1. Simulating the dry scrubbing process

2. Dry scrubbing
 Dry scrubbing involves the reaction
CaCO3(s) +SO2(g) +O2(g)  CaSO4(s) +CO2(g)
 Since Marble slurry contains ~80% CaCO3 (wt % after loss on
ignition), its kinetics was assumed to be similar to dry scrubbing
process at high temperatures.
 Recent study indicates the reaction rate depends on factors such
as SO2 conc., Bed conc. of CaCO3 ,specific humidity,
temperature, roughness factor of bed etc. [1]
 The purpose of the current simulations was to determine the
change in SO2 removal efficiency when pure CaCO3 was
introduced w.r.t. slurry sample (~80% CaCO3 ).
 Hence, all other factors were treated as constants.
 A very detailed approach to the reaction rate expression is
derived in next few slides.

3. Rate expression
 Rate of consumption of CaCO3 –
 Where ,
 Here, qo = 8.7 mol/kg, is the number of moles of CaCO3
available for reaction. Above value is for limestone, which
was taken to be similar for the case of dried slurry, due to
its resemblance to limestone.
 k is the rate constant which depends on relative humidity,
and temperature.
 The article[1] tabulates the values of k for some values of
p/po .
 It is discussed in detail that ‘n’ varies for different values of
p/po , and a typical value is n=0 for p/po =0.69, where,
k(343K) = 1.7 mol/kg-hr.

4. Rate Expression
 Since, by Arhenius relation, which, again, is
confirmed[1],
 So we have k873 =227.494 mol/kg-hr.
 Thus we have finally,
 This is the rate expression that was coded in FLUENT
software, for the solution to be obtained for
comparison of pure CaCO3 and Slurry.

5. Simulation in FLUENT
 The model for the simulation was adopted as-
 Laminar, incompressible, multiphase flow, with
primary phase as gas and secondary phase as packed
bed containing either CaCO3 or CaSO4 formed from
reaction. The gas phase included three fluids, CO2,
SO2, and O2 . Total components of air was not
modelled due to difficulty in solution convergence.
 Each phase solves species-transport equations to find
mass-fraction of component in each phase.
 To model Slurry with 20% impurity, the idea that
CaSO4 once formed acts as only inert impurity in
simulation since back reaction is not modelled, the
initial setup in the packed bed was taken as 80%
CaCO3 by weight.

6. Setup of packed bed

7. Velocity Profile in Gas phase

8. Post-Processing
 To quantify the efficiency of both the samples, the mass
fraction of CO2 at the outlet was calculated when the inlet
flow was assigned as 0.5 m/s (gas phase) and 67% SO2 ,
33% O2 .
 The plot of CO2 m-f Vs. time was obtained for a period of
55 secs.
 It is seen clearly that the production of CO2 , i.e inverse SO2
removal efficiency settles lower in case of Slurry sample
than pure sample, however the argument is that the
percentage drop even over a long period of time is not very
high.
 This gives an indication that the compromise on efficiency
of FGD processes by the use of Marble Slurry is worth
taking in face of the opportunity to prevent a serious
environmental hazard, due to wasteful dumping of Marble
Slurry.

9. Pure CaCO3 sample
Slurry sample (80% CaCO3 )
Plotted in Matlab 6.1

10. Look Ahead…
 Further simulations will be aimed at refining the
current model and cosideration of roughness factor[1],
qo (availability), etc. into consideration.
 Also, the removal efficiency is expected to reach a
steady value after long time[1], which needs to be
found out with further simulations.
 Reference:
[1] Klingspor et al. A KINETIC STUDY OF THE DRY SO2-
LIMESTONE REACTION AT LOW TEMPERATURE.
Chem. Eng. Commun. Vol. 22, pp. 81-103.