Abstract: In a boiler, a space is reserved to expand the boiler by increasing the front and rear walls. To expand the boiler, wall additions are added to increase the width of the front and rear walls. After the expansion, the distance between the front and rear walls is substantially the same distance between the front and rear walls before the expansion.

Abstract: In a boiler, a space is reserved to expand the boiler by increasing the front and rear walls. To expand the boiler, wall additions are added to increase the width of the front and rear walls. After the expansion, the distance between the front and rear walls is substantially the same distance between the front and rear walls before the expansion.

Abstract: An otherwise conventional recovery boiler for burning black liquor requires less capital investment and takes up less space by constructing the economizers in a particular manner. At least first and second economizers are provided for preheating feed water utilizing flue gas from combustion of the black liquor. The economizers are positioned substantially side-by-side, in the width dimension of the recovery boiler furnace, and connected in such a way that flue gas leaving the first economizer passes in a substantially transverse horizontal direction and then through a substantially vertical intra-economizer flow channel into the second economizer. A boiler bank may be connected between the superheater section of the recovery boiler and the first economizer, and any number of economizers may be utilized.