Cattura e stoccaggio a lungo termine della CO 2 via mineralizzazione

Transcription

1 Cattura e stoccaggio a lungo termine della CO 2 via mineralizzazione The long-term sequestration of CO 2 in solid form: the application of nesquehonite as a complementary solution VINCENZO FERRINI, CATERINA DE VITO, SILVANO MIGNARDI Dipartimento di Scienze della Terra, Università di Roma La Sapienza, P.le A. Moro, Roma, Italy vincenzo.ferrini@uniroma1.it, cdevito@uniroma1.it, silvano.mignardi@uniroma1.it 1

2 ...alarming news We are all bombarded daily by alarming news about the rising levels of CO 2 in the atmosphere (about 380 ppm), caused largely by the combustion of fossil fuels, and about the deleterious impact of such loadings on our climate. According to the IPCC (Intergovernmental Panel on Climate Change), new emissions for the period were about 6 Gt C/year (23.5 Gt CO 2 /year). The scientific community must seek to implement quickly the results of research on effective methods of sequestering CO 2. 2

3 Approaches in sequestration The numerous approaches in sequestration of carbon dioxide (CO 2 ) currently in vogue are: ocean terrestrial geological biological chemical 3

4 Numerous approaches to CO 2 sequestration are currently being studied and the retention or sequestration of CO 2 in geological reservoirs is currently the option being applied [e.g., Weyburn, Canada; Sleipner, North Sea]. Complementary technologies applied to CO 2 sequestration, such as neoformation of minerals via reaction of CO 2 with Mg-Ca silicate rocks or of carbonate minerals in aqueous solutions, offer attractive options for the permanent and safe storage of CO 2 in a solid form. There is an advantage in focusing on Mg, because a greater weight proportion of CO 2 is found in Mg carbonates than in Ca carbonates and the higher stability of Mg carbonate than Ca carbonate in order to dispose CO 2 (Teir et al. 2006). 4

5 ...more about reaction of CO 2 with Mg-Ca silicate rocks The carbonation process occurs naturally on a small scale during the weathering of rocks, and has been shown to be important locally in ultrabasic and ophiolitic complexes. Unfortunately, the industrial extraction of Ca and Mg from silicate minerals requires expensive pre-processing, which contributes to the problem rather than to the solution. Furthermore, this option is not at all practical in many countries owing the paucity of exposed basic and ultrabasic rocks. 5

6 Our option We describe here encouraging results of our experiments on the synthesis of nesquehonite [Mg(HCO 3 ) (OH) 2H 2 O]* by flushing CO 2 in a MgCl 2 solution with a view to investigate a possible role for nesquehonite in a CO 2 sequestering process. Our option could result a cost-effective niche CO 2 mineral sequestration process *Nesquehonite is a rare low-temperature carbonate encountered in alkaline soils and in cave deposits. It generally forms euhedral prismatic crystals, but also is found in fibroradial and botryoidal arrays. Ideal formula contains 29.13% MgO, 39.06% H 2 O, 31.81% CO 2 6

7 Sources of magnesium available: Potential magnesium sources involve seawater artificial saltpans evaporitic saline deposits They locally can represent point sources for small-scale industrial applications of the proposed method of carbonation. A massive supply of magnesium could be provided by saline aqueous wastes as a by-product of oil and gas production, the so-called produced water (PW, 70 billion barrels worldwide), as well as reject brines from the desalination process. 7

8 Experimental methods We synthesized nesquehonite by sparging CO 2 through a MgCl 2 6H 2 O solution at 20 ± 2 C. CO 2 + MgCl 2 + 4H 2 O Mg(HCO 3 )(OH) 2H 2 O + 2HCl The product of our synthesis was investigated by SEM-EDS, XRD, FTIR, TG, DTG and DTA. The Mg content in the residual solution was measured by ICP AES. A B Fig. 1 Experimental equipment for the synthesis of nesquehonite (A), pilot plant design (B) and pilot plant (C). 8

9 C 9

10 our idea in a naïve image 10

11 The experiments were carried out: using both doubly distilled and tap water, compressed CO 2 SAPIO (Italy) from analytical grade reagents (MgCl 2 6H 2 O and NH 3, Merck p.a.) the suitable range of ph for the optimum formation of nesquehonite in our experimental conditions ( ) was adjusted by adding about 2% of ammonia solution in thirty-two experiments, we synthesized nesquehonite by sparging CO 2 at a rate of ~100 ml/min through 200 ml of a MgCl 2 6H 2 O solution (~ 7 g/l of Mg) at 20 ± 2ºC the suspension was filtered using 0.20 µm Nucleopore polycarbonate membrane filters, and washed with doubly distilled water and dried in air 11

12 Results the kinetics of the formation of the solid products were followed by sampling the solution at appropriate time intervals and measuring the concentration of Mg Percent Mg in solution Fig Time (min) Fig. 3b Fig. 3a The reaction rate is rapid, with carbonate deposition almost complete in about 10 minutes (Fig. 2). Nesquehonite exhibits well-formed needles up to 0.5 mm in length and 30 µm Ø (Fig. 3a,b). 12

13 6000 Fig. 4 shows a typical XRD pattern (sample AC12); the precipitate has a very high degree of crystallinity. All patterns are in agreement with those reported in JCPDS card for nesquehonite. Relative Intensity Sample AC 12 card theta ( ) The TG-DTG curves (Fig. 5) document the thermal decomposition of nesquehonite during gradual heating, proceeding via dehydration at low temperature (below 350 C) and, above that threshold, complete loss of CO 2 (427 C). 13

14 The efficiency of the CO 2 mineralization process On the basis of these experimental data, 81.7 ± 0.7% of the sparged CO 2 was captured to form nesquehonite About 5% of the starting concentration of Mg was left in the solution after the carbonate formation 14

15 The stability of nesquehonite in an acidic environment Some authors demonstrated that no appreciable CO 2 release will occur if a mixture of Mg hydrate + anhydrous carbonate is leached in solution of ph > 2. Because the ph of acid rain is unlikely to be below ~2.5, we consider the possible release of CO 2 from sites of nesquehonite storage due to acid rain will be unimportant. 15

16 Possible uses for the nesquehonite and by-products The sequestration of CO 2 via carbonation produces a solid material that can be utilized as aggregate in bricks, blocks, mortars, and other building materials. This mineral can be used in the production of eco-cement* concretes because it contributes to strength of the concrete. The ammonium chloride solution produced in the process could be treated for recoverying the salt or decomposed by heat (~ 350 C) to obtain NH 3 and HCl. * F. Pearce, Green Foundations, New Scientist 175 (2002) J. Harrison, Tececo eco-cement masonry product update. 16

17 Why nesquehonite option merits further research? our method as complementary solution could be applied in the countries where the other solutions are not applicable and in those where suitable MgCl 2 sources exist, also as by-product of several industrial processes the process is rapid, simple and environmentally friendly nesquehonite is a light, thermodynamically stable solid product allowing for the long-term storage of CO 2 the starting reactants are easy to be found nesquehonite can be used for industrial and agricultural purposes, and its near surface or underground disposal involves limited environmental risks by-products of the process are sought for a large number of industrial applications 17

18 We are grateful to 1.ENEL S.p.A. (Italian National Electricity Board) for the partial funding provided for the initial research project. 2.Ministry of Education, University and Research (MIUR) through a National Research Program (PRIN _001). 18