SCIENCE INDEX 2000 2001 2002 Atmospheric Sciences

Substituting natural gas for coal offers long-term climate benefits

James E. Kloeppel, Physical Sciences Editor
(217) 244-1073; kloeppel@uiuc.edu

7/3/02

CHAMPAIGN, Ill. — Substituting natural gas for coal in electrical power generating plants could reap greater long-term climate benefits than previously thought.

Switching from coal to gas generally has been considered as a short-term response to climate change. As reported in the July issue of the journal Climatic Change, scientists have found that over the first three decades following implementation, the benefits of switching from coal to gas can be either positive or negative, depending on specific characteristics of the switch.

However, switching to gas was found to provide a consistently lower impact on climate relative to coal use over the long term.
In the past, research on the effects of switching from coal to gas mainly has focused on carbon dioxide, the most important greenhouse gas emitted by human activities. However, the net impact of fuel switching on climate depends on emissions of all substances that affect climate. "Our analysis is unique in that it includes not only carbon dioxide but also estimates the impact of changes in methane, sulfate aerosol, and black carbon emissions resulting from coal-to-gas switching," said Katharine Hayhoe, a research scientist at the University of Illinois at Urbana-Champaign.

Collaborators on the project were Atul Jain and Donald Wuebbles, both professors of atmospheric sciences at Illinois, and Haroon Kheshgi, a scientist with the ExxonMobil Research and Engineering Co. in Annandale, N.J. The study was funded by the U.S. Department of Energy.

The researchers used the Integrated Science Assessment Model to project the impact of substituting natural gas for coal – and the subsequent change in emissions – on global temperature. Developed by Jain in collaboration with the other authors, the model recently was used by the United Nations Intergovernmental Panel on Climate Change to assess the impact of carbon dioxide emissions on the environment. The model calculates the impact of more than 40 greenhouse gases and aerosols on climate and the atmosphere system, including those used in this study.

It is well-known that switching from coal to natural gas would significantly reduce carbon dioxide emissions. The carbon content of natural gas is only 60 percent that of coal per unit of primary energy content. The higher efficiency of state-of-the-art natural gas turbines over older, coal-fired power plants could reduce emissions by an additional 30 percent.

However, coal and gas use also release methane, the second most important greenhouse gas emitted by human activities. During coal extraction, methane trapped in and around coal seams is released to the atmosphere. Methane also is released whenever natural gas escapes during transportation and distribution. Hence, switching from coal to gas would reduce methane emissions from coal mining, but increase natural gas-related emissions.

In addition to greenhouse gases, the combustion of coal can release tremendous quantities of sulfur dioxide. In the atmosphere, sulfur dioxide is quickly oxidized to sulfuric acid, which condenses onto cloud droplets and aerosol particle surfaces to form sulfate aerosols. By scattering solar radiation, these short-lived aerosols tend to cool Earth’s surface. In contrast, black carbon, released in the form of soot from coal-fired power plants, forms the nuclei of infrared-absorbing carbonaceous aerosols that have a short-lived warming effect on the atmosphere. Thus, gas-for-coal substitution would reduce the cooling effect of sulfate aerosols while simultaneously reducing the warming effect of black carbon.
The long lifetime of carbon dioxide relative to aerosols means that coal-to-gas switching will always result in a net long-term benefit, the researchers say. The question is how long it would take before those benefits would be seen.

"When the ‘best guess’ estimates of radiative forcing are applied to global average coal and gas characteristics, the benefits of fuel switching are delayed by about 30 years," Jain said. "The delay is caused by the reduction in sulfate aerosol emissions and increase in natural gas-related methane emissions that occurs when switching from coal to natural gas – creating a net warming effect."

While reductions in sulfur aerosol emissions can indeed postpone the climate benefits achieved by replacing coal with natural gas, "these benefits are merely delayed, they are not eliminated," Hayhoe said.

The overall effectiveness of fuel switching as a mitigation option for a specific power plant depends upon local and regional characteristics. The most important factor is the degree to which sulfur is controlled at the coal plant. Sulfur controls significantly limit production of sulfate aerosols, eliminating the delay before the benefits of switching are observed.

Other factors that affect the outcome of the switch include the sulfur content of the coal, the natural gas leakage rate, and the difference in efficiency between the coal and gas power plants.

Careful selection of the coal plants to be replaced and the gas replacement technology could enhance the potential for fuel switching to reduce the net climate impacts of fossil fuels, Hayhoe said.

"The greatest benefit would occur by converting an inefficient coal plant with low sulfur dioxide emissions to a state-of-the-art gas turbine," Hayhoe said. "In this case, sulfur emissions are already low and the difference in efficiency is substantial, resulting in a significant climate benefit that is only slightly delayed."

In contrast, replacing a coal plant lacking emission controls and burning high-sulfur coal would offer no short-term mitigation benefits, Hayhoe said. This would be particularly true if such a plant were located in a nation without access to the latest natural gas combustion technology, or with leaky natural gas pipelines. In such a case, switching from coal to gas would not result in much of an efficiency increase. In addition, the switch would significantly increase methane emissions, and drastically reduce sulfur dioxide emissions. A net climate benefit would not be seen for over a century.

"Above all, we must recognize that there are other pressing reasons to reduce coal emissions aside from climate concerns," Jain said. "Sulfur and other emissions from coal use are responsible for a number of environmental issues related to human health and air quality."

Coal-fired power plants generate two-thirds of the sulfur dioxide emissions in the United States. Sulfur dioxide controls already exist throughout much of the United States and the rest of the developed world. Controls on these and other harmful substances are expected to become even more stringent in the near future as more and more nations recognize the immediate benefits to the local environment and health.

"In the presence of significant sulfur dioxide emissions controls, the impact of fuel switching will be positive over all time scales," Jain said.