Results from A1 - UNIROMA

The aim of Action A1, with regard to the activities of UNIROMA, was to carry out the preliminary evaluations necessary to define the layout and the operating conditions of the pilot-scale regeneration process to treat the spent solution produced by the biogas upgrading absorption unit. The second objective of the regeneration process is to store the CO2 captured in the absorption solution in a solid, thermodynamically stable and environmentally sound form, such as calcium carbonate. The process selected to regenerate the spent absorption solution is based on the causticizing reaction that is used in the chemical recovery process of the kraft pulping method to produce precipitated calcium carbonate. In this process, calcium hydroxide (Ca(OH)2) is added to an alkaline carbonate (Na2CO3 or K2CO3) solution, producing the corresponding hydroxide (NaOH or KOH) solution and precipitating calcium carbonate (CaCO3). Since the use of raw calcium hydroxide for such a process would not make sense from a CO2 mitigation perspective, as it is manufactured by calcination of limestone, in order to obtain a net reduction of CO2 emissions, in this project industrial waste residues were chosen as alternative alkalinity sources.

The activities carried out by UNIROMA during phase A1 were subdivided into the following sub-actions.

• SUB-ACTION A1.1: Selection and characterization of different alkaline residues for the regeneration process.

This phase consisted in the selection and characterization of different types of alkaline industrial residues, including the bottom ash disposed of at Casa Rota landfill, to use in the regeneration unit. Physical, chemical and mineralogical characterization tests were performed on the selected types of industrial waste to identify and possibly quantify the phases contained in each type of residue of interest in view of the proposed regeneration process. In particular, the elemental composition, loss on ignition, calcite content, acid neutralization capacity and mineralogy of specific grain size fractions of the selected types of residues were determined. The leaching behavior of each type of tested material was also investigated by carrying out standard compliance tests (EN 12457) to identify the most critical contaminants for the management of each type of material with respect to landfill acceptance criteria.

• SUB-ACTION A1.2: Study and optimization of the regeneration process.

The second part of the experimental activities carried by UNIROMA during phase A1, was aimed at investigating the effects of the main operating parameters on the outcome of the regeneration process in terms of the amount of hydroxide regenerated for the absorption unit and of the CO2 uptake obtained in the solid product.

Preliminary regeneration tests were performed using Ca(OH)2 as a reference material with the aim of evaluating the effect of the main operating parameters on process kinetics.

Subsequently, separate regeneration experiments were carried out using each of the different types of characterized solid residues in order to compare the achieved results (hydroxide regeneration in the solution and CO2 uptake in the solid product) and to select the most reactive material for the pilot-scale tests to be performed in the next phases of the project. In particular, the examined residues were classified in two categories: those that exhibited a high content of Ca hydroxide phases (free CaO-rich residues) and those that were mainly made up by silicate phases (Ca silicate rich residues). For each of these two types of materials, the effects exerted by varying the main operating conditions (specifically: temperature and amount of residues) on the kinetics of the regeneration process were investigated.

Batch slurry-phase regeneration experiments were carried out in a 300 ml reactor contacting a fixed amount of residues or Ca(OH)2 with a solution simulating the liquid outlet stream of the absorption unit (eventually containing unreacted hydroxide phases).

Most experiments were carried out feeding a K2CO3/KOH solution to the regeneration reactor, i.e. assuming the reagent (KOH) used in the absorption unit to be the one used in the absorption pilot plant designed and operated by DEF. These experiments were performed to assess the reactivity of the different materials and to identify the operating conditions that allowed to maximize results both in terms of KOH regeneration and of CO2 uptake.

Experiments were then carried out, applying the operating conditions that allowed to obtain the best results and using the type of residues that proved the most reactive, varying the composition of the absorption solution, since DEF is testing the feasibility of using also other reagents for the absorption stage. Regeneration tests were performed with a Na2CO3 solution, as well as with a spent solution sample from the absorption unit.

For the residues that showed the highest reactivity in the lab-scale regeneration tests, the leaching behavior of the solid product obtained by the regeneration process was examined in order to assess whether the treatment exerted relevant effects on trace element mobility and if further treatment of the solid materials would be required prior to its final disposal.

The chemical composition of the liquid products of the regeneration step obtained under selected operating conditions was also analyzed as a further verification of the obtained results and to identify the main elements contained in each effluent of the proposed regeneration process.