Effect of alkaline earth metals on N2 formation during fixed bed pyrolysis of a low rank coal

N2 formation during pyrolysis of Mg-, Ca-, Sr- and Ba-loaded low rank coal at 1200 °C has been examined with a fixed bed quartz reactor in an inert atmosphere. At 10 °C/min, the Ca, Sr and Ba, except for Mg, increase considerably the formation rate between 850 and 1100 °C, and N2 yield reaches ≥60% at 1200 °C. The increase in the yield is almost equal to the decrease in yield of the corresponding char-N, which means that the Ca, Sr and Ba can enhance N2 formation from char-N (and/or precursors) through solid phase reactions. In addition, all of the alkaline earth metals used promote secondary decomposition reactions of tar-N to NH3. When the effects of some factors on the catalytic activity of the Ca for N2 formation are examined, pyrolysis temperature, heating rate and the sulfur in coal affect its performance. The in situ X-ray diffraction (XRD) measurements reveal that the Ca, Sr and Ba added are transformed to nanoscale particles of CaO, SrO and BaO with the mean crystalline sizes of 20–40 nm during heating at 900 °C where their activity for N2 formation appears significantly. The oxides also enhance not only the proportion of crystallized carbon but yields of both H2 and CO. These observations suggest strong interactions between fine particles of the oxides and char substrate. The mechanism of N2 formation is discussed in terms of solid–solid reactions of CaO, SrO or BaO with heterocyclic nitrogen forms in char.