Use of excess off-peak electrical energy to synthesize standard liquid fuels, such as gasoline and jet fuel, could simultaneously address grid stability, domestic oil limitations, climate change, and economic recovery. Simulations have shown that practical innovations should make it possible to reduce CO2 to CO at over 90% of theoretical efficiency limits (under 1.55 MJ/kg-CO). When combined with our other simulated process advances, it should then be possible to synthesize all hydrocarbons and alcohols from point-source CO2 and off-peak wind energy by using currently available catalysts at system efficiencies in the range of 53-61%. Off-peak grid energy averaged under $15/MWhr in the MISO hub in the first four months of 2009. (For reference, the cost of energy in gasoline at $3.60/gal is $100/MWhr.) At such prices, synthesized standard liquid fuels (dubbed "WindFuels") could compete even when petroleum is only $45/bbl. There are sufficient amounts of domestic wind resources and point-source CO2 to produce more than twice our current total transportation fuel usage.
A better alternative for future transportation fuels is needed than those that have previously been advocated - such as biofuels, hydrogen, and methane. When land-use change is properly considered, most biofuels are seen to be only 5% to 20% carbon neutral; and hydrogen has daunting cost challenges with respect to distribution, storage, and end use. It is in our economic and security interests to produce carbon-neutral transportation fuels domestically at the scale of hundreds of billions of gallons annually.
It has long been known that it should theoretically be possible to convert CO2 and water into standard liquid hydrocarbon and alcohol fuels at high efficiency. The problem has been that prior proposals for doing this conversion have had efficiencies of only 25% to 35% for preferred fuels. That is, the chemical energy in the standard liquid fuels produced (gasoline, ethanol, diesel, etc.) would be about 30% of the input energy required. Our work shows that nearly a factor-of-two gain in efficiency should be possible.
Doty Energy is developing novel processes that will allow the production of fully carbon-neutral standard fuels and plastics from waste CO2 and off-peak wind or other low-carbon energy at high efficiency and at prices that should soon be competitive with fossil-derived products. Converting CO2 into fuels can eliminate the need for CO2 sequestration and reduce global CO2 emissions by 40%.
The combination of the major technical advances we have simulated over the past two years should permit conversion of CO2 to fuels to be done at up to 60% efficiency, which is about twice what was expected by most researchers just three years ago. When the input energy is from off-peak wind or nuclear and reasonable consideration is included for climate benefit, WindFuels will sometimes compete even when petroleum is $45/bbl; and most experts are predicting oil will stay over $140/bbl after 2014. Our analysis shows windfuels production per gross land area in good wind regions should exceed biofuels production density in productive farming areas by factors of 4 to 20. Moreover, 99.9% of the land required for the wind farms is generally available for dual use. Switching 70% of global transportation fuels from petroleum to WindFuels should be possible over the next 35 years. The scale-up challenges do not appear to be significantly greater from a technical perspective than similar fuel-synthesis challenges addressed successfully by Germany during WWII.
The WindFuels production system (disclosed in detail in pending patents) will be presented. It is based largely on the commercially proven technologies of wind energy, water electrolysis, and Fischer Tropsch (FT) chemistry. Wind energy is used to split water into hydrogen and oxygen. Some of the hydrogen is used in a process, the so-called reverse water gas shift (RWGS) reaction, that reduces CO2 to carbon monoxide (CO) and water. The CO and the balance of the hydrogen are fed into an FT reactor similar to that commonly used to produce fuels and chemicals from coal or natural gas.
The biggest part of the increase in our projected efficiency comes from improved separations processes in the fully recycled FT loop. Conventional processes for separation of CO2 from the other syngas components have typically required over 6 MJ/kg of CO2. Our simulations indicate a high-pressure cryogenic process can achieve sufficient separation in the FT recycle loop (under 15 molar-%) at under 0.8 MJ/kg. We call the design "full recycle" because essentially all the unreacted H2 and CO are recycled without expansion in the separations processes. To our knowledge, nothing close to full recycle in FTS has been done before. The primary reason it has not been implemented with fossil- or biomass-based FTS is that there is insufficient control flexibility in the H2/CO ratio in the syngas coming from a reforming process to compensate for the variability that will be seen from changes in the water-gas-shift reaction in the FT reactor. Maintaining the desired H2/CO ratio in an RFTS process, on the other hand, is a non-issue, as one has complete and independent control over both the H2 and the CO feed rates in the new syngas.
The next largest gain is expected from an order-of-magnitude advance in cost-effectiveness of gas-to-gas recuperation, which is expected to make 97% effectiveness practical where 75% was previously practical. The third largest gain may be from an optimized RWGS process which promises over 90% efficiency (implying under 1.55 MJ per kg of CO) compared to about 50% for prior demonstrations (about 2.8 MJ/kg-CO). Another significant gain comes from a novel thermodynamic cycle for conversion of the waste heat from the electrolyzer and the FT reactor. A novel approach to RFTS system integration and optimization leads to additional efficiency gains. All the processes have been simulated in detail, and key experiments will soon be carried out to help optimize process conditions.
Windfuels are truly sustainable. The needed CO2 would come from biofuels refineries, ammonia plants, cement factories, ore refining, coal power plants, and other point sources. The water requirements are an order of magnitude less than for biofuels, and the wind will always blow. Windfuels will be over 85% carbon neutral and will flow seamlessly within our current infrastructure.
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See more of this Group/Topical: Topical G: Innovations of Green Process Engineering for Sustainable Energy and Environment