Chemical Composition of Submicrometer Particulate Matter (PM1) Emitted from Combustion of Coals of Various Ranks in O2/N2and O2/CO2Environments

A laboratory-scale investigation has been conducted on the physical and chemical characteristics of particulate matter emissions (ashes) from pulverized coals burning in the air or in simulated oxy-fuel environments. Oxy-fuel combustion is a process that takes place in O2/CO2 gases, using an air separation unit (ASU) to supply the oxygen and a flue-gas recirculation (FGR) stream to supply the carbon dioxide to the boiler. In order to investigate the effects of the background gas on the particulate matter generated by the combustion of coals of different ranks, a bituminous, a sub-bituminous, and a lignite coal were burned in an electrically heated laminar-flow drop-tube furnace (DTF) in both O2/N2 and O2/CO2 environments (21% < O2 <60%). A recent publication by the authors reports on the physical characteristics of the particulate matter; hence, this work focuses on the chemical composition, specifically targeting the difficult-to-capture submicrometer size (PM1) ashes. Particulate matter was collected by a low-pressure multistage cascade impactor and was analyzed for chemical composition by Scanning Electron Microscopy−Energy Dispersive X-ray Spectroscopy (SEM-EDS). Selected samples were also examined by Electron Microprobe Analysis (EMA). Results showed that submicrometer (PM1) ashes of the bituminous, the sub-bituminous, and the lignite coals contained mostly Si, Al, Fe, Mg, Ca, K, Na, and S. Prominent components of large submicrometer particle (PM0.56−1) compositions were Si and Al (Ca in sub-bituminous), whereas small submicrometer particles (PM0.1−0.18) were markedly enriched in S. The mass yields of elemental species found in the submicrometer-size particles from all three coals were lower when combustion occurred in CO2, instead of N2 background gases. The chemical composition of the PM0.56−1 subcategory was not affected by the background gas. To the contrary, the composition of the PM0.1−0.18 subcategory was affected by replacing N2 with CO2, and mass fractions of i, Ca, and Al decreased whereas Na, K, and S increased. Furthermore, in PM0.1−0.18, when the O2 mole fraction increased in either N2 or CO2, the mass fractions of Si, Ca, and Al increased at the expense mostly of Na, K, and S, but also Fe in the case of the sub-bituminous coal. Experimentally derived partial pressures of the volatile suboxide SiO (PSiO) at the char surface were compared with the predictions of an ash vaporization model without and with coupling with a particle combustion model; they were found to be in the range of the model predictions.