Char nitrogen conversion: implications to emissions from coal-fired utility boilers

Char nitrogen conversion: implications to emissions fromcoal-fired utility boilers

A. Molina, E.G. Eddings* , D.W. Pershing, A.F. Sarofim

Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, UT 84112-9203, USA

Received 19 January 2000; revised 21 March 2000; accepted 21 March 2000

Abstract

The contribution of nitrogen present in the char on the production of nitrogen oxides during char combustion was analyzed. Aliterature review summarizes the current understanding of the mechanisms that account for the formation of NO and N2O fromthe nitrogen present in char. The review focused on: (1) the functionalities in which nitrogen is present in the coal and how theyevolve during coal devolatilization; (2) the mechanism of nitrogen release from the char to the homogeneous phase and itsfurther oxidation to NO; and (3) the reduction of NO on the surface of the char. The critical analysis of these three issuesallowed identification of uncertainties and well-founded conclusions observed in the literature for this system.

The existing models for the production of nitrogen oxides from char-N were also reviewed. A critical analysis of theassumptions made in these models and how they affect the final predictions is presented. Finally, a simplified version ofthese models was used to perform a parametric analysis evaluating the impact of several parameters on the total conversion ofchar-N to NO. These parameters include: (1) the rate of NO reduction on the char surface; (2) the rate of carbon oxidation; and(3) early vs. late nitrogen release during the char oxidation process. The results underscore the importance of the reaction of NOreduction on the char surface to the final conversion of char-N to NO.q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Char; Nitrogen oxide; Nitrous oxide; Coal combustion; Computational fluid dynamic; Single particle model

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5082. N containing functionalities in coal and char. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5083. Controlling path in char-N oxidation to NO and N2O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5094. Reaction of NO and N2O on char surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

4.1. Reaction mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5144.1.1. NO reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5144.1.2. N2O reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515

4.2. Effect of other reacting gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5154.2.1. Oxygen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5164.2.2. Carbon monoxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5164.2.3. Other gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517

4.3. Other important parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5174.3.1. Reaction order with respect to NO partial pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5174.3.2. Reaction order with respect to N2O partial pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . 5184.3.3. Kinetic parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518

Progress in Energy and Combustion Science 26 (2000) 507–531PERGAMONwww.elsevier.com/locate/pecs

0360-1285/00/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S0360-1285(00)00010-1

* Corresponding author. Tel.:11-801-585-3931; fax:11-801-585-5607.E-mail address:[email protected] (E.G. Eddings).

4.3.4. Surface area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5184.3.5. Heat treatment and catalytic effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519

5. Models for the production of NO during char combustion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5195.1. Carbon oxidation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5195.2. HCN formation and destruction . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5215.3. NO formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5215.4. NO reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522

6. Influence of kinetic parameters on the amount of char-N converted to NO. . . . . . . . . . . . . . . . . . . . . 5226.1. Reduction of NO on char surface . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5236.2. Carbon–oxygen reaction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5246.3. Rate of nitrogen release during char oxidation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525

7. Implications for coal-fired utility boilers .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5258. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 528Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529

1. Introduction

Nitric oxide is an acid rain precursor and participates inthe generation of photochemical smog through ozoneproduction. In addition, nitrous oxide is a greenhouse gas.Thus, the reduction of emissions of nitrogen oxides iscurrently a major environmental issue. During coal combus-tion, NO may be produced from the oxidation of: (a) themolecular nitrogen present in the combustion air (ThermalNO); (b) compounds produced by the attack of hydrocarbonfree radicals on N2 (Prompt NO); and (c) the organic nitro-gen present in the coal (Fuel NO). The last pathway mayalso be a source for N2O, particularly at low temperatures, asin fluidized bed reactors. This study will focus on the FuelNO route, as it is the major contributor to the NO and N2Oproduced during coal combustion.

Fuel NO and N2O are produced by the oxidation of thevolatiles present in the coal and by the oxidation of thenitrogen present in the residual char. Each mechanism

requires a different control strategy. The nitrogen oxidesproduced from coal volatiles have traditionally beingreduced by modifying the air–fuel mixing in the combustionchamber with acceptable success. In contrast, the NO andN2O produced from heterogeneous oxidation of char are notso amenable to control. This fact, coupled with the increas-ing importance of heterogeneously produced NO and N2Oas the formation of homogeneous nitrogen oxides iscontrolled, provides the motivation for the study of the reac-tions occurring during char-N oxidation.

In this paper, a review of the literature will describe thedifferent characteristics known for the conversion of char-Nto NO and N2O. It will also highlight numerous uncertaintiesthat are yet to be resolved, and at the same time reveals thebasis on which most of the current models for the predictionof the evolution of char-N to NO are founded. A simplifiedversion of one of these models will be used to understand theinfluence of some of the main characteristics of the systemin the total conversion of char-N to NO.

2. N containing functionalities in coal and char

A first approach to understanding the mechanism of NO andN2O formation during char oxidation is to identify the nitrogenfunctionalities present in coal and in char. XPS (X-ray Photo-electron Spectroscopy) of coal, chars, and synthetic chars hasbeen widely used for addressing this problem [1–10]. Someconclusions of these studies are presented by Thomas [11] andmay be summarized as follows:

1. Nitrogen functionalities in coal are basically pyrrolic (50–80%), pyridinic (20–40%) and quaternary nitrogen (0–20%). Small amounts (,10%) of amino groups may bepresent in low rank coals. The nature of the quaternarynitrogen is not clear. Pels et al. [2] suggested that it mayrepresent an N-oxide of pyridinic-N or protonated pyridi-nic-N. Kelemen et al. [6] assigned this peak to pyridinic

A. Molina et al. / Progress in Energy and Combustion Science 26 (2000) 507–531508

Nomenclature

Ci Concentration of speciesi (mol m23)FNO Net conversion of char-N to NOki Coefficient for reaction rate of speciesin, m, j Apparent rate orders in Eq. (27)(N/C) Nitrogen to carbon atomic ratio for the charPi Partial pressure of speciesi (Pa)T Temperature (K)Ri Volumetric reaction rate (mol m23 s21)

Greek symbolsa Relative amount of char-N going to NO

(Fig. 18)h Relative amount of coal-N released as vola-

tiles (Fig. 18)

or basic nitrogen species associated with hydroxyl groupsfrom carboxylic acids or phenols (see Fig. 1).

2. After mild pyrolysis (T , 923 K) there is an increase ofpyridine groups, as the concentration of pyrrole andquaternary nitrogen is reduced.

3. More severe pyrolysis conditions (higher temperatureand longer residence times) tend to reduce pyrroleconcentration, whereas pyridine concentration remainsconstant or tends to increase due to the transformationof pyrrolic nitrogen to pyridinic nitrogen, and the conver-sion of some pyridinic nitrogen to quaternary nitrogen.This quaternary nitrogen is not represented by the struc-tures shown in Fig. 1, but rather by incorporation ofnitrogen in graphene layers, such as those suggested byPels et al. [2] and presented in Fig. 2.

4. The heating rate used for the formation of the chars alsoaffects the distribution of nitrogen functionalities [6].High heating rates (104 K/s up to 923 K) produce nitro-gen functionalities similar to those of chars obtainedunder mild pyrolysis conditions (673 K). In otherwords, a higher heating rate will prevent the formationof quaternary nitrogen of the form shown in Fig. 2, andpyrrolic and pyridinic structures will be favored.

The similar conclusions obtained from different groups andfor different carbonaceous materials suggest that the natureof the nitrogen compounds present in coal is well under-stood. The question that arises now is how this informationmay be related to the production of nitrogen-containinggaseous species.

Two different answers are possible depending on whetherthe release of nitrogen compounds is studied duringpyrolysis or during oxidation. Kambara et al. [1] analyzed

20 coals and their respective chars by XPS as well ascomposition of the gases evolved during pyrolysis by GCand concluded that the variation of the nitrogen function-alities in chars during pyrolysis can be qualitatively relatedwith the yield of nitrogen-containing species. Quaternarynitrogen converts finally to NH3 and some fraction ofpyrrolic and pyridinic nitrogen converts to HCN. Theirexperiments were in an oxygen free environment. Incontrast to the results for pyrolysis, when coals and charshave been oxidized no correlation has been found betweenthe evolved gases and the nitrogen functionalities of coaland chars [5,8,10]. Wo´jtowicz et al. [5] and Stan´czyk [8]didn’t find any correlation between nitrogen functionalityand coal or char-N conversion to NO, N2O or N2 duringlow temperature char oxidation. The reason for this lackof correlation may be that most final nitrogen functionalitiesafter severe pyrolysis correspond to nitrogen present in six-membered rings (quaternary and pyridinic) and no matterwhat groups are present in the original coal, the influence ofthem on the final NOx production is minor [2]. Stan´czyk [8]states that the relation between nitrogen-containing speciesand nitrogen functionalities in char is negligible whencompared to other factors such as surface area of chars,porosity, or reactivity for NOx destruction.

The view of this paper is to agree with the conclusion thatthe functional form of nitrogen present in the char has asecondary effect compared to those of other parameterssuch as catalysts, char surface area, and pore size distribu-tion, when considering char-N conversion to NO.

3. Controlling path in char-N oxidation to NO and N2O

The heterogeneous character of the char-oxygen reactionand the complex gaseous environment surrounding the charduring combustion make it difficult to elucidate the pathwayfor char-N conversion to NO and N2O. Several studies ofthis reaction have been presented [4,7,11–38] Most investi-gators have used indirect techniques to overcome the exper-imental difficulties associated with tracing the fate ofnitrogen atoms bound to char and the high speed of thereactions occurring during oxidation. Thomas and co-workers [18–21,24–28,32] and Ashman et al. [7] employeda probe just above the char particle, in this way they

A. Molina et al. / Progress in Energy and Combustion Science 26 (2000) 507–531 509

Fig. 1. Most likely nitrogen functionalities present in coal. The last four represent possible quaternary structures [1–7].

Fig. 2. Suggested structure of quaternary nitrogen after pyrolysis[2].

minimized homogeneous reactions and were more likely toanalyze the direct products of the char-oxygen reaction.Winter et al. [30,33,36] used iodine injection to reduceradical concentrations to equilibrium levels and thereforeto suppress homogeneous reactions. The combination oftransient kinetics (TK) techniques and reaction modelingwas the tool employed by Goel et al. [23,31], de Soete etal. [12,34] and Croiset et al. [35] The analysis of the influ-ence of char properties and different gas concentrations onfinal emissions was used as a research tool by Feng et al.[29], Amand and Leckner [17] and Abbasi [37].

There is consensus between these studies of the hetero-geneous formation of NO by a direct reaction of organicbounded nitrogen and oxygen. The mechanisms suggestedare similar to the one described by de Soete et al. [18]

O2 1 �2C�1 �2CN� ! �2CO�1 �2CNO� �1�

�2CNO� ! NO 1 �2C� �2�

where species in ( ) refer to solid sites. Basically, oxygenreacts with an active (–CN) site to further produce NO.

In contrast to the agreement when considering a hetero-geneous route for the formation of NO during char combus-tion, the role of HCN as a homogeneous intermediate in theformation of NO and N2O is a matter of debate.

Miller and Bowman [39] describe the mechanisms forhomogeneous production of NO (Fig. 3) and N2O

(Reactions (3) and (4)) from HCN.

NH 1 NO! N2O 1 H �3�

NCO1 NO! N2O 1 CO: �4�The potential formation of NO and N2O from HCN is there-fore clear. This has motivated the study of the existence ofHCN as an intermediate of NO production during charoxidation. Thomas and coworkers [19,20,24–28] foundNO to be the main nitrogen-containing product (excludingN2) detected during their experiments (20–5% O2 in Ar,573–1073 K, and TGA1 MS experimental setup). HCNwas also detected when direct analysis was made (a probeimmediately above the reacting sample), but char-N conver-sion to HCN was low (,15%). When the gases weresampled at the exit of the TGA analyzer (when thehomogeneous reactions were completed) all the HCN wasconverted to NO [24]. Nevertheless, in most of these exper-iments, the determination of N2O and NH3 were hidden by theinterference of the peaks associated with CO2 and OH, respec-tively. However, isotopically labeled carbon allowed theseresearchers to differentiate N2O from CO2 [18]. By impregnat-ing carbon-13 materials with ammonia, carbon compoundswere produced with high carbon content, but low nitrogenand almost negligible hydrogen. Interestingly, temperatureprogrammed oxidation (TPO) of these carbons producedN2O and NO whereas HCN was not reported as product.

Winter et al. [30,33,36] using an iodine addition tech-nique, provided strong evidence that N2O may be only


Fig. 3. Mechanism for homogeneous HCN oxidation to NO and N2. The bold lines indicate the most important reaction path [39].

Fig. 4. Instantaneous conversion to NO, N2O and HCN vs. carbon conversion. 15-mm bituminous coal particle.T� 1023 K, 0.21 O2 in N2.Batch fluidized bed combustor [30].

produced by homogeneous oxidation of HCN. In Winter’sexperiments, coal is combusted at fluidized bed conditions.NO, N2O and HCN are the main N-containing products, notconsidering N2. After in-situ devolatilization, iodine isadded to the combustion chamber and the N2O concentra-tion goes to zero, while NO and HCN concentrationsincrease (Fig. 4). These authors consider that iodinesuppresses the free radical concentration and preventsNCO formation (see Fig. 3) and further reaction of NCOwith NO to produce N2O (Reactions (3) and (4)). Therefore,iodine addition increases HCN concentration by avoiding itshomogeneous conversion to NCO and ultimately N2O.

The main assumption in Winter et al’s [30,33,36] exper-iments is that iodine vapor affects the homogeneous gas-phase chemistry significantly, but not the heterogeneousreaction. To understand the validity of this assumption,Fig. 5 shows the influence of iodine addition on othercompounds. The concentrations of CH4 and CO increasedue to the suppression of the homogeneous oxidationreactions. Although Winter et al. [30,36] consider this asevidence of the effectiveness of iodine addition on eliminat-ing the homogeneous reactions, a question remains on theinfluence of iodine on heterogeneous reactions. Fig. 5 showsthat the addition of iodine results in the destruction of N2O,and not only stops its production. However, Winter et al.[36] consider that this is due to the following heterogeneousreaction:

N2O 1 CO! N2 1 CO2 �over char�: �5�Any contribution of the homogeneous reduction of N2O byCO was discarded on the basis of previous studies [33].Thus, the assumption of the influence of iodine exclusivelyon homogeneous reactions is in this way justified.

The method of iodine addition is just beginning to beexplored in the understanding of combustion reactions.And the understanding of its influence on char oxidationreactions is limited to the information collected by Winteret al. [30,33,36] This review considers that in the refereedliterature there is no available evidence that refutes thevalidity of Winter et al’s [30,33,36] results. However,

further research on the influence of the iodine chemistryon the combustion process is advisable to completely under-stand the effect of iodine on this reacting system.

In contrast to Winter et al’s [30,33,36] results, Sarofimand coworkers [15,16,22,23,31] proposed that N2O isformed from heterogeneous oxidation of char-N. Theirmechanism was developed to explain that N2O formationis greatly enhanced in the presence of O2. In one of theirexperiments [22], they stopped O2 addition to a fluidized bedwhere char combustion was carried out at 1023 K and with275 ppm of NO as background (Fig. 6). After oxygensuspension, the concentrations of CO, CO2 and N2O wentto zero. Once oxygen supply was resumed, all these gasesare produced again. This is, according to the authors, directevidence that oxygen is necessary in order to obtain detect-able N2O production from char nitrogen. The role of theoxygen was to convert the bound nitrogen into a form (–I )that can either evolve to form NO or react with NO to formN2O as illustrated in Fig. 7.

The authors did not identify the intermediate (–I ) andspeculated that it was –CNO or –NCO. In the view ofWinter’s et al. [30,33,36] experiments, the question maybe raised if it could also involve HCN. In further exper-iments, Sarofim and coworkers [31] tried to obtain moreunderstanding on the nature of the intermediate (–I ). Theyinterrupted the O2 addition to a fluidized bed in which charwas combusted and instead added NO to the system (Fig. 8).The authors interpreted the CO and N2O evolution after O2interruption as evidence of the existence of surfacecomplexes (a possible form of the intermediate (–I )) thatheterogeneously reacts with NO to produce N2O. Thepresence of HCN as an intermediate may be discardedsince in the absence of O2, the possibilities of HCN produc-tion are low. And if it is formed by the reduction of NO onchar surface as suggested by A˚ mand and Leckner [17],Abbasi [37], and Ashman et al. [7], the chances of it beenoxidized to N2O in the oxygen free system are low.

Mathematical modeling [23] and the following additionalindirect experimental results [15,16,22,31] were used tosupport the heterogeneous hypothesis.


Fig. 5. Concentration profiles measured at the top of a laboratory scale CFBC burning petroleum coke in continuous mode. Figure presentseffect after addition of N2O, iodine, and after interruption of N2O addition. After Winter et al. [36].

1. The instantaneous conversion of char-N to NO increasesand the instantaneous conversion of char-N to N2Odecreases with char conversion [15,16]. This is explainedaccording to the model presented in Fig. 7 by a reductionof the char surface available for the reaction of NO with

the intermediate (–I ). Therefore, N2O formation and NOconsumption will be reduced.

2. Char samples submitted to longer devolatilization timesbefore oxygen injection do not have differences in overallN2O production [31], which suggests that nitrogen lossduring extended pyrolysis is not important. Thus, if N2Owere only produced through HCN homogeneous oxida-tion, chars with more severe heat treatment wouldproduce less N2O, and this was not the case.

3. Higher NO concentration increases the conversion ofchar-N to N2O [22]. Higher NO concentration willincrease the possibilities of scavenging N atoms fromthe char.


Fig. 6. Concentration profiles during the combustion offive Newland coal particles at 1023 K. The O2 concentration was changed from 4 to 0% after375 s, and back again to 4% at 1500 s. An NO concentration of 275 ppm was used as background during all the experiment (after Ref. [22]).

Fig. 7. Suggested reaction path for heterogeneous N2O formation[31].

Fig. 8. N2O and CO concentration profiles during coal combustion.T� 1123 K, O2� 8% (after Ref. [31]).

4. For small fuel particles (450–1250mm) the ratio char-Nconversion to N2O to char-N conversion to NO is lowerthan the same figure for large fuel particles (630–2000mm) [33]. This may be explained, as in observation1, since larger particles will present more opportunitiesfor the heterogeneous interaction of NO with theintermediate (–I ).

This heterogeneous path is also supported by TK exper-iments combined with heterogeneous kinetic analysisperformed by Croiset et al. [35] and de Soete et al. [38].

To add more complexity to these two completely contra-dictory but apparently valid experimental results, Ashman etal. [7] found that the proportion of the instantaneous nitro-gen release as HCN during the oxidation of chars in a TGAat 873 K and 2% O2 was close to 10% and constant duringchar burnout. This suggests that in Krammer and Sarofim’s[22] experiment the absence of O2 in the system does notimply that O2 is necessary for the production of N2O andwhat may be occurring is that in the absence of O2, HCN isreleased, but it’s not oxidized to N2O. Krammer and Sarofim[22] did not analyze HCN in the exhaust gases.

Ashman et al’s [7] results also suggest that slowdevolatilization is not a source of HCN, but rather that theheterogeneous oxidative process itself is responsible for therelease of HCN from the char. This would explain why charswith more severe heat treatment do not present differencesin the final N2O yield.

Despite the evidence against the heterogeneous produc-tion of N2O, there are some experimental facts that are diffi-cult to explain if only the homogeneous conversion of HCNto N2O is considered, i.e. the decrease of char-N conversionto N2O during the progression of char combustion and thehigher char-N conversion to N2O for larger particles. Winteret al. [33] tried to explain this evidence by only consideringthe homogeneous production of HCN. They proposed thatthis may be explained by different local radical concen-trations close to the fuel particles, but that more knowledgeabout NO and N2O formation is necessary for a correct datainterpretation. In contrast, the explanation of these factswhen the heterogeneous route is considered is straight-forward.

No definitive conclusion on the pathways for theformation of NO and N2O during char oxidation can bemade now. Further experiments can by guided by newanalysis of the system by techniques such as computational

chemistry. These techniques have been extensively used inthe understanding of the carbon oxygen reaction in recentyears [40–46].

Chen and Yang [44] presented an example of the use ofthis technique in understanding the char-oxygen system.These authors proposed, using ab initio molecular orbitalcalculations, a new reaction pathway for the C–O2 reactionas shown in Fig. 9.

In this model, the “epoxy” oxygen, which is the one out ofthe plane or on the basal plane, reduces the strength of theCnCO bond, and CO is released from the molecule(Structure I). The epoxy oxygen flips over to the edge, andtransforms in a new carbonyl structure (II). New oxygenchemisorption will produce structure (III), the carbonylgroups will be released leading to additional oxygen chemi-sorption, this time in the edge plane, transforming structureIV back to structure I.

As an analogy to this study, the use of computationalchemistry has been suggested by Sarofim et al. [47] as analternative technique for evaluating the process of char-Nconversion to NO. Fig. 10 presents possible structures of thechar before oxidation. Pyridinic (N7 and N8) and quaternary(N9) nitrogen represent typical char-N functionalities. Therelease of HCN (C-1 and N7), CN (C3 and N8) and NO (N9and its associated off-plane oxygen) seems possible whenconsidering the weakening of the bonds adjacent to thechemisorbed oxygen. The possibility of the occurrence ofthese reactions may be determined by molecular orbitalcalculations, coupled with the analysis of new experimentalresults.

4. Reaction of NO and N2O on char surface

During the combustion process, NO and N2O destructionreactions on the char surface are as important as a widerange of char-N formation reactions. Correlation of char-Nconversion to NO has been observed with varying charsurface area [13], char reactivity [20,21,26–28] and NObulk concentration [48] at combustion conditions. In orderto explain these results one has to take into account theability of char to reduce nitrogen oxides.

The non-catalytic and catalytic reduction of nitrogenoxides on char surfaces has been extensively investigatedand can be classified into three main groups: (1) those,normally at low temperatures (,573 K), that pursue a better


Fig. 9. Reaction pathway for the graphite–O2 reaction [44].

understanding of the NO and N2O reduction phenomenon inorder to develop materials suitable for the destruction of NOat post-combustion conditions [49–55]; (2) those focused onthe mechanism of nitrogen oxides reduction on char surface[56–84]; (3) and those that search for a kinetic expressionfor this reaction suitable for modeling and design purposes[85–89]. Comprehensive reviews on these reactions can befound in the literature [90,91]. This paper will highlight themost important issues on these reactions, in an attempt toincrease the understanding of its influence when consideringcoal combustion in utility boilers.

4.1. Reaction mechanism

The study of the reduction of nitrogen oxides on the charsurface presents some experimental difficulties associatedwith the two-phase system, that are similar to those ofchar-oxygen and char-N-oxygen reactions. However, thefact that it is two orders of magnitude slower than thechar-oxygen reaction has allowed a better understandingof its mechanism.

Four groups of researchers were identified as those whohave performed extensive studies on this reaction system:Beer and coworkers [86–88,92–95], Suuberg and coworkers[58,61,62,71,80,83,90], Tomita and coworkers [64,66,69,74,75,78,79,81,84] and Linares-Solano and coworkers[59,65,67,68,72,73]. A description is presented of themechanisms proposed in the more recent studies of thethree last groups for NO and N2O reduction on char surfaces,as well as a brief review of their experimental evidence.

4.1.1. NO reductionAll three groups agree that the first step for NO reduction

is its adsorption on char surface. Suuberg and coworkers[58,80] suggested different possibilities for NO adsorptionon char surface (Reactions (6)–(9)). The carbon sites Cp

correspond to rapid turnover sites. The distinction between

rapid turnover sites and normal sites tries to explain the two-temperature regime observed during the NO-char reactionand determined by a break in the Arrhenius plot at tempera-tures near 923 K. The rapid turnover sites will be readilyavailable for NO adsorption, particularly at high tempera-tures when the surface is clean. At low temperatures, theseCp sites won’t be as available, and the reaction will becontrolled by a desorption process. These authors supportthis analysis by mathematical modeling of the process.

2C1 NO! C�O�1 C�N� �6�

C 1 C�O�1 NO! C�O2�1 C�N� �7�

Cp 1 NO! CO1 C�N�1 aCp 1 bC �8�

Cp 1 C�O�1 NO! CO2 1 C�N�1 dCp 1 eC �9�Tomita and coworkers [79] and Linares-Solano and co-workers [73] consider Reaction (10) as the adsorptionprocess, without specific differentiation of adsorbing sites.

2C� �1 NO! C�O�1 C�N� �10�The chemical character of the active sites C( ) is stillunknown. Kyotani and Tomita [81] by ab initio molecularorbital theory determined that the chemisorption of the NOmolecule with its bond axis parallel to the edge plane (Fig.11) gave the most stable chemisorbed species for nitricoxide.

Each group represents the step after NO adsorption, N2

release, in different ways. Suuberg and coworkers [58,80]consider N2 formation as represented by Reactions (11)–(13).

C�O� ! CO1 fCp 1 gC �11�

C�O2� ! CO2 1 hCp 1 iC �12�

2C�N� ! N2 1 jCp 1 kC: �13�


Fig. 10. Possible structure of graphite before nitrogen evolution [47].

The first two reactions are important in the generation ofnew active sites, C and Cp and the third one is responsiblefor N2 formation. The mechanism by which the two activesites C(N) get together to form the N2 molecule was notdetailed.

Tomita and coworkers [79] present direct experimentalevidence for the occurrence of Reaction (14).

C�N�1 NO! N2 1 C�O� or CO: �14�These researchers used isotopically labeled molecules and aTK technique to arrive at this reaction.14N16O/16O2 wasreacted with12C until steady state was obtained. Afterwards,15N18O/18O2 replaced the14N16O/16O2 stream. Since14N15Nwas the dominant species among N2, Reaction (14) wasconsidered the most important path for N2 formation underthe experimental conditions (1123 K, NO 0.05%, O2 0.2%).

Other reactions such as Reaction (15) and (16) may beoccurring as well, but in lesser magnitude.

2C�N� ! N2 �15�

2NO1 2CO! N2 1 2CO2: �16�Linares-Solano and coworkers [73] postulate a reactionsimilar to Reaction (13):

2S�N� ! N2 1 2S �17�where S is a surface site for catalytic reaction, and a carbonreactive site for the non-catalytic reaction.

In summary, the mechanism of NO reduction on charsurface may be a combination of the three previouslydescribed. However, the opinion of this paper is that at1123 K and an O2 concentration of,0.2% (Tomita andcoworkers conditions), the experimental evidence favorsreaction Reaction (14) as the main path for NO reductionon the char surface.

4.1.2. N2O reductionIn contrast to the studies for NO reduction on char

surface, there is agreement on the mechanism of N2O reduc-tion on char surface. In a recent study by Noda et al. [84]where isotopically labeled gases were used, the followingmechanism was proposed:

C� �1 N2O! C�O�1 N2 �18�

C�O�1 N2O! C� �1 CO2 1 N2 �19�

C�O�1 CO! C� �1 CO2 �20�

C�N�1 C�N� ! N2 �21�

C�O� ! C� �1 CO: �22�Reactions (18)–(20) were proposed first in 1953 by Madleyand Strickland-Constable [56] for N2O reduction on a charsurface at 573 K. Noda et al. [84] provide experimentalevidence that strongly support Reaction (18) as the mainpath for N2O reduction at 1123 K. Molecular ab initio calcu-lations also support this [81]. The incidence of Reactions(19) and (20) is only noticeable at low temperatures such asthose of the experiment of Madley and Strickland-Constable[56]. Reaction (21) doesn’t contribute considerably to thereaction system, and reaction Reaction (22) will be thedominant route for the release of the C(O) complexes gener-ated from Reaction (18).

This reaction mechanism is not surprising considering thestrength of the NxN bond present in the N2O molecule.

4.2. Effect of other reacting gases

The influence of other gases on the char-NO reaction hasalso been subject of extensive study. Considering theimportance of CO and O2 on the combustion system, theinfluence of these gases has been studied most extensively.


Fig. 11. Most possible surface nitrogen complex from NO adsorption according to MO calculations. After Kyotani et al. [81].

4.2.1. OxygenAt low temperatures and when active carbons are used for

reducing NO from flue gases, the influence of O2 is im-portant, not only in the char–NO reaction, but also sincethe consumption of char by O2 may be an important issuefrom the economical point of view.

At these conditions (temperature,523 K, O2 concentra-tion typical of flue gas (,3%) and NO concentration,1000 ppm), the presence O2 increases the rate of NOreduction on char surfaces [49,50,53]. This phenomenonhas been explained by the conversion of NO to NO2 onthe char (or char1 catalyst) surface. The reaction betweenNO2 and carbon has been reported to be faster than the O2–carbon reaction [55,60]. Therefore the presence of oxygenincreases NO conversion to NO2 and, in the end, N2production.

At higher temperatures, Tomita and coworkers [79] havealso found a catalytic effect of oxygen in this reaction.Different possible explanations to this are suggested bythese authors: (1) NO conversion to NO2; (2) O2 increasesCO concentration, which in turns reacts heterogeneouslywith NO or provides sites for NO reaction by removingchemisorbed oxygen; (3) Formation of new active sites byO2; and (4) O2 activation of C(N) species. Although theypropose the activation of C(N) sites by O2 as the mostprobable route, they finally conclude that the availableexperimental data are not sufficient to select the dominantprocess. Evidence of the increase of the NO reduction rateby O2 has also been presented by Aarna and Suuberg [83] fordifferent carbonaceous materials at 1023 K in a packed bedreactor. In summary, qualitative experimental evidenceshows that oxygen increases the rate of heterogeneousreduction of NO on char surface. However, no quantitativeexpression for this influence has been reported in thereviewed literature, and the mechanism for this increase inthe rate of NO reduction is unclear.

4.2.2. Carbon monoxideThe enhancement of NO reduction by char in the presence

of CO was identified by Chan et al. [88] who derived aLangmuir–Hinshelwood-type expression to account for

the influence of CO on the NO–char reaction. These authorsconsidered this approximation as an empirical fit of the dataand a more sophisticated model was needed to explain allthe experimental findings. They also hypothesized thatReaction (23) is responsible for this enhancement. HighCO concentration will increase the concentration of carbonactive sites Cf that are necessary for NO reduction on thechar surface, according to Reaction (24). The fact that theinfluence of CO is less noticeable at higher temperaturessupports this hypothesis since a reaction like Reaction(25) will clean the active sites at high temperatures, decreas-ing the importance of Reaction (23).

CO1 C�O� ! CO2 1 Cf �23�

NO 1 Cf ! C�O�1 12 N2 �24�

C�O� ! CO: �25�Furusawa et al. [96] studying char combustion at fluidizedbed conditions, confirmed Chan et al’s [88] hypothesis, butconsidered it to be valid at high temperatures (.1100 K). Atlower temperatures, the catalytic reduction of NO on charsurface (Reaction (26)) will play a dominant role in thissystem.

NO 1 CO! 12 N2 1 CO2 �surface catalyzed�: �26�

Johnson [97] summarized the destruction of NO by CO ondifferent solids (limestone, char, ash, bed material andquartz sand). He concluded that limestone, char and bedmaterial (ash1 quartz sand) are the most active catalystsfor this reaction. More recently, Aarna and Suuberg[80,83,90] addressed this issue. In their initial experiments[80], they tried to explain the different reaction ordersobserved for the reduction of NO on char surface. Thisreaction is traditionally considered first order with respectto NO, however there is more than one study [63,89] thatconsiders a fractional order. Aarna and Suuberg [90] asso-ciated these fractional results with the effect of CO on theapparent order of NO–carbon reaction. Fig. 12 presents theeffect of CO addition on the conversion of NO to N2 on agraphitic surface. When CO is absent from the system, the


Fig. 12. Effect of CO on the apparent order of the NO–graphite reaction. Fixed bed reactor at 1073 K. White points, results in the absence ofadded CO. Black points, results in the presence of 275 ppm of CO. After Aarna and Suuberg [90].

reaction order is one, i.e. there is no influence of the NOconcentration on the NO conversion. However in thepresence of 275 ppm of CO the apparent reaction order is0.56. Aarna and Suuberg [83] conclude that to a goodapproximation the lower bound of unit-order behaviormight be around 60 ppm of NO.

In further experiments, Aarna and Suuberg [83], using TKresponse on a packed bed reactor at 1023 K, have shown thatCO significantly enhances NO reduction. These authorsintroduced 420 ppm CO to a reacting system, in whichNO was reduced on a graphitic surface. The NO reductionrate increased almost instantaneously by one order ofmagnitude. This rapid increase on the reaction rate suggests,according to the authors, that a reaction like Reaction (25) isnot responsible for the enhancement of NO reduction in thepresence of CO, but that an alternative reaction for NOreduction (Reaction (26)) in the presence of CO is themost important path.

These authors attempted to validate the consideration ofReaction (26) as the main path on the reduction of NO onchar surface. They performed a pseudo-steady analysis ofthe reaction of NO on the char when CO was present andtried to determine the influence of CO if Reaction (26) isconsidered as the main route. Eq. (27) considers the rate ofNO reduction on the char surface as a combination of threeparallel reactions, Reactions (26), (28) and (29).k1 accountsfor the combination of Reactions (28) and (29), whilek2

represents Reactions (26). By assumingn�m� 1 andthat j � 0, Aarna and Suuberg were able to determinek2

for the high temperature regime. An Arrhenius plot ofk2

turn out to be linear for two of the samples analyzed, andthe calculated activation energy for Reaction (26) is 116 kJ/mol. However, if it is assumed thatn�m� 0 a similar plotis obtained. This led the authors to conclude that the datacannot be used to determine the true order. An expressionsimilar to Eq. (27) was also proposed by Iisa et al. [98] forblack liquor char. In this case,n, m, and j were set to one.The activation energy was 67 kJ/mol fork1, and 76 kJ/molfor k2 at temperatures below 923 K. With these kinetics, Iisaet al. were able to predict NO reduction on black liquor charwithin 10% of measurements, for NO concentrationsranging from 500 to 1640 ppm, CO from 500 to40320 ppm, and CO/NO ratios from 1 to 100. For tempera-tures above 923 K a similar expression was proposed, withan activation energy of 240 kJ/mol for both kineticconstants. The model predicted the experimental results inreasonable way in the range of temperatures of 973–1273 K; however, for higher temperatures NO reductionwas underpredicted.

r � k1�PNO�n 1 k2�PNO�m�PCO� j �27�

NO 1 C! 12 N2 1 CO �28�

2NO1 C! CO2 1 N2: �29�As was the case for oxygen, CO accelerates the rate of NO

reduction on a char surface. However, satisfactory expres-sions to quantify this effect still need to be developed.

4.2.3. Other gasesThe influence of H2O [86], and CO2 [83] was also studied.

Results suggest that these gases do not affect the rate of NOreduction on char to a great extent. However, new evidence[83] points out that these gases may affect the population ofsurface complexes on a char surface and ultimately the finalreaction rate.

4.3. Other important parameters

Several parameters are important when considering thereduction of nitrogen oxides on the char surface. A briefdescription of the influence of these parameters on thereduction of NO and N2O on a char surface is presentedhere. For a extensive treatment, the reader is referred tothe comprehensive reviews by Hayhurst and Lawrence[99], Johnson [97], Thomas [11], Aarna and Suuberg [90]and Li et al. [91]

4.3.1. Reaction order with respect to NO partial pressureThe reaction of NO with carbon has traditionally been

considered first order with respect to NO partial pressure.However, as stated above, there is more than one paper[63,89] that claims a fractional order. One explanation forthis discrepancy is the influence of CO, as was explainedabove.

Aarna and Suuberg [80] complemented this discussionand concluded that there are two temperature regimes, inwhich different reaction orders may be observed. At hightemperatures (.1000 K, according to their experiments) thereaction order is one with respect to NO. At low tempera-tures the reaction order increases from zero to unity astemperature increases. These authors explain these observa-tions by considering that at low temperatures, a desorptionprocess controls the reaction (represented by Reactions (11)and (12)), while at high temperatures the controlling step isthe direct attack of NO on the char surface (Reactions (8)and (9)). A desorption process should be independent of NOpressure, while the direct attack of NO is obviously firstorder with respect to NO. At intermediate temperaturesthe reaction will be controlled by the sum of the twoprocesses. Aarna and Suuberg [80] further concluded thatprevious studies of this reaction in the low temperatureregime have found the reaction to be first order with respectto NO because they used a cleaned char surface (withoutcomplexes) and they did not allow enough time to obtainpseudo-steady state. In this way these previous studies ineffect measured the order of NO adsorption on a char surface(Reaction (6)), and this is clearly first order with respect toNO.

In contrast to this conclusion, Guo and Hecker [100]determined the order of the char-NO reaction at tempera-tures ranging from 743 to 1173 K (low temperature regime


according to Aarna and Suuberg [80]). In their experiments,Guo and Hecker [100] ran for more than 60 min at 923 K inorder to ensure steady-state (comparable to 20 min at1068 K reported by Aarna and Suuberg [80]) and concludedthat for all the seven coals analyzed (ranging from lignites tographites), the reduction of NO on char surface was firstorder with respect to NO partial pressure. These resultsare in contradiction with the conclusion by Aarna andSuuberg [80] that in the low temperature regime the reactionof NO reduction on char surface presents a fractional orderat pseudo-steady state conditions. In conclusion, the ques-tion of the order of the reaction of NO with respect to NOstill remains unsolved. However, when analyzing the influ-ence of the char-NO reaction at combustion conditions (hightemperatures), the reaction can be considered to be firstorder even if the Aarna and Suuberg [80] hypothesis is valid.

4.3.2. Reaction order with respect to N2O partial pressureAs pointed out by Li et al. [91], the value of the apparent

reaction order with respect to N2O is still unclear. Valuesbetween zero and one are reported in the literature[12,63,101,102] for different experimental conditions. If amechanism as described previously (Reactions (18)–(22)) isconsidered for this reaction, a complex behavior similar tothat described by Aarna and Suuberg [80] may be expected.At low temperatures the desorption of the surface complexesC(O)as given by Reaction (22) may be the controlling step,and a zero order dependency may be expected. At highertemperatures, the desorption step may be less important anda first order behavior may be expected according to Reaction(18). This is only a speculation and further studies may benecessary to provide exact understanding of the reactingsystem.

4.3.3. Kinetic parametersNumerous kinetic studies have been performed on NO reac-

tions witha carbonaceoussystem. Most of them identifya two-regime behavior in the Arrhenius plots. At low temperaturesthe activation energy is lower than in the high temperatureregime. The transition region is observed between 850 and950 K. A possible explanation of this behavior was alreadypresented in the section on reaction mechanisms.

In an attempt to identify a kinetic expression that approxi-mately represents in a reasonable way the reduction of NOon any carbonaceous surface, Aarna and Suuberg [90]analyzed the kinetic results obtained by different researchgroups for twenty-four carbonaceous materials ranging fromlignites to anthracites. For the high temperature regime, theyobtained an expression (Eq. (30)) that was derived by aver-aging the kinetic results at the maximum and minimumtemperature (in the high temperature regime) for eachstudy. These authors claim that this expression representsall the data analyzed within an order of magnitude, with onlytwo exceptions.

k � 5:5 × 106 exp�215;939=T��gNO m22 h21 atm21NO�: �30�

For the low temperature regime, a similar expression wasobtained and is given by Eq. (31).

k � 0:191 exp�23464=T��gNO m22 h21 atm21NO�: �31�

Expressions (30) and (31) should be understood to providean average of kinetic parameters for this reaction for differ-ent carbonaceous materials. They are useful when there isno experimental data available for a specific carbonaceousmaterial. The influence of these expressions on the finalprediction of char-N conversion to NO will be addressedin a later section.

A similar situation to that described for NO exists forN2O. There is uncertainty regarding the values for thekinetics of its reduction on char surfaces. Furthermore, thenumber of kinetic studies on the reduction of N2O on car-bonaceous materials is less than for NO and generalizationssuch as those described for NO were not found in the refer-eed literature. Li et al. [91] present a review of kinetic para-meters for this reaction. The reported activation energyvaried from 65 to 115 kJ/mol. Teng et al. [76] suggest theexistence of a two kinetic regime as that reported for NO. Atlow temperatures (,748 K) the activation energy for thereduction of N2O on a phenol–formaldehyde resin charincreases with burn-off from 57 to 66 kJ/mol. At hightemperatures (.748 K) the activation energy increaseswith burnout, from 170 to 230 kJ/mol. This divergence inresults confirms the need of further research in determiningthe apparent kinetics for the reduction of N2O on charsurfaces.

4.3.4. Surface areaThere is disagreement regarding the influence of surface

area on the rate of NO reduction on a char surface. Severalauthors [13,59,97,103] have considered that the rateconstant is proportional to the internal surface area of thechar. The surface area reported for different chars variedfrom 5 to 895 m2/g. Aarna and Suuberg [90] also foundthat by allowing for differences in BET surface area theywere able to reduce the variability of the NO reactivitieswith char. There is more than one study, however, thatreports that there is no relation between char surface areaand the rate of NO reduction. Mochida et al. [104] did notfind any relation between the surface area of three activatedcarbons (surface area varying from 595 to 1090 m2/g) andchar reactivity towards NO; however, these carbons weretreated with sulfuric acid and it is difficult to separate theeffect of surface area from that of the acidic activation.Ruiz-Machado and Hall [105] analyzed the reactivity ofcellulose char with six different levels of oxidation. Thesurface area, as determined by N2 adsorption, varied from382 to 486 m2/g. They found good agreement between charsurface area and reactivity of chars towards O2 (R� 0.99),whereas no correlation was found between surface area andchar reactivity towards NO (R� 0.20). The authors claimedthat the porosity of the chars plays a different role in O2 and


NO gasification. SAXS (small-angle X-ray scattering) andTPD analysis suggest that for the chars analyzed, some masstransfer limitations will prevent some of the char surfacearea from participating in reactions with NO.

Calo et al. [106] disagreed with the Ruiz-Machado andWall [105] results. They considered that the range of N2

surface area analyzed by Ruiz-Machado and Wall [105](382–486 m2/g) is not sufficiently large to completelyconfirm or deny the relation between surface area and charreactivity. Calo et al. [106] also concluded that Ruiz-Machado and Hall [105] found a correlation betweenTotal Surface Area (TSA) and reactivity towards O2 inthis narrow range because under the experimental condi-tions of Ruiz-Machado and Hall’s study [105], there mayhave been restrictions for the oxygen diffusion. Thesediffusive restrictions will limit O2 access to the microporesthat may not be detectable by N2 surface area measurement.In contrast, NO due to its lower reactivity, may access thesemicropores, and the total reactivity area may not be rep-resented by the adsorption of N2, as this technique neglectsmicropores.

In order to answer the question regarding the relation ofchar reactivity towards NO and surface area, it may be help-ful to briefly analyze prior conclusions on the role of surfacearea on the reaction of other gases (O2 and CO2) withcarbon. Traditionally it has been found that experimentalcarbon gasification rates do not normalize using TSA[107]. In contrast, the Reactive Surface Area (RSA) isproven to give better results when the objective is to normal-ize reactivities. Different techniques have been used fordetermining RSA. The classical experiment by Laine et al.[108] determined Active Surface Area (ASA) by analysis ofCO and CO2 evolution from carbonaceous materialspreviously exposed to O2. More recently [40,109,110], TKtechniques have been used. TK tries to evaluate the exactpopulation of active sites at reaction conditions. The valuesof ASA determined by these techniques typically normalizethe reaction rate in a better way than TSA. Extending theseresults to the NO–char reaction suggests that in order tonormalize the reaction rate, it is necessary to first definewhat area value represents in the best way the truly reactingsurface of the char. Thus, under certain experimental condi-tions (carbons with low microporosity), the reaction rate ofNO on a carbonaceous char surface may be normalized byN2 surface area, as reported by Illa´n-Gomez et al. [59]. Forcarbonaceous materials with high microporosity, CO2

surface area may be valid for normalizing reactivity, aswas the case for Shimizu et al. [13] and Li et al. [103] Itis likely that the reactivity of Ruiz-Machado and Wall [105]chars towards NO should be normalized by CO2 surfacearea, due to the high microporosity of those chars. Finally,under some conditions of high reactivity towards NO, theuse of RSA may be necessary for normalizing the rate of NOreduction on a char surface, as suggested by Li et al. [103].To this end, Guo and Hecker [70] used the CaO surface areadetermined by CO2 chemisorption at 573 K in a thermo-

gravimetric analyzer to normalize the reaction rate vari-ability with respect to burnout. In relating the CaO surfacearea to reactivity, Guo and Hecker [70] are providing anunconventional measure of RSA.

In conclusion, determining the right value of area to beused when NO reacts with char is a difficult task thatstrongly depends on the pore size distribution of the char.However, for the reaction of NO on a char surface, thesimple determination of TSA by N2 and CO2 adsorptionseems to give acceptable results.

4.3.5. Heat treatment and catalytic effectsThe best way to summarize the influence of these vari-

ables on the reduction of NO on char surface is to commentthat this reaction responds to these parameters in a similarway to other char gasification reactions. Briefly (1) the reac-tivity decreases with increased severity of heat treatment,and (2) different minerals, such as calcium [70], potassium[51–53], and some mixtures such as Cu/V/K/Cl [55] cata-lyze the reaction, and therefore demineralized chars exhibitlower reactivity than the raw chars [88].

Since the effect of heat treatment can be incorporated inthe physical parameters (surface area of the char) and theeffect of a specific mineral catalyst in the global reactivity orin the RSA, no further discussion on these issues ispresented.

5. Models for the production of NO during charcombustion

One way to obtain greater insight on char-nitrogenconversion to NO during coal combustion is to model itsproduction from a single char particle. However, from theformer section it is clear that several characteristics of thephysics and chemistry of this system remain to be defined,making it difficult to develop a model. Despite such diffi-culties, different models [13,23,38,47,111,112] have beenproposed to predict the generation of NO from a singlechar particle. As models, they employed engineeringapproximations to address uncertainties in the basicmechanisms and kinetics. This section describes theseassumptions and attempts to define under what conditionsthey are valid.

Table 1 presents the set of reactions used by differentmodels. The assumptions concerning the most importantreactions of these systems are summarized next.

5.1. Carbon oxidation

The carbon in the char is considered to react hetero-geneously with oxygen to produce CO in a one-step reactionin most of the models. Only de Soete et al. [38] considered athree-step heterogeneous reaction system in which CO andCO2 are produced heterogeneously. The homogeneousoxidation of CO to CO2 was also considered by two models


A.

Mo

lina

et

al.

/P

rog

ress

inE

ne

rgy

an

dC

om

bu

stionS

cien

ce2

6(2

00

0)

50

7–

53

1520

Table 1Comparison of some single particle models used in the prediction of NO emissions set of reactions considered

Model Wendt and Schulze [111] Shimizu et al. [13] Goel et al. [23] Visona and Stanmore [112] de Soete et al. [38]

C oxid. C1 12 O2ÿ!k1 CO C1 1

2 O2 ! CO �–C�1 O2ÿ!ko2 CO1 –O �–C�1 O2 ÿ!

ko2 CO1 –O 2�–C�1 O2 ! 2�–CO�–(CO)! CO1 Ca

2(–CO)! CO2 1 Ca

CO oxid. CO1 12 O2ÿ!k4 CO2 CO1 1

2 O2ÿ!k∞ CO2 –

HCN gen. – – 2CN! HCN

HCN oxid. – – HCN1 O2 ÿÿ!kHCN NO

HCN 1 NOÿÿÿÿÿ!kHCN–NO N2

NO gen. Char-N 1 12 O2 ÿ!k2 NO Char-N 1 1

2 O2 ! NO –CN 1 O2 ÿ!k0 –CNO1 –O 12 O2 1 –CN! NO O2 1 (–C)1 (–CN)! (–CO)1 (–CNO)

–CNOÿ!k1 NO 1 –C HCN1 O2 ÿÿ!kHCN NO (–CNO)! NO 1 (–C)

NO reduct. NO1 …ÿ!k3 N2 NO 1 CO! 12 N2 1 CO2 NO 1 –Cÿÿ!kHCN 1

2 N2 1 –CO HCN1 NOÿÿÿÿ!kHCN–NO N2 NO 1 �–C� ! 12 N2 1 �–CO�

NO 1 COÿÿÿÿ!kNO2 CO 12 N2 1 CO2 NO 1 –Cÿ!kNO N2

N2O gen. – –CNO1 NOÿ!k2 N2O 1 –C – NO1 (–CNO)! N2O 1 (–CO)

N2O reduct. – N2O 1 –Cÿÿ!kN2ON2 1 –CO – N2O 1 (–C)! N2 1 (–CO)

a Cp: free carbon site.

[23,111]. Although there is experimental evidence [113] ofsome direct CO2 heterogeneous production, the main path atcombustion temperatures is the heterogeneous production ofCO during char oxidation [114]. It is also well known thatmost of the CO2 produced during combustion comes fromthe homogeneous oxidation of CO. However the possibilityof the occurrence of this reaction inside the pores of the charwhere Knudsen diffusivity may prevail is low. Therefore,only considering the heterogeneous production of COduring char oxidation may be a good approximation.

5.2. HCN formation and destruction

As previously discussed, new experimental evidence [30]suggested the importance of HCN as an intermediate in theproduction of NO. This observation justifies the presence ofreactions for its formation and reduction in Visona andStanmore’s [112] model. These authors not only consideredNO formation from HCN, but also its influence in thereduction of NO. The lack of expressions that predict theproduction of HCN from char oxidation forced these authorsto use an empirically-defined parameter that determines theamount of char-N that evolves as NO or HCN.

The inclusion of HCN as an intermediate in this reactionsystem may be important at: (1) the temperatures typical offluidized bed reactors; and (2) if the prediction of N2Oproduction is the objective. This was the case in the Winteret al. [30,33,36] study. Nevertheless, the rapid oxidation ofHCN to NO, as described for instance in the Jones et al.experiment [24] in which HCN can only be detected if thegas analysis probe is placed immediately on the top of thechar sample, may justify neglecting HCN production duringpulverized coal combustion, where N2O production isnegligible.

5.3. NO formation

All the models consider the formation of NO from theheterogeneous oxidation of the char-N. Goel et al. [23] andde Soete et al. [38] also defined the formation of a hetero-geneous complex (hypothesized to be –CNO) that is respon-sible for N2O formation.

As stated above, the importance of including –CNO (orHCN) as intermediates for NO production during charcombustion decreases at pulverized combustion conditionswhere N2O, either because it is not formed or because it isreduced so rapidly, is not detected as a main product. Whenmodeling combustion reactions in fluidized beds, approxi-mations such as those of Goel et al. [23] and de Soete et al.[38] with –CNO, or that of Visona and Stanmore [112] withHCN as intermediate may be required. But the simpleheterogeneous formation of NO from char-N neglectingany intermediates may be an adequate approximation atpulverized coal combustion conditions. Obviously, thedevelopment of further comprehensive models for this

reacting system should pursue a good representation ofresults at high and low temperatures.

It is also important to consider the rate expression used torepresent NO production from char-N oxidation. The mostcommon approach [13,111,112] is to consider that it isproportional to the rate of carbon oxidation from the char,the proportionality constant being the ratio of nitrogen/carbon atoms (N/C) in the parent char; i.e.,

RNO � �N=C��2RC� �32�whereRC is the rate of carbon consumption andRNO rep-resents the rate of NO formation from char-N oxidation.This approximation, although realistic, is not exact asshown by the data on fifteen coals analyzed by Baxter etal. [115] These authors studied the evolution of nitrogen-containing compounds from coals during coal devolatiliza-tion and char oxidation and concluded that nitrogen evolu-tion is not proportional to the coal burnout rate. Ashman etal. [7] observed that the molar N/C ratio of the product gases(normalized by the N/C ratio of the char) as a function ofchar conversion remained as 1 atT� 1173 K, whereas itvaried from 0.3 to 1.8 atT� 873 K. They obtained theresults during char oxidation in a 2% O2/He mixture carriedout in a TGA. Ashman et al’s [7] results suggest that athigher temperatures there is no selectivity between nitrogenand carbon loss due to oxidation. A similar result wasobtained by Song et al. [116] at temperatures rangingfrom 1250 to 1750 K when burning Montana lignite charat oxygen partial pressures of 0.2 and 0.4 atm.

Fig. 13 presents the results of Ashman et al. [7] and anexample of those of Baxter et al. [115] for one coal. Both arepresented as the ratio of N/C in the gaseous products ofcombustion to that of the parent char. Values above one inthis ratio signify that nitrogen is preferentially released fromthe char during oxidation while values less than one implythat the nitrogen accumulates in the char. As observed inFig. 13, the Baxter et al. [115] study detected that at theonset of the char oxidation there is preferential release ofnitrogen. The authors consider that in the early stages ofcombustion, nitrogen-containing aromatic structures areless stable thermally and may be more susceptible to hetero-geneous oxidative attack. The temperature range of Baxteret al’s [115] experiments was between 1500 and 2000 K[117]. In contrast, Ashman et al’s [7] experiment at 873 Kshows accumulation of nitrogen atoms in the char duringcombustion. These results were confirmed by elementalanalysis of the residual char. For Ashman et al., [7] thegrowing relative amount of pyridinic N and the reductionof pyrrolic N during char oxidation, may be responsible forthe accumulation of nitrogen in the char since pyrrolic ringsmay be preferential oxidized than pyridinic rings. Althoughthe Ashman et al. [7] results are in contradiction to theresults by Baxter et al. [115], the comparison is obscuredby the differences in temperature between both experiments.

A thought experiment would suggest that when a carbon-aceous solid is consumed all elements will be released in


proportion to their concentration. Exception will be thoseelements, such as inorganic elements, that accumulatebecause they are not vaporized. The two conditions underwhich nitrogen will not be released in proportion to carbonare those under which nitrogen is preferentially released bypyrolytic reactions (high temperatures) or when the productssuch as NO are adsorbed (low temperatures).

Obviously at high enough temperatures to burn at diffu-sion limited rate, nitrogen and char will be consumed at thesame rate. The impact of departures from this limit will beexamined later.

5.4. NO reduction

In their pioneering study, Wendt and Schulze [111] onlyconsidered the homogeneous reduction of NO, but theydidn’t specify any mechanism. As pointed out previously,NO is mainly heterogeneously reduced on the char surfaceas in three of the models presented in Table 1. Theadditional reduction of NO by CO catalyzed by char surface(Reaction (26)), is also considered by Shimizu et al. [13] andGoel et al. [23]. Visona and Stanmore [112] also consideredthe homogeneous reduction of NO with HCN.

The models that considered the heterogeneous reductionof NO on char defined the reaction as first order with respectto NO. The value of the kinetic constants used varied foreach model. Visona and Stanmore [112] compared thekinetic constants for the reduction of NO on char surfacereported by de Soete [12], Levy et al., [86] Song et al. [87]and Chan et al. [88] and recommended Chan et al’s [88] asthe one that best fitted their experimental data. Goel et al.[23] applied a least squares optimization method to exper-imental data in order to evaluate the thirteen constants usedin their kinetic model. de Soete et al. [38] also used exper-imental data to evaluate the 25 constants associated withtheir detailed model. de Soete et al’s [38] experiments

also included transient analysis of the reaction system. Allconstants were dependent on the type of char analyzed.

The rate constant for the destruction of NO with COcatalyzed by char (Reaction (26)) was determined fromthe same least square optimization technique by Goel etal. [23] Shimizu et al. [13] on the other hand used Chan etal’s [88] expression for NO reduction in the presence ofCO.

The wide range of approaches to the NO–char reaction isan indication of the uncertainty on the kinetics of the reac-tion and the best way to model it. The influence of thekinetic rate for NO reduction on char will be consideredlater.

6. Influence of kinetic parameters on the amount ofchar-N converted to NO

In order to understand the influence of the rate expressionfor the reduction of NO on the char surface on the finalprediction of char-N conversion to NO, a simplified singleparticle model (SSPM), similar to those of Table 1 was used.A complete description of the model is presented elsewhere[47].

Basically, the model considers the heterogeneous produc-tion of CO from char, the heterogeneous reduction of NO onthe char surface and the direct production of NO fromchar-N oxidation. Reactions (33) and (34) present the rateexpressions.

RvO2� 2kO2

CO2�33�

RvNO � 2�N=C�kO2

CO22 kNOCNO �34�

whereRvi , ki andCi are the volumetric rate of reaction, the

rate constant and the concentration of speciesi respectively.The main assumptions of this model are: (1) all char-Ngoes either to NO or N2 or in other words, N2O and HCN


Fig. 13. Molar N/C ratio of the product gases (normalized by the N/C ratio of the parent char) as a function of char conversion. Squares: dataadapted from Baxter et al. [115] for North Dakota Lignite. Circles: data from Ashman et al. [7] for HVB coal char oxidized at 873 K.

production are neglected; (2) the influence of CO on thereduction of NO on the char surface is also neglected; and(3) all reactions are considered to be first order. Theseassumptions restrict the applicability of the model only tospecific cases. For instance, assumption (1) makes the modelapplicable only at high temperatures (.1400 K), where N2Ois not a product of char oxidation.

This model may provide understanding of the influence ofthe rate of NO reduction on char, the rate of carbon oxida-tion and the nature of nitrogen release during char oxidationon the conversion of char-N to NO. This understanding maysuggest opportunities for further research in this area and forstrategies for NO control.

6.1. Reduction of NO on char surface

Fig. 14 presents a summary of first order rate constants forthe reaction of NO reduction in the high temperature regime.The continuous lines represent the results from Expression(30) and its high and low limits calculated by multiplyingthe rate given by Expression (30) by 10 and 0.1, respec-tively. As discussed above, Aarna and Suuberg’s [90]obtained expression Expression (30) from a least-squaresaverage of results from twenty-four different carbonaceousmaterials and different research groups and claimed that itrepresents the experimental data within one order of magni-tude. The dashed lines represent the kinetic values reportedby Guo and Hecker [100] for two coals (North DakotaLingnite and Pocahontas). These two coals were chosenbecause they were the highest and lowest kinetics in Guoand Hecker’s [100] experiments. It is clear that Expression(30) represents within one order of magnitude the kineticspresented in Fig. 14. Other kinetics (de Soete [12], Levy etal. [86], Song et al. [87] and Chan et al. [88]) were also

found to be within the limits of Aarna and Suuberg’s [90]expression.

Fig. 15 presents the predictions of char-N conversion toNO from the SSPM. Char properties and experimentalconditions were chosen to approximate those of Song etal. [118]. The kinetic rate of Smith [119] was used for thecarbon–O2 reaction.

The constants for the five kinetic rates presented in Fig. 14were used in the SSPM. The data show an increase in theconversion of char-N to NO as the temperature is reducedfrom 1800 to 1000 K. Although a similar trend is reportedby Ashman et al. [7], the results should be interpreted asqualitative, especially at temperatures below 1400 K wherethe production of N2O begins to be important. In fact, deSoete [12] and de Soete et al. [38] present the opposite trend,however their experiments were in the lower range oftemperature (800–1100 K) in which N2O production isconsiderable. At high temperatures, the dependence ofconversion with temperature decreases. This is in accordancewith the results of Pershing and Wendt [120] for charcombustion at pulverized coal conditions. These authorsconcluded that the fuel nitrogen conversion to NO fromchar burned in a pulverized combustor was only a weakfunction of flame temperature. This suggests, as expected,that the predictions of the SSPM may be reasonable at hightemperatures.

As in Fig. 14, the continuous lines represent Aarna andSuuberg’s [90] generalized expression for NO reduction onthe char surface and the limits within one order of magni-tude. The dashed lines are the predictions according to Guoand Hecker’s [100] results.

At the high temperatures in which the SPPM may predictmore realistically the results, the amount of char-Nconverted to NO varied from 0.98 for the lowest value of


Fig. 14. First order rate constants for the reaction of pure NO with coal chars. Char surface area� 530 m2/g. PNO � 10 1325 Pa: rp �1250 kg=m3

: (a) Guo and Hecker [100] (Pocahontas); (b) Guo and Hecker [100] (NDL); (c) 10pAarna and Suuberg [90]; (d) Aarna andSuuberg [90]; and (e) 0.1pAarna and Suuberg [90].

kNO to 0.24 for the highest value. This shows the importanceof the kinetics of NO destruction on char surface on the finalprediction of char-N conversion to NO.

Finally, curves f and g show the predictions of the SSPMwhen the NO concentration surrounding the particle isincreased to 750 ppm. The expression by Aarna andSuuberg [90], and the kinetics of Guo and Hecker [100]for the Pocahontas coal were the equations used for thereduction of NO on a char. A considerable reduction onthe conversion of char-N to NO is predicted. In fact, at1800 K the SSPM predicts a negative value if the kineticsof Guo and Hecker [100] is used. This agrees with Spinti’sresults [48] that showed a reduction on char-N conversion toNO as the NO bulk concentration increases. Higher NOconcentration increases the rate of NO destruction insidethe particle. A negative value of char-N conversion to NOmay be understood thus as a rate of destruction of NO by thechar which exceeds the nitrogen fed in the char.

Fig. 15 illustrates not only the importance of the selectionof adequate kinetic rates for NO reduction, but also the

significance of selecting the right kinetic mechanism. Thesimple mechanism of the SSPM is not accurate enough topredict the results at lower temperatures were HCN and N2Obecome important intermediates.

6.2. Carbon–oxygen reaction

Fig. 16 presents the influence of the rate of carbon reac-tion with O2. The base cases are the results obtained whenthe carbon oxidation kinetics of Smith [119] are used. Thetransformation of the conversion of char-N to NO when theoxidation kinetics is varied within one order of magnitudewas studied for the rate expressions of Aarna and Suuberg[90] and Guo and Hecker [100]. The faster the reaction ofcarbon with oxygen, the higher the conversion of char-N toNO since the rapid consumption of the char will reduce thepossibilities of NO reduction on the surface of the char.Fig. 16 also underscores the importance of the model usedfor the prediction of the coal oxidation.


Fig. 15. Char-N conversion to NO as predicted by the SSPM. Particle size� 38mm. O2 concentration around the particle� 0.21; NO� 0(except for cases f and g). Captions as in Fig. 14. (f) Aarna and Suuberg [90] NO� 750 ppm. (g) Guo and Hecker [100] (Pocahontas)NO� 750 ppm.

Fig. 16. Char-N conversion to NO as predicted by the SSPM. Particle size� 38mm. O2 concentration around the particle� 0.21; NO� 0.Continuous lines: NO kinetics from Aarna and Suuberg [90]. Dashed lines: kinetics of Guo and Hecker [100] for Pocahontas. For a and d,ko2� 0:1 × ko2

by Smith [119]. b and e withko2from Smith [119]. c and f withko2

� 10× ko2by Smith [119].

6.3. Rate of nitrogen release during char oxidation

As discussed before, most of the models for char-Nconversion to NO considered that the rate of NOformation is proportional to the ratio N/C of the parentchar. However, Fig. 18 presented two different experimen-tal results that shows that the ratio N/C varies during charcombustion.

To understand the influence of this variation on theconversion of char-N to NO, empirical expressions of N/Cas a function of char conversion were derived from Fig. 18and applied to the SSPM. The kinetics of Aarna and Suuberg[90] for char-N oxidation and the one of Smith [119] for thecarbon–oxygen reaction were used in all the simulations.

Fig. 17 presents the results. Lines b and c are SSPMpredictions when a constant value of N/C is used for Baxteret al. [115] and Ashman et al. [7] chars. The different finalprediction of char-N conversion to NO is due to the differentchar composition of both experiments. Lines a and d presentthe results when a variable value of N/C is used. Althoughthe experiment of Ashman et al. [7] was carried out at873 K, the results of Fig. 17 are at 1750 K for both expres-sions for N/C to allow comparison.

There is little difference in the final prediction of char-Nconversion to NO when the empirical correlation for N/Cobtained from the Baxter et al. [115] experiments is used(linesa andb). The reason for this is that Baxter et al. [115](Fig. 15) found that most of the N was released at the begin-ning of char oxidation. At these stages of the reaction, theparticle has enough surface area for reducing the NO beingformed. Therefore, although line a exceeds line b at the lowvalues of conversion, the difference between both lines isless than what is expected when Fig. 13 is considered. Athigh conversion, Baxter et al. [115] observed that the releaseof N was less than that predicted according to the value of N/C for the parent char. Therefore it may be expected that theproduction of NO is slow and this phenomenon is observedin Fig. 17. The net sum of these two effects is that the final

conversion of char-N to NO is almost the same with variableand constant value of N/C.

On the contrary, when the empirical expression ofAshman et al. [7] is used, the final conversion of char-Nto NO differs considerably from the one obtained when aconstant value of N/C is used. As observed in Fig. 13,Ashman et al. [7] found that nitrogen accumulated in thechar during combustion. Therefore, during the first stages ofthe oxidation when there is large carbon area for the reduc-tion of NO on char surface, the production of NO is low.Conversely, at high conversions, where the capacity of NOreduction by the char is low, the release of NO is higher, andthus the conversion of char-N to NO is higher.

The results of the SSPM suggest that the final conversionof char-N to NO may be influenced by the time when N isreleased during the oxidation of the char. The retention of Nin the char may increase the amount char-N converted to NOwhereas a premature release of N may reduce it.

7. Implications for coal-fired utility boilers

This review has made the transition from the microchem-istry associated with the elemental mechanisms thatdescribe the gas–solid phase reactions of O2 and NO withchar, to the chemistry and physical process associated withthe burning of a single char particle. However, the combus-tion behavior of single particles can vary considerablywithin coal-fired utility boilers, making it difficult to use asingle particle model to describe overall behavior. To over-come this difficulty, computational fluid dynamic (CFD)codes have been used extensively when modeling combus-tion in coal-fired utility boilers. Particularly in the study ofNO production, various authors have tried to use computer-modeling codes for the prediction of NO emissions duringcoal combustion [121–134]. Agreement between exper-imental data and model predictions varies; however, mostare capable of prediction of NO concentration trends, even if


Fig. 17. Prediction of char-N conversion to NO as a function char conversion according to the SSPM. Particle temperature� 1750 K. Theinfluence of the variation of N/C during combustion is analyzed. Baxter et al [115]: (a) Variable N/C; (b) Constant N/C. Ashman et al. [7]: (c)Constant N/C; and (d) Variable N/C.

exact values at specific points are sometimes not predictedaccurately.

While modeling NO production during pulverized coalcombustion, the most common approach is to develop theNO production routine as a post-processing model for themain comprehensive model. This post-processor usesthe temperature, flow-field and concentration resultsobtained from the main combustion model calculations,and the NO production model is thus decoupled fromthese calculations. This is a reasonable approach since dueto their low concentration, the nitrogen-containing specieshave a negligible influence on the mass, heat and momen-tum balances of the complete furnace.

The most common approach for modeling the conversionof char-N to NO is the one proposed by Smith et al. [135]According to this model, the release of N from the char isproportional to the mass consumption during char combus-tion. All fuel-N is converted to HCN, which is then eitheroxidized to NO or reduced to N2 according to the kineticrates of de Soete [136]. This approach is described by thedashed lines in Fig. 18, and is used extensively in CFD

simulations [121–125,132]. Variations of this approachinclude a fixed efficiency factor for the direct conversionof char-N to NOx, This factor may be zero [129] or avalue between zero and one obtained either empirically[126,128,131] or by considering the reduction of NO withchar or CO [130,137] More refined approaches allow for theseparate formation and destruction reactions as described byJones et al. [138] and in the following paragraphs. Someothers authors not only consider that the nitrogen is releasedas HCN, but also as NH3 [134,139] depending on its func-tional form in the coal. These approximations have beensufficient for predicting with reasonable accuracy the totalNO production from coal combustion. However, as the levelof NO production in the boilers is reduced due to morestringent regulations, the relative importance of the NOproduced from the char increases, and it may be necessaryto use models that approximate in a more accurate way thephysics in this system. Examples of these are the singleparticle models described previously.

One example of the improvements obtained when a moreaccurate char-N description is used was presented byEddings et al. [131]. These authors extended the model bySmith et al. [135] by considering volatile-N and char-N astwo different streams (continuous lines in Fig. 18). Thisvariation in the model improved code predictions asobserved in Fig. 19. By considering nitrogen evolutionfrom the char, independent from nitrogen evolution fromvolatiles, Eddings et al. [131] considerably reduced theover-prediction of NO for all the coals they analyzed(Fig. 19). Visona and Stanmore [134] also compared differ-ent approaches for the model of char-N conversion to NOwhile modeling a swirling pulverized coal flame. Theseauthors did not find an optimum approach, and concludedthat the principal limitation in their model were theuncertainties associated with the model for char-N to NOconversion.

Another attempt to apply a single particle model for the


Fig. 18. Suggested simplifications used in CFD for approximatingthe fuel-N that is transformed to NO. Dashed lines: traditionalmodel by Smith et al. [135]. Continuous lines: model by Eddingset al. [131].h : from experimental results or models.a : fixed value.

Fig. 19. Comparisons of exhaust NO values with model predictions during pulverized coal combustion [131].

prediction of char-N evolution to NO was presented bySarofim et al. [47]. These authors applied the SSPMpreviously described as a post-processor for the predictionof NO formation from coal particle trajectories in a 500 MWutility boiler. The boiler is an opposed-wall-fired pulverizedcoal unit that had undergone a low NOx burner retrofit. Theburner retrofit reduced the measured NOx emissions at theplant from approximately 800 ppm to below 400 ppm. TheSSPM model was used to investigate char N conversionbehavior in a full-scale utility boiler both before and afterthe low NOx burner retrofit, where the environment experi-enced by burning char particles were quite different. Thepre-retrofit burning environment was predominantly oxida-tive; whereas the post-retrofit environment was predomi-nantly reducing. The SSPM model was coupled withGLACIER, a reacting CFD code developed for modelingthree-dimensional, reacting two-phase flows using a Lagran-gian particle cloud tracking technique.

With the input data from the CFD code, the SSPM wasable to predict the cumulative conversion of char-N to NO,FNO. The value ofFNO can be negative if NO productioninside the particle is smaller than the NO consumption bythe reduction reaction, i.e. Eq. (34), 0. Sarofim et al. [47]compared the results for one boiler before and after theimplementation of a NOx control strategy (low NOx burnerswith over-fire air). The information obtained from the CFDcode corresponded to a total of 3456 different particle cloudtrajectories. For each trajectory, the particle size, the oxygenand NO bulk concentration and the particle temperature vs.residence time were known. The SSPM predicted an indi-vidual value of the conversion of char-N to NO,FNoi, foreach trajectory. Fig. 20 presents the predicted values ofFNO

against cumulative mass particle weight. Two characteris-tics of Fig. 20 should be highlighted: (1) the wide range ofvalues predicted forFNO (23 to 100%); and (2) the resultsfor both cases, pre- and post-retrofit, are very similar. The

first point is not surprising, considering the variableatmospheres to which different chars may be exposed in alarge utility boiler. Particles submitted to reducing atmo-sphere, i.e. fuel-rich pockets, may act as reducing agents.Particles in contact with oxygen will be rapidly oxidized andthe possibility for NO reduction will be low. However, thesimilarity of the predictions ofFNO for both cases was unex-pected. Sarofim et al. [47] proposed that this was the resultof a trade-off of two different effects. Table 2 presents massaverage values of the concentration of NO, O2 and particletemperature during the trajectories of all char clouds, forboth cases. As expected, the average NO concentration islower for the post-Retrofit case. This lower concentrationshould reduce the capability of the char for reducing NO(see Fig. 15). However, the technique used for NO reductionin the furnace, not only reduced the local concentration ofNO, but also decreased local O2 concentration. A reductionin the O2 concentration will reduce the rate of carbon oxida-tion, and as shown in Fig. 16, this will reduce the amount ofcoal nitrogen transformed into NO. The predicted value forthe mass average ofFNO is close to previous results reportedby Pershing and Wendt [120], and Chen et al. [140] (10–20%) for similar conditions, providing some confidence inthe calculations.

Regardless of the numerous assumptions associated inSarofim et al’s simulation, their results illustrate thecomplexity of this system and demonstrate the need toinclude the effects of all the main combustion parametersinto char NO calculations.

In the above calculations the NO concenration field wascalculated using a mean char nitrogen conversion effi-ciency. The NO concentration should be calculated inter-actively with the calculation of char nitrogen conversion.Procedures for coupling the homoegeneous and hetero-geneous kinetics include the use of Zone Models asdescribed in Ref. [141].


Fig. 20. Distribution of char-N conversion to NO,FNO, as predicted by a single particle model for two different boilers. From Sarofim et al. [47].

8. Conclusions

Although the conversion of char-N to NO is a complexprocess in which different mechanisms occur simulta-neously, the continuous study of the system in the last 25years has allowed its preliminary characterization. A reviewof these studies has resulted in the following conclusions:

• Char nitrogen contributions to NOx emissions frompulverized coal-fired boilers have grown in importance.Staged air addition for NOx control suppresses theconversion of volatile nitrogen, but has a relativelysmall impact on the char nitrogen conversion to NOsince the char persists into the post-combustion zoneand burns in the leaner combustion products after airaddition. Further, the conversion efficiency of the charnitrogen to NOx increases as the ambient NO concentra-tion decreases.

• XPS analysis of parent coals and chars identified thatnitrogen is present in pyrrolic, pyridinic and quaternarystructures. The abundance of each of these structuresdepends mainly on the thermal history of the char.Despite of the evidence for the existence of differentnitrogen-containing functionalities in the char, the effectof them on the final conversion of char-N to NO appearsto be secondary, when compared to other parameters suchas char surface area, catalyst concentration and pore sizedistribution.

• Although there is agreement on the heterogeneousproduction of NO during char oxidation, the mechanismfor N2O production is still unclear. The existence of HCNas an intermediate has been detected at low temperaturessuch as those offluidized bed combustion. However, thereis also experimental evidence that suggests an alternativeheterogeneous route for N2O production. The exactmechanism for NO and N2O production from char-N istherefore to be established. The recent use of computa-tional chemistry tools on the analysis of the reaction ofcarbonaceous materials may give further insight on thesekinds of systems in which the extremely fast rates ofreactions make the experimental analysis difficult.

• The mechanisms of the NO–char and N2O–char reactionsare fairly well understood; however, the influence of otherreacting gases in the system is still being studied. Carbonmonoxide and oxygen seem to have a strong influence onthe reaction under certain conditions.

• Although the numerous studies carried out on the kineticrates for the reduction of NO on a char surface haveallowed definition of kinetic parameters for differentcarbonaceous materials, there is still uncertainty on: (1)the exact order of the reaction with respect to NO, espe-cially at low temperatures; (2) the influence of the charsurface area; and (3) how to quantify the catalytic influ-ence of other reacting gases, such as oxygen and carbonmonoxide, on the system. The same issues apply to theN2O–char reaction.

• Even though several characteristics of the physics andchemistry of this system remain to be defined, differentmodels that utilize engineering approximations to addressuncertainties in the basic mechanisms and kinetic rateshave been proposed in the literature. Among the differentassumptions of these models, the kinetic parameters forthe NO–char and O–char reactions and the approxima-tion for the rate of nitrogen evolution from the char wereidentified to have a strong influence on the total conver-sion of char-N to NO.

• The NOx/char nitrogen reactions of importance changewith temperature. The reactions are also strongly influ-enced by the presence of oxygen and CO. At low tempera-tures of interest for catalytic reduction (,500 K), char canreduce NO only after the NO has been oxidized to NO2. Athigher temperatures (600–900 K), below the range ofinterest to fludized bed combustion, NO is stronglychemisorbed on char and has different fates dependingupon the char composition; among the mix of productsthat can be formed are HCN, N2, HCNO, N2O, or it mayaccumulate on the surface and be released again as NOwhen the char burns out. At fluidized bed temperatures thepaths to N2, N2O, and HCN dominate. At pulverized coalcombustion conditions, the reduction to N2 is dominant.

• As overall NO emissions from coal-fired boilers move tolower levels, the use of more refined models for the


Table 2Summary of predictions of the SSPM for two different boilers, pre- and post-low NO strategy. After Sarofim et al. [47]

Case Pre-low NOx Post-low NOx Pre/post

Number of trajectories 1536 1920 –Average NOx (ppm) along char cloudtrajectories, normalized by mass

475 327 1.5

Average O2 (%) along char cloudtrajectories, normalized by mass

5.8 1.5 3.9

FNO (%) (average) (wt) 13.2 11.1 –Temperature during char trajectory,normalized by mass (K)

1442 1515 0.95

Predicted Exit NO concentrations (ppm) 803 351 2.3

conversion of char-N to NO becomes necessary toimprove the accuracy of CFD codes in predicting NOformation. Results from applying a single particlemodel as a postprocessor to the predictions of a CFDcode indicates that char particles play an important role,both in NO formation and destruction. The magnitude ofthis role depends on char properties and the environmentsurrounding the char particle.

Acknowledgements

This work was partially sponsored by DOE under grantno. DE/FG62697FT97275. One of the authors (AM) hasreceived support from COLCIENCIAS under the programBecas Cre´dito para estudios en el exterior.

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Char nitrogen conversion: implications to emissions from coal-fired utility boilers

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