In which type of boiler you can find water walls?

A waterwall in a boiler is any furnace wall that is comprised of refractory and tubes in a water-tube boiler. The tubes are evenly spaced and generally connect to headers which in turn are connected back to the main generating bank of the boiler. They serve a two fold purpose here in that they capture heat that would otherwise escape to the atmosphere and help heat the water in the boiler. And they also help keep the refractory cool (relatively speaking) This prevents spalling of the refractory.