Okay. We’re going through this paper, which you can read yourself:
• Stephen Pacala and Robert Socolow, Stabilization wedges: solving the climate problem for the next 50 years using current technologies, Science 305 (2004), 968-972.
The paper lists 15 ‘wedges’, each of which could ramp up to reducing carbon emissions by 1 gigaton/year by 2054. We’re going through all these wedges and discussing them. And the Azimuth Project is lucky to have a new member on board — Frederik De Roo — who is summarizing our discussion here:
• Azimuth Project, Stabilization wedges.
So, let’s get going!
Last time we covered four wedges related to energy conservation and increased efficiency. Wedge 5 is in a category of its own:
5. Shifting from coal to natural gas. Natural gas puts out half as much CO2 as coal does when you burn them to make a given amount of electricity. After all, it’s mainly methane, which is made from hydrogen as well as carbon. Suppose by 2054 we have coal power plants working at 90% of capacity with an efficiency of 50%. 700 gigawatts worth of coal plants like this emit 1 gigaton of carbon per year. So, we can reduce carbon emissions by one ‘wedge’ if we replace 1400 gigawatts of such plants with gas-burning plants. That’s four times the 2004 worldwide total of gas-burning plants.
Wedges 6-8 involve carbon capture and storage:
6. Capturing CO2 at power plants. Carbon capture and storage at power plants can stop about 90% of the carbon from reaching the atmosphere, so we can get a wedge by doing this for 800 GW of coal-burning power plants or 1600 GW of gas-burning power plants by 2054. One way to do carbon capture and storage is to make hydrogen and CO2 from fossil fuels, burn the hydrogen in a power plant, and inject the CO2 into the ground. So, from one viewpoint, building a wedge’s worth of carbon capture and storage would resemble a tenfold expansion of the plants that were manufacturing hydrogen in 2004. But it would also require multiplying by 100 the amount of CO2 injected into the ground.
7. Capturing CO2 at plants that make hydrogen for fuel. You’ve probably heard people dream of a hydrogen economy. But it takes energy to make hydrogen. One way is to copy wedge 6, but then ship the hydrogen off for use as fuel instead of burning it to make electricity at power plants. To capture a wedge’s worth of carbon this way, we’d have to make 250 megatons of hydrogen per year from coal, or 500 megatons per year from natural gas. This would require a substantial scale-up from the 2004 total of 40 megatons of hydrogen manufactured by all methods. There would also be the task of building the infrastructure for a hydrogen economy. The challenge of injecting CO2 into the ground would be the same as in wedge 6.
8. Capturing CO2 at plants that turn coal into synthetic fuels. As the world starts running out of oil, people may start turning coal into synfuels, via a process called coal liquefaction. Of course burning these synfuels will release carbon. But suppose only half of the carbon entering a synfuels plant leaves as fuel, while the other half can be captured as CO2 and injected underground. Then we can capture a wedge’s worth of CO2 from coal synfuels plants that produce 1.8 teraliters of synfuels per year. For comparison, total yearly world oil production in 2004 was 4.7 teraliters.
Now: What are the pros and cons of these four wedges? What is the biggest thing that Pacala and Socolow overlooked?
I’m puzzled about the last wedge. Pacala and Socolow say 1 gigaton carbon/year is the flow of carbon in 24 million barrels/day, or 1.4 teraliters/year. They assume the same value for synfuels and allow for imperfect capture, which leads them to conclude that carbon capture at synfuels plants producing 1.8 teraliters/year of synfuel can catch 1 GtC/year. But this calculation doesn’t make sense to me. If we’re catching just half the carbon, and 1 GtC/year = 1.4 teraliters oil/year, don’t we need to generate at least twice that — 2.8 teraliters synfuel/year — to catch wedge’s worth of carbon?
I’m also unclear what percentage of the carbon you can actually capture while turning coal into synfuels. Can you really capture half of it?
There’s also another funny feature of this last wedge. If we assume people are already committed to making synfuels from coal, then I guess it’s true, they’ll emit less carbon if they use carbon capture and storage as part of the manufacturing process. But compared to making electricity or hydrogen as in wedges 6 and 7, turning coal into synfuels seems bound to emit more carbon, even with the help of carbon capture and storage.
In general, it only makes sense to talk about how much carbon emission some action prevents when we compare it to some alternative action. That’s pretty obvious, but it gets a bit confusing when some of Pacala and Socolow’s wedges look like plausible alternatives to other ones.
Another question: how does carbon capture and storage work, actually? Summarizing Pacala and Socolow, I wrote:
One way to do carbon capture and storage is to make hydrogen and CO2 from fossil fuels, burn the hydrogen in a power plant, and inject the CO2 into the ground.
But I’d like to learn the details!