An accident took place when starting a boiler after a long shutdown period in a power plant (2x 312 MW). The injection of coking gas supplying the boilers put out the flame of the propane igniter. Since the flame surveillance camera was out of control, the roundsman on duty did not see that the flame was extinguished and tried to get the camera running. The operations supervisor did not observe that the propane ignition sequence has been abnormally shortened due to the absence of an alarm. With the available information, the staff thought that the coking gas injection command failed to work and decided to light on another burner. The coking gas already injected into the boiler formed a pocket that exploded when the second burner was ignited. No casualties have been reported but there was significant material damage especially in the boiler and its vicinity. The other segment was not damaged. After the accident was analysed, several malfunctions were observed in addition to the ones already mentioned: absence of flame that did not trigger the closing of the coking gas supply as not adapted to cold engine start (shunt by technician), gas flow rate recorder stayed at ‘0’, switch not in the correct “gas” position. Following the accident and its feedback, several measures have been implemented on an organisational and technical level: ignition using fuel and not exclusively gas, surveillance camera mandatory conditioning start up, automatic cut off of propane and coking gas supply upon defects in the flames on both propane and coking gas burners at the same time.