Clean Coal and Carbon Sequestration

Clean coal, or coal burning combined with carbon capture and storage (CCS) captures carbon dioxide from fossil fuel power plants and stores it in the ground rather than releasing it into the atmosphere. CCS is commercially available but the long term storage of CO2 is a mostly untried. No large scale power plant operates with a full carbon capture and storage system.

Applying CCS to a conventional power plant would reduce CO2 emissions by over 80%. Capturing and compressing CO2 would reduce delivered electricity from the plant by about 25% however, thus raising the cost of such energy by at least such amount.

Clean coal is likely to play some role in a carbon-neutral world. How much of a role is likely to depend on how Americans react to proposals to dramatically expand nuclear energy production. Regardless of how the nation answers that question however, MacKay provides an intriguing option regarding carbon sequestrations possible role in mitigating the effects of air travel on the environment.

As I’ve been researching this issue, the one thing that has most struck me is the massive amount of CO2 released by air travel. One intercontinental jet trip per year uses about 30 kWh per day, or about 10% of the average American’s energy usage. How can air travel co-exist with a carbon free world? MacKay has a potential solution.

Air Travel

How these plans relate to carbon-sucking and air travel. In a future world where carbon pollution is priced appropriately, we are interested in any power scheme that can at low-cost put extra carbon down a hole in the ground. Such schemes might permit us to continue flying at 2004 levels (while oil lasts). In 2004, average emissions of CO2 from flying were about 0.5t per year per person. Accounting for the full greenhouse impact of flying, perhaps the effective emissions were about 1t per year per person CO(e)2. In all five of these plans I assumed that 25% of the UK was devoted to the production of energy crops which were then used for heating or for combined heat and power. If instead we directed all these crops to power plants with carbon capture and storage, the ‘clean coal’ plants that featured in three of the plans, then the amount of extra CO2 captured would be about 1t of CO2 per year. If the municipal and agricultural waste incinerators were located at clean coal plants too so that they could share the same chimney, perhaps the total captured could be increased to 2tCO2 per
year per person. This arrangement would have additional costs: the biomass and waste might have to be transported further; the carbon capture process would require a significant fraction of the energy from the crops; and the lost building-heating would have to be replaced by more air-source heat pumps. But I think it would be worth planning ahead by seeking to locate new clean coal plants with waste incinerators in regions close to potential biomass plantations.

Biomass is Carbon Neutral

What this paragraph doesn’t fully explain is something that I just learned: burning biomass is basically a carbon-neutral activity. As biomass grows, it sequesters carbon in its plant form through the exchange of carbon dioxide for oxygen with the atmosphere. When the biomass dies and rots the carbon is released into the atmosphere and soil via decomposition. This CO2 is then absorbed by existing plants in the giant cycle of life. Burning the biomass simply replaces the decomposition process.

Burning fossil fuels is different. In burning coal or oil we take carbon that has been naturally sequestered in the ground in solid or liquid form and release it into the air in gas form.

For simplicity sake, take a world which burns one unit of biomass, one unit of oil and one unit of coal with no carbon capture. In this example we take one carbon unit from the atmosphere and two from the ground and all three end back up in the atmosphere. If carbon capture technology is used in the burning of the coal then two units end up in the air and one ends up back in the ground.

This is where MacKay’s plan comes in. We can sequester the coal but we can’t capture the carbon emitted by the jet. If instead we capture the biomass emissions we essentially exchange the carbon in the ground for the carbon in the air. We pull carbon out of the air and put it in the ground while simultaneously taking carbon out the ground and putting it in the air.

Interesting indeed.