**4.1 Homogeneous mechanism**

Historically, the first homogeneous mechanism of NO formation is a thermal mechanism. The thermal mechanism of nitrogen oxide formation from molecular nitrogen is a sequence of chemical reactions that occur independently of the combustion process.

#### **Zeldowicz mechanism**

44 Fossil Fuel and the Environment

From the conducted analysis, it may be concluded that a significant part of experimentally confirmed differences in kinetic constants of char-nitrogen conversion into nitrogen oxides result from omission of individual characteristics of investigated char porous structures that

In the process of coal combustion, nitrogen oxides are produced during homo- and heterogeneous reactions of the air, volatile matter and char. In combustion gases, nitrogen oxides occur as NO, N2O and NO2 species. In Figure 3, a simplified scheme of the mechanism of their formation is presented. The basic component of nitrogen oxide emissions in high-temperature processes is NO. Complex mechanisms of nitrogen oxide formation during coal combustion have been classified using the source of nitrogen and divided into two basic groups (Zeldowicz, 1946; Fenimore & Jones, 1957; Fenimore, 1971; Johnsson et al., 1992; Bowman, 1973; Sarofim & Pohl, 1973; de Soete, 1975; Malte & Pratt, 1977; Levy et al., 1978; Fenimore & Fraenkel, 1981; Heyd, 1982; Miller et al., 1984; Glarborg et al., 1986; Fong & Peters, 1986; Niksa, 1988; Cheng et al., 1989; Miller & Bowman, 1989; Muzio et al., 1990; Glarborg et al., 1992; Williams et al., 1992; Tomeczek & Gradoń, 1997;

fuel processes: oxidation of nitrogen compounds that are chemically bound with the

thermal processes: reactions of atmospheric nitrogen with atomic oxygen that is

Fig. 3. A simplified scheme of nitrogen oxide formation during coal combustion

**4. Mechanisms of nitrogen oxide formation during coal combustion** 

strongly affect the mechanism of nitrogen conversion.

Williams et al., 1997):

fuel organic matter

produced at high temperatures.

During his studies on gaseous flames at high temperatures, Zeldowicz (1946) observed that formation of thermal NOs cannot result from direct collisions of N2 and O2 molecules according to a global reaction:

$$\text{N}\_2 + \text{O}\_2 \rightarrow \text{NO} + \text{NO} \tag{1}$$

He proposed a double-reaction mechanism:

$$\text{N}\_2 + \text{O} \rightarrow \text{NO} + \text{N} \tag{2}$$

$$\text{NO} + \text{O}\_2 \rightarrow \text{NO} + \text{O} \tag{3}$$

which is triggered by a reaction of molecular oxygen dissociation:

$$\rm O\_2 + M \rightarrow O + O + M \tag{4}$$

As the mechanism led to lower calculated results than the experimental ones, it was supplemented by a reaction (Fenimore & Jones, 1957; Fenimore, 1971):

$$\text{N} + \text{OH} \rightarrow \text{NO} + \text{H} \tag{5}$$

The rate of NO formation resulting from the Zeldowicz mechanism (1946) mainly depends on the reaction kinetics (5) as well as concentrations of oxygen atoms and nitrogen molecules in the particle neighbourhood. Kinetic constants of the above reactions are presented in scientific papers (Arai et al., 1986; Miller & Bowman, 1989). A detailed review of the kinetic constants of thermal NOs formation are presented in the Ph.D. dissertation by Gradoń (2003), while the issues of the thermal mechanism are included in the papers by, among others: Fenimore, 1971; Johnsson et al., 1992; Bowman, 1973; Sarofim & Pohl, 1973; de Soete, 1975; Malte & Pratt, 1977; Fenimore & Fraenkel, 1981; Glarborg et al., 1986; Miller & Bowman, 1989; Muzio et al., 1990; Glarborg et al., 1992; Williams et al., 1992; Tomeczek & Gradoń, 1997.

#### **Extended thermal mechanism**

After a series of experiments conducted in a flow reactor at 1653 K to 1798 K, Tomeczek and Gradoń (2003) proposed a concept of extended thermal mechanism. Measured NO concentrations were far higher than those calculated according to the Zeldowicz mechanism (1946). The authors proposed the extended thermal mechanism based on five reactions: two reactions [(2) and (3)] were adopted from the Zeldowicz mechanism (1946)

Fuel-N Conversion to NO, N2O and N2 During Coal Combustion 47

As during coal combustion a majority of nitrogen oxides is formed from nitrogen chemically bound with the fuel, heterogeneous reactions of nitrogen release and its conversion to

Nitrogen that is chemically bound with the fuel releases from coal during devolatilisation or during char combustion. In case of devolatilisation, nitrogen mainly moves to the gaseous phase as hydrogen cyanide HCN. Due to rapid oxidation, the compound is primarily transformed into NCO and NHi radicals. Miller et al. (1984) presented a cycle of fuel-

In general, mechanisms of fuel-nitrogen oxide formation are assumed to poorly depend on temperature, contrary to thermal NO formation (Miller & Bowman, 1989). Fuel-NOx plays a key role during coal combustion within the temperature range of 1500–2000 K (Rybak, 1996). The volatile matter is a complex mixture of combustible and non-combustible gases such as: carbon monoxide and dioxide, water vapour, saturated and unsaturated hydrocarbons, sulphur compounds, carbon black and nitrogen compounds (N2, HCN, NH3). The volatile matter composition changes with temperature and devolatilisation duration. Once fuelnitrogen is released as HCN from coal, the compound is rapidly transformed into amino groups NHi (i = 0, 1, 2, 3) which subsequently form NO or N2 through a series of intermediate reactions (Rybak, 1996). Miller et al. (1984) concluded that intermediate chemical reactions which controlled the oxidation rate of HCN were reactions with atomic oxygen. This mechanism is also important when there is oxidant deficiency during combustion. According to many authors (Lavoie et al., 1970; Fraihaut et al., 1982; Miller et al., 1984; Miller & Bowman, 1989), among the mentioned reactions, the most important ones

 **NO N2**

HCN + O NCO + H (13)

NCO + H NH + CO (14)

NCO + H2 NH2 + CO (15)

HCN + OH HNCO + H (16)

HNCO + H NH2 + CO (17)

HCN + OH CN + H2O (18)

CN + OH NH + CO (19)

nitrogen transformations into NO or N2 in the following simplified scheme:

seem to be homogeneous reactions of NCO and NHi formation:

The amino groups NHi (NO precursors) result also from the following reactions:

**Fuel nitrogen N HCN NCO NHi N**

**4.2 Heterogeneous mechanism** 

oxides in the flame are of the greatest importance.

**Mechanism of fuel-nitrogen oxide formation** 

and three reactions [(6), (7) and (8)] were adopted from the N2O mechanism developed by Malte and Pratt (1977):

$$N\_2 + O \to NO + N\tag{2}$$

$$\text{N} + \text{O}\_2 \rightarrow \text{NO} + \text{O} \tag{3}$$

$$\rm N\_2 + O + M \rightarrow N\_2O + M \tag{6}$$

$$\text{NiO} + \text{O} \rightarrow \text{NO} + \text{NO} \tag{7}$$

$$\text{NiO} \uparrow \text{O} \to \text{N}\_2 \downarrow \text{O}\_2 \tag{8}$$

The mechanism is also triggered by dissociation of an oxygen molecule (Reaction 4). Altering the rates of the (7) and (2) reactions, Tomeczek and Gradoń demonstrated that at temperatures below 1770 K, NO is primarily formed through the N2O mechanism (Reactions 6 and 7) where (7) is a key reaction. For temperatures above 1770 K, NO is mainly formed through the Zeldowicz mechanism (Reactions 2 and 3) which is controlled by the (2). The kinetic constants of the extended thermal mechanism are included in the papers by Tomeczek & Gradoń, 1997 and Gradoń, 2003.

#### **Mechanism of NO2 formation**

During combustion, nitrogen dioxide is formed through conversion of primarily generated NO.

Convection of oxygen atoms and NO molecules from the high-temperature flame zone to the cooler post-flame zone results in NO2 formation. The mechanism is based on the following chemical reactions (Cernansky & Sawyer, 1975; Miller & Bowman, 1989):

$$\text{CO} + \text{NO} + \text{M} \rightarrow \text{NO}\_2 + \text{M} \tag{9}$$

$$\rm{NO} + \rm{HO}\_{\rm{B}} \rightarrow \rm{NO}\_{2} + \rm{OH} \tag{10}$$

The presence of HOB in these regions may result from diffusion of this compound from high- to low-temperature regions. At the atmospheric pressure, the reaction (9) becomes significant only below 500 K (Heyd, 1982; Fong et al., 1986; Niksa, 1988). In hightemperature regions, NO2 undergoes rapid re-reduction to NO as a result of a reaction with O and H radicals. At temperatures above 900 K and high concentrations of O and H radicals, NO2 reduction reactions are dominant:

$$\text{NO}\_2 + \text{O} \rightarrow \text{NO} + \text{O}\_2 \tag{11}$$

$$\text{N} \dagger \text{OH} \rightarrow \text{NO} \dagger \text{H} \tag{12}$$

In case of solid fuel combustion, the presence of NO2 in combustion gases that leave a reactor depends on the presence of low-temperature regions in the post-flame zone. The NO2 fraction of the total NOx emission in high-temperature reactors is usually small and falls below a few per cent (Cernansky & Sawyer, 1975). The observed higher NO2 concentrations seem to result from reactions in the measuring probe where rapid cooling of combustion gases during sampling in the flame zone occurs (Miller & Bowman, 1989).

#### **4.2 Heterogeneous mechanism**

46 Fossil Fuel and the Environment

and three reactions [(6), (7) and (8)] were adopted from the N2O mechanism developed by

N2 + O NO + N (2)

N2 + O + M N2O + M (6)

N2O + O NO + NO (7)

 N2O + O N2 + O2 (8) The mechanism is also triggered by dissociation of an oxygen molecule (Reaction 4). Altering the rates of the (7) and (2) reactions, Tomeczek and Gradoń demonstrated that at temperatures below 1770 K, NO is primarily formed through the N2O mechanism (Reactions 6 and 7) where (7) is a key reaction. For temperatures above 1770 K, NO is mainly formed through the Zeldowicz mechanism (Reactions 2 and 3) which is controlled by the (2). The kinetic constants of the extended thermal mechanism are included in the papers by

During combustion, nitrogen dioxide is formed through conversion of primarily

Convection of oxygen atoms and NO molecules from the high-temperature flame zone to the cooler post-flame zone results in NO2 formation. The mechanism is based on the

The presence of HOB in these regions may result from diffusion of this compound from high- to low-temperature regions. At the atmospheric pressure, the reaction (9) becomes significant only below 500 K (Heyd, 1982; Fong et al., 1986; Niksa, 1988). In hightemperature regions, NO2 undergoes rapid re-reduction to NO as a result of a reaction with O and H radicals. At temperatures above 900 K and high concentrations of O and H radicals,

NO2 + O NO + O2 (11)

In case of solid fuel combustion, the presence of NO2 in combustion gases that leave a reactor depends on the presence of low-temperature regions in the post-flame zone. The NO2 fraction of the total NOx emission in high-temperature reactors is usually small and falls below a few per cent (Cernansky & Sawyer, 1975). The observed higher NO2 concentrations seem to result from reactions in the measuring probe where rapid cooling of combustion gases during sampling in the flame zone occurs (Miller & Bowman, 1989).

following chemical reactions (Cernansky & Sawyer, 1975; Miller & Bowman, 1989):

N + O2 NO + O (3)

O + NO + M NO2 + M (9)

NO + HOB NO2 + OH (10)

N + OH NO + H (12)

Malte and Pratt (1977):

Tomeczek & Gradoń, 1997 and Gradoń, 2003.

**Mechanism of NO2 formation** 

NO2 reduction reactions are dominant:

generated NO.

As during coal combustion a majority of nitrogen oxides is formed from nitrogen chemically bound with the fuel, heterogeneous reactions of nitrogen release and its conversion to oxides in the flame are of the greatest importance.

#### **Mechanism of fuel-nitrogen oxide formation**

Nitrogen that is chemically bound with the fuel releases from coal during devolatilisation or during char combustion. In case of devolatilisation, nitrogen mainly moves to the gaseous phase as hydrogen cyanide HCN. Due to rapid oxidation, the compound is primarily transformed into NCO and NHi radicals. Miller et al. (1984) presented a cycle of fuelnitrogen transformations into NO or N2 in the following simplified scheme:

$$\begin{aligned} \text{Fuel nitrogen} \quad \text{N} & \Rightarrow \text{HCN} \Rightarrow \text{NCO} \Rightarrow \text{NH}\_{\text{i}} \Rightarrow \text{N} \overset{\Rightarrow}{\Rightarrow} \text{N}\_{2} \end{aligned}$$

In general, mechanisms of fuel-nitrogen oxide formation are assumed to poorly depend on temperature, contrary to thermal NO formation (Miller & Bowman, 1989). Fuel-NOx plays a key role during coal combustion within the temperature range of 1500–2000 K (Rybak, 1996).

The volatile matter is a complex mixture of combustible and non-combustible gases such as: carbon monoxide and dioxide, water vapour, saturated and unsaturated hydrocarbons, sulphur compounds, carbon black and nitrogen compounds (N2, HCN, NH3). The volatile matter composition changes with temperature and devolatilisation duration. Once fuelnitrogen is released as HCN from coal, the compound is rapidly transformed into amino groups NHi (i = 0, 1, 2, 3) which subsequently form NO or N2 through a series of intermediate reactions (Rybak, 1996). Miller et al. (1984) concluded that intermediate chemical reactions which controlled the oxidation rate of HCN were reactions with atomic oxygen. This mechanism is also important when there is oxidant deficiency during combustion. According to many authors (Lavoie et al., 1970; Fraihaut et al., 1982; Miller et al., 1984; Miller & Bowman, 1989), among the mentioned reactions, the most important ones seem to be homogeneous reactions of NCO and NHi formation:

$$\text{HCN} + \text{O} \rightarrow \text{NCO} + \text{H} \tag{13}$$

$$\text{NCO} + \text{H} \rightarrow \text{NH} + \text{CO} \tag{14}$$

$$\text{NCO} + \text{H}\_2 \rightarrow \text{NH}\_2 + \text{CO} \tag{15}$$

The amino groups NHi (NO precursors) result also from the following reactions:

$$\text{HCN} + \text{OH} \rightarrow \text{HNO} + \text{H} \tag{16}$$

$$\text{HNCCO} + \text{H} \rightarrow \text{NH}\_2 + \text{CO} \tag{17}$$

$$\text{HCN} + \text{OH} \rightarrow \text{CN} + \text{H}\_2\text{O} \tag{18}$$

$$\text{CN} + \text{OH} \rightarrow \text{NH} + \text{CO} \tag{19}$$

Fuel-N Conversion to NO, N2O and N2 During Coal Combustion 49

O2 + 2[C] 2[CO] (21)

[CO] CO + Cf (22)

2[CO] CO2 + [C] + Cf (23)

O2 + [C] + [CN] [CO] + [CNO] (24)

[CNO] NO + [C] (25)

N2O + [C] N2 + [CO] (28)

A disadvantage of this model is omission of diffusion in pores; however, the author noted that above 800 K, NO and N2O formation is controlled by oxygen diffusion in char pores. In their model, Goel et al. (1994) subsequently extended the stage of NO and N2O formation and destruction, adding more reactions as well as introducing "intermediates" and the active centre [NCO]. In their work, they assumed a constant effective diffusion coefficient or they determined it in the procedure of matching the model to the experiments. It means that there is little probability of precise results of modelling of nitrogen oxide emissions during

The authors (Goel et al., 1994) described their model with fifteen reactions and divided it

[CO] CO + Cf (22)

[CO2] CO2 + Cf (33)

[NCO] CO + [N] (35)

[CNO] NO + [C] (25)

coal combustion without previous experiments.

into stages "a" to "f". The "a" stage is CO and CO2 formation:

**A model developed by Goel et al.** 

The "b" stage is NO formation:

NO + [C] 0.5N2 + [CO] (26)

[CN] + [CNO] N2O + 2[C] (27)

NO + 2[C] [CO] + [C … N] (29)

2[C ... N] N2 + 2[C] (30)

0.5 O2 + [C] [CO] (31)

0.5 O2 + [CO] [CO2] (32)

0.5 O2 + [CN] [CNO] or [NCO] (34)

0.5 O2 + [N] [NO] (36)

In this mechanism, nitrogen oxide is produced in a reaction of NH4 and NCO amino groups with O and OH radicals.

Summing up, NO concentration in combustion gases depends on the combustion process organisation as the temperature and oxygen concentration in the combustion zone determine the transition pathways for the reactions of NHi amino groups with O, H and OH.

After volatilisation, a considerable amount of fuel-nitrogen remains in char. The amounts of nitrogen compounds remaining in the char depend on the devolatilisation temperature and level as well as, indirectly, on the ratio of oxygen excess. A part of the total amount of coal nitrogen remaining in the char increases with decreased combustion temperature and decreased ratio of the oxygen excess. Post-devolatilisation nitrogen species that are bound in the char are probably five-membered pyrrole groups which, due to temperature, are transformed into more stable heterocyclic five-membered structures (Rybak, 1996). At present, there is little knowledge of conversions of nitrogen compounds that remain in char. The fraction of nitrogen oxides resulting from char-nitrogen compounds may constitute about 20– 30% of the total amount of generated NOs (Pershing & Wendt, 1979). Time necessary for oxidation of char-nitrogen compounds during char burning is far longer than the combustion duration of the volatile matter. Study results show that during char combustion, char-nitrogen compounds may undergo significant devolatilisation in a parallel manner to their oxidation. Calculated kinetic constants of the above reactions are presented in the papers by Miller et al., 1984; Arai et al., 1986; Glarborg et al., 1986; Thorne et al., 1986; Miller&Bowman, 1989.

There are also clues that a significant part of NOs that are formed during char combustion undergoes reduction to N2 due to a contact with the char surface. If CO is absent, reduction of NO that is formed from the char may be controlled by NO chemisorption on the char surface according to the following reaction (Chan et al., 1983):

$$\text{NO} + \text{C}\_{\text{I}} \rightarrow \text{C(O)} + \text{N}\_{2} \tag{20}$$

The effects of the above heterogeneous reaction become less significant when temperature rises and H2O and O2 levels increase. A part of nitrogen oxide may also undergo reduction as a result of a heterogeneous reaction with surfaces of carbon black particles (Pohl & Sarofim, 1977; Cheng et al., 1989; Kordylewski et al., 1996). The kinetic constant of NO reduction to N2 in the presence of CO is presented in the papers by Arai et al., 1986; Williams et al., 1997.

Mathematical modelling of kinetics of char-nitrogen conversion to NO and N2O species during coal combustion was performed by many researchers (Wendt & Schulce, 1976; Arai et al., 1986; de Soete, 1990, Tullin et al., 1993; Goel et al., 1994; Williams et al., 1997). They developed their models using a few to more than ten chemical reactions and determined kinetic constants of NO and N2O formation. The NO formation kinetics in a heterogeneous reaction during char combustion is extremely complex so the mathematical models significantly simplified the issue (Wendt & Schulce, 1976; Phol & Sarofim, 1977; Arai et al., 1986). De Soete (1990) was the first researcher to develop a heterogeneous model of NO and N2O formation and decomposition on the basis of surface reactions with active centres [CN] and [CNO].

#### **De Soete's model**

De Soete's (1990) model is based on ten reactions of NO and N2O formation and reduction:

In this mechanism, nitrogen oxide is produced in a reaction of NH4 and NCO amino groups

Summing up, NO concentration in combustion gases depends on the combustion process organisation as the temperature and oxygen concentration in the combustion zone determine

After volatilisation, a considerable amount of fuel-nitrogen remains in char. The amounts of nitrogen compounds remaining in the char depend on the devolatilisation temperature and level as well as, indirectly, on the ratio of oxygen excess. A part of the total amount of coal nitrogen remaining in the char increases with decreased combustion temperature and decreased ratio of the oxygen excess. Post-devolatilisation nitrogen species that are bound in the char are probably five-membered pyrrole groups which, due to temperature, are transformed into more stable heterocyclic five-membered structures (Rybak, 1996). At present, there is little knowledge of conversions of nitrogen compounds that remain in char. The fraction of nitrogen oxides resulting from char-nitrogen compounds may constitute about 20– 30% of the total amount of generated NOs (Pershing & Wendt, 1979). Time necessary for oxidation of char-nitrogen compounds during char burning is far longer than the combustion duration of the volatile matter. Study results show that during char combustion, char-nitrogen compounds may undergo significant devolatilisation in a parallel manner to their oxidation. Calculated kinetic constants of the above reactions are presented in the papers by Miller et al.,

the transition pathways for the reactions of NHi amino groups with O, H and OH.

1984; Arai et al., 1986; Glarborg et al., 1986; Thorne et al., 1986; Miller&Bowman, 1989.

surface according to the following reaction (Chan et al., 1983):

There are also clues that a significant part of NOs that are formed during char combustion undergoes reduction to N2 due to a contact with the char surface. If CO is absent, reduction of NO that is formed from the char may be controlled by NO chemisorption on the char

The effects of the above heterogeneous reaction become less significant when temperature rises and H2O and O2 levels increase. A part of nitrogen oxide may also undergo reduction as a result of a heterogeneous reaction with surfaces of carbon black particles (Pohl & Sarofim, 1977; Cheng et al., 1989; Kordylewski et al., 1996). The kinetic constant of NO reduction to N2 in the presence of CO is presented in the papers by Arai et al., 1986; Williams et al., 1997.

Mathematical modelling of kinetics of char-nitrogen conversion to NO and N2O species during coal combustion was performed by many researchers (Wendt & Schulce, 1976; Arai et al., 1986; de Soete, 1990, Tullin et al., 1993; Goel et al., 1994; Williams et al., 1997). They developed their models using a few to more than ten chemical reactions and determined kinetic constants of NO and N2O formation. The NO formation kinetics in a heterogeneous reaction during char combustion is extremely complex so the mathematical models significantly simplified the issue (Wendt & Schulce, 1976; Phol & Sarofim, 1977; Arai et al., 1986). De Soete (1990) was the first researcher to develop a heterogeneous model of NO and N2O formation and

decomposition on the basis of surface reactions with active centres [CN] and [CNO].

De Soete's (1990) model is based on ten reactions of NO and N2O formation and reduction:

NO + Cf C(O) + N2 (20)

with O and OH radicals.

**De Soete's model** 

$$\text{O}\_2 + 2\text{[C]} \rightarrow 2\text{[CO]}\tag{21}$$

$$\text{[CO]} \rightarrow \text{CO} + \text{C} \tag{2}$$

$$\text{2[CO]} \rightarrow \text{CO}\_2 + [\text{C}] + \text{C}\_{\text{A}} \tag{23}$$

$$\text{O}\_2 + [\text{C}] + [\text{CN}] \rightarrow [\text{CO}] + [\text{CNO}] \tag{24}$$

$$\text{[CNO]} \rightarrow \text{NO} + \text{[C]}\tag{25}$$

$$\text{[NO} + \text{[C]} \rightarrow 0.5\text{N}\_2 + \text{[CO]}\tag{26}$$

$$\text{[CN]} + \text{[CNO]} \rightarrow \text{N}\_2\text{O} + 2\text{[C]}\tag{27}$$

$$\text{NiO} + [\text{C}] \rightarrow \text{N}\_2 + [\text{CO}] \tag{28}$$

$$\text{[NO} + 2\text{[C]} \rightarrow \text{[CO]} + \text{[C} \dots \text{N]} \tag{29}$$

$$2\left[\mathbf{C}\dots\mathbf{N}\right]\to\mathbf{N}\_2+2\left[\mathbf{C}\right]\tag{30}$$

A disadvantage of this model is omission of diffusion in pores; however, the author noted that above 800 K, NO and N2O formation is controlled by oxygen diffusion in char pores. In their model, Goel et al. (1994) subsequently extended the stage of NO and N2O formation and destruction, adding more reactions as well as introducing "intermediates" and the active centre [NCO]. In their work, they assumed a constant effective diffusion coefficient or they determined it in the procedure of matching the model to the experiments. It means that there is little probability of precise results of modelling of nitrogen oxide emissions during coal combustion without previous experiments.

#### **A model developed by Goel et al.**

The authors (Goel et al., 1994) described their model with fifteen reactions and divided it into stages "a" to "f". The "a" stage is CO and CO2 formation:

$$\text{0.5 O}\_2 + \text{[C]} \rightarrow \text{[CO]}\tag{31}$$

$$\text{[CO]} \rightarrow \text{CO} + \text{C} \tag{2}$$

$$\text{0.5 O}\_2 + \text{[CO]} \rightarrow \text{[CO}\_2\text{]}\tag{32}$$

$$\text{[CO}\_2\text{]} \rightarrow \text{CO}\_2 + \text{C}\_4\tag{33}$$

The "b" stage is NO formation:

$$\text{[}\text{0.5 O}\_2 + \text{[CN]} \rightarrow \text{[CNO]} \text{ or [NO]}\tag{34}$$

$$\text{[NCO]} \rightarrow \text{CO} + \text{[N]}\tag{35}$$

$$\text{[CNO]} \rightarrow \text{NO} + \text{[C]}\tag{25}$$

$$\text{[}0.5\,\text{O}\_2 + \text{[N]} \to \text{[NO]}\tag{36}$$

Fuel-N Conversion to NO, N2O and N2 During Coal Combustion 51

In the mechanism, concentrations of free Cf and occupied active centres are considered among: [C], [CN], [CO] and [CNO]. Initially, it is assumed that char contains free active centres, Cf*,* and active centres occupied by carbon atoms and by carbon-nitrogen bonds [C] and [CN], respectively, concentrated on the surface. The organically bound nitrogen can react heterogeneously either to produce NO, N2O or N2.. During the presented research, the mechanism proposed by Croiset et al. (1998) was chosen for testing mainly because the rate

1 k

4 k

5 k

6 k

7 k

If the fractions of the occupied active centres are defined as (Croiset et al. 1998 ):

The total concentration of the occupied active centres is equal to

8 k

then we can express the rate of gaseous species formed during the combustion by means of

2 k

3 k

N 2[C] 2[CO] <sup>2</sup> (21)

<sup>f</sup> [CO] [CO] C (22)

2 f 2[CO] CO [ ] C *C* (23)

[CNO] NO [C] (25)

NO [CNO] N O [CO] <sup>2</sup> (44)

NO [C] 0.5N [CO] 2 (26)

N O [C] N [CO] 2 2 (28)

**C** = [C] / S (46)

**CO** = [CO] / S (47)

**CN** = [CN] / S (48)

**CNO** = [CNO] / S (49)

S = [C] + [CO] + [CN] + [CNO] (45)

O [C] [CN] [CO] [CNO] <sup>2</sup> (24)

**5. Kinetics of heterogeneous nitrogen oxide formation during pressurised** 

constants were evaluated by Croiset et al. from enhanced pressure experiments:

**char combustion** 

equations:

$$\text{[NO]} \rightarrow \text{NO} + \text{C}\_{\text{f}} \tag{37}$$

The "c" and "d" stages are related to NO reduction:

$$\text{[NO} + \text{[C]} \rightarrow 0.5\text{ N}\_2 + \text{[CO]}\tag{26}$$

$$\text{NO} + \text{C}\_{\text{f}} \rightarrow \text{[NO]} \tag{38}$$

$$\text{C}\text{O} + \text{C}\_{\text{i}} \rightarrow \text{[CO]}\tag{39}$$

$$\text{[NO]} + \text{[CO]} \rightarrow 0.5\text{ N}\_2 + \text{CO}\_2 + 2\text{ C}\_6\tag{40}$$

The "e" stage is N2O formation:

$$\text{[NO} + \text{[N]} \rightarrow \text{[N\_2O]}\tag{41}$$

$$\text{[NO} + \text{[NCO]} \rightarrow \text{[N}\_2\text{O]} + \text{[CO]} \tag{42}$$

$$\text{[N}\_2\text{O]} \rightarrow \text{N}\_2\text{O} + \text{C}\_\text{l} \tag{43}$$

The "f" stage is N2O reduction:

$$\text{N}\_2\text{O} + [\text{C}] \rightarrow \text{N}\_2 + [\text{CO}] \tag{28}$$

In the model developed by de Soete et al., the combustion mechanism was based on three reaction: two of them were related to CO generating and the other one referred to CO2 formation. Goel et al. (1994) described CO and CO2 formation with four reactions: the first two were related to CO formation and the next two referred to CO2 formation. De Soete developed the mechanism of NO formation with the use of two reactions of CNO formation and decomposition. In the study by Goel et al., NO release was described by five reactions where nitrogen oxide was formed either from CNO or through the active centre [NO]. De Soete described NO reduction be means of one reaction of NO with an active carbon centre [C], while Goel et al. described it with the use of four reactions where the first one was the same as de Soete's and the next three led to formation of N2, CO2 and a free active site. In de Soete's model, N2O generating was based on one reaction of the active centre [CNO] destruction, while Goel et al. described the process by means of three reactions with NO transition to the active centre [N2O] and, subsequently, to a gaseous species N2O. De Soete and Goel et al. described N2O reduction in the same way using one reaction of N2O with the active carbon centre [C]. In de Soete's model (1990), NO and N2O formation is controlled by oxygen adsorption: at temperatures below 800 K, the adsorption is in the kinetic region, while at higher temperatures, it is controlled by oxygen diffusion in pores. The author determined kinetic constants for two chars within 800–1300 K which, in case of oxygen adsorption over the char matrix, depend on the depth of oxygen penetration into the particle. The author did not specify a porous structure of the investigated chars. Goel et al. (1994) extended de Soete's model (1990), allowing for N2O formation in the absence of oxygen through a reaction with an "intermediate" [NCO]. N2O destruction only occurred in a heterogeneous reaction with elemental C in the char matrix. In the mechanism of NO destruction, they included both heterogeneous and homogeneous reaction of NO with CO catalysed by char. The proposed mechanism promotes N2O formation in sites with high NO concentrations. A disadvantage of this model is a stable diffusion constant which neglects variations of porous structure.

[NO] NO + Cf (37)

[N2O] N2O + Cf (43)

 N2O + [C] N2 + [CO] (28) In the model developed by de Soete et al., the combustion mechanism was based on three reaction: two of them were related to CO generating and the other one referred to CO2 formation. Goel et al. (1994) described CO and CO2 formation with four reactions: the first two were related to CO formation and the next two referred to CO2 formation. De Soete developed the mechanism of NO formation with the use of two reactions of CNO formation and decomposition. In the study by Goel et al., NO release was described by five reactions where nitrogen oxide was formed either from CNO or through the active centre [NO]. De Soete described NO reduction be means of one reaction of NO with an active carbon centre [C], while Goel et al. described it with the use of four reactions where the first one was the same as de Soete's and the next three led to formation of N2, CO2 and a free active site. In de Soete's model, N2O generating was based on one reaction of the active centre [CNO] destruction, while Goel et al. described the process by means of three reactions with NO transition to the active centre [N2O] and, subsequently, to a gaseous species N2O. De Soete and Goel et al. described N2O reduction in the same way using one reaction of N2O with the active carbon centre [C]. In de Soete's model (1990), NO and N2O formation is controlled by oxygen adsorption: at temperatures below 800 K, the adsorption is in the kinetic region, while at higher temperatures, it is controlled by oxygen diffusion in pores. The author determined kinetic constants for two chars within 800–1300 K which, in case of oxygen adsorption over the char matrix, depend on the depth of oxygen penetration into the particle. The author did not specify a porous structure of the investigated chars. Goel et al. (1994) extended de Soete's model (1990), allowing for N2O formation in the absence of oxygen through a reaction with an "intermediate" [NCO]. N2O destruction only occurred in a heterogeneous reaction with elemental C in the char matrix. In the mechanism of NO destruction, they included both heterogeneous and homogeneous reaction of NO with CO catalysed by char. The proposed mechanism promotes N2O formation in sites with high NO concentrations. A disadvantage of this

model is a stable diffusion constant which neglects variations of porous structure.

NO + [C] 0.5 N2 + [CO] (26)

[NO] + [CO] 0.5 N2 + CO2 + 2 Cf (40)

NO + Cf [NO] (38)

CO + Cf [CO] (39)

NO + [N] [N2O] (41)

NO + [NCO] [N2O] + [CO] (42)

The "c" and "d" stages are related to NO reduction:

The "e" stage is N2O formation:

The "f" stage is N2O reduction:
