WindFuels – Competitive Transportation Fuels From Wind Energy and Waste CO 2
It is essential for the market to help drive the dramatic cut needed in CO2 emissions to prevent a climate disaster in this century. We show that the economics for producing clean liquid hydrocarbons and alcohols from water and waste CO2 on wind farms improved by an order of magnitude between 2002 and 2008, and we present the scientific and technological basis for another factor-of-two improvement in these economics. Wind energy is by far the most competitive renewable energy resource in many regions. The perceived challenge is getting wind energy from good sites to where and when it is needed, both for the transportation sector and for the power grid. Efficient conversion of wind energy and waste CO2 into clean, stable, liquid fuels – also called WindFuels – solves these problems. Annual WindFuels production per land area in good wind regions should exceed biofuels production density in fertile farming areas by a factor of five. First, water and renewable electrical power are fed into an electrolyzer, which produces the required hydrogen. Then waste CO2 (from coal power plants) and the renewable hydrogen are piped into a novel Reverse Water Gas Shift (RWGS) process, which permits practical reduction of CO2 to the CO needed at efficiencies approaching theoretical limits. The renewable CO and H2 are then fed into a novel Renewable Fischer-Tropsch Synthesis (RFTS) process. There they are converted into mostly mid-alcohols (ethanol, propanol, and butanol) and other valuable chemicals. These fuels may then be readily stored and distributed by conventional means – pipelines and tanker trucks. The electrolyzer also produces a huge amount of oxygen, which may be sold if market conditions warrant, or it may be utilized in novel processes to improve the plant efficiency. From basic thermodynamics, one readily calculates that the theoretical maximum chemical efficiency of synthesis of ethanol from H2 and CO2 is 80.1% without utilization of the excess heat released from the Fischer Tropsch Synthesis (FTS) reactor. If this excess heat is also converted in an ideal heat engine, the theoretical combined-cycle efficiency limit is ~90%. One advantage of ethanol may be appreciated by noting that its synthesis from H2+CO2 results in 1.5 molecules of water per carbon atom in the fuel, while the synthesis of alkanes or alkenes results in 2 molecules of waste water per carbon atom in the fuel. Previously, fossil-based FTS efficiencies for environmentally attractive fuels, such as ethanol and propanol, have usually been under 40%. The need for very efficient recycling of the large amounts of H2, CO, and CO2 in the byproducts from mid-alcohols FTS has possibly been the strongest argument against mid-alcohols FTS compared to gasoline, lubricants, and diesel. A novel plant design is presented, validated by many simulations, that permits order-of-magnitude reduction in energy penalties associated with the major separations in a fully recycled mid-alcohols plant compared to previously published designs. The example wind-driven RFTS plant size chosen for illustration and simulated in detail assumes 250 MW mean input electrical power. It achieves 72% FTS-plant higher heating value (HHV) efficiency in production of mid-alcohols and other products from H2 and recovered (waste) CO2, or about 60% net HHV efficiency when including the electrolyzer at expected near-term performance. At least eight separate, substantial innovations in the system design combine to permit this major advance in RFTS efficiency and cost effectiveness. It is worth pointing out that as recycle efficiency and byproduct utilization both approach 100%, which is the primary thrust in the RFTS optimization, catalyst selectivity and activity both become much less important. Catalysts never affect the thermodynamics – only the kinetics of the various pathways. Hence, further development of catalysts will have limited effect on system efficiency in a plant with 95% recycle efficiency, though of course it will influence profitability by changing the product mix.
Design and Performance of a High-Temperature Regenerator Having Very High Effectiveness, Low Leakage and Negligible Seal Wear
A high-temperature regenerator has been designed under MIT and WTPI patents (figure 1). These call for the ceramic-honeycomb regenerator disk to rotate incrementally, in this case through 90 degrees, for each movement. The seals, which can conform to the profile of the sealing surface, are firmly pressed to the regenerator seal faces during the stationary periods, here about fourteen seconds (figure 2). The seals then lift marginally, just enough to allow free disk movement, during the 0.75 second of the indexing period. Thus no sliding wear can take place. In the demonstrator unit tested the measured effectiveness was over 98%, the pressure drops of the hot and cold flows were under 2%, and the leakage was low within experimental error. The demonstrator was tested at a top temperature of 910° C. (The final temperature after development should be well above 1000°C). The paper describes design methods that are substantially different from those in general use, and discusses design choices for materials and actuation systems. This regenerator was designed to be compatible with a high-temperature 5-kW solid-oxide fuel cell. Our design studies indicate that, when applied (in a considerably larger size) to micro turbines it would enable electrical efficiencies of 50% to be exceeded. Other industrial applications that require too high a temperature for currently available recuperators would also become viable.Copyright © 2006 by ASME
Mechanism of the reverse water gas shift reaction over Cu/ZnO catalyst
Abstract The reverse water gas shift reaction (RWGS) and the reaction with CO2 alone were carried out over a Cu/ZnO catalyst. The surface of the catalyst was characterized by N2O titration, XPS, and FT-IR spectroscopy. CO2 dissociated to give CO and the surface oxygen species. Surface Cu(I) oxide was formed by the reaction with CO2. The oxygen species were hydrogenated to H2O and the surface Cu(I) oxide was reduced to metallic Cu. It is suggested that the RWGS reaction proceeds through surface oxidation and reduction with CO2 and H2, and the dissociation of CO2 is the rate determining step.