Novel model can aid decisions in electricity generation, stream water quality
Switching from coal to natural gas in power plants can reduce how much sulfur dioxide, a gas that smells like a freshly struck match, is emitted into the atmosphere and ultimately how much sulfate pollution enters waterways, according to a Penn State-led research team that has developed a model to detect if the recent switch from coal to gas is affecting streams.
"The number of power plants switching from coal to natural gas is increasing, and sulfur dioxide emissions in Pennsylvania and across the United States are decreasing dramatically," said Xianzeng Niu, assistant research professor in Penn State's Earth and Environmental Systems Institute (EESI). "Both technology and the shift of fuels have contributed to this trend. We wanted to look at how this trend has affected water quality."
Coal- and natural gas-burning power plants emit sulfur dioxide into the atmosphere, but coal has a much higher sulfur content than natural gas and releases more sulfur dioxide when burned. The sulfur particles eventually fall back down to earth and can acidify streams. Too much sulfur, which enters waterways in the form of sulfate, can cause water chemistry to become harmful to ecosystems.
The idea for the project arose when undergraduate students working in a geosciences class with co-author Susan L. Brantley, Barnes Professor of Geosciences and director of EESI, had noticed that some streams near power plants in central Pennsylvania seemed to show improvements in water quality. The students thought that power plants switching to natural gas may explain the positive results. However, small datasets and new technologies that remove sulfur dioxide from emissions made it difficult to identify in the data the more recent shift from coal- to natural-gas-fired power plants. The research team developed a new technology trend model that made it possible to test an idea like that of the students using a big dataset.
The researchers looked at decades of sulfur dioxide emissions from Pennsylvania power plants provided by the U.S. Energy Information Administration (EIA) and stream sulfate concentrations taken by the Pennsylvania Department of Environmental Protection's Bureau of Clean Water. They used the large datasets to search for evidence that water quality improved with the switch to natural gas. They also studied federal regulations, such as the Clean Air Act amendments of 1990 and the acid rain programs of 1995 and 2000, and how power plants adopted technologies to lower their sulfur dioxide emissions as required by the new laws.
They used the data to create a novel technology trend model to see how technological adoption affected emissions from coal-fired power plants and sulfate concentrations in streams within a 60-mile radius of the plants.
"Our model shows what happens when new technologies evolve and power plants adopt them," Niu said. "We see that the trend of sulfur dioxide emissions declined gradually when innovators and early adopters first started using the new technologies, then the decline sped up as a majority of power plants adopted the technologies, and finally it slowed again as the late-adopters began implementing them."
Although not every stream showed improvements in water quality, the model allowed the researchers to see what would happen to emissions and stream sulfate concentrations in Pennsylvania if 30% of the electricity generated by coal-fired power plants in 2017 came from natural gas instead. They found that the shift could have reduced sulfur dioxide emissions by 20.3 thousand tons, which could have decreased stream sulfate concentrations by as much as 10.4%. They reported their findings in the journal Environmental Pollution.
The researchers applied their model to the other 48 contiguous states and found that the model-predicted sulfur dioxide trends largely matched the figures provided by the EIA in all but six states. Climate, soil-type and vegetation-type affect how long it takes sulfate to enter waterways, and scientists and decision makers can change these parameters in the model to make it more accurate for their region, said the researchers.
"To me, what is important here is that whatever energy source we use causes some degradation of the environment," said Brantley. "Most Pennsylvanians worry today about the impact of fracking and development of shale gas. There are some environmental problems that happen during that development. But, as we have switched from coal to natural gas, there are many impacts of coal—mining, transporting, burning—that are now lessened. So, each of us has to decide what source of energy we want to use, and we have to think about all of the characteristics of each energy source."