Coal
and biomass co-combustion in existing utility boilers is a
promising option of mitigating the fossil energy crisis and reducing
the gaseous emissions of NO
x
, SO
x
, and CO2. However, ash-related problems,
including fouling, slagging, and corrosion cause damage to the heat
exchange tube and reduce boiler efficiency. In an attempt to give
better insights into the slagging behavior during coal/biomass combustion,
an experimental investigation was conducted to study the growth of
slag when coal was co-fired with wood and corn stalk in a 300 kW pilot-scale
furnace. For comparison, combustion of pure coal was also conducted.
During the experiments, biomass proportions of 5 and 10% by weight
were examined. Slags formed on an oil-cooled deposition probe were
collected, sampled, and analyzed using scanning electron microscopy
and X-ray diffraction (XRD). The change in slag thickness with time
was obtained by a charge-coupled device monitoring system. With two
thermocouples in the probe, the heat flux through the slag could be
measured. The slag from pure coal combustion showed a layered structure
with different levels of compactness and hardness. The heat flux decreased
by 31.7% as the slag grew to 5.19 mm. The results showed that co-firing
wood significantly inhibited the slagging behavior. Especially in
the 10% wood case, hardly any slag was collected from the probe. Nevertheless,
co-firing corn stalk resulted in severe slagging, with a slag thickness
of 5.5 and 6.1 mm for two blend ratios. The formation of bubbles in
the deposits together with greater deposit thickness caused heat transfer
deterioration. XRD results revealed that the influence of co-firing
biomass and corn stalk caused quite different changes to mineral species
from wood. It was observed that fly ash under different biomass co-firing
conditions differed little on mineral compositions.