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Combustion and Flame 136 (2004) 303–312www.elsevier.com/locate/jnlabr/cn

Nitric oxide destruction during coal and char oxidationunder pulverized-coal combustion conditions

Alejandro Molina,∗,1 Eric G. Eddings, David W. Pershing, and Adel F. Saro

Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, UT 84112, USA

Received 5 March 2003; received in revised form 19 September 2003; accepted 10 October 2003

Abstract

A modified drop-tube reactor that allows particle distribution over the reactor cross-sectional area, and oof chars produced in situ, was used to study the conversion efficiency of char nitrogen to nitric oxide (αNO). Theresults confirm previous findings by other investigators thatαNO decreases as the weight of char burned increaαNO for coal was the same as (at 4% O2) or lower than (at 20% O2) that for an equal mass of char during oxidatioSince coal will yield approximately half its mass as fixed carbon, these results suggest that the local stoichsurrounding the particle is responsible for the observed reduction inαNO as sample size increases. The analof the exhaust gases showed increases in HCN concentration and a decrease in CO2/CO ratio as sample sizincreased, suggesting that local stoichiometry influencesαNO. Additional experiments showed thatαNO decreasedas the background NO concentration was increased, at rates that diminished as the oxygen concentrationindependent of particle size. The steep reduction in NO production as the background NO concentration iwas explained by the destruction of NO in the gas phase. 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: NOx reactions; Coal combustion; NOx pollutants

in-d-

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ays:

ndiare,

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onforedre-to

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Significant progress has been made in thefurnace reduction of NOx emissions from pulverizecoal-fired boilers by techniques such as burners-of-service, flue gas recirculation, overfire air, and lNOx burners. However, as the regulations for Nemissions have become more stringent, the achiment of further NOx reductions with traditional infurnace NOx control techniques has become mchallenging since pathways for NO formation thwere previously neglected have become more imptant. This paper aims to study one of these pathw

* Corresponding author.E-mail address: amolina@sandia.gov (A. Molina).

1 Present address: Combustion Research Facility, SaNational Laboratories, PO Box 969, MS 9052, LivermoCA 94551, USA.

0010-2180/$ – see front matter 2003 The Combustion Institutdoi:10.1016/j.combustflame.2003.10.009

the conversion to NO of the nitrogen that remainsthe char after coal devolatilization under pulverizecoal-combustion conditions.

In 1976, Wendt and Schulze [1] developed a moto study the conditions under which the fuel nitrogin char was converted to NO rather than to N2. Sincethat time, numerous studies have been publishedthe conversion of char nitrogen to nitrogen oxide,both fluidized bed conditions [2–20] and pulverizcombustion conditions [21–28]. However, recentviews [29–31] on the conversion of char nitrogenNO indicate that more research is necessary to arately describe the magnitude of NO formation durchar oxidation.

One of the main difficulties associated with tstudy of char–nitrogen oxidation is the dual charter of char, as an NO generator (R1) and as andestructor (R2), where –C(X) represents a surface a

e. Published by Elsevier Inc. All rights reserved.

304 A. Molina et al. / Combustion and Flame 136 (2004) 303–312

Nomenclature

Ci concentration of speciesi (mol m−3)(N/C)solid atomic ratio of nitrogen to carbon in

the solidNH total nitrogen atoms released by the charNV total nitrogen atoms released by the

volatilesNOH moles of NO produced by the char

NOV moles of NO produced by volatiles

t2 − t1 experimental interval (s)

ν volumetric flow (m3 s−1)

αNO conversion efficiency of nitrogen in the

solid to NO

σαNO standard deviation forαNO

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der ox-e

ar

thedi-ngac-

ed-of

clero-ted

ex-g)

ib-or.

inestoryasu-ent;ng

er-nalp-

gaslemi-theol-ed

tive site. The simultaneous occurrence of reacti(R1) and (R2) makes it difficult to distinguish the cotribution of each mechanism to the net conversionchar nitrogen to NO:

(R1)–C(N) + O2 → NO+ –C(O),

(R2)–C(N) + NO → N2 + –C(O).

Furthermore, the existence of homogeneous retions, either catalyzed at the char surface, such(R3) [32–35], or occurring between nitrogen oxiand any nitrogenous species released during chaidation (e.g., (R4) to (R7)) [36], contributes to thdifficulty of evaluating the total conversion of chnitrogen to NO.

(R3)NO+ COchar−−−−→ 1

2N2 + CO2,

(R4)O+ HCN ↔ NCO+ H,

(R5)NCO+ H ↔ NH + CO,

(R6)NH + H ↔ N + H2,

(R7)N + NO ↔ N2 + O.

This paper studies the conversion of nitrogen inchar to NO under pulverized-coal-combustion contions and from an experimental point of view, takiinto consideration the effect the NO destruction retions have on the net NO production.

1. Experiments

1.1. Experimental protocol

The experiments were conducted in an entrainflow drop-tube reactor with an internal diameter5.1 cm (Fig. 1) designed to minimize particle–partiinteractions and to allow the combustion of char pduced in situ. The method of studying char generain situ follows the setup of Jensen et al. [28]. In theperiments, a metered amount of fuel (4.4 to 5.5 mis injected through a specially designed fuel distrutor (id = 3.2 cm) located at the top of the react

Fig. 1. Experimental setup: entrained batch reactor. Lat the top of the reactor represent expected solid trajecin the distribution zone. (1) Quartz window; (2) carrier ginput; (3) char feeding system; (4) distributor; (5) distribtor radial gas; (6) coal-dispersing disk; (7) heating elem(8) alumina/silica mesh; (9) collection probe; (10) cooliwater; (11) filter; (12) cold trap; (13) vacuum pump.

The distributor section guarantees maximum dispsion of the particles over the reactor cross-sectioarea by generating a highly mixed region in the uper part of the furnace, where a stream of radialflowing in the distributor is used to prevent particdeposition on the side walls. The flow becomes lanar close to the end of the distributor section andparticles continue to flow down to where they are clected over an alumina/silica nonwoven fabric plac

A. Molina et al. / Combustion and Flame 136 (2004) 303–312 305

arted

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00

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sed:

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ed

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Fig. 2. Concentration profiles during the injection of chin the Type I experiments. Char (53–63 µm) was injecat t = 0; oxygen (4%/He) entered the reactor after 60T = 1698 K, (♦) CO2 (%), (•) CO× 10 (%), (�) NO (ppm).

in the middle of the reacting zone. A quartz windoon the top of the drop tube allows visual examinatof the distribution of the particles on the nonwovfabric.

Two kinds of experiments were carried out. Duing Type I experiments, the fuel was injected inHe (110 sccm) and the solid pyrolysis products wcollected on the nonwoven fabric. After a 60-s dvolatilization period, a mixture of O2/He (3990 sccm)was introduced into the reactor, which enabled cobustion to occur. Figure 2 presents the CO, CO2, andNO concentration profiles during a typical Type I eperiment. Just after fuel injection, a small amountCO, CO2, and NO was formed from reaction with thoxygen occluded in the fuel sample. The concention of these species decreased close to backgrolevels as the purge continued. After oxygen injectithe production of CO2, NO, and CO resumed.

In Type II experiments, a gaseous stream (40sccm) of O2 (4–20%), NO (0–750 ppm), and He/N2(balance) was already flowing into the reactor whthe fuel was injected. In this case, the solids beto react in suspension during the time (∼ 1 s) theyflow down to the nonwoven fabric where they are clected, and the reaction proceeds to completion.typical concentration profiles of these experimentssimilar to those in Fig. 2 aftert = 60 s.

The reactor wall temperature was set at 1698 Kall the experiments reported here. This was the mimum temperature for safe operation of the alumtubes and was considered to be representative olower range of temperatures in pulverized-coal cobustors. The difference between gas and wall tperature was less than 20 K during the experimeAfter injection, gases and solid were heated by radtion from the reaction zone. A stable gas temperawas reached 15 cm from the injection.

1.2. Gas analysis

The gases produced during the reactions werelected using a water-cooled stainless steel probeanalyzed for CO, CO2, NO, and NO2 in an FTIR(254-cm3 gas analysis cell with a path of 7.25 m, reolution of 0.5 cm−1). The FTIR calibration was corroborated by a NDIR analyzer for CO and CO2 and bychemiluminescence for NO and NO2. The FTIR alsoallowed for qualitative detection of HCN and lowmolecular-weight hydrocarbons. The estimated elimits for the least-squares regression used in the euation of the FTIR concentrations were±6% for NOand NO2, −15 and+7% for CO, and−12 and+3%for CO2.

The response time of the NDIR, chemiluminecence, and FTIR analyzers was longer (∼ 1 s) than thetypical reaction time (∼ 100 ms), which prevented thtemporal resolution of the experiments. Therefore,experiments were evaluated on an integral basis atal mass of carbon evolved during combustion asand CO2 and total mass of nitrogen evolved as NO

1.3. Fuels

Three different carbonaceous materials were u

(1) Illinois No. 6 coal,(2) the char from this coal produced in a se

sustained pulverized-coal burner described ewhere [37], and

(3) a commercially available activated carbon.

As shown in Table 1, the coal and the char have silar nitrogen contents. The nitrogen content foractivated carbon is 1 order of magnitude lower. TBET surface areas for the char and activated carwere 105 and 1678 m2 g−1, respectively [37].

All of the samples used in this study were rceived as a pulverized solid. Further sample preption was limited to sieving to produce different sifractions. For most of the experiments, a size frtion between 230 and 270 mesh (53–63 µm) was uWhen the effect of particle size was studied, twoditional sizes were considered: particles betweenand 170 mesh (88–108 µm) and particles less thanmesh (< 37 µm).

1.4. Evaluation of fuel N conversion to NO

The conversion of fuel N to NO was evaluataccording to Eq. (E1), wheret2 − t1 is the experi-mental interval (s),Ci is the concentration of speciei (molm−3), ν is the volumetric flow (m3 s−1), and(N/C)solid is the atomic ratio of nitrogen to carbon

306 A. Molina et al. / Combustion and Flame 136 (2004) 303–312

Table 1Proximate and ultimate analysis of the fuels

Proximate analysis (%) Ultimate analysis daf (%)

Moist. Ash Volatile matter Fixed carbon C H N S O

Coal 6.9 5.6 46.4 41.1 80.6 5.3 1.9 1.0 11.2Char 1.1 31.7 5.4 61.7 92.6 1.5 2.0 0.6 3.3Act. carbon 3.5 6.5 12.6 77.4 91.6 2.1 0.1 0.0 6.2

tc-t

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r Ns

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thence

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hem-

.

theuntpleence-NO

s. Inbysedpor-rgeenple

the solid obtained from the elemental analysis:

(E1)αNO =∫ t2t1

νCNO dt

(N/C)solid∫ t2t1

ν(CCO + CCO2) dt.

For Type I experiments,t1 represents the time awhich the O2/He mixture was injected into the reator. For Type II experiments,t1 represents the momenof the injection of the solids.

2. Results

2.1. Effect of sample size

Recent experiments [28] have shown that the cversion of char nitrogen to NO decreases as the sple size increases. Figure 3 presents the variatioαNO with sample size for three different experimenThe lower limit for NO detection (1 ppm) in the gaanalysis system prevented the use of smaller samsizes. The curve with the higher conversion of chato NO is for Type II experiments, in which char wainjected into the reactor with oxygen already presin the system. The results of this curve confirm acrease inαNO as the sample size increases.

αNO was lower for Type I experiments, in whicthe pyrolysis products were purged by helium for 6prior to oxygen injection (Fig. 3), andαNO also de-creased as the distance between the collection pand the char was increased. The carbon balanceType I experiments (51–80%) was lower thanType II experiments (96–101%) and did not show atrend with the variation in sample size. For Typeexperiments, the carbon balance represents a comison between the amount of carbon injected andamount of carbon detected as CO and CO2 in the pe-riod after oxygen injection. Since there are unresolhydrocarbons in the volatiles produced during the fi60 s, it is not surprising that the carbon balanceless than 100% for Type I experiments. What is uexpected is the considerably lower conversion of cnitrogen to NO for the Type I experiments. The coversion is lower by a factor of almost 4 and it is lowfor the experiments in which the collection probe wplaced farther downstream in the reactor.

The FTIR spectra of the exhaust gases duringpurge period of Type I experiments showed evide

Fig. 3. Variation in the conversion of char N to nitric oide (αNO) with sample size (20% O2/He, T = 1698 K,53–63 µm) for three different experiments: (�) Type I ex-periments, probe at 0 cm; (×) Type I experiments, probe a12 cm; (◦) Type II experiments.

Fig. 4. Variation with the sample size of the height of tnitric oxide peak in the FTIR spectra normalized by the saple size injected into the reactor atTg = 1698 K, 53–63 µmType I experiments.

of a peak characteristic of HCN that decreased assample size was reduced from 40 to 5 mg. The amoof HCN released is detectable only for large samsizes. The exhaust gas spectra also showed evidof NO production during the first 60 s of purge. However, as the sample size increased, the peak fordid not increase, but it decreased for some casefact, the height of the peak for NO, normalizedthe mass of char injected into the reactor, increaas the sample size decreased (see Fig. 4). It is imtant to note that the results in Fig. 4 refer to the puperiod in Type I experiments, i.e., prior to oxyginjection. The increase in uncertainty as the sam

A. Molina et al. / Combustion and Flame 136 (2004) 303–312 307

entnts.

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size decreases is related to the complex environmthat occurs after coal injection in these experimeHowever, it is not related to the data presentedFig. 3 and future results at longer times, for whithe NO concentration was recorded under an oxiing atmosphere.

The FTIR spectra and the results in Fig. 4 suggthat after the injection of the char into the reactor,amount of oxygen in the reactor is sufficient to odize all the HCN released to NO when the sample sis low. At larger sample sizes, some HCN escapesidation and it is detected by the FTIR. In other worthe smaller the mass of carbon injected, the higthe possibility that the species released from the care oxidized. As the sample size increases, themosphere becomes more reducing and speciesas HCN that are rapidly oxidized to NO in the preence of O2 at 1698 K can survive until detection.

The decrease in oxygen availability with an icrease in sample size is also manifest by the variaof the CO2/CO ratio during the purge period. At theexperimental temperatures, CO is the main prodof char oxidation [38]; therefore, the CO2/CO ratiogives information on the extent of any CO oxidatiin the homogeneous phase. If the CO2/CO ratio iszero, there is no oxidation of CO. A high valueCO2/CO implies CO oxidation in the boundary layand suggests a more oxidizing atmosphere.

Figure 5 presents this ratio for the Type I expements, when char was injected into the system. It aincludes comparable data for coal. The figure shothat the CO2/CO ratio is considerably lower for coathan for char and decreases as the sample sizcreases.

The amount of carbon-containing species releafrom volatiles after the injection of coal has tohigher than that of char. The FTIR spectra of ehaust gases after coal injection showed that duringpurge period, acetylene, ethylene, methane, and Hwere released in considerable amounts. For char,HCN was detected. This difference explains why

Fig. 5. Variation in the CO2/CO ratio with the samplesize during the purge period for (•) coal and (◦) char,Tg = 1698 K, 53–63 µm. Type I experiments.

CO2/CO ratio is higher for the char: as the amouof carbon-containing material in the gaseous phincreases, the atmosphere surrounding the particlecomes more reducing and less CO is oxidized.

In the same way, the larger the sample size,larger the mass of gaseous material releasedthe injection of the solid, thus the CO2/CO ratio de-creases as the sample size increases. The variatithe CO2/CO ratio is consequently consistent withmore reducing atmosphere for larger sample sizea batch system.

2.2. Effect of nitric oxide concentration

Figure 6 presents the profiles of NO concentratfor the oxidation of coal, char, and activated carbin Type II experiments at 1698 K and 4% O2/He,with the concentration of NO in the reactor as arameter. The three solids produce NO when the baground NO concentration is zero (bottom of Fig.As the background NO concentration increases,net NO production decreases, becoming lesszero at 200 ppm NO for activated carbon and aro400 ppm NO for coal and char.

It is important to note that the experimental secannot resolve the experiments in time. This methat although the profiles in Fig. 6 represent the vation in the NO concentration with time as detectedthe FTIR, they do not correspond to the instantanevariation in the NO profile as the particle is oxidizeNevertheless, the data in Fig. 6 show the dramvariation of NO production as the background Nconcentration changes and allow a relative compson of the response for the three solids used inexperiments. Figure 7 presents the variation inαNOfor the three fuels used in this study with NO cocentration in the background for Type II experimenThe key observables in Fig. 7 are summarized infollowing experimental observations.

(A) For the three solids,αNO decreases as the bacground NO concentration increases.

(B) Within the experimental error of the measument (σαNO = 0.04), the reduction ofαNO withchanges in background NO concentrationserved for coal and char is the same.

(C) When there is no nitric oxide in the backgrounall the nitrogen in the activated carbon is coverted to NO. This value is higher than that otained for coal and char (∼ 0.4).

(D) For background NO concentrations greater thaαNO is lower for the activated carbon than for tchar and coal. Although the high surface areathe activated carbon might be considered a potial explanation for this difference, in reality thdifference is caused by the low value of (N/

308 A. Molina et al. / Combustion and Flame 136 (2004) 303–312

atme-

ofal-

thater-ind

ry

sed

o-ing

edin-

. 7forsis--is

ack-e inndgen

orehe-hanleen

o-ing

ti-are

Fig. 6. Profiles of nitric oxide vs. time. Fuel injectiont = 0, NO concentration in the background as a parater. (•) Coal, (◦) char, (�) activated carbon; O2 = 4%/He,T = 1698 K, 53–63 µm. Type II experiments.

ratio for the activated carbon. If the amountNO reduction by the activated carbon is normized by the nitrogen content in the char,αNO forthe activated carbon becomes comparable toof coal and char, as shown in Fig. 7. This obsvation is consistent with the profiles for NOFig. 6, in which the total amount of NO reduceat high NO concentrations (top of Fig. 6) is vesimilar for the three solids.

These experimental observations will be discusthoroughly under Discussion.

Fig. 7. Variation in the conversion of fuel nitrogen to nitrgen oxide with NO concentration in the background duroxidation (4% O2/He, T = 1698 K, 53–63 µm) of (•) coal,(◦) char, and (�) activated carbon. The values for activatcarbon normalized by nitrogen content in the char arecluded for comparison (�). Type II experiments.

2.3. Effect of oxygen concentration

Figure 8 presents a plot equivalent to that of Figbut for an oxygen concentration of 20%. Resultschar and coal are again very similar, which is content with the results at 4% O2. There is a net production of NO when the background NO concentrationzero, and this net production decreases as the bground NO concentration increases. The decreasnet production of NO with an increase in backgrouNO concentration is less pronounced at 20% oxythan at 4% oxygen.

For activated carbon, the reduction ofαNO with anincrease in the background NO concentration is mpronounced than for the other two solids. Nevertless, the effect is considerably less noticeable tthat at 4% O2 concentration and almost negligibwhen the NO produced is normalized by the nitrogcontent in the char.

Fig. 8. Variation of the conversion of fuel nitrogen to nitrgen oxide with NO concentration in the background durfuel oxidation (20% O2/He, T = 1698 K, 53–63 µm) of (•)coal, (◦) char, and (�) activated carbon. The values for acvated carbon normalized by nitrogen content in the charincluded for comparison (�). Type II experiments.

A. Molina et al. / Combustion and Flame 136 (2004) 303–312 309

sd-ot

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2.4. Effect of particle size

Plots similar to Fig. 7 for particle size fractionof 88–108 and< 37 µm for the three solids showesimilar variation inαNO vs. background NO concentration for the three particle size fractions. It was npossible to find a specific trend in the variation inαNOwith particle size. This lack of an observed trend sgests that the setup used in these experiments wasensitive enough to determine any trend in the cversion of fuel nitrogen to NO with varying particsize.

3. Discussion

The experimental results show evidence of NOstruction due to an increase in sample size (Figand an increase in background NO concentra(Figs. 6–8). The destruction of NO is most likelyoriginate from one of the three mechanisms discusin the Introduction: The heterogeneous reductionNO on the char surface (R2), the reduction of NOCO on the char surface (R3), or homogeneous retions of NO with other nitrogenous compounds (e(R4) to (R7)). In the following sections we compathe experimental data presented above with theseferent mechanisms and address their importancNO destruction for the experimental conditions of tstudy.

3.1. Heterogeneous reduction

It has been traditionally considered that the herogeneous reduction of NO on the char surfacresponsible for most of the NO reduction during choxidation. In fact, experimental observation A isagreement with the calculations by different auth[23,26,39] that illustrated that NO reduction throureaction (R2) increases as the background NO ccentration increases. An enhancement in (R2)also proposed [28] as the cause for an increase indestruction as the sample size increases. The lihood of the occurrence of (R2) increases with increing sample sizes.

Despite the good agreement of some results wan exclusively heterogeneous mechanism, thecontribution of (R2) to NO destruction fails to explaall of the experimental observations. Proof of this iquantitative analysis of the total conversion of nitgen in coal,αCOAL

NO , and char,αCHARNO , to NO, as de-

fined by Eqs. (E2) and (E3), respectively, where NV

and NOH are the moles of NO produced by volatiland char, respectively, and NV and NH are the totalnitrogen contents of volatiles and char, respectivIt follows from experimental observation B that th

t

conversion of char nitrogen to NO has to be the saas the conversion of volatile nitrogen to NO, or,other words, Eq. (E4) (derived from equating (E2) a(E3)) has to be true. The validity of (E4) for the fivdifferent background NO concentrations of the expiments in Fig. 7 is rather unlikely if the mechanismof NO destruction by volatiles and char are differe

(E2)αCOALNO = NOV + NOH

NV + NH ,

(E3)αCHARNO = NOH

NH ,

(E4)αCHARNO = NOV

NV .

In the case of experimental observation C, one woexpect that if a heterogeneous mechanism dominthe destruction of NO during char oxidation, a sosuch as activated carbon with higher surface awould produce higher NO reduction and therefolower αNO than the coal and the char, for which tsurface areas are 1 order of magnitude lower. Hever, the char-nitrogen conversion for activated cbon when there is no background NO is 1, whichhigher than that of the coal and the char.

Furthermore, a heterogeneous process shoulddict that the conversion of fuel N to NO for equamass injection of coal and char will be higher for tcoal since the actual mass of fixed carbon introduwhen the coal is injected is close to half that preswhen the char is injected. However, Fig. 7 illustrathat this is not the case. Moreover, in Fig. 8,αNO isalways lower for the coal than for the char.

It could be argued that the dispersion obtainwith the experimental setup is good enough to guantee negligible particle–particle interaction. In thcase, the expectation would be that differences in cmass do not affect the conversion of char nitrogenNO. However, Fig. 3 shows that in the region undwhich these experiments were carried out (∼ 5 mg),variations in sample size do changeαNO. Therefore,differences in total mass of char injected should affthe total NO production, if heterogeneous proceswere controlling the rate of NO destruction.

In summary, an exclusively heterogeneous mecnism fails to explain the experimental results obserin this study.

3.2. Reduction by CO on the char surface

The failure of the heterogeneous mechanismexplain experimental observation B suggests the etence of an alternative route for NO destruction. Opossibility is the destruction of NO through the raction of CO with NO on the char surface (reacti(R3)). This mechanism is in agreement with a low

310 A. Molina et al. / Combustion and Flame 136 (2004) 303–312

izees atra-5).

B)herforuc-ben-on,for

oner

eentanha-ionont

ef-onox-

sur-cedtrip-on

har.for

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edre-d a3).

onen

inas.

ex-NOndn-

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Oatrate

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conversion of char nitrogen to NO as the sample sincreases (Fig. 3), since a larger sample producmore reducing atmosphere in which CO concention surrounding the particle is higher (see Fig.It can also explain the slope of the plot ofαNO vs.background NO (experimental observations A andgiven that the higher the NO concentration, the higthe rate for (R3). A similar carbon mass injectedthe three solids would guarantee similar NO destrtion since the total integrated CO produced wouldsimilar for the three solids. Higher local CO concetrations are expected for coal and activated carbsince a reducing atmosphere would be generatedcoal by volatile production and for activated carbby the higher surface area, which explains the lowvalues ofαNO for these two solids in Fig. 8, when thactivated carbon is normalized by the nitrogen contin the char. All these findings are in agreement withincreased NO destruction through (R3). This mecnism, however, cannot explain the lower conversof char N to NO as the distance between reaction frand collection probe increases (Fig. 3).

Levy et al. [32] and Chan et al. [33] studied thefect of carbon monoxide on the reduction of NOthe char surface. Both concluded that carbon monide increases the destruction of NO on the charface. They found that the effect was less pronounat high temperatures and proposed an oxide sping reaction (R8) as the mechanism by which carbmonoxide enhances the reduction of NO on the cThe C(O) complexes were proposed as inhibitorsthe reduction of NO on the char surface. Furusawal. [34] obtained similar experimental results and pposed the same mechanism.

(R8)CO+ C(O) → CO2 + C(X).

More recently, Aarna and Suuberg [35] question(R8) as the reason for the enhancement of theduction of NO on the char surface and suggestezero-order expression with respect to CO for (RTheir study was at 1073 K.

The effect of CO on predictions of the conversiof char nitrogen to NO during char oxidation has beexamined by Visona and Stanmore [23] and Molet al. [39] at pulverized-coal combustion conditionThe first authors used a Langmuir–Hinshelwoodpression for the enhancement of the reduction ofreported by Chan et al. [33]. Although Visona aStanmore [23] do not present the value of NO coversion, they state that the results were unsatisfacMolina et al. [39] used the rate expression determiby Aarna and Suuberg [35] for (R8) and found thatthough there is a reduction in the conversion of cnitrogen to NO when (R8) is included in the modthis reduction is small.

The evidence of a relatively small reduction of Nby reaction with CO suggests that it is unlikely ththe presence of CO in the system can increase theof NO reduction to the values observed in the expments. This conclusion is restricted to the temperaat which the present experiments were carried outwhich the effect of CO has been reported to be mi[32].

3.3. Reduction through homogeneous reactions

The detection of HCN in the exhaust gases athe variation in the CO2/CO ratio with sample size(Fig. 5) suggest that homogeneous reactions canan effect on the conversion of char nitrogen to NO acould explain the variation ofαNO with the samplesize, as observed in Fig. 3. If a nitrogenous spesuch as HCN is released during char oxidation,species can be oxidized to NO or reduce backgroNO, depending upon the stoichiometry and tempature of the gases surrounding the particle. It ispected that as the sample size in a batch systemcreases, the gases surrounding the particle willcome more reducing, the rate of destruction willcrease, and the NO production will decrease. Insame way, since the local stoichiometry for Typexperiments is more reducing than for Type II expiments, the char-nitrogen conversion to NO forlatter conditions is higher. Reactions in the gasephase also explain the lower value ofαNO when theprobe is located farther downstream of the reaczone.

A homogeneous mechanism of NO reduction calso explain experimental observations B and C. Ifhomogeneous reactions are responsible for the retion of NO in the background, coal and char shohave a similar slope for the plot ofαNO vs. back-ground NO. The magnitude of NO destruction shodepend on the local stoichiometry. Since local schiometry is more reducing for coal,αNO should bethe same or lower for coal than for char. This iscase in Figs. 7 and 8.

On the other hand, experimental observationseems to contradict the existence of a homogeneroute for NO. Given the lower nitrogen content of tactivated carbon (Table 1), it appears improbablethe reduction of NO by this fuel to be higher than threported for the coal and char, even when the resare normalized by the amount of nitrogen in the chHowever, this argument does not consider the pobility of HCN production from the reduction of NOon the activated carbon surface.

The reaction of NO with char to produce HCN hbeen reported by de Soete [40] (as quoted by Åmand Leckner [41]), by Jones et al. [6], and morecently by Orikasa et al. [42]. Figure 9 presents ad

A. Molina et al. / Combustion and Flame 136 (2004) 303–312 311

al,be

ehatnedson-eAl-ionithusly,acti-NObycon-ousri-low

ithruc-t-

ere-

hanlydings anrbonss

veism

uc-ere

tionct

thew-us

l Nar-erarspse-ases.ases re-genoaltro-

re-en

nceits

al

re-hehisanas

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),

NOtheust

v-ha-

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Fig. 9. Profile of HCN production for the reaction of (•)coal, (◦) char, and (�) activated carbon with 650 ppmNO/He/N2. The fuels were injected into the reactor att = 0.T = 1698 K, 53–63 µm.

tional evidence of the formation of HCN after cochar, and activated carbon injection into the drop tuin the absence of O2 but when a 650-ppm NO/Hstream was flowing at 1698 K. The figure shows tafter the peak of HCN evolution that can be assigto initial devolatilization, HCN production continuefor the three solids. It is also apparent that for the cditions of this experiment, HCN production for thactivated carbon is higher than that of the char.though for the experiment in Fig. 9 the net productof HCN can be affected by the reaction of NO wHCN in the gaseous phase as discussed previothe results show that at these temperatures, thevated carbon can release HCN, in the presence ofin the background. Thus, the destruction of NOactivated carbon can occur by heterogeneous NOsumption to form HCN and subsequent homogenedestruction of NO by this HCN under the appropate conditions. This reduction occurs despite thenitrogen content of activated carbon.

The results in Fig. 8 are also in agreement wa parallel homogeneous mechanism of NO desttion during char oxidation. The more oxidizing amosphere in the experiments at 20% O2 decreases theffectiveness of homogeneous NO reduction; thefore, the slope of the plots in Fig. 8 are less steep tin Fig. 7. Furthermore, coal oxidation, which initialproduces a more reducing atmosphere surrounthe particle due to devolatilization (see Fig. 5), halower value ofαNO in Fig. 8 than char oxidation, evethough, as discussed above, the mass of fixed cainjected with the coal is half the fixed carbon mainjected with the char.

Calculations by different groups [5,23,39] hashown that an exclusively heterogeneous mechanpredicts higher values ofαNO for smaller particlesize, since the surface area available for NO redtion as the NO diffuses to the particle surface is lowfor smaller particle sizes. The low sensitivity of thpresent experiments for the detection of the variaof αNO with particle size and the fact that the effe

of particle size on the homogeneous reaction inboundary layer is not well documented prevent draing conclusions on the validity of a homogeneopathway based on the low variation ofαNO with par-ticle size.

4. Conclusions

The experimental study of the conversion of fueto nitrogen oxides performed in an entrained laminflow reactor, modified to provide solid dispersion ovthe reactor cross section and to allow reaction of chproduced in situ, confirmed results by other grou[28] that the conversion of char nitrogen to NO dcreases as the sample size in a batch reactor incre

The results of this study suggest that an increin oxygen availability as the sample size decreasesults in an increase in the conversion of char nitroto NO. The fact that oxidation of equal masses of cand char produced the same conversion of fuel nigen to NO for coal and char at 4% O2 shows that thevariation of char mass in the batch reactor is notsponsible for the observed reduction of char nitrogconversion to NO with decreasing sample size, sithe char produced by coal is approximately halfmass. A similar conclusion is valid at 20% O2, forwhich the conversion of fuel nitrogen to NO for cowas lower than that of char.

The experimental results also showed a steepduction in the conversion of fuel nitrogen to NO as tbackground NO concentration were increased. Tphenomenon has been previously explained byincrease in the heterogeneous destruction of NOthe concentration gradient between char surfacegas concentration increases [5,39]. However, the silar slope and magnitude for the variation ofαNOwith background NO concentration obtained for eqmass of coal and char, and the evidence of HCN pduction for the reaction of NO with activated carbosuggest that a parallel mechanism of homogeneNO destruction contributes to the steep reductionNO production as the background NO concentratincreases. This homogeneous route is also in agment with a lower slope for the plot ofαNO vs. back-ground NO for a higher oxygen concentration (20%the negligible variation inαNO when the particle sizechanges, the lower conversion of char nitrogen tofor Type I experiments compared to Type II, anddecrease inαNO as the residence time of the exhagas increases.

Although the experiments in this study show eidence of the importance of homogeneous mecnisms for NO reduction during char oxidation, tsimultaneity of heterogeneous and homogeneousactions makes it difficult to establish the relative cotribution of both processes to NO destruction. Fut

312 A. Molina et al. / Combustion and Flame 136 (2004) 303–312

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studies will be focused on comparing both mecnisms from a computational point of view, to providadditional insight into their relative contributions.

Acknowledgment

Major financial support for this research was pvided by the Department of Energy under Grant DFG26-97FT97275.

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