Zero-Emission CH4-poor Biogas Utilization through mineral carbonation (POR CReO FESR 2007-2013)
The companies/organizations involved in the Project are:
- West Systems S.r.l (Project Leader)
- P&I S.r.l.
- STEAM S.r.l.
- MASSA Spin-off S.r.l.
- CNR-IGG Institute of Geo-sciences and Geo-resources
Aim of the Project
Tuscany has a significant number of MSW landfills whose dimensions and/or waste typology don’t allow the production of biogas with sufficiently high methane concentration. Such features don’t allow the production of electricity. Usually this methane poor biogas (<30%), typically present in post-closure landfills, is collected and sent to a torch combustion system. So, in addition to the missed energetic use of methane, a large quantity of CO2 is released into the atmosphere.
Starting from this reference context, ZEBU project (in its current form) will performCO2 collection, through ammonia solution, in a two-column absorption system. By this process the biogas is enriched in CH4 and an ammonium carbonate solution is simultaneously obtained. Enriched biogas is susceptible to energy utilization, while the ammonium carbonate solution is mixed with a magnesium chloride solution inside a carbonation reactor. Here the precipitation of magnesium carbonates occurs and the initially gaseous CO2 contained in the biogas is trapped inside a stable crystal lattice [like that of Magnesite (MgCO3) or Nesquehonite (MgCO3-3H2O).
At present, the necessarymagnesium chloride solution is obtained by simple dissolution of salt into demineralized water. However, in the future, it will be possible to produce it though an acid attack of HCl at 70-80°C on serpentinite rocks, and subsequent adjustment of the pH (as it has been experimentally proved). Moreover the mineral reactant involved in this future technique (serpentine),includes, among its varieties, the fibrous state, better known as asbestos (sadly notorious for its danger to human health). Therefore in the future mineral carbonation processes could also be applied to asbestos treatment and stabilization.
ZEBU project involves both economic advantages (in terms of biogas recoveryfor energetic purposes and of potential use of the produced magnesium carbonate) and a positive effect on the environment (reduction of CO2amount released into the atmosphere). The pilot system (including CO2 absorption columns and carbonation reactor) designed and developed within ZEBU project, is susceptible to industrial adaptation and could be implemented to all regional MSW landfills, with considerable economic and environmental benefits.
The main innovation of this research project is the basic idea: use of methane poor biogas (<30%), typical of post-closure MSW landfills. Currently this biogas is collected and sent to a torch combustion system. Besides the missed energetic use of methane, the process releases large amounts of CO2 into the atmosphere.
A second innovation is the acquired experience on CO2 collection by means of ammonia solution. This technique, already used in other contexts and subject of a renewed attention by scientific literature, has never been applied to biogas.
A third innovation lies in the investigation related to optimal temperature, pressure and PCO2 conditions for the completion of serpentine mineral carbonation reactions. It must be underlined that this process could be applied in the future, to asbestos treatment and stabilization.
Finally, more possible uses of produced magnesium carbonate (for the preparation of catalysts in chemical processes, as insulator, additive for paint and ink, component for glass and ceramic, etc…) will allow to turn current wastes, which heavily affectair quality, into future profitable and ecological resources.
During the ZEBU project no unexpected problems occurred. The only expected difficulty was represented by the approach to serpentine mineral carbonation and by the decision to operate through acid attackrather than by means of gas-solid reaction. This choice was suggested by the results of experimental research carried out within the project (by external heating autoclave, stirred micro-reactor and other instruments). It was showed that the gas-solid reaction required very high pressure (several hundred bars) and therefore it represented an obstacle for the construction of industrial plants based on this approach. One of the possible alternative solutions (serpentinite dissolution in strong acids 2-4M at 70-80° and further adjustment of pH values around 9 and CO2 addition) appeared to be applicable on industrial scale and was therefore adopted.
Due to this process complexityalong withtime and financial constraints, only some parts of the system were implemented(in particular those related to CO2 absorption, to obtain CH4 enriched biogas, and precipitation of magnesium carbonate). These are the two main challenges that will have to be experimentally verified on industrial scale. Engineering the serpentinite grinding phase wasn’t considered appropriate, for the high cost of the necessary equipment to avoid fine fibrous dust dispersion, and because it wouldn’tprovide interesting results for the project. Furthermore, this kind of plant wasn’t authorized by the Municipality of Montespertoli, where the experimental phase was expected to be performed.
Forsimilar reasons the engineering of serpentinite dissolution wasn’t performed, since the problemsthat needed to be solvedwere related only to solids separation (which can be performed by filtration, a rather easy operation):
(i) during incongruent dissolution of minerals in the serpentinite (not only serpentine but also magnetite, pyroxenes, etc..) accompanied by amorphous silica precipitation
(ii) during the subsequent aqueous solution neutralization (up to pH values in the range 8.5-9) by base addiction (NH4OH or NaOH); during this phase different solid precipitations occur, mainly of trivalent iron oxy-hydroxides.
Project outreach: presentation at conferences and publications
Some of the achieved results of ZEBU project have been or will be introduced at the following meetings:
(i) Orlando A., Borrini D., Marmottini F., Montegrossi G., Ruggieri G., Tassi F., Vaselli O., Raco B., Marini L. (2010) Experimental studies on ex-situ carbonation of serpentinites. 89° Congresso SIMP, “L’evoluzione del Sistema Terra dagli atomi ai vulcani”, Ferrara 13-15 Settembre 2010.
(ii) Orlando A., Iacopini M., Lelli M., Marini L., Raco B. (2011) Dissolution and carbonation of a serpentinite: inferences from experiments up to 300 °C and 30 MPa. In: Simposio D6 – Stoccaggio geologico e mineralogico della CO2: stato dell'arte e prospettive future. Geoitalia 2011 - VIII Forum Italiano di Scienze della Terra. Torino, 19-23 settembre 2011.
(iii) Virgili G., Minardi I., Marini L., Raco B., Valleggi G., Masoni L. (2012) Zero-Emission methane-poor Biogas Utilization through mineral carbonation: the ZEBU project. First International Conference on Environmental Challenges in Arid Regions. February 6-8, 2012, Jeddah, Saudi Arabia. (submitted).
In addition, the results of laboratory experiments have been published in the following article in international ISI journals:
Orlando A., Borrini D., Marini L. (2011) Dissolution and carbonation of a serpentinite: inferences from acid attack and high P-T experiments performed in aqueous solutions at variable salinity. Applied Geochemistry, 26 1569–1583.