China’s diesel and gasoline exports surged in the first half of the year as a domestic supply glut and slowing demand growth prompted refiners to sell more fuel abroad.  Diesel shipments jumped almost 21 percent in the first six months compared to the same period a year ago, averaging about 328,500 barrels a day, according to Bloomberg calculations based on data posted Sunday on the website of the General Administration of Customs. Gasoline exports rose 8.1 percent, averaging nearly 222,000 barrels a day.  China’s state-run fuel makers have sent more fuel overseas to draw down stockpiles that have swollen thanks to a refining capacity glut and higher production from independent refiners, known generally as teapots. Meanwhile, the nation’s gasoline and diesel demand growth has been slowed by alternative transportation such as shared bicycles, as well as gas-fed vehicles and electric cars, according to ICIS China, a Shanghai-based commodity researcher.  “Alternative transportation has taken a notable toll on consumption of traditional fuels this year,” Lin Jiaxin, an analyst with ICIS China, said before the data were released. “With new refining units coming online in the second half of the year, refiners will have to ship even more overseas.”