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CMU: Plug-in Hybrids “More Sensible” Use of Coal Than Coal-to-Liquids

Comparing life cycle CO2 emissions from plug-in hybrids, coal-to-liquids gasoline, and conventional gasoline. Click to enlarge.

A study from the Carnegie Mellon Electricity Industry Center (CEIC) concludes that while enacting policies to subsidize the production of coal-to-liquids transportation fuel would enhance national security by lowering oil imports, encouraging plug-in hybrids powered by coal-generated electricity is a less costly policy that also reduces oil imports and does more to lower greenhouse gas (GHG) emissions.

CEIC produced the paper in the context of the current work by the US House Committee on Energy and Commerce on transportation energy legislation, the current draft of which includes significant support for CTL. (Earlier post.)  The CEIC paper compares GHG emissions of CTL gasoline to the emissions of plug-in hybrid vehicles powered with electricity generated from coal on a full life cycle basis.

Although CTL conventionally produces more diesel than gasoline, the process can be altered with catalysts to upgrade some of the diesel and waxes produced in the standard F-T process to gasoline, with an overall efficiency of around 52% (HHV).

The CEIC team used CTL inputs and outputs derived by Bechtel in 1993, and allocated the total emissions factor among the various CTL co-products using the method in the GREET model (by energy content of the co-products).

The allocated worst-case well-to-plant emission factor (no carbon capture and sequestration, current electricity generation mix) is 190 pounds CO2 equivalent per MMBtu of CTL gasoline, and 50 pounds CO2 equivalent per MMBtu of CTL diesel. With 80% CCS and zero-carbon electricity, the allocated factors drop to 50 pounds CO2 equivalent for gasoline and 15 pounds for diesel.

Adding in the other complete lifecycle factors (transportation for distribution, combustion in the engine) resulted in complete well-to-wheel CTL lifecycle emissions of 360 pounds CO2 equivalent per MMBtu of gasoline in the worst-case scenario and 220 pounds CO2 equivalent per MMBtu of gasoline in the best-case scenario.

CEIC then used a fuel consumption figure of 34 mpg and an annual driving distance of 12,000 miles to calculate the annual CTL gasoline emissions: 1.18 lbs/mile (536.7 g/mi) in the worst case; 0.72 lbs/mile (325 g/mi) in the best case.

For plug-ins, the CEIC researchers calculated the impact of both electricity and gasoline. For electricity generation, they used two scenarios: bituminous coal in a pulverized coal power plant and bituminous coal in an integrated gasification combined cycle power plant with carbon capture and sequestration (IGCC w/ CCS).

Well-to-Wheel Greenhouse Gas Emissions
Fuel CO2-equiv.
CTL gasoline 536.7
CTL w/ CCS gasoline 325.1
Gasoline base case 344.0
PHEV, coal generation 264.6
PHEV, coal IGCC w/CCS 105.8

For a vehicle, they assumed a plug-in hybrid built on a Toyota Prius platform in a parallel configuration with an all-electric range of 60 miles. To determine the fraction of vehicle travel powered by electricity or gasoline, they used the percentages resulting from the cumulative distribution function of daily vehicle miles traveled constructed in another paper from CMU (Samaras and Meisterling, “Decarbonized Electricity Needed for Plug-in Hybrids” 2007). The CEIC distribution estimates electricity would power about 85% of average annual vehicle travel for a plug-in hybrid with a 60-mile electric range, assuming vehicles are charged once per day.

The results: total well-to-wheel emissions of 264.6 g/mi for the conventional coal-generated scenario; 105.8 g/mi for the scenario with advanced IGCC with CCS). The conventional gasoline baseline in the study was 344 g/mi.

It can be seen that gasoline derived from CTL plants with no CCS could increase GHG emissions from vehicles by almost 60%. If CCS is available, then a reduction of less than 6% could be obtained. It is important to note, once again, that in this best-case CTL scenario, not only is there CCS at the CTL plant, but also a low-carbon electricity source is used for CTL production. This might not be a very realistic assumption, but is presented here to show that at best we could only obtain a very small reduction in GHG emissions following a path of increased CTL production.

Plug-in hybrids look more promising as a pathway for reduction of GHG emissions. Even if coal electricity without CCS is used, plug-in hybrids could lead to a GHG emissions reduction of almost 25%. This demonstrates the worst case for plug-in hybrids, as GHGs would be further reduced with a low-carbon electricity portfolio. It is important to note however, that this analysis does not include the emissions from manufacturing the storage battery used in plug-in hybrids. If GHG emissions from lithium-ion batteries for plug-in hybrids are included, total annual GHGs from plug-ins would increase by about 800-1,500 pounds of CO2 equivalents, depending if a twelve or eight-year vehicle life is assumed (Samaras and Meisterling 2007). Battery technologies are difficult to predict, but even when emissions from current battery production are included, plug-in hybrids result in substantially lower emissions than CTL pathways.

The Carnegie Mellon Electricity Industry Center (CEIC) was established in August 2001 as one of 20 centers of excellence in different industries that the Alfred P. Sloan Foundation has established at 13 universities. CEIC’s core funding comes jointly from Sloan and from the Electric Power Research Institute (EPRI).




CCS will not happen for a long time, if ever. So, it is realistic to assume the worst case scenario for at least a decade and possibly longer. Those, like Senator Obama, who tout this as a clean alternative, are blowing smoke, literally, into the atmosphere, if not up our ass.

Also, as the study said, only in a few places in the U.S. would the worst case for the PHEV occur because of mixed sources for power geneation. In addition, we should be phasing out coal plants, anyway, unless they can sequester their co2.

Since the study resullts are in, the bill subsidizing these plants should not be allowed to pass. If they want to further study the issue, fine, but no funds should be allowed unless it can be convincingly shown that we could actually improve our co2 situation.

Also note that, even if one includes the embodied energy of batteries, it is still better to go for PHEVs.

Further improvement could possibly be achieved by v2g technology and/or consideration given to recharging these vehicle at night with wind as a way of overcoming the disadvantages of wind variability.

I hope other Senators who oppose this plan bring it up in the next debate.


Of course the catch is that plug ins appear to need lithium based batteries to be doable and those lithium cells are still an R & D project. it will be at least a couple years yet, if then, till they are feasible for mass produced cars and have sufficiently low costs and an acceptable life cycle.

CO2 is very important, but its unrealistic to think Americans will prefer the economic implosion to come with depending on oil to a composite approach using everything thats available while we pursue clean renewable alternatives.



Any CTL plants would take many years before they are complete, so in the short term they won't make a difference anyways.

And if we did allow these CTL plants to gain a foothold, they would be damned near impossible to shut down when we finally did want to get serious about cutting CO2 emissions. Not building them in the first place makes far more sense to me.

And for that matter, I think battery technology is a lot closer than you think.


If Congress accepts the findings of the the Carnegie study, which shows conclusively that PHEV's can generate substantial savings in CO2 over CTL technology, than any $$ subsidies being considered for CTL should be redirected towards improving PHEV battery technology first. That means all the billions of subsidy dollars, not just a few million.


The use of CO2 as a feedstock to grow algae in bioreactors , whatever the configuration, should be aggressively applied to all fossil fueled facilities - power plants, chemical plants, oil sands processors etc., etc. This would increase the total useful output (energy & other raw materials) of the facility, making processes such as oil sands extraction and CTL more environmentally palatable. Most of the CO2 captured by the algae will eventually be released into the air when the fuel created from it is consumed, but at least more useful energy is ultimately extracted from the original fossil fuel, and therefore not as much needs to be used.

Fat Man

Gliders, but schedules will be iffy.

Bud Johns

Another way to cut down on CO2 would be to stop the destruction of the rain forests, and try to revive the ones that have been razed. Is it just me, or why am I suprised I don't hear more about that?


I have a question - what happens to coal fired units at night - do they throttle them back or keep them running producing electricity that no-one wants.

If you had a smart grid signalling system which could turn on the PHEVs when there was excess coal or wind, you could charge them for very little cost and excess CO2.

It strikes me there is a lot of potential in wind IF you can get people to consume it when it is plentiful (or store it [ unlikely ] )

Also, assuming the car gets 34mpg seems a bit rich.


Regarding coal plants, they cut back on production, but there's a still a minimum base load that needs to be maintained. For that matter, we could run a lot of PHEVs on just the excess based load that is not being used. So, until we ramped up total production considerabley, the net co2 impact of the early PHEVs could be zero.


Even if the Carbon footprint were the same, PHEVs using electricity would still be better than CTL in ICE for the simple fact that they don't create as much pollution where people live.




I drive a Citroen Saxo Electrique. The electricity for driving is generated by a CNG powered motor (5kw el/12,5kw th) which is heating my house and generates warm water (neighborhood in summer) same time. Where ist the problem?


This study will hardly be surprising to anyone here, but it will be nice to have a study to point to when someone less informed gives me the old "pollution elsewhere" argument about my BEV.


This study supports the gut feeling of even semi-technical observers. CTL is to be avoided. Use that coal (if you must it at all) to generate electricity.

The needed batteries for PHEVS vehicles are very near. And we have steady increases in ICEs. Stop sweating about every 'whatif'.

People will turn away from those three ton SUVs and Winnebagos. The effect of vehicle downsizing, etc. is a dip in cash. And occasional inconvenience.

e.g. you had planned to drive that whale for fifteen years. But you replace it with a Prius after five, you take the cash hit for the Prius. Then the cash starts coming back at the pump.

As far as electricity loads. I advocate solar for the daytime power bulge and nuclear for base and nights. Night consumption should soar as PHEVs become common.

Coal, NG, and oil are wonderful chemical brews which are far too useful to waste them for heat.

John L.

I am enthusiastic about this study, and about PHEV technology in general. I continue to resist the urge to void the warranty on my Prius, but I don't know how long I'll hold out.

Still, I'm nervous about some of the study's assumptions. The authors assume that PHEV-60 is available. That's a tall order.

Furthermore, I once did a calculation which suggests that Americans can't all drive PHEV-60's at this time anyway, not without building more power plants. I concluded that putting a PHEV-10 in everyone's driveway was practical today, but not much more than that. I'll provide details if anyone wants them.

Building more power plants to service PHEV could defeat the purpose of avoiding CTL. Renewable energy would be best, of course. But we would be likely to see more natural gas and coal plants constructed. These would add to pollution, CO2 emissions, and could decrease our energy independence.

I would have liked to see an analysis which included PHEV-10 and PHEV-25, so that we could see whether the benefits of PHEV can be obtained at an earlier, more achievable stage.

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