Occurrence of Inorganic Elements in Condensed Volatile Matter Emitted from Coal Pyrolysis and Their Contributions to the Formation of Ultrafine Particulates during Coal Combustion

Abstract

Coal pyrolysis is the first step during coal combustion, when the injected coal particles swell to release the volatile matter (VM) at a very short residence time. Simultaneously, the organically bound fraction of inherent metals is likely emitted out too. To prove the presence of organically bound metals in coals, five bituminous coals and one anthracite coal from China were pyrolyzed in N2 in a lab-scale drop tube furnace. The gas temperature in furnace was about 900−1400 K so that almost all the inorganic elements except those containing Na hardly vaporized. The emitted VM was collected by a low-pressure impactor. The results indicate that the condensed VM (CVM) smaller than 1.0 μm has an amorphous carbon structure, which contains the inorganic elements too. Sulfur is the most prevalent, followed by sodium, silicon, chlorine, calcium, and others in the decreasing order. Apart from a portion of sodium in form of NaCl, all the inorganic elements are organically bound with CVM as determined by both TEM-EDS and XPS. These elements disperse highly in CVM; their oxidation and coagulation during VM combustion likely contribute to the majority of ultrafine particulates (PM0.1 smaller than 0.1 μm) formed during coal combustion at a relatively low temperature, 1473 K.