Category: Science

Carbon capture and storage (CCS) is a collection of technologies often mentioned in connection with global warming. Essentially, the idea is to capture the carbon dioxide emitted by things like power plants and then sequester it indefinitely in some sort of geological formation, such as a mined salt dome. While this idea is worthy of discussion in itself, my focus here is a number of approaches often described as CCS, but which do not achieve the same long-term result.

Some people have proposed that, rather than burying carbon underground, we just pump it into the sea. One option I am not going to discuss now is making big pools of liquid carbon dioxide in the very deep ocean. Rather, I will address the idea of using pipelines from shore or trailing from ships to release CO2 about 1000m down. Another alternative with similar effects is to make huge chunks of dry ice and throw them overboard, hoping most of the carbon will sink. Rather than being a type of CCS, these activities migtht be more accurately called ‘oceanic dumping of CO2.’

A matter of equilibrium

The problem here is both fundamental and intuitive. Think about a large plastic bottle of cola. With regards to the carbon dioxide, there is an equilibrium that exists between the amount dissolved in the liquid and the amount that is part of the air at the top of the bottle. As long as the system is closed (the cap is on), the amount of gas in air and water will trend towards that equilibrium point and, once the balance is achieved, stay there. This is what chemists mean when they say that equilibrium states display ‘constant macroscopic properties.’ CO2 from the water is still moving into the air, but it is now doing so at precisely the same rate as CO2 from the air is moving into the water. This is inevitable because if one rate were higher, the relative concentrations would change, and would continue doing so until the equlibrium was reached.

Now imagine that we change the equilibrium. If we take the cap off the bottle, the air inside mixes immediately with the air outside. Since the air inside has more CO2 than the air outside (because some of it has come out of the cola), this mixing causes the concentration of carbon dioxide at the surface of the cola to fall (we are ignoring the effects of atmospheric pressure in this analogy). As a consequence, the cola will start to release CO2, trying to get back to the old equilibrium between cola-dissolved and air-mixed gas. Since there is a lot more air, the equilibrium eventually reached will involve a lot less gas-in-cola. The cola goes flat. In the alternative, if we put a chip of dry ice into the cola and kept the cap on, a new equilibrium would eventually be reached in which both the cola and the air include a higher concentration of CO2.

Consequences

Dumping CO2 in the ocean thereby achieves two first-order effects. Firstly, it carbonates the sea, making it more acidic. Oceanic acidification is worrisome enough without such a helping hand. Secondly, it eventually results in an air-water balance of CO2 that is identical to the one that would have occurred if the CO2 started in the atmosphere. No matter which fluid it begins in, the same amount of CO2 at the same pressure will eventually result in the same balance between air-mixed and water-dissolved gas. It is just a matter of time. This is an important concept to understand, as it is the very heart of physical and chemical equilibria.

One big second order consequence results from this. If we do build such pipelines and do start carbonating the sea, people may decide that very carbon intensive technologies (such as coal generation or, even worse, Coal-to-Liquids) are environmentally acceptable. Using them in combination with oceanic dumping will inevitably have the same long-term atmospheric consequence as dumping the CO2 directly into the air.

Now, there is one reason for which oceanic dumping might be a good idea. Imagine there is some critical threshold for the atmospheric concentration of CO2: stay below it and things are reasonably ok, go above it and things all go wrong. In this scenario, it makes sense to store a bunch of CO2 and release it little by little. Of course, this only makes sense if we (a) only do this with CO2 we were inevitably going to release anyway (no new coal plants) and (b) aggressively cut future emissions so that the slow leak will not make us cross the threshold. Suffice it to say, this isn’t the kind of usage most advocates of CCS have in mind.

Water scarcity is a frequently discussed probable impact of climate change. As glaciers and snowcaps diminish, less fresh water will accumulate in the mountains during the winter; that increases both flooding (during wet seasons) and drought. Higher temperatures also increase water usage for everything from irrigation to cooling industrial processes. Given the extent to which the world’s aquifers are already depleted (see: Ogallala Aquifer), relatively few additional natural sources exist.

The big alternative to natural sources is the desalination of seawater. This is done in one of two ways: using multistage flash distillation or reverse osmosis. About 1,700 flash distillation plants exist in the Middle East already, processing 5.5 billion gallons of seawater per day (72% of the global total). These plants use superheated steam, a by-product of fossil fuel combustion, to pressurize and heat a series of vessels. As salt water flows into each successively lower pressure vessel, it flash boils. Condensers higher in the vessel cause the fresh water to precipitate out from the hot pressurized air solution. This is a simple process, but an energy intensive one.

Reverse osmosis, by contrast, uses a combination of high pressure pumps and specialized membranes to desalinate water. Essentially, the pressure drives fresh water through the membranes more quickly than the accompanying salts. As such, it is progressively less saline with each membrane crossing. In this process, there are both relatively high energy requirements (for high pressure pumping) and the costs associated with building and maintaining the membranes. Because it can be done at different scales, portable reverse osmosis facilities are the preferred option for combat operations or disaster relief.

Unfortunately, both processes are highly energy intensive. Particularly when that energy is being generated in greenhouse gas intensive ways, this is hardly a sustainable solution. Part of the solution is probably to sharply reduce or eliminate water subsidies – especially for industry and agriculture. More transparent pricing should help ensure that the whole business of desalination is only undertaken in situations where the need for water justifies all the expenses incurred.

At a party this weekend, I had a conversation with someone who believed that the energy needs of the future would be solved by hydrogen. Not hydrogen as the input for nuclear fusion, but hydrogen as a feedstock for fuel cells and combustion engines. It’s not entirely surprising that some people believe this. For years, car companies have been spouting off about hydrogen powered vehicles that will produce only water vapour as emissions. The Chevron game mentioned earlier lets you install ‘hydrogen’ electricity generating capacity. The oversight, of course, is that hydrogen is just an energy carrier. You might as well say that the energy source of the future will be AA batteries.

AA batteries are obviously useful things. They provide 1.5 volts of power that you can carry around with you and use to drive all manner of gadgetry, but they are hardly an energy system unto themselves. The chemicals inside them that create their electrical potential had to be extracted, processed, and combined into a usable form. Inevitably, this process required more energy than is in the batteries at the end. The loss of potential energy is a good trade-off, because we get usable and portable power, but there is no sense in which we can say that AA batteries are an energy system.

A similar trade-off may well eventually be made with hydrogen. We may break down hydrocarbons, sequester the CO2 produced in that process, and use the hydrogen generated as fuel for cars. Alternatively, we might use gobs of electricity to electrolyse water into hydrogen and oxygen. Then, we just need to find a way to store a decent amount of hydrogen safely in a tank small, durable, and affordable enough to put in vehicles; build fleets of vehicles with affordable fuel cells or hydrogen powered internal combustion engines; and develop an infrastructure to distribute hydrogen to all those vehicles.

When you think about it, hydrogen seems less like a solution in itself, and more like the possible end-point of solving a number of prior problems. As far as ground vehicles go, it seems a safer bet to concentrate on improvements to rechargeable battery technology.