Negative carbon balance power generation for China

If we want to limit the increase in global temperature to 2 degrees above pre-industrial levels, as stipulated in the Paris Climate Agreement, it will take much more than the transition to sources of energy. carbon neutral energy such as wind and solar. This will require negative carbon technologies, including energy sources to reduce levels of carbon dioxide in the atmosphere.

Although most researchers and climate activists agree that negative carbon solutions will be needed to reach the goal of the Paris agreement, most of these solutions have been considered impractical in the short term, especially for large coal-dependent countries such as China.

Researchers from the Harvard School of Engineering and Applied Sciences John A. Paulson and the Harvard-China project on energy, the economy and the environment, in collaboration with colleagues from Tsinghua University in Beijing and other institutions in China, Australia and the United States, analyzed the technical and economic viability for China to move towards electricity generation. carbon negative.

The research is published in the Proceedings of the National Academy of Sciences.

"This paper boldly suggests that not only can China evolve into negative carbon-emitting energy, but also that it can do so in an economically competitive manner," said Michael McElroy, a professor of economics. Environmental studies at Butler, Gilbert Butler, and co-lead author. paper.

"The system we are describing not only offers a negative carbon alternative for generating electricity in the long run, but also brings a significant indirect benefit in the short term for the reduction of air pollution in China. said Xi Lu, associate professor at Tsinghua University's School of Environment and first author of the newspaper. Lu is also a former graduate student and SEAS postdoctoral fellow.

The strategy outlined by McElroy, Lu and their colleagues involves the combination of two forms of green energy: gasification of coal and bioenergy and carbon capture and storage.

Bioenergy is one of the most important tools in the carbon-negative toolbox.

Bioenergy comes from the best CO₂ scrubbers on the planet – plants. As most of us have learned in elementary school, plants use photosynthesis to convert CO₂ organic carbon and oxygen. Carbon stored in plants can be reconverted into energy by combustion (ka, fire); fermentation, as in the production of ethanol; or through a process called gasification, which converts carbon-rich materials into carbon monoxide, hydrogen, and carbon dioxide for fuels and industrial chemicals.

Process of converting biomass into energy, then capturing and storing CO waste₂ is one of the most discussed strategies for carbon negative energy. This is what is called BECCS, bioenergy with carbon capture and storage. The problem is that in most applications, BECCS is not very efficient and requires huge amounts of land to cultivate the plants needed to feed the planet, which would likely lead to global shortages of food and feed. water.

But what if there is a way to make the process more convenient and efficient?

Lu, McElroy and their international team turned to an unlikely solution for green energy: coal.

"If you're trying to do that with biofuel alone, it's not very effective," McElroy said. "The addition of coal provides a source of energy that is really important.If you combine a biofuel with coal and gasify the mix, you can basically develop a pure source of hydrogen in the process."

By modeling different biofuel / coal ratios, the researchers found that if at least 35% of the mixture was biomass and residual carbon was captured, the power generated would actually reduce CO₂ in the atmosphere. At this level, the researchers found that the discounted cost of electricity would not exceed 9.2 cents per kilowatt hour. A carbon price of about $ 52 per ton would make this system cost competitive compared to current coal-fired power plants in China.

A key element of this strategy is the use of crop residues – leftover plants after harvesting fields – as a biofuel.

Seasonal agricultural fires, when farmers fire their fields to clean stubble after a harvest, are a major source of air pollution in China. The collection of this stubble and its use as biofuels would not only reduce CO₂ but significantly improve the air quality in the country. Gasification also makes it easier to remove air pollutants from the waste stream.

The researchers recognize that it will take time to develop a system to collect biomass and deliver it to power plants, but they claim that the system does not need to be implemented simultaneously. .

"Because we have studied the full range of coal / biomass ratios, we have shown how China could progressively move towards an increasingly carbon-neutral energy source," said Chris P. Nielsen, Executive Director. of the Harvard-China project and co-author of the study. "First, small amounts of biofuel could be used to reduce positive net carbon emissions, then the system could become carbon neutral and eventually become a carbon negative system, and you do not have to do it all at once. departure. "

"This study provides crucial information to policy makers looking to implement negative carbon energy opportunities in China," Lu said.