Key element course-tracked copyrolysis of sewage sludge and biomass for resource recovery and pollution control through kinetic and thermodynamic insights

Abstract

Ameliorating sewage sludge (SS) pyrolysis process with biomass increasingly becomes an effective strategy to utilize SS-entraining resources and depress pollutant evolution, while real-time track of SS and biomass copyrolysis as well associating thermochemical reactions in aspects of kinetics and thermodynamics is yet to be unveiled. Here a combinative thermogravimetry combined with Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry (TG-FTIR-GC/MS) pyrolysis system, lab-scale furnace and theoretical calculation were adopted to develop a key element course-tracked copyrolysis of SS and corn stalk (CS) for simultaneous accommodation of profitable products and derived pollutants. The formation of alkanes and alkene with small carbon number (below 4) was promoted during copyrolysis, while those with heavy carbons (over 5) were contributed by the inherent organic fraction of SS. The release of those nitrogen (N)-/sulfur-/chlorine-involving inorganic pollutants was depressed with the increase in the CS addition. The increased release of CO2 and H2O as well carboxylic volatiles with a high CS addition evidenced the promoted dehydroxylation and decarboxylation. Consequently, the speciation of N and phosphorus (P) and the bioavailability of toxic metals (TMs) were regulated with the increasing CS addition, that is, enhancements of proteins and protein-like N transformation to the other nitrogen species, P within orthophosphate diesters to orthophosphate monoesters, and TM reactivity-dependent bioavailability depression. A low addition (below 20 %) was prone to maintain a congruent mechanism, while the total Gibbs energy tended to decrease with an increasing addition and pyrolysis temperature, leading to a converged pressure-constant heat capacity (∼1430 J/K) at 700 °C. Those thermochemical reactions occurring over sewage sludge ash matrix were pivoted with biomass addition, which can be adopted as a useful strategy for an efficient, cleaner and sustainable mining of waste resources.