Effect of Sodium on the Evolution of Nitrogen Functionalities During Coal Pyrolysis : A Quantitative Experimental and Theoretical Investigation

Abstract

ABSTRACT Understanding the migration and transformation of nitrogen during high-alkali coal pyrolysis and the influence of sodium on the nitrogen-containing components of coal is of great significance for the clean and efficient utilization of coal. In this study, gas chromatography-mass spectrometry (GC-MS), X-ray photoelectron spectroscopy (XPS) and ultraviolet visible spectroscopy (UV-Vis) were used to analyze the conversion of tar-N, char-N and gas-N during coal pyrolysis and the effect of sodium on the nitrogen-containing pyrolysis products of coal. The experimental results were combined with density functional theory (DFT) calculations to investigate the effect of sodium on the internal conversion process of char-N. The results revealed that a large amount of Na (2.5 wt.%, 3.5 wt.%) can reduce the nitrogen content in coal pyrolysis char, promote the formation of amine and amide in tar-N and inhibit the release of gaseous nitrogen. A small amount of Na (0.5 wt.%, 1.5 wt.%) retains nitrogen in char, reducing the total amount of nitrogen-containing species in the gas phase and tar, which is an effective means to inhibit the transformation of nitrogen to volatile matter. In addition, a large amount of Na promotes the conversion of pyrrole nitrogen (N-5) and pyridine nitrogen (N-6) in char to heterocyclic N in tar. A small amount of sodium promotes the hydrogenation and ring opening of heterocyclic-N in tar to HCN. Na promotes the conversion of N-5 and N-6 in char. DFT calculations further confirmed that the addition of Na significantly reduced the energy barrier of the rate determining step by about 36.6 kJ/mol.