A discussion elsewhere brought this issue up, and these numbers should be posted for reference.
Replacing diesel with LNG requires roughly the energy equivalent of methane, plus whatever it takes to purify the gas and convert it to liquid. The info on liquefaction energy is hard to find; Linde Engineering doesn't even mention energy cost in its promotional material on its LNG plants. But I found a paper on Russian stuff which supplies a graph on page 15. This indicates about 250 Wh/kg at typical temperatures. This figure will increase for smaller, less-efficient systems, so figure 0.5 kWh/kg for a truck-stop sized unit. 1 kg of natural gas has 13.83 kWh of energy (47,200 BTU) so it takes about 2.9 kg of LNG to replace a gallon of diesel. This gas takes 1.4 kWh to liquefy. If this is done with electricity supplied from a CCGT powerplant at 50% efficiency delivered, it takes another 0.59 kg of gas per gallon-equivalent; if the power is generated on-site from e.g. a Capstone C60 gas turbine at 30% efficiency, it takes another 0.99 kg of gas per gallon-equivalent.
Replacing 70% of the 3037,000 bbl/day of distillate used for transport in 2007 (46.6 billion gpy @ 138,000 BTU/gal average for US distillate per EIA) would need 114 to 127 billion kg of natural gas, depending on the liquefaction overhead. This is 5.4 to 6.0 quads of gas. The USA produced ~21 trillion cubic feet in 2009; at 1020 BTU/scf, this is 21.4 quads of gas. Substituting for just 70% of diesel with LNG (no gasoline) would require increasing NG production by at least 25%. This may be possible, but it will require much higher NG prices (which are coming anyway).
Electrification needs less. If a dual-mode semi-truck averages 1.5 kWh/mile and traffic is 20% greater than the 2001 figure of 135.4 billion miles, annual electric power requirements would be 244 billion kWh, or about 6% of US demand for a complete replacement of diesel (not just 70%). Supplying this from NG using CCGT's at 50% efficiency delivered to the vehicle would require 3.53 billion kg of natural gas, or 1.67 quads. This is far more efficient, and the electric system can also use electricity from anything else on the grid. Finally, moving trucks to dual-mode rail eliminates pavement damage and cuts road-repair costs. The electric rail system is a better target for policy than converting semis to LNG.
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