Method of generating steam in a forced-through-flow boiler

A method of generating steam in a forced-through-flow boiler, said method including the steps of:

• during operation, conveying water through a preheater and an evaporator, whereby for a forced-through-flow boiler operated with a flow of gas from which waste heat is to be extracted, said evaporator is divided into at least two sections, wherein when viewed in a direction of flow of said gas from which waste heat is to be extracted, a first one of said evaporator sections is supplied with water at a beginning of a start-up phase and is included in a water circulation, while downstream from said first evaporator section when viewed in said direction of flow of said waste heat gas, a second one of said evaporator sections, starting from a dry state, is gradually supplied with water in a controlled manner until, in an operating state, water successively flows through both of said evaporator sections;
• at least during said start-up phase, conveying a water-stream mixture from said first evaporator section to a separation tank, from which steam is withdrawn and separated feed water is supplied to said preheater, which is disposed downstream from said second evaporator section when viewed in said direction of flow of said waste heat gas;
• and
• disposing a three-way valve in a connecting line between said preheater and said evaporator, with said three-way valve having a branch line that acts as a bypass line and leads to said first one of said evaporator sections.