Wednesday, November 26, 2008

Artificial Carbon Cycle

Part of the research for my writing and consulting involves watching for trends or common themes, and one that I've been picking up from diverse sources reflects a growing skepticism about "clean coal" and the processes for capturing and sequestering carbon (CCS) that are central to it. To get a flavor for this, Google on "clean coal" and "oxymoron". Some of these concerns are grounded in the science of thermodynamics, while the balance seem to reflect the long-standing attitude of environmentalists toward the coal industry, which would be the primary beneficiary of a practical CCS scheme. It's worth taking a few minutes examining why CCS is unlikely to be easy, but why, if it can be done cost-effectively on an industrial scale, it would be so beneficial.

It helps to think about CCS in the context of the earth's carbon cycle, in which carbon is exchanged through natural processes among the land, ocean, atmosphere, and living things. The principal issue in anthropogenic climate change is that our activities have upset the balance of this natural cycle, overloading it through the rapid release of vast quantities of stored carbon that had accumulated over geological time in fossil fuels. The goal of climate policy is to reduce the magnitude of that overload and eventually eliminate it by using carbon-intensive energy sources much more efficiently, while working to replace them with carbon-neutral or carbon-free energy. That's why biofuels, wind and solar power are regarded as essential elements of climate change mitigation, though it turns out that current biofuels are not remotely carbon-neutral. The idea behind CCS is to complement the main climate change mitigation strategies by creating an artificial version of the carbon cycle, in which the carbon released from the combustion of fossil fuels is collected and returned to long-term storage, before it can enter the natural carbon cycle.

That sounds simple enough, but to understand why it's so hard to do, consider the amount of coal necessary to produce one kilowatt-hour of electricity. In 2007 the US burned a little more than a billion tons of coal to generate just over 2 trillion kWh of electricity, for an average of 1.0 lb./kWh. Because most of the energy from coal derives from its carbon content, the main chemical reaction involved is very simple: C + O2 → CO2. So unlike the sulfate (SOx) or nitrate (NOx) pollution we have managed for decades, CO2 is neither the result of a fuel impurity nor an inadvertent byproduct of combustion, but rather its primary outcome, along with heat. On average, every lb. of coal yielding a kWh of electricity also emits 2 lb. of CO2 to the atmosphere. In other words, the mass of CO2 leaving coal-fired power plants is double the mass of coal that went in. That's a lot of gas to separate, compress, transport, and dispose of in geological or other storage.

Now consider the energy balance of such a system. Before adding CCS at the back end, you had to mine the coal, ship it to the power plant and burn it, producing heat that was used to make steam to turn a turbine that generated power. The typical thermal efficiency of such a facility is 35-45%, depending on coal quality, plant design and operation. But CCS is inherently energy-intensive, reducing the overall efficiency and the energy return on energy invested (EROEI) for the entire coal-to-power process. If separating the CO2 from the flue gas, compressing it, and putting it back into the ground at some remote location consumes up to a third of the energy generated from the coal, as some estimates suggest, then our artificial carbon cycle doesn't look very impressive, as a net energy source. After referring to the First and Second Laws of Thermodynamics, you might even wonder whether we could produce enough net energy from such a loop to be worthwhile, at all.

I had a hard time finding the EROEI of the standard coal-fired power lifecycle. It appears to fall in the range of 5:1 to 9:1, which compares favorably with conventional oil production and refining, and with the best renewable energy sources. If CCS reduced those returns by one-third, then while the energy balance would remain positive in a physics sense, the economics of some applications might become marginal, because CCS would consume a large helping of the energy surplus that coal-fired power normally creates. Another way to look at that is that the portion of the energy surplus thus consumed was attributable to the non-monetized externality of putting a greenhouse gas into the atmosphere, and thus not sustainable, anyway.

As daunting as all this sounds, there may be some clever ways to overcome the toughest impediments to getting started rounding up the carbon from coal power and stashing it back in the earth. In a new study, a team from MIT has proposed "partial capture": removing only enough CO2 from the flue gas to cut the emissions from a coal-fired power plant to the level of one running on natural gas, about a 35% reduction. This would allow CCS to be introduced incrementally, at a much lower investment cost and a less severe efficiency penalty than full CCS. And as I discussed in another posting, using captured CO2 to enhance the output from productive oil fields creates a positive value for it that offsets at least some of the cost of collecting and transporting it. Work at a Canadian oil field that does this suggests that the stored CO2 can be effectively monitored underground. Even more intriguingly, naturally-occurring mineral deposits called peridodites can act as CO2 sponges. These might be used to increase the efficiency of direct CCS, or to establish indirect CCS--a coal-scale emissions offset that would remove CO2 from the atmosphere without requiring a CO2 pipeline from the emissions source.

Easy or difficult, our motivation for pursuing CCS, instead of abandoning coal as incompatible with alleviating climate change, is based on the reality that we still derive roughly half of our electricity from coal and only about 1% from wind, solar and geothermal power. That means that our annual additions of renewable generating capacity are not yet covering the roughly 1.5% per year growth in US electricity demand we've experienced over the last 5 years, let alone taking market share away from coal or any other carbon-based fuel. Could we advance efficiency and renewables rapidly enough to displace a sizable fraction of our coal use within 10-20 years? Perhaps, though I'd feel a lot more confident about meeting the aggressive emissions-reductions targets the US is likely to take on within the next year or two, if we could tackle coal's emissions directly with CCS.

I'd like to wish my US readers a Happy Thanksgiving. Postings will resume on December 1.

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