The reduction of the accumulation of atmospheric CO2 is the objective of international and national programmes. A number of technologies are available, each characterized by a different level of: (i) knowledge of the effects generated upon application; (ii) scale; (iii) cost; and (iv) experimentation and exploitation (Table 7.1). Such technologies may either reduce the production of CO2 or be used for disposing the produced CO2; in the latter case, the permanence of disposed CO2 becomes a key factor.
Currently, it is impossible to designate a single technology that alone may be able to reduce the CO2 atmospheric loading to such a level that may guarantee that 'nonreturn' points are avoided. More likely, a combination of technologies may help to reach the objective. It is foreseeable that each of the technologies listed in Table 7.1 may vary its amplitude of application with time. By making the right choices now, the amount of CO2 that will be removed from the atmosphere will reasonably increase with time with respect to the actual potential of each technology. However, implementing ten different technologies, each having a reduction potential of 100 million tonnes per year, it will be possible to cut 1 Pg CO2/year, which is a significant amount as a starting point. In the short, medium and long term, each technology will increase its potential for application at a different rate and this will make it meaningful to exclude some possibilities.
This chapter deals with the use of CO2 in the synthesis of chemicals or for technological applications. A specific attention will be paid in the discussion to the assessment of the potential of such options for the control of the CO2 atmospheric loading.
7.1.2 Actual uses of CO2
CO2 is already industrially utilized as:
1. Feedstock for the synthesis of chemicals;
2. Technological fluid;
3. Source of carbon for enhanced production of biomass (mostly marine).
The use of CO2 in the synthesis of chemicals is today applied in only a few cases, with a total amount of CO2 converted equal to ~110 Tg per year, most of which goes into the production of urea (~70 Tg per year) (Ricci, 2003). The use as an additive to CO for the synthesis of methanol is another important
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Table 7.1. Technologies that may be used in the control of the atmospheric level of CO2.
Technologies based on the utilization of fossil fuels
Main issues that generate problems
Efficiency in the production of other forms of energy (thermal, mechanical and electric energy) from fossil fuels
Disposal of CO2 in aquifers, geological cavities and ocean Disposal of CO2 in oil and gas spent fields
Utilization of CO2 in gaseous or liquid hydrocarbons extraction, e.g. enhanced oil recovery (EOR) Enhanced biological utilization of CO2 (production of aquatic biomass) Chemical utilization of CO2
Technological utilization of CO2
Fixation of CO2 into inorganic carbonates
Alternative energies Geothermal energy Solar energy Wind energy
Hydropower Nuclear energy
Site specificity (request of specific conditions) Energy penalty (energy spent in application of the technology with CO2 generation) Cost of exploitation additional to energy cost
Scale (from kt to Gt)
Environmental impact (of the exploitation of the technology) Permanence of CO2 (for the disposal technologies)
Site specificity, cost, total power
Site specificity, cost, low density and intensity
Site specificity, continuity, power per unit, impact Site specificity Existing concerns use (5-6 Tg per year). Minor applications are the synthesis of molecular organic carbonates (0.1 Tg per year) and the synthesis of specialty chemicals such as 2-hydroxy-benzoic acid (40,000 t/year). Specialty inorganic carbonates also represent a large mass application (30 Tg per year). In addition to the utilization in the production of chemicals, CO2 is used as technological fluid or as an additive to beverages (Vansant, 2003) at a level of ~18 Tg per year, with a steady increase in the past years and a 5-10% market expansion rate estimated for the near future. Such CO2 is either collected from natural wells or recovered from chemical processes (mainly the synthesis of NH3) or (in lower amounts) from fermentation processes. Sources such as power plants are not yet exploited. The latter source produces CO2 that is accompanied by pollutants that have to be separated for most applications, as they may cause negative effects on humans or animals, or more generally to biotic and abiotic systems. Large amounts of very pure CO2 (several Tg per year) produced in the sugarcane and other similar industries are vented because of the seasonality of the production, which does not assure a continuous feed over the year to a potential CO2 user.
The relevance of the use of CO2 to the reduction of its atmospheric loading and the estimation of its potential contribution to this loading are key issues for determining the role of such an option in the panorama of CO2 reduction technologies. This aspect will now be discussed in order to put the rest of this chapter in context. It is worth recalling that most of the synthetic processes currently in use produce large amounts of waste as by-products, inorganic and organic salts, as well as waste solvents. The ratio of the mass of waste to the mass of the marketable products is the 'waste factor'. It may range from 2 to 50, or more, according to the process complexity (number of steps and type of reaction). One of the processes with high 'waste factor' is the synthesis of carboxylates. This is not a straightforward procedure, but the ability to introduce the CO2 moiety into an organic substrate using CO2 would greatly reduce the waste production.
The industrial utilization of CO2 includes the use of the entire molecule as a building block in the chemical industry or of its reduced forms (HCOOH, HCHO, CO, CH3OH and, ultimately, CH4). These two uses have different energy requirements and this is a key point in the utilization of CO2. If the recovery of CO2 is implemented on a large scale, CO2 will be more readily available. Two options are then open: its disposal or its utilization. All technologies aimed at disposal are characterized by an energy penalty and this may equal 0.2-0.45 t of CO2 emitted per tonne of disposed CO2. According to some authors (e.g. Mann and Spath, 1997), the implementation of such disposal technologies will cause a net expansion of the extraction of fuels, thus shortening their availability with questionable benefits to the climate. Conversely, chemical, biological and technological uses may avoid CO2 emissions and reduce the need for fossil fuel extraction. For the sake of correctness, one must be aware that this utilization option is not the solution to the CO2 problem by itself, but can at least contribute to wider efforts to reduce global CO2 emissions.
It must also be considered that most of the compounds into which CO2 can be converted will release CO2 on a timescale of months to years, according to their nature and use. Polymers (polycarbonates and polyurethanes) will have a good, but limited for the amount, potential for CO2 storage (decades to centuries). Other chemicals will lead to a reduction in emissions through carbon recycling or, better yet, the implementation of innovative synthetic technologies that will reduce the use of energy and fossil carbon relative to existing ones. Therefore, the reduction of CO2 emission with respect to the existing situation in the chemical industry, will be represented by the amount of avoided CO2 produced by the implementation of new synthetic strategies that use CO2. From this point of view, the utilization of CO2 can be interpreted as a peculiar case of 'efficiency technology'. Interestingly, such technologies may also produce net economic benefits, as they convert a spent chemical into a valuable prod uct. Therefore, the utilization of recovered CO2 in the synthesis of specific chemicals may occur with the reduction of the energy and carbon use associated with traditional chemical synthesis.
The potential benefits of using CO2 can be assessed by comparing innovative to existing technologies. In general, it can be said that the utilization of CO2 will contribute to a reduction in emissions wherever the process meets the following conditions. It must be exoergonic (free energy should be considered more than enthalpy alone, as CO2-based reactions are in general accompanied by a significant change of entropy) and must minimize: (i) the processing energy; (ii) the total carbon balance; (iii) the other materials input; (iv) the CO2 emission; and (v) other emissions.
The patent literature shows that a very large number of reactions utilizing CO2 have been developed in recent years as alternatives to existing processes (Fig. 7.1). These processes are at a range of developmental stages and degrees of exploitation. As Fig. 7.1 shows, two classes of reactions can be distinguished for the conversion of CO2 into other chemicals (Aresta, 1987): the carboxylation and the reduction reactions. The former produce an increase in the C/H ratio, which has been considered by some authors as an effective pathway for CO2 fixation and emission reduction (Pechtl, 1991; Audus and Oonk, 1997).
The quantification of the amount of CO2 avoided is not as straightforward a procedure as it may be for simple CO2 disposal in that it requires a complex calculation methodology, such as a life cycle assessment, to be applied to the processes under assessment. Similar considerations are valid when CO2 is used as a technological fluid (i.e. used directly) rather than it being converted into other chemicals. Whenever it substitutes for other chemicals, the effect of this product substitution can be evaluated by comparing the global warming potential (GWP) of CO2 with that of the chemicals it substitutes - the GWP of these chemicals can often be several thousand times that of CO2 (CO2 has a GWP of 1).
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