The boiler (1) has side tubed walls (2) enclosing an inner space (3) and a device for selective non catalytic reduction (7). The device for selective non catalytic reduction (7) has a lance (8) carrying a hose (9) having at least a nozzle (10) and a hose drive mechanism (11) for driving the hose within the lance. The lance (8) protrudes into the inner space (3) from a side tubed wall (2) of the boiler (1).