Bridging the Carbon Gap: Fossil Fuel Use for the 21st Century
(such as corn ethanol) to bridge the gap, due to the problems of excessive land requirements, high labor intensity, and potentially harmful environmental impacts. 2
Several other strategies should also be considered for extending the lead time for achieving true sustainability. Such strategies include using natural gas to displace coal as the currently dominant fuel for power generation, as well as an interim source of hydrogen for highly efficient, electromotive surface transport and distributed generation. They might also include improved efficiency of the use of petroleum liquids as a transportation fuel. A revival of nuclear fission reactor power might also assist in bridging the gap.
However, the longest extension in lead time for achieving sustainability could be gained by continued reliance on coal for power generation, but augmented by adopting a modification of the Integrated Coal Gasification-Combined Cycle (IGCC) process.
Under the modified IGCC process, coal is first gasified under pressure with steam and oxygen to hydrogen, carbon monoxide and CO 2. These gasification products are then catalytically processed with additional steam to essentially all hydrogen and CO 2. The CO 2 is then removed and sequestered in suitable geological formations or the deep ocean, before the hydrogen is used for power generation or as a transportation fuel. 13, 14, 15 This option would provide a long-term source of carbon-emission-free hydrogen.
Unfortunately, the IGCC process fails to meet the criteria of long-run sustainability. Even coal is exhaustible. Also, we do not know the total capacity for CO 2 sequestration, nor understand all its possible environmental impacts.
Nevertheless, though it is not yet economically competitive, the technology of coal gasification and conversion of coal-derived syngas to hydrogen and CO 2 with subsequent CO 2 removal is commercially available today. Of course, further development is required on the problem of how to sequester CO 2, such as in deep saline acquifers, along with a demonstration on an industrial scale.
Long-Run Sustainability with High-Tech Renewables
The problem with the high-tech renewable power options is that they are inherently intermittent and difficult to integrate with the electric power grid.
Hydrogen is a potential energy storage medium, especially so for direct current (DC) photovoltaic power. That is because excess generation during periods of high insolation could be used for water electrolysis, the high-purity hydrogen stored, and then reconverted to electricity in fuel cells-preferably low-temperature Proton Exchange Membrane (PEM) units-during periods of low or no insolation. However, this is a relatively low-efficiency option, given the combined energy losses of electrolysis and fuel cell power generation. And a key problem is the additional parasitic power requirement for hydrogen compression-from the current PEM electrolyzer levels of 100 to 150 pounds per square inch (psi) to the minimum 3,000 psi required for efficient storage. Also, the investment costs of such electrolytic hydrogen energy storage systems are still quite high. However, Proton Energy Systems, Inc. in Wallingford, Conn. recently has developed systems capable of operating at 2,000 psi and expects to soon reach the 3,000 psi level. 16
Wind Power. Wind turbines are the most technically advanced and most