Performance Assessment of a Vertical Pneumatic Conveying System for High Temperature Solar Particle Receivers Using a New Correlation for Minimum Transport Boundary

Solar
particle receivers for concentrated solar thermal (CST) have been widely
investigated due to the high efficiency of direct heat transfer. In addition,
particles, such as mineral sands and ceramics have high melting temperature,
which allows them to be used directly as heat transfer medium at above >800ºC. However, to
realize the application of particle receivers in tower-mount CST plants
requires the transportation of particles to and from the tower. Pneumatic
conveying systems are proven to be industrially robust, having been widely
implemented for over 100 years, however, no assessment of the potential to
employ pneumatic conveying system under relevant to solar particle receivers has
been reported previously. The minimum transport boundary is an important information
required for the design and operation of a vertical pneumatic conveying system to
ensure both a continuous and steady-state transport with minimum parasitic
losses. The information on the minimum transport boundary for the suitable
particles under CST operating conditions is not available. Beyond this minimum
transport boundary, further increase in solids flow or decrease in gas velocity
could result in accumulation of solids at the bottom, which is defined as Type A chocking. Correlations have been suggested in the
literature to predict Type A chocking. However, most
of these correlations are only valid for particular experimental conditions,
e.g., particle properties and conveying pipe diameter. In particular,
relatively big deviation (>40%) was found by using these correlations for Geldart group B particles.

In the
present study, a correlation, as shown in Eq 1, is
developed to predict Type A chocking velocity (U_ck)
for the selected Geldart group A and B particles, which
suitable under CST environment, with a reasonable accuracy within 30%. The
indexes a, b and c are all assumed
to be governed by the Archimedes number (Ar). The value of b can be calculated for each Ar by the dimensionless
velocity ((U_ck
-U_t)/U_t, U_t is
terminal velocity) and (G/ρ_pU_t) for the selected particle
properties (particle density (ρ_p) and particle size (d_p))
and pipe diameter (D). Similarly the
correlation of c with Ar can calculated
by the dimensionless velocity and (D/d_p)
for each Ar. For each dimensionless
velocity, the value can be calculated for index a using the obtained values of b
and c. Thus, a correlation of a with Ar can be calcuated. The correlations for indexes a, b
and c are shown in Eqs 2-4. Comparison of present calculated
 chocking velocities with
experimental chocking velocities in literature is shown in Figure 1.  Therefore, further assessment on the
performance of vertical pneumatic conveying systems can be conducted using this
correlation.

Uck-UtUt=a×(GρpUt)b×(Ddp)c
                    (1)

a=46000×Ar-2.08    Ar<21412.5×Ar-0.55       Ar>214
          (2)

b=0.06×lnAr+0.08
                       (3)

c=0.2733×lnAr-0.9844
              (4)

Figure 1 Comparison of the calculated chocking
velocities with experimental chocking velocities reported from literature.