He said police responded to the explosion immediately.
Another witness, who gave only his last name, Qing, said there was no one around the man when he set off the bomb.
"It sounded like the sound of big firecrackers," said Qing, who was about 60 meters (66 yards) away from the explosion. "We couldn't see really what happened afterward. There was a lot of smoke and the police arrived very fast."
CCTV identified the man in the wheelchair as Ji Zhongxing, born in 1979 and from the eastern province of Shandong.
It was not immediately clear why the man allegedly set off the bomb, but Xinhua said he was stopped from handing out leaflets airing his complaints before setting off the bomb. Xinhua, which cited an initial police investigation, did not say what his complaints were.
In an online blog entry from 2006, a writer who provides the same name, the same year of birth and the same hometown in Shandong province as Ji complained of his disabilities resulting from cruel beatings by security personnel in the southern province of Guangdong. He said he had been seeking justice and compensation, but to no avail.
"Because we are peasants and we are poor, no one is willing to help us," the author wrote in the entry, which was the latest one in the blog.
The blog, which was available immediately after the explosion, was removed within hours, and it was not possible to verify its authenticity.
Photos posted by CCTV on its microblog showed the area near the arrivals exit empty and filled with smoke. One photo showed medical staff and police officers gathering at one spot, with a wheelchair sitting on its side a few steps away.
Reached by phone, the airport's news office said it was not aware of the explosion, and airport police declined to answer questions.
Terminal 3, which opened in 2008 just ahead of the Beijing Olympics, is the airport's hub for international flights. United Airlines and American Airlines are among the carriers that operate out of the terminal.
United Airlines spokeswoman Karen May in Chicago said the explosion did not affect its operations. American Airlines could not be reached immediately for comment.
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Associated Press TV producer Aritz Parra contributed to this report.