(330c) Windfuels – Competitive Transportation Fuels from Wind Energy and Waste CO2

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 Wind-Fuels ? solves these problems. Annual WindFuels production per land area in good wind regions should exceed biofuels production density in fertile farming areas by more than a factor of five.

Water and renewable electrical power are fed into an electrolyzer, which produces the hydrogen needed in the Reverse Water Gas Shift (RWGS) reactor and in the novel Renewable Fischer Tropsch Synthesis (RFTS) plant design. Waste CO2 may be supplied from a coal power plant, and the novel RWGS process design is shown to permit practical reduction of CO2 to the CO needed in the RFTS process at efficiencies approaching theoretical limits.

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 extensive 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 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 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.

Wind's growth rate is currently beginning to be limited by transmission-grid capacity, but RFTS completely eliminates that problem. If the 28% annual growth rate of wind energy of the past 13 years is maintained for another decade, wind could be providing over 5% of our transportation fuel and 5% of our electrical energy needs in 2018.

The cost of producing chemicals and fuels in an RFTS plant will depend mostly on the quality of the wind site and on the market for the co-produced liquid oxygen. In a Class-5 wind site, ethanol may be profitable at $1.10/gal as long as the local oxygen market is strong and subsidies for renewable fuels are not decreased. In a Class-4 site with no oxygen market and no subsidies for climate benefit, the cost of wind-ethanol should be about $2.70/gal.

A simplified flow diagram discussion is useful for presenting a plant overview and system summary. The first key to competitive performance is obtaining pressurized high-purity hydrogen and oxygen at very high efficiency, which in turn requires operating an electrolyzer at very high pressure. Preheated water is fed into the alkaline electrolyzer that is powered by renewable elec-tricity to produce the oxygen and hydrogen. The pressurized O2 and H2 are then optimally ex-panded before being used. The source hydrogen, at ~4 MPa (near term), further heated using waste heat, is then expanded in a turbo-generator to ~1 MPa. The cleaned, source CO2 is heated and expanded in another turbo-generator. The H2 and CO2 are then further heated before being fed into the RWGS reactor.

The second key is efficient RWGS performance. Two viable approaches ? denoted as ?multi-stage RWGS? and ?recycle RWGS? ? are presented. To drive the reaction equilibrium to the right, most of the water must be efficiently condensed out of the RWGS products as the reaction progresses. Hence, ultra-high-performance gas-to-gas recuperation is central to either approach, and a crucial advance in gas-to-gas recuperation is the subject of a pending patent application. In the recycle case, a CuAlCl4-aromatic complexing method is used to separate the CO and drive the reaction even farther to the right. If there is excessive CO2 in the RWGS products, it needs to be recycled. The CO and H2 from the RWGS reactor are then compressed in a turbo-compressor to produce the pressurized ?new syngas?, with typical molar-% compositions as noted in the flow diagram. This is combined with the preheated recycled syngas and fed into the FTS reactor. A fixed-bed multi-tubular FTS reactor design is shown to have advantages for high-pressure, variable-rate, low-conversion, high-temperature, highly exothermic reactions, as needed for high yield of mid-alcohols.

The third key is achieving dramatically improved efficiency in handling low-conversion FTS processes by using high-pressure condensers for the initial separations. Further compression to 8-14 MPa may be needed to achieve adequate gas and product separations in cryogenic condensers. To achieve adequate FTS-catalyst lifetime, it is necessary to separate much of the WGS-CO2 byproduct from the FTS products for re-conversion to CO in the RWGS reactor. A novel boost-expand separation process is disclosed that allows nearly an order of magnitude lower power consumption than common CO2 separation methods. This is possible partly because of efficient cryogenic recuperation of the cooling capacity in the recycled syngas after its expansion back to the pressure needed in the FTS reactor. The separation also benefits from the recuperator advances mentioned previously, and it benefits markedly from higher FTS reactor operating pressure ? a counter-intuitive discovery.

The fourth key is designing a plant that is inherently compatible with operation over a very wide range of mass flow rates. Variable-angle nozzles, variable-speed motors and generators, and turbine switching assist to this end, along with the use of optimal heat transfer processes. A number of additional features further improve efficiency, including a refrigeration cycle utilizing the free compressed oxygen, a dual-source organic Rankine cycle heat engine, and an improved CH4 separation process.

The fifth key is simplified local upgrading because of the absence of troublesome impurities in the crude products and because of the availability of abundant hydrogen, oxygen, low-grade waste heat, electrical power, and excess cryocooling capacity. Other beneficial aspects of the separations processes allow simplified recovery of all flash gases and avoid the need for any significant purge stream. Many details of these and other innovations, soon to be published in pend-ing patents, will be included in the presentation.

The annual U.S. demand for the various chemicals that are not major fuel components that would come from the RFTS reactors (free of sulfur, salts, metals, halides, and nitrogen) is nearly 100 million tons, and this highly profitable market exceeds 100 billion dollars. The most economically attractive route to a major reduction in green-house gases (GHGs) appears to be synthesis of liquid fuels and chemicals from wind energy and waste CO2. Only these global markets can sup-port the rapid scale-up needed to avert a climate disaster.