Research Shows Carbon-Negative Power Generation can Reduce Atmospheric CO2 Levels in China

The Paris Climate Agreement has specified that global temperature increases must be limited to 2º above preindustrial levels. If that is the case, then it is going to take more than a changeover to carbon-neutral energy sources like solar and wind.

Such a transition calls for carbon-negative technologies, such as energy sources, that essentially reduce the levels of carbon dioxide (CO₂) gas in the atmosphere.

Many climate activists and researchers concede that carbon-negative solutions will be required to fulfill the goal set in Paris, but to date, a majority of these solutions have been perceived as unfeasible in the near term, particularly for China, which is a major, coal-dependent country.

Now, a team of researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Harvard-China Project on Energy, Economy and Environment, in association with coworkers from Tsinghua University in Beijing and other institutions in Australia, the U.S., and China, has examined economic and technical feasibility for China to shift toward the generation of carbon-negative electric power.

The study has been reported in the Proceedings of the National Academy of Sciences.

This paper is making a bold suggestion that not only can China move toward negative carbon power, but that it can do so in an economically competitive way.

Michael McElroy, Study Senior Co-Author and the Gilbert Butler Professor of Environmental Studies, Harvard John A. Paulson School of Engineering

The system we describe not only offers a carbon-negative alternative to generate electricity in the long run, but also brings significant near-term co-benefit to reducing air pollution in China.

Xi Lu, Study First Author and Associate Professor, School of Environment, Tsinghua University

In addition, Lu is a former SEAS graduate student and postdoctoral fellow.

The strategy devised by Lu, McElroy, and their coworkers involves combining two types of green energy—carbon capture and storage and coal-bioenergy gasification.

Bioenergy is considered as an essential tool in the carbon-negative toolbox and it originates from plants—the best carbon dioxide (CO₂) scrubbers on Earth. It is a well-known fact that photosynthesis is used by plants to change CO₂ gas into organic oxygen and carbon. This carbon, which is preserved in plants, can be changed back into energy through various processes like fermentation, as in ethanol production; combustion (also known as fire); or a process called gasification. The latter changes carbon-rich materials into hydrogen, carbon monoxide, and CO₂ for industrial chemicals and fuels.

One of the most frequently discussed approaches for negative-carbon power is the process of changing biomass into energy and subsequently capturing and preserving the waste CO₂. This strategy is called BECCS, short for bioenergy with carbon capture and storage. However, one major issue is that in a majority of applications, BECCS is not a highly efficient process and needs huge amounts of land to grow the required number of plants to power the planet, which can possibly lead to shortages of food and water across the world. But, what if a method is available to render the process more efficient and viable?

McElroy, Lu, and their international group turned to coal, which is an improbable solution for green energy.

If you try to do this with biofuel alone, it’s not very effective. The addition of coal provides an energy source that is really important. If you combine biofuel with coal and gasify the mixture, you can essentially develop a pure source of hydrogen in the process.

Michael McElroy, Study Senior Co-Author and the Gilbert Butler Professor of Environmental Studies, Harvard John A. Paulson School of Engineering

The investigators modeled varied ratios of biofuel to coal and eventually discovered that as long as waste carbon is captured and the mixture contains at least 35% of the biomass, the power produced would essentially lower CO₂ levels in the atmosphere. At that ratio, the team noted that the levelized price of electricity would not be higher than 9.2 cents for each kilowatt hour. Moreover, a carbon cost of about $52 per ton would render this system cost-competitive with existing coal-fired power plants located in China.

The use of crop residue as a biofuel is a major component of this approach. Crop residue is the remains of plants after the fields have been harvested.

In China, one of the main sources of air pollution is seasonal agricultural fires, wherein farmers burn their fields to clear stubble following a harvest. If that stubble is gathered and utilized as biofuel, it would reduce CO₂ levels in the atmosphere and, at the same time, would also enhance the air quality in China. In addition, gasification makes it easier to remove air pollutants from the waste stream.

The scientists admitted that it will take time to develop such a system to gather the biomass and send it to power plants; however, they debated that the system does not necessarily have to be applied immediately.

Because we’ve investigated the whole range of coal-to-biomass ratios, we’ve demonstrated how China could move incrementally toward an increasingly carbon-negative energy source. First, small amounts of biofuel could be used to reduce the net-positive carbon emissions. Then, the system could grow toward carbon neutrality and eventually to a carbon-negative system. You don’t have to accomplish everything from the get-go.

Chris P. Nielsen, Study Co-Author and Executive Director, Harvard-China Project

This study provides critical information for policymakers seeking to implement carbon-negative energy opportunities in China.

Xi Lu, Study First Author and Associate Professor, School of Environment, Tsinghua University

The study was co-authored by Liang Cao, Haikun Wang, Wei Peng, Jia Xing, Shuxiao Wang, Siyi Cai, Bo Shen, and Qing Yang; lead author Xi Lu and three other co-authors based in China are alumni of the Harvard-China Project. It was partly supported by a grant from the Harvard Global Institute.