Earlier this week I ran across a very interesting white paper entitled "Sustainable fuel for the transportation sector" published in March of 2007 by researchers at Purdue University. The paper was written by Agrawal, Singh, Ribeiro and Delgass of Purdue’s School of Chemical Engineering and Energy Center. It advocates synthesizing liquid hydrocarbon fuels like diesel and gasoline by employing a hybrid hydrogen-carbon (H[sub]2[/sub]CAR) process that uses biomass as the carbon source and adds hydrogen gas produced from carbon-free renewable energy, such as solar or wind. Following gasification, CO, H[sub]2[/sub], CO[sub]2[/sub] and H[sub]2[/sub]O pass into a reactor that produces liquid fuel. Unreacted CO[sub]2[/sub], CO and H[sub]2[/sub] are recycled back to the gasifier. The objective is to utilize every carbon atom of the feedstock to produce usable fuel. The researchers believe their process achieves this goal.
According to the paper, other existing processes are able to produce only about 30% of the total transportation fuel used in the U.S. from the available annual biomass production of 1.366 billion tons. By achieving nearly 100% efficiency in carbon utilization, the H[sub]2[/sub]CAR process reduces the amount of arable land needed for sustainable fueling of the entire U.S. transportation sector to less than 40 percent of the requirements of other methods. It therefore has the potential to supply 100% of the fuel needed to run all U.S. transportation from the same amount of biomass. Copies of the full article are available to read or download at http://www.pnas.org/cgi/reprint/104/12/4828
The technique can also be used to process coal into liquids. According to the authors, the same 100% efficiency of carbon conversion applies to coal-to-liquids as to biomass-to-liquids. Consequently, no CO[sub]2[/sub] is released into the atmosphere in connection with the actual conversion process of coal-to-liquids. However, using biomass to produce transportation fuels is a closed loop, in which photosynthesis converts atmospheric CO[sub]2[/sub] to O[sub]2[/sub] and biomass. The latter is converted to fuel and burned in vehicles, releasing CO[sub]2[/sub], which is converted to biomass, etc. Producing liquid fuels from America's enormous reserves of coal is not a cyclical process capable of sustainability. In the long run, using the H[sub]2[/sub]CAR process in this fashion would significantly extend the life of American coal supplies, but probably would not reduce coal usage enough to have a sufficiently major beneficial impact on the problem of increasing levels of greenhouse gases without finding a way to permanently sequester the CO[sub]2[/sub] emissions from burning the fuel.
The preferred fuels of the U.S. transportation sector are liquid hydrocarbons because of their high energy density and easy portability. Unfortunately, the cost of these fuels is projected to rise inexorably over the coming months, years and decades as the available supply fails to meet demand. While hydrogen gas shows some promise for powering our vehicles, it is hindered by the fact that it lacks the energy density of liquid hydrocarbons and, as of yet, has no infrastructure for delivery. It is often described as a means of transporting and utilizing other primary energy sources, such as sunlight, wind and electricity, rather than as an energy source in its own right.
According to the authors of the Purdue paper, there are a number of good reasons for adding extra hydrogen to the gasification mix. In the first place, it can be produced from renewable energy sources. Second, production of hydrogen through solar generation of electricity and hydrolysis has a significantly higher annualized solar energy conversion efficiency than does producing biomass from photosynthesis. Third, adding H[sub]2[/sub] atoms to carbon atoms from coal or biomass, unlike storage of H[sub]2[/sub] in tanks, provides a method of storing massive quantities of H[sub]2[/sub] in an extremely dense and portable form. Thus, in the quest to produce large quantities of liquid hydrocarbons to use as a highly concentrated transportation fuel, hydrogen produced from wind or solar energy would appear to have a significant role to play.
Using gasification of coal or biomass to produce biofuels has its own problems, chief among which is the difficulty of "rigging" the process in order to achieve the most efficient utilization of the feedstock, the goal being to maximize the amount of fuel produced and minimize creation of undesirable byproducts. Some of these byproducts, such as CO and CO[sub]2[/sub], are poisonous and/or contribute to global warming. It may therefore be helpful to consider briefly some of the byproducts derived from the conversion of biomass into fuels.
For example, in addition to the desired fuel, when biomass is heated to greater than 400 degrees Centigrade in an oxygen-free environment, the process also yields 25 percent charcoal and large quantities of dense tars. When air is added to create "producer gas," the output stream typically consists of CO (22%), H[sub]2[/sub] (18%), CH[sub]4[/sub] (3%) and N[sub]2[/sub] (51%). Instead of air, pure oxygen can be used to make "synthesis gas," which is comprised of CO (40%), H[sub]2[/sub] (40%), CH[sub]4[/sub] (3%) and CO[sub]2[/sub] (17%, dry basis). Synthesis gas can be further refined with commercial catalytic methods to create methanol (CH[sub]3[/sub]OH), ammonia (NH[sub]3[/sub]) and diesel fuel.
Some of these byproducts create disposal problems, as in the cases of tars, CO and CO[sub]2[/sub]. It will be necessary either to find uses for these byproducts, to devise practical methods of sequestering them or to discover methods of completely utilizing them in the process of making fuel. It is this latter possibility that is one of the things I find so intriguing about the proposal of the Purdue investigators.
Some experts believe we cannot "farm" our way out of the twin conundra presented by Peak Oil and Global Warming. They say that the magnitude of the problem is just too great and that biofuels are not the answer. I agree that some biofuels, like corn ethanol, are not the solution and that governmental incentives for their production should be repealed. Nevertheless, the Purdue research may present an independent basis of hope for creating sustainable fuels.
T. Boone Pickens' company Mesa Power, LLP recently purchased some 667 wind turbines for a wind farm in Pampa, TX touted by Pickens as the largest of its kind in the world. Other large wind projects are in the construction, development or planning stages throughout the Texas-to-Montana wind corridor and offshore. This would appear to create an opportunity to combine these wind farms with on-site or nearby hydrogen electrolysis plants and H[sub]2[/sub]CAR plants that would have the potential to create enormous quantities of hydrocarbon fuels.
Of course, farming sustainably on the scale envisioned to produce biofuels will create an enormous need for fertilizer. More research is needed on the potential for using some of the wind/hydrogen energy from wind or solar farms to fuel a process for fixing large quantities of ammonia from atmospheric N[sub]2[/sub] gas to aid in the production of the required biomass from renewable sources. This could be done as a wholly separate project or possibly as part of the H[sub]2[/sub]CAR process if some quantity of air is used in producing the fuel, followed by commercial catalysis, as mentioned previously.
A very large potential fly in the ointment with respect to prospects for actually "farming" our way out of the Global Warming and Peak Oil crises and dependence on foreign oil is water availability—or lack thereof. The Ogallala Aquifer is being depleted at alarming rates by agricultural and other uses of water in large parts of the wind corridor mentioned above. Aggressive methods of reducing other water uses may have to be developed, or water may have to be imported, or electricity or hydrogen may have to be exported to other farming regions for this to work on a large scale. In the final analysis, there simply may not be enough land and water to provide sufficient food and water for an inexorably expanding U.S. population and to fuel our burgeoning transportation sector. The effect on food prices of applying enormous acreages to the production of biomass for fuels is obviously a matter of deep concern.
If we fail to realize that overpopulation of this country threatens to overwhelm the potential of the Purdue process and other innovated technologies to create sustainable transportation fuels, we may be doomed to devolve permanently to lower tech, more isolated lifestyles amid pervasive shortages of food, energy and other commodities. On the way down, we are likely to experience much human suffering, strife, violence and even massive population die-offs. This will inevitably occur,not only in the rest of the world, but within the U.S., as well.
It is therefore imperative that we take rising oil prices and gas shortages for what they are—a dire warming of the likely consequences of adopting the Purdue technology and then reverting to business as usual. New technologies like the H[sub]2[/sub]CAR process and large-scale solar and wind farming offer a flickering hope for the future. However, if we do not recognize the need to achieve zero population growth in the near future, it will probably all be for naught.