**2.3 Nitrogen conversion during coal combustion**

A coal particle is introduced into a furnace for pulverised coal and heated up to 1770–1970 K in 1 ms due to heat collection from surrounding gases as well as fire and furnace wall radiation (Stańczyk, 1991). Within 10 ms, volatile matter is released from the particle, ignited and combusted within the subsequent 10–100 ms. Remaining char is combusted within 300 ms (Pershing & Wendt, 1979). Thus, there are three combustion stages:


Nitrogen oxide emissions depend on the heating rate and the presence of the above combustion stages. During combustion under fluid conditions, the presence of the specific stages and their duration are different due to larger coal particles and considerably lower furnace temperature 1100–1170 K, but the listed combustion stages still may be taken into consideration. Some papers show that a high heating rate and small particles may result in heterogeneous particle ignition or simultaneous ignition of volatile matter and a solid (Jüntgen, 1987).

Pershing and Wendt (1979) studies reveal that under typical pulverised coal combustion conditions, about a half of coal nitrogen undergoes pyrolysis. Conversion of the nitrogen into NOx is higher than conversion of char-nitrogen and constitutes about 60–80% of total NOx emissions. Many studies show that main factors affecting the extent of nitrogen emissions are: reaction stoichiometry and (less significant) nitrogen content in coal (Pereira el al., 1974; Cliff & Young, 1985; Midkiff & Atenkirch, 1988). Heterogeneous and homogenous oxidation of coal

Fuel-N Conversion to NO, N2O and N2 During Coal Combustion 41

Reduction of carbon dioxide and nitrogen oxide emissions in pressurised combustion methods is a result of intensive research (Gajewski, 1996). Release of fuel-nitrogen from coal during pressurised combustion, the rate and degree of its conversion to NO and N2O are the data necessary for understanding of formation and destruction mechanisms for

The published studies on the mechanism of fuel-nitrogen release show a lot of discrepancies and contradictions. In case of pressurised combustion, there are no comprehensive kinetic models of NO and N2O formation available. Few papers, mentioned below, require further investigations on modelling of nitrogen conversion in pressurised combustion processes. Weiszrock et al. (1997) conducted studies on pressurised coal combustion in a laser reactor. At various O2 levels, the increase in pressure resulted in NO emission reduction, but N2O

Aho et al. (1995) as well as Aho and Pirkonen (1995) demonstrated reduction of conversion into N2O, which is contradictory to the findings of Weiszrock et al. (1997), while Lu et al. (1992) did not find any effects of the combustion pressure on nitrogen conversion into N2O. Croiset et al. (1996, 1998) proposed a model of NO and N2O formation during pressurised combustion of char. The model was based on eight surface reactions with [C], [CO], [CN] and [CNO] active centres. Modelling of nitrogen oxide emissions using kinetic constants calculated by the authors demonstrates reduction of NO levels with enhanced pressure. Croiset et al. noted that at increased temperature and pressure, reduction of NO to N2O is a more important reaction than reduction of NO to N2. In their calculations, the authors of the model omitted the inner structure and made the reaction kinetic constants dependant on

For 0.2 MPa, De Soete et al. (1999) complemented the model with the efficiency of particle volume application in the reaction of [CO] and {CNO] active centre formation. They also compared kinetic coefficients of individual reactions published by Croiset et al. (1996, 1998) for "Westerholt" coal (at 0.2 MPa) and by de Soete (1990) for "Prosper" coal (at 0.1 MPa), but they did not make any comments on kinetic coefficients for higher pressures presented by

Tomeczek and Gil (2000, 2001) as well as Gil (2000) proposed their model of NO and N2O formation and reduction in pressurised char combustion based on chemical reactions presented by de Soete (1990) and Croiset et al. (1998). They determined their own kinetic coefficient of NO reduction over char where the activation energy of char was 79 kJ/mol within the pressure range of 0.2 MPa to 1.0 MPa. In case of the other coefficients of heterogeneous reactions, they considered the efficiency of particle volume application and made the coefficients non-pressure dependant, which resulted in a good agreement of the model and the conducted experiments within a large pressure range (0.2–1.5 MPa). Moreover, they estimated a total concentration of active centres (not determined yet) and included it in kinetic equations. However, the model requires further investigations and

In case of NO and N2O modelling, the initial pathway of fuel-nitrogen conversion into NO during coal combustion is extremely difficult to explain. During char coal combustion, the

**3. Review of studies on formation of nitrogen compounds during coal** 

**combustion** 

these pollutants.

emissions increased.

Croiset et al. (1998).

experimental studies.

pressure, which arouses some doubts.

nitrogen is included in the model proposed by Midkiff and Atenkrich (1988). In Figure 2, a scheme of coal nitrogen distribution in the process of combustion is presented. The product of primary pyrolysis, nitrogen-containing volatile matter, undergoes secondary pyrolysis to produce HCN, NH3, CN and N2. HCN, NH3 and CN are oxidised to NOx and N2O. One part of N2 is formed directly during pyrolysis, while the other is formed through NOx reduction by hydrocarbon radicals or in a reaction with CO. During the first 4 ms of combustion, 43% of converted nitrogen was transformed into N2, while 57% into NOx and N2O. Thus, about half of nitrogen would be directly converted into N2, while the other part into HCN and NH3 which then would be oxidised into NOx and N2O (Peck et al., 1984).

Fig. 2. Distribution of coal nitrogen during combustion

The composition of nitrogen-containing volatile matter (HCN, NH3, NOx) is influenced by coal types. Studies related to oxygen deficiency ( = 0.5–0.8) revealed that in case of anthracite, only NOx (and not HCN or NH3) was formed in the amount of 17.5%. For bituminous coals, the amount of HCN is higher than that of NH3 and increases with the increase in coal volatile matter content to produce 6–11% of NOx. Low-rank coals (subbituminous coals and lignites) release the highest amounts of NH3 and HCN, but less NOx than bituminous coals. The studies of model nitrogen-containing liquid combustion showed a similar conversion of different types of compounds into NOx (Stańczyk, 1991).

Many studies suggest that emissions of specific coal nitrogen compounds during combustion are strongly related to the reaction stoichiometry. Air deficiency promotes N2 formation, while its excessive amounts lead to NOx formation (Bruisma el al., 1988).

Changes in air excess in furnaces mainly affect the extent of NOx (formed from nitrogen contained in the volatile matter) emission. The main effect of all furnace modifications aimed at multi-stage combustion is limitation of NOx (formed from nitrogen released from coal during the primary pyrolysis) emission. Char-nitrogen, however, is insensitive to these procedures and NOx emissions (formed from char-nitrogen) cannot be limited through furnace aerodynamic modifications (Preshing & Wendt, 1977, 1979).

There is no agreement among researchers on the effects of the nitrogen content in coal on its conversion into NOx. In general, the increase in nitrogen content in coal results in enhanced NOx emissions. However, coals with the same nitrogen contents and the same degree of coalification may significantly differ with respect to nitrogen oxide emissions (Preshing & Wendt, 1977).

nitrogen is included in the model proposed by Midkiff and Atenkrich (1988). In Figure 2, a scheme of coal nitrogen distribution in the process of combustion is presented. The product of primary pyrolysis, nitrogen-containing volatile matter, undergoes secondary pyrolysis to produce HCN, NH3, CN and N2. HCN, NH3 and CN are oxidised to NOx and N2O. One part of N2 is formed directly during pyrolysis, while the other is formed through NOx reduction by hydrocarbon radicals or in a reaction with CO. During the first 4 ms of combustion, 43% of converted nitrogen was transformed into N2, while 57% into NOx and N2O. Thus, about half of nitrogen would be directly converted into N2, while the other part into HCN and NH3 which

The composition of nitrogen-containing volatile matter (HCN, NH3, NOx) is influenced by coal types. Studies related to oxygen deficiency ( = 0.5–0.8) revealed that in case of anthracite, only NOx (and not HCN or NH3) was formed in the amount of 17.5%. For bituminous coals, the amount of HCN is higher than that of NH3 and increases with the increase in coal volatile matter content to produce 6–11% of NOx. Low-rank coals (subbituminous coals and lignites) release the highest amounts of NH3 and HCN, but less NOx than bituminous coals. The studies of model nitrogen-containing liquid combustion showed a similar conversion of different types of compounds into NOx (Stańczyk, 1991).

Many studies suggest that emissions of specific coal nitrogen compounds during combustion are strongly related to the reaction stoichiometry. Air deficiency promotes N2

Changes in air excess in furnaces mainly affect the extent of NOx (formed from nitrogen contained in the volatile matter) emission. The main effect of all furnace modifications aimed at multi-stage combustion is limitation of NOx (formed from nitrogen released from coal during the primary pyrolysis) emission. Char-nitrogen, however, is insensitive to these procedures and NOx emissions (formed from char-nitrogen) cannot be limited through

There is no agreement among researchers on the effects of the nitrogen content in coal on its conversion into NOx. In general, the increase in nitrogen content in coal results in enhanced NOx emissions. However, coals with the same nitrogen contents and the same degree of coalification may significantly differ with respect to nitrogen oxide emissions

formation, while its excessive amounts lead to NOx formation (Bruisma el al., 1988).

furnace aerodynamic modifications (Preshing & Wendt, 1977, 1979).

(Preshing & Wendt, 1977).

then would be oxidised into NOx and N2O (Peck et al., 1984).

Fig. 2. Distribution of coal nitrogen during combustion
