Combined Decarbonization of Electrical Energy Generation and Production of Synthetic Fuels by Renewable Energies and Fossil Fuels

Power generation from renewable energy sources and fossil fuels are integrated into one system. A combination of technologies in the form of a carbon capture utilization (CCU)-combined power station is proposed. The technology is based on energy generation from fossil fuels by a coal power plant with CO₂ recovery from exhaust gases, and pyrolysis of natural gas to hydrogen and carbon, completed by reverse water-gas shift for the conversion of CO₂ to CO, which will react with hydrogen in a Fischer-Tropsch synthesis for synthetic diesel. The carbon from the pyrolysis can replace other fossil carbon or can be sequestered. This technology offers significant CO₂ savings compared to the current state of technology and makes an environmentally friendly use of fossil fuels for electricity and fuel sectors possible.     

Recent advances on the reduction of CO2 to important C2 + oxygenated chemicals and fuels

The chemical utilization of CO₂ is a crucial step for the recycling of carbon resource. In recent years, the study on the conversion of CO₂ into a wide variety of C_2 + important chemicals and fuels has received considerable attention as an emerging technology. Since CO₂ is thermodynamically stable and kinetically inert, the effective activation of CO₂ molecule for the selective transformation to target products still remains a challenge. The well-designed CO₂ reduction route and efficient catalyst system has imposed the feasibility of CO₂ conversion into C_2 + chemicals and fuels. In this paper, we have reviewed the recent advances on chemical conversion of CO₂ into C_2 + chemicals and fuels with wide practical applications, including important alcohols, acetic acid, dimethyl ether, olefins and gasoline. In particular, the synthetic routes for CC coupling and carbon chain growth, multifunctional catalyst design and reaction mechanisms are exclusively emphasized.     

Hydrocracking of Fischer‐Tropsch Wax with Tungstovanadophosphoric Salts as Catalysts

The synthesis of liquid hydrocarbons from CO₂ and H₂, based on renewable energy and H₂O electrolysis, respectively, in a power‐to‐liquid process is a promising concept for the substitution of fossil fuels. Such a process is based on Fischer‐Tropsch synthesis followed by hydrocracking to convert waxy products into transportation fuels such as gasoline and diesel oil. Heteropolyacid cesium salts as catalysts show appropriate activity for hydrocracking, and the selectivity in cracking model hydrocarbons and Fischer‐Tropsch wax can be tuned by the vanadium content of the catalyst. Thermal stability and surface properties were investigated, and the catalysts are compared with a classical H‐Y‐type zeolite used for hydrocracking.     

Komplementäre Dekarbonisierung der Erzeugung von Elektroenergie und Gewinnung synthetischer Kraftstoffe durch Erneuerbare Energien und fossile Energieträger

The concept of complementary decarbonisation of power generation from renewable energy sources and fossil fuels consists of their integration in one system. A technology network in the form of a CCU‐combined power plant is proposed for the energy generation from fossil fuels by a coal power plant with CO₂ recovery from the exhaust gases and a pyrolysis of natural gas to hydrogen and carbon as a basic technology. This technology network is completed by a reverse water‐gas shift reaction for the conversion of the CO₂ to CO, which will react with the hydrogen in a Fischer‐Tropsch synthesis for synthetic diesel. The recovered energy from the exothermic Fischer‐Tropsch synthesis meets the energy needs of CO₂ scrubbing. The carbon from the pyrolysis can replace other fossil carbon or can be sequestered.     

Methane Pyrolysis for CO2-Free H2 Production: A Green Process to Overcome Renewable Energies Unsteadiness

The Carbon2Chem® project aims to convert exhaust gases from the steel industry into chemicals such as methanol to reduce CO₂ emissions. Here, H₂ is required for the conversion of CO₂ into methanol. Although much effort is put to produce H₂ from renewables, the use of fossil fuels, especially natural gas, seems to be fundamental in the short term. For this reason, the development of clean technologies for the processing of natural gas with a low environmental impact has become a topic of utmost importance. In this context, methane pyrolysis has received special attention to produce CO₂-free H₂.     

Large-Scale Hydrogen Production

There is a growing need for hydrogen in processing heavier and dirtier fossil fuels and a future hydrogen economy is widely suggested as the next generation fuel/energy source once fossil fuels diminish in availability. Sustainable fuels are still regarded as too expensive given the large amounts of natural gas and a projected, ample supply of fossil fuels beyond the next twenty-plus years. Today, the steam reforming of hydrocarbons is the most favorable route to H₂. If CO₂ sequestration were ever to become widely practiced, fossil fuels would continue to play an important role in the future hydrogen economy.     

Upgrading of Raw Gas from Biomass and Waste Gasification: Challenges and Opportunities

The depletion of fossil fuel-based resources and concerns for increasing emissions of CO₂ call for new ways of producing environmentally-friendly substitutes for motor fuels and chemicals. Thermo-chemical conversion of biomass and waste using gasification is a strong candidate to meet these challenges. For efficient and cost-effective application of this technique, novel solutions for hot gas cleaning are needed. This review highlights some important areas for improvement of upgrading technologies for pressurised fluidised bed gasification systems using biomass as a fuel.     

Solar Light Photocatalytic CO2 Reduction: General Considerations and Selected Bench-Mark Photocatalysts

The reduction of carbon dioxide to useful chemicals has received a great deal of attention as an alternative to the depletion of fossil resources without altering the atmospheric CO₂ balance. As the chemical reduction of CO₂ is energetically uphill due to its remarkable thermodynamic stability, this process requires a significant transfer of energy. Achievements in the fields of photocatalysis during the last decade sparked increased interest in the possibility of using sunlight to reduce CO₂. In this review we discuss some general features associated with the photocatalytic reduction of CO₂ for the production of solar fuels, with considerations to be taken into account of the photocatalyst design, of the limitations arising from the lack of visible light response of titania, of the use of co-catalysts to overcome this shortcoming, together with several strategies that have been applied to enhance the photocatalytic efficiency of CO₂ reduction. The aim is not to provide an exhaustive review of the area, but to present general aspects to be considered, and then to outline which are currently the most efficient photocatalytic systems.     

Designing Catalysts for Tomorrow’s Environmentally Benign Processes

After first recalling some of the key challenges involved in meeting the demands for energy and chemicals by 2050, brief accounts are given of (a) the present scene in water oxidation photocatalysis with particular reference to TiO₂ and other oxides possessing the appropriate band gap and (b) of band structure engineering of semiconductors for enhanced photoelectrochemical water splitting. The photochemical possibilities of metal–organic frameworks are then briefly outlined as are (c) some of the solids (mainly TiO₂) for the destruction of pollutants and other substances that are environmentally aggressive. Solar-cavity receivers for the catalytic (thermochemical) conversion of CO₂ to CO and H₂O to H₂ are then considered. The principal feature of this article is an analysis, with specific examples, of some existing catalysts that are likely to prove difficult to replace. Biorefineries for producing transportation fuels and bulk chemicals are then considered. The article concludes with the prospects for the design of new catalysts and, in particular, with the fertility of the concept of single-site heterogeneous catalysts in both thermal and photochemical contexts.     

Greenhouse gas emissions of bioenergy from agriculture compared to fossil energy for heat and electricity supply

Increasing the use of bioenergy is one promising option to reduce greenhouse gas emissions. Hence it is important to know the greenhouse gas emissions of bioenergy systems in comparison to fossil fuel systems. A life cycle analyses of biomass and fossil fuel energy systems is made to compare the overall greenhouse gas emission of both systems for heat and electricity supply. Different bioenergy systems to supply electricity and heat from agriculture are analysed for the Austrian situation in 2000. Total emissions of greenhouse gases (CO₂, N₂O, CH₄) along the fuel chain, including land use change and by-products, are calculated. The systems taken into consideration are different conversion technologies and different fuels from agriculture. The methodology was developed within the International Energy Agency (IEA) Bioenergy Task 25 on `Greenhouse Gas Balances of Bioenergy Systems'. In this paper the results of selected bioenergy systems for heat supply and combined supply of electricity and heat shown as emission of CO₂-equivalents per kWh for bioenergy systems in comparison to fossil fuel systems, and as a percentage of CO₂-equivalent reduction. The results demonstrate that some of the bioenergy systems reduce greenhouse gas emission already because of avoided emissions of the reference biomass use and/or because of certain substitution effects of by-products. In general the greenhouse gas emissions of bioenergy systems are lower compared to the fossil systems. Therefore a significant reduction of greenhouse gases is possible by replacing fossil energy systems with bioenergy systems. This comparison should help policy makers, utilities and industry to identify effective agricultural biomass options in order to reach emission reduction targets. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoinduced activation of CO2 on TiO2 surfaces: Quantum chemical modeling of CO2 adsorption on oxygen vacancies

Chemical processes that utilize CO₂ emissions from coal-fired power plants will be required as the world progresses towards reducing CO₂ emissions. The conversion of CO₂ using light energy (CO₂ photoreduction) has the potential to produce useful fuels or valuable chemicals while decreasing CO₂ emissions from the use of fossil fuels such as coal. Computational studies on the initial steps of photoinduced CO₂ activation on TiO₂ surfaces, necessary to develop a mechanistic understanding of CO₂ photoreduction are a focus of this article.The results from previous quantum mechanical modeling studies conducted by the authors indicated that stoichiometric TiO₂ surfaces likely do not promote electron transfer to CO₂. Therefore, the role of oxygen vacancies in promoting the light-induced conversion of CO₂ (CO₂ photoreduction) on TiO₂ surfaces was examined in this study. Two different side-on bonded bent-CO₂ (bridging Ti-CO₂^δ•−-Ti species) were formed on the reduced rutile (110) and anatase (010), (001) surfaces, indicating charge transfer from the reduced surface to CO₂. Further steps in the photoexcitation of these bent-CO₂ species were investigated with density functional theory calculations. Consistent with CO₂ adsorption and photodesorption on other n-type metal oxides such as ZrO₂, the results suggest that the bent-CO₂ species do not gain further charge from the TiO₂ surface under illumination and are likely photodesorbed as neutral species. Additionally, although the formation of species such as CO and HCHO is thermodynamically possible, the energy needed to regenerate the oxygen vacancy on TiO₂ surfaces (~ 7 eV) is greater than that available through band-gap illumination (3.2 eV). Therefore, CO₂ reactions with water on irradiated anatase TiO₂ surfaces are likely to be stoichiometric.     

Emission Reduction of Regenerative Fuel Powered Co-Generation Plants with SCR- and Oxidation-Catalysts

Since the beginning of combustion engine development in this recent century various different fuels have been successfully tested. Diesel engines have been adapted to fuels made from mineral oils because of the rising importance and the cheapness in comparison to other fuels. On the other hand, it is possible to burn regenerative fuels in engines and achieve some significant advantages in comparison to fossil diesel fuel. This is, for example, a closed carbon dioxide (CO₂) cycle which causes no green house effect. It is possible to extract oil from various seeds like rapeseed. It is also possible to burn used oil from the food processing industry or waste grease and oil from food recycling companies. The great advantages: (1) food recycling oils can produce energy instead of use as animal food, and (2) as nobody knows exactly the consistency of the collected oils, poisonous pollution is possible. These regenerative fuels can be burned without any further processing in special adapted diesel engines, for example an Elsbett engine, or in precombustion engines with large swept volumes. Most researchers focused on operating diesel engines with regenerative fuels and reducing the emissions caring only about regulated exhaust components. In comparison to these studies it is necessary to learn more about the emissions beyond the exhaust regulations. Additionally emission reduction is possible by using an SCR-catalyst (selective catalytic reduction) to reduce the NO₂ combined with an oxidation-catalyst which reduces any kind of oxidisable emissions. The TU München, Lehrstuhl für Energie- und Umwelttechnik der Lebensmittelindustrie, operates a small co-generation plant with the ability of analysing the standard emission components (CO, NO₂, HC, particles, CO₂, O₂) and unregulated components (SO₂, NH₃, polycyclic aromatic hydrocarbons (PAH), aldehyde, ketone). The emissions show some significant differences in comparison to fossil diesel fuel which is caused by the diversity of each fuel. Results of an investigation on four different fuels (wastefat methyl ester (WME), rapeseed methyl ester (RME), rapeseed oil and diesel fuel) burned in a small co-generation plant with a SCR- and oxidation-catalyst will be presented. A comparison to the emissions before and after the catalysts will be shown additionally to the results of the different reduction potential of diesel fuel, methyl ester or untreated oils. The combination of regenerative fuel and catalyst shows good potential for reducing the emissions. Furthermore the use of regenerative fuels is a sustainable production of energy with an overall efficiency of almost 90%. Regenerative fuels based on vegetable oils and waste fat are a valuable form of energy and have some significant advantages in comparison to diesel fuel, like an almost closed carbon dioxide cycle, rapid biological decomposition and lower CO, HC and particle emissions. Regenerative fuels should also meet minimum standards discussed in the paper to avoid the risk of engine damage and to reduce emissions.     

Fuels and Energy for the Future: The Role of Catalysis

Abstract

There are many reasons to decrease the dependency on oil and to increase the use of other energy sources than fossil fuels. The wish for energy security is balanced by a wish for sustainable growth. Catalysis plays an important role in creating new routes and flexibility in the network of energy sources, energy carriers, and energy conversion. The process technologies resemble those applied in the large scale manufacture of commodities. This is illustrated by examples from refinery fuels, synfuels, and hydrogen and the future role of fossil fuels is discussed.     

Evaluation of strategies for the subsequent use of CO<Subscript>2</Subscript>

If substantial amounts of CO₂, which according to actual scenarios may in the future be captured from industrial processes and power generation, shall be utilized effectively, scalable energy efficient technologies will be required. Thus, a survey was performed to assess a large variety of applications utilizing CO₂ chemically (e.g., production of synthesis-gas, methanol synthesis), biologically (e.g., CO₂ as fertilizer in green houses, production of algae), or physically (enhancement of fossil fuel recovery, use as refrigerant). For each of the processes, material and energy balances were set up. Starting with pure CO₂ at standard conditions, expenditure for transport and further process specific treatment were included. Based on these calculations, the avoidance of greenhouse gas emissions by applying the discussed technologies was evaluated. Based on the currently available technologies, applications for enhanced fossil fuel recovery turn out to be most attractive regarding the potential of utilizing large quantities of CO₂ (total capacity > 1000 Gt CO₂) and producing significant amounts of marketable products on one hand and having good energy and material balances on the other hand \(\left( {{{t_{CO_2 - emitted} } \mathord{\left/ {\vphantom {{t_{CO_2 - emitted} } {t_{CO_2 - utilized} < 0.2 - 0.4}}} \right. \kern-\nulldelimiterspace} {t_{CO_2 - utilized} < 0.2 - 0.4}}} \right)\). Nevertheless, large scale chemical fixation of CO₂ providing valuable products like fuels is worth considering, if carbon-free energy sources are used to provide the process energy and H₂ being essential as a reactant in a lot of chemical processes (e.g., production of DME: \({{t_{CO_2 - emitted} } \mathord{\left/ {\vphantom {{t_{CO_2 - emitted} } {t_{CO_2 - utilized} > 0.34}}} \right. \kern-\nulldelimiterspace} {t_{CO_2 - utilized} > 0.34}}\)). Biological processes such as CO₂ fixation using micro-algae look attractive as long as energy and CO₂ balance are considered. However, the development of effective photo-bioreactors for growing algae with low requirements for footprint area is a challenge.     

Heterogen katalysierte Herstellung von Furfural aus Xylose

The results of the dehydration of C₅‐raw material to obtain furfural via heterogeneous catalysis in water are discussed. Furfural has the potential to substitute fossil compounds on a large scale for polymers, solvents, fine chemicals and finally fuels. Industrial catalysts were reviewed and new catalysts were synthesized. The main difference to recent work is the abdication of supercritical fluids, organic solvents, phase modifiers and ionic liquids. With this also the contamination of the product is prevented and higher product qualities are obtained.     

Addressing the CO2 dilemma

Perspectives are offered for reducing the impact of huge amounts of CO₂ produced today from power generation and transportation vehicles. The origins of the dilemma between the world’s increasing use of hydrocarbons as an energy source and the cogeneration of CO₂ which results as a co-product are discussed. Hydrocarbons will provide much of the fuel needs for these major, global industries for the next 20 years and meet 60% of the world’s energy demand. With the growth of both power generation and transportation vehicles around the world, CO₂ levels will continue to increase in the atmosphere. Renewables such as wind, dams, and biomass will not be able to handle all the energy demand. Technology breakthroughs are needed to reduce the world’s dependence on fossil fuels, which will be aggravated by the drive to use more coal. Current approaches for removing CO₂ are discussed as well as near term and future options with particular focus on how catalysis can offer some solutions. In particular, solar photocatalysis based approaches offer a potentially viable energy solution.     

Direct conversion of CO2 with diols,aminoalcohols and diamines to cyclic carbonates,cyclic carbamates and cyclic ureas using heterogeneous catalysts

CO₂ has a large effect on global warming by greenhouse gases, and development of an effective technique for the reduction of CO₂ is a crucial and urgent issue. From the chemical viewpoint, CO₂ is regarded as a stable, safe and abundant C1 resource, and the transformation of CO₂ to valuable chemicals is promising not only for reduction of CO₂ but also for production of useful chemicals. This mini‐review focuses on the direct conversion of CO₂ with diols, aminoalcohols and diamines to cyclic compounds such as cyclic carbonates, cyclic carbamates and cyclic ureas, and in particular discusses the mechanisms for these reactions over heterogeneous catalysts. © 2013 Society of Chemical Industry     

CO2 Reduction to Substitute Natural Gas: Toward a Global Low Carbon Energy System

Methane has proven to be an outstanding energy carrier and is the main component of natural gas and substitute natural gas (SNG). SNG may be synthesized from the CO₂ and hydrogen available from various sources and may be introduced into the existing infrastructure used by the natural gas sector for transport and distribution to power plants, industry, and households. Renewable SNG may be generated when H₂ is produced from renewable energy sources, such as solar, wind, and hydro. In parallel, the use of CO₂-containing feed streams from fossil origin or preferably, from biomass, permits the avoidance of CO₂ emissions. In particular, the biomass-to-SNG conversion, combined with the use of renewable H₂ obtained by electrolysis, appears a promising way to reduce CO₂ emissions considerably, while avoiding energy intensive CO₂ separation from the bio feed streams. The existing technologies for the production of SNG are described in this short review, along with the need for renewed research and development efforts to improve the energy efficiency of the renewables-to-SNG conversion chain. Innovative technologies aiming at a more efficient management of the heat delivered in the exothermic methanation process are therefore highly desirable. The production of renewable SNG through the Sabatier process is a key process to the transition towards a global sustainable energy system, and is complementary to other renewable energy carriers such as methanol, dimethyl ether, formic acid, and Fischer-Tropsch fuels.     

Microbial photosynthesis in the harnessing of solar energy

The shortage of fossil fuels restricts the world supply of reduced carbon compounds and energy sources. Biotechnology offers the most feasible route to renewing the supplies of reduced carbon compounds. This involves recycling of CO₂ through photosynthesis. Conventional agriculture has little or no potential for supplying biomass and its derivatives on sufficient scale to offer an alternative to the fossil fuels. The agricultural wastes, on the whole, are intractable to conversion into useful carbon and energy sources and in any case are not available in amounts to provide a significant alternative to the fossil fuels. In contrast, microbial photosynthesis, optimised in photobioreactors, has vast potential to provide organic matter on a scale to match the consumption of fossil fuels. The quantitative study of microbial photosynthesis as a biotechnological route to biomass has been neglected. As a result there is a chaos of conflicting data on fundamental parameters, for example, the photosynthetic efficiency of biomass production. New photosynthetic biotechnology with fully controlled continuous-culture systems is providing unequivocal values for the parameters. For the scale-up of microbial photosynthesis a tubular-loop reactor is proposed.     

Comparing Molecular Mechanisms in Solar NH3 Production and Relations with CO2 Reduction

Molecular mechanisms for N₂ fixation (solar NH₃) and CO₂ conversion to C2+ products in enzymatic conversion (nitrogenase), electrocatalysis, metal complexes and plasma catalysis are analyzed and compared. It is evidenced that differently from what is present in thermal and plasma catalysis, the electrocatalytic path requires not only the direct coordination and hydrogenation of undissociated N₂ molecules, but it is necessary to realize features present in the nitrogenase mechanism. There is the need for (i) a multi-electron and -proton simultaneous transfer, not as sequential steps, (ii) forming bridging metal hydride species, (iii) generating intermediates stabilized by bridging multiple metal atoms and (iv) the capability of the same sites to be effective both in N₂ fixation and in CO_x reduction to C2+ products. Only iron oxide/hydroxide stabilized at defective sites of nanocarbons was found to have these features. This comparison of the molecular mechanisms in solar NH₃ production and CO₂ reduction is proposed to be a source of inspiration to develop the next generation electrocatalysts to address the challenging transition to future sustainable energy and chemistry beyond fossil fuels.