Effects of calcium on the evolution of nitrogen during pyrolysis of a typical low rank coal

Abstract This study aims to investigate the effects of calcium on the migration of nitrogen in coal (coal-N) to N-containing gas species, particularly, NH 3 and HCN (volatile-N) in volatiles, as well as the chemical transformation of the N in char during coal pyrolysis under different temperatures. The pyrolysis experiments of Shengli brown coal and its derived coal samples loaded with different contents of calcium were conducted under 600–800 °C in a novel fluidized bed reactor. The experimental results showed that during coal pyrolysis, the generation of NH 3 is mainly derived from secondary reactions among volatiles, tar and char with the catalytic effect of mineral matter, especially calcium in coal. Increasing pyrolysis temperature from 600 to 800 °C could enhance the release of N in coal to volatiles. Meanwhile, the increased pyrolysis temperature could also inhibit the generation of NH 3 while facilitating the formation of HCN. The release of HCN is more sensitive to pyrolysis temperatures. Specifically, under higher pyrolysis temperatures, more N-containing structures in coal would become thermally unstable and crack into HCN; On the other hand, higher pyrolysis temperature could also enhance the decomposition of N in coal to N-containing species in tar or N 2 , thus reducing the release of HCN and NH 3 . Nitrogen in tar could either undergo secondary decomposition reactions, generating NH 3 , HCN, N 2 and other N-containing species in gas phase, or experience condensation polymerization by forming macromolecular structure and be retained in char at high pyrolysis temperatures. Calcium could significantly restrain the release of N from coal, thus reducing the yields of NH 3 and HCN. During coal pyrolysis, calcium catalytically enhances the fracture and combination of chemical bonds, generating abundant free radicals. These free radicals could continuously attack N-containing structures and consequently release the N-containing gaseous products, such as NH 3 , HCN, N 2 etc., resulting in the decrease of N in char. Calcium also plays important roles in nitrogen transformation in char during coal pyrolysis by catalytically intensifying the transformation of N in char from pyridinic nitrogen (N-6) and pyrrolic nitrogen (N-5) to quaternary type nitrogen (N-Q) during coal pyrolysis.