BEIJING — On a typical day, the air above China’s big cities is so thick it seems chewable-a hazy gray soup that gets its consistency and color from the soot belched by the biggest clump of coal-fired power plants on the planet.

To a growing number of U.S. entrepreneurs, that gray looks like green: the color of money.

These Americans, ranging from startup inventors to big-utility chief executives, see China as a testing ground for technologies that burn coal more cleanly-technologies they hope one day to deploy around the world. They’re banking on a market for a new generation of cleaner coal-fired power plants in the United States many years from now, when a sizable chunk of the nation’s aging coal-fired power plants is likely to wear out and need replacing. But they see little appetite from U.S. investors or policymakers for the risk involved in trying to scale up these technologies today.

So, like all ambitious peddlers of newfangled widgets, they’re going where the buyers are: in this case to China, which over the past decade has become the biggest coal burner and carbon-dioxide emitter on Earth. They’re having a wild ride. But whether they’ll clean up coal on the […]