Kinetics of Flue Gas Desulfurization (Dry Scrubbing)
Simulating the dry scrubbing process
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.  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.
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 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.
Rate Expression Since, by Arhenius relation, which, again, is confirmed, 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.
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.
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.
Pure CaCO3 sampleSlurry sample (80% CaCO3 ) Plotted in Matlab 6.1
Look Ahead… Further simulations will be aimed at refining the current model and cosideration of roughness factor, qo (availability), etc. into consideration. Also, the removal efficiency is expected to reach a steady value after long time, which needs to be found out with further simulations. Reference: Klingspor et al. A KINETIC STUDY OF THE DRY SO2- LIMESTONE REACTION AT LOW TEMPERATURE.Chem. Eng. Commun. Vol. 22, pp. 81-103.