A small Massachusetts start-up has signed a landmark agreement to do a big job in China—it will take already mined coal from the Gobi desert, convert it into natural gas, and transport the gas to the country’s rapidly expanding urban centers. The effort can help China slake its insatiable thirst for energy without exacerbating air pollution. Investors also say it’s the kind of massive clean-tech project that couldn’t have been developed or financed in the U.S. GreatPoint Energy on Friday announced its $1.25 billion partnership with industrial conglomerate China Wanxiang Holdings to build a large-scale plant that should produce about 0.5% of China’s projected energy needs by 2015, according to the companies. GreatPoint’s technology captures carbon dioxide that might otherwise be released into the air during conversion of coal into natural gas. The company then sells it to oil businesses, which use it to improve the yield of already drilled wells.