The multiple influence mechanisms of Fe2O3 on nitrogen evolution during sewage sludge pyrolysis were systematically investigated by correlating temperatures. The results showed that Fe2O3 promoted the pyrolysis of the sludge. The activation energy of the pyrolysis was reduced by 12–110 kJ/mol. Fe2O3 acted oxidatively on sludge nitrogen in the following order Fe2O3→Fe3O4 (ca. 528 K)→FexNy (ca. 763 K)→FexC (ca. 1023 K). The oxidation target of Fe2O3 on nitrogen containing compounds varied as a function of temperature. At temperatures of 528 K and 623 K, Fe2O3 hindered the deamination of proteins in char to NH3, facilitated the condensation of pyridines to nitrile, and impeded the generation of HCN. At temperatures of 763 K and 1023 K, Fe2O3 facilitated the cleavage of amines, heterocycles, and nitrile in char. Additionally, it promoted the breaking of chains and cyclization of long chain amines in tar. This process theoretically leads to the production of a great amount of NH3 and HCN at the front. Overall, Fe2O3 inhibited the release of NOx precursors through inhibition of deamination, fixation of nitrogen in char and consumption of NH3 and HCN. This study will establish the effect mechanism of action of Fe2O3 on the nitrogen conversion of sewage sludge pyrolysis at relevant temperatures. It will provide a theoretical foundation for controlling NOx and the targeted regulation NH3 (carbon-free hydrogen energy carriers) in the high-value conversion of nitrogen-rich biomass. 

Unveiling the complex effect of Fe2O3 on NOx precursors evolution mechanism during sludge protein pyrolysis based on product characteristics

Nitrogen oxides (NOₓ) are primary pollutants produced during biomass combustion. During the devolatilization stage, char nitrogen (char(N)) is formed. In the subsequent char combustion stage, char(N) can decompose directly into NOx precursors or engage in heterogeneous reactions with O2 or NO to form NO and N2. Nonetheless, a comprehensive understanding of the reaction mechanisms and competitiveness of char(N) migration, especially the influence of the alkali metal potassium (K) present in biomass, remains incomplete. Building on the Zigzag char(N) models, the present study delves into the migration reactions of char(N), assessing their competitive dynamics through the integration of density functional theory, electronic structure analysis, and conventional transition state theory. Furthermore, it examines the impact of K on char(N) conversion. The competitiveness of the heterogeneous reactions follows the sequence: heterogeneous reduction of NO to N2 > heterogeneous oxidation of char(N) to NO > decomposition of char(N) to NOx precursors. Moreover, the formation of HCN is more favorable than NH₃ production. The successive conversion from char(N) to NO and then to N2 is the predominant migration route for char(N), with NO generation as the pivotal step. The less preferred char(N) migration route involves decomposition to NH3/HCN, followed by oxidation to NOx within the main combustion zone, which cannot be mitigated by char. K can accelerate NO generation and sustain the primacy of the heterogeneous NO reduction, consequently enhancing the oxidation-reduction process of char(N). As a result, K plays a constructive role in managing NOx emissions during the thermal conversion of char. 

Insight into the fate of nitrogen during char thermal conversion and the influence mechanism of potassium: A theoretical research.

To investigate the control mechanisms of NOx precursors and the synergistic effects of composite catalysts during proline pyrolysis, a systematic series of experiments was conducted utilizing composite catalysts with varying Fe-Ca ratios. Product distribution analysis was employed to elucidate the catalysts' mechanisms in reducing NOx precursor emissions. The synergistic interactions between Fe and Ca were quantitatively assessed through comparative theoretical and experimental release calculations. The results indicate that an increase in the Fe content in the catalyst led to a rise in amine concentrations from 0.9% to 2.95%, implying that Fe facilitates the generation of amine-N through ring-opening and substitution reactions. When the Fe to Ca ratio was balanced at 1:1, nitrogen predominantly participated in the formation of purines via cyclization and substitution reactions. Additionally, all composite catalysts exhibited a suppressive effect on the release of NOx precursors, attributed to their significant enhancement of solid product retention. Fe-Ca composite catalyst synergistically inhibits the release of gaseous nitrogen. Notably, the strongest synergistic effect was observed with a 1:3 Fe to Ca ratio, which reduced the release of NH 

Synergistic Catalytic Effects on Nitrogen Transformation during Biomass Pyrolysis: A Focus on Proline as a Model Compound

With the increase in the world population and economic activity, the production of sewage sludge has grown, and its management has become an environmental problem. The most traditional method of managing sewage sludge is to dispose of it in landfills and on farmland. One way to valorize sewage sludge is to use thermochemical conversion processes to produce added-value products such as biochar, biofuels, and renewable gases. However, due to the high moisture content, thermochemical conversion using processes such as pyrolysis and traditional gasification involves multiple pre-treatment processes such as material drying. Hydrothermal thermochemical processes usually require high pressures, which pose many challenges to their application on a large scale. In this work, the advantages and disadvantages of the different existing thermochemical processes for the recovery of sewage sludge were analyzed, as well as the resulting industrial and environmental challenges. A SWOT analysis was carried out to assess the different thermochemical processes in terms of technical feasibility, economic viability, and broader market considerations.

An Overview of the Thermochemical Valorization of Sewage Sludge: Principles and Current Challenges

Investigation of the mechanism and interaction of nitrogen conversion during lignin/glutamic acid co-pyrolysis

Study on the multiple roles of CaO on nitrogen evolution mechanism of protein inside sewage sludge pyrolysis

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Study on the multiple roles of CaO on nitrogen evolution mechanism of protein inside sewage sludge pyrolysis