Van Der Lans - Parameters on Nitrogen Oxide

Pergamon PN,~. Eiw,qy Comhusc. SC;. Vol. 23, pp. 349-377. 1997

Q 1997 Elsevier Science Ltd Printed in Great Britain. All rinhts reserved

0360- I285197 $29.00

PII: SO360-1285(97)00012-9

INFLUENCE OF PROCESS PARAMETERS ON NITROGEN OXIDE FORMATION IN PULVERIZED COAL BURNERS

R. P. van der Lam*, P. Glarborg and K. Dam-Johansen

Department of Chemical Engineering, Technical University of Denmark. Building 229, 2800 Lyngby, Denmark

Abstract-This paper describes the influence of burner operating conditions, burner geometry and fuel parameters on the formation of nitrogen oxide during combustion of pulverized coal. Main attention has been paid to combustion test facilities with self-sustaining flames, while extensions have been made to full-scale boilers and furnace modeling. Since coal combustion and flame aerodynamics have been reviewed earlier, these phenomena are only treated briefly. 0 1997 Elsevier Science Ltd.

Keywords: nitrogen oxide, pulverized coal, combustion, burner, emission formation, modeling.

CONTENTS

Nomenclature 1. Introduction 2. Coal Combustion

2.1. Coal Characterization 2.2. Devolatilization 2.3. Combustion of Volatile Matter 2.4. Char Oxidation

3. NO Formation 3.1. Sources of NO

3.1.1. Thermal and prompt NO 3.1.1.1. Thermal NO 3.1.1.2. Prompt NO 3.1.1.3. Modeling NO formation

3.2. Fuel NO 3.2.1. Nitrogen release during combustion

3.2.1.1. Rate of release 3.2.1.2. Nitrogen volatile species

3.2.2. Influence of coal properties on the formation of NO 3.2.3. Modeling of gas phase nitrogen conversion

3.3. Stoichiometry and Residence Time 3.4. Effect of Temperature 3.5. Effect of Moisture

4. Flow Pattern and Flame Type 4.1. Swirl Number 4.2. Alternative Swirl Numbers 4.3. Flame Type

5. The Influence of Aerodynamics on NO Formation 5.1. Conventional Versus Low NO, Burner 5.2. Primary Air 5.3. Secondary Air and Swirl Number 5.4. Particle Size and Slip Velocity 5.5. Coal Blends 5.6. Generalization of Results

6. Full Scale 6.1. Reduction of Overall Excess Air Level 6.2. Furnace and Burner Air Staging 6.3. Flue Gas Recirculation 6.4. Fuel Concentration 6.5. Particle Size 6.6. Other Techniques

7. Furnace Modeling 7.1. Reaction Engineering Models 7.2. Computational Fluid Dynamics 7.3. Other Methods

8. Research Needs Acknowledgements References

350 350 350 350 3.50 351 351 352 352 352 352 352 352 353 353 353 354 355 355 356 357 359 359 359 360 360 361 361 362 365 367 368 368 368 369 370 370 370 370 370 371 371 372 373 373 373 374

*Corresponding author.

JPLCS 23.1-o 349

350

L 4 G, GX MR P r, R s, S’, s’ u, w

VPA, VSA, VTA

xv Y, z x P

i &D CSTR HVB, MVB, LVB

IRZ PA, SA, TA PDF PPR SBR SCR SNCR subbit., bit.

R. P. van der Lans et al.

NOMENCLATURE

constant diameter (m) particle diameter (m) angular momentum (kg m*/s*) axial momentum (kg m/s*) momentum ratio ( - ) pressure (Pa) radius (m) swirl numbers ( - ) axial and tangential component of velocity Ws) primary, secondary and tertiary air velocity (m/s) conversion efficiencies stoicbiometric ratio ( - ) density (kg/m’) residence time (s) volume flow (ml/s) computational fluid dynamics continuous stirred tank reactor high, medium and low volatile bituminous coal internal recirculation zone primary, secondary and tertiary air probability density function plug flow reactor solid-body rotation selective catalytic reduction selective non-catalytic reduction subbituminous. bituminous

1. INTRODUCTION

Nitric oxides constitute a major pollutant from coal- fired industrial furnaces, and generally must be controlled to meet governmental standards. Best is to abate NO formation at its source: the burner. Modifications lead to advanced low NO, pulverized coal burners, which use aerodynamically staged air and swirl on the air flow to affect the mixing and combustion pattern and to suppress the formation of NO. Substoichiometry of the near burner zone, where devolatilization takes place, is of primary importance for the reduction of nitrogen oxide emission. The formation of nitrogen pollutants is a complicated process which is influenced by both combustion phenomena and aerodynamics. Coal heat-up, devolatilization, ignition, as well as homogeneous and heterogeneous combustion interact with the mixing process of secondary air and recirculated gases with the pulverized coal. As a result, pollutant formation depends more or less on all these processes.

In the past, reviews have been given on several aspects of pulverized coal combustion: overall combustion phenomena’.‘, pyrolysis3, and char combustion and heterogeneous kinetics4 have been well described. Since aerodynamics are important for swirl burners, reviews on aerodynamics have been published by several autho&*. The main phenomena of pollutant formation during combustion have been described’ including the fundamental principles of NO reduction, but the influence of burner parameters on the formation of NO was not discussed. An overall review on pulverized

coal combustion, which covers most of the phenomena occurring during combustion, including a lot of data, has been published more recently”. However, the review dealt only briefly with nitrogen oxide formation.

The purpose of this paper is to describe the influence of burner and furnace parameters on the formation of nitrogen pollutants from pulverized coal burners. Both coal parameters, operating conditions such as air flows and swirl, and burner configuration will be treated. It must be noted that the list of publications reviewed is not complete, since the amount of literature published in this field is enormous. Main attention is given to combustion test facilities with self-sustaining pulverized coal flames. For detailed information on coal combustion in general or aerodynamics the reader is referred to the reviews listed above. For the sake of completeness a general description is given below.

2. COAL COMBUSTION

2.1. Coal characterization

Coal is a fossil substance formed from the remains of plants and trees. The degree of maturity of a coal represents the extent to which coalification of plant material has progressed and is called the rank of a coal. The process of coalification proceeds from peat over lignite, subbituminous, bituminous and ends with anthracitic coal. The carbon content increases from 65 wt.% for peat to 97 wt.% for anthracite, and during the coalification process moisture is squeezed out. Tem- perature and pressure are the driving forces for these structural changes.

The standard method of determining the rank of a coal is based on tests of the volatile matter content for coal whose rank is medium volatile bituminous or higher, and on tests of the heating value for lower rank coals”. Many different classification systems are in use”, in which criteria have been chosen for historical reasons. Characterization of coal is a tool in predicting the behavior of coal during combustion. Important parameters such as the specific energy, ash content, moisture content, volatile matter content, and elementary composition are expressed in the proximate and ultimate analysis of coal13. The influence of a number of important coal parameters on the combustion behavior of pulverized coal will be described later.

The classic combustion model assumes that combustion takes place in three stages’*: ?? devolatilization ?? combustion of the volatile matter, and ?? char burnout.

2.2. Devolatilization

When a coal particle is introduced into a burner it is first heated up by radiation from the burner quarl and by

Influence of process parameters in nitrogen oxide formation 351

convection of hot, recirculated gases. Convection dominates over radiation for the heat-up of particles. Heating rates of pulverized coal particles in a boiler are known to be of the order 104-IO6 K/s’“, with peak temperatures of the order 1800-1900 K”-lg. During heat-up the particle releases volatile material, which ignites in the presence of oxygen when the temperature is high enough. The amount of volatiles (together with the rank of a coal) influences the ignition and it is therefore important for flame stability. Heterogeneous ignition of the particle may also occur. The yield of volatiles is highly dependent on coal type and combustion process conditions, such as the final temperature reached, heating rate and particle size. Lower rank coals have a larger mass fraction released as volatiles than higher rank coals. For bituminous coals a large proportion is released as tar, with an average molecular weight above 300 g/mole’“,20. NO precursors like HCN and NH3 are formed during devolatilization and secondary cracking of the ta?‘-23.

Different products are released at different temperatures*‘: ?? removal of adsorbed water ( < 150°C) ?? evolution of pyrolytically formed water (300-400°C) ?? softening of coal, accompanied by tar and hydrogen

evolution (4OO-900°C): an open porous structure is produced

?? evolution of carbon monoxide and hydrogen ( >9oo”C).

Rapid loss of volatile matter occurs within the first 30 ms and this is closely associated with a decrease in relative hydrogen content. Char combustion and loss of volatile matter may take place simultaneously. After devolatilization a porous char matrix is left with a high carbon content, which bums heterogeneously in the post- flame region. Pyrolysis times are normally only a small proportion of the burnout time of a coal particle in the combustion chamber; for total burnout in a practical furnace, a residence time of 1 to 3 set is required, determined by the char reactivity, particle size, density and the combustion environment.

The coal composition is important for the formation of NO primarily because it controls the local environment and temperature history in the fumacez4. However, variations in the primary nitrogen species evolved may also have some effect, as discussed in Section 3.

2.3. Combustion of Volatile Matter

Carbon monoxide, C02, HzO, light hydrocarbons such as methane, ethane and ethylene, heavier hydrocarbons, and tar are formed during primary devolatiliza- tion22*23,25*26. Volatile products of low rank coals contain significant amounts of non-condensibles while high rank coals release predominantly tar2’. The tar is subsequently cracked into lighter components and in a region of oxygen deficiency soot will be formed’*2s-30. This decrease in tar content and simultaneous increase in soot concentration is sometimes called secondary devolatilization. The product distribution of the combustible, non- condensible gases becomes dominated by CO, Hz, CH4

and CzH2 *g-3’. Initially tar is converted into soot, while at a later stage mainly lighter components such as CzH2 contribute to the increase in soot yield. The burning rate of the volatiles including tars and soot has been reported to be independent of coal rank3*, while a clear rank dependence has been observed for the combustion of non-condensible fuel compounds as formed by secondary devolatilization. Cho et aZ.33 emphasized the increasing importance of CO and H2 during secondary devolatilization. Differences in burning rates of non- condensibles were related to different proportions of H2 and CO formed during secondary devolatilization of coals from several ranks. More CO and less H2 is produced from lower rank coals.

The uncertainty in devolatilization rate and volatile composition, the wide variety of gaseous components released during primary devolatilization, and the simultaneous process of combustion and secondary devolatilization, render difficult the development of volatile oxidation models. Gas-phase combustion of volatiles is therefore either modeled with a fast chemistry assumption, where local instantaneous gas properties are determined from equilibrium without regard to chemical kinetics, or with simple, global chemical reaction schemes, where the elements (C, H, 0, S) react to CO, C02, HzO, S0234. Calculations on methane combustion in an ideal plug flow reactor which will be described in Section 3 show that after mixing the main reactions are completed in about 1 ms at T= 1500 K, while at higher temperatures, as present in the flame of a pulverized coal burner, typical reaction times are a fraction of a millisecond. This shows that the assumption of fast chemistry may be valid at high temperatures if the time scale of mixing is larger. A few reactions, like the oxidation of CO to CO;?, are slow, but the influence of CO on nitrogen pollutant formation is presumably limited. At high temperatures the interaction of turbulence or mixing rates with chemical reaction rates further complicates the process of gas-phase combustion (see Section 7). This may explain why a substantial amount of literature can be found on devolatilization and char oxidation, but much less on homogeneous gas-phase combustion of coal devolatilization products. It should be emphasized that the gas-phase kinetics to a large extent determine the selectivity of fuel nitrogen to NO or NZ, and that these kinetics must be considered in pollutant modeling. However, as long as devolatilization, soot formation and mixing rates cannot be modeled adequately, use of detailed kinetics cannot be expected to improve modeling accuracy significantly. More research in this area is certainly needed.

2.4. Char Oxidation

Char oxidation in a boiler is a slow process compared to devolatilization and volatile combustion, and it determines therefore the size of the boiler with respect to burnout. The main parameters of importance for the oxidation rate of char are: ?? coal type (including mineral content, pore structure/

development, fragmentation)

352 R. P. van der Lans et al.

?? particle size below. Fuel NO, which is the most important source of ?? temperature (which also influences the char structure NO in pulverized coal combustion will be described in

after devolatilization) more detail in Section 3.2. ?? oxygen concentration.

Oxygen diffuses through the film and into the pores of the particle, and reacts with the carbon to form CO and/ or CO*. Depending on the prevailing temperature, diffusion and/or chemical kinetics may limit the reactions. At low temperatures (usually less than about 600°C) the reaction rate is slow and the oxygen completely penetrates the pores. The rate is controlled by the chemical kinetics and the conversion is proportional to the internal surface area (intrinsic rate). The particle does not shrink, but the overall density decreases. At high temperatures the chemical reaction rate is fast and the diffusion of oxygen through the boundary and into the pores becomes the limiting factor. The particle only bums at the outer surface, and it shrinks in time. Pulverized coal apparently bums under combined diffusion and chemically controlled conditions4. Pore diffusion is usually incorporated into combustion models either at an empirical reaction rate based on the outer particle surface, or in an efficiency factor which is combined with the intrinsic reaction rate.

3.1.1. Thermal and prompt NO

3.1.1.1. Thermal NO. At local temperatures above 1800 K the formation of NO by reaction of Nz with O2 starts to be significant. High temperatures are required to break the strong triple bond in the nitrogen molecule, and that is why NO formed by this mechanism is called thermal NO. The reaction mechanism was discovered by Zel’dovich4*:

N2+O+NO+N (1)

N+02+NO+0 (2)

In fuel-rich flames the following reaction is also of importance43:

The initial particle size distribution is of importance since small particles generally bum out more rapidly. Usually the specific reaction rate (kg coal/m’ outer particle surface per second) decreases with increasing particle size, due to an increase in boundary layer thickness, which reduces the transport rate of O2 to the particle. However, during grinding, maceral dispropor- tionation may occur, giving rise to differences in oxidation rates between the size fractions35’36, and the ash content is higher in the small size ranges’, which may change reaction rates3’. In addition, a change in particle size affects heat generation and heat loss from the particle, resulting in a change in the thermal equilibrium conditions and temperature.

N+OH=NO+H (3)

Low NO, burners are designed to operate below a temperature of 1800 K, and the contribution of thermal NO to the total NO is therefore small, although formation at local peak temperatures still may contribute to the total NO emitted.

3.1.1.2. Prompr NO. A rapid generation of NO in the reaction zone of ethylene-air flames, which could not be described by the thermal NO mechanism, was observed by Fenimore”. This faster, transient formation was called ‘prompt’ NO. Since it was only found in hydrocarbon flames, the initiating step was believed to be the reaction of hydrocarbon radicals with N2 from the combustion air to form the intermediate HCN. This was later verified45 and the most important reaction has been shown to be46*47

The influence of mineral matter on the char combustion rate is included in the overall reactivity. Calcium may increase and phosphor decrease the reactivity of char3s. Mineral matter may also change the macro- porosity of the char39.

CH+N2 =HCN+N _

with a smaller contribution from

(4)

C+Nz +CN+N (5)

The HCN formed can be further oxidized to NO or to Nz depending on the prevailing conditions, as will be discussed below.

3. NO FORMATION

3.1. Sources of NO

Nitrogen oxides are generally termed NO,, which includes NO, NO2 and N20. In pulverized coal fired boilers the N20 and NO1 emissions are usually very low and neglected40*4’.

Prompt NO is proportional to the concentration of Nz and hydrocarbon radicals in the flame. Compared to thermal NO the amount formed is only a weaker function of temperature, and it shows a maximum at fuel-rich conditions”.

During the combustion of coal the production of NO originates from three different sources: thermal NO, ‘prompt’ NO, and fuel NO. These three sources can be distinguished theoretically, but in practice it is often not possible to derive the contribution from each source since the intermediate reaction products are to some extent identical. Thermal and ‘prompt’ NO are described

3.1.1.3. Modeling NO formation. To illustrate the importance of thermal and prompt NO formation, calculations have been performed for the combustion of methane with air in an ideal plug flow reactor. A comprehensive kinetic reaction scheme was applied for the calculation of the homogeneous gas-phase reaction rates, consisting of a hydrocarbon reaction subset4s, an amine reaction subset49, a cyanide reaction subse?’ and a CO/H2/02/N0 X subse?‘. The temperature in the


Fig. 1. Thermal and prompt NO formation in a PFR during combustion of methane in air (1.5 vol.% H20) with and without

addition of 1500 ppm HCN; T = 100 ms; X = 1.2.

Lignite I 8 I 1

High Volatile Bituminous -

0.2 0.4 0.6 0.8 Fractional Mass Loss daf. (-)

Fig. 2. Fractional loss of C and N of a lignite and a high volatile bituminous coal against the fractional total mass loss

(data from “).

reactor was given as an input parameter. For an overall stoichiometry of h = 1.2 the thermal and prompt NO concentrations have been calculated separately as a function of temperature. Discrimination between prompt and thermal NO during plug flow calculations at the temperatures investigated is possible by

removing the reactions for the formation of prompt NO (Eqs (4) and (5)) and the reaction of N2 with 0 (the forward reaction in Eq. (I)), respectively. From the difference between the results from the complete mechanism and the reduced mechanisms the contributions of thermal and prompt NO are known. Addition of 1500 ppm HCN as fuel-N reveals the influence of fuel-N on the formation of prompt and thermal NO (Fig. 1).

The formation of prompt NO takes place in the flame front where the radical concentration is high within about a millisecond, while the thermal NO process is much slower. The formation rate of thermal NO is constant in the time interval used in these calculations. From Fig. 1 it can be observed that prompt NO formation is small in both cases and at all temperatures. Addition of fuel-N reduces the formation of thermal and prompt NO. At 2200 K the thermal NO concentration is reduced from 1750 ppm to 1360 ppm by the addition of HCN.

In flames local peak temperatures may well exceed 1800 K, but the contribution of these fluctuations to the total emissions is difficult to assess.

3.2. Fuel NO

Coal contains usually between 0.2 and 2.5% chemically bound nitrogen on a weight basis. During combustion this nitrogen can react with oxygen to form NO.

3.2.1. Nitrogen release during combustion

3.2. I. 1. Rate oj’release. During devolatilization nitrogen compounds are released as volatiles. The total volatile N- yield can be up to lOO%, depending on the temperature, while the total mass loss during devolatilization is usually smaller than 75 wt.%52*60,69. The rate of nitrogen release in proportion to the rate of total mass loss depends on coal type and temperature. Pohl and Sarofim” pyrolyzed a lignite and a high volatile bituminous coal in an electrically heated furnace, and found that nitrogen evolution paralleled the volatile release after 10 to 15% of the volatiles had been devolatilized with little N-loss. Baxter et al.53 studied the release rates of mass and coal nitrogen in an entrained flow reactor (Fig. 2). They concluded that the fractional nitrogen release from low rank coals is much slower than the fractional mass release and also slower than the initial release of carbon. This is due to the fast release of water and light components during the early stage of devolatilization. At medium rank, the release rates are essentially equal, which confirms the findings of Costa et aZ.54, while at high rank, for low volatile bituminous and anthracite, the fractional nitrogen release is relatively faster than the mass loss. Changes in the rate of nitrogen release are attributed to a combination of differences in both the chemical structure of coal, in temperature histories during combustion, and in char chemistry. Clarke et al.55 studied the nitrogen release rate in an entrained flow reactor for 5 coals. The total

I


Table 1. Nitrogen volatile products under pulverized coal combustion conditions

Results Equipment Conditions

Nitrogen is primarily evolved as HCN

Nitrogen is both evolved as HCN and NH3; HCN is the largest component, but the relative amount of NH3 increases with lower rank

More NH3 is formed than HCN

More HCN is found for high rank coal, while the contribution of NH3 increases towards lower rank coal, and can become larger than HCN

Radiant flow reactora Arc-jet fired entrained flow reactor (7 = 80 ms)57 Heated grid’” Pyroprobe’s Pyroprobe5’ Drop-tube (T= 1-2 s)~’ Laboratory-scale combustor (~=0.15-0.2 s)~’ Electrically heated fumace6’ Flat-flame burner (r=80 ms)63 Laboratory-scale combustor (7 = o-1.5 s)” Laboratory-scale combustor (r=l s)~~

Inert atmosphere; 6 coals 900 ppm Or; bit. and subbit. coal

In Nr; 14 coals Inert atmosphere; 20 coals Inert atmosphere; bit. and subbit. coal In Ar; subbit. and brown coal Substoichiometric; bit. coal

X=0.5-0.8; MVB coal Ar/Os flame; X = 0.4 and 0.6; subbit. coal h = 0.6 and 0.8; bit. and brown coal

Ar/02/C02; 48 coals, various stoichiometries

+NOfl N2

Volatile +0x - HCNKN - NC0 +H

Fuel-N

\ NO +HCCO, CH, (i=O-3)

Fig. 3. The main reaction pattern for the conversion of fuel-N to NO and Nr.

mass loss rate was equal to the N-release rate, except for a bituminous Indonesian coal, which had a relatively faster N-release. This difference could not be explained. Harding er uZ.~~ plotted the % N-release against the percentage of coal burnout for 16 coals from eight different studies, showing that the rates were proportional to each other.

Although the N-release is not always proportional to the mass loss of a coal, the difference is usually small. If the N-release profile of a coal is unknown, it is safest to assume that it is proportional to overall weight loss. In combustion modeling the uncertainty in the devolatilization rate will probably be significantly larger than the error included by this assumption.

3.2.1.2. Nitrogen volatile species. During devolatilization nitrogen is evolved directly as HCN and NH3 or indirectly from tar, which may be released in large quantities. The relative amount of HCN and NH3 in the primary volatiles is still controversial. Listed in Table 1 are selected results from devolatilization at high heating rates ( 103-lo5 K/s), high temperatures (1500- 1800 K), and atmospheric pressure. More details on nitrogen compounds released during devolatilization at other conditions can be found elsewhere&.

The relative yield of HCN and NH3 during devolati-

lization depends on the local stoichiometry6’*63S65. This is also observed from the results in Table 1: more NH3 has been found in experiments with a relatively large amount of OZ. This indicates that NH3 is partly formed from other nitrogen compounds (like HCN) by reaction with 02 or O2 derived radicals. Another trend that can be observed from the pyrolysis experiments under inert conditions is a larger amount of NH3 released from lower rank coals. These contain more hydrogen and oxygen than higher rank coals. The occurrence of secondary reactions conceals the results on the release of primary nitrogen compounds. For the conversion efficiency of fuel nitrogen into NO at high temperatures (1700- 1800 K), however, it is not very important whether the nitrogen is released as HCN or NHs, as discussed below.

From experimental and theoretical studies of homogeneous gas-phase reactions with nitrogen-containing compounds added to the combustion gas in order to simulate fuel-N, it was found that the conversion of fuel-N to NO at high temperatures (1750-2300 K) was nearly independent of the identity of the compound, but strongly dependent on the local combustion environment47’67S68. It was also demonstrated that the fuel-N was converted to HCN or NH3 (or NHi, with i = O-3). which are the main intermediates in the reaction of fuel-N to NO. Coal combustion with NH3 or NO added to the combustion air


confirmed these results65.69. The importance of HCN as a reaction intermediate was emphasized by de Soete6*, and further reaction to amine species in the reaction path to NO or N2 was shown by Fenimore” and Haynes”. Combustion gases doped with NO yielded a reduction of NO via HCN, thus showing the reverse pattern of NO formation’*. Other intermediates like HOCN, HNCO and N20 are likely to exist in much smaller concen- trations50,70,7’.73,74 although in a recent paper” concentrations of HNCO corresponding to 15% of the total nitrogen volatile species were detected by Fourier transform infrared (FTIR) analysis. It should be noted that analysis of NH3 and HCN can be sensitive to errors when using wet detection methods: HNCO is converted to NH, when absorbed in water76, and acetylene interferes with HCN when using selective ion electrodes7’.

A comprehensive reaction mechanism for homogeneous nitrogen chemistry 46,47 involves more than two hundred reactions. A simplified reaction scheme includ-

ing only the most important steps of the formation of NO and N2 from fuel-N is given in Fig. 3. Under fuel-rich conditions the reaction of NO with N to NZ, or the reduction of NO to HCN, prevails. For a more fundamental knowledge of the nitrogen chemistry involved in the early stage of devolatilization, information on the identity of the initial species is still required.

Nitrogen species evolved during devolatilization have been related to functional forms of nitrogen in coal 58~78-8’. With X-ray photoelectron spectroscopy (XPS) the functional groups in coal have been classified into pyrrolic (five-membered ring), pyridinic (six- membered ring), amine and quatemary nitrogen (nitrogen atom with four bonds) groups. A clear trend has not been found and more information is needed to confirm a relation between the functional forms and the nitrogen species released.

3.2.2. Influence of coal properties on the formation of NO

HCN and NH3 formed are subsequently oxidized to NO or to N2, depending on the prevailing conditions. Coal properties influence the effect of NO reduction from staged combustion by the partitioning of nitrogen between the char and the volatiles, since mainly the volatile nitrogen is sensitive to the substoichiometric conditions in the flame. Under unstaged conditions the conversion of volatile nitrogen to NO is more efficient than the conversion to NO of char nitrogen. Increasing volatile amount or lower rank therefore gives higher NO emissions at unstaged conditions82-s4. During staged combustion it is this source of NO that is reduced, the amount of reduction increasing with volatile contents2-84. In this case a higher volatile content gives lower NO emissions. Usually a difference in rank causes also a difference in temperature and residence time in the fuel-rich zone, thus indirectly influencing the nitrogen release and reaction rates.

A relation between the nitrogen content of a coal and

0.5 1.0 1.5 2.0 2.5 % Fuel-N (daf)

Fig. 4. Fuel-NO produced during staged and unstaged combustion of a suite of coals as a function of the fuel-N content (data

from 65),

2800 _C A=&6,NH, added ‘1 9

_ -c 14.6,HCNadded

_ -_I-. 1=O.&NH,added /

- ---j-- A=O.&HCNaddcd

2oGQ- - +- A=1.2,NH3 added

- -,.- A =1.2, HCN added

01,,,,,,,,,,,,,‘,

15&J l&o lioo 1800 1900 2& 2100 2200 TOO

Fig. 5. Total fuel nitrogen species (XN) formed during combustion of CH4 in a PFR at different stoichiometries; 7 = 100 ms.

the NO produced can be found, although it is not more than a very rough trend: more nitrogen gives more NO (Fig. 4)65.85 . The nitrogen content does therefore not give much of an indication of the NO concentrations to be expectedg6. Morgang3 found that under unstaged conditions a clear correlation existed between the nitrogen content of the volatile matter and NO emission. At staged conditions, however, the relation was not clear any more and the variation in emissions was much smaller between the coals. The tests were conducted in a 2.5 MW combustor, with external and internal air- staging concepts. The optimum burner configuration was shown to be a function of coal type.

3.2.3. Modeling of gas-phase nitrogen conversion

Similar to the prediction of prompt and thermal NO in Section 3.1, the relative importance of HCN and NH3 as primary devolatilization products of fuel nitrogen on the formation of NO has been addressed with ideal plug flow reactor (PFR) calculations, using the same mechanism. To the reactant gases (methane and air, with I .5% water) were added 1500 ppm HCN or NH3 in order to simulate


.____ m- 2al- i.__ _ ._

I 7 PFR 1

1: 0 1'1' I’

0.4 0.6 0.8 stoitidcRatio(-;~"

1.2

Fig. 6. Total fuel nitrogen (XN) as a function of stoichiometry for both a CSTR and a PFR, with 1500 ppm of either HCN or

NH3 as fuel-N (T = 100 ms; T = 1800 K).

fuel nitrogen. The excess air ratio was varied from 0.4 to 1.2, the temperature from 1500 K to 2200 K, and the residence time up to 100 ms. Figure 5 shows the concentration of total fuel nitrogen (XN = NO + HCN + NH, + HNCO) as a function of temperature and stoichiometry. At a stoichiometry below 1 a reduction in XN can be. observed, which increases with temperature. For X = 0.6 HCN and NHs are the main components at temperatures below 1900 K. At a stoichiometry of 0.8 and higher, and at temperatures above 1900 K the main nitrogen component is NO at a residence time of 100 ms. HNCO concentrations are always negligible for the PFR. In order to evaluate the effect of mixing mode, calculations for methane combustion have also been performed in a perfectly stirred tank reactor or continuous stirred tank reactor (CSTR). Figure 6 shows the results for both the CSTR and PFR calculations. The total fuel nitrogen concentration (XN) has been plotted as a function of stoichiometry at a temperature of 1800 K and a residence time of 100 ms. The concentration of HNCO in the CSTR could reach up to 25 ppm. The optimum in reduction is at a higher stoichiometry for the CSTR; this shift results in significant differences in XN concentrations at certain stoichiometries. For example, with NHs addition at A = 0.6, twice as much XN is emitted from the CSTR as from the PFR. The explanation has to be sought in the difference in radical pools between the reactors; inside the CSTR the nitrogen species are subjected to a constant radical concentration all the time, while in the PFR a clear flame front exists, after which the radical concentration drops fast.

Our calculations agree well with results from the literature.. Miller and Bowman4’ performed calculations on ethylene combustion in a perfectly stirred reactor with addition of IQ-Is, HCN or NO. Their results compared well with experimental data from Sun et aL8’ for

addition of NH3 to the reactant gases. Calculations for NO and HCN added to the inlet gases in order to simulate fuel-N showed that no significant difference in XN concentrations could be observed between the species added to the combustion gases.

Extrapolation of the present modeling results to low NO, burners indicates that a higher temperature in the reducing zone of the flame will increase the reduction of fuel-N to N2. A longer residence time within this zone will also result in a reduction of the XN concentration, since a difference between a residence time of 10 ms and 100 ms could enhance the decrease in XN emission in a CSTR by 20 to 30%. In a PFR only the thermal NO concentration is affected at residence times above 1 millisecond, but it is believed that a CSTR resembles the recirculation zone of a burner better than a PFR. The next section will show that a longer residence time does favor the reduction of nitrogen pollutants.

3.3. Stoichiometry and Residence Time

Substoichiometric conditions are required to reduce the amount of NO from coal combustion. Investigations discussed below have shown that the optimum of the air/ fuel ratio generally lies around 0.7; more air enhances NO formation, while less air produces more HCN and NH+ which are efficiently oxidized to NO in the post- flame region. Apart from the homogeneous gas-phase conversion of fuel-N to Nz, heterogeneous conversion of NO to Nz may also be important. Reduction of NO on char is too slow to account for a significant reduction of NO in flames, but reaction of carbon blacks in the size range of soot particles with NO have been found to give high conversion rates in shock-tube experimentsggVg9. However, in shock tubes reaction times are small (about 1 ms) and it has been stated that the global reaction rate may slow down when the surface becomes chemically annealed8’. This is supported by the results of Levy et aLw, who found that the reaction was retarded by water vapor and enhanced by CO. This was consistent with the hypothesis that the NO/C reaction is retarded by a chemisorbed layer which can be removed by CO. Further support was obtained from transient experiment?, which showed chemisorption of 0 from NO at low temperatures, followed by decomposition of the chemisorbed layer at higher temperatures to form CO or COz. It should also be noted that soot is present in fuel-rich zones, where the main gaseous nitrogen species are HCN and NHi. In oxygen-rich zones with higher NO concentrations, there is not much soot present. Thus, even with a high reduction rate of NO on soot, its contribution will probably be of minor importance compared to the homogeneous reactions.

Rees et aL6’ investigated the effect of first stage stoichiometry on stack NO level for a bituminous coal. The test rig consisted of a small burner (13.5 kg coal/hr) with a central primary and a coaxial secondary air inlet. A minimum in stack emission occurred at a primary stage stoichiometry of about 0.6 to 0.7, which coincides with a minimum of total nitrogen species (NO, HCN and


Fig. 7. Concentration of first stage XN and stack NO as a function of first stage stoichiometry (data from 6’).

01 0.0 0.5 1.0 1.5 2.0

R&h Tun: (s)

Fig. 8. Influence of residence time on nitrogen species concentration at two stoichiometries (data from 24).

NH3 after the first stage (Fig. 7). The amount of NO produced could be reduced by up to 50%, compared to the emission at a stoichiometry of 1.15 at the first stage. Asay et aL9 measured the amount of NO produced from combustion at different stoichiometries by varying the amount of secondary air in a similar burner. The overall residence time in the furnace was estimated to be 0.1 to 0.2 sec. The overall stoichiometties investigated were 0.57, 0.87 and 1.17, where the lowest amount of NO produced was consistently at the lowest stoichiometry. The total amount of nitrogenous species (NH3, HCN and NO) showed a minimum at a stoichiometry of 0.87. The amount of NO was dependent on swirl number for high stoichiometry, but nearly independent at the lowest stoichiometry. This means that mixing of primary and secondary air has hardly any influence on the pollutant formation at low stoichiometry, since the overall amount of oxygen is too low for the formation of oxygen-rich zones.

Chen et a1.65,92 burned a suite of 48 coals in an air- staged, premixed combustor (2.5 kg/hr), and investigated the influence of coal composition on NO formation under different conditions. They varied the stoichiometry of the first stage while maintaining an excess oxygen level of 5% at the second stage. The exhaust NO concentration showed a minimum at a first stage stoichiometry of about 0.7 for a lignite and a bituminous coal, with a higher dependency on stoichiometry for the lignite. With anthracite no minimum was observed: down to a stoichiometry of 0.45 the NO emission decreased. The residence time in the first stage was about 1 sec.

Wendt and Pershing93 used a small combustor (3 kg/ hr) where they increased the overall stoichiometry by increasing the secondary air velocity. It had no significant effect on NO emissions. Mixing between air and the fuel-rich zone where devolatilization takes place was presumably not enhanced by an increase in air velocity.

The influence of residence time in the reducing zone on the reduction of NO formation was investigated by Bose et uZ.*~ for a bituminous coal and a brown coal in a premixed burner with a coal feed rate of 2 kg/hr. At substoichiometric conditions (A = 0.6 and 0.8) the concentration of NO was clearly a function of residence time in the range of 0.1 to 2 set (Fig. 8). At a stoichiometry of 0.8 the main nitrogenous species is NO, while at X = 0.6 the amounts of NH3 and HCN are significant, and even higher than NO. Similar trends were also found by others6’,85,86.94: the lowest NO concentrations were obtained at the highest residence time.

It can be concluded that more air added to the combustor in the range applied in industrial burners results in higher NO concentrations at the exit, although it is of importance how the air is added. It can partly be added further down in the furnace as over fire air, OFA, or all air can be introduced through the burner itself (internal air staging). In low NO, burners it is not possible to distinguish clearly the fuel-rich zone, since the combustion air entrains gradually. Residence times within the fuel-rich zone are therefore difficult to assess. As shown above, there is an optimum stoichiometry in the lirst stage of combustion, and the amount of air added in the second stage should be kept as low as possible while allowing for satisfactory burnout. The residence time within the fuel-rich zone should be kept high.

3.4. Effect of Temperature

The effect of temperature in the primary combustion zone on the NO emissions depends on the stoichiometry and the coal type. Pershing and Wendt” reported essentially no influence at all on fuel NO, when changing the temperature by variations in 02, CO2 or air preheat for three medium volatile bituminous and a subbituminous coal at an overall stoichiometry of 1.15. Song et al. 96 observed a small decrease in NO emission with increasing temperature in an isothermal electrically heated furnace with two low rank coals, by changing the

358 R. P. van der Lam et al.

gas flow rate of 21% oxygen in helium. At higher temperature the extent of devolatilization increased, but the conversion to NO, of the volatile decreased. The influence on the conversion of char-N to NO was found to be small. Bose et a1.24 investigated the influence of temperature on NO formation for a brown coal and a bituminous coal under fuel-rich conditions by diluting with nitrogen or enriching with oxygen. In all cases an increase in temperature decreased the total volatile fuel nitrogen (NO + HCN + NH3) as a fraction of the nitrogen content of the coal. Taking into account that the amount of volatile increases with temperature, it must be concluded that more fuel-N is converted to N2. This is consistent with the results from Song et aZ.96.

Kremer and Schu1z62 used an electrically heated furnace (staged combustion) and found that the NO emissions decrease with temperature for a lignite, while the emissions increase for anthracite, a medium and a high volatile bituminous coal. The temperature dependence was a clear function of primary zone stoichiometry, variations being less profound at low stoichiometry (h = 0.6), where the high volatile bituminous coal also showed a decrease in emissions with temperature. Char-N conversion increased with temperature but the char was formed at 800-900°C while combustion experiments took place at up to 1500°C. With the relatively large difference in temperature it is expected that a significant amount of volatile will still be released from the char. During single stage combustion NO emissions increased with temperature62*97.

Leithner and Lendt9* changed the cooling rate of the furnace wall to obtain different temperatures in the combustor but they do not report absolute values for the temperatures. Increased cooling results in lower NO emissions in the case when the relative amount of tertiary air is kept below 60%. At higher tertiary air flow rates the NO emissions show the opposite trend. It is not clear what the contribution of aerodynamics is to this change in trend.

The different trends in NO emissions can be explained in terms of the prevailing conditions.

1. At higher temperatures the amount of volatiles is larger, but the conversion to NO can be con- trolled99. When the prevailing conditions are fuel rich with a residence time long enough to establish a reduction in volatile-N species the conversion of volatile nitrogen to NO is low and may be of the same order as the conversion of char nitrogen. In this case a decrease in NO emission with temperature is observed, since the decrease in conversion efficiency to NO for volatiles is more important than the change in volatile yield. At high stoichiometry as observed in flames with fast mixing of air and fuel and with high NO production, the conversion of volatile nitrogen to NO will be substantially higher than that of char nitrogen. In this case an increase in volatile yield will increase the emission of NO. This has recently been confirmed by Spliethoff

;f Volatile-N y NO

Coa’-N A Char-N 2 NO

Fig. 9. Volatile and char fraction of coal and the conversion efficiency of volatile-N and char-N to NO.

et al.lm in an electrically heated entrained-flow reactor.

2. The conversion of char-N to NO at temperatures above 1000°C is only slightly influenced by tem- perature96. The contribution of char-N to the total NO is therefore mainly related to temperature by char yield.

3. Coal type is important since it determines the volatile yield as a function of temperature.

The effect of the combination of these phenomena on the formation of NO can be illustrated with the following example.

Consider a coal which splits during devolatilization in a fraction x of volatile and a fraction (1 - x) of char (see Fig. 9) with nitrogen contents proportional to the mass fractions. The nitrogen content of the volatile and the char are subsequently oxidized to NO with an efficiency of y and z respectively, while the remainder of the nitrogen is converted into Nz. The amount of NO formed is proportional to

NO-xy + ( 1 - x)z (6)

At fuel-rich conditions an increase in temperature will lead to an increase in volatile fraction n and a decrease in conversion efficiency y to NO. The char fraction will decrease, while it is expected that the char-N conversion z remains unchanged. At this increased temperature the amount of NO formed will be proportional to

NO-(x+A~)(y-Ay)+(l -n-AX)Z (7)

Assuming that the product AxAy can be neglected the difference between Eqs (6) and (7) can be expressed as

ANO-aX(y - Z) - xAy (8)

Take for example the conversion efficiency of char-N to NO to be z = 0.3, the fraction of volatile x = 0.5, and the efficiency of volatile conversion y = 0.6. This will lead to

ANO=0.3hx-OSAy (9)

The amount of NO will increase with temperature for AX > 1.67Ay, while it will decrease for AX < 1.67Ay.

In other words: the amount of NO will increase in the case where the increase in volatile fraction is more than 1.67 times larger than the decrease in the conversion efficiency of volatile-N to NO. AX depends on the coal type and the temperature, Ay depends on the stoichiometry, the temperature and the residence time. Since Ay is not directly a function of coal type, it can be concluded that the variations observed between different coals at the same reaction conditions (i.e. temperature and stoichiometry) are mainly due to a difference in volatile yield between the coals.


Both coal type and the conditions in the furnace have to be taken into account in order to be able to determine the influence of temperature on NO emissions, which is not an easy task. However, within the range of operating parameters of low NO, burners the influence of temperature is relatively small compared with the influence of stoichiometry.

3.5. Effkct of Moisture

Moisture in coal may cause a delay in ignition and devolatilization and result in lower boiler temperatures. Its influence on the formation of NO has been investigated by Asay et aL9’ for a subbituminous coal with 4.5 and 25% weight moisture. The coal mass flow was 10.2 kg/hr on a dry basis and unfortunately temperatures were not measured in the furnace. A small decrease in total fuel nitrogen XN was observed

800 I I I 700 + rbied

0.6 0.8 1.0 I .2 Stoichi~c Ratio (-)

Fig. 10. Influence of moisture on total fuel nitrogen (XN) as a function of stoichiometry at three swirl numbers (data from ‘I).

,’

primary air + tie1 +

when the coal was dried (Fig. IO). It is not clear whether the difference is due to a change in temperature or in reaction conditions, i.e. the concentration of radicals.

It can be concluded that moisture has a small effect on the emissions of NO, especially when considering that the variation in moisture content of coal as fired in industrial boilers is relatively much smaller than used in this investigation.

4. FLOW PATTERN AND FLAME TYPE

A pulverized coal burner consists primarily of a central primary and an annular secondary air inlet. In industrial burners the primary air inlet is also annular in shape, due to the presence of a pilot burner or oil lance in the center. Most modern burners are equipped with a tertiary air port, coaxial to the secondary air inlet (as shown in Fig. 14, for example). The coal is mixed with the primary air and transported through the center of the burner at about 15-20 m/s. The velocity is high enough to prevent deposition of the coal in the pipes and flashback of the flame, and low enough for stable ignition. The temperature of the air is usually around 70°C always above the water dew point, and its amount is often about 20% of the total air required”. The main part of the air is added as secondary (or tertiary) air through a coaxial inlet with a velocity of about 30-50 m/s, and it is swirled in order to establish a reverse flow zone. NO emissions are kept low by mixing the air gradually with the fuel. The coal devolatilizes in a fuel-rich environment, where conversion of nitrogen species evolved from the coal to molecular nitrogen instead of NO is favored, as discussed above. This conversion is to some extent dependent on the temperature. In addition, the reverse flow zone is of importance for rapid ignition of the coal; the coal will mix with the recirculated, hot combustion gases and radicals, and increase in temperature. Rapid ignition close to the burner mouth is of great importance for the reduction of NO. as will be shown later.

4.1. Swirl Number

There are different ways to establish swirl: radial

Fig. 1 I. Schematic representation of a moveable block swirler


B ‘SBR 8 Free Vortex Q > w=dr

Fig. 12. Tangential velocity profiles for different vortex types.

vanes, commonly used for industrial applications for secondary and tertiary air; axial vane swirlers, mainly used for primary air, or in small-scale burners; and, less frequently used, tangential entry of the fluid stream into a cylindrical duct. When using guide vanes, the degree of swirl can be altered by adjusting the vane angle. The moveable block swirler”’ is frequently used for pilot- scale test rigs9’.‘02*103, and consists of two annular plates and two series of interlocking wedge-shaped blocks, which form alternate radial and tangential flow channels (Fig. 11). With this swirl generator high degrees of swirl can be established with a low pressure drop. It is important to realize that different swirler types give rise to different tangential velocity profiles: use of radial vanes result in a Rankine type vortex (see Fig. 12), while others may give solid-body rotation, or something in betweentm. A Rankine vortex is a combination of a solid-body rotational core and a free vortex.

To characterize swirling flow, a non-dimensional number is introduced: the swirl number. This number is the ratio between rotational momentum and axial momentum, which are both conserved in swirling free jets’05:

(10)

where GQ is the angular momentum, G, is the axial thrust, and R is the exit radius of the burner nozzle. G+ and G, can be calculated from the velocity profiles:

G+ = I ~(Wrr)pIJl~r dr (11)

I

R

I

R

G,= o lJpU2rrr dr + op2xr dr (12)

Here U and Ware the axial and tangential component of the velocity, respectively, p is the static pressure, and r

the radius. For correct calculation of the axial thrust, the wall pressures have also to be taken into account for confined jets.

4.2. Alternative Swirl Numbers

From the geometry of axial and radial vane swirlers the swirl number can be calculated’05, which is often done in practice. In this case the static pressure term can be omitted, resulting in a modified swirl number S’. Without this pressure term the swirl number is not conserved, but varies from traverse to traverse and the location of measurement of the momentum fluxes becomes important. Usually only the inlet swirl number is given.

Beltagui and Maccallum’06 report that the choice of reference pressure is problematic since the static pressure in a furnace varies. Furthermore, the dimen- sions of the internal reverse flow zone (IRZ) are primarily functions of the furnace diameter for highly confined flows, rather than burner diameter. Hence they propose a new swirl number S* for confined flows, where the pressure term is omitted and the nozzle radius is replaced by the furnace diameter.

The swirl number is not a universal measure for the flow phenomena since flows with the same swirl numbers, which are established with different swirler types, can exhibit varying mixing profiles and reverse flow characteristics. This is due to the fact that the shape of the velocity profile is not reflected in the momentum. Al-Halbouni’07 states that an additional parameter is needed for the characterization of swirling flows, like the vane angle, although Majidi and SO’~* concluded that one should be more concerned about the solid-body rotational core, instead of the vane angle. The impact of different swirler geometry on flame characteristics is still poorly understood.

Since no universal swirl number is present, it is difficult to compare results from different experimental test facilities. However, for jets produced by geometrically similar swirl generators it is a significant similarity criterion.

4.3. Flame Type

Depending on the swirling air and burner geometry such as quarl size, injector blockage ratio, gas velocities and momentum ratio, various characteristic mixing patterns can be achieved. Each of these basic flow mixing profiles produces significantly different NO, emission levels when firing the same coal. The classification system of the International Flame Research Foundation (IFRF) recognizes four different type? (see Fig. 13).

Type 0: this is a long jet, usually without a swirl induced IRZ (not shown in the figure). This type is often used in comer fired boilers. Type 1: the primary air completely penetrates the IRZ, leaving a relatively small annular region of


Fig. 13. Flame types according to the IFRF classification system, showing the burner mouth, internal reverse flow zones and

particle trajectories.

reverse flow. This type of flow, which is a combination of type 0 and type 2 flows, is observed at high gas velocities and gives long flames. Devolatiliza- tion occurs in a fuel-rich environment, which pre- vents excessive formation of NO,. Type 2: this is characterized by a large central IRZ which deflects the primary air radially from the axis. This is observed at relatively low primary air velocities. In practice it gives a short intense flame stabilized on the quarl, and a high NO, production within the mixing region on the recirculation zone boundary. Type 3: the primary jet penetrates the IRZ partially and is then deflected radially. The degree of penetration is increased with increasing velocity of the primary air. A change from type 2 to type 3 flow is usually established by increasing the primary air momentum, or by inserting the primary air injector further downstream into the quarl. Excessive increase of the momentum may lead to complete penetration of the IRZ, and thus to a change to a type 1 flame. In this case the residence time of the coal inside the fuel-rich zone becomes shorter, resulting in higher NO emissions.

Establishing an initial oxygen-deficient zone may result in an unstable flame. Therefore type 2 flames are used in practice for coals that are hard to ignite]‘, despite the larger amount of NO, produced in such flames. In practice there will be a distribution of residence times of coal particles inside the fuel-rich zone, eventually leading to a flame with characteristics in between two of the flame types described above. Besides, it is not possible to relate the flame types to single parameters, since a combination of parameters such as swirl number, flow velocities and burner geometry are all of

importance for the type of flame established. A closer view on the parameters that influence the flame type and the formation of NO will be given in the next section.

A stable or anchored flame is closely attached to the burner and ignites well inside the quarl. Instability of flames leads to lifting of the flame from the quarl, higher entrainment of air into the primary air jet and higher NO emissions. Little work has been carried out on the stability performance of pulverized coal flames and the influence of instabilities on NO emissions. Abbas er al. lo9 investigated the operating boundaries and NO emissions of two burners with lifted and anchored flames as a function of swirl number and momentum ratio (primary air momentum over secondary air momentum). Whether a flame was anchored or lifted depended on a combination of the burner geometry, swirl number, momentum ratio and the amount of excess air.

Fluctuations, another form of flame instability, are an area even less studied. From gas flames it is known that fluctuations increase air entrainment, which leads to lower NO emissions from these flames, because of a reduction in flame temperature”‘.“‘. In coal flames it can be expected that increased air entrainment into the fuel-rich zone, due to fluctuations, will lead to higher NO emissions.

5. THE INFLUENCE OF AERODYNAMICS ON NO FORMATION

5. I. Conventional Versus Low NO, Burners

Swirl stabilized burners were initially designed to stabilize high intensity combustion and to shorten flame length. Recirculation of combustion gases enhanced ignition of the coal, while the swirl induced turbulence caused a rapid mixing of the coal with the air. Devolatilization and combustion of the coal in an oxygen-rich environment were favorable for the formation of NO. These burners are nowadays referred to as ‘high NO,’ or ‘conventional’ burners.

In the last two decades R&D activities have been directed towards reduction of NO emissions. As a result of these efforts a new generation of burners has been developed. A change in fuel injection mode to more concentrated fuel jets, a lower stoichiometry and higher swirl number have significantly lowered NO emissions. Appropriately these burners are called low NO, burners. The conversion of fuel-N to NO is usually minimized in these burners by creating a fuel-rich reverse flow zone. The primary air and coal penetrate this reverse flow zone, which consists of hot, burned gases with a low amount of Oz. Here the devolatilization takes place and hydrocarbons compete with the nitrogen for the available substoichiometric amount qf 02. During the devolatilization of the coal also nitrogen compounds are released as volatiles. In the reducing environment the formation of NO is low and most of the reactive nitrogen is converted to NZ. The secondary air mixes at a later stage with the fuel in order to achieve adequate burnout and low CO emissions.


secondary sir +

primaryairandfucl + ’ -Y

Fig. 14. Burner mouth with annular, contracted coal nozzle, secondary and tertiary air ports.

After devolatilization part of the fuel-bound nitrogen remains in the char. The conversion of char-N to NO is harder to control, since most of the char bums in an oxygen- rich environment. Pershing and Wendt95 estimated the relative amount of NO from fuel-N in pulverized coal burners to be 75% of the total NO emission, but as already mentioned, its contribution cannot be directly measured.

Staged mixing of the air decreases the temperature in the near burner region, thereby reducing the amount of thermal NO simultaneously with fuel nitrogen derived NO. Drawbacks of the change in aerodynamics and stoichiometry are the difficulty of stable ignition and the decrease in burnout. The latter is caused by a reduction in residence time in an oxygen-rich environment at high temperature.

Important features in the change from a high NO, to a low NO, burner design are:

insertion of the coal nozzle into the quarl (mainly used in pilot-scale burners) lower near-burner stoichiometry by introduction of part of the air further downstream into the furnace (furnace air staging or external air staging) lower secondary air flow rate by the use of tertiary air ports (burner air staging or aerodynamically air staging) optimization of swirl level, primary air and secondary air velocities use of coal concentrators to establish better penetration of the IIU.

As a consequence of these changes the flame type will change from a type 2 flame to a type 1 or 3 flame (see Fig. 13); the particles no longer follow the outer boundary of the IIU, but penetrate the IRZ and devolatilize in the fuel-rich zone.

Since aerodynamics have a major influence on the formation of pollutants, it is attempted below to clarify the influence of the main burner operating parameters on the formation of nitrogen oxide. Division between high and low NO, burners has not been made, since there exists no clear boundary between the burner types. The same burner may be able to run in both high and low NO, mode. Most of the results listed are obtained from pilot- scale combustors, having a central primary and a coaxial secondary air inlet, sometimes combined with a tertiary air port (Fig. 14). The test conditions and results are

900 I ’ I ’ I ’ I ’

H- 200

I - I 0 I ’ I ’ 0.0 0.4 0.8 1.2 1.6 2.0

S (-)

Fig. 15. Emission curves for different injector types and 5% excess air. Injector A 115 mm o.d., injectors B, C and F

60 mm o.d. (data from ’ “)

Injector type Outer diameter Primary air (mm) velocity (m/s)

Single hole injector Single hole injector Single hole injector Single hole injector Radial hole injector

115 19

60 19

60 26

52

60 27

summarized in Table 2, and discussed in more detail below. An extension to industrial full-scale burners and boilers is made in Section 6.

5.2. Primary Air

The primary air velocity determines the penetration distance of this stream into the IRZ before it is deflected radially and backwards to mix with the secondary air. Increasing the primary air velocity usually results in a decrease of the recirculated mass flow. A swirl on the primary air increases flame stability, and changes the flow pattern from a type 3 to a type 2 flame. A smaller diameter of the primary air inlet gives similar trends, but a stable type 3 flame can be established at lower secondary air swirl level”. A coal spreader at the exit of the primary air has the same result in that the coal will be spread out wider, and the flame closer attached to the burner.

Often the momentum flux ratio, Ma, between the primary air and the secondary air is taken as a parameter for characterizing inlet conditions:

(13)

Here r is the radius, U the axial (mean) velocity, p the density, subscript p refers to the primary air, and subscript s to the secondary air.

Tabl

e 2.

Sum

mar

y of

inv

estig

atio

ns

on p

ilot-s

cale

bu

rner

s

Bur

ner/o

rific

e C

oal

Ope

ratin

g pa

ram

eter

s In

vest

igat

ion

Res

ults

I .8

MW

ann

ular

/cen

traI/r

adia

1’

“-’

I4

tFR

F*

HV

B

2.5

MW

ann

ular

/cen

tral9

4 IF

RF

HV

B,

MV

B

2.5

MW

cen

tral”

’ IF

RF

HV

B

100

kW c

entra

l”s

BY

U

100

kW c

entra

ls6

BY

U

80 k

W c

entm

19’

BY

U

100

kW a

nnul

ar/c

entra

l”s

IC

100

kW a

nnul

ar/c

entra

l’Og

IC

100

kW c

entra

l”91

C

Max

. 50

0 kW

ann

ular

’20

IVD

0.3

MW

ann

ular

, TA

, O

FAs

IVD

150

kW

‘cen

tral’

with

8 je

ts’*

’ TR

C

470-

730

kW c

entra

l’**

IGT

HV

B

Subb

it.

HV

B

HV

B

HV

B

HV

B

HV

B

(3 c

oals

), LV

B,

blen

ds

HV

B

HV

B

VPA

= 19

-52

m/s

, X

=

1.05

h =

1.15

X

=

1.15

, M

a =

0.08

, V

pA =

7

and

20 m

/s,

VS

A =

20

and

40

m/s

X

=1.0

6,V

pA=3

,15a

nd75

m/s

, V

sA =

5

m/s

V

pA =

15

, 30

m/s

, V

sA =

5

m/s

, M

a =

1.7

VpA

=

I5 m

/s,

VS

A =

3-

8 m

/s

VpA

=

23 m

/s,

VS

A =

38

m/s

, X

=

1.15

, M

a =

0.25

V

P,,

= 17

-38

m/s

, V

SA

=

17-3

9 m

/s

Ma

= 0.

25,

X =

1.

15,

S =

I R

esid

ence

tim

e sc

aled

Con

st.

velo

city

sc

aled

vp,

= 30

m/s

, v,

, =

34 m

/s,

x =

1.15

h=

1.2,

S=1.

67,V

~,=l

5m/s

, V

sA =

3-

4.5

m/s

s (0-

a V

P/v

Q”.P

A (10,

20%

of

tot

al

air)

, N

O v

s. S

depe

nds

on i

njec

tion

mod

e an

d in

ject

ion

mod

e V

P,

S (O

-2).

Ma,

coa

l in

ject

or

posi

tion

S, i

njec

tor

posi

tion,

co

nst.

velo

city

/resi

denc

e tim

e sc

alin

g M

R.

VPA

. S (

O-6

)

5. V

PA.

X,

d,

(50,

14

fun

)

5, X

(-V

,,),

moi

stur

e D

iffer

ence

be

twee

n bu

rner

s

Ma

(S =

I)

, X

, S (

Ms

= 0.

25)

d,

(25,

46,

121

pm)

5, d

, (I

and

10

% >

90

em

)

d,,

S,z,

rpL

r V

sa>

VTA

(pz

=

prim

ary

zone

) d,

(1

2.50

rm

) d,

(7

, 18

.41

rm)

NO

vs.

S or

Ma

depe

nds

on i

njec

tion

mod

e La

rge

decr

ease

in

NO

whe

n in

ject

or

is

inse

rted

into

qua

d M

inim

um

NO

at

inte

rmed

iate

S

No

chan

ge

in N

O w

ith d

,, sm

all

influ

ence

of

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364 R. P. van der Lam et al.

The influence of the primary air velocity and injection mode is well investigated by Heap er al. ’ “-’ 14, whose experiments were conducted in the 1.8 MW IJmuiden No. 1 furnace of the International Flame Research Foundation with a highly volatile coal (coal type is only identified in ‘13). The results of Fig. 15 show clearly that radial injection of the coal through several holes in the wall of the primary air tube, while blocking the central outlet, gives high NO emissions, caused by the rapid mixing of the coal with the secondary air (curve F). Devolatilization takes place in an oxygen-rich environment, where most of the fuel-N is converted to NO. This study also shows that the main parameter in NO reduction is the penetration of the coal and primary air into the fuel-rich reverse flow zone. At low velocities with the single hole injector, the momentum of the primary flow is not high enough to penetrate the IRZ; the coal is deflected and mixes with the secondary air. At higher velocities the penetration becomes large enough to establish significant reduction in NO formation. A large primary air velocity may lift the flame, and air mixes in with the coal giving higher NO emissions. Increasing the swirl on the secondary air increases the strength of the IRZ and the flame will be reattached to the burner, meanwhile reducing the emission of NO (curve A). This shows that the primary and secondary air velocities and the amount of swirl cannot be regarded independently. From curves B and C an initial increase in the NO emission followed by a decrease can be observed. At low swirl the fuel is injected as a straight jet with relatively slow mixing of the gases. This results in intermediate NO levels, and may lead to unstable flames. At higher swirl a reverse flow zone is established, but the jet momentum is not high enough to penetrate this zone. The coal is deflected around the IRZ and mixing with the secondary air gives high NO values. Increasing the swirl strengthens the IRZ, which is now more closely attached to the burner, and the primary air can penetrate the IRZ: consequently a decrease in NO emissions is observed. Even higher swirl increases the turbulence and mixing rate of the IRZ, primary and secondary air. Therefore an increase in NO concentration is often observed at high swirl. It should be noted that the amount of primary air and thereby the primary stage stoichiometry changes with velocity, since injectors B, C, and F have the same diameter. This may have an influence on the NO produced. The diameter of nozzle H was not mentioned in the paper’ ‘*. As stated above, the formation of nitrogen oxide is strongly influenced by the conditions in the fuel- rich reverse flow zone, where the coal devolatilizes. Heap et al.“’ changed the flow rate of primary air by decreasing the diameter of the coal injector at constant velocity (lines A and B in Fig. 15). The dependency of the NO emission on swirl number is not straightforward and the sudden decrease in emission in line A may be due to the higher jet momentum which facilitates penetration of the reverse flow zone. Use of an annular injector altered the dependency on swirl number and decreased the influence of primary air flow rate on NO. When using an annular injector more air will entrain with the coal,

0.6

5 0.5

$ $ 0.4 c4 % .B 0.3

d 0.2

I I I h = 1.32

----- h=l.OO

0.0 1.0 2.0 3.0 s C-1

Fig. 16. Effect of swirl, stoichiometric ratio and primary air velocity on the fractional conversion of fuel-N (data from 56). Solid symbols: primary air velocity 15 m/s. Open symbols:

primary air velocity 30 m/s.

giving more NO, and the influence of the primary air flow rate decreases.

Smith et ai.’ I5 used three different diameter primary air injectors (diameter 0.01 m, 0.023 m and 0.05 m, while keeping the air volume flow constant, which resulted in different velocities. In a vertical down fired combustor with a feed rate of 13.6 kg/hr subbituminous coal (about 100 kW) they obtained a minimum in NO emission for the medium diameter primary air injector. During the tests the same cross-sectional area of the secondary air inlet was used. The velocities in the test are not realistic for industrial burners, since the primary air velocity was respectively about 75, 15 and 5 m/s, while the secondary velocity was around 5 m/s. In full-scale burners these values would usually be. about 20 m/s and 30-50 m/s respectively. At the lowest primary air velocity the coal does not fully penetrate the IRZ, while at high velocities the flame will be lifted and secondary air entrained with the coal.

Harding et al.56 found only a small influence of the primary air velocity when it was increased from 15 to 30 m/s, while using a high-volatile bituminous coal (13 kg/hr) at different swirl intensities (Fig. 16). A higher primary air velocity increases the emission of XN (XN = NO + HCN + NHs). The secondary air velocity is not known, but is probably equal to the rate used by Smith er al.“‘.

Kimoto et al. ‘I6 investigated four different burners (750 kW) with an annular coal inlet and a tertiary air port, two of them being different only in primary nozzle design: one straight outlet and one contracted, which caused the primary fuel to concentrate towards the burner axis (Fig. 14). Swirl on the primary air in the same or in the opposite direction of the secondary and tertiary air swirl had no significant effect on NO emissions, but it did increase the amount of unburned carbon in the fly ash. Modem full-scale burners usually

Nelson

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Nelson

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Nelson

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Nelson

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Nelson

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have swirl on the primary air, although its effect is diminished by coal concentrators at the burner mouth, which divide the coal into a number of separate jets. The swirl is in this case used to concentrate the coal at the outer radius of the primary air nozzle. Not much information is publicly available on the impact of different coal concentrators on NO formation or flame stability.

A method to accomplish penetration of the coal into the IRZ is to insert the primary air injector further downstream into the quarl. A lower velocity will now be sufficient for good penetration and low NO emissions, while maintaining a low primary air floods. Excessive insertion of the primary air/coal injector causes the particles to penetrate the IRZ completely, and the lower residence time within this zone causes a higher NO production.

The main importance of coal injector design and primary air momentum is the establishment of sufficient penetration and residence time within the IRZ. A high primary air velocity and a central nozzle stimulate penetration. Too-high velocities cause the flame to lift, resulting in a large increase in NO emissions by the extensive mixing of coal with secondary air before penetrating the reverse flow zone. Due to practical limitations (e.g. the presence of a gas pilot burner or an oil lance in the center of the burner) annular nozzles are more frequently used. This way, air entrainment is greater and penetration depth lower. However, this can be overcome to a large degree by inserting the primary air nozzle into the quarl, or applying an appropriate degree of swirl, as will be seen in the next section, where the influence of secondary air and swirl number will be discussed. Coal concentrators, concentrating the coal in a number of jets, may also improve the degree of particle penetration.

The influence of the primary air velocity on the formation of NO cannot be regarded as independent of the other burner parameters. However, the available results indicate that an optimum velocity can be found for the reduction of NO.

5.3. Seconday Air and Swirl Number

In most pilot-scale studies the secondary air is swirled, and the primary air unswirled. Since the secondary air stream supplies most additional air to the combustion process necessary to create a certain overall stoichiometric ratio, the influence of secondary air flow is directly related to the stoichiometry. A change in secondary air flow rate is therefore not possible without changing stoichiometry unless tertiary air is applied. Since the secondary air is swirled, many studies contain a parametric study on the influence of swirl number on combustion parameters. Usually only the swirl on the secondary air is mentioned and investigated, although the total swirl number including primary air will not deviate much from the secondary air swirl number due to the relatively low momentum of the primary air. As discussed above, the influence of primary air flow on NO JplCS ?,-,-I

formation is also directly related to the secondary air flow and the swirl number. Despite these interactions, a description of the influence of secondary air velocity and swirl number will be given in this section. Table 2 contains a summary of the operating conditions and parameters investigated by the authors referred to.

One of the first studies dealing with the influence of secondary stream swirl on the formation of NO in a pilot- scale combustor has been described in the section above ’ 12-’ j4. Results on the influence of swirl combined with stoichiometry obtained with a smaller laboratory- scale combustor are given by Harding et al.56. From experiments with high volatile bituminous coal it was clear that the swirl number had a major influence on the fractional conversion of fuel-N to XN (XN = NO + HCN + NH3). Changing the swirl number from zero upwards, they were able to reduce the XN emissions with about 50% at an overall stoichiometry of 1 .OO and 1.32, respectively. At these stoichiometries a minimum could be observed in XN formation (mainly NO at these stoichiometries), since the concentration increased slightly at a high swirl level. Lower stoichiometries resulted in lower amounts of XN, and the influence of the swirl became less important between S = 1.0 and 3.0, both in relative and absolute number (see Fig. 16). At h = 0.66 large amounts of HCN and NH3 were present in the gas phase. At this substoichiometric condition a minimum could not be observed within the range of swirl intensities applied in the tests56.9’. Smith et ~1.“~ also found a minimum in NO emission at a swirl number of about 2 for a subbituminous coal at an overall stoichiometry of 1.06. It must be noted that these three studies used the same test rig, under similar conditions, i.e. with a high momentum ratio.

Smart and Weber94 showed in a 2.5 MW semi- industrial-scale furnace, with an aerodynamically air- staged burner for a high volatile bituminous coal and a swirl number of 1, that a change in momentum ratio from 0.095 to 0.435 increased the amount of NO produced. A higher momentum ratio Ma established deeper penetration of the primary air into the IRZ which would be expected to reduce NO formation. However, counter- balancing this effect, air entrainment increased with increasing momentum ratio, and especially with the annular injector mode it seemed to dominate the formation of NO. Therefore in this configuration a higher NO emission resulted from a higher momentum ratio. An increase in swirl number from I .O to 2.6 for MR = 0.042 had only a small effect on the NO concentration. An air-staged precombustor burner was also used with the same coal injected through a central oriJice, while staged air was added further downstream. For a primary stage stoichiometry of 0.8 and a swirl number of 1.3 the NO emission decreased when increasing Ms from 0.043 to 0.101. Air entrainment is obviously less important for this burner type than for the aerodynamically air-staged burner, since the overall stoichiometry in the near burner region is low.

Schnell et al. I20 observed a doubling in NO with swirl for an increase in swirl number from 0.1 to 1.3. They

Nelson

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I200 ’ ’ ’ ’ ’ ’ ’ !k=O.6

200 1

constant velocity

0’ , -3

1200 I ’ I ’ I I’

a i s=l.O

0 I ’ I t 1’1 I II 0.0 0.1 Nwneliilnj~Pasii~4(~o) 0.2 0.5 0.6

Fig. 17. Influence of injector position on NO, for a constant residence time and a &&ant-velocity scaled burner at swirl

numbers 0.6 and 1 .O (data from ’ I’).

used an annular coal injector with up to 50% of the total air introduced as core airlz3 . The swirl was dependent on the ratio of core air and secondary air, which makes it difficult to evaluate the influence of swirl alone. When the core air was swirled and the secondary air unswirled the swirl number had hardly any influence on the NO emission.

Abbas et a1.‘09 studied the influence of swirl and momentum ratio for two different type combustors (100 kW) for a high volatile bituminous coal. The coal was introduced either through an annular injector (single annular orifice, SAO) typical for an industrial burner, or a central orifice (single central orifice, SCO) with the same cross-sectional area. At all swirl numbers the SAO burner produced higher NO concentrations than the SC0 burner. This was explained by a difference in mixing pattern; the coal from the annular orifice did not penetrate the IRZ well, but followed the outer part of the reverse flow zone (type 2 flame), thus being in an environment richer in oxygen than that for the coal from the SC0 burner, which penetrates the IRZ deeply (type 3 flame).

The momentum ratio of primary and secondary air was changed from 0.1 to 0.6 for the same burners by Abbas et al.‘” and a minimum in NO was found between 0.3 and 0.5. At a low ratio the momentum of the primary air is not large enough to penetrate the IRZ, while at a higher ratio the flame will lift off and more air is entrained into the fuel jet. It can be speculated that Smart and Weber94 would have found the same trend if a larger range of momentum ratio had been investigated.

Pilot-scale combustors are usually scaled-down versions of large industrial burners. Since it is not

possible to achieve similarity in all fluid dynamic and chemical processes, there are several ways to scale down a burner. Many burners are scaled according to the constant residence time criterion, relating the mass flow throughput to the burner throat diameter, thus reducing the velocities, although most burner manufacturers use constant-velocity-scaled burners. Using the last method the mixing intensity is kept constant. Constant residence time scaling has the advantage that the degree of particle penetration into the IRZ is kept constant, and that the NO emissions better represent full-scale values. Since the constant residence time scaling results in a decrease in velocity with burner diameter, these burners are larger than the constant velocity burner. Both methods have their advantages and disadvantages, so they are both in use, depending on the ease of operation and preference of the designer.

Smart et al. ’ ” used both constant residence time and constant velocity scaling of a conceptual 50 MW burner to a 2.5 MW burner, in order to investigate the importance of scaling parameters. The velocities of the primary and secondary air were 7 and 15 m/s, respectively, for the constant-residence-time-scaled, and 20 and 40 m/s for the constant-velocity-scaled burner. The velocities in the last case are of course identical to those in the full-scale 50 MW burner. For the two swirl numbers (0.6 and 1.0) investigated, the constant-velocity-scaled burner resulted in higher NO emissions than the constant-residence-time-scaled burner at injector position 0.0 (Fig. 17). This is due to the higher mixing rate between fuel and air which is proportional to the product of the characteristic burner velocity and the characteristic burner diameter Usa. The coal (a high volatile bituminous) ignited on the outside of the IRZ in both cases, and NO levels were around 1000 ppm. When the primary air injector was inserted downstream into the quarl, a decrease in NO was first obtained by the velocity-scaled burner. The higher velocity caused a larger slip velocity in the IRZ, and a deeper penetration of the coal into the IRZ than in the case of the residence-time-scaled burner. When the injector was inserted further, the NO emissions from the residence-time-scaled burner became smaller than for the constant-velocity-scaled burner, since the coal penetrated the IRZ with less entrained air.

5.3.1. Tertiary air

Tertiary air (TA) is air supplied through an annular inlet at the burner, parallel to the secondary air (Fig. 14). Sometimes the air injected further downstream in the furnace is called tertiary air (here termed over fire air, OFA). Tertiary air provides the possibility of changing the secondary air velocity independently of the overall stoichiometry and delaying mixing by increasing the initial distance between the fuel and the air at the burner.

It is generally found that increasing the flow rate of tertiary air, while decreasing the secondary air flow rate, results in a reduction in NO. Maier et a1.86 used secondary air velocities of 25, 40 and 55 m/s combined


with tertiary air velocities of 0, 40, and 80 m/s, and detected the lowest emission with the lowest secondary air velocity (v = 25 m/s). Kimoto et ~1.“~ obtained the best results at a ratio of SA/TA = 0.145, with most of the air injected through the tertiary air inlet. A low secondary air flow delays mixing of the coal and the secondary air, thus decreasing local stoichiometry and increasing residence time of coal particles in the reducing zone. The same result was found by Leithner and Lendt9s, who showed that the optimum amount of tertiary air was a function of the heat extracted at the furnace wall.

It can be concluded that the influence of swirl depends on the near burner stoichiometry and the air inlet conditions (geometry, momentum). However, there is no general trend to be found: swirl increases mixing and stabilizes the flame, but the formation of NO depends on the combination with the geometry of the coal injector and air velocities. Reduction of NO is usually possible by application of swirl to the secondary air. It should always be kept in mind that penetration of the coal into the reverse flow zone is the main parameter to be fulfilled in order to keep the NO emissions low, and that the flame is closely attached to the burner.

5.4. Particle Size and Slip Velociry

For most bituminous coals a fineness of 70-75% passing a 200-mesh (75 pm) sieve is recommended*, as an optimum between milling effort and burning behavior. For these small particles it is often assumed that they follow the streamlines of the gas phase. The question is whether this is true or not, and how it depends on particle size and solid loading of the jet. This is important since the spread of coal particles and entrainment of secondary air is related to availability of oxygen and the formation of NO.

Wall et aLIz studied the influence of particles on entrainment and jet spread of a cold flow jet in a stagnant fluid. Eight size classes, between 0 and 180 pm, and different particle densities were investigated. The jet with the smallest particle size (calcium carbonate, p = 2700 kg/m3), O-20 pm, spread out wider compared to a clean jet. All jets with solids of larger particle size distributions showed the opposite effect. A balance between the diffusion characteristics of the particles and the attenuation of turbulence by the presence of particles characterizes the net effect. According to Field et al.’ the effect on entrainment depended on the diameter of the nozzle. Smaller eddies emerged from a smaller nozzle, and caused a higher slip velocity, and a decrease in entrainment.

Weber et aLIz measured in-flame velocity profiles with laser Doppler anemometry, and from the amplitude of the signal they could get a qualitative impression of the size of particles passing the measurement volume. This particle discrimination technique demonstrated that there was no difference between the velocities of small and large particles in the IRZ. Within the flame region, which includes the fuel jet, a significant difference in

velocities was found, indicating that the slip velocity cannot be neglected. This means that laser Doppler anemometry measurements in flames using coal particles as seeding have to be treated with care, since they do not always give an exact view of the gas velocities in the flame. On the other hand, gas velocities cannot be used to determine the flow pattern of coal particles within the flame region. Cold flow measurements by Jensen et al. lz7 on coal particles (100% passing 150 pm sieve, 50% passing 45 pm on mass basis) and 5 pm diameter alumina particles showed a difference within 10% in velocity at a low particle load (2 kg/hr). They suggested that the coal particles would follow the gas during combustion (high particle load, 160 kgkr). The influence of coal load on the particle slip velocity is not known. However, according to Wall et a1.1z5, spread of a particle-laden jet is suppressed by increasing concentration, which means that the slip velocity is higher at higher loads. It indicates that conclusions such as those drawn by Jensen et a1.lz7 have to be regarded with care.

A difference in particle heat-up and devolatilization rate caused by a variation in particle size may change the flame conditions and aerodynamics and thereby NO emission. Laboratory experiments in flat-flames’28Z’2’ and a laminar flow reactor9’ revealed lower NO emissions from larger particles in the early state of combustion. This was explained by the delayed but more intense volatile release of the larger particles. When volatiles are released as jets emerging from the particle surface they will bum further away from the particle surface. O2 diffusion to the particle surface is in this case inhibited, and NO may be reduced on its way out of the particle, thus lower NO emissions result. It should be noted that at longer residence times in the substoichiometric primary zone of the combustor or at low stoichiometry there was no difference any more. NO reduction in the burnout zone was smaller for larger particles, resulting in equal outlet concentrations for all particle sizes. At short residence times in the reducing zone of a combustor, the faster release of volatiles from small particles compared to release from larger particles may reduce emission of NO.

Pictures from Abbas et al. ’ I9 showed that the flame structure and ignition patterns are dependent on the degree of spread of the particles and the rate of devolatilization. The reaction environment and oxygen availability change, thereby leading to variations in NO emissions. However, from pilot-scale combustion studies a relation is not readily apparent. Briceland et al. “’ and Harding et aLs6 found no difference when firing a finer grind; Barratt and Roberts”’ and Schnell et al. Izo observed higher emissions for smaller particles (up to 30% more NO), with an initial temperature difference of 500°C. due to faster ignition and devolatilization of the fine grind. Maier et aLs6 found for one coal higher NO concentrations for particles larger than standard, although for another coal an optimum was found within a range of grinds, all finer than standard. Unfortunately the size ranges do not overlap. Abbas et al. ‘I9 detected lower emissions for both a finer and a

368 R. P. van der Lans er al.

coarser grind. Knill et aLL3’ found a dependence on injector type; no influence of particle size was found for an annular injector, while smaller particles gave higher NO concentrations for an oblique injector (dividing the primary air and coal into four jets). The injector type is directly related to the mixing pattern of the particles with the air, and the conditions in the near burner region where the particles devolatilize. Chen et aZ.13’ investigated the influence of particle size on mixing conditions by fast mixing all the air with the particles under excess air conditions, and by creating a long jet with slow mixing (axial diffusion flame). Fast mixing yielded higher NO emissions for smaller particles, while the opposite was true for the axial diffusion flame.

A closer look at the operating conditions does reveal some more details: in two investigations where no influence of particle size was found, the primary air velocity was several times the secondary air velocity. A large momentum ratio makes penetration into the IRZ easier, and the size of the particles is apparently less important. Maier et aLE6 showed that under fumace- staged conditions (staging air introduced further down the furnace) the influence of particle size decreases with residence time in the primary zone and with lower stoichiometry of the primary zone. This is consistent with the laboratory results mentioned above. Mixing becomes less important since the overall stoichiometry of the near burner area is substoichiometric. That large particles may form more NO is due to a lower residence time in the primary zone, combined with a lower burnout when they reach the oxygen-rich second stage.

Under burner-staged conditions, with all the combustion air introduced through the burner, generally higher NO, emissions have been found for smaller particles, in agreement with the laboratory experiments mentioned above. Besides, small particles will increase the spread of the jet, and mix faster with the secondary air. Abbas et al.‘19 found the opposite trend, while using a single central orifice. This shows the importance of the orifice to particle size dependency, since annular and oblique (coal jet split into several jets) injectors were used in the other investigations.

It can be concluded that both the orifice and the momentum ratio are of major importance to the penetration of particles into the IRZ. In studies where the momentum ratio was high no change in emissions with particle size was observed. The annular and oblique orifices spread particles wider than the single orifice; this is most pronounced for smaller particles. For this reason these orifices produce higher NO emissions for small particle sizes since more O2 is entrained during the early stage of combustion.

5.5. Coal Blends

Coals are often blended in full-scale boilers in order to minimize the effect of variations in coal properties on boiler operation. It is therefore interesting to investigate the influence of blends on the emission of nitrogen oxide.

In a semi-industrial combustion facility (2.5 MW), operating under both high NO, and low NO, conditions (by inserting coal injector into quarl), four pure coals and six blends were investigateds“, ranging from high volatile bituminous to semi-anthracite. A linear trend was observed between the NO concentrations obtained from independent firing of each coal and the mass fractions of the coals in the blend. During unstaged conditions the NO emissions were highest for high volatile coals, while under staged (low NO,) conditions, the low volatile coals created the highest NO levels. Also Miyamae et aLs2, blending semi-anthracite with medium volatile bituminous in a 1 MW combustor, and Sato et al. 13* , firing several blends of medium to high volatile coal in a 750 kW installation, found a linear relation between the NO emissions and the blending ratio under furnace-staged conditions.

Maier et aLS6 concluded that a clear relation cannot be found for blends of a high and a low volatile bituminous coal; with a stoichiometry of 0.6 in the primary zone the NO emission of the blend was higher than that from each individual coal. At higher stoichiometry (0.9 and 1.2) the emissions were linear with blending ratio and in between the values of the individual coals. The experiments were conducted in a scaled-down version of an industrial burner (constant velocity scaling). Also, Rozendaal et al.‘33 in a 1 MW combustor with an annular coal injector’34 found no clear trend when burning two blends of different high volatile coals with a low volatile coal under both unstaged and furnace-staged conditions (up to 40% of the air staged); for one of the blends the NO emission was lower than that of the individual coals, while for the other the concentrations were mainly in between those of the pure coals. They ascribed this to a difference in particle size between the two high volatile coals, since the proximate and ultimate analysis of the coals were almost identical.

The effect of blends on the NO emissions is not clear from these results. Mostly the trend is linear with blending ratio, but this is not a general trend. More work is required in order to get an understanding of the influence of blends on NO formation.

5.6. Generalization of Results

The influence of operating parameters on NO formation in pulverized coal burners cannot be easily general- ized. Similar changes in burner parameters in various studies may cause different directional effects. A general statement can be made about the influence of burner, coal and mixing parameters: those parameters that tend to decrease NO under low NO, conditions (like particle diameter and volatile content) increase NO under well- mixed high NO, conditions. Although this looks simple, there is one factor that compiicates the story. A change in a certain parameter may cause a change in mixing intensity, and thus a clear distinction between low and high NO, cannot be made. The different directional effects are directly related to changes in mixing pattern in the flame. The prediction of mixing pattern from

Influence of process parameters in nitrogen oxide formation

Table 3. NO, reduction measures

369

Control method NO, reduction potential* Investigation and baseline

Low excess air level

Over fire air

Low NO, burners

Flue gas recirculation

Rebuming 50% 60-70%

SNCR 30-80% SCR 60-90%

O-15% lo-15%

15-30% 25% 20% 15% 15-50% 23-40%

50%

55% 40-70% up to 30%

9-13%

(Guidelines for retrofit; study sponsored by EPRI”‘) Lowering from 6 to 4% O2 in several industrial installations with brown coal; baselines from 200-350 ppm’“’ (Review”‘) Baseline 220 ppm in a 300 MW, boiler with brown coal”’ Baseline about 500 ppm in a tangentially fired furnace”’ Baseline about 300 ppm in a 600 MW,s opposed-fired boiler’40 Typically 15% for wall-fired and 25% for tangentially firedlX6 Two tangentially fired and two front wall-fired boilers; 40% reduction from baselines of X0-650 ppm, 23% reduction for 350 ppm as baseline14’ 500 and 250 MW,s wall-fired, baseline about 900 and 600 ppm respectively’42 600 MW, opposed-fired, baseline about 800 ppm”’ (Guidelines for retrofit; study sponsored by EPRI’s6) For wet bottom boilers; only few percent for dry bottom boilers (hard coal)‘38 150,300 and 600 MW, boiler with brown coal; baselines about 120 ppm, 10% recirculationi3’ 300 MW, opposed-fired, baseline about 600 ppm’44 71 MW, tangentially fired (baseline about 550 ppm) and 158 MW, wall-fired (baseline about 400 ppm)‘45 (Review’38) Most installations have been designed for a reduction around 60-70%. but over 90% can be reached at high initial NO concentration”’

*Compared to baseline value.

l_/_ mu tertiary +

air

secondary air +

primary air ---*

and fuel

Fig. 18. Flame stabilizing ring.

burner, coal and operating parameters is still difficult, and for this reason many phenomena can be explained but not predicted.

6. FULL SCALE

Prior to low NO, burners, conventional burners were designed to obtain fast mixing between the coal and air, leading to short, hot flames and a high burnout. The coal ignites and devolatilizes on the outer boundary of the IRZ which is favorable for the creation of NO. In order to reduce the NO level so called low NO, burners and other air staging techniques have been developed. The technology described in the previous sections has been successfully applied to full-scale boilers, and it is largely focused on modifications for retrofit’35. Most burner modifications include installation of low NO, burners

OFA OFA -j c

fuel + ri air W

(b)

Fig. 19. Concept of furnace air staging (a) and burner air staging (b).

with internal air staging and over fire air (OFA), which is added above the burner belt in order to allow for a reduction of the oxygen level at the burners. The techniques used in full-scale boilers are summarized below and in Table 3.

When scaling up a (full-scale) single burner to a furnace including several burners, interaction between the burners will occur. Temperatures in the flames of the outer burners in a furnace may be lower than those in between other burners, and coal burned in the lower burner levels has a longer residence time in the furnace than coal from the upper burner rows. The interaction phenomenon is poorly understood, due to practical limitations. The size and accessibility of the boiler often restrict the possibilities of detailed mapping of the boiler environment.


6.1. Reduction of Overall Excess Air Level

The easiest and cheapest way to reduce NO emissions is a reduction in excess air. A lower amount of oxygen will be available for the formation of NO. A reduction of about 15% in NO can be obtained, but its application is limited, due to the risk of low burnout and corrosiont3’. A reduced overall excess air level is used in combination with other methods in almost all low NO, applications, and care is taken to minimize its drawbacks. A lower excess air level may cause flame instability, and to overcome this a coal spreader or flame stabilizing ring is applied (Fig. 1 8)146-148 . The coal will ignite closer to the burner since the local mixing is enhanced, but the NO emissions are still low. Although the coal is spread out, the entrainment of air is less when the IRZ is more closely attached to the burner. These two phenomena partly compensate each other. The method is often used to obtain higher burnout levels and stable operation at lower overall excess air levels, which increases the overall plant efficiency. In addition, slagging will also be reduced. Auxiliary air ports located further away from the fuel jet are applied to protect the wall against reducing conditions. A curtain of air is established between the flame and the wall.

6.2. Furnace and Burner Air Staging

Air staging is applied to create fuel-rich zones in which devolatilization takes place. The secondary air is swirled, mixing of coal and air is delayed and a fuel-rich zone is created close to the burner. This is called burner air staging or internal air staging. As shown in Fig. 19, lo-20% of the combustion air can be added as over fire air (OFA) above the burner belt, thus reducing the stoichiometry of the primary zone. A reduction in NO up to 30% can be achieved by external air staging or furnace air staging’36-‘40, which demonstrates its important role in NO abatement. Air staging is further implemented by over burner air, added just above the burner, and by tertiary air, often injected coaxially to the secondary air, which protects the wall against the corrosive environment of the flame.

Tangentially fired boilers are often equipped with non- swirling secondary air registers, although later versions include swirl, and tertiary air ports. The reduction of NO is accomplished by the doughnut-shaped fire ball in the furnace. Combination with OFA ensures a reducing environment in the burner belt, while auxiliary air is directed away from the furnace diagonal to reduce the entrainment of air by the fuel jet, to increase the swirl by a factor two, and to protect the wall against reducing conditions’39. This secondary air port is often located between fuel compartments.

OFA ports and burners can sometimes be tilted, primarily to regulate reheat steam temperature. The influence on NO is mainly through a change in the residence times in the fuel-rich and the burnout zone. An increase in angle between burner tilt (varied between 0 and -20’) and OFA tilt (between -5’ and 30’) could

decrease NO up to 25%‘49, although a minimum amount of NO at a burner tilt of 0” has been reported, when changing the tilt over a range of positive and negative values’50. Both these studies were carried out in comer- fired boilers. Position of tilts is selected as a compromise between NO, emissions, minimum 02, combustion uniformity, and boiler efficiency.

6.3. Flue Gas Recirculation

Recirculation of flue gas reduces the local oxygen accessibility by dilution, and may also reduce the local temperature. Decrease in NO emissions of about 10% can be achieved by this method when recirculating 10% of the total flue gas13’, although often no influence at all on NO emissions is observedt3s. Introduction of the flue gas may occur either as separate jets below the burner belt, or as a mixture with combustion air (with auxiliary air in tangentially fired boilers). The jet momentum can increase the mixing rate of gases in the furnace, thus contributing to a lower burnout.

6.4. Fuel Concentration

Shortening of the burner belt leads to an increase in the concentration of fuel and a decrease in local stoichiometry. This method can be used to decrease NO emissions’37. By decreasing the burner belt from four burner levels to three levels while maintaining the same load, a decrease of about 10% in NO could be achieved.

6.5. Particle Size

It has been shown in a previous section that the influence of particle size on the formation of NO was unclear. For full-scale boilers a systematic trend cannot be observed either. Some investigations show that NO emissions decrease with particle size’S’*‘52 while others found no effect when increasing fineness from 70 wt.% passing 200 mesh (74 pm) to 82%‘53.

6.6. Other Techniques

Apart from NO, control through combustion measures other methods exist to reduce already formed NO in the burnout zone or in the flue gas’38*‘54.

Rebuming: burning a relatively small amount of fuel under reducing conditions above the burner belt produces hydrocarbon radicals for the destruction of NO via HCN. The fuel must be easy to ignite and mix rapidly with the combustion gases. The amount of fuel required varies between 10 and 20% of the net heat input. Selective non-catalytic reduction @NCR): ammonia or urea is introduced above the burner belt as a reducing agent in order to destroy NO. Good mixing and a well-specified temperature window are required to obtain significant reduction of NO. Mixing can be problematic due to the high viscosity of the flue gas and to the location of available injection points’38.


The temperature window for the most common che- micals is 900-I 100°C. Below these temperatures ammonia can be formed and emitted with the flue gas, while at higher temperatures even more NO can be produced. The conversion takes place at relatively high temperatures where undesirable side reactions cause simultaneous formation of NO. Selective catalytic reduction: the flue gas is passed over a catalyst, which reduces the concentration of NO by promoting reaction with an agent, for instance NH+ Using this method undesirable side reactions that cause simultaneous production of NO at the relatively high temperature of the SNCR process are avoided. The advantage of this method over the SNCR process is the high conversion of NO to N2 (80-90 vol.%) and the lower consumption of chemi- cals. Zeolites and activated coals as well as titanium oxide, iron oxide and vanadium oxide based catalysts are in use.

The costs of these NO, control methods are summarized by Hjalmarsson’38.

7. FURNACE MODELING

Mathematical models may help us to understand the combustion process and support the design and optimization of burners and furnaces. In addition, they help to predict the influence of coal quality on combustion performance in existing boilers, which is important for the purchase of coal by the power generating industry. Practical testing is expensive and time consuming, and consequently considerable effort is made to build models that can predict combustion performance. Temperatures, residence times and composition of the gaseous environment are needed in order to estimate NO emissions. Comprehensive models have to be developed, including devolatilization and fuel- nitrogen release, homogeneous gas-phase reactions, heterogeneous char reactions, mixing and radiation. In the evaluation of mathematical models two groups will be considered: ?? chemical reaction engineering models, and . simultaneous flow field and combustion models

using computational fluid dynamics (two- and three-dimensional).

7.1. Reaction Engineering Models

The primary problem in modeling any practical combustion system is simultaneously accounting for mixing and chemical reaction. One way to do this is by treating the performance of a combustor only in terms of mixing rates (fast chemistry) or in terms of chemical kinetics (fast mixing). Another approach is to approx- imate the combustion zone as a continuous stirred reactor with a smaller volume, making the assumption that the deviation of a practical furnace from the reactor performance is solely due to incomplete use of the volume available. By connecting different reactor types,

such as plug flow reactors and stirred tank reactors, including bypass, recirculation and dead-space, one could empirically fit such a model to a wide range of variations in the operating variables. This is the starting- point from which many reactor configurations are formed. This way of modeling was frequently used two or three decades ago, when computer capacity was low.

Hottel et al. ‘55 examined different models for a high- output gas burner, consisting of two well-stirred reactors with recirculation. Thring and Masdin’56 used a cascade of perfectly stirred reactors for kerosine combustion including air staging. In both papers only a theoretical survey was included without verification or comparison with experimental data. Beer and Lee15’ studied the relation between the residence time distribution in the combustor and the performance of a combustor. They measured residence times of a salt tracer solution in water in a cold-scale model combustor with a tangential inlet swirler, and found from the detected concentration profile that the furnace could be regarded as a combination of a continuous stirred tank reactor (CSTR) and a plug flow reactor (PFR) in series. Variation of the swirl number changed the residence time in the well-stirred region of the flame, and thus changed the ratio of the mean residence time between the CSTR and PFR. During an increase in swirl, the proportion of the stirred tank went through a minimum around a swirl number of one. Experiments were compared with argon tracer measurements during anthracite combustion on a semi-industrial scale in a combustor geometrically similar to the cold model. The results obtained from the combustor showed good agreement with the cold model for cases when the degree of swirl was the same. From the reactor system the combustion efficiency (based on burnout) was calculated and it compared favorably with the experiments.

Burnout in a furnace has been predicted based on the residence time in a full-scale furnace by application of detailed char oxidation models for the coal particles’58. Quite good results are obtained from these models. A few authors have stated long ago the difficulties of such engineering models for the prediction of NO from gas- fired burners. Using a CSTR and a PFR in series with the residence times derived from tracer measurements leads to erroneous results due to the fact that the IRZ and ERZ (external recirculation zone) are inseparable”3. The problem of reactor volume estimation was emphasized earlier’59. Information on flow field, amount of recirculation, temperature and mixing rate is required to make these models suitable for the prediction of pollutants. Accurate predictions rely on the volumes of the reactors, which again depend on the operating parameters and burner geometry. If these difficulties are overcome considerable progress may be possible by choosing or developing better submodels for micro mixing and homogeneous gas-phase reactions. Until now the concept has not been tested for NO, prediction from coal-fired burners with state-of-the-art chemistry. The advantage of the flow field simplification is the


allowance for complex chemistry and combustion submodels. Combination with computational fluid dynamics or measured mixing parameters for reactor volume estimation may increase the accuracy of the method considerably. The simplicity of these models makes it relatively easy to understand the impact of certain parameters on the combustion process, and the complex chemistry may give good qualitative results, which can contribute to a better understanding of pulverized coal combustion in general. However, accurate quantitative results cannot be obtained by models that greatly simplify the mixing pattern in flames16’. Taking it into account that even volatiles do not always bum in a homogeneous mode, but also in a diffusion mode in or around the coal particle as recently indicated in a single particle study16’, application of detailed kinetics will not be sufficient to model NO formation accurately with engineering reaction methods. However, the potential of this concept needs to be assessed in the near future.

7.2. Computational Fluid Dynamics

In the last decade great improvement has been made in computational power. Empirical and global models as used in the reaction engineering approach have to a large extent been superseded by local, phenomenological models. Comprehensive numerical packages are available to solve for both the flow pattern of the gas and particulate phases, and the chemical reactions occurring throughout the furnace. Numerical codes have been developed since 1970, and they are all based on the same principles using a finite-volume or finite-difference discretization scheme. The basis for these models is the conservation equations for mass, momentum, energy and species for stationary flows. The differences between the models can be found in the application of the submodels. Most models use the k-e model for closure, Lagrangian calculations for the particles and fast chemistry for the gas phase. Submodels for ignition, devolatilization, char oxidation and heat transfer are implemented. Since the concentration of NO is relatively small, initial combustion calculations are performed neglecting the presence of NO. The formation of NO is calculated afterwards with a post-processor, using the results from the combustion calculations. This requires that the initial coal combustion calculations must be correct, providing accurate temperature, oxygen and combustion product concentrations and burnout predictions. Turbulence remains a problem to be solved, as do the homogeneous gas-phase reactions. The products of devolatilization are complex and to a significant extent unknown. Part of the volatiles is released as tar, which is further cracked into lighter components. Due to the complexity of the tar and volatile chemistry, detailed chemical modeling of the full chemistry cannot be performed at present. It is possible to simplify the components and to assume that the volatiles react to light components such as CH4, Hz and CO which can be dealt with kinetically in a stirred tank reactor or a plug flow reactor, but combination with

a CFD code is still not possible due to inadequate computer power. Global kinetic models have been developed for the combustion of hydrocarbons16*. Parameters for these models have been fitted for a specific case, and they are only valid for a narrow range of conditions. Even when burning the same fuel in a different flame, the temperature and transport phenomena can change reaction mechanisms or the relative importance of the fundamental reactions’62. This means that one should be very careful when applying these models.

The assumption of fast chemistry is frequently applied in combustion modeling, assuming that all species are in thermodynamic equilibrium, and using the mixing rate as the observed rate of reaction. Although this assumption holds for many reactions, it cannot be applied to all species. In particular the CO concentrations are often erroneously predicted by using this approach. Weber et al. ‘63 obtain in the best case a substantial overprediction of the CO concentrations. Brewster et a1.34 use two global rate expressions from Howard et al. ‘61 and Dryer and Glassman’65. Even though these global expressions were used outside their applicability range, the result was a significant improvement in prediction. This shows that global rates are often more realistic than the equilibrium assumption. It must be noted that more precise devolatilization and gas temperature predictions are needed before the effects of CO/CO2 kinetics can be accurately evaluated or justifiably included. As a start the global rate expressions could be evaluated with a comprehensive kinetic model.

A short summary of comprehensive modeling is given in Brewster et al.34; modeling between 1970 and 1983 is described by Smoot’66 and deals with the basic model elements; three-dimensional modeling is treated in brief by Gillis and Smith16’. A more general review on the modeling of swirl in turbulent flow systems can be found in Sloan et al.16*. Most models only deal with combustion, without taking into account the formation of NO. Models have been developed for both pilot-scale furnaces ‘8*‘69-173 and full-scale furnaces, both wall-fired and tangentially fired’9~‘67*‘74-‘77.

Validation of the full-scale predictions is almost impossible due to a lack of accurate measurement data”*. Boilers are large and difficult to access with measurement equipment. Usually burnout, 02 concentrations and temperature profiles can be obtained from a furnace, but near-burner processes, which have a significant influence on the total flow and temperature field, are very difficult to monitor in a large furnace. Validation of submodels and the influence of grid refinement and numerical routines are best investigated from pilot-scale data. An advantage of the CFD models over the one-dimensional models is the potential ability to predict furnace temperatures, and in-furnace concentration and flow profiles. In particular the near burner zone is of importance for the reduction of NO, and detailed knowledge of this zone is required for burner development and scaling purposes.

NO post-processors have been developed and applied to the initial combustion model. Comparison of


prediction of NO concentrations with measurements has been made in the quarl zone of a semi-industrial bumerlz6, in pilot-scale bumers’19~‘20~‘79~180, and in full-scale boilers34*‘8’*1*2. All NO post-processors make use of the same basic principles for fuel-NO as first developed by Smith et nZ. ‘83 and evaluated by Hill et al.lw . It is assumed that nitrogen is evolved as HCN at a rate proportional to the rate of total mass loss. The conversion of HCN into NO and N2 follows the global reaction rates as proposed by de Soete@‘. It is assumed that the individual instantaneous reaction rates are only functions of the stoichiometry, and the time-mean reaction rate can therefore be obtained from the appropriate probability density functions (PDFs). The stoichiometry is calculated from the mixture fraction and the local amount of off-gas, and the PDFs can be created from the root mean square fluctuations of these variables. Apart from de Soete’s model three other models have been tested’s6, indicating that the simple de Soete model gave as good predictions as the other, more complicated models. The rate of formation of NO from heterogeneous reaction of the char is modeled by a mean rate proportional to the mean particle burnout. Reduction of NO on char is taken into account by an Arrhenius function with first-order dependence of particle density and NO concentration. The reduction of NO on char and the simultaneous formation of NO from char have also been modeled by assuming that a constant quantity of the nitrogen in the char is converted to NO”‘.

Due to the numerous engineering and mathematical approximations made in the subprocesses, in the coupling of different processes, and in the numerical solution of the equations, continuous validation of results obtained from the individual submodels is needed. The requirement of large computational power, combined with the confusing amount of assumptions and submodels, raises some questions about the reliability of the modeling’s5. Quantitative results are only reliable in cases where the model has already been validated for a similar case. Significant changes in geometrical or operating parameters cannot be handled with accuracy by these models, although often qualitative results are obtained. A good description of the possibilities of comprehensive modeling is given by Weber et al. 163.

7.3. Other Methods

Estimation of NO emissions from utility boilers by empirical relations was briefly reviewed by Smouse et aZ.‘*’ with the conclusion that no reliable method is publicly available. They indicate that more advanced techniques using neural networks show some promise. However, this may not directly contribute to a better understanding of the combustion phenomena in boilers.

Another method used by Afonso et a1.‘8* assumes independent effects of fuel properties, boiler design, and boiler operating parameters. The influence of each parameter is taken into account as a change in NO from a reference condition. The three independent effects are further divided into sub-influences, which

are handled in the same way. Influence of fuel-N content, primary zone stoichiometry, primary air velocity, heat release rate, and coal fineness have been described. It appears from the results that the amount of data used is not sufficient, and also that this model is not reliable.

Although detailed kinetic models cannot be implemented in comprehensive fluid dynamic models simply because of limitations in computer capacity, a compromise has been made by Bdrls9. Before applying an NO post-processor he divided the flow field in relatively large volume cells, which were treated as plug flow reactors and well-stirred reactors with turbulent exchange between the zones. This concept was first developed by Sadakata and Be&‘% for gas flames, and it is in fact a combination of a CFD-code with a chemical reaction engineering approach. Examples of applications show promising results, but the lack of validation and information on the model make it difficult to draw any conclusions. Also, this model relies on combustion information obtained from CFD-calculations, but application of more detailed chemistry may well improve existing models.

There is still a need for simple models, which can be used by the boiler and burner manufacturers and by the power generating industry. As long as the comprehensive models cannot be used reliably by non-experts, the search for simple models will proceed.

8. RESEARCH NEEDS

Until it is possible to predict combustion behavior from coal quality, and to scale up burners with adequate accuracy, a combination of experimental and theoretical research in pulverized coal combustion is needed. As long as existing models cannot predict combustion behavior satisfactorily it is not enough to conduct experiments for model verification only. There are still phenomena involved in combustion that cannot be well described by submodels. Modeling of NO formation requires the improvement of several submodels, particularly turbulence-chemistry interaction, turbulence models, devolatilization and homogeneous gas-phase combustion. Since the modeling of flow field and temperature distribution gives problems, the NO post-processor must be applied very cautiously.

A more systematic parameter analysis is required regarding burner geometry and operating parameters. A considerable amount of research results is not publicly available due to commercial considerations. It is expensive to run pilot-scale or full-scale facilities and in many publications only a minimum of information is given, enough to explain some qualitative results but not enough to allow a detailed comparison between different studies. Apart from burner manufacturers only a few institutions possess a burner. A better understanding will be obtained when comprehensive in-furnace data are collected, so that the influence on flow pattern and species can be monitored as well. Such data are useful for increasing the reliability and development of both


fundamental submodels and empirical models, eventually supported by CFD programs.

The influence of flame stability, and fluctuations in flames, which can be aerodynamically induced or induced by burner slagging, on NO emissions needs to be assessed. Also, the interaction between burners in full-scale plant and the effect of differences in operating parameters of those burners are largely unknown.

Some of the needs listed above will be difficult to realize, due to expense or to the complexity of the problem. Until crucial combustion processes can be modeled adequately, practical experience is of major importance.

Acknowledgements-This work is part of the research program CHEC (Combustion and Harmful Emission Control) funded by the Danish Technical Research Council, Elsam (the Jutland- Funen Electricity Consortium), Elkraft (the Zealand Electricity Consortium), and the Danish and Nordic Energy Research PrOgMl.

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