Contents

Contents...........................................................................................................................................1

Abstract ......................................................................................................................................2

1. Introduction..............................................................................................................................3

2. Experimental studies of the fate of char-N during combustion...............................................6

2. 1 Experimental technique...................................................................................................6

2. 2 Effect of operation conditions on the formation of NO during char combustion...........11

2. 3 The formation of other N-containing gas species during char oxidation.......................21

3. Mechanistic studies of char-N conversion.............................................................................24

3.1 The mechanisms of the conversion of NO during char combustion...............................25

3.2 NO reduction over char with oxygen and effect of other reacting gases.........................29

4. Model studies of nitric oxides formation from char combustion..........................................34

4.1 Single particle model.......................................................................................................34

4.2 The SKIPPY (Surface Kinetics in Porous Particles) approach.......................................42

5. Concluding remarks..............................................................................................................45

References..................................................................................................................................46

1

The fate of fuel nitrogen during combustion of char: A review

Zhou H., Jensen A., Glarborg P.,

Abstract:

The conversion of char-N to NO is the main contribution of the formation of nitric oxides

and is the most unsolved modeling problem during the char combustion. This literature review

summarizes the current understanding of the fate of fuel-N during combustion of char. The

review focused on the following three topics: (1) Experimental findings of the split of the fuel-N

under single and batch char particle combustion conditions. Approximately 100 % of fuel-N was

found to be converted to NO for fine particle under single particle conditions for fixed bed,

fluidized bed, and pulverized combustion systems. The finding implies that the formation of

minor N-containing species such as NH3, HNCO, and HCN may be caused by the secondary

reaction after the release of NO. The operation conditions such as the reactor temperature, the

particle size, and the coal rank may affect differently the formation and destruction of NO at

different combustion systems; (2) The most useful mechanism for describing the formation and

destruction of the NO and N2O during char combustion was proposed by De soete (1990). The

effects of major and minor species such as O2 and H2 on the NO reduction of char have not been

well elucidated both experimentally and theoretically; and (3) Different simplified models have

been proposed at single particle level for describing the fuel-N conversion during char

combustion.

2

Keywords: char, combustion, nitrogen oxides, emission, model

1. Introduction

Nitrogen oxides (NO, NO2, and N2O) emission has drawn much attention by researchers

and public health authorities due to the serious environmental impact of NOx (Kraushaar and

Ristinen, 1993). Combustion systems are one of the major sources of pollution due to NOx.

Fossil fuels such as coal, natural gas, petroleum, as well as renewable bio-fuels and municipal

solid waste for instances agricultural residuals (straw, cotton stalk), and forest debris are

commonly used in the systems (Smoot, 1993, Saxena, 1994, Samuelsson et al. 2004, Zhou et al.

2006). The fuels, particularly solid fuels, as listed in Table 1, contain significant amounts of

nitrogen and give rise to the NOx emission during combustion.

Table 1. Typical nitrogen contents of various fuels

Fuel Nitrogen (% w/w)Natural gasCrude oilHeavy fuel oilLight fuel oilCoalBiomass

May contain 1-5 % molecular N2

0.1 – 0.80.2 – 0.50.003 – 0.010.5 - 2.00.2 – 2.1

Three mechanisms have been identified for the production of NOx in combustion

systems: (a) Thermal NO - significant NO may be formed by this route when the temperature is

in excess of 1573 K. Therefore control of the thermal NO can be achieved by decreasing either

the combustion temperature or the residence time at high temperature; (b) Prompt NO -

hydrocarbon radicals produced when the fuels being burned react with N2 to produce HCN or

3

CN which subsequently be oxidized to NO. Under most practical combustion conditions, the

contribution of the prompt NO to total NO formation is small; and (c) Fuel NO - the fuel NO is

formed by combustion of fuel nitrogen. In modern combustion systems, fuel NO contributes

more than 80 % of the total emissions. N2O may also be produced during fuel nitrogen

conversion particularly at low temperature as in fluidized bed combustors.

The nitrogen functionalities present in coal and in char are: pyrrolic (50-80 %), pyridinic

(20-40 %), and quaternary nitrogen (0 – 20 %) (Thomas, 1997). The group may rearrange during

pyrolysis and at higher temperature the formation of quaternary nitrogen in graphene layer has

been postulated (Wojtowicz et al. 1995). The effect of the nitrogen functionalities on the rate of

NO formation is believed to be small (Pels, et al. 1995, Stanczyk, 1999).

Previous studies have shown that the effect of volatile and char nitrogen on the

transformation of fuel-N to NO is significant. Volatile nitrogen has been identified to form HCN,

soot-bound nitrogen, and NH3 as intermediate species during the combustion of pyrolysis

products. A large fraction of char-N may form NO directly. It is believed that char nitrogen is a

greater contributor to NO and N2O formation than the volatiles (Tullin et al., 1993, b).

Technologically, the released volatile nitrogen is more amenable to control by modification of

combustion zone aerodynamics than is char nitrogen (Wendt, 1993). The fate of the nitrogen that

remains within the char is crucial when determining the ultimate NO emissions (Coda et al.

1998). Phong-Anant et al. (1985) identified that the contribution to total NOx from char-N in low

NOx burners is greater than 60 %. Wendt (1993) further described that the formation of NOx

from coal char as the most important unsolved NO modeling problems. An improved knowledge

of the parameters that determine NOx formation from the coal char is imperative in order to

generate efficient control methods as more stringent emission regulations to be applied.

4

Reviews of the understanding of nitrogen conversion are given by Miller and Bowman

(1989), Johnsson (1994), Thomas (1997), Aarna and Suuberg (1997), Molina et al. (2000), and

Glarborg et al. (2003). Different authors restricted to specific topics of NO formation and

destruction during solid fuel combustion. Miller and Bowman (1989) discussed mechanisms and

rate parameters for the gas-phase reactions of nitrogen compounds. Johnsson (1994) tabulated

data on the fraction of char-N to NO and N2O in fluidized bed combustion. He concluded that the

conversions of char-N to NO vary between 20-80 %, largely because of wide variations in

experimental techniques and the influence of devolatilization temperature, combustion

temperature, coal type, particle size, and nitrogen content of the char. Thomas (1997) reviewed

the influence coal and char structural characteristics on the release of nitrogen oxides during the

combustion of chars. Aarna and Suuberg (1997) elegantly summarized the kinetics of the carbon-

NO reaction, and they pointed out that this reaction may involve the possible initial

chemisorption of NO and also the reaction of surface complexes. The reduction reaction with

pure NO is generally found to be first order with respect to the NO concentration. The reaction is

known to be enhanced in the presence of CO and inhibited in the presence of water. Molina et al.

(2000) focused their review on the mechanism and model of nitrogen release from the char to the

homogeneous phase and its further oxidation to NO, and the reduction of NO on the surface of

the char. Glarborg et al. (2003) gave a general review of solid fuel (coal and biomass) nitrogen

conversion in fired systems. They emphasized on discussing the homogeneous and

heterogeneous pathways in fuel NO formation and destruction and evaluating the effect of fuel

characteristics, devolatilization conditions and combustion mode on the oxidation selectivity

towards NO and N2.

5

To further elucidate the mechanism of NO formation during the char combustion, the

following three topics are reviewed below: (1) The experimental findings of the split of fuel

nitrogen at single particle condition and during batch combustion for different combustion

systems; (2) The mechanisms of the conversion of fuel-N proposed by different researchers. We

will focus on the review of the formation of HCN during the char combustion and the reduction

of NO on the char surface in the presence of oxygen; (3) The modeling work of the fate of fuel-N

during char combustion at single particle condition. The main purposes of the review are to

present how the fuel-N is split during the char combustion at different reaction systems, and how

to explain the experimental findings from the mechanisms and the model point of view.

2. Experimental studies of the fate of char-N during combustion

The heterogeneous char-oxygen reaction, the complex char structure, the surrounding gas

species and particles, as well as the very uncertain fluid dynamics in the practical combustion

system make it very difficult to elucidate the pathway for char-N conversion during the char

combustion. Therefore, different experimental techniques for different combustion systems have

been used by researchers in order to understand the conversion of fuel-N to NO during the char

combustion.

2. 1 Experimental technique

Table 2 summarizes the experimental studies of char-N conversion during combustion.

Test rigs include pulverized coal combustor (PC), fixed bed reactor (FB) including

Thermogravimetric Analysis (TGA), and fluidized bed reactor (FBC). It can be observed that the

percents (W/W) of fuel-N converted to NO, N2O, and HCN are in the ranges of 3-90% , 0.2-5.7

6

%, 0.5-10 % depending on coal type, char preparation, residual volatile matter content,

temperature, reactor type, stoichiometry, and particle size. The NO is the major product of fuel-N

converted and the ratio of fuel-N conversion to NO is remarkably different for different

researchers.

Traditional experimental method, for instance, a steady-state technique (de soete, 1990),

is commonly employed to determine the global rates of char oxidation, NO and N2O formation

during char oxidation and heterogeneous NO and N2O reduction on the char surface.

Some new and promising techniques have been applied and are very helpful to improve

the understanding of the pathway of fuel-N conversion. Winter et al. (1996) applied an approach

called an iodine addition technique, in which the purpose of the addition of iodine is to reduce

radical concentrations to equilibrium levels and therefore to suppress homogeneous reactions,

but not affect the heterogeneous. Their experiments provided strong evidence that N2O may be

only produced by homogeneous oxidation of HCN in fluidized bed combustion conditions.

However, the iodine addition technique has not been used by other researchers mainly due to the

influence of the iodine chemistry on the combustion process is not known yet. Miettinen (1996)

used tracer technique to trace the path of fuel-N during the char combustion. The advantage of

the technique is that it may distinguish the source of N. A 15N-isotope-marked NO was used in

the inlet gas by to investigate N2O formation during fluidized bed char combustion. Her

experiments confirmed three paths of the possibilities of N2O formation during char combustion,

i.e. (1) heterogeneous formation of N2O (one nitrogen atom from the char and the other from

NO); (2) homogeneous formation of N2O (two nitrogen atoms from the inlet gas containing NO),

7

and (3) primary formation of a cyano compound (HCN or HNCO) then further reacts with O2

and / or NO to form N2O ( , ).

The NO formed from a particle can be subsequently reduced by surrounding char

particles or gas species which may make the reaction system very complicated and very hard to

elucidate the NO formation during char combustion. To reduce the possible secondary reaction, a

few special techniques were developed. To minimize the temperature fluctuation and the second

reaction of NO with char, a pulse technique was adopted by Orikasa and Tomita (2003). In their

experiments, a small amount of char sample (about 2.5 mg of char, roughly 10-15 particles) was

used to minimize the heat generation and a series of a little O2 pulse was fed to the sample bed.

Furusawa et al. (1982), Aihara et al. (2000), and Jensen et al. (2000) adopted a single particle

technique, in which very small amount of char was placed into fixed bed reactors during char

combustion. In this case, the char particles situated on the reactor distributor is only one layer.

Therefore, the interactions between particles can be neglected. The experiments thus may be

regarded as single particle experiments. Their experiments showed that most of fuel-N is

converted to NO, and in some cases the conversion is near 100 %.

8

Table 2. Summary of experimental char-N conversion studies

Reactor Parent coal Char preparation Nitrogen(N/C, w/w)

Fraction of char-N to NOx, N2O, and HCN

Operation conditions Reference

1 PC FMC coal char From FMC-COED coal gasification process

0.0136 150.0 , stoichiometric 1.02-1.25, O2/Ar

Pershing and Wendt (1976)

2 PC Montana lignite Char produced at temperatures of 1250 and 1750 K

0.0072-0.0162 21%O2/He, 1250 K Song et al. (1982)

3 FB 2 coals Char produced at 1273,1373,and 1473 K

0.008-0.022 65-1125 , 0.02-5 g, 5%O2/Ar, 1073 and 1173K

Ninomiya et al. (1989)

4 FB 3 coals Char produced in argon at 1450 K

0.0145-0.0178 35-500 , 50-100 mg, 2-21 %O2/Ar

De soete (1990)

5 FB 9 coals Char produced in fixed and fluidized bed at 1173 K

0.00758-0.02275

500-590 , 5-10 mg, 1163 K, 3.15O2/Ar, single particle

level

Shimizu et al. (1992)

6 FBC Kentucky No. 9 bituminous coal

Char produced in the FBC at 1223 K

0.021 1.02-4.48 mm, 0.2-1.2g, 10%O2, 753-1003 K

Yue et al. (1992)

7 FB 2 subbituminous coal 1 lignite

Char produced in the FB up to 1223 K

0.0144-0.0188 90-106 ,0.2 MPa, 6-20 % O2, 0-50 %CO2

Croiset et al. (1995)

8 FBC 3 coals – Tilmanstone, Holditch, and Baddesley

coals were put into the FBC Not available 1.0/2.5 g (1-2 cm), single particle level and

1.0g batches of small particles (1.4-1.7 or 2.0-2.4 mm)

1073 K/1173 K

Hayhurst and Lawrence

(1996)

9 FBC Bituminous and sub-bituminous coal

coals were put into the FBC 0,0126-0.018 5,10,15 mm, 600-1173 K, 5-21%O2/N2, at a particle level

Winter et al. (1996)

10 FB (TGA)

20 coals from anthracite to bituminous coal

Produced in entrained flow reactor at 1273 K

0.0124-0.031 37-75 , 5 mg, 20%O2/Ar, 873-1323 K

Harding et al. (1996)

11 FBC Newland coal Produced in fluidized bed at 1123 K

Not available 200 mg(4-5 particles),1092 K, 8% O2/He

Goel et al.(1996)

12 Fluidized bed

Anthracite and bituminous coals

Produced by rapid pyrolysis at 1123 K

0.0157-0.017 0.25-1.5mm, 1123 K, 2.5-21%O2/N2, single or a few

Klein and Rotzoll (1999)

9

particles13 FB

(TGA)Bituminous coals Produced in a quartz reactor up

to 1123 K25 mg, 20%O2/Ar, 773-1473

K.Arenillas et al.

(1999)14 FB Blair Athol and

bituminous coalsProduced in fluidized bed at

1273 K70-150 , 50-250 mg, 0.55-

2.0 %O2/He, up to 1273 KAihara et al.

(2000)15 FB

(TGA)Bituminous coal Produced in entrained flow

reactor45-75 , 60 mg, 2 % O2/He,

873 KAshman et al.

(2000)

16 FB bituminous and anthracite coals

Produced in fixed bed at 1123 – 1423 K

10-20 , 0.1-20 mg, 10 % O2/N2, 1123, 1323, 1423 K, experiments were performed

at particle level

Jensen et al. (2000)

17 FB bituminous and lignite coals

Produced in fluidized bed at 1273 K

0.25-0.5 mm, 25 mg, 5 %O2/N2

Lin et al. (2002)

18 PC 4 coals Produced in an entrained flow reactor

0,017-0.021 45-75 ,CO2/H2O/O2, 1473 K

Nelson et al. (2002)

19 PC 5 coals of various rank – bituminous,

subbituminous, and lignite coals

Produced in the pc combustor at temperatures of 1700-1820 K

~200 , 64%Ar/15%CO2/21%O2,

1800-2020 K

Spinti and Pershing (2003)

20 FB (TGA)

9 coals Produced in drop-tube reactors at 1273 and 1623 K

0.0174-0.0206 Ar/20%O2, 750-1250 K

Jones et al. (2004)

21 FB Australian bituminous coal

Produced in an entrained flow reactor

0.021 , 0.1 ~ 1.0MPa, ,, 873 K, 2% O2

Park et al. (2005)

when loading is close to

zero

0.1 MPa, 1-120 mg

10

2. 2 Effect of operation conditions on the formation of NO during char combustion

Table 2 provides an overview This section summarizes experimental investigation of the

effect of the type of combustion system, the properties of char, the operation conditions such as

reactor temperature, stoichiometry, particle size, and the amount of char used on the NO

formation during char combustion.

2.2.1 Fuel-N conversion to NO at a single particle condition

(a) – Experimental findings under fixed bed conditions

The fuel-N conversion to NO at single particle condition provides important information

on the mechanisms of NO formation and reduction during char combustion. However, only a few

researchers have paid attention to this point mainly due to the measurement difficulty. Figure 1

plots the fraction of the conversion from fuel-N to NO against char loading. It is observed that

more fuel-N is converted to NO with the decreasing of the char loading. In the case of the single

particle condition, i.e. the char loading is close to zero, most of fuel-N is converted to NO.

General agreement reached by different researchers is that the fraction of fuel-N to NO is

independent of O2 concentration (Jensen et al. 2000, Park et al. 2005) at single particle

conditions, and the smaller particle size used, the more fuel-N is converted to NO. The effects of

parent coal rank and the temperature on the fuel-N to NO reported by different researchers are

inconsistent. Jensen et al. (2000) found that the fraction of fuel-N to NO depends on the reactor

temperature. They estimated that around 65 % of nitrogen in the char evolved as NO at 850 °C

and it was almost 100 % at 1050 and 1150 °C under single particle condition. More fuel-N is

converted to NO at the higher reactor temperature. However, as shown in Figure 2, experiments

by Ninomiya et al. (1989) indicated that the effect of temperature on the conversion of fuel-N to

11

NO is small. The different findings by Jensen et al. (2000) and by Ninomiya et al. (1989) may be

explained as following: the char-O2 reaction becomes diffusion limited and the surface reaction

dominates the char-O2 reaction at the high reactor temperature. The formed NO at the char

surface may escape the char surface rapidly and the contribution of the secondly reaction of NO

reduction over char surface is negligible. Almost all fuel-N is therefore converted to the NO.

While O2 may diffuse to the internal side of the char particle, part of the formed NO inside the

particle may be reduced by the char when the NO diffuses from inside to the out side at the low

temperature. Less fuel-N is therefore converted to NO. In Jensen’s experiments, the applied

temperatures are range from 1123 to 1423 K, the control reaction may change from kinetic to

diffusion, while in Ninomiya’s experiments, the reaction is only in either kinetic control or

diffusion control in the temperature range of 1073 to 1173 K..

Jensen et al. (2000) found that the fuel-N to NO is independent of the parent coal for

making coal char. Very similar tendencies of the fuel-N to NO as functions of char loading were

obtained when bituminous coal char or anthracite char was applied. While experiments by

Ninomiya et al. (1989) showed that the lower rank the parent coal was, the higher conversion of

fuel-N to NO was obtained as shown in Figure 1. In their experiments, the applied Taisei coal

has much higher volatiles than that of the Taiheyo. It should be noted that the loading shown in

Figure 1 by Ninomiya et al. (1989) is estimated on the particle size and the projected area

provided by Ninomiya et al. (1989).

12

Figure 1: The conversion of char-N to NO as function of the char loading in the fixed bed reactor

Figure 2: The conversion of char-N to NO as function of the particle size under single particle

condition at two different reactor temperatures (Ninomiya et al. 1989)

13

(b) – Experimental findings under fluidized bed conditions

Fuel-N conversion to NO under fluidized bed conditions has been found to be less than

60 % at single particle conditions (Johnsson et al. 1994). The particle size used in these

experiments was always at mm level and the reactor temperature was at around 1123 K, which

means the char-O2 reaction is always under both kinetic and diffusion controls. The internal

secondary reaction of NO+C therefore may be the main reason of the low fuel-N conversion to

NO. Moreover, due to the measurement accuracy, generally a few numbers of char were used in

the experiments, the well mixed char particle in the fluidized bed reactor may also lead to the

external secondary reaction.

Experiments indicated that the fuel-N conversion to NO decreases monotonically with

increasing the particle size (Tullin et al. 1993, a). The larger the particle size, the more formed

NO during C(N)–O2 reaction inside the particle may be reduced by C-NO reaction when the NO

diffuses from the inside of the particle to the surrounding gas. The fuel-N conversion to NO

increases with the increasing of the bed temperature as shown in Figure 3. With the increase of

the bed temperature, the C-O2 reaction transfers from the kinetic control to the diffusion control.

Under diffusion control, the surface reaction may dominate the C-O2 reaction. That is, most of

the NO formed by the fuel-N-O2 reaction may released directly to the surrounding gas which

leading to the higher fuel-N to NO.

The effects of O2 concentration on the fuel-N to NO reported by different authors are

inconsistent as shown in Figure 4. Klein and Rotzoll (1999) reported that the fuel-N to NO

decreases with the increasing the inlet oxygen concentration which may be attributed to the high

char particle temperature leading to the fast NO-C reaction. However, Winter et al. (1996) found

that the conversion decreases initially when the inlet O2 concentration increases from 5 to 10 %,

14

and then the fuel-N conversion increases again when the applied O2 concentration is 21 %.

Experiments by Hayhurst and Lawrence (1996) indicated that less NOx and N2O are released

during burning of char when the concentration of O2 in fluidizing gas was reduced. Moreover,

Hayhurst and Lawrence (1996) also reported that coals of higher rank yiel more N2O and more

NOx during char combustion. Therefore, more experiments should be conducted to determine the

effect of inlet O2 concentration on the fuel-N conversion in the fluidized bed combustion of char.

500 600 700 800 900 10000,0

0,2

0,4

0,6

0,8

1,0 Winter et al. (1996) Hayhurst and Lawrence (1996) Klein and Rotzoll (1999)

Fuel

-N to

NO

/ %

Bed temperature / oC

Figure 3. The effect of bed temperature on the conversion of char-N to NO

15

Figure 4. The effect of oxygen concentration on the conversion of char-N to NO

(c) – Experiments findings under pulverized conditions

In the case of pulverized coal combustion, Haussmann and Kruger (1990) are the sole

authors who reported that for Rosebud coal (Proximate analysis: volatile matter: 42.4 %, 10.1 %

ash; Ultimate analysis: 67.2 % C, 4.4 % H, 0.78% N, 0.52 % S, and 17.0 % O2 ) under single

particle conditions at oxygen concentrations as low as 0.5 %, nearly 100 % of the released fuel

nitrogen is converted to NO during combustion. They inferred that under such conditions,

individual particles remain surrounded by an oxygen environment, with nitrogen reduction

chemistry confined to the volatile cloud surrounding individual coal particles. This suggests that

for the Rosebud coal, fuel nitrogen conversion to molecular nitrogen requires conditions under

which released volatile matter is able to form aggregate clouds, in regions devoid of oxygen. In

16

contrast, their experiments indicated that only 55 % evolved fuel-N is converted to N2 during

combustion of Pittsburg # 8 coal under similar conditions. The low fuel-N conversion to NO may

be attributed to the secondary heterogeneous of NO on tar/soot due to the high yield of tar and

soot during the Pittsburgh # 8 coal combustion. Experiments by Haussmann and Kruger (1990)

also confirm 100 % fuel-N may be converted to NO under the condition of without any

secondary reactions.

(d) - Implications of the experimental results

Experiments indicate that the NO/N2 selectivity strongly depends on the extent of the

secondary reaction of NO with char, in other word, all fuel-N may be converted to NO and then

the formed NO reacts with its surrounding species or solid by homogeneous and heterogeneous

reactions to form N-containing species such as HCN, HNCO, and NH3 etc. If the conclusion can

be further verified experimentally under fine and single particle combustion conditions, then the

existing mechanisms of the formation of N-containing species during char combustion should be

reconsidered.

2.2.2 Fuel-N conversion to NO at batch particle condition

Table 3 lists the effect of the operation conditions such as the inlet oxygen concentration,

the particle size, the coal rank, the reactor temperature, and the reactor pressure on the fuel-N

conversion to the NO under batch coal particle conditions. Obviously, in most cases the changes

of the fuel-N conversion to NO reported by different authors are consistent.

17

Table 3. The effect of operation conditions on fuel-N to NO

OxygenConcentration

Particle Size

Coal rank

Rector temperature Reactor pressure

Fixed bed Consistent Inconsistent Consistent consistent

↔ ↔ ↑ ↑ ↑ ↓ ↑ ↓ ↑ ↓De soete (1990)Ashman et al.

(1999)

De soete (1990)

De soete (1990)

Harding et al. (1996) Harding et al. (1996) Ashman et al. (1998)Orikasa and Tomita

(2003)Jones et al. (2004)

Croiset et al. (1995) Lin et al.

(2002)Park et al. (2005)

FluidizedBed

Consistent Consistent Consistent Consistent Consistent↑ ↓ ↑ ↓ ↑ ↑ ↑ ↑ ↑ ↓

Tullin et al. (1993)Gao (2003)

Hayhurst and Lawrence, 1996,

Yue et al. 1992

Hayhurst and Lawrence (1996)

Shimizu et al. (1992

Hayhurst and Lawrence, 1996, Yue et al. 1992,

Amand and Leckner, 1991

Laughlin et at. (1994)

Shen and Yoshizo (2001)

Pulverized combustor

Consistent Consistent Consistent↑ ↓ ↑ ↓ ↑ ↑ ↑ ↓

Pershing and Wendt (1976)

Song et al. (1982)Nelson et al.

(2002)

Haussmann et al. (1990) Chen et al. (1982) Song et al. (1982)Spinti and Pershing

(2003)

Note 1. ↑ ↑ represents the conversion of fuel-N NO increase with increasing of the parameter;

2. ↑ ↓ represents the conversion of fuel-N NO increase with decreasing of the parameter;

3. ↔ no noticeable tendency

Many experiments have been performed as shown in Table 2 in the cases of batch coal

particles were used in the fixed bed reactor. Both de Soete (1990) and Ashman et al. (1999)

found that changes of the inlet oxygen concentration have little effect of the fuel-N conversion to

NO. Contradictory trends were reported by de Soete (1990) of the effect of particle size on the

fuel-N conversion to NO. He found that the fuel-N conversion may either increase or decrease

with the increase of particle size depending on different char used. 20 coals, covering a wide

18

range of rank from anthracite to high volatile bituminous coal, were used by Harding et al.

(1996). A lower conversion of char-N to NO was observed for coal chars produced from high

ranks coals. They attributed to the finding were resulted from the low surface area and low

reactivity of char produced from low rank coals. Consensus results have been obtained on the

effect of reactor temperature and the bed pressure. Less fuel-N is converted to NO with the

decreasing of the reactor temperature and increasing the reactor pressure. Furthermore, Jones et

al. (2004) found that the conversion of fuel-N to NO decreases to an approximate constant value

around 20 % at high bed temperature when diffusion becomes important. They further concluded

that the quite constant conversion of fuel-N to NO may be applicable to a wide range of

combustion systems.

Quite consensus results have been obtained under the fluidized bed combustion

conditions as illustrated in Table 3. The fraction of char-N released as NO decreased when the

particle size, the O2 concentration, and the reactor pressure were increased. It was found that the

increase of operation pressure remarkably decreased the NO emission. The higher the rank of the

coal and the reactor temperature, the greater is the fraction of char-N that was released as NO.

However, some experiments of Hayhurst and Lawrence (1996) showed the formation of NO is

not sensitive to the temperature and so it appears that no clear trend can be observed.

Under pulverized coal combustion conditions, the char-N to NO conversion decreases

with increasing the fuel equivalence ratio as shown in Figure 5. The fractional conversion of

fuel-N to NO are in the ranges of 10-70 % under fuel-lean conditions, and it is 5-10 % under

fuel-rich conditions. It seems that the effect of increasing O2 on fuel-N to NO in the fuel lean

zone is stronger than that in the fuel rich area, for example, about 68 % fuel-N is converted to

NO at Φ=0,3, it is 0.39 at Φ=1, and 0.32 at Φ=3.6, where Φ is the equivalence ratio.

19

Figure 5. Effect of equivalence ratio on the conversion of char-N to NOx

In actual coal flame, more or less NO exists in the gas phase before the onset of char

oxidation mainly formed from volatile-N. A few experiments have been performed to study the

effect of the initial NO concentration in the gas phase on the char-N conversion during char

oxidation. Experiments by Spinti and Pershing (2003) indicated that the conversion of char-N to

NO depends strongly on the initial NO concentration regardless of char rank and nitrogen

content. The apparent conversion (defined as the ratio of formed NO to char-N plus N in added

NO) decreases dramatically as the initial NO rises. Different from the observations of Spinti and

Pershing, Nelson et al. (2002) found that the conversions are quite similar in the cases of with or

without doping NO in the gas phase.

20

Experimental findings of the effect of fuel-N content on the conversion are different.

Pershing’s data (1990) exhibits a slight decrease in fuel-N conversion to NO with increasing

fuel-N while Spinti and Pershing’s findings (2003) show opposite tendency.

The effects of individual operation conditions on the fuel-N conversion to NO are not

always uniform for different reactors as shown in Table 3. Under fluidized bed combustion

conditions, more fuel-N was found to be converted to NO when the bed temperature was

increased, while contrary tendencies were found under fixed bed and pulverized combustion

conditions. The effect of oxygen concentration on fuel-N conversion is insignificant in the fixed

bed combustion, while less fuel-N is converted to NO in the cases of fluidized bed and

pulverized combustion conditions, and so on.

2. 3 The formation of other N-containing gas species during char oxidation

Char-N is released predominately as N2 and NO with small amounts of N2O, HCN, NH3,

and HNCO. The formed minor species as N2O may significantly impact the environment. The

experimental determination of the formation these species are summarized as following:

2.3.1 Formation of N2O

N2O is regarded as one of greenhouse gases, which plays an important role in destruction

of the ozone layer and gives rise to the greenhouse effect. The fuel-N conversion to N2O is

important only when the temperature is low in the ranges of 1123-1273 K, for example when the

fluidized bed combustor is used. The N2O emission may reach as high as 20 – 250 ppm. The fuel

fed into the fluidized bed combustor undergoes drying, devolatilization, and char combustion.

The species evolved during these processes react in a complex homogeneous and heterogeneous

reaction system. Moreover, the different solids in the bed material act as effective catalysts. Thus,

21

it is difficult to explain the whole mechanisms of formation and reduction of N2O. Detail reviews

of the mechanism of formation and destruction of the N2O was given by Johnsson (1994).

2.3.2 Formation of HCN

In literature, strong evidence exists that when char-nitrogen is oxidized the main products

are not only NO, N2, and N2O but significant amounts of HCN is also formed ranging typically

from 1 to 10% of char-nitrogen. This HCN can be further oxidized to nitrogen oxides and N2.

Kramlich et al. (1989) proposed that the devolatilization or gasification of char nitrogen

followed by gas-phase reaction could lead to N2O formation. They suggested that HCN might be

the species that once released from the char is oxidized in the homogeneous phase to produce

N2O. A simple HCN release model was proposed (Eqs.1 to 2)– but the mechanism is unknown.

(1)

(2)

where .

A similar hypothesis was proposed by Amand and Leckner (1993) when explaining why

the conversion of char nitrogen to N2O increased during CH3CN addition to a fluidized bed

reactor. De Soete (1992) reported the formation of HCN during the reaction of coal char with

NO, but there were no further discussions about its formation mechanism. It is not clear whether

nitrogen in HCN was from inherent nitrogen in char matrix or from gaseous NO. Jones et al.

(1995) reported HCN formation during coal char gasification in the presence of 20 % O2 at

above 500 oC. Winter et al. (1996) conducted the char combustion at 750 oC. They added iodine

to the reactant gas stream to suppress the radical formation and found the increment of HCN

22

formation. They assumed the release of HCN is proportional to the carbon conversion, it was

arbitrarily taken as 4 % w-%. Ashman et al. (1998, 2000) reported HCN formation during the

oxidation of coal char at 600 oC. They suggested that a portion of HCN may be formed through

the secondary reaction of NO, which were derived from the oxidation.

Orikasa et al. (2002, 2003) found that the formation rate of HCN was greatly affected by

the available hydrogen. They thus proposed the following mechanism of HCN formation:

(3)

(4)

They further postulated that direct formation of HCN from C(N) might also be possible,

and that the exact nature and structure of the C(H,N) species remains unclear. In this scheme,

C(N) is formed by reaction of NO with the char surface .

The fate of the depends on the availability of hydrogen. Hydrogen may be supplied by gas

phase H2, a gas phase H atom, and/or a H atom on the char surface. In this scheme, N2 formation

becomes predominant when there is insufficient hydrogen available, and results from the

reaction of C(N) with NO: . This mechanism is in

agreement with the results of isotopic labeling experiments by Chambrion et al., (1997, a and b,

1998). Their experiments indicated that the mixed isotopic composition of N2, HCN, and HNCO

were formed when 15NO is added to the gas phase. Experiments by Park et al. (2005) showed that

the presence of H2O in the reactant gas also gave rise to HCN. Based on the observation, they

proposed that the source of H in Eqs. (3) and (4) may also be provided by steam.

2.3.3 Formation of HNCO

23

FTIR measurements by Nicholls and Nelson (2000) indicated the formation of detectable

concentration of HNCO in a laboratory scale quartz fixed bed reactor at temperatures of 600 and

900 oC. Ashman et al. (2000) identified that 12 % fule-N may be converted to HNCO at low

temperature of 873 K in 2 % O2. They proposed that the formation of HNCO may have similar

process as that of the HCN.

2.3.4 Formation of NH3

Park et al. (2005) found that NH3 was produced in the presence of H2O. The formation of

NH3 increases by increasing of the pressure and the concentration of H2O in the feed gas. They

found the major nitrogen-containing products are NH3, HCN, and N2. The conversions of fuel-N

to NH3, HCN, and N2 change slightly varying with H2O content. The fractional conversion of

char-N to NH3, HCN and N2 are around 50, 25, and 20 %. No NO and HNCO was detected in the

presence of H2O alone. The results suggested that NH3 may be a secondary product of the char-

H2O reaction, and probably arises from interactions of the initial product of HCN which reacts

with the char surface which is modified by the uptake of water in some form. The mechanism of

the formation NH3 is unknown.

3. Mechanistic studies of char-N conversion

The production of NOx from coal char combustion is a balance between the oxidation of

char-N and its subsequent reduction at the char surface and its surrounding environments.

Different mechanisms have been proposed although there is no evidence to prove that a given

mechanism is valid each or is useful in the development of rate equation.

24

3.1 The mechanisms of the conversion of NO during char combustion

The most comprehensive mechanism of NO and N2O from coal was proposed by De

soete (1990). A steady-state technique was used to determine the global rates of char oxidation,

NO and N2O formation during char oxidation and heterogeneous NO and N2O reduction on the

char surface. A transient-state technique, created by sudden suppression of O2 in the reactant

gases, was employed to determine the rate of different adsorption and desorption reactions.

Consequently, the flowing mechanisms were proposed:

(a) Char oxidation

(5)

(6)

(7)

(b) Formation of NO

(8)

(9)

(c) Formation of N2O

(10)

(d) Destruction of NO and N2O

(11)

(12)

Reaction 6 is the mass balance of the following two reactions, from which 9 is rate

controlling:

25

(13)

(14)

where atoms or atom groups placed between brackets are solid bound atoms.

The De soete approach (de soete 1990) is quite useful in that it also incorporates the

formation of N2O at the char surface and thus it can be extended to the conditions of N2O is

significant.

Table 4 lists the different mechanisms proposed by researchers except by de soete (1990).

Chan et al. (1983) proposed the possible mechanisms of the enhancement of NO reduction over

char in the presence of CO. They thought that 2 reasons may be attributed to: (1) NO-CO

reaction may be catalyzed by the carbon surface; and (2) CO may attack the chemisorbed

oxygen, C(O), the subsequent release of CO2 from the C surface provides active carbon atom for

further reaction with NO. Suzuki et al. (1994) tried to explain the role of O2 in the C-NO reaction

since their experiments indicated that the presence of a certain amount of O2 may significantly

increases the rate of NO-C reaction. With the presence of O2, a number of oxygen-containing

complexes, C(O), may be produced. The decomposition of COx may provide active site for NO-

C reaction. Additionally, the formed nitrogen-containing compound, C(N), on the carbon surface

may be attacked by either O2 or NO to form N2. However, two important factors, i.e. the rise of

char particle temperature and the produced CO due to the presence of O2, on the enhancement of

C-NO reaction were not accounted for by Suzuki et al. (1994). Besides the mechanisms of NO

and N2O formation suggested by de Soete (1990), Krammer and Sarofim (1994) proposed a

possible pathway of NO and N2O. They believed that the bounded C-N must first be broken due

to the attack of O2 to form intermediate active bound nitrogen, -N, and the –N may further reacts

26

with O2 or NO to form NO or N2O. They also proposed the intermediate produced by C-N-O2

reaction may be CNO, and the CNO may be attacked by surrounding NO to form N2O. Their

proposed mechanisms seem to be in agreement with experiments of Tullin et al. (1993, a).

Harding et al. (1994) proposed that the mechanisms for NO formation and reduction on the char

surface are similar to those of carbon oxidation. The main point in their mechanisms is that the

surface reaction by adjacent complex such as C(N) and C(NO) plays important role in the NO

and N2O formation and reduction. They believe that the mechanisms are not reasonable for the

following reasons: let do a simple estimation, a char has ultimate analysis of the fixed carbon

79.5 and N 1.25 % respectively. The numbers of molecular C is 50 times higher than that of N. If

we assume that elements C, H and N distributed uniformly in the solid, then we may say the

elements H and N should be separated by C and the opportunity of N-N in neighborhood is rare.

Chambrion et al. (1998) and Aihara et al. (2000) proposed that the NO may react with complex

C(N) to form N2O. More recently, Park et al. (2005) suggested that the formation of HCN is due

to the attacks of the surface C(N) by the presence of the surrounding H2 or by the adjacent C(H)

and C(N). The author also believes the possibility of C(N) and C(H) is very low due to the low

very low H content in chars.

27

Table 4. Different mechanisms of char oxidation, and NO and N2O formation and destruction from char-N, and the values of reaction rate

Char oxidation NO formation N2O formation HCN formation and destruction

NO and N2O destruction

Chan et al. (1983)

Suzuki et al. (1994)

Krammer and Sarofim (1994)

or

Harding et al. (1996)

(◊)

Chambrion et al. (1998)Aihara et al. (2000)Park et al. (2005)

28

3.2 NO reduction over char with oxygen and effect of other reacting gases

The chemistry of NO reduction by carbon or in the presence of CO is excellently

reviewed by Thomas (1997) and Aarna and Suuberg (1997). It is generally recognized that the

reaction of pure nitric oxide with carbons is first order with respects to NO and the NO-carbon

reaction was enhanced significantly with the presence of surrounding CO. Therefore, instead of

repeating these aspects, we hereafter summarize how O2, CO2, H2, and H2O present in practical

combustors will affect the NO-char reaction.

3.2.1 NO-O2-C system

In the char combustion process, NO gas produced during combustion will further react

with char. Generally there is some oxygen in these practical processes. There is consensus that

the presence of oxygen alone promotes NO reduction during NO/char reactions.

In a pioneering work of Smith and co-workers (1959), six reaction (15-20) steps as

following were proposed to explain experimental observations that O2-preteatment increased the

NO-carbon reaction rate whereas H2 treatment played the opposite role. They postulated that the

first step was the formation of C-O surface complexes, and these complexes were very important

intermediates in the reactions. Furthermore, they assumed that C-O complexes formed by O2

were greater in quantity and somewhat different in nature from those produced by NO. When the

carbon was treated with O2 prior to the addition of NO, the surface has more the usual

concentration of and , and thus the NO reduction rate is greater.

(15)

(16)

(17)

29

(18)

(19)

(20)

(In the figure : α represents the concentration ratio of oxygen to NO)

Figure 6. Temperature dependence of reaction rate by Kunii et al. (1980)

The addition of small amounts of oxygen increased the rate of NO destruction over char

as shown in Figure 6. Kunii et al. (1980) found that the rate of NO reduction over char was

significantly enhanced by the presence of O2 in a range of lower temperatures (< 1023 K) while

30

the rate was not significantly reduced at 1073 K. At the low temperature, the desorption of C-O

surface complexes is regarded as a controlling reaction of the overall rate, and at the high

temperature, the reaction with the coadsorbed O atoms may keep the carbon surface continuously

rich in C-O complexes thus maintaining the constant reaction rate. The mechanism was further

developed by Yamashita et al. (1993). They postulated that the C-O complexes consists of

reactive C(O) and stable C-O, and both of them are increased by the presence of O2 caused by

the reaction: . The desorption of oxygen from C(O) produces

highly reactive Cf sites which is easily attacked by NO.

Chan et al. (1983) and Rodriguez-Mirasol et al. (1994) ascribed the enhancement of the

NO reduction over char to the action of CO formed by C-O2 reaction. The rate of the O2-char

reaction is approximately two orders of magnitude greater than that of the NO-char reaction. For

this reason, when NO and O2 are passed over carbon under conditions of no diffusional

resistance, either the O2 will be completely consumed early in the reactor before any appreciable

NO reduction is achieved, or a negligible fraction of NO reduction will take place with an

appreciable fraction of the O2 escaping unreacted. In this case, O2 may both accelerate and inhibit

the NO-CO reaction. Under diffusional control conditions, the O2 is consumed close to the

particle surface, excepting for the smaller particles of for low-temperature operation. Most of the

NO reduction will occur therefore in the porous of the solid where negligible amount of O2 are

present. The effect of O2 will then be to produce CO which will codiffuse with NO and

accelerate the rate of NO reduction. The effect of O2 is to enhance NO reduction by generating

CO to inhibit the reaction. De soete (1992) found that the effect of O2 in NO-C-CO2 system

depends on CO2/CO ratio. The oxygen inhibits NO reduction when the CO2/CO ratio in the

31

oxidation products is greater than one, and it promotes NO reduction when the ratio is smaller

than one. Chen and Tang (2001) reported that NO-C reactivity decreases about 2 – 4 orders to

magnitudes in the natural logarithmic scale in typical reburning environments (CO2 and O2 in

feed are 16.8 and 1.95 % respectively).

Suzuki et al. (1994) proposed that not only the formation of C-O complex, but also the

removal of nitrogen complex in the presence of O2 as: ,

.

Another plausible role of O2 in NO-carbon reaction is the formation of NO2 as

intermediate. Ahmed et al. (1993) found that NO2 was more readily reduced to N2 than was NO

whatever in the presence or absence of O2. They proposed more NO could be oxidized to NO2

by increasing the gas-phase O2 concentration as: , and . The

mechanism was confirmed by Kong and Cha (1999) that NO2 is more readily chemisorbed than

that of NO on the carbon surface. However, experiments of Suzuki et al. (1994) indicated that

the major product of C-NO2 reaction is NO not N2 in the absence of O2.

Contrarily, O2 inhibits N2O reduction with carbon or carbon catalysts (Noda et al. 1999;

Zhu et al. 2000). The opposite effect of O2 on NO-C and N2O-C systems can be attributed to the

different manners of N2O and NO adsorption on the carbon surface (Zhu et al. 2001).

Theoretical calculation indicated that in the case of excess O2, NO is more likely to chemisorb in

the form of NO2 and NO chemisorption is more thermodynamically favorable than O2

chemisorption. N2 can form through –C-ONO-C or –C(N) complexes (the former one is the main

pathway). Oxygen remains on the carbon edge site during the formation of N2 since it is not

necessary to weaken or break the adjacent C-C bounds. New active sites are quickly produced

32

through the formation of epoxy oxygen and consequently the NO-C reaction is greatly improved.

By contrast, it was found that N2O chemisorption is much less thermally stable either on the

consecutive edge sites or edge sites isolated by semiquinone oxygen. Although the presence of

excess O2 produces a large amount of new active sites, the produced active sites are occupied by

O2 again. Therefore, the N2O-C reaction rate is depressed by the presence of O2.

3.2.2 NO-CO2-C system

CO2 is assumed essentially inert in NO-C reaction system. However, Suuberg and Aarna

(1998) reported that the NO reduction over graphite is strongly inhibited by the addition of CO2,

and the reaction rate in the NO/CO2 graphite system is, on average, lower by a factor of 1.7. In

the NO/CO2-wyodak coal char system, they reported that the NO reduction over the char is not

affected by the presence of CO2 at supplying CO2 partial pressure below 2 kPa, while the

inhibiting effect of CO2 is obvious when the CO2 partial pressure is around 101 kPa. They thus

concluded that the extent of inhibition of the NO-C reaction by CO2 depends on the natural of

carbon itself, as well as on CO2 partial pressure.

3.2.3 NO-H2O-C system

There has been no detailed study of the NO-C reaction in the presence of water vapor.

Levy et al. (1981) reported that the NO-C reactivity is reduced by water vapor at high

temperature above 1000 oC.

3.2.4 NO-H2-C system

The effect of the presence of H2 on NO-C reactivity is inconsistent. Smith et al. (1959)

found that the NO-carbon reaction rate was reduced by the pretreatment of H2. They found that

N2 was formed instead of and (see Eq. 15) in which less C-O complexes appeared in

the char surface and thus leads to the lower NO-C reaction rate. Furusawa et al. (1982) and

33

Johnsson (1994) indicated that, in the presence of H2, char catalyzes the NO reduction, and the

consumption of carbon in the char is nearly negligible.

In coal combustion system, NO-C reactivity is more or less affected by the presence of

O2, CO, CO2, H2, and H2O. The reached consensuses results are: the NO-C reactivity is

significantly enhanced in the presence of CO, and the reactivity may be increased with the O 2

addition. The mechanisms of enhancement of NO-carbon reaction in the presence of O2 have not

been investigated systematically yet as described above, and no global kinetic rate regarding the

influence of O2 on NO-carbon reaction has been developed. The effect of the presence of H2O,

H2 and SO2 on NO-C reactivity has not yet been well elucidated theoretically and experimentally.

4. Model studies of nitric oxides formation from char combustion

4.1 Single particle model

Following the pioneering work of Wendt and Schulze (1976), Sarofim and coworkers

(Goel et al., 1994; Goel, Zhang et al., 1996; Krammer and Sarofim, 1994; Tullin, Goel et al.,

1993; Tullin, Sarofim et al., 1993; Tullin et al., 1995), and Visona and Stanmore (1996) used

exclusively heterogeneous processes to explain and model the main experimental findings on the

conversion of char-N to nitrogen oxides.

The formation of NO during a char particle includes the release of char-N inside the

particle and further reaction at the boundary layer of the particle. The general transport equation

for description the process in symmetric, spherical coordinate (r terms only) is used via:

(21)

34

where is the density, t is the time, is the process variable (energy, gas species concentration,

……), r is the radial position, v is the velocity, is the transport coefficient, S is the source

term. The first term on the right side represents the rate of change of per unit volume, the

second term is the convection flux of , the first term on the right side stands for the diffusion

flux of , and the last term is rate of generation of . The can have a positive or negative

value.

To model the formation of nitric oxides, simplified mechanisms of the production of

major species of CO and CO2, and the formation and destruction of minor species of HCN, NO

and N2O have been proposed by different authors as listed in Table 5. CO and CO2 are two major

products during char combustion. General, it is assumed that CO is initially formed by surface

reactions and then is further oxidized to CO2 either in the pores of the particle or in the gas phase.

Modeling of char combustion was reviewed (Smith, 1982, Smoot and Smith, 1985) and

modeling of CO oxidation around the particle (Goel et al. 1996, Mitchell et al. 1990). Modeling

of char combustion, CO oxidation, the formation and reduction of NO and other N-containing

species both internal and external of a char particle is complicated due to the high temperatures

involved, the complex coupling between the partial differential equations governing mass and

energy conservation, and stiff gas phase chemistry as indicated in Eq. (21).

35

Table 5. Mechanisms used for modeling the release of NO from a single char particle modelChar oxidation NO formation N2O formation HCN formation and

destructionNO and N2O destruction

Wendt and Schulze (1976)

NI NI

Hahn et al. (1983)

NI NI

Arai et al. (1986)

NI NI

Shimizu et al. (1992) where

NI NI

Tullin et al. (1993)

NI

Goel et al (1993)

NI

Visona and Stanmore (1996)

NI

Sarofim et al. (1999)

NI NI

* NI – not included

36

In general, the following approaches have been used to simply the description of NO

release during a single char particle combustion: (1) NO formation: NO production is

proportional to the nitrogen/carbon atomic ratio of char and the char combustion rate (Wendt and

Schulze, 1976, Hahn et al. 1983, Shimizu et al. 1992, Tullin et al. 1993, Goel et al. 1994,

Sarofim et al. 1999). Arai et al. (1986) assumed that char-N is converted competitively to NO

and NHi at the surface of the particles during char combustion. Tullin et al. (1993) assumed that

fraction of the formed NO may be further converted to N2O; (2) NO reduction: Different

approaches have been applied to describe the NO reduction of the formed NO inside the particle,

on the particle surface, and with surrounding species. Wendt and Schulze (1976) believed that

the NO disappearance is controlled by a reverse Zeldovich reaction, Hahn et al. (1983) regarded

that the NO reduction over char occurs both on the char surface and inside the char particle and

the effect of CO on NO reduction is neglected, while the presence of CO on NO reduction of the

internal surface of char is included by Shimizu et al. (1992); (3) The NO formation and

destruction inside and outside of the char particle was described separately (Wendt and Schulze,

1976, Visona and Stanmore, 1996) or only the reactions inside the particle was considered (Goel

et al. 1994); (4) Mass transfer from the particle surface to the external gas phase is driven by

molecular diffusion and convection; and (5) The pore size and surface area are constants (Wendt

and Schulze, 1976, Arai et al. 1986, Tullin et al. 1993, Goel et al. 1994, Sarofim et al. 1999) or

change during char combustion (Hahn et al., 1983; Shimizu et al. 1992).

The importance of physical properties of char on the selectivity of fuel-N toward NO

depends on combustion conditions (Wendt and Schulze, 1976). Pore size is important at low

temperature fluidized bed combustion, while the particle size significantly affects and the

temperature has very weak impact on the NO formation. Hahn and Shadman (1983) concentrated

37

their work on investigating the effect of the changes of porosity and surface area on the rate of

NO formation during char combustion. They found that better predictions as compared with

experimental data were given with the char structure effects were included, and the model

provided possibility of explaining why fuel-N is only partially converted to NO even when there

is excess O2 in the bulk gas. Twenty-five heterogeneous and homogeneous reaction steps have

been adopted to model the overall formation of char-NO during the combustion of a single

particle of coal char by Arai et al. (1986). Two distinct features can be identified in the model:

(1) char-N is converted to both NO and NHi at the particle surface during the single particle char

combustion; and (2) homogeneous reactions after different species released from char particle

were described using detailed chemical kinetic reactions (18 steps). Furthermore, the model was

validated by experimental data of the transient formation of char-NO and for the overall fraction

conversion of char-N into char-NO for three kinds of chars. Shimizu et al. (1992) used a very

simple model to explain the observations of their experiments. In their model, NO reduction on

the internal surface of a single char particle was included. In Visona and Stanmore’s (1996)

model, the intrinsic char combustion developed by Smith (1976) was employed and the NO or

HCN may be released directly from char-N. They suggested that there are two possible

mechanisms for the conversion of char-N to NO at high temperature: (1) char-N may first go

completely to NO, and subsequently be reduced on the char surface. The conclusion is in

agreement with the latter experimentally findings by Jensen et al. (2000); and (2) The char-N

may go completely to HCN, and is then be oxidized and reduced by secondary reactions with

surrounding atmosphere. The most detailed model regarding the mechanisms of the formation

and destruction of NO at a single particle level has been developed by Sarofim and his

colleagues (Tullin et al. 1993, Goel et al. 1993, Sarofim et al. 1999). Based on Goel’s model, a

38

simple energy balance model was added and a modified partial differential equations solution

method was used by Kilpinen et al. (2002) to investigate the nitrogen chemistry in fluidized bed

char combustion conditions.

The Goel model (1993), includes 15 reaction steps which are grouped into 4 main parts,

i.e. (1) the reaction of O2 with the carbon to form CO and CO2; (2) the formation of NO from

char bound nitrogen; (3) destruction of NO on char surface and subsequent formation of N2O;

and (4) destruction of N2O on the char surface. The model is based on char-C oxidation and char-

N oxidation and reduction as illustrated in Figure 7.

Figure 7. Reaction network proposed by Goel et al. (1993)

The Goel’s model single particle model solves for the diffusive transport and the kinetics

of the reactants and products in the shrinking char sphere. The main assumptions are:

1. The char particle is spherically symmetric,

2. The system is at pseudo steady state,

3. Ash falls from the particle surface during the oxidation,

4. The diameter of the particle decreases continuously while its density remains constant.

The governing mass conservation equations of the single particle oxidation are:

C

Char

N

CO, CO2

O2

NO

N2O

O2

N2

Char/CO

39

(22)

The boundary conditions are

(23)

(24)

where is the effective diffusion coefficient (m2/s). and are the concentrations of

species i in the bulk gas and at the particle surface, respectively, kg is the external mass transfer

coefficient (m/s), and r is the radial position inside the particle (m). The term Ri represents the

rates of the chemical reactions occurring inside the char particle: char-C oxidation and char-N

oxidation and reduction.

Figure 8. Comparison of the experimental and model predicted fuel-N to NO and N2O (Goel et

al. 1994)

40

Figure 8 is an example of the comparison of the experimental and model predicted of

fuel-N to NO and N2O. The model prediction is in good agreement with the measurement which

suggests that the kinetic model is able to explain the experimental observations and that it can be

used as a predictive tool for other experimental observations.

One of the main limitations of Goel’s model is that the energy due to the C-O2 reaction

was not taken into account. Kilpinen et al. (2002) extended the Goel’s model with the energy

balance and tested different input parameters such as kinetic rate of C-O2 and C-NO. Simulation

by Konttinen et al. (2002) shows that the temperature of char particles reaches around 24 K

higher than its surrounding gas temperature while the temperature profile inside the particle is

quite uniform except near the particle surface when the applied O2 is 8 % (vol), char particle size

is 4 mm, and the surrounding gas temperature is 1123 oC. The NO-char reaction was found to be

around 25 % higher as the energy balance was accounted when the C-NO reaction kinetic given

by Goel was used, which indicated that the temperature change due to the C-O2 reaction may

greatly affect the formation and destruction of NO. The prediction obtained by the improved

model of Kilpinen et al. (2002) matches better with experimental data as compared with Goel’s

model.

The mentioned model for describing NOx formation and destruction during single particle

char combustion can be ascertained as stationary model since the effect of fluid dynamics on the

combustion process is not accounted for. With the development of the computer capacity, a new

approach to track the motions of individual particles including chemical reactions in fluidized

bed combustion of char has been employed by Rong et al. (2000) and Zhou et al. (2004). They

found that the evolution of the release of fuel-N during the char combustion is not only affected

by the physical properties of char but also the local fluid hydrodynamics of gas and particle

41

particularly their temperatures. The changes of particle temperature greatly affect the NOx

emission. Unfortunately, the global kinetic rate instead of very detailed mechanism model were

applied by Rong et al. (2000) and Zhou et al. (2004) to describe the char combustion and the

formation and destruction of NO and N2O. More detailed mechanisms for describing the

formation and destruction of the conversion of fuel-N to NO together with Computational Fluid

Dynamic (CFD) approach describing the gas particle flow may be applied in the future.

4.2 The SKIPPY (Surface Kinetics in Porous Particles) approach

The SKIPPY approach, developed by Haynes at The University of Sydney (Ashman et al.

1999), may be applied to study fuel-N conversion during coal combustion. The SKIPPY may

calculate species and temperature profiles for the reaction of a porous solid in a reacting gaseous

environment at pseudo-steady state, it accounts for detailed finite-rate elementary chemical

kinetics and molecular transport. The relationship between the various components of the

program is depicted in Figure 9. The SKIPPY code employs data and subroutines from the

CHEMKIN package (Kee et al., 1989), the SURFACE CHEMKIN package (Coltrin et al., 1990)

and the Sandia transport packages (Kee et al., 1986) to handle the chemical kinetics,

thermodynamics, and transport properties calculations.

Ashman et al. (1998) reported their modeling work of fuel-N conversion during char

combustion in a TGA system using the SKIPPY model. The network of the reaction of char

oxidation and fuel-N conversion chemistry are listed in 25 to 30:

2Csolid + O2 2CO (25)

2Nsolid + O2 2NO (26)

Ysolid + O2 HCN + 2CO (27)

42

In accordance with accepted models for the reactions of NO with chars, secondary

heterogeneous reactions of this species were modeled as

Nsolid + NO N2O (28)

Nsolid + NO + CO N2 + CO2 (29)

Finally, the well known reduction of N2O by carbon is represented as

Csolid + N2O N2 + CO (30)

In Reaction 27 , Y represents the HCN precursor “Y” .

ChemkinInterpreter

Gas PhaseReactions

ThermodynamicData Base

ChemkinLink File

Surface DataFile

TransportData Base

TransportFitting Code

SurfaceInterpreter

TransportLink File

Surface LinkFile

TransportLibrary

SurfaceLibrary

ChemkinLibrary

SKIPPYKeywordInput File

RestartFile

PrintedOutput File

BinarySave File

Figure 9. Schematic diagram showing the various components of the SKIPPY program and the

relationships between each of them.

Figure 10 compares the experimental and predicted distribution of the char nitrogen into

gas-phase products. The predictions are in fairly good agreement with the experimental data

43

although the poor state of knowledge of the fundamental kinetics of the heterogeneous reactions

of carbon, no attempt has been made to optimize the fit of the predictions to the experimental

points.

The SKIPPY approach was adopted by Molina et al. (2002) to investigate char-N

conversion to NO, N2O, and HCN at the boundary layer during chat combustion. They found that

the predictions are in agreement with most of the qualitative experimental trends. The SKIPPY

approach is a new method which can be used to predict fuel-N conversion during char

combustion even in the case of the mechanisms of the formation and destruction of nitric oxides

during the char combustion are unclear.

0.00E+00

1.00E-01

2.00E-01

3.00E-01

4.00E-01

5.00E-01

6.00E-01

7.00E-01

8.00E-01

9.00E-01

1.00E+00

800 900 1000 1100 1200 1300 1400

Temperature (K)

Frac

tion

of C

har N NO

N2O

HCN

N2

Figure 10. Comparison of experimental and predicted fate of char N in TGA experiments

44

5. Concluding remarks

To develop more effective NOx control technology, a large amount of experiments of NO

formation and destruction during char combustion at different operation conditions have been

performed and different mechanisms to describe the relevant reactions have been proposed.

Consensus observations as well as inconsistent results were reported by different research

groups. The following conclusions can be reached after a review of these studies:

(1) NO is a primary nitrogen-containing product in the char-nitrogen combustion. In the

case of without secondary reaction (small particles under single conditions), approximately 100

% of fuel-N is converted to NO. Globally, less fuel-N is converted to NO as the char particle

loading, the particle size, or the bed height increase which can be attributed to the occurrence of

secondary reaction on the char surface. The observations implies that all fuel-N may be first

converted to NO in the presence of O2, and then the formed NO may further react with the char

itself or surrounding particles or gas to form N-containing species such as N2O, HCN and NH3.

The mechanisms of the fuel-N conversion during char combustion should be re-accounted if all

fuel-N is converted to NO applicable for all chars without secondary reactions. More

experiments should be conducted to confirm the experimental observations.

(2) In the practical combustion system, gas species in the reacting system are complicated

which may contain major species of CO, CO2, H2O, CO2 and O2, and minor species NO, N2O,

HCN, and HNCO. The presence of the major species might significantly affect the kinetics of the

reactions of C with N-containing species. The lack of the systematical studies of such effects

may inhibit the understanding of the mechanisms and modeling the formation and destruction of

NO during char combustion.

45

(3) The effects of operation conditions on the changes of NO formation in some cases

depend on the reactor systems. For example, the conversion of fuel-N to NO increases with

increasing of the fluidized bed temperature, while contrary tendencies were observed for the

fixed bed reactor and pulverized combustor.

(4) The presence of H2 and H2O seems important to give rise to the formation of N-H

containing species such as NH3, HCN and HNCO. The mechanisms of the formation of such

kind of minor N-containing species are unclear.

(5) Models for describing the fuel-N conversion at single particle level have been

developed based on different assumptions. The Goel’s model seems the best model to describe

the fuel-N conversion although there are some limitations for instance the change of particle

temperature during the char combustion is neglected;

(6) The SKIPPY is a new approach to elucidate the fuel-N conversion in the case of the

lack of the fundamental kinetics of the heterogeneous reactions of carbon and detailed

mechanisms of the formation and destruction of NO.

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