Chemical Engineering Journal 179 (2012) 63–71 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal j ourna l ho mepage: www.elsevier.com/locate/cej Regeneration of a spent alkaline solution from a biogas upgrading unit by carbonation of APC residues Renato Baciocchi a,∗ , Giulia Costa a , Renato Gavasci a , Lidia Lombardi b , Daniela Zingaretti a a Department of Civil Engineering, University of Rome “Tor Vergata”, Via del Politecnico 1, 00133 Rome, Italy b Department of Energy Engineering “Sergio Stecco”, University of Florence, Via di Santa Marta 3, 50139 Florence, Italy a r t i c l e i n f o Article history: Received 4 August 2011 Received in revised form 17 October 2011 Accepted 18 October 2011 Keywords: Carbonation Causticization CO2 storage Landfill gas upgrading Leaching behavior a b s t r a c t This work deals with the regeneration of a KOH solution to use in a chemical absorption process for biogas upgrading. The proposed method consists in the caustic recovery of the spent absorption solution by carbonation of selected industrial residues and allows to store the separated CO 2 in a solid phase (calcite). This paper presents the main results of the lab scale experiments carried out to evaluate the effects of selected operating parameters on the carbonation reaction so to identify the conditions that allow to maximize the CO 2 uptake of the solid residues and the percentage of KOH that can be regenerated for the absorption process. Furthermore, the effects of the tested treatments on the leaching behavior of the residues are also examined. The results presented in this paper were obtained within the UPGAS-LOWCO 2 project (LIFE08 ENV/IT/000429) funded by the European Commission. © 2011 Elsevier B.V. All rights reserved. 1. Introduction According to the current European waste policy, which is based on the waste-management hierarchy concept, landfilling should be considered as the last option to adopt once all valorization oppor- tunities have been fully exploited. However, waste landfilling can also be viewed as a energy recovery treatment due to the possibil- ity of using the biogas produced from anaerobic digestion. Indeed, the major components of landfill gas are methane (40–60%), car- bon dioxide (35–50%) and nitrogen (0–20%) [1]. Currently this gas is mainly used for heat and power generation, nevertheless in some EU countries, such as Switzerland and Sweden, landfill gas is being increasingly used as vehicle fuel [2]. The commercial recovery of CH 4 from landfill gas has been practiced at full scale since 1975 and currently globally exceeds 105 Mt CO 2 -equivalent/year [3]. For some applications, such as injection in the gas grid and utilization as vehicle fuel, biogas pre-treatment is required for removing harmful trace components such as H 2 S, O 2 and dust (cleaning process) and for increasing the calorific value and relative gas density (upgrad- ing process) [4]. In particular this latter process consists basically in the removal of CO 2 from landfill gas and is mainly performed by physical or chemical absorption, pressure swing adsorption or membrane separation [4]. It should be noted that the CO 2 separated from the upgraded gas with any of the above mentioned methods is generally released into the atmosphere. Since this carbon dioxide is ∗ Corresponding author. Tel.: +39 06 72597022; fax: +39 06 72597021. E-mail address: baciocchi@ing.uniroma2.it (R. Baciocchi). of biogenic origin, its emissions may be regarded as climate neutral and, therefore, its definitive storage may be considered an interest- ing strategy in view of achieving the goal of negative emissions. In a former LIFE+ project funded by the European Commis- sion (GHERL-GreenHouse Effect Reduction from Landfills, LIFE05 ENV/IT/000874) the feasibility of a process based on chemical absorption with an aqueous solution of potassium hydroxide (KOH) for upgrading landfill gas and capturing CO 2 was demonstrated with the construction and operation of a pilot-scale plant in a landfill site in Italy [5]. This process leads to the production of a spent absorption solution containing dissolved potassium carbon- ate (K 2 CO 3 ); no further CO 2 storage processes or reuse options for the K 2 CO 3 solution were investigated in the GHERL project. A sim- ilar reaction was used in several processes developed to separate carbon dioxide from atmospheric air by absorption with alkaline solutions (e.g. [6–13]). The main limit to the application of this technique as a separation method for atmospheric air or landfill gas is the high cost of the alkaline reagent that implies the need of regenerating the spent absorption solution, rich in potassium carbonate. Regarding CO 2 capture from air, Lackner et al. [8,9], for example, proposed a process whereby carbon dioxide is absorbed by a sodium hydroxide solution forming sodium carbonate that is then regenerated by adding calcium hydroxide and precipitating calcium carbonate. Thereafter, the precipitate is conveyed to a cal- ciner where quicklime and carbon dioxide are obtained, whereas the sodium hydroxide containing mother liquor is recycled back to the absorption column. The most energy intensive step of this process is calcium carbonate calcination since a large energy cost is required to undo the strong binding of the CO 2 [14]. 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.10.051