Seven years have passed since the world's first low mass flux vertical tube once-through furnace was put into operation by Doosan Babcock at the YaomengThermal Power Plant Unit 1 in China. That boiler replaced a boiler of another design that had become unreliable. The operating experience with the Posiflow design has been so positive that the owner has since ordered and commissioned a replacement for Unit 2's boiler. Here's what makes this furnace design unique.