CO2 conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO2 utilization as a measure for global CO2 mitigation. Whereas the latter focuses on reducing the end-of-pipe problem “CO2 emissions” from todays’ industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO2-based chemistry does...

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

Because solar energy is available in large excess relative to current rates of energy consumption,
effective conversion of this renewable yet intermittent resource into a transportable and
dispatchable chemical fuel may ensure the goal of a sustainable energy future. However, low
conversion efficiencies, particularly with CO_2 reduction, as well as utilization of precious
materials have limited the practical generation of solar fuels. By using a solar cavity-receiver
reactor, we combined the oxygen uptake and release capacity of cerium oxide and facile catalysis
at elevated temperatures to thermochemically dissociate CO_2 and H_2O, yielding CO and H_2,
respectively. Stable and rapid generation of fuel was demonstrated over 500 cycles. Solar-to-fuel
efficiencies of 0.7 to 0.8% were achieved and shown to be largely limited by the system scale
and design rather than by chemistry.

http://science.sciencemag.org/content/sci/330/6012/1797.full.pdf

High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria

To improve the sustainability of transportation, a major goal is the replacement of conventional petroleum-based fuels with more sustainable fuels that can be used in the existing infrastructure (fuel distribution and vehicles). While fossil-derived synthetic fuels (e.g. coal derived liquid fuels) and biofuels have received the most attention, similar hydrocarbons can be produced without using fossil fuels or biomass. Using renewable and/or nuclear energy, carbon dioxide and water can be recycled into liquid hydrocarbon fuels in non-biological processes which remove oxygen from CO2 and H2O (the reverse of fuel combustion). Capture of CO2 from the atmosphere would enable a closed-loop carbon-neutral fuel cycle. This article critically reviews the many possible technological pathways for recycling CO2 into fuels using renewable or nuclear energy, considering three stages—CO2 capture, H2O and CO2 dissociation, and fuel synthesis. Dissociation methods include thermolysis, thermochemical cycles, electrolysis, and photoelectrolysis of CO2 and/or H2O. High temperature co-electrolysis of H2O and CO2 makes very efficient use of electricity and heat (near-100% electricity-to-syngas efficiency), provides high reaction rates, and directly produces syngas (CO/H2 mixture) for use in conventional catalytic fuel synthesis reactors. Capturing CO2 from the atmosphere using a solid sorbent, electrolyzing H2O and CO2 in solid oxide electrolysis cells to yield syngas, and converting the syngas to gasoline or diesel by Fischer–Tropsch synthesis is identified as one of the most promising, feasible routes. An analysis of the energy balance and economics of this CO2 recycling process is presented. We estimate that the full system can feasibly operate at 70% electricity-to-liquid fuel efficiency (higher heating value basis) and the price of electricity needed to produce synthetic gasoline at U.S.D$ 2/gal ($ 0.53/L) is 2–3 U.S. cents/kWh. For $ 3/gal ($ 0.78/L) gasoline, electricity at 4–5 cents/kWh is needed. In some regions that have inexpensive renewable electricity, such as Iceland, fuel production may already be economical. The dominant costs of the process are the electricity cost and the capital cost of the electrolyzer, and this capital cost is significantly increased when operating intermittently (on renewable power sources such as solar and wind). The potential of this CO2 recycling process is assessed, in terms of what technological progress is needed to achieve large-scale, economically competitive production of sustainable fuels by this method.

Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy

The U.S. Department of Energy and the National Renewable Energy Laboratory are developing technologies to produce hydrogen from renewable, sustainable sources. A cost goal of $2.00–$3.00 kg−1 of hydrogen has been identified as the range at which delivered hydrogen becomes cost competitive with gasoline for passenger vehicles. Electrolysis of water is a standard commercial technology for producing hydrogen. Using wind and solar resources to produce the electricity for the process creates a renewable system. Biomass-to-hydrogen processes, including gasification, pyrolysis, and fermentation, are less well-developed technologies. These processes offer the possibility of producing hydrogen from energy crops and from biomass materials such as forest residue and municipal sewage. Solar energy can be used to produce hydrogen from water and biomass by several conversion pathways. Concentrated solar energy can generate high temperatures at which thermochemical reactions can be used to split water. Photoelectrochemical water splitting and photobiology are long-term options for producing hydrogen from water using solar energy. All these technologies are in the development stage. Copyright © 2007 John Wiley & Sons, Ltd.

Renewable hydrogen production

Nanofluids—a simple product of the emerging world of nanotechnology—are suspensions of nanoparticles (nominally 1–100 nm in size) in conventional base fluids such as water, oils, or glycols. Nanofluids have seen enormous growth in popularity since they were proposed by Choi in 1995. In the year 2011 alone, there were nearly 700 research articles where the term nanofluid was used in the title, showing rapid growth from 2006 (175) and 2001 (10). The first decade of nanofluid research was primarily focused on measuring and modeling fundamental thermophysical properties of nanofluids (thermal conductivity, density, viscosity, heat transfer coefficient). Recent research, however, explores the performance of nanofluids in a wide variety of other applications. Analyzing the available body of research to date, this article presents recent trends and future possibilities for nanofluids research and suggests which applications will see the most significant improvement from employing nanofluids.

Small particles, big impacts: A review of the diverse applications of nanofluids

Likely near-term solar-thermal water splitting technologies

Solar-thermal production of renewable hydrogen

The steam-gasification of biochar with concentrated solar radiation is experimentally investigated using a 3 kW solar reactor prototype consisting of a cylindrical cavity-receiver containing an opaque tubular absorber. Particles of beech charcoal are used as the biomass feedstock in a continuous steam-particle flow through the tubular absorber. A reactor model that couples radiative, convective, and conductive heat transfer to the chemical kinetics is formulated and validated by comparing numerically computed and experimentally measured temperatures and carbon conversions. The simulation model is further applied to examine the thermal performance of 100 kW and 1 MW scaled-up solar reactor containing multiple tubular absorbers, yielding a theoretical maximum solar-to-chemical energy conversion efficiency of 39% and 50%, respectively. Major sources of irreversibility are associated with re-radiation losses through the cavity's aperture.

Solar-driven biochar gasification in a particle-flow reactor

A cavity-receiver containing a tubular absorber for high-temperature thermochemical processing using concentrated solar energy

High temperature biomass gasification has been performed in a prototype concentrated solar reactor. Gasification of biomass at high temperatures has many advantages compared with historical methods of producing fuels. Enhancements in overall conversion, product composition ratios, and tar reduction are achievable at temperatures greater than 1000°C. Furthermore, the utilization of concentrated solar energy to drive these reactions eliminates the need to consume a portion of the product stream for heating and some of the solar energy is stored as chemical energy in the product stream. Experiments to determine the effects of temperature, gas flow rate, and feed type were conducted at the high flux solar furnace at the National Renewable Energy Laboratory, Golden, CO. These experiments were conducted in a reflective cavity multitube prototype reactor. Biomass type was found to be the only significant factor within a 95% confidence interval. Biomass conversion as high as 68% was achieved on sun. Construction and design considerations of the prototype reactor are discussed as well as initial performance results.

Christopher Perkins

Papers

Likely near-term solar-thermal water splitting technologies

Solar-thermal production of renewable hydrogen

Solar-driven biochar gasification in a particle-flow reactor

A cavity-receiver containing a tubular absorber for high-temperature thermochemical processing using concentrated solar energy

Rapid High Temperature Solar Thermal Biomass Gasification in a Prototype Cavity Reactor