Blast furnace, - wherein the blast furnace has a shaft-like furnace wall which encloses a process chamber in which iron ore is smelted to form iron, - wherein the furnace wall has, on its circumference, openings via which hot blast is fed to the process chamber, - wherein the furnace wall is surrounded by a ring line in which the hot blast is conducted, - wherein branch lines extend from the ring line, which branch lines end at the openings of the furnace wall, - wherein at least the branch lines are composed of a steel pipe which is lined on the inside with a fire-resistant material, - wherein injection lances end in the region of the openings, by means of which injection lances coal dust is added to the hot blast, such that the coal dust is burned within the process chamber, - wherein the branch lines are assigned sensor devices for observing the combustion of the coal dust, - wherein the sensor devices each have a detection device by means of which the combustion of the coal dust added to the hot blast can be observed, - wherein the sensor devices each have a transmission device which is connected in terms of data transmission to the respective detection device and by means of which data relating to the combustion observed by means of the respective detection device can be wirelessly transmitted, - wherein the sensor devices each have a thermoelectric converter which is thermally coupled to the respective branch line and which generates electrical energy from the temperature difference between the respective branch line and the surroundings of the blast furnace, by means of which electrical energy the respective sensor device is supplied with electrical energy, and - wherein the steel pipe of the respective branch line has a recess in a partial region, and the thermoelectric converter is arranged in the region of the recess, wherein the fire-resistant material has, in the region of the recess, a depression such that a resulting residual thickness (d') of the fire-resistant material in the region between the thermoelectric converter and the hot blast conducted in the respective branch line is smaller than a normal thickness (d) that the fire-resistant material has in the remaining region, wherein, in the region of the recess, there is arranged an intermediate element which is connected in gas-tight fashion to the steel pipe, and in that the thermoelectric converter makes contact with the intermediate element. - wherein the blast furnace has a shaft-like furnace wall which encloses a process chamber in which iron ore is smelted to form iron, - wherein the furnace wall has, on its circumference, openings via which hot blast is fed to the process chamber, - wherein the furnace wall is surrounded by a ring line in which the hot blast is conducted, - wherein branch lines extend from the ring line, which branch lines end at the openings of the furnace wall, - wherein at least the branch lines are composed of a steel pipe which is lined on the inside with a fire-resistant material, - wherein injection lances end in the region of the openings, by means of which injection lances coal dust is added to the hot blast, such that the coal dust is burned within the process chamber, - wherein the branch lines are assigned sensor devices for observing the combustion of the coal dust, - wherein the sensor devices each have a detection device by means of which the combustion of the coal dust added to the hot blast can be observed, - wherein the sensor devices each have a transmission device which is connected in terms of data transmission to the respective detection device and by means of which data relating to the combustion observed by means of the respective detection device can be wirelessly transmitted, - wherein the sensor devices each have a thermoelectric converter which is thermally coupled to the respective branch line and which generates electrical energy from the temperature difference between the respective branch line and the surroundings of the blast furnace, by means of which electrical energy the respective sensor device is supplied with electrical energy, and - wherein the steel pipe of the respective branch line has a recess in a partial region, and the thermoelectric converter is arranged in the region of the recess, wherein