Trade-offs between cutting air pollution and worsening climate damage

According to new research conducted by researchers from the IIASA Air Quality and Greenhouse Gases Program in collaboration with scientists in China and the USA, synthetic natural gas represents a trade-off between reducing air pollution and increasing greenhouse gas emissions in China.

China’s industrial regions have been plagued by severe air pollution in recent decades and the situation has received worldwide attention thanks to photos of Beijing and other smog-blanketed Chinese cities. More than just an eyesore, China’s smog has created a public health crisis that has led the Chinese government to declare war on air pollution. Air pollution contributes to cardiovascular disease, lung cancer and emphysema, among other illnesses and in China specifically, causes about 1.6 million people to die prematurely each year. This accounts for more than a quarter of annual deaths that can be attributed to air pollution.

Driven by this public health emergency and a desire to rely on its large supplies of coal, China has plans to substantially expand its production of synthetic natural gas (SNG). Around 40 SNG plants have been approved or are currently under construction in the country. In addition, as part of the Paris climate agreements, China has committed to peaking its CO₂ emissions by 2030 or sooner.

SNG is a fuel derived from coal that is relatively free of conventional air pollutants. For China, this fuel source represents a trade-off. Using SNG instead of coal could improve air quality and public health by reducing illness and premature mortalities due to air pollution, but it would also markedly increase CO₂ emissions, because creating synthetic natural gas from coal produces more CO₂ than burning the coal directly.

Given China’s plans to produce SNG, the new study [1] examined possible ways for China to maximize improvements in air quality while minimizing the additional CO₂ emitted from the production and use of this fuel source. Yue Qin, a former participant of the Young Scientists Summer Program (2016), along with Fabian Wagner and other colleagues at IIASA developed a sophisticated modeling approach to estimate both health outcomes and carbon emissions under various SNG-use scenarios. Their goal was to determine whether the Chinese government might have options for optimizing deployment of SNG to minimize the climate penalty, while maximizing the health benefits of reduced air pollution.

The study examined the impact of switching from coal to SNG in three broad areas: electricity production, industry, and residential use. They found that switching to SNG in industry and electricity production would have little impact on smog-related deaths and cause a major increase in CO₂ emissions. However, the results also showed that switching from coal to SNG for residential uses, such as heating and cooking, would substantially reduce deaths due to air pollution and increase emissions of CO₂ to a lesser extent than the alternative. The smaller environmental impact of using SNG in the residential sector results because residential coal combustion in small stoves is inherently inefficient and has uncontrolled emissions of air pollutants. Coal combustion in the power and industrial sectors on the other hand, is more efficient and pollution control devices reduce emissions of health damaging air pollutants, thus reducing the benefits of switching to SNG.

Comparison of CO₂ emission changes and avoided premature mortality under various synthetic natural gas (SNG) allocation scenarios relative to the use of coal. Results for standard SNG scenarios all find increases in CO₂ emissions while results that include Carbon Capture and Storage (CCS) and the use of conventional natural gas (NG) all result in reductions in CO₂ emissions. Solid circles represent SNG without applying CCS, and open circles represent SNG with CCS applied during SNG production. Solid triangles represent the outcome when conventional natural gas is used without CCS. We assume that electricity provided for SNG_CCS is from natural gas combined cycle (NGCC) power plants using CCS.

SNG plants convert coal to a form of natural gas by a variety of methods that are typically energy intensive. As a result, it is a less efficient fuel than coal because of the energy involved in the gas’s creation. The researchers found that using synthetic natural gas to generate electricity, for example, results in 60% more CO₂ emissions than using coal-fired generators.

One benefit of SNG is that toxic sulfur and nitrogen compounds are removed from the coal as part of the production process. Compared to coal-fired electric plants, this difference is relatively minor, as modern coal plants remove much of these pollutants. When SNG is however used in the residential sector, it produces far less pollution than the use of coal.

The study concluded that deploying synthetic natural gas in the residential sector would substantially improve air quality and reduce premature deaths associated with outdoor air pollution with the smallest increase in CO₂ emissions compared with the power and industrial sectors. However, the results also indicate that in no case can SNG simultaneously meet the desire to improve air quality while reducing carbon emissions.

References

[1] Qin Y, Wagner F, Scovronick N, Peng W, Yang J, Zhu T, Smith KR, & Mauzerall DL (2017). Air quality, health, and climate implications of China’s synthetic natural gas development. Proceedings of the National Academy of Sciences 114 (19): 4887-4892.