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 Si, 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.
