2.2. Fuels and Bed Materials. The overbed feeding system is
employed. The fuels stored in two hoppers fall directly through
the screw feeders and an air lock and, then, by a 70° cooled chute
into the fluidized bed chamber at 0.6 m above the distributor. Rice
husk, soybean, and high sulfur sub-bituminous coal are used as
the feeding material. The total heat (wet base) feed is kept as 150 00
kcal/h. The proximate and ultimate analyses of feeding material
are listed in Table 1.
Flue gas leaving the combustor enters two cyclones arranged in
series for the primary cleaning. Ash and unburned char drop from
each cyclone into a sealed vessel for removal and analysis. After
exiting from the cyclone, the flue gas enters a shell-and-tube heat
exchanger and then passes to a venturi scrubber. Finally, the flue
gas is discharged via the stack.
Silica sand is employed as the bed material in this study. The
sand particle size range is 163–920 μm, and the mean size of the
sand is 486 μm in diameter. The operating conditions for experiments
are shown in Table 2. It should be noted that we change the
total excess air by changing the primary or the second air flow rate
in our experiment. The following definitions were used:13,14
stoichiometry)(total air flow rate)/(stoichiometric air flow rate)
excess air (%))(stoichiometry-1)·100
3. Results and Discussion
3.1. Effects of Bed Temperature. A typical temperature
distribution within the VFBC at different bed temperature is
presented in Figure 3. Figure 3 shows that the temperature within
the VFBC increases with an increasing bed temperature. The
temperature in the freeboard is higher than that within the bed,
and the highest temperature is found at the position above the
second air injection. Figure 4 shows the radial temperature
distribution at the freeboard. It can be seen that the temperature
below the second air injection is lower than that above the
second air injection. In the case of the second air injection by
vortexing, the contact opportunity between oxygen and volatile
increases due to the turbulence and, consequently, the temperature
increases. A clear tendency also can be seen, i.e., the
temperature close to the wall is lower than that close to the
center of the freeboard. This can be attributed to the heat loss
from the combustor wall.
Figure 5 shows the effect of the bed temperature on NO and
N2O emission when rice husk is used as the feeding fuel.
Usually, the NO emission increases with the bed temperature.
However, from Figure 5, we can find that it exhibits a levelingoff
trend. This can be attributed to the adequate residence time
within the combustor to reach the equilibrium of the formation
and reduction reactions of NO.