CH2 98 CH + NO f HCN V C + NO f HCN Production of additional HCN directly eliminates NO, then the newly formed HCN replenishes the supply of

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Figure 9. The intrinsic reburning mechanism.

N which, in turn, reduces even more NO into N2. The reburning process that makes HCN in conjunction with the process that converts HCN into N2 via NO comprises the closed loop scheme in Figure 9 that explains how fuel-N can be converted into N2 with virtually no NO. Primary devolatilization and secondary pyrolysis generate HCN, char-N, and soot-N. As long as hydrocarbon radicals are present, the HCN supply will be replenished and N2 is the only stable repository for fuel-N in the gas phase. Even if char-N and soot-N are released as NO, hydrocarbon radicals and HCN will reduce it to N2. Unfortunately, HCN production shuts down once the hydrocarbons are consumed, at which point the NO concentration rises above the threshold that will quickly deplete the pool of N, thereby shutting down N2 production. With O2 levels beyond this point, there is no mechanism to restore a favorable competition between the production of N2 and NO, so the NO conversion efficiency increases dramatically as char-N and soot-N are released as NO, and any surviving HCN and NH3 are converted into NO. The intrinsic reburning mechanism in Figure 9 also rationalizes very high early NO levels from the flat flame burner in Figure 8c. These tests were run with inlet O2 levels of 23%, more than 50% greater than the most oxidizing cases in our experiments. It appears that NO is expelled before HCN in these tests because the HCN was rapidly converted into NO by the abundance of O2 and O. Some N2 is also produced but with a selectivity that quickly falls as the abundance of NO depletes the concentration of N. Gaseous hydrocarbons were not monitored in these tests. But measured CO profiles rise on the same time scales at the NO levels, suggesting that NO production is inversely proportional to the hydrocarbon concentrations, consistent with intrinsic reburning. Predicted NO levels based on an elementary mechanism for the gas phase overpredicted the early NO levels, perhaps because the calculated rates of NO reduction by CHi species were too slow. Our hypothetical “intrinsic reburning” mechanism for early fuel-N conversion incorporates only the wellestablished pathways for N-species conversion in gas-

Niksa and Cho

eous hydrocarbon combustion systems30 because HCN conversion plays the leading role throughout. Even though tar is the major fuel-N shuttle during devolatilization,25 it releases its nitrogen as HCN prior to combustion, even in premixed suspensions. Additional HCN is formed very rapidly from C2N2 with lowvolatility coals or released from chars of any rank during secondary pyrolysis on long time scales. In our product distributions, even the NH3 produced by the subbituminous coal was probably derived from HCN in the gas phase, because it was observed only at low O2 levels and is clearly not a secondary pyrolysis product.5 Thus, we have no evidence that any of the nitrogen functionalities in coal are predisposed to release compounds that promote NO production. To the contrary, it appears that HCN conversion in the gas phase governs fuel-N conversion for all coal types, regardless of the N-species released into the solid fuel/gas interface. It remains to be shown that the mechanism in Figure 9 is quantitatively consistent with the tendency for larger carbon conversions at the point of NO inception with coals of lower rank, and these calculations are in preparation. But some additional qualitative implications are worth considering: To minimize near-burner NO in coal-fired utility burners, it is imperative to release as much fuel-N as possible while there are hydrocarbons in the gas phase that will reburn NO into HCN and, ultimately, into N2. Fuel-N devolatilization determines the initial extent of volatile-N release and, more importantly, the amount of tar-N available for reincorporation into soot. The relatively modest extent of total fuel-N release during the devolatilization of lowvolatility coals is seriously compounded by the relatively very high fraction of the total weight loss which is tar for these coals, because proportionately more of the volatile-N species will be incorporated into soot. But char reactivity is the key to the NO inception point. In our tests with the subbituminous and lowvolatility coals, all but one-third of the fuel-N was retained in char and soot at the end of secondary pyrolysis in both cases. However, at the point where all gaseous hydrocarbons were burned away, less than a third remained in the subbituminous char and soot, whereas nearly two-thirds remained with the lowvolatility char and soot. This difference could be responsible for the poor performance of aerodynamic NOx abatement strategies with low-volatility coals, because char-N and soot-N processed in hydrocarbon deficient environments are subject to very high NO conversion efficiencies, at least under near-burner operating conditions. Conclusions Detailed product distributions from the oxidative pyrolysis and combustion of various coal types under simulated conditions in primary flame zones are the basis for the following conclusions: 1. The gaseous and aerosol fuel components burned in premixed coal suspensions are products of secondary volatiles pyrolysis, not primary devolatilization. 2. As long as detectable amounts of CH4 and C2H2 are present, NO is absent with all coal types and TFN is composed of HCN with bituminous coals, plus appreciable amounts of NH3 during intermediate stages (30) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287.


with low-rank coals. TFN concentrations fall until the NO inception point is achieved, reaching concentrations as low as 100 ppm with Pittsburgh No. 8 coal. 3. The extent of carbon conversion up to the NO inception point is the primary coal rank index for early NO production in coal flames, diminishing for coals of progressively higher rank. Char oxidation rates that are fast enough to compete for O2 with noncondensible fuels and soot are responsible for the extensive release of fuel-N from low-rank coals prior to the NO inception point. 4. For carbon conversions greater than the NO inception point, HCN and NH3 are absent and most of


the remaining char-N and soot-N are converted into NO, so NO comprises all of TFN. 5. “Intrinsic reburning” determines NO levels out of the primary zones of coal flames in two stages of a continuous chemical cycle: First, fuel-derived HCN is converted into NO; then, NO is reduced by C and CHi radicals into additional HCN. As long as this cycle is sustained in the gas phase by hydrocarbon radicals, N2 is the only stable product of fuel-N conversion, regardless of the functional form of nitrogen in the parent coal or the molecular N-species in the gas phase. EF950117M

Conversion of Fuel-Nitrogen in the Primary Zones of Pulverized Coal

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