Carbon Dioxide and the Fundamentals of Heat Transfer

With the recent push for the regulation of carbon dioxide emissions from new and existing power plants (see “Turning the Heat Up on Carbon Emissions” in the October 2013 issue of POWER), the science behind this action needs to be revisited.

The regulation of carbon dioxide emissions will apply to all fossil fuel energy sources; however, previous actions by the Environmental Protection Agency have tailored the regulations in a way that fuel sources such as natural gas readily comply while coal-powered plants require substantial investments to remain operational. Coal still dominates power generation, as over 40% of the power produced in the United States comes from this fuel source.

As a fuel source, coal is broken down into varying classifications (anthracite, bituminous, subbituminous, lignite) with widely varying ranges of carbon content. The carbon contained within coal is the primary driver for the thermal energy produced during the combustion process. During this combustion process it is highly desirable to covert as much fuel-bound carbon into carbon dioxide as possible. The chemical energy released from a single carbon atom binding to a single oxygen atom, thus creating a carbon monoxide molecule, is approximately 3,965 Btu/lb-carbon. Pushing that carbon monoxide molecule to react with another oxygen molecule to generate carbon dioxide will liberate an additional 10,125 Btu/lb-carbon.

From an efficiency standpoint, it is obvious that heat released from the conversion of carbon into carbon dioxide is very desirable. From a permitting and human health standpoint, carbon monoxide is regulated as a Prevention of Significant Deterioration (PSD) pollutant and has been shown to be detrimental to the health of human beings. Carbon dioxide, on the other hand, is a natural byproduct of many life forms on Earth.

Heat Transfer Fundamentals

The issue of carbon dioxide being classified as an air pollutant due to its higher ability to reflect solar radiation is complex, which is why it leads to so much debate between climatologists. To better understand how an increase in atmospheric carbon dioxide can potentially increase global temperatures, one must go back to the basics of heat transfer. There are three primary modes of heat transfer that occur in the world we live in: conduction, convection, and thermal radiation.

Conduction refers to a temperature gradient that exists in a stationary medium (solid or fluid) and the heat transfer that occurs across that medium. Convection, on the other hand, is when a temperature gradient exists between a surface and moving fluid and the heat transfer between the two. Thermal radiation is the transfer of energy in the form of electromagnetic waves between two surfaces.

Because of the void of space between Earth and the Sun, conduction and convection are not modes of heat transfer that can warm Earth’s surface. Energy is transmitted from the hot plasma of the Sun to Earth by means of thermal radiation. The peak intensity of the thermal radiation from the sun comes in the form of visible light, which is defined as short wave thermal radiation. When this short wave thermal radiation hits Earth’s atmosphere, a small portion of the energy is absorbed by gases present in the atmosphere, but the majority of it passes through to reach Earth’s surface. Particulate and water vapor will react differently to the short wave radiation and reflect most of that energy back into space (think of a cloudy day).

Properties of Carbon Dioxide

The solar radiation that makes its way to Earth is either absorbed by the surface of the Earth or reflected back to space. When solar radiation is reflected from Earth’s surface back into space, the wavelengths of the solar radiation become elongated and shift toward the infrared region of the spectrum. This is where the problem with gases such as carbon dioxide comes into play. At longer radiation wavelengths, thermal radiation no longer passes through the molecule back into space; rather, the energy is absorbed, and then reflected back to Earth’s surface.

It’s at this point where global warming advocates and skeptics start to disagree with the impact of greenhouse gases (GHGs) on the environment. Global warming advocates point to scientific data showing the propensity for carbon dioxide to reflect more thermal radiation back to Earth’s surface, thus increasing the total energy absorbed at the surface.

Those who call themselves skeptics regarding carbon dioxide’s effect on global warming, on the other hand, point to the fact that carbon dioxide constitutes such a small percentage of the atmosphere that it has little effect on the total energy balance of the environment. Here’s the breakdown of molecules and their average percentage in Earth’s atmosphere, according to Mark’s Standard Handbook for Mechanical Engineers, 11th edition:

  • Nitrogen: 78.08%
  • Oxygen: 20.95%
  • Argon: 0.93%
  • Carbon dioxide: 0.035%
  • Methane: 0.00017%

Instead, slight variations in the natural solar cycle have a more profound effect on the energy transmitted and retained by Earth than the reflective nature of carbon dioxide.

Other Heat-Trapping Sources

Carbon dioxide is often looked at as the driving force for absorption of solar energy and global warming. However, there are multiple gases that exhibit heat-trapping properties. In general, water vapor is a far more abundant and potent GHG than carbon dioxide. The lifecycle of water vapor in the atmosphere is relatively short (it can usually be measured in days) as weather events, such as rain, will remove it from the atmosphere. Carbon dioxide has a much longer atmospheric lifecycle (typically measured in years), which theoretically allows excess concentrations to build.

In May of this year, the weather station located in Mauna Loa, Hawaii, recorded carbon dioxide concentration of 400 ppm for the first time. Even with this record measurement, data from the National Oceanic and Atmospheric Administration has shown a leveling of surface temperatures over the past 15 years. Although the fundamentals of heat transfer and carbon dioxide are well known, the role of carbon dioxide in the multi-variable complex environment we live in needs to be reevaluated.

—Brandon Bell, PE is a project manager at Valdes Engineering and a POWER contributing editor.

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