**2.1 Nitrogen in hard coal**

Fuel-nitrogen is found in plants, animal proteins and nitrogen-rich bacteria. Reactions of amines with carboxylic groups or aldehyde groups resulted in nitrogenous species present in coal. Coal is a heterogeneous, complex mineral where nitrogen is typically bound to organic matter. During a coalification cycle, nitrogen content in coal substance only slightly changes so the nitrogen fraction (in per cent) in the substance increases as peat loses oxygen (Stańczyk, 1991). Nitrogen content in coal is 0.5% to 2% (Tingey & Morrey, 1973), reaching maximum for elemental carbon Cwaf = 85% (Rybak, 1996). Coal typically contains 1% to 2% of nitrogen with bituminous coals usually containing 1.5–1.75% and anthracites mostly containing less than 1%.

The presence of nitrogen in coal has not been fully understood and described yet. There is far better knowledge of the structures of sulphur and oxygen than those of fuel-nitrogen. Due to difficulties encountered in investigations of nitrogen content in solid fuels, indirect methods of analysis are used and structures present in coal extracts or high-temperature coal tars are determined. There are few methods of direct analysis of nitrogen-containing groups in coal.

Fuel-N Conversion to NO, N2O and N2 During Coal Combustion 39

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

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

ms (Pershing & Wendt, 1979). Thus, there are three combustion stages:

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

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

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

**2.3 Nitrogen conversion during coal combustion** 

rapid heating to release volatile matter

heterogenous combustion of char

(Jüntgen, 1987).

homogenous combustion of the volatile matter

methods for many years.

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

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