Combustible impurities in oxygen-containing offgases are burnt according to a method and by an apparatus of the type in which at least some of the heat of combustion is recovered by a regenerative heat exchange in two identical heat exchange zones (10,11) containing a solid heat exchange material and separated by a combustion chamber (15). The air or gas to be purified flows through both of the heat exchange zones and by means of valves (1,2,3,4) the direction of flow is changed periodically so that the two zones are alternately heated and cooled in periods of 0.1-60 minutes. The risk of discharge of unburnt combustible contaminants to the atmosphere is minimized by dividing the purified gas stream in the first 1-50% of each period into two part streams of which one is discharged directly from the combution chamber (15) to a recipient (22) whereas the other is passed through the heat exchange zone (10 or 11) being heated and from there recycled through a line (25 or 24) controlled by a valve (7 or 6) and combined with unpurified gas being passed to the heat exchange zone (11 or 10) being cooled.
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