China’s coal problem: Is synthetic natural gas a solution?

April 26, 2017

Harbin in smog, China

Ambient air pollution contributes to more than a million preventable deaths annually in China, and burning coal is by far the biggest contributor to air pollution. But one possible solution proposed by the Chinese government—synthesizing cleaner-burning natural gas from coal—will increase CO2 emissions, negatively impacting the climate. A recent study estimated the impacts of synthetic natural gas (SNG) substitution strategies and recommends that China should focus its use in residential areas in order to achieve maximum health benefits at minimum climate cost.

The study, published April 24 in Proceedings of the National Academy of Sciences was jointly authored by researchers at UC Berkeley, Princeton, and Peking University. They found that using all produced SNG in the residential sector would decrease outdoor-air-pollution-associated premature deaths by approximately 32,0000 a year, a reduction 10 times greater than if they used the SNG in the industrial sector and 60 times greater than deploying it in the power sector. Encouragingly, they also found that using SNG residentially results in only 20 to 30 percent of the carbon penalty associated with using it in the industrial or power sectors.

“Coal is used in industry and power plants, but it’s also used in households for space heating and cooking,” says Kirk R. Smith, professor of Global Environmental Health at the UC Berkeley School of Public Health and coauthor of the study. “Although this fact is well known, it hasn’t been a big part of the policy effort in China. There’s this concept that air pollution is an urban phenomenon. But air pollution doesn’t pay attention to political boundaries.”

Emissions from households make up about 30 percent of total air pollution in China. As well, household combustion of coal is particularly inefficient and uncontrolled—as opposed to in industrial settings—so switching from coal to gas in residential areas results in the largest thermal efficiency improvement, offsetting some of the carbon penalty associated with producing the SNG.

“There is also more health impact in household areas because much of the coal is burned right next to people and they have greater exposure,” says Smith. “Indeed, the total health benefit is even larger than indicated in this study as it did not consider the reduction in indoor air pollution in households using coal.”

Using an integrated assessment approach, the authors looked at how air quality, human health, and climate change would be affected by the year 2020. The outcomes differ if China were to use SNG, conventional natural gas, or coal, and they also differ depending on where the various fuel types would be employed—the power sector, industrial sector, or residential sector—the three areas where 85 percent of natural gas in China is consumed. The largest discrepancies were among the number of estimated premature deaths (which is a measure of health linked to air quality) and in the amount of change in CO2 emissions (which is linked to climate change).

China’s most recent Five-Year Plan includes specific goals for the amount of particulate matter (PM2.5) in the air for the first time. In the Copenhagen Accord of 2012, China pledged to curb carbon emissions, in part by turning from coal to natural gas.

Though it has abundant resources of coal, China lacks a large supply of natural gas, relying mostly on imports. SNG, which is made from coal, has become an attractive option for China to reduce air pollution and dependence on natural gas imports.

“The conversion of coal to natural gas is an old process. And you produce more CO2 than by just burning the coal directly,” says Smith. “There’s a climate penalty but there’s a health benefit. The Chinese government has agreed to change out power plants from coal to natural gas. I’m not sure that SNG should count, since it’s made from coal, too. It’s better for air pollution, but it’s not necessarily better for the climate.”

The authors found that even when focusing on residential areas, SNG substitution for coal results in an increase of 28 million tons of CO2 emissions. They suggest the possibility of employing carbon capture and storage (CSS).

“The SNG plants have a stream within them of nearly pure CO2, just by the process,” says Smith. “So if you’re going to employ carbon capture and storage, this is a pretty good technology to start with.”

But even with CCS used for SNG production, the study found that net CO2 emissions are still 22 to 40 percent higher than occurs with the same amount of conventional natural gas. Thus they conclude that “SNG cannot simultaneously address the multiple objectives facing China.” However, allocating the SNG to residences as opposed to power or industry would provide substantial air quality improvement with relatively small climate impacts. And save lives.

Funding for this study was provided by a graduate fellowship from the Woodrow Wilson School of Public and International Affairs at Princeton University, postdoctoral support from the Climate Futures Initiative at Princeton University, and National Natural Science Foundation Committee of China Grants.

By Jaron Zanerhaft

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