Page 140 - New Trends In Coal Conversion

Coal and biomass cofiring: CFD modeling 103

Such a model for tracking large, nonspherical particles in dilute two-phase flow is
described in detail in (Yin et al., 2003). The model is validated by experiments before it
is refined for biomass cofiring modeling (Yin et al., 2004). Here, a few results are given
as examples. Fig. 4.7 shows the model-predicted and experiment-observed motion
pattern of a cylindrical PVC particle (5.41 mm in diameter, 50 mm in length, and
3
1366 kg/m in density) released from rest in originally stagnant water and the final wa-
ter flow due to the coupling between the particle and the water. The cylindrical particle
is found to have a remarkable lateral motion along its settling in the water tank.

(a)
0 (c)
–0.1
Vertical position, Z(m) –0.3 (solid triangle)
–0.2
–0.4
Measured
–0.5
–0.6
–0.7
0 1 2 3 Calculated (line) 5 6
4
Time (s)
Translation motion
Initial
(b) particle
150 position
Calculated incidence angle
Angle (degree) 120
90
60
30 Angle between particle z’-axis and inertial y-axis Z
Measured
Calculated
0 Y
0 1 2 3 4 5
Time (s) Final water flow pattern and
Rotation motion the particle trajectory
Figure 4.7 Motion of a large cylindrical PVC particle in originally stagnant water. (a) Particle
positions; (b) Particle orientation; (c) Final water flow pattern and the particle trajectory.

4.4.3 Special modeling issue: conversion of large biomass
particles

A fuel particle experiences different subprocesses in its conversion, such as drying, py-
rolysis, combustion of the released volatiles, and char reactions, as seen in Fig. 4.8a.In
the traditional modeling method, the sequential conversion pattern is used by assuming
particles under an isothermal condition, as illustrated in Fig. 4.8b. Such an assumption
may not be valid for large, highly nonspherical biomass particles, in which the
different subprocesses can occur simultaneously, as seen in Fig. 4.8c.
To investigate the impact of intraparticle heat and mass transfer on conversion of
large biomass particles, a 1D particle conversion model is developed and implemented