CO2: A Valuable Source of Carbon

The utilization of CO₂ as a feedstock for producing chemicals is an interesting challenge to explore new concepts and new opportunities for catalysis and industrial chemistry. It is an excellent possibility to inject renewable energy in the energy and chemical production chains, but a major current hurdle for a large-scale use is the need to further improve production routes for renewable H₂ by improving electrocatalysts and device technology in current electrolyzers. However, when cheap electrical energy from renewable sources is available, the use of CO₂ could be already economic. In the organic synthesis and polymer chemistry, new routes for activating CO₂ and producing valuable chemicals and/or materials are being developed. Electrocatalysis is also offering new possibilities, either to produce small organic molecules (fuels) to be used in conjunction or integrated with solar devices (for artificial leaf type systems), or as a valuable synthetic procedure. The main relevant aspects of these routes are summarized to present the status and outlooks, as well as the strategies, for carbon dioxide (re)use.

A competitive and sustainable chemical industry requires developing new strategies for resource and energy efficiency. We present here the concept that the use of CO₂ offers innovative possibilities to achieve this objective. The routes, opportunities, and barriers in converting CO₂ using renewable energy and their impact on the chemical and energy value chains are discussed after introducing the general aspects of this topic evidencing the tight integration between the CO₂ use and renewable energy insertion in the value chain of process industry. The specific challenge of using CO₂ for the production of light olefins (ethylene, propylene), as specific example of value of carbon dioxide as carbon source to meet both resource and energy efficiency, is discussed. The conversion of CO₂ back to fuels using sunlight (solar fuels) is also discussed to evidence how is a relevant opportunity to develop effective energy vectors for the storage of solar energy which integrates into existing energy infrastructure and allow a smooth, but fast transition to a more sustainable energy in future.

Biogas produced by the anaerobic digestion of biomass can be exploited directly as a fuel for small-to-medium-scale combined heat and power production, or as a renewable carbon source for the production of synthesis gas and/or hydrogen for industrial syntheses or energetic purposes. Since biogas contains CH₄ and CO₂ as two main components, it could be processed to a syngas according to a well-reported technological process called CO₂ reforming of methane (dry reforming). We highlight the dry reforming of biogas as one area of activity where catalysts are already a significant focus of worldwide research efforts. Nickel catalysts are highly active for reforming reactions, and their cost is much lower compared with noble metals, which makes them suitable for a cost-effective commercial reforming process. For this reason, Ni-based catalysts are extensively studied, with emphasis on the effect of catalyst composition, preparation method, and pre-treatment. Unfortunately, nickel is more prone to carbon deposition. Improvement in the performance of Ni-based catalysts by incorporation of a second metal to catalyst composition and use of different Ni catalyst precursors is discussed in some detail. The challenges for catalysts applied to the dry reforming of biogas (activity, sulfur poisoning, carbon formation, and sintering) are also examined in order to reveal the specific needs and responses for the reforming process. A brief account of strategies and approaches adopted in the search for catalysts that respond to the above challenges is given here.

The basics of the process architecture to produce methanol from CO₂ using renewable hydrogen are discussed and integrated within a process scheme to analyse the effects of variables such as capital investment (CI), variable operating and CO₂ at site costs, electric power need for Nm³ of produced H₂. These estimations are used to provide a comparison of the overall production cost with conventional hydrocarbon-based technology.

The emission of CO₂ from fossil fuels is the object of an increasing worldwide attention, and although the development of new emission-poor or emission-free energy sources must be the long-term goal, for the near future the development of efficient CO₂ capture technologies remains the sole strategy to control the CO₂ level. A relatively recent approach to the removal of CO₂ from gas streams employs ionic liquids (ILs), a broad class of compounds composed exclusively of ions that exist in the liquid state at room temperature or below. An overview on the different ILs and techniques used to this purpose is here reported.

Biodiesel from microalgae is one of the most promising options of carbon capture and utilization (CCU) because of the direct utilization of solar radiation and the higher biomass and lipid areal productivity in comparison with marine or terrestrial plants. LCA studies on this subject are rather difficult to compare because of inhomogeneous assumptions for oil extraction (the critical step), fertilizer consumptions, use of residual biomass, use of primary energy needed for electricity, use of different fossil fuel for process heat supply, availability of waste heat from fossil or biomass power stations, and climate of the cultivation areas. Nevertheless, GHG savings appear in line with the current EU standards for automotive fuel only under the most favourable conditions (biomass productivity at 25 gm^-2 day^-1, lipid content at 40–45 %, water and nutrient recycle, favourable climate conditions, use of low energy wet oil extraction processes still to be checked on large scale).

This chapter describes the hydrogen production by solar-powered steam reforming. This process allows some upgrading of the reformed fuel (in terms of heat value and environmental impact) and significant reduction (40–50 %) in CO₂ emission to the atmosphere, with respect to the conventional steam reforming process. Additionally, solar steam reforming is presented as an emission-free process: the only carbon-containing by-product stream is well suited for the application of CCS technologies, and the overall process can also be considered as a “fuel pre-combustion decarbonization route”. Application of pressurized membrane reactors for low-temperature steam reforming improves the overall process efficiency and enhances the recovery of CO₂ thanks to its relatively high partial pressure in the by-product stream. The application of SERP technology for in situ CO₂ separation is discussed too.

Methanation is an alternative route to treat CO₂, which allows the enhancement of carbon in the molecule, through its conversion to methane. A very wide catalytic system is available to operate the transformation. A review of the materials and their performances is presented together with some industrial applications. These technologies are useful methods to store energy as chemical energy.

Methanol plays a key role in a CO₂ economy scenario. This contribution analyzes recent developments in this area from the catalyst and process perspectives, in terms of both current industrial production of methanol from fossil fuels via syngas, and changes need to progressively go to CO₂-rich feedstock and finally to use pure CO₂ and renewable H₂ as raw materials. Aspects discussed regard the methanol industrial production, the catalysts for methanol synthesis (especially regarding recent advances), the reaction mechanism, and role of CO₂, and the catalyst needs for methanol synthesis directly from CO₂.

An innovative process scheme to produce methanol from carbon dioxide is here presented and assessed via simulation. In this configuration, the syngas stream, composed by CO, CO₂, and H₂ and fed to the methanol synthesis reactor, is produced by means of a reverse-water–gas-shift by which a CO₂ stream is partially converted in carbon monoxide. In the chapter, the best catalyst to support the reverse reaction is selected; then a simulation model is applied to define the proper operating conditions to achieve syngas composition targets. The simulation results show that the plant configuration represents a feasible way to produce methanol using carbon dioxide, competitively with the traditional process in which the syngas is produced by a natural gas steam reforming unit.

This chapter reports an overview of German funding programme on CO₂ utilization (CCU) as a valuable example of how the integration of all stakeholders together with a push from governmental side can bring a large advancement in science in this area and new industrial opportunities for the companies. Many German companies in chemical and power supply have identified CO₂ utilization as promising area and are openly advertising their activities in the field. Policy makers have taken up the topic and are strongly engaged to promote CCU also on European level. The scientific–technical community has been stimulated, and the number of conferences and workshops in the CCU area is rapidly increasing. Last but not least, the projects have met tremendous press and media attention, leading to many publications that increase public awareness. Other BMBF funding programmes, for example, on energy storage have been issued that take up the topic of CO₂ utilization in new power to gas projects, ensuring sufficient continuity of public support for this important research field.