Mechanism and product analysis of sludge pyrolysis driven by double alkali synergy

China's extensive sludge production has necessitated the development of advanced pyrolysis modification techniques, particularly those involving alkaline metals such as CaO and KOH. Previous studies have shown that CaO effectively captures phosphorus, whereas KOH promotes organic cracking. This study investigated dual-alkali-driven sludge pyrolysis processes for the production of biochar with stabilized heavy metals and enriched phosphorus contents. To elucidate the mechanisms underlying the pyrolysis process, advanced analytical techniques, including thermogravimetric analysis-Fourier transform infrared-gas chromatography–mass spectrometry (TG-FTIR-GC–MS), were employed to analyze the gas emissions from the CaO/KOH-modified sludge. Furthermore, infrared spectroscopy and X-ray diffraction phase analysis were used to characterize the resulting biochar. The results demonstrate that the influence of alkali metals on biochar composition and structure is closely related to the content of Lewis acids and bases in the sludge, and the reaction paths of CaO/KOH with typical functional groups (such as –COOH, –C–N and-C–OH) are given. For instance, CaO facilitates nitrogen volatilization, reducing its content by 17%–38%, whereas KOH retains nitrogen, increasing its content by 17%. The synergistic application of CaO and KOH significantly enhances the apatite phosphorus (AP) and total phosphorus (TP) ratio from 24%–35% to 87%–88%, and the AP to non-apatite inorganic phosphorus (NAIP) ratio from 0.32–0.54 to 5.74–43.77, thereby enabling the tuning of soil phosphorus availability and slow-release properties. Additionally, while CaO promotes phosphorus accumulation and assists in the sequestration of heavy metals, particularly Cr and Zn, KOH activates certain metals in the slag, potentially reducing their retention in the biochar.

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