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Working toward cleaner coal: Part 2

This article first appeared in the St. Louis Beacon, July 29, 2009 - In the push for cleaner air, public attention has now turned to the main product of burning organic matter, the "greenhouse gas" carbon dioxide. The United States emits more than 6 billion tons of carbon dioxide annually, one-third each from coal and natural gas and one-third from transportation and petroleum. Scientists and policy makers generally agree that this greenhouse carbon dioxide is causing the average temperature of the Earth to rise. The question is just how to reduce drastically the amounts of carbon dioxide added to the atmosphere.

Today, lots of money and research are aimed at sending carbon dioxide from combustion back where it came from. The process is called sequestration. Sequestration means capturing the carbon dioxide that results from burning fossil fuels, pumping it underground and storing it there permanently.

What's going on

A visit to the DOE-National Energy Technology website at www.netl.doe.gov gives an idea of research underway and being considered with regard to coal, oil and gas technology.

The DOE's main website, www.energy.gov , describes government funded research in all fields of energy.

That's not as simple as it sounds.

  • The carbon dioxide has to be purified and compressed into a liquid-like state called a supercritical fluid.
  • The fluid carbon dioxide must be pipelined and pumped very deep underground.
  • The underground storage site must be porous. It must also have a natural cap of non-porous rock like slate or shale that will keep the carbon dioxide from escaping.

This technology is not completely new. Carbon dioxide is already pumped into oil fields to enhance oil recovery. In addition, natural gas is pumped through pipelines and stored underground until needed.
Other than oil fields, deep salt-water reservoirs, porous rock such as sandstone, and even some depleted coal fields could be used for storage. The Sleipner project off the coast of Norway has been pumping a million tons of carbon dioxide annually into a salt-water reservoir since 1996.

Finding storage for carbon dioxide in Missouri will be difficult, since its porous rock is closer to the surface. City Power in Springfield is using a Department of Energy-funded project to explore pumping carbon dioxide into a formation about 2,000 feet below the surface, reports Runar Nygaard, a professor at Missouri University of Science and Technology in Rolla.

An Expensive Fix

The challenges of purifying the carbon dioxide are enormous. The problem is to separate it from the nitrogen that makes up 80 percent of air, as well as small percentages of other gases including oxygen. Scientists are testing to see if:

  • The flue gas could be bubbled through a liquid that would react chemically with the carbon dioxide, and release it later in another reaction.
  • Metal-organic "sponges" could cyclically absorb and release huge amounts of the gas.
  • Similar "absorb and release" compounds could be painted on silicon wafers.

Ironically, to power any of these schemes, more coal will need to be burned. It takes power to run belts, fans and conveyers. Techniques for removing sulfur and nitrogen oxides may need to be modified. But carbon would be returned to the earth, from whence it came.
One more approach, considered very promising, works with gasified coals. If coal is burned with limited oxygen, it becomes "synthesis gas" or "syngas" consisting of carbon monoxide and gaseous hydrogen. Syngas can then be burned like natural gas. This method has been used in two commercial-scale plants since the 1990s. Of course, these plants still produce carbon dioxide, but in lesser quantities per unit of electricity.

Syngas is also used as a raw material in making Tylenol, Nutrasweet, Craftsman tool handles and Kodak film.

Maximizing Carbon Sequestration

Taking the gas produced from burning coal and sequestering it underground would prevent more greenhouse carbon dioxide from entering Earth's atmosphere. But what if the same process could actually remove existing carbon stores from the air?

All plant materials eventually decompose with carbon dioxide as a by-product. Washington U.'s Axelbaum has proposed taking biomass from trees and grasses and combusting it together with coal, and then getting all the resulting carbon dioxide underground. This process could result in a net reduction of carbon dioxide in the atmosphere.

On its way underground, the carbon dioxide waste stream could be used as food for growing algae as a source of oil, he proposes. Some algae, grown under the proper conditions, can contain 35-40 percent oil. Theoretically, according to Gary Stacey, a professor at the Center for Sustainable Energy at the University of Missouri-Columbia, 1 percent of current crop land could generate enough algae to supply biodiesel for all the trucks in the country.

Paul Nam at Missouri University of Science and Technology and his wife, Keesoo Lee, at Lincoln University, have begun a demonstration project with some central Missouri electric cooperatives in which flue gas is bubbled through open pools of algae. The problem in open systems is contamination by microbes. Midwest Research Institute in Kansas City is trying the same general experiment in a closed system. The closed system can be kept sterile, but is much more costly.

Rising Prices Could Spur Innovation

We expect our electricity to be uninterrupted and inexpensive. Coal-powered electricity is very inexpensive, and every added step in its processing will add to our electric bills. Ameren plans to spend $2.2 billion to $2.6 billion between now and 2018 to upgrade its four coal-powered plants to comply with environmental regulations. This expense will surely be passed on to the consumer.

"The public has to decide how much it's willing to pay for the cost of cleaning things up," said Axelbaum of the Consortium for Clean Coal Utilization.

Rising prices will no doubt have an impact. The cost of generating electricity from coal may force the development of new, clean,and renewable energy sources. Note how gasoline at $4 a gallon changed the automobile market, especially while the price lasted.

Understanding fossil fuels

Burning any organic matter creates carbon dioxide. Carbon (C) + oxygen (O2) = carbon dioxide (CO2).

  • Organic matter includes fossil fuels such as coal, gas and oil. It also includes wood, biomass such as switch grass, ethanol and oils extracted from plants or algae.
  • Burning fossil fuels adds to the net carbon in the atmosphere because the fuel has been buried underground.
  • Burning of biomass is carbon neutral, because the carbon has been stored as biomaterial above ground and will naturally decay if not destroyed prematurely.
  • Different fossil fuels produce different amounts of carbon dioxide per unit of electricity.
  • Burning coal in most of today's electrical plants is 35 percent efficient. Coal is burned in a boiler, converting chemical energy to thermal energy. The thermal energy is transferred to steam turbine tubes wrapped around the boiler. The stream drives the turbine, converting thermal energy to mechanical energy. Finally, the turbine drives the generator, converting mechanical energy to electricity. Each conversion wastes energy. A state-of-the-art coal-burning plant built today would have 40 percent efficiency.
  • Natural gas produces less carbon dioxide than coal per electrical unit because it is burned with 60 percent efficiency. Gas makes electricity in two steps. First, it burns in a gas turbine similar to jet engine that is hooked up to a generator. The hot exhaust gases then run through a steam turbine for a second round of electrical generation.

Jo Seltzer is a freelance writer with more than 30 years on the research faculty at the Washington University School of Medicine and seven years teaching tech writing at WU's engineering school.