The following document is a slightly modified version of a "Statement of Interest" that was submitted to the DOE Climate Change Technology Program Office in response to a request published in the Federal Register.
by Robert E. Uhrig
On behalf of The University of Tennessee and Oak Ridge National Laboratory, I hereby submit this "Statement of Interest" in research on innovative climate change technologies. Our approach involves enhancing the hydrogen content of hydrocarbon transportation fuels as an interim, but important, step along the road to the "hydrogen economy."
Replacing the hydrocarbon content of fuel with hydrogen proportionally reduces the amount of greenhouse gases and disproportionally reduces SOx and NOx pollutants. Currently, the emissions from U.S. transportation systems are contributing over a third of the pollutants and greenhouse gases that adversely influence atmospheric conditions, weather, solar radiation received, and melting of glaciers, i.e., our climate.
In summary, the benefits of increased hydrogenation of refined oil products, if implemented on a large scale, can impact the climate in many beneficial ways. This approach is also consistent with achieving the non-petroleum fuel-constituent goals of the Energy Policy Act as well as pending sulfur reductions in gasoline and diesel fuels.
HYDROGENATION OF OIL PRODUCTS
High-quality crude oil has an overall hydrogen-to-carbon (H/C) ratio of about 1.5, while poor-quality crude oil may have an overall H/C ratio as low as 0.8. When crude oil arrives at a refinery, it is first separated into various fractions, which may be processed further. This further processing can involve adding hydrogen, a process called hydrogenation. The specific hydrogenation process used depends upon the quality of the feed to the process and the upgrading objective. The essence of what is being proposed here is that the hydrogen content of many of the separated components be increased until the overall H/C ratio of the refinery products is about 2.0. This effectively gives the various refined products increased energy per unit mass, which, of course, comes from the added hydrogen.
The chemistry and physics of adding hydrogen to an oil-based fuel, thereby increasing its H/C ratio, is not simple. First, the addition of hydrogen changes the density of the refined product and increases the energy per unit mass. Second, the structure of the resultant product is changed, resulting in changes in its physical and chemical properties. Third, the combustion characteristics change because hydrogen tends to burn differently from oil-based fuels.
Hydrogen-enhanced fuels have several characteristics that make it worth consideration as a component of national energy policy of the United States. The increase of energy per unit mass by the addition of hydrogen means that less mass of crude oil will be required to produce a given amount of energy for transportation systems. Large-scale use of hydrogen-enhanced fuel could reduce oil imports and balance-of-payment deficits significantly. Other advantages are that the hydrogenation and refining technologies are well developed, are in use, and that the existing fuel distribution infrastructure could be used. Indeed, hydrogen-enhanced fuel could be sold at existing gasoline service stations in lieu of a "premium" fuel.
Perhaps equally important, hydrogen-enhanced transportation fuels could be implemented on a nationwide scale in a decade, well before advanced hydrogen-based systems currently under consideration, particularly fuel cells replacing current engines in transportation vehicles, could be implemented. While hydrogen-enhanced fuels can be competitive under a number of special circumstances, the cost of producing hydrogen must be reduced to be competitive generally.
Using electricity in electrolysis to split water into hydrogen and oxygen is a mature technology, and units as large as 10 MWe are available commercially today. Use of electricity generated using nuclear power provides a non-carbon source of hydrogen. Credit for the sale of oxygen and heavy water, as well as for using "spinning reserve" to generate hydrogen (electrolyzers can be switched instantaneously to pick up load dropped by plant failures) may make electrolysis competitive under some circumstances. Recent studies on high temperature-high pressure electrolysis have also been encouraging.
Thermo-chemical splitting of water using very high temperature (over 850°C) heat, particularly using the sulfur-iodine process, is considered as a means of using nuclear energy to produce hydrogen. However, the feasibility of the sulfur-iodine process has been demonstrated only on small-scale units. High-temperature nuclear reactors can provide the required heat, again providing a non-carbon source of hydrogen, but only one such high-temperature reactor is in operation today. Nuclear reactors capable of providing greater than 850°C heat could be available by the time (in about a decade) that the thermo-chemical splitting of water to produce hydrogen is available. The overall thermal efficiency of producing hydrogen with such a system would be about 50 percent.
Methane reforming is by far the most common and most economical method of producing hydrogen today, but it has two drawbacks. About 20 percent of the energy of the methane is used to produce heat for the process, resulting in the emission of both pollutants and greenhouse gases (about 6.7 pounds of CO2 for every pound of H2). Furthermore, the methane feedstock is a premium hydrocarbon fuel, and as such its price has been volatile and generally is tied to the price of oil. Compromising the environmental advantages of hydrogen-enhanced fuel makes this option unattractive unless the endothermic heat required can be provided by an external non-carbon source. Recent studies of this option in Japan using a high-temperature nuclear power plant have been encouraging.
EXAMPLE OF HYDROGEN-ENRICHED FUELS
Recent economic analyses indicate that hydrogen-enhanced fuels may be competitive with oil-based fuels in some special situations. When the overall H/C ratio of oil products is increased, the overall energy per unit mass is increased, and the amount of crude oil required for a given amount of energy is correspondingly decreased. Let us consider two hydrogen-enhanced fuels. The energy contents of oil and hydrogen used are 18,040 BTUs/lb. and 51,690 BTUs/lb. (lower heating value), respectively.
The first fuel, called HE-15, could probably be used in the engine of almost any current transportation vehicle without modification of its engine. It consists of 0.85 lb. pound of oil (containing 15,335 BTUs) to which we add enough hydrogen (0.0523 lb.) to supply the energy of the missing 0.15 lb. of oil (2,705 BTUs). After hydrogenation, we have 0.9023 lb. of this new HE-15 fuel containing 18,040 BTUs. Hence, the specific energy content of HE-15 fuel is 19,995 BTUs/lb. The percentages of total energy coming from oil and hydrogen are 85 percent and 15 percent, respectively. The percentages of materials by weight are 94.2 percent oil and 5.8 percent hydrogen.
The second hydrogen-enhanced fuel, called HE-30, consists of 0.85 lb. of oil to which we add enough hydrogen to supply the energy of the missing 0.15 lb. of crude oil, as well as an equal amount of hydrogen to provide an additional 15 percent energy, or 2,705 BTUs. After hydrogenation, we have 0.9546 lb. of this new HE-30 fuel containing 20,745 BTUs. Hence, the specific energy content of HE-30 fuel is 21,730 BTUs per pound. The percentages of total energy coming from oil and hydrogen are 73.92 percent and 26.08 percent, respectively. The percentages of materials by weight are 89.0 percent oil and 11.0 percent hydrogen. To use HE-30 fuel, transportation vehicles may need some modifications or computer control of their engines of the type used on some premium vehicles today. The properties of HE-15 and HE-30 fuels are summarized in Table 1.
||HE-30 Fuel |
|Specific Energy Content BTUs/lb
|Percent Energy from Crude Oil
|Percent Energy from Hydrogen
|Percent Oil by Weight
|Percent Hydrogen by Weight
TABLE 1. PROPERTIES OF OIL, HE-15 AND HE-30 FUELS
ECONOMICS OF HYDROGEN-ENHANCED FUELS
We can make reasonable assumptions regarding energy costs and the performance of hydrogen generation systems and calculate the costs of HE-15 and HE-30 fuels on a per-pound and per-MBTU basis. The basic energy costs for this example are $29 per barrel of oil and $0.04 per kilowatt-hour of electricity at the bus bar for electrolysis. This cost of electricity may seem low, but it is the bus bar cost without transmission costs or losses.
The following calculations are based on weight because the volumetric behavior of oil products with increased hydrogenation is significantly influenced by the chemistry and physics involved. In calculating the fuel costs, it was assumed that the oil fractions were already being hydrogenated to reach the required overall H/C ratio of 1.5, and hence the only significant additional cost is the cost of hydrogen. As compensation for such an assumption, no credit is taken for the value of the oxygen or heavy water produced or for the significantly improved (but unquantified) environmental benefits. The results are shown in Table 2.
|Oil at $29 per barrel
|Hydrogen produced by electrolysis ($0.04/KWeHr)
|Hydrogen produced by thermal-chemical process
|Hydrogen produced by methane reformation
|HE-Fuel with H2 by electrolysis ($0.04/KWeH)
|HE-Fuel with H2 produced by thermal-chemical process
|HE-Fuel with H2 produced by methane reformation
TABLE 2. COSTS OF OIL, HYDROGEN AND H-E FUELS
Crude oil prices may vary widely over time, and a range from $13 to $65 per barrel is used in Figure 1 to compare the costs of HE-15 fuel with crude oil. Similarly, the cost of electricity may vary with time and conditions, and a range from $0.01 to $0.08 per kWh was used in Figure 2 to compare the costs of HE-15 fuels with the cost of electricity.
Perhaps the most significant features of these plots is the "crossover" values, where the cost of HE-15 fuel using hydrogen produced by the three principal methods are equal to the cost of oil on a $/MBTU basis. For HE-15 fuel, these crossover points occur at $65 per barrel for hydrogen produced by a thermo-chemical process, and at $45 per barrel for hydrogen produced by methane reforming. There is no crossover for HE-15 fuel in the range investigated using hydrogen produced by electrolysis. Similar plots for HE-30 fuel show these crossover points occur at almost the same values for all methods of producing hydrogen.
The crossover points with HE-15 fuel using hydrogen produced by electrolysis are $0.010/kWh for oil, $0.015/kWh for the methane reformed process, and $0.025/kWh for the thermo-chemical process. The crossover point with HE-30 using hydrogen produced by electrolysis are $0.020/ kWh for oil, $0.025/kWh for the methane reforming process, and $0.030/kWh for the thermo-chemical process.
CRUDE OIL SAVINGS
By using HE-15 and HE-30 fuels, we can replace 15.0 percent and 26.1 percent, respectively, of the energy of the crude oil with the energy of hydrogen, thus saving 15.0 and 26.1 percent of the crude oil previously used. If the energy of half of the 19 million barrels of oil per day used in the United States were replaced with energy from HE-15 or HE-30 fuels, the reduction in crude oil required would be 1.40 million barrels a day (the amount of oil the U.S. imports from Saudi Arabia) for HE-15 fuel and 2.48 million barrels a day (the amount of crude oil imported from the whole Middle East) for HE-30 fuel.
To generate the hydrogen using electrolysis to produce enough HE-15 or HE-30 fuel to replace 1.43 or 2.48 million barrels a day of crude oil would require generating capacity of 125,430 or 218,290 MWe. This means that about 126 or 218 1,000-MW nuclear electric plants would be required to produce enough hydrogen by electrolysis to reduce the crude oil imports by 1.43 or 2.48 Mbbl/day oil. With the thermo-chemical production of hydrogen, only 107 or 186 2,000 MWt (1,000 MWe equivalent) nuclear power plants would be required. These requirements exceed the combined capacity of the current fleet of commercial nuclear plants in the United States. However, building five to 11 nuclear plants per year for 20 years is a prudent investment for the benefits of hydrogen-enhanced fuels. Indeed, it has been almost 30 years since the 1973 OPEC oil crisis. Had such a program as proposed here been implemented in the mid-1970s, the U.S. oil supply situation, as well as environmental conditions, would be much improved today.
SUMMARY AND RECOMMENDATIONS
The enhancement of the hydrogen content of transportation fuels is clearly a small, but logical, step forward on the road toward the hydrogen economy. The technology of hydrogenation is well-developed and could be implemented in a relatively short period of time. The chemical and physical properties of hydrogen-enhanced fuels are generally understood, but the compatibility of such fuels, particularly as the hydrogen content increases, with gasoline, diesel, and jet engines remains to be demonstrated. The economics of hydrogen-enhanced fuels has been explored on a "first approximation" basis. It is clear that the economic feasibility (without subsidies) is dependent upon the relative cost on a per unit energy basis (e.g., $/MBTU basis) of oil vs. the cost of producing hydrogen and carrying out the hydrogenation process.
What is needed now is a comprehensive study of all aspects of this whole concept of hydrogen-enhanced hydrocarbon transportation fuels by a qualified organization, such as the National Transportation Research Center operated jointly by The University of Tennessee and Oak Ridge National Laboratory. Several actions taken in series (generally based on the success of the previous action) seems appropriate at this time:
1. The information provided here should be reviewed and examined in detail by petroleum specialists.
2. Evaluation of the physical, chemical, and combustion characteristics of hydrogen-enhanced fuels over a wide range of hydrogen contents and operating conditions should be undertaken.
3. Testing in laboratory combustion facilities (single-cylinder engines, single-cylinder diesel engines, and simple jet engines) should be undertaken to determine the operating and combustion characteristics of hydrogen-enhanced fuels.
4. Environmental emissions from test engines should be used to evaluate impact (benefits) to the atmosphere and on climate of using varying amounts of hydrogen enhancement of hydrocarbon fuels.
5. A comprehensive analysis of the economic aspects of the beneficial and adverse effects of all aspects of hydrogen-enhanced fuels (production, storage, distribution, use in transportation vehicles, environmental effects, and health effects on communities and people) should be undertaken.
6. Industrial organizations should be involved in the evaluation and testing of hydrogen-enhanced fuels.
The above discussion may seem more relevant to the implementation of hydrogen-enhanced transportation fuels than it does to innovative climate change. However, climate changes come as a result of reducing the greenhouse gases and pollutants going into the atmosphere, and that is perhaps the principal benefit of using hydrogen-enhanced fuels. If implemented on a large scale, the impact on the climate could be major, because the transportation system in the United States contributes more than one-third of the greenhouse gases and pollutants going into the atmosphere.
The fact that hydrogen-enhanced transportation fuels do not completely eliminate atmospheric emissions should not be an excuse for not proceeding with an evaluation and testing of the concept and its benefits. Indeed, the petroleum industry is currently using increased hydrogenation of crude oil to overcome the deteriorating quality of the available crude oil and to reduce sulfur and nitrogen oxides. Increasing the hydrogen content beyond present-day levels is a simple step, if the additional costs are warranted by the benefits or by meeting increasingly severe environmental requirements or meeting the non-petroleum goals of Energy Policy Act.
The proposed program could be carried out by faculty members and graduate students at The University of Tennessee and by scientists and technicians at Oak Ridge National Laboratory. Test facilities and staff of the National Transportation Research Center could be used as appropriate. We would be pleased to provide additional information about this proposed innovative research program that can provide improvements in the climate, while achieving other desirable energy and environmental goals at the national level.
Robert E. Uhrig
University Distinguished Professor Emeritus, University of Tennessee
Distinguished Scientist Emeritus, Oak Ridge National Laboratory
The author, Robert E. Uhrig, is a Life Fellow of ASME and recently retired from a joint appointment with The University of Tennessee and Oak Ridge National Laboratory.