Monday, 8 January 2018

Carbon Capture and Artificial Photosynthesis

(Apparently I haven't written anything here for over three years. That's what being absorbed in one's day job does ...)

I only recently learned that, whatever we do to reduce carbon emissions, we are almost certainly going to have to figure out large-scale atmospheric carbon capture soon, or else make Earth a catastrophically crap place to live for a long time.

From a Quartz article last month:
"The foremost authority on the matter, the Intergovernmental Panel on Climate Change, has modeled hundreds of possible futures to find economically optimal paths to achieving these goals, which require the world to bring emissions down to zero by around 2060. In virtually every IPCC model, carbon capture is absolutely essential—no matter what else we do to mitigate climate change."
By way of background, it's perhaps worth looking again at why carbon dioxide makes such a big difference, despite contributing only about 0.04% of the atmosphere's volume. In short, CO2 absorbs a spectrum of wavelengths that very neatly covers the range in which the planet loses much of its heat to space. Most other atmospheric gases don't absorb in that spectrum. Increased atmospheric carbon dioxide levels mean that heat that would previously have been radiated by the ground straight through the atmosphere into space is now being absorbed by atmospheric CO22 instead.

So we need some way to process carbon dioxide and turn it into something solid or liquid, so that it won't interfere with Earth's temperature.

Plants capture carbon as they grow (and there is a lovely video of Richard Feynman explaining how plants are made out of air). So in principle we could just grow biofuels and store them. A major problem with biofuels is that they need soil—and particularly fixed nitrogen, in the form of fertiliser—to grow. So that only works in places where spare soil exists, and for as long as the soil hasn't been ruined by intensive farming. And even when it works, we need to put in plenty of energy just to fix the nitrogen to produce the fertiliser.

Another limitation with biofuels is that photosynthesis isn't very efficient, because chlorophyll only absorbs a very small fraction of sunlight. In fact, in a day we can capture more sunlight from a square metre of land by using solar panels than by growing plants.

A second possibility would be to run large solar (or nuclear) plants, and use the resulting electricity to drive some chemical process that emits hydrocarbons or carbohydrates. A difficulty of this kind of approach is that CO2 makes up so little of the atmosphere, and it's hard to find processes that work well in low concentrations. Some research is looking at using this kind of approach where concentrations are highest, in the chimneys of power plants. But turning burnt natural gas back into (synthetic) fossil fuels again is surely going to consume more energy than was produced by burning the gas in the first place. Unless I'm missing something, this is only a sensible thing to do if you want to produce electricity at a predictable rate and then later convert the exhaust gases back to fuel whenever the sunlight happens to be available. And it doesn't even begin to deal with numerous, small sources of carbon dioxide, like cars or planes.

A third way would be to engineer microbes that perform carbon capture. They could use metabolic processes similar to those of plants, but they would excrete the resulting fuels instead of building more plant with them. That means the volume of microbes can be fixed, so for a fixed output rate there's no need to supply nitrogen to grow more. Imagine you have a shallow tank of bacteria, which you just leave in the sun. All you do is water it occasionally, and over time it produces oil for you. On the other hand, the microbes are alive, so if you do want to increase production then you can let them reproduce easily. Got one solar bacterium tank? Just set up a new tank, add nutrients, breed your existing population in it, and double your output! You don't need viable soil, or concentrated carbon dioxide.

All we need for this is the right microbes. It's fortunate, then, that artificial photosynthesis is on its way.

Turning sunlight straight into oil obviously has the desirable overall outcome of reversing the effects of fossil fuel use. But it also means that the notoriously hard-to-predict availability of solar energy doesn't matter much any more: on a very sunny day, you don't need to have a large, expensive battery to store electricity, because you can have a large, cheap tank to store oil instead. And on a rainy day you can send some of that tank of oil to a regular power station. Your solar plant can be a base load plant.

And that in turn might yield some handy geopolitical consequences: every country could use its land to achieve energy independence, without having to swap oil pipelines for electrical grids and without having to swap petrol engines for electric vehicles.