CO₂ Capture and Storage

7. How can CO₂ be stored in other materials?

• 7.1 Can CO₂ be transformed and stored in solid form?
• 7.2 What are the industrial uses of CO₂ and can they reduce CO₂ emissions?

7.1 Can CO₂ be transformed and stored in solid form?

The source document for this Digest states:

Mineral carbonation

This section deals with two rather different options for CO₂ storage. The first is mineral carbonation, which involves converting CO₂ to solid inorganic carbonates using chemical reactions. The second option is the industrial use of CO₂, either directly or as feedstock for production of various carbon-containing chemicals

Mineral carbonation: technology, impacts and costs

Mineral carbonation refers to the fixation of CO₂ using alkaline and alkaline-earth oxides, such as magnesium oxide (MgO) and calcium oxide (CaO), which are present in naturally occurring silicate rocks such as serpentine and olivine. Chemical reactions between these materials and CO₂ produces compounds such as magnesium carbonate (MgCO₃) and calcium carbonate (CaCO₃, commonly known as limestone). The quantity of metal oxides in the silicate rocks that can be found in the earth’s crust exceeds the amounts needed to fix all the CO₂ that would be produced by the combustion of all available fossil fuel reserves. These oxides are also present in small quantities in some industrial wastes, such as stainless steel slags and ashes. Mineral carbonation produces silica and carbonates that are stable over long time scales and can therefore be disposed of in areas such as silicate mines, or re-used for construction purposes (see Figure TS.10), although such re-use is likely to be small relative to the amounts produced. After carbonation, CO₂ would not be released to the atmosphere. As a consequence, there would be little need to monitor the disposal sites and the associated risks would be very low. The storage potential is difficult to estimate at this early phase of development. It would be limited by the fraction of silicate reserves that can be technically exploited, by environmental issues such as the volume of product disposal, and by legal and societal constraints at the storage location.

The process of mineral carbonation occurs naturally, where it is known as ‘weathering’. In nature, the process occurs very slowly; it must therefore be accelerated considerably to be a viable storage method for CO₂ captured from anthropogenic sources. Research in the field of mineral carbonation therefore focuses on finding process routes that can achieve reaction rates viable for industrial purposes and make the reaction more energy-efficient. Mineral carbonation technology using natural silicates is in the research phase but some processes using industrial wastes are in the demonstration phase.

A commercial process would require mining, crushing and milling of the mineral-bearing ores and their transport to a processing plant receiving a concentrated CO₂ stream from a capture plant (see Figure TS.10). The carbonation process energy required would be 30 to 50% of the capture plant output. Considering the additional energy requirements for the capture of CO₂, a CCS system with mineral carbonation would require 60 to 180% more energy input per kilowatt- hour than a reference electricity plant without capture or mineral carbonation. These energy requirements raise the cost per tonne of CO₂ avoided for the overall system significantly (see Section 8). The best case studied so far is the wet carbonation of natural silicate olivine. The estimated cost of this process is approximately 50–100 US$/tCO₂ net mineralized (in addition to CO₂ capture and transport costs, but taking into account the additional energy requirements). The mineral carbonation process would require 1.6 to 3.7 tonnes of silicates per tonne of CO₂ to be mined, and produce 2.6 to 4.7 tonnes of materials to be disposed per tonne of CO₂ stored as carbonates. This would therefore be a large operation, with an environmental impact similar to that of current large-scale surface mining operations. Serpentine also often contains chrysotile, a natural form of asbestos. Its presence therefore demands monitoring and mitigation measures of the kind available in the mining industry. On the other hand, the products of mineral carbonation are chrysotile-free, since this is the most reactive component of the rock and therefore the first substance converted to carbonates.

A number of issues still need to be clarified before any estimates of the storage potential of mineral carbonation can be given. The issues include assessments of the technical feasibility and corresponding energy requirements at large scales, but also the fraction of silicate reserves that can be technically and economically exploited for CO₂ storage. The environmental impact of mining, waste disposal and product storage could also limit potential. The extent to which mineral carbonation may be used cannot be determined at this time, since it depends on the unknown amount of silicate reserves that can be technically exploited, and environmental issues such as those noted above.

Source & ©: IPCC  Carbon Dioxide Capture and Storage: Technical Summary (2005)
7. Mineral carbonation and industrial uses, p. 39

7.2 What are the industrial uses of CO₂ and can they reduce CO₂ emissions?

The source document for this Digest states:

Industrial uses of CO₂ include chemical and biological processes where CO₂ is a reactant, such as those used in urea and methanol production, as well as various technological applications that use CO₂ directly, for example in the horticulture industry, refrigeration, food packaging, welding, beverages and fire extinguishers. Currently, CO₂ is used at a rate of approximately 120 MtCO₂ per year (30 MtC yr-1) worldwide, excluding use for EOR (discussed in Section 5). Most (two thirds of the total) is used to produce urea, which is used in the manufacture of fertilizers and other products. Some of the CO₂ is extracted from natural wells, and some originates from industrial sources – mainly high-concentration sources such as ammonia and hydrogen production plants– that capture CO₂ as part of the production process. Industrial uses of CO₂ can, in principle, contribute to keeping CO₂ out of the atmosphere by storing it in the “carbon chemical pool” (i.e., the stock of carbon-bearing manufactured products). However, as a measure for mitigating climate change, this option is meaningful only if the quantity and duration of CO₂ stored are significant, and if there is a real net reduction of CO₂ emissions. The typical lifetime of most of the CO₂ currently used by industrial processes has storage times of only days to months. The stored carbon is then degraded to CO₂ and again emitted to the atmosphere. Such short time scales do not contribute meaningfully to climate change mitigation. In addition, the total industrial use figure of 120 MtCO₂ yr-1 is small compared to emissions from major anthropogenic sources (see Table TS.2). While some industrial processes store a small proportion of CO₂ (totalling roughly 20 MtCO₂ yr-1) for up to several decades, the total amount of long-term (century-scale) storage is presently in the order of 1 MtCO₂ yr-1 or less, with no prospects for major increases.

Another important question is whether industrial uses of CO₂ can result in an overall net reduction of CO₂ emissions by substitution for other industrial processes or products. This can be evaluated correctly only by considering proper system boundaries for the energy and material balances of the CO₂ utilization processes, and by carrying out a detailed life-cycle analysis of the proposed use of CO₂. The literature in this area is limited but it shows that precise figures are difficult to estimate and that in many cases industrial uses could lead to an increase in overall emissions rather than a net reduction. In view of the low fraction of CO₂ retained, the small volumes used and the possibility that substitution may lead to increases in CO₂ emissions, it can be concluded that the contribution of industrial uses of captured CO₂ to climate change mitigation is expected to be small.

Source & ©: IPCC (WGI)  Intergovernmental Panel on Climate Change IPCC
Carbon Dioxide Capture and Storage: Technical Summary (2005)
7. Mineral carbonation and industrial uses, p. 40