aMass of carbon emitted divided by mass of hydrogen produced.
(Mt) of hydrogen per year.3 Assuming possible future technologies, without CCS, the associated annual carbon emissions from natural gas production would be 255 Mt C, and 518 Mt C from coal plants. CCS technology reduces emissions to 42 Mt C and 83 Mt C for natural gas and coal, respectively (less than 100 percent of the CO2 will be removed by any competitive technology). Thus, the scale of the task of capturing and storing most of the CO2 associated with an all-hydrogen U.S. fleet of light-duty vehicles in 2050 could be approximately 200 to 400 Mt C/yr (assuming insignificant hydrogen production from nuclear or renewable energy sources).
Today’s annual U.S. CO2 emissions from light-duty vehicles, about 320 Mt C, are about 20 percent of U.S. CO2 emissions and 5 percent of global emissions from all sources. It was beyond the scope of this report to generalize beyond transportation to the role of CCS in hydrogen production for use in other sectors of the U.S. economy (hydrogen use in industry and in buildings, for example).
Capturing and storing 200 to 400 Mt C annually by 2050 is a huge task. On the order of a thousand projects the size of the first two CO2 geological storage demonstration projects (discussed below, in the subsection “CCS Demonstration Projects”) would be required. These projects each capture and store about 300,000 metric tons C/yr. This is also the scale of CCS planned for the proposed FutureGen Project (see below).
Carbon Emissions Associated with Current Hydrogen Production
At the present time, global crude hydrogen production relies almost exclusively on processes that extract hydrogen from fossil fuel feedstock (see Figure 7-1). It is not current practice to capture and store the by-product CO2 that results
FIGURE 7-1 Feedstocks used in the current global production of hydrogen. SOURCE: Courtesy of Air Products and Chemicals (2003).
from the production of hydrogen from these feedstocks. Consequently, more than 100 Mt C/yr are vented to the atmosphere as part of the global production of roughly 38 Mt of hydrogen per year.4
Some but not all of the CO2 is emitted in concentrated streams at elevated pressure. At an intermediate step in the production of hydrogen from fossil fuels, there is a mixture of CO2 and H2 under pressure, together with impurities. At this stage, there are two options:
Extract CO2 at high purity and elevated pressure, which is well matched to CCS technology. Left behind is a less-than-pure hydrogen, but it is of high enough concentration to be commercially useful. Such hydrogen is sometimes called refinery hydrogen. It is suitable for applications such as hydrogen-fueled internal combustion engines, but some CO2 and other impurities will be emitted.
Extract hydrogen at high purity; such hydrogen is sometimes called fuel-cell-grade hydrogen. The residue still contains some hydrogen, which can be burned to produce steam or electricity. If the residue is burned in air, the exhaust stream contains the CO2 from the conversion, but also a large fraction of nitrogen from the atmosphere. Then, the CO2 is dilute and more difficult to capture than in option 1.
There is a recent trend in refineries away from the first option and toward the second, because of the falling cost of hydrogen separation from gas mixtures via pressure-swing absorption. If the CO2 from refinery production of hydrogen
Corresponding to 7.9 trillion vehicle miles of annual travel (three times the current value) at an average fuel economy equivalent to 80 mpg. There is similar energy content in 1 kg H2 and 1 gallon of gasoline.
Based on estimated global annual production of hydrogen of 15.9 trillion cubic feet (ORNL, 2003).