D5.1: Kinetic rates for calcination and carbonation of Vernasca raw meal

This Deliverable reports the results obtained from the tests carried out with samples of Vernasca raw meal (and derived solids) supplied by Buzzi. The goal of the tests is to quantify the kinetic rates of calcination and carbonation under conditions as close as possible to those expected in the Cleanker Calcium Looping pilot, which will use Vernasca raw meal as solid feed. This task has been accomplished taking into account the different behaviour of the three main components of Vernasca raw meal in the calcium looping system: the fraction of CaCO₃-rich particles (limestone), the fraction of Ca-Si particles (the marl) and the fraction of inert materials that do not participate in the reactions with CO₂ (but that may sequester part of the free CaO to form stable compounds). Vernasca is mainly a binary mixture 0.72 wt/wt of a marl (Marine) and 0.28 wt/wt of a limestone (Calcare).

Two thermogravimetric set ups have been used to determine the evolution with time of both carbonation and calcination conversions of Vernasca at different temperatures and partial pressure of CO₂, as well as the belite formation and the decay constants that describe the drop in the maximum carrying capacity of the sorbent with the number of calcination-carbonation cycles. The decay constants determined for the Vernasca limestone fraction (Calcare) (k = 0.57 and Xr = 0.060) are shown to be comparable to typical values reported for other standard limestones. However, the raw meal has yielded much more complex behaviour with the operating conditions (i.e. the CO₂ carrying capacities was shown to be especially sensitive to calcination times and temperatures) due to belite formation (and other silicates) detectable by XRD analyses. Thus, the belite formation rate was investigated independently and fitted to the Jander´s kinetic model, resulting in a calculated activation energy of 336 kJ/mol, which is similar to that reported in the literature for CaO-SiO₂ mixtures (i.e. no raw meals) at higher temperatures (between 1000 and 1500 ºC).

On the other hand, the activation energy derived for the calcination reaction,  obtained from TG experiments with Vernasca solids (85 kJ/mol) has also been found to be consistent with some previously published data. However, the pre-exponential factors (and hence the overall reactions rates) are subjected to great uncertainty if only the TG measurements are used as a basis to derive kinetic parameters, as there are signs indicating substantial undesired effects in the TG reaction environment compared with the entrained bed results. Regarding the carbonation kinetics, the parameters obtained are an activation energy of 26 kJ/mol and a pre-exponential factor of 4.51 s^-1.

A drop tube furnace has been commissioned to simulate the reaction environments of the solids as close as possible to those expected in the pilot and to generate calcined solid samples of Vernasca and other raw meals in time scales of 1-5 seconds, thereby decoupling the calcination reaction from the belite formation reactions. Although results from this facility were not originally planned for this deliverable, it is important to note here that the initial results have confirmed the high activity and CO₂ carrying capacity of the calcined raw meal of Vernasca when calcination is sufficiently fast to minimise reactions of CaO with other solid species in the marl component of the raw meal. From these preliminary tests, it was also found that calcination kinetics can be an order of magnitude faster than the kinetics predicted by the simplified kinetic correlations derived for this reaction in the TGA. Therefore, a re-adjustment of the kinetic parameters has been carried out after the experimental campaign in this reactor,  to tune the calcination model to the observed experimental results. As planned in the workplan for WP5, future experimental and modelling work with other materials and raw meals, at relevant calcination and carbonation conditions and reaction times, should improve the understanding of the results described in this deliverable. However, the information reported here for Vernasca raw meal should be taken into account in the design and operation strategies of the Cleanker pilot.