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Getting the Carbon Out of Hydrocarbon Fuels

We Americans love our cars, and our cars love gasoline. Like most love triangles, this one is not without its problems. Namely, the transportation sector, which runs almost exclusively on oil, accounts for a full one-third of U.S. fossil fuel greenhouse gas (GHG) emissions. These greenhouse gases are likely responsible for a large portion of global warming that has occurred in the past several decades. And an even greater cause for concern in many people’s view is the nation’s dependence on imported oil — in 2009, the U.S. imported 2.2 barrels of oil for every barrel produced domestically — not exactly a ratio for self-sufficiency.

A paper published online in the journal Energy and Fuels on December 6th, reporting on research I did with colleagues at Princeton University, identifies technologies that just might help solve both problems simultaneously. The paper builds on one we published earlier this year.

We designed and analyzed systems that would use technologies that are mostly already available at commercial scale to co-produce low-carbon petroleum-like fuels and electricity. These systems would use coal, an abundant domestic energy source, as the major energy input.  

How can hydrocarbon fuels be low carbon, you might ask, with coal as the starting material?  After all, the fuels are hydrocarbons and coal is the most carbon-intensive of all fossil fuels.

There are two key features of these systems that would enable low carbon emissions — one involves capturing and storing byproduct carbon dioxide (CO2), which is a key greenhouse gas, and the other involves converting some biomass to fuels along with the conversion of the coal.

Coal-to-Liquids

Technology for making gasoline and diesel-like fuels from coal (“coal-to-liquids” or Fischer-Tropsch liquids) is well known — in use in South Africa since the 1950s, beginning to be used today in China, and being discussed for other countries, including the U.S. With this technology, about half the carbon in the coal ends up in the liquid. The rest is released to the atmosphere as more-or-less pure CO2. The Sasol coal-to-liquids facility in South Africa emits about 20 million tons of CO2 per year. If this CO2 were to be captured and stored underground 

— a process that scientists and engineers are actively working to develop as a large-scale option by the end of this decade — the only CO2 from the starting coal that would reach the atmosphere would be from the fuels when they are burned in a vehicle. Per gallon of fuel used, that amount of CO2 is about as much as would be released if regular petroleum-derived gasoline or diesel had been used instead.

So, using this method, there would be no reduction in GHG emissions relative to our current transportation fuels. But since we don’t have to import coal, making liquid fuels this way would help reduce our dependence on foreign oil.

Coal Plus Biomass to Liquids

Illustration of the coal/biomass systems described in the new paper in the scientific journal Energy and Fuels, as well as a preceding paper. Credit: Eric Larson.

Now add in some biomass, of the type that is produced sustainably (like forest thinnings or crop residues). Such biomass can be converted to petroleum-like fuels with similar technology to what I’ve described above for converting coal. Unlike coal, however, all of the carbon in biomass, and hence in the liquid fuel made from it, is carbon that has recently been extracted from the atmosphere during biomass growth. So any CO2 released by converting this biomass and burning the resulting liquid fuel is simply being returned to the atmosphere, and will be removed again by next year’s biomass growth. The end result: no net accumulation of CO2 in the atmosphere.

As with coal, converting biomass to fuel results in a CO2 byproduct. If this CO2 is not released to the atmosphere, but instead is captured and stored underground, some CO2 has been effectively removed from the atmosphere.

In a conversion facility that uses some coal and biomass, and captures byproduct CO2 for underground storage, the “negative emissions” associated with the biomass would offset the unavoidable emissions from coal. The picture below illustrates the various flows of GHG emissions to and from the atmosphere that have to be taken into account in assessing the carbon-intensiveness of such a combined coal/biomass system. With the right ratio of coal to biomass, the GHG flows into the atmosphere can be balanced by the flows out of the atmosphere, in which case the resulting liquid fuels would contribute no net emissions to the atmosphere.

Fuels made using only biomass and without carbon capture are also low-GHG fuels (with some notable exceptions). However our research found that making “pure” low-carbon biofuels, such as “cellulosic ethanol” requires two to three times as much biomass per gallon of fuel made as do the coal/biomass systems described above. This is a significant benefit of the coal/biomass systems, since biomass is a limited resource.

Economics and Policies

The coal/biomass systems we have described, if widely deployed, may be effective in reducing U.S. GHG emissions and oil imports, but what are they going to cost? This is a question we also examined in detail in our research.

One discovery we made was that the economics of these systems will often be better when the liquids-producing plants are designed to co-produce some electricity along with the fuel, since the revenues from electricity sales help offset production costs.

We also found that producing zero GHG emissions fuels is not cost-competitive unless oil prices are higher than today’s price, or unless there are policies in place that require reductions in GHG emissions, for example a carbon tax or a cap-and-trade system.

The higher the oil price is, the lower the GHG emission price can be, and vice versa. With no GHG emission price (i.e., with no carbon policy in place), we found oil prices would have to climb to at least $110 per barrel in order for zero-carbon liquids fuels to be cost competitive with petroleum-derived fuels. With a relatively modest GHG emission price of $20 per ton of CO2 equivalent, the zero-carbon liquid fuels would be competitive with petroleum fuels when the oil price is about what it is today ($90 per barrel).

Our work at Princeton illustrates that there are technologies that can be deployed to reduce energy-related GHG emissions, even with continued fossil fuel use, without breaking the bank. But demonstrations of the viability of CO2 capture and storage technologies, along with effective carbon mitigation policies, are needed before investors are likely to put money into such systems.

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