Nitrogen migration and transformation during the pyrolysis of corn straw with iron-based catalyst

Abstract

The pyrolysis of renewable biomass into biofuels and high-value chemicals showcases remarkable application value and extensive market potential. However, the pyrolysis of biomass with high nitrogen content results in the production of NOX precursors that contribute to environmental pollution. Therefore, it is proposed to introduce iron-based catalysts to change the formation pathway of NOX precursors and control their production.The study elucidates the influence of iron-based catalysts on the conversion of nitrogen-containing products between gas, liquid and solid phases during corn straw pyrolysis. First, the effects of iron-based catalyst types on pyrolysis kinetics of corn straw were estimated using Thermogravimetry-mass spectrometry (TG-MS). Based on this, the effects of catalyst type and dispersion on the migration path of three-phase nitrogen-containing products were investigated, and the chemical morphology of the catalyst during pyrolysis was monitored by in-situ XRD to clarify the main catalytic active phase for N2 formation. Results showed that the addition of Fe, FeCl3 and Fe2O3 changed the distribution ratio of three-phase nitrogen-containing products, and the gas phase yield was significantly improved. Notably, the chemical form of Fe2O3 will change and transform into FeOOH, Fe3O4 and α-Fe during pyrolysis. It was confirmed that α-Fe was the main active phase for catalyzing the conversion of nitrogen-containing substances. Higher N2 conversion can be achieved by preparing high purity α-Fe. The mechanism is that α-Fe catalyzes the secondary decomposition of tar nitrogen into NH3 and HCN. Simultaneously, α-Fe reacts with char nitrogen, NH3, and HCN to form FeXN, which decomposes to produce environmentally friendly N2. The research findings offer theoretical guidance for the control of nitrogen pollutant emissions and the targeted acquisition of pyrolysis products (For example, NH3 can be used as a carbon-free hydrogen-rich fue).