Current strategies to reduce CO₂ are founded on a number of well-known measures and technologies (energy efficiency, renewable energy sources, substitution of CO₂-intensive energy sources). Recently, approaches have been discussed – with a high media profile, too – regarding how it would be possible to employ technical means to intervene in the CO₂ cycle, with the aim of easing the CO₂ burden on the atmosphere. Two routes are at the foreground here.

The active removal of CO₂ from the atmosphere

To achieve this, one can for instance take advantage of the ability of green plants to fix CO₂ and to use the biomass thus created in such a way that no (or little) CO₂ gets back into the atmosphere. Filtering the surrounding air using technical processes can, in principle, also remove CO₂ from the atmosphere.

The advantage of these plans – the possibility of actively reducing the CO₂ content of the atmosphere – faces the challenge that due to the low atmospheric concentration of CO₂ (approx. 0.03%), the procedural and technical effort, the energy requirement and the costs for extracting CO₂ from the atmosphere will not be inconsiderable.

In order to achieve a positive contribution to climate protection, it must be ensured that the CO₂ does not leak back into the atmosphere. In addition to deposition in geological layers (sequestration), there are various other options for this. For instance, it can be used to build up humus layers or be bound up long term in carbonates (mineralization). The option of not "disposing" of CO₂ but exploiting it usefully is discussed in the following.

Exploiting CO₂-streams for useful products and applications

These concepts proceed from the idea of considering the CO₂ produced during the use of fossil fuels to be a resource instead of a waste product. Compared with deposition in geological formations, which is currently being discussed and developed as a way of reducing CO₂, effective utilisation of CO₂ seems attractive at first sight. Options here consist, for instance, in its direct use (e.g. as solvent and detergent, as an inert gas, in the soft drink industry), its use as a base substance in the chemical industry, where it can be processed into a more valuable product (e.g. urea, methanol etc.), in converting it to energy carriers, in particular fuels (e.g. biodiesel, ethanol) or to accelerate plant growth (especially microalgae) for the purpose of biomass production for material or energetic use.

Before these concepts can contribute to the reduction of CO₂, considerable challenges must first be overcome. For instance, so far there are only a few forms of use where CO₂ is permanently removed from the atmosphere. If, for example, CO₂ is used to produce fuel, it is released back into the atmosphere when the fuel is used (during general combustion). Furthermore, there is a problem with the amounts: currently approximately 110 million tonnes CO₂ are used in chemical processes globally. In comparison, CO₂ emissions amount to about 25 billion tonnes (about 800 million tonnes in Germany alone). Finally, CO₂ is a relatively stable chemical molecule (e.g. the thermal decomposition into carbon monoxide and oxygen only occurs at temperatures above 2400°C). For this reason, a considerable amount of energy is required to convert the CO₂ into other substances. This energy must come from low-CO₂ sources so that there is a chance of achieving a positive CO₂ balance. Here the question arises as to whether the direct use of this energy input would not be more efficient in the overall balance.

Despite these restrictions there are several interesting conceptual ideas in which positive contributions to CO₂ reduction might be possible, e.g. if synergy effects with other areas could be used. For example, certain microalgae could fix CO₂ and at the same time satisfy their nutrient requirements from liquid effluents, in this way contributing to waste-water treatment. This would be a real leap towards sustainability since the nutrients contained in sewage would not be destroyed as they are now (i.e., precipitated and disposed of) but used for the production of green algae. The vision is that at the end of a long optimization phase there would be a processing technique in which waste-water treatment and CO₂ fixation in biomass are combined. The resulting biomass could be used in various ways (directly as a fuel or indirectly in the production of biodiesel, methane, or ethanol).

Objectives and approach

Many of the technologies and processes to be investigated which might possibly be used in the future in the management of the CO₂ cycle are currently at the stage of basic research or indeed only exist as conceptual ideas. The aim of the TAB report would be to provide an overview of the technological options for CO₂ management.

Since the data basis for many of the technologies to be considered is foreseeably scanty, it will hardly be possible to conduct complete life-cycle analyses, energy balances, or cost-benefit assessments. The best that thus could be provided is rough estimates on technical feasibility, on the options and advantages on the one hand, and on the limitations and problems on the other.

Certain aspects should be excluded from the goal of the suggested TAB project, in particular CO₂ capture from the exhaust stream from power plants and industrial plants and storage of this in geological formations since these aspects were already extensively examined in the recently published TAB working report "CO₂ capture and storage".

At present, the Research Centre Jülich is working in cooperation with the RWTH Aachen on an overview of processes for using CO₂, especially material conversion (in building block chemicals, polymer blocks or similar substances), physical use (e.g. enhanced oil/gas recovery) and use, for instance, in the soft drinks industry, as a solvent, or as inert gas. In the focus of the relevant analyses are the technical feasibility and the potentials of the individual forms of use, but also for example their energy requirements. The project has just begun and is expected to be concluded in late summer 2009. To avoid duplication of work it thus seems proper to first await the results of this project.

TAB thus suggests conducting the project in two phases: first, in the context of a preparatory investigation, the available literature should be examined and the scientific and political debate on the theme of "CO₂ removal from the atmosphere" worked through. The aim is to provide an overview of the status of knowledge and identify knowledge gaps and possible research deficits. In this preliminary phase, it should for example be examined whether topics such as "Land Use and Forestry" (or, to use the terminology of the Intergovernmental Panel on Climate Change (IPCC), "Land Use, Land Use Change and Forestry, LULUCF") and "Geo-engineering" (e.g., large-scale fertilisation of oceans to stimulate the growth of algae which fix CO₂) come into question for an advanced analysis.

Suggested for the field of CO₂ use is to evaluate the report being prepared by the Research Centre Jülich, which will presumably be available in late summer, 2009, and identify topics that were either not or insufficiently covered but deserve a more detailed consideration in the TAB project. Even now, it can be anticipated that, for example, the synergy potentials between waste-water treatment and the CO₂ fixation by biomass with the subsequent use of the biomass as energy could be interesting.

The results of this preliminary examination will be documented in a background paper, which is to be presented to the technology assessment rapporteurs in December, 2009. This informal report is to contain suggestions as to which subject areas could be appropriate for a more detailed analysis in the main phase. On the basis of this information, the then newly constituted Committee for Education, Research, and Technology Assessment can decide at the beginning of 2010 whether and, if yes, how – with which cut and which priorities – the project is to be continued.