**2.2 Nitrogenous species in coal**

In coal substance, there are the following nitrogen functional groups: pyridine and pyridinederivative nitrogen, pyrrole and pyrrole-derivative nitrogen, nitriles, amines and amides. Nitrogen-containing molecules differ in size and occur as mono- and polycyclic compounds up to nine fused aromatic rings. In a heteroaromatic compound molecule, there is mostly a single nitrogen atom, but there may be two or even three atoms and the repetitive structures are tri- to pentacyclic compounds such as carbazole and acridine (Ostman & Colmsjo, 1988). A majority of nitrogen compounds in coal may be nitriles; however, during pyrolysis, they react with hydrogen to produce basic nitrogen; a product of some nitrile reactions may be ammonia. The presence of amine and cyano groups has not been proved but there is no evidence of their absence either as small amounts of amine groups are probably present due to their reactivity (Attar & Hendrickson, 1982). Studies by Burchill and Welch (1989) demonstrate the issue of nitrogen in coal in a slightly different manner and indicate pyrrole nitrogen to be dominant in coal. The contents of pyrrole and pyridine nitrogen change also during the coalification process so the pyrrole-pyridine nitrogen ratio changes with the degree of coalification. Pyrrole nitrogen reaches maximum in coal with Ca = 84%, while pyridine nitrogen – in coal with Ca = 90%. Hence, it may be concluded that heterocyclic six-membered structures are more stable in later coalification stages. The studies of 182 coals with coalification degrees Ca = 79–95% showed that mean nitrogen contents reach their maximum in coals with coalification degrees Cwaf = 84–85%. The increase in N/C ratio within Ca = 79–81% is explained by decarboxylation which is completed in coals with a slightly smaller coalification degree than Ca = 80%. A decrease in N content in coals with the coalification degree above Ca = 85% occurs due to a poorer stability of nitrogenous species than that of the main aromatic structures in the coal matrix. A structural formula of one nitrogen binding in the coal matrix is shown in Figure 1 below (van Krevelen, 1981).

Maceral studies revealed non-uniform nitrogen content in macerals. The highest amounts of nitrogen are in vitrinite and then in liptinite, while the lowest amounts are found in inertinite where there is the highest level of pyridine nitrogen (van Krevelen, 1981; Given et al., 1984).

At present, two standardised methods used for quantitative determination of nitrogen in organic compounds are the Kjeldahl method and the Dumas method. In the Kjeldahl method (Krzyżanowska&Kubica, 1978), analysed organic matter is decomposed by sulphuric acid at 393 K to 423 K. Nitrogen in the matter is converted into ammonium

The knowledge of nitrogenous species in coal would allow for a more effective application of this material in many processing technologies. The significance of the problem is clearly seen

Analyses of coal extracts or analyses of pyrolysis, oxidation or hydrogenation products yield data on nitrogen in coal. In coal extracts and tar (a coal depolymerisation product), basic and neutral nitrogen compounds are found. Their fractions depend on the nitrogen atom environment. During pyrolysis, a conversion of nitrogen compounds occurs: some of basic nitrogen compounds are formed during the process while some are released as ammonia. This depends on the process temperature and coal humidity. In acids, amines and nitrogen contained in a six-membered ring are dissolved. Five-membered rings do not dissolve. Solubility decreases with the increase in a molecular weight and the presence of oxygen

In coal substance, there are the following nitrogen functional groups: pyridine and pyridinederivative nitrogen, pyrrole and pyrrole-derivative nitrogen, nitriles, amines and amides. Nitrogen-containing molecules differ in size and occur as mono- and polycyclic compounds up to nine fused aromatic rings. In a heteroaromatic compound molecule, there is mostly a single nitrogen atom, but there may be two or even three atoms and the repetitive structures are tri- to pentacyclic compounds such as carbazole and acridine (Ostman & Colmsjo, 1988). A majority of nitrogen compounds in coal may be nitriles; however, during pyrolysis, they react with hydrogen to produce basic nitrogen; a product of some nitrile reactions may be ammonia. The presence of amine and cyano groups has not been proved but there is no evidence of their absence either as small amounts of amine groups are probably present due to their reactivity (Attar & Hendrickson, 1982). Studies by Burchill and Welch (1989) demonstrate the issue of nitrogen in coal in a slightly different manner and indicate pyrrole nitrogen to be dominant in coal. The contents of pyrrole and pyridine nitrogen change also during the coalification process so the pyrrole-pyridine nitrogen ratio changes with the degree of coalification. Pyrrole nitrogen reaches maximum in coal with Ca = 84%, while pyridine nitrogen – in coal with Ca = 90%. Hence, it may be concluded that heterocyclic six-membered structures are more stable in later coalification stages. The studies of 182 coals with coalification degrees Ca = 79–95% showed that mean nitrogen contents reach their maximum in coals with coalification degrees Cwaf = 84–85%. The increase in N/C ratio within Ca = 79–81% is explained by decarboxylation which is completed in coals with a slightly smaller coalification degree than Ca = 80%. A decrease in N content in coals with the coalification degree above Ca = 85% occurs due to a poorer stability of nitrogenous species than that of the main aromatic structures in the coal matrix. A structural formula of one nitrogen binding in the coal matrix

Maceral studies revealed non-uniform nitrogen content in macerals. The highest amounts of nitrogen are in vitrinite and then in liptinite, while the lowest amounts are found in inertinite where there is the highest level of pyridine nitrogen (van Krevelen, 1981; Given et al., 1984).

At present, two standardised methods used for quantitative determination of nitrogen in organic compounds are the Kjeldahl method and the Dumas method. In the Kjeldahl method (Krzyżanowska&Kubica, 1978), analysed organic matter is decomposed by sulphuric acid at 393 K to 423 K. Nitrogen in the matter is converted into ammonium

in the amounts of nitric oxide emissions during coal combustion (Stańczyk, 1991).

functional groups (Attar & Hendrickson, 1982; Stańczyk, 1991).

**2.2 Nitrogenous species in coal** 

is shown in Figure 1 below (van Krevelen, 1981).

sulphate which is next decomposed by NaOH to produce ammonia. The amount of ammonia is determined using a titration method with 0.01n H2SO4 and the acid-base indicator. In the Dumas method (Krzyżanowska & Kubica, 1978), a mixture of analysed substance and a catalyst (cupric oxide) is combusted in a quartz tube purged with carbon dioxide. Nitrogen oxides that form during combustion are decomposed into elemental nitrogen over incandescent copper. A received mixture of carbon dioxide and nitrogen is then introduced into a KOH-filled nitrometer where CO2 is absorbed and the volume of nitrogen is determined. A factor that strongly influences result accuracy is the temperature of combustion. At present, C, H, N elemental analysers are applied with an electronic data processing programme and an autosampler for multi-sample analysis. The most common C, H, N elemental analysers are produced by Perkin - Elmer, Carlo Erba Strumentazione and Hewlett Packard companies. The above techniques of elemental analysis have been standard methods for many years.

Fig. 1. A structural formula of one nitrogen binding in the coal matrix
