Effects of red mud on emission control of NO x precursors during sludge pyrolysis: A protein model compound study

Keke Xiao, Ruonan Guan, Jiakuan Yang*, Hongsen Li, Zecong Yu, Sha Liang, Wenbo Yu, Jingping Hu, Huijie Hou, Bingchuan Liu

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

53 Scopus citations

Abstract

The nitrogen-containing gases pyrolyzed from sewage sludge can be converted into NO x compounds, which would cause severe environmental pollution. This study developed a new strategy to reduce the emission of NO x precursors such as ammonia (NH 3 ) and hydrogen cyanide (HCN) using red mud. The highest reduction efficiencies (15.10% for NH 3 and 24.72% for HCN) were achieved at 900 °C while compared with those pyrolyzed from raw sludge without the addition of red mud. The transformation and distribution of nitrogenous compounds in three-phase pyrolysates were studied at 400–800 °C for pyrolysis process of a model soybean protein compound. The nitrogenous compounds, i.e., amine-N, heterocyclic-N, and nitrile-N, were identified as the three main intermediates related with the production of NO x precursors. Ferric oxide (Fe 2 O 3 ) and calcium oxide (CaO) presented in red mud were identified as the driving force which facilitated nitrogen stabilization in char (e.g., at 800 °C, 21.63% increase of char-N after addition of Fe 2 O 3 , and 41.54% increase of char-N after addition of CaO). These metal oxides possibly reacted with protein-N to form Fe x N and CaC x N y , inhibited the secondary cracking of amine-N compounds in tar (e.g., at 800 °C, 2.33% increase of amine-N after addition of Fe 2 O 3 , and 0.38% increase of amine-N after addition of CaO), and reduced the production of nitrile-N (e.g., at 800 °C, 30.41% reduction of nitrile-N after addition of Fe 2 O 3 , and 27.40% reduction of nitrile-N after addition of CaO) and heterocyclic-N compounds (e.g., at 800 °C, 21.60% reduction of heterocyclic-N after addition of Fe 2 O 3 , and 13.98% reduction of heterocyclic-N after addition of CaO). Hence, the emission of NH 3 and HCN in gas phase can be controlled. Moreover, Fe 2 O 3 showed better capability in controlling the emission of NO x precursors than CaO (higher reduction of NH 3 -N and higher reduction of HCN-N). These results indicate that red mud is an efficient catalyst to reduce emission of NO x precursors through controlling intermediates at 400–800 °C.

Original languageEnglish
Pages (from-to)452-463
Number of pages12
JournalWaste Management
Volume85
DOIs
StatePublished - 15 Feb 2019
Externally publishedYes

Keywords

  • Metal oxides
  • NO precursors
  • Pyrolysis
  • Red mud
  • Sewage sludge

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