UNIVERSITY PARK, Pa. — Researchers say burning a mixture of coal and crop residue biomass might provide a cost-effective, net carbon-negative electricity source that can be scaled to commercial levels in China in order to meet global temperature objectives by mid-century.

According to The United Nations Framework Convention on Climate Change, the Paris Agreement’s main goal is to strengthen the global response to the threat of climate change by keeping the global temperature rise this century below 2 degrees Celsius above pre-industrial levels. Many scenarios have been investigated for meeting this goal, but a common feature across all of them is that large-scale application of carbon-negative technologies, especially bioenergy with carbon capture and storage, will be necessary.

“This is really the common consensus in the field,” said Wei Peng, assistant professor in environmental engineering and international affairs at Penn State. “Until now, there have not been that many negative-emissions technologies that are being deployed on a commercial scale.”

Peng and her colleagues believe that an integrated gasification cycle system combined with carbon capture and storage (CCS) would be one of the most viable net carbon-negative technologies in certain regions around the world, especially in China, the world’s top carbon emitter. The method burns coal and crop residue biomass together using a gasifier (coal and biomass co-combustion with CCS, or CBECCS), which creates a clean stream of carbon dioxide that can then be captured and stored in deep geological formations.

In order to further investigate this technology, the researchers evaluated the cost performance, carbon mitigation potential and air-quality benefits of deployment of CBECCS systems using crop residues in China. Based on simulations of the CBECCS systems using Aspen Plus, energy flow and carbon footprints were evaluated. The team then assessed the cost competitiveness compared with coal-powered plants under various carbon prices. In addition, they measured the air-quality benefits of deploying CBECCS systems in mainland China based on the projected scale of future plant additions, which utilizes about 24.3 percent of available crop residues.

“Based on China's situation, we identified under which circumstances we can achieve zero carbon emissions, both in terms of direct emissions during the electricity production process as well as lifecycle emissions,” Peng said.  

The researchers found that if the crop residue ratio was greater than 35 percent, CBECCS systems could generate electricity with net-zero lifecycle emissions. Second, when the carbon price reached $52 per ton, net-zero CBECCS systems became economically competitive compared with traditional coal-fired power plants. Finally, deployment of CBECCS systems can significantly reduce air pollutant emissions and improve air quality.

While CBECCS systems currently have relatively high costs compared to traditional coal-powered plants, the team reported that air pollution concerns in China provide an additional incentive for early deployment and may facilitate long-term cost reduction as learning progresses.

“China is in a unique position to demonstrate and deploy negative carbon emissions for a few reasons,” Peng said. “Sixty to 70 percent of power generation still comes from coal in China, so it would be incredibly challenging to tell China they need to stop using coal immediately. With this technology, China can use coal and biomass simultaneously, but gradually over time, transition to more biomass and less coal, which will provide a smoother transition into more environmentally-friendly electricity generation.”

Peng said there are still several challenges to overcome with this technology. Logistically, researchers have to figure out plausible locations suitable to deploy these plants since they require both coal and crop residue biomass. Additionally, current electricity market and carbon pricing in China do not provide the economic incentives to favor base-load, low-carbon electricity generated from CBECCS because electricity prices are not managed through real-time pricing and day-ahead markets like in the U.S. The team hopes that once the markets catch up, this issue will resolve itself.

“Negative emissions technologies are important, and they're also understudied,” Peng said. “When you look at The Intergovernmental Panel on Climate Change report, you'll see that in order to achieve 1.5 degrees, we almost definitely need negative emission technology by mid-century, and that's not too far away.”

Peng’s next steps include follow-up analyses in China as well as working with colleagues to better understand the role of negative-emissions technology in the U.S. to achieve decarbonization goals.

This work, published in April in Proceedings of the National Academy of Sciences, was supported by the National Key R&D Program, National Natural Science Foundation of China, State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Collaborative Innovation Centre for Regional Environmental Quality, State Key Joint Laboratory of Environment Simulation and Pollution Control, and Volvo Group in a research project of the Research Center for Green Economy and Sustainable Development, Tsinghua University, and by a grant from the Harvard Global Institute.

Additional researchers include Xi Lu, School of Environment, Tsinghua University; Liang Cao, School of Environment, Tsinghua University and School of Chemical Engineering, The University of Queensland; Haikun Wang, School of the Environment, Nanjing University; Jia Xing, School of Environment, Tsinghua University; Shuxiao Wang, School of Environment, Tsinghua University; Siyi Cai; School of Environment, Tsinghua University; Bo Shen, Lawrence Berkeley National Laboratory; Qing Yang, Department of New Energy, Science and Engineering and School of Energy and Power Engineering, Huazhong University of Science and Technology, John A. Paulson School of Engineering and Applied Sciences, Harvard University; Chris P. Nielseni, John A. Paulson School of Engineering and Applied Sciences, Harvard University; and Michael B. McElroy, John A. Paulson School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University.