RICERCA ITALIANA     

Contribution to atmospheric CO2 mitigation at the point source

Università degli Studi di Roma "La Sapienza"

Abstract

The constant increase in atmospheric carbon dioxide caused by human activities has to be reduced by developing methodologies to control this increase. Among the different approaches that can be implemented to have an effort on net CO2 emissions, the CO2 sequestration through mineral carbonation could play a main role.
The aim of the research program is to study the reactions of CO2 carbonation by the synthesis of magnesium hydrated carbonates (nesquehonite and hydromagnesite) using magnesium chloride.
The experimental work will be carried out according to two stages. During the first year, a set of tests will be aimed at studying the carbonation reactions, whereas in the second year a set of experiments will be focused on the collection of data for optimising the process design for its per-industrial development. The first set of experiments will be carried out under conditions of room temperature and atmospheric pressure following a process route suitable for industrial application. The synthesis of the carbonates will be executed sparging gaseous CO2 through magnesium chloride solutions while stirring and controlling pH. The second set of experiments will be carried out according to the above experimental procedure in reactors applying operating conditions up to 150°C and pCO2 20-30 bars. The mineralogical and chemical data of the products will be obtained by XRD, FTIR, SEM-EDS and ICP-AES analyses. Some sample could also be analysed by EXAFS.
The results of the experimental work will be useful to evaluate the efficiency of the proposed carbonation process and for planning its pre-industrial scale application. <<<

Principal Investigator

Vincenzo Ferrini Università degli Studi di ROMA "La Sapienza"

Research Objectives

The research program is focused on the mineral carbonation of CO2 in aqueous solution. The program is being carried ut together with a research program of a research group of chemical engineering, which has the aim to obtain CO2-hydrates, an ice-like combination of CO2 and water. The disposal of CO2-hydrates in a geological environment is a main problem with long-term disposal, to solve which the present research group would contribute.
The dramatic and constant increase in atmospheric carbon dioxide since the industrial revolution has caused concerns about global warming. The data on CO2 concentration show an approximate constant CO2 concentration of 280 ppm until 1800, when an increase began and the level reached about 380 ppm in 1999. Moreover, sustaining a growth in world energy consumption of 2.3% per annum with the current fossil fuel mix would drive total emissions to 2300 GtC for 100 years. Absorbing that amount of carbon by natural processes without environmental consequences is not possible. The atmosphere, biomass, soil and ocean are all limited in their uptake capacity.
There are different approaches that can be implemented in concert to have an effort on net CO2 emissions: a) energy consumption must be decreased by conservation and by improving the efficiency of energy utilization and conversion system; b) to switch to fuels that are less carbon-intensive and to expand power generation with renewable energy such as wind, solar, geothermal, etc.; c) sequestration of CO2. Although conservation and increased energy efficiency can contribute to reduce or stabilize the atmospheric levels of CO2, sequestration will also play a main role.
Knowledge obtained by the local research unit studying the genesis of ore deposits allow to develop a process at laboratory scale at first and than at pilot-scale for the mineral carbonation of CO2 directly at the power station. In fact, mineral carbonation is a process that follows nature’s example of atmospheric carbon dioxide sequestration. This process requires the neutralization of the carbonic acid with base ions. The obvious base ions to be found in nature are alkali or alkaline earth ions. Alkaline earth metals are more suitable for carbonation. Some non-alkali and non-alkaline earth metals (e.g., Li, Na, K, Rb, Cs, Be, Sr, Ba, Mn, Fe, Co, Ni, Cu, Pb, and Zn) can also be carbonated, but most of them are too rare or valuable, such as iron. The most common ions are Ca and Mg and this latter can immobilize more CO2 than Ca (52% with respect to 43%).
The carbonation reactions are common in nature but the idea of copying this process for the disposal of CO2 based on the chemical fixation of the carbon dioxide in the form of carbonate minerals has been proposed in 90s using calcium in brines and then by means of the carbonation of magnesium-bearing minerals from ultramafic igneous rocks. The main problems of this latter approach is the fact that the reaction kinetics for magnesium silicates tends to be too slow and that environmental concerns are associated with mining and processing very large amounts of magnesium-bearing rocks. For example, the carbonation of 10000 ton/day of CO2 would require about 23000ton/day of rocks.
The aim of the present research program is to develop in laboratory tests a CO2 carbonation process using magnesium chloride derived from the production process of NaCl from seawater. The production of magnesium chloride is at the moment scarcely widespread, however, the increasing demand arising from the use of the product for CO2 sequestration could produce an increase in the production of the chloride.
A further advantage of this method for CO2 sequestration is the safe and long-term storage of the carbon dioxide as the carbonates that will be synthesized are stable up to 500°C. Moreover, the product of the proposed process can be used as construction materials and in many industrial processes.
In short, the main advantages of the carbonation process by using magnesium chloride solutions are the speed of the carbonation reactions, the stability of the products and the low cost of the starting Mg-bearing material. <<<

Timescale

24 months

National and international background

The increasing concentration of “greenhouse gases”, including carbon dioxide (CO2), methane (CH4), and water vapor, in the earth’s atmosphere has provoked the reactions of researchers to solve the over-emission of CO2. So in light of this problem many scientists have been testing several methodologies on sequestering carbon dioxide (e. g. Dunsmore 1992, Herzog 2001, Herzog et al. 2003, IEAGHG 2001, Yegulalp et al. 2001).
While the basic physics and principles of climate forcing are well established, forecasts of global climate evolution are highly uncertain. Atmospheric CO2 concentrations have increased from an estimated 180 ppm 25,000 years ago, during the most recent glacial maximum, to 280 ppm 200 years ago, to the current concentration of over 380 ppm (Houghton et al. 2001). In parallel with current scientific research is a diverse and evolving body of policy options for dealing with climate change through preventive, mitigation, remediation, and adaptive measures. In the long-term, fuel switching to lower or noncarbon fuels, hydrogen as an energy carrier, renewable sources of energy, efficiency improvements, and energy conservation are the most promising alternatives. Even under the most aggressive projections of technology development and progressive policy regimes, the transition from the current dependence on fossil fuels would take many decades or longer. Predicting the shift in energy usage is also complicated by the uncertain factors of population and economic growth (McCarthy et al 2001).
In the meantime, potential global climate impacts associated with carbon inputs to the atmosphere must be addressed. There is no known or expected panacea to negate the climate change issue, so any response must inevitably include a broad array of policy measures and technology options. The sequestration of carbon by various means is one class of mitigation strategies, which has great near-term promise as a way to hedge against the potential severity of consequences and to buy time to develop further currently uncertain options (Hepple &amp; Benson 2004).
Sequestration is the effective removal of carbon from the atmosphere and its storage in forms such as dissolved carbon in the ocean or subsurface formation waters, in terrestrial biomass, as a fluid in deep geologic traps, or as mineral carbonates. The three basic types of sequestration are: ocean, terrestrial, and geologic storage of CO2 in underground formations such as depleted or nearly depleted oil and gas reservoirs, deep sedimentary brine-filled formations, and deep unmineable coal beds (Keith 2000; Holloway 2001).
To address the question, “How much CO2 might be stored underground and for how long?” zeroth-order estimates for the annual amount of CO2 that would need to be sequestered to meet atmospheric stabilization targets of 350, 450, 550, 650, and 750 ppm were developed (Liu &amp; Zhao 2000). The difference was calculated between (1) the six IPCC SRES (Intergovernmental Panel on Climate Change, Special Report on Emissions Scenarios) marker emissions scenarios (Nakicenovic et al 2000) and (2) the economically optimized (Wigley et al 1996) allowable emissions for a range of long-term atmospheric CO2 stabilization targets. For each of the scenarios, the assumption was that geologic sequestration would be used as a bridging technology, allowing for the gradual phase out of fossil fuels over a period of up to 300 years.
It is possible to sequester carbon dioxide in natural water bodies if one is willing to accept a permanently higher level of carbon dioxide in the air. The only reservoirs of significance are the oceans, but they are so large that left to themselves they would take many centuries to equilibrate with the atmosphere and in the process of equilibration absorb 70% to 80% of the excess carbon dioxide (Lackner 2002).
In geologic carbon storage, carbon dioxide would be captured from large point sources such as power plants, compressed, transported by pipeline, and injected into geologic reservoirs. Some pilot projects are already underway with Statoils Sleipner Saline Aquifer Carbon Dioxide Storage (SACS) Project in the North Sea, the Weyburn enhanced oil recovery project in Alberta, Canada, and several U.S. Department of Energy sponsored projects in the U.S. (NETL 2001; IEAGHG 2001). The carcon dioxide is a gas, and therefore its long-term storage requires physical barriers to keep it out of the atmosphere. Good storage sites are likely underground or the deep ocean where the necessary pressure is easily maintained. This approach requires some important tricks, the first one is the reducing pressure prior to injection of the gas to eliminate over-stressing, preferential ways to escaping, and then eliminate the exposure to air because this promote a risk of carbon dioxide escaping. Just as for gaseous carbon dioxide one must maintain a physical barrier that separates the solution from the air (Lackner, 2002). The probability that storage of carbon dioxide in deep geologic formations will become an important climate change mitigation strategy depends on a number of factors in which the cost of geologic storage represent the main obstacle. Whether or not a site is suitable will be determined by establishing that it can meet a set of performance requirements for safe and effective geologic storage. To date, no such performance requirements have been developed. Establishing effective requirements must start with an evaluation of how much CO2 might be stored and for how long the CO2 must remain underground to meet goals for controlling atmospheric CO2 concentrations. Answers to these questions provide a context for setting performance requirements for geologic storage projects.
According to the results presented in the literature, geologic storage could be an effective method to ease the transition away from a fossil-fuel based economy over the next several centuries, even if large amounts of CO2 are stored and some small fraction seeps from storage reservoirs back into the atmosphere. An annual seepage rate of 0.01% would ensure the effectiveness of geologic carbon storage for any of the projected sequestration scenarios explored, even those with the largest amounts of storage (1,000 s of gigatonnes of carbon-GtC), and still provide some safety margin. Storing smaller amounts of carbon (10 s to 100 s of GtC) may allow for a slightly higher seepage rate on the order of 0.1% .
Carbon sequestration strategies based on the formation of carbonate or bicarbonate require base ions to neutralize CO2. These ions are monovalent eg. sodium and potassium, or divalent calcium and magnesium. There is a large class of sequestration methods that could be developed on this principle (O’ Connor et al. 2001). An important feature is that all the methods neutralize carbonic acid before or during sequestration. The disposal which uses carbonates or bicarbonates rather than carbonic acid (O’ Connor et al. 2000) reduces environmental impact and raises the capacity, permanence, and safety of potential carbon sinks.
The major carbonate disposal strategies are: introduction of bicarbonate salts into the ocean; injection of carbonate or bicarbonate brines into undergrounds reservoirs and third is form solid carbonates that can be stored either on surface or underground. If the solid are soluble in water they would be stored in salt cavern, or insoluble in the ocean floor or in the terrestrial surface. However the source of magnesium and calcium can be different. If the disposal product is carbonate brine or solid carbonate the source of alkaline ions is likely a silicate mineral magnesium- and calcium–rich. The stronger carbonic acid drives silicic acid out of its salts resulting in carbonates and silica. Because carbonate are thermodynamically favored, the process occurs naturally in rock weathering (Lackner et al 1995). There are several magnesium and calcium silicates and for some the chemical conversion to carbonate by industrial process is possible. For example these ions can be extracted from serpentine or generally from mafic-and ultramafic rocks that through the carbonation of these rocks are stabilized Mg-carbonate minerals.So, it is of environmental interest because its ability to fix anthropogenic carbon dioxide (Seifritz 1990).
The progressive interaction between ultramafic rocks variably affected by serpentinization and meteoric waters accompanied by precipitation of magnesite and silica minerals, in area of high terrestrial CO2 fluxes such as Southern Tuscany , represent field evidence supporting the feasibility of this methodology of CO2 sequestration (Cipolli et al. 2004). However, dissolution of serpentine accompanied by precipitation of magnesite and chalcedony brings about a progressive reduction in the effective porosity of the acquifer, at least under closed-system conditions, and the situation could be much worse if the amorphous silica precipitates instead of chalcedony. These effects could represent serious obstacles for implementation of this methodology of CO2 sequestration and their importance must be evaluated by laboratory experiments and fields tests (Lackner et al 1995, Goff &amp; Lackner 1998, Goldberg &amp; Walters 2002, Cipolli et al. 2004).
Listwanite, a carbonate-altered serpentinite also represents a natural analogue to CO2 sequestration via in situ carbonation of minerals. The reaction pathways and permeability structure controlling listwanite formation are preserved and exposed at Atlin, British Columbia. The overall mineralogical transformation is the same as that being considered for industrial sequestration of CO2. In nature this reaction proceeds via subreactions that are fossilized as spatial distinct zones. Serpentine+olivine + brucite reacted with CO2 to form serpentine + magnesite, then magnesite+talc and finally magnesite +quartz. These mineralogical transformation are achieved isochemically, except for the volatile species H2O and CO2. Although the first stage of the reaction only accounts for about 5-15% of the carbonation potential of the serpentinite may have sequestered a significant portion of the total bound CO2 (Hansen et al. 2005).
The presence of low-temperature carbonated ultramafic rocks in nature suggests that conditions favourable for mineral carbonation exist at shallow levels of the crust. However these rocks occur in smaller outcrops rather than other suites also useful for the CO2 sequestration via carbonation of minerals or solutions.
Globally mineral carbonation offers virtually unlimited capacity and the promise of safe, permanent storage of CO2, with little risk of accidental release (Guthrie et al. 2001). But, though it would be possible to use alkaline ions extracted from silicate minerals, this would require more expensive pre-processing in order to prepare the input materials. But these disposal is not practicable in Italy because the outcrops of mafic and ultramafic rocks are limited and for this sequestration are necessary tons and tons of rocks. Several problems exist within mining area, so before to plain and mine any deposit it is necessary to examine the problems regarding to socio-economic and environmental impacts. Questions about location, accessibility, land degradation, future of the mining industry governed by mining legislation must be considered.
As an alternative to the direct carbonation of minerals discussed so far, the magnesium and calcium content could first be extracted in aqueous solution. The extraction could proceed by way of various agents, such as hydrochloric acid, caustic soda, sulphuric acid or steam. Carbon dioxide sequestration by MgCl2 process can happen or using solid or for the process to industrial scale sea water that contains 1,3 g/l of magnesium. In the first case the availability is lie to the deposits of Messinian age and, therefore, to the mines of Sicily, Calabria and the Tuscany.
Other magnesium chloride source is represented by the product of salt pans occurring in Sardinia and Puglia. <<<