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Thu, 02 Jul 2009

Carbon capture from coal plants

The U.S. and Canada are spending billions of dollars on Carbon Capture and Storage projects, hoping that in this way the fossil fuel industry will seem to emit less greenhouse gases into the air. I say seem to, because once the effects of the money are all counted, it is likely that it will have increased total emissions, not decreased them.

The province of Alberta has just announced the projects that will receive 2 billion dollars of funding, for which they will capture 4 million tonnes of carbon dioxide. Yes, that's a subsidy of $500 per tonne. The projects calls for the building of new coal-fired power plants, while most other developed economies are shutting them down, and of new bituminous sands upgraders. So no actual reductions in GHG emissions are planned, only the building of new plants of the type that are the greatest emitters in the world: coal-fired plants emit more than virtually all the alternatives for producing electricity, and upgrading bitumen similarly emits more than all other methods of producing fuel for transportation.

Most of the carbon dioxide that is being captured will be used for Enhanced Oil Recovery (EOR). Traditionally, EOR pumps liquefied CO2 into the ground to be used as a solvent to wash extra oil out of the oil-bearing rock. Nearly half of the CO2 then comes up to the surface again. Much of it is often vented, but with careful engineering and energy use, much of it can be separated once again and re-injected. The irony of using CO2 that was separated, compressed, transported and pumped at great cost in order to produce oil that will be burned in the atmosphere is completely lost.

Removing carbon dioxide from existing coal-fired plants is the Holy Grail of CCS. These are probably the greatest fixed sources of emissions and represent a huge investment that is unlikely to be retired early. Unfortunately, removing the CO2 from existing coal-fired plants is almost certainly a lost cause. Our hands are tied by the laws of thermodynamics. The gas coming out the chimney is hot and only has about 10% CO2.

The physical limit to the amount of energy required to separate and compress the CO2 is about 0.2 kWh/kg. If you look up the reference, it is for pulling CO2 out of the air, but if you redo the calculation for the higher partial pressure and higher temperature for post-combustion gas, you get roughly the same figure. In addition to this, you must also separate out sulfur dioxide and other pollutants in order for the process to be possible at all, which requires more energy. This is the theoretical efficiency; industrial processes never get anywhere near 100% efficiency. The efficiency of coal-fired power plants is around 35% of the theoretical maximum, which is not bad for a mature industrial process. Figures of 20-30% are common in the real world.

How much extra energy is required to capture and store CO2 from coal-fired plants? The emissions from coal-fired plants are about 1 kg/kWh, in round numbers. The theoretical lower limit to the energy required for CCS is 0.2 kWh/kg. Say that after all processes and losses are accounted for, you have an efficiency of 20%. That means that you will use 1 kWh to capture and store the CO2 emitted by producing 1 kWh. Are you just doubling the amount of coal you need? No, because producing that extra 1kWh of electricity has just emitted 1 kg of CO2, which has to be captured at the cost of 1 kWh, and so on.

So any amount of CCS at 20% efficiency will require all the coal in the world, and have infinite cost. You need much higher efficiency for any of this to be worthwhile. You can be confused by figures where they say that the energy used to do CCS emits less than 1 kg/kWh, since the average utility emits less. That is because the average utility does not use coal alone, but other fuels and renewables as well. But think about it for a second. How much sense does it make to use 1 kWh of renewable energy to clean the emissions from 1 kWh of coal-fired electricity? You would just use the renewable energy and not bother with the coal at all. Those types of figures are just meant to mislead.

From the point of view of energy companies, CCS subsidies are a great deal: all other planned coal power plants in North America have been cancelled, and the Alberta government is actually paying someone to build a new one. Similarly, bitumen upgraders were facing an uncertain future. And if you own an oilfield at the end of its life and the government is willing to pay you to to extract more oil through EOR, how sweet a deal is that?

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