Will Carbon Capture and Sequestration Save Our Climate?

by Dick Walton (Billings, MT)

Dick Walton is retired after 40 years as a college physics professor. For years he taught a course on Energy and Society approximately once a year. This course covered the gamut of issues regarding energy sources and uses.

What is the Problem?

The burning of coal to produce electricity is a major source of the carbon dioxide (CO2) which is causing climate change.  People who understand that this is a serious problem, but who don’t believe civilization is possible without burning coal, sincerely promote the idea of “clean coal.”  Climate change deniers, who consider CO2 from coal as only a perceived political problem, promote “clean coal” more cynically.  To make coal “clean” it would be necessary to eliminate the release of CO2 into the atmosphere (in addition to removing pollutants like sulfur dioxide and mercury etc.).  People in coal industries, and many politicians, commentators, and reporters promote “Carbon Capture and Sequestration” (CCS) as the solution to coal’s dirty CO2 problem.

Critics of CCS discuss the economic costs of CCS, and argue that it would increase the cost of coal generated electricity well beyond a price competitive with cleaner sources of electricity.  (Even without CCS electricity from coal plants is already losing that competition.)   If we knew with some certainty the additional cost per kWh for CCS, that might help resolve the issue.  But CCS is complex and still experimental.  We hear vague cost estimates from competing experts, but no clear answer.  A different approach to the issue may be more helpful.

Most of the public have little understanding of what CCS involves.  This is also true of some (many?) reporters, commentators and politicians.  Others “know” but do not “notice” the realities of CCS.  And some are willfully deceptive.  A basic understanding of the nature and magnitude of the physical and chemical challenges of CCS might temper people’s naive optimism regarding CCS.

Many people assume and expect that smart scientists and engineers will invent ways to greatly reduce the costs of CCS – and thus save the climate.  People believe science can do anything.  (Sometimes these are the same people who do not accept the science of climate change.)   But some things can’t be done.  The laws of physics limit what is possible.

What is CCS?

“Carbon Capture” is a misnomer.  It should be “Carbon Dioxide Capture.”   If we captured carbon, that would be a useful material that could be sold and used for many purposes, or burned to generate more electricity.  But it is not carbon that must be captured.  After coal is burned it becomes CO2.  The only way to capture the carbon would be to leave it where it was – coal captured in the ground.  And, of course, we can’t split CO2 back to carbon and oxygen without using more energy than we got by burning the carbon in the first place.  The first and second laws of thermodynamics make that clear.  But some people may not understand this.

Some people may think that if we can just capture and remove the carbon before we burn coal, we would then have “clean coal” – with no CO2.  They may think carbon is an impurity in the coal that can be removed like sulfur, arsenic and mercury etc.  But coal is between 50% and 90% carbon depending on the grade of coal.   It is primarily the carbon that burns to generate electricity.  To produce each megawatt hour of electricity, more lower grade coal is needed to have the same amount of carbon to burn.

CO2 is not a “side effect” of burning coal.  It is the unavoidable result of burning coal.  This chemical reaction is the source of the energy.  Burning one ton of carbon produces 3.67 tons of CO2. (Ask any high school chemistry student.)  So burning one ton of 70% grade coal produces 2.5 tons of CO2.

If you capture the CO2 from burning a ton of coal, then you have 2 ½ tons of climate changing rattlesnake in your hands.  And the CO2 is a “squirming” gas rather than a solid like the coal.  What do you do with it?   You can’t just pile it in a landfill or sweep it under a rug.  It won’t stay there.  At standard temperature and pressure the volume of this CO2 gas is approximately one thousand times the volume of the burned carbon that produced it!

There is no commercial use for the huge amount of CO2 produced every year by burning coal.  And any use that does not permanently sequester the CO2 will not help the problem.  The only solution, other than not burning coal, is to capture the CO2 gas and pump it deep into the earth at geologically suitable locations where it will not leak back to the surface on a human time scale.  We should call it “Carbon Dioxide Capture and Permanent Sequestration” (CDCPS).  This is the “Sequestration” part of CCS.  The CO2 must be “sequestered” permanently.   This may be feasible, but….

How Big, and Expensive, is the Task?

In 2017 the U.S. burned 665 million tons of coal in electric power plants, producing 1,330 million tons of CO2. At standard temperature and pressure this is about 22 trillion cubic feet of CO2 gas.  (This does not count the CO2 produced by burning gasoline, natural gas and another 52 million tons of coal burned for other purposes.)    This is the same scale as the 27 trillion cubic feet of natural gas used in the U.S. in 2017.  Wells are drilled to obtain the natural gas.  The gas is compressed and pumped through pipeline networks constructed from the gas wells to our houses and factories.  All that infrastructure and transportation costs considerably in both energy and dollars.  We pay for that when we buy natural gas.

A similar sized, but reversed, infrastructure with similar energy and dollar costs would be required for CCS.  The CO2 gas from coal fired power plants will need to be captured, compressed and pumped through new pipelines constructed from the power plants to geologically appropriate locations that won’t leak.  Wells will need to be drilled and the CO2 injected down for permanent sequestration.  The difference is that the natural gas has a value and is sold to its users; The CO2 gas has no value and can’t be sold.  It is hazardous waste and must be permanently buried deep in the earth.  Obviously the CCS infrastructure and process, like that for natural gas, will be costly.  Who will pay?  The cost of CCS will be added to the price everyone pays for electricity generated by the burning of coal.

Addendum

Are Natural Gas Power Plants Better?  Maybe….

Natural gas is replacing coal for electricity generation.  The primary reason is the current low price of natural gas compared to the price of coal.  But there is another potential advantage also.  Natural gas emits about 50% to 60% less CO2 than coal does to generate the same amount of electricity.  Burning coal produces CO2:   C + O2 -> CO2.  Natural gas is mostly methane.  Burning methane (CH4) produces CO2 and water:  CH4 + 2O2 -> CO2 + 2H2O.   Approximately half the heat energy produced comes from burning the hydrogen.   This CO2 is still a climate changing problem, but the task, and cost, of CCS would be reduced compared to burning coal.  However, there is another problem….

Methane’s Dirty Secret.

If unburned methane escapes to the atmosphere it is a much worse greenhouse gas than CO2 is.  How much worse is complex.  Over a twenty year period methane traps about 86 times the heat that CO2 does.  Natural processes remove methane from the atmosphere faster than CO2, so over a 100 year period the greenhouse impact of methane is about 34 times worse than CO2.  Fortunately there is much less methane in the atmosphere than CO2.

Small methane leaks anywhere between the natural gas well and the power plant have a big effect.  The leaks can make burning natural gas as bad a cause of greenhouse warming as burning coal.  Leakage of about 1.5% of the methane used will eliminate the greenhouse gas advantage of burning natural gas instead of coal for twenty years.  Leakage of 3% to 4% will eliminate the advantage of natural gas for 100 years!  Recent studies show a current leak rate of 2.3%. There is only a small economic incentive for gas companies to eliminate a mere couple percent of leakage.  But the climate change impact is huge.  This is why it is unconscionable that the Trump administration has rolled back the Obama era regulations on methane leakage.

 

Fact Sheet: Carbon Capture and Sequestration

  • Coal fired power plants are a major source of climate changing carbon dioxide, CO2.
  • Bituminous coal is about 70% carbon (plus or minus) depending on the grade of coal. [1]
  • The energy comes from burning the carbon to make CO2.  The rest of the coal is mostly irrelevant in terms of energy production.  But can cause serious air pollution.
  • Every ton of carbon burned produces 3.7 tons of CO2.  Depending on the grade, a ton of coal produces about 2 ½ tons of CO2. [2]
  • The volume of the CO2 is about one thousand times the volume of the carbon that produced it at standard temperature and pressure (STP). [3]
  • In the U.S. we burned 665 million tons of coal in electric power plants in 2017. [4]
  • Burning coal in power plants produced 22 trillion cubic feet of CO2 (1,330 million tons) in 2017. [5]  That is the same scale as the 27 trillion cubic feet of natural gas [6] we burned in 2017 in the U.S.
  • Coal can not be “clean” unless the CO2 is captured and permanently sequestered.
  • Carbon Capture and Sequestration (CCS) is the process of capturing the CO2 from burned coal, transporting it to a geologically appropriate location that will not leak, and injecting it down wells and where it will remain permanently.
  • Capturing CO2 is very different from capturing SO2, mercury and other toxic pollutants in coal, because those other pollutants occur in very small quantities compared to carbon.
  •  Natural Gas (mostly methane) emits 50% to 60% less CO2 than coal (mostly carbon) for each unit of electricity generated by new and efficient power plants. [7]
  • Over a twenty year period methane has 86 times the greenhouse gas impact as CO2.  It has 34 times the impact over a one hundred year period. [8]
  • If 1.2% of the natural gas (methane) leaks at the well, from pipelines, or at power plant, then gas fired power plants are just a damaging to the climate as coal fired plants for 20 years.  If 3% of the methane leaks gas power plants are as bad as coal for 100 years. [9]
  • The EPA estimates a methane leak rate of 1.4%.  Recent studies show the leak rate is 2.3%. [10]

[1] https://en.wikipedia.org/wiki/Bituminous_coal

[2] Atomic mass of Carbon: 12.  Atomic mass of oxygen: 16.  Molecular mass of CO2:  12 + 2(16) = 44.

44/12 = 11/3 = 3.67.   70% of 3.67 = 2.57

[3] Computed using the densities of carbon and CO2.

[4] https://www.eia.gov/coal/annual/

https://www.statista.com/statistics/184333/coal-energy-consumption-in-the-us/

[5] Computed using the density of CO2, 2 kg/m3, and 1,207 metric tons of CO2. https://www.eia.gov/tools/faqs/faq.php?id=77&t=11

[6] https://www.eia.gov/tools/faqs/faq.php?id=50&t=8

[7] https://www.ucsusa.org/clean-energy/coal-and-other-fossil-fuels/environmental-impacts-of-natural-gas#.XCZp6i2ZNjs

[8] https://www.ucsusa.org/clean-energy/coal-and-other-fossil-fuels/environmental-impacts-of-natural-gas#.XCZp6i2ZNjs

The difference between 20 years and 100 years is because CO2 remains in the atmosphere longer than methane does.

[9] Computed using the above data and assuming a 50% reduction in CO2 emission from gas power plants compared to coal.

[10] “…based on the results of the new study … the methane leak rate is 2.3 percent.”

https://www.pbs.org/newshour/science/the-u-s-natural-gas-industry-is-leaking-way-more-methane-than-previously-thought

https://www.cnbc.com/2018/07/06/the-us-natural-gas-industry-leaking-way-more-methane-than-ever-before.html

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