China is testing use of giant chimney to combat urban pollution

In Xian, one of China's most polluted cities, a giant 60 meter chimney located among high-rise buildings is actually an outdoor air-purifying system that filters out noxious particles and emits clean air into the atmosphere.

The system, currently being tested as a solution to China's growing city air pollution crisis, is solar powered. The Chinese scientists who designed the prototype claim that the system could cut the level of air pollution in urban areas in China and elsewhere.

Xian

Since the 1990's Xian has participated in the economic revival of inland China in the northwest region. It has emerged as an important industrial, cultural and educational center for the northwest region. It has facilities for research and development, national security and the space exploration program.

In 2015 the city had a population of 8,705,600 but the metro area had 12.9 million.

Results of the XIan project so far are promising

The technology used in the project has excited Chinese researchers who are looking for solutions to the challenge of heavy pollution in Chinese cities. Early results are yet to be published but Cao Junji, a chemist at the Chinese Academy of Sciences' Key Laboratory of Aerosol Chemistry and Physics in Xian said those results look promising.

An American atmospheric scientist Donald Wuebbles at the University of Illinois said: “This is certainly a very interesting idea. I am not aware of anyone else doing a project like this one.”

Proposal made for another tower 300 meters tall

The prototype was built with $2 million funding from the provincial government.

The project was visited by Bai Chunli the president of the Chinese Academy of Sciences last month. He said that Chinese leaders are eager to find new innovative solutions to the widespread pollution in Chinese cities as it is a widespread public health problem. A 2015 study found that pollution contributed to 1.1 million premature deaths in China just in that one year.

Cao submitted a proposal for another 300-metre high tower in Xian and also is negotiating proposals with cities in Guangzhou, Hebei and Henan.

The stack effect of the chimney

The system looks and works as follows:"The concrete chimney sits atop a large open structure with a glass roof. Solar radiation hitting the glass heats the air, causing it to rise towards the base of the tower. The air then passes through a wall of industrial filters before billowing out the chimney. The system is inspired by renewable-energy power plants that generate electricity from solar heat."

Renaud de Richter, a chemical engineer at the Ecole Nationale Superieure de Chimie de Montpelier who himself has worked on solar energy towers said that the prototype was both well-designed and well-made. Richter said that Cao's success could convince investors to support other applications that are based on the flow of solar-powered air through chimneys. The technology is known as solar updraft.

Tests of the system

Cao first tested the air filters of the system during two weeks in January when pollution was at its peak. He placed monitors at the tower and ten monitoring stations over a 10 square kilometre area. They measure particulate matter under 2.5 micrometer (PM2.5) in diameter, a type of pollution in many Chinese cities.

Cao found that the tower was expelling between 5 and 8 million cubic meters of filtered air each day. The surrounding air showed a decrease of 19.5 percent of PM2.5 concentrations when compared with monitors in other parts of the city.

More larger chimneys are needed

Since the impact was just local Cao is proposing arrays of about half a dozen larger chimneys that would be distributed in different areas of cities. He also designed a much larger 500 meter high tower. Only multiple systems will have a significant effect and reduce air pollution Cao claims.

Critics of the system

Neil Donahue, of Carnegie Mellon University in Pittsburgh Pennsylvania agrees that pulling large volumes of air through high efficiency particulate filters would clean the air. However, he is concerned about the benefits as compared to the costs: "I would like to see an assessment of the power and resource use for the filtration." He claims that turning the power used into clean electricity or not emitting the pollution in the first place could also attain the goal of reducing pollution.

Wuebbles of the University of Illinois also worried that the filters take out only particulate matter and not precursors to it such as sulfur dioxide gas and nitrogen oxides. Both are also dangerous to human health. Wuebbles said the sky may look cleaner but the air quality could still be quite bad.

In reply, Cao noted that the system did already remove nitrogen oxides. Cao also said concerns about costs were exaggerated with the costs of running the pilot project about $30,000 a year.