Nitrogen migration in coal during the chemical looping gasification reduction process using a nickel-based oxygen carrier

Abstract

The release of nitrogen from a chemical looping gasification (CLG) system has always been a great concern. In this work, the thermal reactivity of samples obtained from the Jinfeng district, which were involved in the CLG process, was investigated via thermogravimetric–mass spectrometric (TG–MS) analysis. The migration mechanism of gas-phase nitrogen was characterized online, and changes in solid-phase nitrogen were analyzed during different stages using X-ray photoelectron spectroscopy. Results showed that nitrogen mainly existed in the solid and gas phases during different temperature stages of CLG; however, its migration varied at different stages. During the low-temperature pyrolysis stage (320 °C–795 °C, without adding steam), the addition of a nickel-based oxygen carrier accelerated the decomposition of the aliphatic structure and increased the hydroxyl content in the sample. Further, the release of nitrogen precursors was promoted by the accelerated release of hydrogen-containing free radicals from the aliphatic structure. The interaction between the nitrogen precursor and hydroxyl structure induced NO release at 350 °C. During the high-temperature pyrolysis stage (795 °C–950 °C, without adding steam), the reaction between the oxygen carrier and char exposed protic pyridine (N-Q) in the macromolecular structure. Furthermore, during the gasification stage (670 °C–950 °C, with steam addition), the addition of the oxygen carrier decreased the degree of graphitization of char and enhanced its gasification performance. As the structural units of char became smaller, N-Q in its macromolecular structure underwent further decomposition. Thus, the addition of the oxygen carrier at high temperatures benefited N-Q decomposition.