What is the working principle of natural water circulation in CFB boilers?

　　The CFB boiler water circulation system consists of a steam drum, downcomer, lower header and riser (ie water wall tube group), and upper header, forming a circulation loop. Because the riser absorbs heat inside the furnace, part of the water becomes steam, and the density of the steam-water mixture is low, while the downcomer is not heated outside the furnace, it is filled with water and the working fluid has a high density, the steam-water mixture flows upwards along the riser through the upper header and enters the steam drum. The water in the steam drum flows into the lower header through the riser and enters the water wall, thereby forming a natural circulation of water.