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

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

$$\text{CN}\_2 + 2\text{[C]} \xrightarrow{\text{k}\_l} 2\text{[CO]} \tag{21}$$

$$\text{[CO]} \xrightarrow[\text{I}]{\text{k}\_2} \text{[CO]} + \text{C}\_\text{f} \tag{22}$$

$$2\,\text{[CO]} \xrightarrow{\text{k}\_{\text{J}}} \text{CO}\_{2} + \text{[C]} + \text{C}\_{\text{f}} \tag{23}$$

$$\text{CO}\_2 + \text{[C]} + \text{[CN]} \xrightarrow{\text{k}\_4} \text{[CO]} + \text{[CNO]}\tag{24}$$

$$\text{[CNO]} \xrightarrow{\text{k}\_{\sharp}} \text{NO} + \text{[C]} \tag{25}$$

$$\text{NO} + \text{[CNO]} \xrightarrow{\text{k}\_6} \text{N}\_2\text{O} + \text{[CO]} \tag{44}$$

$$\text{NO} + \text{[C]} \xrightarrow{\text{k}\_{\text{\textquotedblleft}}} \text{0.} \text{SN}\_2 + \text{[CO]} \tag{26}$$

$$\text{N}\_2\text{O} + \text{[C]} \xrightarrow{\text{k}\_8} \text{N}\_2 + \text{[CO]} \tag{28}$$

The total concentration of the occupied active centres is equal to

$$\text{S} = \text{[C]} + \text{[CO]} + \text{[CN]} + \text{[CNO]} \tag{45}$$

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

$$\boldsymbol{\Theta}\_{\mathbb{C}} = \left[ \mathbb{C} \right] / \mathbb{S} \tag{46}$$

$$\boldsymbol{\mathfrak{A}\_{\text{CO}}} = \left[ \text{CO} \right] / \,\mathrm{S} \tag{47}$$

$$\boldsymbol{\Theta\_{\rm CN}} = \left[ \rm CN \right] / \,\rm S \tag{48}$$

$$\mathfrak{g}\_{\text{CNO}} = \left[ \text{CNO} \right] / \text{ S} \tag{49}$$

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

Fuel-N Conversion to NO, N2O and N2 During Coal Combustion 53

evaluated using Croiset et al. (1998) data as a mean value within the experimental pressure range of 0.2–1.0 MPa. A very difficult problem of the effectiveness factor for

easy, the combustion reactions were replaced by an overall reaction O2 + [C] CO2 with the rate constants of reaction (21). This enabled calculation of the effectiveness factor from

and k = k1(1-)/MC, = p/RT, - char density, MC – carbon molar mass, p – pressure, R – gas constant, T – temperature. The effective diffusion coefficient was expressed as a function of the oxygen-helium molecular diffusion coefficient D and the Knudsen diffusion

In order to calculate the rate of CO, CO2, NO, N2O and N2 formation by means of equations (50–54), it is necessary to find the fractions of the occupied active centres [CO] and [CNO]

+ k φ - θ η η p ,

<sup>=</sup> <sup>k</sup> <sup>φ</sup> - <sup>θ</sup> 1- <sup>φ</sup> - <sup>θ</sup> <sup>p</sup> - <sup>k</sup> <sup>θ</sup> - <sup>k</sup> <sup>θ</sup> <sup>p</sup> . dt

The 90s experiments were conducted at pressures of 0.2 – 1.5 MPa in a pressurised reactor. A single layer of char particles 0.1– 0.8 g of initial diameter 0.9 mm was placed in a canthal tray and heated from the bottom and the top at a rate of 100 Ks up to the final temperature 1073–1373 K. The final heating temperature and the samples residence time at this temperature were automatically controlled. The maximum deviation from the adjusted temperature within the isothermal period was smaller than 40K. The temperature of the sample was measured by a NiCr-NiAl thermocouple and continuously recorded. The char was produced at 1373 K in atmospheric pressure helium during 20 minute devolatilisation of subbituminous "Siersza", "Janina" and "Piast" coals, whose characteristics (water content in the analytical state – Wa, volatile matter content in the water ash free state – Vwaf, ash content in the water free state – Awf, carbon content in the water ash free state – Cwaf, hydrogen content in the water ash free state – Hwaf, sulphur

4 CO CNO O 5 CNO 6 CNO NO

2

8 CO 8 1

η η p + k θ η η p + k φ - θ η η p +

<sup>=</sup> <sup>2</sup> <sup>k</sup> <sup>φ</sup> - <sup>θ</sup> <sup>p</sup> - <sup>k</sup> <sup>θ</sup> - <sup>2</sup> <sup>k</sup> <sup>θ</sup> <sup>+</sup> <sup>k</sup> <sup>φ</sup> - <sup>θ</sup> 1-<sup>φ</sup> - <sup>θ</sup> dt


CO O 2 CO 3 CO

O 6 CNO 4 1

2

2

2

NO 7 CO 7 1

N O -1

2

*1* = *4*, *6 = 4* and *7 = <sup>8</sup>* = 1.

, 1 1 η η 1 4 3[(tanh(Th)) (Th) ]/(Th) (59)

Th 0.5d kp /D , <sup>e</sup> (60)

(Laurendeau, 1979):

1 12 De K <sup>ε</sup> /(D D )<sup>τ</sup> . (61)

NO -1

(62)

(63)

4 CO CNO

*<sup>4</sup>* for the dual site reactions (21) and (24) is not

the particular reactions was simplified by the assumption:

coefficient DK through the particle pores of tortuosity

from the following kinetic equations:

CNO


2

1

d θ

CO

d θ

*1* and 

Because calculation of the values

where the Thiele module is:

the equation:

$$\mathbf{k}\_{\rm CO} = \mathbf{k}\_2 \theta\_{\rm CO} \mathbf{S}\_{\rm mC} \eta\_{\rm l},\tag{50}$$

$$
\boldsymbol{\mathfrak{K}}\_{\rm CO\_2} = \boldsymbol{\mathfrak{k}}\_3 \boldsymbol{\mathfrak{H}}\_{\rm CO}^2 \, \mathrm{S}\_{\rm H\_2} \boldsymbol{\mathfrak{q}}\_{\rm I}, \tag{51}
$$

$$\dot{\mathbf{R}}\_{\rm NO} = \mathbf{k}\_{5}\theta\_{\rm CNO}\mathbf{S}\_{\rm mC}\eta\_{4} \cdot \mathbf{k}\_{6}\theta\_{\rm CNO}\mathbf{S}\_{\rm mC}\eta\_{4}\mathbf{p}\_{\rm NO} \cdot \mathbf{k}\_{7} \{\boldsymbol{\upphi}\cdot\theta\_{\rm CO}\}\mathbf{S}\_{\rm mC}\eta\_{7}\mathbf{p}\_{\rm NO},\tag{52}$$

$$\mathbf{R}\_{\mathrm{N}\_{2}\mathrm{O}} = 2\mathbf{k}\_{6}\,\boldsymbol{\varrho}\_{\mathrm{CNO}}\,\mathrm{S}\_{\mathrm{m}\_{\mathrm{C}}}\boldsymbol{\eta}\_{4}\,\mathrm{p}\_{\mathrm{NO}}\,\mathrm{-}2\mathbf{k}\_{8}(\boldsymbol{\varphi}\cdot\boldsymbol{\varrho}\_{\mathrm{CO}})\mathrm{S}\_{\mathrm{m}\_{\mathrm{C}}}\boldsymbol{\eta}\_{8}\,\mathrm{p}\_{\mathrm{N}\_{2}\mathrm{O}},\tag{53}$$

$$\dot{\mathbf{R}}\_{\text{N}\_2} = (\boldsymbol{\upmu} \cdot \boldsymbol{\upmu}\_{\text{CO}}) \mathbf{[k}\_7 \,\eta\_7 \,\mathbf{p}\_{\text{NO}} + \mathbf{2} \,\mathbf{k}\_8 \,\eta\_8 \,\mathbf{p}\_{\text{N}\_2\text{O}}] \mathbf{S}\_{\text{m}\_\text{C}}.\tag{54}$$

For modelling, a crucial problem is the concentration of occupied active centres which cannot be eliminated from the kinetic equations. In the past, that problem was overcome by determination of products of the surface concentration of active centres and the rate constant. It may be successful when we are only interested in the char particle reaction rate. However, in order to model individual species, separation of active site concentrations from the rate constants is necessary. Considering all the difficulties, it is proposed to assume the concentration of the total active centres as a number of carbon atoms in a monolayer of active centres on the particle internal surface. For a particle of an internal surface area *A*, diameter *d* and porosity , the total concentration of the occupied active centres can be described using the equation by Gil (2002):

$$\mathbf{S} = \mathbf{K} \mathbf{A} \mathbf{d}^3 (1 \cdot \boldsymbol{\varepsilon}) ,\tag{55}$$

where K = 1.95 1028 a.c. (active centres) m-5 or K = 32.32 kmola.c. m-5.

It was assumed that the total fraction of the active centres **[C]** and **[CO]** is a dimensionless, equal value (Croiset et al., 1998):

$$
\clubsuit\_C + \clubsuit\_{CO} = \spadesuit \tag{56}
$$

The total fraction of the active centres **[CN]** and **[CNO]** is:

$$
\clubsuit\_{\text{CN}} + \clubsuit\_{\text{CNO}} = 1 \cdot \spadesuit \tag{57}
$$

where is defined as the probability that a C atom will not bind to a nitrogen atom during combustion (Croiset et al., 1998):

$$\mathbf{q} = \mathbf{1} - \frac{\mathbf{N}\_{\mathbf{K}}^{\mathbf{a}}}{\mathbf{C}\_{\mathbf{K}}^{\mathbf{a}}} \frac{\mathbf{M}\_{\mathbf{C}}}{\mathbf{M}\_{\mathbf{N}}} \tag{58}$$

In the kinetic equations of the above mechanism, there are two key parameters: rate constants ki of reactions (21–26, 28, 44) and the mode of reaction expressed by the effectiveness factor I, yielding a part of the particle volume available for the i-th reaction. The rate constants of reaction (26) were found to be independent of pressure within the range of 0.2–1.5 MPa (Tomeczek & Gil, 2001). Based on this finding, it was assumed that for all the other reactions, the rate constants were independent of pressure and they were evaluated using Croiset et al. (1998) data as a mean value within the experimental pressure range of 0.2–1.0 MPa. A very difficult problem of the effectiveness factor for the particular reactions was simplified by the assumption: *1* = *4*, *6 = 4* and *7 = <sup>8</sup>* = 1. Because calculation of the values *1* and *<sup>4</sup>* for the dual site reactions (21) and (24) is not easy, the combustion reactions were replaced by an overall reaction O2 + [C] CO2 with the rate constants of reaction (21). This enabled calculation of the effectiveness factor from the equation:

$$\mathfrak{n}\_{\parallel} = \mathfrak{n}\_{\parallel} = \mathfrak{Z}\left[\left(\tanh(\mathrm{Th})\right)^{-1} - \left(\mathrm{Th}\right)^{-1}\right]/\left(\mathrm{Th}\right),\tag{59}$$

where the Thiele module is:

52 Fossil Fuel and the Environment

R = k θ S m η , <sup>C</sup> <sup>1</sup> 2

For modelling, a crucial problem is the concentration of occupied active centres which cannot be eliminated from the kinetic equations. In the past, that problem was overcome by determination of products of the surface concentration of active centres and the rate constant. It may be successful when we are only interested in the char particle reaction rate. However, in order to model individual species, separation of active site concentrations from the rate constants is necessary. Considering all the difficulties, it is proposed to assume the concentration of the total active centres as a number of carbon atoms in a monolayer of active centres on the particle internal surface. For a particle of an internal surface area *A*,

It was assumed that the total fraction of the active centres **[C]** and **[CO]** is a dimensionless,

where is defined as the probability that a C atom will not bind to a nitrogen atom during

In the kinetic equations of the above mechanism, there are two key parameters: rate constants ki of reactions (21–26, 28, 44) and the mode of reaction expressed by the effectiveness factor I, yielding a part of the particle volume available for the i-th reaction. The rate constants of reaction (26) were found to be independent of pressure within the range of 0.2–1.5 MPa (Tomeczek & Gil, 2001). Based on this finding, it was assumed that for all the other reactions, the rate constants were independent of pressure and they were

a K C a K N N M

φ 1

diameter *d* and porosity

equal value (Croiset et al., 1998):

combustion (Croiset et al., 1998):

The total fraction of the active centres **[CN]** and **[CNO]** is:

where K = 1.95 1028 a.c. (active centres) m-5 or K = 32.32 kmola.c. m-5.

described using the equation by Gil (2002):

R = k θ Sm η - k θ Sm η p - k φ - θ Sm η p , NO <sup>5</sup> CNO <sup>C</sup> <sup>4</sup> <sup>6</sup> CNO <sup>C</sup> <sup>4</sup> NO <sup>7</sup> CO <sup>C</sup> <sup>7</sup> NO (52)

<sup>R</sup> <sup>=</sup> 2k <sup>θ</sup> Sm <sup>η</sup> <sup>p</sup> - 2k (<sup>φ</sup> - <sup>θ</sup> )Sm <sup>η</sup> <sup>p</sup> , N2O <sup>6</sup> CNO <sup>C</sup> <sup>4</sup> NO <sup>8</sup> CO <sup>C</sup> <sup>8</sup> N2O (53)

<sup>R</sup> <sup>=</sup> <sup>φ</sup> - <sup>θ</sup> <sup>k</sup> <sup>η</sup> <sup>p</sup> <sup>+</sup> <sup>2</sup> <sup>k</sup> <sup>η</sup> <sup>p</sup> SmC. N2 CO <sup>7</sup> <sup>7</sup> NO <sup>8</sup> <sup>8</sup> N2O (54)

, the total concentration of the occupied active centres can be

<sup>3</sup> S = KA 1 - d ε , (55)

**C** + **CO** = (56)

**CN** + **CNO** = 1 - (57)

<sup>C</sup> <sup>M</sup> (58)

R = k θ Sm η , CO <sup>2</sup> CO <sup>C</sup> <sup>1</sup> (50)

CO <sup>2</sup> <sup>3</sup> CO (51)

$$\text{Th} = 0.5 \text{d} \sqrt{\text{kp}} / \text{D}\_{\text{e}\prime} \tag{60}$$

and k = k1(1-)/MC, = p/RT, - char density, MC – carbon molar mass, p – pressure, R – gas constant, T – temperature. The effective diffusion coefficient was expressed as a function of the oxygen-helium molecular diffusion coefficient D and the Knudsen diffusion coefficient DK through the particle pores of tortuosity (Laurendeau, 1979):

$$\mathbf{D}\_{\mathbf{e}} = \varepsilon \left/ (\mathbf{D}^{-1} + \mathbf{D}\_{\mathbf{K}}^{-1})\mathbf{r}^{2}\right. \tag{61}$$

In order to calculate the rate of CO, CO2, NO, N2O and N2 formation by means of equations (50–54), it is necessary to find the fractions of the occupied active centres [CO] and [CNO] from the following kinetic equations:

$$\begin{split} \frac{\mathbf{d}\,\theta\_{\rm CO}}{\mathbf{d}t} &= 2\,\mathrm{k}\_{\mathrm{l}}(\boldsymbol{\upmu}\cdot\boldsymbol{\upmu}\_{\rm CO})^{2}\,\mathrm{p}\_{\rm O\_{2}}\cdot\mathrm{k}\_{2}\,\theta\_{\rm CO}\cdot 2\,\mathrm{k}\_{3}\,\theta\_{\rm CO}^{2} + \mathrm{k}\_{4}\,(\boldsymbol{\upmu}\cdot\boldsymbol{\upmu}\_{\rm CO})\mathrm{l}\cdot(\boldsymbol{\upmu}\cdot\boldsymbol{\upmu}\_{\rm CO})\cdot \\ &\cdot\boldsymbol{\upmu}\_{\rm I}\,\boldsymbol{\upmu}\_{\rm I}^{\uparrow\cdot}\,\mathrm{p}\_{\rm O\_{2}} + \mathrm{k}\_{6}\,\boldsymbol{\upmu}\_{\rm CO}\,\boldsymbol{\upmu}\_{\rm I}^{\uparrow\cdot}\,\mathrm{p}\_{\rm NO} + \mathrm{k}\_{7}\,(\boldsymbol{\upmu}\cdot\boldsymbol{\upmu}\_{\rm CO})\,\boldsymbol{\upmu}\_{\rm I}^{\cdot\cdot}\,\mathrm{p}\_{\rm NO} + \\ &\quad + \mathrm{k}\_{8}\,(\boldsymbol{\upmu}\cdot\boldsymbol{\upmu}\_{\rm CO})\,\boldsymbol{\upmu}\_{\rm S}\,\boldsymbol{\upmu}\_{\rm I}^{\uparrow\cdot}\,\mathrm{p}\_{\rm N\_{2}O}, \end{split} \tag{62}$$

$$\frac{\mathbf{d}\,\theta\_{\rm CNO}}{\mathbf{dt}} = \mathbf{k}\_4 \left(\boldsymbol{\upphi}\cdot\theta\_{\rm CO}\right) \mathbf{l}\,\mathbf{\upphi}\cdot\theta\_{\rm CNO} \left(\mathbf{p}\_{O\_2}\,\mathbf{\upphi}\_5\theta\_{\rm CNO}\,\mathbf{\upphi}\cdot\mathbf{k}\_6\theta\_{\rm CNO}\,\mathbf{p}\_{NO}\right) \,\tag{63}$$

The 90s experiments were conducted at pressures of 0.2 – 1.5 MPa in a pressurised reactor. A single layer of char particles 0.1– 0.8 g of initial diameter 0.9 mm was placed in a canthal tray and heated from the bottom and the top at a rate of 100 Ks up to the final temperature 1073–1373 K. The final heating temperature and the samples residence time at this temperature were automatically controlled. The maximum deviation from the adjusted temperature within the isothermal period was smaller than 40K. The temperature of the sample was measured by a NiCr-NiAl thermocouple and continuously recorded. The char was produced at 1373 K in atmospheric pressure helium during 20 minute devolatilisation of subbituminous "Siersza", "Janina" and "Piast" coals, whose characteristics (water content in the analytical state – Wa, volatile matter content in the water ash free state – Vwaf, ash content in the water free state – Awf, carbon content in the water ash free state – Cwaf, hydrogen content in the water ash free state – Hwaf, sulphur

Fuel-N Conversion to NO, N2O and N2 During Coal Combustion 55

Siersza 27.1103 79.3 0.91 40.0 1210 Janina 16.7103 80.1 0.79 59.3 1390 Piast 12.5103 86.6 0.78 45.7 1320

In Figures 4 and 5, comparisons of the model curve and experimental points for enriched "Siersza", "Janina" and "Piast" coals are shown. The rates of nitric and nitrous oxide

Fig. 4. Measured and modelled (kinetic constants Table 2) rates of nitrous oxide emission as

a function of time during char combustion

**A0 Ca0 Na0 <sup>0</sup> <sup>0</sup>**

m2kg-1 % % % kg1m-3

**Char** 

Table 3. Characteristics of chars

content in the analytical state – Sa, nitrogen content in the water ash free state – Nwaf, solid particle porosity – **0**) are presented in Table 1. A mixture of oxygen and helium of initial volume content O2 = 21% was used. After reactor cooling, the concentrations of NO and N2O were analysed as well as a mass of the remaining char and its nitrogen content. Gas sampling was calibrated on an empty reactor.


Table 1. Characteristics of coals

The rate constants of reactions (21–26, 28, 44) are presented in Table 2. The constants of reactions (21) and (24) were evaluated using Croiset et al. (1998) data, assuming average values of ki within the experimental pressure range 0.2–1.0 MPa.


Table 2. Rate constants of reactions

The initial properties (surface area – A0, carbon content in analytical state – Ca 0, nitrogen content in analytical state – Na0, solid particle porosity – **0**, real density **– 0**) of "Siersza", "Janina" and "Piast" chars used for testing of the mechanism and the kinetic constants are presented in Table 3. The constant value of char pores tortuosity was = 1.83 (Tomeczek & Mlonka, 1998). During particles burning, the internal surface area A as well as the porosity altered because most combustion reactions take place within the pores (Stanmore, 1991): A = Ao (1 + 2.5 B) (1 – B) and = o + B (1 - o), where B is the particle burn-out.


Table 3. Characteristics of chars

content in the analytical state – Sa, nitrogen content in the water ash free state – Nwaf, solid particle porosity – **0**) are presented in Table 1. A mixture of oxygen and helium of initial volume content O2 = 21% was used. After reactor cooling, the concentrations of NO and N2O were analysed as well as a mass of the remaining char and its nitrogen content. Gas

Siersza 2.6 37.7 6.9 77.0 5.1 2.5 1.51 15.8 Janina 3.3 33.9 8.1 77.6 3.3 1.1 1.34 14.6 Piast 5.7 30.8 7.1 84.2 3.9 1.2 1.42 15.1

The rate constants of reactions (21–26, 28, 44) are presented in Table 2. The constants of reactions (21) and (24) were evaluated using Croiset et al. (1998) data, assuming average

**Rate constant Author Pressure k = k0 exp(-ER-1T-1)** 

k1, s-1Pa-1 Croiset et al., 1998 0.2 - 1.0 *MPa* 6.410-6exp(-42R-1T-1)

k2, s-1 Gil, 2003 0.2 - 1.5 *MPa* 8.4102exp(-90R-1T-1)

k3, s-1 Gil, 2003 0.2 - 1.5 *MPa* 8.2105exp(-51R-1T-1)

k4, s-1Pa-1 Croiset et al., 1998 0.2 - 1.0 *MPa* 8.110-4exp(-58R-1T-1)

k5, s-1 Gil, 2003 0.2 - 1.5 *MPa* 2.2102exp(-84R-1T-1)

k6, s-1Pa-1 Tomeczek & Gil, 2001 0.2 - 1.5 *MPa* 1.1102exp(-110R-1T-1)

k7, s-1Pa-1 Tomeczek & Gil, 2001 0.2 - 1.0 *MPa* 210-4exp(-79R-1T-1)

k8, s-1Pa-1 Gil, 2003 0.2 - 1.0 *MPa* 410-5 exp(-75R-1T-1)

content in analytical state – Na0, solid particle porosity – **0**, real density **– 0**) of "Siersza", "Janina" and "Piast" chars used for testing of the mechanism and the kinetic constants are presented in Table 3. The constant value of char pores tortuosity was = 1.83 (Tomeczek & Mlonka, 1998). During particles burning, the internal surface area A as well as the porosity altered because most combustion reactions take place within the pores (Stanmore, 1991):

0, nitrogen

The initial properties (surface area – A0, carbon content in analytical state – Ca

A = Ao (1 + 2.5 B) (1 – B) and = o + B (1 - o), where B is the particle burn-out.

values of ki within the experimental pressure range 0.2–1.0 MPa.

**Wa Vwaf Awf Cwaf Hwaf Sa Nwaf <sup>0</sup>**

*% % % % % % % %* 

sampling was calibrated on an empty reactor.

**Enriched coal** 

Table 1. Characteristics of coals

Table 2. Rate constants of reactions

In Figures 4 and 5, comparisons of the model curve and experimental points for enriched "Siersza", "Janina" and "Piast" coals are shown. The rates of nitric and nitrous oxide

Fig. 4. Measured and modelled (kinetic constants Table 2) rates of nitrous oxide emission as a function of time during char combustion

Fuel-N Conversion to NO, N2O and N2 During Coal Combustion 57

a clear increase in N2O emission, and significantly reduced emissions of N2 could be obtained. The model was very sensitive to the change of activation energy *E8* because its

Pressurised combustion abates char-N conversion into NO and enhances conversion into N2O. Within the analysed ranges of pressure and temperature, reduction of NO by char is controlled by both chemical kinetics and diffusion in pores, with the activation energy equal to 79 kJ/mol. The rate constants of the heterogeneous mechanism of char-N conversion are independent of the combustion pressure if the chars are produced at the same pressure. About 60% of char-N is converted into N2 through reduction of NO and N2O over char;

Aho, M.J., Paakkinen, K.M., Pirkonen, P.M., Kilpinen, P. & Hupa M. (1995). The effects of

Anthony, D.B., Howard, J.B., Hottel, H.C. & Meissner, H.P. (1975). Rapid devolatilization of

Arai, N. & Hasatani, M. (1986). Model of simultaneous formation of volatile-NO and char-

Arai, N., Hasatani, M., Ninomiya, Y., Churchill, S.W. & Lior, N. (1986). A comprehensive

Bliek, A., van Poelje, W.M., van Swaaij, W.P.M. & van Beckum, F.P.H. (1985). Effects of

Bowman, C.T. (1973). Kinetics of nitric oxide formation in combustion processes. *14th* 

Bruisma, O.S.L., Geertsma, R.S., Oudhuis, A.B.J.,Kapteijn, F. & Moulijn J.A. (1988).

Buchill, P. & Welch, L.S. (1989). Variation of nitrogen content and functionality with rank for

Attar, A. & Hendrickson, G.G. (1982). *Coal Structure*. Academic Press, New York. Benson, S.W. (1968). *Thermochemical Kinetics.* John Wiley & Sons, New York.

and NO2 from pulverized coal. *Combustion and Flame* Vol.102: 387-400. Aho, M.J. & Pirkonen, P.M. (1995). Effects of pressure, gas temperature and CO2 and O2

pressure, oxygen partial pressure, and temperature on the formation of N2O, NO

partial pressure on the conversion of coal-nitrogen to N2O, NO and NO2. *Fuel* Vol.

pulverized coal.. *15th Symposium (International) on Combustion.* The Combustion

NO during the packed-bed combustion of coal char particles under a NH3/O2/Ar gas stream. *21st Symposium (International) on Combustion.* The Combustion Institute,

kinetic model for the formation of char-NO during the combustion of a single particle of coal char. *21st Symposium (International) on Combustion.* The Combustion

intraparticle heat and mass transfer during devolatilization of single coal particle.

*Symposium (International) on Combustion.* The Combustion Institute, Pittsburgh, pp.

Measurement of C, H, N - release from coals during pyrolysis. *Fuel* Vol. 67: 1190-

small growths caused a very large increase of N2O emissions.

however, modelling of this pathway needs further investigations.

**6. References** 

74: 1677-1681.

729-738.

1196.

Institute, Pittsburgh, pp. 1303-1317.

Institute, Pittsburgh, pp. 1207-1216.

*AICHE Journal*, Vol. 31: 1666-1681.

some UK bituminous coals. *Fuel* Vol. 68: 100-104.

Pittsburgh, Presented in the Poster Session.

Fig. 5. Measured and modelled (kinetic constants Table 2) rates of nitrous oxide emission as a function of time during the char combustion

emissions were presented as a function of time during char combustion for the pressure range of 0.2 MPa - 1.5 MPa and 1373 K. The agreement of the kinetic model and the experimental points is better for "Siersza" than for "Janina" and "Piast" coals because the kinetic data in Table 2 were evaluated only on the basis of the first. The average correlation coefficients for the studied chars within the pressure range of 0.2–1.5 MPa at 1373 K were approximately 0.80 for NO emission and 0.75 for N2O emission. The best compatibility of the modelled NO emission rate and recorded experimental values was achieved for the constant *k07* **=** 210-2 s-1Pa-1, and then drastically decreased correlation of N2. The model is not significantly sensitive to the activation energy of reaction (R7), *E7*, since a change in the range of 30 kJ mol-1 to 150 kJ mol-1 did not give a clear difference in the results. By changing the preexponential factor of constant k08 in the range of 4 10-5 s-1 Pa-1 to 2.1 103 s-1 Pa-1, a clear increase in N2O emission, and significantly reduced emissions of N2 could be obtained. The model was very sensitive to the change of activation energy *E8* because its small growths caused a very large increase of N2O emissions.

Pressurised combustion abates char-N conversion into NO and enhances conversion into N2O. Within the analysed ranges of pressure and temperature, reduction of NO by char is controlled by both chemical kinetics and diffusion in pores, with the activation energy equal to 79 kJ/mol. The rate constants of the heterogeneous mechanism of char-N conversion are independent of the combustion pressure if the chars are produced at the same pressure. About 60% of char-N is converted into N2 through reduction of NO and N2O over char; however, modelling of this pathway needs further investigations.
