Employing a step-wise titration method under semi-slow reaction regime for evaluating the reactivity of limestone and dolomite in acidic environment

Carbonate rocks are commonly utilized in Wet Flue Gas Desulfurization, WFGD, because of their capability to release calcium ions and precipitate as solid gypsum in an acidic environment. Studies on the reactivity of carbonate rocks and dissolution models can be employed for optimizing the WFGD process. The correct evaluation of limestone reactivity is therefore necessary for the design of the WFGD scrubbing process and for plant operation. In this study, after statistical considerations on evaluating the sample size threshold, a mathematical model and a detailed procedure are given for the estimation of the reaction rate constant and mass transfer coefficient. Results are reported from testing limestone and dolomite samples with different formation periods and geological backgrounds. Samples were tested in a Batch Stirred Tank Reactor (BSTR) with a stepwise titration method using hydrochloric acid and non-steady state conditions. In the experiments particles were shown to be completely immersed in a defined viscous sub-layer. A parametric evaluation for the reaction rate was performed at each titration step using an implemented software procedure that handles hundreds of pH values and more than fifty particle size ranges. The experimental data were accurately fitted to the model. The second order model for dolomite and limestone samples accounts for both mass transfer and reaction rate terms, yielding values for the mass transfer coefficients that are congruent with values estimated by fluid-dynamics inspection.