The tested fuel was burned sep. as well as after addn. of Fe/Mg catalyst (50 ppm) in the boiler burners under air excess (1.23-1.52). The O-2 concn. in exhaust gases was detd. by electrochem. method and the concn. of remaining gases by non-dispersive IR spectroscopy. The combustion efficiency was detd. according to the relevant std. The use of a catalyst resulted in reduction of CO emission and an improvement of the boiler efficiency irrespective of the amt. of the air excess used.