An evaporation cycle of a natural circulation steam generator. An evaporator is in flow connection with a downcomer pipe and includes a first evaporative section and a second evaporative section connected in parallel with the first evaporative section and arranged at a higher level than the first evaporative section. The evaporator cycle is not associated with another external source of motive force than heat from the gas flow to assist the flow of the water in the evaporator, and the evaporator has a vertically extending outlet collector for collecting the steam and water from the first and second evaporative sections to the riser pipe. The outlet collector includes a lower portion and an upper portion above the lower portion. The first evaporative section is in direct flow connection with the lower portion and the second evaporative section is in direct flow connection with the upper portion.

A pressurized fluidized bed combustor (PFBC) and method of operation. A heated diluent is used alone or in combination with adjustments to a combustor gas velocity, to manage the bed temperature and keep it within allowable ranges. The diluent can be taken from the combustor flue gas, and recycled and reheated through the fluidized bed.

Proposed is a heat pipe unit and a waste heat recovery boiler including the same, wherein a heat pipe unit includes a plurality of heat pipes provided inside a casing part of a waste heat recovery boiler and a plurality of connection pipes connecting the plurality of heat pipes to each other. The heat pipes are disposed in a plurality of rows along a direction in which the exhaust gas flows. The connection pipes include a plurality of first connection pipes disposed in oblique directions and connecting the plurality of heat pipes disposed in corresponding front and rear side rows, and a plurality of second connection pipes connecting the plurality of heat pipes disposed in corresponding front and rear side rows and disposed not to be parallel with the plurality of first connection pipes.