Emission characteristics of condensable particulate matter during the production of solid waste-based sulfoaluminate cement: Compositions, heavy metals, and preparation impacts

Abstract

Recycling solid waste for preparing sulfoaluminate cementitious materials (SACM) represents a promising approach for low-carbon development. There are drastic physical-chemical reactions during SACM calcination. However, there is a lack of research on the flue gas pollutants emissions from this process. Condensable particulate matter (CPM) has been found to constitute the majority of the primary PM emitted from various fuel combustion. In this study, the emission characteristics of CPM during the calcination of SACM were determined using tests in both a real-operated kiln and laboratory experiments. The mass concentration of CPM reached 96.6 mg/Nm³ and occupied 87% of total PM emission from the SACM kiln. Additionally, the mass proportion of SO₄^2- in the CPM reached 93.8%, thus indicating that large quantities of sulfuric acid mist or SO₃ were emitted. CaSO₄ was one key component for the formation of main mineral ye'elimite (3CaO·3Al₂O₃·CaSO₄), and its decomposition probably led to the high SO₄^2- emission. Furthermore, the use of CaSO₄ as a calcium source led to SO₄^2- emission factor much higher than conventional calcium sources. Higher calcination temperature and more residence time also increased SO₄^2- emission. The most abundant heavy metal in kiln flue gas and CPM was Zn. However, the total condensation ratio of heavy metals detected was only 40.5%. CPM particles with diameters below 2.5 μm and 4-20 μm were both clearly observed, and components such as Na₂SO₄ and NaCl were conformed. This work contributes to the understanding of CPM emissions and the establishment of pollutant reduction strategies for waste collaborative disposal in cement industry.

Keywords: Condensable particulate matter; Heavy metal; Solid waste utilization; Sulfoaluminate cement; Sulfur.